Wear and degradation analysis of light management nanofabricated layers

Outubro de 2024 Marisol Farinha Assis Wear and degradation analysis of light management nanofabricated layers Outubro de 2024 Marisol Farinha Assis Wear and degradation analysis of light management nanofabricated layers Dissertação de Mestrado Mestrado em Engenharia Mecânica Área de Especialização em Manufatura Avançada Trabalho efetuado sob a orientação do: Professor Doutor Pedro Filipe Lima Marques Professor Doutor Vítor Emanuel Rebelo Lopes i DIREITOS DE AUTOR E CONDIÇÕES DE UTILIZAÇÃO DO TRABALHO POR TERCEIROS Este é um trabalho académico que pode ser utilizado por terceiros desde que respeitadas as regras e boas práticas internacionalmente aceites, no que concerne aos direitos de autor e direitos conexos. Assim, o presente trabalho pode ser utilizado nos termos previstos na licença abaixo indicada. Caso o utilizador necessite de permissão para poder fazer um uso do trabalho em condições não previstas no licenciamento indicado, deverá contactar o autor, através do RepositóriUM da Universidade do Minho. Licença concedida aos utilizadores deste trabalho Atribuição-NãoComercial CC BY-NC https://creativecommons.org/licenses/by-nc/4.0/ ii ACKNOWLEDGEMENTS The completion of this dissertation requires the acknowledgement of those who contributed to its achievement, as well as the success of my entire academic journey. My determination has been crucial throughout this five-year journey, but I certainly owe my gratitude to those who constantly supported me. Firstly, I would like to express my deep appreciation to my supervisors, Professor Doctor Filipe Marques and Professor Doctor Vítor Lopes, for the constant support and guidance throughout the completion of this project. I am grateful for all the knowledge transmitted during the course of this work and for having provided me with an unsurpassed encouraging environment. This experience has truly led me to become a more complete engineer. I would also like to express my gratitude to Professor Doctor Pedro Salomé, for providing me with all the necessary conditions for the completion of this work, and for constantly investing in my training during my stay at the Nanofabrication, Optoelectronics and Energy Applications (NOA) group at the International-Iberian Nanotechnology Laboratory (INL). I am truly honored by the opportunity to work side by side with some of the greatest minds of our generation and to be able to witness the development of cutting-edge technology. I would like to extend my gratitude to all members of the NOA group, for the constant support, namely to Enzo, for guiding me during the cleanroom nanofabrication processes. Your dedication was fundamental for the completion of this work. My profound appreciation is extended to my parents, my sister and my grandmother. It may be a cliché to say I could not have done it without you all, but it is the truth. I am forever thankful for the constant support, for providing me with the best education and for being the perfect role models. Lastly, I would like to thank Diogo. Your brilliance, insightfulness and companionship have undoubtedly contributed to the success of this work. I am indefinitely grateful to the INL for the opportunity to get to know someone like you. iii STATEMENT OF INTEGRITY I hereby declare having conducted this academic work with integrity. I confirm that I have not used plagiarism or any form of undue use of information or falsification of results along the process leading to its elaboration. I further declare that I have fully acknowledged the Code of Ethical Conduct of the University of Minho. University of Minho, 30th of October 2024 iv ABSTRACT Wear and Degradation Analysis of Light Management Nanofabricated Layers The growing global energy demand and crescent search for fossil fuel independence has prompted the development of sustainable energy sources. Photovoltaic technology stands out as a promising solution to meet this demand, with production costs being constantly mitigated by the development of increasingly thinner solar cells, which aim to reduce the use of raw materials and increase production. However, this thickness reduction leads to incomplete light absorption, triggering the need to implement light management strategies. One of these strategies consist of adding nanoscale texturization on the top surface of solar cells, such as a bio-inspired moth-eye pattern, which confers broadband, omnidirectional and angle-independent anti-reflection properties to the surface. This strategy involves the exposure of the nanostructures to external conditions, as they will be present at the top surface of solar modules. Thus, a tribological characterization of this layer is required to quantify the wear and degradation mechanisms. Hence, this study focuses on performing a tribological characterization of silicon moth-eye nanostructured surfaces, obtained through nanoimprint lithography, in a cleanroom environment. This study was conducted using a ball-on-plate configuration for the friction tests and a spherical 5 mm radius counterface, made of bearing steel. The surfaces were tested for two loads, 0.1 N and 0.5 N, as well as for various test cycles (from 1 to 50 cycles). The results of these experiments show that the friction curves possess a repetitive morphology for both loads, being possible to detect the pre-sliding and sliding phases. The higher load tests present an overall longer pre-sliding phase, in comparison with the lower load tests, due to the more mechanically aggressive interaction, which leads to the creation of an intermediate rough surface and delays the polishing mechanism. It is also evident from the chemical analysis of the samples that the presence of iron on the surfaces promotes adhesive mechanisms, resulting in an overall higher friction coefficient for the 0.5 N tests. This evidence only appears after 15 cycles for the surfaces tested with 0.1 N, a point from which friction values begin to rise. Through optical microscopy it was found that the width of the test track was larger for 0.5 N tests and increased with the number of cycles for both loads. Moreover, an additional study conducted on two types of moth-eye structures with similar optical performances was performed, being concluded that shorter connected pillars, which have a smaller roughness, showcase lower friction than taller individual pillars. KEYWORDS FRICTION COEFFICIENT, LIGHT MANAGEMENT STRATEGIES, SOLAR CELLS, TRIBOLOGICAL CHARACTERIZATION, WEAR MECHANISMS v RESUMO Análise do Desgaste e Degradação de Camadas Nanofabricadas para Gestão de Luz A crescente procura global por energia e pela independência dos combustíveis fósseis tem impulsionado o desenvolvimento de fontes de energia sustentáveis. A tecnologia fotovoltaica destaca-se como uma solução promissora para atender a essa procura, sendo que os custos de produção têm diminuído com o desenvolvimento de células solares mais finas, levando à redução do uso de matériasprimas. Contudo, a diminuição de espessura leva a uma absorção incompleta de luz, sendo necessário implementar estratégias de gestão de luz. Uma dessas estratégias consiste na adição de nano-estruturas na parte superior das células solares, tais como padrões inspirados nos olhos de traças ( moth-eye ), que conferem propriedades de anti-reflexão em banda larga, omnidirecionais e independentes do ângulo à superfície. Esta estratégia implica que as nano-estruturas estejam expostas a condições externas, já que estarão presentes na superfície superior. Portanto, é necessário efetuar uma caracterização tribológica, de modo a quantificar os mecanismos de desgaste e degradação. Assim, este estudo foca-se na realização de uma caracterização tribomecânica de superfícies de silício com nano-estruturas moth-eye , obtidas por litografia por nanoimpressão, em ambiente de sala limpa. Para os testes de atrito, utilizou-se uma configuração de esfera sobre placa e um contra-corpo esférico de aço de rolamento, com 5 mm de raio. As superfícies foram testadas para duas cargas, 0,1 N e 0,5 N, bem como para vários ciclos de teste (de 1 até 50 ciclos). Os resultados mostram que as curvas de atrito possuem uma morfologia repetitiva para ambas as cargas, sendo possível detetar as fases de pré-deslizamento e deslizamento. Os testes com carga maior apresentam uma fase de pré-deslizamento mais longa, devido a uma interação mecânica mais agressiva que forma uma superfície rugosa intermédia e atrasa o fenómeno de polimento. A análise da composição química revelou a presença do mecanismo de adesão, tendo sido detetado ferro nas superfícies, justificando os valores de atrito superiores para os testes de 0,5 N. As superfícies testadas com 0,1 N revelam a presença de ferro apenas após 15 ciclos, ponto a partir do qual os valores de atrito aumentam. Através de microscopia ótica, constatou-se que a largura da pista de teste foi maior para 0,5 N e aumentou com o número de ciclos para ambas as cargas. Foi também realizado um estudo adicional em dois tipos de estruturas moth-eye com desempenhos óticos semelhantes, concluindo-se que pilares conectados e mais curtos possuem menor atrito do que pilares individuais mais altos. PALAVRAS-CHAVE CARACTERIZAÇÃO TRIBOLÓGICA, CÉLULAS SOLARES, COEFICIENTE DE ATRITO, ESTRATÉGIAS DE GESTÃO DE LUZ, MECANISMOS DE DESGASTE vi TABLE OF CONTENTS Acknowledgements ......................................................................................................................... ii Abstract ........................................................................................................................................ iv Resumo ......................................................................................................................................... v Table of Contents .......................................................................................................................... vi List of Figures ..............................................................................................................................viii List of Tables................................................................................................................................ xii List of Symbols .............................................................................................................................xiii Nomenclature .............................................................................................................................. xv 1. Introduction............................................................................................................................ 1 1.1. Motivation ..................................................................................................................... 2 1.2. State of the art .............................................................................................................. 5 1.3. Objectives ................................................................................................................... 11 1.4. Structure of Dissertation............................................................................................... 11 2. Nanoscale texturization in Solar Cells ..................................................................................... 13 2.1. Solar Cells .................................................................................................................. 13 2.2. Light Management ....................................................................................................... 14 2.3. Nanoimprint Lithography .............................................................................................. 15 2.4. Characterization Techniques......................................................................................... 18 3. Frictional Characterization ..................................................................................................... 23 3.1. Friction Coefficient ....................................................................................................... 23 3.2. Stick-slip phenomenon ................................................................................................. 24 3.3. Pre-sliding and sliding Behavior .................................................................................... 25 3.4. Theoretical Friction Models........................................................................................... 27 4. Wear Characterization ........................................................................................................... 31 vii 4.1. Adhesive Wear............................................................................................................. 32 4.2. Abrasive Wear ............................................................................................................. 33 4.3. Tribochemical Wear ..................................................................................................... 34 4.4. Surface Fatigue ........................................................................................................... 35 5. Experimental Characterization ............................................................................................... 37 5.1. Test Description .......................................................................................................... 37 5.2. Optical and Morphological Characterization ................................................................... 39 5.3. Tribological Characterization......................................................................................... 43 5.4. Structural-Tribological Correlation.................................................................................. 54 6. Concluding Remarks ............................................................................................................. 59 6.1. Summary and Conclusions ........................................................................................... 59 6.2. Contributions............................................................................................................... 62 6.3. Future Work ................................................................................................................ 63 References .................................................................................................................................. 64 Appendices ................................................................................................................................. 76 Appendix A: Characterization of the Moth-eye Texturization ............................................................. 77 Appendix B: Roughness Calculations ............................................................................................. 80 Appendix C: Optical Microscopy Measurements ............................................................................. 87 Appendix D: Pre-sliding Measurements .......................................................................................... 88 Wear and degradation analysis of light management nanofabricated layers xiv ε1 - Strain to the failure in one loading cycle ξ m Critical thickness of the reaction layer ρ m Thickness of the reaction layer θ rad Angle between conical asperity and surface σ0 N/m Bristle stiffness σ1 N.s/m Damping coefficient Wear and degradation analysis of light management nanofabricated layers xv NOMENCLATURE AFM Atomic Force Microscopy AR Anti-reflection ASL Anti-sticking Layer CA Contact Angle CdTe Cadmium Telluride CG Coarse-grained CIGS Copper Indium Gallium Selenide CIS Copper Indium Selenium CVD Chemical Vapor Deposition EDS Energy Dispersive X-ray Spectroscopy GaAs Gallium Arsenide GNS Gradient Nanostructured GNG Gradient Nanograined HFW Horizontal Field Width IPS Intermediate Polymer Stamp ME Moth-Eye NG Nanograined NIL Nanoimprint Lithography NT Nanotubes OM Optical Microscope PP Percentual Point PV Photovoltaic Sccm Standard Cubic Centimeters per Minute SD Standard Deviation SEM Scanning Electron Microscopy UV-vis Ultraviolet-Visible Wear and degradation analysis of light management nanofabricated layers 1 1. INTRODUCTION The world has been facing an escalating energy crisis, driven by not only industrialization, but also population growth. The aggravation of this crisis has become more prominent as years go by, through an exponential increase of energy consumption worldwide during the last century, in which fossil fuels have the largest share, as shown in Figure 1.1. The alignment of this constant need with the crescent search for fossil fuel independence has triggered the development of alternatives such as renewable energies. These alternatives consist of innovative approaches, based on the harvesting of energy through natural resources, such as water, sunlight, wind and biomass, which aim to not only tackle economic challenges, but also the pressing environmental issues. Figure 1.1 - Global primary energy consumption by source [1]. According to the Energy Institute [1], the share of total power generation by renewable energy sources increased from 29% in 2022, to more than 30% in 2023, as shown in Figure 1.2. In this scope, photovoltaic technologies have stood out, having been developed solar cells able to reach conversion efficiencies of 26.41% [2]. In order to convert solar energy into a more sustainable and economically viable option, it is imperative to reduce the use of rare materials in solar cells. This necessity has led to the development of increasingly thin technologies, which suffer from incomplete light absorption, that is, an increased reflectance. Wear and degradation analysis of light management nanofabricated layers 2 Figure 1.2 - Global electricity generation in 2023. Adapted from [1]. 1.1. MOTIVATION The decrease of the observed reflectance in these developing solar cells requires the comprehension of the optical phenomena which take place. When light passes from one medium to another, a portion of light is reflected at the interface due to the refractive index between two media, a phenomenon known as Fresnel reflection [3-4]. The mitigation of this phenomenon is fundamental to the performance of optoelectronic devices and can be achieved through the implementation of light management strategies [5-6], which are crucial to increase the number of photons absorbed in the active layer [7]. One of these strategies consists of implementing sub-wavelength surface structures, which have a graded refractive index profile. This gradual variation of spatial effective refractive index from air to a substrate significantly reduces the Fresnel reflection [3]. Henceforth, the broadband and omnidirectional anti-reflection (AR) characteristics of graded index nanostructures consist of an effective light management strategy. Researchers have explored novel AR strategies to implement on the front contact of solar cells, having taken inspiration in nature, namely on certain moth species, which possess subwavelength structured arrays of tapered pillars on their eyes and wings. The addition of this moth-eye pattern as nanoscale texturization has been proven to promote large-angle light refraction, as opposed to a flat surface, enabling a prolonged optical path length [7-13]. In addition to moth-eye, other types of Wear and degradation analysis of light management nanofabricated layers 3 nanostructures have also been studied, such as nanocones [14-15], nanospheres [16-17], nanowires [18-19] and so forth [20-21]. Although these structures also revealed an attractive AR performance, its fabrication has proven to be costly. Other AR strategies have also been researched previously, such as the implementation of coatings with broadband and omnidirectional characteristics. The thickness of the AR layer is typically designed to a quarter of wavelength, making the phase difference of the incident light half a wavelength. Thus, reflection is suppressed through destructive interference [22]. Chhajed et al [23] presented a study in which it is shown that the reflection losses are reduced to 5.9% by applying a threelayer graded-index AR coating to silicon, which typically reflects around 37% of incident radiation. However, the effectiveness of these coatings is dependent on not only the wavelength, but also on light’s incidence angle. This indicates that if the incident light is oblique, the AR effect will also be weakened due to the change of the light’s travel path [22]. Furthermore, AR coatings rely on chemical or physical deposition processes, which increase production costs [24-25]. Hence, nanostructures with broadband light trapping capabilities constitute an improved light management approach, capable of significantly suppressing the front-surface reflection [22,26]. The presence of moth-eye nanostructures at the front contact of solar cells implies its exposure to external conditions, such as environmental stresses, but also to mechanical loads, which can decrease its performance. Therefore, incorporating tribological testing into the evaluation of these architectures is a valuable approach to ensure that both optical and mechanical requirements are achieved. Tribological tests can assume various configurations, such as pin-on-disk, ball-on-diskballon-plate and pin-on-plate, as seen in Figure 1.3. Figure 1.3 - Illustration of tribological testing configurations: pin-on-disk (on the top left), ball-on-disk (on the top right), ballon-plate (on the bottom left) and pin-on-plate (on the bottom right). Wear and degradation analysis of light management nanofabricated layers 4 The pin-on-disk configuration consists of a fixed pin and a rotating disk, resulting in a circular test path, with the frictional and tangential force being measured by the load cells and sensors. In this configuration, the loading force is applied on the pin, while the disk is driven by a servo motor with a certain rotational speed. The ball-on-disk configuration is similar to the previous one, with the main difference being the use of a ball instead of a pin, as the name suggests. The ball-on-plate and pin-onplate configurations are also alike, both being designed for reciprocating sliding motion tests. Thus, the ball or pin performs an alternating linear movement back and forth over the plate. As seen in literature, bio-inspired texturizations have also proven to be tribologically beneficial, as they exhibit a decrease in adhesion and friction forces due to the reduction of the real area of contact, as well as to their hydrophobicity [27-30]. This last feature occurs due to the fact that the nano-patterns seem to act as composite surfaces, with air being trapped between the pillars, leading to the increase of the water contact angle [29]. The tribological characterization of nanostructures has been reported in previous works. Afshar-Mohajer et al [31] has reported the effects of the employment of micro/nanohierarchical structures, with the aim of gaining new insights regarding their mechanical behavior. This study mainly regarded the coupling between the mechanics and deformations of different lengths scales, as well as the frictional response of the structures before and after the start of sliding. This study focused on squared cross-section micro-pillars with “nano-hair” at its top base, which were obtained through a two-photon lithography process, as explained in the study of Afshar-Mohajer et al [31]. The tribological tests showed that the no-hair structures presented the highest friction, whereas the short-hair structures had the lowest friction values. The friction values of the long-hair structures were in between both previous structures. The deformation resistance of the structures has revealed that the bending was more severe for the no-hair and long-hair structures, in comparison with the short-hair structures, which only presented minor bending. These differences uprise from the fact that the actual contact area at the interface depends on the nano-hair structure. The short-hair structure provides a smaller area of contact, compared to that of the no-hair structures, due to the spacing between the short hairs. The long-hair structures can undergo buckling, resulting in an increased area of contact between the buckled hairs and the counterface, in comparison with the short-hair structure. From the investigation performed by Afshar-Mohajer et al [31], it is possible to conclude that for applications where a low friction is desired, short-hair nanostructures are desirable, as they presented not only the lowest bending for the highest applied load, but also the lowest friction values, in comparison with no-hair and long-haired surfaces. Wear and degradation analysis of light management nanofabricated layers 5 Another study which highlights the tribological advantages of the implementation of nanostructures was conducted by Singh et al [29]. In this study, a tribological characterization of bio-inspired lotus-leaf silicon surfaces is conducted. The tested surfaces underwent textural and chemical modifications, being posteriorly submitted to a normal load of 0.3 N through a ball-on-disk test configuration. This investigation has revealed that the implementation of these nanostructures is accompanied by low friction properties, as well as high wear durability, due to the reduction of the surface energy and the real contact area. The investigation proposed in this dissertation aims to evaluate the tribological properties of silicon moth-eye nanostructures, obtained through nanoimprint lithography. The frictional characterization is performed under specific conditions in laboratorial context and is complemented by advanced characterization techniques, such as scanning electron microscopy (SEM) and ultraviolet-visible (UV-vis) spectroscopy. 1.2. STATE OF THE ART During the last decade, the tribological characterization of nanostructures has gained particular attention. The continuous development of strategies to improve wear and confer particular frictional properties to materials and surfaces has prompted tribological investigations in various areas, such as physics, chemistry, surface engineering, nanotechnology, biomedical engineering, and so forth [32]. Considering the aim of this dissertation, a brief review of research performed in the scope of tribology is provided, with particular focus on the addition of nanoscale texturization and its influence on tribological properties. This review of academic papers was conducted through the use of search platforms such as Scopus and Google Scholar , utilizing the keywords “tribology” and “nanostructures” to search within article titles, abstracts and keywords. The results from this research are exposed in Table 1.1, which contains the main conclusions from each article, as well as the respective tribological testing parameters. The selected articles were published from 2008 to 2022 and are displayed in alphabetical order of each material’s name. Additionally, it is presented the type of nanostructure and the respective materials. In an attempt to offer a broad overview of this research, a statistical analysis is performed. This investigation focused on the identification of trends regarding the tribological testing of nanostructured surfaces. Thus, several parameters were chosen for analysis, such as the type of test configuration and the shape of the nanostructures, as well as the material employed. This statistical analysis is presented through 2D pie charts to better illustrate and identify the trendy behaviors in tribological testing. Wear and degradation analysis of light management nanofabricated layers 6 Table 1.1 - State of the art regarding tribological testing of nanostructures. Material Nanostructure Frictional testing Main conclusions Year Ref Alumina Nanograins Thickness: 117 - 241 μm Equipment: Tribometer Configuration: Pin-on-disk Counterface: 0.5 mm radius silicon nitride pin Normal load: 5, 10 and 15N Enhanced friction and wear resistance with nanograins - Lower friction coefficient (approximately 0.9 times lower) - Lower wear (up to 1.84 times lower) 2013 [33] Aluminatitania Nanograins (cauliflower-like agglomerates) Size: 10 - 500 nm Equipment: Tribometer Configuration: Ball-on-disk Counterface: 3 mm radius 100Cr6 steel ball Normal load: 5 N Enhanced friction resistance with nanograins - Lower friction coefficient (up to 0.8 times lower) - Higher wear resistance (up to 3 times higher) 2014 [34] Carbon Nanocrystals Size: 1-2 nm Spacing: 0.336 nm Equipment: Tribometer Configuration: Pin-on-disk Counterface: 0.5 mm radius silicon nitride pin Normal load: 400 mN Enhanced friction resistance with nanocrystals - Lower friction coefficient (µ=0.07) and wear (0.04 µm3) - Increased hardness 2012 [35] Carbon / polyimide Multiwalled nanotubes (NT) Equipment: Tribometer Configuration: Ball-on-disk Counterface: 1.5 mm radius bearing steel (GCr15) ball Normal load: 10 N Enhanced friction and wear resistance with NT - Higher weight percentage of NT improves tribological properties - Lower friction coefficient (2.3 to 10.8 times lower than without NT) - Improved wear resistance (smaller wear scar) - Lower wear rate (up to 18 times lower) 2020 [36] Wear and degradation analysis of light management nanofabricated layers 7 Coppersilver alloy Nanograins (Nanograined (NG), coarsegrained (CG) and gradient nanograined (GNG) structures) Average grain size: 20 μm Equipment: Tribometer Configuration: Ball-on-plate Counterface: 5 mm radius WCCo ball Normal loads: 30 to 90 N Enhanced friction and wear resistance for GNG - Lower friction coefficient for 90 N (1.88 times lower than NG and CG) - Lower wear volume (4.8 and 10.4 times less than NG and CG, respectively) - Lower wear rate (4.7 and 10.3 times less than NG and CG, respectively) - Unaltered surface roughness after tests 2016 [37] Silicon Micro and nanogrooves Depth: 80 nm Width: 2.6 μm Equipment: Tribometer Configuration: Ball-on-plate Counterface: 3 mm radius Stainless steel ball Normal load: 60 mN - Micro and nanofriction decreased as groove area density increased - Improved wear resistance (nearly indistinguishable scratch) 2010 [38] Silicon Nanogroove array Width: 50 nm Periodicity: 125 nm Equipment: Atomic force microscope (AFM) Cantilever: Flat squared (2 µm x 2 µm) tip Enhanced friction resistance with nanopattern - Friction coefficient of patterned area is approximately half - Linear dependence of the lateral force on the applied load - Highest load provokes non-negligible wear effect 2008 [39] Silicon Random nanotextures Equipment: Tribometer Configuration: Ball-on-disk Counterface: 0.8 mm radius 100Cr6 steel ball Normal load: 250 mN Enhanced friction and wear resistance with nanotextures - Lower friction coefficient (up to 2.5 times lower than flat samples) - High dependence of friction coefficient on roughness parameters - Improved wear resistance (no wear scars detected) 2011 [40] Wear and degradation analysis of light management nanofabricated layers 8 Silicon Low-spatial frequency laser induced periodic nanostructures Height: 230 nm Period: 730 nm Equipment: Nanotribometer Configuration: Ball-on-plate Counterface: 1.5 mm radius polytetrafluoroethylene balls Normal load: 5, 10 and 25 mN Enhanced friction resistance with nanostructures - Lower friction coefficient (which decreased with increasing load) - Friction coefficient decreased with testing time 2019 [41] Steel Gradient nanostructured surfaces (GNS) and coarse-grained (CG) surfaces Thickness: 340 -530 nm Equipment: Tribometer Configuration: Ball-on-plate Counterface: 5 mm radius WCCo ball Normal load: 30 to 90 N Enhanced friction and wear resistance for GNS - Lower friction coefficient for 50 N (1.2 times lower) - Decrease of friction with increasing load - Lower wear volume for 30 N (4 times lower than CG) - Lower wear volume for 90 N (3.4 times lower than CG) - Smooth worn surface - Accommodation of large plastic strain, due to finer grains 2018 [42] TiMe1Me2CN/TiAlSiN Nanostructured superlattice coatings Thickness: 6.5, 5.2, 5.5 μm Equipment: Tribometer Configuration: Ball-on-disk Counterface: 3 mm radius 100Cr6 steel ball Normal load: 3 and 5 N Enhanced friction and wear resistance with nanostructured coating - Lower friction coefficient (up to four times lower) - High wear resistance (up to 28 times more resistant) 2022 [43] Titanium dioxide Nanotubes Diameters: 30, 50, 70 and 100 nm Equipment: Nano-indenter Counterface: 100 nm radius three-sided pyramidal tip Normal load: 500 µN Enhanced friction resistance with nanotubes - Lower friction coefficient (for smaller nanotubes) - Higher deformation resistance for smaller-diameter nanotubes 2018 [44] Wear and degradation analysis of light management nanofabricated layers 15 the reduction of the absorber layer, which causes an efficiency gap in comparison to standard thin-film solar cells. Moreover, solar cells are subject to optical losses that result from shading by metallic grids and parasitic absorption in other layers which constitute the solar cells [7]. These optical losses can also be tackled with the implementation of light management schemes. There are different approaches, which can be divided in two categories: front light management and rear light management. In summary, front light management consists of the application of an AR coating or nanoscale architectures at the top surface, to achieve broadband, omnidirectional and angleindependent AR [12]. Rear light management is implemented to increase the optical path length in the absorber by enhancing light scattering and reflectivity at the back contact interface [7]. Contrary to front light management, rear light management strategies can differ according to the type of solar cell. In this work, only front light management strategies will be considered, which can be implemented regardless of the type of solar cells. These strategies focus on minimizing the reflection at the top surface, with the most common AR strategy being the use of an AR coating. Although this strategy allows a conversion efficiency increase of 1 percentual point (PP) [7], it also has some downsides, such as being only effective in a narrow-wavelength value range, and performing optimally for certain light incidence angles [25]. These disadvantages led to the testing of nanoscale texturization, which enables an improved and broadband AR effect, due to the multiple interactions with the rough surface before being reflected [58,59]. Various patterns have been researched, namely an architecture referred to as moth-eye (ME), which consists of a bio-inspired array of closely tapered nanopillars, whose AR results from a gradually changing refractive index as light travels from air to the bottom of the structure [60], as mentioned in Chapter 1.1. These structures are considered to be a homogeneous medium, in accordance with the effective medium theory, which states that if the period of the features on a textured surface is much smaller than the incident wavelength value, the wavefront interacts with the structure as a homogeneous surface, meaning the nanopillars/air interface is perceived as one material [61]. Since the ME structure consists of tapered nanopillars, with more material being present towards the bottom, the effective refractive index increases as light travels further down the structure, reducing reflection drastically and allowing more light to enter the solar cell [62]. 2.3. NANOIMPRINT LITHOGRAPHY Nanoimprint lithography (NIL) is a contact lithography process in which a master stamp is pressed into a resist layer, conferring the stamp’s reverse pattern by physical deformation, and with nanoscale resolution [63]. This method requires certain elements, such as a master stamp, containing the moth- Wear and degradation analysis of light management nanofabricated layers 16 eye pattern, an anti-sticking layer (ASL), an intermediate polymer stamp (IPS), a photoresist, and a silicon substrate, to which the pattern will be transferred [64]. The implementation of an anti-sticking layer is justified due to the criticality of the demolding step, to ensure the pattern’s quality. Since the nanoscale features increase adhesion forces between the stamp and the resist layer, it is imperative to confer antiadhesive properties to this interface [65]. The implemented ASL in this study consists of a teflon-like C4F8 polymer. The IPS is implemented to avoid direct contact between the master stamp and the samples, enhancing its lifetime. Additionally, the IPS is associated to a lower cost, higher mechanical flexibility, and high resolution [64]. The fabrication of the master stamp containing the moth-eye pattern involves the use of an anisotropic silicon substrate, to which is applied a gold nanoparticles mask for a posterior etching process, as illustrated by Figure 2.2. The procedure starts with the cleansing of the silicon substrate on a microwave plasma asher (PVA TePla Gigabatch 360M system), with 500 W 2.45 GHz plasma, with 600 standard cubic centimeters per minute (sccm) 𝑂2 and 50 sccm argon (𝐴𝑟) flow, followed by the deposition of gold (step 1 in Figure 2.2) by sputtering [66] [67] on a coating system (100 W DC Kenosistec UHV PVD system), at room temperature. Then, a solid-state thermal dewetting [68-70] of the deposited gold film is performed (step 2) in an oven (Termolab Chamber Furnace Type MLM), at 800 ºC for 2 h, creating the desired mask. Lastly, a reactive ion etching is performed (step 3) for 40 and 60 s, with a simultaneous C4F8 and SF6 plasma. The remaining gold nanoparticles are removed utilizing a mix of hydrochloric acid with nitric acid (HCl:HNO3), on a 3:1 ratio. Figure 2.2 - Illustration of the master stamp fabrication process. The pattern transferring process is described in Table 2.1 and Figure 2.3 and starts with the cleansing of the master stamp, which is achieved on the microwave plasma asher (PVA TePla Gigabatch 360M system), with 500 W 2.45 GHz plasma, with 600 sccm O2 and 50 sccm Ar flow, followed by the deposition of a 5 nm SiOx layer on the master stamp by plasma-enhanced chemical vapour deposition (CVD), using the SPTS MPX CVD system at 200 ºC, in a 30 W 13.56 MHz plasma with 1420 sccm N20, 10 sccm SiH4, and 392 sccm N2. This thin layer aids the posterior demolding step, due to its antiadhesive properties, and is considered part of the ASL. Nevertheless, the ASL is completed with a final Wear and degradation analysis of light management nanofabricated layers 17 layer of 8 nm of the Teflon-like C4F8 polymer (step 1 in Figure 2.3), which is deposited on the stamp through an adapted reactive ion etching equipment (SPTS Pegasus), with a 2000 W 13.56 MHz plasma, 0 W platen power, 200 sccm C4F8 flow at 20 ºC, and 25 mTorr. Table 2.1 - Overview of the moth-eye nanofabrication process. Step Process Equipment 1 Cleaning the silicon substrate PVA TePla Gigabatch 2 Gold deposition by sputtering Kenosistec UHV PVD system 3 Thermal dewetting Termolab Chamber Furnace Type MLM oven 4 Reactive ion etching SPTS Pegasus 5 Cleaning the master stamp PVA TePla Gigabatch 6 Deposition of anti-sticking layer SPTS MPX CVD system + SPTS Pegasus 7 Transferring of pattern to IPS Obducat Eitre 8 8 Cleaning silicon substrate Ultrasound bath 9 Dehydration of silicon substrate Vapour Prime YES-310TA oven 10 Spin-coating the substrate with resist Suss Microtec Gamma Photoresist cluster system 11 Patterning of the resist layer Obducat Eitre 8 12 Removal of residual resist SPTS Pegasus 13 Reactive ion etching SPTS Pegasus 14 Removal of remaining resist PVA TePla Gigabatch After the deposition of the ASL, the master stamp’s pattern is transferred to the IPS with the aid of a simultaneous thermal and UV (STU) NIL system on the Obducat Eitre 8 equipment (step 2), at a constant pressure of 15 bar for 60 s, at 22 ºC, followed by 240 s of UV exposure to cure the IPS. Then, the patterned IPS was coated with an 8 nm ASL (step 3), and the silicon substrate was cleansed with an ultrasound bath for 15 min on acetone, and for 10 min on isopropyl alcohol and 5 min on de-ionized water. Next, the silicon substrate’s surface was dehydrated in a vacuum, in an oven (Vapour Prime YES310TA), at 150 ºC, for 300 s. This was performed as a surface treatment to promote adhesion of the posterior resist layer. This step was followed by the spin-coat of a resist layer on the substrate, which consisted of 250 nm mr-Nil diluted in ma-T 1045 on a 1:2 weight ratio. This was performed in a Suss Microtec Gamma Photoresist Cluster system, at 2000 rpm for 30 s, and baked at 90 °C for 60 s. Wear and degradation analysis of light management nanofabricated layers 18 The next step is the patterning of the resist layer through NIL (step 5), which consists of heating up the substrate and the IPS at 65 ºC for 10 s, applying constant pressure of 15 bar for 100 s, and exposing to UV for 180 s, to cure the resist. This process is performed on the Obducat Eitre 8 equipment. After this, it is necessary to remove the residual resist (step 6), having been chosen to utilize an O2 plasma, for 12 s, at 20 ºC on a 85 mTorr 13.56 MHz plasma, with a coil power of 25000 W and 180 ºC platen power, and an O2 flow of 115 sccm on the SPTS Pegasus equipment. Lastly, the substrate was submitted to a Si deep reactive ion etching (step 7) on the SPTS Pegasus equipment, at 20 ºC, using a 13.56 MHz plasma, at a pressure of 15 mTorr with 2000 W coil power, 45 W platen power, 160 sccm C4F8 and 120 sccm SF6 flow. This process was followed by the removal of the remaining resist performed by an O2 plasma ashing process, with 500 W 2.45 GHz plasma, 600 sccm O2 and 50 sccm Ar flow, on the PVA TePla Gigabatch 360M system, for 8 min. Figure 2.3 - Schematic of the fabrication process of the moth-eye nanostructures. 2.4. CHARACTERIZATION TECHNIQUES Prior to conducting a tribological characterization, the moth-eye nanostructures must undergo a morphological and optical characterization, in order to compare the results. Hence, it is necessary to define a systematic characterization protocol which allows an accurate analysis of the obtained data. The optical characterization is performed after the nanofabrication of the moth-eye structures, to ensure that the optical requirements are achieved. These optical measurements are performed through UV spectroscopy, which allows the study of the interaction between matter and electromagnetic radiation. During this interaction, several processes can occur, such as absorbance, reflectance, scattering, photochemical reactions, and also fluorescence [71]. Since light is a form of energy, its absorbance provokes the energy increase of matter’s molecules, which can origin transitions between different Wear and degradation analysis of light management nanofabricated layers 19 electronic energy levels, depending on the specific wavelength of the incident light [72]. As seen in Figure 2.4, the UV-vis spectrophotometer contains several components such as a light source, with the ability to generate a broadband of electromagnetic radiation across the UV-visible spectrum, a dispersion device which separates broadband radiation into wavelengths, detectors to measure the intensity of the reflected radiation, and a sample area [73]. In this investigation, the optical performance will be obtained through the reflectance measurements, performed for a wavelength range of 300 nm to 1100 nm, and a slit width of 2 nm. All measurements are in relation to a baseline and were performed on a PerkinElmer Lambda 1050+ equipment. The resulting data from this technique requires correction, which is achieved through the multiplication between a correction factor and the relative total reflectance obtained from the measurements. The correction factor consists of the division between the literature values of the reflectance for silicon, as this is the material which constitutes the samples, and the experimental data. Figure 2.4 - Schematic of the UV-vis spectrophotometer [73]. The morphology of the nanostructures is analyzed through a scanning electron microscope (SEM NovaNano 650). This characterization technique allows the study of a sample’s external morphology, and chemical composition, as well as its crystalline structure and orientation of the materials of which it is composed. This is possible through a focused beam of high-energy electrons which generate a variety of signals at the surface of solid specimens as a result of the electron-sample interactions [74-75]. As illustrated in Figure 2.5 (A), the SEM is composed of an electron source, electron lenses, detectors, sample stage, display, and data output devices [76]. The infrastructure requirements of this equipment are a power supply, a vibration-free floor, a vacuum system, and a room free of ambient magnetic and electric fields. When introducing the samples in the equipment, they must be attached to a support wafer with copper tape, as shown in Figure 2.5 (B), and later placed on the sample stage of the equipment. Through this technique, the moth-eye nanostructures can be analyzed before and after tribological testing. Wear and degradation analysis of light management nanofabricated layers 20 Figure 2.5 - (A) Illustration of the SEM constituents [76] and (B) Samples attached to a wafer prior to SEM analysis. The SEM allows for an additional method, denominated energy dispersive X-ray spectroscopy (EDS), which enables the analysis of the chemical composition of the samples. EDS is a technique based on the measurement of the energy and intensity of the X-rays which are emitted by a sample when exposed to the electron beam of the SEM. The energy of the emitted X-rays is unique for each element, thus enabling the assessment of the sample’s chemical composition [77]. In addition to SEM and SEM-EDS, the tested sample surface was also analyzed through optical microscopy (OM) using a Leica DM8000 M microscope for inspection of the tested area, enabling the measurement of the track widths. Another technique which can be utilized to analyze the morphology of samples is the atomic force microscopy (AFM) (Bruker AFM Dimension Icon Atomic Force Scanning Probe Microscope). This technique is a high-resolution form of scanning probe microscopy that is frequently used to visualize small-scale structures, at a scale ranging from below a nanometer, up to one micrometer [78-79]. Additionally, AFM can also measure electromechanical strain, chemical structure, adhesion, and electrical conductivity. This method scans a 3D profile of the surface by measuring any type of force detected by a sharp tip, due to its proximity to the surface of the sample [80-81], provoking a deflection of an elastic cantilever, which is connected to the sharp tip. Through the measurement of this deflection, it is possible to estimate the interactive forces between the tip and the surface of the sample, such as the interatomic and electromagnetic forces. A simplified illustration of the AFM constituents is presented in Figure 2.6. Wear and degradation analysis of light management nanofabricated layers 21 Figure 2.6 - Simplified illustration of the AFM constituents [81]. For the investigation performed in this study, the AFM could not be utilized to characterize the moth-eye-patterned surface, due to the reduced space between the nanostructures, which causes the tip to detect repulsive forces upon approach and, consequently, its retraction, obtaining an inaccurate profile. To employ this characterization technique, it would be necessary to utilize a reduced-size tip. Nevertheless, this method constitutes a valuable characterization tool for nanostructured profiles, with various applications. As a part of the surface characterization, contact angle (CA) measurements have also been performed. This characterization technique provides information regarding surface characteristics, such as wettability, hydrophilicity, surface charge and tension, as well as interaction energy [82]. The CA is known as the angle between a solid/liquid/gas interface on a surface, as illustrated by Figure 2.7, and can be defined as the ability of a liquid to wet the surface of a solid. If the measured angle is between 0° and 90°, then the surface is considered hydrophilic. For angles between 90° and 180°, the surface is considered hydrophobic. Any angle which is close to 180° can be considered ultrahydrophobic. Figure 2.7 - Representation of the contact angle. In this work, the CA measurements are performed with the aim of assessing the hydrophobic features of the moth-eye nanostructures. These measurements are performed on a Drop Shape Analyzer Wear and degradation analysis of light management nanofabricated layers 22 (Kruss DSA100), shown in Figure 2.8, in which a drop is located at the tip of a needle. The measurements are obtained through the recording of an image of the drop on the sample’s surface, which is then transferred to the drop shape analysis software, calculating the CA through a grey-scale analysis of the image, as well as a geometrical model of the drop shape. Figure 2.8 - Drop Shape Analyzer (Kruss DSA100). Wear and degradation analysis of light management nanofabricated layers 23 3. FRICTIONAL CHARACTERIZATION Friction is considered a complex non-linear phenomenon, resulting from the contact and sliding of two surfaces against each other. This phenomenon is dependent on several parameters, such as the surface topography, the contacting materials, the presence and type of lubrication and the relative motion [83]. At the contact interface, a frictional force acts to oppose the sliding motion. All solid surfaces have various irregular asperities, which interlock during the contact between the two surfaces. Based on this phenomenon, it was proposed that the real contact area changes according to the normal load, although the apparent contact area remains the same. This asperity interlocking phenomenon was considered the main cause of friction by earlier researchers, such as Leonardo da Vinci, Amonton and Coulomb [84]. Coulomb presented one of the first friction models, stating that the magnitude of friction is proportional to the normal contact force [85]. This early model depends on the direction of the relative velocity but does not specify a friction force at zero velocity, which can lead to perturbations upon the analysis of a dynamic system’s response. However, this model is still employed to simulate friction behavior, due to its simplicity, as it only requires the coefficient of friction. Various relevant friction models and concepts have been developed in the following years, being presented in the succeeding sub-sections. 3.1. FRICTION COEFFICIENT The friction coefficient, 𝜇, is a dimensionless scalar value that can be described as the ratio of the tangential force between two contacting surfaces and the normal force pressing them together, as defined by the following equation: 𝜇 = 𝐹f 𝐹N ( 1 ) where, 𝐹𝑓 is the frictional force, and 𝐹𝑁 corresponds to the normal force. Two types of friction coefficients can be distinguished: static and kinetic. The static friction coefficient represents the friction opposing the start of relative motion, whereas the kinetic friction coefficient represents the friction opposing the continuance of relative motion once it has started. As a result, the static friction coefficient is larger than the kinetic, with its values depending on the pair of surfaces in contacts. As illustrated in Figure 3.1 friction can be considered as a mechanical system. Upon a close examination of the sliding process, it is Wear and degradation analysis of light management nanofabricated layers 24 verified the occurrence of two friction regimes, a pre-sliding regime and a sliding regime, which will both be explained in the following sub-sections. Figure 3.1 - Representation of the mechanical system formed during the sliding process. 3.2. STICK-SLIP PHENOMENON The stick-slip phenomenon concerns the dynamic interaction between two surfaces and is characterized by the alternation of the friction force of the contact spots between adhesion and slip during the sliding movement [86-87], as illustrated by Figure 3.2. In other words, the sliding surface will remain stationary until the external tangential force reaches a magnitude sufficient to overcome the static friction force. This is an unstable effect which is repeated in rapid succession, being able to cause noticeable low frequency vibration of the slide [88]. For this phenomenon to occur, the system must be flexible enough in order to enable a change in the speed of the sliding body, and also the friction coefficient must be variable [89]. Figure 3.2 - Representation of the stick-slip phenomenon, showcasing the transitions between adhesion and sliding [52]. Wear and degradation analysis of light management nanofabricated layers 31 4. WEAR CHARACTERIZATION Wear can be defined as the surface damage or removal of material from one or both of the contacting surfaces in a system of relative motion [105-106]. This phenomenon is a function of the tribosystem, which is composed by several elements, such as the body, counterbody, interfacial element and environment [107], as illustrated in Figure 4.1. Thus, a material’s susceptibility to wear is dependent on not only its mechanical properties, but also on environmental factors, namely the conditions to which it is exposed. Figure 4.1 - Illustration of the tribo-system. The main properties which influence wear are the sliding speed, hardness, temperature and the material composition. The applied load can also influence wear, as it leads to the increase of the shear force, as well as the frictional momentum, resulting in the increase of the wear rate [108]. The wear rate can be defined as the amount of material per unit of time. The worn material resulting from the sliding between two bodies can be quantified through Archard’s law [109]. This law states that the volume of worn material (𝑉) is proportional to the normal load and the sliding distance (𝐿), and inversely proportional to the material hardness (𝐻), as shown through the following equation: 𝑉 = 𝐾w𝐹N𝐿 𝐻 ( 6 ) in which 𝐾𝑤 denotes a dimensionless constant corresponding to the wear coefficient. Wear and degradation analysis of light management nanofabricated layers 32 There can be various types of wear mechanisms, such as adhesion, abrasion, tribochemical reaction and surface fatigue [110], as illustrated in Figure 4.2, which will be explained in the following sub-sections. Figure 4.2 – Schematic representation of wear mechanisms [107]. 4.1. ADHESIVE WEAR Adhesive wear occurs when the atomic forces between the materials of contacting surfaces under relative pressure outweigh the inherent material properties of each surface [111-112] In other words, adhesion occurs at the asperity contacts at the interface, which are sheared by relative motion [113]. This can result in the detachment of a fragment from one surface and attachment to the other. As sliding continues, loose wear particles may form, as illustrated in Figure 4.3. Figure 4.3 - Representation of the adhesive wear mechanism [87] A detailed list of steps which lead to the formation of adhesive-wear particles is provided below, based on information from [114]: 1. Deformation of the contacting asperities 2. Removal of the surface films 3. Formation of the surface films 4. Failure of the junctions and material transfer 5. Modification of transferred fragments 6. Removal of transferred fragments and creation of loose wear particles The volume of material removed through adhesive wear can be estimated from Archard’s law, Eq. (6). Wear and degradation analysis of light management nanofabricated layers 33 4.2. ABRASIVE WEAR Abrasive wear takes place when the asperities of a hard rough surface slide on a softer surface, provoking damage on the interface due to fracture or plastic deformation. The abrasive wear caused by plastic deformation can occur in various deformation modes, which include ploughing, wedge formation and cutting [115]. There can be two types of abrasive wear, according to the type of contact, two-body abrasion and three-body abrasion. The term two-body abrasion is given when the ploughing is caused by an asperity from the harder material, whereas the term three-body abrasion is conferred when the ploughing occurs through the contact with a wear particle which is trapped at the interface of both contacting bodies [107,116], as shown in Figure 4.4. Figure 4.4 - Schematic of the formation of two and three body abrasive wear [117]. Within the category of abrasive wear, there can be three main mechanisms: plowing, cutting and fragmentation [117-118]. Plowing consists of the displacement of material to the sides, resulting in ridges adjacent to grooves, which may be removed by the passage of abrasive particles. This formation of grooves does not involve direct material removal [118]. Cutting can be defined as the removal of material from the surface in the form of microchips, with little or no material being displaced to the sides of the groves. Lastly, fragmentation is characterized as the separation of material from the surface due to cutting and crack formation, leading to a localized fracture and crack formation around the grooves. This results in an additional material removal by fragmentation, in which the debris are produced in the form of flakes [117]. Wear and degradation analysis of light management nanofabricated layers 34 The volume of material removed during this process (𝑉) can be estimated by considering the diagram from Figure 4.5, in which a hard conical asperity is traversing a softer surface with an angle (𝜃), under the action of a normal load, leading to the following equation: 𝑉 =2tan𝜃 𝜋𝐹N𝐿 𝐻 ( 7 ) Figure 4.5 - Representation of a conical asperity traversing a softer surface under the action of a normal load [114]. Similarly to Eq. (6), this equation constitutes a simplified model, as it only considers hardness as a material property. 4.3. TRIBOCHEMICAL WEAR Tribochemical wear occurs when there is a chemical interaction between the mating surfaces. Chemical reactions induced by friction are mainly influenced by the environment and its active interaction with the materials in contact [119]. Typically, the contacting surfaces react with the environment, creating reaction products which are deposited on the surfaces. Then, these reaction products are removed due to crack formation and abrasion, leading once again to the exposure of the surfaces to environmental damage. This process can lead to an increased reactivity due to increased temperature, accelerating the formation of the reaction product. Moreover, it can also lead to an increased brittleness, as a result of the heavy work-hardening [114]. The amount of material loss due to this wear mechanism (𝑉) can be estimated by the following equation, in which it is considered that the surface layers formed by a chemical reaction are removed from the contact zone after a certain critical thickness [113]: 𝑉 = 𝑘𝑑 𝜉2𝜌2𝐹N𝐿 𝐻 ( 8 ) where 𝜉 is the critical thickness of the reaction layer, 𝜌 is the thickness of the reaction layer, 𝑘 is the velocity factor of oxidation and 𝑑 is the diameter of asperity contact, as illustrated by Figure 4.6. Wear and degradation analysis of light management nanofabricated layers 35 Figure 4.6 - Representation of a conical asperity traversing a softer surface under the action of a normal load [114]. 4.4. SURFACE FATIGUE Surface fatigue wear occurs due to the growth and formation of cracks, being associated with rolling contacts due to the cycling nature of the load [120]. In sliding contacts, the asperities are subject to cycling stress, leading to stress concentration effects, as well as the propagation of cracks. A detailed list of steps which lead to the generation of wear particles is shown below, based on information from [114]: 1. Transmission of stresses at contact points 2. Growth of plastic deformation 3. Crack formation and propagation 4. Creation of wear particles For sliding contacts, the volume of removed material due to fatigue (𝑉) can be obtained through [114]: 𝑉 = 𝐶𝜂𝛾𝜀1 2𝐹𝑁𝐿 𝐻 ( 9 ) where 𝜂 corresponds to the distribution of asperity heights, 𝛾 is a particle size constant, 𝜀1 is the strain to failure in one loading cycle and 𝐶 is the fatigue ductility coefficient. Overall, the analysis of the acting wear mechanisms can provide valuable information for various applications, namely regarding the material selection, the need to improve the design of products and the prediction of failure, aiding the comparison of performance between different materials or designs. For this study, considering the environmental factors involved, one of the key wear mechanisms would be abrasion, as the nanostructured surfaces could be subjected to contacts between particles, such as dust, which can lead to surface degradation, and thus, a possible decrease of their AR properties. In addition, adhesive wear is also a relevant mechanism, as the mechanical stress caused by the interaction between external conditions can lead to an attachment or detachment of material, which could also affect the performance of the nanostructures. Moreover, tribochemical wear could also be an important aspect to Wear and degradation analysis of light management nanofabricated layers 36 consider, as the nanostructured surfaces could be exposed to elements such as high humidity, as well as to pollutants, leading to chemical degradation. However, in this study, the samples will not be subjected to these types of conditions, hence this wear mechanism will not be considered. Wear and degradation analysis of light management nanofabricated layers 37 5. EXPERIMENTAL CHARACTERIZATION The tribological characterization of the moth-eye nanostructures constitutes a valuable approach, as it allows the evaluation of the wear over time, making it possible to verify the existence of significant degradation. The wear of these nanostructures could affect their AR properties, as they depend on the existence of a gradually variating refractive index, provided by the presence of nanostructures, which could be altered by degradation. Moreover, the assessment of the behavior of the structures to wear could provide insights regarding the susceptibility to damage caused by cleaning processes, aiding the selection of appropriate cleaning methods which preserve the surface’s integrity. The adhesion properties of the nanostructured surfaces are also relevant to their cleaning, as it can help identify how strongly contaminants such as dust can adhere to the surfaces, and, consequently, the selection of appropriate cleaning agents and techniques. These adhesion properties can be influenced by the roughness of the surfaces, and thus the selection of adequate cleaning protocols. Factors such as the cleaning frequency could also be analyzed through a tribological characterization, and it could also guide the development of novel nanostructured surfaces, based on specific tribological properties. This chapter presents a detailed description of the testing parameters, namely regarding the utilized equipment, the selected test counterface, the applied loads and the number of cycles. Moreover, the morphological and optical characterization of the nanofabricated samples is showcased, specifically the reflectance spectra of nanostructured surfaces, in comparison with a flat surface, as well as SEM images from the moth-eye structures. In addition, the surface roughness and CA of the nanostructures is characterized. The results of the performed tribological tests are also shown in this chapter, as well as a detailed discussion of the acquired data. 5.1. TEST DESCRIPTION The assessment of tribological properties of the moth-eye structures was achieved through friction testing, on a tribometer (CETR-UMT2, USA), which is shown in Figure 5.1 (A), utilizing a ball-on-plate test configuration, as presented in Figure 5.1 (B). This equipment performs the acquisition of data every 0.01 s and can detect frictional forces as low as 0.001 N. The selected test counterface consists of a 10 mm diameter bearing steel (AISI 52100) sphere, and the resulting data consists of the variation of the friction coefficient throughout the test length. This test counterface was selected, as this material is well characterized tribologically. Additionally, a spherical surface means that there is only one point in which there is contact with the sample’s surface, which facilitates both the characterization and calibration Wear and degradation analysis of light management nanofabricated layers 38 process. For these tests, two loads were applied, 0.1 N and 0.5 N, having been defined various test cycles: 1, 2, 3, 4, 5, 10, 15, 20, 25, 30, 35, 40, 45 and 50 cycles. These test parameters were defined with the purpose of understanding, not only the impact of the applied load, but also the behavior of the structures when exposed to successive cycles. Each test was performed three times, for each load, and cycle has an approximate duration of 17 s and the length of the route performed by the counterface is approximately equal to 15.8 mm. Figure 5.1 - (A) CETR-UMT2 tribometer utilized for testing and (B) Sample positioned on the tribometer for testing. To perform the experimental tests, it was necessary to develop a sample support, ensuring an adequate connection to the equipment, as well as a secure positioning of the samples. An image of the sample support printed in polylactic acid (PLA) is shown in Figure 5.2. As seen on the image, the squared central cavity is used to place the sample, containing two additional cavities to accommodate the user’s fingers when inserting it. In addition, the support contains four symmetrical mounting holes, which allow the adjustment of the connection to the tribometer through bolts and nuts. Figure 5.2 – Design of the sample support used in this work, obtained by 3D printing, Wear and degradation analysis of light management nanofabricated layers 39 5.2. OPTICAL AND MORPHOLOGICAL CHARACTERIZATION The nanofabrication of the silicon moth-eye structures gave place to samples with an appearance as the one presented in Figure 5.3. The square at the center of the samples corresponds to the area with the nanopattern, obtained through NIL processes, which is approximately equal to 25 x 25 mm. The silicon substrate has an area of 50 x 50 mm. Figure 5.3 - Silicon moth-eye sample obtained through NIL. These nanostructured surfaces require a morphological characterization, to ensure that the structures are in compliance with the expected results. This characterization was performed through SEM images of the nanostructured surfaces, as shown in Figure 5.4 (A), (B) and (C), from a top view, and Figure 5.4 (D), (E) and (F), with a 30° tilt. According to the images, the structures consist of randomly arranged pillars, with variable diameters, but roughly with uniform height. Further SEM images of the nanostructures can be found in Appendix A: Characterization of the Moth-eye Texturization. Figure 5.4 - SEM images of Moth-eye structures before tribological testing, (A), (B) and (C) from a top view and (D), (E) and (F) with a 30° tilt. Wear and degradation analysis of light management nanofabricated layers 40 The mean height of the structures was measured through the image-processing software Image J , and five SEM cross-section images, such as the one shown in Figure 5.5, being obtained an average height of 434 ± 32 nm. Figure 5.5 - SEM cross-section image of moth-eye sample utilized for measurements. Subsequent to the nanofabrication of the structures, an optical characterization is required, as the purpose of the implementation of this nanopatterning is the decrease of reflectance. Thus, in order to verify that the optical performance of these architectures is in compliance with the expected results, the total relative reflectance was measured, being presented in Figure 5.6 the results for the total relative reflectance of four samples. The silicon reference refers to a bare silicon sample, without the moth-eye nanostructures. Figure 5.6 - Reflectance spectra of four nanopatterned moth-eye samples and a bare silicon sample. Wear and degradation analysis of light management nanofabricated layers 47 counterface. However, this could not be proved due to the complexity of the roughness calculation of the spherical counterface, as it would have to be performed based on SEM imaging, in which it is difficult to focus on a round surface’s asperities. The friction evolution curves as a function of the test time are presented in Figure 5.13 (A). These curves correspond to the average of three tests performed for one cycle and are representative of the frictional behavior of the moth-eye nanostructured surfaces, as this curve morphology repeats throughout the various test cycles. As can be seen in Figure 5.13 (B), each friction coefficient curve is characterized by two distinct phases: (I) a pre-sliding phase, and (II) sliding phase. The pre-sliding phase is characterized by an initial rapid increase in friction, in which mechanical interlocking of the asperities occurs, giving place to a strong abrasive interaction, due to the relative motion of the contacting surfaces. The curve relative to this phase showcases a relation between its duration and the applied load, having been detected that for a higher load (0.5 N), phase I lasts longer (average of 2.09 s) than for the lower applied load (0.1 N), which lasts for approximately an average of 1.79 s. This is due to a more mechanically aggressive truncating of the asperities’ extremities provoked by a higher load, which delays the beginning of the polishing effect gradually verified on the increasingly longer tests. As the tests continue, the friction coefficient values tend to stabilize, although some small oscillations are verified, possibly due to the random distribution of differently shaped pillars, as seen in the previous section. The pre-sliding phase duration measurements can be seen in Appendix D: Pre-sliding Measurements. Figure 5.13 - Friction coefficient curves, as a function of the test time, (A) for 0.1 N and 0.5 N, (B) illustrating pre-sliding (I) and sliding (II) phases. The sliding phase of the friction coefficient has also been analyzed, through the average friction values of the three tests, with the resulting data being presented in Table 5.3. For this study, it would also be interesting to analyze frictional mechanisms such as the stick-slip phenomenon. This analysis can be Wear and degradation analysis of light management nanofabricated layers 48 performed if the system would have to be flexible enough to enable a change in the velocity of the sliding body, which is the case of this study. However, this evaluation could not be accomplished, as the utilized tribometer does not allow the measurement of the counterface’s velocity during testing, preventing a correlation between the friction coefficient variation and the velocity profile. In addition, it would also be of relevance to analyze further acting mechanisms, such as the Stribeck effect, which manifests as a nonlinear dependence of friction force on the sliding speed, and the frictional lag, which is characterized by a lag in the friction force relative to the sliding velocity. Seen as in the sliding phase the friction force becomes predominantly a function of the velocity, it was also not possible to perform this analysis. Hence, the analysis of the frictional behavior during phase II of the tests was performed through the data presented in Table 5.3. Table 5.3 - Mean friction coefficient values relative to the sliding phase of the tribological tests, for both tested loads (0.1 N and 0.5 N). The friction data resulting from the tests performed with the lower load (0.1 N) reveal that the backward stroke (2 cycles) showcased higher friction values, in comparison with the forward slide (1 cycle). These differences between the forward and backward friction are common observations at the micro and nanoscale [126], and have been reported previously [31]. This phenomenon occurs due to the Cycle number 0.1 N 0.5 N 1 0.42 ± 0.06 0.85 ± 0.22 2 0.48 ± 0.10 0.70 ± 0.09 3 0.47 ± 0.08 0.74 ± 0.09 4 0.40 ± 0.07 0.71 ± 0.08 5 0.41 ± 0.01 0.77 ± 0.09 10 0.44 ± 0.08 0.76 ± 0.07 15 0.56 ± 0.06 0.87 ± 0.13 20 0.48 ± 0.07 0.82 ± 0.08 25 0.52 ± 0.08 0.93 ± 0.12 30 0.48 ± 0.07 0.88 ± 0.08 35 0.52 ± 0.06 0.97 ± 0.06 40 0.48 ± 0.08 0.87 ± 0.06 45 0.51 ± 0.07 0.97 ± 0.05 50 0.50 ± 0.06 1.01 ± 0.04 Wear and degradation analysis of light management nanofabricated layers 49 fracture and deformation of the structures during the first slide, which results in an increased true area of contact during the backward stroke. This is shown in Figure 5.14 (B), in which horizontally stacked fractured pillars can be observed, contrary to Figure 5.14 (A), which mainly shows vertically stacked fractured pillars. Figure 5.14 - SEM cross-section images of samples tested for 0.1 N and (A) 1 cycle and (B) 50 Cycles. When analyzing the following frictional behavior, it was verified a significant increase of the friction coefficient at 15 cycles, approximately equal to 1.3 in comparison with the 10-cycles tests, corresponding to the highest detected friction of the lower load tests. This can be justified by the chemical composition of the sample’s tested area. Through EDS (Figure 5.15 (C) and (D)), it was found that at 15 cycles, the tested surface revealed the presence of oxygen and iron, which arose from the test counterface, in addition to silicon from the substrate. When analyzing the surfaces tested for less than 15 cycles, it was only verified the existence of silicon (Figure 5.15 (A) and (B)). Therefore, it seems that the adhesion mechanism between both sliding surfaces is the main cause for the increase of friction for the lower load tests. It is important to note that EDS is a qualitative method, which depends greatly on the selected point of the sample. Thus, the objective with the employment of this technique in this study was to get an idea of the sample’s composition and not to perform a quantitative study regarding the atomic composition of the samples. The chemical composition of the tests that are longer than 15 cycles also revealed an increase of the adhesion with the number of cycles, as shown in the SEM images contained in Figure 5.15. For these tests, the overall friction coefficient values were higher than the ones obtained previously, presenting average values within the range of 0.50 ± 2. Wear and degradation analysis of light management nanofabricated layers 50 Figure 5.15 - EDS spectrum of moth-eye samples tested with 0.1 N for (A) 1 cycle, (B) 3 cycles, (C) 15 cycles and (D) 50 cycles. A trend in the frictional behavior has been detected throughout the test cycles. As seen in Figure 5.16, it is observed that the friction coefficient increases for the odd cycle numbers and decreases for even cycle numbers. This occurs for both applied loads, except for the 50 cycle tests performed with 0.5 N. The observed phenomenon could be justified by the type of test setup, as a slight tilt of the samples, provoked by an uneven sample support, could have been the cause for the decrease in friction in a certain direction, as there is less contact with the sample. Another possible justification could be due to the fracture of the pillars to same side as the movement is performed on the first passage. As the counterface performs the returning movement, it will have to overcome more resistance from the structures, due to its tilt to the opposite side, hence the repetitive increase and decrease in friction coefficient. Wear and degradation analysis of light management nanofabricated layers 51 Figure 5.16 - Average friction coefficient values for the various test cycles, for a load of 0.1 N and 0.5 N. The higher load tests (0.5 N) reveal a significant increase of the friction coefficient. Contrary to the lower load tests, the 2-cycle tests showcased lower friction than the first stroke (1 cycle). This phenomenon can be justified by the stronger abrasive interaction between the asperities during the 1cycle test, which led to the facture and severe deformation of the nanostructures, as shown in Figure 5.17 (A). This event lead to the delay of the polishing effect, as the worn structures now form an intermediate rough surface. With the increase of the test cycles, a polishing phenomenon becomes progressively prominent, as seen in Figure 5.17 (B), leading to gradually higher friction values, as the real contact area increases with each pass of the sphere. Figure 5.17 - SEM cross-section images with a 10° tilt of samples tested for 0.5 N and (A) 1 cycle and (B) 50 Cycles. Wear and degradation analysis of light management nanofabricated layers 52 When analyzing the chemical composition of the samples, the presence of iron and oxygen was detected for all tested cycles, as shown in Figure 5.18. This further justifies the significant increase in friction, in comparison with the lower load tests. Figure 5.18 - EDS spectrum of moth-eye samples tested with 0.5 N for (A) 1 cycle, (B) 3 cycles, (C) 15 cycles and (D) 50 cycles. A morphological analysis of the tested surfaces revealed that as the number of cycles increases, the adhesion of counterface material became more prominent, as shown in Figure 5.19. It is possible to determine a relation between the number of cycles and the compression of the structures. Through the Figure 5.19 (A), it is possible to observe that the adhesion is mainly located towards the center of the track, with this area presenting a higher visible compression of the structures, manifested through the appearance of plateaus. These plateaus are a result of a self-polishing mechanism, that is, the fractured remains of the structures became trapped in the interstices between the structures. This phenomenon is better illustrated in Figure 5.20 through high magnification images of the structures after (A) 3 cycles and (B) 15 cycles. Wear and degradation analysis of light management nanofabricated layers 53 Figure 5.19 - SEM top view images of moth-eye samples tested with 0.5 N for (A) 3 cycles and (B) 15 cycles. Figure 5.20 - SEM top view images of moth-eye samples tested with 0.5 N for (A) 3 cycles and (B) 15 cycles. As contact and sliding between the structures continued, the surface eventually became smoother, as a result of the compression and subsequent flattening of the pillars, as seen in Figure 5.19 (B) and Figure 5.20 (B). However, with the increase of cycles it was observed that both adhesion and the compression of the structures became more prominent on the sides of the track (Figure 5.19 (B)). Regarding the optical performance after tribological testing, it was not possible to assess the influence of the wear caused by the applied loads and number of cycles on the AR properties of the nanostructured surfaces. This can be due to the fact that the wear track resulting from the tests does not possess a large enough area to perform the optical measurements on the UV-vis equipment. Wear and degradation analysis of light management nanofabricated layers 54 5.4. STRUCTURAL-TRIBOLOGICAL CORRELATION An additional investigation has been conducted into different types of moth-eye structures. During the nanofabrication phase of this project, a calibration of the etching equipment was necessary, which also required a recalibration of the moth-eye pattern nanofabrication. During this process, two new types of structures were obtained, with one consisting of agglomerated pillars, as shown in Figure 5.21, which appear to be shorter than the previously obtained structures, as shown in Figure 5.22. The other obtained structures are morphologically similar to those structures characterized in Chapter 5.2, except for the height, as they appear to be taller than the previous structures. Figure 5.21 - SEM top view images of moth-eye sample (Structure B) for different horizontal field widths (HFW): (A) 30 μm HFW; (B) 15 μm HFW, (C) 5 μm HFW and (D) 2.5 μm HFW. Figure 5.22 - SEM images of a moth-eye sample (Structure B) with a 30° tilt, for (A) a HFW of 15 μm and (B) a HFW of 5 μm. Wear and degradation analysis of light management nanofabricated layers 55 Through SEM cross-section images (Figure 5.23), the heights of the individual (Structure A) and of the agglomerated (Structure B) pillars were measured, obtaining average values of 578 ± 58 nm and 290 ± 34 nm, respectively. These measurements were performed on the image-processing software Image J and were the result of 1400 measurements. Figure 5.23 - SEM cross-section images for a HFW of 5 μm of (A) Structure A and (B) Structure B. An optical characterization was performed to analyze the influence of both types of structures on the total relative reflectance, as shown in Figure 5.24. The structures present very similar optical performances, with Structure B showcasing a slightly better performance, equivalent to 1 PP, in comparison with Structure A. This was a positive finding, as it is predicted that both structures will present very different tribological behaviors, due to their significant morphological differences. Therefore, an ideal structure can be selected, solely based on their tribological performances. Figure 5.24 - Reflectance spectra of moth-eye samples with Structure A and B, in comparison with a bare silicon sample. Wear and degradation analysis of light management nanofabricated layers 56 Regarding the morphological characterization, the average pillar diameter for both structures was analyzed through the software ImageJ and SEM top view images. This analysis was performed through an average of 30 measurements, which are presented in Appendix A: Characterization of the Moth-eye Texturization, and concluded that the average diameter of structures A and B was equal to 163 ± 65 nm and 509 ± 218 nm, respectively. The roughness of both structures was also analyzed through the method described previously in Chapter 5.2. The average values of the roughness parameters are presented in Table 5.4 and represent an average of 30 measurements, which are shown in Table B.1 and Table B.2 of Appendix B: Roughness Calculations. From these results, it is possible to conclude that structure A possesses a larger roughness, in comparison with the other type of structures. The conclusions drawn in Chapter 5.2 regarding the surface roughness have stipulated that a lower roughness could indicate a better wear resistance, in comparison with rougher surfaces. Hence, it could be predictable that structure B will confer better tribological properties to the surface. Table 5.4 - Mean roughness parameters and the respective SD for Structure A and B. Parameter Mean value (nm) ± SD Structure A Structure B 𝑹𝒂 250 ± 15 138 ± 8 𝑹𝒒 278 ± 17 160 ± 11 𝑹𝒕 979 ± 112 614 ± 89 𝑹𝒗 337 ± 60 226 ± 44 𝑹𝒑 641 ± 75 388 ± 59 Regarding the tribological testing, two samples of each type of structure were tested, with a load of 0.1 N, being performed from 1 to 50 cycles, as in the previous tests. The ball-on-plate test configuration was also employed, utilizing a spherical bearing steel (AISI 52100) counterface. The results of these tests are presented in Table 5.5 and show that the friction coefficient is overall lower for structure B than for structure A, signifying that these structures are ideal if lower conditions are desired. These results are in agreement with experiments performed by Afshar-Mohajer et al [31], which have been mentioned in Chapter 1.1. Similarly to the conclusions drawn in this study, the shorter pillar-like structures showcased the lower friction values, in comparison with the taller structures. Wear and degradation analysis of light management nanofabricated layers 63 Lastly, a comprehensive tribological characterization of bio-inspired anti-reflective nanostructured surfaces was performed. For this, a sample holder was developed, as well as a systematic testing procedure, which establishes comparison parameters and enables the analysis of wear and frictional behavior. 6.3. FUTURE WORK The tribological characterization of nanostructured surfaces is an area which holds significant promise, as advancements in nanotechnology are continuously made. Understanding friction and wear can be crucial to enhance the performance and lifespan of nanostructured materials. Hence, it would be valuable to continue the investigations carried out in this study, through the utilization of test counterfaces with different geometries, materials, sliding velocities and applied loads. 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Wear and degradation analysis of light management nanofabricated layers 79 The average diameters of the nanofabricated moth-eye structures A and B were obtained through 30 measurements, which are presented in Table A.1. Table A.1 - Measurements of the diameter of pillars from structure A and B. Diameter of the pillars (nm) Structure A Structure B 1 225 487 2 243 208 3 204 654 4 220 227 5 208 681 6 191 207 7 257 265 8 148 498 9 145 778 10 118 383 11 192 621 12 147 654 13 164 532 14 129 1038 15 135 163 16 68 520 17 152 782 18 229 443 19 100 487 20 78 664 21 121 522 22 393 660 23 116 399 24 158 207 25 121 140 26 143 525 27 152 391 28 106 573 29 111 716 30 107 836 Wear and degradation analysis of light management nanofabricated layers 80 APPENDIX B: ROUGHNESS CALCULATIONS As previously mentioned, the roughness calculations were performed with the aid of a Matlab code, which is presented below, in Figure B.1, Figure B.2 and Figure B.3. Figure B.1 – Matlab code developed to calculate the roughness of the moth-eye texturization Wear and degradation analysis of light management nanofabricated layers 81 Figure B.2 - Matlab code developed to calculate the roughness of the moth-eye texturization (continuation). Wear and degradation analysis of light management nanofabricated layers 82 Figure B.3 - Matlab code developed to calculate the roughness of the moth-eye texturization (continuation). Wear and degradation analysis of light management nanofabricated layers 83 A step-by-step illustration of the roughness calculation is provided by Figure B.4 and Figure B.5. Figure B.4 - Execution of the matlab generated code. (A) showcases the input of the image and the selection of the scale. (B) illustrates the row of pillars which will be analyzed. Wear and degradation analysis of light management nanofabricated layers 84 Figure B.5 - Execution of the matlab generated code. (A) showcases the selection of minimum and maximum values, which will be correlated with the actual height values. (B) presents the corrected surface profile, which will be utilized to calculate the roughness parameters. Wear and degradation analysis of light management nanofabricated layers 85 The results of the 30 performed measurements for Structure A and B are included in Table B.1 and Table B.2, respectively. Table B.1 - Results of the performed roughness measurements for Structure A. 𝑹𝒂 𝑹𝒒 𝑹𝒕 𝑹𝒗 𝑹𝒑 1 276.606 294.171 926.495 341.814 584.682 2 253.837 271.316 858.590 297.037 561.553 3 246.196 272.067 946.240 287.498 658.741 4 231.872 256.622 854.546 287.941 566.605 5 249.449 270.379 846.262 320.074 526.188 6 222.738 254.719 880.215 269.934 610.281 7 221.157 245.407 890.643 259.285 631.358 8 268.270 283.702 915.680 334.190 581.490 9 254.330 274.703 906.716 317.735 588.981 10 248.106 271.895 952.466 316.326 636.141 11 241.231 274.900 911.824 319.734 592.090 12 237.879 269.295 944.922 292.266 652.656 13 230.452 260.153 1004.620 292.261 712.363 14 266.933 292.796 1030.700 394.080 636.621 15 246.973 270.688 1092.870 311.400 781.474 16 245.962 271.510 930.628 290.016 640.612 17 225.702 256.718 874.095 279.155 594.940 18 252.843 276.139 974.417 388.963 585.454 19 241.234 270.113 989.763 278.981 710.781 20 279.610 298.663 1044.120 392.337 651.779 21 240.795 274.292 978.458 313.671 664.787 22 258.766 322.586 1301.740 539.201 762.538 23 240.222 261.069 843.922 308.980 534.942 24 275.286 290.370 935.068 377.477 557.591 25 242.266 277.746 1019.720 379.250 640.472 26 256.935 307.728 1303.000 455.575 847.423 27 271.398 296.690 1117.250 350.638 766.616 28 265.982 295.317 1046.430 324.190 722.242 29 263.009 291.264 1031.540 417.203 614.335 30 261.751 294.126 1019.340 376.579 642.757 Wear and degradation analysis of light management nanofabricated layers 86 Table B.2 - Results of the performed roughness measurements for Structure B. 𝑹𝒂 𝑹𝒒 𝑹𝒕 𝑹𝒗 𝑹𝒑 1 134.355 149.337 554.087 194.920 359.168 2 138.871 154.586 540.043 202.194 337.849 3 143.121 158.784 558.202 183.761 374.441 4 132.030 155.277 604.352 249.712 354.640 5 149.805 174.663 687.071 254.618 432.453 6 134.553 157.402 653.858 207.544 446.314 7 137.527 156.072 524.616 216.960 307.656 8 129.150 143.226 498.837 180.770 318.067 9 133.392 161.327 663.546 274.325 389.221 10 162.642 190.293 724.231 264.031 460.199 11 133.583 158.516 613.838 259.673 354.165 12 132.179 148.917 532.554 170.217 362.337 13 138.223 157.281 608.385 204.301 404.084 14 148.403 168.857 635.848 207.118 428.730 15 143.678 175.507 714.203 285.152 429.051 16 134.969 149.388 497.000 196.144 300.856 17 135.566 161.595 689.221 235.327 453.894 18 138.173 157.017 620.821 183.844 436.977 19 123.834 154.051 673.523 299.524 373.999 20 130.523 148.593 558.991 212.993 345.998 21 147.032 166.570 618.986 192.430 426.557 22 141.895 169.688 672.797 212.815 459.981 23 148.319 168.109 597.295 243.817 353.478 24 127.213 145.104 571.724 171.605 400.119 25 146.467 163.422 547.545 223.493 324.052 26 136.040 151.979 544.918 189.567 355.351 27 135.388 153.246 552.331 199.298 353.033 28 150.083 191.002 957.352 369.719 587.633 29 121.552 157.587 657.150 286.425 370.726 30 144.302 160.764 562.887 219.417 343.470 Wear and degradation analysis of light management nanofabricated layers 87 APPENDIX C: OPTICAL MICROSCOPY MEASUREMENTS Table C.1 and Table C.2 contain the measurements of the test track width for tests performed with 0.1 N and 0.5 N, respectively. Table C.1 - Test track width measurements of tests performed with 0.1 N. Number of cycles 1 2 3 15 50 1 113.02 132.20 179.94 232.73 250.28 2 118.05 141.08 178.07 213.29 257.20 3 124.29 150.22 140.86 200.58 252.41 4 124.32 132.92 174.68 218.57 229.17 5 104.37 140.35 159.79 171.56 260.35 6 105.09 140.13 156.44 207.05 253.13 7 107.49 132.45 137.91 208.03 232.74 8 109.41 133.40 110.90 236.81 264.39 9 110.37 152.35 157.64 189.54 257.94 10 114.47 148.51 155.78 178.27 259.12 Table C.2 - Test path width measurements of tests performed with 0.5 N. Number of cycles 1 2 3 15 50 1 167.47 210.18 235.61 363.04 333.02 2 161.27 232.52 238.01 366.22 368.76 3 153.07 214.97 276.47 377.88 379.81 4 157.88 293.19 258.64 369.27 398.28 5 159.55 299.03 231.10 376.95 400.02 6 161.72 199.40 272.58 369.59 410.29 7 162.43 186.45 284.81 317.69 404.75 8 175.88 232.51 293.91 314.81 403.80 9 148.53 223.13 308.82 297.81 405.71 10 154.34 194.36 299.00 284.08 376.20 Wear and degradation analysis of light management nanofabricated layers 88 APPENDIX D: PRE-SLIDING MEASUREMENTS The values resulting from the measurements of the pre-sliding phase length of the friction coefficient variation for both tested loads (0.1 N and 0.5 N) are contained in Table D.1. Table D.1 - Measurements of the pre-sliding phase durations for 0.1 N and 0.5 N. Time (seconds) 0.1 N 0.5 N 1 1.048 1.533 2 1.371 2.258 3 1.442 2.631 4 1.572 2.731 5 2.025 2.803 6 0.232 2.309 7 2.580 2.609 8 2.591 2.451 9 2.520 3.054 10 2.510 1.018 11 2.561 2.147 12 1.240 0.645 13 1.209 1.301 14 1.744 1.311 15 2.220 2.614