Change in Dimensions and Surface Roughness of 42CrMo4 Steel after Nitridation in Plasma and Gas

Citation: Dobrocky, D.; Pokorny, Z.; Joska, Z.; Sedlak, J.; Zouhar, J.; Majerik, J.; Studeny, Z.; Prochazka, J.; Barenyi, I. Change in Dimensions and Surface Roughness of 42CrMo4 Steel after Nitridation in Plasma and Gas. Coatings 2022,12, 1481. https:// doi.org/10.3390/coatings12101481 Academic Editors: Ivan A. Pelevin and Dmitriy Yu. Ozherelkov Received: 13 September 2022 Accepted: 4 October 2022 Published: 6 October 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). coatings Article Change in Dimensions and Surface Roughness of 42CrMo4 Steel after Nitridation in Plasma and Gas David Dobrocky 1, Zdenek Pokorny 1,* , Zdenek Joska 1, Josef Sedlak 2, Jan Zouhar 2, Jozef Majerik 3, Zbynek Studeny 1, Jiri Prochazka 1and Igor Barenyi 3 1Department of Mechanical Engineering, Faculty of Military Technology, University of Defense, 662 10 Brno, Czech Republic 2Department of Machining Technology, Institute of Manufacturing Technology, Faculty of Mechanical Engineering, Brno University of Technology, 616 69 Brno, Czech Republic 3Department of Engineering Technologies and Materials, Faculty of Special Technology, Alexander Dubˇcek University of Trenˇcín, 911 50 Trenˇcín, Slovakia *Correspondence: [email protected]; Tel.: +420-973-442-989 Abstract: The influence of plasma nitriding and gas nitriding processes on the change of surface roughness and dimensional accuracy of 42CrMo4 steel was investigated in this paper. Both processes almost always led to changes in the surface texture. After plasma nitriding, clusters of nitride ions were formed on the surface of steel, while gas nitriding very often led to the new creation of a formation of a “plate-like” surface texture. In both cases of these processes, a compound layer in specific thickness was formed, although the parameters of the processes were chosen with the aim of suppressing it. After the optimizing of nitriding parameters during nitriding processes, it was found that there were no changes in the surface roughness evaluated using the Ra parameter. However, it turned out that when using a multi-parameter evaluation of roughness (the parameters Rz, Rsk and Rku were used), there were presented some changes in roughness due to nitriding processes, which affect the functional behavior of the components. Roughness changes were also detected by evaluating surface roughness profiles, where nitriding led to changes in peak heights and valley depths. Nitriding processes further led to changes in dimensions in the form of an increase of 0.032 mm on average. However, the magnitude of the change has some context on chemical composition of material. A larger increase in dimensions was found with gas nitriding. The change in the degree of IT accuracy is closely related to the change in dimension. For both processes, there was a change of one degree of IT accuracy compared to the ground part (from IT8 to IT9). On the basis of the achieved dimensional accuracy results, a coefficient of change in the degree of accuracy IT was created, which can be used to predict changes in the dimensional accuracy of ground surfaces after nitriding processes in degrees of accuracy IT3–IT10. In this study, a tool for predicting changes in degrees of accuracy of ground parts after nitriding processes is presented. Keywords: plasma nitriding; gas nitriding; dimensional accuracy; roughness; functional properties 1. Introduction The assurance of the necessary quality of manufactured components is one of the key responsibilities of engineering technology. The term “quality” covers a fairly broad range, including shape and dimensional correctness, surface roughness, and changes to the material’s surface and surface layer following the completion of technological operations. Their operational reliability and service life, which combines a number of other indicators, such as service life, repairability, wear resistance, corrosion resistance, fatigue strength, and others, is a comprehensive criterion for evaluating the impact of technological operations on the quality of components. The increasing demands for component reliability necessitate either the introduction of new materials, such as high-strength steels, to replace current ones, or the modification Coatings 2022,12, 1481. https://doi.org/10.3390/coatings12101481 https://www.mdpi.com/journal/coatings Coatings 2022,12, 1481 2 of 18 of existing ones’ surfaces to enhance their usable qualities. Surface modification can be accomplished in a variety of ways, including hard layer deposition [ 1 ], chemical–thermal diffusion technologies [2], or a combination of these two approaches [3,4]. Cr-Mo steel 42CrMo4 is suited for surface hardening and heat treating. This steel is frequently employed in the manufacturing of barrel and breechblock mechanisms for armaments as well as extremely stressed machine elements (shafts and connecting parts) that call for both high strength and high toughness [ 5 ]. Unfortunately, with this steel, friction causes surface degradation and shortens the useful life of the functional surfaces that come into contact. The tribological characteristics of this steel must therefore be improved for this reason. One of the most used methods of modifying the surface properties of 42CrMo4 steel is the process of chemical-heat treatment by nitriding [ 6 , 7 ]. Nitriding is the surface saturation of steel by nitrogen in a gaseous or liquid state. The purpose of nitriding is the creation of a surface layer containing highly dispersed nitrides of alloy elements with a high affinity to nitrogen, which mainly include Al, Cr, Ti, W and V [ 8 ]. The condition for nitriding is that the working atmosphere in which nitriding takes place must contains nitrogen in the nascent state, which has an atomic form. The most accessible method to obtain atomic nitrogen is the decomposition of ammonia. Since decomposition takes place directly on the metal surface of nitrided steel parts at temperatures of 480–600 ◦ C, atomic nitrogen combines with iron and the previously mentioned elements with a high affinity for nitrogen to form nitrides of the respective metals. The resulting Fe4N (so-called γ’) or Fe2-3N (so-called εphase) has neither exceptional hardness nor special thermal stability. However, the resulting nitrides of Al, Cr, V and others are finely precipitated in the ferrite and will cause considerable hardness in steels, especially if the ferrite is present in such a fine form as in sorbite [ 9 ]. Diffusion nitriding technology leads to increasing the surface hardness, corrosion resistance, fatigue failure and tribological properties [ 10 – 12 ]. These properties can be influenced by process parameters. In Ref. [ 13 ], it is stated that the characteristics of the nitrided layer are explicitly dependent on the parameters of the nitriding process and on a material of substrate well. These are specifically the process temperature, processing time, current density and composition of the nitriding atmosphere. By controlling these parameters, we can, for example, eliminate the formation of a compound (so-called white) layer on the surface of the components [ 14 ]. The phase composition of the surface layer in the nitrided component can be derived from the isothermal section of the Fe-C-N ternary diagram for C = const. With regard to the present phases, it is divided into two basic areas [15]: • The pure nitride region (compound layer) is formed by nitrides, or carbonitrides of type ε (Fe 2-3 N) and γ ’(Fe 4 N) of iron and alloying elements. Only exceptionally is ξ -type nitride (Fe 2 N) present. Its actual structure is influenced by the technology of the saturation process and the composition of the steel. A frequent phenomenon is its porosity as a result of the metastability of nitrides, the release of atomic nitrogen and the exothermic reaction during its fusion. • The diffusion layer is the structure of the layer consisting of ferrite and nitrides (carbonitrides) of Fe and alloying elements, possibly carbides. The formation of nitrides is essentially a precipitation process from nitrogen-saturated ferrite. The consequence of its time-dependent progress in several stages is the formation of both coherent and incoherent nitrides. For tribological applications, it is suitable to suppress the formation of a compound layer, which is fragile and during the initial sliding contact of the friction pairs, it can separate and form hard abrasive particles, which can significantly affect wear resistance [16]. A very important aspect of the surface technology application is their influence on shape, dimensional accuracy and surface roughness. The mentioned parameters significantly affect the functional behavior of the friction pairs and thus the service life and reliability of the components. The nitriding process generally leads to a change in the dimensional accuracy and surface roughness of many materials [ 17 ]. The change in dimensional accuracy and surface roughness is caused by an increase in the volume (up to 5%) Coatings 2022,12, 1481 3 of 18 of the compound layer. This increase is mostly caused by an increase in the volume of newly formed nitride and carbonitride phases [ 18 ]. Another aspect is the presence of coherent precipitates, mainly of the FeCr(NC) type. The consequence is the distortion of the matrix lattice and the formation of local internal stresses. Due to the stabilizing effect of nitrogen on austenite, the carbon in the austenite is transformed into martensite and further transformed into the γ ’-Fe 4 N form. However, the bcc lattice structure remains unchanged. The diffusion of nitrogen results in an increase in volume [ 19 ]. The quality of the surface is further affected by the nucleation of Fe 4 N 1−x nitrides on the steel surface. Nitride nucleation is energetically more advantageous at the grain boundaries; however, as the incubation time increases, nitride nucleation also occurs on the outer surface of the grains, in the crystal lattice, and their further growth occurs. Small Fe 16 N 2 precipitates in the form of dark protrusions, and long needles of Fe 4 N 1−x are then visible on the steel surface in the cross section [ 20 ]. In particular, the surface roughness of nitrided components is affected by the formation of porosity on the surface and in the surface layer. The formation of pores is associated with the desorption of nitrogen molecules. The area with the highest nitrogen content is the surface and surface layer; therefore the highest probability of thermally activated pore nucleation is here. Pore nucleation is preferentially initiated at energetically favorable locations, such as grain boundaries in the nitrided layer. The pore formation process is described in detail in [ 18 ]. The creation of pores leads to a deterioration of roughness, both due to the presence of pores and the expulsion of grains from the surface due to the action of very high pressures in the pores, which lead to volume expansion and local grain separation [ 21 ]. The last reason for the change in dimensional accuracy and surface roughness of nitrided components is the increase in lattice parameters in the surface layer by increasing the concentration of nitrogen and the formation of dispersed iron nitrides (γ’-Fe4N) [22]. For example, the volume of the lattice increases more than two-fold. The purpose of the research was to evaluate changes in dimensional accuracy and surface roughness of 42CrMo4 nitriding steel after gas and plasma nitriding processes. This issue is currently not adequately addressed, and it mainly concerns precise parts of weapons (barrels, breechblocks, etc.). Nitriding parameters were adjusted to suppress compound layer formation. The research was therefore focused on improving the tribological properties of the components. The importance of the research lies in the comparison of two nitriding technologies, the setting of suitable process parameters, the evaluation and interpretation of the detected changes. Based on the achieved results, the coefficient of change in the accuracy of IT components was determined, and the change in surface roughness was described. 2. Materials and Methods Samples with dimensions of 30.3 mm × 20.3 mm × 90.3 mm were milled from a 42CrMo4 steel blank on a MAS MCV 1000 machining center (MAS, Sezimovo Ústí, Czech Republic). A total of 5 samples were produced for each nitriding technology. The chemical composition of the steel was measured by optical emission spectrometry (OES) on a Q4 Tasman spectrometer (Bruker, Billerica, MA, USA): C: 0.39, Mn: 0.72, Si: 0.31, Cr: 1.09, Ni: 0.06 , Mo: 0.19, P: 0.011, S: 0.015 (wt%). The analyzed surfaces of the samples were ground on a surface grinder BPH20 NA (TOS, Hostivaˇr, Czech Republic), with a Norton 3SG46JVS grinding wheel with dimensions of 200 mm ×20 mm ×32 mm. Spindle speed n= 2400 rpm , engagement depth h= 0.03 mm. The requirement for surface roughness was Ra = 1.6 µ m. In both machining operations, semi-synthetic cutting fluid TRIM MicroSol 515 was used as a cooling medium. The experimental samples were thermally processed, i.e., normalized and heat-treated in a LAC L70V laboratory furnace (LAC, Židlochovice, Czech Republic). The normalizing annealing was carried out at a temperature of 860 ◦ C, for 45 min, with air cooling. Hardening was carried out at a temperature of 840 ◦ C, while staying at the temperature for 45 min in water. This was followed by tempering at 600 ◦ C for 100 min with water cooling. Before Coatings 2022,12, 1481 4 of 18 individual heat treatment processes, the samples were provided with a layer of Kalsen 3 protective coating against surface decarburization. The heat treatment of the samples was followed by grinding of the functional surfaces to the final dimensions of 30 mm × 20 mm × 90 mm. The grinding parameters and the grinder used remained the same. The roughness requirements for ground surfaces were set at a value of Ra = 0.8 µ m. This was achieved by finishing grinding without feed motion (sparking out). Plasma nitriding was performed at a process temperature of 520 ◦ C, for 14 h, in an atmosphere of 1H 2 :3N 2 , at a pressure of 500 Pa, in a Rübig PN 100/80 device (Rübig, Marchtrenk, Austria). Nitriding in gas was carried out at a temperature of 530 ◦ C, for 7 h, at a pressure of 400 Pa, in an atmosphere of NH 3 /N 2 /H 2 , in a Nitrex 80/200 device (Nitrex, Montreal, QC, Canada). After nitriding, the samples were cooled to ambient temperature in the furnace. For both nitriding technologies, the parameters were set to suppress the formation of the compound layer. A scanning electron microscope, Tescan Mira 4 (Tescan, Brno, Czech Republic), was used to analyze the surface structure of ground and nitrided samples. Metallographic analysis of cross sections of the samples was performed on an Olympus DSX500i inverted opto-digital metallographic microscope (Olympus, Šindžuku, Japan). The case depth of the formed nitrided layers was measured using microhardness curves on an automated microhardness tester AMH55 (Leco, St. Joseph, MI, USA), using a Vickers indenter and a load of 0.98 N (HV0.1). Surface hardness was measured with a Zwick ZHU 2.5 universal hardness tester (Zwick Roell Group, Ulm, Germany), with a load of 29.42 N (HV3). The evaluation of the surface roughness was carried out on a Talysurf CCI Lite coherence correlation interferometer (Taylor Hobson, Leicester, UK), at an evaluated length of 4 mm, and cut-off of 0.8 mm. A Gaussian filter was used to filter the data. The change in sample dimensions was measured on a 3D CNC coordinate machine Werth ScopeCheck ® S (Werth, Giessen, Germany), 5 times on each sample. The measurement was made with a touch sensor of length l= 20 mm, with a ruby ball of diameter d= 3 mm. The results were evaluated using WinWerth ® 8 software. An area of 27 mm × 17 mm was measured on the fronts of the samples, and 25 measurement points were taken from this area. The measurement took place at a constant temperature of 22 ± 1 ◦ C and a relative humidity of 55%. The maximum measurement error for the sample length l= 90 mm was 3.25 µ m, the measurement uncertainty for the sample length l= 90 mm was ±0.001 mm. To determine the dependence of the change in dimensional accuracy and surface roughness after nitriding processes, their mutual relationship was evaluated. The relationship between dimensional tolerance and surface roughness was discussed, for example, in work [ 23 ]. An overview of the recommended surface roughness depending on the degree of accuracy and size is given in [ 24 ]. It is recommended to determine the surface roughness value from the relationship: Rz =K×T, (1) where Kis a coefficient including the effects of various factors, and Tis the dimension tolerance. The interrelationship between surface roughness and dimensional tolerance can be expressed by the maximum clearance: T=Dmax –Dmin, (2) T 2=T1 2+Rt, (3) T=T1+2Rt, (4) Surface roughness and dimension tolerance and their mutual relationship according to the type of storage of the components is shown in Figure 1. Coatings 2022,12, 1481 5 of 18 Coatings 2022, 12, 1481 5 of 19 𝑇 = 𝐷𝑚𝑎𝑥 – 𝐷𝑚𝑖𝑛, (2) 𝑇 2=𝑇1 2+𝑅𝑡, (3) 𝑇 = 𝑇1+2𝑅𝑡, (4) Surface roughness and dimension tolerance and their mutual relationship according to the type of storage of the components is shown in Figure 1. Figure 1. Surface roughness and dimensional tolerance. If chosen, 2𝑅𝑡 = 𝑇1 3, (5) 𝑇1=6×𝑅𝑡, (6) 𝑇 2=6×𝑅𝑡 2+𝑅𝑡=4×𝑅𝑡⇒𝑅𝑡≦ 1 8×𝑇, (7) The surface roughness therefore depends on the tolerance. The value 𝑐=1 8×𝑇 was determined for bearing with clearance (moveable): 𝑅𝑡 =(0.11÷0.20)×𝑇, (8) If the frictional surfaces of the functional components are considered, then as the wear progresses, the bearing area of the profile increases. The rate of wear is inversely proportional to the size of the bearing surface for the same surface load. The bearing area is given by 𝑅 = 𝑓(𝑧), (9) The rate of wear is determined by 𝑣0 = 𝑐× 1 𝑓(𝑧), (10) where 𝑐 is a constant. At the beginning, the rate of wear v0 is high, then it decreases and takes on a linear course. The time t during which the roughness wears down to the Rt value can be expressed by the equations 𝑅𝑡 = 𝑑𝑧 𝑑𝑡 ⇒𝑑𝑡=𝑑𝑧 𝑅𝑡, (11) Figure 1. Surface roughness and dimensional tolerance. If chosen, 2Rt =T1 3, (5) T1=6×Rt, (6) T 2=6×Rt 2+Rt=4×Rt⇒Rt51 8×T, (7) The surface roughness therefore depends on the tolerance. The value c=1 8×T was determined for bearing with clearance (moveable): Rt =(0.11 ÷0.20)×T, (8) If the frictional surfaces of the functional components are considered, then as the wear progresses, the bearing area of the profile increases. The rate of wear is inversely proportional to the size of the bearing surface for the same surface load. The bearing area is given by R=f(z), (9) The rate of wear is determined by v0=c×1 f(z), (10) where cis a constant. At the beginning, the rate of wear v 0 is high, then it decreases and takes on a linear course. The time tduring which the roughness wears down to the Rt value can be expressed by the equations Rt =dz dt ⇒dt =dz Rt, (11) dt =f(z) cdz, (12) t=1 c× Rt Z 0 f(z)dz, (13) During the time t max , the entire roughness Rt will wear out. One can also encounter the term wear half-time t max /2, after which the surface roughness generally drops from the Rt value to the Rt 1/2 value. However, this value is influenced by the shape of the roughness profile. The roughness of the surface must be such that during operational wear of the surfaces, the difference in the largest clearances of adjacent bearing types is completely or partially suppressed. The criterion is such a value of wear when the clearance difference increases to 1/2. If it were the case that the unevenness of the surface would wear out Coatings 2022,12, 1481 6 of 18 completely (for a time t max ), the clearance of the neighboring bearing types would increase by the value v=2×Rthole +2×Rtsha f t, (14) During tmax/2 (wear half-life), v1/2 =v×k=∆v 2÷3, (15) where ∆ vis the clearance, and kis the coefficient indicating the dependence of wear on time, which is determined according to the relationship k=v1/2 vmax , (16) The surface roughness may, therefore, be maximal such that the wear value is less than 1/2 to 1/3 of the bearing play difference. The values of the coefficient k, indicating the dependence of wear on time, range from 0.4 to 0.7 [25]. In the case of some steels, the nitriding process causes significant changes in the dimensional accuracy and surface roughness [ 26 ], and these changes then affect the dimensional tolerances of the part and have an effect on their functional properties and service life. 3. Results The prepared samples were metallographically analyzed after the application of nitriding, mainly for the purpose of measuring the thickness of the compound layer. Furthermore, the surface hardness and the depth of the diffusion layers were measured using the microhardness curves so that it was possible to compare the changes in the mechanical properties of the two selected nitriding processes. In terms of demonstrating surface texture changes (roughness) and dimensional changes (tolerance), the experimental activities listed below were carried out. 3.1. Metallographic Analysis Micrographs of the cross-section of the nitrided layers after plasma nitriding and gas nitriding, taken with an opto-digital microscope, are shown in Figure 2. Cross sections were provided due to their fundamental description and subsequent evaluation of the thickness of the compound layer. Coatings 2022, 12, 1481 7 of 19 such as ε, ε + γ, γ′, γ′ + α or α, are regulated by changing the voltage, pressure and composition of the atmosphere. The nitriding parameters, especially the nitriding potential, then depend on whether a single-phase layer consisting of only γ′-nitride is formed. If the nitriding potential in the gaseous atmosphere is high enough before a closed γ′-nitride layer is formed, additional ε-nitrides of iron ε (Fe2-3N) are formed on the γ′-nitrides. The lateral growth of these twins eventually leads to the formation of a two-phase layer consisting of γ′- and ε-nitrides of iron in the binary iron–nitrogen system. If the base material is alloy steel, the γ′- and ε-nitrides typically appear mixed, often with the γ′-grains embedded in a continuous ε-phase. Additional alloying elements can lead to the formation of additional nitrides depending on their Gibbs energy [27,28]. The large difference in the thickness of the compound layer after the gas nitriding process can be explained by the fact that adding nitrogen to ammonia can create a higher concentration gradient of nitrogen on the steel surface, which can promote the diffusion rate and thereby increase the thickness of the compound layer and the diffusion zone. The nitrogen content in the gas mixture therefore has an obvious effect on the formation of the compound layer, which was also confirmed in the paper [29]. In Ref. [30], it is stated that the higher surface concentration of nitrogen, which occurs in gas nitriding, is favorable for the formation of ε (Fe2-3N) nitrides. (a) (b) Figure 2. Microstructure of cross-sections of samples after nitriding: (a) in plasma; (b) in a gas. 3.2. Measurement of Surface Hardness By measuring the hardness of the surface with a load of 3 kg (29.42 N), the hardness of both the compound layer and the diffusion layer, which takes over the function of the base under this load, was evaluated. Therefore, the hardness of the surface of the compound layer + diffusion layer system is measured. The ground surface of the heat-treated samples reached a surface hardness of 278.26 ± 12.45 HV3. The plasma nitriding process led to an increase in surface hardness to 709.16 ± 11.92 HV3. Surface hardness after gas nitriding reached 693.80 ± 33.51 HV3. A comparison of both nitriding processes shows that plasma nitriding led to higher hardness values compared to gas nitriding; however, it is evident that the increase is not significant. 3.3. Evaluation of the Depth of the Diffusion Layer The profiles of the plasma and gas nitrided 42CrMo4 steel samples are shown in Figure 3. It can be observed that the microhardness of the surface and the depth of the diffusion layer increased with the nitriding time. The depth of the diffusion layer (NHD) is dependent on the duration of the process according to the relationship given in [31]: Figure 2. Microstructure of cross-sections of samples after nitriding: (a) in plasma; (b) in a gas. The steel structures after both nitriding processes consist of sorbite and tempered martensite. The thickness of the compound layer created by plasma nitriding was 6.4 ±1.1 µm , while in gas nitriding, the thickness of the compound layer reached a value of 18.4 ±0.5 µm. In Coatings 2022,12, 1481 7 of 18 both processes, the nitriding parameters were chosen so that the formation of the compound layer was suppressed. Various required structures of the surface layer, such as ε , ε + γ , γ0 , γ0 + α or α , are regulated by changing the voltage, pressure and composition of the atmosphere. The nitriding parameters, especially the nitriding potential, then depend on whether a single-phase layer consisting of only γ0 -nitride is formed. If the nitriding potential in the gaseous atmosphere is high enough before a closed γ0 -nitride layer is formed, additional ε -nitrides of iron ε (Fe 2-3 N) are formed on the γ0 -nitrides. The lateral growth of these twins eventually leads to the formation of a two-phase layer consisting of γ0 - and ε -nitrides of iron in the binary iron–nitrogen system. If the base material is alloy steel, the γ0 - and ε -nitrides typically appear mixed, often with the γ0 -grains embedded in a continuous ε -phase. Additional alloying elements can lead to the formation of additional nitrides depending on their Gibbs energy [ 27 , 28 ]. The large difference in the thickness of the compound layer after the gas nitriding process can be explained by the fact that adding nitrogen to ammonia can create a higher concentration gradient of nitrogen on the steel surface, which can promote the diffusion rate and thereby increase the thickness of the compound layer and the diffusion zone. The nitrogen content in the gas mixture therefore has an obvious effect on the formation of the compound layer, which was also confirmed in the paper [ 29 ]. In Ref. [ 30 ], it is stated that the higher surface concentration of nitrogen, which occurs in gas nitriding, is favorable for the formation of ε(Fe2-3N) nitrides. 3.2. Measurement of Surface Hardness By measuring the hardness of the surface with a load of 3 kg (29.42 N), the hardness of both the compound layer and the diffusion layer, which takes over the function of the base under this load, was evaluated. Therefore, the hardness of the surface of the compound layer + diffusion layer system is measured. The ground surface of the heat-treated samples reached a surface hardness of 278.26 ± 12.45 HV3. The plasma nitriding process led to an increase in surface hardness to 709.16 ± 11.92 HV3. Surface hardness after gas nitriding reached 693.80 ± 33.51 HV3. A comparison of both nitriding processes shows that plasma nitriding led to higher hardness values compared to gas nitriding; however, it is evident that the increase is not significant. 3.3. Evaluation of the Depth of the Diffusion Layer The profiles of the plasma and gas nitrided 42CrMo4 steel samples are shown in Figure 3. It can be observed that the microhardness of the surface and the depth of the diffusion layer increased with the nitriding time. The depth of the diffusion layer (NHD) is dependent on the duration of the process according to the relationship given in [31]: NHD =K√t, (17) where tis the time of the process in hours and Kis the temperature factor given, for example, in Ref. [32]. The surface hardness of the samples after plasma nitriding was 753 ± 11 HV0.1, with a diffusion layer depth of 0.319 ± 0.009 µ m. The samples after nitriding in gas reached a surface hardness of 637 ± 8 HV0.1 and a diffusion layer depth of 0.224 ± 0.026 µ m. One of the reasons for the higher hardness of the nitrided surface after plasma nitriding is again the longer process time, during which the surface is more intensively saturated with nitrogen and the compound layer containing the phase γ0 (Fe 4 N) + ε (Fe 2-3 N) is formed and the diffusion layer is strengthened. As the ε / γ0 ratio increases, the surface hardness increases [ 33 , 34 ]. The degree of supersaturation of the surface adsorption layer is exponentially related to temperature and depends on the time of the process and the saturation potential of the environment. As the process time increases, the hardness of the surface part of the diffusion layer increases. Likewise, a longer process time leads to an increase in its depth. Coatings 2022,12, 1481 8 of 18 Coatings 2022, 12, 1481 8 of 19 𝑁𝐻𝐷 = 𝐾√𝑡, (17) where t is the time of the process in hours and K is the temperature factor given, for example, in Ref. [32]. Figure 3. Microhardness profiles of samples nitrided in plasma and in gas. The surface hardness of the samples after plasma nitriding was 753 ± 11 HV0.1, with a diffusion layer depth of 0.319 ± 0.009 μm. The samples after nitriding in gas reached a surface hardness of 637 ± 8 HV0.1 and a diffusion layer depth of 0.224 ± 0.026 μm. One of the reasons for the higher hardness of the nitrided surface after plasma nitriding is again the longer process time, during which the surface is more intensively saturated with nitrogen and the compound layer containing the phase γ′(Fe4N) + ε (Fe2-3N) is formed and the diffusion layer is strengthened. As the ε/γ′ ratio increases, the surface hardness increases [33,34]. The degree of supersaturation of the surface adsorption layer is exponentially related to temperature and depends on the time of the process and the saturation potential of the environment. As the process time increases, the hardness of the surface part of the diffusion layer increases. Likewise, a longer process time leads to an increase in its depth. 3.4. Surface Texture Analysis The diffusion processes changed the morphology and texture of the surface. Figure 4 shows an SEM image of the surface of 42CrMo4 steel after heat treatment and grinding to the required roughness Ra = 0.8 μm and after nitriding in plasma and in gas. The image of the ground surface (Figure 4a) shows traces of the grinding tool, in the form of a directed structure (peaks and valleys). The structure of the ground surface is determined by the size of the grinding grains, their shape and the distance between them as well as the cutting speed of the wheel, the speed of the workpiece, longitudinal displacement and sparking out. The grinding wheel dressing conditions also have their influence. The dimensions of the grinding grains are determined by the grit, where it is true that a smaller grain size (smaller grit) is chosen when less surface roughness is required. The shape of the grains is determined by the type of abrasive used. The distance between the grains is determined by the structure of the wheel (its porosity), which is chosen according to the material being machined, the shape of the workpiece, the grinding method, the size of the contact area between the wheel and the workpiece, the amount of material removed, and the type of grinder. It is generally known that the resulting surface roughness depends on 0 100 200 300 400 500 600 700 800 050 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 Hardness HV 0.1 Case Depth [µm] Ground Plasma nitriding Nitriding in gas Figure 3. Microhardness profiles of samples nitrided in plasma and in gas. 3.4. Surface Texture Analysis The diffusion processes changed the morphology and texture of the surface. Figure 4 shows an SEM image of the surface of 42CrMo4 steel after heat treatment and grinding to the required roughness Ra = 0.8 µ m and after nitriding in plasma and in gas. The image of the ground surface (Figure 4a) shows traces of the grinding tool, in the form of a directed structure (peaks and valleys). The structure of the ground surface is determined by the size of the grinding grains, their shape and the distance between them as well as the cutting speed of the wheel, the speed of the workpiece, longitudinal displacement and sparking out. The grinding wheel dressing conditions also have their influence. The dimensions of the grinding grains are determined by the grit, where it is true that a smaller grain size (smaller grit) is chosen when less surface roughness is required. The shape of the grains is determined by the type of abrasive used. The distance between the grains is determined by the structure of the wheel (its porosity), which is chosen according to the material being machined, the shape of the workpiece, the grinding method, the size of the contact area between the wheel and the workpiece, the amount of material removed, and the type of grinder. It is generally known that the resulting surface roughness depends on the mean thickness of the chip removed by the abrasive grain. It is true that the smaller the mean thickness of the chip, the smaller the surface roughness, quantified, e.g., by the Ra value, must be. By reducing the cutting speed of the grinding wheel and increasing the transverse feed speed, the surface roughness increases. The effect of sparking out softens the profile of the ground surface. The method of dressing the wheel has a direct effect on the surface roughness; in the case of bad dressing, a periodic component of the surface profile can be created, and the surface roughness can increase. The steel surface after plasma nitriding is shown in Figure 4b. Compared to the ground surface, the nitrided surface shows a rugged morphology and contented texture with clusters of spherical particles (nitride ion clusters) in the range of 0.5–1 µ m. The occurrence of these particles is regular and forms a compact, continuous layer. Plasma nitriding resulted in a change in the surface texture and thus the suppression of the original structure after the grinding tool. Clusters of ions on the surface are created as a result of the nitriding process in the plasma and are presented, for example, in [ 35 ]. It is also stated here that the chemical composition of the steel has no significant effect on the surface texture after plasma nitriding. This is closely related to the process parameters, especially temperature, nitriding time or micropulse parameters. Clusters of ions create partial porosity within the surface part [ 36 ], which can be a negative parameter with regard to corrosion resistance. Coatings 2022,12, 1481 9 of 18 In work [ 37 ], it is stated that the concentration of elements in nitrided steel 42CrMo4 was 80% Fe, 11% C and 4% N at the same parameters of the nitriding process. Coatings 2022, 12, 1481 9 of 19 the mean thickness of the chip removed by the abrasive grain. It is true that the smaller the mean thickness of the chip, the smaller the surface roughness, quantified, e.g., by the Ra value, must be. By reducing the cutting speed of the grinding wheel and increasing the transverse feed speed, the surface roughness increases. The effect of sparking out softens the profile of the ground surface. The method of dressing the wheel has a direct effect on the surface roughness; in the case of bad dressing, a periodic component of the surface profile can be created, and the surface roughness can increase. (a) (b) Ground Plasma nitrided Nitrided in gas (c) (d) Figure 4. SEM images of the surface of the samples: (a) after grinding; (b) after plasma nitriding; (c) after gas nitriding; (d) surface roughness profiles. The steel surface after plasma nitriding is shown in Figure 4b. Compared to the ground surface, the nitrided surface shows a rugged morphology and contented texture with clusters of spherical particles (nitride ion clusters) in the range of 0.5–1 μm. The occurrence of these particles is regular and forms a compact, continuous layer. Plasma nitriding resulted in a change in the surface texture and thus the suppression of the original structure after the grinding tool. Clusters of ions on the surface are created as a result of the nitriding process in the plasma and are presented, for example, in [35]. It is also stated Figure 4. SEM images of the surface of the samples: ( a ) after grinding; ( b ) after plasma nitriding; (c) after gas nitriding; (d) surface roughness profiles. The gas nitriding process (Figure 4c) resulted in a highly fragmented surface with a large number of plate-like particles, which are caused by the presence of different substances. Traces of the grinding tool in the form of dominant peaks are visible on the surface texture. In the central part of the image, the newly created surface creates valleys in the form of craters. Compared to the surface of the sample after the plasma nitriding process, the sample after gas nitriding has a rougher surface texture and it is visible as a presented rougher morphology. As part of the created surface structure, the nitride phases of the alloying elements were formed, and the surface compound layer was formed. On this surface, the presence of pores can be identified; the principle of their formation during nitriding was presented, for example, in Ref. [ 38 ]. As a result of nitriding in gas, there is very often a more significant development of porosity related to the subsequent brittleness of the surface layers in gaseous environments. Coatings 2022,12, 1481 16 of 18 Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: This work was supported by the specific research project 2020 “SV20-216“ at the Department of Mechanical Engineering, University of Defence in Brno, by the Project for the Development of the Organization “DZRO VAROPS” and by the project with the grant Modern technologies for processing advanced materials used for interdisciplinary applications “FSI-S-22-7957”. Conflicts of Interest: The authors declare no conflict of interest. References 1. Naeem, M.; Diáz-Guillén, J.C.; Khalid, A.; Guzmán-Flores, I.; Mu´noz-Arroyo, R.; Iqbal, J.; Sousa, R.R.M. 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