High temperature oxidation resistance of (Ti,Ta)(C,N)-based cermets

1 High temperature oxidation resistance of (Ti,Ta)(C,N)-based cermets E. Chicardi*1,2, J.M. Córdoba1,3 and F. J. Gotor1 1Instituto de Ciencia de Materiales de Sevilla (CSIC-US), Américo Vespucio 49, 41092 Sevilla, Spain. 2Departamento de Ingeniería Metalúrgica y de Materiales, Universidad Técnica Federico Santa María, Valparaíso 1680, Chile. 3Departamento de Química Inorgánica, Universidad de Sevilla (US), Calle Prof. García González 1, 41012, Sevilla, Spain Abstract Cermets based on titanium-tantalum carbonitride were oxidized in static air between 800 ºC and 1100 ºC for 48 h. The thermogravimetric and microstructural study showed an outstanding reduction in the oxidation of more than 90% when the Ta content was increased. In cermets with low Ta content, the formation of a thin CoO/Co3O4 outer layer tends to disappear by reacting with the underlying rutile phase, which emerges at the surface. However, in cermets with higher Ta content, the formation of an external titanate layer, observed even at a low temperature, appears to prevent the oxygen diffusion and the oxidation progression. Keywords: A. Ceramic matrix composites; A. Cobalt; A. Titanium; B. SEM; B. X-ray diffraction; C. Oxidation. * Corresponding Author. Tel: +34 954489217. Fax: +34 954460165. E-mail: [email protected] / [email protected] 1. Introduction. The materials used in various applications, such as cutting tools (rock and earth drilling tools), drawing and sheet metal-forming tools and dies, wear parts (nozzles, plunges, paper cutters, etc.) and other structural components (mechanical seals, boring 2 bars, etc.), can encounter extreme conditions (high temperature, pressure, and an oxidizing atmosphere, among others) during use. For this reason and because of constantly increasing industrial demands, the search for novel materials and designs with improved performance remains a challenge. Materials for these applications are expected to possess not only high abrasive and impact wear resistance but also excellent high temperature corrosion or/and oxidation resistance. Oxidation in non-oxidic structural materials is a major disadvantage because it typically controls the operational properties. Oxidation can reduce ductility, tensile strength and erosion and corrosion resistance, among other properties [1-3]. Oxidation is not always a slow process and can progress significantly over a short service time in certain practical applications [4]. Hard materials used in the metal working industry for high-speed machining must withstand a broad range of thermal, mechanical and chemical stresses. These materials are primarily based on tungsten carbide (cemented carbides) or titanium carbonitride (cermets). The desired properties can be achieved through complex alloying with other transition metals, such as Ta and Nb [5-7]. In this context, (Ti,Ta)(C,N)-based cermets have been successfully utilized as heat-resistant structural materials, including wear parts and, especially, cutting tools in high-speed finishing and semi-finishing applications due to their excellent physical and chemical properties, such as a favorable hot hardness, excellent wear resistance, perfect chemical stability, low friction coefficient to metals and great thermal deformation resistance [8-15]. However, while in service, (Ti,Ta)(C,N)-based cermets are often exposed to elevated temperatures in oxidizing environments [16]; therefore, their oxidation behavior plays a key role in cutting performance and tool life. In a previous study [17], it was determined at a fixed temperature of 900 ºC that the oxidation resistance of the (Ti,Ta)(C,N)-based cermets was significantly dependent on the Ta content. The goal of the present work is to report a systematic study of the outstanding oxidation improvement of TixTa1-xC0.5N0.5-Co cermets over a wide temperature range (800 ºC-1100 ºC) to address the lack of available experimental data in the literature on this topic. This study also attempts to provide new insights into the key role played by Ta in the excellent oxidation resistance of cermets. Note that the temperatures studied cover the maximum cutting tool work temperatures for the most demanding processes [18]. It must be considered that cutting tool oxidation at these 3 temperatures is more important than the mechanical abrasion in determining the tool life in many cases. 2. Experimental. Five cermets with nominal compositions of 80 wt% TixTa1-xC0.5N0.5 – 20 wt% Co and variable Ta atomic content, x = 1, 0.99, 0.95, 0.90 and 0.80 (labeled as Ti100, Ti99Ta1, Ti95Ta5, Ti90Ta10 and Ti80Ta20, respectively), were synthesized by the mechanochemical process called the Mechanically Induced Self-Sustaining Reaction (MSR) from Ti, Ta and C powder mixtures and N2 gas. A powder mixture containing 46.5 g elemental Ti (99% purity, <325 mesh, Strem Chemicals), Ta (99.6% purity, <325 mesh, Alfa-Aesar), C (as graphite, <270 mesh, Fe < 0.4%, Merck) and Co (99.8% purity, <100 mesh, Strem Chemicals), together with 13 tempered steel balls (d = 20 mm, m = 32.6 g), were put into a tempered steel vial (300 ml in volume) and milled using a modified planetary ball mill (P4, Fritsch) at a spinning rate of 400 rpm under 6 atm N2 gas (H2O and O2 < 3 ppm, Air Liquide). The planetary ball mill enabled the MSR reactions to be monitored by continuously measuring the pressure inside the vial. When the MSR reaction associated with the synthesis of the TixTa1-xC0.5N0.5 carbonitride solid solution phase occurs, the temperature increases due to the release of heat from the exothermic formation reaction, which consequently increases the total pressure. The ignition time, i.e, critical milling time required to induce the MSR process, can then be determined from the spike in the recorded time-pressure data. After ignition, the milling is continued to ensure full conversion and homogenization. The ignition time was about 45 min for all cermets and the total milling time was 75 min. Subsequently, the powdered cermets were shaped using a uniaxial press (2 tons, 5 min) and compacted by cold isostatic pressing (200 MPa, 10 min). The green bodies were then sintered at temperatures between 1450 ºC and 1550 ºC, optimized for each Ta/Ti ratio [19], for 60 min under a flowing Ar (H2O < 3 and O2 < 2 ppm, Linde) atmosphere in a horizontal tubular furnace (IGM1360 model No. RTH-180-50-1H, AGNI) to obtain cylindrical cermets 13 mm in diameter and 9 mm in height. After sintering, the cermets consisted of the mentioned carbonitride phase (TixTa1-xC0.5N0.5) and two Ti-Ta-Co intermetallic compounds, namely, TixTa1-xCo2 and TixTa1-xCo, acting 4 as binder instead of the initial Co. A detailed description of the microstructure and properties of cermets can be found in a previously published study [19]. To perform the oxidation experiments, parallelepiped specimens of these five sintered cylinder cermets with similar dimensions (3±0.1 mm x 4±0.1 mm x 5±0.1 mm) and surface area (94±0.5 mm2) were obtained by cutting from the center part of the sintered body. The surfaces of cermets were subjected to successive grinding and polishing steps (final step with 3 m diamond powder suspension). Finally, the specimens were ultrasonically cleaned with ethanol. The oxidation tests were performed in static air under isothermal conditions from 800 ºC to 1100 ºC (heating rate of 20 ºC/min and free cooling) for 48 h. The weight gain due to oxidation was continually monitored using a CI Robal electrobalance (C.I. Electronics Ltd.) with a maximum weight allowed of 5 g and a sensitivity of 1 μg; the balance was attached to the support frame of a high-temperature vertical furnace (Severn Thermal Solutions Ltd.) designed for use up to 1500 ºC. The specimens were placed on a platinum wire coil to maximize the contact of the surfaces with the surrounding atmosphere. X-ray diffraction (XRD) diagrams were collected on the surfaces exposed to oxidation to get information about the phases present in the oxide scale of the oxidized cermets. XRD patterns were also obtained once the oxidized specimens (bulk + oxide scale) were crushed and reduced to powder by hand-grinding to obtain information on the extent of oxidation. A PANalytical X’Pert Pro instrument equipped with a θ/θ goniometer, a Cu K radiation source (40 kV, 40 mA), a secondary K filter and an X’Celerator detector was used. The diffraction patterns were collected from 20º to 80º (2θ) in a step-scan mode at a step of 0.02º and a counting time of 275 s/step and were compared with those in the PDF-4+ database from the International Centre for Diffraction Data (ICDD). Lanthanum hexaboride powder (Standard Reference Material for powder diffraction 660b, NIST) was used to calibrate the positions of the diffraction lines. Lattice parameters were calculated from the complete set of peaks in the XRD pattern using the DICVOL04 software. The polished cross sections of the oxidized specimens were examined via scanning electron microscopy (SEM) performed on a Hitachi S-4800 SEM-FEG microscope coupled with an X-ray energy dispersive spectrometry (XEDS) detector 5 (Quantax-EDS, Bruker Corporation), which was used for the chemical semiquantification of the ceramic, binder and oxidized phases. 3. Results and Discussion. 3.1. Dependence of oxidation on temperature and tantalum content. The weight gain normalized to the surface area of the material (per unit area) after 48 h of oxidation for all of the temperatures and cermets is shown in figure 1. A significant decrease in the extent of oxidation is clearly observed for all of the temperatures when the Ta content is increased. Cermet without Ta (Ti100) exhibited a non-negligible oxidation (18.6 mg/cm2), even at the lowest oxidation temperature tested (800 ºC). However, cermet Ti90Ta10 with a higher Ta content required a substantially higher temperature (1100 ºC) to produce a similar oxidation level (19.5 mg/cm2). At 1100 ºC, cermet Ti80Ta20 showed a low oxidation of 9.5 mg/cm2, which was lower than that of cermet Ti100 at 800ºC. At 800 ºC, the oxidation of cermets Ti95Ta5, Ti90Ta10 and Ti80Ta20 was practically imperceptible. In contrast, cermets Ti100 and Ti99Ta1 were extensively oxidized at 1100 ºC. To quantify the improvement in the oxidation resistance of cermets when Ti is partially substituted by Ta, the percentage reduction of the weight gain per unit area for each cermet at each temperature using the Ti100 cermet as reference is presented in table 1. By replacing only 1 at% of Ti with Ta (Ti99Ta1), oxidation was reduced by 67.2% at 800ºC. This improvement remained high for Ti99Ta1 until 1100 ºC (46.3%). At this high temperature, the enhancement of oxidation resistance increases with increasing Ta content, reaching a remarkable 92.8% for Ti80Ta20. An improvement of at least 90% is also achieved in cermets Ti95Ta5 and Ti90Ta10 but at lower temperatures of 900 ºC and 1000 ºC, respectively. To corroborate the thermogravimetric results and to analyze the phases formed during the oxidation tests, XRD patterns of oxidized cermets reduced to powder are compared in figure 2. The XRD pattern corresponding to oxidized Ti100 cermet at a low temperature (800 ºC) clearly shows the presence of rutile TiO2 (tetragonal structure, P42/mnm, 136) due to the cermet oxidation. The intensity of the TiO2 reflections increases with the oxidation temperature as the oxidation progresses. Moreover, new 6 reflections that can be assigned to cobalt methatitanate (CoTiO3, rhombohedral structure, R-3, 148) are detected from 900 ºC. The existence of this Co-containing phase is due to the presence of Co in the binder phase, which is also oxidized. Simultaneously, the intensity of the original ceramic and binder phases is considerably reduced. At 1000 ºC, the unoxidized phases are only slightly detected in the XRD patterns, suggesting a high degree of oxidation, in accordance with the thermogravimetric results. At 1100 ºC, the XRD pattern is practically dominated by the TiO2 rutile reflections. Note that as described in the literature [19], the binder phase in cermets developed by MSR was composed of a mixture of TiCo (cubic structure, Pm-3m, 221) and TiCo2 (cubic structure, Fd-3m, 227) intermetallic phases. Therefore, the presence in the oxidized specimen XRD patterns of reflections corresponding to a α-Co alloy (cubic structure, Fm-3m, 225) implies the preferential oxidation of Ti vs Co in the binder phase. The XRD patterns of oxidized Ti99Ta1 cermet (figure 2) show a similar trend to that of Ti100 cermet as the oxidation progresses with temperature. However, the improvement in the oxidation resistance by substituting only 1 at% of Ti with Ta is evidenced by the delay in the evolution of oxidized phases, corroborating the thermogravimetric results in figure 1. A comparison of the XRD patterns of the oxidized cermets reveals that the patterns are similar but shifted by 100 ºC, e.g., XRD patterns at 900 ºC, 1000 ºC and 1100 ºC for Ti99Ta1 cermet are equivalent to those at 800 ºC, 900 ºC and 1000 ºC for Ti100 cermet, respectively. This result indicates that oxidation was deferred by approximately 100 ºC. For Ti95Ta5 cermet, the XRD patterns in figure 2 indicate that the oxidation remained at a low level until 1100 ºC in which the reflections of the original unoxidized phases were still clearly visible. The XRD pattern of Ti95Ta5 cermet at 1100 ºC was similar to that of Ti99Ta1 and Ti100 cermets at 900 ºC and 800 ºC, respectively, showing again the positive effect of Ta substitution on oxidation resistance. In this case, when Ti was replaced with 5 at% of Ta, the oxidation was delayed by approximately 300 ºC compared with cermets without Ta. For Ti90Ta10 and Ti80Ta20 cermets, with the highest Ta contents of 10 and 20 at%, respectively, the XRD patterns in figure 2 show an even greater oxidation resistance. The XRD patterns of Ti80Ta20 cermet are dominated by the unoxidized 7 phases over the entire temperature range studied. Only at 1000 ºC, the most intense reflections of the rutile structure begin to emerge from the background signal. Note that single tantalum oxides, such as Ta2O5 or TaO2, were not detected in the XRD patterns of oxidized cermets (figure 2). However, displacements toward the lower 2θ angles were observed in the reflections corresponding to the rutile structure when the Ta amount was increased. This effect is illustrated in figure 3 in which the XRD patterns over the range of the (110) and (211) reflections for TiO2 and TaO2 are shown for the five different cermets oxidized at 1100 ºC. The observed shift is equivalent to an increase in the unit cell parameters as shown in table 2 and suggests that Ta is introduced into the TiO2 lattice, resulting in the formation of a TixTa1-xO2 solid solution with rutile-type structure (tetragonal structure, P42/mnm, 136). 3.2. Surface characterization of the oxidized cermets. To obtain precise information on the nature of the oxide scale formed in oxidized cermets, XRD patterns of the surfaces exposed to oxidation were recorded and are shown in figure 4. In the XRD pattern of Ti100 cermet oxidized at 800 ºC, CoO (cubic structure, Fm-3m, 225) and Co3O4 (cubic structure, Fd-3m, 227) are the only phases observed. However, these oxides were not detected at this temperature in the XRD pattern of figure 2 in which only unoxidized phases and low intensity peaks corresponding to TiO2 were identified. These differences are due to the penetration depth of the X-ray and the fact that both of the cobalt oxides must be located precisely at the outermost layer of the oxide scale, preventing the detection of other inwardly located phases. At 900 ºC, CoO appears as the major phase because of the instability of Co3O4, according to reaction (1), although the marked (200) preferred orientation observed (2 ≈ 42.4º) could also be at the origin of this predominance in the XRD pattern. 243 OCoO6OCo2 (1) Note that the presence of external CoO and Co3O4 implies the migration of cobalt through the TiO2 oxide layer, which is supposed to be the main product of oxidation, towards the surface of cermets and its subsequent oxidation. The formation of an outermost region of nearly pure cobalt oxide has been frequently observed during the oxidation of Co-base alloys [20, 21]. 8 At higher temperatures (1000 ºC and 1100 ºC) (figure 4), when the oxidation has significantly progressed, TiO2 emerges at the surface of the cermet, while cobalt oxides disappear by reacting with TiO2 rutile to form cobalt methatitanate (CoTiO3) or cobalt orthotitanate (Co2TiO4, cubic structure, Fd-3m, 227), according to reactions (2-5), as follows [22, 23]: 32 CoTiOTiOCoO  (2) 23243 OCoTiO6TiO6OCo2 (3) 422 TiOCoTiOCoO2  (4) 242243 OTiOCo3TiO3OCo2 (5) It is possible to consider the formation of Co2TiO4 from CoTiO3, according to the following reaction (6): 423 TiOCoCoTiOCoO  (6) The XRD patterns of oxidized surfaces of Ti99Ta1 cermet (figure 4) are generally similar to those corresponding to Ti100 cermet (with the same shift in temperature as previously observed in figure 2), although certain differences are also noticed. The first difference is the presence of rutile and small reflections of cobalt titanates (CoTiO3 and Co2TiO4) at 800 ºC. For Ti99Ta1 cermet, cobalt oxides dominate the XRD pattern of the surface at higher temperatures, over the 900-1000 ºC temperature range. Second, the XRD pattern of the oxidized surface at 1100 ºC shows the presence of intense reflections of Co2TiO4 and CoTi2O5 (orthorhombic structure, Cmcm, 63). This result is in contrast to the pattern for Ti100 cermet at this temperature in which only TiO2 was detected. According to the binary CoO/Co3O4-TiO2 phase diagram [22], cobalt dititanate (CoTi2O5) is stable at temperatures above ~1140 ºC, and its formation at these temperatures can occur by reactions (7-10), as follows: 5223 OCoTiTiOCoTiO  (7) 2522 OOCoTiTiO2CoO  (8) CoOOCoTiCoTiO2 523  (9) 9 52422 OCoTi2TiOCoTiO3  (10) The existence of a higher proportion of cobalt titanates, especially Co2TiO4, at lower temperatures becomes more apparent with increasing Ta content (Ti95Ta5, Ti90Ta10 and Ti80Ta20 in figure 4). These observations suggest that the substitution of Ti4+ by Ta5+ in the rutile phase and the formation of Ti3+ in the structure for charge compensation [24] must increase the reactivity of this phase and promote the formation of cobalt titanates at lower temperatures through reactions (2-5). Moreover, note that cobalt oxides (CoO and Co3O4) were not observed in the oxidized surfaces of Ti90Ta10 and Ti80Ta20 cermets over the entire temperature range studied. For both of the cermets, only the rutile, Co2TiO4 and CoTiO3 phases are observed. It is then possible to propose for these cermets the direct reaction between cobalt and rutile to produce cobalt titanates at the surface through reactions (11 and 12) instead of reactions (2-5). The stoichiometries proposed in reactions (11 and 12) are based on the XEDS results by SEM (shown later), showing that cobalt titanates do not contain Ta and revealing the existence in the same specimen of a TixTa1-xO2 solid solution with different Ti/Ta ratios. 213221 OTaTiCoTiOO21OTaTiCo yyxx   (11) 2142221 OTaTiTiOCoOOTaTi2Co yyxx   (12) The XRD pattern in figure 4 for the Ti80Ta20 cermet at 1100 ºC only shows the presence of Co2TiO4 and CoTiO3. Maintaining these cobalt titanates as an outermost layer in the oxide scale at higher temperatures should be related to the improvement in oxidation resistance of cermets and would indicate a more protective effect than that provided by the rutile layer only. Furthermore, it has been shown that Ta5+ doping in the rutile phase decreases the oxygen vacancy concentration, hindering the oxygen mass transport [25] through the oxide scale, which should also decelerate the oxidation of cermets [26]. Finally, note that except for Ti80Ta20 oxidized at 800 ºC, the XRD patterns of oxidized surfaces for Ti90Ta10 and Ti80Ta20 cermets do not show the presence of unoxidized phases, although TG measurements evidenced a low degree of oxidation. This fact is related again to the penetration depth of the X-rays that as confirmed by 16 [9] P. Ettmayer, H. Kolaska, W. Lengauer, K. Dreyer, Ti(C,N) Cermets - Metallurgy and Properties, International Journal of Refractory Metals and Hard Materials, 13 (1995) 343. [10] H. Pastor, Titanium-carbonitride-based hard alloys for cutting tools, Materials Science and Engineering, 105-106 (1988) 401-409. [11] S. Bolognini, G. Feusier, D. Mari, T. Viatte, W. Benoit, High temperature mechanical behaviour of Ti(C,N)-Mo-Co cermets, International Journal of Refractory Metals & Hard Materials, 16 (1998) 257-268. [12] C. Mroz, Titanium diboride, American Ceramic Society Bulletin, 74 (1995) 158-159. [13] C. Martin, B. Cales, P. Vivier, P. Mathieu, Electrical-discharge machinable ceramic composites, Materials Science and Engineering a-Structural Materials Properties Microstructure and Processing, 109 (1989) 351-356. [14] V.J. Tennery, C.B. Finch, C.S. Yust, G.W. Clark, Science of Hard Materials, Plenum, New York, 1983. [15] R.A. Alliegro, Encyclopedia of electrochemistry, Reinhold, New York, 1964. [16] B.V.M. Kumar, J.R. Kumar, B. 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Rizzo, M.J. Monteiro, The corrosion of two Co-Nb alloys under 1 ATM O-2 at 600-800 degrees C, Corrosion Science, 38 (1996) 193-211. [22] K.T. Jacob, G. Rajitha, Role of entropy in the stability of cobalt titanates, Journal of Chemical Thermodynamics, 42 (2010) 879-885. [23] A. Przepiera, J. Plaska, K. Przepiera, M. Jablonski, A. Konratowska, Preparation of cobalt titanates via co-precipitation while using industrial intermediates as titanium precursors, Polish Journal of Chemical Technology, 11 (2009) 51-54. [24] T. Yamamoto, T. Ohno, Screened hybrid density functional study on Nb- and Ta-doped TiO2, Physical Review B, 85 (2012). [25] V. Guidi, M.C. Carotta, M. Ferroni, G. Martinelli, M. Sacerdoti, Effect of dopants on grain coalescence and oxygen mobility in nanostructured titania anatase and rutile, Journal of Physical Chemistry B, 107 (2003) 120-124. [26] M. Pfeiler, C. Scheu, H. Hutter, J. Schnoeller, C. Michotte, C. Mitterer, M. Kathrein, On the effect of Ta on improved oxidation resistance of Ti-Al-Ta-N coatings, Journal of Vacuum Science & Technology A, 27 (2009) 554-560. 17 FIGURE CAPTIONS. Figure 1. Weight gain per unit area after 48 h of oxidation for all of the cermets and oxidation temperatures. Figure 2. XRD patterns of oxidized cermets reduced to powder after the 48 h oxidation tests. (a) 800 ºC; (b) 900 ºC; (c) 1000 ºC; (d) 1100 ºC. Phases: () Unoxidized ceramic phase: TixTa1-xC0.5N0.5; () Unoxidized binder phase: TixTa1-xCo; (▼) Unoxidized binder phase: TixTa1-xCo2; () TixTa1-xO2 with rutile structure; () CoTiO3; () -Co. Figure 3. XRD patterns of oxidized cermets reduced to powder after the 48 h oxidation tests at 1100 ºC over the range of (110) and (211) reflections for TiO2 and TaO2 with the rutile structure. a) Ti100; b) Ti99Ta1; c) Ti95Ta5; d) Ti90Ta10; and e) Ti80Ta20. Figure 4. XRD patterns of the surface of the oxidized cermets after the 48 h oxidation tests. (a) 800 ºC; (b) 900 ºC; (c) 1000 ºC; (d) 1100 ºC. Phases: () Unoxidized ceramic phase TixTa1-xC0.5N0.5; (o) Co3O4; (Δ) CoO; () TixTa1-xO2 with rutile structure; () CoTiO3; (+) Co2TiO4; () CoTi2O5. Figure 5. Cross-sectional SEM images of the different cermets oxidized at 800 ºC, showing the approximate measurements of the oxidation penetration depth. The dotted lines separate the completed oxidation zones, the internal degradation zones and the unoxidized cermet areas. Figure 6. Cross-sectional SEM images of the cermet Ti95Ta5 oxidized at 800 ºC, 900 ºC, 1000 ºC and 1100 ºC, showing the approximate measurements of the oxidation penetration depth. The dotted lines separate the completed oxidation zones, the internal degradation zones and the unoxidized cermet areas. Figure 7. A characteristic XEDS-SEM mapping of Ti100 cermet oxidized at 800ºC, corroborating the clear differentiation between the oxidized / unoxidized phases and used to measure the oxidation penetration depth showed in figures 5 and 6. Figure 8. Representative cross-sectional SEM images of cermets Ti100 and Ti99Ta1 showing the oxidized phases in the completed oxidation zone as determined by XEDSSEM. a) Ti100 at 800 ºC; b) Ti100 at 1000 ºC; c) Ti99Ta1 at 800 ºC; d) Ti99Ta1 at 18 1000 ºC; e) Ti99Ta1 at 1100 ºC. (1) CoO; (2) Co3O4; (3) TiO2; (4) CoTiO3; (5) Co2TiO4; (6) CoTi2O5. Figure 9. Representative cross-sectional SEM images of cermets Ti95Ta5, Ti90Ta10 and Ti80Ta20 showing the phases in the completed oxidation zone as determined by XEDS-SEM. a) Ti95Ta5 at 900 ºC; b) Ti99Ta5 at 1100 ºC; c) Ti90Ta10 at 800 ºC; d) Ti80Ta20 at 1000 ºC; e) Ti90Ta10 at 1000 ºC. (1) CoO; (3a), (3b) and (3c) TixTa1-xO2; (4) CoTiO3; (5) Co2TiO4. Figure10. Representative cross-sectional SEM images of the internal degradation zone [a) Ti100 at 1000 ºC; b) Ti99Ta1 at 1000 ºC; c) Ti95Ta5 at 1100 ºC] and the oxidation front [d) Ti100 at 900 ºC; e) Ti80Ta20 at 1100 ºC] for the different oxidized cermets, showing the phases in these regions as determined by XEDS-SEM. (3) TixTa1-xO2 (7) - Co. Figure 11. Representative semi-quantitative XEDS-SEM spectra of oxidized phases found in the different areas analyzed and previously detected by XRD. 19 TABLES. Table 1. Percentage reduction of the weight gain per unit area for each cermet at each oxidation temperature compared to the weight gain per unit area of the Ti100 cermet (reference material) after the 48h oxidation tests. Cermet 800ºC 900ºC 1000ºC 1100ºC Ti100 - - - - Ti99Ta1 67.2 67.2 48.0 46.3 Ti95Ta5 89.2 90.7 84.9 72.8 Ti90Ta10 94.1 94.2 89.3 85.6 Ti80Ta20 95.6 96.3 94.0 92.8 20 Table 2. Lattice parameters for the TixTa1-xO2 solid solution with rutile-type structure determined from the X-ray diffractograms of cermets oxidized at 1100ºC. Cermet a=b (Å) c (Å) Ti100 4.5933 2.9592 Ti99Ta1 4.5935 2.9608 Ti95Ta5 4.6025 2.9666 Ti90Ta10 4.6139 2.9761 Ti80Ta20 4.6314 2.9823 21 Table 3. Punctual semi-quantitave compositions calculated from XEDS-SEM spectra performed in different zones of the oxide scale and internal oxidation region of the oxidized cermets. The numbers in figures 8-10 mark the analyzed zones. The nature of phases that best suits the results of the analyses is also shown. Zone Ti (at%) Ta (at%) Co (at%) O (at%) Approximate phase stoichiometry 1 - - 50.4 49.6 CoO 2 - - 58.4 41.6 Co3O4 3 34.3 - - 65.7 TiO2 3a 28.1 1.7 - 70.2 TixTa1-xO2 3b 25.5 4.8 - 69.7 3c 20.8 8.8 - 70.4 4 24.7 - 22.7 52.6 CoTiO3 5 15.8 - 29.6 54.6 Co2TiO4 6 25.4 - 13.1 61.5 CoTi2O5 7 3.9 1.3 94.8 - α-Co 0 5 10 15 20 25 30 35 0 20 40 60 80 100 120 140 Ti80Ta20 Ti90Ta10 Ti95Ta5 Ti99Ta1 Ti100 Ta (wt.%) m48 (mg/cm2) 800ºC 900ºC 1000ºC 1100ºC Figure 1 20 30 40 50 60 70 80  Ti90Ta10 I (ncps) 2 (º)  ·                     · · · · · · · · · · · · · · · · · · ·  (d) (c) (b) (a) 20 30 40 50 60 70 80 * · © © Ti100 (d) (c) (b) (a)  ©     · · · · ¨   ·   © © * * * * * * *       ¨          · · · · ·              © 2 (º) I (ncps) 20 30 40 50 60 70 80 Ti99Ta1 I (ncps) 2 (º) *    · ¨ · ·  · · · * *  · ·   · ¨ © ©             ¨   · · · ¨ · ·         · © ¨     ·   (d) (c) (b) (a) 20 30 40 50 60 70 80  © I (ncps) 2 (º) Ti95Ta5   ¨  · ·         ¨ ¨ ¨            · · · · · · · · · · · · · · · · · · (d) (c) (b) (a) 20 30 40 50 60 70 80 Ti80Ta20                       · · · · · · · · · · · · · · · · · · · ·  I (ncps) 2 (º) (d) (c) (b) (a) Figure 2 Figure 3 52 53 54 55 56 57 TiO2 2 (º) I (ncps) TaO2 e) d) c) b) a) (211) 25 26 27 28 29 30 e) d) c) b) a) (110) TaO2 TiO2 2 (º) I (ncps) 20 30 40 50 60 70 80   * +            2 (º) I (ncps) Ti100 * * +                (a) (d) (c) (b)   20 30 40 50 60 70 80                  I (ncps) 2 (º) Ti99Ta1  + + + + +  +    +      * +     (a) (d) (c) (b) 20 30 40 50 60 70 80 +           *  I (ncps) 2 (º) Ti95Ta5  * * + * * + ** +       + + + +      * + + + + + + + + *   + + + + + +     + + * +* * * + + (a) (d) (c) (b) 20 30 40 50 60 70 80 Ti90Ta10 I (ncps) 2 (º) * + + + + *  * *    + + *   + *  + + +  * + +          + * * * * + + +     · · · + + * * * * (a) (d) (c) (b) 20 30 40 50 60 70 80 * I (ncps) 2 (º) * * * * Ti80Ta20 + + + +  * *    * * · · * * + + + + * * * + + + +   +   + * + + + + + + + * * * *      · ·  * *   (a) (d) (c) (b) Figure 4 0 2 4 6 8 10 12 0.0 0.2 0.4 0.6 0.8 1.0 1.2 CoO OK CoL CoK I (ncps) E (keV) 0 2 4 6 8 10 12 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Co3O4 CoK CoL OK I (ncps) E (keV) 0 2 4 6 8 10 12 0.0 0.2 0.4 0.6 0.8 1.0 1.2 TixTa1-xO2 TaL TaM TiK I (ncps) E (keV) OK 0 2 4 6 8 10 12 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Co2TiO4 OK · · TiL CoL TiK CoK I (ncps) E (keV) 0 2 4 6 8 10 12 0.0 0.2 0.4 0.6 0.8 1.0 1.2 CoTi2O5 OK I (ncps) E (keV) CoL · TiK CoK · TiL 0 2 4 6 8 10 12 0.0 0.2 0.4 0.6 0.8 1.0 1.2 · CoTiO3 I (ncps) E (keV) OK · TiL CoL CoK TiK 0 2 4 6 8 10 12 0.0 0.2 0.4 0.6 0.8 1.0 1.2 -Co TiK CoK CoL I (ncps) E (keV) 0 2 4 6 8 10 12 0.0 0.2 0.4 0.6 0.8 1.0 1.2 TiO2 · OK · TiLTiK I (ncps) E (keV) Figure 11