Densification,microstructures, and mechanical properties of (Zr,Ti)(C,N) ceramics fabricated by spark plasma sintering

Dense (Zr, Ti) (C, N) ceramics were fabricated by spark plasma sintering (SPS) at 1900–2000 °C using ZrC, TiCN and ZrH₂ powders as raw materials. A single Zr-rich (Zr, Ti)(C, N) solid solution was formed in Zr_0.95Ti_0.05C_0.975N_0.025 and Zr_0.80Ti_0.20C_0.90N_0.10 ceramics (nominal composition). A Ti-rich solid solution appears in Zr_0.50Ti_0.50C_0.75N_0.25 ceramics. The coaddition of TiCN and ZrH₂ promoted the densification of (Zr, Ti) (C, N) ceramics by forming solid solutions and carbon vacancies, which could reduce critical resolved shear stress (CRSS) and promote carbon and metal atom diffusion. ZrC-45 mol% TiCN-10 mol% ZrH₂ (raw powder composition) possesses good comprehensive mechanical properties (Vickers hardness of 24.5 ± 0.9 GPa, flexural strength of 503 ± 51 MPa, and fracture toughness of 4.3 ± 0.2 MPa·m^1/2), which reach or exceed most ZrC-based (Zr, Ti) C and (Zr, Ti) (C, N) ceramics in previous reports.     

Microstructure and ablation behavior of carbon/carbon composites infiltrated with Zr–Ti

Carbon/carbon(C/C) composites infiltrated with Zr–Ti were prepared by chemical vapor infiltration and reactive melt infiltration. Their microstructure and ablation behavior at different temperatures and time were investigated. The results show that C/C composites infiltrated with Zr–Ti have good interface cohesion between carbon fibers, pyrocarbon and carbide. Compared with C/C composites and C/C–ZrC composites, the synthesized sample with Zr_0.83Ti_0.17C_0.92 and Ti_0.82Zr_0.18C_0.92 exhibits better ablation resistance at 2500 °C due to the newly formed protective layer composed of ZrTiO₂ pinned by ZrO₂ grains after ablation. The ablation resistance of the sample with Zr_0.57Ti_0.43C_1.01 increased gradually with the decrease of temperature from 3000 °C to 2000 °C, whereas the ablation resistance of the sample with Zr_0.83Ti_0.17C_0.92 and Ti_0.82Zr_0.18C_0.92 first increased obviously and then decreased slightly. In addition, the work indicates that surplus particles or liquid phases of oxides cannot protect the matrix, and that the liquid oxides may even cause severe ablation. Furthermore, a protective layer of oxides tends to be formed with the increase of ablation time.     

Zirconium Incorporation into CaTiO3 Perovskite Prepared from Xerogels and Implication for the Fate of (Ca,Sr)TiO3 Nuclear Waste Ceramics

CaTiO₃ perovskite has been proposed as a ceramic waste form for immobilization of ⁹⁰Sr. Nonradioactive coprecipitated xerogel powders with nominal atomic ratios of Ca:Zr:Ti = 0.75:0.25:1.00 were synthesized to mimic the fate of (Ca_0.75⁹⁰Sr_0.25)TiO₃ solid solution after complete decay of the Sr and its intermediate product Y to stable Zr when an excess B⁴⁺ (Ti and ⁹⁰Zr) cations will present. Ca:Ti = 1.00:1.00 samples were used as a reference. The powders were heated to various conditions to explore the thermodynamic stability of its oxides. The heated Ca:Zr:Ti = 0.75:0.25:1.00 samples formed a major orthorhombic Ca(Zr_1?xTi_x)O₃ perovskite phase. The Ti/(Ti + Zr) ratio of the perovskite preserves its nominal ratio at 600°C. The Zr rejects from the Ca(Zr_1?xTi_x)O₃ with further increasing the temperature, following the formation of Ca–Ti–Zr–O secondary phases. This study indicates a tendency of the Zr to segregate from an original (Ca,Sr)TiO₃ waste form when the stoichiometry is controlled by the conversion of Sr to Zr (in normal oxidation states).     

High‐Strength ZrC Ceramics Doped with Aluminum

High‐strength ZrC ceramics with relative density above 98% were prepared by reactive hot pressing of ZrC and Al at 1900°C. The reaction between ZrC and Al resulted in the formation of ZrC_1?x, Zr₃Al₃C₅ and Zr–Al compound such as AlZr₃ and Al–C–Zr. The intermediate product AlZr₃ below 1600°C and remained Al–C–Zr phase could form liquid phase and promoted the first stage of densification process. The improvement in densification behavior at higher temperatures (1800°C–1900°C) could be attributed to the formation of nonstoichiometric ZrC_1?x. Adding 5 wt% and 7.5 wt% Al to ZrC, the formed ZrC_0.85–Zr₃Al₃C₅ and ZrC_0.80–Zr₃Al₃C₅ based ceramics had 3‐point bending strength as high as 757 ± 79 MPa and 967 ± 50 MPa, respectively, with hardness and fracture toughness being 16.2–18.3 GPa and 3.3–3.5 MPa m^1/2, respectively.     

Pyroelectric energy harvesting using low–TC (1–x)(Ba0.7Ca0.3)TiO3–xBa(Zr0.2Ti0.8)O3 bulk ceramics

Lead‐free ferroelectric ceramics (1–x)(Ba_0.7Ca_0.3)TiO₃–xBa(Zr_0.2Ti_0.8)O₃ (BCTZ100x) with x = 0.20, 0.30, 0.40, 0.50, 0.60, 0.70, and 0.80 were evaluated for their pyroelectric energy harvesting performance, using the Olsen cycle. As the composition ratio x increased, the crystal phase changed to tetragonal, orthorhombic, rhombohedral, and cubic; the phase boundaries crossed each other in the vicinity of BCTZ70. The crossover phase transition behavior between first‐order and diffuse phase transition changed to only the diffusion phase transition with increasing x. A pinching effect occurred because an increase in dielectric constant was also observed. Energy densities N_D of 229 mJ/cm³ and 256 mJ/cm³ for BCTZ50 and BCTZ80 were obtained, respectively, in temperature of 30°C‐100°C and an electric field of 0‐30 kV/cm. These N_D values are over two times higher than that of soft–Pb(Ti,Zr)O₃ (PZT), which exhibits piezoelectric performance equivalent to BCTZ50 at room temperature. Compared with soft–PZT, BCTZ50 and BCTZ80 exhibited larger N_D values owing to their lower Curie temperatures (T_C ~ 50°C‐110°C). We conclude that low–T_C ferroelectrics are useful for pyroelectric energy conversion based on the Olsen cycle even if they are unsuitable for piezoelectric applications at high temperatures.     

Oxidation behaviors of (Ti1-xNbx)C ceramic solid solutions

(Ti_1-xNb_x)C (x=0, 0.2, 0.5 and 0.8) ceramic solid solutions were prepared by spark plasma sintering and their oxidation behaviours were investigated at 1000 °C in air. The niobium content was found to exhibit a remarkable influence on the oxidation resistance of (Ti_1-xNb_x)C solid solutions. An optimization of oxidation resistance was achieved in (Ti_0.8Nb_0.2)C. After oxidation at 1000 °C for 4 h, the oxidation layer thickness of (Ti_0.8Nb_0.2)C is less than 1/4 of the oxidation layer thickness of monolithic TiC. The higher oxidation resistance can be ascribed to the presence of Nb-doped rutile TiO₂ phase in the oxidation layer of (Ti_0.8Nb_0.2)C. The (Ti,Nb)O₂ phase suppressed the abnormal grain growth and the formation of cracks in the oxide layer, more importantly, it could effectively sluggish the outward diffusion of titanium during oxidation.     

Reaction,densification, and mechanical properties of Ti2AlCx ceramics at low applied pressure and temperature

Ti₂AlC_x ceramic was produced by reactive hot pressing (RHP) of Ti:Al:C powder mixtures with a molar ratio of 2:1:1–.5 at 10–20 MPa, 1200–1300°C for 60 min. X-ray diffraction analysis confirmed the Ti₂AlC with TiC, Ti₃Al as minor phases in samples produced at 10–20 MPa, 1200°C. The samples RHPed at 10 MPa, 1300°C exhibited ≥95 vol.% Ti₂AlC with TiC as a minor phase. The density of samples increased from 3.69 to 4.04 g/cm³ at 10 MPa, 1200°C, whereas an increase of pressure to 20 MPa resulted from 3.84 to 4.07 g/cm³ (2:1:1 to 2:1:.5). The samples made at 10 MPa, 1300°C exhibited a density from 3.95 to 4.07 g/cm³. Reaction and densification were studied for 2Ti–Al–.67C composition at 10 MPa, 700–1300°C for 5 min showed the formation of Ti–Al intermetallic and TiC phases up to 900°C with Ti, Al, and carbon. The appearance of the Ti₂AlC phase was ≥1000°C; further, as the temperature increased, Ti₂AlC peak intensity was raised, and other phase intensities were reduced. The sample made at 700°C showed a density of 2.87 g/cm³, whereas at 1300°C it exhibited 3.98 g/cm³; further, soaking for 60 min resulted in a density of 4.07 g/cm³. Microhardness and flexural strength of Ti₂AlC_0.8 sample were 5.81 ± .21 GPa and 445 ± 35 MPa.     

High‐Energy Density Dielectrics and Capacitors for Elevated Temperatures: Ca(Zr,Ti)O3

The Ca(Zr_1?xTi_x)O₃ (CZT) solid solution system is a linear dielectric that is of interest for high‐temperature power capacitor applications. This dielectric was synthesized by conventional solid‐state processes and prototyped into single‐layer CZT capacitors, which were fabricated with interdigitated electrodes cofired between dielectric layers ~10‐μm thick. The dielectric properties of these capacitors were extensively investigated as a function of Zr/Ti ratio on the CaZrO₃ side of the solid solution. The electrostatic energy density and breakdown strength of the CZT capacitors were investigated by measuring polarization–electric field curves as a function of temperature from room temperature to 250°C. The Ca(Zr_0.80Ti_0.20)O₃ capacitors show high electrostatic energy density of 4 J/cm³ at 250°C. Highly accelerated life testing (HALT) was also performed on these dielectrics, and we quantified the thermal stimulated depolarization current (TSDC) to access the major point defects in the CZT system.     

Mesoporous CuO/TixZr1 − xO2 catalysts for low-temperature CO oxidation

Mesoporous CuO/Ti_xZr_1 − xO₂ catalysts were prepared by a surfactant-assisted method, and characterized by N₂ adsorption/desorption, TEM, XPS, in-situ FTIR and H₂-TPR. The catalysts exhibited high specific surface area (S_BET = 241 m²/g) and uniform pore size distribution. XPS and in-situ FTIR displayed that Cu⁺ and Cu²⁺ species coexisted in the catalysts. The CuO/Ti_xZr_1 − xO₂ catalysts presented obviously higher activity in CO oxidation reaction than the CuO/TiO₂ and CuO/ZrO₂ catalysts. Effect of molar ratios of Ti to Zr and calcination temperature on catalytic activity was investigated. The CuO/Ti_0.6Zr_0.4O₂ catalyst calcined at 400 °C exhibited excellent activity with 100% CO conversion at 140 °C.     

The synthesis and mechanical properties of high pure Ti2Al(Sn)C solid solution

High pure Ti₂Al_(1?x)Sn_xC (x = 0‐1) powders were synthesized using Ti, Al, Sn, and TiC powders as raw materials by pressureless sintering method. The influence of sintering temperature and raw material ratio on the purity of Ti₂AlC and Ti₂Al_0.8Sn_0.2C powders were investigated. The results show that pure Ti₂AlC and Ti₂Al_0.8Sn_0.2C powders were obtained from the mixed raw materials ratio of Ti:1.1Al:0.9TiC and Ti:0.9Al:0.2Sn:0.9TiC at 1450°C, respectively. Subsequently, fully dense Ti₂AlC and Ti₂Al_0.8Sn_0.2C bulks were prepared using mechanically alloying and hot pressed sintering method. The Vickers hardness of Ti₂AlC and Ti₂Al_0.8Sn_0.2C approaches approximately about 6 GPa and 4 GPa, the flexural strength was measured to be 650 ± 36 MPa and 521 ± 33 MPa, respectively. Microstructural analysis reveals that grain delamination, kink bands, and crack deflection occurred around the indentation area and at the fracture surface.     

Chlorobenzene total oxidation over palladium supported on ZrO2, TiO2 nanostructured supports

Two series of supported Pd catalysts were synthesized on new mesoporous–macroporous supports (ZrO₂, TiO₂) labelled M (Zr and Ti). The deposition of palladium was carried out by wet impregnation on the calcined TiO₂ and ZrO₂ supports at 400 °C (Pd/Zr₄, Pd/Ti₄) and 600 °C (Pd/Zr₆, Pd/Ti₆) and followed by a calcination at 400 °C for 4 h. The pre-reduced Pd/MX catalysts were investigated for the chlorobenzene total oxidation and their catalytic properties where compared to those of a reference catalyst Pd/Ti-Ref (TiO₂ from Huntsman Tioxide recalcined at 500 °C) and of a palladium supported on the fresh mesoporous–macroporous TiO₂ (Pd/Ti). Based on the activity determined by T₅₀, the Pd/Ti and Pd/Ti₄ catalysts have been found to be more active than the reference one. Moreover activity decreased owing to the sequence: Pd/TiX  Pd/ZrX and in each series when the temperature of calcination of the support was raised. The overall results clearly showed that the activity was dependant on the nature of the support. The better activity of Pd/TiX compared to Pd/ZrX was likely due to a better reducibility of the TiO₂ support (Ti⁴⁺ into Ti³⁺) leading to an enhancement of the oxygen mobility. Production of polychlorinated benzenes PhCl_x (x = 2–6) and of Cl₂ was also observed. Nevertheless at 500 °C the selectivity in HCl was higher than 90% for the best catalysts.     

Synthesis and Microstructural Characterization of Substoichiometric Ti2Al(CxNy) Solid Solutions and Related Ti2AlCx and Ti2AlN End‐Members

Ti, TiC, Al and AlN powders were mixed to synthesize Ti₂Al(C_xN_y) (x + y < 1) solid solutions, Ti₂AlC_x (x < 1) and Ti₂AlN‐related end‐members by hot isostatic pressing at 1400°C/80 MPa for 4 h. For the pure carbides, it is demonstrated that single‐phased samples can only be obtained when about 15% of substoichiometry on the carbon site is applied. Such a result likely implies that Ti₂AlC_x can only exist in a narrow range of carbon composition. Ti₂AlN nitride can be synthesized with y = 1. Assuming that vacancy content varies linearly from 0 to 0.15 going from Ti₂AlN to Ti₂AlC_0.85 in the solid solutions, element concentrations have been calculated to synthesize different solid solutions. Thus, it is demonstrated that single‐phased and fully dense Ti₂Al(C_0.23N_0.71), Ti₂Al(C_0.45N_0.45), and Ti₂Al(C_0.66N_0.22) carbonitrides can be synthesized.     

Preparation and tunability properties of Ba(ZrxTi1−x)O3 thin films grown by a sol–gel process

Ba(Zr_xTi_1−x)O₃ (BZT) thin films were deposited via sol–gel process on LaNiO₃, as buffer layer, and Pt-coated silicon substrates. The BZT films were perovskite phase and showed a (1 0 0) preferred orientation dependent upon zirconium content. The grain size decreased and the microstructure became dense with increasing zirconium content. The addition of Zr to the BaTiO₃ lattice decreased the grain size of the crystallized films. The temperature dependent dielectric constant revealed that the thin films have relaxor behavior and diffuse phase transition characteristics that depend on the substitution of Zr for Ti in BaTiO₃. The dependence of electrical properties on film thickness has been studied, with the emphasis placed on dielectric nonlinear characteristics. Ba(Zr_0.35Ti₆₅)O₃ thin films with weak temperature dependence of tunability in the temperature range from 0 to 130 °C could be attractive materials for situations in which precise control of temperature would be either impossible or too expensive.     

Oxidation Behavior of MAX Phase Ti2Al(1−x)SnxC Solid Solution

MAX phase Ti₂Al_(1?x)Sn_xC solid solution with x = 0, 0.32, 0.57, 0.82, and 1 was synthesized by pressureless sintering of uniaxially pressed Ti, Al, Sn, and TiC powder mixtures. Annealing in air atmosphere at 200°C–1000°C triggered a sequence of oxidation reactions which reveal a distinct influence of solid solution composition on the oxidation process. With decreasing Al/Sn ratio, the characteristic temperature of accelerated oxidation reaction of A‐element was reduced from 900°C (x = 0) to 460°C (x = 1). SnO₂ was formed at temperatures significantly lower than TiO₂ (rutile) and Al₂O₃. Substitution of A‐element in MAX phase solid solution by low‐melting elements such as Sn may offer potential for reducing oxidation‐induced crack healing temperatures.     

Enhanced high-temperature strength in textured (Ti1/3Zr1/3Hf1/3)B2 medium-entropy ceramics via strong magnetic field

This study prepared textured (Ti_1/3Zr_1/3Hf_1/3)B₂ medium-entropy ceramics for the first time that maintain enhanced flexural strength up to 1800°C using single-phase (Ti_1/3Zr_1/3Hf_1/3)B₂ powders, slip casting under a strong magnetic field, and hot-pressed sintering methods. Effects of WC additive and strong magnetic field direction on the phase compositions, orientation degree, microstructure evolution, and high-temperature flexural strength of (Ti_1/3Zr_1/3Hf_1/3)B₂ were investigated. (Ti_1/3Zr_1/3Hf_1/3)B₂ grain grows along the a,b-axes, resulting in a platelet-like morphology. Pressure parallel and perpendicular to the magnetic field direction can promote the orientation degree and hinder the texture structure formation, respectively. Reaction products of W(B,C) and (Ti,Zr,Hf)C between (Ti_1/3Zr_1/3Hf_1/3)B₂ and WC additive can efficiently refine the (Ti_1/3Zr_1/3Hf_1/3)B₂ grain size and promote grain orientation. (Ti_1/3Zr_1/3Hf_1/3)B₂ ceramics doped with 5 vol.% WC yielded a Lotgering orientation factor of 0.74 through slip casting under a strong magnetic field (12 T) and hot-pressed sintering at 1900°C. Furthermore, cleaning the boundary by W(B,C) and introducing texture can enhance the grain-boundary strength and improve its high-temperature flexural strength. The four-point flexural strength of textured (Ti_1/3Zr_1/3Hf_1/3)B₂-5 vol.% WC ceramics was 770 ± 59 MPa at 1600°C and 638 ± 117 MPa at 1800°C.     

Stability of High‐Temperature Dielectric Properties for (1 − x)Ba0.8Ca0.2TiO3–xBi(Mg0.5Ti0.5)O3 Ceramics

Ceramics in the solid solution system, (1 ? x)Ba_0.8Ca_0.2TiO₃–xBi(Mg_0.5Ti_0.5)O₃, were prepared by a conventional mixed oxide route. Single‐phase perovskite‐type X‐ray diffraction patterns were observed for compositions x < 0.6. A change from tetragonal to single‐phase cubic X‐ray patterns occurred at x ≥ 0.1. Dielectric measurements indicated relaxor behavior for x ≥ 0.1. Increasing the Bi(Mg_0.5Ti_0.5)O₃ content improved the temperature sensitivity of relative permittivity ?_r at high temperatures. At x = 0.5, a near‐plateau relative permittivity, 835 ± 40, extended across the temperature range, 65°C–550°C; the permittivity increased at x = 0.6 to 2170 ± 100 for temperatures 160°C–400°C (1 kHz). The corresponding loss tangent, tanδ, was ≤0.025 for temperatures between 100°C and 430°C for composition x = 0.5; at x = 0.6, losses increased sharply at >300°C. Comparisons of dielectric properties with other materials proposed for high‐temperature capacitor applications suggest that (1 ? x)Ba_0.8Ca_0.2TiO₃–xBi(Mg_0.5Ti_0.5)O₃ ceramics are a promising base material for further development.     

Oxyacetylene ablation of (Hf0.2Ti0.2Zr0.2Ta0.2Nb0.2)C at 1350 − 2050 °C

Ablation resistance of a multi-component carbide (Hf_0.2Ti_0.2Zr_0.2Ta_0.2Nb_0.2)C (HTZTNC) was investigated using an oxyacetylene flame apparatus. When the surface temperature of the HTZTNC was below 1800 °C, (Nb, Ta)₂O₅, (Hf, Zr)TiO₄, and (Hf, Zr)O₂ were found to be the main oxidation products, while at higher temperature, formation of (Hf, Zr, Ti, Ta, Nb)O_x was favored and its content gradually increased with the increase in ablation temperature. Based on the ablation results and thermodynamic simulation analysis, a possible ablation mechanism of HTZTNC was proposed. Active oxidation of TiC and outward diffusion of TiO were demonstrated to occur during the ablation process, which constitute the critical steps for the ablation of HTZTNC. These results can contribute to the design of ablation resistant ultra-high-temperature ceramics.     

The influence of phase transition on electrocaloric effect in lead‐free (Ba0.9Ca0.1)(Ti1−xZrx)O3 ceramics

In this paper, the influence of phase evolution on polarization change and electrocaloric response in lead‐free (Ba_0.9Ca_0.1)(Ti_1?xZr_x)O₃ ceramics (BCTZ) was systematically investigated. With increasing Zr/Ti ratio, the phase structure and phase transition behavior were greatly changed, resulting in various temperature and electric field dependence of electrocaloric responses. For x=0.05, a peak electrocaloric temperature change 1.64 K (at 130°C) and corresponding entropy change 1.78 J·kg^?1·K^?1 were obtained for 0‐7 kV·mm^?1 electric field. Negative electrocaloric temperature change in ?0.1 K was obtained below Curie temperature (T_c), which may be induced by the orthorhombic‐tetragonal ferroelectric phase transition. With the increase in x, the peak value of the electrocaloric response decreased but much better temperature stability was observed. Simultaneously the negative electrocaloric response gradually disappeared with the disappearance of the low temperature ferroelectric‐ferroelectric phase transition. For x=0.2, electrocaloric response showed good temperature stability ranging from room temperature to 130°C, attributing to the relaxor ferroelectric feature.     

New insight into the formation and oxygen barrier mechanism of carbonaceous oxide interlayer in a multicomponent carbide

Early transition metal carbides are considered to be superior candidate materials for oxidizing environments at temperatures exceeding 2000°C. Generally, the remarkable oxidation resistance is largely attributed to a carbonaceous oxide interlayer (eg, Hf–O–C, Zr–O–C, and Ta–O–C), located at the interface between the external oxide layer and internal carbide (eg, HfC, ZrC, and TaC), acting as the primary oxygen barrier. However, the oxygen barrier mechanism of the carbonaceous oxide interlayer remains unclear. Herein, through studying the oxidation behavior of a novel multicomponent carbide Hf_0.5Zr_0.3Ti_0.2C in oxidizing environments up to 2500°C, the oxygen barrier mechanism of the carbonaceous oxide was recently revealed. We found that the oxygen barrier resulted from the slow oxygen diffusion through the inner grains of Hf-Zr–Ti–O due to the presence of carbon formed at the grain boundaries because of the existence of compact external oxide layer, beneath which the Hf–Zr–Ti–O–C interlayer possesses much lower oxygen activity and temperature that allow carbon to exist stably. This as-formed carbon strongly retarded the fast diffusion of oxygen along the grain boundaries of oxides. Additionally, desirable synergisms of the designed multicomponent system, particularly, the outward short-circuit diffusion of Ti, lead to the self-healing of the external oxide layer, evidently enhancing integral protection performance against oxidizing environments.     

Fabrication,Mechanical Properties,and Tribological Behaviors of Ti3Al0.8Sn0.4C2 Solid Solution by Two‐Time Hot‐Pressing Method

A nearly pure, dense Ti₃Al_0.8Sn_0.4C₂ solid solution bulk has been prepared by two‐time hot‐pressing sintering a purity Ti₃Al_0.8Sn_0.4C₂ powders at 1450°C with 30 MPa for 30 min in Ar atmosphere. The Ti₃Al_0.8Sn_0.4C₂ powders have been synthesized by sintering a mixture of Ti, Al, Sn, and TiC powders with a molar ratio of 1:0.8:0.4:1.85 at 1450°C for 5 min. The Vickers microhardness and flexural strength of the Ti₃Al_0.8Sn_0.4C₂ have been measured to be 3.51 GPa and 620 MPa, respectively. Buckling and kinking of the layered structure as well as grain delamination crack deflection have been extensively observed around indentations and on the fracture surfaces. The tribological behaviors have been investigated by dry sliding a low‐carbon steel disk. Ti₃Al_0.8Sn_0.4C₂ bulk has a friction coefficient of 0.3–0.48 and a very low wear rate of 0.4–4.3 × 10^?6 mm³/Nm for the test conditions. These tribological properties are attributed to the presence of a compact self‐generating film, which covers uniformly over the friction surface of Ti₃Al_0.8Sn_0.4C₂.