Effect of TiC0.4 Addition on Microstructures and Properties of Ti3SiC2 Matrix Composites


 TiC0.4-Ti3SiC2 composites were manufactured using spark plasma sintering (SPS) at 1400°C for 10 min, with an applied pressure of 40 MPa. The effect of TiC0.4 additions on microstructures and properties of Ti3SiC2 matrix composites was investigated. The addition of TiC0.4 induced the formation of TiCx (0.4༜x≤1) and promoted the decomposition of Ti3SiC2, yielding TiCx and Ti5Si3. The mechanical and tribological properties of the TiC0.4-Ti3SiC2 composite improved with the TiC0.4 content due to C vacancy present in TiC0.4, although the friction distance in the initial stage of friction test was increased. The 30 vol.% TiC0.4-Ti3SiC2 composites exhibited the highest hardness and fracture toughness of 14.87 GPa and 5.45 MPa⋅m1/2. At the same conditions, the wear rate at room temperature reached a minimum value of 4.32×10−7mm3N−1m−1, while the friction coefficient was 0.74. The friction distance in the initial friction stage of 30 vol.% TiC0.4-Ti3SiC2 was about 38 m, and the wear mechanism of the composite at room temperature was mainly adhesive wear.


Introduction
Ti 3 SiC 2 ternary ceramic embodies the advantages of both ceramics and metals, such as high Young's modulus (343 GPa) [1], high fracture toughness (7.20 MPa·m 1/2 ) [2], high corrosion resistance [3], good thermal and electrical conductivity (43 W/m·K and 9.6⋅10 6 Ω −1 m −1 ) [4], thermal shock resistance [5], etc. Its layered crystal structure is comparable to that of well-known graphite and MoS 2 solid lubricants, implying that it may be an excellent solid lubricant material with a low friction coe cient [6]. Thus, Ti 3 SiC 2 could nd potential applications in bearings, turbines, cutting tools, electrical contacts, heat exchangers [7][8][9][10][11], etc. However, this is severely hampered by low hardness (4.54 GPa) and wear resistance (0.21-1.87×10 −3 mm 3 /N·m) of Ti 3 SiC 2 ceramics [12]. Many reinforcing phases, including Al 2 O 3 , SiC, TiC-TiS 2 , and TiC-TiB 2 , were applied to improve the hardness and wear resistance of Ti 3 SiC 2 due to the nailing of the softer phase by the hard phase [13][14][15]. However, we still lack the solution for the weak interfacial bonding between these reinforcing phases and the Ti 3 SiC 2 matrix, limiting the reinforcing phases' enhancement effect. TiC 0.4 is a representative of nonstoichiometric TiC x (0.3<x<0.9) compounds, exhibiting a NaCl-type crystal structure [16][17][18]. It possesses high hardness , and the existence of vacancies improves the interfacial bonding between the reinforcing phase and the matrix [19][20][21]. Recently, TiC x was used to improve the mechanical properties of the Al or Cu matrix composite, showing that the hardness and bending strength were increased by 84.7-116.3 and 80.7-122.4%, respectively, compared to the Al or Cu matrix composite without TiC x [21,22].
In this study, we prepared the TiC 0.4 (10, 20, and 30 vol.%) particles-reinforced Ti 3 SiC 2 matrix composites using SPS. Meanwhile, the effect of TiC0.4 content on the microstructure, mechanical properties, and friction and wear behaviour of the composites was systematically investigated, and the related mechanism was discussed.

Experimental Details
TiC 0.4 powder was synthesized via mechanical alloying (MA) using the production method described in Ref. 19. TiC 0.4 (purity 99.6%, size 180-200 nm) and Ti 3 SiC 2 (purity 99%, size 30 µm) powders were used as raw materials. The TiC 0.4 volume fraction was xed at 10, 20, and 30 vol.% in the whole raw material, and the corresponding sintered samples were denoted as TC0 (10 vol.% TiC 0.4 ), TC1 (20 vol.% TiC 0.4 ), and TC2 (30 vol.% TiC 0.4 ). These raw materials were well dispersed using the high-energy ball-milling method, and then the mixed powders were sintered using an SPS system (LABOX TM -110, Japan) at a sintering temperature of 1400°C and a holding time of 10 min under a pressure of 40 MPa in a 30 Pa vacuum. The heating rate was set to 100°C/min. The bulk density of the TiC 0.4 -Ti 3 SiC 2 composite was measured by Archimedes' method. The theoretical density of TC0, TC1, and TC2 was 3.78, 4.39, and 4.64 g/cm 3 , as calculated using the method described in Ref. 23. The hardness of the Ti 3 SiC 2 matrix composite was measured by a Vicker's hardness instrument (FM-ARS9000, China) with a dwell time of 15 s at a load of 500 gf. Fracture toughness (K IC ) was determined by the indentation method proposed in Ref. 24 with a load of 5000 g and a holding time of 15 s. Friction and wear tests were carried out using a ball-on-disc system (TRB, Switzerland). A ball sliding on a linear reciprocating athletic at specimen (5 mm × 15 mm × 5 mm) was adopted. The specimens were cut by electrical discharge method and the test surface was mechanically polished down to 2000# SiC paper. The Si 3 N 4 balls were used as the friction counterparts with a diameter of 4 mm and a hardness of 15 GPa. Disc-specimens and Si 3 N 4 balls were cleaned by acetone solution before frictional wear tests. All tests were carried out under dry sliding conditions at room temperature (25°C) in the air. The wear track diameter was typically 11-13 mm. The tribological tests were performed at an applied load of 5 N, at a sliding speed of 0.1 m/s, and a testing distance of 100 m. The friction coe cient was continuously measured and automatically recorded by the computer system of the friction tester, while the wear rate was calculated using Archard's equation [25]: 3 ) is the wear volume obtained by measuring the weight loss in a microbalance after ultrasonic cleaning and drying and the measured density, P (N) is the applied load, and L (m) is the total sliding distance.

Results Ad Discussion
The TiC 0.4 -Ti 3 SiC 2 composite is composed of Ti 3 SiC 2 (Fig. 1a, marked  and Ti 3 SiC 2 composite, indicating a good wettability between TiC 0.4 and Ti 3 SiC 2 (Fig. 2). The Si content in the reaction layer is higher than in the Ti 3 SiC 2 region, while the C content is lower than in the Ti 3 SiC 2 and TiC 0.4 regions. According to the phase composition of the TiC 0.4 -Ti 3 SiC 2 composite (Fig. 1a), Ti 5 Si 3 and TiC x are formed in the reaction layer. The C vacancies can improve the wettability between TiC 0.4 and Ti 3 SiC 2 by enhancing the atomic diffusion ability of the elements, yielding the formation of TiC x and Ti 5 Si 3 by the Ti 3 SiC 2 decomposition. This can change the interfacial structure between TiC 0.4 and Ti 3 SiC 2 and promote interfacial bonding, enhancing the mechanical and tribological properties of the composite [21,22].
The bulk density, relative density, hardness, fracture toughness, friction coe cient, and wear rate of 1c-e) [32,33]. Although the fracture toughness of the TiC 0.4 -Ti 3 SiC 2 composites is close to the mean value reported in the previous studies [7,14,30,[34][35][36][37][38][39], the TiC 0.4 -Ti 3 SiC 2 composites exhibit an unprecedentedly high hardness of ~14.87 GPa (Fig. 3e). TiC x particles can effectively pin the Ti 3 SiC 2 matrix to prevent the Ti 3 SiC 2 detaching during the friction process due to the high hardness and strong interfacial bonding between TiC x and Ti 3 SiC 2 (Fig. 1) [14]. In addition, the decrease of grain size lowers the wear rate due to the mitigation of multiple risks caused by plastic deformation, fracture, fragmentation, and oxidation of grains [40]. The detached particles partly form a lubricating lm on the friction surface of TiC 0.4 -Ti 3 SiC 2 composite, while others yield wear debris, which is later removed [32]. As the TiC 0.4 addition increases from 20 to 30 vol.%, the friction coe cient increases although the wear rate decreases, implying that the excessive TiC x particles destroy the continuity of the lubrication lm formed by the Ti 3 SiC 2 matrix (Fig. 4b-d). It is worth noting that TiC 0.4 -Ti 3 SiC 2 composites show a wear rate as low as 4.32-6.49×10 −7 mm 3 N m −1 , which is much lower than the values reported for pure Ti 3 SiC 2 and other Ti 3 SiC 2 -based ceramics [14,37,[41][42][43][44][45][46].
The relationship between friction coe cient and friction distance of the TiC 0.4 -Ti 3 SiC 2 composites can be divided into two stages, i.e., initial friction and stable friction (Fig. 4a). The friction distance of the TiC 0.4 -Ti 3 SiC 2 composites in the stable friction stage is from the end of the initial friction stage to the end of the friction process (Fig. 4a)  However, it affects the tribological properties mainly through the formation of the lubricating lm on the surface of the TiC 0.4 -Ti 3 SiC 2 composites. As the formation of the lubricating lm is in a dynamic balance with its loss during the friction process, the lubricating lm is formed on the entire friction surface of the TiC 0.4 -Ti 3 SiC 2 composites, stabilizing the friction coe cient ( Fig. 4 and Table 1). The lubricating lm integrity is improved by the pinning effect of TiC particles, decreasing the friction coe cient and wear rate. Still, the pinning effect of TiC particles is limited by the continuity of the lubricating lm. If TiC x particles in the TiC 0.4 -Ti 3 SiC 2 composites are present in excess, the continuity of lubricating lm will decrease although the pinning effect of TiC particles is improved, yielding an increase of friction coe cient (Fig. 3d, Fig. 4, and Table 1). This work provides a theoretical basis for studying other materials enhanced by the addition of nonstoichiometric compounds and broadens the application spectra of the Ti 3 SiC 2 matrix composites under severe environments.