Microstructures and Mechanical Properties of Ti3SiC2/Al2O3 and Ti3SiC2-TiC/Al2O3 Composite Ceramics by Modifying Al2O3 Content Via in-situ Synthesized


 Ti3SiC2/Al2O3 and Ti3SiC2-TiC/Al2O3 composite ceramics with various Al2O3 fractions were prepared by Ti, Si, Al, TiC and Al2O3 powers via in-situ reaction. Al2O3 contents in raw materials could influence the reaction process of Ti3SiC2 generated. It would lead to react completely when the Al2O3 volume percentage between 50% to 70%, otherwise TiC as an impurity would be found. Finally the Ti3SiC2/Al2O3 composites with 54.4 wt% Ti3SiC2, and Ti3SiC2-TiC/Al2O3 composites with 54.7 wt% Ti3SiC2 and 9.2 wt% TiC were fabricated successfully. There were two types of TiC existed in composites, called intragranular and intergranular particles. The Ti3SiC2-TiC/Al2O3 composite ceramics with 9.2 wt% TiC showed the higher mechanical properties than Ti3SiC2/Al2O3 composite. From the analyzing of microcrack propagation paths, the dispersed TiC particles as reinforcement agents would cause significant crack deflection, improving the flexural strength and fracture toughness up to 492 MPa and 7.8 MPa.m1/2 respectively.

properties, making them possible candidates used as structural materials at high temperatures and/ or in corrosive environments [1][2][3]. Ti 3 SiC 2 is the most extensively studied MAX phase, has been synthesized by various pressure sintering methods at high temperatures, which can be mainly conducted by the following three reactions [4][5][6] However, it is not facile to fabricate the single-phase Ti 3 SiC 2 due to its narrow phase region as well as impurity of TiC would be generated inevitably [7,8]. In order to address this problem, Al is often used in in-situ synthesis of Ti 3 SiC 2 as sintering additive [9][10][11][12]. Molten Al could remove oxygen, fill gaps and provide a bridging effect among the reacting powder particles, all of which could promote the diffusion reaction [13]. As a result, TiC as impurity will decrease obviously when suitable contents of Al are introduced into in-suit synthesis [13][14][15].
Although Ti 3 SiC 2 has to be seen as a potential structure/functional material as its combined metallic-and ceramic-like properties, the relatively low strength and hardness limit its further application. Al 2 O 3 ceramics with superb performances such as low price, high hardness, good corrosion resistance and stability at elevated temperatures [16], has been incorporated into Ti 3 SiC 2 to improve the mechanical properties.
Composite ceramics can be synthesized by various of sintering methods, such as self-propagating high-temperature sintering (SHS) [17], spark plasma sintering (SPS) [18] and hot pressing (HP) [19,20]. Qi et al. [21] fabricated Ti 3 SiC 2 /Al 2 O 3 composites with various volume contents of Although our work is fundamental, it is rare to see the high-purity Ti 3 SiC 2 /Al 2 O 3 composites with 50 vol% Ti 3 SiC 2 through in-situ synthesis.
The composition, micro-structure and mechanical properties of all samples were further investigated.

Mechanical properties
The mechanical properties including Vickers' hardness, flexural strength, and fracture toughness of samples 1-5 are described in Figure 5.
With the reducing of Al 2 O 3 , the Vickers' hardness of the samples1-4 decline simultaneously (Fig. 5a).

Microcrack analysis
Samples 4 and 5 were selected for microcrack analysis because of the superior toughness. As can be found in Fig. 6, both samples showed irregular fracture paths, clearly indicating that they fractured in both intergranular and transgranular modes. It should be pointed out that transgranular fracture and particle pullout were mainly observed in laminar Ti 3 SiC 2 grains, which absorbed most of the crack propagation energy and caused significant deflection and increase in the crack propagation path. In addition, there is a pit of square particle with the diameter of 200nm in Fig. 6c, which most likely result from the pullout of TiC nano-particle. TiC grains with such small size preferred being pulled out rather than cleaved. Moreover, TiC as intergranular particle would result in crack deflection. When a crack met with the TiC particle, the crack direction changed due to the hindering effect of TiC. The deflection would increase the demand of fracture energy, which is beneficial to improve the fracture toughness of composite ceramics.

Conclusion
In this research, Ti 3 SiC 2 /Al 2 O 3 and Ti 3 SiC 2 -TiC/Al 2 O 3 composite ceramics with different Al 2 O 3 contents were synthesized by in-situ reaction via hot pressing sintering. Based on the analysis above, the conclusions of this paper can be summarized as follows: (1) The volume fractions of Al 2 O 3 in raw materials could affect the reaction process of in-situ synthesis of Ti 3 SiC 2 . When the Al 2 O 3 volume percentage between 50% to 70%, the reaction will finish completely, otherwise TiC as an impurity would be found.
(2) Based on the quantitative method, the maximum of Ti 3 SiC 2 mass fractions is 54.4% in Ti 3 SiC 2 /Al 2 O 3 composites. To the best of our knowledge, this is the first report on Ti 3 SiC 2 /Al 2 O 3 composites with high contents of Ti 3 SiC 2 by in-situ reaction.