Effects of Silicon and Silicon-molybdenum Alloys on Pressureless Reaction Sintering of Silicon Carbide

Silicon carbide (SiC) ceramics, with their superior physical properties, are widely utilized in various industrial sectors, particularly under harsh conditions such as high temperatures and pressures. Reaction-bonded silicon carbide (RBSC), favored for its ease of manufacturing and cost-effectiveness, has traditionally required vacuum sintering, which limits production capacity and increases costs. This study is aimed to study the feasibility of pressureless reaction sintering of SiC through infiltration of silicon melt and molybdenum-silicon (Mo-Si) alloy under argon (Ar) protection, in order to enhance process efficiency and reduce costs.

In this study, the effects of different melt infiltrations on the sintering behavior of SiC ceramics were systematically examined. Mo-Si alloy was prepared by mixing and melting molybdenum and silicon powders according to the phase diagram, followed by crushing into small pieces under Ar at 1500 ℃ in BN crucible. Ceramic samples were fabricated using α-SiC powder, carbon black, phenolic resin, and other additives, which were mixed and pressed into rectangular bars. After pre-firing to remove binders, the green bodies were then infiltrated with either silicon or Mo-Si alloy melts at temperatures between 1530 ℃ and 1600 ℃. Density, microstructure, flexural strength, and hardness of the sintered samples were analyzed by using the Archimedes method, scanning electron microscopy (SEM), electronic universal testing machine, and Vickers hardness.

It is found that the infiltration effect of silicon melt on physical properties of the sintered samples was highly sensitive to the processing temperature. For instance, the flexural strength was increased from 58.6 MPa at 1530 ℃ to 317.0 MPa at 1570 ℃ for the silicon-infiltrated samples. Meanwhile, Mo-Si alloy-infiltrated samples achieved a relative density exceeding 94% at 1530 ℃, due mainly to the higher polarizability of the Mo-Si alloy, reducing the contact angle between silicon and carbon. The mechanical properties of the Mo-Si alloy-infiltrated samples remained stable across the tested temperature range but were lower than those of silicon-infiltrated samples at 1570 ℃ due to the segregation of MoSi₂.

It has been demonstrated that pressureless reaction sintering of SiC is feasible under Ar protection, especially when using Mo-Si alloy as the infiltration material, allowing for nearly full densification at relatively low sintering temperatures. However, the presence of MoSi₂ led to decreased mechanical properties as compared with silicon melt. While introducing Mo-Si alloy offers advantages in terms of reducing sintering temperature, further optimization is necessary to address the issue of MoSi₂ segregation and improve the overall performance of the SiC ceramics.