Investigation Of MoSi₂ And Its Composites By Field-activated And Pressure-assisted Combustion Synthesis

With moderate density of 6.24g·cm-3, high melting point up to 2030℃, relatively low thermal expansion coefficient of 8.1×10-6K-1, and good thermal and electrical properties, MoSi2 exhibits promising potential in fields of aeronautic and astronautic industries, power and chemical engineering and metallurgical machineries. In the present study, Field-activated and pressure-assisted combustion synthesis (FAPACS) was employed to improve mechanical properties of MoSi2 and its composite, in which in situ synthesis and densification of composite could be achieved simultaneously. The effects of processing parameters on microstructure evolution were investigated. Mechanical properties and high temperature wear-resistance capability of the products were analyzed. Process of FAPACS was modeled by means of computer aided simulation.The following conclusions were made with this work:For Mo-Si system, microstructure of the product shows that Mo5Si3 distributes at grain boundaries on matrix of MoSi2, with production of small amount of SiO2. Mo5Si3 is a product of interfacial diffusion reaction between Mo and Si at relatively lower temperature. SiO2 results from oxidation of Si.In FAPACS, sintering temperature plays an important role in microstructure morphology and degree of densification. When it is higher than melting point of Si (1410℃) , the molten Si wraps particles of Mo. It helps to synthesize substantial fine grained MoSi2 with higher density. Analysis with sintering dynamics shows that combination of environment higher than melting point of Si, no less than heating rate of 100℃/min, and sufficient sintering time basic condition ensure obtaining of pure and dense bulk MoSi2.That was realized in experiments with sintering temperature 1500℃, heating rate 100℃/min, pressure 35Mpa and holding time of 10min. As regards system of Mo-Si-C, SiC was obtained from Si and C with spark plasma activated by electrical field, which is difficult to synthesize under conventional environment. SiC particles were scaled over 100nm ~ 3um. Coarse particles distributed at grain boundaries of MoSi2. Whereas the finer ones dispersed within grains.Existence of carbon is favor of elimination of SiO2 since it is free in synthesized MoSi2-SiC composite. However, inter phase of Mo5Si3C produced, which is unstable and transitive. With extension of holding time, reaction of Mo5Si3C + 8Si=5MoSi2 + SiC took place, resulting in decreasing or even elimination of its contention. In the experiments, a simultaneous in situ synthesis and densification of MoSi2-SiC composite was achieved under sintering temperature 1500℃, pressure 30Mpa and holding time of 30min.For mechanical properties of MoSi2-SiC, The micro-hardness and fracture toughness of the synthesized MoSi2-SiC composites improved with increasing SiC contents. When the volume fraction of SiC was 30%, the fracture toughness of the MoSi2-SiC composite, K1C , was up to 5.58 MPa ? m, which is 33% higher than that of MoSi2.Further study shows MoSi2 is characterized by brittle transgranular fracture with cracks within grains. Whilst, MoSi2-SiC composite possesses a hybrid fracture attribute with both intergranular and transgranular cracks around or within grains. Its mechanisms of toughness mainly lie in refinement of grains, deflection and bridging of cracks, and pinning effect of fine SiC particles.The high temperature wear resistances of MoSi2 and its composites were studied. The friction coefficient of MoSi2 is 0.35~0.53 over the range 25~700℃, and 0.26~0.44 for MoSi2-SiC composite. Moreover, the wear resistance of the latter was enhanced by 36% ~59.6% higher than that of the former.For MoSi2-SiC composite, under certain load and speed of sliding, worn process could be staged with increasing temperature as: 1) decrease of friction coefficient with slightly dropping of wear rate over room temperature to 300℃; 2) continuing enhancement of friction coefficient with gradually increasing temperature up to 600℃, and a saturation of increasing of wear rate kept over 400-600℃; and 3) leveling off of friction coefficient and wear rate to temperature of 700℃.The study of wear mechanism shows that adhesions, oxidation and fatigue fracture are coactively cause the worn of MoSi2. However, those are mainly oxidation, shifting and tearing off of friction layer for MoSi2-SiC composite.The FAPACS modeling was made and a numerical simulation was done by means of computer aided finite element method. The results show that FAPACS temperature field was determined by an interaction among Joule heating of electric field, heat released by chemical reaction, and heat transferring characteristics of die-sample system. Due to overlay of heats by Joule effect and chemical reactions, the highest temperature was in the center of the sample and a radius temperature gradient was established. That significantly took effects to uniformity of microstructure and densification degree. Therefore, it is important to adjust temperature gradient within the sample of FAPACS to prepare dense and fine grained bulk materials.