Development Of Mo-W-Si-N Coated Molybdenum Based On High-temperature Diffusion Of Si

Molybdenum is usually used as furnace electrode material for atuo glass manufacture due to its excellent properties, such as high melting point, good corrosion resistance, admirable high-temperature strength and rigidity et al. However, the applications of Mo as electrodes at high temperatures are hindered by its poor oxidation resistance in air. The awful oxidation resistance of Mo is due to the generation of volatile oxide during its applications. Therefore, MoSi₂ coatings were applied on its surface to ensure the effective use of it. However, the lifetime of MoSi₂-coated Mo electrodes are influence seriously by the diffusion between Si element and Mo substrate. Thus, it is of great importance to study the diffusion regularity of Si as well as to find out the effective ways to block the diffusion.Therefore, the present study is to illustrate the diffusion regularity of Si elment, the MoSi₂-Si₃N₄/Mo and MoSi₂-WSi₂-Si₃N₄/Mo diffusion couples were prepared via spark plasma sintering（SPS） and annealed in Al2O3 tube furnace to study the diffusive regularity of Si element. The transition of intermediate phases in diffusion couples were observed by XRD, SEM and EDS. Furthermore, the silicon diffusion coefficient and diffusion activation energy in diffusion couples were calculated based on diffusion theory. The effect of Si₃N₄ and tungsten addition on hindering Si element diffusion was analyzed, the results came as follow:The homogeneous MoSi₂-Si₃N₄ and MoSi₂-WSi₂-Si₃N₄ composite powders were gained by ball milling and then sintered in the SPS equipment. The powder mixtures were simultaneously pressed with 25 MPa under vacuum（10-1 Pa）, then followed by heating up to 1550 °C with constant external pressure. The heating rate was 60 °C / min, and the holding time was 10 minutes for all samples. The relative densities of the SPSed samples with different content of Si₃N₄ or W were 93⁹5%. Besides, cracks were not found in as-prepared samples.Based on law of conservation of mass, Fick’s first law of diffusion and parabolic growth law, the growth kinetics of intermediate layers in diffusion couples were studied at temperatures of 1200-1500℃. The results showed that the growth rate constants of Mo5Si3 layers in MoSi₂-Si₃N₄/Mo and MoSi₂-WSi₂-Si₃N₄/Mo diffusion couples were the minimum when the content of Si₃N₄ and WSi₂ were 3.5vol% and 5.0vol%, respectively. Two different formation mechanisms of intermediate layers Mo5Si3 were found:（1） the degradation of MoSi₂ phase into Mo-rich phases,（2） the reaction of freed Si and Mo.The diffusion activation energy（Q） and silicon coefficient（D） in MoSi₂-3.5%vol.Si₃N₄/Mo and MoSi₂-5.0%vol.WSi₂-3.5%vol.Si₃N₄/Mo diffusion couples were calculated based on diffusion theory, the Q value were 278±19 kJ/mol and 316±23 kJ/mol, and the expression of D were 1.28exp[（-278 ± 19）/RT]cm2/s and 5.64exp[（-316 ± 23）/RT] cm2/s, respectively. The Q value of MoSi₂-5.0%vol.WSi₂-3.5%vol.Si₃N₄/Mo diffusion couples was 38kJ/mol more than that of MoSi₂-3.5%vol.Si₃N₄/Mo diffusion couples, which revealed that the diffusion of Si was hindered by W element, and the diffusion of Si was more difficult in the diffusion couples contained WSi₂.The high temperature oxidation resistance properties of pure MoSi₂ material, MoSi₂-3.5%vol.Si₃N₄ and MoSi₂-5.0%vol.WSi₂-3.5%vol.Si₃N₄ composite materials were assessed at 1450℃ in air, the oxidation rate of these materials after oxidation for 384 h were 0.065 g/（m2·h）, 0.025 g/（m2·h）and 0.017 g/（m2·h）, respectively. All materials exhibited excellent high temperature oxidation resistance, the high temperature oxidation resistance performance of MoSi₂-based composites were improved by Si₃N₄ and WSi₂ phases, and WSi₂ phase achieved better results.