Microstructures And Properties Of In-situ MoSi₂ Reinforced Si₃N₄ Matrix Composites And Tribological Behavior Of Si₃N₄-Mo₅Si₃ Composites

As one of the most important structural ceramic materials, Si₃N₄ possess a combination of high temperature strength, low thermal expansion coefficient, low density, and high-temperature resistance to oxidation. However, low fracture toughness, the difficulty in machining and high wear rate limit the widespread use in engineering fields. MoSi₂-reinforced Si₃N₄ composites have been paid attention since the end of last century because of the improved high temperature strength, fracture toughness and machinability.In this work, the reactivity of (Si₃N₄+Mo) and (Si₃N₄+MoO₃) to form MoSi₂ was evaluated by adding powders of Mo and MoO₃ from ammonium heptamolybdate to silicon nitride, and the powder mixture were sintered at different temperatures. Results of X-ray diffraction (XRD) indicated that both MoSi₂ and Mo₅Si₃ were formed by reaction of Mo and Si₃N₄ at 1600℃and 1680℃, and MoSi₂ was formed only at 1750℃; there is no reaction of MoO₃ and Si₃N₄ at 1680℃, and only MoSi₂ was formed at 1750℃, no Mo₅Si₃ was detected. According to the analysis of thermodynamics, both the free energy of formation of MoSi₂ and Mo₅Si₃ from Mo and Si₃N₄ has a negative value above 1600℃, and the former is more negative; the free energy of formation of MoSi₂ from MoO₃ and Si₃N₄ at 1750℃can be negative when the gas of MoO₃ possess certain pressure. The reaction sequence of MoSi₂ like (Mo+ Si)→(Mo₅Si-3 + Si)→MoSi₂ between Mo-Si is the reason for the formation of Mo₅Si₃ at 1600℃and 1680℃.Si₃N₄-MoSi₂ composite was fabricated by adding powders of Mo to Si₃N₄ and employing HP technique. X-ray diffraction (XRD), scanning electron microscope (SEM), EDS, material test machine was used to evaluate samples structure, density, hardness and fracture toughness. The results showed that all samples were densificated comparatively, relative density were more than 95%; MoSi₂ distributed uniformly in the matrix; the existence of MoSi₂ in the matrix did not reduce the hardness of the composites; fracture toughness of Si₃N₄-MoSi₂ increased gradually with the increases of MoSi₂ content, achieving the maximum value of 7.6 MPa·m^1/2 at 30vol% MoSi₂ with the increases of 36%, and then decreased as MoSi₂ further increased. The improvement of fracture toughness should be attributed to residual stress generated around MoSi₂ which was caused by the thermal expansion mismatch between MoSi₂ and matrix. The increasing tendency for crack running in the MoSi₂ phase induced the decrease of fracture toughness when more MoSi₂ formed.Because of the brittleness of MoSi₂, the wear rate of Si₃N₄-MoSi₂ was not less than that of Si₃N₄. Considering that M05S13 can easily be oxidized to form crystalline MoO₃ whicch exhibits a lower friction coefficient at elevated temperature, Mo₅Si₃ was added to Si₃N₄ ceramics directly and Si₃N₄-Mo₅Si₃ composites were fabricated by HP sintering method. Friction and wear properties were evaluated at the temperature range of 20℃to 500℃at the condition of dry friction, constant load of 10N, sliding velocity of 0.4m/s with the counterpart of Si₃N₄ ball. The result showed that wear rate of Si₃N₄-Mo₅Si₃ decreased with a maximum magnitude of an order as the Mo₅Si₃ increases and the friction coefficient also decreased. By the means of SEM, and XPS analysis, it was found that the wear mechanism of Si₃N₄ were inter-crystalline cracking and surface cracking at 20℃and 500℃respectively, which lead to the abrasive wear and the rapid increase of wear rate. With addition of Mo₅Si₃, however, the wear mechanism was trioxidation and plastic deformation, which contribute to the improvement of the wear insistence of Si₃N₄-Mo₅Si₃.