Phase Evolution,Mircostructure And Properties Of SiC-MgAl₂O₄ Composites Sintered In Nitrogen Atmosphere

SiC and MgAl₂O₄show excellent physical and chemical properties and are widely used in high temperature industries.Previous studies showed that silicon carbide-spinel composites have the potential to be a chromium-free substitute for the lining of coal gasification furnaces.However,low strength of the silicon carbide-spinel composites depresses the application in coal gasification.The key may lie in the effective bonding between oxides and non-oxides,which is also the core problem in the preparation of the composites.SiC-MgAl₂O₄composites with Mg-Sialon as the main bonding phase were prepared,and the appropriate metal Al addition,the nitriding temperature and the SiC-MgAl₂O₄ratio were optimized.Effects of the Al/Siratio,the Y₂O₃addition and theα-Si₃N₄addition on the phase and the microstructure of Mg-Sialon bonded SiC/MgAl₂O₄were studied.The microstructure and the mechanical properties of SiC-MgAl₂O₄composites with Mg-Sialon bonded aggregates were investigated.The conclusions are summarized as follows:The SiC-MgAl₂O₄composites with different additions of metal Al were heat-treated at 1200℃,1300℃,1400℃and 1500℃for 3 h in nitrogen atmosphere.The treated specimens were analyzed by XRD and the fracture morphology and microstructure of the specimens were observed by SEM.The study shows that the addition of metal Al treated at1500℃can form the Mg-Sialon phase with crystal form of 15R.The increasing metal Al content can promote the transformation fromβ-Sialon to Mg-Sialon,and the micromorphology of Mg-Sialon evolves from strip to sheet.Another possible gas-solid reaction:Al₂O（g）and SiO（g）form flaky Mg-Sialon precipitation with N₂（g）under the condition of high temperature and low oxygen.When the heat treatment temperature is1650℃and the ratio of SiC/MgAl₂O₄is increasing,the crystal form of Mg-Sialon changes to 12H and 21R.In order to obtain the Mg-Sialon phase with better sintering performance and micromorphology,the addition of Al powder should be controlled at 10 wt%,the ratio of SiC/MgAl₂O₄should not be higher than 6:4,and the nitride temperature should be about1500℃.Sipowders were introduced into the SiC-MgAl₂O₄-Al system to change the Al/Siratio.The specimens were heat-treated at 1500℃ for 3 h under nitrogen atmosphere.With the increase of Siaddition,the formation of（15R）Mg-Sialon phase gradually decreases,and the formation ofβ-Sialon phase gradually increases.The results show that the addition of Siis detrimental to the formation of Mg-Sialon phase.When the head treatment temperature increases to 1650℃,with the Siaddition rising,obvious liquid sintering sign can be observed,Mg-Sialon crystal development becomes poor.Thus,Siaddition shall be avoided or minimized.Different amounts of sintering aid Y₂O₃and seed crystalα-Si₃N₄were introduced into the SiC-MgAl₂O₄-Al system and the specimens were heat-treated for3 h under nitrogen atmosphere at 1350℃,1450℃and 1550℃.The addition of Y₂O₃andα-Si₃N₄can effectively reduce the formation temperature of Mg-Sialon phase to the experimental temperature of 1350℃,and the content of Mg-Sialon increases with the treatment temperature increasing.With the addition of Y₂O₃andα-Si₃N₄,the cell constant and the cell volume of Mg-Sialon increase slightly.At 1550℃,the Mg-Sialon content of the specimen added with 4 wt%seed crystalα-Si₃N₄is up to 33%.The specimen formulation with 4 wt%α-Si₃N₄are selected as the matrix formula,and SiC/MgAl₂O₄aggregates were added and heat treated for 3 h under nitrogen atmosphere at 1550℃.The results show that the HMOR of the SiC-MgAl₂O₄specimens with Mg-Sialon as the bonding phase are nearly 8 times higher than that of SiC-MgAl₂O₄specimens without modified matrix.