Preparation, Microstructure And Properties Of AlON And Its Composites

Spinel-structure aluminum oxynitride (AlON) has been considered as a potential ceramic material for high-performance structural, advanced refractory and window materials due to its excellent mechanical, chemical and optical properties, such as low density, low thermal expansion coefficient, high hardness, high rigidity and good chemical stability. However, its relatively low flexural strength and poor fracture toughness somewhat limit its applications. The purpose of this project is, therefore, to synthesize dense AlON ceramic and develop a novel type of SiC-AlON and ZrN-AlON composites in an attempt to enhance AlON ceramic with improved strength and toughness via hot-pressed sintering method, and to evaluate their sintering behaviors, phase compositions, microstructures, mechanical properties, the resistance to oxidation, thermal shock and molten steel, and the underlying strengthening and toughening mechanisms. The present study might open up a new approach to the development of AlON matrix composites with improved mechanical properties for further applications.The AlON powders prepared using AI2O3and A1N as raw materials were then used to fabricate high purity AlON ceramic using hot-pressed sintering method. It was found that the relative density, microhardness, Young’s modulus, flexural strength, fracture toughness of sintered AlON ceramic were97.6%,13.8GPa,237GPa,296MPa and1.5MPa·m1/2, respectively.SiC-AlON composites with excellent properties were fabricated successfully using hot-pressed sintering. The addition of SiC nano-particles resulted in a reduction in the grain size due to the pinning effect of SiC nano-particles positioned at the grain boundaries or triple junctions of micro-sized AlON particles. With the increase in SiC contents, the microhardness, Young’s modulus, flexural strength and fracture toughness all increased initially and then decreased. When the SiC content is8wt%, the mechanical properties reach the maximum values, and the flexural strength and fracture toughness are399MPa and1.9MPa·m1/2, respectively. This represents an increase of the flexural strength and fracture toughness by35%and23%in comparison with pure AlON ceramic, respectively. The major strengthening and toughening mechanisms for SiC-AlON composites are grain refinement and the mismatch of thermal expansion behavior between AlON and SiC.The oxidation behaviors of AlON and SiC-AlON composites were investigated in the present study. It was observed that the AlON matrix and SiC particles near the sample surface were oxidized to form Al2O3and SiO2, respectively. However, no mullite phase formed within the temperature range between700℃and1200℃. The oxidation products appeared in the order of needle-like, flake-like and layer-like forms with increasing temperatures during the oxidation process at high temperatures. The starting oxidation temperature of pure AlON ceramic was about1000℃. With the addition of nano-sized SiC particles, the starting oxidation temperature decreased to about700℃, the oxidation resistance at higher temperatures above1100℃was also considerably enhanced. This is attributed to the formation of dense oxidation layers due to the presence of SiO2stemming from the added nano-sized SiC particles. Such oxidation layer protects and suppresses the further penetration of oxygen, thereby effectively improves the high temperature oxidation resistance of the SiC-AlON composites.A new type of ZrN-AlON was also prepared in the present study. It was of interest to observe from XRD results that all of the added ZrO2powders were reacted with AlON matrix and changed into ZrN phase after the hot-pressed sintering at a high temperature of1850℃. With an increasing amount of ZrN nano-particles up to2.7%, the relative density, hardness, Young’s modulus, flexural strength, and fracture toughness all increased. When the amount of ZrN nano-particles exceeds2.7%, the density, hardness and Young’s modulus continued to increase moderately, while the flexural strength and fracture toughness displayed a slight decrease due to the occurrence of agglomerates of ZrN nano-particles. When the amount of ZrN particles is2.7%, the flexural strength and fracture toughness are555MPa and2.5MPa·m1/2, respectively. This represents an increase of the flexural strength and fracture toughness by88%and60%in comparison with pure AlON ceramic, respectively. It was observed that the presence of ZrN nano-particles led to the pinning effect of ZrN nano-particles positioned at grain boundaries or triples junctions of micro-scale AlON particles, and restrained the growth of AlON grains during sintering and thus reduced the grain size in the ZrN nano-particulate reinforced A1ON composites. In addition, some ZrN particles positioned in the A1ON grains also played a pinning role in the dislocation movements. A variety of strengthening and toughening mechanisms in the ZrN-AlON composites were observed, such as grain refinement strengthening, crack deflection and crack branching. Hence, the considerably increased crack propagation resistance results in a significant improvement in the flexural strength and fracture toughness of the composites.The thermal shock behaviors and the corrosion behaviors to molten steel for A10N ceramic and its composites were also investigated in the present study. The results show that the critical thermal shock temperature of AlON ceramic is about200℃. Both of SiC and ZrN particles enhance the resistance of thermal shock for AlON ceramic, and the critical thermal shock temperature of SiC-AlON and ZrN-AlON are about225℃. Besides, the thickness of corrosion layer by molten steel is about150μm, indicating AlON ceramic has excellent resistance to molten steel corrosion. However, due to the reaction with molten steel, SiC reduced the corrosion resistance of AlON ceramic to molten steel. Additionally, the addition of ZrN particles to AlON ceramic has no noticeable effect on the corrosion resistance to molten steel.