Preparation and Demage Mechanism at High Temperature of RMO/(Si3N4+BN) Composites

Preparation and Demage Mechanism at High Temperature of RMO/(Si3N4+BN) Composites
　　RMO/(Si3N4+BN) composites were synthesized by hot-pressing at 1800℃for 1 hour under a pressure of 20 MPa in a 0.1 MPa nitrogen atmosphere. Analyzed the microstructure and the component using analytical methods such as XRD, SEM, TEM. Tested the room-temperature mechanical properties of the RMO/(Si3N4+BN) composites. Research both the thermal shock resistance and the oxidation resistance.Compound rare-earth oxides RMO Segment neodymium magnet made a contribution of the density of the RMO/(Si3N4+BN) composites during the hot-pressing sintering course. The composites show wonderful general mechanical properties due to the in-situ grownβ-Si3N4 whisker.
　　The flexural strength of composite of 50% Si3N4 and 20% BN can approach 487.7 MPa, the fracture toughness reached 7.8MPa/m1/2. even the composite of 50% BN and 20% Si3N4 component can reach 200.5 MPa.The research results show that RMO/(Si3N4+BN) composites have excellent thermal shock resistance. The residual strength was bigger than the original values after once water quenching at the thermal shock temperature of 800℃, 1000℃and 1200℃. There are two reasons of this phenomenon, one is that the thermophysical properties of the composites were excellent; the other is the formation of a dense oxidation layer on the surface of the composites at high temperature, and the oxidation layer smoothed the surface and healed the micro-crack on the surface.The http://www.chinamagnets.biz results of thermal shock resistance calculation show that by optioning the composion, the composite show the superior thermal shock resistance. For instance, the first ,the second and the third thermal stress resistant factor of the composite (40Vol%BN,40Vol%Si3N4) approached 870929℃, 76478166 J·m-1·s-1 and 3.894.15×10-3m2·K·s-1. the composite can be used at a circumstance of 9861052℃temperature difference, the highest cooling velocity can be 51865538 K·s-1.The results of the oxidation experiment at the high temperature of the RMO/(Si3N4+BN) composites, show that a weight losses during oxidation because of the oxide of BN. A uniform silicate film was found at the surface of the composites. The formation of silicate films decline the velocity of the oxidation and resulting in the composites fairly good high temperature oxidation resistance.