Ultrafine Al <sub> 2 </ Sub> Preparation Of The O <sub> 3 </ Sub>-tic - Co Composite Powders And Composites

To obtain the ultra fine ATC ceramic with fine, uniform grain and high mechanical properties and solve the problem of brittle of ceramic, the technique of electroless cobalt plating under low temperature and ultrasonic was used to get a cobalt deposition on nano/micro Al2O3, TiC ceramic. The nano/micro Al2O3-TiC-Co composite powder with different cobalt content was prepared by changing the load. The composite powder was then hot-pressed into a ultra fine composite ceramic. The technique of electroless plating and hot-press, the microstructure, mechanical properties and toughen mechanism of the ultra fine composite ceramic were investigated.The best parameters of electroless plating were observed. Results demonstrated that by utilizing the ultrasonic into the electroless plating, the reaction can reacted in lower temperature and the self-decomposition of the solution can be prevented. The ultrasonic can also afford the energy that the electroless plating need so that the coating speed is rapider than the normal low-temperature electroless plating, then reduce the cost. The ultrasonic has great advantage in dispersion compared with the normal mechanical mixing.The mechanical properties of micro ATC ceramic is much higher than the AT ceramic and ATC ceramic prepared by normal electroless plating. The analysis on the micro structure demonstrated that the cobalt can prevent the ceramic's growth and the reaction between different ceramic. The cobalt along grain boundary can change the "hard" type bonding of ceramic interface and improve the toughness of the composite ceramic by plastic deformation. The loading and hot stress of the dispersed brittle ceramic can be relaxed by the cobalt which is in three-dimensional net structure. The cobalt can also be strengthened by the difference of the hot expansion coefficient between cobalt and ceramic. These all can improve the strength and toughness of thebulk ceramic.The research on the hot-press parameter of the nano ATC composite powder demonstrated that the ceramic prepared under vacuum has better mechanical properties than the ceramic prepared under N2. The hardness of the ultra fine ATC ceramic grows first and then fall down with the growing sinter temperature. The bending strength and fracture toughness of the ultra fine ATC ceramic all fall down with the growing sinter temperature. When the cobalt content grows, the hardness and bending strength fall down and the fracture toughness fall down first and then grow.The main fracture model of ultra fine ATC ceramic is intergranular fracture. The analysis on the microstructure of the ultra fine ATC ceramic demonstrated that the ceramic is intergranular-transgranular structure. The residual hot stress caused by the difference of the hot expansion coefficient between cobalt and ceramic can strengthen the grain boundary and disperse the energy of crack. The cobalt in the grain causes stress concentration and makes the crack through the sub-grain-boundary, which can increase the path of crack.