Study on Thermal Shock in Si3N4/SiC platelets

Abstract

Intensive investigations have been carried out to improve the mechanical behavior of silicon nitride. Studies show an increase of mechanical resistance and creep resistance through the incorporation of reinforcement compounds such as ®bers, whiskers and platelets [1±6], as well as through the use of additives, which lead to a partial crystallization of the grain boundary vitreous phase [7±9]. In recent years, platelets have been used more than whiskers due to the toxicity of the latter. Alumina and silicon carbide platelets have been used to reinforce monolithic ceramic matrices [1±3, 10±12] and vitreous ceramics [13]. In both cases, the mechanical properties showed an improvement over the base material. Silicon nitride is a material with great potential under high temperatures. It is therefore important that it should resist thermal gradients. While there are several works in the literature related to thermal shock resistance of pure silicon nitride, none report the effects caused by sudden temperature changes on the residual mechanical resistance of silicon nitride reinforced by silicon carbide platelets. The purpose of this work is to investigate the damage occurring in this composite material when submitted to thermal shock. The material studied was fabricated by conventional hot-pressing of a silicon nitride powder (SNE 10, Ube Corp., Japan) containing as sintering additives 5 and 2 wt % of aluminum and yttrium oxide, respectively. As the reinforced phase, 20 v=o of monocrystalline silicon carbide platelets were added (32±50 im, American Matrix, Knoxville, USA). Fig. 1 shows a scanning electronic microscope (SEM) micrograph of monocrystalline silicon carbide platelets used in this work. One can easily notice that the platelets present a hexagonal format. The powder mixture of silicon nitride and silicon carbide platelets was pressed at 1800 8C in graphite dies. Details on the preparation of this composite have been described elsewhere [1, 14]. Thermal shock treatment was undertaken by heating the samples in a vertical tubular furnace to a particular temperature. The temperature was measured by a cromel±alumel thermocouple beside the samples. The samples (having dimensions of 4 3 4 3 50 mm) considered for mechanical resistance measurement were ground and polished and had rounded edges. After 10 min under constant temperature, the samples were released and allowed to fall into a water reservoir at 25 8C. After thermal shock, residual exure strength tests by the four-point bending test were performed using a cross-head speed of 0.05 mm miny1 and a standard testing machine (Instron), with inner and outer spans of 20 (e) and 40 mm (l), respectively (Fig. 2). To evaluate the thermal shock behavior of the composite, two parameters, R and R, were calculated. The determination of these two parameters are based on Hasselman's theory [15]. After the rupture, the width and length of the samples were measured in the region of the fracture.