Elastic Modulus of Oxidation-formed Silicon Nitride Composite

Abstract

Silicon nitride ceramics are generally used in high temperature oxidizing environments. The phase composition and microstructure of silicon nitride ceramics change during oxidation resulting in changes in the mechanical properties of materials. The oxidation behaviour of silicon nitride is dealt with in numerous studies [e.g. 1±7]. In these papers the effects of the oxidation on the room-, and the high-temperature strength and fracture toughness were studied [1±5]. The phase composition and the thickness of the surface oxide layer also studied by several authors [6, 7]. The published studies in this ®eld concentrated on the mechanical properties relevant for utilization but little attention was paid to the elastic properties. It is known that the Young's modulus gives valuable information about the structural changes of ceramics during sintering [8], therefore it was assumed that the measurement of the modulus of oxidized samples would contribute to the understanding of the complex process of oxidation. In this paper, the Young's modulus of oxidized silicon nitride ceramics is studied. The sample oxidized at elevated temperatures is considered as a composite material that consists of an internal part and a surface oxide layer. The elastic modulus of the whole sample was measured by the standard four point bending method, that of the internal region and the surface oxide layer was determined by the depth sensing indentation (DSI) method. A formula was derived for the calculation of the modulus of the composite from the moduli of the parts. Mixtures of Si3N4, AlN, Al2O3 and Y2O3 powders were milled in ethanol in a planetary type alumina ball mill. The composition of materials is given in Table I. Samples were compacted by dry pressing at 220 MPa. The materials were sintered in high purity nitrogen by a two-step sinter-HIP method using BN embedding powder. The dimensions of as-sintered specimens were approximately 3:5 mm 3 5 mm 3 50 mm. Oxidation took place in static air at 800 8C, 1000 8C, 1150 8C, 1300 8C and 1400 8C for up to 48 h. The Young's modulus of the samples was determined before and after oxidation by a room temperature four point bending test with spans of 40 and 20 mm. The elastic moduli for different compositions and temperatures of oxidation are shown in Fig. 1, the one standard deviation is less than 5 GPa. The Young's modulus of the samples began to decrease at an oxidation temperature of 1300 8C for materials X, A and F and a drastic reduction occurred at 1400 8C for all the compositions investigated. On the surface of the samples oxidized at 1300 8C and 1400 8C a white, rough oxide layer appeared. The role of this oxide layer in the change of the elastic modulus will be analysed in the following. The oxidized rectangular specimen can be considered as a composite material that consists of an internal part and an oxide layer (Fig. 2a). The Young's modulus of the internal region and the surface oxide layer of the materials oxidized at 1300 8C and 1400 8C was measured by the depth sensing indentation (DSI) method. The surface layer was removed by grinding before the measurement of the modulus of the internal part of the material. The DSI measurements were carried out by a Shimadzu dynamic ultra-micro hardness tester (DUH 202). Each DSI test consisted of a loading±unloading cycle. During the loading period a Vickers indenter penetrated into the surface of the sample at constant loading rate (70 mN sy1) and the same rate was applied in the unloading period when the pyramid moved backwards. The maximum load in each test was 500 mN. The Young's modulus of the samples was calculated from the indentation curves using the method of Oliver and Pharr [9]. The average modulus values of the internal region (Ei) and the surface oxide layer (El) are shown in Table II (the one standard deviation is about 10 GPa). The Young's modulus of the oxide layer of material B oxidized at 1400 8C is much lower than that of other studied materials. The reason for this deviation is under investigation. A simple relationship between the modulus of the