THE EFFECTS OF THERMAL SHOCKING ON THE RESIDUAL-STRESS IN SI3N4 CERAMIC COMPOSITES

In this work, synchrotron radiation X-ray diffraction results on Si3N4 ceramic composites are presented and interpreted in order to analyze the variations of the residual stress in the samples as the preparatio n and the thermal treatment methods are changed. Along with the elasti city theory, the shifts of the diffraction peak positions when the sam ple is tilted at various angles with respect to the incoming beam, are used to determine the stress in the sample. A detailed comparison of the residual stresses between the as-machined and the thermally shocke d samples are made and the effects of thermal shocking temperature on the residual stress and the strength of the materials are discussed. A ngle dispersive X-ray diffraction results indicated a correlation betw een the residual stress and the thermal shocking temperature as the co mpression increased in both directions parallel and perpendicular to t he surface reaching a maximum around 1100 degrees C. The increase in t he compression near the surface layers is a favorable situation from t he point of crack initiation on the surface since the existing cracks can be reduced in size by thermal shocking, and the developed compress ive stresses can play a role in preventing the initiation and propagat ion of the cracks. The X-ray diffraction results were compared with va rious strength tests results on these materials. (C) 1997 Published by Elsevier Science B.V.