Stresses developed in reaction-bonded ceramics

A physical model is presented that predicts the stress distribution created in a particle during its reaction with a surrounding reactant to form a un iform layer of reaction product on its surface, when the reaction involves a volume change. The results of the model are applied specifically to the c ase of silicon reacting with nitrogen to form Si3N4. The model predicts the generation of a high, tensile hydrostatic stress in the Si core as well as high tensile radial stress and compressive tangential stress in the nitrid e layer. Although the model is restricted to elastic deformation only and t herefore predicts unrealistically high stresses in some cases, the results are anyway of relevance in the consideration of possible non-elastic proces ses such as creep and fracture and also in assessing the possible effect of stress on the reaction equilibrium. It is predicted that the nitride react ion layer would fracture during the nitridation process. A second model is also presented predicting the residual stresses arising during cooling of a partially reacted particle as a result of the difference in thermal expans ion of the reactant core and the reaction product layer. In the case of the reaction of silicon to silicon nitride these thermal expansion mismatch st resses are significant but small compared to the stresses due to the chemic al reaction. (C) 1999 Elsevier Science Limited. All rights reserved.