CREEP EFFECTS ON THE SPALLATION OF AN ALUMINA LAYER FROM A NICRALY COATING

A finite-element analysis has been performed of alumina spallation by wedge cracking from a flat Nil6Cr6A1Y coating substrate during cooling from 1100 degrees C. The analysis allowed for creep deformation withi n both the coating and oxide but it was found that only creep within t he coating influenced the growth of the interfacial wedge crack by rel axing stresses at the crack tip. As a result, crack growth was suppres sed during the early stages of cooling. Computed values of the critica l temperature drop to cause spallation are presented for various cooli ng rates and values of the coefficient of thermal expansion of the coa ting. The results show that the effective fracture energy for growth o f the interfacial crack in the presence of creep is some 1-2 orders of magnitude greater than previously determined values for elastic fract ure of the oxide and demonstrate the important role of coating creep. These results are used to produce a first spallation map for this syst em. (C) 1997 Elsevier Science S.A.