Multiple cracking in functionally graded ceramic/metal coatings

Functionally graded coatings such as thermal barrier coatings on advanced metals and alloys can sustain multiple cracking upon thermal and mechanical loading. A special feature pertaining to the fracture behavior of coating is the coating gradation which can be characterized by the local volume fraction of metal. To guide the design of coating gradation so that cracking damage can be minimized, a fracture mechanics study is made of the crack driving force for multiple cracks in functionally graded ceramic/metal coatings. The metal substrate and the ceramic/metal coating are taken as linear elastic, with the elastic properties of the coating varying through the film thickness. Systematic finite element calculations are made for the energy release rate of the cracks in the coating as determined by the coating gradation, crack length, and the crack density; both mechanical and thermal loads are considered. It is found that compared with the pure ceramic coating, gradation of the coating can significantly reduce the crack driving force. It is also found that under mechanical loading the effect of different gradations on the crack driving force is relatively small. However, under thermal loading the influence of coating gradation can be significant. The implications of the results for the material design of functionally graded coatings are discussed.