Thermal fracture behavior of metal/ceramic functionally graded materials

Thermal fracture behavior of metal/ceramic functionally graded materials (FGMs) was evaluated by a well controlled burner heating method using a H-2/O-2 combustion flame, which simulated real environment. Partially stabilized zirconia (PSZ)/IN100 FGMs having finely mixed microstructures and PSZ/Inco718 FGMs having rather coarse microstructures were prepared by a slurry dipping and HIP sintering process. Also, three types of functionally graded thermal barrier coatings (TBCs) as well as duplex coatings, each designed to have the same thermal resistance, were fabricated by an air plasma spraying process. The fracture mechanism has been discussed on the basis of the crack morphology, the analysis of acoustic emissions and the variation of effective thermal conductivity. The thermal shock fracture behavior is discussed on PSZ/In100 FGMs and PSZ/Inco718 FGMs, while the cyclic fracture behavior is discussed on plasma sprayed coatings. The cyclic fracture behavior is found to be: orthogonal crack formation on the top surface during cooling, then transverse crack formation in the graded layer during heating, and subsequent growth of transverse cracks and their coalescence which eventually causes the ceramic coat to spall. Compared to duplex coatings, it has been revealed that functionally graded TBCs possess the desirable effect for improvement of spallation life under cyclic thermal loads. The dependence of spallation life on composition profile in functionally graded coatings has been discussed. (C) 2002 Elsevier Science Ltd. All rights reserved.