Thermal properties of plasma-sprayed functionally graded thermal barrier coatings

Plasma-sprayed thermal barrier coatings often have the problems of spallation and cracking in service owing to their poor bond strength and high residual stresses. Functionally graded thermal barrier coatings with a gradual compositional variation from heat resistant ceramics to fracture-resistant metals are proposed to mitigate these problems. In this paper, functionally graded yttria stabilized ZrO2/NiCoCrAlY coatings were prepared using pre-alloyed and spheroidized composite powders as the feedstock and the thermal properties of the coatings were studied. Results show that the coefficient of thermal expansion and thermal conductivity/diffusivity changed gradually through the five-layer functionally graded coating. The thermal diffusivity and conductivity were found to increase with an increase in NiCoCrAlY content and heating temperature. The resistance of functionally graded coatings to thermal fatigue was approximately five times better than that of the duplex coatings. Results also show that the graded coating was superior to the duplex coating in long-term oxidation resistance. (C) 2000 Elsevier Science S.A. All rights reserved.