Thermal properties of plasma-sprayed functionally graded thermal barrier coatings

Plasma-sprayed thermal barrier coatings often have the problems of spallati on and cracking in service owing to their poor bond strength and high resid ual stresses. Functionally graded thermal barrier coatings with a gradual c ompositional variation from heat resistant ceramics to fracture-resistant m etals are proposed to mitigate these problems. In this paper, functionally graded yttria stabilized ZrO2/NiCoCrAlY coatings were prepared using pre-al loyed and spheroidized composite powders as the feedstock and the thermal p roperties of the coatings were studied. Results show that the coefficient o f 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 cont ent and heating temperature. The resistance of functionally graded coatings to thermal fatigue was approximately five times better than that of the du plex coatings. Results also show that the graded coating was superior to th e duplex coating in long-term oxidation resistance. (C) 2000 Elsevier Scien ce S.A. All rights reserved.