INFLUENCE OF PHASE-STABILITY ON THE RESIDUAL-STRESS IN PARTIALLY-STABILIZED ZIRCONIA TBC PRODUCED BY PLASMA SPRAY

Thermal barrier coatings (TBCs) of yttria-partially-stabilized zirconi a were plasma sprayed on aluminium substrates under controlled atmosph ere and temperature conditions (ATCS process) at 45 degrees C and 155 degrees C. Residual stress and phase composition were deeply influence d by substrate temperature and atmosphere during deposition. Spraying in Ar atmosphere promoted the formation of highly disordered and under -stoichiometric ceramics with unstable phase composition. Reducing atm osphere and low substrate temperature caused the retention of excess o xygen vacancies which stabilize a high density cubic phase (p approxim ate to 6.16 g cm(-3)), which transformed to the non-transformable tetr agonal phase (rho approximate to 6.05 g cm(-3)) after low temperature ageing. Phase composition and residual stress field in TBCs were studi ed by two different techniques, X-ray diffraction (XRD) and coating le ngth change after detaching from the substrate (CD); the two technique s gave complementary results, since the former measured surface strain , whereas the latter gave information on average bulk strain. The aver age in-plane stress was compressive, with a positive gradient leading to a low surface tensile stress; at higher deposition temperatures coa tings were increasingly in compression, During ageing and heat treatme nt, the volume expansion associated with the cubic to tetragonal trans formation led to a considerable increase of the compressive stress, de monstrating that high residual stresses can develop after phase transf ormations.