Stress-Relaxation and Creep Behavior of Heat-Treated Stand-Alone Plasma-Sprayed 7 wt.% Y₂O₃-ZrO₂ Coatings

Plasma-sprayed 7 wt.% Y₂O₃-ZrO₂ (YSZ) stand-alone coatings were subjected to 10, 50 and 100-hr heat treatments at 1200°C, followed by mechanical testing in compression at 25°C, 1050°C, and 1200°C. The mechanical tests performed on the samples included cyclic loading/ unloading, stress-relaxation, and creep. In cyclic compression tests at 25°C, it was observed that the YSZ coating that had been heat-treated for 50-hr at 1200°C demonstrated a higher modulus and more strain hysteresis as compared to an as-sprayed sample. As heat-treatment time increased, the YSZ stand-alone coating demonstrated less relaxation of the initial applied stress for stress-relaxation tests run at 1050°C and 1200°C. The steady-state creep rate was observed to decrease with increasing heat-treatment time prior to testing; as expected, the steady-state creep rate increases when the testing temperature was increased from 1050°C to 1200°C. Density (via Archimedes’) and phase analysis (via X-ray diffraction) were performed on YSZ coatings before (i.e. as-sprayed) and after heat-treating at 1200°C. Porosity was observed to decrease only slightly (~1%) after a 100-hr heat-treatment at 1200°C as compared to the as sprayed porosity. The high-yttria metastable tetragonal phase observed in the as-sprayed coatings was observed after a 100-hr heat-treatment at 1200°C. The same metastable tetragonal phase was also observed after a 100-hr heat-treatment at 1200°C followed by a 3-hr stress-relaxation test at 1200°C.