Stress Relaxation of Compression Loaded Plasma‐Sprayed 7 Wt% Y ₂ O ₃ –ZrO ₂ Stand‐Alone Coatings

Stand‐alone plasma‐sprayed tubes of 7 wt% Y ₂ O ₃ –ZrO ₂ made from the same starting powder but at two different sites were subject to stress‐relaxation testing in axial compression at temperatures of 25°, 1000°, 1050°, 1100°, and 1200°C and at an initial stress of 10–80 MPa. A time‐dependent stress response was observed for both coatings at all temperatures. For example, a 20 MPa stress applied at 1050°C relaxed to ∼3 MPa in 180 min. When the same initial stress was applied at 1200°C, the coating fully relaxed in 32 min. For all experimental conditions evaluated, an initial fast stress‐relaxation regime was observed (<10 min), followed by a slower second stress‐relaxation regime at later times (>10 min). Coatings with higher as‐sprayed densities exhibited a lengthened fast relaxation regime as compared with less dense coatings. A Maxwell model was modified in order to provide an accurate fit to the experimental stress‐relaxation curves. From scanning electron microscopy experiments and mechanical data, the mechanism for stress relaxation from 25°C through 1200°C, particularly during fast relaxation, was proposed to be the formation of cracks parallel with respect to the applied load. In addition to this mechanism, stress relaxation that occurred in specimens tested at 1000°C through 1200°C was proposed to be due to partial or complete closure of cracks oriented perpendicular to the applied stress.