Effect of heat-treatment on stress relaxation behavior of plasma-sprayed 7 wt.% Y2O3–ZrO2 stand-alone coatings

The present work studied the effect of heat-treatment temperature (1000 °C and 1200 °C) and time (10, 50, and 100 h) on the compressive stress relaxation behavior of plasma-sprayed stand-alone 7 wt.% Y2O3–ZrO2 (YSZ) coatings at test temperatures of 1000 °C, 1100 °C, and 1200 °C, from stresses of 60 and 20 MPa. As-sprayed coatings were also stress relaxed in the baseline condition at room and elevated temperatures. All coatings demonstrated a two-stage relaxation behavior: fast relaxation (stage I) in the first 10 min and much slower relaxation in the final 170 min of the test (stage II). Stage I relaxation, as measured by percentage of the original stress relaxed, accounts for at least 50% of the total stress relaxed despite occurring in only 5–10 min and was attributed to lamella sliding and compaction, and permanent intralamellar crack closure (for tests conducted at higher temperatures). Stage II relaxation behavior is dominated by diffusion creep mechanisms, where prior densification at 1200 °C resulted in reduced relaxation rates compared to coatings heat treated at 1000 °C and in the as-sprayed condition. The 1200 °C test temperature greatly influenced the percentage of relaxation in the coating, more so than the prior coating heat-treatment conditions.

Research Highlights
► Complete characterization of stress relaxation behavior under simulated service
► Stage I and stage II stress relaxation mechanisms differentiated
► Strategies for improved coating designs are discussed