Direct measurement of strain behavior of compression loaded plasma-sprayed yttria-stabilized zirconia

Direct measurement of the stress–strain behavior of stand-alone plasma-sprayed 7 wt.% Y2O3–ZrO2 (YSZ) coatings was made at room temperature. YSZ coatings were evaluated in the as-sprayed condition, and after heat-treatments for 10, 50, and 100 h at 1200 °C using both monotonic and two different cyclic uniaxial compression loading profiles. Heat-treatments were used to change the primarily mechanically interlocked system of lamella to a chemical interlocked system. Monotonic loading of the coatings indicated three different moduli. The first modulus was lower (~ 10 GPa) than observed typically in a plasma sprayed coating. The relatively easy deformation associated with the low modulus was attributed to minor sample crushing near its ends. The second modulus observed was ~ 24 GPa and is due to closure of cracks oriented perpendicular to the stress and lamella sliding and compaction. The third modulus measured ranged from 40 to 48 GPa and represents the elasticity of the material after the easy crack closure and lamella sliding events have occurred. In repeated cyclic loading to a compressive stress of 60 MPa, the coatings were observed to demonstrate large amounts of non-recoverable strain during the first several load/unload cycles, presumably due to permanent sliding and compaction of lamella. Permanent strain tended to a minimum amount after ~ 20 load/unload cycles. Increased cyclic loading from 20 to 320 MPa in 20 MPa increments revealed significant anelastic behavior as evidenced by stress–strain hysteresis loops, particularly in the as-sprayed coating. The elastic moduli of the coatings upon reloading were observed to increase after each subsequent 20 MPa increase in stress applied to the coating to values as high as ~ 80 GPa. This was attributed to an increase in the volume fraction of coating sustaining the load. The amount of non-recoverable strain was highest in the heat-treated coatings indicating a loss of strain tolerance of YSZ coatings during simulated service conditions.