Deformation Mechanisms in Compression‐Loaded, Stand‐Alone Plasma‐Sprayed Alumina Coatings

Abstract

Cylindrical, stand-alone tubes of plasma-sprayed alumina were tested in compression in the axial direction at room temperature, using strain gauges to monitor axial and circumferential strains. The primary compression-loading profile used was cyclic loading, with monotonically increased peak stresses. Hysteresis was observed in the stress-strain response on unloading, beginning at a peak stress of 50 MPa. The modulus decreased as the maximum applied stress increased. The stress-strain response was only linear at low stresses; the degree of nonlinearity at high stresses scaled with the stress applied. One-hour dwells at constant stress at room temperature revealed a time-dependent strain response. Using transmission electron microscopy and acoustic emission to investigate deformation mechanisms, the stressstrain response was correlated with crack pop-in, growth, and arrest. It is proposed that the numerous defects in plasmasprayed coatings, including porosity and microcracks, serve as sites for crack nucleation and/or propagation. As these small, nucleated cracks extend under the applied stress, they propagate nearly parallel to the loading direction along interlamellae boundaries. With increasing stress, these cracks ultimately link, resulting in catastrophic failure.

In Situ Observation of Crack Behavior in Compressively Loaded Plasma‐Sprayed 7‐wt%‐Yttria‐Stabilized Zirconia

“…It has been hypothesized that the increase in modulus is caused by cracks oriented perpendicular to the applied load becoming partially or completely closed. 2 Figures 3 and 4 seem to confirm that this explanation is plausible in that cracks oriented perpendicular to the loading application exhibit a decrease in width. The observed increase in modulus with partial crack closure can be explained by the fact that, as cracks oriented perpendicular to the stress close, a larger area fraction of the load is carried by the YSZ.…”

“…This has been observed in plasma-sprayed 8 wt% yttriazirconia 1 and alumina. 2 It has been speculated that the increase in modulus is a result of microcrack closure caused by loading of the plasma-sprayed coating. Another characteristic feature of plasmasprayed materials is loading/unloading hysteresis; this behavior has been attributed to the formation of microcracks during loading 2,3 that do not cause catastrophic failure.…”

Stress Relaxation of Compression Loaded Plasma‐Sprayed 7 Wt% Y2O3–ZrO2 Stand‐Alone Coatings

“…In a prior study, acoustic emission sensors attached to a plasma-sprayed alumina tube continued to detect the formation of microcracks after the peak stress was reached. 19 The orientation of these new cracks with respect to . Plot of modulus as a function of time at 10001C for three initial stress levels in YSZ S1 coatings.…”

“…2) cannot be attributed to any thermally activated mechanism within the YSZ as the temperature is too low. In a prior study, acoustic emission sensors attached to a plasma‐sprayed alumina tube continued to detect the formation of microcracks after the peak stress was reached 19 . The orientation of these new cracks with respect to the loading direction was directly observed by Levin et al .…”

“…It has been previously shown 19,20 that cracks oriented perpendicular to a uniaxial load in plasma‐sprayed coatings will partially close at 25°C; these cracks will open up again upon unloading, as detected in tests on plasma‐sprayed alumina coatings by acoustic emission sensors 19 . However, at elevated temperatures, when these cracks are partially closed by a mechanical load, crack tips and adjacent surfaces in contact would be expected to close by sintering 21 .…”