Deformation mechanisms in compression-loaded, stand-alone plasma-sprayed alumina coatings

Cylindrical, stand-alone tubes of plasma-sprayed alumina were tested in com pression 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 stresse s. Hysteresis was observed in the stress-strain response on unloading, begi nning at a peak stress of 50 MPa. The modulus decreased as the maximum appl ied stress increased. The stress-strain response was only linear at low str esses; the degree of nonlinearity at high stresses scaled with the stress a pplied. One-hour dwells at constant stress at room temperature revealed a t ime-dependent strain response. Using transmission electron microscopy and a coustic emission to investigate deformation mechanisms, the stress-strain r esponse was correlated with crack pop in, growth, and arrest. It is propose d that the numerous defects in plasma-sprayed 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 propag ate nearly parallel to the loading direction along interlamellae boundaries . With increasing stress, these cracks ultimately link, resulting in catast rophic failure.