Effects of pores on mechanical properties of plasma-sprayed ceramic coatings

The effects of pore sizes, shapes, and orientations on the mechanical prope rties of thermally sprayed ceramic coatings are investigated. The analysis is conducted using detailed finite-element models with geometries similar t o those of actual ceramic coatings containing many embedded pores. These mi crostructural models include many randomly placed pores of different sizes and shapes and are loaded in tension to determine their effective elastic m oduli along the spray and transverse directions. We modeled coatings with s tatistical distributions of pore sizes and shapes that followed those of ac tual Al2O3-TiO2 coatings. Because the pores in such a model are of differen t sizes and shapes, the model must be large enough to contain sufficient po res before the average modulus obtained from uniaxial loading can be identi fied as an effective property. Using differently sized models, we determine d the variability of the average moduli, Such information is valuable when homogenized or continuum material models are used in the stress analyses of coatings. Our computed results show that a model must be large enough to c ontain 50-100 pores before the averaging of properties is accurate, Using t he Al2O3-TiO2 models, we also simulated microindentation tests. Unlike the results determined from uniaxial loading, the elastic moduli estimated from indentation possessed large variations. Apparently, the morphology of the pores immediately beneath the indentation or within the zone of influence h as a significant effect on the response of the indenter and the measured mo dulus, The implications of these results and the computational capability t o predict the mechanical properties of porous, plasma-sprayed ceramic coati ngs are discussed here.