Meridian crack test strength of plasma-sprayed amorphous and nanocrystalline ceramic microparticles

We implement the meridian crack test method to measure the surface and subsurface flaw-controlled strength of spherical near-eutectic plasma-sprayed alumina-zirconia-silica particles of average diameter near 30 μm. The particles are tested in two states, namely as-sprayed amorphous, or nanocrystalline as obtained after a supplementary annealing step. The test consists in uniaxial compression testing of individual particles between a pair of elasto-plastic steel platens, the hardness of which is tailored to achieve relative contact radii (defined as the ratio between the radius of the projected particle-platen contact area to the particle radius) above 0.6 at the moment of particle failure. Results show that nanocrystalline particles exhibit characteristic Weibull strength 1490 MPa, which is approximately 30% higher than for amorphous particles while the Weibull modulus is, relative to the test precision, comparable, being equal to 6.0 and 7.8 for nanocrystalline and amorphous particles, respectively. This result is an indication that the flaw size distribution is not significantly affected by the annealing step, the strength increase resulting from an increase in fracture toughness upon nanocrystallisation. This conclusion is consistent with fractographic observations. The principal strength-limiting defects identified for both amorphous and nanocrystalline particles were micropores formed during plasma spraying.