Abstract
Observations on fracture and crack bifurcation phenomenon of plasma spray Zr coating is studied in current paper with new understanding based on new recognized information.A new model is developed for Zr coating layers and cracks. Analytical fundamental characterization is carried out regarding mechanism under high thermal stresses on outer face with cooling back side of the substrate. Results are new such as crack branches at 120°, layered cracks, stepped crack surface and variable crack width. The zirconia coating has hexahedral shape as a honeycomb microstructure. Due to effect of heating-cooling, the first coating layer and near layers to substrate surface of zirconia coating will be contracted or do not be expanded while outside coating layers will be extended producing bi-interfacial de-bonding between particles and propagation to other crystals producing cracks and branching. Depth and width of the crack will be extended and increased in the coating layers producing stepped and layered cracks with branches. Propagation angles of branches are always 60°. Cracking load will be maximum at main crack orientation angle of β = 45° for equal stresses in both X and Y directions while for β<30, β>60 cracking load will be minimized meaning that crack will carry high stresses at 30>β > 60.
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