Effect of coating thickness on crack initiation and propagation in non-planar bi-layers

Hertzian contact damage is studied in glass coatings (thickness range 160 μm to 1 mm) on polycarbonate polymer substrates. Both planar and non-planar geometries are considered, subjected to indentation by fixed size spherical indenters of radius 4 mm. Finite element analysis is carried out to evaluate the stress distribution in the bilayer structure. Radial cracking initiating at the coating undersurface directly under the indenter is the primary focus of this investigation, and cone cracking at the top surface of the coating (inner and outer cone cracks) is also considered.It is concluded that crack propagation is facilitated in coatings of an intermediate thickness. Thick (1000 μm) coatings resist deflection, decreasing tensile stresses at the coating undersurface, while thin (160 μm) coatings deflect massively, causing a compression zone beneath the indenter which also limits undersurface tension.