Designing damage-resistant brittle-coating structures: II. Trilayers Original Research Article

An extension of the FEA-based damage analysis for bilayers in Part I is presented for trilayers consisting of a functional outer layer on an underlying substrate with an intermediate inner core layer. The inner core layer may be used to enhance bonding or load support, but is itself vulnerable to subsurface radial cracking (brittle interlayer) or yield (soft interlayer). A stress analysis is conducted by reducing the trilayer system to an effective bilayer in which the core layer is regarded as either part of the coating or substrate, depending on the damage interface (i.e. outer/inner or inner/substrate). The stress solutions are used to determine generic relations for the critical loads to induce radial cracking or plasticity (or quasiplasticity) in the outer or inner layers, and even in the substrate. A quadratic relationship between critical load and effective coating thickness is preserved; and strength and hardness of the constituent layers remain principal, linearly dependent, material parameters. However, dependence on relative outer/inner layer thicknesses and elastic moduli are not generally amenable to exact solution, limiting useful explicit relations to radial cracking in the inner layer. Use of the analysis in constructing design diagrams is again considered.