Role of flaw statistics in contact fracture of brittle coatings

• Published in: Acta materialia 2001-10, Vol.49 (18), p.3719-3726
• Main Authors: Miranda, P., Pajares, A., Guiberteau, F., Cumbrera, F.L., Lawn, B.R.
• Format: Article
• Language: English
• Subjects: Brittle; Coating; Condensed matter: structure, mechanical and thermal properties; Exact sciences and technology; Flaw statistics; Mechanical and acoustical properties; Physical properties of thin films, nonelectronic; Physics; Surfaces and interfaces; thin films and whiskers (structure and nonelectronic properties)
• ISSN: 1359-6454; 1873-2453
• DOI: 10.1016/S1359-6454(01)00280-4

A flaw statistics analysis is here developed to account for systematic differences between experimentally observed and theoretically predicted critical loads for the initiation of contact-induced radial cracks in brittle coatings on compliant substrates. Specific attention is drawn to deviations in critical load ( P R) data from ideal quadratic dependence on coating thickness ( d), i.e. P R∝ d 2, especially at low d values. It is postulated that these deviations are attributable to the existence of distributions in flaw size and location, in relation to the bell-shaped tensile stress fields responsible for initiation of the radial cracks at the coating lower surface. A statistics-based expression is derived for the mean values of P R in terms of flaw density and size distribution. Data from model bilayers consisting of glass plates of different thicknesses d bonded to polycarbonate substrates are used as an illustrative case study. Controlled pre-abrasion flaws are introduced into the lower glass surfaces before joining into the bilayer configuration, to enable a priori characterization of distribution parameters by image analysis. Finite element modelling is used to determine the tensile stress distribution at the coating lower surface. The predicted statistics-based P R( d) function is shown to fit the data within uncertainty bounds. Implications concerning the continued usefulness of the ideal, P R∝ d 2 relation for designing ceramic coatings for failure resistance are considered.