Fracture Modes in Brittle Coatings With Large Interlayer Modulus Mismatch
Herzl Chai, Brian R. Lawn, S Wuttiphan
Fracture modes in a model glass/polymer coating/substrate system indented with hard spheres are investigated. The large modulus mismatch between the glass and polymer results in distinctive transverse fracture modes in the brittle coating: exaggerated circumferential (C) ring cracks that initiate at the upper coating surface well outside the contact (as opposed to the near-contact Hertzian cone fractures observed in monolithic brittle materials); and median-radial (M) cracks that initiate at the lower surface (at the substrate interface) on median planes containing the contact axis. Bonding between the coating and substrate is sufficiently strong as to preclude delamination in our system. The transparency of the constituent t materials usefully enables in situ identification and quantification of the two transverse fracture modes during contact. The morphologies of the cracks and the corresponding critical indentation loads for initiation are measured over a broad range of coating thickness (20υm to 5.6 mm), on coatings with like surface flaw states, here ensured by a pre-bonding abrasion treatment. There is a well-defined intermediate range where the indented coating behaves more like a flexing plate than a Hertzian contact, and where the M and C cracks initiate in close correspondence with a simple critical stress criterion, for example, when the maximum tensile stresses exceed the bulk strength of the (abraded) glass. In this intermediate range the M cracks from the first--only when the flaws on the lower surface are removed (by etching) do the C cracks form first. Finite element modeling is used to evaluate the critical stresses at crack initiation and the surface locations for the crack origins. Departures from the critical stress condition occur at the extremes of very thick coatings (near-monolith limit) and very thin coatings (thin-film limit), where stress gradients over the flaw dimension are high. Implications of the results concerning practical coating systems are considered.