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 within the brittle coating f exaggerated circumferential (C) ring cracks that initiate at the upper coating surface well outside the contact (as opposed to the near-contact Heltzian cone fractures observed in monolithic brittle materials); median-radial (M) cracks that initiate at the lower surface (i.e., 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 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 thicknesses (20 pm to 5.6 mm), on coatings with like surface flaw states, here ensured by a prebonding abrasion treatment. There is a well-defined, broad intermediate range where the indented coating responds 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, i.e., when the maximum tensile stresses exceed the bulk strength of the (abraded) glass. In this intermediate range the M cracks generally form first--Only when the flaws on the lower surface are removed (by etching) do the C cracks f
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