Mechanical behavior of stiff coating on glass under sliding contact

In this paper, we investigate the failure mechanism under scratch testing of glass samples coated with an Si3N4 monolayer deposited by magnetron sputtering. We demonstrate that applying a thin Si3N4 layer considerably compromises the effective glass strength. Contact and fracture mechanics conspire to produce this result when a stiff and well adherent film is deposited on a more compliant substrate. Three major phenomena have been identified: (i) the Si3N4 layer generates a high friction coefficient which enhances the tensile stress at the trailing edge of the contact zone, (ii) the stiff film enhances the in-plane tensile stress due to elastic mismatch and (iii) when a through-thickness crack hits the interface, the elastic mismatch provides a strong driving force for the crack to cross the interface and penetrate into the glass substrate. A specific type of damage, where a hierarchy of cracks is observed in the trail of the indenter instead of the more usual homogeneous series of so-called Hertzian cracks, is also discussed.