Indentation-induced subsurface tunneling cracks as a means for evaluating fracture toughness of brittle coatings

Abstract

Whenaplategluedtoacompliantsubstrate is subject to indentation, cracks may initiate from its subsurface due to flexure. Upon increasing the load, the damage develops into a set of tunnel radial cracks whichpropagatestablyunderadiminishingstressfield. This phenomenon is utilized here to extract fracture toughness KC for brittle materials in the form of thin plates orfilms.Experiments show that the SIF at the tip of the subsurface radial cracks is well approximated as K ∼ P/c 3/2 ,where P istheindentationloadand cthe meanlengthofthecrackfragments.Usingatransparent substrate, c can be easily determined after unloading, from which KC is found. This simple and economic conceptisappliedtoawidevarietyofthinceramiccoat- ings, yielding toughness data consistent with literature values. Because the tip of the tunneling cracks are well removed from the contact site, the method circumvents certain complications encountered in common top-sur- face radial cracking techniques such as the effect of plastic deformation, residual stresses and crack exten-