CONTACT-INDUCED TRANSVERSE FRACTURES IN BRITTLE LAYERS ON SOFT SUBSTRATES - A STUDY ON SILICON-NITRIDE BILAYERS

An analysis of transverse cracks induced in brittle coatings on soft s ubstrates by spherical indenters is developed, The transverse cracks a re essentially axisymmetric and geometrically conelike, with variant f orms dependent on the location of initiation: outer cracks that initia te at the top surface outside the contact and propagate downward; inne r cracks that initiate at the coating/substrate interface beneath the contact and propagate upward; intermediate cracks that initiate within the coating and propagate in both directions, Bilayers consisting of hard silicon nitride (coating) on a composite underlayer of silicon ni tride with boron nitride platelets (substrate), with strong interfacia l bonding to minimize delamination, are used as a model test system fo r Hertzian testing. Test variables investigated are contact load, coat ing/substrate elastic-plastic mismatch (controlled by substrate boron nitride content), and coating thickness, Initiation of the transverse coating cracks occurs at lower critical loads, and shifts from the sur face to the interface, with increasing elastic-plastic mismatch and de creasing coating thickness, This shift is accompanied by increasing qu asi-plasticity in the substrate, Once initiated, the cracks pop in and arrest within the coating, becoming highly stabilized and insensitive to further increases in contact load, or even to coating toughness, A finite element analysis of the stress fields in the loaded layer syst ems enables a direct correlation between the damage patterns and the s tress distributions: between the transverse cracks and the tensile (an d compressive) stresses; and between the subsurface yield zones and th e shear stresses, Implications of these conclusions concerning the des ign of coating systems for damage tolerance are discussed.