An analysis of transverse cracks induced in brittle coatings on soft substrates by spherical indenters is developed. The transverse cracks are essentially axisymmetric and geometrically conelike, with variant forms dependent on the location of initiation: outer cracks that initiate at the top surface outside the contact and propagate downward; inner 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 nitride with boron nitride platelets (substrate), with strong interfacial bonding to minimize delamination, are used as a model test system for Hertzian testing. Test variables investigated are contact load, coating/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 surface to the interface, with increasing elastic-plastic mismatch and decreasing coating thickness. This shift is accompanied by increasing quasi-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 systems enables a direct correlation between the damage patterns and the stress distributions: between the transverse cracks and the tensile (and compressive) stresses; and between the subsurface yield zones and the shear stresses. Implications of these conclusions concerning the design of coating systems for damage tolerance are discussed.

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A model of contact damage accumulation from cyclic loading with spheres and ensuing strength degradation in relatively tough, heterogeneous ceramics is developed. The damage takes the form of a quasi-plastic zone beneath the contact, consisting of an array of closed frictional shear faults with attendant "wing" microcracks at their ends. Contact fatigue takes place by attrition of the frictional resistance at the sliding fault interfaces, in accordance with an empirical degradation law, allowing the microcracks to extend. At large numbers of cycles or loads the microcracks coalesce, ultimately into radial cracks. Fracture mechanics relations for the strength degradation as a function of number of cycles and contact load are derived. Indentation-strength data from two well-studied coarse-grain quasi-plastic ceramics, a micaceous glass-ceramic and a silicon nitride, are used to evaluate the model. Comparative tests in static and cyclic contact loading confirm a dominant mechanical component in the fatigue. At the same time, the presence of water is shown to enhance the fatigue. The model accounts for the broader trends in the strength degradation data, and paves the way for consideration of key variables in microstructural design for optimum fatigue resistance.

A study is made of the damage resistance of silicon nitride bilayers consisting of a hard overlayer (coating) on a soft underlayer (substrate). The two layers are fabricated with different starting powders, to provide distinctive elongategrain microstructures, and are cosintered, to provide strong interfacial bonding and thus to minimize subsequent delamination. Contact testing with spherical indenters is used to characterize the damage response. The elasticplastic mismatch between the layers is sufficiently high as to produce distinctive damage modes in the two layers: predominantly cone cracking in the coating, and quasiplasticity in the substrate. However, the mismatch is also sufficiently low as to preclude secondary transverse cracks of the kind observed in other bilayer systems to initiate immediately beneath the contact at the coating/substrate interface and propagate upward within the coating. The dominant damage mode shifts from coating fracture to substrate quasi-plasticity with increasing contact load and decreasing coating thickness. Significantly, the presence of the soft underlayer inhibits growth of the coating cone cracks as the latter approach and intersect the interface. The underlayer also substantially diminishes strength losses from the contact-induced damage, especially in bilayers with thinner coatings. The implication is that bilayer structures with thin, hard coatings can preserve benefits from the inherent toughness of soft substrate materials, and at the same time afford surface protection (high wear resistance) to the underlayer.