Damage-Resistant Brittle Coatings

Abstract

Laminate structures consisting of hard, brittle coatings and soft, tough substrates are important in a wide variety of engineering applications (cutting tools, electronic multilayers, laminated windscreens), biological structures (teeth and dental crowns, shells, bones), and traditional pottery (ceramic glazes). A hard outerlayer variously offers increased loadbearing capacity, wear resistance, thermal and corrosion protection, electrical insulation, and aesthetics; a compliant underlayer offers stress redistribution and damage tolerance. But hard layers are susceptible to cracking, especially in surface-concentrated loads from static or cyclic contacts. In natural or restorative tooth structures, for instance, forces in excess of 100 N operate at contacts between opposing cusps of characteristic radii 2±4 mm over 10 cycles, leading to occasional premature failures. The stress field in the supported brittle coating is Hertzian-like in the near-contact region and flexure-like in the far-contact region, with resultant competing modes of fracture and damage, including some new modes not observed in monolithic materials. These damage modes determine the useful lifetime of the layer structure. Conventional design of brittle layer structures is based on a philosophy of acrack containmento, i.e., preventing already well-developed cracks in the coating from penetrating into adjacent layers, so increasing the effective atoughnesso of the composite structure. Several approaches of this kind have been proposed: i) Crack deflection along weak interfaces between hard layers. Cracks are deflected out of the main tensile field along weak orthogonal interlayer interfaces. ii) Crack inhibition from intralayer residual compression. Thermal expansion mismatch introduces compression stresses into one or other of the layers during fabrication, inhibiting crack extension, with possible lower-limit thresholds for interlayer crack penetration. iii) Crack arrest in tough substrates. Cracks enter a toughened sublayer where they are slowed and arrested. iv) Crack confinement by stress redistribution. Incorporation of a soft underlayer with a strong interface redirects and confines cracks within the boundaries of the outer coating. Generally, effective crack containment calls for thinner brittle layers and higher interlayer mismatch. Crack-containment approaches are viable where limited cracking in brittle outerlayers is tolerable (e.g., automobile windshields, thermal shock applications). But in many applications one cannot sustain even a single crack (e.g., dental crowns, ceramic-coated cutting tools, corrosion protective coatings, electronic multilayer devices). Such cracks may occur at much lower loads than those needed to cause ultimate failure. The problem is exacerbated in prolonged or cyclic loading, where even the smallest of cracks or other inelastic damage can evolve steadily but inexorably into a catastrophic failure. A more conservative approach is called forÐone must design against crack initiation rather than crack propagation. The idea is not so much to contain cracks once they start, but to prevent them from starting in the first place. This can lead to conflicting requirements in layer dimensions and material properties. In this study we introduce a new approach to the design of damage-resistant brittle coatings, based on a combination of new and existing relations for crack initiation in well-defined contact-induced stress fields. Consider a brittle layer of thickness d on a compliant substrate, with a well-bonded interface, subjected to contact at load P with a sphere of radius r, Figure 1. The coating is considered to be thin enough that it flexes under the applied contact. Since we are interested in first damage in the brittle coating, we may treat the system as elastic up to the critical load. There are two main fracture modes, both of which initiate from some surface flaw: Cone cracks: This kind of fracture is well documented from decades of research on monolithic brittle solids. The crack first develops from the top surface outside the contact circle, where the tensile stress is maximum, as a shallow, stable surface ring within a rapidly diminishing subsurface tensile field; it then pops in to its ultimate (truncated) cone-like geometry at a critical load

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Cite this paper

@inproceedings{Lawn2000DamageResistantBC, title={Damage-Resistant Brittle Coatings}, author={Brian R. Lawn and Kee Sung Lee and Herzl Chai and Antonia Pajares and Do Kyung Kim and Sataporn Wuttiphan and Irene M. Peterson and Xiaozhi Hu}, year={2000} }