Deep-Penetrating Conical Cracks in Brittle Layers from Hydraulic Cyclic Contact Yu Zhang,* Jun-Kwang Song, † Brian R. Lawn Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 Received 13 August 2004; accepted 8 September 2004 Published online 25 January 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jbm.b.30195 Abstract: A study is made of fracture from cyclic loading of WC spheres on the top surfaces of thick (1 mm) brittle layers on polymeric substrates, as representative of repetitive occlusal contact on dental crown structures. The advantage of glass layers is that internal cracks can be followed in situ during the entire cyclic loading process. The glass surfaces are first given a surface-abrasion treatment to control the flaw state, such that the strengths match those of dental porcelains. Cyclic contact tests are carried out at prescribed maximum loads and frequencies, in water. In addition to conventional cone cracks that form outside the contact circle, additional, inner cone cracks form within the contact in the water environment. These inner cones are observed only in cyclic loading in water and are accelerated at higher frequencies, indicating a strong mechanical driving force. They tend to initiate after the outer cones, but subsequently catch up and penetrate much more rapidly and deeply, ultimately intersecting the underlying coating/substrate interface. Comparative tests on glass/polymer bilayers versus monolithic glass, in cyclic versus static loading, in water versus air environ- ment, on abraded versus etched surfaces, and with glass instead of WC indenters, confirm the existence of a dominant mechanical element in the inner-cone crack evolution. It is suggested that the source of the mechanical driving force is hydraulic pressure from intrusion and entrapment of liquid in surface fissures at the closing contact interface. This new type of cone cracking may limit dental crown veneer lifetimes under occlusal fatigue conditions, especially in thicker layers, where competing modes—such as undersurface radial cracks—are sup- pressed. © 2005 Wiley Periodicals, Inc.* J Biomed Mater Res Part B: Appl Biomater 73B: 186 –193, 2005 Keywords: biomechanical ceramics; cone cracks; contact damage; cyclic fatigue; fracture modes; hydraulic fracture; glass INTRODUCTION Ceramic-based layer systems are used in many engineering and biomechanical applications. Important examples are all- ceramic crowns (replacing enamel) on tooth dentin 1–3 and ceramic acetabular liners in total hip replacements. 4,5 The ceramic layers afford mechanical protection to compliant/soft support underlayers. However, ceramics are subject to life- time-threatening cracking from concentrated contact stresses, especially in sustained and cyclic loading. There is a need to understand how different modes of fracture and deformation compete under such extenuating conditions. Several damage modes induced by curved indenters in ceramic layers on compliant substrates have been identified and analyzed. 1,2,6 –10 These can be divided into two catego- ries: top-surface damage from near-contact stresses, and bot- tom-surface damage from flexural stresses. Generally, top- surface modes dominate when the coating thickness d is large and sphere radius r is small, especially in sharp-particle contacts; conversely, bottom-surface modes dominate when d is small and r is large. One of the most deleterious fracture modes is radial cracking, usually at the bottom surface, but also, especially in softer ceramics, from quasiplastic defor- mation zones at the top surface. Such radial cracks are ori- ented normal to the plate surface and are therefore susceptible to any superposed tensile stresses generated during biome- chanical function. Another top-surface fracture mode associated with curved indenters is that of classical Hertzian cone cracking. 11–16 In single-cycle loading, ring cracks form just outside the con- tact. Because they tend to remain shallow and at a low angle Information of product names and suppliers in this article does not imply endorse- ment by NIST *On leave from: New York University College of Dentistry, 345 East 24th Street, New York, NY 10010 † On leave from: Machine & Material Center, Korea Testing Laboratory, Guro- Dong, Guro-Gu, Seoul 152-848, Korea Correspondence to: Brian R. Lawn (e-mail: brian.lawn@nist.gov) Contract grant sponsor: National Institute of Dental and Craniofacial Research; contract grant number: PO1 DE10976 Contract grant sponsor: Korea Institute S & T Evaluation and Planning (KISTEP) through the National Research Laboratory © 2005 Wiley Periodicals, Inc. *This article is a US Government work and, as such, is in the public domain in the United States of America. 186