Cracking of porcelain coatings bonded to metal substrates of different modulus and hardness
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Brian R. Lawn Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 (Received 13 December 2000; accepted 1 March 2001)
A preceding study of contact damage in a bilayer system consisting of a porcelain coating on a stiff Pd-alloy substrate is here expanded to investigate the role of substrate modulus and hardness. Bilayers are made by fusing the same dental porcelain onto Co-, Pd-, and Au-alloy metal bases. Indentations are made on the porcelain surfaces using spheres of radii 2.38 and 3.98 mm. Critical loads to initiate cone fracture at the top surface of the porcelain and yield in the substrate below the contact are measured as a function of porcelain thickness. Radial cracks form at the lower surface of the coating once the substrate yield is well developed. By virtue of its controlling role in the metal yield process, substrate hardness is revealed to be a key material parameter—substrate modulus plays a secondary role. A simple elasticitybased analysis for predetermining critical loads for a given brittle/plastic bilayer system is presented.
I. INTRODUCTION
Ceramic coatings on metal substrates are of technological importance in engineering and biomechanical applications requiring high-wear resistance and mechanical, thermal, or chemical insulation. Dentistry, where ceramic coatings on metal underlayers form the basis of traditional crown and bridge design,1,2 is an illustrative case in point—a porcelain coating layer provides wear resistance and aesthetics, a metal core provides mechanical support for the porcelain, and the composite ceramic– metal bilayer provides protection for the inner tooth dentin structure. Such coating/substrate structures tend to be highly vulnerable to fracture in concentrated loads,3–9 raising the issue of the most desirable material properties for ensuring crack-free structures. In a recent study of Hertzian indentation of a brittle porcelain coating on a Pd-alloy substrate10 the critical role of the metal substrate in the fracture of the ceramic overlayer was demonstrated. The metal may indeed provide a relatively stiff support, yet may actually promote certain kinds of fracture in the coating by yielding beneath the contact—the coating then flexes like an axisymmetric plate on a compliant foundation, initiating a)
Guest scientist at the National Institute of Standards and Technology. J. Mater. Res., Vol. 16, No. 5, May 2001
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radial cracks at the lower surface (i.e., at the coating/ substrate interfaces)—these cracks subsequently spread subsurface in elongate fashion on median planes containing the load axis. Radial cracks are dangerous because they can grow at relatively low loads, and can be difficult to detect in opaque materials. They are considered to be a primary source of failure in dental crowns.2 These cracks are in addition to the conventional cone cracks that initiate from the top coating surface in the Hertzian contact zone. Cone cracks tend to
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