Deformation and fracture of mica-containing glass-ceramics in Hertzian contacts
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The Hertzian indentation response of a machinable mica-containing glass-ceramic is studied. Relative to the highly brittle base glass from which it is formed, the glass-ceramic shows evidence of considerable "ductility" in its indentation stress-strain response. Section views through the indentation sites reveal a transition from classical cone fracture outside the contact area in the base glass to accumulated subsurface deformation-microfracture in the glass-ceramic. The deformation is attributed to shear-driven sliding at the weak interfaces between the mica flakes and glass matrix. Extensile microcracks initiate at the shear-fault interfaces and propagate into the matrix, ultimately coalescing with neighbors at adjacent mica flakes to effect easy material removal. The faults are subject to strong compressive stresses in the Hertzian field, suggesting that frictional tractions are an important element in the micromechanics. Bend-test measurements on indented specimens show that the glass-ceramic, although weaker than its base glass counterpart, has superior resistance to strength degradation at high contact loads. Implications of the results in relation to microstructural design of glass-ceramics for optimal toughness, strength, and wear and fatigue properties are discussed.
I. INTRODUCTION Hertzian indentation of homogeneous brittle materials, such as glasses and single crystals, has received extensive attention in the literature.1"20 Above a critical load, a ring crack initiates in the weakly tensile region just outside the circle of contact with the indenting sphere, propagates downward as a surface-truncated cone, and finally arrests at a depth approximately equal to the contact radius. 615 This is the so-called Hertzian cone crack.21 Hertzian fracture is of interest to materials scientists for its uncommon insight into the stability of strength-degrading flaws6-20 and for its intrinsic relation to material toughness.6'15 Few Hertzian indentation studies have been made of the newer generation of tougher polycrystalline and twophase ceramics, where microstructure plays a critical role in the fracture behavior. Some cone fracture observations have previously been reported on low-toughness finegrain monophase ceramics15'22 and brittle lithium-silicate glass-ceramics.23'24 Higher long-crack fracture toughness occurs in those ceramics with larger grains and greater internal mismatch stresses25'26 and is attributable most commonly to crack-interface "bridging" from interlocka
)Guest Scientist on leave from Department of Materials Science and Engineering, Lehigh University, Bethlehem, Pennsylvania 18015. b ^Guest Scientist on leave from Department of Applied Physics, University of Technology, Sydney, New South Wales 2007, Australia. 762
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ing grains or particles.20'27 31 Recently, examination of Hertzian indentations in a coarse-grain polycrystalline alumina32 has revealed a radical departure from the classical fracture pattern; the cone crack is suppressed in favor of distributed dam
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