Conventional Vickers microhardness testing can be used to determine mode I fracture toughness, KIc, as well as hardness, H. The crack length versus indenter load formulation previously used in geology indicates that KIc for quartz increases with decreasing load. This analysis is unreliable because it neglects residual stresses, which depend on the modulus to hardness ratio (E/H). A recent analysis incorporating this effect gives results compatible with published bend-test determinations. Earlier results systematically under- or overestimate KIc depending on whether (E/H)1/2 is greater or less than 4.15.An extensive SEM study to determine three-dimensional indent–fracture morphology reveals a region of high back-scattered electron contrast immediately adjacent to the indenter contact. TEM analysis shows this is an intensely deformed region with very high fracture density and evidence of partial melting. A submicrometre-scale blocky structure, perhaps resulting from cataclasis, occurs between fractures. The analysis shows that "plastic" deformation associated with indentation in quartz is controlled largely by fracturing, although the absence of voids suggests some crystal–plastic accommodation process may also operate.

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