In this article we review the nature and mechanics of damage induced in ceramics by spherical indenters, from the classical studies of Hertz over a century ago to the present day. Basic descriptions of continuum elastic and elastic-plastic contact stress fields are first given. Two distinct modes of damage are then identified: Hertzian cone cracks, in relatively hard, homogeneous materials, such as glasses, single crystals, fine-grain ceramics (tensile, brittle mode); and diffuse subsurface damage zones, in relatively tough ceramics with heterogeneous microstructures (shear, quasi-plastic mode). Ceramographic evidence is presented for the two damage types in a broad range of materials, illustrating how an effective brittle-ductile transition can be engineered by coarsening and weakening the grain structure. Continuum analyses for cone fracture and quasi plasticity, using Griffith-Irwin fracture mechanics and yield theory, respectively, are surveyed. Recent micromechanical models of the quasi-plastic mode are also considered, in terms of grain-localized shear faults with extensile wing cracks. The effect of contact-induced damage on the ensuing strength properties of both brittle and quasi-plastic ceramics is examined. Whereas cone cracking causes abrupt losses in strength, the effect of quasi-plastic damage is more gradual-so that more heterogeneous ceramics are more damage tolerant. On the other hand, quasi-plastic ceramics are subject to accelerated strength losses in extreme cyclic conditions (contact fatigue), because of coalescence of attendant microcracks, with implications concerning wear resistance and machinability. Extension of Hertzian contact testing to novel layer structures with hard, brittle outer layers and soft, tough underlayers, designed to impart high toughness while preserving wear resistance, is described.

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Indentation of Ceramics with Spheres: A Century after Hertz

Preparation and mechanical properties of chitosan/carbon nanotubes composites.

Biopolymer chitosan/montmorillonite nanocomposites: Preparation and characterization

Nanoindentation load-displacement curves provide a “mechanical fingerprint” of a materials response to contact deformation. Over the last few years, much attention has been focused on understanding the factors controlling the detailed shape of unloading curves so that parameters such as true contact area, Young's modulus, and an indentation hardness number can be derived. When the unloading curve is well behaved (by which we mean approximating to linear behavior, or alternatively, fitting a power-law relationship), then this approach can be very successful. However, when the test volume displays considerable elastic recovery as the load is removed [e.g., for many stiff hard materials and many inhomogeneous systems (e.g., those employing thin hard coatings)], then the unloading curve fits no existing model particularly well. This results in considerable difficulty in obtaining valid mechanical property data for these types of materials. An alternative approach, described here, is to attempt to understand the shapes of nanoindentation loading curve and thus quantitatively model the relationship between Young's modulus, indentation hardness, indenter geometry, and the resultant maximum displacement for a given load. This paper describes the development and refinement of a previous approach by Loubet et al1 originally suggested for a Vickers indenter, but applied here to understand the factors that control the shape of the loading curve during nanoindentation experiments with a pointed, trigonal (Berkovich) indenter. For a range of materials, the relationship P = Kmδ2 was found to describe the indenter displacement, δ, in terms of the applied load P. For each material, Km can be predicted from the Young's modulus (E) and the hardness (H). The result is that if either E or H is known, then the other may be calculated from the experimental loading curve. This approach provides an attractive alternative to finite element modeling and is a tractable approach for those cases where analysis of unloading curves is infeasible.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400024432

J. Menčík

Papers

Mechanical property characterization of thin films using spherical tipped indenters