Lanhua Wei

Effect of microstructural coarsening on Hertzian contact damage in silicon nitride

The critical role of grain size in determining the nature of damage accumulation in silicon-nitride ceramics is evaluated using Hertzian contact testing. Single-cycle tests are conducted on materials of two grain sizes, 0.5 Μm (fine) and 2.0 Μm (coarse). Damage patterns for these two materials are compared and contrasted using a special bonded-interface specimen to investigate the subsurface regions. Optical and thermal wave-imaging techniques provide complementary pictures of the damage patterns: whereas the optical image maps elements of both deformation and fracture, the thermal wave image maps only the fracture. Taken together, these two imaging methods disclose a fundamental transition in the mechanical response in the two silicon nitrides, from cone-crack-dominated in the fine material to distributed-microcrack-dominated in the coarse material. Scanning electron microscopy (SEM) confirms the incidence of microfracture in the latter case. Thermal-wave measurements also allow a quantitative evaluation of the microfracture damage. Multiple-cycle tests on the coarse material show a build up of subsurface damage with increasing number of cycles, indicating mechanical fatigue. The results are discussed in terms of a shear-fault model, in which subsurface microcracks initiate from intrinsic planes of shear weakness in the microstructure. Implications concerning the microstructural design of silicon nitride ceramics for strength and wear applications are briefly considered, with reference to countervailing resisting and driving forces in the long-crack and short-crack toughness characteristics.

Mechanical characterization of plasma sprayed ceramic coatings on metal substrates by contact testing

Abstract Hertzian indentation testing is used to generate contact damage in plasma sprayed ceramic coatings on metal substrates. Two basic ceramic/metal coating/substrate systems are examined: alumina on steel and zirconia on superalloy. Macroscopic mechanical responses are measured via indentation stress-strain curves, which quantify the relative role of the coating and substrate in the net deformation and facilitate evaluations of elastic moduli and yield stresses. Micromechanical damage processes within the coating and substrate subsurface layers are studied using a “bonded-interface” specimen. Degradation occurs primarily by delamination and other cracking at the coating/substrate interface or in the coating, but plastic deformation of the metal substrate contributes importantly to the crack driving force.

Grinding force and microcrack density in abrasive machining of silicon nitride

The relationship between grinding forces and the material`s resistance to microfracture is investigated in abrasive machining of silicon nitride ceramics. Surface grinding is performed on two forms of silicon nitride with different microstructures, and the grinding forces are measured. In addition, single-point scratching is performed on polished surfaces to amplify the damage associated with the action of an individual abrasive particle in grinding. A thermal wave measurement technique in then used on the cross sections to characterize the density of subsurface microcracks associated with scratching. Compared to a fine-grain silicon nitride, the density of microcracks in a coarse-grain silicon nitride is significantly larger, while the grinding force is smaller. The smaller grinding force for the coarse-grain silicon nitride is attributed to the ease of local intergranular microfracture and grain dislodgement during grinding. {copyright} {ital 1995} {ital Materials} {ital Research} {ital Society}.

THERMAL WAVE ANALYSIS OF CONTACT DAMAGE IN CERAMICS : CASE STUDY ON ALUMINA

Thermal waves are used to image damage accumulation digitally beneath Hertzian contacts in ceramics. Alumina ceramics over a range of grain size 3–48 μ m are used in a case study. The nature of the images changes with increasing alumina grain size, reflecting a transition in damage mode from well-defined cone fracture in the finer-grain materials to distributed subsurface microfracture in the coarser-grain materials. Quantitative determinations of microcrack densities are evaluated in the latter case by deconvoluting thermal diffusivities from the image data. These determinations confirm the grain-size dependence of degree of damage predicted by fracture mechanics models. Potential advantages and disadvantages of thermal waves as a route to damage characterization in ceramic systems are discussed.

EFFECT OF MECHANICAL DAMAGE ON THERMAL CONDUCTION OF PLASMA-SPRAYED COATINGS

A thermal wave methodology for monitoring the thermal conduction of ceramic coatings with accumulating mechanical damage is described. Tests are conducted on a model alumina coating containing laminar defect intralayers. Controlled subsurface damage introduced with a spherical indenter is observed using a presectioned specimen. Microcrack damage accumulates progressively with increasing contact load and number of cycles. Associated changes in thermal diffusivity, specifically in the through-thickness direction, are imaged and quantified point-by-point using laser-generated thermal waves. The effective thermal resistance of the coating increases with crack density, up to the point of failure.