Material removal and damage formation mechanisms in grinding silicon nitride
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Material removal and damage formation mechanisms in grinding silicon nitride Hockin H. K. Xu,a) Said Jahanmir, and Lewis K. Ives Ceramics Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 (Received 28 September 1995; accepted 2 February 1996)
Surface grinding was performed on two silicon nitrides with different microstructures. The ground surfaces of both materials were observed with scanning electron microscopy (SEM) to consist of areas of microfracture, smeared areas, and areas covered with fine debris particles. It was determined that microfracture is the primary mechanism for material removal. Subsurface grinding damage was revealed by a bonded-interface technique to take the form of median-type cracks extending from the plastic zones. Distributed intergranular microcracks and intragrain twin/slip bands were observed within the plastic zones. The strengths of transverse-ground specimens were measured in four-point flexure. For the silicon nitride with a fine grain size and a mildly rising toughness-curve, grinding damage resulted in a drastic strength degradation compared to polished specimens. In contrast, the silicon nitride with large and elongated grains and a steeply rising toughness curve showed relatively little strength loss. The relationship between the ceramic microstructure and the damage tolerance in abrasive machining is discussed in light of these results.
I. INTRODUCTION
Structural ceramics such as silicon nitride are now being increasingly used in bearings, valves, rotors, and other applications where a close dimensional tolerance is required. Abrasive machining by means of grinding with diamond wheels is the primary process used in achieving the desired tolerances and surfaces integrity.1 However, due to their high hardness and brittleness, the ceramic materials are difficult to machine, and the surface and subsurface damage produced by abrasive machining can be detrimental to the strength and performance of ceramic components.2–5 Several previous studies have been performed to evaluate the effect of grinding conditions on the strength of ground specimens.6–9 The purpose of this paper is to examine the microstructural aspects of the damage formation and material removal processes in grinding. The ceramic microstructure has been shown in recent studies in our laboratory10–16 to play a key role in the material removal processes in single-point scratching and in surface grinding. Subsurface damage produced by scratching of a fine-grain alumina was observed to take the classical form of median and lateral cracks extending from a plastic zone.10,11 However, damage in a coarse-grain alumina took the form of distributed intergranular microcracks and intragrain twin/slip bands, with the absence of well-defined median and lateral cracks. a)
Guest scientist, from School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0405.
Similarly, scratching and grinding produced macroscopic median
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