Initiation of Dislocation Systems in Alumina Under Single-point Scratching
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Initiation of dislocation systems in alumina under single-point scratching Irena Zarudi and Liangchi Zhanga) Department of Mechanical and Mechatronic Engineering, The University of Sydney, New South Wales 2006, Australia (Received 24 November 1997; accepted 26 October 1998)
This paper aims to investigate the initiation and distribution of dislocations and twins in the subsurface of alumina subjected to single-point scratching and to gain a deeper understanding of the mechanisms of ductile-regime grinding of alumina. It found that there generally exist three regions of dislocation and twin systems in the scratched alumina. The first region contains five independent slip systems so that macroscopic plastic flow is possible there. In the second and third regions, not all the systems can be activated, and then microcracking in the subsurface may occur easily. The distribution of these regions varies with the grain size of alumina. In the 25 mm-grained alumina all three regions appear. Thus, in this case, microcracking is difficult to avoid. In the 1 mm-grained alumina, however, only the first region appears, indicating that the material may be scratched under a real ductile mode without microcracking. A comparison shows that theoretical predictions are in good agreement with experimental observations.
I. INTRODUCTION
Alumina is known as a brittle material at temperatures below 1100 ±C because of its mixed ionic and covalent bondings. These bondings limit the generation and development of independent slip systems that are essential to the formation of macroscopic plastic flow.1 It has been concluded that plastic flow in alumina at ambient temperatures can be activated only by methods involving large hydrostatic stresses such as indentation,2,3 as in these cases, plastic zones with high density of dislocations and twins can be observed. Theoretical analysis of the initiation of plastic flow in alumina subjected to indentation has received extensive attention. The modeling was based on the estimation of the possibility of activating primary slip systems by examining the effective shear stresses.4,5 An important development has been to take into account the hardness anisotropy and crystallographic structure of the material,5 which permits obtaining valuable information on the initiation of plastic flow in the vicinity of the indentation zone. Unfortunately, the structure of the plastic zone has not been characterized and its dimensions have not been quantified. In the case of single-point grinding, such studies are even more lacking. This is what the present paper is aiming to do. II. EXPERIMENTAL PROCEDURE
Polycrystalline alumina specimens were made from commercially available alumina powder of 99.99% purity (Morimura Brothers Inc., Tokyo, Japan). Sintering a)
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J. Mater. Res., Vol. 14, No. 4, Apr 1999
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was carried out in air at 1700 ±C for 10 h. The average grain diamet
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