Influence of substitutional nitrogen in synthetic saw-grade diamond on crystal strength
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Influence of substitutional nitrogen in synthetic saw-grade diamond on crystal strength W. E. Jackson and Steven W. Webb GE Superabrasives, 6325 Huntley Road, Worthington, Ohio 43085 (Received 29 July 1996; accepted 14 January 1997)
The amount and defect type of substitutional nitrogen in synthetic diamond strongly influences crystal strength. There is an optimum amount of nitrogen that yields the highest compressive fracture strength for crystals derived from common growth conditions. It is postulated that the role of nitrogen is to charge-balance vacancies created during growth. If too little nitrogen exists in the diamond, vacancies are not charge-balanced and may serve as crack initiation and/or propagation sites. Excess nitrogen above that required to charge-balance vacancies may weaken the lattice by adding local strain to the crystal. IR microscopy indicates that most of the substitutional nitrogen in synthetic diamond is increased in the vicinity of the intersections of growth sectors on the crystal surface. Most surface IR-visible nitrogen is biased toward the (111) –(100) intersection. The bias in incorporation of substitutional nitrogen at external growth sector intersections (i.e., edges and corners) of an industrial high-grade saw diamond crystal influences the progression of fatigue by microfracture during cutting of hard stone.
I. INTRODUCTION
Reduction in metal and other inclusions in highgrade industrial synthetic grit diamond has been achieved through improved control of the high-pressure, hightemperature (HPHT) synthesis process. As a result, nitrogen has become the dominant impurity that influences crystal compressive fracture strength (CFS). Fracture strength is an important property that predicts grinding, drilling, and sawing performance of diamond tools. Yet, little is known of the mechanism by which substituted nitrogen affects diamond strength and fatigue resistance. Nitrogen is normally found substituted in the diamond lattice at concentrations up to 1200 ppm. Nitrogen exists in a singlet state which may be either a C-center (IR peak centered at 1130 cm21 ) or a nitrogen-plusvacancy complex (denoted NV or NV2 ). It may also be observed in an aggregate state as an A-center (IR peak centered at 1282 cm21 ) or a multi-nitrogen-plusvacancy complex (denoted N2 V). In addition, it may also be present in discrete inclusions as metal-nitride. The sum of included and substituted nitrogen is referred to as total nitrogen. Nitrogen in synthetic diamond has been studied extensively because it is easy to observe by a variety of spectroscopic methods. However, the effect of nitrogen on crystal strength and fatigue relevant to industrial diamond sawing, drilling, and grinding has received little attention. This work focuses on the IR characterization of nitrogen in synthetic diamond and the correlation 1646
http://journals.cambridge.org
J. Mater. Res., Vol. 12, No. 6, Jun 1997
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of nitrogen content, state, and spatial distribution
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