Diamond Technology

 

Understanding and Controlling Stresses in Polycrystalline Diamond Cutters for Enhanced Duribility

K. Bertagnolli & R. Vale

Abstract

Residual stresses in PDC cutters arise from the difference in thermal expansion between the polycrystalline diamond layer and the supporting tungsten carbide substrate after sintering at high pressure and temperature. If not managed correctly, these stresses can significantly reduce the toughness of the cutters, especially as the diamond-layer thickness increases. Current industry trends favor thick diamond cutters to take advantage of increased cutter life. This paper outlines the potential benefits of using thick diamond layers and details a mechanism for mitigating the related high residual tensile stresses. Finite-element models, laboratory tests, and field results are presented.

Summary and Conclusion Highlights

Early attempts at US Synthetic to manufacture thick diamond PDC cutters with a flat interface resulted in delamination-type failures of the finished product. Residual stress was assumed to be responsible for these failures and cracks visible on the outer diameter of finished thick-diamond parts. Finite-element analysis was utilized to model the residual stresses present in PDC. Experimental measurements of surface residual stress agreed well with FEA model predictions when the model was subjected to a –650 °F temperature change. FEA models of thick diamond PDC showed a high tensile stress region in the diamond table above the interface on the cutter outer diameter. The location of this high tensile stress correlated well with observed cracks and fracturing after side impact loading. The models also revealed that a diamond hoop around the cutter perimeter with a portion of substrate material internal to the diamond table eliminated this harmful residual stress zone. Laboratory heavy-wear tests of .160-in thick PDC incorporating the diamond hoop produced a significant improvement in wear life, nearly five-times greater than a .030-in thick cutter. Subsequent field tests of the .160-in thick cutter confirmed the extended wear life observed in the lab. In addition, the delamination-type failure of flat-interface thick diamond was not observed with the hoop interface.

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