Residual stress and failure in polycrystalline diamond cutters

Thesis

Polycrystalline diamond (PCD) is an ultra hard material used in cutters for oil and gas drilling. As the technology for making PCD has developed, a wide number of material property tests have been conducted, but little work has been done when considering the geometry as used in practice. This work aims to change that.

PCD cutters are made with a PCD layer above a substrate. When they are cooled to room temperature from the manufacturing temperature the difference in the thermal expansion coefficients between the layers results in residual stresses. This residual stress was measured through neutron diffraction on ENGIN-X at ISIS. The in-plane stress at the centre of the cutter was found to be -380 ± 50 MPa. This stress in the PCD undergoes hysteresis through a heating cycle. Both of these results are understood in terms of a three layer system consisting of the PCD , the substrate and a cobalt rich interface layer between them of 40 µm. This is confirmed with finite element modelling which predicts the post sintering stress to be -346 MPa and also predicts the hysteresis behaviour.

PCD cutters can fail through impact and smooth wear. Single impact failures were replicated using an impedance matched direct impact Hopkinson bar. The impact failure threshold force was found to be 7 kN. This threshold failure force increases with interaction speed. The increased strength is attributed to a confining pressure within the cutter during dynamic loading. The fracture is predominantly trans-granular at all interaction speeds. Under static loading individual cracking events can be observed visually and through acoustic emission. Smooth wear failure was produced by replicating rock drilling. Leaching is shown to prevent graphite formation in smooth wear failure and reduce the wear rate. 2D maps of the graphite show it is not omnipresent on the rough wear surface. The cutting temperature is estimated to be 850-1300 °C and only the grains within 300 µm of the contact with the rock achieve the graphitisation temperature.

Authors: Laurence, R

• DOI: 10.5287/ora-aqo9yypxr
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: stable crack growth; high strain rate; high speed photography; fracture; graphite; graphitisation; dynamic failure; digital image correlation; low strain rate; finite element; residual stress; ABAQUS; failure force; ceramics; single impact faiilure; diamond; polycrystalline diamond cutters; direct impact; neutron diffraction; raman spectroscopy; static loading
• Subjects: impact failure; Ceramics; Residual stresses; polycrystalline diamond cutters