Laser shock processing of polycrystalline diamond: experimental and numerical studies

Synthetic polycrystalline diamond (PCD), favoured for its advantageous characteristics, has seen widespread adoption in industries, including oil and gas, particularly in the manufacturing of tools. These tools, when subjected to challenging environments, often experience degradation and wear, leading to issues such as micro-chipping and gross fracturing. To address these, a common practice is to apply coatings to strengthen their resistance to chemical wear. In addition, conventional laser shock processing/peening (LSP) is a technique renowned for enhancing the mechanical and wear properties of metals and more recently ceramics. However, this thesis focuses on an alternative solution, i.e., an in-depth understanding of the impact of an unconventional laser shock processing technique at low energy and high frequency regime on the mechanical and wear characteristics of PCD composites. Various techniques (profilometry, Vicker’s micro indenter, Scanning Electron Microscope and Energy Dispersive Spectroscopy, Raman spectroscopy, X-Ray diffractometer) have been used to analyse the influence of mechanical and microstructural changes occurring as a consequence of low energy, high frequency laser shock processing without coatings (LE/HF-LSPwC and with coatings (LE/HF-LSP). This process enabled the identification of the laser treated surfaces and evaluate the nanometric changes that occurred. A significant hardening of the PCD processed at LE/HF-LSPwC was measured by 14% relative to the commercially available PCDs. In addition, the laser treatment resulted in 18.42% reduction in cobalt volume compared to the conventional LSP involving coatings (increased in the range of 16.4 and 66.3%). Any fluence exceeding the energy threshold led to a softening of the material. Transitioning from tensile to compressive stress was noticeable when the PCD was processed using vinyl and quartz. XRD and Raman spectroscopy results revealed graphitisation and deformation of cobalt. A correlation between the surface modification, microhardness and phase shifts in PCD was evident, thereby establishing a connection between phase transition, hardness and laser fluence.

This thesis brings forth a multi-scale approach that is integrated with dry surface analyses for the characterisation of PCD composites before and after laser shock processing, combining atomic force microscopy (AFM) at nano scale. The component level investigation at microscale is conducted with AFM in lateral force mode, employed to analyse the nanoscale characteristics of asperity level contacts. AFM is also used with stiff cantilevers to determine the wear properties of the sample surfaces, which provides information about the specific correlation between the laser process parameters and the resulting changes in the wear properties of PCD composites. A correlation between the surface roughness, hardness, coefficient of friction and wear have been noticed.

This research aims to validate the improvements in mechanical and microstructural properties of laser treated PCD in high-speed machining (orthogonal turning test) versus the commercially available PCD cutting tool. The laser processed tools demonstrated reduced cutting forces as compared to the commercial tool. The prevalent wear mechanism found in the analysis of the commercial tool was the slice chipping indicating the occurrence of a fast wear, on the other hand, no chipping observed in the laser treated tool. The average major flank wear of the LE/HF-LSPwC PCD tools were decreased by 21% (averaged) and LE/HF-LSP with vinyl and quartz by 32% when compared to the commercial tool. Over the sliding distance of 15.8 km, reduction in workpiece surface roughness of 18% was proved by the LE/HF-LSPwC at fluence 19.98 J/cm2 PCD tool when compared to the commercial tool. Overall, K15 (LE/HF-LSPwC at fluence 19.98 J/cm2) and V22 (LE/HF-LSP with vinyl and quartz at fluence 9.02 J/cm2) showed a better durability and performance in machining operations, giving a potential practical use of these unconventional laser processing techniques with the cutting tool industryIn order to predict the residual stresses in the surface and sub-surface layers of PCD after laser shock processing, finite element models were developed using Abaqus CAE. In this study, 2-D microstructures from SEM images have been used to generate 3D polycrystalline structure. In addition, influence of laser fluences, overlap of laser spots on the effect of temperature, heat flux, and stress components distributions have been predicted. Tensile stress was found to be reduced at 0% overlap when compared to 50% and 75% overlap of laser spots. The model was validated using the residual stress results obtained from XRD analysis. The simulation results were found to be in within the range of the experimental data and statistically proven relevant. This predictive model was essential not only for low energy LSP of PCD composites but also other varieties of composites to be studied in future.