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Title: Pulsed laser ablation (PLA) of ultra-hard structures : generation of damage-tolerant freeform surfaces for advanced machining applications
Author: Pacella, Manuela
ISNI:       0000 0004 5354 5370
Awarding Body: University of Nottingham
Current Institution: University of Nottingham
Date of Award: 2014
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The current methods for manufacturing super-abrasive elements result in a stochastic geometry of abrasives with random three-dimensional abrasive spatial locations. This thesis covers the laser generation of novel micro-cutting arrays in ultra-hard super-abrasive composites (e.g. polycrystalline diamond, PCD and polycrystalline cubic boron nitride, PCBN). Pulsed laser ablation (PLA) has been used to manufacture repeatable patterns of micro cutting/abrasive edges onto micro structurally different PCD/PCBN composites. The analysis on the influence of microstructural factors of the composite materials in the use of laser ablation technology has been carried out via a novel technique (Focused Ion Beam/High Resolution Transmission Electron Microscopy/Electron Energy Loss Spectroscopy) to identify the allotropic changes occurring in the composite as a consequence of PLA allowing the laser ablated PCD/PCBN surfaces to be characterized and the nanometric changes evaluated. The wear/failure characteristics/progression of the ultra-hard laser generated micro cutting/abrasive arrays has been studied in wear tests of Silicon Dioxide workpiece shafts and the influence of the microstructural factors in the wear properties of the super-abrasive micro cutting edges has been found. Opposing to these highly-engineered microcutting/abrasive arrays, conventional electroplated abrasive pads containing diamond and CBN abrasives respectively have been chosen as benchmarks and tested under the same conditions. Contact profiling, Optical Microscopy and Environmental Scanning Electron Microscopy have been employed for the characterization of the abrasive arrays/electroplated tools before/during/after the wear/cutting tests. In the PCD abrasive micro-arrays, the type of grain and binder percentage proved to affect the wear performances due to the different extents of compressive stresses occurring at the grain boundaries. In this respect, the micro-arrays made of PCD with mixed diamond grains have shown slower wear progression when compared to the electroplated diamond pads confirming the combination of the high wear resistance typical of the fine grain and the good shock resistance typical of the coarse grain structures. While PCD laser manufactured arrays indicated edge break as typical wear mechanism, the abrasive pad confirmed flattening of grits as main wear mechanism. Mixed grained PCD arrays performed 25% better than fine grained arrays. The improved wear performances of laser manufactured arrays when compared to industrial benchmark is proved by the different wear failure mechanism in the array and in the electroplated pad: in the first one the edges break creating new sharp edges during testing, while in the latter grit flattening is the main wear mechanism. This increases up to 60% the life of the laser manufactured array when compared to the benchmarked pads. As for the PCBN abrasive micro-arrays, two are the main wear mechanisms experienced by the arrays: edge flattening for the high CBN content array and edge breaking for the medium CBN content array. The wear performance of the high content PCBN array is directly comparable to the electroplated boron nitride pad, because they both worn out with edge/grit flattening. The increase of metallic binder and the presence of metalloids in the medium content-CBN specimens have shown to produce higher contact pressure with the workpiece when compared to the electroplated specimen, causing fracturing as the main wear mechanism; while the PCBN micro-array with purely a metallic binder phase has shown better wear performances and lower contact pressure in comparison to the electroplated CBN specimen. In particular, the laser manufactured array proved to perform 50% better than the electroplated ones in terms of wear resistance. Among all of the tested arrays, the mixed grained PCD and the purely metallic binder phase PCBN micro-arrays have shown slower wear when benchmarked to the electroplated pads, giving a possible application of their use in the cutting tool industry.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: TA Engineering (General). Civil engineering (General)