Yes / Alumina-cubic boron nitride (cBN) composites were prepared using the spark plasma sintering (SPS) technique. Alpha-alumina powders with particle sizes of ∼15 µm and ∼150 nm were used as the matrix while cBN particles with and without nickel coating were used as reinforcement agents. The amount of both coated and uncoated cBN reinforcements for each type of matrix was varied between 10 to 30 wt%. The powder materials were sintered at a temperature of 1400 °C under a constant uniaxial pressure of 50 MPa. We studied the effect of the size of the starting alumina powder particles, as well as the effect of the nickel coating, on the phase transformation from cBN to hBN (hexagonal boron nitride) and on the thermo-mechanical properties of the composites. In contrast to micro-sized alumina, utilization of nano-sized alumina as the starting powder was observed to have played a pivotal role in preventing the cBN-to-hBN transformation. The composites prepared using nano-sized alumina reinforced with nickel-coated 30 wt% cBN showed the highest relative density of 99% along with the highest Vickers hardness (Hv2) value of 29 GPa. Because the compositions made with micro-sized alumina underwent the phase transformation from cBN to hBN, their relative densification as well as hardness values were relatively low (20.9–22.8 GPa). However, the nickel coating on the cBN reinforcement particles hindered the cBN-to-hBN transformation in the micro-sized alumina matrix, resulting in improved hardness values of up to 24.64 GPa.

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<p> Case hardening of steels is extensively used throughout general engineering to produce components with a hardened layer whilst retaining a tough core. This is usually accomplished using different sources of energy, e.g. flame and induction being the most common. In recent years, a new case hardening technology, named 'Grind-Hardening' has surfaced. In this method, the heat dissipated during grinding is utilized to induce martensitic phase transformation in the surface layer of a component. Therefore it is possible to incorporate grinding and surface hardening into a single operation to develop a cost-effective production method. The grinding process then becomes an integrated heat treatment process.</p> <p> In the present study on 'grind hardening', a numerical thermal model has been developed to compute the temperature distribution beneath the ground surface to predict the extent of surface hardening and the case depth. Grinding experiments were conducted in order to examine the influence of various process variables such as wheel depth of cut, feed speed, and wheel preparation. AISI 52100 and 1045 steels were used in this study to evaluate the behavior of plain and alloy steels during grind hardening. Effective case depth was measured using a Vickers hardness tester and was found to be over 0.5 mm for a target hardness of 513 Hv. Microstructure was analyzed using optical and scanning electron microscopes. The microstructure was observed to have fine martensitic laths which give rise to remarkable high hardness.</p> / Thesis / Master of Applied Science (MASc)