Summary in English. / Bibliography: pages 102-118. / Synthetic diamonds are successfully substituting for natural diamonds in the area of industrial application. Synthetic diamonds increased their market share from 10% in 1960 to 50% in 1968 and to 90% in 1994. The success of synthetic diamonds may be ascribed largely to technological advance in the area of diamond manufacture. Two technologies in particular contributed to this advance: (i) High pressure and high temperature (HPHT) processes for crystallising carbon material and (ii) chemical vapour deposition (CVD) of these materials. The substitution of synthetic for natural diamond occurred in a systematic and predictable manner. Further technological advance could threaten the concept of diamond as a unique and desirable substance in the minds of the consumers and may require the repositioning of its image.

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A dissertation submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in partial fulfillment of the requirements for the degree of Master of Science in Engineering, School of Mechanical, Industrial & Aeronautical Engineering, 2017 / An investigation of polycrystalline diamond compact (PDC) cutter failures, which are industrially known as spalling, was conducted by exploring changes in the diamond layer architecture and edge geometry of the cutter. Layer architecture was investigated through the use of layered functionally graded (FG) structures. Twenty one FG variations were prepared by the tape casting method and sintered using a high-pressure, high-temperature press. The vertical borer test (VBX), a laboratory test method, was used to gauge the improvement in spall resistance of the FG specimens against the benchmarks. Due to cost constraints associated with VBX testing, of the 21 available specimens, only four variations were tested for spalling. Contrary to expectation, it was found that all four specimens spalled during VBX testing despite showing a slight improvement in the spall area. For this reason, this route was abandoned. It was concluded that the use of layered structures is not effective in resolving the spalling problem. The use of novel edge geometry was investigated by taking three standard products and creating new geometric profiles on the specimens using a spark erosion machine. Each profile comprised a depression on the front face of the cutter. The specimens with novel geometry were also tested on the VBX. The spall was found to be confined between the chamfer breach and the depression feature. The depression appeared to have stopped the spall from propagating beyond the allowable spall limit of 1.2mm. On the basis of this 3 finding, it was concluded that spalling was successfully resolved. It is recommended that further optimization of this solution should be explored in field testing. In addition, a cost-effective way to fabricate the geometric profiles on the cutters should be further investigated because creating specimens using the spark erosion machine was quite expensive. Therefore, it is not viable for fabrication of large production volumes / CK2018

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Titanium (Ti) and its alloys have been widely used in aerospace, biomedical, chemical processing, marine facilities, and sports equipment because of their low density, very high tensile strength and toughness, and high corrosion resistance. However, the poor tribological properties has been a major problem and limited their widespread applications. Deposition of wear/corrosion resistant diamond-like carbon (DLC) coatings on Ti alloys is promising to significantly enhance the durability and service performances of these materials. However, the adhesion between DLC coatings and Ti alloy substrates is too weak to meet the application requirements. Up to now, approaches including optimization of deposition conditions, surface treatment of the substrate, deposition of an interlayer, and incorporation of metallic or nonmetallic elements have been used for adhesion enhancement of DLC on Ti alloys. In this research, a new method, nanodiamond particles incorporation, was developed for adhesion enhancement of DLC coatings on Ti alloys. In order to achieve high diamond nucleation without damaging the Ti alloy, nucleation enhancement of diamond on Ti alloys by nanodiamond seeding, tungsten (W) interlayers, and high methane concentration were studied. Diamond, DLC and W deposition were carried out by microwave assisted chemical vapor deposition, direct ion beam deposition and hot filament assisted chemical vapor deposition, respectively. Scanning electron microscopy, Atomic force microscopy, X-ray diffraction, Raman spectroscopy and synchrotron-based near edge extended X-ray absorption fine structure spectroscopy were used to characterize the microstructure and chemical bonding of the as-deposited particles and films, and indentation testing was used to evaluate the adhesion of the as-deposited coatings. By nanodiamond seeding or applying a W interlayer, significantly enhanced diamond nucleation has been obtained on Ti alloys, and consequently high quality nanocrystalline diamond thin films have been obtained on Ti alloys at decreased deposition temperature and reduced deposition time, which mitigates the deterioration of Ti alloy substrates due to hydrogen diffusion during diamond deposition and also enhances the adhesion of diamond on Ti alloys. Based on these results, nanodiamond particles (NDP) with high nucleation density and high adhesion were deposited on Ti alloys initially to enhance the adhesion of DLC films on Ti alloys. Results show that the pre-deposited NDP can significantly increase the adhesion of DLC on Ti6Al4V, probably due to the increased interfacial bonding, mechanical interlocking, and stress relief by the incorporation of NDP into DLC to form NDP/DLC composite films.

The 400°C limitation to the most common nanoindenter material, diamond, is not due to diamond changing to graphite in air, which can happen above 750°C, but to the reaction of the nanoindenter with the sample, causing a change in the geometry. The nanoindentation methodology is very sensitive to a change in nanoindenter geometry, thus the typical solution for measurements above 400°C is to use a cBN nanoindenter. However, the cBN indenter that is commercially used at temperatures above 400°C is too soft for measuring hard coatings such as machine tool coatings. There is limited published research on improving nanoindentation measurements in this way. Thus, the objective of this thesis is to address whether a coated diamond could be used for nanoindentation between 400-750°C. Due to the results of early experiments PVD titanium is the focus of this thesis as it will adhere to diamond by forming a carbide interlayer, TiC. A methodology to determine the best coating based on resistance to oxidation and robustness of the coating at temperature was used, allowing the exploration of several different titanium based alloys and coating thickness. The methodology used is as follows: 1. PVD coatings of titanium and titanium based alloys TiAl, TiN, and TiAlN were oxidized at 500, 575, and 650°C. Measurements by SEM and EDS were taken after the oxidation at each temperature. Of the coatings tested, the pure titanium coating was determined to be the best coating. 2. The coating thickness of 0.25, 0.50, and 1.0 um were evaluated with the same static oxidation test applied to the different alloys. It was determined that 0.50 um was the best thickness. A duplicate 0.50 um thick sample had a cross-section machined by FIB, and was examined by STEM, HAADF and EELS. The results confirmed that TiC was being formed at the expected rate. 3. To determine whether a coated nanoindenter could be used for measurements, a nanoindenter was first calibrated, coated by PVD with 0.50 um thick titanium, and calibrated again. The results confirmed that a coated nanoindenter could be used for performing nanoindentation measurements. 4. To determine whether the titanium PVD coating would adhere to the diamond at temperature, a coated nanoindenter was used to measure fused silicon at 450°C. After each measurement, the nanoindenter was examined by SEM and EDS. The results confirmed that the titanium coating adhered to the diamond. This thesis demonstrates that a titanium PVD coating can protect a diamond nanoindenter during measurements between 400-750°C. The primary contributions are that coated diamond nanoindenters can be used for nanoindentation measurements, and that titanium PVD coated nanoindenters can be used for nanoindentation measurements between 400-750°C. Additional contributions include the testing of adhesion of titanium PVD coating to diamond between 400-750°C, and a methodology of evaluating coatings. / Thesis / Doctor of Philosophy (PhD)

Diamonds have many extreme physical properties that lead to a host of technological applications. Their main use as a tool or abrasive for the machining of hard materials involves diamonds rubbing against the material being worked and often against each other. The strength and frictional properties of diamond are thus of great practical, as well as academic interest. Studies of the reciprocating sliding of one diamond on another for extended periods have shown that the changes in friction and wear are linked with the formation of debris. Analysis of the debris shows it does not consist of diamond fragments. Investigations of the effect of low-pressure gases on diamond friction also indicate debris plays an important role, and that the friction and wear are markedly dependent on the state of adsorbants on the diamond surface. Details are also given of the friction and wear of steel, glass and sintered diamond in contact with diamond. An associated study of the solid particle erosion of diamond has shown that cracking can occur at impact velocities of 25 m s<SUP>-1</SUP>. To facilitate the friction studies two existing friction apparatus have been automated. Computer control now allows data to be collected and analysed more rapidly and thoroughly than was possible before. The equipment has also been adapted to extend the conditions under which friction studies can be made.

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