Diamond reinforced metal-matrix composites (MMCs) are utilised for cutting, drilling, grinding and polishing a variety of materials, in many cases being the most efficient and economic choice. The increased cost of synthetic diamond abrasives has led to constant search for ways to extent diamond tool life. This has been realised by introducing chemical reactions at the interfaces in order to develop chemical bridges between diamonds and metals that prolong the retention of crystals at the operating surfaces of the tools. Alloying the matrix with carbide forming metals is a way to introduce interfacial reactivity, but involves problems with concentrating the alloying element at the interfacial region and may cause alteration of the wear resistance characteristics of the binder, which may be an undesirable effect. A recent development and alternative method to alloying is the coating of the diamonds with carbide forming metals, offering unique advantages. Although metal-coated diamonds are commercially available, the effectiveness of their usage and the understanding of interfacial phenomena occurring in composites reinforced with such abrasives still remain unexplored. The work carried out in this research has examined the interfacial bonding in diamond MMCs reinforced with metal-coated crystals. The work described in this thesis included a preliminary study on diamond/metal reactivity serving the need to identify the mode and intensity at which synthetic diamonds and elemental metals interact at various conditions. This was achieved by examining the changes occurring to diamond surfaces when crystals were heated in the presence of various elemental metals. The latter were brought in contact with the diamonds either in the form of loose or hot-pressed metallic powders or in the form of thin metal coatings deposited onto the crystals by vapour deposition methods. Results showed that metals, depending on their electronic configuration, either catalyse the graphitisation of diamond surfaces and dissolve carbon or react at the diamond surfaces to form carbide crystallites. Dissolution of the diamond occurred by formation of oriented hexagonal/triangular and rectangular pits on octahedral {111} and cubic {100} surfaces respectively. Intensity of interactions strongly depended on heating temperature and time. Metal coatings were found to efficiently react with the diamonds only after annealing at temperatures of the order of 1000°C subsequent to the deposition. The diamond impregnated MMCs investigated in this research were reinforced with various types of metal-coated and metal-powder encapsulated diamonds of the carbide forming metals of Ti, Cr and W. The tested composites included two types of metal-matrices that of standard plain cobalt as well as some selected alloyed matrices typically employed in practice. Interfacial bonding characterisation and assessment of the potential capability of the metal-coatings to offer enhanced diamond retention has been made by determining the mechanical properties of the composites and by conducting extensive microscopic analysis of the developed fracture surfaces. The results suggested that incorporating metal-coated crystals could be beneficial in improving the diamond retention, provided that consolidation temperature is sufficiently high to favour diamond/metal reactions. Results showed improvements in mechanical properties to be achieved when reinforcing with the coated diamonds compared to non-coated grit. The characteristics of the interactions at the diamond surfaces in the composites conformed to the findings of the preliminary study on the fundamentals of diamond/metals interactions. Reactions on crystal surfaces took place at the locations where prior dissolution of the diamond had occurred. Metal coatings were found to provide excellent protection to the diamonds against catalysed dissolution by aggressive binders. Thin coatings suffered from loss of continuity in systems were the coating metal atoms were readily soluble in the metal-matrix. This was avoided with thicker coatings that also appeared to provide a supplementary mechanical effect in addition to the chemical bonding in improving the retention of the diamond crystals. Encapsulation of diamond with carbide forming metals was a hybrid method between alloying the metal-matrix and coating the crystals. Although encapsulation provided sufficient levels of chemical interactions, it was shown that diamonds could not be efficiently protected from aggressive binders. In addition, composites impregnated with powder-encapsulated diamonds suffered from inadequate sintering of the carbide forming metal zones surrounding the crystals when consolidation was performed at relatively low temperatures which was reflected in inferior mechanical properties.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:272755 |
| Date | January 2003 |
| Creators | Margaritis, Dimitris-Peter |
| Publisher | University of Nottingham |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://eprints.nottingham.ac.uk/13237/ |
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