Designing the next generation of high performance cutting tools

The successful introduction of hard ceramic coatings considerably increased the performance of cutting tools. However, limitations of these hard coatings, such as brittleness and high internal stresses, have led to the need to improve tool coatings. The aim of this thesis was to investigate coatings that could be used as a component in “functionally graded coatings” in which the coating grades smoothly from tool steel substrate to a hard top layer thereby overcoming the property mismatch between the two materials. The development of functionally graded coatings may lead to the development of thicker coatings that can be better integrated into the tool design, leading to an overall increase in tool performance.<br><br>A variety of thin film materials suitable for use in functionally graded coatings were energetically deposited using a filtered cathodic arc deposition system. The first thin film material explored was vapour deposited high speed steel with the aim of producing metallic thin films which may form a suitable grading layer from bulk high speed steel tools to other coatings. This work was then extended to the reactive deposition of high speed steel. A range of microstructures were obtained under different deposition conditions including fine columnar grain growth at low deposition energy, coarser grains at high temperature and a dominant amorphous phase at high deposition energy. The films with the columnar grain microstructure exhibited the highest hardness values, however the amorphous structure may also be useful as an interlayer material in functionally graded coatings.<br><br>The ductile phase toughening of TiN was also explored. TiN-Ni thin films were characterised to determine structure-property relationships using a combination of in-depth microstructural characterisation using transmission electron microscopy and mechanical property analysis using nanoindentation. It was found that films deposited at low energy had a fine microstructure comprised of small crystallites of TiN and a disordered Ni phase. When the deposition energy or temperature of the substrate was increased, the Ni phase formed crystals with the size increasing with an increase in deposition energy or temperature. A decrease in internal stress, decrease in hardness and increase in ductility was observed for films with larger Ni crystals. These films may be useful in forming functionally graded coatings from a single cathode. This investigation provided a detailed analysis of the types of microstructures and properties that are produced under various deposition conditions.<br><br>A novel method of producing multi-element cathodes using cold-spray technology was investigated. Several cathode designs with different combinations of elements were tested and the most successful cathode design was used to deposit high speed steel-titanium nitride nanocomposite thin films. The films produced displayed a range of microstructures and properties depending on the deposition conditions, with a nanocrystalline film forming at low energy and films becoming increasingly crystalline with an increase in either deposition energy or substrate temperature. The more crystalline films exhibited higher hardness values than the nanocrystalline film. These nanocomposite films are promising for use as a graded interface between a steel cutting tool and a titanium nitride coating in a functionally graded coating.