Ceramic tool materials : structure and properties relevant to wear
This thesis is concerned with the microstructures and wear characteristics of eight ceramic tool materials which have been, or are currently, commercially available as indexable inserts for lathe tools. Two of the materials are alumina-based and the other six are derived from silicon nitride. The opening chapters of the thesis outline the essential features of metal cutting, the properties required of a good tool material, the development of ceramic tools and possible wear behaviour as discussed in the literature. Subsequent chapters describe the experimental procedures adopted in this work and both present and discuss the results obtained. The eight ceramics have been microstructurally characterised by the use of X-ray diffractometry, scanning electron microscopy and transmission electron microscopy. Machining tests have been performed using five common workpiece materials, spanning a range of nickel and iron contents: mild steel, stainless steel, two nickel-based superalloys and commercial purity nickel. The two alumina-based materials were found to wear in a different, less severe manner to the silicon nitride-based tools. The mechanical response to surface contacts was established using identation techniques to give hardness (as a function of contact size and temperature) and fracture toughness (as a function of temperature) to test the correlation between these properties and wear behaviour, but this proved to be unfruitful. Since other classes of tools can be subject to dissolution/diffusion wear and little is known about the compatability of these ceramics with molten metals, a second type of investigation was instigated. The involved immersing pieces of ceramic in samples of molten workpiece materials, followed by cooling, sectioning and examining in the scanning electron microscope. Vast differences in the behaviour of the materials were observed and results from these tests correlated well with machining data, indicating the importance of high temperature stability with respect to the hot workpiece. Drawing on the experimental observations made during the project, the thesis concludes with suggestions for improving and optimising ceramic tool materials for turning purposes.