A collaborative research project was set up to study peripheral diamond wheel machining damage in silicon nitride ceramics. The objective of the work to be carried out at the University of Surrey was to study the nature and depth of machining damage in 3 point flexural rupture test bars made from 5 wt% Y2O3 sintered hot isostatically pressed silicon nitride. The bars were machined to three surface finishes. The work carried out at Rolls-Royce by Mr R Quinn concentrated on the effects the machining damage had on the fracture strengths of the test bars. Work at the University of Surrey has identified in detail the nature and depth of machining damage in "coarse" 0. 4mum centre line average roughness surfaces. Deep grooves up to 2mum depth, 18mum width are found to have been superimposed on the general surface roughness by singularly large diamonds in the 350 grit diamond wheel. Sub-surface median cracks normal to the machining direction were clearly identified in bar cross-sections using oblique, diffuse "penumbra" illumination in an optical microscope, an as yet undocumented technique. Cross-section views of the machined surface and sub-surface were made possible by the very difficult and delicate technique of producing sandwich cross-sections of the machined bars. An analysis of machining-induced median cracks has not been carried out in such detail before. Semi-elliptical in shape the median cracks extend from 6mum - 45mum below the machined surface, and range from 19mum to 101mum in length parallel to the machining direction. They initiate at the focal point of a tributary system of microcracks at an average depth of 4mum - 5mum below the machined surface. It is believed that the median cracks initiate at the plastic/elastic boundary of a plastically deformed surface layer. Therefore a residual compressive layer, formed by the overlap of localised residual stresses from multi-particle contact events. and bound by an underlying tensile field, is thought to have an average depth of 4mum - 5mum. A very innovative technique was used to reveal sub-surface deformation, where TEM X-ray microdiffraction spots were distorted by mechanical damage in the ceramic grain structure. The "arcing" or "streaking" of the diffraction spots tended to disappear at a depth of 4mum - 5mum below the machined surface. This is further evidence of the existence of a thin layer in residual compression, which has an average depth of 4mum - 5mum. This technique is not known to have been used before. Fine diamond machining with a 600 grit wheel produced a centre line average roughness of 0.01/0.02 mum. However, evidence of machining damage is still present in the form of "remnant tracks" which lie parallel to the machining direction and consist of material pull-out. They are remnants of machining damage under grinding grooves introduced in previous machining stages. Single point Vickers pyramid diamond scratches were implemented at different loads on a polished surface. The morphology of the grooves and material fragmentation and the sub-surface median cracks were examined. Many features were found to resemble the deformation/fracture formed under a deep grinding groove in the coarse machined surface. Work carried out at Rolls-Royce by R Quinn showed that an increase in the quality of surface finish is accompanied by an increase in the mean strength and Weibull modulus of the machined bars. Furthermore a distinct anisotropy in the fracture strengths parallel and normal to the "coarse" machining direction is evidence of anisotropy in machining damage formed by a peripheral diamond grinding wheel. X-ray diffraction tests carried out at the CEGB by P E J Flewitt showed that machining damage produces a long range biaxial residual compressive field with the highest component acting normal to the machining direction. These results are consistent with the nature of machining damage identified at the University of Surrey, namely the strength-controlling median cracks which lie parallel to the machining direction and the residual compressive stress which exists as a thin 4mum - 5mum layer below the machined surface. Processing flaws were discovered in the as-hipped billets received for the project. Their elemental composition and likely origin were examined. A three dimensional "cellular network" flaw ranging from 400mum to 2.1mm in size (in different production batches) is believed to have been formed as a result of flocculation clustering during processing. Clusters of 1mum - 3mum metallic particles were also identified. They range from 5mum - 45mum in size. The contaminant particles are steel and were introduced as a result of the original ceramic powder ball milling process which employed a steel ball mill.