Polycrystalline diamond is a man-made diamond material used for rock drilling and machining operations. Its vastly superior hardness to any other material allows it to cut through challenging materials such as rock, high-strength metals and carbon fibre composites whilst itself wearing away very slowly. However, at high temperature, it suffers from mechanical breakdown due to an unknown mechanism. It is suspected that one of its constituents, cobalt metal, reacts with the diamond causing it to convert to graphite. This experiment will aim to discover the temperature at which this begins by observing changes in the crystal structure of the diamond with neutron diffraction as temperature is increased. The stress put on the surrounding diamond material by this crystal structure change will also be measured and used to determine how the material breaks down at high temperature.