Elevated temperature erosion of range of composite layers of Ni-Cr based functionally graded material

Functionally graded materials can be used in aggressive environments at elevated temperature because they provide the possibility of minimising wastage of materials. Gradation of the volume fraction of hard particles through the layers means that thermal cycling effects are less severe than for many conventional metal-substrate systems. Because such materials may provide resistance to wear and corrosion (by using a corrosion resistant matrix), it is thought that they may be applications to environments at elevated temperatures, in which materials selection involves a compromise between corrosion resistance and high yield strength. The object of the present study was to investigate to erosion resistance of the various layers of a candidate functionally graded material which consisted of WC particles in a Ni-Cr matrix. The performances of the various composite layers were considered separately in order to establish to variation of erosion rates through the graded structure. The effects of temperature, volume fraction of hard particles, and erodent size were investigated in a laboratory simulated fluidised bed erosion rig. Scanning electron microscopy and thickness loss measurements were used to characterise the surfaces following exposure. The results showed that the erosion rate at room temperature was at a minimum at intermediate volume fractions of WC particles. However, this behaviour reversed for erosion with larger particle sizes. Although the thickness losses increased with increasing temperature for all volume fractions of reinforcement particles, a reduction in the thickness loss at the highest temperature studied was observed for exposure to both large and small erodents (600 and 200 mu m alumina). The results are explained in terms of the transition between erosion regimes for the various graded layers of the material