ELEVATED-TEMPERATURE EROSION OF RANGE OF COMPOSITE LAYERS OF NI-CR BASED FUNCTIONALLY GRADED MATERIAL

Functionally graded materials can be used in aggressive environments a t elevated temperature because they provide the possibility of minimis ing wastage of materials. Gradation of the volume fraction of hard par ticles through the layers means that thermal cycling effects are less severe than for many conventional metal-substrate systems. Because suc h materials may provide resistance to wear and corrosion (by using a c orrosion resistant matrix), it is thought that they may be application s to environments at elevated temperatures, in which materials selecti on involves a compromise between corrosion resistance and high yield s trength. The object of the present study was to investigate to erosion resistance of the various layers of a candidate functionally graded m aterial which consisted of WC particles in a Ni-Cr matrix. The perform ances of the various composite layers were considered separately in or der to establish to variation of erosion rates through the graded stru cture. 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 meas urements were used to characterise the surfaces following exposure. Th e results showed that the erosion rate at room temperature was at a mi nimum at intermediate volume fractions of WC particles. However, this behaviour reversed for erosion with larger particle sizes. Although th e thickness losses increased with increasing temperature for all volum e fractions of reinforcement particles, a reduction in the thickness l oss at the highest temperature studied was observed for exposure to bo th large and small erodents (600 and 200 mu m alumina). The results ar e explained in terms of the transition between erosion regimes for the various graded layers of the material. (C) 1996 The Institute of Mate rials.