Tribological Properties of Sintered Graphite-Steel Composites Containing Lignite Bottom Ash

Abstract

Lignite bottom ash is an industrial byproduct from the combustion of lignite coal in electric power plants. The ash is composed of various hard metal oxides, and therefore may be suitable for use as a low cost friction modifier in friction materials. This research studied the effect of lignite bottom ash additions (up to 20 weight percent) on the tribological properties of a graphite-steel composite (5 weight percent graphite and 95 weight percent high carbon steel). The powder compositions were uniaxially pressed with 300 MPa applied pressure to produce disc shaped samples. The samples were sintered at 1,100 °C for 30 minutes in a reducing atmosphere of 90 percent nitrogen and 10 percent hydrogen. The friction coefficients were measured using a ball-on-disc tribometer. It was found that the addition of bottom ash increased the friction coefficients of the samples due to the increased abrasiveness provided by the bottom ash. The density of the samples was reduced due to the lower theoretical density of the bottom ash compared to the steel that it replaced. The hardness of the samples were found to be independent of the amount of lignite bottom ash, possibly as a result of a hard particle reinforcement effect.