Effect of natural anhydrite content on the microstructure and hardness of bronze-based powder metallurgy parts

In this study, the effect of natural anhydrite (CaSO4) powder additions on the microstructure and hardness of a Cu-8Sn bronze composite, prepared by a powder metallurgy process, were examined. Natural anhydrite powder, ranging in content from 2 to 8 percent by weight was added to a premixed powder composed of copper and tin. The samples were uniaxially compacted into a disc shape with a diameter of 20 mm and a thickness of 3 mm under an applied pressure 281 MPa. The samples were sintered at 650 °C for 30 minutes in a reducing atmosphere. It was found that the green density of the samples decreased with increased anhydrite content. The sintered densities were higher than the green densities owing to densification of the samples. From microstructural images, the specimens were composed of twinned grains of bronze, pores, and anhydrite particles. The average hardness of the samples ranged between 65 to 80Hv. The hardness deviation was due to the porous microstructure of the multiple phase material. There was a slight increase of hardness with an increase of anhydrite content within the range examined. Phase analysis by X-ray diffraction found that the samples contain calcium sulfite instead of calcium sulfate anhydrite after sintering.