PM processing and characterisation of new ZrO₂-fly ash composites

In this study, new nanocrystalline partially stabilised zirconia (3Y-ZrO₂) composites reinforced with fly ash (0, 5, 10 and 20 wt-%) were prepared through powder metallurgy route. To improve composites densification, 2 wt-% of MgO was added. The powders were ball milled for 4 h, then cold isostatically compacted and finally sintered at 1350°C for different sintering times. Particle morphology and microstructure of the sintered samples were characterised by means of a field emission gun scanning electron microscope. The density and hardness of samples were evaluated as a function of sintering temperature, sintering time and fly ash content. Ball milling for 4 h deceased particle size and yielded a homogenous mixture. Also, it reduced crystallite size of the tetragonal phase from 37.73 to 29.85 nm in the unreinforced sample. For samples containing fly ash, the crystallite size of the tetragonal phase was reduced and the reduction was less in samples containing higher content fly ash. Overall, the addition of fly ash decreased the density of the composites because of the lower density of fly ash than zirconia which resulted in light weight composite materials. The addition of 5 wt-% fly ash to 3Y-ZrO₂ increased the hardness while the addition of 10 and 20 wt-% decreased it. The hardness of the composites increased with increasing sintering time to maximum values depending on fly ash content owing to the elimination of porosity and then decreased owing to grain growth.