Enhancement of Strength through Sinter Forging

Packing defects in powder compacts can develop into microcracks during the sintering process. For example, an agglomerate which sinters differently than its surroundings can open up large, cracklike defects which are difficult to remove in free sintering. Traditionally, hot‐pressing or hot isostatic pressing has been used to prevent the formation of such defects. Here we show that processing defects can be virtually eliminated by sinter forging. In sinter forging the powder preform is compacted under a uniaxial compressive stress without any lateral constraint. In this way both densification and shear strain are imposed on the specimen. We have found that the shear strain can eliminate large pores by changing their shape. Eventually the flattening of the pore under compressive strain causes it to disintegrate into small pores which are then removed by sintering, leading to an increase in the strength of the ceramic. In alumina, a sigmoidal curve for the change in strength with strain is obtained where the strength begins to increase at 20% shear strain and reaches a maximum value at 60% strain. The shear strain required for the improvement in strength is compared with theoretical estimates.