Effect of HfO2 addition on sintering of Si3N4

The effect of HfO2 addition on sintering of Si3N4-Y2O3-AlN system was inves tigated. Although the densification was improved by the addition of HfO2, t he behavior was divided into two types according to the selected firing tem perature: below 1800 degreesC, the maximum density was achieved with 1.5 ma ss% HfO2 addition; on the other hand, above 1800 degreesC density continuou sly increased with increasing HfO2 addition. This densification behavior co uld be explained according to phase diagrams of the HfO2-Y2O3 . SiO2 (eutec tic temperature: 1800 degreesC) and HfO2-Y2O3 . 2SiO(2) (eutectic temperatu re: 1600 degreesC). As a tentative mechanism for densification, it was cons idered that,in the initial stage of sintering, the formation of some liquid phases with low melting point promoted densification. Then precipitation o f c-HfO2 including Y, Si elements and of HfN including O occurred. This mec hanism was supported by X-ray diffraction (XRD), transmission electron micr oscopy (TEM), and energy dispersive spectrometry (EDS) analysis, The phase transformation from alpha -Si3N4 to beta -SiAlON was found to be enhanced b y HfO2 addition. Consequently, sintered bodies obtained from the Si3N4-Y2O3 -AlN-HfO2 mixtures were consisted of not only elongated beta -SiAlON grains , and grain-boundary phases composed of Hf, Si, Al, Y and O, but also isola ted spherical grains of both c-HfO2 and HfN (200-400 nm in diameter).