Studies hot-pressing of Si//3N//4 have suggested the use of an additive which both promotes densitification and forms a more refractory silicate grain-boundary phase, thus improving high-temperature strength. This concept suggested that zircon (ZrSiO//4), with a reported melting point as high as 2500 degree C and solidus temperatures of greater than equivalent to 1775 degree C (compared to greater than equivalent to 1500 degree C for magnesium silicate and greater than equivalent to 1300 degree C for magnesium silicate with a few wt% CaO), might be a better additive for Si//3N//4. The poor resultant density ( approximately equals 85%) appears consistent with the failure of some powders, including those apparently the purest, to densify adequately with ZrO//2 additions in the present study. It may be that ZrO//2 is an effective additive only for powders with a particular character impurity; if so, the relation between cost and properties remains to be determined. However, neither of the powders successfully densified is particularly impure. Thus, either limited impurity effects or the observed milling behavior would appear to correlate with densification. In any event, Zr-based additions to some Si//3N//4 powders have produced densification and promising properties.
