Densification maps for spark plasma sintering of nanocrystalline MgO ceramics

Densification maps for spark plasma sintering (SPS) of nanocrystalline MgO powders were constructed using hot-isostatic pressing (HIP) model. Effects of the grain size, applied pressure, vacuum level, SPS temperature, and duration on densification were determined. Plastic flow and diffusion were the dominating mechanisms at the first and the final stages of densification, respectively, whereas power-law creep did not contribute to densification. Most impressive was the effect of the grain size on the SPS duration needed for full densification. At 800 °C and 150 MPa, the 20 nm particle size powder compact may be fully densified within 1 min. Significant densification is expected by the plastic yield if a threshold pressure is applied. Small temperature decrease of 50 °C may increase the densification duration from tens of seconds to tens of minutes. Relative densities as high as 75% may be reached within 1 min, irrespective of the initial particle size. However, threshold particle size exists (i.e. 50 nm at 800 °C and 100 MPa) for full densification at workable SPS durations. The densification maps highlight the presence of a temperature window between 750 °C and 850 °C within which fast densification of nanocrystalline MgO may be achieved by SPS. Reasonable agreement between the calculated and the experimental SPS results confirms the validity of the HIP model for description of the SPS process.