For example, traditionally hafnium oxide (HfO2) is sintered at temperatures around 1800℃,instead similar relative densities can be achieved by HRS at temperatures in between 900 and 1000 ℃.HRS, therefore, is a relatively mild temperature sintering method [63], and is characterized by three distinct stages: (i) hydrothermal (by supercritical water) oxidation of the metal, (ii) diffusion of the H2 gas from the capsule, and (iii) sintering of the oxide powder consolidate [64].