Study of densification mechanisms during Spark Plasma Sintering of co-precipitated Ho:Lu2O3 nanopowders: Application to transparent ceramics for lasers

This work was focused on the determination of densification mechanisms during Spark Plasma Sintering (SPS) of Ho:Lu2O3 nanopowders. Strong variation of the stress exponent n was evidenced during the sintering process. At low relative density (i.e. rho < 66 %), n = 3 and powder particles rearrangement and coalescence take place because of high value of effective stress and low size of primary nanoparticles. Then, for rho between 66 % and 85 %, the stress exponent decreases to n = 2 then n = 1. Such values were related to Rachinger then Lifshitz sliding mechanisms, the last one was associated with an average activation energy of 565 kJ.mol(-1). At the final densification stage (rho > 85 %), the stress exponent suddenly increases to 4 in accordance with a power-law creep. From these investigations, an optimized thermomechanical cycle was proposed to obtain highly transparent Ho: Lu2O3 ceramics suitable for laser applications.