Thermal Barrier Coating on Diamond Particles for the SPS Sintering of the Diamond-ZrO₂ Composite

The aim of this work was to obtain a protective ZrO₂ coating on diamond particles, which was to protect diamond from oxidation and graphitization, enabling sintering of diamond at higher temperatures and lower pressures than its thermodynamic stability in atmospheric conditions. The coatings were obtained by mixing diamond with zirconium and oxidizing in air or oxygen. Mixtures of diamond and 80 wt% zirconium were sintered by SPS method at temperatures of 1250 °C and 1450 °C. To stabilize the tetragonal structure of ZrO₂, 3 mol% Y₂O₃ was added to zirconium before the milling process. The composition of powder phases, morphology, and microstructures of sintered materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectrometry (EDS). Diffraction studies show the presence of zirconium monoclinic and tetragonal oxides in coatings, after oxidation in air, and in oxygen. Oxidation in oxygen flow is possible for lower temperatures (75 °C), which results in the presence of unreacted zirconium. In ZrO₂ doped with yttria after the oxidation process in oxygen, there is no monoclinic ZrO_2. It is possible to sinter the ZrO₂-diamond composite at 1250 °C using the spark plasma sintering method without graphitization of the diamond. The sintered material consists of monoclinic and tetragonal ZrO₂ structures.