Spark plasma sintering of yttria-stabilized zirconia/titanium nitride composites

Full text: Composites of 8 mol% yttria-stabilized zirconia (YSZ) and titanium nitride (TiN) were fabricated. Such composites exhibit interesting mechanical and electrical properties and have been investigated for different high-temperature applications. Commercially available starting powders of YSZ (Tosoh, Japan) and TiN (Sigma-Aldrich) were mechanically mixed in a ball-mill using zirconia-based balls and ethanol. After drying under mixing, composite powders YSZ / w TiN, with relative weight fractions w = 0, 25, 50, and 75% of TiN were analyzed by X-rays diffraction (XRD). Samples were sintered by spark plasma (SPS) using graphite dies and punches (14 mm diameter), which were covered with a carbon paper to avoid direct contact with the samples. Maximum SPS temperature was 1450 °C for 5 min, with 100 °C min^-1 heating rate and applied pressure of 30 kN cm^-2 up to 1250 °C and 60 kN cm^-2 for higher sintering temperatures. Final samples dimensions were ~14 mm diameter and 1 mm thickness. Estimated apparent densities were ~98% of the theoretical density calculated by the rule of mixtures. The sintered pellets were analyzed by XRD. The XRD data revealed a pronounced superficial carbon contamination, as usually observed in SPS. However, in the present composite the covalent nature of TiN rules out carbon removal by thermal treatment. Indeed, the carbon oxidation was confirmed by thermogravimetric analysis to occur at temperatures higher than the conversion of TiN into TiO₂ (~500 °C). Therefore, YSZ/TiN pellets were mechanically rectified in a diamond saw, which removed ~300 μm of each parallel surface of the samples. Rectified pellets were analyzed by XRD and only the characteristic diffraction lines corresponding to YSZ and TiN were found, indicating that significant carbon diffusion was limited to thickness < 350 μm. Moreover, by comparing the XRD data before and after SPS indicated that no significant reaction between the phases occurred. (author)