Wear-resistant, super hard ceramic composites based on cubic boron nitride (cBN) are of great interest to industry. However, cBN is metastable under sintering conditions at normal pressure and converts into the soft hexagonal BN (hBN). Therefore, efforts are being made to avoid this process. Besides short sintering times, the use of coated cBN-particles is a way to minimize this process. Therefore, the thermal stability of TiN coated cBN powders in high purity argon and nitrogen atmospheres up to temperatures of 1600 °C was investigated by thermogravimetry, X-ray phase analysis, scanning electron microscopy and Raman spectroscopy. The TiN coating was prepared by the atomic layer deposition (ALD)-method. The investigations showed that the TiN layer reacts in Ar at T ≥ 1200 °C with the cBN and forms a porous TiB2 layer. No reaction takes place in nitrogen up to temperatures of 1600 °C. Nevertheless, the 20 and 50 nm thin coatings also undergo a recrystallization process during heat treatment up to temperatures of 1600 °C.
Keywords: TiN; cBN; coating; cubic boron nitride; hBN; hexagonal boron nitride; microstructure; phase transformation; thermal stability; titanium nitride.