The nanostructured phase transition of superhard Ti-Si-C-N coatings

Ti-Si-C-N coatings with different Si contents were synthesized by means of pulsed direct current plasma-enhanced chemical vapor deposition using a TiCl(4), SiCl(4), CH(4), N(2), H(2) and Ar gas mixture. A complex phase transition has been identified which is strongly controlled by the silicon content in the coatings. Transmission electron microscopy (TEM) and x-ray diffraction (XRD) results indicate that increasing Si content leads to a phase transition from a solid solution (Ti, Si)(C, N) to a dual-phase Ti (C,N)+TiSi(2) structure, and then to a two-phase Ti (C,N)+SiC structure, and that the changes in grain size and lattice parameter coincide with the phase transition. The Ti-Si-C-N coatings with high Si contents (≥4.3 at.%) possess a superhigh hardness (43-52 GPa) due to grain refinement/grain boundary hardening and compressive stress/dispersion hardening of the hard, nanosized crystalline TiSi(2) or SiC dispersed in the matrix.