An expanding thermal plasma of argon and nitrogen into which acetylene is injected, was used to deposit amorphous hydrogenated carbon nitride (a-C:N:H) films. In the absence of an external bias high growth rates (up to 37 nm/s) were achieved. The growth rate and refractive index of the films were studied in situ with HeNe-ellipsometry. Fourier transform infrared (FTIR) absorption spectroscopy was used to investigate the different bondings. The changes observed in the infrared absorption spectrum with increasing N-2/C2H2 ratio in the plasma indicate an increase in dehydrogenation of the films leading to polymerization, and possibly also to graphitization. The microhardness and Young's modulus, as determined by nano-indentation measurements, show a strong respectively overall increase with increasing acetylene flow, and appear to decrease on addition of N-2. This is attributed to the increase in the polymerization of the films on incorporation of nitrogen. Evaluation of the chemical composition by Rutherford back scattering (RBS) and elastic recoil detection analysis (ERDA) reveals that a deposition mechanism should be considered in which N and H are in competition with each other during growth.