There is currently considerable interest in producing new materials with extreme combinations of mechanical properties such as high hardnesses and moduli. One example of such a material is crystalline C3N4, which has been predicted to have a bulk modulus higher than that of diamond. In this paper we report on experiments carried out to synthesize CN(x) thin films. The films were grown in an unbalanced magnetron-sputtering system by reactive sputtering of C in N2 discharges. Si(001) substrates with the native oxide removed by thermal desorption and then kept at temperatures ranging from 150 to 600-degrees-C and substrate bias voltages V(s) between 7.5 and -200 V were used. The films were analysed using X-ray diffraction, transmission electron microscopy (TEM). Auger electron spectroscopy, Rutherford backscattering and nano-indentation tests. Typically the films were grown at rates of 5 nm s-1 to total thicknesses of 300 nm. Owing to an extensive re-sputtering, only low negative bias voltages (-80 < V(s) < 7.5 V) could be used while still retaining reasonable growth rates. Films with a maximum N concentration of -28 at.% were obtained under the growth conditions used. Films analysed using TEM proved to be crystalline with a turbostratic structure in both high resolution imaging and electron diffraction. Nano-indentation tests showed load vs. deflection curves with an extremely large elastic recovery (compared with the Si substrate alone) of between 82% and 74% of the total indentation displacement for loads of 1, 5 and 10 mN. The "load-on" (i.e. with no allowance for recovery of the substantial elastic component of the displacement) hardness values calculated from the measurements were between 7 and 9 GPa.