Effect of Si/O doping on the thermal stability of non-bonded hydrogenated diamondlike carbon coatings

Diamond-like carbon coatings containing Si and O (a-C:H:SiOx) were deposited onto 316L steel and silicon wafer substrates using a mixture precursor of hexamethyldisiloxane (HMDSO) and acetylene under a high bias voltage. Then, the coated coupons were annealed in an argon media. The hydrogen content in the coating was measured by 2 MeV 4He+ elastic recoil detection analysis (ERDA). The structure and morphology evolution during annealing were characterized by scanning electron microscopy (SEM), mass spectrometry (MS), Fourier transformation infrared spectroscopy (FTIR), Raman spectra, and X-ray photoelectron spectroscopy (XPS). The mechanical properties were investigated using a residual stress tester and nanoindentor. The results showed that under a high bias voltage a large amount of the hydrogen of the a-C:H:SiOx coatings did not bond with carbon. Si and O preferred to form SiC bonds and SiO bonds respectively to enhance C sp3 fraction, relax the residual stress, and depress the graphite cluster growth. During annealing, combining with C sp3 rehybridization, the transformation of the SiC bond to SiO or Si-C-O bonds caused hardness and residual stress variation. The a-C:H:SiOx coatings still held more than 12 GPa hardness after annealing at 590 °C for 4 h, while the hardness was approximately 8 GPa for the pure a-C:H coating.