Superior hardness and Young’s modulus of low temperature nanocrystalline diamond coatings

Nanocrystalline diamond (NCD) coatings with thickness of about 3 mu m were grown on silicon substrates at four deposition temperatures ranging from 653 to 884 degrees C in CH4/H-2/Ar microwave plasmas. The morphology, structure, chemical composition and mechanical and surface properties were studied by means of Atomic Force Microscopy (AFM), X-Ray Diffraction (XRD), Raman spectroscopy, nanoindentation and Water Contact Angle (WCA) techniques. The different deposition temperatures used enabled to modulate the chemical, structural and mechanical NCD properties, in particular the grain size and the shape. The characterization measurements revealed a relatively smooth surface morphology with a variable grain size, which affected the incorporated hydrogen amount and the sp(2) carbon content, and, as a consequence, the mechanical properties. Specifically, the hydrogen content decreased by increasing the grain size, whereas the sp(2) carbon content increased. The highest values of hardness (121 +/- 25 GPa) and elastic modulus (1036 +/- 163 GPa) were achieved in NCD film grown at the lowest value of deposition temperature, which favored the formation of elongated nanocrystallites characterized by improved hydrophobic surface properties