Mechanical properties of ultrananocrystalline diamond films modified by hydrogen concentration in deposition atmosphere

Abstract Ultrananocrystalline diamond (UNCD) films on silicon were prepared by microwave plasma chemical vapor deposition method using argon-rich CH 4 /H 2 /Ar plasmas. The grains with different sizes were obtained by deposition in different atmosphere and the sizes of grain were measured after the deposition by X-ray diffraction (XRD). The influences of the grain size on the development of the morphology of ultrananocrystalline diamond (UNCD) films and their mechanical properties have been investigated by variation of hydrogen content from 5% to 20% in the deposition atmosphere. Their morphology and topography have been characterized by scanning electron microscopy (SEM) and surface profilometer. The influences of the hydrogen concentration on the mechanical properties of the deposited UNCD films are investigated by using nano-indentation and nano-scratch tests. It was found that the grain size, growth rate and surface roughness are increased with the increase of the hydrogen concentration. This changes the morphology of the films from granular to needle-like clusters. It can also be seen that the elastic modulus is increased with the addition of hydrogen; the hardness increased monotonically from 5% to 15% hydrogen, then decreased for 20% hydrogen with the maximum at 15% hydrogen; and the elastic recovery is 72–78%. Hardness and toughness are all decisive for protecting nature of the coatings. The scratch tests proved a strong adhesion of the UNCD coatings up to 45 mN and their protective effect on silicon substrates. The hydrogen has significant influence on the mechanical properties. Detailed experimental results and failure mechanisms for UNCD film deposition in argon-rich plasma are discussed. The deposited highly smooth UNCD film is also expected to be applicable in medical implants, surface acoustic wave (SAW) devices and micro-electromechanical systems (MEMS).