Influence of seeding pre-treatments on mechanical properties of ultrananocrystalline diamond films on silicon and Si3N4 substrates

Abstract

Ultrananocrystalline diamond (UNCD) films on silicon and Si3N4 substrate were prepared by microwave plasma chemistry vapor deposition method using argon-rich CH4/H2/Ar plasmas. The influence of the nucleation density on the development of morphology of UNCD films and their mechanical properties has been investigated by varying pretreatments. Their morphology and topography have been characterized by scanning electron microscopy and surface profilometer. The influences of pretreatments on mechanical properties of the deposited UNCD films are investigated by using nano-indentation and nano-scratch tests. It was found that grain size, hardness, and elastic modulus are less dependent on nucleation density change, which is caused by different pre-treatments. Changes in grain size are more dependent on deposition temperature and plasma atmosphere. As compared to the nucleation density pretreated by the suspension of diamond (0.25μm) powder mixed with nano-diamond (3–5nm), nucleation density is obviously enhanced by pretreatments with W powder (0.25μm) or Ti powder (0.25μm) mixed with nano-diamond (3–5nm). It was also observed that there is no difference in nanohardness and elastic modulus with different substrates (Si and Si3N4 substrates). But the UNCD's adhesion on Si3N4 substrate is obviously higher than that on Si substrate. The determined hardness was about 54–58GPa for all samples under investigation, the elastic modulus 542–667GPa, and the elastic recovery 74–81%. The scratch tests proved a strong adhesion of the UNCD coatings and their protective effect on silicon substrates and Si3N4 substrates. Detailed experimental results and mechanisms for UNCD film deposition in argon-rich plasma are discussed.