Influence of the nucleation density on the structure and mechanical properties of ultrananocrystalline diamond films

摘要

The influence of the nucleation density on the development of the morphology of ultrananocrystalline diamond/amorphous carbon (UNCD/a-C) composite films and their mechanical properties has been investigated by variation of the substrate pre-treatment used to enhance the nucleation. The films have been prepared by microwave plasma chemical vapour deposition from 17 percent CH_4/N_2 mixtures on silicon substrates. Their morphology and topography have been characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). It is shown that by successive addition of ultradispere diamond powder (3-5 nm average grain size) to the suspension of nanocrystalline diamond powder (250 nm average grain size) in n-pentane used for the ultrasonic pre-treatment, the nucleation density can be enhanced by two orders of magnitude from 1 X 10~8 cm~(-2) to higher than 1 X 10~(10) cm~(-2). This changes the morphology of the films from individual nodules to uniform coatings and reduces the thickness required to achieve closed films. The mechanical properties of these films have been investigated by nanoindentation and nanoscratch tests. The determined hardness was about 25-28 GPa for all samples under investigation, the elastic modulus 262-271 GPa, and the elastic recovery 62-65 percent, revealing the influence of the amorphous carbon matrix. The scratch tests proved a strong adhesion of the UNCD/a-C coatings and their protective effect on silicon substrates. The nucleation density has no distinct influence on the mechanical properties of the closed films.