Interfacial adhesion and toughness of nanostructured diamond coatings

Microwave plasma chemical vapor deposition was used to deposit ultrasmooth nanostructured diamond films (roughness of 14 nm) on a titanium alloy (Ti-6 Al-4V) by employing a feedgas mixture containing a high methane fraction (1 5% by volume) in nitrogen and hydrogen. Of particular interest in this stud y is the exceptional adhesion of the 4-mu m-thick diamond coatings to the m etal substrates as observed by indentation testing up to 150 kg load with a 1/8-in.-diameter tungsten carbide ball. No film delamination was observed up to 150 kg indentation load for each of 5 samples grown under the same pr ocessing conditions. Scanning electron microscopy of the Film surrounding t he indentations revealed circumferential microcracking beginning at loads r anging from 60 kg to as high as 150 kg. The strain to cause film microcrack ing was estimated from calculations of indentation surface areas to be as h igh as 1.9 +/- 0.2%, which represents a significant improvement in toughnes s over other ceramic coatings.