Microwave plasma chemical vapor deposition was used to deposit ultrasmooth nanostructured diamond films (roughness of 14 nm) on a titanium alloy (Ti-6 Al-4 V) by employing a feedgas mixture containing a high methane fraction (15% by volume) in nitrogen and hydrogen of particular interest in this study is the exceptional adhesion of the 4-μm-thick diamond coatings to the metal 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 processing conditions. Scanning electron microscopy of the film surrounding the indentations revealed circumferential microcracking beginning at loads ranging from 60 kg to as high as 150 kg. The strain to cause film microcracking was estimated from calculations of indentation surface areas to be as high as 1.9 ± 0.2%, which represents a significant improvement in toughness over other ceramic coatings. © 2000 Materials Research Society.