Diamond film coated tool and process for producing the same

A diamond coated tool comprising:

• a substrate;
• and
• a polycrystalline diamond coating formed on a surface of the substrate, wherein
• said substrate is made of a cemented carbide or a cermet,
• said polycrystalline diamond coating is an aggregate of secondary diamond grains, each of which includes an aggregate of primary diamond fine grains, the primary diamond fine grains having a first crystal structure and the secondary diamond grains having a second crystal structure different from the first crystal structure,
• said secondary diamond grains constituting a growth surface of said polycrystalline diamond coating has an average grain size of about 1.5 micrometers or below,
• grain boundaries are formed between each of said secondary diamond grains,
• said primary diamond fine grains are elongated in a growth direction of said polycrystalline diamond coating,
• said primary diamond fine grains have a major axis diameter ranging from 0.01 micrometer to 1 micrometer,
• said polycrystalline diamond coating has a thickness ranging from about 0.1 micrometer to 20 micrometers, and
• said polycrystalline diamond coating has an average surface roughness Ra ranging from about 0.01 micrometer to 0.2 micrometer.