AbstractPolycrystalline diamond films were deposited on to Si3N4coated silicon substrates using thermal chemical vapour deposition. The defects on the amorphous Si3N4layer played an essential role in diamond nucleation. After deposition, very few diamond crystallites were found on the untreated Si3N4coating, while a diamond film had formed on the ultrasonic diamond powder treated Si3N4coating. During the deposition process, the substrate temperature drastically affected the integrity of the intermediate Si3N4layer. At a substrate temperature above 850°C, the Si3N4layer began to degrade. The lower CH4concentration produced better diamond films in terms of crystallinity and purity. The adhesion of the diamond film to the Si3N4coated silicon substrate and the friction coefficient between the diamond stylus and the diamond film decreased as the CH4concentration increased. The corrosion resistance of the diamond film on the Si3N4coated silicon substrate was much stronger than that of the Si3N4film on the silicon substrate.MST/3177