Diamond-coated cemented carbide cutting tools have great potential applications in machining efficaciously non-ferrous materials such as Al-Si alloys, ceramics and various composite materials because of their high hardness, good toughness, long lifetime, free shape and low price. However, poor adhesion of diamond coatings to cemented carbide cutting tools is one enormous technological obstacle to the wildly commercial applications of the diamond coated tool inserts. How to improve the adhesion of the diamond coatings on cutting tools is the basic problem that has to be solved today.In the present work, diamond coatings were deposited on the cobalt-cemented tungsten carbide (WC-6%Co) tool inserts by microwave plasma CVD (MPCVD) method. We improved the adhesion of diamond coatings considerably by optimizing the conditions of diamond coating deposition and using novel methods to modify the substrate surfaces.1. Optimizing the conditions of diamond coating depositionThe effects of methane concentration and substrate temperature on the adhesion of diamond coatings were studied. The results showed that diamond nucleation, diamond coating"s quality and residual stress all have important effects on the adhesion strength. Our experiments show that the adhesion between diamond coatings and cemented carbide substrates is strong when methane concentration is 0.8% and substrate temperature is 800 if other conditions are the same. By using a two-step method, that is, using different deposition conditions respectively during the diamond nucleation and growth periods, we can increase the nucleation density remarkably, at the same time, insure the purity of the diamond coatings. The limit load of the diamond coatings deposited by the two-step method is more than 60kgf.2. Effect of Mo ion-implantation on the adhesion of diamond coatingsCemented carbide substrates were etched by HNO3 solution and some of them were implanted by Mo ions. The indentation test showed that the adhesive strength between CVD diamond coatings and the substrates implanted with Mo ions was improved to some extent. The roles of the Mo ions implanted into the surfaces of cemented carbide substrates are as follows: (1) react with the binder Co to form the CoMoO4 compound; (2) form Mo-C chemical bonds at the diamond coating/substrate interfaces.3. Deposition Ti and Ti/TiNx interlayers between diamond coatings and cemented carbide substratesUsing the reactive magnetron sputtering method to deposit Ti and Ti/TiNx interlayers (about 0.05um) on the surfaces of cemented carbide substrates, we studied the effects of interlayers on diamond nucleation density, diamond coating quality and it"s adhesion strength. The results showed that CVD diamond nucleation density was enhanced by three orders of magnitude after introduing the interlayers. There is a lot of graphic carbon at the interfaces between diamond coatings and substrates on which the interlayers have not been deposited, and the main adhesion mechanism of the diamond coatings is mechanical interlocking. Ti and Ti/TiNx interlayers can not only act as diffusion barriers of the binder Co, but also help to form Ti-C chemical bonds at the diamond coating/substrate interfaces. The limit load of the diamond coatings is more than 100kgf after introducing Ti and Ti/TiNx interlayers.Designing proper interlayers between diamond coatings and cemented carbide cutting tools may be the most prospective ways to improve the adhesion of CVD diamond coatings.