The extreme hardness, wear-resistance, high thermal conductivity, and low friction coefficient of diamond films, combined with the ductility and toughness of Co cemented tungsten carbide (WC-Co) allow the development of coated tools and components with new characteristics. The typical domain of diamond-coated WC-Co tools should include application to the machining of materials such as metal-matrix composite (MMC) materials, Al-Si alloys, nonferrous metals, wood, fiber-reinforced plastics, and graphite. 1-4 Unfortunately, there are several problems associated with depositing good quality and strongly adherent diamond coatings onto WC-Co based components. In fact, the presence at the interface of cobalt hinders the heterogeneous nucleation of diamond. 5-8 In addition, cobalt induces the formation of a nondiamond type of carbon layer at the substrate surface during the early stages of deposition, on which weakly adherent diamond grows later. 9,10 These problems can be reduced by modifications of the WC-Co substrate surface prior to diamond deposition. The various pretreatments reported in the literature include the removal of cobalt by chemical etching, 11,12 the surface decarburization after the etching of the binder, 13 and the deposition of interlayers which act as diffusion barriers for carbon. 14 Moreover, the increase of the interface contact area by roughening the substrate surface has also been proposed as a key factor for achieving satisfactory adhesion, 3,15,16 Besides substrate pretreatments, deposition conditions might influence the mechanical and cutting properties of diamond-coated WC-Co tools. Haubner and Lux 17 found that diamond films deposited at relatively low temperatures (760 Ϯ 10ЊC) by hot filament chemical vapor deposition (HFCVD) showed improved adhesion onto hard metal substrates. Taher et al. 18,19 recently performed an extensive investigation on the effect of methane concentration in the gas phase on the mechanical properties of cutting inserts deposited at 850ЊC by HFCVD.The focus of this work is to examine the combined effects of substrate pretreatments, deposition temperature, and methane concentration on diamond nucleation and film adhesion, in order to determine which are the factors that mainly contribute to the achievement of a good film/substrate interface toughness.Experimental 10 ϫ 10 ϫ 3 mm WC-5.8 wt % Co substrates were sintered using WC with an average grain size of 6 m, which is larger than the grain size (1-2 m) conventionally used to improve the hardness of commercial tools. The substrates were provided by FILMS SpA. The as-sintered samples were ground and then rinsed with acetone and ethanol in an ultrasonic bath. In order to enhance diamond nucleation, to suppress the deleterious effects of the binder, and to increase the contact area at the interface, the substrates were submitted to surface modifications using the mechanical and chemical pretreatments listed and labeled in Table I. Following each pretreatment, the substrates were washed with acetone and deionized water ...