Nucleation and Adhesion of Diamond Films on Co Cemented Tungsten Carbide

Abstract

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 ...

Recent Advances in the Deposition of Diamond Coatings on Co-Cemented Tungsten Carbides

“…Laser treatments of WC-Co can be therefore as effective as the widely recognized Murakami's treatment insofar as the adhesion of diamond coatings on WC-Co substrates is concerned. These findings also agree with several data reported in the relevant literature, that is, that proper substrate surface roughening is a crucial issue affecting diamond film adhesion [8,34,35].…”

“…Average (dX/dP) −1 parameter measured on diamond deposited on laser-treated WC-Co substrate was ∼0.437 ± 0.03 kg/μm and very close to the value of 0.452 ± 0.03 kg/μm achieved on diamond deposited on MP-treated WC-Co substrate. Such results are in accordance with results measured for films of similar thickness and grown onto MP-treated WC-Co substrate [34,35]. Laser treatments of WC-Co can be therefore as effective as the widely recognized Murakami's treatment insofar as the adhesion of diamond coatings on WC-Co substrates is concerned.…”

Fluidized bed micro-machining and HFCVD of diamond films onto Co-cemented tungsten carbide (WC-Co) hardmetal slabs

“…Although a clear correlation between diamond nucleation density and adhesive toughness has not been established yet, data from our laboratory [19] have shown that nucleation densities in the range 0.02 -0.3 Am À 2 did not significantly affect the adhesion of diamond films deposited using same CVD conditions onto WC-Co substrates previously submitted to the same pretreatment. Nevertheless, the larger nucleation densities caused by FB treatment with diamond powders allowed growing fine-grained smooth diamond films, which should allow better chip evacuation in dry cutting operations and exhibit lower friction coefficient and better wear resistance as well.…”

Chemically vapour deposited diamond coatings on cemented tungsten carbides: Substrate pretreatments, adhesion and cutting performance

“…Total chamber pressure was 18 Torr (2.4 kPa) and the gas phase was activated using a tantalum filament heated to 2350 -C. No diamond crystals nucleated at the WC-Co surface because of the low concentration of methane and the high substrate temperature [51]. However, as described in ' 2.3 and shown in Fig.…”