Early Stages of Diamond‐Film Formation on Cobalt‐Cemented Tungsten Carbide

Abstract

The surface composition of cemented tungsten carbide (WC-5.8 wt% Co) was studied by X-ray photoelectron spectroscopy (XPS), during the early stages of diamondfilm deposition, by hot-filament chemical vapor deposition (HFCVD). The nucleated diamond films were analyzed by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and automatic image analysis (AIA). The evolution of the surface composition of cemented tungsten carbide during the early stages of diamond-film deposition was strongly dependent on the substrate temperature. At relatively low temperature (750°C), cobalt-rich particles started to segregate at the substrate surface after a few minutes of diamond deposition. The conspicuous segregation of the binder partly inhibited the formation of stable diamond nuclei, through intense carbon dissolution or carbon segregation at the binder surface, but did not affect nucleic growth. At higher temperatures (940°C), no cobalt-rich particles formed at the substrate surface, even after 2 h of deposition. However, XPS results demonstrated the presence of cobalt in a surface layer, although in a lower amount than at 750°C. Nevertheless, the nucleation density of diamond at 940°C was much lower than at 750°C. Gaps between WC grains formed within 10 mins. Therefore, intergranular cobalt was removed at 940°C, a finding attributed to the etching performed by monohydrogen, rather than to binder evaporation. The time evolution of the substrate area fraction covered by diamond islands, S(t), was well described by Avrami kinetics for twodimensional phase transformations, suggesting that diamond formation took place via a heterogeneous nucleation process. The S(t) functions exhibited a similar trend at 750°a nd 940°C, because the higher growth rate of diamond crystallites at higher temperature counteracted the slower nucleation rate at the higher temperature.

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

“…Consequently, the nucleation and growth of highquality diamond films, with reduced interfacial sp 2 -carbon formation, is possible onto WC-Co materials. However, under certain CVD conditions, the binder can diffuse through the ''etched layer'' up to the substrate surface and form Cocontaining clusters, as demonstrated by combined use of SEM and X-Ray Photoelectron Spectroscopy (XPS) [28]. Therefore, process conditions that favour either binder mobility or Co clustering during the diamond nucleation stage must be carefully avoided.…”

“…Samples were ultrasonically seeded for 15 min in 1/4 Am diamond suspension (Struers, DP-Suspension, HQ), then cleaned with ethanol in an ultrasonic bath. Diamond films were deposited for 9 h in a stainless steel hot-filament chemical vapor deposition (HFCVD) chamber described elsewhere [28]. The gas phase was composed of a mixture of methane and hydrogen with a CH 4 /H 2 volume ratio fixed at 1.0% and was activated using a tantalum filament (2170 -C).…”

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

“…Nonetheless, the built-up of diamond films on WC-Co substrates is quite tricky because of the harmful effects of Co binder, which leads to the formation of a graphitic carbon layer at the substrate surface during the early stages of deposition, on which diamond grows later [3]. Mostly, the problem is overtaken by covering the WCCo substrate with interlayers, which act as diffusion barrier [4], etching the WC-Co substrate surface to wash superficial Co out [5], and decarburizing the substrate surface after the etching of binder [6].…”

Cobalt diffusion in different microstructured WC-Co substrates during diamond chemical vapor deposition

“…Polini et al [26] showed that the nucleation of diamond on WC-Co substrates, which were pretreated in a way that was similar to the pretreatments used in this investigation, was independent of substrate grain size. However, in their study they used a conventional hot-filament CVD system to deposit the diamond films.…”

“…This observation has been frequently reported by other workers. [25][26][27] Despite the Co fraction volume, with both types of substrates being the same the coarse-grain substrate showed greater density of such drops. Because the temperature of the coatings is below the liquid phase formation during cemented carbide sintering, it is expected that the drop formation (Fig.…”