|M.Sc Student||Hojman Elad|
|Subject||Properties of Diamond CVD Films Deposited onto Surface|
Modified WC-Co Substrates
|Department||Department of Nanoscience and Nanotechnology||Supervisor||Professor Hoffman Alon|
|Full Thesis text|
Polycrystalline diamond coatings deposited onto cemented WC-Co substrates using the Chemical Vapor Deposition (CVD) method are of great interest for tribological applications. The performance of such composites as machining tools, however, exhibits several limitations regarding the final diamond quality in the diamond film and its adhesion to the underlying substrate. These limitations result mainly due to the chemical interaction between the Co in the substrate and the CVD environment, which catalyzes the formation of sp2-bonded carbon both at the interface and within the diamond film.
In the present work, several substrate-surface modification methods for suppression of the activity of Co during the CVD process were examined. The modified substrate-surfaces were then pre-treated by either single-step (i.e. seeding) or dual-step (i.e. carburization and seeding) for enhancement of the diamond nucleation and growth at the early stages of the CVD process.
The findings of this work show that substrate-surface modification by direct thermal nitridation resulted in poor stabilization of the Co, mainly due to the thermodynamically preferred formation of W-N phases. Yet, dual-step pre-treatment of the substrate showed to partially suppress the Co activity during the following deposition of the diamond film.
Alternatively, substrate-surface modification by prior deposition of a ~1.5 µm Cr layer followed by thermal nitridation resulted in formation of a Cr-N interlayer that successfully performed as a diffusion-barrier for Co. The interlayer was found to include mainly the CrN phase which limited the mobility of Co due to its fully occupied interstitial sites and relatively high density structure. The CrN phase microstructure was found to enhance mechanical interlocking between the diamond film and the interlayer due to its ‘net-like’ microstructure. Moreover, the presence of a Cr2N phase containing region adjacent to the Cr-N interlayer/WC-Co substrate enabled the formation of Cr3C2 and CoCr intermetallic phases, which were suggested to enhance the chemical binding between the interlayer and the substrate. Further examination showed that the formation of a stable CrN phase sucessfully prevents the formation of an oxide Cr2O?3 phase within the interlayer, which was suggested to deteriorate the interlayer’s mechanical properties.
The formation of phases within the diamond film and underlying material after the substrate-surface modifications and the CVD process; and evaluation of the diamond quality in the deposited films were investigated by complementary characterization techniques: SEM, XRD, XPS, SIMS and Raman spectroscopy. The adhesion properties were evaluated according to the indentation tests and fracture morphology.