ICMCTF2010 Session D2-1: Diamond and Diamond-Like Carbon Materials

Like many other platform technologies, Ultrananocrystalline diamond has faced many hurdles on its way from the laboratory into products. In this talk I will highlight our efforts over the past several years to transfer the technology out of Argonne National Laboratory and into the real world. UNCD®, ADT's trademarked technology, spans a family of diamond materials that all share two attributes: THIN and SMOOTH. The critical value building exercise of developing and implementing a set of reproducible production processes to make UNCD films has resulted in ADT being able to launch three separate product lines (wafers, mechanical seals, and NaDiaProbes®) in the past 1-2 years into three very different markets. How ADT looks at wafer-scale production of UNCD-based MEMS devices going forward as well as our technology roadmap for UNCD-enabled products under development will be reviewed. Particular emphasis will be placed on the development of UNCD coatings and MEMS devices for bio-related applications.

The deposition of diamond coatings onto Co-cemented tungsten carbide (WC-Co) substrates can be extremely troublesome as Co can catalyze the formation of graphitic carbon. CrN-based interlayers deposited by PVD-arc technique have been recently proposed to prevent the formation of graphitic carbon. CrN films act as physical (diffusion) barriers to the Co, interposing themselves between the underlying WC-Co and the growing diamond. Yet, the adhesion of diamond coatings onto CrN interlayers is still poorly investigated. In the present investigation, Fluidized Bed Machining (FBM) is applied to pre-treat CrN interlayers onto WC-Co substrates to promote the growth on them of highly adherent diamond coatings. During Fluidized Bed Machining (FBM), CrN interlayers are submitted to high speed impacts of loose abrasives. The action of their cutting edges is able to deeply change the starting morphology of the as-deposited PVD CrN interlayers, thus promoting the establishment of a highly corrugated surface on which to grow HF-CVD diamond coatings.

Growth, morphology, adhesion and wear resistance of the CVD deposited diamond coatings onto Fluidized Bed Machined (FBM) CrN interlayers were looked into and compared to diamond coated WC-Co substrates with untreated CrN interlayers or pre-treated with two-step chemical etching (Murakami’s reagent and Caro’s acid, MC-treatment). Fluidized Bed Machining proved to be an effective technique to tailor the surface morphology and roughness of CrN films deposited by PVD-arc technique, and was found to be very useful in improving the adhesion and wear resistance of CVD diamond onto CrN-interlayers.

Diamond deposition on cemented carbides is widely used to improve the lifetime of tools, blades and components for abrasive applications. These deposition processes are performed in CVD coating units. For better initial growth and a good adhesion of the deposited films to the cemented carbide, the substrate surfaces are seeded with diamond powders before deposition.

In this work 3 different diamond cluster sizes were used for the seeding pretreatment (0 – 4 nm, 0 – 250 nm, 0 – 500 nm). Diamond Hot Filament CVD (HFCVD) was carried out on polished tungsten plates to study the initial growth steps. Further on, cemented carbide substrates with different cobalt content (6 – 12 % Co) were coated to study the growth rate and adhesion of the deposited layers. For the 4 nm powder SEM pictures showed a closed diamond film on the polished tungsten plates after 12 min deposition time. With 250 nm pre-treatment the diamond film is not completely closed yet and with 500 nm powder only few diamond crystallites can be found on the surface.

The cemented carbides are chemically etched before deposition to remove the cobalt binder from the surface. After 12 min of diamond deposition we confirmed the initial growth results of the polished tungsten discs. The deposition rate and morphology in long time deposition processes is almost the same for all pre-treatment particle sizes. The adhesion of the diamond film in the blast wear test was the same for all powders on cemented carbides with low cobalt content (6 % Co). With higher cobalt content (10 % Co) the adhesion of the diamond film increase with the decrease of particle size in the seeding powder.

We attribute this to the faster covering of the surface with a closed diamond film at the beginning, which comes along with the following benefits:

- better interlocking of diamond with the cemented carbides due to a higher number of small diamond crystallites

- fast passivation of the cobalt phase in the cemented carbide, which means less interaction of cobalt with the gas phase in the deposition process

In this study we found, that the seeding cluster size before diamond deposition has enormous effect on the initial growth rate on polished and etched surfaces. Furthermore the adhesion of the deposited diamond film depends on the cluster size for high cobalt cemented carbides.

Thermo chemical computing validates the stability of different nitrides against Co, Mo, and methane up to 1150K, showing the highest chemical stability against carburization for ZrN and TaN under static conditions.

Single zirconium and tantalum nitrides layers have been sputtered onto WC-Co substrates as diffusion barriers and buffer layers under specific reactive sputtering conditions. To improve the nuclei density of diamond during CVD processing, a thin Mo extra layer has been added (<500 nm). Nano crystalline diamond has been grown under negative biased substrates.

Metallic and ceramic materials have been widely used as intermediate layers for diamond coating on various substrates to enhance diamond nucleation, growth and adhesion. Properties of diamond formed are strongly dependent upon the chemical nature, thickness, and complex reaction between the diamond-interlayer and interlayer-substrate interfaces. Steels are known as difficult-to-coat substrates for diamond, due to the low diamond nucleation density and low interfacial bonding strength. The interlayers in use are usually as thick as several micrometers. We will report on the successful application of ultrathin dual metal interlayers, to obtain continuous, dense and well adhering diamond coatings on stainless steels.