Nanocrystalline diamond deposited on tungsten carbidecobalt substrates using hot filament chemical vapour deposition technique

Abstract

Diamond coatings on cutting tools provide the advantages of the properties of diamond in terms of high hardness, excellent wear resistance, and general chemical inertness. The main limitations of diamond coating are coating adhesion problems, high surface roughness and low production rate due to the use of coating equipment with small chamber. There is also a need to have a better understanding on the growth mechanism of diamond coatings on tungsten carbide with cobalt binder (WC-Co). Thus the motivation for this work is to obtain low surface roughness of diamond coatings while achieving good adhesion to substrates through the use of large chamber hot filament chemical vapour deposition (HFCVD) method. In this research, nanocrystalline diamond (NCD) coatings were deposited on WC-Co substrates using the HFCVD method. WC-Co was selected because it is used widely in the current tooling market. The cutting method was varied between precision cutting and electrical discharge machining EDM (Wire-Cut). It was found that precision cutting produce generally planar substrates and was the preferred method of cutting. To achieve good adhesion between the diamond coating and WC-Co substrates, the WC-Co substrates were pretreated before being deposited with diamond and some of these pretreatments parameters were varied. It was found that 20 minutes of Murakami agent treatment, 4560 seconds of acid etching and <0.25 ^m natural diamond seeding in ultrasonic bath were the best pretreatment method. The substrates were then deposited with diamond in the HFCVD chamber. Four batches of deposition were ran namely batch A, B, C and D. The overall results show that the deposited diamonds were nanocrystalline in size with cauliflower or ballas NCD morphology with various crystallite arrangements. Batch A produced generally four different types of morphologies. Type 1 was planar diamond coating morphology attributed to precision cutting effects. Type 2 was planar diamond coating morphology with micro features attributed to rough bench grinding. Type 4 was the extension of type 3 where EDM cut produced about a third tier morphology separating islands of diamond ballas aggregates. Three-tier ballas morphology improves the adhesion property where boundaries hinder failure path of the diamond coating. Batch B produced an obvious microcrystalline diamond layer under the NCD layer. Batch C produced a single layer of very thin NCD layer of only 1.7 microns. Batch D produced a layer NCD coating of about 4 microns thick by changing oxygen pulsing rate and time. X-ray diffraction (XRD) and grazing XRD showed that the diamond layer was in compression. Raman Spectrometer identified the presence of NCD. Atomic force microscope (AFM) showed the ultra-low roughness of the diamond coatings with <200 nm. Nanoindentation revealed that the NCD coating has high hardness of 10-60 GPa and reduced modulus of 40 - 300 GPa. The adhesion strength is good as indicated from the indentation test. Electron microscopy results showed the ballas consist of elongated radial grains in accordance to the thickness of the NCD coating. Further magnifications revealed diamond twins that contributed to the properties and nano size of diamond crystallites. Transmission electron microscopy analyses also indicate that the NCD nucleated and grew on the tungsten carbide (100) planes in the (1 1 1 ) direction, forming (111) planes.