Such Characterisation of the CVD grown PCDs is very essential besides its synthesis. Some techniques which are frequently used for powder samples like, BET surface area analysis, TGA-DSC, Zetasizer particle size measurements are being reported here for the first time, for CVD polycrystalline diamond coatings. Moreover, black and white grades of diamond have been grown and characterised so far, but their corresponding growth defects have not frequently been reported . Here, authors have tried to elucidate the CVD growth defects that are present in their CVD grown diamonds. Afterwards, the PCD samples were polished to make them effective for tribological applications. Later on, such polished diamond surfaces have been mechanically characterised for studying tribological action against silicon nitride balls under machine oil lubricants for the first time.

2. Materials and methods

2.1 Processing of polycrystalline diamond (PCD)

Polycrystalline diamond samples were fabricated with CSIR-CGCRI based DT1800 microwave plasma enhanced CVD (MPCVD) reactor on single crystal Si wafers [115]. Deposition was carried out for 4–10 days to grow 0.5–0.8 mm thick diamond coatings. Reactor pressure and temperature were maintained in between 110 and 120 Torr and 900–1100°C, respectively. 1–3% methane in hydrogen gas mixture was used for growing diamond coatings with 9 kW input microwave power. Afterwards, PCDs were made freestanding by wet chemical etching of Si wafers with 1:1:1 ratio solution of HF:HNO3:CH3COOH [116]. Such freestanding diamond wafers were cut in 6 mm diameter smaller discs by laser cutter (Hallmark Plus model. Nd-YAG lamp pumped laser with 10 watt power at 1064 nm wavelength). The diamond sample was then hot mounted to polish the as-grown rough surface [117]. Mechanical polishing was done using Leco, Germany polisher and subsequently CP4 model, Bruker, USA machine was used for chemo-mechanical polishing of the PCD samples. Metal bonded diamond disc and Cu-bonded diamond discs were used with different grit sizes for mechanical polishing with water as cooling agent. On the other hand, the chemo-mechanical polishing of diamond samples was carried out against alumina ceramic disc using K2S2O8 solution and concentrated sulphuric acid along with diamond pastes. Separately, annealing of few diamond samples was also done at 600°C for 1 h in standard air furnace.

2.2 Physical characterizations of PCDs

The detonation nano diamond (DND) particle size of the seeding slurry was measured by Malvern make Zetasizer instrument. Brunauer-Emmett-Teller (BET) surface area analyser (Quantachrome Instruments) was used to measure the surface area; and nitrogen adsorption isotherm (Barrett-Joyner-Halenda or BJH method) was used to measure the pore size and volume in the freestanding diamond coatings. The samples were degassed under vacuum at 100°C for 1 h prior to measurement. Thermogravimetric (TGA) and differential scanning calorimetry (DSC) analysis of the freestanding diamond sample was done at a heating rate of 10°C min−1 on a simultaneous thermal analyser (STA 449F, Netzsch, Germany). X-ray diffractograms (XRD) were recorded in the 2θ range 30°−100° at a slow scanning rate of 1° min−1 by an X-ray diffractometer (Philips X’Pert, The Netherlands) with Cu Kα radiation (at 40 kV and 40 mA). Transmission electron microscopy (TEM-Tecnai G2 30ST, FEI Company, USA) was used to evaluate the defects present inside the diamond lattice. FTIR/FIR spectrophotometer (Model: Frontier IRL 1280119, Perkin Elmer) attached with reflectance measurement arrangement with incident and reflectance angles set at 22.5°, was used for studying opaque and translucent PCD samples. The photoluminescence (PL) studies including emission and excitation measurements were carried out on a fluorescence spectrophotometer (Model: Quantum Master, enhanced NIR, from Photon Technologies International) using a xenon arc lamp of 60 W as a pump source. Raman spectra were obtained using a STR500, Cornes Technologies, (formerly known as Seki Technotron) micro-Raman spectrometer, with excitation by argon ion (514.5 nm) laser. The resolution was about 1 cm−1 with 1200 grating size. In all the Raman experiments, the laser spot size was 1–2 μm and an exposure time of 20 s was used which was repeated thrice to acquire the Raman signals during each measurement.