Abstract

Characterisations of polycrystalline diamond (PCD) coatings have routinely been done over the past three decades of diamond research, but there is less number of reports on some of its very unique properties. For example, diamond is the hardest known material and, in probing such hard surfaces with any indenter tip, it may lead to damage of the instrument. Due to such chances of experimental accidents, researchers have performed very few attempts in evaluating the mechanical properties of PCDs. In the present work, some of these very special properties of diamond that are less reported in the literature are being re-investigated. PCDs were characterised by photoluminescence (PL), Fourier transform infrared (FTIR) spectroscopy, transmission electron microscope (TEM), and X-ray diffraction (XRD) techniques. The diamond surface was also polished to bring the as-grown micron level of surface roughness (detrimental for wear application) down to few hundreds of nanometer. The tribological properties of such polished and smooth surfaces were found to be appropriate for wear protective coating application. This chapter revisits some of the unreported issues in the synthesis and characterisation of PCD coatings grown on Si wafer by the innovative 915 MHz microwave plasma chemical vapour deposition (MPCVD) technique.

1. Introduction

CVD grown diamond is an important class of material . The characteristic of such material is very much dependant on the CVD processing conditions  and as well as on the post-processing steps that are equally significant for the efficient use of this material in engineering applications . There are environments where it is exposed to heat under extreme conditions or it is rubbed against hard ceramic surfaces. Diamond is the hardest known material with very low co-efficient of friction which naturally makes it a suitable tribological surface .

But the as grown diamond surface is very rough in nature which is required to be planarised before putting into real environments. It is well understood nowadays that the diamond nucleation and growth takes place first by coalescence of the seeded layer on the substrate surface and then on top of which diamond crystals grown in columnar fashion. Such vertical growth of crystals causes the top surface to be very rough and simultaneously there may be some inherent porosity present due to rise of such vertical columns. On the other hand, the nucleation side of the freestanding diamond may have some porosity due to random coalescence of the islands during CVD processing. There is no such report of studying the surface area and porosity of CVD grown polycrystalline diamond (PCD). Here, it has been attempted for the first time to evaluate the pore size that may be present in the diamond coatings grown by CVD, using Brunauer-Emmett-Teller (BET) technique.

Moreover, tribological action may cause rise of temperature under humid atmospheric condition for diamond material to degrade. So it is necessary for the CVD grown diamond to be well characterised for the wear and friction applications [30, 31, 32, 33, 34]. Thermal stability has been studied so far by many authors [35, 36, 37] but it lacks proper assessment of oxidation temperature [38, 39, 40] since only standard furnaces have been used so far. Here, thermo-gravimetric analysis (TGA) and differential scanning calorimeter (DSC) techniques have been used for the first time ever to pinpoint the augmentation of oxidation.

substitutional defects are studied by  photoluminescence (PL) , Fourier transform infra-red (FTIR) spectroscopy techniques [59], whereas the point, line, plane or volume defects during CVD growth can be viewed under transmission electron microscopes (TEM) . Post-processing steps such as annealing may be required to remove some of these defects . Annealing also makes the diamond to become purer in respect of graphitic inclusions. Raman spectra  reveal the phase purity of CVD grown diamonds. Polishing is routinely done to reduce as-grown diamond surface roughness. Tribology of such polished surfaces has been studied in detail but there is lack of literature  against silicon nitride ceramics  under machine oil lubrication. Moreover, due to very hard and rough top surface of CVD grown PCD, researchers are hesitant to probe the as-grown PCD surface with their expensive Berkovich or other indenters . In effect, there is not much literature  available which describes the mechanical properties  of CVD grown PCDs.

Methane or other precursor gas compositions  determine the defects present inside the diamond crystals. For example, high methane concentration may cause many CVD growth defects which render the grown coating to become black/grey; whereas the cleaner processing conditions of low methane percentages etc. may cause the diamond coating to be white or transparent. Similarly boron or nitrogen in the precursor gases make the diamond blue  or yellow  in colour due to the substitutional atomic defects introduced in the diamond lattice .