There are several processes to generate thin films on diamond particles. These include electrochemical methods (eg electroplating and electroless-plating), chemical vapour deposition (CVD) or physical vapour deposition (PVD). All these processes have advantages and disadvantages. Electrochemistry has limited application with respect to the choice of the coating materials. PVD requires extra effort to obtain homogenous coatings. CVD needs suitable precursor gases and a fluidised bed to obtain homogenous coatings.

An Austrian research group has looked at coating diamonds with chromium, molybdenum and tungsten using a thermo-diffusion technique developed by Yu Hui at ARC in Seibersdorf. The team used a focused ion beam in conjunction with energy dispersive spectroscopy (EDS) as part of the analysis of the coatings.

Current limitations for commercial coated diamonds lie both in the coating materials - only titanium, chromium or silicon - and in diamond sizes - mostly from 30 to 60 US mesh . Considering the requirements of future work on diamond-based composites, molybdenum and tungsten were specially selected to investigate the effects of the process parameters on the coating for coarse and fine diamonds of 40/45, 120/140, and 325/400 US mesh. Phase determination of the coated diamonds was performed by means of X-ray powder diffractometry (XRD).

Before the publication of this report, detailed information on the effects of heating conditions on the composition and the thickness of the diamond coating, both very important factors influencing thermal performance of diamond-based composites, was sparse.

The metallic chromium, molybdenum, and tungsten were deposited on the diamond surfaces by thermo-diffusion under different coating conditions in order to modify the composition and the thickness of the metal coatings.

The heating conditions were 60 minutes, 10-6 mbar, and the temperature was in the range from 750°C to 950°C. It can be seen that the starting formation temperature depends on the type of the metal, and the full coating temperature is 950°C for any metal for a fixed holding time of 60 minutes.

When heating conditions were 950°C, 10-6 mbar, and corresponding holding time from 30, 60 to 180 minutes, it can be seen that the 60 min is a good time for any metal at a fixed temperature of 950°C. It is evident that a short holding time results in either a thin or no coating and there is a certain dependence of the coating formation on the diamond size observed.

The surface morphology of the Cr, Mo, and W-coated diamonds heated at 950°C, 10-6 mbar for 60 min was observed by an optical microscopy. These coatings evenly cover the diamond surface.

Since all three elements are carbide forming and the deposition takes place at high temperatures, a certain reaction between the coating element and the diamond is expected. X-ray diffraction was used to identify the phase composition of the layers formed. It was seen that the metal carbides such as Cr3C2, Mo2C, and W2C are successfully formed on the diamond surfaces for the Cr, Mo, and W metal coatings respectively.

The focus ion beam offers the ability to design, sculpt, mill or pattern nano- and micro-structures on different materials with a spatial resolution down to 20 nm. The principles of operation of FIB and SEM are conceptually identical [5]. At the top of the electro-optical columns there is a source of charged particles: ions for the FIB and electrons for the SEM. A series of electrodes, electron lenses and mechanical apertures extract the charged particles and focus them into a beam with the desired characteristics: either a "large" beam of high current or a "small" beam of low current. The position of the beam over the sample is controlled by the deflection plates which provide the scanning feature of both instruments. Finally, the amplitude of a secondary signal, generated by the beam-sample interaction, is displayed synchronously with the beam position to form an image of the scanned area. By SEM navigation the exposed cross-section will be obtained by FIB milling over the dashed rectangular zone.

In order to analyse coated diamonds it is necessary to prepare a cross section of an individual coated diamond grain. A focused ion beam system (FIB) attached to an EDX scanning electronic microscope was employed for the layer analysis related to the morphology and elemental analysis of the diamond coatings heated at 950°C, 10-6 mbar for holding times of 30 or 60 min. The diamonds used were 40/45 mesh.
The cross section on an individual diamond grain was prepared by the ion beam milling and polishing for 3-8 hours which depends on the layer thickness. The morphology of the cross section was observed by SEM. The elemental analysis of the coating was made by energy dispersive spectroscopy through a line scan.

Alternation of content
The layer morphology and element analysis results from the line scan of the chromium coating, (holding time: 60 min) are shown in SEMs where the lower black zone is a diamond, and the upper zone is the coating. A vertical white line indicates the scanned area, and the scan direction is from the top of the coating to the surface of a diamond. A red line corresponds to a line distribution of carbon content, and a green line corresponds to chromium. The alternation of an elemental content can be observed from the upper to the lower.

Inner structure accessible
A scan result of the interface between the upper white zone and the lower diamond, shows there are gradually reducing chromium and enhancing carbon from the upper to the lower. This indicates that this zone or inner layer is the mixture of metallic chromium and chromium carbide.

Meanwhile, as a comparison, the chromium coated diamonds supplied in the market were also investigated in detail. With FIB technique the inner structure of a zone becomes accessible and the buried layers and interfaces are exposed showing the lower black zone is a diamond.

There are two layers observed on the diamond's surface. According to the results of SEM and EDX the upper layer or outer layer is pure metallic chromium, and the bellow layer or inner layer comprises of the metallic chromium and the chromium carbide. For the W-coated diamonds, diamonds (40/45 mesh) were coated at 950°C for different holding times. The white line indicates the scanned zone, and the direction of a line scan is from the top of the coating to the surface of a diamond. The change of an elemental content can be observed from the upper to the lower.

After a coated diamond was obtained after holding for 30 minutes, one layer of pure metallic tungsten is present on the diamond surface. It is evident that a relatively short holding time does not allow significant carbide formation. By increasing the holding time to 60 minutes, one layer of partial metallic tungsten plus partial tungsten carbide is observed on the diamond surface.

According to the investigations described above, the phase compositions of layers will be modified when the holding time is gradually increasing under a fixed, correct heating temperature.

The FIB method was testified to be very effective in analysing the interface and the coating composition of a coated diamond crystal. The main achievements and conclusions can be drawn as follows:
(a) Coating formation is determined by the metal type. The phase transformation of the coating is thermo-diffusion-controlled; the phase composition of the coating depends on the heating temperature and the heating time.
(b) With increasing the heating time the composite phases of the coating will be modified from a pure metallic layer to a pure carbide layer, and to two layers (the inner layer is the mixture of the metal and the carbide, the outer layer is the pure metal).
(c) Optimum heating conditions were found at the 950°C, 60 minutes, 10-6 mbar experimental testing conditions used.

Future work will be focused on the usage of coated diamonds in the diamond based composites and on the characterisation of their effects on the mechanical and thermal behaviours of the composites prepared.