Performance Of Diamond Coatings Research Articles

Research on the innovative pretreatment to improve the adhesion and performance of diamond coatings

In order to improve the quality of diamond coatings, this paper combined the chemical etching and buffer layers to deal with cemented carbide with cobalt (Co) content of 8%. Diamond coatings were prepared with hot filament chemical vapor deposition (HFCVD). The surface morphology, quality and wear property were explored by using scanning electron microscopy, X-ray diffraction, laser Raman spectrometer, and friction and wear tester. Compared to the sample with no pretreatment, samples after pretreatment have better surface morphology. There are no obvious impurities as well. Diamond crystals are uniform and compact. Friction and wear test indicated that the friction coefficient is lower than 0.1. Samples after pretreatment are much lower and steady among them. The sample using tungsten(W)-molybdenum(Mo) layer has the best surface morphology and the lowest friction coefficient. The result shows that the pretreatment combined the chemical etching and buffer layers cannot only act as the diffusion barrier, but also can improve the adhesion between substrate and diamond coatings. The effect of composite buffer layer is better than single metal buffer layers.

Research on the growth state and performance of diamond coatings based on different carbon source concentration and temperature

In this paper, diamond coatings with different microstructure were prepared on cemented carbide substrates by HFCVD technique. The changes of grain growth mode and size of diamond coating under different substrate temperature (800 °C, 850 °C, 900 °C) and carbon source concentration (2%, 4%, 6%) were studied, and the differences of coating performance were explored. Both substrate temperature and carbon source concentration have an effect on grain size. When the grain size is in micron scale (0.5 ∼ 6 μm), the growth mode is columnar crystal competing growth. When the substrate temperature reaches 900 °C and the carbon source concentration is 6%, the diamond grain will be fully secondary nucleated, and the grain size will reduce to the nanometer scale (20 ∼ 50 nm). The growth mode is particle cluster accumulation growth and the rate of growth increases significantly. The increase of grain size will increase the adhesion of the coating and its resistance to wear and external loading. However, it may lead to larger friction coefficient, which is about 0.1. Due to the small grain size, the adhesion force of nanocrystalline diamond coating is obviously smaller than micrystalline diamond coating, but the friction coefficient is only 0.03.

Run-in behavior of nanocrystalline diamond coatings studied by in situ tribometry

The friction performance of nanocrystalline diamond coatings was evaluated using in situ tribometry with sapphire counterfaces. Coatings were grown by microwave plasma assisted chemical vapor deposition in an Ar–H–CH 4 plasma, with H ranging from 0 to 36%. In situ examination of the sliding contact, combined with ex situ analysis of the sapphire counterface revealed that the velocity accommodation mode was interfacial sliding of a carbonaceous transfer film versus the coating wear track. For most tests, the contact diameter increased during the first 50 sliding cycles and then remained constant. The in situ measure of the contact diameter was found to correlate confidently to ex situ measurements of counterface wear. The performance of the diamond coatings, characterized by quick run-in to low friction was best when a small but detectable graphite peak was present in the X-ray diffraction (XRD) profile. The relative intensity of the XRD graphite peak was also found to directly correlate with the peak position of the C1s → π* transition as measured by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Increasing the relative amount of graphite-bonded sp 2 carbon in the NCD films decreased run-in cycles to low friction.

Preparation and performance of diamond coatings on cemented carbide inserts with cobalt boride interlayers

Abstract A novel two-step process has been developed, with a view to improving the reproducibility of techniques for preparing diamond-coated cemented carbide inserts. Firstly, diffusing boron into WC–6% Co inserts produced an interlayer of cobalt boride, and then diamond films were deposited on this interlayer using a DC arcjet plasma system. Investigations showed that the formation of the cobalt boride interlayer favored the growth of high quality diamond films, and the interlayer functioned effectively as a diffusion barrier layer, inhibiting outward Co migrations from the insert interior to the surface. Adhesion of the boronized interlayers as well as the composite coatings was evaluated by using Rockwell A indentation and end-milling tests, respectively. It was demonstrated that both the Co–B interlayer and the diamond coating thereon were very adherent to the substrates, and a 10-fold increase in tool life could be expected with the coated inserts compared with uncoated ones.