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Strengthening the Research and Technological Capacity in the Field of Nanostructured Thin Films, Hard and Superhard Coatings

Final Report Summary - RTCNANOHARD (Strengthening the Research and Technological Capacity in the Field of Nanostructured Thin Films, Hard and Superhard Coatings)

The improvement of the technology capability of deposition of nanostructured thin films, hard and superhard coatings has been achieved by activities related to upgrading the equipment available at IAP and realisation of feed-back between characterisation and technology of the coatings. After the equipment renewal and upgrade, the work of the team was focused on obtaining the reproducible technological regimes for deposition of nitrides and carbides of transition metals and their combinations being of interest for the industry.

The coatings have been obtained by cathodic arc deposition (CAD) technique as a PVD variant outperforming the other techniques by its higher degree of ionisation, the coating density and adhesion and the overall process efficiency, a crucial issue in industrial applications. However, the presence of macroparticles in the layers deposited by this method deteriorates their quality.

To improve the coating morphology by decrease of the macroparticle size and quantity and increase the process productivity, two new arc evaporators with controlled magnetic field (CMF) have been mounted into the CAD machine available at the Institute of Applied Physics in addition to the existed permanent magnetic field (PMF) evaporators.

Technology for deposition of TiN, ZrN and CrN single-layered coatings has been developed. These coatings have passed the R&D stages and they are ready for industrial applications. Various binary and ternary metallic coatings such as TiAlN, TiCN, ZrNC, TiZrN have been obtained and investigated also. Both single-layered and compounds were deposited on different types of substrates, namely stainless steel, high-speed steel, tool steel and silicon.

Special attention was paid on obtaining the optimal technological regime for each developed coating and its reproducibility. Therefore several sets of experiments for deposition of Ti, Cr and Zr nitrides and carbides as well as their combination ((TiAl)N, TiCN, ZrNC, TiZrN) were performed at different technological regimes. Strong dependence of coating properties on the physical characteristics of the vacuum arc and induced plasma, which is determined significantly by the design of the vacuum arc deposition system, has been obtained.

Different coatings such as TiN, TiNC, AlTiN, ZrN, ZrNC, deposited by permanent magnetic field and controlled magnetic field evaporators have been characterised and compared. The technology conditions such as vacuum pressure, gas ratio, deposition temperature, deposition time, cathode current and the effect on the coating properties were investigated. This dependence has been observed for all developed coatings. The characterisation was performed in accordance with the project physical indicators by controlling the following main coating characteristics: thickness, hardness, adhesion, surface morphology.

The coating thickness was determined using a Leitz incident-light microscope interferometer with accuracy of 90 nm. Control measurements were made with a tip profilemeter in order to precise the measurement. The found deviations between both type measurements were in the measurement accuracy. Control specimens with coatings of nitrides, carbides and carbonitrides for thickness measurement were prepared in the same technological cycle together with the coatings deposited on stainless steel and intended for other characterisations.

The thicknesses varied in the range 0.5 - 5 µm depending on the deposition time and were found to be with very good reproducibility. The measurements of a number of films showed stable linear dependence of the coating thickness on the deposition time at full control over the other parameters of the deposition process. The deposition ratio was also calculated. It depends strongly on the cathode material and cathode current, while it does not affected by the vacuum pressure and gas ratio.

The coatings hardness was determined using a Vickers hardness tester with a load ranging from 20 to 200 g. The coating image was observed on a Leitz microscope. The hardness of the coatings deposited on stainless steel substrates (H = 4 GPa) varies in the range 13 - 37 GPa depending on the chemical composition of the coatings, i.e. the ratio of the reactive gases, and the film thickness. At small film thickness the influence of the substrate is significant and the determined hardness is lower, respectively. The coating thickness increase leads to increase the measured hardness, i.e. the effect of the substrate decreases and at very thick films could be eliminated. Hence the usual thickness of coatings for industrial applications should not be lower than 2.0 - 2.5 µm. The experiments carried out showed that the coating adhesion depends on the film properties (thickness, microhardness) as well as the substrate characteristics (material, surface quality, hardness). Surface morphology and composition have been examined using scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy.

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