ICMCTF2013 Session B1-3: PVD Coatings and Technologies

TiN and CrN based binary or ternary layers have been used for many years in order to extend the service life of tools for machining. Increasing demands in the industrial use require more efficient layer systems. According to recent studies, silicon opened promising opportunities to influence the characteristics of thin titanium or chromium-based coatings for the better. So nanocomposites as TiAlSiN have an extremely high hardness and a fine grain structure . Furthermore, by the addition of silicon, the oxidation resistance as well as the tribological properties can be increased and improved.

Since these coatings are exposed to high temperatures used in machining processes, the influence of different silicon contents of the Ti/TiN/TiAlSiN multilayer system on mechanical and tribological behavior should be investigated at room temperature and elevated temperatures. In order to adjust different coating compositions and thus modify different silicon contents, various production parameters were systematically varied and tested their effects in detail . Within these studies, the hot working steel AISI H11 was used as the substrate . This steel substrate was previously plasma nitrided to raise the hardness and hence the supporting effect of the substrate opposite to the coating. The structure, the morphology and the different chemical compositions of the silicon-doped coating were investigated by means of scanning electron microscopy with additional device for energy dispersive X-ray spectroscopy. Scratch tests were also performed in order to characterize the quality of the adhesion between the substrate and the multilayer system. The mechanical and tribological properties such as hardness, Young´s modulus, friction and wear coefficient, which were determined using nanoindentation and ball-on-disk tests, as well as the thermal fatigue behavior, which was analyzed by means of an impact tester, were examined at elevated temperatures up to 500°C.

Crystalline chromium carbide (Cr-C) thin films are known to possess excellent corrosion resistance in acidic and saline media. Growth of crystalline Cr-C by sputtering typically requires a substrate temperature above 500 °C, which prevents hardened steels and other temperature-sensitive substrates from being coated without adversities. We have observed that sputtered low-temperature films can be described as an amorphous nanocomposite with two non-crystalline phases: amorphous CrC_x and amorphous carbon (a-C). One interesting feature of amorphous Cr-C is the lack of grain boundaries which would eliminate grain boundary corrosion. On the other hand, large stable crystallites mean that corrosive attacks need to overcome a high energy threshold. The absence of carbide grains in amorphous films may therefore lower the overall corrosion resistance. The objective of this study is to investigate the influence of the deposition temperature and the composition on the microstructure of Cr-C films, as well as their corrosion resistance.

Cr-C thin films with 25-85 at% C were deposited at 20-700 °C by direct current magnetron sputtering from elemental targets under UHV conditions. XRD and TEM show that films deposited at 20–300 °C are amorphous. An increase of the temperature to 500 °C leads to a transformation from amorphous to nanocrystalline Cr₇C₃ or nanocomposite Cr-C/a-C films depending on composition. XPS shows that the amorphous CrC_x phase has a composition close to crystalline Cr₇C₃. There is no shift in the C1s core level binding energy between amorphous and crystalline Cr₇C₃ films, which indicates a similar short-range chemical order. XRD reflections from Cr₃C₂ were observed at 700 °C and this phase was accompanied by a positive shift in the C1s spectra. The corrosion resistance has been investigated by polarization curves, chronoamperometry, XPS, and SEM. The amorphous films are less oxidation resistant than the more crystalline films. Generally, higher deposition temperature results in lower accumulated charge in chronoamperometry. Films deposited at 500 °C have a 50% reduction in charge when compared to films deposited at 20 °C, and post-corrosion XPS shows a similar trend in the amount of surface oxides. More a‑C phase also lowers the corrosion current. Raman spectroscopy shows that the hybridization of the a-C phase is predominantly sp², which contribute to a relatively low hardness around 7‑10 GPa. Reactively sputtered nanocomposite films have previously been shown to yield a low coefficient of friction (0.13), which suggests that the studied films are candidates to enhance properties of steel in combined corrosive and tribological environments.

CrN monolayers along with CrN/SiC and CrN/DLC duplex layers up to 5 μm thick were grown by accelerated plasma arc deposition on 4340 steel substrates. The layers exhibit a dense and defect free microstructure. The tribological properties were evaluated with dry ball-on-disk sliding tests against an alumina counterface at room temperature. The CrN monolayer exhibits a coefficient of friction, μ = 0.28, while CrN/SiC and CrN/DLC duplex exhibit much lower μ = 0.08 and 0.06, respectively. The low friction is attributed to the top layers of amorphous SiC and DLC, respectively. The wear rates of the coatings were very low and measured as 5.3x10^-7, 7.3x10^-7, and 7.9x10^-7 mm³/Nm, respectively, for CrN, CrN/DLC, and CrN/SiC. Corrosion resistance was evaluated in both cyclical environmental testing as well as quantitatively via potentiodynamic corrosion tests in sodium chloride solution. The corrosion resistance in both cases is compared to standard electroless Ni and electroplated Cr coatings. The highest level of corrosion resistance was observed for the CrN monolayer and CrN/SiC duplex layer. Both exhibit improved corrosion resistance over electroplated Cr coatings and comparable corrosion resistance to electroless Ni coatings. This high level of corrosion resistance is remarkable for physical vapor deposited thin nitride layers. The accelerated plasma arc deposited materials show great promise for applications in which a combination of low friction, wear resistance, as well as corrosion resistance are required.

Deposition from the filtered vacuum arc plasma is the widely used effective method of manufacturing high quality protective wear resistant coatings. Ti-Al-Y-N coatings with small Y percentage have demonstrated high hardness and excellent oxidation resistance which provides their application for protection the machine parts which operate under extreme environmental conditions. In our recent work [1] we investigated structure and properties of such coatings prepared by the plasma immersion ion implantation and deposition (PIII&D) using filtered vacuum-arc plasma source.

A high-voltage pulsed bias applied to the substrate permits the deposition of thicker coatings with good adhesion and low residual stresses. I n the present work we examined the cavitation and abrasion resistance of PIII&D deposited Ti-Al-Y-N coatings doped with yttrium (≤ 1 at.%) and analyzed correlation between their properties and structure.

Ti_0.5‑xAl_0.5Y_x alloys (x = 0, 0.002, 0.004, 0.01) produced by vacuum-arc remelting were used as cathodes in the vacuum-arc plasma source. Ti-Al-Y-N coatings with thickness of 5-6 micron were deposited on the 302 stainless steel substrates from filtered vacuum-arc plasma at nitrogen pressure of 0.1 Pa.

The substrate potential was either DC (-150 V) or negative pulsed, the amplitude A_U varied in the range of 0-2.5 kV.

The erosion resistance of the coatings was evaluated on the measured mass loss during cavitation treatment in distilled water. The tests were continued until visually watched open-ended pores in the coating were formed. Abrasion wear was determined in the scheme substrate plane – rotating abrasion disk. Adhesion properties, friction coefficient and wear resistance were determined using equipment by CSM Instruments.

The substrate potential during deposition process and Y content in the coatings were found to be important factors influencing the rate and character of their cavitation damage. The coating deposited at A_U=0 was subjected to pitting erosion. Long cracks prevailed on the surface of the cavitation treated coating deposited at DC (-150 V) potential. High voltage pulsed substrate potential contributed to decrease by 3-5 times in the rate of the mass loss under cavitation, quantity of the erosion defects on the treated surface diminishes sharply. Increase of Y content resulted in improvement of wear durability. Mean rates of cavitation and abrasion wear of (Ti,Al)N+1at.%Y were several times lower than that of (Ti,Al)N and TiN.

[1] V.A. Belous et al. Surf. Coat. Technol. 206 (2011) 1720.