ICMCTF2015 Session F4-2: Functional Oxide and Oxynitride Coatings

Oxidation contributes to wear phenomena of any component or tool exposed to extreme environments. Driven by the steady increase in metal machining efficiency, the conditions of use of coated cutting tools give rise to higher and higher cutting edge temperatures, which tend to promote oxidative wear.

Even basic ceramic coatings deposited by CVD or PVD are much better suited to handle oxidation compared to the base material, typically cemented carbide. This substrate needs to be efficiently protected by the coating, which thus needs to provide a combination of diffusion barrier properties, moderate to high temperature abrasion resistance and a sophisticated level of surface oxidation resistance. Of particular interest for rotary cutting tools, and to an increasing amount as well for indexable cutting inserts, are high-temperature resistant PVD coatings, that do not challenge the substrate by adverse thermal effects during deposition.

For many years already, and both for Ti and Cr-based nitride coatings, the addition of aluminum alone or in combination with elements such as silicon, yttrium or vanadium has been very successful for cutting tools. However, from a certain usage temperature point onwards, even these advanced nitride-based approaches fail and a more efficient oxidation barrier needs to be found. Such enhanced PVD layers were identified in the family of oxynitride and oxide coatings, which have been introduced to industrial use during the recent years.

This contribution outlines the fundamental differences in high-temperature behavior between nitrides and oxygen-containing coatings such as AlCrON and other oxynitrides on the one hand, and pure oxides such as α-(Al,Cr)₂O₃ or Zirconia on the other hand. Coatings produced by different PVD techniques, with an emphasis on reactive cathodic arc evaporation, will be compared in terms of phase stability, stress and hardness changes, and wear resistance. The main factors governing high-temperature stability and oxidation resistance are outlined. These physical data are complemented by well-controlled high-temperature tribology tests.

Optimized oxynitride and oxide coatings were deposited on industrial platforms using the sophisticated target surface poisoning control provided by the lateral (LARC^®) cylindrical rotating arc cathodes PVD technology. Cuttings test of high-performance tools at elevated temperatures, such as dry rough milling of mild and tool steels using indexable cutting inserts, will be shown in order to demonstrate the potential of oxidation- and high-temperature wear control of these coatings.

α-Al₂O₃ made by PVD-processes has been a long sought goal in coating development for the cutting tool industry. Different approaches have been taken throughout the years, such as alloying with materials stabilizing the corundum phase, i.e. Cr, as reported in [1, 2, 3]. Having achieved α-(Cr, Al)₂O₃ phase from compound targets, there are still many problems to solve with respect to process stability and coating quality. The reactive arcing of mixed AlCr targets in oxygen creates a large amount of droplets as well as target poisoning [4], which leads to a reduced film quality and uneconomical use of target material, especially on an industrial scale. A recent discovery shows that Si can be added to the AlCr-material in order to stabilize the process, creating a more even erosion of the cathode, without Si being added to the film [5]. We find, however, that Si can be incorporated into the film under similar conditions as in [5], which changes the coating properties, such as crystallographic structure and hardness. There is also an ongoing discussion about where Si is located in Si-doped pure Al₂O₃. Nahif et al [6] propose that segregation of SiO to the grain boundaries is the most plausible option.

The aim of this work is to study the effects of Si incorporation in the oxide coatings for two different amounts of Si and two Al/Cr ratios, as well as the resulting coating properties. XPS and EDX show that Si is incorporated into the coating with concentrations close to target levels of 5 and 10 at% Si with Al/Cr ratios of 1 and 2.8. The α-(Cr, Al)₂O₃ coating, free from silicon, change phase to alleged cubic B1-like (Cr, Al)₂O₃ upon incorporating silicon. The coating hardness is reduced when adding Si to the coating.

Our preliminary XPS analysis shows that the Si is present in the film in several different chemical environments; bonded to oxygen, as well as to Cr, possibly in CrSi intermetallic compounds. We also use high-resolution TEM to elucidate the location of Si in the ternary (Al, Cr, Si) oxide alloys.

[1] Ramm, J., et al., Surface & Coatings Technology, 2007. 202(4–7): p. 876-883.

[2] Khatibi, A., et al. Acta Materialia, 2013. 61(13): p. 4811-4822.

[3] Edlmayr, V., et al., Thin Solid Films, 2013. 534(0): p. 373-379.

[4] Pohler, M., et al., Surface and Coatings Technology 2011. 206(6): p. 1454-1460.

[5] Paulitsch, J., et al., Vacuum, 2014. 100: p. 29-32

[6] Nahif, F., et al., Surface & Coatings Technology, 2013. 235: p. 250–258

Closed loop reactive process control has been used for many years to reactively sputter materials such as aluminum oxide (Al₂O₃) and silicon dioxide (SiO₂) at high deposition rates. The introduction of a second reactive gas, however, such as nitrogen, can cause difficulties with process control and maintenance of coating stoichiometry. These difficulties are caused by the different rates of reactivity of the individual gases with the target material as well as their combined influence on the intensity of optical emission lines in the plasma itself. Using a spectrometer, several optical emission lines could be used for process control, however, integration times tend to be long and can result in the lack of process stability. Direct measurement of optical emission lines at the target face also presents difficulties due to plasma fluctuations in batch systems related to substrate movement. Using a Penning gauge as a remote source of plasma in combination with fast optical detection is a robust method for creating coatings such as aluminum oxynitrides (AlO_xN_y) with virtually any stoichiometry. HIPIMS discharges can also be difficult to monitor directly due to low duty cycles as well as plasma fluctuations. Two control schemes for reactive HIPIMS sputtering will be also presented.

Hydrogen pressurized hollow glass micro spheres in combination with NaBH₄ (sodaborohydrid) hydrolysis bear the potential of storing hydrogen in feasible amounts and could reach storage densities up to 12 wt% or 85 kg/m³ (theoretically). Therefore the approximately 40 µm diameter spheres are heated up and pressurized with hydrogen at 70 MPa, so hydrogen diffuses into the spheres. After the spheres are cooled down, hydrogen can be stored at room temperature without excessive security measures. To release the trapped hydrogen heat has to be applied again to reach the temperatures of about 120 °C. To reach this temperature an exothermal chemical reaction can be used, in this case a NaBH₄-water reaction, which produces hydrogen as a most welcome by-product. This chemical reaction has to be initialized by a catalyst deposited on the hollow glass microspheres.

To apply the catalytic coatings on the hollow glass microspheres by magnetron sputtering a special coating system was used. The catalytic behaviour, the film thickness and the film durability of Ru, Pt, Ti, TiO₂ and combinations from them were investigated. To characterise the catalyst the amount of released hydrogen and the reaction temperature were measured with a custom built experimental setup. After one test the spheres were washed, dried and reused to investigate the stability of the catalyst. Since the activity of the catalyst decreased a method to reactivate it was needed to be found.

Ruthenium films showed the best catalytic performance of the tested coatings, e.g. 90% H₂-yield with ruthenium as compared to 20% H₂-yield with Pt. However, it became clear that ruthenium films have very bad adhesion behaviour, which was solved with an interlayer between the catalyst and the hollow sphere. Current work focuses on the long time stability of the catalyst and on finding an inexpensive catalyst material.

This work was supported by the Austrian Science Fund (FWF), project "CatSphere", grant number P-22718.

In this work, we present the structural, chemical, morphological and optical properties of BiVO₄ thin films produced by dual magnetron sputtering using both Bi₂O₃ (a-phase, 99.98% purity) and V (99.9 % purity) targets under and argon-oxygen atmosphere. A wide range of deposition conditions were tried changing basically the power applied to the individually driven targets (RF for Bi₂O₃ and DC for V) and the substrate temperature (T_S). The structural characterization of the films done by X-ray diffraction indicated that none of the as-deposited films showed the BiVO₄ monoclinic phase. However, after thermal annealing at 400 ^oC, the films deposited with the V target power about 3-3.5 times the power applied to the Bi₂O₃ target, presented the monoclinic phase. Thus, films were deposited on both glass and transparent conductive oxide substrates and submitted to the thermal annealing to obtain the monoclinic-BiVO₄ phase. Two photoactivated processes were studied; the photocatalytic degradation of methyl orange dye and the photo electrochemical current under white light illumination. The photocatalytic activity was not as good as expected, while large photocurrents (above 0.5 mA/cm²) were obtained.

Smart self-healing coatings attract increasing research interest for they could provide great cost reduction of maintenance and longer service life. In this research, plasma electrolytic oxidation (PEO) coatings are produced on magnesium alloy in a halloysite nanoclay modified alkaline electrolyte comprising 12g/L Na2SiO3, 4g/L NaF, 2g/L KOH and 10g/L halloysite nanoclay. Benzotriazole was loaded into halloysite nanoclay as the healing agent assuming that the repairing process will be triggered by the dissolution of PEO coatings under the condition of corrosion, resulting in the release of the healing agent. Scanning electron microscopy (SEM) studies show that the halloysite nanoclay has been successfully incorporated in the matrix of the PEO coatings, which according to the XRD phase analysis is mainly composed of MgO and Mg2SiO4. To better understand the self-healing capacity of PEO coatings, the corrosion process was studied using electrochemical impedance spectroscopy (EIS) with frequency swept from 10-2 to 105 Hz, as well as long term corrosion test in a 3.5 wt.% NaCl aqueous solution. It is found that the produced PEO coatings can effectively reduce the degradation rate and provide excellent corrosion resistance.

Keywords: plasma electrolytic oxidation, anti-corrosion, self-healing, halloysite nanoclay