ICMCTF2014 Session B1-2: PVD Coatings and Technologies

Arc deposition from compound cathodes is today a common method for synthesis of a wide range of functional multi-element coatings. It is known, that the kinetic ion energy is a crucial plasma parameter for the structural evolution of the film. Therefore, an increased understanding of the plasma generation from compound cathodes is highly motivated. In this work, unfiltered DC arc plasma from Ti-C, Ti-Al, and Ti-Si cathodes were characterized with respect to charge-state-resolved ion energy (velocity). We found that the most likely ion velocity is independent on mass, and hence the same for all ion species and ion charge states originating from the same cathode. Therefore, measured difference in kinetic energies can be inferred to the difference in ion mass, as E=mv²/2. It was also discovered that plasma generated from the cathodes with increased concentrations of C and Si resulted in more energetic ions. This effect may be explained by the “Cohesive energy rule”, where single-element (C) and binary (Ti_1-xC_x, Ti_1-ySi_y) phases of higher cohesive energy generally result in increasing ion energies. The collected results are consistent with a gas-dynamic mechanisms of ion acceleration based on pressure gradients, and suggest that the addition of C, and Si into Ti cathodes increase the pressure in the arc spots which in turn lead to increased ion velocities/energies. This is supported by an obtained most likely ion velocity of 1.47, 1.76, and 1.81 (∙10⁴ m/s) for ions from Ti, Ti_0.75C_0.25, Ti_0.75Si_0.25, respectively. For Ti ions this correspond to an ion energy of 53.8 eV, 77.1 eV and 81.5 eV, respectively.

TiN films were deposited using four cathode coating system equipped with power supplies for High Power Impulse Magnetron Sputtering (HIPIMS) and power supplies for Direct Current Magnetron Sputtering (DCMS). In this study, the coatings were produced using different combinations of HIPIMS and standard magnetron sputtering sources. Increasing the number of HIPIMS sources involved in the combined process proved to be an effective tool to manipulate the ionisation degree in the plasma. Optical emission spectroscopy revealed that the intensity ratio of Ti¹⁺:Ti⁰ in the plasma increased with increasing the number of HIPIMS sources involved in the process from 0.09 to 1.93 for pure DCMS and pure HIPIMS respectively.

TiN coatings phase composition, microstructure texture as well as residual stress, and corrosion properties were studied by number of surface analyses techniques such as X-Ray Diffraction (XRD), Fracture Cross Sectional Scanning Electron Microscopy, (XSEM), Transmission Electron Microscopy, (TEM) as well as RAMAN spectroscopy.

It was revealed that in mixed HIPIMS - DCMS processes the residual stress can be controlled in vide range from -0.21 GPa to -11.35 GPa by smart selection of the degree of HIPIMS utilisation, strength of the electromagnetic field of the unbalancing coils of the machine as well as the bias voltage applied to the substrate.

XSEM analyses revealed that the fracture morphology changes from open columnar to dense almost glassy morphology when the number of HIPIMS sources in the process increases due to the higher ionisation and therefore higher mobility of the condensing species. Furthermore, higher ionisation with HIPIMS dominated processes leads to change in preferred film orientation from (111) to (200).

Significant improvement in corrosion performance was achieved by increasing the number of the HIPIMS sources involved in the process. Raman spectroscopy was used to analyse the nature of the corrosion products as well as to estimate the extent of the corrosion damage. When polarised around E_corr values (-1 V to + 200 mV), pure DCMS and 1 HIPIMS + 3 DCMS coatings exhibited iron and chromium oxide (Fe₂O₃, Cr₂O₃) peaks at ~495 cm-1 and ~ 550 cm-1 in the spectra indicating that the corrosion has reached the substrate. No corrosion products were found in the above potential range for 2 HIPIMS + 2 DCMS and pure HIPIMS coatings demonstrating the advantages of the denser structures.

Mixing HIPIMS with DCMS is also seen as an effective tool for improving the productivity of the deposition process.

Many recent studies have been investigating ternary carbonitrides including metals or metalloids (like TiCN, NbCN and BCN) for their potential as wear resistant coatings in several applications. The adjustability of their properties, such as hardness, coefficient of friction, wear resistance, and temperature stability is especially promising. Different deposition methods are used to grow thin films for wear protection purposes and it is agreed that structure has a strong influence on their mechanical and chemical properties.

The short pulses with very high power density applied in High Power Impulse Magnetron Sputtering (HiPIMS) result in a higher ionization rate of the sputtered material compared to conventional DC magnetron sputtering. Therefore, HiPIMS technology can have a beneficial influence on the film structure and provides a promising approach to tailor the properties of ternary coating systems.

This work focuses on the influence of chamber pressure and nitrogen to argon ratio during the deposition of ternary carbonitride coatings on their composition and structure. The films were grown on high speed steel and silicon substrates with DC-MS and HiPIMS and were characterized using X-ray photoelectron spectroscopy and X-ray diffraction. Topography and morphology were examined by scanning electron microscopy. Furthermore, hardness and adhesion were measured using nano indentation and Rockwell indentation and the correlation with the structure was investigated. The differences in composition, structure and hardness we observed between DC-MS and HiPIMS sputtered coatings are discussed within this publication.

An overview will be given about the development of both technologies during the last 20 years from basic processes in the laboratory scale to the industrial applicable deposition source. Just as well the combination of both technologies in form of the laser assisted pulsed arc deposition process (Laser-Arc) will be presented. The advantages of this combination are presented with resprect of introduction for high volume coating of parts and tools.

Mainly advantages of the Laser-Arc technology are to have a very controlled pulsed arc deposition technology with a high deposition rate (2 µm/h twofold rotating axes of a planetary). By the laser controlling of pulsed arc evaporation a longtime using of the applied rotating graphite cathodes is guaranteed and ta-C films with a thickness up to 10 microns can be deposited. By integration of a filter unit for separation of particles from the carbon plasma, an improved ta-C film quality can be obtained, regarding their roughness, hardness and Young´s modulus with an acceptable loss of the deposition rate.

The nature of the Laser-Arc-Module system is, that this carbon ion source can be integrated in commercial available coating machines, independently of producer.