ICMCTF2016 Session B1-2: PVD Coatings and Technologies

Titanium aluminium nitride (Ti,Al)N is one of today’s most used hard coatings in the cutting tool industry. The requirements for the tool industry with high productivity and the impact of a critical wear at elevated operating temperatures, typical for such cutting conditions, motivates the study of phase stability and microstructure evolution.

Recently, a modified synthesis route for microstructural control, in particular grain size, of polycrystalline high-Al (Ti,Al)N coatings has been reported [1]. Another way to reach similar growth conditions during arc evaporation is to apply a tunable magnetic field (B_z) at the backside of the arc source in the direction normal to the cathode surface. By changing B_z, the plasma characteristics and hence the film growth are strongly altered [2, 3]. Here we present results from a high temperature in situ synchrotron x-ray study of arc evaporated (Ti,Al)N coatings with high-Al content, deposited with different field strengths, B_z. Evolved structures are also characterized at sub-nm resolution by atom probe tomography (APT).

(Ti,Al)N coatings were grown at different magnetic field (B_z), with an observed grain coarsening as B_z increases. Coatings grown at B_z=0 display a fine-grained nanocrystalline microstructure, while B_z=6 mT results in a coarse columnar microstructure with grain width of about 200 nm. The fine-grained coatings contained a phase mixture of cubic c-(Ti,Al)N and hexagonal h-AlN. Clearly, the onset to h-AlN already occurred during growth, while the coarse-grained coatings contained exclusively a solid solution of c-(Ti,Al)N.

During in situ annealing at 1000 °C, only minor changes were observed in the amount of h-AlN for the fine-grained coatings. The coarse-grained, however, shows a formation of h-AlN starting at about 800 °C followed by an increase of the amount of h-AlN during the subsequent 3-hour annealing at 1000 °C. In addition, a difference in the c-lattice parameter of the hexagonal phase is observed between the two types of coatings, suggesting the formation of chemically different h-AlN, here studied via APT by investigating the Ti content in Al-rich domains. The onset and growth of h-AlN for the fine and coarse-grained materials will be discussed with respect to the local chemical compositions and mechanical properties.

[1] J.M. Andersson, J. Vetter, J. Muller, J. Sjölen, Surf Coat Tech, 240 (2014) 211-220.

[2] D. Kurapov, S. Krassnitzer,W. PCT, US 2013/0126347 A1, Oerlikon Trading AG, 2013.

[3] S. Krassnitzer, J. Hagmann, U.S.P.A., WO 2013/045039 A2, Oerlikon Trading AG, 2013.

Metastable NaCl-structure of TiAlN shows high hardness and oxidation resistance, which provides excellent machining performance. It is well known that arc ion plating (AIP) is suitable for manufacturing of hard coated tools with dense, fine-grained TiAlN films. In AIP process, however, macroparticles which are liquid or solid metallic particles are produced at cathode spots on power supplied cathode surface [1]. Macroparticles are incorporated with the AIP coated films and negatively influent properties of deposited films, creating mechanical and tribological defects and poor surface roughness.

In order to reveal its origin and control its generation, in this study, observation of macroperticles with low pressure coating process of TiAlN in discharging AIP chamber was carried out. To achieve this, the quasi-bright field particle detector of HYT9020 with the two orthogonally polarized laser beams was employed to avoid optical noise from non-polarized lights of plasma discharges [2]. A particle passing through the laser beams is detected by the phase shift between the two beams. The measurements were carried out during deposition process of TiAlN films by using Ti_0.5Al_0.5 alloying cathode in N₂ atmosphere with the pressure from 0.5 to 20 Pa. We found the relation between the number of particles counted by the detector and the obtained surface state of TiAlN films, such as roughness and suffered area by macroparticles. The number of particles by the detector N_p can be described by power law of arc current I_arc as N_p=A* I_arcⁿ. The power law would represent that the origin of the detected particles is the arc spots which are in the field of chaos and self-organization [1].

[1] A. Anders, Surf. Coat. Technol. 257 (2014) 308, and references therein.

MAX phase thin films are very interesting for high temperature applications. The nanolaminate structure causes a combination of metallic and ceramic properties. The objective of the work is to prepare Cr₂AlC-MAX phase films by means of the dc arc evaporation, which is a widely used PVD process for the thin film deposition, especially for wear protection. Various routes were tested to achieve the MAX phase films. The carbon was added to the process as the reactive-gas C₂H₂. The relation between chromium and aluminum was adjusted by the position between two evaporators (Al and Cr respectively Cr and AlCr), the bias voltage and the cathode currents. A one cathode setup was also tested using the cathode composition in combination with the bias voltage to adjust the composition of the film. Additionally, films without reactive gas were prepared using the pure cathodes Al, Cr and C. The temperature of the substrate during the coating process was varied in the range of 200°C up to 900°C. Structure, surface morphology and microstructure evolution were investigated by SEM, TEM and XRD.

Due to the high plasma activation the composition of the films is strongly affected by the coating distance and also by the angle of the plasma flow. Due to the many stable phases in the Cr-Al-C system, small changes in the gas flow of the reactive gas, the total pressure, the substrate position, the bias voltage and the substrate temperature resulted in large variation of the composition of the films. At temperatures below 600°C no MAX phase was found at the films deposited with moderate bias voltages (<300 V). At bias voltages >300 V small proportion of the max phase could be detected in the films. At temperatures above 700°C nearly homogeneous MAX phase films were obtained.

In this paper the producibility of Cr₂AlC - MAX phase films, deposited by means of vacuum arc technology, could be verified. Perspectives for industrial applications of these types of films can be predicted.

One of the most important design criteria in state of the art mechanical engineering is the ability of materials to withstand high temperature and oxidative environments. In general, oxidation processes and their kinetics depend on various parameters and depict a complex topic. Nevertheless, for highly oxidation resistant coatings (especially in wide temperature ranges) major aspects in the oxide scale formation are the adherence of the scale (induced stresses), a non-volatile behavior, continuous coverage, and retarded diffusion processes within the scale (short circuit mechanism).

Considering the above mentioned criteria combined with the standard requirement for excellent mechanical properties, we present within this study the innovative design of multilayered Mo_1-x-ySi_xB_y/Ti_1-xAl_xN coatings. Within the coating architecture, Ti_1-xAl_xN provides its well-known superior thermomechanical properties and Mo_1-x-ySi_xB_y its outstanding oxidation resistance – as one of the new high-temperature structure materials.

The influence of the individual layer thickness (bilayer period variation from 20 to 200nm) on oxidation resistance and kinetics is investigated applying differential scanning calometry (DSC/TGA) in the temperature range from 400 up to 1600°C. These results and the individual coating characteristics such as hardness, structure and thermal stability are correlated with detailed transmission electron microscopy studies. In a further consequence the chemical composition of the Mo_1-x-ySi_xB_y layers is changed to investigate the oxidation kinetics, especially regarding the pesting phenomena.

We report the growth and characterization of epitaxial Zr and ZrC thin films deposited on Al₂O₃(0001) substrates via dc magnetron sputtering. All of the Zr and ZrC layers were grown in an ultra-high vacuum deposition system using Ar (99.999%) (10 mTorr) and Ar/C₂H₄ (8.0/2.0 mTorr) gas atmospheres at substrate temperatures T_s between 973 – 1123 K and 1073 – 1673 K, respectively. The as-deposited film surface structure and composition were determined in situ using low-energy electron diffraction and Auger electron spectroscopy, respectively. The in-plane and out-of-plane lattice parameters were measured as a function of T_s using a combination of ω-2θ x-ray diffraction spectra and high-resolution symmetric as well as asymmetric reciprocal space maps. Microstructures of the layers and the film-substrate interfaces were determined using cross-sectional transmission electron microscopy. We find that all the Zr/Al₂O₃(0001) films grow epitaxially with {0001} orientation at 973 < T_s < 1123 K, while ZrC(111) layers were obtained only at T_s > 1473 K.

Tungsten carbide is one of the hardest man-made metallic composites. In powder form, it can be pressed and formed into various shapes, also as a thin layer onto the surface of goods through thermal cementing of the tungsten carbide fine powder on the surface of metals of a lower melting point. An alternative method for generation of thin layers is film deposition through vacuum arc, which allows generation of metallic flux even from refractory materials. In this work, we explore plasma generation from a tungsten carbide (W_0.50C_0.50) cathode in a DC vacuum arc deposition system. Plasma chemistry and charge-state-resolved ion energy distributions are analyzed and compared to corresponding data from pure W and C cathodes, showing a change in charge states as well as ion energies compared to the single-element cathodes. An intense macroparticle generation from the W-C cathode is also demonstrated. A new mechanism for the droplet generation is suggested based on scanning electron microscopy of the cathode surface as well as of collected macroparticles, and from the W-C phase diagram, which indicates an intense sublimation of carbon from the carbide melt on the cathode surface. The plasma analysis and macroparticle generation is correlated to the resulting film composition, and angular dependence thereof. The results are of importance for an improved control of the thin film synthesis process, and for prospects of future tuning of droplet generation.