ICMCTF2015 Session B1-3: PVD Coatings and Technologies

Granular Materials have many applications in the field of material science and catalysis, especially if coated with functional surface layers. With Physical Vapor Deposition (PVD) processes in general and DC magnetron sputtering in special, as used within this work, various layers ranging from metals via oxides and nitrides to complex composite materials such as nanocomposites and multilayered heterostructures can be deposited. Due to the line-of-sight nature of magnetron sputtering each particle surface has to be exposed equally to the vapour beam to achieve homogeneous layers. Therefore continuous intermixing is necessary with special attention to not damage fragile granulate and avoid agglomeration.

A device has been designed to coat up to one liter of granular material with particle diameters ranging from 20 µm to 500 µm. Basically this device is an upscaled version of special coating geometries developed and tested in former projects at the Vienna University of Technology [1,2] and has similarities to a concrete mixer regarding rotation of the mixing bowl and gear mechanism.

The most efficient way to suppress agglomeration during sputter process is to execute strokes against the bowl containing the granular materials. The optimisation of the deposition rate and coating uniformity is made possible by using two sputtering sources which can be tilted in different angles to obtain a vapour beam ideally directed exactly towards the exposed surface of the particle ensemble. In addition the sputter rate varies with the distance between sources and substrate, which leads to higher deposition rates at higher amounts of granulate.

Within this work hollow glass micro spheres are the most used substrate. To characterize the deposition device and the behaviour of the micro spheres under vacuum conditions, the films deposited are mostly Copper and Aluminium due to the high sputter yield. The filmthickness and filmthickness distribution of mono metallic layers can be determined by using a light microscope and a software developed at the Vienna University of Technology [3]. The uniformity of the films was also verified with the scanning electron microscope (SEM).

The financial support of the Austrian Science Fund (FWF) under Grant Nr. TRP-281-N20 is gratefully acknowledged.

References

[1] G. H. S. Schmid, C. Eisenmenger-Sittner, J. Hell, M. Horkel, M. Keding, H. Mahr, Surface & Coatings Technology 205(7) (2010) 1929.

[2] G. H. S. Schmid, C. Eisenmenger-Sittner, Surface & Coatings Technology 236 (2013) 353.

[3] M. Horkel, H. Mahr, J. Hell, C. Eisenmenger- Sittner, E. Neubauer, Vacuum 84(1) (2009) 57.

This study focuses on the preparation and characterization of Fe- and N-doped TiO₂ thin films by metal plasma ion implantation (MPII) and N₂ annealing treatment. The band gap of anatase TiO₂ (3.2eV) only absorbs UV light, which is less than 10% of the solar energy and results in the limitation of practical applications. The band gap of TiO₂ can be controlled by tuning the position of the impurity level, which can then significantly increase the practical application and result in many industrial applications. As the Fe concentration ranges from 5× 10¹⁵ to 1× 10¹⁷ ions/cm², the films revealed the characteristic anatase TiO₂. The Fe content of the Fe-implanted TiO₂ films was found to be 0.5-1.5at%. The band gap of Fe-doped TiO₂ was ranged from 2.87 to 3.14eV. The photocatalytic activity of Fe-implanted TiO₂ is higher than that of as-deposited TiO₂ (5.6%), and the removal ratio was increased to 14.1-17.8%. The N-doped TiO₂ films were prepared by using N₂ thermal annealing. As the annealing temperature increased from 400°C to 600°C, the films revealed the characteristic anatase TiO₂. The N content was found to be 0.1-0.3at%. The band gap decreased with increasing N concentration. The band gap of N-doped TiO₂ decreased to 2.16-2.30eV. The N-doped TiO₂ films exhibited excellent visible photocatalytic activity, and the removal ratio was increased to 9-14.1%.

For (Fe, N) co-doped TiO₂ films, the Fe content was 0.5at% and the N content was found to be 0.3-0.5at%. The band gap decreased with increasing N concentration. The band gap of (Fe, N) co-doped TiO₂ was ranged from 1.97 to 2.22eV. The (Fe, N) co-doped TiO₂ design conforms to the requirements for highly effective photocatalytic activity, and the removal ratio was 16.4-30.1%. The narrowing of the band gap in the substitutional N dopant was more effective than that in the substitutional Fe dopant. The substitution of Fe and N for Ti and O atoms can form the impurity levels near conduction band (CB) and valence band (VB), respectively. The distance from CB or VB to impurity can be controlled by changing the dopant concentration, and the tunable region was approximately 0.45eV and 0.48eV, respectively. The band gap decreased along with the concentration of Fe and N dopants. However, when the dopant concentration exceeds the optimal value, the photocatalytic activity of the film significantly decreases due to the formation of deep impurity.

The tuning band gap of the TiO₂ system can yield not only a single-element-doped TiO₂, but also a multi-element TiO₂ films, which have great industrial applications.

Transition metal diborides are ceramics with potential applications as hard protective thin films and electrical contact materials. We investigate the possibility to obtain age hardening through isostructural clustering, including spinodal decomposition, or ordering induced precipitation in ternary diboride alloys. By means of first-principles mixing thermodynamics calculations, 45 ternary M¹_1-xM²_xB₂ alloys with AlB₂ type structure are studied. Several alloys, among them Al_1−xTi_xB₂, are found to be of interest for coherent isostructural decomposition with a strong driving force for phase separation, while having a weak concentration-dependence of a and c lattice parameters. The mixing thermodynamics results are explained by revealing the nature of the electronic structure in these alloys. In particular the pseudogap at E_F in TiB₂, ZrB₂, and HfB₂ was found to originate from d-d interaction between metal sites in the hexagonal structure, further enhanced by metal-boron interactions. The theoretical work is compared to results from PVD synthesis of ternary diboride thin films made by magnetron sputtering and cathodic arc synthesis methods.

Young active patients who require joint replacement surgery are driving research into developing new implants that can potentially last a lifetime. The cost of revision operations is also an important driving factor. In order to extend the lifetime of joint implants, a suite of novel CrN-Ag coatings have been developed. Through extensive wear testing on an orbital hip simulator system, CrN-Ag-coated hip implants have been shown to possess excellent wear resistance properties leading to a significant reduction in metal ion release and a reduction in the volume of wear debris generated. The mechanical and structural properties of CrN-Ag were also investigated and the results revealed that although the coating hardness decreases with increasing silver content, a uniform structure is observed in all cases. Furthermore, the presence of Ag is shown to improve the antimicrobial properties of the CrN-Ag coatings which will be beneficial for reducing the risk of post-operative infections.

Al-Zr thin films were deposited on glass slides by dc magnetron sputtering with Zr content ranging between 0 and 25 at %. Microstructure, morphology, hardness and intrinsic corrosion resistance in NaCl solution were investigated.

Microstructural characterisation reveal the formation of a supersaturated solid solution for Zr content up to 12 at % and a single amorphous phase for higher Zr contents. Microhardness is found to increase with the increase of Zr content up to 18 at %. The electrochemical measurements, in 5 wt % NaCl solution, show an increase of corrosion resistance with addition of Zr and are correlated with microstructures of as-deposited films. The improved pitting corrosion resistance is ascribed to compact structure of alloys and also to enrichment of Zr in the passive film.

The lecture starts with a brief review of the state of the art in pulsed laser deposition equipment. A novel deposition system for the production on industrial scale of super hard tetrahedral amorphous carbon (ta-C) coatings will be presented. The resulting layers characterized by low internal stress using a combination of pulsed laser deposition (PLD) and pulsed laser stress relaxation (LR) as deposition method. The system has been designed on the basis of scientific findings showing the possibility to remove the high mechanical stresses in as- deposited ta-C films with optimum sp³-content and hardness by irradiating them after deposition with nanosecond laser pulses of 248 nm wavelength, where the maximum thickness of ta-C must not exceed 100 nm, however. Thick ta-C films can be prepared by applying this stress relaxation method alternating to the PLD method, for which nanosecond pulses of 248 nm wavelength and high pulse energy are used, too.

The general method as it has been used in research and the parameters allowing the preparation of ta-C films with hardness up to 60 GPa will be outlined.

The main part of the talk will be directed to the integration of the method in an industrial deposition system with the focus on general aspects and modifications required for up-scaling and industrialization of the process. This includes:

- Target and substrate handling,

- In-situ cleaning of the windows through which the laser beams are guided into the chamber,

- In-situ monitoring of coating quality

- The controlling of the system and the system and the production process.

Some examples of ta-C coated parts prepared in the system and the properties of the coatings will also be presented.

Finally, a summary of coating rates and productivity attainable with the system and the costs of ta-C coatings produced with it, will be given.

Keywords: PLD, ta-C, industrial Systems,