ICMCTF2010 Session B6-3: Hard and Multifunctional Nano-Structured Coatings

Colour derived from Au nanoparticles embedded in a dielectric matrix is a phenomenon well known from the Middle Age, used for producing coloured glass windows for cathedrals. Depending on either the size, content and distribution of the Au nanoparticles or the dielectric constants of the used matrix a large variety of colours can be obtained due to the so called Surface Plasmon Resonance (SPR) effect. SPR is related to the cooperative oscillation of the free electrons of Au which promote strong absorption of particular bands of visible light giving rise to intense colour.

The aim of this research was to use this “old” idea for producing hard and coloured coatings for decorative applications. It is intended to produce a matrix of a dielectric material with high hardness, in order to guarantee a high abrasion and wear resistance, and therein insert in a controlled way Au nanoparticles. Different matrixes based on oxides are being studied (Al-O, Ti-O, W-O and Sn-O) which allow to interfere directly and indirectly (by determining the ability for the precipitation of Au nanoparticles) on the final colour of the films. The Au parameters are being controlled by the Au amount added to the matrix, the deposition conditions and the post-deposition thermal treatment.

The coatings were deposited by sputtering from a metal (Al, W, Ti, Sn) target embedded with thin gold plates, in a reactive environment containing oxygen. The O₂ flow was high enough for working in compound mode producing transparent dielectric films. The Au content was varied from 0 up to 20 at.%. In the as-deposited conditions structures varying from amorphous, through nanocrystalline up to crystalline could be achieved. Subsequently, the films were heat treated to promote further segregation of the Au metallic nanocrystals. The precipitation is easily enhanced for the (nano)crystalline films but revealed quite difficult in case of amorphous matrixes. The size and distribution of the Au nanoparticles is being studied by X-ray diffraction and transmission electron microscopy. Hardness as high as 20 GPa is measured in some cases, being the hardest matrix the Al-O. The best optical behaviour from the point of view of varied colours is being obtained with coatings from the Ti-O and W-O systems.

A two-phase nanocomposite system that consists of inclusions of silver in a vanadium nitride matrix (VN/Ag) was investigated as a potential adaptive coating with reduced friction coefficient from 25 to 1000°C. This nanocomposite structure was selected based on the premise that silver will migrate to the surface of these coatings to act as a lubricant in the 25 to 500 °C temperature range. At higher temperatures, silver and vanadium were expected to react with oxygen to form a lubricious silver vanadate film on the surface. The VN/Ag coatings were deposited using unbalanced magnetron sputtering as a function of silver content and their elemental composition was evaluated using X-Ray photoelectron spectroscopy. The friction coefficients were measured to be 0.35, 0.30, 0.10 and 0.2 using a pin-on-disc tester at 25°, 350°C, 700°C, and 1000°C, respectively. Raman spectroscopy and x-ray diffraction (XRD) revealed the formation of silver orthovanadate (Ag₃VO₄) as well as vanadium oxide on the surface of these coatings. Finally, real time Raman spectroscopy and XRD were utilized to understand the structural and chemical changes that these materials go through as a function of temperature. These techniques revealed very useful information regarding processes that occur with an increase in temperature that include phase segregation, phase changes, and melting. These observations were correlated to the tribological properties of these materials under the selected testing conditions.

Application of thin films in the biomedical engineering field represents an attractive challenge due to the multiple situations where they may improve or even functionalize a certain part of human body. Implants are one of such cases, representing one of the most active fields within the so-called biomaterials R&D. Implant failure is a huge problem for both the patient and governmental agencies, once it involves repeated surgeries and consequently considerable economical resources as well as patients’ death. This failure can be attributed to excessive wear and wear debris and also to microbial infection. Additionally, the use of several kinds of carbon/nitride-based thin films has been carried out in the group, with some promising results. Thus, the main aim of the present work is to study such carbon/nitride-based thin films, namely in what concerns the Ag addition to well-known TiCN thin films by DC unbalanced reactive magnetron sputtering.

The obtained Ag-Ti(C,N) based coatings were characterized in terms of composition and structure as well as in terms of biological properties. Mechanical/tribological resistance of the films was achieved by hardness testing in combination with friction and wear measurements. The tribological response was studied under biological fluid using reciprocating pin-on disk configuration. Biological properties were assessed both in terms of cytotoxicity, by the evaluation of animal cell death induced by the material, and of microbial colonization, through the analysis of biofilm formation on the samples.

There have been efforts for many years to establish cathodic arc evaporation for oxide deposition. Issues associated with process stability and droplet formation prevented the utilization of this technology on a large scale in production. Although cathodic arc technology is well understood and the dominant technology in the PVD tool coating business for conductive layer materials, the deposition of oxides utilizing this technology has been only recently enabled by the development of a dedicated production technology (P3e^TM) [1]. The robustness of this technology is based on its inherent broad process window which permits the arc operation in pure reactive gas, a variation in process pressure over several decades, the freedom in the selection of reactive gases and target materials, and the easiness of adjusting deposition rates. All these aspects make this technology unique amongst PVD technologies.

The straightforwardness in process control allows the deposition of oxides under very different conditions resulting in a variation of the coating microstructure. This is illustrated in a cross-sectional scanning electron microscopy analysis for the Al-Cr-O and Zr-Y-O coatings which demonstrate that the layer morphology can be adjusted from “glassy” to “columnar” in a simple way. XRD investigations show the influence of target composition and reactive gas flow on crystal structure and crystallite size of the synthesized oxides.

In additional investigations, an attempt has been made to correlate the deposition parameters with processes at the surface of the composite Al-Cr targets and the nucleation and phase formation of Al-Cr-O layers at the substrate surface. The oxygen partial pressure and pulsing of the arc current influence the formation of intermetallic phases and/or solid solutions at the target surface. The nucleation of the ternary oxides at the substrate site appears to be, to some extent, controlled by the composition of the intermetallics or solid solutions formed at the target surface. This in turn suggests that the formation of phases in the synthesized layer can be influenced by the processes at the target surface. This hypothesis is supported by the X-ray diffraction analysis of the layers as well as of the target surface. It is also confirmed by the results of cross-sectional transmission electron microscopy investigations of the synthesized oxide layers, especially by the results obtained from selected area electron diffraction.

[1] J. Ramm, M. Ante, T. Bachmann, B. Widrig, H. Brändle, M. Döbeli, Surf. Coat. Technol. 202 (2007) 876

TiCN coatings were synthesized using a large area filtered arc deposition (LAFAD) technique from a Ti targets at an atmosphere of mixed N₂ and CH₄ gases. The hardness, elastic modulus, and tribological behavior of the TiCN coatings were characterized using nanoindentation and pin-on-disk tribometer. To investigate the influence of the C content in the coatings on the mechanical and tribological properties, CH₄ gas volume fractions in the gas were varied from 0 to 50%. It was found that with increasing C content in the coatings, the hardness and elastic modulus increase to a maximum at a C content of 2.8 at.%, then decreases rapidly with the further increase in the C content in the coatings. Adding a small amount of C into the TiN coatings leads to a significant decrease in the plasticity. Tribological test results show that there is no significant change in the friction coefficient (0.78-0.88) of the TiCN coatings against Al₂O₃ balls when C content in the coatings is below 4.6 at.%, but the friction coefficient decreases rapidly to 0.21 with the further increase in the C content in the coatings to 9.3 at%. With increasing C content in the coatings from 0 to 2.8 at.%, the wear rate decreases from 2.5×10^-6 mm³/Nm to 9.5×10^-7 mm³/Nm. The further increase in the C content in the coatings to 9.3 at.% leads to a gradual decrease in the wear rate to 5.3×10^-7 mm³/Nm, which is 21% of the wear rate of the pure TiN coatings.