ICMCTF2016 Session B1-4: PVD Coatings and Technologies

A combination of appropriate metallic elements may result in a formation of amorphous metallic alloys with properties superior to polycrystalline counterparts. If these alloys exhibit the glass transition before crystallizing, they are called metallic glasses. In a bulk form, they possess higher tensile yield strengths and higher elastic strain limits than polycrystalline metal alloys, but their ductilities and fatigue strengths are lower. These drawbacks can be overcome by a reduction of size or thickness below a critical value, which leads to an enhanced plasticity and fatigue resistance of thin-film metallic glasses. Magnetron sputter deposition as a non-equilibrium process with high cooling rates (higher than 10⁶ K/s) is a proper technique for the preparation of such metastable materials.

In the present study, we focus on the preparation of amorphous metallic alloys from the Zr-Cu system by non-reactive magnetron co-sputtering and on detailed investigation of the effect of the elemental composition and process parameters on film properties. The Zr/Cu-based films were deposited using two unbalanced magnetrons equipped with Zr and Cu targets in pure argon gas. The magnetron with the Zr target was operated in a dc regime while the Cu magnetron in a pulse regime either at low or high density discharge conditions. The Zr and Cu contents in the films were controlled in a wide range (from 10 to 90 at.% Cu) by adjusting the dc power and/or the pulse repetition frequency. The films were deposited onto substrates held at a floating potential and mounted on a rotating substrate holder. Particular attention is paid to characterization of the structure, microstructure, surface and mechanical, electrical and thermal properties by means of X-ray diffraction, electron microscopy, atomic force microscopy, drop shape analysis, nanoindentation, standard four-point technique and differential scanning calorimetry.

Preliminary results show that amorphous Zr/Cu-based films can be prepared by magnetron co-sputtering in a very wide composition range (from 18 to 90 at.% Cu). The glass transition is, however, observed only in a limited composition interval. An increasing Cu content in the films results in (a) a shift of a very broad diffraction peak to higher 2θ values, (b) an increase of the hardness reaching a maximum (~7 GPa) between 70 to 80 at.% Cu, and (c) an increase of the crystallization temperature. In addition, the films exhibit very smooth surface and high water contact angle (>100°). Further experiments, including an addition of other elements, are still in progress and will be presented as well.

High-quality coatings on the inner surfaces of cylinders are required in many fields of mechanical engineering, sensor technology and applied optics. Protection layers on sleeve bearings, sockets and motor cylinders minimize wear and friction, and thin film strain gauges in the interior of machine parts allow their permanent control and protection against damage by excessive stress. Magnetron sputtering is a promising method to fabricate thin inner coatings of a wide range of materials. A rod-like sputtering target is drawn coaxially through the cylinder that has to be coated, and a magnetic field is applied to increase electron density and enhance deposition rate. Configuration and strength of the magnetic field are important experimental parameters and determine the quality and homogeneity of the obtained coatings.

We have constructed and set up a magnetron sputtering system for homogeneous inner coating of narrow cylinders. Along the entire length of the cylinders, the process chamber is enclosed by a coil system. Five coils, which can be triggered electrically and separately, generate a magnetic field which is subdivided along the substrate axis into five sectors. In this way, the plasma density can be spatially varied and used for the local adjustment of the film properties. A Hall probe array, which is inserted into the process chamber, allows to monitor the magnetic field configuration and to regulate the process. With this approach, the film homogeneity is improved by the compensation of edge effects and film characteristics can be modified tailor-made.

Narrow cylinders with inner diameters of 16 mm have been provided with thin copper, aluminium and gold coatings. First results of magnetic field quantification and film characterization are presented.

The future of metallic or ceramic sputter coated layers on top of a variety of substrates is looking bright. Besides current applications in architectural glass, display, electronics, solar, decorative, optical and hard coatings; other promising opportunities are emerging. New stacks with added-value functionalities require however a higher level of flexibility in the coater set-up, the use of the right target material and they depend on accurate tuning of the deposition process for realizing the desired layer properties.

The use of rotating cylindrical targets has been well accepted over the last decades as a good alternative for sputtering from planar targets. Besides contributing significantly to a reduction in cost-of-ownership (higher target material consumption, more system throughput and up-time, …), it opens new perspectives in advanced coating applications where tolerances are narrow and requirements are high. Initially, rotatable sputtering was mainly geared towards larger area coatings while using a limited number of materials. Presently, this technology has been well adopted for use in smaller coaters and especially because of specific advantages that may be offered.

Some recent developments within various application areas will be presented.

The formation of oxide layers does not necessarily require sputtering from an insulating target in RF power mode or being confronted with a sensitive hysteresis behavior between metallic and poisoned mode; very often substoichiometric and conductive ceramic target tubes exist, giving an easy and stable sputter process combined with enhanced deposition rates.

Optical coatings typically require really accurate control of layer thickness and composition in order to achieve multilayer interference stack with the desired optical response, both on smaller and larger substrate sizes. Remote tuning of the local magnetic field strength of the plasma race-track across a portion of the target surface provides unique capabilities of controlling spatial target material flux, independently of partial pressure gas distribution. In this way, layer uniformities better than +/1 % may be achieved and controlled over larger campaign lengths on substrates up to 100” wide.

In some cases, specific coater geometries are designed for allowing or preventing bombardment of charged species on the substrate, for enhancing or reducing heat load to the substrate surface. Planar cathodes are more limited in capabilities, while rotatables may move to unbalanced sputtering or off-axis directional sputter deposition without jeopardizing target utilization, even in stretched pressure ranges.

1. Introduction

Under the premise of an increasing scarcity of raw materials and increasing demands on construction materials, the mechanical properties of steels and its joints are gaining highly important. In particular high- and highest-strength steels are getting in the focus of the research and the manufacturing industry. To the same extent, the requirements for filler metals are increasing as well. At present, these low-alloy materials are protected by a copper coating (<1µm) against corrosion. In addition, the coating realizes a good ohmic contact and good sliding properties between the welding machine and the wire during the welding process. By exchanging the copper with other elements it should be possible to change the mechanical properties of the weld metal and the arc stability during gas metal arc welding processes and keep the basic functions of the coating nearly untouched.

2. Experimental Results

On a laboratory scale solid wire electrodes with coatings of various elements and compounds such as titanium oxide were made and processed with a Gas Metal Arc Welding process. During the processing a different process behavior between the wire electrodes, coated and original, could be observed. The influences ranges from greater/shorter arc-length over increasing/decreasing droplets to larger/smaller arc foot point. Furthermore, the weld metal of the coated electrodes has significantly different mechanical and technological characteristics as the weld metal from the copper coated wire. The yield strength and tensile strength can be increased by up to 50%. In addition, the chemical composition of the weld metal was influenced by the application of coatings with layer thicknesses to 5 microns in the lower percentage range (up to about 2%). Another effect of the coating is a modified penetration. The normally occurring “argon finger” can be suppressed or enhanced by the choice of the coating.

3. Conclusion

With the help of the presented studies it will be shown that Gas Metal Arc Welding processes are significantly affected by thin film coatings on solid wire electrodes for Gas Metal Arc welding. The influences are regarding the stability of the arc, the properties of the weld metal in terms of geometric expression, chemical composition and mechanical properties, the composition of the arc-plasma and the dynamics of the molten metal.

The use of Nitrogen as an alloying element in steel is limited either to small quantities (1-2 wt%) in bulk materials (where it is used primarily as an austenite stabiliser and property enhancer), or to much larger quantities (>14 wt%) in thermochemical surface engineering treatments (where it is used to create a hard, corrosion-resistant surface diffusion layer - commonly referred to as “Expanded Austenite”). This study examines the effects of Nitrogen incorporation in austenitic steels at levels that lie between the two abovementioned extremities with the intention of improving the mechanical and wear properties of austenitic steels without compromising the inherent high corrosion resistance which many such alloys possess.

Thick coatings of Austenitic-Mn steels containing different levels of interstitial Nitrogen have been deposited by reactive sputtering in an Argon-Nitrogen plasma. The resulting microstructures were characterized by optical micrography and SEM; the mechanical properties have been analyzed by nanoindentation evaluation of hardness and modulus (including coating cross-sectional measurements).