ICMCTF2016 Session G4: Pre-/Post-Treatment and Duplex Technology

Magnetron sputtering is used to deposit multi-layer structures for photovoltaic (PV) panels, flat panel displays (FPD), architectural and automotive glass, and flexible webs. Initially, sputtering magnetrons were driven by DC power sources. As process needs evolved they were driven also by AC and pulsed power supplies, with corresponding advantages and process considerations. Pulsed power can be used for reactive deposition of dielectrics. It enables reactive sputtering of dielectrics that are essentially impossible with straight DC, owing to periodic discharge of the voltage on dielectric films deposited on the target itself, preventing target arcs and helping to mitigate anode coverage. The concept was introduced in the 1970s. Pulsed reversal was developed industrially in the 1990s. Further innovation broadened the solution space to include control of material characteristics such as morphology and crystallinity, and process measurement and control. Dual magnetron sputtering (DMS) processes were first powered with AC supplies; today pulsed DMS supplies offer increased flexibility in process development and operation. Stable processes are possible at lower frequencies due to faster arc handling and reduced arc energy. Greater rate in reactive processes can be achieved by running incrementally higher on the transition curve, by controlling the working point of each magnetron in the DMS pair. Process measurement and control are enhanced by quasi-DC conditions accessed by quasi current source pulsed power solutions.

Sputtering processes are prone to arcing, which can cause damage to the work piece. Therefore, arcs must be detected and extinguished in a timely manner to minimize damage. Arc handling is now a standard feature of many PVD power supplies; it has been the focus of considerable development effort. The sputtering process arcing problem will be reviewed, including references to early experimental work implying the influence of oxides in the glow to arc transition as well as selected history of the evolution of arc handling for sputtering processes.

Key developments, solutions, and opportunities driven by precision process power capabilities will be presented.

In the thin film deposition process the type of the substrate used has often a large influence on the layer properties, therefore one of the most important and the first steps is the preparation of the surface substrate before the deposition. Different, in-situ and ex-situ processes can chemically and physically change the surface of the substrate in order to enhance the quality of the film such as organic solvent cleaning, etching (chemically by acids or physically by plasma), annealing under different atmosphere, etc[1,2]. Many different techniques can be used for good cleaning/preparation but they can be time consuming.

This study focuses on the ex-situ wet cleaning of (001) MgO substrates, which are highly reactive forming surface hydroxides and carbonates. Time-of-Flight Secondary spectroscopy (TOF-SIMS) and X-ray Photoelectron Spectroscopy (XPS) analyses of the as-received substrates and after different cleaning processes showed a low efficiency of the commonly used organic solvents (acetone, isopropanol) leaving magnesium hydroxide at the surface. In contrast, cleaning with alkaline liquid detergent allows to achieve magnesium oxide surface free of hydroxides / carbonates contaminants. Those different cleaning processes has been further studied, specifically their influence on the quality of the scandium nitride based thin films deposited by sputtering.

[1] S. Chakrabarti et al, Materials Letters 57 (2003) 4483.

[2] J. Du, S. Gnanarajan et al, Superconductor Science and Technology 18 (2005) 1035.

As having been greatly considered recently for use in mobile devices and computer appliances, anodized aluminum was requested to provide colorful surface whilst mechanically strong of the surface. Conventional anodizing in combination with dyeing process, though fulfill these surface requirement, yet not good enough. Micro-arc oxidation (MAO) on the other hand, provides very strong surface mechanical properties but incompatible to dyeing process. In this study, MAO was carried out on aluminum to produce oxide coating, followed by nanolization and dyeing process. Chromaticity of the treated samples was measured by means of a colorimeter, while steel wool test was carried out to reveal color durability. Conventional anodization was also compared. Experimental results show that uniform dyeing coloration can be obtained for the micro-arc oxidized aluminum enhanced by surface nanolization, which exhibits the highest surface area of nanoscale porous structure for dyeing process and thus increases dye-absorbing efficiency. Both dyeability and mechanical durability of the MAO-Al is demonstrated.

When using thin films to practical parts, not only the film properties such as hardness and lubricity, it is also necessary consider such adhesion and surface texture. There are dependent on the substrate, it is necessary to develop in consideration of properties of the substrate, such as hardness and fatigue strength. Surface modification is an effective way as the method to control the properties of the substrate material. It introduces FPB (Fine Particle Bombarding) as one of the surface modification methods.

FPB is a kind of the shot peening, but is a method of projecting particles finer( about 50um) than normal at high speed (several 100m / s). The projection of high-speed fine particles can bring large plastic deformation on the substrate surface. Plastic deformation causes giving of compressive remaining stress and formation of micro-dimples in a substrate material surface, and improves fatigue strength and the oil retention. Moreover, the FPB, by projecting the soft particles to hard substrate, it is possible to coating the projection material. In addition, by projecting the ductile particles in ductile materials, it is also possible formation of a mixed layer by the mechanical alloying.

There are the following effects when FPB is used as substrate pre-treatment of a hard thin film. In the case of ceramic thin films, from brittle properties, the wear resistance is improved, but decrease of fatigue strength occurs. By using FPB as substrate pre-treatment, to improve the fatigue strength of the parts. Plastic deformation of the substrate surface by FPB, not only compressive residual stress, resulting in the micro-crystallization of the metallographic structure and metal structure of the surface-hardened layer. The formation of the surface-hardened layer improves effective adhesion of the film by decreasing of the substrate deformation, reduction of shearing forces on film-substrate interface. Also, the micro crystallization of substrate to suppress abnormal growth in thin films. DLC coating of high adhesion becomes possible to form mixing layers with aluminum and tungsten in aluminum alloy surface by FPB . Typically, the micro-dimples formed by FPB has a depth of about 2μ in diameter about 20μ. The surface texture of the range is suitable for improving the sliding properties and retention of lubricants and release agents, and an effect is used in such the sliding parts and the mold.

Long term surface peening, such as the Ultrasonic Shot Peening (USP), was developed to upgrade directly the mechanical properties of the materials as well as a surface activator prior to chemical treatments such as Plasma Nitriding [1]. The High Current Pulsed Electron Beam (HCPEB) technique is also a recent technique that has been proved to increase surface hardness as well as improve wear and corrosion properties [2]. These techniques create a deformed graded surface for which the grain size reduction, the increased grain boundary density and the introduction of structural defects (twins, dislocations, vacancies …) improve directly the properties. It was also suggested that USP promote the diffusion of nitrogen and thereby, the reductions in the nitriding temperature and/or duration, leading to avoid the formation of nitrides which affect the corrosion behavior of stainless steels.

In the present work, the AISI 316L stainless steel was treated by various duplex treatments consisting of USP or HCPEB followed by Plasma Nitriding. The evolutions of the structure in the nanocrystalline surfaces and sub-surfaces were analyzed at the light of a quantitative analysis of the deformed state using a recently developed procedure based on the analysis of Geometrically Necessary Dislocation (GND) obtained from EBSD orientation maps [3]. This procedure, which was initially developed to study quantitatively the surface and sub-surface microstructural changes issued from USP, is here extended to the analysis of these duplex treatments. The structure and properties of the nanostructured nitrided surfaces will be detailed and the effect of the processing conditions discussed. The comparative effectiveness of the various duplex treatments in terms of surface and subsurface hardening as well as corrosion resistance will be highlighted.

[1] Lu K, Lu J. Nanostructured surface layer on metallic materials induced by surface mechanical attrition treatment. Mater Sci Eng A-Struct Mater Prop Microstruct Process 2004;375:38–45.

[2] Zou J, Grosdidier T, Zhang K, Dong C. Mechanisms of nanostructure and metastable phase formations in the surface melted layers of a HCPEB-treated D2 steel. Acta Mater 2006;54:5409–19.

[3] Samih Y, Beausir B, Bolle B, Grosdidier T. In-depth quantitative analysis of the microstructures produced by Surface Mechanical Attrition Treatment (SMAT). Materials Characterization 2013;83:129–38.

Titanium diboride (TiB₂) and titanium carbide (TiC) are metallic ceramics with high hardness, excellent wear and corrosion resistance similar to ceramics; as well as high electrical and thermal conductivity, resembling metals. The purpose of this research is to obtain a multilayered-coating structure composed of TiB₂/TiC by using Cathodic Arc Physical Vapour Deposition (CAPVD) and electrochemical boriding (CRTD-Bor). To achieve this aim; firstly titanium was coated on the medium carbon steel substrates via CAPVD. Subsequently, the Ti-deposited steels were electrolytically borided in a non-toxic oxide-based molten salt mixture. The cross-sectional SEM and EDX investigations revealed that during boriding treatment the molten salt bath served as the boron source for the growth of top TiB₂ layer, whereas, the carbon content of the steel acted as the carbon source for the formation of underneath TiC layer. In the light of experimental results, it was found that there is a critical role of TiC zone on the structural and mechanical integrity of this TiB₂/TiC bi-layer.