ICMCTF2016 Session B4-4: Properties and Characterization of Hard Coatings and Surfaces

Titanium diboride (TiB₂) can be considered as an extraordinary metallic ceramic material due to its extreme hardness and high melting point as well as exceptional oxidation and corrosion resistance. In this study, the co-deposition of TiB₂ was carried out from a simple oxide-based molten salt electrolyte containing both titanium and boron chemicals. During this high temperature electrolysis, parallel reduction reactions of titanium and boron take place to form a stoichiometric TiB₂ coating on the substrate in a relatively economical and environmentally friendly way compared to the vapor deposition techniques and other halide electrolysis. The growth of TiB₂ layers was performed at various current densities (50-150 mA/cm²), temperatures (800-1000 °C), and time (30-360 min) on cathodically polarized nickel in borax electrolyte with the addition of sodium titanate and calcium fluoride serving as a titanium source and an activator, respectively. At all applied parameters, TiB₂ formations were confirmed by thin film X-Ray diffraction (XRD) method. Scanning electron microscopy (SEM) investigations revealed that the high growth rate dependency of TiB₂ on chosen process parameters. The most dense, continuous and coherent TiB₂ layers were obtained at 70 mA/cm² and 850 °C with the thickness of 3-40 μm enlarged with longer electrolysis time. Vickers micro hardness test of cross-sectioned TiB₂ coated samples showed that the formed TiB₂ layer was hard as 3000±200 HV.

Aluminium alloys are widely used in the transport industry due to their high specific strength leading to significant weight savings and ultimately to lower fuel consumption. However, because of their lower hardness and corrosion resistance, these alloys often present a less than adequate resistance to surface degradation. In order to improve the tribo-corrosion behaviour of these materials, Hard Anodizing (HA) coatings have been extensively used historically but, more recently, protective coatings formed by Plasma Electrolytic Oxidation (PEO) have been shown to offer better wear performance through increased hardness due to the formation of hard crystalline phases during the coating formation process. In addition, PEO coatings, although harder, have been shown to present ductile modes of deformation and very little fracture when compared to HA coatings. In this paper, we investigated this particular multi-scale mechanical behaviour on Al₂O₃ coatings formed on a 6082 aluminium alloy using a pulsed bipolar PEO process.

In order to understand the evolution of microstructure and phase content during formation, coatings of three different thicknesses were evaluated (approximately 15, 20 and 30 microns). The microstructure was characterised by cross-sectional Scanning Electron Microscopy (SEM) while the phase composition was evaluated by depth-resolved grazing angle X-Ray Diffraction (XRD). With the aim of understanding the multi-scale mechanical behaviour of the three coatings, the surface response to large scale indentation and scratch testing was analysed using conventional optical and SEM imaging while coating cross-sections were extracted using Focused Ion Beam (FIB) milling to investigate the microstructure of the sub-surface plastically deformed volume by Electron Back-Scattered Diffraction (EBSD) and X-ray Computed Nano-Tomography. Moreover, in order to observe the effects of thermal gradients during coating deposition on the occurrence, size and distribution of the different alumina phases (alpha, gamma and amorphous phases) within the coating microstructure, we carried out EBSD 3D tomography on the coating cross-sections using Xe⁺ ion Plasma FIB serial-sectioning and in higher resolution around regions of interest such as near discharge channels. The mechanical properties obtained by depth-sensing indentation mapping over the coating cross-sections we then correlated to the EBSD results illustrating the contribution of the different phases to the mechanical response of the coatings.

Applications of austenitic stainless steel for medical instrumentation and food industry are to certain extent limited due to its inherently low hardness and wear resistance. Also resistance to corrosion and in particular to pitting corrosion needs to be improved. Numerous attempts to enhance these properties had been made by classical and plasma enhanced low temperature nitriding or carburizing. As a result of these processes so called nitrogen or carbon stabilized expanded austenite layers are formed. Their hardness is higher than that of austenite and can easily be tuned up to 14-17GPa by the amount of nitrogen or carbon interstitially dissolved in the Fe-Cr-Ni matrix.

Similar surface modification of stainless steels can be achieved with deposition of expanded austenite coatings by means of reactive sputtering of stainless steel targets in nitrogen or hydrocarbon containing atmosphere.

However, in both cases, single surface treatment is not sufficient to meet the requirements of still more advanced applications. It is a reason why numerous attempts had been made to design duplex processes leading to formation of hybrid coatings, offering combination of desired properties, including enhanced load bearing capacity as well as wear and corrosion resistance of stainless steels.

In this work a state of the art in the field is presented. Discussed are also recent results of research on surface modification of stainless steels by deposition of single and hybrid coatings.

Among austenitic steels, some high manganese steels have the particularity of being mechanically metastable. Indeed, the addition of some alloying elements in the steel may result in a decrease of the materials staking fault energy, inducing a modification of the plastic deformation mechanism from classical dislocation glide to twinning or martensitic transformation. Theses steels are designated as TWIP (Twinning Induced Plasticity) and TRIP (Transformation Induced Plasticity) steels and show at different extents a very high strength and an excellent ductility. These materials are thus particularly appropriate for the forming industry.

However, in the particular field of micro deep drawing, these steels were never employed despite their interesting characteristics for the forming of complex parts. Indeed, foils of these steels are not highly available due to the difficulty of rolling down TWIP and TRIP steel sheets to the desired thickness (15 - 30 µm), which implies to regularly recover the original austenitic structure through several heat treatments. Moreover, most of the technical literature on the topic deals about samples with thicknesses around 1 mm. Hence, very little is known about the behavior of TWIP and TRIP steels foils at the micrometer scale.

In this study, magnetron sputtering was used to manufacture 30 µm 25Mn-3Si-3Al steel foils and the influence of the process characteristics, in particular the target power, the roughness of the substrate and several annealing treatments on the tensile properties and the microstructure of theses foils was investigated. Scanning electron microscopy was employed to reveal the changes of morphology while the evolution of the microstructure and dislocation density was followed by x-ray diffraction.

It was shown that after deposition the foils have a ferritic structure that could only become fully austenitic after annealing over 973 K and that an increasing annealing temperature results in a better ductility due to higher grain size and lower crystal defects density. The quality of the substrate surface was also revealed as being a critical characteristic for the fabrication of dense freestanding thin films with high density, resulting in simultaneously a higher tensile strength and a better deformation of the foil after annealing. However, the reached deformations at fracture were limited compared to those reached by macroscopic samples and the statistical dispersion of the deformation at fracture, was very important after annealing at higher temperature. Several scaling effects, in particular the grain size to sample size ratio could explain this particular behavior in the micrometer scale.

A considerable increase in scientific exploration concerning hydrogen production and storage has occurred. Numerous technologies are being developed to provide competitive alternatives to fossil fuel energy technologies. One issue for successful commercial implementation of these technologies is the ability of structural materials for process vessels and piping, storage containers, and engineered components to resist embrittlement from hydrogen.

Major structural component materials for hydrogen service applications are austenitic stainless steels such as Type 304, 316L, etc. Some metallurgical coatings have been applied to minimize the potential impact from hydrogen exposure. They have shown hydrogen permeation reductions that range from 10 to 10000 times. However, this variation is due to several factors such as inherent permeation resistance and microstructure or nanostructure of coatings. In this study, effect of nanostructure of metallurgical coatings such as TiN, TiC, BN and Al2O3 on hydrogen permeation is evaluated.