Interrelationship between atomic species, bias voltage, texture and microstructure of nano-scale multilayers

A matrix of binary and ternary nitrides containing lighter elements (Al, Ti, V and Cr) with atomic mass 89 has been formulated. These have been grown as nano-scale multilayer coatings (bilayer thickness approx. 3.0 nm) on stainless steel substrates using an industrial size multiple-target ABS coater. When lighter elements are incorporated into the multilayer at a lower bias voltage (U-B = -75 V) pronounced {111} or {110}, textures develop which are determined by the dominating species present. A {111} or {110} texture develops when TiAlN or VN and or CrN dominates the matrix, respectively. In contrast when a heavier element is incorporated a {100} texture is observed. Additionally, there is a strong indication that in the case when heavy elements (>89) are involved in the growth process, which evolves by continuous re-nucleation. Conversely, when only light elements (<52) are involved then the coating evolves by competitive growth. This observation is limited only for the lower bias voltage range of U-B -75 to -120 V However, as the bias voltage is increased (up to U-B = -150 V) the texture becomes increasingly sharp and in all cases a {111} texture develops. A lower residual compressive stress (typically -1.8 GPa) is observed when one of the bi-layers is dominated by a heavier element. The stress increases (up to -6.8 GPa) in these coatings when the bias voltage is increased to U-B = -150 V which is always systematically lower than in coatings containing only lighter elements which are typically up to -11.7 GPa at the same bias voltage. In parallel this results in an increase in plastic hardness (80 GPa) and in the sliding wear coefficient by an order of magnitude regardless of the type of lattice growth observed

Some thoughts on the construction of erosion-corrosion maps for PVD coated steels in aqueous environments

In studies of erosion-corrosion of materials in aqueous conditions, there have been a few attempts to evaluate the performance of coatings. This is despite the fact that the use of coatings provides a relatively economic method of reducing erosion-corrosion damage in such environments. There has also, as a consequence, been no attempt to define a rationale for coating selection for exposure to various erosion-corrosion environments. The object of this work has been to address the above issues by investigating the effect of increasing velocity on a range of physical vapour deposition (PVD) coated and uncoated steels, at various applied potentials, in a carbonate/bicarbonate slurry solution containing alumina particles. The performance of two coatings, Ti2N and CrN was evaluated. The results were compared to those for the uncoated steel (mild steel) and 304 stainless steel, The results showed that the both the corrosion and passivation potentials for the coatings occurred at different values for the individual coatings. Hence, the erosion-corrosion "regime" maps differed for the different coating systems. Materials performance maps were constructed from the results, providing a basis by which the use of such coatings may be optimized for exposure to aqueous erosion-corrosion conditions. (C) 1999 Elsevier Science S.A. All rights reserved

Preferential erosive wear of droplet particles for cathodic arc/unbalanced magnetron sputtering CrN.NbN superlattice PVD coatings

In a previous letter [1], we have demonstrated that corrosion of a physical vapor deposition (PVD) coating substrate system can be induced by coating defects, resulting in premature damage to the usually hard and corrosion resistant coatings. Shrinkage pin holes, one of the most common defects, allow access of solution to substrate to cause galvanic corrosion [2, 3], and this may be prevented by forming a pin-hole interrupting under or sandwich layers of passivating metals such as Al or Ti [4, 5]. Since the increasing application and development of plasma arc in various PVD techniques, owing to the capability of the arc technique to achieve relatively high metallic vapor ion valence states, surface and atom mobility and diffusivity, and consequently the greater coating-substrate adhesion [6], more attention has been paid to the study of the arc-related formation of droplet or macroparticle coating defects.Due to the nature of its formation, a macroparticle is the product of a droplet of cathode metal induced by plasma arc heating, which, after being ejected from the cathode, solidifies and becomes embedded within the coating after incomplete reaction (due to its size) within the deposition chamber gas (N-plasma, for example)during its flight to the substrate [7]. That is why a macroparticle is different, in addition to its great size,in chemical composition (N-content) not only from the adjacent coating matrix, but also from the particle exterior to interior parts [8]. The composition inconsistency of the droplets rendered the defects to galvanic coupling (anodic to the adjacent coating matrix) to corrode first upon contact with aqueous solutions, and with progression, this eventually led to the penetration of solutions to the substrate to cause more severe crevice corrosion [9]. The severity of such droplet-induced corrosion depends on many factors but generally droplets are regarded to be detrimental as far as their effect on the overall coating's corrosion performance is concerned [10-12]

The influence of low concentrations of chromium and yttrium on the oxidation behaviour, residual stress and corrosion performance of TiAlN hard coatings on steel substrates

Ti0.43Al0.52Cr0.03Y0.02N films, which have been shown to exhibit a fine grain near equiaxed microstructure were found to exhibit a compressive residual stress of - 6.5 GPa in contrast to conventional columnar Ti0.44Al0.53Cr0.03N coatings which demonstrated - 3.8 GPa compressive stress. Novel coatings with this modified microstructure were also found to possess improved resistance to both dry oxidation and wet aqueous corrosion. Glancing angle parallel beam geometry X-ray diffraction (GAXRD) studies showed that in conventional Ti0.44Al0.53Cr0.03N films, severe oxidation initiated above 850 degrees C whilst oxidation of Ti0.43Al0.52Cr0.03Y0.02N started close to 950 degrees C. In an alkaline aqueous medium, Ti0.43Al0.52Cr0.03Y0.02N coatings deposited on steel showed an extended passive potential range and a significantly lower passive current compared with Ti0.44Al0.53Cr0.03N films of similar thickness. A similar improvement was evident in sulphuric acid where yttrium containing coatings passivated at high potential (Ti0.44Al0.53Cr0.03N films did not passivate). These effects may be ascribed to reduced porosity in the fine-grained Ti0.43Al0.52Cr0.03Y0.02N as well as the well-known effects of low concentrations of yttrium on high-temperature oxidation performance. (C) 1999 Elsevier Science Ltd. All rights reserved

General discussion

Publication of a general discussion as part of the soft nanotechnology issu