Metal finishing: enhanced performance achieved through plasma processing

Abstract

This paper provides an overview on how plasma deposited coatings can significantly enhance the surface properties of metallic components. Two plasma processing technologies known as physical vapour deposition (PVD) and plasma enhanced chemical vapour deposition (PECVD) are introduced. Examples of the coatings deposited using these two processes are discussed. Among the coating application areas outlined are their use to increase the wear life of tools, to improve decorative finish and to enhance medical device biocompatibility.     

Influence of SiC + WC addition to Cr(VI) bath on corrosion behaviour of coatings in sulphate solution

Abstract

Cr–WC–SiC coatings were deposited from Cr(VI) baths containing a mixture of both WC and SiC particles in the ratio of 1 : 1. X-ray photoelectron spectroscopy data indicated relatively low percentages of both WC (about 2˙68–2˙85 at.-% of W4f) and SiC (0˙16–0˙45 at.-% of Si2p) particles in the top layers of the Cr–WC–SiC coatings. However, these particles have some effect on the morphology and corrosion properties of the Cr coating. Protective properties of the coatings obtained were studied at different exposure times of samples in sulphate solution (pH=2˙8), using the electrochemical impedance spectroscopy method (EIS). Equivalent circuits, based on the electrophysical model describing the electrochemical corrosion at the coated surface, were proposed. The simulation of EIS data with the proposed equivalent circuit models made it possible to reveal the details of the corrosion processes occurring at coated systems (steel/Cr–WC–SiC coatings) immersed in the sulphate solution. Based on EIS data, diffusion in the coating was found to be a significantly controlling factor in the corrosion process for the system under investigation. The electrochemical impedance spectroscopy tests indicated a better corrosion resistance of Cr–WC–SiC coating than that of Cr. Analysis of the EIS data suggests that the enhanced corrosion resistance of Cr–WC–SiC coating was due to the microstructural features of these coatings, presumably containing smaller pores than Cr coating.     

Investigations on role of HVOF sprayed Co and Ni based coatings to combat hot corrosion

Abstract

This paper aims to investigate the hot corrosion resistance of high velocity oxy-fuel (HVOF) sprayed cobalt based (Stellite-6) and nickel based (Ni–20Cr) coatings deposited on the superalloy Superni-718 (Ni–19Cr–18˙5Fe–5˙13Ta–3˙05Mo–0˙9Ti–0˙5AI–0˙18Mn–0˙18Si–0˙15Cu–0˙04C) in the Na₂SO₄–60%V₂O₅ salt environment at 900°C under cyclic conditions. The X-ray diffractometry, scanning electron microscopy/energy dispersive analysis and electron probe microanalyser techniques were used to study the corrosion products with respect to their morphology, phase composition and element concentration. The thermogravimetric technique was used to establish the kinetics of corrosion. The bare alloy underwent severe hot corrosion attack. The Ni–20Cr coating shows excellent hot corrosion resistance with negligible spallation, whereas Stellite-6 coating reveals less hot corrosion resistance and more spallation. The hot corrosion resistance of Ni–20Cr coating has been attributed to the formation of oxides of chromium, nickel and spinel of nickel chromium. The oxides of silicon, chromium, cobalt and spinels of cobalt–chromium and nickel–chromium have contributed for hot corrosion resistance of Stellite-6 coatings.     

Origin of hard and soft zone formation during cladding of austenitic/duplex stainless steel on plain carbon steel

Abstract

Weld cladding of corrosion resistant materials is an attractive alternative to bulk usage. Such processes are generally associated with potential deterioration of mechanical properties at the clad/metal interface, often attributed to the formation of hard and soft zones which is usually minimised by control of dilution and use of a buffer layer. To ascertain the origin of the hard and soft zones, weld cladding of austenitic stainless steel (309L) was attempted on plain carbon (0.28 wt-%) steel base metal at various heat inputs, and correspondingly different dilution percentages for the weld metal were obtained. The formation of hard and soft zones was observed mainly on the austenitic stainless steel (i.e. weld metal) side. Typically close to the welding interface (i.e. visible fusion line) a hard zone was observed, followed, but only at higher dilution, by a soft zone. In general, with increasing dilution, the peak hardness and width of the hard zone increased, and the chromium and nickel concentrations of the soft zone significantly decreased. The hard zones were martensitic regions which, at higher dilution, contained chromium carbide precipitates. These precipitates may explain the chromium depletion in the adjacent soft zone. The nickel depletion may be a result of the positive interaction coefficient between nickel and carbon. Since hard/soft zone formation was least at lower dilution, cladding of duplex and superduplex stainless steel was conducted on the same plain carbon base with and without a 309L buffer at an optimum dilution of about 15%. Even at this dilution, direct cladding always resulted in hard and soft zone formation, which was significantly reduced or eliminated by indirect cladding. The hard zones were identified as regions with extensive carbide precipitation (no evidence of martensite formation was found), which occurred in duplex/superduplex grades even at 15% dilution.     

Microstructure and polarisation behaviour of TiN–TiB2 matrix coating prepared by reactive plasma spraying

Abstract

Two TiN–TiB₂ matrix coatings were prepared by reactive plasma spraying with two spraying powders (Ti+B₄C+Cr, Ti+B₄C). Their microstructure, phases, microhardness and polarisation behaviours in 3·5 wt-% NaCl solution have been investigated by SEM, XRD, hardness tester and electrochemical analyser. The thermodynamics analysis for reactive plasma spraying was discussed and the effect of addition of Cr on adiabatic temperature of TiB₂ forming reaction was also analysed. The thermodynamics analysis shows that the reaction temperature during reactive spraying process is >2030 K. To satisfy the criterion of self-propagating high temperature TiB₂ forming reaction, the addition of Cr should be >192·6 wt-% of the total weight of Ti and B₄C. The experimental results show that the addition of Cr decreases the stress in the coating so as to the cracks of coating at the expense of microhardness, yet the microhardness of both two coatings is a high value. The corrosion resistance of TiN–TiB₂ matrix coating was greatly improved. The anodic polarisation curve of TiN–TiB₂ coating includes a narrow activation zone and a broad passivation zone, which indicates the stability of coating is very excellent. The addition of Cr increases the corrosion potential of coating, yet an overpassivation zone appears, which is due to the breakage of passivation film.     

Kinetic equations for non-equilibrium grain boundary segregation induced by applied tensile stress

Abstract

Based on an earlier model, a set of kinetic equations is derived to formulate the process of non-equilibrium grain boundary segregation induced by a low tensile stress. These kinetic equations allow excellent simulation of the grain boundary segregation of phosphorus and sulphur observed in steels subjected to low tensile stresses. The simulation results justify both the earlier model and the present kinetic equations. They also show that an applied tensile stress can increase the diffusion rate of solute-vacancy complexes and decrease that of isolated solute atoms significantly, and can also bring forward the critical time of non-equilibrium segregation for phosphorus and delay that for sulphur.     

Controlled synthesis of microarc oxidation coating on Ti6Al4V alloy and its antifriction properties

Abstract

Ceramic coatings were fabricated on a Ti6Al4V alloy surface by microarc oxidation (MAO) in Na₂ SiO₃ – (NaPO₃)₆ aqueous solutions with and without NaAlO₂ additive using an AC power supply. The effect of NaAlO₂ on microstructure, composition, and homogeneity of ceramic coatings were characterised using SEM, XRD, and EPMA. The antifriction property of the coatings with optimised microstructure sliding against SAE 52 100 steel ball was investigated on a pin-on-disc friction and wear tester. The results show that the addition of NaAlO₂ into Na₂ SiO₃ – (NaPO₃)₆ solution assists the formation of more dense, uniform, and thicker coatings and increases rutile TiO₂ content in the coatings. The optimised coating sliding against the steel has a friction coefficient as low as 0.2 – 0.3 at an applied load of 0.5 N and sliding cycle below 2500, which is much smaller than that of uncoated Ti6Al4V against the same counterpart. The transferring of material from the softer steel ball onto the coating surface is the main wear event, while the microarc oxidation coating is characterised by slight abrasive wear and adhesive wear.     

Transport of radioactive material in Bangladesh: a regulatory perspective

Abstract

Radioactive material is transported in Bangladesh in various types of packages and by different modes of transport. The transport of radioactive materials involves a risk both for the workers and members of the public. The safe transport of radioactive material is ensured in Bangladesh by compliance with Nuclear Safety and Radiation Control (NSRC) Act-93 and NSRC Rules-97. The Bangladesh Atomic Energy Commission is the competent authority for the enforcement of the NSRC act and rules. The competent authority has established regulatory control at each stage to ensure radiation safety to transport workers, members of the general public and the environment. An overview is presented of the activities related to the transport of radioactive material in Bangladesh. In particular, the applicable legislation, the scope of authority and the regulatory functions of the competent authority are discussed. The categories of radioactive materials transported and the packaging requirements for the safe transport of these radioactive materials are also described.     

Effect of substrate surface polishing on galvannealing characteristics of a mild IF and an HSIF steel

Abstract

Mechanical properties and visual appearance of galvannealed steel sheets are strongly influenced by steel substrate surfaces before dipping into molten Zn. It has been known that oil contamination, iron fragments and surface enrichment of oxides deteriorate the Fe–Zn reaction and result in various harmful surface defects on the galvannealed coating. In order to prepare the optimal galvannealed coating for automotive exposed panels, the steel substrate surfaces were mechanically polished by SiO₂ polishing brushes.

The types of brush and pressure between the brush roll and strip are major factors for removing the appearance of visual defects on the surfaces of galvannealed coatings. The microstructure of the polished substrate, galvannealed coating and whiteness index on the surface of steel substrate were characterised by optical microscopy, scanning electron microscopy and colour difference meter.     

Adhesion Improvement of Powder Coating on Medium Density Fibreboard (MDF) by Thermal Pre-treatment

In the powder coating of non-conventional substrates such as engineered wood, natural fibre composites or synthetic polymers, several technological problems must be resolved which result from the substantial differences between such coating substrates and metallic ones. Unlike metals, non-conventional carrier materials show low temperature stability, much rougher and more irregular surface texture, large dependence of their dimensional stability on the moisture content of the surrounding environment and significantly less electrical conductivity. Hence, when powder technology is transferred from coating metals to coating fiberboards, for example, the surfaces of the boards need to be engineered in order to provide ideal adhesion for the coating layer. One major problem is the lack of electrical conductivity. Since the powder coatings are preferably applied using electrostatic spraying equipment, sufficient electrical conductivity of the surface is a major requirement and the correct moisture content plays an important role as well. In the present study, a pre-heating process was used to improve the powder application during powder coating of medium density fibreboards (MDFs). Electrical resistance, treatment temperature and moisture content were systematically studied to better understand the complex physical mechanisms leading to an improvement in powder application by such a pre-heating process. To this end, a new sophisticated procedure was developed to measure the electrical resistance (surface and core resistance) during pre-heating. The results show, that the electrical resistance of MDF is influenced by board temperature and moisture content. Moreover, it is confirmed that pre-heating proves to be an efficient method to improve the powder application onto non-conventional substrates.