Showing papers in "Surface Engineering in 2014"

Cold spray coating: review of material systems and future perspectives

Abstract: Cold gas dynamic spray or simply cold spray (CS) is a process in which solid powders are accelerated in a de Laval nozzle toward a substrate. If the impact velocity exceeds a threshold value, particles endure plastic deformation and adhere to the surface. Different materials such as metals, ceramics, composites and polymers can be deposited using CS, creating a wealth of interesting opportunities towards harvesting particular properties. CS is a novel and promising technology to obtain surface coating, offering several technological advantages over thermal spray since it utilizes kinetic rather than thermal energy for deposition. As a result, tensile residual stresses, oxidation and undesired chemical reactions can be avoided. Development of new material systems with enhanced properties covering a wide range of required functionalities of surfaces and interfaces, from internal combustion engines to biotechnology, brought forth new opportunities to the cold spraying with a rich variety of material ...

Cold spray coating process for corrosion protection: a review

Abstract: Cold spray deposition is an emerging technology that addresses many classic shortcomings of more traditional thermal spray processes. Efforts are under way as of this writing to develop a more comprehensive scientific basis for the cold spray process as well as a wider range of suitable materials and its applications for corrosion protection. This paper presents a tutorial introduction to the cold spray coating process, highlighting the coating formation mechanisms, its characteristics and applications. It includes the comparison of cold spray technique with some popular thermal spray techniques. A thorough survey on the combinations of coatings deposited on diverse substrates with their features has also been detailed with emphasis on its applications for oxidation/corrosion protection.

Mainstreaming cold spray – push for applications

Abstract: Modern cold spray, first implemented 30 years ago, has since expanded rapidly in system designs and applications. While initially considered to be a method to deposit a ductile metal coating onto another metal, subsequent innovations in design and use have allowed new applications to areas as diverse as medical and electronics. This paper describes some new methods and applications that are currently being developed. Twenty-four new applications are presented, chosen to represent the diversity of actual and potential uses of cold spray technology.

Modelling of impact behaviour of cold spray particles: review

Abstract: Cold spraying (CS) is a coating technique, which has developed rapidly in the last two decades and shows great potential in the industrial community due to its advantages of low temperature deposition as well as no oxides forming in the coating. This review’s focus is on the behaviour of particles impacting and the prediction of critical velocity for particle deposition during CS as calculated by numerical simulations according to the open literature. The first part presents an introduction of CS and its particle bonding mechanism. The second part briefly introduces the typically employed numerical computation methods and compares these methods. The third part discusses the effect of particle parameters on particle deformation behaviour. Finally, the current problems and prospects existing in numerical simulations of the impact of CS particles are explored.

Preparation of CuO nanostructures coating on copper as supercapacitor materials

Abstract: Copper oxide nanostructures with diverse morphology were prepared using Cu foil as substrate via a solution route. The as grown copper oxide nanostructures were characterised using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy analysis. The XRD and FTIR analysis confirm the formation of copper oxide. Scanning electron microscopy analysis shows the formation of hierarchical structures of copper oxide with different morphologies. Application of the prepared copper oxide was evaluated as supercapacitive material in 1M Na2SO4 solution using cyclic voltammetry (CV). The specific capacitance has been calculated using CV curves. It has been found that the pseudo-capacitor performance of copper oxide can be tuned via employing different samples.

Experimental and SPH study of cold spray impact between similar and dissimilar metals

Abstract: In this study, the critical, maximum and optimum velocity of a single cold sprayed (CS) particle is estimated using the smoothed particle hydrodynamics (SPH) method by evaluating the impact shape coefficient of restitution, as well as the rebound and deposit energy ratio. The contact surfaces of the particle and the substrate are modelled as intersurface forces using the Dugdale– Barenblatt cohesive zone model. The application of SPH allows the simulation of the CS process without the use of mesh, thereby avoiding the disadvantages of the traditional numerical method in handling large deformations and tracing moving interfaces. The impact of CS particles is simulated using various powder and substrate materials. The influence of the materials on the optimum velocity and the coating quality is discussed. The reliability of the model is verified with CS experiments.

Corrosion protection of magnesium and its alloys by metal phosphate conversion coatings

Abstract: This work describes a group of metal phosphate films, including calcium, manganese and strontium phosphate conversion coatings, for corrosion protection of magnesium alloys. These films have been developed by a process that is simple for implementation, chemically stable and non-toxic. The preparation and characterisation of these coatings, along with evaluation of their corrosion performance in 0·1M NaCl, are systematically described. Phase diagram of calcium phosphate was produced to elucidate the coating formation mechanism, which depends upon the chemistry of coating solution, in particular pH and ion concentration. A discussion of the advantages and disadvantages of the coatings is presented, along with perspectives for industrial applications.

Effect of cutting speed and CVD multilayer coating on machinability of Inconel 825

Abstract: The current study aims at investigating the effect of cutting speed (51, 84 and 124 m min−1) as well as tool coating deposited using chemical vapour deposition (CVD) on machined surface roughness, tool wear characteristics, chip morphology and chip reduction coefficient (ζ) during dry machining of Inconel 825. Turning operation was carried out using ISO P30 uncoated cemented carbide and CVD multilayer coated (TiN/TiCN/Al2O3/ZrCN) insert with a constant feedrate of 0·198 mm rev−1 and depth of cut of 1 mm. Chip morphology and tool wear were studied using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and optical microscopy. Results indicated CVD multilayer coated tool resulted in more machined surface roughness compare to it uncoated counterpart. However, coated tool demonstrated excellent resistance to crater and flank wear, and decrease in chip reduction coefficient during dry machining of Inconel 825.

Which powders for cold spray

Abstract: Since the major technological issues in cold spray are now controlled, the key parameter for successful coating relates to the powder. The influence of the main characteristics of the starting powder on the final coating properties remains rather unknown. This includes primarily metallurgical, morphological and physico–chemical characteristics of the particles. This article focuses on the so-called local approach to these characteristics to show that the current global approach is insufficient. The discussion is based on many examples of cold spray results, including recent work on Ag-based and Ta cold sprayed-coatings. This results in proposed specifications for powders which are claimed to be tailored for cold spray.

Significant influence of carrier gas temperature during the cold spray process

Abstract: Carrier gas is known as the medium to inject the cold sprayed powders into the main driving flow inside the nozzle. Hence, the properties and conditions of carrier gas should be of great importance to the particle motion behaviour and then particle deposition process. In this study, the effect of carrier gas temperature on the particle acceleration and deposition in cold spray process was investigated by both numerical and experimental methods. It is found that the supersonic driving gas flow and the consequent particle acceleration behaviour are significantly influenced by the carrier gas temperature, more specifically, higher carrier gas temperature results in higher particle impact velocity. In addition, because the carrier gas has additional heating effect on the powder particles before injection, the final impact temperature also increases with the carrier gas temperature, which leads to the reduction in the critical velocity. The increase in particle impact velocity and reduction in critical velocit...

Microstructural characterisation and microhardness distribution of HVOF sprayed WC–10Co–4Cr coating

Abstract: The aim of this work was to study the detailed microstructures, thermostability and microhardness distribution of high velocity oxygen fuel sprayed WC–10Co–4Cr coating using X-ray diffraction, scanning electron microscope, energy dispersive spectroscopy, differential scanning calorimeter and high resolution transmission electron microscope. The coating has a dense structure and is well bonded to the substrate. The coating is mainly composed of WC phase and W2C phase. Owing to the high cooling rates of molten droplets and the multicomponent system of feedstock powder, amorphous phase is obtained in the coating. The crystallisation temperature of the amorphous phase is 651°C. The Weibull analysis of Vickers microhardness of the coating exhibits a bimodal distribution under indentation load of 300 g and a monomodal distribution under indentation load of 100 g. Such distribution is attributed to the presence of the poorly bonded splats and pores within the coating.

Wear and high temperature oxidation behaviour of wire arc sprayed iron based coatings

Abstract: In the present work, FeCrB(CSi) cored wires (C17, C21 and C25) were designed and deposited as coatings by wire arc spraying. The microstructure, microhardness, wear and high temperature corrosion behaviour of the new coatings were investigated in comparison with a commercial Fe–Cr–Al coating. The FeCrB(CSi) coatings presented lamellar microstructures with some pores and microcracks and a few oxide inclusions. The microhardness of the coatings is much harder than those of the substrate and the commercial Fe–Cr–Al coating. The wear resistance of the coatings is much better than that of the uncoated substrate. Thermogravimetric analysis was used to evaluate the high temperature oxidation behaviour of the coatings at a temperature of 650°C under cyclic oxidation conditions. The results show that the high temperature oxidation resistance of the C17 coating was not as good as the Fe–Cr–Al coating, while the C21 and C25 coatings exhibited better oxidation resistance than the FeCrAl coating.

Current Design and Performance of Cold Spray Nozzles: Experimental and Numerical Observations on Deposition Efficiency and Particle Velocity

Abstract: Coating technologies play a critical role in the worldwide manufacturing industry. The ability to form layers of specific materials onto engineering components to enhance mechanical and physical properties has numerous applications, ranging from corrosion protection, repair, hardfacing, down to purely aesthetic purposes. Cold spray is an innovative technology, which allows for the manufacturing of coatings in a solid state manner; hence, feedstock properties can be fully preserved. Its working principles relies upon the acceleration of powders up to supersonic velocities, and the subsequent generation of high energy impacts on a substrate which triggers the coating formation. This paper presents deposition efficiency (DE) results from four different supersonic nozzles when using titanium as feedstock material. DE is the most critical parameter to assess the performance of cold spray nozzles. A theoretical analysis through computational fluid dynamics (CFD) is carried out so to compare numerical re...

Influence of current density and electrolyte concentration on DC PEO titania coatings

Abstract: Titania films, 5–9 μm in thickness, were produced on commercial purity titanium by a direct current (DC) plasma electrolytic oxidation (PEO) process using 5 to 20 A dm−2 current densities in 5 to 15 g L−1 trisodium orthophosphate electrolytes. Phase analyses (composition and crystallite size) were carried out using X-ray diffraction and TEM techniques. Residual stresses associated with one of the crystalline coating phases (anatase) were evaluated using the X-ray diffraction Sin2ψ method. SEM and TEM techniques were utilised to study the cross-section and surface morphologies and nanofeatures of the PEO titania coatings. Variations in the proportions of anatase and rutile phases, surface morphologies, coating thicknesses and associated internal stresses are correlated with the current density and electrolyte concentration used during the DC PEO process on titanium. Direct and shear stresses in the anatase are found in the range −70 to −331 and −17 to −167 MPa respectively.

Electrodeposition of Ni matrix composite coatings containing ZrC particles

Abstract: In this paper, Ni matrix composite coatings containing either micro-ZrC or nano-ZrC particles were successfully prepared by direct current electrodeposition. The microstructure of the coatings, including crystallographic texture and grain size, were studied by X-ray diffraction analysis. Energy dispersive spectrometer and scanning electron microscope were used to investigate the chemical composition and morphology of the coatings. The microhardness of the coatings was measured by a Vicker microhardness tester. Grain refinement strengthening and dispersion strengthening mechanisms were applied to interpret the evolution of the microhardness. The corrosion resistance of the Ni matrix composite coatings was investigated by corrosion polarisation method. The composite coatings exhibit better mechanical property and corrosion resistance than pure Ni coating.

Microstructural and mechanical study of PCL coated Mg scaffolds

Abstract: In recent years, attention has been focused on the magnesium (Mg) as a promising material in biodegradable metallic scaffolds for bone tissue engineering. Since an orthopedic scaffold is supposed to repair and regenerate fractured bones, its mechanical integrity is vital throughout the healing process. In this study, a biocompatible polymeric layer made of polycaprolactone (PCL) in different concentrations of 3% w/v and 6% w/v was coated on the surface of Mg scaffolds. The structural characteristics and mechanical behaviour of the Mg scaffolds during the immersion in physiological saline solution (PSS) were investigated. According to our results, the PCL coating hindered the diminution of mechanical stability of scaffolds to provide adequate support for bone healing. Specifically, scaffold coated with 3% w/v and 6% w/v PCL demonstrated 24 and 100% improvement in the elastic modulus and 41 and 83% enhancement in compressive strength respectively, after 24 h immersion in PSS, compare to the uncoated scaffol...

Structure and corrosion properties of cold sprayed coatings: a review

Abstract: Coating technology is growing because of its important role in improving, e.g. corrosion resistance, thermal and electrical conductivities, and other properties of material in order to decrease costs and increase service life and safety. Cold spraying is the technique to produce coatings from powder feedstock by spraying solid particles on the substrate, forming the coating. Cold spraying is a relatively new spraying method, which has many advantages over other forms of thermal spraying. The advantages are dense and pure structures, high deposition efficiency, low residual stresses, minimal heat input to substrate, phase and compositional stability, and little need for masking. Furthermore, cold spraying is a cost effective and environmentally friendly alternative to, e.g. soldering, electroplating, and painting. One potential application field of cold sprayed coatings is corrosion protection due to the possibility to produce fully dense and impermeable coatings. From this aspect, this review is f...

Biodegradable magnesium alloy coated by fluoridated hydroxyapatite using MAO/EPD technique

Abstract: In this paper, a nanostructured fluoridated hydroxyapatite (FHA) coating was used on AZ91 magnesium to improve its corrosion resistance and in vitro bioactivity. The coating was made through micro arc oxidation (MAO) and electrophoretic deposition (EPD) method. After characterising phase composition and surface morphology, the corrosion and the in vitro bioactivity behaviours of the coated materials were investigated by electrochemical measurements and immersion tests in simulated body fluid (SBF). The results confirmed the enhancement of the corrosion resistance and the bioactivity of the AZ91 alloy coated with nanostructured FHA. Therefore, the surface treatment introduced in this paper may be an apt approach to make magnesium alloys more suitable for orthopedic applications.

Annealing behaviour of 6061 aluminium deposited by high pressure cold spray

Abstract: Aluminium 6061 deposited by high pressure cold spray was analysed using a transmission electron microscope (TEM) to characterise its microstructure and response to annealing to 450°C in the plane of the deposition and perpendicular to the deposited layers. The cold sprayed deposition was also analysed with scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) analysis and differential scanning calorimeter (DSC). Segregation of the solute atoms (Mg and Si) at the grain boundaries during cold spraying was seen to play a crucial role in stabilising the deformation substructure until specific temperatures, and was found to have a significant effect on the annealing behaviour of the microstructure in the two different directions.

Determination of high temperature mechanical properties of thermal barrier coatings by nanoindentation

Abstract: A nanoindentation method is employed to measure the high temperature mechanical properties, including hardness H, Young’s modulus E and fracture toughness Kc, of 8YSZ (ZrO2+8 wt-%Y2O3) thermal barrier coatings (TBCs) under as sprayed condition and after subjecting them to thermal oxidation. The tests are conducted on the surfaces of YSZ coatings at temperatures of 250 and 450°C. The expressions for indentation H and E are deduced analytically, and the influence of testing temperature and thermal oxidation on them is discussed. An expression for estimating the fracture toughness of the coatings is also derived. The estimated fracture toughness values for the as sprayed YSZ coatings in terms of stress intensity factor at 250 and 450°C are determined to be 1·58 and 0·84 MPa m1/2 respectively. For the oxidised YSZ coatings, these values are 1·35 and 1·10 MPa m1/2 respectively. This study proves that nanoindentation tests can be performed at elevated temperatures for YSZ based TBCs.

Process optimisation of cold spray Al coating on AZ91 alloy

Abstract: Cold spray technology continues to attract increasing attention in surface modification of light metals due to its low temperature operation. A method for selecting optimum process parameters is presented for cold spraying of aluminium coatings upon magnesium alloy AZ91. In the present study, particle deposition behaviour is investigated via modifying spraying parameters. Subsequently, based on the relationship between bonding ratio and particle velocity, the critical particle velocity for successful adhesion is determined. Finally, the influence of particle velocity on key coating characteristics is examined, including coating porosity, hardness, deposition efficiency and bond strength.

Promise of multiobjective genetic algorithms in coating performance formulation

Abstract: Genetic and evolutionary algorithms are inspired by natural biological processes. In these nature inspired computing paradigms, even non-biological problems are solved constructing a pseudobiologic...

Investigation of boriding kinetics of AISI D2 steel

Abstract: A diffusion model was suggested to analyse the growth kinetics of Fe2B layers formed on AISI D2 steel by the pack-boriding process. It was used to estimate the boron diffusion coefficients of Fe2B in the temperature range of 1123–1273 K by applying the mass balance equation at the (Fe2B/substrate) interface. The proposed model was validated experimentally at 1253 K for a treatment time of 5 h by comparing the experimental Fe2B layer thickness with the predicted value. Furthermore, the pack-borided AISI D2 steel was characterised by optical microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction analysis. A contour diagram describing the evolution of Fe2B layer thickness as a function of the process variables was also proposed. In addition, the boron activation energy for AISI D2 steel was found to be equal to 201·50 kJ mol−1, on the basis of our experimental results.

Innovative surface modification of orthopaedic implants with positive effects on wettability and in vitro anti-corrosion performance

Abstract: In this work, sol–gel derived bioactive glass/zirconium titanate coatings were uniformly deposited on stainless steel orthopaedic implants, by using carboxymethyl cellulose as a particulate dispersant in the sol. The surface features, wetting, and in vitro electrochemical corrosion behaviour of the coated samples were evaluated. It was found that, by applying the coating on the substrate, the water contact angle was decreased, which is indicative of an improvement in the implant hydrophilicity. Also, the coating improved the corrosion resistance of the metallic implant, as realised by an increase in the corrosion potential and a decrease in the corrosion current density. Indeed, this coating acted as a physical protective barrier which retards the electrolyte access to the metal surface and thereby electrochemical processes.

Influence of plasma coatings on Inconel 617 for gas turbine applications

Abstract: The Inconel 617 with plasma sprayed Al2O3–40%TiO2 (A40T) and NiCr–Cr2O3 coatings were investigated for the hot corrosion behaviour at 1000°C for gas turbine applications. The behaviour of the alloy under the mixed salt environment with and without V2O5 in Na2SO4 and NaCl was analysed by means of mass change per unit area, optical and SEM images along with X-ray diffraction and EDAX. Microstructural and EDAX analyses revealed presence of oxides, rich in Ni and Mo on the surface of the samples. The coated sample exhibited better corrosion resistance in A40T and NiCr – Cr2O3 coatings than bare alloy.

Influence of magnetic fields on cobalt electrodeposition

Abstract: Magnetic fields parallel to electrodes were introduced during plating process to prepare cobalt films from baths without additives. Effects of magnetic intensities on the nucleation process, electrochemical mechanism and surface morphology were investigated. It was found that limiting current and deposition mass increased gradually with the rise in magnetic intensities. Magnetohydrodynamic phenomenon (magnetic fields can induce currents in a moving conductive fluid, which in turn creates forces on the fluid and also changes the magnetic field itself) caused by Lorentz forces could agitate to decrease thickness of diffusion layers, which contribute to the increase in deposition rate. Reduction of cobalt on copper substrates without magnetic fields showed instantaneous nucleation process. However, cobalt reduction with 1 T magnetic intensity deviated from instantaneous nucleation process as a result of micromagnetohydrodynamic (micro-MHD) flows. Films of smaller grain size and compact surface could be obtained under 1 T magnetic intensity as a result of MHD effects.

Dry erosion wear performance of Inconel 718 microwave clad

Abstract: This paper reports on the dry erosive wear performance of Inconel 718 clads deposited on SS-304 substrates through microwave hybrid heating technique. Clads were deposited using a domestic microwave applicator at 2·45 GHz and 900 W. The microstructural observations of the Inconel 718 clad indicate good metallurgical bonding with the substrate and revealed no visible interfacial cracking. The microhardness of the clads was assessed using a Vicker’s microhardness tester and the average microhardness in the clads was 564±22 HV. Erosive wear performance of the clads was evaluated using an air jet erosion test setup as per ASTM G76 standard. The effect of varying impact angle was studied; the results have been discussed with the help of SEM images of the worn surfaces. It was observed that the improved erosive performance of the Inconel 718 clads was due to presence of strengthening intermetallic phases (Ni3Ti, Ni3Al) in the tough Ni–Fe–Cr matrix.

Corrosion resistance of EPD nanohydroxyapatite coated 316L stainless steel

Abstract: The effect of nanohydroxyapatite (HA) coating on corrosion resistance of 316L stainless steel was investigated. The stainless steel samples were pretreated in different volume concentrations of H2SO4 solution and were electrophoretically coated for various coating time spans and voltages. The coated samples were sintered in a vacuum furnace at different temperatures to modify the coating structure. The quality and morphology of the pretreated samples and the coating were microscopically characterised, and the compounds presented in the coatings were determined. The electrochemical corrosion test was performed in simulated body fluid to evaluate the coating impact on corrosion behaviour of the samples. The results showed that the deposition of nano-HA using moderate voltage and appropriate time span provides a smooth and almost cracked free coating, improving the corrosion resistance of the samples. Increasing the temperature of sintering modified the HA structure, resulting in the formation of denser coating and enhanced corrosion resistance of the samples.

Microstructure and mechanical properties of surface layer of M50NiL steel plasma nitrided

Abstract: The quenched M50NiL steel specimens have been plasma nitrided at temperatures from 460 to 590°C for 4 h under a constant mixture gaseous supply of 0·05N2–0·4H2 L min−1. The effects of the treatment temperature on the microstructure, microhardness and wear resistance of the surface nitrided layers are studied. The results show that the plasma nitrided layer includes only the diffusion layer without conventional compound layer. The surface hardness is significantly enhanced. α′-Fe and γ′-Fe4N phases in surface layer are formed at relatively low nitriding temperatures (460–560°C), while a low nitrogen phase FeN0·076 forms when the nitriding temperature exceeds 575°C. With the increase in nitriding temperature, the nitrided layer thickness increases, whereas both the surface and core microhardness of the nitrided specimens decreases. The wear resistance of specimens can also be improved significantly by plasma nitriding.

Chlorine based hot corrosion study of cold sprayed NiCrAlY coating

Abstract: In this work, the NiCrAlY coating was deposited by cold spray deposition technique on Fe based Superfer 800H superalloy. The cold sprayed coating was found to be dense and hard and adhered strongly to the substrate superalloy. In the present investigation, the cyclic hot corrosion behaviour of the coated and bare specimens under the Na2SO4–10%NaCl environment at 900°C is described. It is found that unlike bare superalloy, uniform, dense and adhered scale layer formed on top of the coated specimen. The various characterisation techniques X-ray diffraction, SEM/EDAX and elemental X-ray mapping were used to analyse the corrosion products. It was found that the coating deposited by this coating technique successfully provided protection to the substrate against the hot corrosion degradation by reducing the weight gain of the superalloy.