Low-pressure Cold Spray Research Articles

Particle deformation and microstructure evolution during cold spray of individual Al-Cu alloy powder particles

This paper examines the single particle impact process for a series of Al-Cu alloy powder particles with 2–5 wt% copper, performed using a low-pressure cold spray system with helium as the carrier gas. The cold spray deposition process is fundamentally controlled by the deformation processes, which occur during single particle impacts. Single particle depositions on steel substrates were produced for each composition over a range of gas temperatures (225–325 °C). Cross sections from single, impacted particles were produced using focused ion beam methods. The deformation microstructure in individual cold sprayed particles was studied using precession electron diffraction (PED) and transmission Kikuchi diffraction (TKD) techniques. For Al-Cu alloy particles, the amount of deformation estimated using a compression ratio did not show a significant difference with varying copper alloy additions. The single particles experienced large deformation upon impact, and ultrafine grains were formed at the particle/substrate interface via dynamic recrystallization. The particles were bonded to the steel substrate via a thin amorphous layer at the interface.

Aligned Carbon Nanotube-polymer Composite Film Deposited by a Low-pressure Cold Spray Process

Aligned Carbon Nanotube-polymer Composite Film Deposited by a Low-pressure Cold Spray Process

Characterization and comparison of copper coatings developed by low pressure cold spraying and laser cladding techniques

Abstract Copper coatings of 3-mm thickness were developed by both cold spraying and laser cladding techniques on SS316 steel. The developed coatings were then characterized with various techniques to evaluate metallurgical and mechanical properties. Tensile strength (170 MPa) of the cold sprayed coating was found to be comparable with that of the laser cladded coating, whereas %age elongation of laser cladding (11%) was almost double (6%) of that for the cold sprayed coating. Porosity was found to be low in the cold sprayed coating as compared to that in the laser cladding, which indicates that the former technique could give a denser coating. This difference in porosity led to relatively higher electrical and thermal conductivity in the cold sprayed coating as compared to the laser cladding. X-ray diffraction and electron disperse spectroscopy analyses revealed the absence of oxide formation in the cold sprayed coating, however oxides were seen in the laser cladding. Also, a large heat affected zone (HAZ) was detected in the laser cladding, whereas no HAZ was observed in the cold sprayed coating. Hardening was observed along the interface and HAZ of the cladded steel, which may be attributed to the diffusion of carbon from the substrate

Cold Spray of Mixed Sn-Zn and Sn-Al Powders on Carbon Fiber Reinforced Polymers

Carbon fiber reinforced polymers (CFRPs) have been increasingly used in the latest generations of aircraft and helicopters for lightweight purposes, but this leaves vulnerability against lightning strike. Cold spray is one coating approach to metallize the polymers, thus making them lightning strike proof. It has been reported that direct cold spray of metals onto CFRP is difficult. However, research at McGill University has shown that tin coatings can be cold sprayed on CFRP, but the deposition efficiency is very low. In this work, aluminum and zinc powders were mixed with tin to investigate the effect of mixing on deposition efficiency of the coating. The mixed metal powders were cold sprayed on CFRP with a low-pressure cold spray system at various conditions. It was found that the addition of aluminum or zinc resulted led to increased deposition efficiencies compared to pure tin, but there are many differences in the details of the effect of Al and Zn additions on the deposition characteristics. The deposition mechanism of the mixed metal powders on CFRP is discussed, and the effect of mixing powders on the deposition efficiency is evaluated.

Microstructure, Mechanical Properties and Dry Sliding Wear Behavior of Cu-Al2O3-Graphite Solid-Lubricating Coatings Deposited by Low-Pressure Cold Spraying

In this study, Cu-Al2O3-graphite solid-lubricating coatings were deposited on the 304 stainless steel substrate by low-pressure cold spray technique. The influences of Cu-coated graphite content on the microstructure, mechanical properties and dry sliding wear behavior of the Cu-based solid-lubricating coatings were investigated. The results showed that the mixture addition of Al2O3 and Cu-coated graphite is an effective strategy to improve the solid lubricity of Cu-based coatings while simultaneously retaining favorable mechanical properties and bonding strength. When the Cu-coated graphite content increased from 0 to 20 wt.% in the spray powder, the Brinell hardness and the interface bonding strength of the Cu-based coatings declined, but the solid-lubricating property improved obviously. The Cu-based solid-lubricating coating with 10 wt.% Al2O3 and 10 wt.% Cu-coated graphite exhibited the lowest friction coefficient of 0.29, which was attributed to the formation of a lubricating tribo-layer and the reduction in cracks on the worn subsurface. Additionally, the wear mechanism changed from adhesive wear to abrasive wear and delamination with the increase in graphite content.

Low-Temperature Transformation of Amorphous Sol\u2013Gel TiO2 Powder to Anatase During Cold Spray Deposition

Nanocrystalline TiO2 coatings were successfully deposited on metallic substrate surfaces through low-pressure cold spray. Sol–gel amorphous TiO2 dry gel was for the first time used as a feedstock powder. Agglomerated morphology of the amorphous powder with high-density packing of particles and reduced pores number was an important factor for successful low-pressure cold spraying. However, in the present study one more factor contributing to anchoring of TiO2 particles by cold gas spraying onto a substrate was indicated. This factor enabling effective powder deposition and coating production is based on an exotic for conventional ceramic materials’ plastic deformation of amorphous powder. Furthermore, the detailed analysis of SEM, Raman and XRD measurements clearly demonstrated that amorphous titanium dioxide is transformed into crystalline anatase form during the process of low-pressure cold spraying.

Application of interlayers in the soldering process of graphite composite to aluminium alloy 6060

The paper presents a problem of joining graphite-copper composite to aluminium alloy 6060 by soldering method. This type of joints is used in electricity transmitting devices, e.g. in current receivers of rail vehicles. Therefore, the joints should show good electrical conductivity. Soldering process of graphite to aluminium directly is impossible and requires application of metallic interlayers. The problem was solved by using interlayers deposited by electroplating and low pressure cold spraying (LPCS) prior to soldering process. Various methods of substrate preparation were used. The results of shear strength and metallographic analysis were described. The highest shear strength of 17.6MPa showed sample prepared by grit blasting and cold plasma, simultaneously.

Preliminary studies of TiO2 nanopowder deposition onto metallic substrate by low pressure cold spraying

Ceramic coatings, e.g. TiO2, can be produced by various methods, such as flame spraying, plasma spraying, PVD, CVD, etc. According to the literature, cold spraying enables deposition of plastic materials and therefore is dedicated to metals and polymers. Ceramics are hard and brittle due to the complex crystalline structure. However, very recently it was shown that chosen ceramics can be deposited by cold spraying. In this work, various polymorphs of sol-gel TiO2 powders, e.g. amorphous, anatase and rutile, were used as a feedstock for the cold spray (CS) process. The preliminary study of coating formation indicated that the key parameter for the process seems to be temperature of working gas. However, coatings were deposited form all prepared powders. Selected parameters enabled deposition of titania coatings with the thickness about 100 μm. What is even more interesting, the increase in the temperature of working gas results in nanocrystallization of the amorphous powder into anatase coating.

Microstructure and Mechanical Properties of Zn-Ni-Al₂O₃ Composite Coatings.

Zn-Ni-Al2O3 composite coatings with different Ni contents were fabricated by low-pressure cold spray (LPCS) technology. The effects of the Ni content on the microstructural and mechanical properties of the coatings were investigated. According to X-ray diffraction patterns, the composite coatings were primarily composed of metallic-phase Zn and Ni and ceramic-phase Al2O3. The energy-dispersive spectroscopy results show that the Al2O3 content of the composite coatings gradually decreased with increasing of Ni content. The cross-sectional morphology revealed thick, dense coatings with a wave-like stacking structure. The process of depositing Zn and Ni particles and Al2O3 particles by the LPCS method was examined, and the deposition mechanism was demonstrated to be mechanical interlocking. The bond strength, micro hardness and friction coefficient of the coatings did not obviously change when the Ni content varied. The presence of Al2O3 and Ni increased the wear resistance of the composite coatings, which was higher than that of pure Zn coatings, and the wear mechanism was abrasive and adhesive wear.

Analytical study of the heat transfer coefficient of the impinging air jet during cold spraying

A semi-empirical analytical model was developed to determine the convective heat transfer coefficient of an impinging air jet generated by a cold spraying nozzle. A low-pressure cold spraying unit was utilized to produce hot air jets that impinged upon a flat substrate surface. An infrared camera was used to measure the surface temperature of the substrate at different time intervals. A method involving Green's functions was employed to solve a transient two-dimensional heat conduction problem to obtain an expression for the temperature distribution within the substrate. By coupling the analytical results of temperature distribution and experimental surface temperature data, the radial variation of the non-dimensional heat transfer coefficient of the impinging air jets upon the substrate was estimated. The results showed that the maximum values of the heat transfer coefficient were present close to the stagnation point of the air jets. It was found that the heat transfer coefficient was independent of the time that the cold spray nozzle remained stationary over the substrate surface. It was found further that by increasing the stand-off distance of the nozzle, the radial variations of the heat transfer coefficient became negligible, compared to those for small stand-off distances. The close agreement between the experimental results and the predictions of the model suggested that the estimated heat transfer coefficient of the cold spray gas jet can be used to estimate the surface temperature of the substrate at any time.

Experimental Study and Numerical Investigation of the Phenomena Occurring During Long Duration Cold Spray Deposition

Low Pressure Cold Spray process is a relatively new additive technique allowing to create high quality metallic coatings, through the high velocity spraying of particles, on both metallic and non-metallic substrates. The adhesion mechanism of the particles is, to date, not fully understood and the phenomena occurring during long depositions (which are the more interesting for industrial processes) are not known and studied in details. Aiming to fulfill this lack of knowledge, the phenomena occurring during long deposition were studied through both careful experimental evaluations and numerical approach. Particular attention was paid at the working conditions of the De Laval nozzle system which accelerates the particles, a computational fluid dynamics model focusing on both geometrical features and spray behavior was proposed. Micron-sized aluminum powders and compressed air as carrier gas were used in this experimentation, numerical simulations are performed to predict the gas flow regime. It has been found that the deposition efficiency tends to decrease due to the critical phenomena occur into the spray nozzle during long time depositions. The sprayed particles tend to stick to the walls of the nozzle and, moreover, shock waves occur inside the nozzle further promoting the particles stick phenomena, making the system inefficient.

Metallisation of polycarbonates using a low pressure cold spray method

ABSTRACTIn many industries, polymers are in great demand due to their low weight and good mechanical properties. However, films and coatings are sometimes applied to improve polymer surface properties. This work presents a study on the metallisation of polycarbonates using a low pressure cold spray (LPCS) method. Two types of commercial powders were selected for the deposition of coatings: (i) tin and (ii) aluminium. The coatings were applied (i) on an unmodified substrate and (ii) on a substrate with an interlayer, which enabled the formation of good adherent aluminium coatings. A light and scanning microscopy was used to analyse the microstructure of coatings. Electrical conductivity was determined using the four-probe method and is discussed in relation to the oxygen content. Moreover, bond strength and microhardness were investigated.

Three-dimensional numerical simulations of the particle loading effect on gas flow features for low pressure cold spray applications

This paper reports numerical investigations to develop a high quality simulation model for the Cold Spray (CS) process as part of the project, Supersonic Spray Advanced Modelling (SSAM). Cold spray is a process in which unmelted solid particles are accelerated through a De Laval nozzle toward a target surface and deposited, which avoids or minimises the detrimental effects that arise from melting. The gas flow injected allows the particles to achieve velocities up to 1000 m/s until they impact onto a substrate, bonding with plastic deformation. Analytical and numerical tools were used in the recent years to study the most important parameters of the fluid dynamic of the process. These parameters are often overestimated or non-validated, as many authors tend to neglect inter-phase couplings. The interactions between the gas flow features and particulate phase are important for the present multiphase flows, and many of the connected phenomena lack physical comprehension. For this purpose, a detailed 3D model for RANS simulations has been realised and the gas and particle dynamics in the nozzle and jet were analysed. Advancements for future computational methods are derived from this study, in order to build a comprehensive and coherent numerical model for cold spray applications.

Effect of Substrate and Process Parameters on the Gas-Substrate Convective Heat Transfer Coefficient During Cold Spraying

The final quality of cold-sprayed coatings can be significantly influenced by gas-substrate heat exchange, due to the dependence of the deposition efficiency of the particles on the substrate temperature distribution. In this study, the effect of the air temperature and pressure, as process parameters, and surface roughness and thickness, as substrate parameters, on the convective heat transfer coefficient of the impinging air jet was investigated. A low-pressure cold spraying unit was used to generate a compressed air jet that impinged on a flat substrate. A comprehensive mathematical model was developed and coupled with experimental data to estimate the heat transfer coefficient and the surface temperature of the substrate. The effect of the air total temperature and pressure on the heat transfer coefficient was studied. It was found that increasing the total pressure would increase the Nusselt number of the impinging air jet, while total temperature of the air jet had negligible effect on the Nusslet number. It was further found that increasing the roughness of the substrate enhanced the heat exchange between the impinging air jet and the substrate. As a result, higher surface temperatures on the rough substrate were measured. The study of the effect of the substrate thickness on the heat transfer coefficient showed that the Nusselt number that was predicted by the model was independent of the thickness of the substrate. The surface temperature profile, however, decreased in increasing radial distances from the stagnation point of the impinging jet as the thickness of the substrate increased. The results of the current study were aimed to inform on the influence and effect of substrate and process parameters on the gas-substrate heat exchange and the surface temperature of the substrate on the final quality of cold-sprayed coatings.

The connections between powder variability and coating microstructures for cold spray deposition of austenitic stainless steel

This article explores the variability in austenitic stainless steel feed-stock powders and its effect upon low-pressure cold spray deposition. Four, commercial, austenitic stainless steel powders were deposited using cold spray deposition with helium gas at a temperature of 230°C and a pressure of 1.7MPa. Electron microscopy and X-ray diffraction were used to characterize the microstructure of both the powders and the resultant coatings. The particle size for the powders ranged from 17 to 40μm, while the crystallites ranged from 2 to 6μm in size. Only the 304L stainless steel powder was purely austenite in phase; while the three 316L powders had ferrite contents ranging from 23 to 50%. The deposition efficiency of the four powders varied considerably from 12 to 42%, but this variation did not correlate with the ferrite content alone. The spatial location of the ferrite in the cold sprayed material was highly dependent upon the powder type used: ferrite primarily on the outside of austenite particles, mixtures of wholly ferrite and wholly austenite particles, and particles with both ferrite and austenite internally. After cold spray deposition, all of the coatings exhibited a composite microstructure containing highly deformed prior particle centers with small (300nm) crystallites near the prior particle boundaries.

Microstructural Characteristics and Oxidation Behavior of Low-Pressure Cold-Sprayed CoNiCrAlY Coatings

CoNiCrAlY coatings were deposited by low-pressure cold spraying and subsequently heat-treated at 1050 °C for 4 h in a vacuum environment. The microstructural characteristics and oxidation behavior of CoNiCrAlY coatings were investigated. The as-sprayed coating exhibited low porosity and oxygen content. The high plastic deformation of the sprayed particles led to significant refinement of γ-matrix and dissolution of β-(Ni,Co)Al phase in the as-sprayed coating. After heat treatment, the single phase (γ) in the as-sprayed coating was converted into a γ/β microstructure, and a continuous single α-Al2O3 scale was formed on the coating surface. Vacuum heat treatment can postpone the formation of spinel oxides within 100 h. After being oxidized at 1050 °C for 400 h, the heat-treated coating exhibited better oxidation resistance than the as-sprayed coating. The reduced growth rate of the oxide scale and the suppression of the formation of spinel oxides can be attributed to the vacuum heat treatment, as well as the intrinsic microstructure of the cold-sprayed coating. Finally, the effects of the microstructural changes induced during the cold spraying process on the growth of the thermally grown oxide and the oxidation mechanisms of the CoNiCrAlY coatings were discussed.

Wear behavior of corroded Al-Al2O3 composite coatings prepared by cold spray

Al-based Al2O3 particle-reinforced composite coatings were fabricated onto steel substrates by low pressure cold spray technology (LPCS). The content of Al2O3 in the feedstock was 25vol%. The microstructure of the coating was characterized using scanning electron microscopy (SEM) and X-ray diffraction. It was found that the composite coating contained a uniform distribution of Al and Al2O3 particles without evidence of oxidations. The coating exhibited a very dense microstructure with strong adhesion to the substrate. Neutral salt spray (NSS) and electrochemical tests revealed that the Al-Al2O3 composite coating was still efficient in providing a corrosion protection for the steel substrate, although a porous top layer formed in the coating after 960h corrosion. Corroded coatings were then subjected to dry sliding wear tests from which the wear tracks were observed. The results indicated that increased damage accumulation by corrosion reduced cohesive strength between particles and resulted in high volume losses and poor worn appearances.

Novel Al-based Self-lubricating Wear-resistant Coatings by Low Pressure Cold Spray

To improve the wear resistance of Al coating, the Al-based self-lubricating wear-resistance coatings were fabricated by Low pressure cold spray(LPCS) technology onto Q235 carbon steel. The right amount of hard phase(Al2O3) and a lubricant(MoS2) were added to improve the wear resistance of the nonvel coating. The friction and wear behaviour were carried out using the UMT-3 fricition and wear tester by line-contact reciprocating sliding. The microstructure of coating and corresponding wear traces of these novel Al-based self-lubricating wear-resistant coatings were investigated by means of scanning electron microscopy (SEM), with the energy dispersive spectrum (EDS). The results showed that the average friction coefficient and the weight loss of the novel Al-based composite coatings with the contents of 5wt% MoS2 is lower than Al-Al2O3 composite coating. The novel Al-based composite coating showed an improved wear resistance and the wear mechanism was abrasive and adhesive wear.

Effect of type of reinforcing particles on the deposition efficiency and wear resistance of low-pressure cold-sprayed metal matrix composite coatings

Low-pressure cold spraying was used to deposit boron carbide (B4C), titanium carbide (TiC), and tungsten carbide (WC) based metal matrix composite (MMC) coatings. Nickel (Ni) was used as the matrix and each carbide powder was mechanically blended with Ni powder prior to spraying. The average velocity of the carbide particles and their momentum during the cold spray deposition were estimated using a mathematical model. The effect of the carbide particle momentum on the Vickers micro-hardness and wear resistance was evaluated. The model showed that the average momentum of WC particles was more than two times greater than that of B4C particles and almost six times greater than that of TiC particles. The higher momentum of the WC particles led to a higher level of work hardening of the matrix, which resulted in improvement of the hardness and wear resistance of the MMC coatings. This led to similar hardness values for the deposited MMC coatings (400kg/mm2) despite the difference in hardness of the selected reinforcing carbides. Furthermore, it was found that the high momentum and high fracture toughness of the WC particles increased the roughness of the coating surface and compacted the coating, which led to higher deposition efficiency for this carbide-metal powder blend on previously deposited coating layers. The lowest wear resistance was achieved by the WC-Ni MMC coatings due to the higher fracture toughness of WC particles and also work hardening of the Ni matrix. The results obtained emphasize the importance of the carbide fracture toughness and particle momentum on the deposition efficiency, hardness, and wear resistance of cold-sprayed MMC coatings.

Corrosion behavior of low pressure cold sprayed Zn-Ni composite coatings

In this study, Zn-Ni composite coatings with different Ni contents were deposited on low-carbon steel using Low pressure cold spray (LPCS) method. The morphology, element distribution and phase compositions of powders and as-sprayed coatings were analyzed by using scanning electron microscopy, energy dispersive spectrometer and X-ray diffraction respectively. The corrosion protection mechanism of these composite coatings were investigated using electrochemical technique and their corrosion morphologies. The results show that Zn-Ni composite coatings exhibited a better sacrificial anodic protection and physical shielding effect, where the corrosion resistance of the coating with 10 wt% Ni in the feedstock was best. The corrosion products were mainly composed of Zinc hydroxide Zn(OH)2, basic zinc chloride hydrate ZnCl2·4Zn(OH)2·H2O, basic zinc carbonate hydrate Zn4·CO3(OH)6·H2O and the corrosion products is not mixed with alumina. The corrosion process mainly includes active dissolution of zinc, zinc cathodic protection, physical shielding caused by corrosion products.