Multi-walled carbon nanotube-reinforced copper nanocomposite coating fabricated by low-pressure cold spray process

Multi-walled carbon nanotube (MWCNT)-reinforced copper (Cu) nanocomposite coatings were successfully deposited on aluminum (Al) substrate by a cold spraying process at a low pressure. The microstructure and the Raman spectrum of the low-pressure-cold-sprayed MWCNT–Cu nanocomposite coating showed that the MWCNTs maintained their tube structure in the Cu matrix, even though structural damage to the MWCNTs increased slightly. MWCNT–Cu nanocomposite-coated Al exhibits higher thermal diffusivity than pure-Cu-coated Al with a comparable hardness. The higher thermal diffusivity of the MWCNT–Cu coating could be explained by the dispersion of MWCNTs within the clean and closed CNT/Cu interfaces, which were achieved with the aid of compressive stress during the cold spraying.     

Critical particle velocity under cold spray conditions

Cold spray process is an emerging technique that produces high density coatings. Particles (1 to 50 μm in diameter) are carried by a supersonic gas stream through a de Laval nozzle and, finally, impact on a substrate with high kinetic energy. Low gas temperatures (< 600 °C) make it possible to maintain sprayed material in solid state during the whole process. If the particles reach a given velocity, called “critical velocity”, they can bind to the surface and create a coating. This velocity is clearly dependent on both sprayed material and substrate properties. This work presents an imaging technique that allows a fast measurement of critical velocity. The measuring method is first evaluated by comparing the critical velocity of copper (sprayed on copper substrate) found in the literature, with the measured one. Its accuracy is then tested with other materials and, finally, some improvements of the method are proposed.     

Laser-assisted spraying and laser treatment of thermally sprayed coatings

In the present study, surface engineering research related to thermally sprayed wear and corrosion resistant coatings modified by in-situ and post-treatment by novel high power lasers (Nd:YAG and diode lasers) has been carried out. The main aim of the study was to create experimental based information and knowledge on simultaneous remelting of thermally sprayed (plasma, HVOF, flame) single layers for production of metallurgically bonded, dense corrosion and wear resistant coatings on surfaces of steels. The main focus of research was to estimate the needed processing parameters, which enable simultaneous laser-assisted thermal spraying. The microstructures and hardness-profiles for the coatings prepared by laser-assisted thermal spraying are also introduced.     

Properties of alumina-titania coatings prepared by laser-assisted air plasma spraying

Studies have shown that microstructures formed by post-laser remelting of air plasma sprayed coatings exhibit densification but also numerous macrocracks due to the rapid cooling and thermal stresses. In laser-assisted air plasma spraying (LAAPS) process, the laser beam interacts simultaneously with the plasma torch in order to increase the temperature of the coating and possibly remelt the coating at the surface. As a result, the microstructure is partially densified and macrocracks, which are generally produced in the post-laser irradiation treatment, may be inhibited. In this paper, LAAPS was performed to improve the hardness and wear resistance of Al₂O₃-13%TiO₂ coatings. These coatings prepared by air plasma spraying (APS) are widely used to protect components against abrasive wear at low temperatures. The coating microstructure was characterized by SEM and X-ray diffraction. The mechanical characterization was done by hardness measurements, erosive wear tests and abrasion wear tests. Results showed that laser assistance may improve the microstructural and mechanical properties. Phenomena involved in LAAPS of alumina-titania coatings are discussed in this paper.     

Thermal fatigue behavior of thermal barrier coatings with the MCrAlY bond coats by cold spraying and low-pressure plasma spraying

The thermal fatigue behavior of thermal barrier coatings (TBCs) with the NiCoCrAlTaY bond coats deposited by cold spraying and low-pressure plasma spraying (LPPS) was examined through thermal cyclic test. The TBCs were subjected to the pre-oxidation before the test in an Ar atmosphere. The results show that a more uniform TGO in both thickness and composition forms on the cold-sprayed bond coat than that deposited by LPPS. The TBCs with the cold-sprayed bond coat exhibit a longer thermal cyclic lifetime than that with the LPPS bond coat. The differences in oxidation behavior and thermal cyclic behavior between two TBCs were discussed based on the evident difference in the surface morphology of two MCrAlY bond coats deposited by cold spraying and LPPS.     

Development of multimaterial coatings by cold spray and gas detonation spraying

The basic objective is the development of multifunctional multimaterial protective coatings using cold spraying (CS) and computer controlled detonation spraying (CCDS).As far as CS is concerned, the separate injection of each powder into different zones of the carrier gas stream is applied. Cu-Al, Cu-SiC, Al-Al₂O₃, Cu-Al₂O₃, Al-SiC, Al-Ti and Ti-SiC coatings are successfully sprayed. As to CCDS, powders are sprayed with a recently developed apparatus that is characterized by a high-precision gas supply system and a fine-dosed twin powder feeding system. Computer control provides a flexible programmed readjustment of the detonation gases energy impact on powder thus allowing selecting the optimal for each component spraying parameters to form composite and multilayered coatings. Several powders are sprayed to obtain composite coatings, specifically, among others, WC-Co-Cr + Al₂O₃, Cu + Al₂O₃, and Al₂O₃ + ZrO₂.     

Oxidation behaviour of CoNiCrAlY bond coats produced by plasma, HVOF and cold gas dynamic spraying

This paper examines and compares the microstructure and oxidation behaviour of CoNiCrAlY coatings manufactured by the APS, HVOF and CGDS deposition techniques. The coatings microstructural features were characterized by means of SEM and XRD analyses. Coating samples were then subjected to isothermal heat treatments at 1000 °C. Oxide growth rates were obtained from a series of mass gain measurements while oxide scale compositions were determined from SEM, XRD and EDS analyses. Results obtained in this study show that the as-sprayed CGDS and HVOF coatings exhibit similar microstructures, whereas the APS coating features high levels of visible defects and oxide content. Oxidation experiments revealed low oxide growth rates for both the CGDS and HVOF coatings as a result of low porosity and oxide content. The oxide scale on the CGDS and HVOF coatings after 100 h of oxidation were composed mainly of alumina without the presence of detrimental fast-growing mixed oxides. The presence of Cr₂O₃ and dispersed NiO was however also observed for the HVOF coating. As expected, the APS coating featured the onset of mixed oxides in the early stages of oxidation. From these results, it appears that potential improvements to the bond coat oxidation behaviour can be achieved using low-temperature processing methods such as CGDS.     

Preparation and characterization of magnesium coating deposited by cold spraying

Magnesium (Mg) and its alloys have a great potential as structural materials due to their beneficial combination of high strength to weight ratio, high thermal conductivity and good machinability. However, few works about Mg coatings fabricated by cold spraying can be found in the literature. Thus, Mg coatings prepared at different main gas temperatures by cold spraying were investigated as well as their microstructure, phase structure, oxygen content and microhardness. The critical velocity of the particle was evaluated through numerical simulations. The particle deformation behavior and bonding mechanism were discussed. The result of the oxygen content measurement shows that the oxygen contents of coatings did not increase compared with that of the feedstock powder. The simulation results show that the critical velocity of Mg particles was in the range from 653 m/s to 677 m/s. The observation of the coating fracture morphology shows that the formation of the coating was due to the intensive plastic deformation and mechanical interlocking. The microhardness of the coating increased with the increase of the main gas temperature from 350 °C to 450 °C due to the decrease of the coating porosity.     

The Laser-assisted Cold Spray process and deposit characterisation

Laser-assisted Cold Spray (LCS) is a new coating and fabrication process which combines the supersonic powder beam found in Cold Spray (CS) with laser heating of the deposition zone. LCS combines some advantages of CS: solid-state deposition, high build rate and the ability to deposit metals onto a range of substrates, with reduced operating costs which arise from not using a gas heater and replacing helium with nitrogen as the process gas. A system has been developed to impact metallic powder particles onto a substrate which is locally heated using a diode laser. A pyrometer and control system are used to record and maintain impact site temperature. In this study, < 50 µm powder particles are measured to be traveling at around 400 ms^− 1, and heated to temperatures between 450 °C and 900 °C when they impact the substrates. Build rates of up to 45 g min^− 1 were achieved for deposits with less than 1% porosity. Oxygen and nitrogen content in the deposits were measured to be less than 0.6 wt.% and 0.03 wt.% respectively.     

电爆喷涂方法的研究进展

电爆喷涂是一种新兴的热喷涂方法,它是利用高电压对喷涂材料脉冲放电,瞬时大电流将其加热并发生爆炸,产生高温粒子伴随冲击波喷射到基体表面形成涂层。其特点是喷射粒子速度高,设备尺寸小,适用于孔腔内壁喷涂。本文综述了电爆喷涂方法的发展现状,其经历了自由、定向和约束电爆喷涂。自由喷涂的电极直接接触,烧损严重,并对管径有限制,仅用于小直径管/孔内壁;定向喷涂的约束腔常用陶瓷材料,在爆炸冲击时易破裂或烧蚀;约束喷涂采用消融材料制作约束腔,气体放电导入电流,使这些问题得以解决。其次,阐述了该方法制备的涂层特性,即涂层与基体呈冶金结合,形成了超细晶、纳米晶结构的致密涂层,且具有良好的耐磨和抗腐蚀性及较高的硬度;给出了过程参数与涂层的关系,能量密度和喷涂距离是影响涂层性能的主要因素。最后分析了将来需要研究的问题,展望了电爆喷涂方法的发展趋势。     

Carbon nanotube reinforced aluminum composite coating via cold spraying

Multiwalled carbon nanotube (CNT) reinforced aluminum nanocomposite coatings were prepared using cold gas kinetic spraying. Spray drying was used to obtain a good dispersion of the nanotubes in micron-sized gas atomized Al-Si eutectic powders. Spray dried powders containing 5 wt.% CNT were blended with pure aluminum powder to give overall nominal CNT compositions of 0.5 wt.% and 1 wt.% respectively. Cold spraying resulted in coatings of the order of 500 μm in thickness. Fracture surfaces of deposits show that the nanotubes were uniformly distributed in the matrix. Nanotubes were shorter in length as they fractured due to impact and shearing between Al-Si particles and the Al matrix during the deposition process. Nanoindentation shows a distribution in the elastic modulus values from 40-229 GPa which is attributed to microstructural heterogeneity of the coatings that comprise the following: pure Al, Al-Si eutectic, porosity and CNTs.     

Influence of annealing treatment on microstructure and properties of cold sprayed stainless steel coatings

304 stainless steel coatings had been deposited on carbon-steel substrate by cold spray technique, vacuum annealing treatment was applied to the coatings with different temperatures, and the influence of annealing treatment on the microstructure and electrochemical behavior of the coatings in 3.5% NaCl were analyzed. The results indicated that, the cold sprayed coating was constituted by the flattened particles, and the interfaces were clearly observed between the deposited particles. It was also found that...     

Study of Ti deposition onto Al alloy by cold gas spraying

The Cold Gas Spray method has become a novel thermal-spray process which offers an amount of interesting advantages such as no oxidation and dense coatings; it involves the acceleration of the powder particles by injection into a high-velocity gas stream and these are then impinged upon suitable substrates. Common consensus says that the main parameter that controls the process is the critical particle velocity (v_c), above which, the particles are able to adhere and form the coating; v_c depends mainly on the material composition, particle size and substrate materials.In the present work, the influence of the propellant gas temperature and pressure and, of the powder feeding rate has been studied for the deposition of titanium powder onto aluminium substrates by Cold Gas Spray. Different sets of parameters have been tested in order to optimize the process regarding porosity and adherence of the deposits. The interest arose as a way to protect the aluminium alloy 7075-T6, highly used in aeronautic engineering, from corrosion. It has been also studied as an alternative to other deposition techniques such as Electrodeposition, Chemical Vapour Deposition and Vacuum Plasma Spray, which are more expensive.     

Deposition behavior of thermally softened copper particles in cold spraying

Finite-element analysis (FEA) combined with experimental observation was conducted on preheated Cu particles deposited on Cu substrate to clarify the deposition behavior of thermally softened particles in cold spraying. An explicit FEA code, ABAQUS, was used to predict the deformation features of the thermally softened particles. The experiment was performed by a home-made cold-spray system with a powder preheating device. Considering the possible serious oxidation of the cold-sprayed particles under high-temperature conditions, the preheating temperature was limited to 300 °C for each test. Based on the numerical and experimental results, a new concept called the thermal softening zone within which thermal softening occurs is proposed in the present work. It is found that thermally softened particles deform more intensively compared to non-preheated particles, and a more prominent metal jet can be achieved at the rim of the deformed particles with higher initial temperature. Moreover, the results also reveal that increasing the particle preheating temperature can stimulate the occurrence of thermal softening. For non-preheating or low-temperature preheating particles, thermal softening mainly occurs at the interfacial region. If the preheating temperature is sufficiently high, the whole particle can experience thermal softening. In addition, it is also found that preheated particles are more likely to deposit on the substrate surface than non-preheated particles. In addition, particle preheating is also found to facilitate the coating formation process, enabling the coating to be very thick. The coating microhardness decreases with increasing particle preheating temperature due to the elimination of work hardening by thermal softening.     

Corrosion resistance of cold-sprayed Ta coatings in very aggressive conditions

This study deals with the corrosion behaviour of high-pressure cold-sprayed Ta coatings, compared with Ta bulk material and inert-atmosphere plasma-sprayed Ta coatings. Electrochemical polarization and electrochemical impedance spectroscopy measurements are carried out in 1 M KOH solution; immersion tests are performed in 1 wt.% HF solution, and the resistance to high-temperature oxidation and molten salt attack is evaluated by hot corrosion tests. Moreover, the effect of substrate material (steel, Al and Cu) on the coating formation is investigated using FESEM.     

Effect of the spraying process on the microstructure and tribological properties of bronze-alumina composite coatings

In the present paper, aluminum bronze-alumina composite coatings have been applied on mild steel substrate using conventional plasma spray and a new route based on cold spray techniques. The microstructure, phase distribution, microhardness, adhesion strength and tribological behavior of the composite coatings, consisting of alumina reinforcing phase distributed in a bronze matrix, were studied. Ball-on-disk dry sliding wear tests, Rubber Wheel tests and Erosion tests were conducted to examine the tribological behavior of the composite coatings. The tribological properties of the bronze coatings were improved by the addition of alumina. Friction coefficient of coatings depends strongly on the reinforcing particles content and spraying process. Wear mechanisms of the composite coatings, such as ploughing and particle delamination, were considered. In the case of abrasive wear test, the wear rate was greatly reduced due to the reinforcing ceramic particles. Relationships between size and volume fraction of the ceramic reinforcement Al₂O₃ and the wear rate are discussed. On the other hand, erosion wear behavior of coatings with higher bronze content showed the best results.     

Ablation laser and heating laser combined to cold spraying

Cold spraying is particularly suitable for the elaboration of coatings sensitive to heat and oxidation. As spraying particles are not subjected to melting, the adhesion and formation of the coating is due to the kinetic energy transmitted to the particles by accelerating gas. Bonding mechanisms are not only strongly dependent on the particle velocity but also on the state of the substrate surface. The presence of surface pollutants inhibits the coating elaboration. Surface modifications are necessary to reach a high adhesion between the coating and the substrate. A laser ablation and a laser heat treatment are proposed to prepare the substrate prior to cold spraying. Ablation laser is used to eliminate adsorbed pollutant molecules and heating laser is employed to improve the contact between substrate and particles and coating substrate adherence. The bonding of aluminium coating on aluminium 2017 alloy was evaluated by tensile adhesion tests and demonstrated the strong influence of the laser treatments in comparison with conventional processes (grit blasting and degreasing).     

Residual stress development in cold sprayed Al,Cu and Ti coatings

Residual stresses play an important role in the formation and performance of thermal spray coatings. A curvature-based approach where the substrate–coating system deflection and temperature are monitored throughout the coating deposition process was used to determine residual stress formation during cold spray deposition of Al, Cu and Ti coatings. The effect of substrate material (carbon steel, stainless steel and aluminium) and substrate pre-treatment (normal grit blasting, grit blasting with the cold spray system and grinding for carbon steel substrate) were studied for all coating materials with optimized deposition parameters. Mainly compressive stresses were expected because of the nature of cold spraying, but also neutral as well as tensile stresses were formed for studied coatings. The magnitudes of the residual stresses were mainly dependent on the substrate/coating material combination, but the surface preparation was also found to have an effect on the final stress stage of the coating.     

Fabrication of automotive heat exchanger using kinetic spraying process

The conventional manufacturing process of the automotive brazed heat exchanger includes complex preparation processes before brazing: aluminum brazing filler alloy is pre-claded on both sides of a fin by an extrusion method, and holed aluminum tubes are coated on both sides with Zn for corrosion protection by a wire arc spraying process.The intent of this study is to simplify the preparation process by kinetic spraying using all of the components, including Al-12%Si (for the brazing filler metal), Zn (for corrosion protection), and KAlF4 (flux powder). Four kinds of blended powders were evaluated with and without flux. The bond properties and composition distribution of composition at the braze joint area were evaluated by scanning electron microscope (SEM) and electron probe micro analyzer (EPMA).In this study, kinetic spray condition was optimized, in order to fabricate the heat exchanger. It was observed that the joints of the brazed specimens at each side were sounder than that achieved by the conventional methods. It is necessary to control the Zn content to get the high corrosion resistance and good brazeability of the aluminum heat exchanger. Further, the kinetic sprayed heat exchanger showed acceptable corrosion protection.     

Ball milled Ni–Ti powder deposited by cold spraying

A 50Ni–50Ti powder was produced by solid state synthesis via mechanical alloying from elemental Ti and Ni powders. The product was characterized using X-ray diffraction, scanning electron microscopy and optical microscopy.Cold spraying makes possible to deposit the coating of temperature sensitive materials, such as nanostructured material, without any significant change in the microstructure of feedstock. The preliminary results of this work showed that the ball milled Ni–Ti coating can be deposited by cold spraying. The coating presents a multilayer structure consisting of the alternate flattened Ni and Ti particles.