Low-pressure Cold Spray Research Articles

Mechanical optimisation of Ni coatings produced by low-pressure cold spray

ABSTRACT Nickel-based coatings are widely used as thermal barriers in several sectors thanks to their remarkable corrosion and wear resistance and outstanding stability at high temperatures. Recently, Ni coatings, produced with thermal spraying and vacuum techniques, have been investigated for solar power energy applications. In the present manuscript, low-pressure cold spray (LPCS) was used to deposit pure Nickel onto a steel substrate. The influence of gas temperature, nozzle stand-off distance, and advancing speed on morphological and mechanical properties were studied. The optimal deposition conditions were derived by ANOVA analysis. The hardness and the adhesion strength were approximately 160 HV and 26 MPa, respectively. The highest thickness obtained under the optimised deposition with a single pass was around 900 µm.

Low-Pressure Cold Spray Deposition Window Derived from a One-Dimensional Analytical Model

Low-pressure cold spray (LPCS) coating deposition requires the consideration of multiple parameters to define spraying conditions. The use of a parameter window provides an integral approach to achieving this goal. In this work, an LPCS deposition window for zinc powders was obtained through the development of a one-dimensional analytical model of fluid and particle interaction. The model considers powder particle injection downstream of the nozzle and follows the particle from injection to impact. The model equations relate the particle velocity (vp) to the process parameters, such as the gas pressure (P0) and temperature (T0), particle size (dp) and stand-off distance (SoD). The values of the particle velocity (vp) at the nozzle exit and during the “free-jet”, as well as the drag coefficient (Cd), were calculated using experimental spraying conditions for Cu and Al that have previously been documented in LPCS studies. The model’s accuracy and applicability to other materials were confirmed upon comparing the results with those in the aforementioned studies. Moreover, the definition of the model equations allowed for the identification of three new parameters: (γ) the maximum ideal particle velocity, (β) the capacity to accelerate the powder particle inside the nozzle and (α) the deceleration of the particle in the free-jet zone. These parameters have not previously been published and allow for comparative evaluation between LPCS processes.

Independent Control of Particle Temperature and Velocity using a Novel Powder Preheater Design for Low-Pressure Cold Spray

Independent Control of Particle Temperature and Velocity using a Novel Powder Preheater Design for Low-Pressure Cold Spray

Localised surface modification of high-strength aluminium–alumina metal matrix composite coatings using cold spraying and friction stir processing

ABSTRACT This study presents a novel approach for surface repairs of high-strength, cold-worked aluminium (Al) alloys, without negatively affecting the base material strength. Low-pressure cold spraying was used to fabricate Al–Al2O3 metal matrix composite overlays, with varying concentrations of Al2O3 deposited on an Al alloy. Friction stir processing (FSP) was employed to disperse and consolidate the overlay coating on the base material. The FSP was found to improve the Al2O3 particle distribution in the coating and reduce the mean free path between Al2O3 particles. The coating hardness increased after FSP. The post-FSPed overlays also exhibited lower wear rates compared to the as-sprayed condition. Remarkable improvement was observed in the tensile properties of the coatings, which were attributed to the improved dispersion of Al2O3 particles in the matrix, while the enhanced ductility was attributed to the possible grain refinement that occurred due to the recrystallisation of Al during FSP.

Effect of Ball-Milled Feedstock Powder on Microstructure and Mechanical Properties of Cu-Ni-Al-Al2O3 Composite Coatings by Cold Spraying

Cu, Ni and Al powders mixed in a certain stoichiometric proportion were ground via ball milling and deposited as coatings using low pressure cold spraying (LPCS) technology. The effect of particle morphology on the powder structure as well as the microstructure, composition and mechanical properties of the coatings was studied. The results revealed a core–shell structure of ball-milled powders. Compared with a mechanically mixed (MM) coating, coatings after ball milling at a rotation speed of 200 rpm exhibited the most uniform composition distribution and a lower degree of porosity (by 0.29%). Moreover, ball milling at 200 rpm was conducive to a significant increase in the deposition efficiency of the sprayed powder (by 10.89%), thereby improving the microhardness distribution uniformity. The ball milling treatment improved the adhesion of the coatings, and the adhesion of the composite coating increased to 40.29 MPa with the increase in ball milling speed. The dry sliding wear tests indicated that ball milling treatment of sprayed powder significantly improved the wear properties of the coatings. The coating after ball milling at a speed of 250 rpm showed the lowest friction coefficient and wear rate, with values of 0.41 and 2.47 × 10−12 m3/m, respectively. The wear mechanism of coatings changed from abrasive wear to adhesive wear with the increase in ball milling speed.

Effect of WC-17Co content on microstructure, mechanical properties and tribological behavior of low-pressure cold sprayed tin bronze composite coating

As an important anti-wear material, tin bronze coatings and their composites have been widely used as bimetallic bearings, sliding plates and ship propellers. In this work, the dense CuSn10/WC-17Co composite coatings were deposited by low-pressure cold spraying (LPCS) technology. The effect of WC-17Co cermet content on the microstructure, mechanical properties and tribological behavior of the LPCS CuSn10/WC-17Co coatings was investigated. The results showed that the deformation of the particles that deposited inside the coatings was enhanced by the increasing peening effect of the WC-17Co cermet in feedstock powders. During the coating deposition process, the cold spraying can effectively avoid the decomposition or oxidation of the feedstock powders. Meanwhile, in the wake of the increasing WC-17Co content in the coatings, the hardness and bonding strength of the composite coatings were improved. In addition, the evolution of the coating fracture surfaces was analyzed. It is gratifying that the wear rate of the CuSn10/WC-17Co composite coating was significantly decreased to 3.75 × 10−5 mm3/N·m. Correspondingly, the wear mechanisms of the coatings were all elucidated, respectively.

Fabrication of In Situ rGO Reinforced Ni-Al Intermetallic Composite Coatings by Low Pressure Cold Spraying with Desired High Temperature Wear Characteristics.

In this study, the surface of aluminum powder was uniformly coated with in situ reduced graphene oxide (r-GO) sheets (Al/r-GO). The Ni powder, Al2O3 powder, and Al/r-GO powders were mixed uniformly in a mass ratio of 20:6:4. In situ rGO-reinforced Ni-Al intermetallic composite coatings were successfully prepared using low-pressure cold spraying and subsequent heat treatment. The microstructure and phase of the composite coatings were characterized using X-ray diffraction (XRD), Raman spectroscopy, and scanning electron microscopy (SEM). The high-temperature wear test was conducted at 200 °C, 400 °C, and 600 °C to understand the mechanism. The results indicate that the in situ rGO-reinforced Ni-Al intermetallic composite coatings exhibit a 33.3% lower friction coefficient and 26% lower wear rate in comparison to pure Ni-Al intermetallic coatings, which could be attributed to the generation of an easy-shearing transferred film between the coating and grinding ball.

Metallization of carbon-fibre reinforced composites via a metal-epoxy biphasic sublayer and low-pressure cold spraying

This study investigates the suitability of a sublayer-assisted low-pressure cold spraying for metallizing Carbon Fibre-Reinforced Polymer (CFRP) composites. The metal-based polymeric sublayer consists of a Cu-based epoxy sublayer, made of Cu spherical micron-sized powders and an epoxy phase, and which is embedded onto the CFRP composite surface using a vacuum moulding process. Then, a cold spray deposition onto the sublayer is performed to generate a coating. Different cold spray powders (copper, aluminium, zinc, and tin) were used to explore the optimal coupling powder/sublayer/CFRC substrate under different spraying conditions. The Cu powders require a high velocity so that they are difficult to be compatibilized with the CFRC structure without an erosion during the cold spray deposition. Soft powders like Al, Zn and Sn can lead to an onset of coating formation via the bonding of these powders onto the embedded sublayer. The Sn powders generate an adhesion onto the Cu-epoxy embedded sublayer via a metal-to-epoxy bonding (Sn-to-epoxy zone) and a metal-to-metal bonding (Sn-to-Cu), under the gas working condition of 9 bars and 25 °C. As conclusion, the present work demonstrates the feasibility of a sublayer-assisted cold spray process to metallize a CFRC structure partly made of metal-to-metal bonding.

Dry abrasive wear and solid particle erosion assessments of high entropy alloy coatings fabricated by cold spraying

Recent advances in advanced metallic alloys have enabled the development of high entropy alloys (HEAs) for extreme engineering environments. HEAs are characterized by their mixing of elements with equimolar concentrations or near to equimolar concentrations, which enables the formation of solid-solution structures due to inherent high configurational entropy. With the advancement of HEAs, the attractiveness of these alloys as cold-sprayed coatings for wear resistant surfaces has been increasing. In the present study, a low-pressure cold spraying technique was employed to fabricate two new mechanically alloyed AlCoCrFeMoW and AlCoCrFeMoV HEA coatings. The phase formations, microstructure evolutions, and average microhardness of the fabricated HEA coatings were systematically investigated, in order to assess their resistance to dry abrasive wear and solid-particle erosion. The results showed that the cold-sprayed HEA coatings exhibited refined microstructure and low porosity, with the presence of body-centered cubic (BCC1 and BCC2) structures and CrMo intermetallic phases. Microhardness results revealed that AlCoCrFeMoV HEA coatings achieved the highest average hardness of around 561±63HV, such that it was 35% and 52% higher than that of AlCoCrFeMoW and AlCoCrFeMo HEA coatings, respectively. Dry abrasive wear results showed lower wear rates were achieved for AlCoCrFeMoV HEA coatings (590×10−6mm3/Nm), followed by AlCoCrFeMoW (660×10−6mm3/Nm) and AlCoCrFeMo HEA coatings (720×10−6mm3/Nm). However, damage during solid particle erosion was found to be higher in case of AlCoCrFeMoV HEA coatings compared to the other two coatings, suggesting no apparent correlation with microhardness values. These results suggest that the existence of high weight fractions of solid-solution phases in conjunction with refined microstructure and low porosity resulted in improved resistance to abrasive loading for the AlCoCrFeMoV HEA coatings but undermine performance and resistance to solid particle erosion damage.

EIS and potentiodynamic polarization studies of arc-sprayed aluminum coating on Q235 steel surface

The use of aluminum coating is advantageous due to its low potential and protective properties against corrosion. Three aluminum coatings were applied to the surface of Q235 steel using flame spraying, arc spraying, and low pressure cold spray. Microscopic analysis revealed that the flame sprayed and arc sprayed coatings had a rough surface, while the cold sprayed aluminum coating had a uniform color and a smooth surface with no visible defects. The corrosion resistance of the arc sprayed aluminum coating was tested using simulated seawater, and the results of the polarization test showed that the original aluminum coating had a passivation phenomenon, which was attributed to the strong adhesion of the gel corrosion product Al(OH)3 and the formation of Al2O3 film. Electrochemical impedance spectroscopy (EIS) testing revealed that the main form of failure of the aluminum coating in 3.5 % NaCI solution was pore corrosion.

The effect of process temperature and powder composition on microstructure and mechanical characteristics of low-pressure cold spraying aluminum-based coatings

The effect of operating gas temperature and powder type on microstructure and mechanical characteristics of cold spraying coatings deposited on EZ33A-T5 magnesium alloy was studied. Three aluminum-based cold spraying powder mixtures Al + Zn, Al + Al2O3 and Al + Zn + Al2O3 were used for the investigation. Deposition was performed using D423 low-pressure cold spray system at operating gas pressure of 1.0 MPa and different temperatures –300 °C, 450 °C, and 600 °C. The coatings microstructure was investigated with optical and scanning electron microscopy. Mechanical properties of the coatings were characterized through standard test methods for adhesion and cohesion strength, and standard test methods for Vickers hardness of thermal spray coatings. The results demonstrate that with increasing initial gas temperature at spraying nozzle inlet from 300 °C to 600 °C, an increase in the porosity of the coatings of all investigated powder mixtures can be observed. Microstructure characterization showed an increase in porosity from 2.3% to 4.1% for Al + Zn powder mixture, from 2.1% to 3.5% for Al + Al2O3 powder mixture, and from 2.5% to 5.6% for Al + Zn + Al2O3 powder mixture. The minimum porosity was obtained at 450 °C for all investigated powder mixtures. Adhesion and cohesion strength and microhardness of coatings were reach their maximum value at 450 °C. The best performance was obtained for Al + Al2O3 powder mixture: coating adhesion—31.9 MPa (was limited by the bonding strength of the glue), cohesion—93.5 MPa, microhardness—81 HV0.15. The influence of Al2O3 particles in the powder mixture on the above-mentioned parameters was also established. The results show that the presence of ceramic particles in powder mixtures can positively effect porosity level and mechanical characteristics.

Experimental study on aluminium bronze coatings fabricated by low pressure cold spraying and subsequent heat treatment

Among bronze alloys, strength and corrosion resistance are the distinguishing features of aluminium bronzes. CuAl alloys coatings are commonly prepared using thermal spraying technologies, but low-pressure cold spray, unpopular for this purpose so far, seems to be a good alternative in terms of manufacturing and facility costs. The research studies the effect of aluminium addition on low-pressure cold spray deposition of mixed CuAl feedstock powders (up to 30 wt% of Al) instead of bronze particles. Additionally, hard alumina particles (of the same weight as CuAl feedstock powders together) were added to lower coating porosity, provide good adhesion, and tailor the mechanical properties of coatings. After the implementation of thermal treatment, fine mixing of feedstock powders within the coating facilitated the formation of copper-rich solid solution and Cu4Al9 intermetallic phase for high amounts of Al (≥25 wt%). The coatings were characterized through light and scanning electron microscopy to examine cross-section morphology; energy-dispersive X-ray spectrometry and X-ray diffraction to investigate elemental and phase composition before and after thermal treatment. Additionally, to quantify the individual phases in sprayed coatings Rietveld refinement was performed. For coatings after thermal treatment were bronze formation was observed, the maximum solubility of aluminium in the copper structure was also determined using Vegard's linear dependence.

Influence of Coating Thickness on Adhesion Strength of Aluminum Coatings on Aluminum Nitride Substrates by Low Pressure Cold Spraying

Influence of Coating Thickness on Adhesion Strength of Aluminum Coatings on Aluminum Nitride Substrates by Low Pressure Cold Spraying

Surface-Metallized CNTs/Cu Composite Coating Prepared by Laser-Assisted Low-Pressure Cold Spray

Surface-Metallized CNTs/Cu Composite Coating Prepared by Laser-Assisted Low-Pressure Cold Spray

Interfacial oxidation and boundary amorphization deposition mechanisms of GaN powder on metallic substrate by low-pressure cold spraying

Understanding powder adhesion mechanism in cold spray is primordial to control the coating formation. Previous studies have suggested that metallic coatings are governed by metallurgical bonding, but no consensus has been established regarding the bonding mechanisms of ceramic particles. In this study, the deposition mechanism of agglomerated gallium nitride (GaN) particles cold sprayed on stainless steel substrates was investigated. The evolution of the oxide layer thickness and coating/particle microstructures were analyzed by X-ray photoelectron spectroscopy and transmission electron microscopy. The results revealed that although mechanical interlocking and coating thickness were enhanced by the substrate surface roughness, grain refinement was not the key factor for the brittle particle deposition, considering the diameter of the nano-sized particles. Instead, a new interfacial oxide layer was formed because of the destruction and removal of native oxide films upon impact. Local heteroepitaxy, which occurred near the interface of the newly formed gallium oxide, enhanced the coating formation. Compared with the GaN feedstock, the coating exhibited boundary amorphization and a high dislocation density induced by the high strain rate. Thus, this study proposed considerable insight into the formation mechanisms of cold-sprayed GaN coatings, which can support a more expanded range of ceramic/metal combinations in the future.

A new approach to flux deposition for brazing aluminium by low pressure cold spraying

Abstract This paper presents the results of study on the possibility of Nocolok flux deposition to aluminium substrates using the low-pressure cold gas spraying (LPCS) method. An innovative method of applying flux in the form of a powder without organic adhesive additives was proposed, allowing strict control of the deposited material. The influence of the flux powder feeding rate (PFR – 4.5 g/min, 6.3 g/min and 8.5 g/min) on the efficiency of the brazing process of AA3003 aluminium alloy plates was investigated. The results of energy dispersive spectrometry (EDS) analysis of the deposited flux coatings are presented for various process parameters. The wettability tested by spreading the B-AlSi12 filler metal on flux-covered aluminium substrates increased significantly with increasing PFR – the contact angle decreased from 21.0° to 4.6°. Microstructure analysis confirmed the high quality of the brazed joints, which were devoid of braze incompatibilities.

Nozzle Geometry and Particle Size Influence on the Behavior of Low Pressure Cold Sprayed Hydroxyapatite Particles

Low-pressure cold spray (LPCS) technology has attracted interest for the deposition of ceramic coatings due to the thermo-kinetic conditions experienced by the sprayed particles. Unlike conventional thermal spray techniques, the spraying conditions in LPCS can be controlled to avoid the formation of undesired phases. However, ceramics deposition through this process is still challenging. The present study includes a finite element analysis and simulation study of the kinetic conditions of ceramic particles in the LPCS process based on experimental data. The analysis seeks to discuss the effect of nozzle geometry on the kinetic and thermal energy of the sprayed particles at impact and elucidate how the particle travels within the high-velocity jet to be deposited onto a metallic surface. This work examines the behavior of hydroxyapatite particles as a function of particle size and nozzle geometry during LPCS deposition. Interestingly, the results from this research suggest that particle size and nozzle geometry have an influence on the deposition of hydroxyapatite particles. Inertia of large particles proved to be beneficial in keeping their trajectories, allowing them to contribute to the formation of the coatings. Nozzle geometry modifications produced changes in the jet profile and affected the homogeneity of the coatings obtained. This finding contributes to a better understanding of the deposition of hydroxyapatite particles by cold spraying.

Mechanical and Tribological Study on Aluminum Coatings with High-Pressure and Low-Pressure Cold-Spray Processes

Cold spray is a promising approach to repair all damages and defects in aluminum (Al) constituent elements. The study aims to investigate the mechanical and tribological properties of Al coatings deposited using high-pressure cold-spray (HPCS) and low-pressure cold-spray (LPCS) techniques. Al powder was sprayed on a cold-rolled plate of aluminum 1100, which was used as the substrate. The results showed that the micro-hardness of the LPCS Al coating reached up to 196.6 HV before the wear test compared to that of HPCS (174.3 HV). Moreover, more low friction coefficients obtained by LPCS (0.798) than HPCS (0.807) indicated good tribological properties with a high amount of oxide composition. Meanwhile, the wear studies reveal that the specific wear rate of the Al coating of LPCS (0.008) was lower than the HPCS (0.009) as the load increased from 3 N to 5 N, thus providing excellent wear resistance. Therefore, the results exhibited greater mechanical and tribological characteristics for Al coatings produced by the LPCS process than by the HPCS process.

Multiscale Simulation of Shot-Peening-Assisted Low-Pressure Cold Spraying Based on Al-Zn-Al2O3 Coatings

Low-pressure cold spraying has gained much significance for its good economy in recent years. However, compared with high-pressure cold spraying, the unsatisfactory performance of coatings prepared by this method is a key problem restricting its further development. To improve the properties of the coating without incorporating severe conditions, the paper proposed an original shot-peening-assisted low-pressure cold-spraying method (i.e., SP-LPCS). By proceeding with cold spraying and shot peening alternately, SP-LPCS was proved to enhance the mechanical property of the coating effectively. Mixed particles of Zn, Al, and Al2O3 were adopted as the coating powder. Effects of shot-peening pressure, flow rate, and shot size on the micromorphology and the microhardness variance were studied. Results shows that the thickness of the plastic deformation layer stabilizes as the impact time increases to 6. The microscopic simulation of the deformation shows that according to the different metal characteristics of the powder, brittle grains fracture while plastic grains go through deformation and refinement. Meanwhile, the porosity decreases greatly after the impacts, resulting in a higher denseness of the coating. Several factors mutually contribute to the performance improvement of the coating. The microhardness of the material was increased after SP-LPCS, and obvious strengthening belts were observed, with the highest microhardness being 90.93Hv.

Parametric Study to Repair Leaks in Water Pipe Using the Low-Pressure Cold Spray Technique

Parametric Study to Repair Leaks in Water Pipe Using the Low-Pressure Cold Spray Technique