A magnesium coating with a low porosity and high microhardness was elaborated using cold spraying. However at present, a poor bonding strength between the coating and substrate limits its application. This paper aims at improving the bonding strength between the coating and substrate using substrate preheating. Aluminum substrates were heated to 100, 200 and 300°C respectively by a flame prior to cold spraying. The results show that substrate preheating can significantly increase the bonding strength. The bonding strength increased from 3.3±0.8 MPa to 11.6±0.5 MPa when the substrate temperature increased from room-temperature to 200°C. The fracture analyses show that the coating fracture occurred within the coating when the substrate was preheated at 200°C.

A previous study indicated that dense thick Cu-4Cr-2Nb coatings could be formed by cold spraying, and the post-spray heat treatment could significantly influence the microstructure and microhardness of the as-sprayed Cu- 4Cr-2Nb coatings. In this study, the tensile strength and fracture performance of the Cu-4Cr-2Nb coatings after annealing were investigated. The vacuum heat treatment was conducted under 10-2 Pa at 850°C for 4 h. Results showed that the heat treatment had a great contribution to the healing-up of the incompleteness of the interfaces between the deposited particles. In addition, the coating microhardness decreased from 156.8±4.6 Hv0.2 for the as-sprayed coatings to 101.7±4.5 Hv 0.2 for the annealed ones. The mean tensile strength of the annealed coatings was approximately 298.8±31.5 MPa compared to that of 45±10.5 MPa for the as-sprayed ones, which results from the partially metallurgically bonded zones between the deposited particles inducing by the heat treatment process.

This study investigates the effect of substrate conditions on supersonic jet flow pattern, particle acceleration and substrate preheating process in cold spraying. Computational fluid dynamic (CFD) approach is employed in the present work to achieve this objective. The simulated results show that substrate diameter has some effects on gas flow regime and thus particle acceleration. When the substrate diameter is smaller than the nozzle exit diameter, the size of the bow shock formed in front of the substrate is rather small, which contributes to increase the particle impact velocity. With increasing the substrate diameter gradually, the flow regime becomes more and more insensitive to the substrate diameter and the pattern becomes uniform. Moreover, the current numerical work also reveals that substrate preheating process can be significantly influenced by substrate thickness. With the increment in substrate thickness, the preheating effect becomes increasingly undesirable and the substrate surface temperature presents a downward trend.

Numerical simulations focused on the impacting behavior of cold-sprayed particles are usually conducted with the Lagrangian method. However, the calculated output is much dependent on the mesh size due to the mesh distortion and/or the element-averaged variables in simulations. While the Eulerian method is attractive to overcome the mesh distortion. In this study, an investigation on the impacting behavior of cold-sprayed particles using the Eulerian formulation available in ABAQUS/Explicit was conducted with typical copper material. The results show that a jet can not be formed at the impacting velocities of 200-300m/s, but a continuous and smooth jet composed of the particle and substrate materials can be formed as the impacting velocity is in the extent of about 300-400 m/s which could be a theoretical value of the critical velocity for a successful bonding. At this velocity extent, the maximum PEEQ almost keeps unchanged accompanying with a temperature fluctuation. In addition, the jet presents discontinuous and the splashing causes the loss of material as the impacting velocity exceeds this velocity extent. Therefore, this model could be also used to predict the critical velocity of other materials besides the copper.

Magnesium coatings were deposited upon aluminum and stainless steel substrates by cold spraying. Three Mg powders with different particle size distributions were used as feedstock. The microstructures of as-sprayed coatings were evaluated by optical microscopy, and scanning electron microscopy. The coating observations show that a main gas temperature has an important effect on the deposition behaviour of particles. Changing the gas temperature from 350°C to 630°C involves an increase of the deposition efficiency from 1.57% to 19.57%. The effects of the particle size distribution and substrate material on the deposition efficiency of particles were also investigated. The results show that the particle size distribution has a significant effect on the deposition efficiency of particles which increases from 19.57% to 59% when the mean particle size decreases from 63 µm to 38 µm under gas temperature of 630°C. However, the deposition efficiency of particles was slightly influenced by the substrate material. In addition to these experimental results, the in-flight particle velocities were simulated by FLUENT software to point out the effects of the gas temperature and particle size distribution.

In this paper, a commercial AZ91D magnesium alloy powder and its mixture with 30 vol.% SiC powder were used to deposit coatings by cold spraying. Two types of converging-diverging nozzles with different cross-sectional shapes were employed. The velocity and temperature of in-flight particles under different operating conditions were simulated using the FLUENT software. The simulated results show that the particle velocity through the rectangular cross-section nozzle is the same with that through the circular one. However, the coating observation shows that the AZ91D coating and its composite could only be deposited using the rectangular cross-section nozzle. The increase of gas temperature has little effect on the coating microstructure, porosity and microhardness. Furthermore, the observation of the composite coating produced under the gas temperature of 600°C shows that the SiC content in the composite is about 23 vol.%. The microhardness of the composite is improved to about 140 HV 0.3 due to the enhancement of SiC particles, compared to that of about 100 HV 0.3 for the AZ91D coating.

In this research, large gas-atomized copper powder was selected as the feedstock. Some powder was annealed in a vacuum circumstance to avoid to the greatest extent the effect of grain boundaries on the high velocity impact behavior of particles during cold spraying. Some powder was oxidized in a resistance furnace to clarify the effect of surface oxide films. Both the annealed and the oxidized Cu powders were deposited by cold spraying with respect to the single impacts and coating deposition under the same gas condition. In addition, the rebounded copper particles were collected for morphology analysis compared to the adhered particles. The results show that the average size of the rebounded particles is apparently increased compared to the starting powder because of the rebound of the larger particles and the intensive plastic deformation of particles. For the deposited particles, obvious plastic deformation causes the higher hardness of the coatings. The last but not the least finding in this study is the rebounded particles also experienced large deformation and possible shear instability at the impact interfaces.

Critical velocity is an important parameter in cold spraying. It determines the deposition efficiency under a given spray condition. It depends not only on material types, but also particle temperature and oxidation conditions. In this present work, three types of materials including copper, 316L stainless steel, and Monel alloy were used to deposit coatings by cold spraying. The critical velocities of spray materials were determined using a novel measurement method. Oxygen content in three powders was changed by isothermal oxidation at ambient atmosphere. The effect of oxygen content on the critical velocity was examined. It was found that critical velocity was significantly influenced by particle oxidation besides material properties. The critical velocity of Cu particles increased from about 300 m/s to over 610 m/s with a change in oxygen content in the powder. The results suggest that with a severely oxidized powder, critical velocity tends to be dominated by the oxide on the powder rather than material properties.

In this study, cold spay particle impact behavior of a typical copper material is examined using ABAQUS finite element analysis software. Various combinations of calculation settings are explored, such as element type, adaptive meshing, contact interaction, and material damage, with the main focus on element distortion and its effect on output. The influence of mesh size on modeling accuracy is also discussed as are fundamental aspects of modeling cold spray particle deformation.

The previous studies indicate that the fabrication of metal matrix composites (MMCs) by cold spraying is effective and promising. When light materials, such as SiC and Al 2 O 3 , were used as reforcements, it is difficult to obtain a high volume fraction of hard phase in the composite just through the simple powder mixture. Therefore, in this study, a Ni-coated Al 2 O 3 powder, which was produced through hydrothermal hydrogen reduction method, was employed aiming at increasing the volume fraction of ceramic particles in the deposited composite coating. It was found that a dense Ni-Al 2 O 3 composite coating could be deposited with the Ni-coated Al 2 O 3 powder under the present spray conditions. X-ray diffraction analysis indicated that the composite coating had the same phase structures as the feedstock. The volume fraction of Al 2 O 3 in the composite was about 29±6%, which is less than that in the feedstock (nominal: 40-45%) due to the rebound of some Al 2 O 3 particulates upon kinetic impacting. The microhardness of the composite coating was about 173±33Hv 0.2 .

In this study, the impact melting phenomenon at the interfaces of particles deposited by cold spraying and its effect on coating microstructure were examined. Different powders with various thermal and mechanical properties were selected as feedstock. They are respectively Al2319, Ti, Ti-6Al-4V, Ni and MCrAlY powders. The results showed that most of the sprayed materials possibly experienced local melting at the contact interfaces of particles under certain gas conditions. Low melting point and reaction with the atmosphere are the two main factors contributing to the impact fusion during cold spraying. The results indicated that the local melting would benefit the formation of the metallurgical bonding between the deposited particles and enhance the coating adhesion.

In this paper, the microstructure and microhardness of the Al-12Si coating produced by cold spraying with a relatively large powder were investigated. It is found that a thick, dense and well bonded Al-12Si coating could be produced by cold spraying with a relatively large powder through the control of spray conditions. The critical velocity for large Al-12Si particles was about 500 m/s under the spray conditions in this study. The as-deposited coating had the same phase as the Al- 12Si powder. The localized interface melting occurred resulting from both the adiabatic shearing upon impact and the thermal effect of hot gas. The precipitation of fine Si particles occurred in α-Al matrix in the coating deposited at an elevated gas temperature because of the thermal effect of hot gas during coating deposition. The dispersed Si particles in the coating improved the coating microhardness.

The particle critical impact velocity is a key parameter in cold spraying. The appropriate estimation of this critical velocity benefits the optimization of coating deposition in terms of deposition efficiency. The critical velocity can be estimated experimentally through measuring the relation between deposition efficiency and particle velocity. A recent study has revealed that the critical velocity can be estimated by the velocity resulting in the onset of adiabatic shear instability through numerical simulation of particle impact. However, it was found that the critical velocities of a pure metal material obtained by different investigators through simulation were significantly different. No reasonable explanation was provided to such difference. In the present paper, the critical velocities reported in the literature for Cu powder as a typical material were reviewed. The factors influencing the critical velocity were examined. The numerical simulation methods employed in the limited literature were also examined for a better understanding of the factors that influence the simulated results. A reasonable estimation method was proposed by using the numerical critical velocity.

It is difficult to deposit dense intermetallic compound coatings by cold spraying directly using the compound feedstock powders due to their intrinsic low temperature brittleness. A method to prepare intermetallic compound coatings in-situ employing cold spraying was developed using a metastable alloy powder assisted with post-annealing. In this study, a nanostructured Fe/Al alloy powder was prepared by ball-milling process. The cold sprayed Fe/Al alloy coating was evolved in-situ to intermetallic compound coating through a post-annealing treatment. The microstructural evolution of the Fe-40Al powder during mechanical alloying and the effect of the post-annealing on the microstructure of the cold sprayed Fe(Al) coatings were characterized by optical microscopy, scanning electron microscopy and X-ray diffraction analysis. The results showed that the milled Fe-40Al powder exhibits lamellar microstructure. The microstructure of the as-sprayed Fe(Al) coating depends significantly on that of the as-milled powder. The annealing temperature significantly influences the in-situ evolution of the intermetallic compound. The annealing treatment at a temperature of 500oC results in the complete transformation of Fe(Al) solid solution to FeAl intermetallic compound.

In this study, a comprehensive examination of deformation of Al particles impacting on Al substrate was conducted by using the Arbitrary Lagrangian Eulerian (ALE) method to clarify the deposition characteristics of Al powder and effect of surface oxide films in cold spraying. It was found that the deformation behaviour of Al particles is different from that of Cu particles under the same impact conditions owing to its lower density and thus less kinetic energy upon impact. The results indicate that the Al particles need a higher velocity to reach the same compression ratio as that of Cu particles. On the other hand, the numerical results show that the oxide films at particle surfaces influences the bonding condition between the particles and substrate. The inclusions of the crushed oxide films at the interfaces between the depostied particles inhibit the deformation.

Yttria stabilized zirconia and lanthanum strontium manganate (YSZ/LSM) have been employed to fabricate the composite cathode functional layer for solid oxide fuel cells (SOFCs). In the present study, the YSZ/LSM composite coating was deposited by atmospheric plasma spray (APS). The electrical conductivity of the composite coating was measured by the means of Direct Current (DC) measurement in a temperature range of 500-900 °C. The electrical conductivity of YSZ- 50%LSM coating changed from 2.17 S/cm to 3.60 S/cm along the direction parallel to the coating surface at the temperature range. For the same specimen, the electrical conductivity in the perpendicular direction is less than one-tenth of that in the parallel direction. The anisotropy of the electrical conductivity is attributed to the phases of different properties in the composite coating and the APS coating structure characteristics. The activation energy of the composite coating at both directions was calculated. The results showed that the electrical conduction of the composite was strongly influenced by the YSZ content.

The velocity of cold spray particles was measured by a diagnostic system for thermal spray particles based on thermal radiation. A laser beam was employed to illuminate the cold sprayed particles in cold spraying for obtaining a sufficient radiant energy intensity for detection. The measurement was carried out for Cu particles of different mean particle sizes. The particle velocity was also estimated using the previously developed two-dimensional axisymmetric model. It was found that the measured results agreed well with the calculated ones. The proposed measurement method in this paper is reliable. On the other hand, it is confirmed that the particle acceleration behavior in cold spraying can be accurately predicted through the simulation method developed previously. The optimization of cold spray process can be conducted following the simulation method.

A convergent-barrel (CB) cold spray nozzle was designed through numerical simulation. It was found that the main factors influencing significantly the particle velocity and temperature include the length and diameter of the barrel section, the nature of the accelerating gas and the operating gas pressure and temperature, and the particle size. Particles can achieve a relatively low velocity but a high temperature under the same gas pressure using a CB nozzle compared to a convergent-divergent (CD) nozzle. The experiment results with Cu powder using the designed CB nozzle confirmed that the deposition can be realized under a lower gas pressure with a CB nozzle.

Yttria doped zirconia has been widely employed as electrolyte materials for solid oxide fuel cells (SOFCs). Plasma spraying is a cost-effective process to deposit YSZ electrolyte. In this study, the 8 mol % Y 2 O 3 stabilized ZrO 2 (YSZ) layer was deposited by low pressure plasma spraying (LPPS) and atmospheric plasma spraying (APS) with fused-crushed and agglomerated powders to examine the effect of spray method and particle size on the electrical conductivity and gas permeability of YSZ coating. The microstructure of YSZ coating was characterized by scanning electron microscopy and X-ray diffraction analysis. The results showed that the gas permeability was significantly influenced by powder structure. The gas permeability of YSZ coating deposited by fused-crushed powder is one order lower in magnitude than that by agglomerated powder. Moreover, the gas permeability of YSZ deposited by LPPS is lower than that of APS YSZ. The electrical conductivity of the deposits through thickness direction was measured by potentiostat/galvanostat based on three-electrode assembly approach. The electrical conductivity of YSZ coating deposited by LPPS with fused-crushed powder of small particle size was 0.043 S × cm-1 at 1000°C, which is about 20% higher than that of APS YSZ with the same powder.

For high velocity oxy-fuel (HVOF) sprayed Cr 3 C 2 -NiCr coating, an experimental method to evaluate the carbon and carbide loss of the particles in the stages of both in-flight and impacting on a substrate was proposed. The carbon loss in these two stages was determined by comparing the carbon content in the starting powder, the collected powder sprayed into water and the coating deposited on the substrate. The carbide loss caused by carbide rebounding, dissolving into NiCr matrix and oxidizing in-flight can be determined by evaluating quantitatively the carbide content of the collected powder and the deposited coating. According to the experimental results, it was revealed that the rebounding off of large carbides during particle impact on the substrate is mainly responsible for the carbon loss. The carbide rebounding off and dissolving are two main reasons for the carbide reduction in the coating. The carbon and carbide loss caused by oxidizing during the in-flight of particles is very limited.