In this study, TiB 2 -40Ni and TiB 2 -50Ni powders are deposited on mild steel substrates by HVOF spraying in order to investigate the influence of Ni on coating hardness and corrosion, wear, and thermal shock resistance. The surface morphology and cross-sectional microstructure of the ball-milled powders and composite coatings are examined, and various tests are conducted to measure properties of interest. The findings are presented and discussed in the paper.

This study investigates the effects of feedstock composition and annealing temperature on cold-sprayed aluminum-iron deposits. Commercially available Al and Fe powders mixed in ratios of 55:45, 75:25, and 85:15 were cold sprayed on aluminum substrates, producing dense coatings that were subsequently annealed at 500 and 550 °C. Solid diffusion reactions between Fe and Al produced Al 5 Fe 2 intermetallic compounds, the morphology and content of which were found to depend on annealing temperature and the composition of the as-sprayed deposit. The Fe particles in the Al matrix were fully consumed via compounding reaction with Al at 550 °C. At higher temperatures, however, the intermetallic particles begin to crack possibly due to large tensile stresses.

This study evaluates the effect of annealing on the microstructure, hardness, and wear resistance of FeAl-WC coatings obtained by cold spraying. As-sprayed deposits exhibited a dense microstructure with uniformly dispersed WC particles in the iron matrix. The Fe(Al) solid solution was transformed to an FeAl intermetallic compound at around 650 °C. Further increases in temperature were found to improve the composite microstructure with a slight decrease in microhardness. Wear resistance peaked at 750 °C, and at 950 °C, a diffusion layer appeared at the bottom of the coating close to the substrate.

Nanostructured WC-Co powders were cold sprayed on different substrate materials at different accelerating gas temperatures. Splat morphology and microstructure were examined, showing that the splats are partially embedded in plain carbon and stainless steel substrates with a contour similar to that of the feedstock powder. Gaps and revers were observed around the splats and corrugations or ripples were found on the surface. In contrast, splats on the surface of WC-Co substrates are relatively flat with ejectas on the periphery. A comparison of splats also shows that particle deformation increases with increasing gas temperature and substrate hardness.

The aim of this work is to fabricate a particle-reinforced FeAl composite coating by cold spraying. Fe, Al, and WC powders were placed in a ball mill and mechanically alloyed for up to 36 h in order to obtain a nanostructured Fe(Al) solid solution reinforced with a high volume fraction of WC particles. The powder was examined and then cold sprayed on stainless steel substrates using N 2 as the accelerating gas. The as-sprayed deposits exhibited rough surface morphology and dense cross-sectional microstructure with dual-scale WC dispersoids distributed uniformly in the Fe(Al) matrix. The coatings were annealed at 650 °C and subsequently reexamined. In-situ phase transformation from the solid solution to an intermetallic compound occurred after the post-spray treatment along with an improvement in microstructure.

In this study, Al-SiC composite coatings are produced by cold spraying ball-milled Al powders with different volume fractions of SiC particles. The morphology and microstructure evolution of the powder during ball milling are evaluated along with the effect of SiC content on the microstructure and wear behavior of the coatings. The results show that dense Al-SiC coatings with different volume fractions of SiC particles can be fabricated by cold spraying and that abrasive wear resistance is improved by raising the volume fraction of SiC particles. Wear surfaces indicate that the predominant wear mechanism is gouging of the soft Al matrix in the early stages and cracking and spalling of SiC particles in the latter stages. The dispersed SiC particles serve to protect the matrix from wear products thus raising the wear resistance of the coatings.

In the present study, a nanostructured FeAl coating was prepared by cold spraying of ball milled powder. Annealing treatment was applied to the coating to investigate its effect on the phase structure, grain size and microhardness of the cold-sprayed nanostructured FeAl coating. The results showed that the FeAl phase was kept unchangeable when the coating annealed at the temperature above 500°C. Annealing temperature significantly influenced the microstructure and microhardness of cold-sprayed FeAl coating. With raising annealing temperature, the lamellar structure in the as-sprayed coating disappeared and a dense coating microstructure with fully bonding of deposited particles at their interfaces was achieved after annealing at 950°C. Nanograin growth of the FeAl phase occurred at an annealing temperature higher than 800°C. The microhardness of cold-sprayed FeAl coating remained about 400 Hv 0.1 at the annealing temperature below 800°C and decreased to 300 Hv 0.1 at 1100°C.

In this paper, an iron/aluminum composite coating was prepared by cold spraying using iron and aluminum powder mixture and then annealed to aim at forming iron aluminides by suitable annealing treatment. The annealed coating was characterized using X-ray diffraction (XRD) to determine the coating phases and scanning electron microscope (SEM) with an EDXA energy dispersive spectroscopy (EDS) to examine the coating microstructure evolution. Results showed that the Fe 2 Al 5 intermetallic layer along some regions of the aluminum-iron boundaries forms after annealing at a temperature of 450°C, where true metal to metal contact had occurred. The content of Fe 2 Al 5 phase increased with raising annealing temperature. It was observed that some cracks were developed in Fe 2 Al 5 layer after annealing treatment at a high temperature of 600°C.

FeAl intermetallic compound coating was prepared by cold spraying a mechanically alloyed Fe(Al) alloy powder followed by post-spray annealing at 950°C. The high-temperature abrasive wear test was carried out for the annealed cold-sprayed FeAl at a temperature range from room temperature to 800°C. The high temperature abrasive wear of a heat-resistant stainless steel 2520 was performed for comparison. The results showed that the annealing treatment of the as-sprayed Fe(Al) alloy coating at a temperature of 950°C results in the formation of dense FeAl intermetallic compound coating with no particle boundaries. It was found that with the increase of the test temperature the wear rate of the stainless steel increased at the temperature higher than 400°C, while the wear rate of cold sprayed FeAl coating tended to decrease at the temperature higher than 400°C. The high temperature abrasive wear resistance of the cold-sprayed FeAl intermetallic compound coating increased with the increase of the abrasive wear temperature in a temperature range from 400°C to 600°C and changed little in the temperature range from 600°C up to 800°C. The wear resistance of cold-sprayed FeAl coating was higher than that of heat-resistant 2520 stainless steel under 800°C by a factor of 3.

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.

Nickel titanium is promising cavitation erosion resistant material. Using NiTi in bulk for components might not be feasible due to its poor workability, as well as the high material and processing costs. Surfacing components with its coating is effective for utilizing the good erosion properties of NiTi intermetallic compounds. In this study, a method to prepare NiTi intermetallic compound coatings in-situ through annealing of the cold-sprayed Ni(Ti) metastable coating was investigated. A nanostructured Ni(Ti) solid solution alloy powder was prepared by ball-milling process. The cold sprayed Ni(Ti) alloy coating was used as the precursor coating. The effect of annealing temperature on the microstructure in-situ evolution of Ni-Ti intermetallic compound in cold-sprayed coating was investigated. The morphology and phase composition of the powders milled for different durations and the microstructure of the as-sprayed coating were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that after annealing at 950°C the Ni(Ti) alloy was transformed to intermetallic phases. NiTi, Ni 3 Ti and NiTi 2 intermetallic phases coexisted in the annealed coating.

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.

Nanostructured NiCrAlY coating was deposited by cold spray, using a milled powder for applications as a bond coat to thermal barrier coating. A shot-peening treatment was then applied to the as-sprayed coating to modify the coating surface morphology. The oxidation behavior of the coating with the shot-peened surface was investigated under isothermal oxidation at 900°C and 1,000°C for different times. The oxidation behavior of the coating was characterized through surface morphology and cross-sectional microstructure by scanning electron microscopy and X-ray diffraction analysis. It was found that a uniform oxide layer was formed on the surface of the shot-peened nanostructured NiCrAlY coating during oxidation at temperatures of 900°C and 1,000°C. The nanostructure of the initial coating possibly promoted rapid formation of α-Al 2 O 3 oxide. It was clearly revealed that the surface morphology of the coating significantly impacted the morphology of the oxide. The surface geometry of the cold-sprayed MCrAlY coating must be modified to promote formation of a protective oxide film during oxidation, through application of a post-treatment process such as shot-peening.

Intermetallic materials have excellent high temperature oxidation resistance and erosion, cavitation resistances and are promising coating materials with many potential industrial applications. In this study, the formation of Fe-Al intermetallic compound-based coating was performed by cold spraying assisted by a post-annealing treatment. Fe-Al alloy composite powder containing 20wt% WC-Co was produced by ball milling process. Nano-structured Fe-Al alloy coating was deposited through cold spraying. The coating was annealed at different temperatures. The microstructure of the coating was characterized by scanning electron microscopy, optical microscopy and x-ray diffraction analysis. It was found that the microstructure of the as-sprayed coating depended significantly on the microstructure of the powder. A Fe-Al intermetallic phase was formed during the annealing at a temperature higher than 500°C. Moreover, grain growth occurred with the increase of the annealing temperature. The results showed that the microhardness of the as-sprayed coating reached 600HV and more. The effect of the annealing treatment on the coating microstructure and hardness was examined.

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.