In this study, finite element analysis and experimental observation are used to evaluate the effect of substrate hardness and spray angle on the deposition of cold-sprayed Ti particles. It is found, in the case of Cu substrates, that both the particle and substrate deform during impact, resulting in a large contact area. Metallurgical bonding is highly likely under such conditions, facilitating formation of thick coatings. In the case of Al substrates, although the contact area is smaller, Ti particles are trapped by the softer substrate material, resulting in mechanical interlocking and a relatively thick coating. In the case of stainless steel substrates, mechanical interlocking does not occur due to the relative hardness of the material, which limits coating thickness. The results of the study also show that decreasing the spray angle reduces interfacial contact area and coating thickness, while increasing porosity.

In the present work, a coupled thermomechanical-Eulerian model was developed to calculate particle impact behavior during cold spraying. The model is based on temperature-displacement elements and is capable of nonlinear transient analysis of impact deformation and heat conduction. Simulation results show that heat conduction has a major effect on temperature distribution within the particle, but little influence on particle deformation. The FEA model is also used to calculate critical velocities of commonly sprayed materials and the results are evaluated based on the morphology of deformed particles and plastic strain analysis.

To implement image-based numerical modeling of the thermal conductivity of coatings, a YSZ coating was considered as a network of two phases, namely the coating material and pores. The variation of the thermal conductivity of the gas trapped within the pores caused by their small size was considered by complying with the Knudsen law. In the present work, the quantification of the Knudsen effect on the effective thermal conductivity of coatings was achieved with the help of image analysis. The thicknesses of the pores were determined by combining the use of the Scion image software with a new in-house algorithm coded in C language. The Knudsen effect was quantified by applying a finite-difference model for both 2D images and a 3D image. In both cases, a decrease of the computed thermal conductivity was found while considering the Knudsen effect. Furthermore, the Knudsen effect was also taken into account in a finite-element model applied on the same images. Despite differences were noticed on the computed thermal conductivities obtained with the two methods, a quite similar decrease was calculated by comparing with data computed with the FDM method.

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.

Lanthanum silicate coatings were deposited onto stainless steel substrates by atmospheric plasma spraying (APS) using mechanically mixed (type A) and calcined feedstock (type B) powders. The phase composition, microstructure, density and porosity of coatings prepared from the two types of powder were compared.

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.

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.

FeAl iron-aluminide based materials with the ordered B2 structure are excellent candidates for use in high temperature applications because of the combination of good mechanical properties, low density, low cost and availability of raw materials, and improved oxidation resistance. The aim of this article is to produce an ultra-fine grained FeAl coating by HVOF thermal spraying of milled powders and characterize the fine scale features of its microstructure. Comparison is made with a more conventional coating obtained by projection of powders obtained by atomization. Starting powders and coatings were investigated using X-ray diffraction and transmission electron microscopy. It was observed that the coating obtained from milled powders had a microstructure essentially characterised by a nanometer grain size and the presence of a disordered FeAl phase.