Heinrich Kreye

Bonding mechanism in cold gas spraying

Cold gas spraying is a relatively new coating process by which coatings can be produced without significant heating of the sprayed powder. In contrast to the well-known thermal spray processes such as flame, arc, and plasma spraying, in cold spraying there is no melting of particles prior to impact on the substrate. The adhesion of particles in this process is due solely to their kinetic energy upon impact. Experimental investigations show that successful bonding is achieved only above a critical particle velocity, whose value depends on the temperature and the thermomechanical properties of the sprayed material. This paper supplies a hypothesis for the bonding of particles in cold gas spraying, by making use of numerical modelling of the deformation during particle impact. The results of modelling are assessed with respect to the experimentally evaluated critical velocities, impact morphologies and strengths of coatings. The analysis demonstrates that bonding can be attributed to adiabatic shear instabilities which occur at the particle surface at or beyond the critical velocity. On the basis of this criterion, critical velocities can be predicted and used to optimise process parameters for various materials.

An analysis of the cold spray process and its coatings

In this study, computational fluid dynamics (CFD) and extensive spray tests were performed for detailed analyses of the cold spray process. The modeling of the gas and particle flow field for different nozzle geometries and process parameters in correlation with the results of the experiments reveal that adhesion only occurs when the powder particles exceed a critical impact velocity that is specific to the spray material. For spherical copper powder with low oxygen content, the critical velocity was determined to be about 570 m/s. With nitrogen as the process gas and particle grain sizes from 5–25 µm, deposition efficiencies of more than 70% were achieved. The cold sprayed coatings show negligible porosity and oxygen contents comparable to the initial powder feedstock. Therefore, properties such as the electrical conductivity at room temperature correspond to those of the bulk material. The methods presented here can also be applied to develop strategies for cold spraying of other materials such as zinc, stainless steel, or nickel-based super-alloys.

The Cold Spray Process and Its Potential for Industrial Applications

Cold spraying has attracted serious attention since unique coating properties can be obtained by the process that are not achievable by conventional thermal spraying. This uniqueness is due to the fact that coating deposition takes place without exposing the spray or subtrate material to high temperatures and, in particular, without melting the sprayed particles. Thus, oxidation and other undesired reactions can be avoided. Spryy particles adhere to the substrate only because of their high kinetic energy on impact. For successful bonding, powder particles have to exceed a critical velocity on impact, which is dependent on the properties of the particular spray material. This requires new concepts for the description of coating formation but also indicates applications beyond the market for typical thermal spray coatings. The present contribution summarizes the current “state of the art” in cold spraying and demonstrates concepts for process optimization.

Microstructural and macroscopic properties of cold sprayed copper coatings

Cold spraying is a coating technique in which the formation of dense, tightly bonded coatings occurs only due to the kinetic energy of high velocity particles of the spray powder. These particles are still in the solid state as they impinge on the substrate. This study correlates optimized deposition parameters with the corresponding microstructure as well as mechanical and conductive behavior of cold sprayed copper coatings in order to explain possible bonding mechanisms. In addition, the performance of cold sprayed copper coatings is compared to that of cold rolled copper and to coatings prepared by thermal spray methods.

New developments in cold spray based on higher gas and particle temperatures

In cold spraying, bonding is associated with shear instabilities caused by high strain rate deformation during the impact. It is well known that bonding occurs when the impact velocity of an impacting particle exceeds a critical value. This critical velocity depends not only on the type of spray material, but also on the powder quality, the particle size, and the particle impact temperature. Up to now, optimization of cold spraying mainly focused on increasing the particle velocity. The new approach presented in this contribution demonstrates capabilities to reduce critical velocities by well-tuned powder sizes and particle impact temperatures. A newly designed temperature control unit was implemented to a conventional cold spray system and various spray experiments with different powder size cuts were performed to verify results from calculations. Microstructures and mechanical strength of coatings demonstrate that the coating quality can be significantly improved by using well-tuned powder sizes and higher process gas temperatures. The presented optimization strategy, using copper as an example, can be transferred to a variety of spray materials and thus, should boost the development of the cold spray technology with respect to the coating quality.

Microstructural bonding features of cold sprayed face centered cubic metals

Cold spraying is a coating technique, in which the formation of dense, tightly bonded coatings occurs only due to the kinetic energy of high velocity particles of the spray powder. These particles are still in the solid state as they impinge on the substrate. This study correlates physicomechanical properties of the fcc metals Al, Cu, and Ni with the respective microstructures of their cold sprayed coatings in the light of scanning and transmission electron microscopical investigations. It is found that the microstructures of these coatings differ substantially from each other, and the results are discussed in the light of melting temperatures and stacking fault energies.

Oxidation of stainless steel in the high velocity oxy-fuel process

The high velocity oxy-fuel (HVOF) spray process has been primarily used for the application of wear-resistant coatings and, with the introduction of new, more powerful systems, is being increasingly considered for producing corrosion-resistant coatings. In this study, the influence of various spray parameters for the JP-5000 and Diamond Jet (DJ) Hybrid systems on the oxidation of stainless steel 316L is characterized. Experimental results reveal that coating oxygen contents of less than 1 wt.% can be more easily attained with the JP-5000 than the DJ Hybrid systems because of the former’s design. In both cases, however, the low particle temperatures necessary for low oxygen content coatings may impair bond and cohesive strength. Heat treating the coatings after processing reduces hardness, metallurgically enhances bond strength, and enables the spheroidization of oxide layers surrounding unmelted particles.An empirical model describing oxidation in the thermal spray process was expanded to explain the oxidation in the HVOF spraying of stainless steel. It was concluded that for these oxygen-sensitive materials, maintaining a relatively low particle temperature throughout the spray process minimizes oxygen pickup by preventing an autocatalytic oxidation process and particle fragmentation upon impact. For the DJ Hybrid systems, understoichiometric fuel settings are selected, whereas for the JP-5000, oxygen-rich mixtures are preferred.

The mechanism of pseudo-intercrystalline brittleness of precipitation-hardened alloys and tempered steels

The term pseudo-intercrystalline brittleness is proposed to describe a fracture mechanism which can occur in poly-crystalline alloys which contain a fine dispersion of a second phase. If narrow particle-free zones develop along grain boundaries, separation can occur after large amounts of plastic strain, which is highly localized to the vicinity of grain boundaries. Since the hardened grain interior does not contribute to plastic deformation the total plastic deformation to fracture and fracture toughness remain small. Quantitative models are proposed to interpret the micromechanism of fracture and to describe the grain-size dependence of fracture toughness. The fracture of precipitation hardening aluminum alloys, creep resistant and structural steels are discussed in terms of the models. Finally an interpretation of the mechanism of stress-relief cracking in steel weldments is given.

Present Status and Future Prospects of Cold Spraying

Cold spraying is a fairly new coating technique, which within the last decade attracted serious attention of research groups and spray companies. As compared to thermal spraying, the low process temperatures in cold spraying result in unique coating properties, which promise new applications. Since particles impact with high kinetic energy in the solid state, new concepts to describe coating formation are requested to enable the full potential of this new technology. The present contribution gives a brief review of current models concerning bonding, supplying a description of the most influential spray parameters and consequences for new developments. With respect to spray forming by cold cold spraying, microstructures and thick, further machineable structures are presented.

Tailoring nanocrystalline materials towards potential applications

Abstract The high interface area in nanocrystalline materials leads to advanced structural and functional properties that are interesting for a variety of applications and products. Three distinct examples for potential applications are given: Nanocrystalline and submicron-sized light-weight intermetallics based on γ-TiAl exhibiting favourable deformation behaviour at reduced temperatures, nanocrystalline cermet coatings produced by thermal spray process exhibiting improved hardness and wear resistance, and nanocrystalline Mg hydride-based composites for hydrogen storage in future mobile applications exhibiting extremely high, reversible storage capacity and fast kinetics.