Frank Gärtner

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.

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.

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.

Analysis of Thermal History and Residual Stress in Cold Sprayed Coatings

Residual stress in coatings has significant effect on their performance. In cold-sprayed coatings, in which particles impact the substrate at high velocity in solid state, in-plane residual stresses are usually conceived to be compressive. In this research, analysis of residual stresses in cold-sprayed deposits is performed by analytical and numerical modeling. The influence of various parameters such as the dimensions and elastic properties of the coating and the substrate on the residual stress are analyzed. In addition, the amount of heat input as a key parameter in the build-up of the residual stress is examined. It has been found that the heat input and the associated thermal history have a major influence on the final distortion and the residual stress, to an extent that the in-plane stress can in some cases change from compressive to tensile. Based on these results, a simple model is put forward for the prediction of the final state of the stress and distortion in cold-sprayed flat components.

Cold Spraying of Cu-Al-Bronze for Cavitation Protection in Marine Environments

Traveling at high speeds, ships have to face the problem of rudder cavitation-erosion. At present, the problem is countered by fluid dynamically optimized rudders, synthetic, and weld-cladded coatings on steel basis. Nevertheless, docking and repair is required after certain intervals. Bulk Cu-Al-bronzes are in use at ships propellers to withstand corrosion and cavitation. Deposited as coatings with bulk-like properties, such bronzes could also enhance rudder life times. The present study investigates the coating formation by cold spraying CuAl10Fe5Ni5 bronze powders. By calculations of the impact conditions, the range of optimum spray parameters was preselected in terms of the coating quality parameter η on steel substrates with different temperatures. As-atomized and annealed powders were compared to optimize cavitation resistance of the coatings. Results provide insights about the interplay between the mechanical properties of powder and substrate for coating formation. Single particle impact morphologies visualize the deformation behavior. Coating performance was assessed by analyzing microstructures, bond strength, and cavitation resistance. These first results demonstrate that cold-sprayed bronze coatings have a high potential for ensuring a good performances in rudder protection. With further optimization, such coatings could evolve towards a competitive alternative to existing anti-cavitation procedures.

Cold Spraying of Amorphous Cu50Zr50 Alloys

A new range of applications in cold spraying is expected for bulk metallic glass (BMG) coatings. For retaining amorphous structures in cast multi-component BMG parts, typically high purity raw material must be used. The present investigation explores an alternative approach, where cold spraying is used to deposit a technical-grade binary amorphous alloy. This approach is shown to be potentially cost-effective and suitable for rapid manufacturing. For this purpose, amorphous Cu50Zr50 was chosen as a model alloy system, and cold spraying was performed using nitrogen as process gas. By a systematic variation of the spray parameter sets, the critical velocities for coating formation were determined experimentally. Based on the current models of bonding of amorphous Cu50Zr50 powder in cold spraying, a new, more comprehensive concept of bonding and rebound is presented, which also considers the presence of liquefied interfaces and quenching rates for resolidification. Results concerning impact morphologies and coating formation demonstrate that under suitable choice of spray conditions, well-adhering coatings with amorphous structure of the Cu50Zr50 powders can be obtained by cold spraying.

Patterned CoCrMo and Al2O3 surfaces for reduced free wear debris in artificial joint arthroplasty

Surface wear of corresponding tribological pairings is still a major problem in the application of artificial joint surgery. This study aims at developing wear reduced surfaces to utilize them in total joint arthroplasty. Using a pico-second laser, samples of medical CoCrMo metal alloy and Al2 O3 ceramic were patterned by laser material removal. The subsequent tribological investigations employed a ring-on-disc method. The results showed that those samples with modified surfaces show less mass or volume loss than those with a regular, smooth surface. Using calf serum as lubricating medium, the volume loss of the structured CoCrMo samples was eight times lower than that of regular samples. By structuring Al2 O3 surfaces, the wear volume could be reduced by 4.5 times. The results demonstrate that defined surface channels or pits enable the local sedimentation of wear debris. Thus, the amount of free debris could be reduced. Fewer abrasives in the lubricated so-called three-body-wear between the contact surfaces should result in less surface damage. Apart from direct influences on the wear behavior, less amounts of free debris of artificial joints should also be beneficial for avoiding undesired reactions with the surrounding soft tissues. The results from this study are very promising. Future investigations should involve the use of simulators meeting the natural conditions in the joint and in vivo studies with living organisms.

Advances in cold spraying

Developments for different thermal processes or cold spraying commonly aim to minimise coating defects as porosity, disturbing oxides or undesired phase transformations as side effects. Whatever spray process is considered, a certain amount of energy is requested for building up coatings. Comparing thermal spray techniques in Fig. 1, ranging from arc spraying (AS), plasma spraying (PS) and conventional flame spraying (FS) to high velocity oxyfuel flame (HVOF) spraying, a trend to reduced particle temperatures and increased particle velocities is observed with respect to attaining optimum coating properties. The balance of requested energy for successful coating formation by thermal heat or kinetic impact energy is highly dependent on the type of coating material. For cermets and metallic spray materials, new developments in HVOF spraying already shifted the balance towards higher kinetic energy. All that has to be tuned to the wide variety of different feedstock types, which are available for one spray material. In that comparison, cold spraying represents the far edge, operating at temperatures far below the melting temperatures of metallic materials and at very high particle impact velocities, ranging more than 1000 m s 21 with respect to different process conditions. The major benefits of cold spraying are given by negligible amounts of side reactions during deposition. Therefore, microstructures and properties of feedstock materials can be retained in the coatings. 1 As compared with thermal spraying, also the particle beam with typical diameters from 6 to 8 mm is much smaller, reducing overspray and therefore costs as well as efforts for masking. In addition, the localised deposition allows to build up solid structures which is interesting for rapid prototyping or repair work. Cold spraying was developed around 20 years ago by a group of scientists at the Institute of Theoretical and Applied Mechanics, Novosibirsk, Russia. 2,3 Already these early work reported conditions for building up coatings of different spray materials. Moreover, they postulated a so called critical velocity for successful bonding in cold spraying. In the western world, cold spraying attracted rapidly increasing interests in the late 1990s, as demonstrated by the number of contributions to the annual meetings of the Thermal Spray Society. To facilitate applications, spray systems were developed which meet industrial standards. So far, about 20 to 30 units of such commercial spray systems are operated worldwide. About two-thirds of them are used by research centres or universities. 4 Companies operate the other third for various, mainly high tech, applications. As compared with the units of commercial equipment, a probably similar number of own design ‘home made’ cold spray units are operated at universities or research centres.

Coating formation, fracture mode and cavitation performance of Fe40Al deposited by cold gas spraying

The present study investigates the coating formation by cold spraying Fe–40 at.-%Al intermetallics and evaluates their cavitation performance. Coating properties identify them as good candidates for application in many industrial environments. Despite their low ductility, after optimising the spraying parameters, dense layers were achieved. Fracture surfaces after bending tests indicate a combination of ductile and brittle failure, revealing some information on bonding features. The high microstructural homogeneity of these coatings, similar to that of bulk materials, is beneficial for their resistance to cavitation damage. The high strength of cold sprayed intermetallics makes them an interesting alternative to other materials and coating solutions.