Advances in cold spraying

Abstract

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.