| Abstract: |
In principle, coatings made by brazing is a process variant of brazing which can be classified as high-temperature brazing. It is a thermal process which is carried out either without flux in a vacuum or under inert gas with tapes/slurries/pastes/powders with a liquidus temperature generally above 900 °C. In this process, no components are firmly connected to each other but brazeable materials are applied which, after the heat treatment process, produce a metallic, material-tight coating. The most commonly used filler matrix materials are nickel-based ones, cobalt, iron, copper-based ones or corresponding alloys. Hard materials are mixed in depending on the coating function. Carbides, silicides, borides, oxides, diamonds, CBn or hard material mixtures can be used as hard materials. Common industrial used hard materials are WC, CrC or NbC. Hard material proportions in the coating can be up to 80 Vol.%. Actual developments show contents up to 90 Vol.% and more. Depending on the application, layer matrix hardnesses are flexibly adjustable from 20-30 HRC to 62-65 HRC. The coating produced can therefore perform various functions. For this reason, they are also called functional coatings. For example, hard material particles introduced into the matrix can be firmly brazed onto the surface of the component and thus take on a wear protection or gripping function. Alternatively, worn components such as moulds or turbine blades can be recontoured by brazing suitable materials as tapes or slurries into the wear areas and then reworking them. The coatings are very dense and crack-free and are therefore also very suitable for corrosion protection, even at high temperatures. In contrast to deposit welding, the deposit-brazed coatings are relatively smooth and often do not need to be reworked or ground. The strength of high-temperature brazed hard material coatings can reach the strength of the base materials. This results in a highly stressable layered composite. 2D and 3D geometries can be coated both internally and externally. Coating thicknesses are usually ranging from 1.0 up to 4.0 mm. Minimum layer thicknesses of 0.05-0.1 mm up to 10 mm and more can be achieved. Recent developments also show the possibility of locally brazing on applied tapes or suspensions using laser energy without having to heat the entire component. By selecting appropriate morphologies of the starting powders for braze matrix materials and hard materials, the coating system can be specifically optimised and adapted for the respective application. In addition, work is being carried out on systems in which "signaling elements" are incorporated into the coating in order to record the condition of a surface during operation. For example, forces, wear or temperature. The lecture gives an overview of selected processes, materials and applications and an outlook on new developments.
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