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.

Depending on the size and type defects of nickel-based alloy turbine blades two procedures are used mainly: cladding and high temperature brazing. The repair brazing of turbine blades is used to regenerate cracks and surface defects and is the focus of this work. In this contribution a two stage hybrid repair brazing process is presented which allows reducing the current process chain for repair brazing turbine blades. In the first stage of this process the filler metal (NiCrSi) then the hot gas corrosion protective coating (NiCoCrAlY) and finally the aluminium are applied in this order by atmospheric plasma spraying. In the second stage of this hybrid technology the applied coating system undergoes a heat treatment in which brazing and aluminising are combined. The temperature-time regime has an influence on the microstructure of the coating which is investigated in this work.

This study investigates the use of cold gas spraying (CGS) for depositing braze filler coatings. In the experiments, pure Cu layers were sprayed onto Mg alloy substrates, which were then joined to AlSi steel by contact reaction brazing in a vacuum furnace. The bonding temperature influenced the dissolution of Cu as well as the eutectic reaction between the coating and substrate. The thickness of the brazed seam was found to be 300 μm although the initial thickness of the Cu layer was just 50 μm. The shear strength of the joint peaked at 37 MPa, corresponding to a brazing temperature of 530 °C. Intermetallic phases and interfacial defects of various types were responsible for the low strength of the joints.

In this research project a hybrid technology is developed to repair turbine blades. This technology incorporates procedural and manufacturing aspects like raising the degree of automation or lowering the effort of machining and includes materials mechanisms (e.g. diffusion processes) as well. Taking into account these aspects it is possible to shorten the process chain for regenerating turbine blades. In this study the turbine blades of the high pressure turbine are considered and therefore nickel-based alloys are regarded. To repair or regenerate turbine blades the following methods are employed: welding and brazing and a subsequent aluminizing CVD-process. The focus in this work lies on the brazing method and the required filler-metal is applied together with the hot-gas corrosion protective coating by means of thermal spraying and represents the first stage of this hybrid technology. In the second stage of this hybrid technology the brazing process is integrated into the aluminizing CVD-process and a first effort is presented here.

The aim of the research project is to combine repair brazing with protective coating against hot-gas corrosion into a common integrated process. Both the braze-metal as well as the hot-gas corrosion protection coating is applied by means of thermal spraying. The material layout is to be realized as far as possible to the near net shape by using thermal spraying. The processes are to be performed in such a way that the brazing is integrated into the CVD diffusion annealing process as a transient liquid phase bonding (TLP bonding) process which, as a consequence, can then be eliminated as a separate processing step. The thermal spraying processes of atmospheric plasma spraying (APS), high velocity oxygen fuel spraying (HVOF) and cold gas spraying (CGS) are to be qualified for this purpose. Thus the project working hypothesis is to be able to transform thermal coating and joining processes into a common integrated hybrid process and, in doing so, obtain both high-quality and economic advantages. The importance of combining these processes lies in reducing the effort of grinding as well as economizing on the vacuum brazing, which is currently a separate process step, and consequently lowering the production costs.

SOFCs for mobile applications require short starting times and capability of withstanding several and severe cycles. For such applications metallic cassette type cells with low weight and thermal capacity are beneficial where the active cell part is set in interconnects consisting of two sheets of ferritic steel. These cells are stacked serially to get higher voltage and power. This approach needs interconnect sheets that are electrically insulated from each other to prevent electrical short circuit. The technology discussed here is to use brazed metals, as sealants, and ceramic layers, as electrical insulators, which are vacuum plasma sprayed on the cassette rims. For reliable insulating layers, a variety of deposits were developed, starting from cermet-spinel multilayers with various compositions and constituents, where reactive metals (such as Ti, Zr) were part of the coatings, to pure ceramic layers. The qualities and characteristics of these coatings were investigated which included electric insulation at room temperature and at 800 °C (SOFC operating temperature), wettability of different brazes towards these deposits, phase stability and peeling strength. The single steps of development, characteristics of the insulating layers for SOFCs as well as some challenges that have to be taken into account in the process are described.

Heat exchangers play a vital role in ongoing efforts to conserve energy. Plate-type heat exchangers typically consist of two flat separated flow paths in which heat transfer enhancing matrices are inserted. The combined effects of small irregular hydraulic diameters along with elevated heat transfer areas results in highly-efficient heat transfer to the external fluid. This allows for very versatile and compact heat exchanger designs. Typical plate-type heat exchanger fabrication methods such as brazing are labour intensive and limit post-processing operations like welding. In this paper, a novel micro-heat exchanger fabrication method using recently patented technologies is presented. The approach uses thermal spray processes such as Pulsed Gas Dynamic Spraying (PGDS) as an alternative to brazing for the production of a pressure barrier and integration of flow headers. Mesh wafer surfaces sealed using PGDS

Superabrasive composite materials are typically used for grinding stone, minerals and concrete. Sintering and brazing are the key manufacturing technologies for grinding tools production. But restricted geometry-flexibility, absence of repair possibilities for damaged tool surfaces as well as difficulties in controlling materials interfaces are main weaknesses of these production processes. Thermal spraying offers the possibility to avoid these restrictions. In this research work a fabrication method based on the detonation flame spraying technology has been investigated to bond large superabrasive particles (150 – 600 µm, needed for grinding minerals and stones) in a metallic matrix. Layer morphology and bonding quality are evaluated with respect to superabrasive material, geometry, spraying- and powder-injection-parameters. Influences of process temperature and possibilities of thermal treatment of MMC-layers are analyzed.

In order to overcome the disadvantage of local carburizing of steel components in contact with light-weight graphite or carbon fiber reinforced ceramic racks alumina based thermal spray coatings are produced as diffusion barriers with improved life time compared to rapidly degrading alumina or boron nitride pastes. The powder flame sprayed coatings are also capable to prevent damage by excess filler material in high temperature brazing processes effectively. Besides graphite also C/C racks are coated with pure alumina, Al 2 O 3 -TiO 2 and Al 2 O 3 -Cr 2 O 3 . Conventional powder flame spraying is applied in order to provide a low-cost solution for realization of diffusion barriers. Coatings are characterized by means of optical microscopy and SEM with regard to the interface to the substrates and their porosity. Coated racks are used in field tests for case hardening of steel components. The life time of thermal spray coatings is compared to alumina and boron nitride based pastes. Comparative liquid metal corrosion tests are carried out with NiCr7Si4.5B3.1Fe3 filler at 1,050 °C.

The use of modern thermal spraying techniques for filler metal application may offer new solutions for brazing complex metal components without fluxing agents, when the spraying processes are fitted to the requirements of the following brazing process. The necessary coating parameters always depend on the kind of base material, the geometry of the joint, the used filler metal, and the chosen heating process for brazing (heating in vacuum or protective atmosphere furnaces, inductive or flame heating etc.). Copper and nickel based alloys are typical filler metals for brazing complex components made of stainless steel (heat exchanger, fuel pipe systems, exhaust systems, catalytic devices etc.). Using these examples, the results of brazing experiments and technical aspects of thermally sprayed braze coatings compared with conventional filler metal application techniques are discussed.

Extensive laboratory testing and field usage have shown that innovative surfacing techniques have produced cost effective maintenance systems and are providing long-term benefits. Self-fusing (sometimes known as self-fluxing) alloys containing tungsten carbide (WC), applied by PTAW, HVOF and SF (Spray Fusion) brazing processes are investigated. The process used and the effect of process parameters on the wear resistance of these coatings is evaluated. The test results show that the same self-fusing alloy applied by SF compared to PTAW have proven superior in severe erosive and abrasive applications. The case histories presented will cover a variety of applications including the use of HVOF versus hard chrome plating and the improvement in wear resistance of SF applied self-fused coatings versus PTAW. These comparisons are useful in providing new, higher performance solutions, in helping to overcome today's tougher surfacing and environmental requirements

Traditional brazing alloys do not wet ceramics and are therefore unusable for metal-ceramic bonding. One way to overcome the problem is by depositing a metal layer onto the ceramic prior to brazing. The approach taken in this paper was to plasma spray copper onto different ceramics (Al2O3, AlN, SiAlON) and then assess the wettability of potential brazing alloys (AgCu, AgCuTi). Interface analysis showed that silver and titanium segregation occurs at the ceramic surface and that, conversely, sprayed copper diffuses into the brazed joint.

Both bonding strength of coating to substrate in low pressure plasma spraying and the effect of reverse transferred arc treating before spraying are studied in this paper. It is difficult to obtain the bonding strength precisely in low pressure plasma spraying by standard testing methods, such as, ASTM Standard C633-79 and JIS H8666-80. Therefore, for the bonding strength test rather than using a conventional adhesive, we believe a vacuum brazing process using Ag-Cu-In-Ti active filler metal at 1023 K should be used. We have also confirmed the practicality of this step. By the above test method, it has been proven that the bonding strength of low pressure plasma sprayed coating is over 100 MPa. Also, that reverse transferred arc treating after blasting enhances the bonding strength of low pressure plasma sprayed coating. It is also believed that the projections formed on the substrate surface by reverse transferred arc treating are buried into the coating and perform the pile effect.