HVOF-sprayed carbide based coatings such as WC/Co or Cr 3 C 2 /NiCr are industrially well established for wear and corrosion protection applications. Due to their high carbide content of typically 75 wt.-% and more, they are providing a very high hardness and excellent wear resistance. Unfortunately costs for matrix materials like Ni or Co underlie strong fluctuations and are significant higher compared to iron. Therefore an alternative concept to the conventional carbides is based on TiC-strengthened low cost Fe-base materials, which are already in use for sintering processes. Depending on the carbon content the Fe-base material can additionally offer a temperable matrix for enhanced wear behaviour. Within this study the sprayability of TiC-strengthened Fe-powders with a gaseous and a liquid fuel driven HVOF-system has been investigated. The resulting coatings have been analysed with respect to microstructure, hardness and phase composition and compared to galvanic hard chrome, HVOF-sprayed and remelted NiCrBSi and HVOF-sprayed Cr 3 C 2 /NiCr (80/20) coatings as well as sintered Fe/TiC reference materials. Furthermore the Fe/TiC coatings have been heat treated to proof the retained temperability of the Fe-matrix after thermal spray processing. For determination of wear properties tribometer tests have been conducted. Currently the corrosion resistance of the sprayed Fe/TiC coatings is investigated as well the wear behaviour in a practical hydraulic test bench.

Newly developed iron based hard alloy powders with high chromium and vanadium contents are used for coating production by means of HVOF and LPPS. Crack free and dense coatings with fairly homogeneous microstructure are possible for both spraying methods. XRD analyses of sprayed coatings prove phase compositions similar to those of the powder feedstock when using HVOF systems. In contrast LPPS coatings contain a large share of amorphous phase. Microhardness of LPPS and HVOF coatings is about 1,200 HV0.3 and 800 - 950 HV0.3 respectively. The higher microhardness of LPPS coatings is attributed to the presence of the amorphous phase. However, LPPS coatings are brittle and tend to crack under mechanical load. Wear resistance of coatings is determined by means of corundum grinding disk and ASTM G65 wear test. Corrosion behavior is characterized by means of salt fog test and electrochemical measurements. Cermet and stainless steel 316L coatings are used for comparative purposes in the investigations.

MoSi 2 -TiB 2 coatings were applied on Ti6Al4V substrates by means of low pressure plasma spraying with and without a surface pre-treatment with plasma-transferred-arc. The coatings were characterized by optical and scanning electron microscopy, X-ray diffraction analysis and hardness measurements. No difference in the microstructure between the coatings was detectable. The microstructure shows the typical lamellar structure of thermal spray coatings with a good embedding of the titanium borides in the matrix. At the interface between matrix and some borides a reaction zone is visible. In comparison to the feedstock powder, the phase composition of the coatings has change, because a great amount of the tetragonal MoSi 2 phase transformed into the hexagonal high temperature modification. The content of titanium diboride is lower in the coating. Coatings on substrates with a pre-treatment show a good adhesion to the substrate, while the adhesion of the coatings on the pre-treated substrates is poor.

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.

In some applications like car armouring for personal safety, in bulk containers in pebbles working, in off-shore applications or in offroad-vehicles, impact of bodies with different sizes and high velocities has to be calculated in material design. A possibility to design under economical aspects is to use to low-cost material as substrate with a protective coating on its surface instead of using expensive bulk materials. One conceivable material system to be used as protective coating is WC-CoCr, which is usually applied by HVOF and shows high hardness combined with a good toughness of the matrix. These properties are very important for dissipation of the impact energy and for high fracture toughness. In the presented case study, WC-CoCr coatings with different carbide sizes were produced with the JP5000 system from TAFA. Projectiles with high velocities of more than 250 m/s were shot by an experimental shooting device on the coatings and for comparability on the uncoated substrate. Impact tests were done with balls and cylinders with varying impact angles between 10° and 90°. The powders and the coatings were characterized by means of microscopy and XRD. Furthermore, the hardness of the coatings was measured. After impact tests, the coatings were investigated with optical microscopy. In the case of ball impact, a significant dependency can be found between the impact behaviour and carbide size. Coatings with coarse carbides show the lowest impact depth. The dependency of the impact depth of coatings with one carbide size on their hardness level is relatively low. No dependencies of the carbide size or the hardness level can be found for impact with cylinders.

Molybdenum disilicide (MoSi 2 ) is a suitable material for high temperature applications especially because of its excellent high temperature oxidation resistance. For several high temperature applications MoSi 2 shows high potential to be used as a protective coating. The oxidation behaviour of HVOF sprayed MoSi 2 coatings is studied at 1500 °C. The oxidation tests are carried out in a simultaneous thermogravimetric device and the mass change is measured in dependence on the oxidation time. The microstructure of the coatings before and after oxidation is examined by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDXS). The mass of the coating increases according to a parabolic function. During the oxidation test the microstructure changes significantly from a typical thermal spray coating microstructure with lamellae, pores and a phase mixture of MoSi 2 and Mo 5 Si 3 to a two phase system with sharply separated grain boundaries. On the surface of the coating a silicon dioxide layer with a thickness of less than 10 µm is formed.

The laser-flash method is used to determine the thermal diffusivity of HVOF sprayed WC-Co(Cr) and Cr 3 C 2 -Ni20Cr as well as APS sprayed Cr 2 O 3 and electroplated hard chromium coatings in the temperature range between RT and 600°C. Additionally bond and/or corrosion protective coatings like Ni5Al, Ni20Cr and 316L are characterized taking into account the different manufacturing methods twin wire arc spraying, HVCW and HVOF. With respect to the application example of drying rollers in paper industries the Taber-Abraser wear test is applied to evaluate the wear resistance. Finally the coatings are characterized concerning their corrosion resistance by salt fog test and by exposure to humid SO 2 environment. For WC-CoCr feedstock the effect of carbide size and micro hardness on thermal, wear and corrosion properties are studied. WC-CoCr coatings with maximum micro hardness and fine carbides show the best thermal conductivity. The use of coarse carbide feedstock permits manufacturing of coatings with the highest resistance against dry abrasive wear, but the protective function depends severely on the processing conditions.

During the last decades, the application of copper alloys has become very prominent in engineering. Judging from the properties of bulk materials or galvanic copper coatings, a thermal sprayed coating shows significant disadvantages. The reason for this effect is the build up of a thermal sprayed coating by individual droplets. The main aim of this work is to improve the properties of the twin wire arc sprayed copper alloys, thereby expanding the application of these kind of coatings to the areas where galvanic copper plating technique are mainly used. A copper-cobalt-beryllium alloy has been investigated and the possibility of its application in the twin wire arc process evaluated. The arc sprayed coatings were classified based on the properties of bulk material. The improvement of properties like hardness is based on an investigation of several spraying parameters of the arc spray process. To achieve a maximum value of thermal or electrical conductivity, minimum porosity of the coatings was the aim. Furthermore, a post heat treatment of the sprayed coatings, with the aim of reducing residual tensile stresses within the coating and to improve the wear resistance by means of hardening effects, was carried out. The investigations involved metallographic examination of the coatings using optical microscopy and scanning electron microscopy. Phase composition and residual stresses were detected by X-ray diffraction analysis. Microhardness was measured in the as sprayed as well as in the heat treated state.

There is a variety of components, which are subject to high wear and/or corrosion stress on the one hand and are used to transfer heat on the other hand. Two examples are drying cylinders in paper production and condensing boilers. Up to now there are no data available for the thermal design of thermal spray coated components except for some MCrAlY and thermal barrier coatings for turbine applications. Also guidelines for the optimization of thermally sprayed coatings concerning heat transfer including the effect on the wear resistance are missing. HVOF sprayed cermet coatings are widely used for combined wear and corrosion protection these days. In addition to WC-CoCr 86 - 10 4 and 75 Cr 3 C 2 - 25 Ni20Cr conventional Ni5Al and Ni20Cr bond coats are evaluated concerning their thermal conductivity in the range between room temperature and 600 °C. Also the thermal contact resistance is determined depending on the substrate material: mild steel S355J2G3 (1.0570), grey cast iron GG25 (0.6025) and austenitic stainless steel X5CrNi18-10 (1.4301, AISI 304). The applied Laser- Flash method requires knowledge of the heat capacity, thermal expansion and density, which are determined before. HVOF spraying has only negligible influence on the heat capacity of WC-CoCr feedstock, as the temperature depending functions are almost identical. The use of spraying feedstock with average WC particle sizes of 800 nm, 3 µm and 5 µm permits to investigate the influence of the specific surface area of the hard phases both on the thermal conductivity and wear resistance. Furthermore the influence of the coating porosity is determined. In accordance to the drying cylinder application the wear resistance is determined by Taber-Abraser wear tests. Bond coats are produced by HVOF, HVCW and arc spraying and compared concerning microstructure and thermal conductivity. A comparison to the properties of electroplated hard chromium coatings is drawn.

Molybdenum silicides have the potential as protective coatings for high-temperature applications because of their high melting point and their high-temperature oxidation resistance. Reinforcing MoSi2 with SiC shows an improvement of its low toughness at room temperature and low creep resistance at temperatures above the brittle-ductile transition temperature of approximately 700-1000 °C. A new kind of powder processing was used to produce MoSi2 and MoSi2-SiC as a feedstock for thermal spraying. Mixtures of the elemental powders, molybdenum and silicon, were prepared by milling and subsequent heat treatment to get highly dispersed, pre-reacted powders. As high-energy milling equipment, a planetary ball mill was used to prepare the powders. In the case of reinforcement, SiC was mixed to the pre-reacted MoSi2 at the end of the milling process, that means before heat treatment. On these as-milled powders, X-ray diffraction characterization (XRD), scanning electron microscopy (SEM), electron probe micro analysis (EPMA) and determination of the oxygen level were carried out. Vacuum plasma spraying has been used to deposit the powders onto a carbon steel substrate. Evaluated coating characteristics were the microstructure (SEM), phases (XRD), EPMA, oxygen content, microhardness and surface roughness. Tests at high temperatures will be considered in future work.