The present work summarizes the most important results of a research project dealing with the comprehensive!! investigation of the bonding mechanisms between cold sprayed Al coatings and various poly- and monocrystalline ceramic substrates (Al 2 O 3 , AlN, Si 3 N 4 , SiC, MgF 2 ). Due to their exceptional combination of properties, metallized ceramics are gaining more and more importance for a wide variety of applications, especially in electronic engineering. Cold spraying provides a quick, flexible and cost-effective one-step process to apply metallic coatings on ceramic surfaces. However, since most of the existing cold spray-related publications focus on metallic substrates, only very little is known about the bonding mechanisms acting between cold sprayed metals and ceramic substrates. In this paper, the essential factors influencing the bonding strength in such composites are identified. Besides mechanical tensile strength testing, a thorough analysis of the coatings and especially the metal/ceramic interfaces was conducted by means of HRTEM, FFT, STEM, EDX, EELS, XRD and EBSD. The influence of substrate material, substrate temperature and particle size is evaluated with regard to the observed bonding behavior. The results suggest that, apart from mechanical interlocking, the adhesion of cold sprayed metallic coatings on ceramics is based on a complex interplay of different mechanisms such as quasi-adiabatic shearing, static recrystallization as well as heteroepitaxial growth.

The present work builds on investigations of cold gas-sprayed Al coatings on Al 2 O 3 , which strongly indicated that heteroepitaxial growth is a possible mechanism supporting the adhesion between metal and ceramic at their interface. The present study was focused on the deposition of Al on further ceramic substrates (AlN, Si 3 N 4 and SiC). In particular, it should be clarified whether the different ionicity of the chemical bonding in these substrate materials influences the interface formation or not. Aluminum coatings were deposited alternatively by using cold-gas spraying (CGS) and magnetron sputtering. In CGS coatings, the effect of substrate roughness, substrate temperature and powder fraction on the adhesion of the coating was investigated. The magnetron-deposited coatings were used to evaluate the role of the heteroepitaxy in the interface formation and to identify microstructure defects in the metal/ceramic interface, which are caused solely by the lattice misfit between the counterparts and not by the impact-induced deformation that is typical for cold gas-sprayed coatings. Interface characterization was conducted by scanning electron and high resolution transmission electron microscopies combined with XRD.

Thermal barrier coatings typically incorporate a YSZ topcoat and a metallic bond coat. During service, a reaction zone consisting of different thermally grown oxides forms at the interface. Although most such oxides are detrimental, one (α-Al 2 O 3 ) improves service life due to its barrier effect on oxygen diffusion. In this study, Al and AlOx films are deposited on metallic bond coats by dc magnetron sputtering prior to topcoat deposition. The resulting TBCs were thermally cycled to determine the effect of the interlayer films on service life and TGO formation. It is shown that the Al films transform in situ into dense Al 2 O 3 layers that act as oxygen diffusion barriers. TBCs with interlayer alumina, whether deposited directly or formed in situ, showed less cracking and were more mechanically stable during thermal cycle tests.

This work investigates the adhesion mechanisms associated with cold gas sprayed metallic coatings on ceramic substrates. Aluminum layers were deposited on sintered corundum plates and single-crystalline sapphire substrates with different lattice orientations. Examination of the interface region showed that adhesion was the result of recrystallisation and heteroepitaxial growth. Cold spray aluminum coatings were also deposited on Al 2 O 3 , AlN, Si 3 N 4 , and SiC substrates to determine if ionic bonding plays a role in interface formation. For comparison, aluminum coatings produced by physical vapor deposition were also examined.

This study evaluates the friction and wear behavior of iron-base coatings produced by arc spraying using experimental cored wires. Coating microstructure was analyzed and various wear tests were performed. The results show that the tribological properties of the ferrous coating materials are greatly affected by porosity, oxide inclusions, particle shape, and microhardness.

In this study, aluminum is deposited by cold gas spraying on smooth and rough alumina substrates. SEM examination of single particle splats on polycrystalline Al 2 O 3 suggests that mechanical interlocking is the primary bonding mechanism for cold gas sprayed metals on rough ceramic surfaces. Coating cross-sections on monocrystalline, atomically smooth sapphire substrates were examined by means of high-resolution TEM, revealing nanosized Al grains close to the interface, which could be an indicator of recrystallisation induced by local heating and the deformation energy stored in the particle during impact.

Thermally sprayed cermet powder coatings as well as bulk cermet materials sintered of carbide/metal powder blends are widely used in applications with severe abrasive wear conditions. A cost-saving alternative can be provided by using iron-based melt-atomised hard alloy powder feedstocks. Among them, commercial alloys containing high amounts of vanadium and carbon obtain outstanding wear resistance due to their high volume fraction of finely dispersed, hard vanadium carbides. However, their performance is still exceeded by cemented carbides. A further improvement of the wear properties of hard alloys basically can be attained by increasing their carbide content, concurrently considering the limitations of the melting and atomisation process regarding the melting temperature. A possible solution can be provided by alloying the basic system Fe-V-C with an additional strong carbide former like niobium. Subject of this work is the comparing investigation of the technologically important melting equilibria in the systems Fe-V-C and Fe-V-C-Nb.

Thermal spray coatings of austenitic materials are mainly used under corrosive conditions. The relatively poor wear resistance strongly limits their use. A selective enrichment of the surface layer region with carbon by means of thermochemical heat treatment improves the residual stresses and increases the wear resistance. The interstitial deposition of carbon causes strong compressive residual stresses and a high surface hardness. The low process temperature of the thermochemical heat treatment avoids the precipitation of chromium carbide, whereas the corrosion resistance is not affected. Increases in the service life of existing applications or new material combinations with face-centred cubic friction partners are possible. In the absence of dimensional change, uniform as well as partial carbon enrichment of the thermal spray coating is possible. In comparative studies between carburized and untreated thermal spray coatings, the influence of the carbon enrichment on the coating properties and the microstructure was investigated. Carburized coatings demonstrate a significant improvement in adhesive wear resistance and an extremely high surface hardness. The cross section micrograph of the carburized coating shows the S-phase formation in the surface layer region. The depth profile of the carbon concentration was determined by GDOS analysis.

Cermets like WC/Co or Cr 3 C 2 /Ni20Cr are well-established materials for thermally sprayed wear protection coatings. However, their high price and the adverse health effects of nickel and cobalt cause the motivation for the development of novel materials with excellent wear resistance. Within the AiF/DFG research cluster “Thermal Spraying”, a multi-institutional cooperation of various German research centres, the focus is put on particle-reinforced iron-based composite alloys. High-alloyed steels serve as matrix materials into which hard CrB 2 particles are incorporated by means of high-energy ball milling (HEM). By adjusting appropriate milling parameters, the microstructure of the powder and its level of amorphisation can be influenced effectively. The high-velocity oxygen fuel process (HVOF) allows a transfer of the desired nanocrystalline structure from the particles to the coatings. The deposited coatings exhibit low porosity and high microhardness values of more than 1000 HV0.3. The wear resistance of the coatings was determined by means of Miller test (ASTM G75-01) and compared with conventional wear protection materials and coatings produced with agglomerated and sintered powders. The obtained outstanding results qualify particle-reinforced iron-based materials as a promising alternative for a wide range of applications.

A novel process to produce dense, well adherent aluminium coatings on ceramic materials is cold gas spraying (CGS). The mechanical, physical and chemical processes leading to the bonding of cold-sprayed coatings on ceramic substrates have only been described rudimentarily. A survey of the literature on adhesion mechanisms of cold spray coatings is given, where influences on bond strength are discussed and parameters identified. Using the example of coating Al 2 O 3 and AlN substrates with pure aluminium via cold gas spraying, a process and substrate parameter variation is presented. A significant raise in tensile coating strength was seen at elevated substrate temperatures and after subsequent annealing. Tensile strength also depended on chemical composition and roughness of the substrate. The results allow the discussion of the bonding mechanisms of cold spray aluminium on ceramic substrates as a function of deposition and annealing parameters.

Several model Fe- and Ni-based alloys with increased content (up to 30 at.-%) of protecting scale forming elements were developed. High temperature corrosion resistance of bulk alloys as well as thermally sprayed coatings and welded overlays were investigated under the waste power plants simulated atmosphere (500 °C, 100 hours, 75N 2 -20O 2 -4.9Ar-0.1Cl 2 ). Arc and HVOF spraying as well as PTA overlay welding were used to produce the coatings. After an exposure the samples were examined with electron probe micro-analysis (EPMA). It was shown that the protection behaviour of overlay welds depends on the content of alloying elements, although the last is limited because of weldability decrease by high alloying level. High temperature corrosion resistance of thermally sprayed coatings is determined by their porosity, which can be varied over a very broad range depending on the applied spray method. The arc sprayed coatings need an additional post-treatment to close a porosity. Two methods were applied, pre-oxidation treatment in the air and sealing with the commercial sealant. Newly developed iron-based coatings with increased aluminium content (< 20 wt- %) sprayed with HVOF-spraying with powders obtained by means of high energy ball milling demonstrate high corrosion resistance. Selected coatings were evaluated for 1000 h exposure under chlorine-containing salt deposits at the higher temperature (600 °C).

Solid-particle erosion of metals and alloys at elevated temperatures is one of the main reasons of the damage of components used in the energy production and utilization industries. Application of protective coating systems can be an attractive and economically reasonable solution for preventing the failure and increasing the durability of the components working in severe conditions of high-temperature corrosion and erosion. However, thermal spraying of intermetallic materials that have excellent high-temperature corrosion resistance is limited because of their low ductility. Present work reports the results of the investigation of abrasion wear resistance at elevated temperatures of combined coatings, which include the intermetallic layer. Such iron aluminide layers have been formed as a result of diffusion during the heat post-treatment of arc-prayed metallic coatings combining Fe- and Al-based layers. Post-treatment of arc-sprayed coatings was carried out by means of infrared radiation and induction heating. It was shown that the abrasion resistance of the developed coating tested at elevated temperatures (T > 500 °C) is considerably higher than that of low-alloyed steel and some nickel-based alloys and depends on the test load condition. The high performance of intermetallic-based graded coatings at elevated temperatures makes them interesting for applications as a low-cost erosion-corrosion-resistant material.

Thermally sprayed alumina coatings are widely used in a range of industrial applications to improve wear and erosion resistance, corrosion protection and thermal insulation of metallic surfaces. These properties are required for many components for production processes in the paper and printing industry. By means of efficient and adjustable coating processes, long-term use of the refined surfaces is obtained. It can be seen that cost-efficient arc-sprayed Al coatings post-treated by plasma-electrolytic oxidation (PEO) form Al 2 O 3 -layers with outstanding hardness, bonding strength, abrasion and corrosion resistance as well as extended service time. These coatings are designed to partially replace hard chromium.

This paper presents a new strategy for improving the quality of HVOF sprayed coatings as well as the deposition efficiency of the process. The highly turbulent expanding gas jet is stabilized and focused with the aid of a helical gas shroud. The effect of the design modification is demonstrated by numerical calculations and through the use of a prototype torch.

In this work, mechanically alloyed Al–12Si/TiB 2 /h-BN composite powder was deposited onto an aluminum substrate by atmospheric plasma spraying. The results revealed that the mechanical alloying (MA) process has a significant effect on composite powder morphology and in-situ reaction intensity between the selective powders during plasma spraying. In addition, hexagonal boron nitride (h-BN) powder incorporated as a solid lubricant, which has excellent lubricating properties, decomposed into B and N and formed a solid solution after a long period of milling. More specifically, during plasma spraying a large amount of h-BN reacted with Al to form AlN. Unlubricated ball-on-disk testing ring was used to examine the anti wear performance of the coatings. The worn surfaces were examined using scanning electron and energy dispersive spectroscopy to elucidate the wear mechanisms operating at the sliding interface.

Well-adjusted processing conditions are the basics for a high quality of thermal sprayed coatings. For HVOF spraying that means super sonic particle velocities and moderate particle temperatures. To fulfil these requirements, a certain speed of the gas jet is necessary. If low fuel and gas consumption is aspired, this can only be realised with adapted nozzle designs. Some efficient ways of nozzle design optimisation are shown in this paper. Novel methods and strategies in computed fluid dynamics are explained and correlated to experimental results. For this purpose, gas jet and particle velocities are investigated by means of laser optical measurements.

The thermal spray industry requires universal and economical HVOF systems for the production of high-quality coatings with high deposition efficiencies. In the last years classic HVOF guns have been adapted insufficiently to these requirements. This paper shows how modern numeric simulation and new inventions in gas safety engineering enable the development of a spray system for powder and wire feedstock that is optimized especially for the needs of the market. The new IBEDA TopGunAirJet is equipped with an air-cooled ‘de Laval’-like nozzle. The optimized expansion of the gas leads to high gas and particle velocities as well as to moderate flame and particle temperatures. Advantages of the TopGunAirJet are the achievable high coating quality, the low energy consumption (propane, ethene), the utilization of powder or wire and the efficient air cooling of the thermally loaded nozzle parts. Additionally, as a result of the axial powder injection, the free jet divergence and the nozzle wear are minimized. By varying the powder gas flow, the dwell time of the particles inside the flame can be influenced in order to avoid fusing of hard phases. As an example, WC-Co 88-12 coatings are presented and compared to coatings sprayed with a standard HVOF system of the 3rd generation. Different 316L coatings are produced and tested in comparison to standard HVCW coatings to show the workability of solid and cored wires. Finally, extensive LDA measurements are used for in-flight particle analysis, and investigations concerning the achievable deposition efficiencies are carried out.

The thermal spray application of inert gas atomised iron based powders for combined wear and corrosion protection prospectively offers important economical advantages compared to the well-established cermet coatings due to their lower price. Recent studies revealed basic knowledge about the thermal spray processing of these materials. For protecting the substrate from corrosive media, coatings have to be dense and impermeable to fluids. Especially poor bonding, occurring between partially melted or unmelted spray particles, leads to open porosity. Hence a certain degree of melting of particles is required. The GTV K2 spray gun allows the use of different nozzles to vary process temperature and velocity in a wide range. This paper shows the influence of applicated nozzles and process conditions on coating characteristics. Powder and coating characterisation is carried out by means of optical microscopy, digital image analysis, SEM and XRD. Additionally, some results regarding microhardness and wear behaviour are given.

Development of new arc sprayed iron based coatings for protection against gas abrasive wear at room and elevated temperatures are of the great interest because of permanently increasing pressure to reduce production and repair costs of power production facilities. Two cored wires in steel cover with Fe-Cr-B-Al and Fe-Cr-N-Al filling are proposed as an alternative choice for self fluxing and cermet coatings that are considered nowadays for protection of screen tubes of boilers of power stations that are operated under the temperatures 500-600 °C. Oxidation behaviour of arc sprayed coatings is estimated by gravimetric measurements. Abrasive wear resistance at elevated temperatures after 1 hour is investigated by means of laboratory unit that alloys a rotation of coated specimens in heated quartz sand. It is shown that abrasion wear lost of carbon steel increases 1.5 times when test temperature increases from 20 °C to 550 °C. For all investigated coatings the 20-25% decrease of wear lost is observed at higher temperature. Arc sprayed coatings of both investigated systems improve significally the abrasive wear resistance of carbon steel. At room temperature the improvement by factor 1.3-2.2 times and at the temperature 550 °C by factor 2.7-4.6 is observed depending on chemical composition of coatings.

Different post treatment methods are developed up to now to improve the properties of thermally sprayed coatings. In this work, arc sprayed aluminium coatings on aluminium substrates are post-treated by plasma electrolytic oxidation. To estimate the wear resistance of resulting oxide coatings, two abrasive wear tests (ASTM G65 and ASTM C1624) are carried out. Worn surfaces are examined by scanning electron microscopy in order to establish the wear mechanisms. These results of the abrasive wear tests are correlated with the parameters of the PEO process and the hence resulting micro structures of the coatings.