Boiler tube failure is the number one source of forced outages in all coal-fired and biomass-fired power generation plants. It is estimated that plants lose approximately 6% of their power generation annually, due to boiler tube leaks. The major causes for premature tube failure are excessive fireside boiler tube erosion and corrosion caused by impact, abrasive wear, oxidation and molten corrosion of low eutectic alloys.

The interaction between thermal spray parameters and coating characteristics of atmospheric plasma sprayed WC-12Co was investigated by means of statistically designed experiments. A central composite design was employed to study the influence of the process parameters’ ratio Ar/H 2 , plasma gas volume flow, powder feed rate and stand-off distance on the deposition efficiency, tensile strength, porosity, roughness, particle temperature, particle velocity and the percentage of molten carbides. The process parameters’ volume flow and powder feed rate showed a considerable impact on the microstructure and morphology of the coatings, which could be traced back to a change in the melting degree of the powders as well as a change in particle temperature and velocity. On the basis of the statistical experiments, mathematical expressions were found to describe the coating characteristics. The quality of the mathematic model was scrutinized in a subsequent evaluation stage.

Thermal spraying technology still suffers from a lack of reproducibility due to uncontrollable factors during the process. Current methods of process control by means of observing process parameters like gas- and powder flow are insufficient to guarantee a constant quality of coatings, while a direct analysis of the deposited layer is time- consuming and can only be conducted after the process. Furthermore, recently developed mathematical models which correlate process parameters to coating properties are not applicable for all materials. As the particles’ behavior during the process affects the coating properties, a direct process control by the observation of the particles seems expedient. This method is applicable on running processes and thus avoids defective production. In this study, HVOF spraying experiments were conducted. The in-flight particles’ behavior was investigated using an optical diagnostic system, while coating properties were analysed by metallographical methods.

This is the first of two papers concerning the intrinsic mechanical properties of arc-sprayed WC-FeCSiMn coatings. In part 1 the elastic and plastic forming behavior of the layers are investigated by indentation, bending and tensile tests. They were performed on coated mild steel substrates as well as freestanding as-sprayed samples with different geometries. Considering the coatings microstructure, element and pore distribution, as well as the local microhardness the results of the indentation, bending, and tensile tests were evaluated. The critical role of pores and inhomogeneities within the sprayed coating was examined in detail. Micro- and macrocracking were investigated by SEM after the indentation and tensile tests. In-situ surface observation by optical 3D-microscopy was used to study the onset of cracking during the 3-point bending test.

Functionally graded coatings (FGC), in which the composition and properties vary gradually from the bond layer to the top layer, are introduced in this study. Using a mixing nozzle, a graded coating was generated employing twin wire spraying (TWAS) process. The deposition started with a base layer of massive metallic wires for better adhesion to the substrate. At following top layers hard material, in form of powders, was injected to the massive wires to enhance the wear protection. The results show that microstructure, porosity, and compositions are gradually varied in the coatings. This is a clear indication for the better performance in as-sprayed FGC than coatings sprayed by means of cored wires. The approach was to grade the coating composition from pure metallic to composite with higher content of hard material particles. The goal of the study is to articulate the needed coating performances by customizing the layers deposited regarding to their position.

The characterization of thermal sprayed coatings is often limited to microstructural analysis to evaluate the coatings morphology. Indentation is commonly used to determine the mechanical properties of different kind of engineering materials. However, due to the complex structure of thermal sprayed coatings few results have been obtained so far. This is the second of two papers concerning the intrinsic mechanical properties of arc-sprayed WC-FeSiCMn coatings. In part 2 experimental nanoindentation tests and simulation results are compared. The experimental indentation tests show scattering in the force-deformation data due to the complex structure of the arc-sprayed coating which is investigated by means of an indentation test simulation. Based on these results the effective Young's modulus as well as further properties are identified. A general procedure is presented to predict the effective mechanical properties of different coating composites based on the microstructure, porosity and properties of the chemical composition after thermal spraying.

Detonation spraying provides the opportunity to produce superabrasive diamond grinding tools under atmospheric conditions. In this study, several methods are used to assess the effects of the spraying process on diamond particles, including SEM analysis, energy dispersive X-ray spectroscopy, differential thermal analysis, thermogravimetric analysis, X-ray diffraction, Raman spectroscopy, and friability and fracture force testing. It was found that under optimized conditions, the thermal and mechanical impact of the detonation can remain low enough to ensure the reliability of the diamonds with no adverse effects.

The intent of this study is to gain a better understanding of the twin wire arc spray process, particularly the correlation between spray path or gun movement and coating quality. Coatings were deposited on steel substrates using solid nickel and cored WC-Cr wire while moving the gun in various directions relative to the orientation of the electrodes. The coatings obtained are evaluated based on porosity, hardness, thickness, adhesion strength, and microstructure. It is observed that gun movements cause inhomogeneities in the spray cone that affect different coating properties in different ways depending on the direction of the movement and gas pressure.

HVOF spraying of fine feedstock powder allows the deposition of cermet coatings with outstanding properties, but selecting and optimizing process parameters can be difficult. In this study, investigators employ a design of experiments (DOE) approach to identify the most relevant process parameters in the spraying of 2-8 μm Cr 3 C 2 -NiCr powders. In a screening step, all parameters were assessed in a 12-run Plackett-Burman experimental design and linear models were used to estimate their effect on coating properties and deposition efficiency. The five most influential parameters were then analyzed in a 16-run fractional factorial set of experiments in order to determine their effect, along with all possible two-way interactions, on coating quality.

Thermal spray coatings produced from hard materials show good potential for extending the service life of sheet metal forming tools. However, post-process methods currently used to improve coating surface quality are slow and expensive, especially on complex surfaces. A novel approach, demonstrated in this study, is the optimization of thermally sprayed coatings by roller burnishing. This process shows a high potential to not only reduce surface roughness and porosity, but also increase hardness even on free-formed surfaces. In the experiments, atmospheric plasma-sprayed CrC-NiCr coatings were produced and the burnishing process was performed on a CNC milling machine. Both the spraying and surface finishing processes are described and the results are presented and discussed.

This paper presents an investigation of the influence of the spray angle on thermally sprayed coatings. Spray beads were manufactured with different spray angles between 90° and 20° by means of atmospheric plasma spraying (APS) on heat-treated mild steel (1.0503). WC-12Co and Cr 3 C 2 -10(Ni20Cr) powders were employed as feedstock materials. The spray beads were characterized by a Gaussian fit. This opens the opportunity to analyse the influence of the spray angle on coating properties. Furthermore, metallographic studies of the surface roughness, the porosity, the hardness and the morphology were carried out and the deposition efficiency as well as the tensile strength were measured. The thermally sprayed coatings show a clear dependence on the spray angle. A decrease in spray angle changes the thickness, the width and the form of the spray beads. The coatings become rougher and their quality decreases. In addition, the spray process becomes inefficient as the deposition efficiency declines. Especially, below 30° for Cr 3 C 2 -10(Ni20Cr) and below 50° for WC-12Co a significant change in microstructure was observed.

Grinding applications for the machining of stone and concrete require composite tools where large diamonds are perfectly embedded into a metallic matrix. With the detonation flame spraying process it is possible to manufacture these superabrasive composites. Excellent embedment of the voluminous superabrasive particles into the matrix coating material can be realized in order to produce high quality composite layers for grinding applications of stone and concrete. In this paper different diamond sizes as well as different volume contents of diamond in matrix are compared. Especially, the influence of particle size on its implantation efficiency is investigated and the influence of process and substrate temperature is analyzed. The thermal sprayed grinding tools are evaluated in the sense of their morphology as well as their grinding abilities. Compared to sintered diamond-bronze samples the results of an adaptively designed grinding test for the machining of concrete are presented and analyzed.

Thermal spraying technologies take part in the current worldwide scene as serviceable and profitable technologies for a wide range of intents. The growing adoption of those techniques has driven companies to progressive investments in efforts to reach innovative designs and market needs. Wear resistance is among the desired features which certain components must be able to provide while performing. In this context tribology has an outstanding importance, where the adoption of suitable materials and operational parameters jointly contribute for a longer lifetime of parts. This contribution presents a comparison of AISI 1045 (DIN C45) steel surfaces coated by means of arc spray process, employing cored wires as feedstock. Corresponding light microscopy, SEM-EDX, 3Dprofilometry, hardness and ball-on-disk analyses were conducted to understand the tribological behaviour of two unlubricated iron-based layers with and without carbides reinforcement. The results point out equivalent thickness, porosity and as-sprayed roughness for both coatings. Higher hardness and lower friction coefficients for the carbide free coating as well as a downward trend considering the coefficients of friction for coated and uncoated surfaces under heavier loads.

Deep-drawing is a widely used sheet metal forming process in the aircraft and automotive industry. The manufacturing of modern parts with complicated shapes and curvatures requires forming tools with highest shape accuracy even at complex surface geometries. However, the application of novel, high-strength sheet metals combined with a continuous increase in productivity impose high tribological demands on forming tools and finally lead to increasing wear. In order to minimize the high costs for the repair and maintenance of such tools it is crucial to enhance their service life by an appropriate surface modification, which is able to preserve the high shape accuracy. Conventional coatings obtained by thermal spraying of coarse grained feedstock materials are not suitable to achieve this aim. In this collaborative study, the feeding and HVOF spraying of WC-Co submicron powders (- 8 + 1 µm) have been investigated to manufacture superfine structured, wear resistant near-net-shape coatings with improved macroscopic properties and smooth surfaces. Special equipment for the powder feeding and a novel HVOF flame spraying system designed by Thermico (optimized for the processing of fine-scaled powder fractions) have been employed. Correlations related to the process dynamics at varying HVOF gas compositions, the thermokinetic particle behavior in-flight and corresponding coating properties have been analyzed.

To use the manifold possibilities that arc spraying offers to deposit wear resistance layers, the knowledge of the particle formation and their characteristics are necessary. The work is focused on studying the particle trajectories during arc spraying with cored wires. Different cored wires under various spraying parameters are investigated by means of a high speed camera. Particle properties in-flight, such as velocity and temperature are determined. Correlation between particle trajectories and particle characteristics at different spraying conditions are established. At the same time, the particle-laden gas flow is simulated numerically and the computed solutions are used to illustrate the utility of the proposed CFD model and compared with experimental results. The employed mathematical model represents a system of macroscopic conservation laws for the continuous gas phase and for the gas-solid mixture. This formulation makes it possible to circumvent the numerical difficulties associated with the implementation of a (potentially ill-posed) two-fluid model. The discretization in space is performed using a high-resolution finite element scheme based on algebraic flux correction in terms of local characteristic variables. The artificial diffusion operator is constructed on the discrete level and fitted to the local solution behavior using a multidimensional flux limiter of TVD type.

Asymmetric melting behaviour of the electrodes is a process related feature of the twin wire arc spraying (TWAS) technique since the heating of the negative connected wire is different from that of the positive connected wire. Due to these differences in melting behaviour a tracking of particle velocity and temperature for both electrodes individually is very important. Particle velocity and temperature have been recorded from anode and cathode by positioning the tracking device on each side of the spraying gun. To draw the whole picture of the spraying jet particles have been tracked also from the top side of the spray gun. The goal of this study is to have an experimental data set-up for model building and simulation of depositing process in TWAS. Corresponding measuring devices have been employed to investigate the TWAS process by spraying of massive and cored wires.

To use the manifold possibilities that the arc spraying offers to deposit carbide layers, the knowledge of the particle characteristics are necessary. This work is focused on understanding the particle formation during arc spraying with cored wires. The influence of primary and secondary atomization gas on the particle formation as well as on the particle shape, particle dimension and grain size are analyzed. Melting behaviors of different cored wires under various spraying parameters are investigated. Correlations between coating properties and particle characteristics are established. Experimental approaches to determine the in-flight properties of the sprayed particles in combination with metallographic analyses were employed to establish these correlations.

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.

The electromagnetic forming (EMF) is a process for forming sheet metals or thin metal tubes with a high electrical conductivity such as Al-alloys. Extremely high forming velocities can be achieved by employing this process based on pulsed electro-magnetic fields. To exploit the advantages of this process in forming sheet materials with a limited electrical conductivity thermal spraying methods have been applied. The essential requirement is based on the design and development of an electrically conductive layer system, which can act as a “driver“ for materials to be formed. Therefore, the layer has to fulfil high demands on its density as well as on the adhesion to the substrate. In this contribution a new coating concept for thin sheets to be electromagnetically formed is presented. The influences of pre-cleaning processes, coating parameters, layer properties and sheet thickness on the deformation are scrutinized. Accompanying metallographic, mechanical and microscopic investigations of the coated layers help to understand the mechanisms of materials behaviour during EMF processes.

Nowadays the use of light weight materials increases rapidly. Owing to growing requirements regarding material properties and corresponding production costs new material designs and novel production concepts are needed. The low density of aluminium and its alloys is accompanied by lower Young’s modules and lower strengths compared to steel. These disadvantages regarding to stiffness and strength can be overcome by using a composite material consisting of aluminium and embedded endless reinforcing elements. In this work a novel technology based on the thermal spraying process to manufacture endless reinforcing elements for extrusion molding of Al-profiles will be discussed. A specific handling system for arc-spraying Al-alloys onto steel wires has been developed. The influence of the coatings materials and coating parameters on the subsequent extrusion moulding process has been studied.