Three kind of commercial ceramics powders, Al 2 O 3 , ZrO 2 and TiO 2 were deposited on 304 stainless steel plates by plasma spraying technique. Their porosities were evaluated with digital image analysis method. As the accuracy of this method depends significantly on metallographic preparation and metallography procedure for coating sample, we studied the effects of surface roughness, magnification and number of fields of view on the porosity. The proper values for the three parameters are discussed.

The deposition efficiency (DE) of a particular powder for a particular thermal spray process is very important factor when coating economics is being considered. There are many coating applications, however, where it is also important to know how the deposition efficiency changes during a longer coating process. Normally the DE is determined as mass ratio of powder fed into the process and corresponding weight gain of the sample. In this work the deposition efficiency has been determined for aluminum oxide powder in atmospheric plasma spraying using different spray parameters and electrode wear states. The coating process and in-flight particles were monitored using a fast non-intensified CCD-camera. Using digital image analysis the relative hot particle concentrations and velocity distributions were calculated from images. The possibility to use a CCD camera based monitoring system for in-situ measurement of DE is discussed. Additional laser illumination and PTV measurements were performed to verify the cold particle flux unseen by the plain CCD camera.

In order to reduce the manufacturing costs of SOFC layer systems the atmospheric plasma-spray (APS) process is an attractive technique. Especially the deposition of high performance cathodes, which are essential for a high power density of the SOFC, are of major importance. The cathode functional layer (CFL) has to provide a high amount of triple-phases boundaries to allow the cathodic oxidation reaction. A typical materials mixture used for such a CFL is the perovskite (La0.65,Sr0.3)MnO3 (LSM) and the fully Y 2 O 3 -stabilized zirconia (YSZ). A sufficiently porous layer has to be formed where both components are mixed on a sub-micrometer scale. In addition to the tough challenge concerning the creation of a suitable microstructure of a CFL deposited by an APS process two additional difficulties have to be solved: i.) a decomposition of the perovskite and ii.) a chemical reaction between the LSM and the YSZ during the atmospheric plasma-spray process must be avoided. Suitable spray processing strategies were developed to overcome these problems. Cathode functional layers were produced by an atmospheric plasma-spray process using a multi-cathode Triplex-APS torch. Coatings are characterized by SEM, EDX, XRD and the porosity was determined by digital image analysis. SOFC layer systems with the new developed CFL are tested in single fuel cell tests.

The main application of HVOF processes is the production of wear and/or corrosion protective coatings based on cermets (e.g. WC-Co). To reduce the weight of the sprayed volume, to increase the oxidation resistance and to reduce the costs of consumables SiC based cermets are of particular interest. The outstanding mechanical, thermal and chemical properties of SiC have made this material attractive for thermal spray applications. Thermal spraying of pure SiC is impossible, because of its decomposition at elevated temperatures. The development of suitable alloys as metallic binders enables the wetting of the SiC particles with reduced reactivity between the matrix and the hard particle during the thermal spray process. In the present work starting from the metallurgical concept of the metallic matrix to the production route from the atomization of the metallic matrices, the spray drying and sintering as well as the processability of the composites is discussed. The coatings are characterized concerning porosity and SiC content using optical and scanning electron microscopy, digital image analysis, X-ray diffraction and micro hardness testing. The wear resistance of the coatings is determined by oscillating wear test. Microstructure and performance of the SiC composite coatings are discussed with respect to technical applications.

Thermal spraying of pure SiC is difficult due to decomposition issues at elevated temperatures. However, the development of suspension plasma spray opens a new path to investigate the deposition of this material since the liquid carrier can hinder this phenomenon. The present work investigates a new route for producing SiC submicron structured coating by suspension plasma spraying (SPS). Classical SiC manufacturing routes using suspension (i.e: spray drying, tape casting) are studied regarding their feasibility to be used on suspension plasma spraying. Aqueous-based suspensions containing 10 wt.% SiC powder (0.60 µm) along with sintering additives are dispersed and stabilized. Both suspensions are sprayed on martensitic stainless steel substrate (AISI 440C) to achieve finely structured and dense coatings. Digital image analysis, X-ray diffraction and scanning electron microscopy are utilized to characterize the coating microstructures. Their dependency on suspension characteristics and spray operation parameters are discussed with respect to the final coating performance.

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.

The size and shape of individual particles are essential characteristics to control their behavior during spraying. Thanks to recently available technology in powder image analysis, this paper investigates the correlation between the geometric properties of several thousands of particles and their flowability (Hall flowmeter) and apparent density (Scott volumeter). Four different chromium carbide Cr 3 C 2 -NiCr 75/25 powders used for HVOF wear / oxidation resistant coatings are studied. The coating quality obtained for each powder by the HVOF spraying process is further investigated using metallographic image analysis.

Thermal conductivity plays a critical role in the thermal transport of thermal sprayed coatings. In this paper, a combined image analysis and finite element method approach is developed to assess thermal conductivity from high-resolution scanning electron microscopy (SEM) images of the coating microstructure. Images are analyzed with a collection of image processing algorithms to reveal the microscopic coating morphology. The processed digital image is used to generate a two-dimensional finite element meshing in which pores, cracks and the bulk coating material are identified. The effective thermal conductivity is then simulated using a commercial finite element code. Results are presented for three coating material systems: yttria stabilized zirconia (YSZ), molybdenum and NiAl, and results are found to be in good agreement with experimental values, obtained using the laser flash method. YSZ coatings are also annealed and the analysis procedure repeated to determine if the technique could accurately assess changes in coating morphology.

This study investigates the influence of segmentation crack density on the strain tolerance of thermal barrier coatings produced by atmospheric plasma spraying. A Triplex II plasma gun is used to spray fused and crushed yttria-stabilized zirconia, forming thick deposits with high segmentation crack densities, low porosity, and low branching crack density, which is necessary for good interlamellar bonding. Thermal cycling and burner rig tests yield promising results in terms of lifetime and strain tolerance behavior and microstructural analysis shows that the segmentation crack network was stable during thermal shock testing. The main failure mechanism was delamination and horizontal cracking in the vicinity of the TBC-TGO (thermally grown oxide) interface.

WC-Ni metal-matrix composite coatings were deposited by low-pressure cold spraying using feedstock powders with different amounts of carbide. Uniaxial quasi-static tensile testing was conducted on the as-sprayed coatings to investigate the effect of porosity, particle size, and mean free path on mechanical properties. The evolving strain fields were measured via digital image correlation and image analysis was used to characterize coating microstructure. The coatings with higher carbide content exhibited better tensile properties, which is attributed to significant consolidation of the matrix, increased interfacial area, smaller average carbide size, and reduced mean free path between carbide particles.

During plasma spraying in-flight particle characteristics are influenced by the many operating parameters associated with the deposition process. The distinctive temperature and velocity signals given by particles as they exit the plasma torch can be used to develop processing maps for defining the optimal operating envelope. Knowing the temperature and velocity history of the particles, the evolution of the microstructure, the amount of porosity and the phase composition can potentially be predicted. In this paper, the relationship of system parameters (stand-off distance, torch power, plasma gas composition and process gas flow) was correlated to in-flight particle characteristics of yttria stabilized zirconia and compared to the resulting coating features such as thickness, microstructure, porosity and phase composition. The appearance of the coating (i.e., color) was also compared after the deposition process. Yttria stabilized zirconia was deposited on grit blasted samples using an F4 (Sulzer Metco) plasma torch. Before depositing each sample on the substrate, the particle properties were measured at the desired stand off distance perpendicular to the particle jet covering an area of 18x18 mm 2 using the Tecnar DPV2000 inflight particle analyzer. The coatings were cross-sectioned for microstructure analysis, thickness measurements and deposition efficiency. Free standing films were used for mercury intrusion porosimetry. Grey levels of the coatings were obtained by optical microscopy and subsequent digital image recording. X-ray-diffraction analysis was also used to obtain the phase composition. Results showed that different particle temperature and velocity conditions lead to specific porosity and varying colors of the deposit. The color of the deposit was correlated directly to the amount of monoclinic phase in the as-deposit material.

This study investigates the effect of mounting materials on microstructure and property measurements obtained from thermal spray coatings. Various epoxies and embedding techniques are used and a wide range of layers are examined, including HVOF sprayed WC-Co, Cr 3 C 2 -NiCr, and Al 2 O 3 -TiO 2 ; plasma sprayed Cr 2 O 3 , YSZ, WC-Co, and Ni-Al; and arc sprayed copper and silicon bronze. Image analysis measurements of area percent porosity, thickness, lamellar spacing, and unmelted particles and the results of hardness tests show substantial variation relative to the method used to encapsulate soft and porous coatings. Results indicate that the ideal mounting system for thermal spray coatings would consist of a low viscosity epoxy, to maximize penetration depth, and a high cured hardness, for adequate protection of surfaces and open porosity of hard coating materials. Paper includes a German-language abstract.

In this work, digital image processing and finite element analysis are used to predict physical properties of thermal barrier coatings based on microstructure. Factors that affect thermal conductivity such as porosity, crack morphology, and interface defects are systematically studied. It is shown that transverse cracks can significantly retard the transfer of thermal flux and that the effective TCE and elastic modulus at the coating interface is determined mainly by composition, with interface morphology having little effect. Furthermore, no anisotropy was found at the interface. Unlike traditional property-prediction methods, the methodology presented in this paper reflects real coating microstructures, thus providing more accurate results.

HVOF sprayed WC/Co(Cr) and Cr 3 C 2 /Ni20Cr composite coatings have gained high acceptance in many industrial applications for protection of components against wear. The achieved coatings have quite good corrosion resistance for use of chromium containing matrices. Present research in the field of PTA-Surfacing resulted in the development of high chromium and high vanadium containing iron based hard alloys with simultaneous improvement of abrasive wear and corrosion resistance. These properties of the PTA-Surfaced coatings were studied and it was found that the newly developed alloys have nearly same wear resistance and improved corrosion resistance compared to Co-based alloys reinforced with Fused Tungsten Carbides (FTC). One major advantage of high chromium and high vanadium containing iron based coatings is machinability by turning and milling processes. These features make it attractive to be used in thermal spraying. The absence of substrate melting in HVOF-spraying is advantageous, as the coatings preserve the properties of the alloy due to prevention of dilution with substrate material in contrast to PTA-Surfacing. High chromium and high vanadium containing iron based atomized powder was used for HVOF spraying and deposition efficiency was measured. The sprayed coatings were studied metallographically by optical microscopy, SEM, XRD and micro-hardness measurements. Later, abrasive wear and corrosion properties of the coatings are investigated.

In HVSFS (High Velocity Suspension Spraying), a suspension is axially fed into the combustion chamber of a modified HVOF torch. The particle formation is strongly influenced by the phenomena occurring during the short dwell time in the combustion chamber and expansion nozzle, i.e. break up and evaporation of the liquid jet, particle formation, sintering and melting. Optimization of the spray torch has already led to an improved coating process but is still an important topic for the future. A deeper understanding of the suspension liquid / flame interaction is still necessary to control coating properties. It turns out, that among other process parameters, rheological properties and agglomeration behaviour of the suspension have a strong influence of the particle formation and hence resulting coating properties. As a result of using nano particles HVSF sprayed coatings show partly different microstructures compared to HVOF sprayed coatings. To gain a deeper understanding of the process particles were collected in-flight to get information about the size distribution and melting degree during the spray process. The paper will give an overview of the present state of HVSFS development at the IFKB and present experimental results.

Modern thermal spray equipments and diagnostic tools give a lot of relevant process data. Currently only single process parameters (e.g. particle temperature) are recorded and analysed in order to keep the parameters constant. However, even this simple kind of process control is not state of the art in every spray shop. The issue of this publication is the development of a method for prediction of material- and component-properties for thermal spraying. This method will allow an offline process control of the complex coating process. For the development of this kind of process control numerous experiments will be been carried out with both HVOF- and APS-processes using the Particle-Flux-Imaging diagnostic tool (PFI). The PFI-system is the basis for the process control to be developed. In comparison to other diagnostic tools it is easy to handle and cheaper. The set up of experiments and the data evaluation were carried out by means of statistical DOE [1]. The first series of experiments were designed to determine the significant interrelations between these influencing parameters. Based on the results from the first series of experiments, the significant parameters were further manipulated during spraying the second series of experiments The correlation between the coating properties (hardness, porosity) and the main influencing parameters are the basis for the creation of an offline process control.

This paper describes the development of a diagnostic system that monitors in-flight particle diameters, velocities, and temperatures during thermal spraying. The system is based on a low-cost CCD camera and user-developed software. The camera incorporates a 732 x 282 pixel sensor with high sensitivity in the near IR range where the only radiation is that of the particles. User-developed software modules handle signal processing, image analysis, calibration, and data visualization. In video images, particles appear as light tracks of varying length, width, and intensity, corresponding (respectively) to velocity, diameter, and temperature. A test case in which Cr 2 O 3 powder is sprayed in a plasma jet demonstrates the capabilities of the diagnostic system. Paper includes a German-language abstract.

Effect of plasma spraying parameter on the strain of plasma sprayed YSZ coatings is investigated by changing the spraying power and spraying distance. The porosity is measured through using image processing method. The strain of coatings is calculated and characterized through using the digital image correlation (DIC) software system. It can be found that the porosity of YSZ coatings increases with the increment of spraying distance and the decrement of spraying power. The strain on the far field of the cracking tip shows a fluctuation tendency when the tensile forces increase. The slope of strain-force is related with the coating porosity. DIC method can be used to characterize the coating deformation in 2-dimension.

Ceramic coatings are applied in mechanical components subject to high temperature conditions, normally are deposited by plasma thermal spraying process. In this work, the porosity of YSZ ceramic coatings, deposited with different parameters conditions were analyzed by optical microscopy, scanning electron microscopy using back-scatter electron (SEM-BSE) detector and ultrasonic technique. It was verified that porosity measurement by optical and scanning electron microscopy is very sensitive with respect to metallographic preparation, mainly cutting process, and gray level adjustment. SEM-BSE technique showed less scatter results with easier porosity visualization, compared with optical microscopy. The porosity of the coatings was also measured by ultrasonic technique. It was observed that the ultrasonic velocity increase with porosity reduction. Ultrasound technique showed a good correlation with OM and SEM porosity measurement.

In nuclear fusion reactors, the first wall is the name given to the surface which is in direct contact with the plasma. A part of it is the divertor which is a device that removes fusion products from the plasma and impurities that have entered into it from the vessel lining. It is covered with water cooled tiles which have to withstand high temperatures and high heat fluxes. Moreover, resistance to neutron bombardment, low tritium absorption and low hydrogen permeation are additional demands. One materials concept under research is the application of a Reduced Activation Ferritic Martensitic Steel (RAFM) as a structural material with a tungsten protective coating. Since there is a considerable thermal mismatch between, a functional graded materials (FGM) concept was proposed. As the formation of undesired intermetallic Fe-W phases as well as oxidation should be avoided, cold gas spraying was chosen as manufacturing process. Two powder blends of EUROFER97 RAFM steel and a fine tungsten powder cut on the one hand and a coarser one on the other hand were tested in different ratios. The coatings were characterized with respect to their porosity and surface structure. Furthermore, the deposition efficiencies for steel and tungsten were determined each. It turned out, that the deposition process is a complex mixed situation of bonding and erosion mechanisms as the deposition windows of these very different materials obviously diverge. Thus, a lower working gas temperature and pressure was advantageous in some cases. Unexpectedly, the coarser tungsten powder in general enabled to achieve better results.