Abstract The porous architecture of coatings has a significant influence on the coating performances and thus should be properly designed for the intended applications. For simulating the coating properties; it is necessary to determine the numerical representation of the coating microstructure. In this study; YSZ coatings were manufactured by suspension plasma spray (SPS). Afterwards; the porous architecture of as-prepared coatings was investigated by the combination of three techniques; imaging analysis; Ultra Small Angle X-ray Scattering (USAXS); and X-ray transmission. A microstructural model for reconstructing the porous architecture of the SPS coating was subsequently computed according to the collected experimental results. Finally; the coating thermal properties were simulated based on the model and were compared with the experimental results.

Abstract In order to properly characterize the entire deposition process, evaluation of the coating, including a reliable metallographic preparation technique which reveals the true microstructure, must be performed. Often, recommended metallographic sample preparation methods for thermally sprayed coatings are generic and are not tailored to specific materials. They are time-consuming and, in some cases, may provide inaccurate details (pull-outs, smearing, etc). This could lead to a wrong interpretation of the coating quality. The aim of the investigation was to develop new metallographic sample preparation procedures tailored to different types of coatings (metallic, ceramic, multilayer and composites), in order to reveal a more representative microstructure. A comparative study of different preparation procedures for the examination of various as-sprayed coatings is presented using an optical microscope. The coatings were deposited by atmospheric and vacuum plasma spray (APS and VPS) and high velocity oxygen fuel (HVOF) processes. A separate approach is recommended for choosing the right metallographic preparation procedure for ceramic, metallic, or composite coatings. Applied load and positioning of the mounted sample during preparation are identified as key factors in developing proper procedures. The microhardness of the coating must be considered when determining the applied load. Interesting practical trends in preparation procedures that may lead to superior coating representation and, in some instances, cost and time savings are presented.

Abstract Thermal Barriers Coatings (TBC) are mass-produced in several industrial fields: aeronautic, automotive or energetic industries. All production requires the same constant level of fest and reproducible quality control at the lowest cost. The metallographic process is the primary way to evaluate thermally sprayed coatings but it must be both highly very repeatable and fast, especially when metallography is used to keep the production at a constant level. Therefore the management and the organization of a metallographic laboratory is of prime importance in order to reduce the cost and to provide a quality structure. The present approach to the whole chain of characterization is based on the user's point of view. Generally speaking, metallographic control of widely used parts often seems to be considered to be an uninteresting and obtuse subject, since it has been in practice for so long. Despite the lack of the prestige associated with the subject, optimization of an appraisal post can provide very concrete and more importantly profitable, results.

Abstract Thermally-sprayed coatings are applied to various materials to provide protection for beat-loaded parts as well as corrosion and wear. They also serve as a thermal barrier to reduce metal substrate temperatures. Microstructural analysis is used to characterize the substrate, the coating, and the adherence of the coating to the substrate. Accurate evaluation or these coatings depends heavily upon the quality of the polished surface produced by metallographic preparation. A well-prepared surface must first be free of plucked material and have minimal relief between the hard and soft constituents. The entire surface must be flat to allow a clear view of the interface and any reaction products that might be present in the coating or the substrate. Furthermore, the finished polished specimen must be scratch-free and have no resolution destroying films or stains. Various metal and oxide coatings on ferrous substrate material are used to demonstrate the excellent microstructural detail that is revealed when correct polishing procedures are used. These special techniques successfully address the problems normally encountered when preparing hard, brittle coatings or softer metallic coatings and substrates. As a result, clear microstructures are revealed across the specimen surface and valuable information is obtained.

Abstract Deformation stages and specific features were studied by mesomechanical methods under tension of plasma sprayed coating-steel matrix composites. The effect of coating quality on the deformation of the matrix at the mesolevel was revealed. The results obtained make it possible to predict reliability and durability of this kind of coating and of the entire composite.

Abstract Graded metal-to-ceramic coatings as thermal barrier coating (TBC) have been studied to improve the surface properties of the coating on high-temperature components. The atmospheric and low pressure plasma spray experiments are carried out with partially stabilized zirconia (YSZ and MSZ) and MCrAlY for mixing of functionally gradient materials. Especially, three-layer and five-layer graded TBCs are designed and produced to match gradually the material properties of a substrate and a top coating for reducing thermal stresses. A fully-saturated fractional factorial test is employed to determine optimum process conditions related to the thermal plasma generation, powder feeding, and substrate handling. Microstructural analysis using optical microscope and SEM, compositional analysis by XRD, bond strength test and thermal shock tests are carried out for material evaluation of fabricated TBCs. According to the results of material evaluation, the spray process is optimized for improving coating quality, and the proper raw materials are selected as a result of microstructural analysis. Especially, it is found that the durability and strain tolerance to thermal shock are remarkably increased by the gradation of TBCs and the improvement of graded TBC results from the fracture mechanism related to the vertical crack.

Abstract The results of low stress, pin-on-disc and high stress grinding abrasive wear tests on coatings produced by plasma and oxy-acetylene flame spraywelding are presented. FNil5A and FNiWC35 Ni-based self-fluxing alloys were selected as typical spraywelding materials for abrasive wear resistance. The abrasive wear resistance mechanisms of welded overlays produced by various materials and processes were also characterized by hardness tests, microstructural and compositional analyses, and through analysis of the effect of different kinds of abrasive on the wear resistant of Ni-base self-fluxing spraywelding overlays. Results showed that FNiWC35 overlays exhibited improved resistance under low stress abrasion, but the relative wear resistances of FNiWC35 and FNil5A still depended primarily on the type and hardness of the abrasive medium used. For the same material, the abrasive wear resistance of oxyacetylene flame sprayed overlays was higher than that produced by plasma spraywelding. The wear resistance of the plasma spraywelding overlays depended not only on the material, but also strongly on the spraywelding process parameters.

Abstract In corrosive media the wear resistance of ceramic-metallic coatings is dependent on the corrosion resistance of the metal matrix. Other factors that will affect the coating deterioration are the corrosivity of the medium and any galvanic interaction from the surrounding material. This paper presents results from a study where different types of WC(Co/Cr/Mo/Ni) powders have been sprayed by HVOF, Diamond Jet 2600 Hybrid equipment. The properties of the sprayed coatings have been verified by metallographic studies and by erosion-corrosion testing both under corrosive and non-corrosive conditions. The results clearly demonstrate the importance of having a metal matrix at least as corrosion resistance as the surrounding materials. When wear exposed components in pipe systems, pumps or valves are coated with a WC type coating, the corrosion resistance of the metal matrix should be compatible to the material of the rest of the system. This is especially important when the surrounding materials are corrosion resistant alloys as stainless steels, where the coatings otherwise will act as an anode.

Abstract Chromium carbide-nickel chromium coatings produced by HVOF spraying are widely used for high temperature wear and erosion resistant applications. Examination of the literature shows that whilst the mechanical properties of these coatings have been widely investigated, there has been little research into the physical processes occurring during HVOF spraying of this system, such as carbide dissolution, liquid-metallic phase oxidation, decarburisation and rapid solidification. The purpose of the present work has been to perform a systematic characterisation of the chromium carbide-nickel chromium system in both the initial powder and as-sprayed states with a variety of spraying conditions using optical, scanning and transmission electron microscopy, electron microprobe and X-ray diffraction. The presence of amorphous and nanocrystalline phases has been demonstrated. The nanocrystalline structures tend to be Ni rich, with the amorphous phases rich in Cr. Carbides of the form Cr 3 C 2 were found to be dissolved slightly during spraying, increasing the Cr and C contents of the liquid metallic phase. There was no evidence of chromium carbide oxidation.

Abstract It is widely held that most of the oxidation in thermally sprayed coatings occurs on the surface of the droplet after it has flattened. The evidence in this paper suggests that, for the conditions studied here, oxidation of the top surface of flattened droplets is not the dominant oxidation mechanism. In this study, a mild steel wire (AISI 1025) was sprayed using a high-velocity oxy-fuel (HVOF) torch onto copper and aluminum substrates. Ion milling and Auger spectroscopy were used to examine the distribution of oxides within individual splats. Conventional metallographic analysis was also used to study oxide distributions within coatings that were sprayed under the same conditions. An analytical model for oxidation of the exposed surface of a splat is presented. Based on literature data, the model assumes that diffusion of iron through a solid FeO layer is the rate limiting factor in forming the oxide on the top surface of a splat. An FeO layer only a few thousandths of a micron thick is predicted to form on the splat surface as it cools. However, the experimental evidence shows that the oxide layers are typically 100x thicker than the predicted value. These thick, oxide layers are not always observed on the top surface of a splat. Indeed, in some instances the oxide layer is on the bottom, and the metal is on the top. The observed oxide distributions are more consistently explained if most of the oxide formed before the droplets impact the substrate.

Abstract New near-net-shape structures of alloy Inconel 718 processed by HVOF spraying require optimum mechanical properties. Dominant factors defining the material quality are the particle properties velocity and temperature adjusted by the HVOF process parameters. Based on theoretical analysis of the HVOF process, experiments were performed with a defined variation of primary process parameters, producing coating samples of alloy 718 and measuring the particle velocities. Microstructural and X-ray analysis shows that in coatings with a high fraction of molten phase and high velocity, mainly divalent and spinell-type oxides are formed during particle impact on the substrate. Due to severe oxidation of the y'/y''- forming elements Ti, Al and Nb, precipitation-hardening effects of In 718 coatings are low. This leads to merely mediocre mechanical properties. The reduction of the molten phase to nearly zero leads to a drastic decrease of the oxide formation. The hardening γ'/γ'' phases are precipitated homogeneously in the Ni-base matrix. Strength values comparable to cast and wrought alloy In718 are attained by spraying with a low molten-phase fraction and high particle velocity. However, extensive intergranular 8-phase precipitation due to too high an Nb content of the powder causes only mediocre fracture elongation. Coatings up to 10 mm thick have been sprayed. The construction effort and hence the costs and weight of combustion chambers for hypersonic propulsion systems are to be reduced through direct thermal spraying of the loadbearing metallic pressure jacket onto the tubular cooling system. As a semifinished product, the selected Inconel 718 alloy exhibits good mechanical properties in the cryogenic temperature range as well as under higher thermal loads, and is commercially available in powder form. Aging serves to increase the strength up to the range of 1,200 N/mm2. For the sprayed In718 version, coating thicknesses in the centimeter range, a porosity < 1% and mechanical properties comparable with those of the cast version are required. The objective of the research work is to optimize spray-process control so that the resultant structural thick layers meet the design as well as the material requirements with respect to combustion-chamber technology. This necessitates elaborating the dominant microstructural parameters influencing the mechanical properties and the effect on them of the spraying process, and correlating them with the particle-condition parameters and the process parameters [1].

Abstract Two boron-rich Fe/Cr based gas atomised powders (Armacor M and Armacor C) have been thermally sprayed using the HVOF process and the resultant deposits subsequently characterised, using X-ray diffraction, scanning electron microscopy (SEM), plan view transmission electron microscopy (TEM), and microhardness measurements. The wear and corrosion characteristics of the two alloy coatings have also been investigated by three body abrasive wear (utilising cross-sectional TEM to examine the worn surfaces) and potentiodynamic corrosion testing respectively. Results from microstructural analysis of the as-sprayed deposits revealed the presence of small chromium-iron boride precipitates within a predominantly amorphous matrix in the Fe-based Armacor M coating. The Fe-Cr-based Armacor C coating, however, consisted mainly of regions of nano- and microcrystalline material interspersed with chromium boride precipitation. Iron-chromium oxides have been observed within both of the alloy coatings studied. Both of these alloys exhibit good abrasive wear resistance when compared with other metallic based HVOF sprayed coatings. Both Armacor M and Armacor C also exhibit extensive passivation on exposure to an acidic solution. The wear and corrosion test results are related to the microstructural observations.

Abstract The metallographic process for evaluating thermally sprayed coatings is sometimes viewed as a variable process in the scope of coating evaluation. There is always a question as to whether the failure of a coating is polishing related or an actual change in the spray production process. The use of metallographic standards similar to hardness calibration can be implemented to provide assurance of a repeatable metallographic polishing. Development and use of the standards will be discussed and examples given of the standards principle.

Abstract The characterization of a thermal barrier coating (TBC) using routine metallographic methods has had investigators uncertain as to the authenticity of the microstructure concerning porosity, cracking and other features. Mounting is a very important step in the process. A greater understanding of relationships between high and low viscosity mounting epoxies exposed to vacuum impregnation, pressure impregnation procedures (PIP), and the combined methods is the goal of this paper. This study will attempt to show how variations in sample mounting procedures will affect the final results in conjunction with varied metallographic procedures.

Abstract If the microstructure of thermally sprayed coatings is to be accurately evaluated, proper metallographic specimen preparation techniques need to be utilized to reveal the true coating characteristics. Variations of the materials used and the method of application on a substrate requires variations in the metallographic preparation procedure. The ASTM and Best Recommended Practices Committee of TSS are developing metallographic standards for thermally sprayed coatings using semi-automatic or automatic preparation equipment There are laboratories that continue to prepare coating specimens manually due to an inability to upgrade their equipment, and they must meet industry standards of evaluation. The purpose of this paper is to assist these laboratories in manual preparation that will enable them to produce accurate coating characteristics.

Abstract Experimental studies of the subsonic combustion process have been conducted in order to determine the quality and economics of polyester, epoxy, urethane, and hybrid polyester-epoxy coatings. Thermally sprayed polymer coatings are of interest to several industries for anti-corrosion applications, including the infrastructural, chemical, automotive, and aircraft industries. Classical experiments were conducted, from which a substantial range of thermal processing conditions and their effect on the resultant coating were obtained. The coatings were characterized and evaluated by a number of techniques, including Knoop microhardness tests, optical metallography, image analysis, and bond strength. Characterization of the coatings yielded thickness, bond strength, hardness, and porosity.

Abstract Composite coatings are increasingly applied for the protection against wear in mechanical constructions. Especially, in the case of abrasion these coatings offer the possibility to protect the base material. The matrix is ductile and the reinforcements cause the higher strength and hardness. A research project presented in this paper dealt with the manufacture of carbon-short-fibre-reinforced aluminum composite coatings by vacuum plasma spraying. The basis of the processing is the agglomeration of aluminum powder and carbon fibres. During the spraying process the aluminum melts, covers the fibres, and so, contributes to the creation of the composite coating and/or the composites. The processing times are so short that the damaging formation of carbides can be suppressed mostly. For the creation of free standing bodies it is necessary to find a qualified core material which allows the removing of the sprayed composites. The investigations on the composites are focused on the metallographical judgement regarding the fibre and void content, the fibre distribution, the characterization of the interface as well as the determination of mechanical properties and the wear resistance.

Abstract An experimental study of twin-wire electric arc spraying of zinc and aluminum coatings demonstrates the suitability of the process for anticorrosion applications. Experiments were conducted using Box-type full-factorial designs. Operating parameters were varied around the following process parameters: nozzle diameter, nozzle geometry, and system pressure. A systematic design of experiments displayed the range of processing conditions and their effect on the resultant coatings. The coatings were characterized with hardness and deposition efficiency tests, and optical metallography. Coating properties are quantified with respect to roughness, hardness, porosity, thickness, bond strength, and microstructure. The features of the coatings are correlated with the process changes. Selected analytical calculations and process diagnostics of the meltpool dynamics are presented.

Abstract The influence of alumina substrate temperature and phase structure : columnar gamma phase, columnar alpha phase and granular alpha phase on splat formation and crystal growth has been studied by SEM and Atomic Force Microscopy. X Ray Diffraction at low angle has allowed to obtain informations on phase structure of layered splats according to substrate phase structure and coating thickness. Column sizes of splats are correlated to a ID model of splat cooling showing the influence of substrate thermal properties and splat thickness on crystal growth kinetic. Finally, coatings adhesion-cohesion values function of spraying parameters are in good agreement with splat morphology and microstructural evolution.

Abstract The heterogeneous or composite nature of the thermal spray deposit can make choosing a metallographic preparation process very difficult. Hard vs. soft phases, brittle ceramics, and mixtures of many phases can create a variety of issues. When a procedure is developed, it becomes a necessity to document all the parameters or intangibles that define the process. Examples of procedure definition will be discussed and a suggested list of critical intangibles such as polishing abrasives (concentration vs. frequency of application), lubricant (pH, type), and many other factors will be highlighted.