Microstructure of plasma-sprayed thermal barrier coating has an influence on the lifetime under service conditions in aero engine. This paper focuses on problems of preparing and comparing ceramic samples and characterising microstructure features such as porosity and cracks within coating cross sections by image analysis. The results can be used for developing standards and best practices for characterization plasma-sprayed coatings. The methodology was evolved to relate coating microstructure to spraying parameters and properties of coating. It would help to develop new ceramics coatings or optimise them.

This study was undertaken to investigate the influence of the initial powder particle size on the microstructure development in a Cu-3wt% Ag alloy obtained by vacuum plasma spraying of a mixture of copper and silver powders. Various mixtures of powder particles were used where the copper powder particle size was kept as –80/+40 μm, while the silver powder particle size fraction was varied. Vacuum plasma spraying (VPS) in inert gas atmosphere was used to elaborate thick coatings (2 mm) with limited oxygen contamination. Significant variations in microstructure were obtained depending on the Ag powder size fraction that was used. These differences in behavior are discussed taking into account differences in the solid state precipitation mechanisms in the alloy.

By means of homemade ultrasonic arc spraying equipment patented, under the condition of the optimum arc spraying parameters, TiAl alloy coatings were manufactured on the aluminum alloy (LY12) matrix surface. The ultrasonic arc spraying forming process and microstructure of TiAl compounds alloy coatings were investigated by the color metallurgical microscopy, SEM, XRD, EPMA and chemical composition quantitative analysis (ICP). The results showed that TiAl compounds composite coatings can be synthesized by ultrasonic arc spraying system, the coatings possess typical lamella character, consist of TiN (or TiO), TiAl, Ti 0.7.1.1 Al(N,O), Ti 2.4 Al(N,O), Ti 2.4 Al and aluminum alloy solid solution.

Relationships between the properties of thermally sprayed nickel based alloys and their microstructure (density, porosity, oxide phase content) are investigated. Cross-property- correlation of physical properties such as electrical conductivity and elasticity were examined. The experimental results of the structures and properties of the different coatings are discussed with respect to their pore surface area obtained by small angle neutron scattering (SANS) measurements. As wide as possible range of thermally sprayed microstructures of NiCr and NiCrAlY was produced by vacuum - and atmospheric plasma spraying, flame spraying, HVOF and water stabilized plasma spraying. Commercially available powders with process specific grain size distributions were used as feedstock materials resulting in a wide range of microstructures of the coatings depending on the spraying technique and, to much less extent, on the variation of the process parameters. In this work the examination of the pore structure was carried out by optical microscopy on metallographic cross sections. Phase composition and distribution were investigated by hot gas extraction for oxygen and nitrogen determination and by Scanning Auger Microscopy on polished cross sections and fracture surfaces. The properties of the coatings were characterized by their wear (ASTM G75) behavior, reflecting application-oriented properties. Significant and varying amount of anisotropy of the coating properties - electrical conductivity and elastic modulus - was found in the sprayed microstructures. This anisotropy could be directly linked to microstructure anisotropy as characterized by Small-Angle Neutron Scattering.

Technological properties of thermally sprayed deposits are to a great extent related to the underlying microstructure. The present project aims to relate macroscopic properties of metallic coatings to their microstructure. For this purpose, thermally sprayed deposits of nickel based alloys (NiCr, NiCrAlY) were manufactured by various spraying techniques - atmospheric and vacuum plasma spraying, flame spraying, high velocity oxygen fuel and water-stabilized plasma spraying. One of the key microstructural features is the void system. This system is usually characterized by the total volume of voids, the so called porosity. An additional characteristic parameter of the void system is the specific surface area. The method of anisotropic Small Angle Neutron Scattering (SANS) in the "Porod Regime" allows the determination of the anisotropic specific surface area of the complex void system that consists of intralamellar cracks and interlamellar pores. In contrast to optical microscopy, the SANS technique is capable of resolving the pore structure down to the nanometer scale, and the measured specific surface area represents a statistically relevant average value for the whole illuminated sample volume which is usually a few mm 3 . Besides the presence of voids and cracks the performance of thermally sprayed coatings is also significantly influenced by residual stresses. In the present work residual strains were determined by the technique of neutron diffraction as well as by bending tests, i.e. laser profilometry of the substrate before and after the spraying process. The specific surface area and the residual stresses are discussed with respect to total porosity, the presence of secondary phases like oxides and wear behavior. Special attention is drawn to the anisotropy of the apparent surface area, which is discussed with respect to the anisotropy of macroscopic properties like electrical resistance.

The aim of the present work is to study the microstructure of high temperature oxidation and corrosion resistant MCrAlY coatings prepared by high-velocity oxy-fuel (HVOF) spraying and to compare them to vacuum plasma sprayed (VPS) MCrAlY coatings. In this work, CoNiCrAlY, NiCoCrAlYTaReSiHf, and NiCrAlY coatings were prepared on nickel based superalloys by VPS spraying and by two types of HVOF spray process (DJ Hybrid 2600 and JP-5000). The influence of spray parameters, i.e. oxygen flow rate and spray distance, in one of the HVOF processes (DJH 2600) on coating structures and properties were studied also in more detail. The coatings sprayed with different spraying processes were studied in their as-sprayed state and after heat-treatment by optical microscope and scanning electron microscopy (SEM). The results show that selection of appropriate particle size of the spray powder has marked importance in the HVOF process; presence of fine powder particles produce oxides in the coatings and ease clogging of the nozzle in the HVOF gun. The use of appropriate size distribution of the spray powder enables preparation of HVOF MCrAlY coatings with low oxide content and dense structure. The oxide content of HVOF sprayed coatings should be kept as low as possible in order to make the post heat-treatment active in producing a dense microstructure with correct phase structure. The oxide content can be controlled e.g. by flame temperature control by means of oxygen flow rate and by appropriate spray distance selected individually for each powder. Unacceptable high oxide contents in the as-sprayed structures tend to remain unchanged after heat-treatment, whereas well-developed coating microstructure and phase structure are obtained with HVOF sprayed coatings with low oxide content. Microstructures developed in heat-treated VPS and HVOF sprayed coatings are compared.

The aerospace industry currently applies high velocity oxy-fuel (HVOF) coatings to turbine engine, structural, and landing gear components. An increasing demand for HVOF wear resistant coatings to replace electrolytic hard chrome (EHC) on landing gear components has renewed focus on the spray limitations of HVOF WC/CoCr. One such limitation resulting from the line-of-sight HVOF process is the spray angle. In this study, HVOF WC/CoCr coatings were sprayed at several angles while maintaining consistent combustion characterisitics and standoff distance. The measured responses included tensile bond strength, microhardness, residual stress, coating surface roughness, and dry fretting wear resistance. Fatigue response was also of interest, but no results were available at the time this paper was written. The microstructure of each coating was examined, both normal to the surface and in cross section. Coatings sprayed at 90° exhibited the highest microhardness and most compressive residual stress, both considered favorable for good wear response. But these coatings also exhibited the highest as-sprayed roughness, least homogeneous microstructure normal to the surface, and lower wear resistance compared to the off-angle coatings; however, the off-angle coatings apear to cause greater wear of the contacting surface. The microstructural differences among the coatings are related to the measured responses.

Spraying distance (SD) is one of the main parameters that can affect the spraying process - its efficiency as well as deposit's character. The reason lies in a different thermal history of particles corresponding to different spraying distances. Variation in the structure, preferred orientation, variation of the phase contents and/or in the chemical composition of deposits can be then found for different SDs. Consequently, properties of coatings can greatly vary, not only in the resistance state but also on deposit's annealing. Some materials are, however, more and some less sensitive to that effect. The goal of this work is to compare CaTiO 3 samples produced by plasma spraying with WSP at SD = 350 and 450mm with sintered samples. The following properties were compared: microhardness, thermal expansion coefficient, permittivity and reflectivity. Porosity and the differential thermal analysis of resistance and annealed deposits were also compared. Perovskite CaTiO 3 belongs among materials very stable during spraying - neither chemical nor phase differences were found between the feedstock powder, free-flight particles, as-sprayed deposit and annealed deposit. Despite that there are significant differences in behavior of deposits and freestanding parts sprayed from different SDs. All the recorded differences for CaTiO 3 plasma sprayed deposits with varying SDs must be therefore accounted to the deposit's structural differences, such as pore and splat sizes and shapes and preferred orientation. Additional differences on annealing can be probably attributed to the different amount of "stored" thermal energy in deposits due to the varying SD. However, there is a lower limit for SD assuring a quality deposit on spraying.

Cored wires expand the spectrum of coating materials, that can be processed from wire feedstock. In particular they allow to provide a custom-made alloy composition. For many industrial applications a low thermal expansion coefficient is desirable to adapt the expansion behavior to specific needs. Bulk material alloys within the system Fe-Ni-Co show thermal expansion coefficients below 5 10 –6 K –1 at temperatures below 600 °C. Thermal spraying is a suitable technology to manufacture coatings, that provide a gradient of the thermal expansion behavior, or free standing bodies for components with low shape changes due to thermal interference. Detailed research has to be done on the influence of the alloy composition regarding the specific metallurgical requirements on wire feedstock material for arc spraying. Fe-Ni(-Co) coatings are produced from cored wires by arc spraying. Guidelines for the choice of the optimum process parameters with respect to coating properties and economical effects are worked out. The coatings are characterized with concern to porosity, oxide content, surface roughness, deposition efficiency, power consumption and deposition rate. The coefficient of thermal expansion is determined by dilatometry.

Titanium powder has been sprayed with nitrogen or Ar/H 2 d.c. plasma jets flowing in air. Particles have been collected at several distances downstream of the nozzle exit. In the first 40- 60 mm, convective movements created within the liquid droplets entrain homogeneously nitrogen and oxygen in the particle cores. Farther downstream, convection is less important and absorption of nitrogen and oxygen is controlled by diffusion from the particles surface. After solidification induced by high quenching rates (in the order of 10 K/s) due to different cooling means, particles are composed by a superficial layer which is an oxi-nitride of titanium and in their core by a solid solution α-Ti containing both nitrogen and oxygen.

Hardness and microstructure of Cr 3 C 2 -NiCr coating formed by vacuum plasma spraying process (VPS coating) were investigated in compare with that formed by High Velocity Oxygen Fuel Spraying process (HVOF coating). The results are as follows. (1) The hardness of VPS coatings in as sprayed condition was HV1243 ± 80, which was much higher than that of HVOF coatings, HV958±44, and never went down under HV1100 even after heat treatment at 1273K for 3.6ks. (2) VPS coating presents dense lameller structures composed of Cr 2 C 3 , Cr 7 C 3 and γ-NiCr phase, while HVOF coating presents lameller structures with many fine gaps, composed of Cr 2 C 3 , Cr 7 C 3 , γ-NiCr phase and relatively large amounts of Cr 2 O 3 . (3) The reason why such high hardness was obtained in VPS coating, has been considered due to their dense lamella structures.