Thermal spray coatings exhibit a wide variety of microstructural characteristics that lead to variation in their functional properties. A complete understanding of the plasma spray process includes examination of the particle-flame interaction, particle impact (to form the splats), and the particle-substrate interaction during coating deposition. The links between these process parameters and coating properties has been established by using diagnostic tools in conjunction with a splat collection shutter and an in-situ curvature measurement instrument. In this study, a commercial grade molybdenum (Mo) powder was plasma sprayed; the spray stream was characterized in relation to the resulting particle state. A "splat map" was deposited through a "spray stream guillotine" to capture the fingerprint of the plume cross section. Subsequently, coatings were deposited at these spray conditions on a newly developed in-situ curvature measurement instrument to measure coating stresses and to estimate the coating modulus. Splats and coatings were subsequently characterized by micro-diffraction (for splat residual stresses), by nano and micro-indentation for elastic and elastic-plastic properties, and by electron microscopy. This complete history of the process followed by splat and coating characterization provides insight into the correlation between processing parameters, resultant particle states, and final coating properties. The role of particle temperature and velocity on the splat (and coating) morphology and residual stress is explained in the results.

A new method for evaluating the tensile adhesive strength of thermal sprayed coatings has been developed using a test specimen that incorporates an artificially introduced circumferential crack to control the stress intensity factor at the crack tip along the interface between the coating and the substrate. FEM-analysis is carried out to calculate a correction factor and the stress intensity factor for the test specimen. When the results of tensile test are sorted out using the stress intensity factor at which failure occurred, constant values should be obtained regardless of changes in geometry such as crack length and the diameter of test specimen. The method was first applied to air plasma spray (APS) coatings and then to HVOF sprayed coatings. Effects of substrate preparation such as surface roughness and preheating temperature on the resultant coating adhesion were studied experimentally.

Cryomilling was applied to conventional gas-atomized CoNiCrAlY powder to produce powder with nanocrystalline grains. The cryomilled powder and gas-atomized powder were HVOF sprayed onto mild steel sheets to prepare two coatings with fine-grained (~15 nm) and coarse-grained (~one micron) microstructure, respectively. Isothermal oxidation tests in air at 1000°C were conducted for the two coatings for up to 330 h. The morphology and composition of the thermally grown oxides (TGOs) formed on the two coatings were characterized with XRD, SEM, and EDS and compared with each other. After oxidation for up to 24 h, a pure alumina scale formed on the cryomilled coating, while a mixed oxide layer formed on the conventional coating. However, after oxidation for 330 h, non-alumina oxides formed also on the cryomilled coating. These results indicate that, while a fine-grained microstructure can promote the formation of a pure alumina scale after short-term oxidation by increasing the Al diffusion rate toward the surface, it can also accelerate the Al depletion by increasing the Al diffusion rate toward the substrate, which results in the formation of non-alumina oxides after long-term oxidation. The mechanisms governing the oxide formation are discussed in terms of atomic diffusion and thermodynamic stability.

Fe-B composite powder was produced by spray-drying of elemental iron and boron powders. Fe-ZrB 2 composite powder was made by ball-milling of elemental iron and ZrB 2 powders. These powders were plasma-sprayed in low-pressure argon atmosphere to produce iron matrix composite deposits with dispersed boride particles. As-sprayed deposits formed using Fe-B composite powder are composed of ferrite (α) and austenite (γ) phase that is supersaturated with boron due to the high cooling rate of molten particles on a substrate. Heat treatment of the deposit at 673 K leads to the formation of Fe 3 B. The deposit heat-treated at 1073 K is made up of α, Fe 2 B, and FeB. With increasing heat-treatment temperature up to 1073 K, the hardness of the deposit decreases. As-sprayed deposit produced using Fe-ZrB 2 composite powder is composed of α, ZrB 2 , Fe 2 B, and Fe 3 Zr. Heat treatment of the deposit at 1073 K results in the formation of FeB. Heat treatment at 1073 K lowers hardness of the deposit.

Nanostructured and conventional hydroxyapatite (HA) feedstocks were evaluated to determine the effects of feedstock structure on the processing, properties and performance of coatings produced by atmospheric plasma spraying. The structure of the feedstocks was characterized using scanning electron microscopy (SEM). It was found that the nanostructured feedstock particles were formed by an agglomeration of nanostructured fibers having dimensions of less than 500 nm in length and below 100 nm in width. The average particle temperatures and velocities were measured during plasma spraying and found to be ~2650°C and ~315 m/s for the both feedstocks. The mechanical, microstructural and biocompatibility characteristics of coatings deposited on Ti-6Al-4V substrates were evaluated. The hardness was measured using Vickers indentation. The bond strength was determined via a tensile adhesion test. Microstructural characteristics of the coatings and their porosity levels were evaluated using SEM and image analysis. Phase analysis was carried out via X-ray diffraction (XRD) and aided by energy-dispersive X-ray analysis (EDS). The in-vitro behavior of these coatings was investigated in a simulated physiological solution in an attempt to simulate the environment of an implant in the human body.

To address drawbacks with conventional destructive adhesion tests, two advanced nondestructive methods were developed to characterize the acoustic response of coating-substrate bonding. These new tests, immersion ultrasonic testing and laser shock adhesion testing, are discussed in this contribution. The paper describes the test process and results as compared to traditional destructive testing. The advantages and drawbacks of the techniques are addressed.

TiO 2 coatings were prepared on polybutylene succinate (PBS) plastic by plasma spraying and High Velocity Oxygen Fuel (HVOF) spraying using agglomerated sub-micron TiO 2 powder (200nm). Since PBS is well known material as bio-degradable plastic, the photo-catalytic TiO 2 /bio-degradable PBS composite material will exhibit purification of air and water without environmental damage. It was found that TiO 2 agglomerated powders could be injected into PBS surface and anatase ratio greater than 60% could be achieved by 1pass plasma spraying. The anatase phase ratio decreased gradually with increasing pass count. By finding out adequate plasma spraying condition as a result of many experiments, TiO 2 /PBS composite material could be formed without large damage of PBS substrate. HVOF spraying could also provide TiO 2 coating of 100% anatase ratio on PBS for 1~6 pass spraying. This means that 200nm agglomerated powders showed lower susceptibility to heat effect from gas flame and provided higher photo-catalytic activity as compared to plasma sprayed coatings. These coatings showed photo-catalytic degradation of 100ppm gaseous acetaldehyde (CH 3 CHO). As a result of photo-catalytic experiments, TiO 2 coating formed by HVOF spraying showed a higher degradation rate because of higher anatase ratio. It means that 100ppm gaseous acetaldehyde was decomposed to less than 5 ppm after 7.2ks. Thermal spraying on PBS plastic in this investigation could imply both new application of thermal spraying and new possibilities of surface modification of low-melting point plastic.

The crack propagation characteristics obtained by observing the microstructure of bent coatings at different stages of applied strain were conducted. At the same time, acoustic emission (AE) features of released events were recorded. Generated cracks were found to emit weak acoustic emission events at the initiation (creation) of the crack at the coating surface, and then become strong releasing a uniform acoustic emission energy per event, when propagating transversally in the coating. However, cracks propagating at the interface with the substrate could generate both weak and strong events. It was also found that increasing the thickness of the coating leads to a decrease in the number of events and an increase of their released acoustic emission energy.

A typical feature of plasma sprayed zirconia-based coatings is that their depostion efficiencies are low. Thus, from the economic view of reducing cost, how to improve the depostion efficiency of plasma sprayed zirconia-based coatings is very important for practical application. In this paper, spray-dried nanostructured powders and conventional powders were used as feedstocks to deposit nanostructured and conventional zirconia coatings by atmospheric plasma spraying(APS). Deposition efficiencies for both nanostructured and conventional zirconia coatings deposited under different spraying parameters were comparatively measured. The obtained results revealed that the spray-dried nanostructured powders possessed higher deposition efficiency than that of conventional powders. In addition, it was found that spraying parameters had strongly influence on microstructure and microhardness of zirconia coatings. Therefore, in order to obtain high quality zirconia coating, proper spraying parameters must be considered.

Pb-Sn alloys tend to form string-like abrasion products during compressor operations, and the products cause malfunctions at the down streams of the seals and may affect the operation efficiency of the compressors. In this article, a thermal spraying method is developed for forming an abradable labyrinth seal of powdered abradable alloys to cope with the problems for improving the product quality and the operation life of the compressors. The article evaluates the abradable properties of the thermal sprayed coating labyrinth seals of the RIK-type compressors that have higher rotational speed than the other type. Various abradable properties of the sprayed coating are tested by the following test methods: microstructure observation, bond strength, machinability, bending ductility, and corrosion resistance. The article reports the results obtained from these test methods and compares them with those of the cast alloy.

Spray coatings composed of zirconia, zirconium boride, and zirconium boroxide were reactively produced by Electrothermally-Exploded-Powder-Spray (ELTEPS) technique by using a mixed powder of zirconium and boron oxide particles. Substrates used were mild steels on which a Ni-20%Cr coating was plasma-sprayed as an undercoat for such a composite coating. Energy-controlled multiple sprays brought the hetero-structure of a top layer of Zr-O-B ceramic composite and a graded layer which composition continuously changed from the ceramics to the metal under-coat. The microstructure of the top ceramic layer was composed of Zr-O-B ceramic particles the size of which was on the order of several nanometers to a few hundred nanometers.

Optical selective surfaces, i.e., surfaces with optical properties varying according to the frequency of the impinging radiation, have been exploited in several technical fields. These surfaces consist generally of doped semiconducting films, such as mixed oxides of Indium and Tin as well as Aluminium and Zinc. Thay are currently obtained by physical vapour deposition or sol-gel techniques. The present work aimed at demonstrating that coatings retaining optical selectivity can be obtained also by plasma spraying. Powders of Indium Tin Oxide (ITO) were prepared by an agglomeration technique and sprayed with a plasma torch under air and inert gas atmospheres. Both powders and coatings were characterised by X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. Optical reflection coefficients of the coatings were determined in the wavelength range 0.3-20 µm, i.e., in the visible and in the infrared regions of the spectrum. The experimental results indicated that it was possible to deposit, by plasma spraying, coatings possessing optically selective properties.

Instrumented indentation is well-suited as a technique for probing mechanical behaviour of coatings; it requires minimal specimen preparation, can be performed multiple times on a single specimen, and can measure behaviour over various length scales, by recourse to appropriate indenter tip selection. However, the complex nature of the deformation fields under the tip, coupled with complex (e.g. anisotropy, nonlinearity) coating properties necessitates robust means of interpretation in order to extract stress-strain behaviour. Here we present a simple method, based on empirical work by Tabor, for converting coating indentation force-depth data to stress-strain curves. Using this method, results of indentation can thus be used to quantify mechanical behaviour of coatings deposited by different processes, and subjected to post-processing treatments, providing a powerful supplement to microstructural characterization. In addition, we show how anisotropic elasticity of coatings, measured via indentation, can qualitatively identify the existence of different microstructual features. Finally, we explore subsurface strain mapping as a technique to quantitatively compare elastoplastic behaviour of coatings to bulk, isotropic materials.

Pure alumina coating obtained by thermal spraying can find applications as electrical insulating layer. Thermally-sprayed ceramic coatings exhibit a complex lamellar structure with a network of interconnected pores, inter-lamellar and intra-lamellar cracks. In this work, the influence of the microstructure on electric properties for plasma-sprayed alumina coatings was investigated. Coatings have been sprayed with different pressures and gases using a CAPS (‘Controlled Atmosphere Plasma Spraying’) as well as different alumina feedstock powders. Detailed quantitative image analysis of cross-section views allowed to select six microstructures with different porosity levels and cracks orientation distributions. In order to assess the behaviour of the electrical insulation and the influence of local defects on electric properties, the so-called Scanning Electron Microscope Mirror Effect (SEMME) method has been applied on outer surfaces and on cross-sections of the different selected alumina coatings. This method, originally developed to study the ability of a bulk insulating material in trapping of charges from an electron beam irradiation in a SEM, revealed to be successfully feasible for porous materials such as thermally-sprayed ceramic coatings. It has been shown that cracks orientation modified both propagation and trapping of charges and therefore the electric properties of plasma-sprayed alumina coatings.

Boron carbide has been successfully deposited on Ti alloy by vacuum plasma spraying (VPS). Mechanical properties of the deposited structure were assessed by micro-hardness and nano-hardness indentation. Chemical and phase compositions of the starting powder and the as-sprayed structure were characterized using hot gas extraction (LECO), x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), and Raman spectroscopy. The microstructure consisted of equiaxed boron carbide grains, microcrystalline boron carbide particles, and amorphous carbon regions at the grain boundaries. The amount of boron oxide and amorphous carbon increased during spraying. Carbon segregation to grain boundaries in the as-deposited B 4 C was observed. The measured micro-hardness was slightly higher than values previously reported (1033 ± 2009 HV). There was significant variation of nano-hardness from point to point in the material due to the existence of multiple phases, splat boundaries, and porosity in the deposited structure.

Wollastonite and its composite coatings with zirconia and titania have been deposited by plasma spraying. The bioactivity of coatings was evaluated using a simulated body fluid soaking test. The fetal rat’s osteoblasts were seeded on the surface of the coatings to examine their biocompatibility. The SEM and XRD technologies were used to examine the morphologies, structure and composition of the surface of the coatings soaked in the simulated body fluid. The results obtained showed that carbonate-containing hydroxyapatite was formed on the surfaces of wollastonite, wollastonite/ZrO 2 and wollastonite/TiO 2 coatings, while was not formed on the surfaces of ZrO 2 and TiO 2 , indicating the wollastonite improved the bioactivity of ZrO 2 and TiO 2 coatings. Osteoblasts are able to survive and proliferate on the surfaces of wollastonite, wollastonite/ZrO 2 and wollastonite/TiO 2 coatings. It is enough to prove that these coatings possess excellent biocompatibility.

The effects of the plasma gas composition (Ar-N 2 , Ar-H 2 ), arc current and voltage on the temperature and velocity of a low power (5 kW) plasma torch in the arc field free region have been investigated using an enthalpy probe. Coatings of Al 2 O 3 -13TiO 2 were deposited under different conditions. The results show that in the Ar-N 2 plasma, the enthalpy, temperature and velocity change little with arc current and voltage when regulating the nitrogen proportion in the plasma gas, and the hardness of the resulting coatings is 800-900 HV. For Ar-H 2 plasma, however, increasing the H 2 content in the gas mixture remarkably enhanced the velocity and heat transfer of the plasma jet with the result that the coatings showed the high hardness up to 1200 HV.

An important and often neglected factor in determining the final properties of a plasma sprayed deposit is its phase composition. It is well known that spraying of alumina results usually in the formation of phases other than the desired hard alpha-phase (corundum) unless a stabilizing admixture is added to the feedstock. In this work, chromia was used for stabilization and feedstocks with varying amounts of chromia were used to produced coatings by water spray plasma coating. Two types of feedstocks were used: fused/crushed feedstock and a mixture of alumina and chromia powders. The effect of the chromia content on phase composition, residual stresses, hardness, and wear abrasion of coatings was studied. Microanalysis indicates that a certain amount of chromium is transferred into alumina particles during the "in-flight" reaction of both powders in the plasma jet. Complex "alloy" oxides form where part of the aluminum is replaced by chromium and the alpha-phase is partially stabilized. However, any amount of chromia in coatings significantly changes the wear resistance and hardness even when the stabilization of corundum is only partial.

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 aim of this study was to compare different processes for a spraying NiCrBSi powder in order to understand how the coating properties are influenced by the particle characteristics. For that, three processes were tested: atmospheric plasma, HVOF and flame spraying. The plasma torch was a classical F4 gun from Sulzer-Metco, the HVOF gun was the CDS system manufactured by Sulzer-Metco and the flame spray gun was a Castolyn 8000 system. For each spraying process, the particles were analyzed using the DPV 2000 (Technar Ltd) at the distance corresponding to the coating build up. Particles velocity and temperature data were then correlated to the coating quality. Results indicate that the choice of the spraying technique induces important modifications in the coating properties in terms of adherence, porosity level, hardness and Young modulus. Changes in particle velocity and temperature are the key parameters determining the coating properties.