Hydroxyapatite (HA)/titania composite coatings were deposited on titanium alloy substrates using the high velocity oxy-fuel (HVOF) technique. Chemical reactions between the mechanically blended HA and titania particles in the HVOF stream were analyzed. Qualitative phase analysis through X-ray diffraction (XRD) on the composite coatings showed that the chemical reaction between titania and HA occurred during the impingement stage. High temperature differential scanning calorimeter (DSC) analysis revealed that the reaction temperature was 1410 C. The activation energy of the chemical reaction between HA and titania demonstrated a value of 5441.46 kJ/mol obtained through the multiple-heating-rate method. Chemical bonding caused by the reaction between the components was suggested, which may be mainly responsible for the trapping of titania particles during the impingement. Transmission electron microscope (TEM) observation identified the reaction zone and phase distribution area within the HA/titania composite coatings. It demonstrated that the reaction products located around titania were beneficial for the improvement of coating structure. Furthermore, in vitro bioactivity of the HA/titania composite coatings in simulated body fluid (SBF) was revealed. Results showed that the coatings were fully covered by a bone-like apatite layer after 7 days’ incubation.

Cu coating was deposited by microplasma spraying system under a low power of 2.8 to 4.2kW. The effects of the main processing parameters including plasma arc power, operating gas flow and spray distance on particle velocity during spraying, and the microstructure and properties of the coating were investigated. The coating microstructure was examined with optical microscopy. The coating properties were characterized by cross sectional microhardness. The particle velocity during in-flight was examined using a particle velocity/temperature measurement system based on thermal radiation. The experiment results showed that particle velocity was increased with the increase in operating gas flow, and was not influenced significantly by plasma arc power and spray distance. Moreover, the microhardness of the coating was increased with the increase in arc power and with the decrease in spray distance. The operating gas flow showed no significant influence on the microhardness of the coating. The analysis suggested that the microhardness of the coating is influenced significantly by particle temperature. The comparison showed that the microhardness of the Cu coating deposited by microplasma spray is comparable to that of the coating deposited by conventional plasma spray system at power level of 30kW.

Mechanical and tribological properties of layer composites like hardness, bonding strength, friction and wear coefficients as well as operation performance, component reliability and finally the system lifetime are mainly influenced by residual stresses after the thermal spraying and machining process. The residual stress situation in thermally spray coated composites is superimposed by different stress mechanisms occurring during the manufacturing process based on heat and mass transfer during the coating deposition. Using the microhole milling and drilling method, residual stress fields can be measured in a quasi non destructive way over the drilling depth with appropriate resolution. In several drilling and micromilling operations a circular, cylindrically shaped microhole is brought step by step into the component surface. The residual stresses are locally relieved due to material removal, deform the surface around the drilled microhole and are measured by high resolution measurement tools, e.g. strain gauges (DMS), for every drilling step in form of relaxed surface strains. Using calibration curves and material data (E, μ) the measured surface strains are converted into nominal strains at the bottom of the drilled hole for every drilling step. Out of the differentiated strains, in-plane stress fields can be incrementally determined by Hooke's law. This study describes residual stress measurement features, the FEM-calculation and idealization of calibration curves as well as results of exemplary stress measurements.

By modification of parameters in plasma spraying it is possible to alter porosity of the deposits only in a relatively small range. Industrial applications may require to adjust the porosity in wider range, to double or triple the original value in some cases. Such changes can be achieved only by special procedures. One of them is plasma spraying of ceramic/metal deposit followed by removal of the metal within the coating. The material removal is performed by dissolving, by leaching or by its extraction with an appropriate process. The paper describes preparation of very porous Cr 2 O 3 coatings by this method from a composite Cr 2 O 3 /Al deposit. The initial porosity of approximately 13% (total porosity) was increased to 37% or 51% respectively by changing the initial volume of aluminum in the ceramic/metal plasma-sprayed composite. The discussion is complemented by observation of the resulting coating structure and detailed characterizations of the pore structure and porosity.

Thermally sprayed ceramic coating consists generally of porosity up to over ten percent. The existence of voids in the coating will degrade mechanical performance and corrosion resistance. The infiltration technique has been used to enhance the coating performance. In the present work, the adhesives of high strength are used as infiltrator for plasma sprayed alumina coatings to investigate the effect of the infiltration on the adhesive strength and erosion resistance. The adhesive strength of coating after infiltration is tested following ASTM-C-633 standard with the same adhesive. The commercial adhesives used are liquid types of adhesives. Alumina coatings are deposited by plasma spraying to a thickness about 500ìm. The adhesive strength of as-sprayed coating was tested using another epoxy resin adhesive. The test pieces are prepared without infiltration of adhesive into the coating. The results revealed that the infiltration of adhesive to alumina coating can improve significantly the adhesive strength and erosion resistance. The adhesive strength from 40 to 55 MPa can be obtained in spite of deposition conditions. The erosion resistance of the coating at 90 degrees can be improved by a factor about 3.

For electrical or thermal insulation, the porosity of an air plasma sprayed (APS) coating is an important property to control. Moreover in aggressive environment the interconnected porosity is responsible for the substrate corrosion. To solve, at least partially, this problem, deposition by mutitechniques (APS and a PECVD) was used to close interconnected and opened porosities. In this study, titanium alloy (TA6V) substrates were coated by alumina using either one or both deposition processes. Electrochemical characterization technique was used to evaluate the open porosity in alumina coatings. It consists of evaluating the polarization resistance of the reference sample surface (uncoated substrate) and to compare it to coated ones. After different tests for selecting the electrolyte solution, the influences of different parameters (thickness and relative position) of each deposition process on coating porosity were examined.

Two kind of the feedstock materials were used to produce the atmospheric plasma sprayed coatings. One is the spraying dried powder and the other is the nano-particle dispersed sol precursor. Coating properties were evaluated by the XRD, Raman spectroscopy, SEM and TEM. Also, the photodecomposition efficiency was examined by measuring the decomposition rate of the MB(methylene blue) in a water-cooled solution during the UV radiation and compared to the decomposition efficiency of the optimum coating produced using spraying dried particle. Both the anatase phase fraction and the photodecomposition rate of the coating using the sol precursor were higher than that of the optimum coating using the spraying dried powder.

Titanium dioxide (TiO 2 ) is an attractive material for numerous technological applications such as photocatalytic applications. The photocatalysts allow the environmental purification of air and water by the decomposition of toxic organic compounds and removal of harmful gases. This work was focused on the production and evaluation of the environmental properties of titanium dioxide coatings obtained by the plasma spray technique. To carry out the step of validation of the TiO 2 coatings for their environmental functionalities, a control test of the photocatalytic effectiveness was performed with respect to nitrogen oxides. A custom-designed test chamber has been developed. The photocatalytic properties of different coatings were studied as a function of various parameters (porosity, anatase/rutile ratio, nature of the substrate).

An experimental study is conducted to determine the property fields of 40 MHz argon radio frequency inductively coupled plasma using optical emission spectroscopy. The pure argon plasma was operated at the input power of 0.3 kW and under atmospheric pressure. 29 atomic argon lines with upper level energies ranging from 12.9 to 15.5 eV, continuum emission and line width are used to evaluate plasma parameters such as temperature and electron number density. Since 40 MHz plasma is in almost complete nonequilibrium, the validaty and accuracy of most usual spectroscopic methods are questioned. Analysis based on the Boltzmann diagram, line-to-continuum intensity ratio, population of continuum extrapolated level, and continuum intensity reveals the departure from thermodynamic equilibrium in the plasma. Among these methods, the Boltzmann diagram method is shown to provide reliable plasma excitation temperature as long as the Boltzmann plot is drawn based on enough spectra lines covering from infrared to ultraviolet regions. The continuum emission at wavelengths within visible region can give good estimation of the electron density by using excitation temperature in the continuum relation. The line-to-continuum is not a reliable method for the temperature measurement of nonequilibrim plasma. The electron density obtained from the Saha plot can provide rough estimation of the electron density. It is shown that the electron-atom interaction contribution to the continuum radiation is more important than being expected before for the argon plasma in our study. The non-axisymmetric distribution of the emission was found to exist within the coil zone of the plasma, which may affect the estimation of the local emission coefficient, and consequently the measured plasma fields.

The electronic structure of the bulk and the surface of rutile titanium dioxide (TiO 2 ) are analyzed from band calculation based on quantum mechanics formalism. The surface density of states and charge distributions of bulk and (001) surface are compared to better understand the mechanism of adsorption and photodegradation processes.

The aim of this experimental study was to comprehend the relative performance and failure modes of WC-NiCrBSi Thermal Spray coatings in As–Sprayed and HIPed (Hot Isostatically Pressed) conditions in rolling/sliding contact. Recently a number of scientific studies have addressed the fatigue performance and durability of Thermal spray coatings in rolling/sliding contact, but as of yet there have been no investigations on Thermal Spray Coatings which have undergone the post treatment HIPing. The understanding of the mechanisms of failure in rolling /sliding contact after HIPing is therefore critical in optimising the parameters associated with this post treatment to achieve superior performance. Coatings were deposited by a JP5000 system and HIPing was carried out at two different furnace temperatures of 1123K and 1473K. At both HIPing temperatures the rate of cooling was kept constant at 8°C/minute. Rolling Contact Fatigue tests were conducted using a modified four ball machine under various tribological conditions of contact stress, configuration and lubrication. Results are discussed in terms of as-sprayed and HIPed surface examination of rolling elements using Scanning Electron Microscope (SEM) and Light Microscope.

It is essential to understand and clarify what causes adhesive strength between the thermally sprayed deposit and the substrate. It is known that the adhesive bond strength is strongly related to the roughness of the substrate surface, however, we are yet to know why and how the roughness affects the strength. Although the roughness works as a socalled anchor or interlocking effect, we cannot show how the effect quantitatively relates to the strength. Before we use the word “roughness”, we need to define roughness as a strict meaning in connection with the adhesive strength to avoid any ambiguity in expressing the roughness itself. Seeking the true roughness to relate to the bond strength, we have introduced a newly developed roughness indicator correlated closely with the adhesive strength. This indicator is derived by theory on the basis of the assumption that the bond strength is primarily caused by the mechanical friction between the deposit and the substrate. It was supposed that chemical and physical adhesions were secondary effects except that the molten particle, the substrate temperature or both of them are high enough to form these bonds by thermal interaction. The increase in contact area between the splat and the substrate could raise the friction force between them. The higher substrate temperature could increase the contact area when the molten particle interacts with the substrate, because the molten particle could wet better on the high temperature substrate. The substrate temperature influence on the adhesive strength was also investigated.

Nondestructive techniques for evaluating and characterizing coatings have been extensively demanded by the thermal spray community; nonetheless, few results have been produced in practice due to difficulties in analyzing the complex structure of thermal spray coatings. Of particular interest is knowledge of the elastic modulus values and Poisson’s ratios, which are very important when seeking to understand and/or model the mechanical behavior or develop life prediction models of thermal spray coatings employed in various applications (e.g., wear, fatigue and high temperatures (TBCs)). In the present study, two techniques, laser-ultrasonics and Knoop indentation, were used to determine the elastic modulus of thermal spray coatings. Laser ultrasonics is a non-contact and nondestructive evaluation method that uses lasers to generate and detect ultrasound. Ultrasonic velocities in a material are directly related to its elastic modulus value. The Knoop indentation technique, which has been widely used as a method for determining elastic modulus values, was employed in order to compare and validate the measurements of the laser-ultrasonic technique. The determination of elastic modulus values via the Knoop indentation technique is based on the measurement of elastic recovery of the dimensions of the Knoop indentation impression. The approach used in the present study was to focus on evaluating the elastic modulus of very uniform, dense and near-isotropic titania and WC-Co thermal spray coatings using these two techniques. Four different coatings were evaluated: two titania coatings produced by APS and HVOF and two types of WC-Co coatings, conventional and multimodal (nanostructured and micro-sized particles), deposited by HVOF.

Fatigue behavior and Young’s modulus of plasma sprayed gray alumina on low-carbon steel substrates were investigated. The investigation of the properties of composite material “coating-substrate” included the measurements of microhardness profile, residual stress on the top of the coating and residual stress profile in substrate. Fatigue samples were periodically loaded as a cantilever beam on a special testing machine. Failed samples were observed in SEM to determine failure processes in the coating. The Young’s modulus of the coating was measured by the four-point bending method. Samples were tested both in tension and compression at low (300 N) and high (800 N) loads. Our experiments revealed that the average fatigue lives of coated specimens were nearly 2 times longer than those of the uncoated specimens. The Young’s modulus of the coating varied between 27 and 53 GPa with an average value of 43 GPa. Loading in tension caused decrease in Young’s modulus of the coating while loading in compression lead to increase in Young’s modulus. Increase in the lifetime of coated samples was likely due to compressive residual stresses in the substrate, originating from the spray process. Failure of the coating consisted of several processes, among which the most important are splat cracking, splat debonding and coalescence of cracks through the voids in the coating.

Relationships between the electrical properties of thermally sprayed titania coatings and their microstructure have been investigated. As far as possible, a broad range of microstructures was produced by using various processes of plasma spraying with different powder size ranges and variations of the plasma operating parameters. The two spraying processes consisted of DC plasma spraying and RF plasma spraying. Physical properties of plasma-sprayed coatings are generally influenced by their microstructure. But the electrical properties of plasma-sprayed titania coatings are known to be strongly influenced by their stoichiometry. It is the reason why coatings with identical stoichiometry were compared. It was found that electrical resistivity was directly linked to the quality of the contact between the splats and their density through the titania plasma-sprayed coatings.

The visualization of the detailed microstructure of plasma sprayed ceramic coatings is carried out through quantitative analysis of the distribution of the infiltrated element as a tracer. A method to characterize quantitatively the microstructure of spray coating is proposed based on an idealized layer structure model and the distribution of the tracer in the coating. The microstructural parameters include mean thickness of lamellae, bonding ratio between flattened lamellae, distribution and average lamellar interface gap, and vertical crack related parameters. The experiment is performed for plasma sprayed alumina coating with the infiltration of Cr 2 O 3 through chromium acid. The Cr distribution along the lines perpendicular to lamellae is obtained by EDAX line analysis in the form of digital signal at intervals of 2 µm. The processing method of the signal to evolve the structural parameters is introduced.

Titanium dioxide (TiO 2 ) has emerged as an excellent photocatalyst material for environmental purification about two decades ago. In order to avoid the use of TiO 2 powder, which requires later separation from the treated liquid, few researchers reported on ways of fixing TiO 2 . In these cases, many difficulties must be overcome such as maintaining the photoactive anatase phase as well as the mechanical stability and avoiding the mass transfer limitations. Thermal spraying seems to be an appropriate process to provide ceramic coatings exhibiting such characteristics. The present work assesses the influence of several spray parameters on the morphology of deposits, dominated by two void systems – interlamellar pores and interlamellar cracks – and on their crystalline structures. TiO 2 was atmospherically plasma sprayed by systematically varying the process conditions including torch power, plasma gas mixture Ar/H 2 and Ar/He, and cooling device. An analytical procedure based on XRD patterns was established for the determination of the relative amounts of anatase and rutile in the coatings. An Ar/He mixture as compared to an Ar/H 2 one seems to enhance the formation of anatase from 38vol% to 44vol%. In both cases, a reduction of the plasma power increases the amount of anatase. Several methods of porosity determination were performed: image analysis, Archimedean porosimetry and calculation from phases, mass and thickness. Generally, the results indicate a total porosity varying between 15 and 25% depending on the spraying conditions.

The development of amorphous and nanocrystalline materials took a significant part in search on materials these last years. Indeed, the magnetic, chemical and mechanic properties of materials are greatly modified when the size of crystallites becomes nanometric. The absence of crystal structure involves a macroscopic behaviour of the alloy, which is completely different from the same alloy in a polycrystalline state, particularly magnetic and mechanical properties. We have carried out coatings by APS plasma thermal spraying on a copper substrate using three types of powders, FeB (17,5% wt-B), FeSi (6,5% wt-Si) and FeNb (67,2% wt-Nb). Structure of these coatings was characterized by SEM and X-rays. We have also tested the magnetic properties of these deposits. Results obtained showed that the FeNb alloys are amagnetic with a partially amorphous structure, however FeSi and FeB alloys presented a microcrystalline structure with soft magnetic properties.

Wollastonite coating deposited on Ti-6Al-4V substrate was prepared using atmospheric plasma spraying technology. Scan electron microscopy, transmission electron microscopy and X-ray diffraction were used to investigate the microstructure, surface morphology and phase compositions of coatings. Simulated body fluid tests were carried out to evaluate the bioactivity of the coating. Osteoblasts culturing test was used to examine the cytocompatibility of wollastonite coatings. The tensile bonding strength of the coating was measured with ASTM C-633 methods. The results obtained indicated that the plasma sprayed wollastonite coating had a rough surface and lamellar structure including some pores and microcracks. The primary crystalline phase of the coating was triclinic wollastonite. Glassy phase with nanocrystals was also found in the coating. The bonding strength of wollastonite coating reaches to about 39 MPa. This was explained in term of that the thermal expansion coefficient of wollastonite coatings is close to the Ti-6Al-4V substrate. The apatite layer was formed on the surface of plasma sprayed wollastonite coating soaked in SBF solution, which indicates wollastonite coating had a good bioactivity. Osteoblasts survive and proliferate on the surface of wollastonite coating, which indicates plasma sprayed wollastonite coatings possess excellent cytocompatibility.

Thermal spray processing involves a multitude of interdependent process variables resulting in a complex layered microstructure with each layer composed of several splats separated by splat-splat interfaces. Each splat contains columnar/equiaxed grains with multiple orientations. A typical microstructure also contains defects such as globular pores, and interlamellar pores. This gamut of features encompasses a range of macro, micro and nano length scales. This study involves a thorough microstructural characterization across length scales of Ni-5wt%Al coatings to understand the dependence of microstructural variability on processing conditions and its profound influence on coating properties. Studied here are coatings produced by four different spray methods, namely twin wire-arc, air plasma, high velocity oxyfuel and cold spraying with distinct variations w.r.t. nature of feedstock, method of material injection, amount of melting, and process parameters such as particle velocity and temperature in flight. Macro-scale, Micro-scale and Nanoscale microstructural characterization of these Ni5Al coatings elucidates the variations in oxide content, splat structure, grain structure and phases obtained, based on differences in processing parameters. Through characterization of these bond-coats, using Optical microscopy, Image analysis, Scanning and Transmission electron microscopy techniques, variations in coating properties such as porosity and thermal conductivity can be explained with respect to coating microstructure and process characteristics.