Ghislain Montavon

Structure and wear behaviour of HVOF sprayed Cr3C2–NiCr and WC–Co coatings

Abstract Hard chrome plating is used to restore the original dimensions to worn surfaces of gas turbine shafts. However, its use is about to decrease due to some intrinsic limitations of its deposits and the toxic and carcinogenic characteristics of the hexavalent chromium. During the last decade high velocity oxy-fuel (HVOF) thermal sprayed cermet coatings play an important role in industrial applications where exceptional friction and wear resistance are required. The purpose of this study is to investigate and to compare the microstructure, wear resistance and potentials of HVOF sprayed Cr 3 C 2 –NiCr and WC–Co coatings for a possible replacement of hard chromium plating in gas turbine components repair. It has been shown that coatings exhibit high hardness with a high volume fraction of carbides being preserved during the spraying, and have different wear behaviour.

Effects of the spray angle on splat morphology during thermal spraying

Abstract The effects of spray angle on the morphology of thermally sprayed particles impinging on polished substrates have been studied by implementing several statistical tools (i.e., Gaussian analysis, Weibull distribution and the t -test). Nickel-based alloy (Astroloy) particles were vacuum plasma-sprayed onto copper plates at normal (i.e., 90 °) and several off-normal spray angles (i.e., 75, 60, 45 and 30 °). Different geometric shape factors (i.e., referring to an equivalent diameter, elongation factor and degree of splashing) were determined using image analysis. The spray angle had a strong effect on these geometric properties, in particular on the elongation factor of the shapes.

Effects of Vacuum Plasma Spray Processing Parameters on Splat Morphology

Several statistical tools (i.e., Gaussian and Weibull distribution analyses, the t-test, and analysis of the variance) were used to examine relationships between vacuum plasma spray processing parameters and the morphology of flattened particles (splats) on a smooth, polished substrate. Astroloy, a nickel-base powder, was vacuum plasma sprayed onto polished copper substrates under various processing conditions. Different flattened particle shape factors, including equivalent diameter, elongation factor, and degree of splashing, were determined using image analysis. The spray parameters (i.e., current intensity, chamber pressure, argon mass flow rate, etc.) strongly influenced splat formation and morphology and thus deposit microstructure and properties.

Surface preparation and thermal spray in a single step: The PROTAL process—Example of application for an aluminum-base substrate

Thermal spray techniques can fulfill numerous industrial applications. Coatings are thus applied to resist wear and corrosion or to modify the surface characteristics of the substrate (e.g., thermal conductivity/thermal insulation). However, many of these applications remain inhibited by some deposit characteristics, such as a limited coating adhesion or pores or by industrial costs because several nonsynchronized and sequential steps (that is, degreasing, sand blasting, and spraying) are needed to manufacture a deposit. The PROTAL process was designed to reduce the aforementioned difficulties by implementing simultaneously a Q-switched laser and a thermal spray torch. The laser irradiation is primarily aimed to eliminate the contamination films and oxide layers, to generate a surface state enhancing the deposit adhesion, and to limit the contamination of the deposited layers by condensed vapors. From PROTAL arises the possibility to reduce, indeed suppress, the preliminary steps of degreasing and grit blasting. In this study, the benefits of the PROTAL process were investigated, comparing adhesion of different atmospheric plasma spray coatings (e.g., metallic and ceramic coatings) on an aluminum-base substrate. Substrates were coated rough from the machine shop, for example, manipulated barehanded and without any prior surface preparation. Results obtained this way were compared with those obtained using a classical procedure; that is, degreasing and grit blasting prior to the coating deposition.

COMBINATION OF INVERSE AND NEURAL NETWORK METHODS TO ESTIMATE HEAT FLUX

ABSTRACT This article deals with the estimation of the heat flux transmitted to a workpiece from a flame gun during the preheating process that is implemented quite often in thermal spraying. Internal temperature measurements permit determination of the temperature distribution in the workpiece. The heat flux is then determined by solving the inverse problem and the correlations among some processing parameters and the heat flux are recognized by implementing an artificial neural network.

A Two-Dimensional Heat Transfer Model for Thermal Barrier Coating Average Thermal Conductivity Computation

ABSTRACT The present article is devoted to quantifying the contribution of pores and cracks in the decrease of the effective thermal conductivity of thermal barrier coatings (TBCs). A finite-difference-based model is used for the computation of heat transfer through a porous structure. Each pixel of a binary picture describing the pore network is interpreted as a cell of integration of the heat conduction equation. A thermal gradient is applied and a large system of linear Equations is subsequently derived. Since it has a strong influence on the required CPU time, particular attention is given to the solving procedure.

Microstructural Index to Quantify Thermal Spray Deposit Microstructures Using Image Analysis

The basic metallographic analysis leads to qualitative interpretation of the structural characteristics of a microstructure, for example the presence of phases, and the description of singularities such as inclusions. On the contrary, microstructural characterization which implements image analysis leads to a quantified analysis of structural characteristics. A method is described to assess thermal spray deposit microstructures using image analysis by means of a metallographic index. This index is based on the determination of several stereological and morphological parameters by primary referee to the size-shape distributions of the features, the fractal dimension of the deposit upper surface, and the Euclidean distance map of the bodies of interest. This work employs quantitative metallography on a much wider scale to provide better quality control of deposit microstructures.

Designing expert system using neural computation in view of the control of plasma spray processes

Abstract This paper aims at integrating the artificial intelligence methodologies in a quality control of ceramic coating fabrication using the atmospheric plasma spray (APS) process. In such a way, the average velocity, temperature and diameter of thermally sprayed Al 2 O 3 -13 wt.% TiO 2 particles before impinging the work piece and forming a deposit are monitored. Then, as these particle characteristics represent the most pertinent indicators of the coating properties and characteristics reproducibility, they are chosen as the output of an expert system based on neural computation. The model is built also considering at the system input the plasma and particle powder injection-processing parameters. After an optimisation procedure, the predicted results are compared to the results of experimental data resulting from a non-intrusive sensor conventionally used by industrials to control the coating quality. The good agreement found between these results permits to establish the overall effect of each processing parameter on the in-flight particle characteristics.

On the implementation of the fractal concept to quantify thermal spray deposit surface characteristics

A methodology is suggested to accurately describe the surface state of thermally sprayed coatings, which is mostly related to the process operating conditions and to the feedstock characteristics. Up to now, standard methodologies and their related descriptors are not sufficient to give simultaneously global and local information about deposit roughness. An approach is proposed, taking into account the multi-scale complexity of the profile. This approach is based on a description of the surface in terms of fractal geometry. Several aspects of the fractal methodology are discussed and some results related to the substrates and upper surface deposit profiles are presented. Comparisons with conventional methodologies do not permit a direct correlation: the fractal methodology proves to be sensitive to the measurement scale and to the calculation protocol.

Thermal and Cold Spray: Recent Developments

Thermal spraying consists in a technology aiming at producing coatings whose thicknesses range from 10 μm to a few millimeters onto mechanical components to confer them specific and unique functional properties, such as wear and corrosion resistances, friction coefficient adaptation, thermal and electrical insulation, biocompatibility, repair, etc., among the principals. Thermal spraying consists in injecting in a viscous enthalpic jet (animated by a momentum) powder with particles which average size ranges from 0.01 to 100 μm. These particles are melted and simultaneously accelerated towards the surface of the part to be covered. They form, after impact, spreading and solidification, near-circular lamellae the stacking of which form the coating. Due to the versatility of the available processes exhibiting a wide range of enthalpic and momentum contents, virtually any kind of material exhibiting congruent melting behavior can be processed, from alloys and ceramics to polymers, ever since its melting temperature differs from its vaporization or decomposition temperature by at least 300 K and that it can be processed previously under the form of powder particles or wires. Thermal spray techniques offer the unique capability to manufacture a large variety of coatings on components of a large variety and geometry. However, thermal spraying constitutes a special process for which the coating service properties derive mostly from the structure and indirectly from the selection of the operating parameters. Very significant improvements over the past years permitted to diagnose the in-flight particle characteristics, mostly in terms of velocity and temperature. Recently, these new capabilities have made possible the development of on-line process controls. This should participate to a drastic increase in coating reliability. In convetntional thermal spraying processes, a pulverulent feedstock (i.e., powder particles) is injected within the plasma jet via a carrier gas. This approach does not permit to process small diameter particles; i.e., nano-sized particles, which could permit to form finely grained coatings. Replacing gas by liquid to carry particles offer the unique possibility to process nano-sized particles. Cold gas spraying may appear as an alternative process to reach the same goal. Indeed, thermal spray processes experienced very significant developments over the past years, opening new doors to manufacture coatings with a high reliability and superior properties. This papepr indend at presenting some of those developments.