François Borit

Two-dimensional (2D) and three-dimensional (3D) analyses of plasma-sprayed alumina microstructures for finite-element simulation of Young’s modulus

Thermally sprayed ceramic coatings such as plasma-sprayed alumina exhibit a composite microstructure actually due to the presence of defects such as pores, inter-lamellar and intra-lamellar cracks. These second phase-typed features influence the mechanical behaviour of the coating dramatically. In this study, a microstructure simulation of plasma-sprayed alumina was developed for the optimizing of component properties such as electrical tool used in the oil industry. This approach consisted of a finite-element analysis of mechanical properties from simulated microstructures. Several composite microstructures were tested from air plasma spraying of alumina. Various degrees of porosity and cracks could be obtained from different spraying conditions. Every composite microstructure was studied using a quantitative image analysis of scanning electron microscope (SEM) cross-sections. A finite-element model based on the actual microstructure was developed. First, two-dimensional (2D) finite elements meshes were created from SEM images of microstructures. Then, in order to have a realistic representation of the three-dimensional (3D) microstructure, pictures were obtained using X-ray microtomography. Volume tetrahedral grids were generated to simulate the properties of alumina coatings. This work studies the contribution of every part of the alumina coating to the final properties and shows potentials and limitations of the 2D and 3D computational approach.

Three-Dimensional Simulation of Porosity in Plasma-Sprayed Alumina Using Microtomography and Electrochemical Impedance Spectrometry for Finite Element Modeling of Properties

Moving from a 2-dimensional to a 3-dimensional (3D) approach to microstructure and properties has been expected eagerly for a long while to result in a dramatic increase in the knowledge of thermally sprayed coatings. To meet these expectations, in the present study, microtomography and electrochemical impedance spectroscopy (EIS) were carried out to simulate the microstructure of plasma-sprayed alumina. As-sprayed and excimer laser-processed deposits were studied. Some unexpected but relevant results, e.g., regarding pore orientation in the coatings, could be obtained. EIS led to the establishment of an equivalent electrical circuit representation of the microstructure which enabled modeling of the insulating properties as a function of interfaces and pore interconnection. The pore interconnection was studied by microtomography. From this 3D simulation, a finite element analysis of Young’s modulus properties was developed and compared to experiments. Using this approach, excimer laser surface processing was shown to be an innovative process to modify insulating characteristics of plasma-sprayed alumina.

Lasers and Thermal Spray

Basically, thermal spray and laser processing can be considered as half brothers since they show many common features due to the use of a (more or less) high-energy source for both. Their combination can therefore be very fruitful and prominent to achieve coatings, which results in their most recent and advanced applications. In the materials processing development story, the laser will thus have moved from cutting to coating. This keynote presentation focuses on the recently-developed coupling of laser processing to cold spray). In this dual process, a cold spray gun is combined to a laser head in a single device, e.g. on a robot. Series of coating experiments using various laser irradiation conditions, primarily pulse frequency, were carried out for Al-based and Ni-based alloys. Laser pre-treatment of the substrate just prior to cold spray, was shown to be beneficial for adhesion of cold-sprayed coatings. Adhesion improvement was exhibited and studied from LASATesting (LASAT for “LAser Shock Adhesion Test”). Incidentally, through LASAT also, the role of lasers in the development of thermally-sprayed coatings can be considered as major. Results are discussed in the light of a TEM (Transmission Electron Microscope) study of the coating-substrate interface with and without laser pre-treatment.

Effect of substrate preparation on flattening of plasma sprayed aluminium bronze powders

Abstract Plasma spray coatings are produced by introducing powder particles of the material into a plasma plume, which melts and forwards them to the substrate. The flattening process of these individual molten droplets is one of the most critical factors as the coating quality strongly depends on the deposition of individual particles. Powders of aluminium bronze, a fine (−53 + 11 μ m) and a coarse one (−125 + 45 μm) were plasma sprayed onto stainless steel substrates (AISI 304L) under atmospheric condition with three different substrate temperatures (25, 165 and 270°C). Two different ranges of surface roughness Ra were used: mirror polished substrates with about 0·01–0·03 μm and grit blasted substrates with about 1·89–2·43 μm. A scanning electron microscopy equipped with energy dispersive spectrometry system and an electron probe microanalysis, were used in order to study the splat morphology and the chemical composition of the splats.

Microstructural Study of Copper and Copper/Alumina Composite Coatings Produced by Cold Spray Process

Cold spraying is a novel coating method, by which many composites have been successfully sprayed. The objective of this study was to thoroughly characterize the microstructure of copper as well as copper/alumina composite coatings obtained using this technique. Composite coatings which differed to the percentage of Al2O3 in the initial powder blend as well as the Al2O3 particle size were studied. Coatings presented Ra of the order of 6-8 μm, with the copper coating having the highest value. Using optical microscopy the deformation of the copper particles of the cross sections was evident. Thickness ranged from 170 to 450 μm for the different coatings. The surfaces and the cross sections of the coatings were studied by means of scanning electron microscopy. Craters and Al2O3 particles that were embedded in the copper particles were observed. The XRD analysis of the powders and the respective coatings presented no differences.

Oxidation and particle deposition modeling in plasma spraying of Ti-6Al-4V/SiC fiber composites

Plasma spraying is known to be a promising process for the manufacturing of Ti/SiC long-fiber composites. However, some improvements remain for this process to be applied in an industrial route. These include: oxygen contamination of the sprayed material through that of titanium particles before and during spraying, damage to fibers due to a high level of thermal stresses induced at the spraying stage, adequate deposition of titanium-base powder to achieve a low-porosity matrix and good impregnation of the fiber array. This article deals with work that resulted in a threefold study of the process. Oxidation was studied using electron microprobe analysis of elementary particles quenched and trapped into a closed box at various given flight distances. Oxygen diffusion phenomena within the particles are discussed from a preliminary theoretical approach coupled with experimental data. Isothermal and thermomechanical calculations were made using the ABAQUS code to determine stresses arising from contact of a liquid Ti-6Al-4V particle onto a SiC fiber. On the scale of the sprayed powder flow, a two-dimensional new type of model simulating the deposition of droplets onto a substrate was developed. This new type of model is based on a lattice-gas automaton that reproduces the hydrodynamical behavior of fluids.

Effect of the cold-sprayed aluminum coating-substrate interface morphology on bond strength for aircraft repair application

This article is dealing with the effects of surface preparation of the substrate on aluminum cold-sprayed coating bond strength. Different sets of AA2024-T3 specimens have been coated with pure Al 1050 feedstock powder, using a conventional cold spray coating technique. The sets were grit-blasted (GB) before coating. The study focuses on substrate surface topography evolution before coating and coating-substrate interface morphology after coating. To study coating adhesion by LASAT® technique for each set, specimens with and without preceding GB treatment were tested in load-controlled conditions. Then, several techniques were used to evaluate the effects of substrate surface treatment on the final coating mechanical properties. Irregularities induced by the GB treatment modify significantly the interface morphology. Results showed that particle anchoring was improved dramatically by the presence of craters. The substrate surface was characterized by numerous anchors. Numerical simulation results exhibited the increasing deformation of particle onto the grit-blasted surface. In addition, results showed a strong relationship between the coating-substrate bond strength on the deposited material and surface preparation.

Multiscale Experimental and Numerical Approach to the Powder Particle Shape Effect on Al-Al2O3 Coating Build-Up

Aluminum (Al) powders with spherical and irregular particle shapes were mixed with two alumina (Al2O3) powders with either a spherical or an angular particle shape to achieve high-performance cold-sprayed coatings onto steel. Two effects of the aluminum particle shape were observed. First, coating microstructure observation showed impinging heterogeneity depending on particle shape. Second, particle jet differences depending on particle morphology were shown by velocity maps. From the latter, SEM and XRD, three effects of the alumina particle shape were also shown, i.e., higher in-flight velocity of angular particles, fragmentation of spherical hollow particles and embedding of alumina particles with aluminum. Numerical simulation of particle impacts was developed to study the densification of Al coating due to Al2O3 addition through elucidation of Al-Al2O3 interaction behavior at the scale of the coating. Al/Al and Al/Al2O3 interfaces were investigated using TEM to understand coating strengthening effects due to alumina addition at the scale of the particle. As a whole, Al and Al2O3 particle shape effects were claimed to explain coating mechanical properties, e.g., microhardness and coating–substrate bond strength. This study resulted in specifying criteria to help cold spray users in selecting powders for their applications, to meet economic and technical requirements.

Innovative surface microcracking treatment of polymeric substrate to be coated with plasma sprayed stainless steel

Abstract Coating of organic substrates to improve properties has attracted interest because of the potential industrial applications for these materials. Thermal spraying is an attractive coating technology for this purpose owing to its high deposition rate relative to competing processes; however, actual industrial applications remain limited because of poor bonding between the polymeric substrate and the thermal sprayed deposit. Consequently, specific surface preparation of the polymer is required in most cases. In the present study, an innovative surface treatment of a polymeric material allowing subsequent buildup of a plasma sprayed coating is reported. This patented process, known as Pinpro, was successfully applied to coat an organic stereolithography substrate with an AISI 316 stainless steel thick (1 mm) deposit. The process consists in creating a population of surface microcracks in the polymer to provide mechanical anchoring for the steel deposit. Methods of surface preparation were investigated and optimised. The main surface modification mechanisms were elucidated and a phenomenological model of coating buildup on the treated substrate is proposed.