The study of both the interface strength and residual stresses within a plasma sprayed ceramic coating is of great interest which main purpose is a better understanding of the mechanical properties of metal/ceramic systems. In this work, experiments involving a LASAT facility (LASAT: Laser Shock adhesion test) were implemented in order to analyse the adhesion and the damaging behaviour (debonding and buckling) of alumina coatings onto Co-based alloy. Similar alumina coatings were deposited using same plasma parameters with various surface preparations: smooth or severe grit blasting, with and without pre-oxidation. The non-destructive analyses (Optical and IR imaging) of the buckled region after LASAT have allowed to compare and discuss the interface strength of the studied coated samples. Further discussion was carried out by analysing the blister, resulting from the release of residual stresses within the coating after LASAT. It was thus evidenced that the residual stress state is a key parameter on resulting adhesive properties. This explorating work suggests using the LASAT method to analyse the adhesion and residual stresses within thermal sprayings.

Cold spray is continuously expanding for the repair of parts made of aluminum-based alloys. Beyond repair applications, the process is now expected to be exploited efficiently for the additive manufacturing of shaped parts. However, up to now, cold spray is limited to the achievement of rather simple shapes due to a lack of basic knowledge on coating build-up mechanisms to result in dimension-controlled deposition. The objective of this work is to fill that gap through an experimental and modeling study of the coating build-up in cold spray for this specific application. Experimentally, Al-based coatings were deposited for a large range of particle velocity due to the use of low-pressure, medium-pressure and high-pressure cold spray facilities. Particle velocity was monitored as a function of cold spray conditions. Two different types of Al 2024 (Aluminium 2024 Alloy) powders were tested. Coating porosity and microhardness were studied as a function of (both morphological and metallurgical) powder characteristics and spray conditions, primarily in the light of particle velocity. Various correlations could be exhibited. Finite element (FE) simulations of particle impacts were developed, including particle velocity from experimental measurements. These will be used as inputs in an in-house morphological model, the first stages of which could be established successfully.

In this work, two agglomerated hydroxyapatite (HA) powders, with and without heat treatment, were cold sprayed using various spraying parameters on metallic (Ti-6Al-4V) and polymeric (PVA) substrates. The structure of the agglomerated powders and corresponding features of the coatings were examined. For both types of substrates, it was shown that submicron HA powders produce homogenous layers with submicron HA grains. In the case of non-heat treated particles, thick layers could be obtained due to the binding action of residual by-products. HA layers were also found to be adherent after immersion in water, which could potentially lead to the fabrication of ceramic coated hydrogels.

In this work, copper and PEEK powder mixtures are cold sprayed onto carbon fiber-reinforced polymer (CFRP) substrates with the aim of producing a well-adhered conductive layer. The composite coatings were optimized through the study of the deposited mass and its dependence on process parameters and Cu powder morphology. A morphological model based on Cu phase data was developed to better understand coating microstructures. Coatings synthesized from irregular Cu particles were found to be electrically conductive, while those containing spherical Cu particles were insulating. These phenomena are explained using the developed simulation tools coupled with the investigation of coating build-up and microstructure.

This study assesses the influence of powder morphology on the microstructure and bond strength of cold-sprayed aluminum. Aluminum powders with spherical and irregular particle shapes were deposited on shot-peened steel. The feedstocks were mixed with alumina powders, either spherical or angular in shape, to improve coating properties. Coating samples and powder mixtures were examined by means of SEM and XRD analysis and pull-off tests were conducted to evaluate coating adhesion. It was found that alumina addition reduces porosity and increases hardness and that aluminum-alumina mixtures with the same particle shape are more suitable for producing dense coatings with high bond strength.

This study deals with the influence of spray angle on the deposition of cold-sprayed Al particles. Spray trails were conducted in parallel with finite element simulations of particle deformation and coating build-up as a function of spray angle, powder size, substrate roughness, and surface configuration. Coating cross-sections and splats were examined by SEM; bonding strength and particle adhesion were determined via laser shock adhesion testing. Experimental as well as modeling results show that splats deposited at spray angles less than 60° are highly deformed and poorly adhered. Based on the findings, several conclusions are drawn with regard to the potential use of cold spraying for the repair of aircraft components.

Numerous automotive and aircraft/aerospace applications involve metallic coating of organic matrix composite materials, e.g. for aesthetic, electric, or engineering functions. In the thermal spray process family, cold spray is very attractive for the achievement of metallization of low-temperature resistant materials such as organic composites, due to its “cold” characteristic. However, despite the current (and justified) craze for cold spray, little is still known about the potential of this process for this type of application. The work demonstrated the feasibility of cold spray for satisfactory metallization of PA66-matrix composites with Al. This paves the way for using cold spray as an advantageous substitute process for the industrial protection of polymer-based composites.

In this work, the microstructure of silver coatings obtained by cold spraying with a fine and a rather coarse powder were investigated. Cold-sprayed microstructures strongly depend on the deposition process parameters. These parameters have an influence on porosity, plastic strain, particle-to-particle bonding mechanisms. The originality of this study rests on the use of X-ray microtomography, which is shown to be a very powerful technique to investigate into cold-sprayed coatings. Several samples were machined from coatings by electro-discharge machining (EDM) then scanned using microtomography. Porosity fraction was determined by three-dimensional image analysis and compared to classical two-dimensional analysis of micrographs. A difference between the two methods was exhibited, which showed sample preparation effects. Consequently, X-ray microtomography seems to be well adapted to study cold-sprayed coatings thoroughly. Another result from this work was the characterization of the morphology of pores and deformed particles. Bonding mechanisms in cold spray could therefore be discussed. In addition, the knowledge on deformed particles allowed to simulate cold-sprayed coatings with build-up models. This will be done in a further work

Cold gas dynamic spraying, namely cold spray, is an innovative coating process in which powder particles are injected in a supersonic gas flow to be accelerated above a certain critical velocity. Even though particles adhesion onto the substrate has not be yet elucidated, it appears clearly that it is influenced by particle impact velocity, which results from spraying conditions, diameter of particles and their positions from the center of the particle jet. Particle velocity can change dramatically depending on particle position from the core to the rim of the jet. In the present work, an original experimental set-up was designed to discriminate the particles as a function of the levels of velocity to investigate the influence of this parameter on adhesion. Particles at given positions in the jet could therefore be observed using SEM (Scanning Electron Microscopy), which showed different morphologies and microstructures as a function of impact velocity. High pressure and tangential velocity at the interface during impact were calculated from numerical simulations using ABAQUS. TEM (Transmission Electron Microscopy) analyses of thin foils were carried out to investigate into resulting local interface phenomena. These were correlated to particle impact velocity and corresponding adhesion strength which was obtained from LASAT testing (LAser Shock Adhesion Test).

During the deposition of metallic cold sprayed coatings, it could be observed that only a thin layer is formed on the substrate and further building-up of a thick coating is not enabled. As for other thermal spray techniques, the formation of cold sprayed coatings can be divided to two stages: the creation of the first layer onto the substrate and the building-up of the coating itself onto as-sprayed layers. This two-stage build-up process was evidenced according the study of two Ti-6Al-4V powders exhibiting different characteristics (particle size, morphology, oxygen content, hardness, etc) which were sprayed by cold gas dynamic spraying onto substrates of different nature with various hardnesses (Ti-6Al-4V, AISI 304L, Al-alloy 2017). The phenomenology of the two-stage process is investigated in the present study. Cold spray conditions with pure nitrogen or pure helium as processes gas were applied to achieve a significant difference for particle velocities. The first stage of the process was completed by both powders with the formation of a first coating layer onto the various substrates. However, very different features for particle-substrate interactions (penetration depth and comparative deformation) were observed. For the particle-particle interaction (the second stage of the process), despite similar spraying conditions for both powders, the results were completely different since the formation of thick coating was achieved only with one of the powders. It was found that the intrinsic ductility of the material powder is the main parameter to promote the successful completion of both stages in order to achieve thick coatings.

Moving from a 2-dimensional to a 3-dimensional 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 processes and coatings. To meet these expectations, in the present work, 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 simulation led to the establishing of an electrical circuit equivalent to the microstructure which simulated the insulating properties as a function of interfaces and pore interconnection. The latter was studied by microtomography. From this 3-dimensional simulation, a finite element analysis of mechanical properties was developed and compared to experimental measurements. Using this approach to microstructure and properties, excimer laser surface processing was shown to be an innovative process to modify insulating characteristics of plasma-sprayed alumina.

The demand of industry for metallic thermal sprayed coatings with controlled porosity until now is fulfilled by the spraying of metallic powders mixed with additives (organic element in many cases) which play the role of pores. The new technology of cold spray can lead to the formation of innovate coatings of controlled porosity by using pure metallic (or alloy) powder without any further addition. A fine Al-12Si powder (<45 µm) was sprayed with a cold spray system (CGT Kinetic 3000-M) on stainless steel substrate under different spraying conditions. In the present study, the new polymeric nozzle PBI-33 of CGT was used for the formation of al-based coatings. The microstructure, the porosity, the Vickers microhardness and the superficial Rockwell hardness (R15Y) of the produced coatings are examined.

A good adhesion of plasma sprayed hydroxyapatite (HA) coating on Ti-based alloy is crucial for ensuring highly-reliable non cemented implants in the biomedical industry. In the present work, the laser shock adhesion test, namely LASAT, has been applied to investigate the interface strength of plasma sprayed HA coatings. This contact less method allowed a rapid assessment of the HA coating adhesion on simple coated plates. Varying the laser energy to impact the substrate and to generate the interface decohesion, a LASAT adhesion threshold can be determined for the highest laser fluence (J/m²) for which no debonding of the coating occurred. This qualitative and discerning LASAT procedure has been carried out on HA coatings to investigate the role of various interfaces on the adhesive property of the HA/Ti bond. According to the LASAT analysis, a surface roughness prepared with medium or coarse grit-blasting did not influence drastically the adhesion threshold while smooth pre-oxidized specimens LASAT threshold were near to those obtained with a Ti bond-coat. These thresholds also corresponded with the highest adhesion measured in this study. In addition, pre-heating treatment of substrates just prior to spraying up to 270°C did not exhibit a significant difference with grit-blasted HA/Ti interface. Further investigations (SEM, XRD) was also achieved to investigate the interface characteristics before and after the laser treatment. Sample cross-sections of laser shocked specimens were examined in detail, right at the impact location and within the debonding area to assess the fracture feature. This complementary materials analysis permitted to establish the relevance of the LASAT test as a fast and easy-to-use method devoted to the design or the control of highly adhesive HA coatings. Preliminary experiments to apply the LASAT method in liquid environment is described. Further work is on progress to implement an in situ adhesion testing of HA coating in simulated body fluid.

It is well known that residual stresses in plasma-sprayed coatings play a prominent role on coating-substrate adhesion in particular. This is all the more prominent because the coating is thick and adhesion intricate. The control and measurements of residual stresses in plasma-sprayed coatings onto organic-based substrates result in an issue of very high concern, even though very little (not to say nil) was published in this specific area. In this work, thick coating of a polyurethane resin with Inconel 625 was achieved by plasma spraying coupled with Atmosphere and Temperature Control (ATC), i.e. using cryogenic cooling to limit thermal degradation of the substrate when spraying. Residual stresses were determined by X-ray diffraction at the coating surface due to low X-ray penetration in nickel. In addition, residual stress in-depth profiles were obtained using the incremental hole drilling method. The investigation mainly focused on the measurement of residual stresses. In this paper, residual stresses were studied as a function of plasma spraying conditions. Results are discussed in the light of the coating microstructure. Residual stress measurements resulted in optimizing and controlling coating deformation and adhesion which are crucial for applications.

This work assesses the properties of vacuum plasma sprayed YSZ coatings for potential use in solid oxide fuel cells. The results of the investigation show that low-porosity layers of yttria-stabilized zirconia can be produced by using a fine powder and by adjusting plasma gas composition. Under optimized spraying conditions, YSZ layers with a thickness of 10-20 μm, a porosity less than 1%, and an average roughness of 1 μm are achievable. Paper includes a German-language abstract.

The adhesion of copper on aluminum depends on the presence of intermetallic phases. Such phases can form during spraying at the interface between the layer and substrate. This paper deals with the formation mechanism of the intermetallic phases and their influence on adhesion. The type, size, and distribution of the intermetallic phases are investigated as a function of spray parameters and bonding strength is determined by laser shock adhesion testing. Paper includes a German-language abstract.

Monocrystalline lithium fluoride (LiF) windows are used in mineral physics research. Optical Pyrometric Measurements (OPM) during shock tests are done through LiF windows on which a few mm thick plates of tested materials are fixed. Current OPM requires the plates to be glued on the LiF windows. However, this glue is an undetermined source of error for OPM. This study deals with plasma spraying to deposit a tin coating directly on to the polished LiF windows as a substitute to the plate/glue/LiF system. Although seldom used for low melting point materials, plasma spraying was shown to be the appropriate technique for this original material couple and application especially for its flexibility. The study confirmed the feasibility of plasma spraying of tin on polished surfaces of LiF windows. Scanning electron microscopy (SEM), electron probe micro-analysis (EPMA), hydrostatic weigh measurements (HWM), optical and confocal microscopy were used to characterize the coating and substrate.

Thermal plasma spray processes with their various operating parameters can be considered as flexible technique to carry out appropriate ceramics coatings. This work deals with plasma spraying of several ceramics powders (hydroxyapatite (HA), Al 2 O 3 -TiO 2 , Al 2 O 3 , ZrO 2 -Y 2 O 3 (YSZ) and Cr 2 O 3 ) with suitable parameters using a CAPS system ("Controlled Atmosphere Plasma Spraying"). The HPPS (High Pressure Plasma Spraying), APS (Air Plasma Spraying) and IPS (Inert Plasma Spraying) modes were applied in order to obtain the suitable microstructure. The microstructures and phase compositions allowed to establish that surrounding high-pressure in the CAPS chamber is leading to a good heating of the powder and a good quality for the coatings.

In the present work thin Al 2 O 3 coating was obtained mainly by low pressure plasma spray (LPPS). Low-porosity and low-roughness deposits resulted from the optimized spray conditions, i.e. plasma parameters, grit blasting, powder feed rate and specimen rotation speed. Results showed that LPPS processing was highly beneficial for densifying the ceramic coatings, especially when coupled to a moderate powder feed rate. Coating average surface roughness (Ra) ranged from 1.5 to 2.5 µm for a coating thickness of less than 30µm and an original substrate Ra of 1.1µm. The spray conditions were optimized particularly for a low feed rate and a high specimen rotating speed to lower surface roughness. Moreover, a specific atmosphere/temperature control device was developed (using local gas injection close to the specimen to be coated). This resulted in improving cooling efficiency, which reduced microcracking in the deposits. Mechanical pull-off adhesion test was also carried out to evaluate these low-roughness thin coatings. Adhesion was shown to be satisfactory for direct coating (i.e. without any bond coat) of a low-roughness (Ra=1.1µm) AISI 316L substrate.

The use of plasma spraying can be very beneficial in a rapid prototyping/manufacturing route for producing industrial metallic parts, e.g. tools and molds. Compared to the most recent and advanced laser-based or flame/arc spray-based methods, plasma spray basically shows a higher efficiency and process control in addition to a wider range of coating materials and better coating properties. This work deals with plasma spraying of AISI 316 stainless steel onto conventional MVA 200 acrylate resin as the second stage of a rapid prototyping route involving stereolithography plus plasma spray. This resulted in a 3 key issues : - Achieving a good coating/substrate adhesion, notwithstanding the preventing from previous grit blasting of the resin and poor physico-chemical bonding between the organic substrate and stainless steel ; - Good building-up of the sprayed particles up to a rather high thickness (i.e. of a few mm) through adequate plasma spray conditions, which limits the residual stress level; - Coating conforming closely to the substrate geometry, which precludes from surface damaging under spraying. The work showed that the process can meet all the previously-mentioned requirements combined with the achievement of high-quality coatings (i.e. with a low porosity in particular). A major part consisted in optimizing the plasma spray conditions using a CAPSATC ("Controlled Atmosphere/Temperature Plasma Spraying") unit. This included the development of an original thermal pre-treatment of the substrate (patented), namely "PINPRO", to promote coating/substrate adhesion. SEM, EDS, EPMA, FTIR, QIA (Quantitative Image Analysis), ... were particularly employed to study microstructures and interfaces. Phenomenological approaches to the involved adhesion mechanisms and coating build-up are discussed. For the latter, the first steps in the application of Lattice-Gas Modeling (LGM) of stainless steel layer build-up were made. LGM is a new and powerful simulation tool for the spray operator or user.