The tribological properties of cold sprayed Ni-WC metal matrix composite (MMC) coatings were investigated under dry sliding conditions from room temperature (RT) up to 400°C, and during thermal cycling to explore their temperature adaptive friction and wear behavior. Characterization of worn surfaces was conducted using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and Raman spectroscopy to determine the chemical and microstructural evolution during friction testing. Data provided insights into tribo-oxide formation mechanisms controlling friction and wear. It was determined that the steady-state coefficient of friction (CoF) decreased from 0.41 at RT to 0.32 at 400˚C, while the wear rate increased from 0.5×10-4 mm3/N·m at RT to 3.7×10-4 mm3/N·m at 400˚C. The friction reduction is attributed primarily to the tribochemical formation of lubricious NiO on both the wear track and transfer film adhered to the counterface. The increase in wear is attributed to a combination of thermal softening of the coating and a change in the wear mechanism from adhesive to more abrasive. In addition, the coating exhibited low friction behavior during thermal cycling by restoring the lubricious NiO phase inside the wear track at high temperature intervals. Therefore, cold sprayed Ni-WC coatings are potential candidates for elevated temperature and thermally self-adaptive sliding wear applications.

Military aircraft that require high maneuverability, durability, ballistic protection, reparability, and energy efficiency require structural alloys with low density, high toughness, and high strength. Also, repairs to these aircraft demand a production process that has the flexibility to be relatively in-situ with the same high-performance output. Materials produced by the cold spray process, a thermo-mechanical powder consolidation technique, meet many of the requirements. In accordance with President Obama’s 2011 Materials Genome Initiative, the focus of this effort is to design customized aluminum alloy powders which exploit the unique behavior and properties of the materials created by the cold spray process. Analytical and computational models are used to customize microchemistry, thermal conditioning, and solidification behavior of the powders by predicting equilibrium and non-equilibrium microstructure and resulting materials properties and performance. Thermodynamic, kinetic, and solidification models are used, including commercial software packages Thermo-Calc, Pandat™, and JMatPro®, and TC-PRISMA. Predicted powder properties can be used as input into a cold spray process impact model to determine the consolidated materials’ properties. Mechanical properties of powder particles are predicted as a function of powder particle diameter and are compared to experimental results.

Lors de la projection dynamique par gaz froid, nommée " cold spray ", les particules sont accélérées aérodynamiquement et maintenues à l'état solide jusqu'au moment de l'impact sur le substrat. L'empilement de ces particules constitue le revêtement dont l'adhérence représente un paramètre déterminant quant à la qualité des pièces industrielles. L'enjeu de cette étude est la compréhension et la quantification de l'adhérence interfaciale par l'analyse de la phase d'accrochage des premières particules dispersées à la surface du substrat, en fonction des conditions d'impact. Vitesse d'impact, température de préchauffage et oxydation des surfaces mises en contact ont, en effet, une influence directe sur les phénomènes survenant à l'interface donc sur l'adhérence. Cette étude propose une méthode permettant de relier les conditions de projection aux conditions d'impact, aux mécanismes survenant entre la particule et le substrat durant l'impact et, enfin, au niveau d'adhérence des particules correspondantes. Pour cela, les conditions d'impact ont été déterminées par l'utilisation conjointe de techniques expérimentales et de modélisations numériques. Les températures locales et les pressions de contact alors induites à l'interface durant l'impact ont été calculées par simulation numérique. Des analyses fines par transmission ont été réalisées afin d'étudier les phénomènes survenant le long de l'interface, en fonction des conditions d'impact. Enfin, les niveaux d'adhérence correspondants ont été obtenus par une utilisation originale de l'essai LASAT (LAser Shock Adhesion Test). Les phénomènes responsables de l'adhésion ont ainsi pu être mis en évidence et leur influence quantifiée.

La projection à froid est une technologie en plein essor pour le dépôt de matériaux à l'état solide. Le procédé de dépôt des particules par pulvérisation à froid est simulé par la modélisation de l'impact à haute vitesse de particules sphériques sur un substrat plat dans diverses conditions. Pour la première fois, nous proposons une approche numérique par couplage Euler-Lagrange (CEL) afin de résoudre ce problème à haute vitesse de déformation. Les capacités de l'approche numérique CEL pour la modélisation du processus de dépôt de projection à froid sont évaluées par une étude paramétrique de : la vitesse d'impact, la température initiale des particules, le coefficient de frottement et le choix des matériaux. Les résultats de la simulation à l'aide de l'approche numérique CEL sont en accord avec les résultats expérimentaux publiés dans la littérature. La méthode CEL est généralement plus précise et plus robuste dans des régimes de déformations élevées. Un nouveau modèle d'empilement de type CFC, inspiré de la structure cristalline, est construit afin d'étudier le taux de porosité des particules déposées et les contraintes résiduelles dans le matériau de substrat pour diverses conditions. Nous pouvons observer non seulement la géométrie 3D de porosités, mais aussi leur répartition et leur évolution pendant les impacts successifs. Pour les particules, une vitesse d'impact et une température initiale élevées, sont des avantages pour produire des revêtements denses par projection à froid. Des contraintes résiduelles de compression existent à l'interface entre les particules et le substrat. Ces dernières sont causées par les grandes amplitudes et vitesses de déformation plastique induites par le procédé. Un second modèle moins complexe pour la modélisation de l'impact multiple oblique a été créé afin de simuler l'érosion de surface. Une forte érosion de surface est le résultat : d'une plus grande vitesse d'impact, d'un coefficient de frottement élevé et d'un angle de contact réduite. Pour un matériau ductile comme le cuivre, il y a deux modes de rupture : le mode 1 de traction et le mode 2 de rupture par cisaillement. Le premier survient principalement en dessous de la surface du substrat et à la périphérie de impacts, tandis que le second intervient de manière prédominante à la surface des impacts. On observe quatre étapes lors de la propagation des fissures : la formation de porosités, de fissures, la croissance de ces dernières, puis une dernière étape de coalescence et rupture. Un critère simple, où la vitesse d'érosion est fonction de l'angle de contact et de la vitesse critique d'érosion lors d'un impact de vitesse normale , est proposé sur la base des résultats des simulations afin de prédire l'initiation de l'endommagement. La déformation plastique équivalente est également un paramètre clef pour identifier l'initiation de l'endommagement, une valeur critique de 1,042 a été trouvée dans notre étude pour le cuivre. / Cold spray is a rapidly developing coating technology for depositing materials in the solid state. The cold spray particle deposition process was simulated by modeling the high velocity impacts of spherical particles onto a flat substrate under various conditions. We, for the first time, proposed the Couple Eulerian Lagrangian (CEL) numerical approach to solve the high strain rate deformation problem. The capability of the CEL numerical approach in modeling the Cold Spray deposition process was verified through a systematic parameter study, including impact velocity, initial particle temperature, friction coefficient and materials combination. The simulation results by using the CEL numerical approach agree with the experimental results published in the literature. Comparing with other numerical approaches, which are Lagrangian, ALE and SPH, the CEL analyses are generally more accurate and more robust in higher deformation regimes. Besides simulating the single particle impact problem, we also extended our study into the simulation of multiple impacts. A FCC-like particles arrangement model that inspired by the crystal structure was built to investigate the porosity rate and residual stress of deposited particles under various conditions. We observed not only the 3D profiles of voids, but also their distributions and developments during different procedures. Higher impact velocity and higher initial temperature of particles are both of benefit to produce a denser cold spray coating. The compressive residual stresses existed in the interface between the particle and substrate is mainly caused by the large and fast plastic deformation. Another simplified model for multiple impacts was created for the simulation of surface erosion. A severe surface erosion is the result of a high impact velocity, a high friction coefficient and a low contact angle. Two element failure models suitable for high-strain-rate dynamic problems were introduced in this study. For a ductile material as Copper, it followed two fracture modes in our study, which are tensile failure mode and shear failure mode. The former one mainly occurred beneath the substrate surface and the periphery of substrate craters, nevertheless the latter one was found predominately at the surface of craters. Four steps were found during the propagation of crack: void formation; crack formation; crack growth; coalescence and failure. A simple criterion equation was derived based on the simulation results for predicting the initiation of damage, which the erosion velocity v_{ero} is a function of contact angle and erosion velocity for normal impact v_{pi/2}. The equivalent plastic strain could also be a parameter for identifying the onset of damage, identified as being 1.042 for Copper in our study.

Cold spray is a novel coating method. Due to its low-temperature character, it has a potential to replace the high-temperature thermal spray processes in some applications. The presented work analyzes the microstructure and the phase composition of pure metal coatings deposited by cold spraying and evaluates selected mechanical properties. Specimens prepared by readily used HVOF industrial technology were used for comparison purposes.

The impact phenomena of high velocity micron-size particles, although commonly considered and described as detrimental in numerous engineering applications, can be used in a beneficial way if properly understood and controlled. The Cold Gas Dynamic Spray (CGDS) process, known as a surface modification, repair and additive manufacturing process, relies on such high velocity impacts. In the process, solid particles are accelerated by a supersonic gas flow to velocities up to 1200 m/s and are simultaneously heated to temperatures lower than their melting point. When propelled under proper velocity and temperature, the particles can bond onto a target surface. This bonding is caused by the resulting interfacial deformation processes occurring at the contact interface. Hence, the process relies heavily on the gas/particle and particle/substrate interactions. Although numerous experimental and/or numerical studies have been performed to describe the phenomena occurring during particle flight and impact in the CGDS process, numerous phenomena remain poorly understood. First, the effect of substrate surface topographical condition on the particle deformation and ability to successfully adhere, i.e. atomically and/or mechanically, has not been thoroughly investigated such that its influence is not well understood. Another aspect of the process that is generating the largest gap between experimental and numerical studies in the field is the lack of particle in-flight temperature measurements. Obtaining such data has proven to be technically difficult. The challenges stem from the short particle flight time, low particle temperature and small particle size preventing the use of established thermal spray pyrometry equipment. Relatedly, lack of such measurements precludes a proper experimental study of the impact related phenomena at the particle/substrate interface. As a result, the effect of particle size dependent temperature on overall coating properties and atomic bonding relies currently on estimates. Finally, the effect of particle impact characteristics on interfacial phenomena, i.e. grain size and geometry, velocity/temperature, and oxide scale thickness, on adhesion and deformation upon single particle collision has also been scarcely studied for soft particle depositions on hard substrate. Hence, the current research work aims at studying fundamental aspects of particle/gas heat transfer and particle/substrate impact features in goals to improve the understanding of the CGDS process. Different surface preparation methods will be used to create various surface roughness and topographical features, to provide a clear understanding of the target surface state influence on coating formation and adhesion. Additionally, new equipment relying on novel technology, i.e. high-speed IR camera, will be utilized to obtain particle in-flight temperature readings with sequence recordings. Subsequently, the experimental particle in-flight temperature readings will be used to develop a computational fluid dynamics model in goals to validate currently used Nusselt number correlations and heat transfer equations. The particle size-dependent temperature effect on the particle’s elastic and plastic response to its impact with a targeted surface and its ability to successfully bond and form a coating will be studied experimentally. A thorough CFD numerical work, based on experimental findings, will be included to provide full impact characteristics (velocity, temperature, size and trajectory) of successfully deposited particles. Finally, the numerical results will be utilized in the ensuing study to correlate single particle deformation, adhesion and interfacial features to impact characteristics. A finite element model will be included to investigate the effect of particle size dependent temperature on single particle interfacial pressure, temperature and bonding ability.

This work deals with issues of preparation of intermetallics based on iron, nickel and titanium aluminides. It works with an idea of preparation of bulk material by reaction synthe-sis from kinetic spraying deposits by cold spray. Theoretical part is concerned with phases and compounds of these aluminides for structural applications, their characteristics and present fabrication. In experimental part there are studied microstructures created by annealing of deposits.

Titanium powder was deposited into low-carbon steel specimens using three thermal spray technologies: plasma spray, cold spray (USA, Singapore) and warm spray (denoted as PS, CSU, CS-S, and WS, respectively, in further text). The aim was to determine the influence of the coatings on the fatigue lives of the specimens. The experimental work was carried out in a symmetrical cantilever-beam bending setup using a computer-controlled SF-Test loading device. Further to that, the micro-morphology of coatings structure, their respective porosity content and the fracture surfaces of both coatings and substrate materials was carried out.

Properties of samples and their coatings may be affected by the electron beam. This paper deals with the analysis of the microstructure, phase and chemical composition and the determination of mechanical characteristics of inconel steel substrate and CoNiCrAlY coatings deposited via different types of thermal spraying (HVOF, cold spray), in combination with modifications by the electron beam technology. During the study it was found that the deposition did not change the chemical composition. Further it was found that the interaction of the electron beam with the material did not change the chemical composition, but there is a change in the structure and a reduction of porosity and surface roughness, resulting in a change of mechanical properties such as decreasing hardness or increase of the modulus of elasticity.

Cold spray has seen rapid advancement since its inception and has shown significant potential as a method of additive manufacturing. However, the large plastic deformation and repeated heating/cooling cycles that the material undergoes during the cold spray process can result in gradients in material structure and large residual stresses. The purpose of this study is to extend the existing EMMI material model to include anisotropic material response through the use of orientation distribution functions to predict residual stresses and anisotropy resulting from cold spray and similar additive manufacturing processes. Through the use of a finite element simulation, yield surfaces for a two-step tension problem were generated and analyzed to capture the effects of the four coaxiality parameters that govern the model.