High temperature durability of a bond-coatless plasma-sprayed thermal barrier coating system with laser textured Ni-based single crystal substrate

Thermal barrier coating systems are usually build-up with bond coats to ensure a good adhesion of the ceramic top coat and to protect the substrate against oxidation and corrosion. Such system is often subjected to complex thermo-mechanical loading. Because of the very different damage processes encountered during service operations, a simplified system was investigated by removing the bond-coat. Recently adhesion bond strength was enhanced using laser surface texturing of the substrate in thermal spraying processes. Atmospheric plasma spray yttria-stabilized-zirconia thermal barrier coating system was deposited on the Ni-based AM1 single crystalline superalloy without bond coat. Adhesion bond strength was already increased compared to conventional processing method. Top coat durability was evaluated at high temperature and damage mechanisms were studied. Isothermal and cyclic oxidation tests showed durability of 1000 h and 400 cycles at 1100 °C. The oxidation mechanisms at the substrate/top coat interface changed due to fast solidification during the laser texturing process. Then, TBC system was studied under high temperature mechanical solicitation in tension creep. The textured interfaces were not damaged after 1% creep strain while top-coat/substrate interfacial cracking was observed for grit-blasted specimens. Moreover, no preferential crack development in the substrate was observed. Patterns provided an enhanced adhesion by changing the stress distribution near the interface

Laser surface patterning to enhance adhesion of plasma sprayed coatings

In thermal spraying, adhesive bond strength is a feature of surface properties. An adapted surface is studied with prior-surface treatments to enhance interface energy. This study deals with Ni–Al coatings on 2017 aluminum alloy substrate produced by atmospheric plasma spraying. The adherence was evaluated with several controlled surface topographies obtained by grit-blasting and laser surface texturing technique. Adherence has been tested with two different techniques: pull-off test and LASer Adhesion Test. They induce different stresses at the interface. The results showed that the adhesive strength is mostly controlled by a contact adhesion area. A large contact area increases the energy release rate at the interface during coating failures. The bond strength tendency for the two adherence tests is similar: apparent adherence is tripled thanks to laser surface patterning. Fracture propagation is stopped nearby laser-induced holes due to the complex shape and has to deviate inside the coating to maintain crack propagation (inter-splat cracks). The energy at the interfaces being stored locally due to pattern: pattern morphology, pattern localization and powder feed rate are important factors that control the adhesion strength of the thermally sprayed coatings

Laser adhesion test for thermal sprayed coatings on textured surface by laser

The laser shock adhesion test (LASAT) is a technique allowing the generation of high tensile stresses in materials. The LASAT consists in focusing a pulsed laser beam on a water-confined target. The laser pulse crosses the water transparent layer and is absorbed by the target. High energetic plasma is created at the surface of the sample. As a response to the expansion of the plasma, a shock wave is generated and propagates through the sample. This shock wave leads to the generation of high tensile stresses in the sample. These stresses allow the interface solicitation in order to evaluate the dynamic adhesive bond strength of coated systems. In order to determine interface strengths, this technique has already proven its feasibility. In this paper, the adhesion strength of coated system was evaluated using LASAT for two surface pretreatments of substrates obtained by grit-blasting and laser surface texturing techniques. The generation of the high-intensity shock wave by laser plasma in the water-confinement regime has been performed at 7.1 ns at 532 nm with the new Nd:YAG laser facility HEPHAISTOS. This paper shows that surface treatments have a great influence on the adherence results of the coated systems obtained with laser adhesion test. However, the LASAT is efficient on thin coating. In that sense, thicker industrial coatings are not adapted for the conventional LASAT anymore. Therefore, a new technique was designed to improve and extend the conventional technique. This technique consists of varying the delay Δt between two incident pulses to adjust the location of the maximum tensile stresses near the interface. Some preliminary results on the improved configuration are presented in this paper and the problematic of the laser-matter interaction with two time-delayed laser pulses which has arisen is discussed

Laser Patterning Pretreatment before Thermal Spraying: A Technique to Adapt and Control the Surface Topography to Thermomechanical Loading and Materials

Coating characteristics are highly dependent on substrate preparation and spray parameters. Hence, the surface must be adapted mechanically and physicochemically to favor coating–substrate adhesion. Conventional surface preparation methods such as grit blasting are limited by surface embrittlement and produce large plastic deformations throughout the surface, resulting in compressive stress and potential cracks. Among all such methods, laser patterning is suitable to prepare the surface of sensitive materials. No embedded grit particles can be observed, and high-quality coatings are obtained. Finally, laser surface patterning adapts the impacted surface, creating large anchoring area. Optimized surface topographies can then be elaborated according to the material as well as the application. The objective of this study is to compare the adhesive bond strength between two surface preparation methods, namely grit blasting and laser surface patterning, for two material couples used in aerospace applications: 2017 aluminum alloy and AISI 304L stainless steel coated with NiAl and YSZ, respectively. Laser patterning significantly increases adherence values for similar contact area due to mixed-mode (cohesive and adhesive) failure. The coating is locked in the pattern

Optimisation de l'adhérence de revêtements d'alumine projetés sur matériaux composites à matrice céramique (application à l'instrumentation extensométrique)

BELFORT-BU L. FEBVRE (900102102) / SudocSudocFranceF

Etude de l'association laser-projection thermique pour l'optimisation de revêtements

Afin d'améliorer la qualité des dépôts réalisés par projection thermique, diverses techniques connexes aux procédés de projection ont été développées. Parmi celles-ci, on peut noter les procédés laser qui peuvent présenter l'avantage de dispenser un traitement localisé en surface, simultané à la projection. Comparés aux techniques de préparation couramment employées, ces procédés sont également plus respectueux de l'environnement. Le sujet de thèse repose donc sur la compréhension et la maîtrise des prétraitements laser (préchauffage, ablation) afin d'améliorer les propriétés finales des revêtements réalisés et en particulier l'adhérence des dépôts sur le substrat.In order to improve coating elaborated by thermal spraying, several processes are employed. Pulsed laser irradiation is a promising approach to remove the contaminant film with a limited damage of the metallic substrate surface. It reduces some serious drawbacks of the traditional means where chemical solvents and aggressive operations are usually employed. Furthermore, this process can be used simultaneously with the spraying process. The aim of this work is to investigate laser processes on surface before the elaboration of coatings in order to improve the final coating properties, and particularly the adhesion strength between coating and substrate.BELFORT-UTBM-SEVENANS (900942101) / SudocSudocFranceF

Caractérisation et développement du procédé Protal® (le couplage d'un laser impulsionnel et d'une torche de projection thermique pour un procédé de traitement de surface éfficace et respectueux de l'environnement)

Les techniques de préparation de surface avant projection thermique par dégraissage et projetage d'abrasifs sont soumises à de nombreuses limitations. Le respect des normes environnementales et les coûts induits, la protection sanitaire des opérateurs, la modification des propriétés mécaniques des matériaux ductiles en sont les principales contraintes. Le procédé Protal® est une alternative aux techniques conventionnelles, puisqu'il assure la préparation de surface par un décapage laser de façon simultanée à la projection thermique de matière. Le développement industriel du procédé passe par la maitrîse et la compréhension du rôle des paramètres opératoires dans l'obtention d'une adhérence élevée du revêtement. L'approche suivie deans ce travail permet, tout d'abord, d'appréhender les effets produits par le décapage des substrats tant d'un point de vue topographique qu'énergétique. L'analyse des morphologies de particules écrasées précise ensuite les conditions opératoires favorables à la réalisation de revêtements métalliques (NiA1) ou céramiques (A12O3-TiO2) sur différentes natures de substrats (2017, TA6V, 2C22). Enfin, l'optimisation du protocole opératoire par une analyse factorielle permet de définir des conditions où les niveaux d'adhérence atteints sont supérieurs à ceux obtenus par la procédure de préparation conventionnelle.The Protal® process has been developed as a palliative technique to degreasing and grit-blasting prior to thermal spraying. Although widely used, both techniques suffer from disadvantages such as environmental considerations, security and mechanical aspects (e.g., decrease of fatigue properties). By implementing a Nd:YAG laser to ensure the cleaning step simultaneously to thermal spraying, these constraints can be avoided. The industrial growth of Protal® requires the control of the processing parameters to obtain high quality coatings. The understanding of the mechanisms involved in the laser cleaning of metallic surfaces is achieved on topographic and energetic points of view. Particles shape factor analysis leads to the parameter settings allowing the manufacturing of metallic (NiA1) and ceramic (A1203-TiO2) coatings on various substrates natures (2017, TA6V, 2C22). At last, higher adhesion levels than those obtained by the conventional procedure are reached by implementing fractional factorial designs of experiments for the optimization of the processing parameters.BELFORT-BU L. FEBVRE (900102102) / SudocBELFORT-UTBM-SEVENANS (900942101) / SudocSudocFranceF

Traitement de surface par texturation laser (une alternative "propre" de préparation de surface pour la projection thermique)

La préparation de surface avant projection thermique est une étape très importante pourl adhérence des revêtements. Conventionnellement, le dégraissage et le sablage sont les deuxprocédés utilisés pour ce type de préparation, mais l'impact environnemental important de cesprocédés conventionnels, ainsi que les couts désormais associés, et la modification despropriétés des matériaux ductiles ont mené au développement de nouvelles méthodes.Le procédé de texturation par laser est alors apparu comme une alternative intéressante et"propre" à la technique conventionnelle. Ce procédé permet la préparation de la surface parablation de la matière jusqu à création de microcavités de forme conique à la surface dusubstrat. Cette texturation permet alors d augmenter la surface de contact entre le matériau etle revêtement et de mieux ancrer mécaniquement le dépôt. Ce procédé permet également letraitement de la surface dans un temps très court, et surtout il n engendre aucun déchet dansl environnement.L approche suivie dans cette étude, a permis de caractériser les effets de chaque paramètreopératoire du laser à travers un protocole d optimisation par plan d expériences. La démarcheconsiste, tout d abord à apprécier le niveau de modifications morphologiques de la surface dusubstrat, ainsi que l effet thermique induit par l irradiation laser avant d'évaluer lesperformances des texturations réalisées en termes adhérence et de ténacité d'interface. Cetteapproche a pour objectif de définir les conditions opératoires qui proposent la meilleureadhérence du revêtement et a permis d'atteindre des niveaux supérieurs à ceux proposés parle procédé conventionnel.Enfin, l analyse de l impact environnemental du procédé de traitement de surface partexturation laser permet de définir le niveau de respect de l environnement, de la santé ainsique l écosystème du procédé en comparaison au procédé conventionnel.The surface preparation before thermal spraying is a very important step for coating adhesion.Conventionally, degreasing and sandblasting are the two processes used for this surfacepretreatment, but the significant environmental impact of these conventional methods, theircosts and the possible modification of the properties of ductile materials lead to thedevelopment of new methods.The laser texturing process appears as an attractive and "clean" alternative to the conventionaltechnique. The method allows the surface preparation by ablation of material to create microcavities with a conical shape at the surface of the substrate. This texturing process willincrease the surface of contact between the substrate surface and the coating and lead to abetter mechanical anchoring of the coating. This process also allows the pretreatment of thesurface in a very short time without generating any waste.The approach followed in this study aims at characterizing the effects of each operatingparameter of the laser through a optimization protocol by experimental design strategy. Theapproach consists in assessing the level of modifications of surface substrate morphology aswell as the thermal effect induced by laser irradiation before evaluating the performance oftexturing by carrying out tenacity and interface adhesion tests. This approach aims atdetermining the operating conditions that provide the best adhesion of the coating and allowto reach adherence levels higher than those proposed by the conventional methods.Finally, analysis of the environmental impacts of the laser texturing pretraitment process isused to define its effect on the environment, health and ecosystem in comparison with theconventional methods.BELFORT-UTBM-SEVENANS (900942101) / SudocSudocFranceF

Etude de l oxydation de l aluminium induite par traitement laser Nd :YAG (1064 nm, 10 ns) (conséquences sur les propriétés d adhérence d un revêtement d alumine élaboré par projection thermique)

Le procédé PROTAL® permet la combinaison du nettoyage par impulsion laser (10 ns) d une surface métallique et le dépôt instantané par projection thermique d un revêtement protecteur. Si la validation technologique est bien avancée, les états physiques et chimiques des surfaces, après décapage et avant dépôt sont encore mal compris. Lors du nettoyage laser d une surface métallique à l ambiante, une oxydation liée à l échauffement transitoire peut se manifester permettant de modifier les propriétés d adhésion de la surface métallique. L objectif de cette étude est de mettre en évidence ce phénomène pour un substrat d aluminium et d en évaluer l impact sur l adhérence d un revêtement d Al203 élaboré par projection plasma. Le couplage de plusieurs techniques d analyses de surface a permis de caractériser les mécanismes d oxydation induits par l irradiation laser. Ceux-ci ont été reliés à l amélioration de l adhérence du dépôt d alumine mis en évidence par la méthode laser-ultrasons .The PROTAL® allows combining the laser cleaning with short pulses (10 ns) of a metallic surface with a step of thermal spraying. The technology is well advanced but the chemical and physical states of the surface after laser irradiation and before coating are still misunderstood. During laser cleaning of a metallic surface in ambient atmosphere, a limiting oxidation can occur due to transient heating and modify the adhesion properties of this surface. The aim of this study was to highlight this phenomenon and to assess its influence on the adherence of Al2O3 plasma sprayed coating. The oxidation mechanisms induced by laser irradiation were characterized using several surface analysis techniques. These mechanisms were then correlated with adherence improvements of the alumina coating using the laser-ultrasonic method.DIJON-BU Sciences Economie (212312102) / SudocSudocFranceF