Laser Shock Adhesion Test (LASAT) of electron beam physical vapor deposited thermal barrier coatings (EB-PVD TBCs)

International audienceDamage prediction, adhesion strength and remaining lifetime of TBC are highly important data for understanding and preventing TBC spallation on blades. LAser Shock Adhesion Test (LASAT) is a powerful method to measure adhesion of coating due to its rapidity, simplicity and capabilities to distinguish different strength levels and the easy damage observation in case of TBCs. A new protocol of LASAT has been introduced in order to measure the adhesion level of the ceramic coating from the exploitation of the two-dimensional effects that promotes a shock wave pressure-dependent size of the damage. Finite element modeling, taking into account the TBCs dimensions, showed the edges effect on interfacial stress applied by laser shock

Etude par choc laser de l'adhérence de barrières thermiques aéronautiques

National audienceL'estimation de la durée de vie des barrières thermiques aéronautiques (BT) déposées sur les aubes de turbines haute pression nécessite de comprendre les mécanismes induisant l'endommagement entre la sous-couche et la zircone. Le LASAT, LAser Shock Adhesion Test ou essai d'adhérence par choc laser, est une technique exploitant tout son potentiel dans le cas des BTs, en particulier par sa rapidité, simplicité et au comportement optique de la couche de zircone. Ce dernier avantage permet de dimensionner l'endommagement généré sans réaliser de coupes métallographiques. Un nouveau protocole de l'essai LASAT a été mis en place utilisant les ondes de choc bidimensionnelles et exploitant la dépendance entre l'intensité du choc appliqué, le niveau d'adhérence du dépôt, et la dimension de la fissure générée

Behaviour of metals at ultra-high strain rate by using femtosecond laser shockwaves

The mechanical behavior of materials under extreme conditions can be investigated by using laser driven shocks. Actually, femtosecond (fs) technologies allow to reach strong pressures over a very fast duration. This work is dedicated to characterize metals behavior in this ultra-short mode, (aluminum, tantalum), leading to an extreme dynamic solicitation in the target (>107s−1). The study includes the validation of experimental results obtained on the LULI 100TW facility by comparison with numerical model. Three modeling steps are considered. First, we characterize the pressure loading resulting from the fs laser-matter interaction, different from what happens in the classical nanosecond regime. Then, the shock wave propagation is observed through the target and particularly its pressure decay, strong in this regime. The elastic-plastic influence on the shock attenuation is discussed, particularly for tantalum which has a high elastic limit. Dynamic damage appears with spallation. Experimentally, spallation is characterized by VISAR measurements and post-test observations. Shots with different thicknesses have been carried out to determine the damage properties in function of strain rate. We show in this work that a simple instantaneous rupture criterion is not sufficient to reproduce the damage induced in the sample. Only the Kanel model, which includes damage kinetics, is able to reproduce experimental data (VISAR measurements, spall thickness). A generalization of this model to any strain rate can be performed by confronting these results to other shock generators data (ns laser driven shocks, plate impacts). One remarkable result is that every Kanel parameters follows a power law with strain rate in dynamic regime (105 to 108s−1) for both aluminum and tantalum

Behaviour of metals at ultra-high strain rate by using femtosecond laser shockwaves

The mechanical behavior of materials under extreme conditions can be investigated by using laser driven shocks. Actually, femtosecond (fs) technologies allow to reach strong pressures over a very fast duration. This work is dedicated to characterize metals behavior in this ultra-short mode, (aluminum, tantalum), leading to an extreme dynamic solicitation in the target (>107s−1). The study includes the validation of experimental results obtained on the LULI 100TW facility by comparison with numerical model. Three modeling steps are considered. First, we characterize the pressure loading resulting from the fs laser-matter interaction, different from what happens in the classical nanosecond regime. Then, the shock wave propagation is observed through the target and particularly its pressure decay, strong in this regime. The elastic-plastic influence on the shock attenuation is discussed, particularly for tantalum which has a high elastic limit. Dynamic damage appears with spallation. Experimentally, spallation is characterized by VISAR measurements and post-test observations. Shots with different thicknesses have been carried out to determine the damage properties in function of strain rate. We show in this work that a simple instantaneous rupture criterion is not sufficient to reproduce the damage induced in the sample. Only the Kanel model, which includes damage kinetics, is able to reproduce experimental data (VISAR measurements, spall thickness). A generalization of this model to any strain rate can be performed by confronting these results to other shock generators data (ns laser driven shocks, plate impacts). One remarkable result is that every Kanel parameters follows a power law with strain rate in dynamic regime (105 to 108s−1) for both aluminum and tantalum

Spallation and microjetting in laser-shock-loaded aluminium and gold

Dynamic fragmentation of shock-loaded metals is an issue of considerable importance for both basic science and a variety of technological applications, such as inertial confinement fusion, which involves high energy laser irradiation of thin metallic shells. In this context, we present an experimental and numerical study of fragmentation and debris ejection in laser shock-loaded aluminium and gold, under both nanosecond and sub-picosecond laser pulses. Such fragmentation is mainly governed by two distinct processes: microjetting, that is ejection of thin jets upon shock breakout at the (rough) free surface, and spall fracture, which occurs upon tensile loading due to wave interactions inside the sample. Experimental results consist of time-resolved velocity measurements, transverse optical shadowgraphy of ejected debris, and post-shock observations of recovered targets. They are compared to numerical computations performed with two hydrocodes, and a correct overall consistency is obtained

Study of spallation by sub-picosecond laser driven shocks in metals

Spallation induced by a laser driven shock has been studied for two decades on time scales of nanosecond order. The evolution of laser technologies now provides access to sources whose pulse duration is under the picosecond, corresponding to characteristic times of numerous microscopic phenomena. In this ultra-short irradiation regime, spallation experiments have been performed with time-resolved measurements of the free surface. In this solicitation type, damage occurs at small scale, leading to micrometric spalls. The VISAR measurements have been complemented with post-test observations and microtomography and compared with numerical simulations to check the models consistency of the laser-matter interaction, shock wave propagation and the dynamic damage criteria ability to reproduce spallation at this ultra-short time scale, inducing strong tensile stress states at very high strain rates

LAser Shock Adhesion Test (LASAT) of Electron Beam Physical Vapor Deposited Thermal Barrier Coatings (EB-PVD TBCs)

Abstract. Damage prediction, adhesion strength and remaining lifetime of TBC are highly important data for understanding and preventing TBC spallation on blades. LAser Shock Adhesion Test (LASAT) is a powerful method to measure adhesion of coating due to its rapidity, simplicity and capabilities to distinguish different strength levels and the easy damage observation in case of TBCs. A new protocol of LASAT has been introduced in order to measure the adhesion level of the ceramic coating from the exploitation of the two-dimensional effects that promotes a shock wave pressure-dependent size of the damage. Finite element modeling, taking into account the TBCs dimensions, showed the edges effect on interfacial stress applied by laser shock

Study of spallation by sub-picosecond laser driven shocks in metals

Spallation induced by a laser driven shock has been studied for two decades on time scales of nanosecond order. The evolution of laser technologies now provides access to sources whose pulse duration is under the picosecond, corresponding to characteristic times of numerous microscopic phenomena. In this ultra-short irradiation regime, spallation experiments have been performed with time-resolved measurements of the free surface. In this solicitation type, damage occurs at small scale, leading to micrometric spalls. The VISAR measurements have been complemented with post-test observations and microtomography and compared with numerical simulations to check the models consistency of the laser-matter interaction, shock wave propagation and the dynamic damage criteria ability to reproduce spallation at this ultra-short time scale, inducing strong tensile stress states at very high strain rates

Bond strength determination of hydroxyapatite coatings on Ti-6Al-4V substrates using the LAser Shock Adhesion Test (LASAT)

International audienceAn adhesion test procedure applied to plasma-sprayed hydroxyapatite (HA) coatings to measure the “LASAT threshold” (LAser Shock Adhesion test) is described. The good repeatability and minimal discrepancy of the laser-driven adhesion test data were ascertained for conventional plasma sprayed HA coatings. As a further demonstration, the procedure was applied to HA coatings with diverse characteristics on the ceramic/metal interface. Different preheating and grit blasting conditions and the presence of a thick plasma-sprayed Ti sublayer or a thin TiO2 layer prepared by oxidation were investigated through LASAT. It was assessed that a rough surface can significantly improve the coating's bond strength. However, it was also demonstrated that a thin TiO2 layer on a smooth Ti-6Al-4V substrate can have a major influence on adhesion as well. Preheating up to 270°C just prior to the first HA spraying pass had no effect on the adhesion strength. Further development of the procedure was done to achieve an in situ LASAT with in vitro conditions applied on HA coatings. To that end, different crystalline HA contents were soaked in simulated body fluid (SBF). Beyond the demonstration of the capability of this laser-driven adhesion test devoted to HA coatings in dry or liquid environment, the present study provided empirical information on pertinent processing characteristics that could strengthen or weaken the HA/Ti-6Al-4V bond

Comparison between experiment and molecular dynamic simulation of spallation by ultra-short laser shock driven on micrometric Tantalum targets

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