Laser Shock Adhesion Test Research Articles

Driving forces in thermal barrier coatings blistering

Thermal barrier coating (TBC) systems are known to be extremely sensitive to the applied thermo-mechanical loading in terms of both microstructure evolution and of subsequent damage, leading to final failure by ceramic top-coat layer spallation. The analysis of the relationship between mechanical behavior and final failure is limited by the scatter in the experimental results. Based on a typical TBC system for columnar top-coat, this study focuses on blister evolution for pure thermal cycling. The blister is processed by LAser Shock Adhesion Test (LASAT) method. It is observed experimentally that prior to significant further interfacial debonding, the height of the blister increases with the number of applied cycles. To clarify this, a finite element analysis was developed to account for multilayer system behavior and growth strain associated to a thermal-grown oxide layer evolution. Finally, this study demonstrates that rough interfaces, elasto-viscoplastic behavior of the metallic bond-coat, and oxide growth strain are needed to model the experimentally observed evolution of the blister. Thus, the driving forces acting on bond-coat rumpling, are similar to driving forces for blister height increase and final failure of the TBC.

On the cyclic fatigue of adhesively bonded aluminium: Experiments and molecular dynamics simulation

Direct bonding of metals with resin plays a critical role in the jointing of dissimilar materials. The adhesion strength is known to be dependent on strain (loading) rate due to the strain rate sensitivity of polymeric resin. This study evaluated the adhesion durability of an epoxy resin adhesive on an aluminum alloy under cyclic impact loading. First, we used a repetitive laser shock adhesion test (LaSAT), which enabled us to evaluate the impact strength of interfacial fractures (i.e., adhesion and its durability). In this method, a pulsed YAG laser was used to generate strong elastic waves, which resulted in the interfacial fracture of the adhesive/aluminum alloy. We prepared two types of specimens with different curing temperatures (20 °C and 100 °C) and found that the specimen with the higher curing temperature showed higher adhesion strength and durability. We revealed that the adhesion strength showed cyclic fatigue characteristics, and that higher curing temperature improved fatigue strength. To elucidate this mechanism on the molecular level, we conducted a molecular dynamics (MD) simulation for the epoxy resin adhesive on aluminum alloy. The model featured an all-atom model of the Al2O3/epoxy resin interface, and we performed cyclic tensile deformation until delamination. We found that the number of loading cycles until delamination increased when the applied tensile stress was lower. We also found that the 400 K curing model had greater adhesion strength than the 300 K curing model. This tendency was confirmed by the results of the LaSAT experiments. Our comprehensive study incorporating LaSAT experiments and MD simulations evaluated adhesion strength and revealed its fracture mechanism.

Adhesion Strength of Al/Epoxy Resin Interface over a Wide Range of Loading Rates

This study evaluated the interfacial adhesive strength between aluminium alloy and epoxy resin (Al/epoxy resin) over a wide range of strain rates (loading rates). We conducted three types of tests with different loading rate, i.e., a quasi-static tensile test for the range of lower loading rate, a Split Hopkinson Bar (SHB) for the range of middle loading rate, and Laser Shock Adhesion Test (LaSAT) for the range of higher loading rate. LaSAT is a unique test of adhesion evaluation, since laser induced shock wave is employed to lead interfacial fracture. In parallel, finite element method (FEM) is conducted in order to calculate stress distribution at the interface during LaSAT. As a result, it was found that the interface between the aluminium alloy and the epoxy resin interface shows significant loading rate dependency of the adhesion strength and this tendency is very similar to that of bulk epoxy materials.

Evaluation of adhesion durability of Ni–P coating using repeated Laser Shock Adhesion Test

This study aims to evaluate the adhesion strength and durability of NiP coatings in a non-contact manner using the Laser Shock Adhesion Test (LaSAT). A hard coating of electroless NiP deposited on carbon steel substrate (SKD11) was prepared, and the effect of post heat treatment on adhesion was comprehensively investigated in this study. With the LaSAT, strong elastic waves induced by pulsed YAG laser irradiation applies tensile stress to the coating/substrate interface resulted in interfacial delamination. Computation of the elastic-wave propagation using finite element method (FEM) was carried out to estimate the interfacial tensile stress. In addition, the adhesion durability was investigated by repetitive pulsed-laser irradiation, and the relationship between adhesion strength and the number of laser irradiation cycles until delamination was obtained. Finally, the effects of heat treatment on the adhesion strength and durability of the NiP coating were discussed. Owing to the post-heat treatment, it is found that the hardness of coating, adhesion strength and durability were improved. Indentation test reveals that the coating hardness increases from 792 HV to 1163 HV. LaSAT quantitatively evaluated the adhesion strength with increasing from 84.4 MPa to 143.9 MPa. In addition, repetitive LaSAT reveals that adhesion durability improves twice or more. This result was verified using the tensile adhesion test and microscopic observations of the interface. Therefore, the present LaSAT is a quick measurement for the adhesion durability of hard coating, and it may shed some light on quality control of surface coatings.

Laser shock adhesion test numerical optimization for composite bonding assessment

The present work presents the latest development of laser shock adhesion test (LASAT) technology, targeting the weak bond detection in bonded aeronautic structures. This problematic is still holding back a wider use of bonding, which could however be a significant breakthrough in the way of assembling parts. By mechanically loading the bondline thanks to laser-induced shock waves, LASAT acts as proof test to reveal the presence of local weaknesses. In the present paper, focus is made on the optimization of the laser shock parameters regarding the assembly to test. Objective is to avoid loading too much the composite, thus avoiding damage, to increase the test performances. Numerical modelling is used, following a specific methodology, to understand the phenomena and identify the key parameters.The basic laser shock configuration was first investigated. Due to the stress distribution, this setting allows one to test a bond whose strength is equal or below 40% of composite inter-laminar strength. The effects of the laser focal spot on the stress distribution are also quantified. A 4 mm diameter shows good performances for the assembly to test. For the first time, three different optimizations are proposed: tunable pulse duration, double pulses on the front face and symmetrical laser shocks. They are first theoretically described. Numerical results then support these configurations’ performances. The double pulse solution makes it possible to test a bond strength equal or inferior to 80% of composite inter-laminar strength, when symmetrical pulses enable to reach 100% thanks to a sharp stress distribution. These results are validated by experimental evidence that is also presented. Finally, the present work offers helpful information for the development and deployment of LASAT for aeronautic bonded structures.

Development of a numerical code for laser-induced shock waves applications

The recent development of LAser Shock Adhesion Test (LASAT) as quantitative Non-Destructive Testing (NDT) process for evaluation of structural bonded assemblies brings new challenges. Applicative assemblies composed of complex materials with poor transverse mechanical properties and highly resistant bonded joints require laser parameters optimization and a more accurate control on the whole process. The development of a numerical tool is then necessary to ensure laser parameters specification to evaluate the bond mechanical strength for a given assembly. In this document, the ability of ESTHER code for the description of laser-matter interaction on aluminum and ablation pressure prediction is exposed. The influence of the target initial reflectivity on ablation pressure is investigated. In this paper, validation of the code in both direct (1-500GW/cm2) and water-confined (0,2-7GW/cm2) irradiation regimes is achieved with comparison to suitable sets of experimental data. Experiments were led on two laser facilities: the transportable laser shock generator (GCLT) at the CEA/DAM/DIF and the Hephaïstos facility at the Processes and Engineering in Mechanics and Materials laboratory (PIMM lab). Numerical models developed in this work are compared to previous experimental data and to reference models. Ablation pressures defined by our predictive models can then be coupled to other codes which are able to describe 2D/3D shock propagation, in order to model the entire LASAT process on complex assemblies. Characterization of a 6061 Aluminum/ Epoxy/ 6061 Aluminum assembly is achieved using ESTHER, showing its ability to master the phenomena involved in the LASAT process. For the first time, results open the full numerical design of laser adhesion test with the same code.

Thermal cycling damage monitoring of thermal barrier coating assisted with LASAT (LAser Shock Adhesion Test)

One of the major challenges for thermal barrier coating (TBC) system is to determine the time to failure of ceramic layer during aging. The aim of this paper is to propose new methodologies for experimental assessment of adhesion and for analysing its link to lifetime to spallation. Recently, the use of LAser Shock Adhesion Test (LASAT) with multi-shock experiments have shown its capability for both ranking different coating solutions in terms of adhesion and evaluating its evolution with aging. The methodology developed in this study is based on single shock experiments using the same laser parameters in order to measure non destructively and compare directly the resulting crack sizes and corresponding interface strength levels during interrupted thermal cycling. This cycle-by-cycle non destructive methodology was also enriched by the assessment of the evolution of the interfacial damages that were first initiated by LASAT and further propagated during subsequent thermal cycling. The evolution of interfacial damage during thermal cycling was shown to be consistent with interfacial delamination measured post-mortem by cross-sectioning. Besides, the influence of dwell time at high temperature has been clearly established, confirming that short dwell time was more detrimental as compared to longer and conventional dwell used for thermal cycling tests at high temperature.

Buckling and interface strength analyses of thermal barrier coatings combining Laser Shock Adhesion Test to thermal cycling

Laser Shock Adhesion Test (LASAT) is a non-contacting technique that can be applied to evaluate the interfacial adherence of ceramic layer on metallic substrate. This study aims at combining the laser-induced delamination and blistering when applied to a conventional EB-PVD TBC submitted to thermal cycling. Besides, the use of white spot diagnostic as a NDT technique and 3D profilometer measurement along thermal cycling were fully consistent. The LASAT analysis of as-deposited samples with various initial substrate roughness is discussed regarding the corresponding life to top-coat spallation under air thermal cycling. Mixing thermal cycling with LASAT has enabled to show that the adherence was sensitive to process condition consistently with the performance measured by conventional thermal cycling tests. Moreover, the evolution of the adherence has also been be assessed by LASAT applied to TBC samples with different interrupted thermal cycling conditions. According to this methodology, LASAT could also be used to introduce an interfacial defect known in location, size of debonded area, height of the blister and number of cycles. After further interrupted thermal cycling, the crack propagation and the buckling were monitored. Such measurements allowed to derive the interfacial toughness corresponding to these various aging conditions.

Three-Dimensional Characterization of Cracks in a Columnar Thermal Barrier Coating System for Gas Turbine Applications

Thermal barrier coatings (TBC) are multilayered systems comprising a metallic oxidation protection layer or so-called bond coat, a ceramic topcoat, and a thermally grown aluminum oxide developing at the interface. The coating systems fail typically by delamination cracking near this interface, which has a complex three-dimensional morphology influencing the crack path. This study combines laser shock adhesion test to introduce an artificial interfacial crack, known in size and location, and focused ion beam coupled with scanning electron microscopy 3D serial sectioning tomography. Methods for proper segmentation of cracks and adjacent materials and quantitative evaluation of the complex crack system are proposed and applied for analyzing the crack tip region. Finally, derived from the three-dimensional segmentation, a finite element model has been achieved and used for thermal analysis highlighting the crucial role of local damage on thermal conductivity of the TBC.

Interfacial Strength Evaluation of Oxide Films on Carbon Steel by Using the Laser Shock Adhesion Test

We investigate the adhesion strength (interfacial strength) of the oxide film formed on a carbon steel substrate by using a laser shock adhesion test (LaSAT). A rolled carbon steel (SS400) plate is first heated at high temperature (700 °C) under the atmosphere (heat treatment) to cause the formation of oxide films about 20-50 μm thick on the steel surface. The film consisted of multiple layers, including wustite (FeO), magnetite (Fe3O4), and hematite (Fe2O3). Next, the x-ray diffraction method was used to measure the residual stress in the film. We find that compression stress develops in the film because of a mismatch of thermal strain in between the film and substrate after the heat treatment. With LaSAT, strong elastic wave generates by a pulsed laser ablation of the substrate backside, and results in interfacial fractures in the oxide films. To monitor the normal (out-of-plane) displacement of the top surface film, a laser ultrasonic interferometer is simultaneously utilized, such that we detect film delamination and determine the critical laser energy for interfacial fracture to obtain the critical stress (interfacial strength). Furthermore, additional pulsed laser irradiation is conducted against a pre-delaminated specimen in order to examine the growth of delamination area and evaluate the interfacial fracture toughness. Finally, elastic wave propagation in the sample is computed using the finite element method (FEM) in order to evaluate the interfacial stress field, accurately. For the FEM, multiple layers in the oxide film are modeled, and the residual stress due to thermal strain mismatch is introduced. The FEM computations reveal the stress distribution around the interface and evaluate the critical stress and critical stress intensity factor for delamination. We thus quantitatively evaluate the interfacial strength and interfacial fracture toughness of the oxide film formed on carbon steel. The mechanism of interfacial fracture is also discussed on the basis of microstructures in a film with multiple layers. This result may be useful for understanding the adhesion quality of oxide film during the hot-rolling process.

A Laser Shock Approach to Cold Spray

Cold spray utilizes supersonic jets of compressed gas to accelerate powder particles at high velocities. A coating is formed on a substrate by the impact and deformation of particles. Laser Shock consists in illuminating a sample with a pulsed laser to generate a high pressure shock. Cold spray and laser shock are extremely dynamic processes (time scales of about 10-100 ns). In this paper, applications of laser shock for the study of cold spray are presented. A powder particle of a given size and morphology can be laser shock accelerated at cold spray velocities, finally hitting a substrate in a controlled experimental simulation of the process. Results allow also the characterization of powder materials, through the comparison of deformed particles with numerical impact simulations and the fitting of a plasticity model. Two main advantages compared to the split-Hopkinson pressure bar emerge: deformation rates are closer to cold spray conditions and powders are directly tested, rather than macroscopic cylinders. Laser shock can also be used to measure adhesion and internal cohesion of cold-sprayed coatings (LAser Shock Adhesion Test, LASAT). Cold LAser Shock Spray (CLASS), consisting in laser shocking a coating to re-spray it, can be used to characterize property gradient within a coating or as a new spraying process. Laser shock techniques can prove beneficial for the knowledge of powder materials, which is key for advances in cold spray and other powder based processes. Moreover, the combination of the two techniques could lead to hybrid processes.

Development of the symmetrical laser shock test for weak bond inspection

This study aims to assess the capability of the LAser Shock Adhesion Test to detect weak bonds in assemblies made of carbon fibre reinforced polymer laminates as well as understand the behaviour of different bonded composite structure under a shock load. A specific setup based on symmetrical laser shocks has been used. After each test, ultrasounds are used to determine if the bond has been damaged or not. At first, samples with two contaminants - de-icing fluid and finger prints - were studied. Then, the bond quality of two partially contaminated aircraft parts were investigated. These original results demonstrate the efficiency of the symmetrical laser shocks method as a Non-Destructive Test for bonded carbon fibre reinforced polymer assemblies.

Interfacial Strength Evaluation of Cu Electroplating by Using Laser Shock Adhesion Test

Interfacial Strength Evaluation of Cu Electroplating by Using Laser Shock Adhesion Test

Numerical modeling of laser-induced shock experiments for the development of the adhesion test for bonded composite materials

In this work, laser shock experiments on composite material are modeled. Focus is made on the development of a reliable numerical model to be used for the laser shock wave adhesion test of bonded composites. Technique principle is explained as well as the laser shock experiment procedure. Then, the numerical investigations are presented. A calibration method is given to set the model input parameters, and the modeling choices are detailed. Dynamic material parameters are identified thanks to experimental results, and validated through a complete campaign of laser shocks on various carbon fiber reinforced plastic (CFRP) materials (monolithic and bonded). Finally, numerical results for bonded composites are discussed. They enable to understand the stress distribution within the composite assembly during the wave propagation. This is a key step toward the development of a reliable and controlled laser shock adhesion test.

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.

Development of the laser shock wave adhesion test on bonded CFRP composite

Purpose – The purpose of this paper is to develop a laser shock adhesion test (LASAT) and evaluate its ability to reveal various bond qualities of stuck carbon fiber reinforced polymer (CFRP) industrial assemblies. Design/methodology/approach – Four grades of adhesion were prepared by release agent contamination of CFRP prior to assembly. Laser shots were performed at different intensities on these samples. Findings – To characterize and quantify the damage created by the propagation of shock waves in the bonded material, several diagnoses were used (confocal microscopy, ultra-sound inspection and cross-sections microscopy). These three post-mortem techniques are complementary and provide consistent results. Originality/value – The combination of these diagnoses along with the LASAT technique provides relevant information on the bond quality in agreement with GIC values measured by the University of Patras.

Development of a laser shock adhesion test for the assessment of weak adhesive bonded CFRP structures

Adhesive bonding has a great potential for future lightweight high-loaded structures in the aeronautic industry. A prerequisite for such an application is that the bond quality of the adhesive joint can be assessed in a non-destructive way. However, the use of classical Non-Destructive Techniques (NDT) does not allow the evaluation of the adhesion strength of an adhesive bond yet. This paper presents an investigation made on weak composite bonds in order to develop a laser shock wave adhesion test. First, the procedure to produce controlled weak bonds is described. CFRP bonded samples are prepared in a specific way and characterized by ultrasonic techniques to assess the absence of any detectable defect. Then, for some of the samples, their bond strength is evaluated by mechanical destructive tests and other samples are loaded by various intensity lasers shocks. The obtained results help to understand the behavior of the composite bonds under laser shock loading, thanks to two post-mortem techniques. The correlation between the laser parameters and the induced damage is demonstrated. The potential of the laser shock technique to discriminate different bond qualities is shown, and the need for the test optimization is discussed.

Laser Shock Adhesion Test (LASAT) of EB-PVD TBCs: Towards an industrial application

The assessment of the interface strength of EB-PVD thermal barrier coating (TBC) is a key issue to control the production and better understand the ceramic spallation that will occur during life duration of coated turbine blades. The Laser Shock Adhesion Test (LASAT) involving bi-dimensional shock wave propagation, namely the LASAT-2D, consists in measuring the interfacial crack diameter when implementing a set of laser shocks with increased laser power densities. From the resulting “LASAT-2D plots”, adhesion strength could be obtained from tensile stress calculated at the interface with a finite element model of shockwave propagation combined to interface cracking criterion. This work is focusing on the qualitative approach to compare adhesion on TBC systems directly through LASAT-2D plots. Two different testing modes were investigated: the conventional MS mode when the laser irradiation is carried out on the metallic side of a TBC coupon and the newly developed CS mode for which the laser shock is implemented onto the ceramic side. The interfacial crack is revealed by the presence of a spot that could be measured on a top-view optical image of the ceramic. In this work, the actual relationship between spot diameter and interfacial inner crack size is clearly established for both MS and CS modes. It was achieved experimentally by examinations of SEM cross-sections or non-destructive piezospectroscopic measurements involving lifetime decay map of the photoluminescence signal. From LASAT-2D curves, the role of the laser conditions like laser shock diameter and substrate thickness is discussed. This discussion is illustrated through the results of the qualitative LASAT-2D method applied on two different TBC samples (as-coated and thermally-cycled). The final objective of this work is to enable a sound and relevant application of the LASAT on TBC systems for both modes. With this goal, the LASAT-2D CS was successfully applied directly on a TBC coated turbine blade.

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

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.

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

An 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 TiO(2) 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 TiO(2) 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.