Sophie Barradas

Physical approach to adhesion testing using laser-driven shock waves

This paper deals with an adhesion test of coatings using laser-driven shock waves. Physical aspects concerning laser–matter interaction, shock wave propagation and interface fracture strength are described. This comprehensive approach using two numerical codes (HUGO and SHYLAC) allows the determination of mechanisms responsible for coating debonding and a quantitative evaluation of fracture strength. From this description, a coating test protocol is also designed. To diagnose coating debonding, it is based on the analysis of experimental rear free surface velocity profiles measured by velocity interferometer system for any reflectors (VISAR). Ni electrolytic coating (70–90 µm) deposited on a Cu substrate (120–190 µm) is used for the experimental validation of the test. The fracture strength is 1.49 ± 0.01 GPa for a laser pulse duration of 10 ns at 1.064 µm.

A comparative study of three adhesion tests (EN 582, similar to ASTM C633, LASAT (LASer Adhesion Test), and bulge and blister test) performed on plasma sprayed copper deposited on aluminium 2017 substrates

The aim of this study was to compare three adhesion tests carried out on plasma-sprayed copper coatings on aluminium substrates. The first test, the bond pull test, designated EN 582 or ASTM C633, involves a uniaxial static stress and is commonly used in the coating industry. The second test, the LASAT (LASer Adhesion Test), is a recently developed technique based on spallation phenomenon due to laser induced shock waves. In this test, the coating delamination results from spallation at the coating/substrate interface due to uniaxial tensile stress. The last test, the bulge and blister test, involves a quasi-static measurement of the crack propagation energy at the coating/substrate interface. These three techniques have been used to evaluate the influences of different process parameters involved in the coating adhesion such as aluminium surface roughness, substrate pre-heating and plasma spray conditions.

Study of damage phenomena induced by edge effects into materials under laser driven shocks

Spallation of materials induced by laser driven shock waves is generally produced under uniaxial (one-dimensional (1D)) deformation by irradiating a spot of diameter much greater than the sample thickness. Here, two-dimensional (2D) effects are introduced in shock wave propagation by drastically reducing the loaded spot. Experiments performed on aluminium samples detect the effect of lateral wave propagation, both on recovered samples and on time-resolved VISAR measurements. Damage zones are localized completely differently from that under uniaxial condition, according to the presence of 2D effects, and the signature of these 2D effects can be read on VISAR signals. Numerical simulations provide a full understanding of wave propagation and resulting damage in 1D or 2D configuration. Comparisons with experimental VISAR signals show the possibility of validating more accurately the dynamic damage criteria, including the 2D effects.

Damaging of materials by bi-dimensional dynamic effects

Laser shocks are most often used to produce uniaxial stress and strain into materials by irradiating a spot diameter conventionally admitted at least three times larger than the thickness of the shocked sample. By reducing the laser spot versus the sample thickness, 2D lateral waves are created earlier and their crossing during propagation stages generates traction which can yield to voids into materials (near the front loaded face). This phenomenon has been evidenced by an experimental study, including VISAR measurements which exhibit the signature of the fracture generated by these 2D effects. Numerical simulations with the explicit finite element code RADIOSS clearly evidence the origin of the 2D effects on VISAR measurements. This different mode of damaging materials by laser lateral waves can act simultaneously with the classical laser spallation produced by the uniaxial propagation (shock wave reverberation crossing the unloading). This opens new discussed prospects for the development of 2D damage ...

VISAR Pull‐Back Signals as a Diagnostic for the LASer Adherence Test Applied to Copper Coating on Aluminum Substrate

The LASer Adherence Test so‐called LASAT aims to control the interface adhesion of a coating deposited over a substrate which is irradiated by a high power laser intensity. The shock waves propagation yields traction at the interface, likely to debond the coating. This technique has been applied to a copper plasma sprayed coating on an aluminum substrate. During the test, the coating’s pull back VISAR signals clearly evidence characteristic patterns according to the debonding or the integrity of the system. This shows the possibility to use this non‐intrusive diagnostic for assessing the bond quality of the coating. The numerical simulations of the experiments performed provide an estimate of the debonding parameters associated with a cumulative damage criterion for the interface of this system.

Use of a macroscopic model for describing the effects of porosity on shock wave propagation

Materials are manufactured by sintering involve porosity. Some material processes, like laser peening, consist in applying shocks onto the surface of a porous material surface to induce permanent densification that will increase its resistance to corrosion and wear. An estimation of the residual compaction and stresses within the material after treatment requires a good knowledge of shock wave propagation in such media. To investigate the effects of porosity on this propagation, we have performed velocity interferometer system for any reflectors measurements on laser shock-loaded samples of sintered steels with 10%−28% porosity. The records do not agree with the predictions of a simple P−α model from the literature. Hence, a formulation of the compaction process is proposed to improve the correlation between experimental and simulated velocity profile.

Benefits of the Impedance Mismatch Technique for Laser Shock Adhesion Test (LASAT)

The LASer Shock Adhesion Test (LASAT) is based on coating‐substrate spallation induced by a power pulsed YAG laser. While the laser induced shock wave propagates in the target, pull back velocity VISAR signals provide a diagnostic about the coating debonding. However, for targets over the millimeter range, the hydrodynamic decay can reduce the tensile stress created at the interface, down to below the debonding threshold. A numerical study shows that the application of a Low‐Impedance Layer (LIL) on the impacted face increases stresses at the coating‐substrate interface. It is experimentally evidenced by VISAR signals obtained on a 110 μm electro‐deposited chromium on copper. Possibilities of pushing up thickness limits of LASAT tested samples are shown.

The use of a macroscopic formulation describing the effects of dynamic compaction and porosity on plasma sprayed copper

Coatings processed by thermal deposition techniques involve porosity. The Laser adhesion test developed for testing bond strength of a coating on its substrate requires a good knowledge of shock wave propagation in such media. Experiments carried out on plasma sprayed copper samples, about 14% porous, with velocity interferometer system for any reflector measurements display the discrepancy of previously used models. Hence, a one-dimensional formulation of the compaction process, based on a simple P-α model, is proposed to improve the correlation between experimental and computed data signals obtained on a plasma sprayed copper under dynamic loading. Besides, this improvement allows the estimation of the bond strength of a plasma sprayed copper on aluminum substrate.

2D effect during the propagation of schock wave induced by laser plasma : Application for laser adherence test (LASAT)

Using numerical simulations (RADIOSS), 2D mechanical effects induced at laser spot edges have been studied. Results show that during the propagation in material, tensile lateral waves are generated behind the main compressive shock. The main shock wave is attenuated and/or the interface coating-substrate can be debonded by this tensile wave. Experimental validation has be done using pure Al target and Cu thermal spraying coating on Al substrate. Free rear velocity profiles measured by Doppler Velocimetry technique are in rather good agreement in both configurations. New perspectives are open with the 2D-LASAT test for massive target and so, new industrial applications.Using numerical simulations (RADIOSS), 2D mechanical effects induced at laser spot edges have been studied. Results show that during the propagation in material, tensile lateral waves are generated behind the main compressive shock. The main shock wave is attenuated and/or the interface coating-substrate can be debonded by this tensile wave. Experimental validation has be done using pure Al target and Cu thermal spraying coating on Al substrate. Free rear velocity profiles measured by Doppler Velocimetry technique are in rather good agreement in both configurations. New perspectives are open with the 2D-LASAT test for massive target and so, new industrial applications.