Applications of laseR shoCk waves with variable pulse duratiOns to the anaLysis of coatings adhEsion – ARCOLE

The ARCOLE project uses several experimental diagnostics or numerical methods described here below:
For the experimental part:
- Generation of shocks with short pulse lasers (ns), and two pulses delayed of a Dt time
- Measurement of free surface velocities- shock pressures with VISAR Doppler velocimetry for identifying pressure loading and debonding stress thresholds
- Analysis of confined plasma with ombroscopy and photodiodes to understand absorption mechanisms of laser in plasmas with two pulses
- Laser texturing (120 ns) of substrates before thermal projection
- Thermal spraying
- Microstructural investigations (optical and electronic microscopy)
- Mechanical tests (adhesion)
- Thermal or thermo-mechanical (Maatre set-up) ageing
For the numerical part:
- Simulation of laser-matter interaction in confined regime (ESTHER Code)
- Simulation of shock wave propagation in bi-materials (ESTHER and HUGO Codes in 1D, ABAQUS in 2D or 3D)
- Thermo-hydrodynamic simulation of laser texturing (FLUENT Code)

The following results have been obtained:
1) VISAR characterization of laser-shock loading -Pressure P0 (GPa) versus intensity I0 (GW/cm²) curve with single pulse or double pulse). Results confirma maximum pressures obtained (6 GPa) in single pulse at lambda=0.53 µm, with a P0 = 2 (I0)0.5 tendency. Tests carried out on Aluminium foils with two pulses and a front irradiation show a tendency to a reduction of the pressure pulse when the time delay between pulses increases
2) VISAR analysis of shock wave propagation in AM1 monocrystals with various crystal orientations [001], [110], [111]. A 20 % evolution of sound velocity with crystal orientation is clearly evidenced
3) LASAT Adhesion testing of Ni-Al coatings on 2017 Al alloy with or without surface texturing. Those preliminary tests confirm a beneficial effect of laser texturing on adhesion strength
4) Installation of the TGA device in PPRIME for characterizing thermal ageing of TB
5) Optimization of laser texturing on various substrates (Al alloy, 316L steel, AM1). Idnfluence of laser parameters on hole morphology.
6) Correlation between a surface ratio parameter at the interface between coating and substrate and the adhesion strength, due to mechanical anchoring.

Oncoming tests will aim at characterizing the behaviour of thick AM1 substrates under laser-shock loading, at investigating the adhesion strength of zirconia coatings on laser-textured stainless steel substrates, with the use of double shock front loading, to make preliminary testing with opposite shock loading (2 shocks delayed of Dt time, applied on opposite faces of the system to be tested).
Also, Young's moduli will be characterized using ultrasound method, and first thermo-oxidative ageing will be carried out at PPRIME Lab on TB/ steel systems with or without laser texturing.

International conferences
1. D. Courapied et al., ICALEO'2014 (San Diego, 19-23 Oct 2014 USA)
2. L. Berthe et al., ICALEO'2014 (San Diego, 19-23 Oct 2014 USA)
3. R. Kromer et al., ITSC 2015 (Long Beach mai 2015, USA)

National Conference
1. D. Courapied et al., Matériaux 2014 , (24-28 Nov 2014, Montpellier, Fr)
2. R. Kromer et al. Ingédoc 2014, journée des doctorants de l’UTBM

The use of laser-induced shock waves (LSW) for characterizing coatings’ interfaces was first developed in USA, but validated a few years ago in France. The main physical principle deals with the generation of a local and intense stress loading inside materials, due to the crossing of incident and reflected release waves. The localization of tensile applied stresses is directly associated with the pressure pulse duration: the longer the pressure pulse, the deeper the applied tensile stress. This means that long pressure pulses (due to long laser pulses) will allow characterizing thick materials (including coating and substrate) whereas short pulses will be restricted to thin systems, with stresses applied very near free surfaces. This new technique of characterization allows quantifying damage thresholds (debonding stresses) at interfaces by the use of a combined experimental (VISAR velocimetry) and numerical approach. However, the use of constant pulse durations in the different laser systems, is really restrictive to test a large range of coatings’ thicknesses. The ARCOLE project, propose by three main partners : PIMM (UMR 8006 CNRS – Arts et Métiers Paris-Tech), LERMPS (EA3316 – UTBM) and PPRIME (UPR 3346 CNRS), aims at using laser-shock waves with adjustable pulse durations, to test substrate-coating interfaces, with a specific focus on thermal spray coatings. The investigation of interface debonding will be carried out using a dual experimental (VISAR velocimetry) + numerical approach in order to understand precisely the physical processes involved, and to reach quantitative stress values.
Four aspects will be considered: (1)The characterization of laser-matter interaction and pressure loading P=f(t), on a large range of pulse durations (5 ns – 100 ns), never investigated before in water confined regime, (2) The experimental and numerical analysis of shock wave propagation in heterogeneous materials (including interfaces such as Al substrate / NiAl plasma sprayed coating), and the validation of interface testing for different systems thicknesses, corresponding to different pulse durations, (3) The use of LSW to test the adhesion of thermal barrier with various thicknesses. Considering 100 µm to 1000 µm thick classical ZrO2-Y2O3 system, with or without sub-layers , we will investigate the influence of different substrates pre-treatments versus adhesion strengths. For similar coating thickness, the objective will be to identify the most influent parameters (physico-chemistry, morphology, mechanics) conducing to the best adhesion properties. A comparison with conventional adhesion tests will also be carried out, (4) The use of LSW for analyzing the thermal or thermo-mechanical ageing of thermal barrier systems, including the influence of oxidation, sintering or phase transformation during the lifetime of coatings.
For this work, ARCOLE project requires two doctoral positions: one dedicated to laser-matter analysis (PIMM), and the second one on the influence of pre-treatments and ageing on adhesion strengths (LERMPS, PPRIME). This project is a completely modified version of 2011’ submitted project CLASSE. It aims at supporting the new laser-shock Ile de France platform (already granted by Sesame Hephaistos and labeled by Francilian Fed of Mechanics). More widely, it will contribute to the scientific excellence of the French community of laser-shock waves, worldwide well known already.