The efficiency of aero-engines combustion chambers with thermal barrier coating (TBC) is improved when numerous cooling holes are laser drilled with inclined angles. However, during the laser drilling process, especially in the percussion mode, a detrimental crack can be generated at the TBC interface. Thus, each hole could be edged with a non-visible delaminated area underneath the ceramic top-coat. The present work is focused on the thorough study of the delamination induced by laser percussion drilling when interrupted drilling conditions are presented. Shallow angle drilling was applied on separated holes with 1 to 4 laser pulses respectively and various acute incident angles. Crack length was assessed by conventional metallographic preparation. A special experimental method was carried out in order to inspect the delaminated interface and the lateral edge of a semi-hole. This non-destructive assessment of the delamination of laser drilled TBC was complemented by a 3D imaging of a semi-hole using X-Ray microscopy. Results are presented with attention on both crack initiation and propagation during the laser percussion drilling of plasma-sprayed TBC.

The adhesion mechanisms involved in the cold spray coatings are not still well elucidated. The quality of the deposit does depend mainly on particles and dynamic characteristics (which result from nozzle type, nozzle-substrate distance, etc.). The present work is based on the study of particle-substrate and particle-particle interfaces in the tantalum-copper coating-substrate system. The content focuses on the influence of the oxygen content in the starting powder on interface features, consequently on coating properties. Tantalum powders with different oxygen levels were studied using SEM (Scanning Electron Microscopy) and EPMA (Electron Probe Microanalysis). Laser shock spallation of cold-sprayed Ta coatings was developed as a reliable and flexible process to achieve Ta spalls to be deposited at a high-velocity onto Cu targets. The velocity due to the laser shock could be controlled to be similar to that of particles in conventional cold spray. This results in Ta-Cu interfaces, the study of which was carried out to go into interface phenomena involved in cold spray, using TEM (Transmission Electron Microscopy) in particular. Results were compared to those obtained from laser shock spallation of Ta bulk specimens (i.e. made of a conventional Ta sheet). The role of powder oxidation on interface soundness was exhibited. Adhesion was shown to be all the lower as powder oxygen content was higher, using LASAT (“ Laser Shock Adhesion Test”) in addition to direct observation of interfaces. Results were exploited to discuss properties of the corresponding Ta coatings onto Cu, i.e. which were cold sprayed using powders with different oxygen contents.

A good adhesion of plasma sprayed hydroxyapatite (HA) coating on Ti-based alloy is crucial for ensuring highly-reliable non cemented implants in the biomedical industry. In the present work, the laser shock adhesion test, namely LASAT, has been applied to investigate the interface strength of plasma sprayed HA coatings. This contact less method allowed a rapid assessment of the HA coating adhesion on simple coated plates. Varying the laser energy to impact the substrate and to generate the interface decohesion, a LASAT adhesion threshold can be determined for the highest laser fluence (J/m²) for which no debonding of the coating occurred. This qualitative and discerning LASAT procedure has been carried out on HA coatings to investigate the role of various interfaces on the adhesive property of the HA/Ti bond. According to the LASAT analysis, a surface roughness prepared with medium or coarse grit-blasting did not influence drastically the adhesion threshold while smooth pre-oxidized specimens LASAT threshold were near to those obtained with a Ti bond-coat. These thresholds also corresponded with the highest adhesion measured in this study. In addition, pre-heating treatment of substrates just prior to spraying up to 270°C did not exhibit a significant difference with grit-blasted HA/Ti interface. Further investigations (SEM, XRD) was also achieved to investigate the interface characteristics before and after the laser treatment. Sample cross-sections of laser shocked specimens were examined in detail, right at the impact location and within the debonding area to assess the fracture feature. This complementary materials analysis permitted to establish the relevance of the LASAT test as a fast and easy-to-use method devoted to the design or the control of highly adhesive HA coatings. Preliminary experiments to apply the LASAT method in liquid environment is described. Further work is on progress to implement an in situ adhesion testing of HA coating in simulated body fluid.

The adhesion strength of a ceramic coating deposited through direct spraying on a roughened substrate is a key issue in the manufacture of high-quality coatings on industrial components. The purpose of this work was to develop a rapid and discerning procedure for establishing adhesion level of a ceramic coating on a metallic substrate. The Laser Shock Adhesion Test, namely LASAT, was successfully applied to ceramic coatings with irradiation impact on the metallic side. Suitable parameters were found to determine the LASAT adhesion threshold using a standard Nd:YAG laser source. With a laser-irradiated area of several millimetres in diameter, it allowed assessment of the coating threshold on several areas of a coated plate sample. A control procedure for a qualitative assessment of coating adhesion was developed. This testing procedure could be easily used in industry, with possible location of the LASAT unit near to the spraying booth, for a direct production control on coated sample to improve the tracability of manufactured parts. Additional work was carried out to investigate a quantitative approach of the LASAT test to ceramic coating. The purpose was to simulate the shock wave propagation with the RADIOSS® code (a 3D software originally developed for car crash simulation). This code was implemented to calculate the velocity of the material and corresponding pressure throughout the substrate and the coating during the shock wave release (less than 2 ms). Experimental VISAR profiles ('Velocity Interferometer System for Any Reflector') were monitored in the straight direction of the laser-irradiated area on the rear side. These experimental signals (velocity measures) of the ceramic coating could be fitted and compared with a fairly good agreement with simulated profiles obtained by RADIOSS®. This modelling work was the first step towards a more comprehensive coating adhesion strength calculation in the future.

Coating-substrate adhesion in cold spray is a paramount property, the mechanisms of which are not yet well elucidated. These mechanisms are governed by metallurgical and morphological phenomena occuring when cold-sprayed particles impinge on the substrate. To go into these mechanisms, due to the intrinsic characteristics of the cold spray process, i.e. the low-temperature and high velocity of the particles, direct observation and control of inflight particles and related phenomena (especially when impinging) cannot be done easily. For this reason, an experimental simulation of the particle-substrate reactions at the particle impingement was developed. This simulation is based on original filter impact experiments from laser shock acceleration of plates/foils (fliers). These were applied to the Cu-Al metallurgically-reactive system to simulate Cu cold-sprayed onto Al. The velocity of the plate was selected in the range of actual cold spray velocities. Relevant Cu-Al interaction phenomena were featured and studied as a function of filter impact conditions, i.e., primarily, shearing, plastic deformation, phase transformation (including rapid melting/solidification and formation of intermetallics). These phenomena were shown to be similar to those involved in cold spray. This was ascertained by a parallel study of cold-sprayed Cu coating of Al using SEM, TEM, EPMA, and an energy balance and diffusion calculations. In addition, this simulation can be used to feed FE modeling of cold spray particle impingement on the substrate. Preliminary results are discussed from modeling using the “RADIOSS®” code. More generally, laser shock flier impact experiments were demonstrated to result in a powerful tool capable of simulating cold spray coating-substrate interface mechanisms. Major assets rest on their high significance, reproducibility, flexibility and potential for substituting for direct laborious cold spray optimization testing.

To address drawbacks with conventional destructive adhesion tests, two advanced nondestructive methods were developed to characterize the acoustic response of coating-substrate bonding. These new tests, immersion ultrasonic testing and laser shock adhesion testing, are discussed in this contribution. The paper describes the test process and results as compared to traditional destructive testing. The advantages and drawbacks of the techniques are addressed.

This paper discusses the principles of laser shock adhesion testing, a nonintrusive method for measuring the bonding strength of coatings. The technique uses a laser to generate a thermal shock in the substrate, while Doppler laser interferometry is used to detect detachments. An application example is presented in which the method is employed to measure the adhesion strength of a plasma-sprayed copper coating on an aluminum substrate. The results are compared with numerical simulations of shock wave propagation. Paper includes a German-language abstract.

The adhesion of copper on aluminum depends on the presence of intermetallic phases. Such phases can form during spraying at the interface between the layer and substrate. This paper deals with the formation mechanism of the intermetallic phases and their influence on adhesion. The type, size, and distribution of the intermetallic phases are investigated as a function of spray parameters and bonding strength is determined by laser shock adhesion testing. Paper includes a German-language abstract.