Thermal-sprayed (i.e. LPPS or HVOF) MCrAlY coatings are widely used for land-based gas turbine applications against high-temperature oxidation and hot corrosion. However, due to requirement for further improvement of turbine efficiency, dense and stable coatings are necessary. The cold spray (also referred to as cold gas dynamic spray) makes it possible to increase coating density, due to high velocity particle impact during spraying. However, deposition mechanisms of cold spraying have not been elucidated yet. In this study, we investigated the deposition mechanisms focused on the behavior of interface between a coating and a substrate. The mechanisms were evaluated by the spray impact phenomena simulation tests, namely laser shock flier impact tests, and STEM-EDX elemental analyses at the interface between the substrate and the cold sprayed coating. From the results of STEM-EDX for as-sprayed coating and of SEM-EDX of the flier specimen, the bonding between the CoNiCrAlY coating and the substrate occurred at the only particular phase combination.

The studying of splats in thermal spray is prominent to improve coating properties due to better understanding of coating build-up mechanisms. Most of studies, however, devoted to splats rest on physical aspects of the build-up, which feature the behaviour of a single particle impinging on a substrate. These studies involve the nature, velocity and intrinsic characteristics of the particle. The present work deals with the study of the splat-substrate interface using an ultra-sonic (U.S.) technique. This technique is based on the determining of splat-substrate adhesion (for an elementary splat or a group of splats) to help in the optimising of spraying conditions. APS copper splats were studied for Al-based substrates. This U.S. technique was already tested successfully for adhesion control of thermally-sprayed coatings in Basel, Switzerland. Further development then related to the specific experimental set-up for splat analysis using several transducers. This involved the use of various frequencies, various sizes for the focal spot and various scan velocities. In this study, results from this U.S. technique were discussed in the light of the observation of cross-sections of U.S.- controlled splats. U.S. C-Scan images were shown to fit well with cross-sectional images. In a general conclusion, the work showed the feasibility and efficiency of advanced U.S. control of splats to promote thermally-sprayed coatings whatever the process, i.e. plasma spray, flame spray, HVOF or cold spray.

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.