Laser-induced damage in cold-sprayed composite coatings

Abstract

Ag–Ni composite coatings were achieved using the cold spray process. In this work, two blends were sprayed to lead to coatings with different mechanical properties, which corresponded to either a fine or a coarse microstructure. These coatings were studied using SEM and X-ray microtomography (XMT) to determine the best coating quality through characterization of particle-to-particle bonding. The fine microstructure showed better properties than the coarse microstructure.

To avoid time-consuming characterization techniques such as SEM and XMT, an innovative laser characterization test was developed. The principle of this test is based on the thermal shock-induced by a laser pulse at the surface of the coating. Thus, particle interfaces were stressed and cracking damage could occur. SEM observation of cross-sections and top views was carried out to rank the coatings as a function of the generated damage. The influence of heat treatments was also studied.

To enhance the approach, thermomechanical simulations have been performed on a real meshed microstructure. The influence of layering effect in the cold spray was shown to be detrimental due to local concentrated stresses.

Introduction

Cold spray is a relatively new coating process. Powder particles are injected and accelerated in a supersonic heated gas flow to reach a critical velocity, which allows the coating of a substrate. This deposition process is based on high plastic deformation of particles upon impact. Particles are not heated enough to be melted. They remain at the solid state during their flight which involves very low oxidation only, no phase transformation, and no detrimental phenomena as those encountered in conventional thermal spray [1].

Another interest in this process is the capability in obtaining composite coatings such as metal/metal [2], [3] or ceramic/metal composites [4], [5], [6], [7], [8] for a wide range of industrial applications. Cold spray is expected to substitute for processes such as extrusion, casting, sintering [12] or plasma and HVOF spray [11], [10], to achieve composite materials. This process could therefore be applied in the field of electrical contacts, which needs fully-dense composite materials with no oxidation. Several processes were studied to deposit composite materials by cold spray. The most common method is to start from a powder mixture [2], [3], [4], [6], [7], despite the particle addition loss [6], [7] due to intrinsic process effects. Consequently, researchers tried to overcome this drawback by working on the starting powder [5] or to control powder injection when spraying [9].

In the present study, composite coatings were deposited by cold spray using a silver powder for the matrix mixed with a nickel powder. A large variety of microstructures was elaborated by modifying powder granulometry to obtain a fine or a coarse microstructure and by applying or not an annealing post-treatment.

The purpose of this work is to develop a laser-based method suitable for discriminating the cold-sprayed coatings through their thermo-mechanical properties as a function of their microstructure. A classical microstructural study using conventional cross-section and hardness measurements is not sufficient to select the best deposit due to a lack of data on particle-to-particle adhesion within the coating. Several methods based on mechanical testing can be employed to determine the bonding strength within coatings. The most classical method is to perform tensile tests on samples machined from thick deposits [13], [14], [15]. It can be easily seen that these mechanical tests are not very easy to carry out to obtain bonding properties and characterize microstructures. Another way for assessing coating properties consists in performing corrosion tests to exhibit open porosity in the coatings. These corrosion tests are based on open-cell potential measurements as done in [16]. Nevertheless, these methods are time-consuming and relatively far from evaluating the in-service coating behavior.

Considering this situation, a pulsed YAG laser was used to develop an innovative approach to the mechanical behavior of cold-sprayed coatings. This test relies on the study of the degree of the damage caused by a laser pulse. A YAG laser is therefore a tool to characterize the microstructural response of the coatings to an energy pulse. This can be claimed to be original, economical and efficient compared to mechanical testing. Lasers are fairly widely used in the field of thermal spray for post-treatments such as coating densification [17], remelting [18], or pre-treatments to enhance coating adhesion [19], coating deposition efficiency [19], [20], or density [20].

For the development of a characterization tool to discriminate the microstructural response from a rather unknown coating process, a wide range of microstructures was achieved. The influence of major cold spray parameters such as the powder size for the matrix and the applied heat treatment was studied. Laser testing of these microstructures consisted in irradiating the material surface with a single pulse at given duration, spot size and location. The resulting damage, i.e. cracking, material removal or melting, has been investigated carefully using SEM. Furthermore, 3D investigation by X-ray microtomography which is now known to be very suitable for coating analysis [21], [22], [23], was carried out to complement coating characterization. In a final stage, a thermomechanical simulation on a real-meshed microstructure is proposed to characterize the zones which were modified by the laser irradiation pulse. The study of the real-meshed microstructure is based on the computational analysis of SEM images. More generally, the development of pulsed laser surface testing was shown to be a powerful tool to study damage mechanisms, which resulted from thorough experimental observation at various scales.