Combining effects of ablation laser and laser preheating on metallic substrates before thermal spraying

Abstract

Pulsed laser irradiation is a promising approach to remove the contaminant film with a limited damage of the metallic substrate surface. It reduces some serious drawbacks of the traditional means where chemical solvents and aggressive operations are usually employed. But the quality of thermally sprayed coatings depends on a lot of other parameters. The temperature, the wettability and the geometry play important roles in the coating formation and particularly from the adhesion point of view. So, several surface treatments before thermal spraying can be often used in order to obtain good deposits. If the laser ablation effects on metallic substrates are mostly known as well as the substrate temperature impact, the modifications of the surface morphology induced by the combination of laser cleaning and temperature need to be investigated. In this paper, the surface modifications on aluminium 2017 polished substrates induced by laser irradiation at several surface temperatures are proposed and their influence on plasma-sprayed Ni–Al splats was examined. It was confirmed also that a favorable surface condition tends to suppress the splashing and to promote the occurrence of disc-shaped splat.

Introduction

Many factors influence the spreading and splashing of plasma-sprayed particles impacting a surface [1]. Lots of interacting parameters have to be controlled in order to obtain good coatings and ensure the adhesion between the deposit and the substrate. The adhesion of the layer on the substrate or on a previously deposited layer is fundamental for a good cohesion of the coating and a successful application. To create appropriate conditions for anchoring coatings on substrate, suitable surface preparations are required [2]. A clean surface is a key parameter for the contact quality which depends on the droplet wetting and desorption of the pollutants adsorbed on the surface or on the underlying layer [3], [4]. Mostly to remove grease, oil or dust from the surface, chemical degreasing and grit blasting are used. But those processes present some disadvantages: the elimination of the chemical wastes from the cleaning requires precautions for environmental considerations and grit blasting causes a fatigue property decrease of the material, (and grit inclusions which are harmful for the adhesion [5]). Other cleaning processes have also been developed to eliminate those defects [6]. One of them, the PROTAL® process [7], is interesting as it uses laser nanosecond pulses to ablate surface contaminants and improve the wettability of the substrate that is good for the flattening of the melting particles [8]. This technique allows an association between decontamination and thermal spraying contrary to the classical processes for which these steps are decoupled.

To decrease the particle splashing and to enhance the adhesion between impacting particles and substrate, the decontamination of the surface is necessary but it has been established that the substrate temperature is also a key parameter. Pershin et al. [9] sprayed nickel powder onto stainless steel and found that coating adhesion strength increased as surface temperature was raised. Increasing the surface temperature changes the splat morphology: on substrate at room temperature, the splats present splashing shape with irregular morphologies, whereas on heated surfaces splats have disk shape morphologies which leads to a better adhesion of the coating [10]. A transition temperature which depends on the couple substrate/powder materials has been identified [11]. The splat morphology modification occurs beyond this transition temperature. Several explanations were proposed. Heating the surface clears volatile contaminants adsorbed on the surface, improving contact between particles and the substrate. It was suggested that the growth of a nano-scale thick oxide layer improved the wettability and contact between the splat and substrate [9]. The formation of nano-scale peaks was observed on stainless steel (304L) substrate [12] which can explain the rise of surface wettability and a better thermal contact between impinging particles and substrate. The substrate heating before thermal spraying is traditionally made by flame of plasma but to get a better control on the treated area, a laser can be employed. The combination of the two techniques: laser cleaning and laser preheating has been already used and encouraging results were obtained on TA6V [13]. Splats of Ni/Al show disk shape on samples which were prepared with these two processes. Several configurations depending on the position between lasers have been tested and it has been established that a better droplet flattening occurs when the laser spots are superposed. The objectives of this study were to: (i) observe splat of plasma-sprayed nickel–aluminium (Ni–Al) particles on substrate at room temperature but cleaned by ablation laser, (ii) observe splats on substrate heated by conduction (with a heating plate) and cleaned by laser, (iii) observe splats on substrate heated by laser and cleaned by ablation laser (iv) determine the effect of the different heating processes (heating plate/laser).