Effect of standoff distance on coating deposition characteristics in cold spraying

Abstract

In this study, the effect of standoff distance on coating deposition characteristics in cold spraying in cold spraying was investigated by the experiment and numerical simulation of particle acceleration. Al, Ti and Cu powders of different sizes were used as feedstocks. It was found that the deposition efficiency was decreased with the increase of standoff distance from 10 mm to 110 mm for both Al and Ti powders used in this study. However, for Cu powders, the maximum deposition efficiency was obtained at the standoff distance of 30 mm, and then the deposition efficiency decreased with further increasing the standoff distance to 110 mm. The standoff distance had a little effect on coating microstructure and microhardness for these three powders. Both the stain-hardening effect of the deposited particles and the shot-peening effect of the rebounded particles take the roles in coating hardness. It was also found that the surface of substrate or previously deposited coating could be exposed to a relatively high gas temperature at a short standoff distance.

Introduction

Cold spraying is an emerging coating process. In this process, spray particles are injected into a high speed gas jet in a de Laval type nozzle and accelerated to a high velocity (300–1200 m/s). The deposition of particles takes place through the intensive plastic deformation upon impact in a solid state at a temperature well below the melting point of spray material. Consequently, the deleterious effects such as oxidation, phase transformation, decomposition, grain growth and other problems inherent to conventional thermal spraying processes can be minimized or eliminated [1], [2]. Therefore, it becomes more and more attractive for its high deposition efficiency and volume production of many metallic coatings and composites [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], and even cermets [10], [19] and nanostructured coatings [19], [20].

It has been widely accepted that particle velocity prior to impact is one of the most important parameters. Generally, for a given material, there exists a critical velocity resulting in a transition from erosion of the substrate to deposition of the particle. Only those particles achieving a velocity higher than the critical one can be deposited to produce a coating. It has been reported that the critical velocity was associated with properties of spray materials [2], [3], [12], [13], [16], [17], [21] and substrate [3], [4], [5], particle conditions prior to impact such as particle temperature [5], [6], [13], [18], size [16], [21], [22] and its surface oxidation state [6]. As particle velocity is higher than the critical one, the deposition efficiency increases with increasing the particle velocity [2], [3], [7], [8], [9], [10], [13], [14], [17], [18], [23]. Consequently, in order to realize a high deposition efficiency, the majority of spray particles have to be accelerated to a velocity higher than the critical one.

According to the reported results obtained by both the experiment and numerical simulation, many factors influence the particle velocity in cold spraying, including nozzle geometry, accelerating gas conditions and properties of particles [3], [7], [13], [14], [16], [17], [18], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31]. For a converging–diverging nozzle, the increase of nozzle divergent section length will lead to the significant increase of particle velocity [18], [25]. There exists an optimal expansion ratio (area ratio of nozzle throat to exit) for particle acceleration under different spray conditions owing to the presence of show waves outside the nozzle exit [11], [12], [13], [16]. As the nozzle dimensions are fixed, with increasing either the gas temperature or pressure, particle velocity will be increased. When helium is used, the particle can reach to a higher velocity than that using nitrogen or air. Moreover, the particle velocity increases with the decrease of particle size and a higher velocity can be obtained for a particle of lower density under the same gas conditions. The previous study [25] and other results in the literature [7], [10] also showed that when the nozzle dimensions are fixed, the standoff distance from nozzle exit to substrate influenced the particle velocity and thus the deposition efficiency [3], [8]. However, there are few reports focusing on this issue. Therefore, in this study, the effect of standoff distance on coating deposition characteristics was investigated through both the experiment and numerical simulation of particle acceleration aiming at the optimization of cold spray process.