Elsevier

Surface and Coatings Technology

Volume 235, 25 November 2013, Pages 108-116
Surface and Coatings Technology

Microstructure, mechanical properties and corrosion performance of 7075 Al matrix ceramic particle reinforced composite coatings produced by the cold gas dynamic spraying process

https://doi.org/10.1016/j.surfcoat.2013.07.020Get rights and content

Highlights

  • According to XRD analyses cold sprayed coatings consist of finer grains.

  • Presence of ceramic particles improved hardness and wear performance.

  • Amount of ceramic particle in coatings has no strong effect on wear performance.

  • Among ceramic particles B4C have slightly better contribution to wear performance.

  • Coatings showed more noble corrosion potentials but higher current densities.

Abstract

In this study, microstructural evolutions, mechanical properties and corrosion performance of coatings made of 7075 Al matrix with B4C or SiC reinforcement deposited on T6 6061 Al alloy using the cold gas dynamic spraying process were investigated. Microstructural surveys have shown that coatings with no discontinuity at the interface as well as with fine grains were obtained and the addition of ceramic particles enhanced the coating density for a prescribed set of spray parameters and nozzle configuration. The presence of ceramic particles in the 7075 Al matrix improved the coatings hardness and wear resistance when compared to unreinforced 7075 Al coatings. Although coatings hardness increased with increasing ceramic particle content, the effect on the coatings wear performance is not that significant. B4C reinforced composite coatings exhibited slightly better wear performance compared to SiC reinforced composite coatings. The cold sprayed coatings showed more noble corrosion potentials but higher corrosion current densities than those of the T6 6061 Al substrate. The addition of ceramic particles into 7075 Al matrix led to increased corrosion current densities when compared to that of unreinforced 7075 Al coating.

Introduction

In engineering applications, the material used in the design can fail when its surface cannot sufficiently withstand the external forces or its surrounding environment. Therefore, surface damages are very important since they affect the system reliability and thus materials surface properties are as important as their bulk properties. However, it is challenging to manufacture a material which can provide both the required bulk and surface properties at the same time, in particular due to issues related to economics. In this context, surface modification of engineering materials has been of interest and surface modification techniques that enhance surface properties such as corrosion and wear resistance have been developed [1]. In these techniques, the underlying material, which meets the required bulk properties is coated with a material that can provide the required surface properties. Manufacturing of a material by these techniques is more economical than producing it as a bulk material through conventional methods.

For instance, in many engineering applications metallic parts are coated with Aluminum (Al) or its alloys to increase the surface corrosion resistance by means of anodic protection due to formation of a very thin and impervious aluminum oxide layer [2]. Although Al based coatings increase the corrosion resistance of metallic parts, these coatings usually do not protect adequately the coated surface from mechanical forces applied during operation due to their poor mechanical properties [3], [4]. The low mechanical properties of Al and its alloys can be greatly enhanced by incorporating hard ceramic reinforcing particles. Al matrix composites (AMCs) have been of importance for a long time due to their superior mechanical properties as compared to conventional monolithic Al without sacrificing the corrosion resistance of Al [5], [6]. The improved properties of AMCs are strongly related to the characteristics of the matrix and reinforcement as well as the reinforcement/matrix interface. B4C, SiC and Al2O3 are excellent reinforcement materials for AMCs due to their high hardness, chemical and thermal stabilities [5], [6]. The success of AMCs in providing a good combination of mechanical strength and corrosion resistance has attracted much attention in the coating industry [6], [7], [8], [9], [10]. Al based composites are usually coated on metallic parts using thermal spraying techniques [6], [7]. These techniques involve high temperature which results in heating of the spray material (usually in powder form) to molten or semi-molten state. The sprayed particles flatten and cool rapidly upon impact on the substrate to form a coating. High process temperatures in thermal spraying tend to favor undesirable effects such as oxidation and grain growth. Conversely, rapid cooling of the molten particles can lead to porosity, formation of thermal stress, distortion and weakened bond strength [11].

In the case of AMC coatings, it is challenging to obtain a homogenous distribution of ceramic particles in the Al matrix combined with low porosity level and good adherence between the substrate and the coating due to the melting of Al during spraying. Furthermore, reactions between ceramic particles and molten Al and/or dissolving of ceramic particles in molten Al at high temperatures lead to reduced ceramic particle efficiency [12].

In recent years, the success of the emerging cold gas dynamic spraying (CGDS) technique in eliminating many of the drawbacks associated with thermal spraying has been demonstrated and the technique can be considered to produce AMC coatings. This technique was discovered in the 1980s and is a solid state deposition technique [13]. Simply, in CGDS, sprayed particles are accelerated by a supersonic inert gas stream and deform plastically upon impact on the substrate to form either a mechanical or metallurgical bond with the substrate [14], [15]. CGDS relies on the particle kinetic energy at impact as opposed to thermal energy of a sprayed particle to produce a coating. Therefore, the most important distinguishing feature of CGDS from thermal spraying techniques is its ability to spray the particles without any melting. Due to the low temperature characteristic of CGDS there is no material loss through melting. Furthermore, low temperature in CGDS allows minimizing the deleterious effects of high temperature such as oxidation, melting, recrystallization and debonding generally encountered in thermal spray processes [3], [11], [16], [17]. As such, CGDS offers similar or better control of the coating properties as compared to thermal spraying techniques.

This success of CGDS in depositing metal coatings has attracted much attention and has given a rise to many efforts in metal matrix composites production using this technique. Cold sprayed composite coatings of Al-Al2O3 [12], [18], [19], [20], [21], Al-SiC [21], Al12Si-SiC [8] and Al-B4C [22] have been reported. However, most of the reported studies mainly focused on microstructural evaluations of these coatings. Previous studies revealed that addition of ceramic particles into metal matrix led to denser coatings [18], [19], [20] improved hardness [8], [18], [20], [21], [22] and bond strength [8], [18], [19], [20] without any phase transformation [12], [22] during spraying.

In the light of this short review, the aim of this study is to produce 7075 Al matrix B4C or SiC reinforced coatings on T6 6061 Al alloy via CGDS. It is believed that CGDS could lead to better microstructures compared to similar coatings produced using thermal spray techniques. Comparison of the effects of B4C and SiC particles on the microstructural evolutions, mechanical properties as well as corrosion performance of the cold sprayed coatings will be performed.

Section snippets

Experimental Studies

Within the scope of this study, B4C (Atlantic Equipment Engineers, USA, d0.5 = 7 μm) and SiC (Atlantic Equipment Engineers, USA, d0.5 = 28 μm) powder particles were mixed separately with 7075 Al (Valimet Inc., USA, d0.5 = 15 μm) powder particles at volume ratios of ceramic particles of 10% and 20% to produce the feedstock powder mixtures. Fig. 1 shows the typical morphologies and particle size distributions of 7075 Al, B4C and SiC powder particles and reveals that 7075 Al particles have a spherical

Results and discussion

BSE micrographs of the typical cross sectional and coating/substrate interface views of unreinforced 7075 Al and composite coatings, produced with feedstock powder mixtures containing 20 vol.% ceramic particles, are shown in Fig. 2. Interface micrographs of the coatings reveal that the coatings adhered to the substrate without any discontinuity. Some porosities can be observed in the cross section images of the unreinforced 7075 Al coatings. The average porosity content was measured as 0.5 ± 0.1 

Conclusions

The results of this study can be state as follows:

7075 Al alloy matrix ceramic particle (B4C and SiC) reinforced composite coatings were successfully produced on T6 6061 Al alloy substrate without any discontinuity at the coating/substrate interface and with uniform distribution of ceramic particles in the 7075 Al matrix.

According to the XRD patterns of the coatings beside the main phases of Al, B4C and SiC there are no other phases in the coatings. It can be concluded that no phase

Acknowledgement

The first author wishes to acknowledge Istanbul Technical University (ITU) for supporting a scholarship to be a visiting researcher at the University of Ottawa. The authors from ITU are grateful to financial support provided by Rectorate of ITU under the project number 34237.

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