Cavitation erosion and jet impingement erosion mechanism of cold sprayed Ni–Al2O3 coating

https://doi.org/10.1016/j.nucengdes.2011.09.038Get rights and content

Abstract

A composite coating was deposited on Inconel 600 substrate by cold spray method using pure Ni powder (60 wt.%) blended with α-Al2O3 (40 wt.%) as feedstock. It is expected to be applied to repair the bellows eroded by the liquid droplet erosion (LDE). Microstructure of the coating was observed using optical microscope (OM) and scanning electron microscopy (SEM). Microhardness of the coating was determined by Vickers hardness tester. Cavitation erosion (CE) experiments were carried out in the distilled water. Jet impingement erosion (JIE) experiments were performed in slurry containing 1 wt.% quartz particle with the flow velocity of 15 m/s at impingement angles of 30°, 60° and 90°, respectively. Cumulative mass loss vs. testing time was used to evaluate the erosion rate of the coating. The erosion mechanism was analyzed by OM, SEM, X-ray diffraction (XRD) and the microhardness measurement. The results show that the composite coating has compact microstructure and relatively high hardness. The resistance to CE of the coating is not as good as that of Inconel 600 substrate due to the weak bonds of the Al2O3 particles. However, the results of the JIE test indicate that the slurry erosion resistance of the coating is better than that of Inconel 600 at the impact angles of 30° and 60°, but not at the normal impact angle.

Highlights

► The cold sprayed Ni-α–Al2O3 coating has a compact microstructure and high hardness. ► The coating resistance to CE is not as good as that of the Inconel 600 substrate. ► The low resistance to CE is due to the low bonds between alumina particles. ► The coating resistance to JIE is better at oblique but not normal impact angles.

Introduction

Bellows expansion joints are designed to absorb the thermal expansion motion and vibration in a connecting system. Wall thinning of the bellows caused by liquid droplet erosion (LDE) or liquid impingement erosion (LIE) threaten the security of the nuclear power plant. The problem is common in high-speed wet steam environments and often causes unexpected outages and millions of dollars in lost revenue. Therefore, it is significant to study and manage to overcome the problem. Generally, three corresponding measures can be applied to deal with this problem: optimizing the design of the bellows to alleviate LDE (Hu et al., 2011); substituting current bellows materials for that with better resistance to LDE (Hu et al., 2010); repairing the eroded bellows by using some coating technique in situ.

The present work partly focuses on the feasibility of in situ restorations of nuclear plant bellows by coating. Two important aspects need to be determined: the selection of appropriate coating technique and coating materials.

Cold spraying or more precisely cold gas dynamic spraying is a superior candidate for the repair in this case. The most notable characteristic of the technique is the low-heat input to the substrate, in which coating formation occurs via plastic impact at high velocity (300–1200 m/s) and ambient temperature to 700 °C which is far below the melting point of the feedstock powder. In addition, cold spraying cannot only provide a coating with lower coating porosity and less oxidation, but also less deformation of the substrate (McCune et al., 2000, McCune et al., 1995, Van Steenkiste et al., 1999) than the conventional coating technique (e.g. flame, arc, plasma and HVOF spraying).

Selection of coating materials is another key problem in the in situ repair. Although Inconel alloy is the first candidate material as a repair coating for its similar chemical compositions and mechanical properties with the substrate, its low hardness may be not good enough for the requirement of high erosion resistance. As a result, the commonly used Ni–Al2O3 composite coating with hardening particles may be suitable to protect the nickel-based Inconel 600 against LDE. Ceramic particles as an intensifying phase can enhance the hardness of the nickel-based substrate as an intensifying phase in the nickel-based material. Furthermore, a microstructure with a highly interconnected ceramic and metal network may achieve some unique characteristics, since the continuous metal and ceramic phase could improve the fracture strength and creep resistance. Because of these advantages, the Ni–Al2O3 composite were used as coating materials by many coating techniques, such as the sol–gel method (Rodeghiero et al., 1995), the hydrogen reduction and hot pressure method (Li et al., 2003), vacuum infiltration casting technology (Yang et al., 2008), brush plating technique (Du et al., 2005), and electroplating (Badarulzaman et al., 2009, Gül et al., 2009, Szczygiel and Kolodziej, 2005). However, most of these conventional coating techniques are unsuitable for the in situ repair of the bellows due to the high heat-input to the thin bellows and the great demand of working space for their large equipment assemblies and complex operation. In addition to the conventional coating techniques, Ni–Al2O3 coating has been manufactured by cold spraying technique (Li et al., 2008). Similar metal–ceramic composite coatings manufactured using cold spraying were also conducted, such as Al–Al2O3 (Lee et al., 2005, Tao et al., 2009, Wang et al., 2010), Ti–Al2O3 (Novoselova et al., 2006) and Cu–Al2O3 (Sudharshan Phani et al., 2007). These work mainly proved that the amount of the brittle and hard Al2O3 phase in ductile substrate had a significant effect on the coating density, hardness and deposition efficiency. However, up to now no report on the resistance of Ni–Al2O3 composite coating to cavitation erosion (CE) and LDE has been found.

One more question is how to evaluate the resistance of the coatings to LDE after determining the coating technique and materials. When the droplet impacts on a target the liquid behaves in a compressible manner generating ‘water-hammer’ pressure, which is responsible for most of the damage resulting from liquid impact. Moreover, the strong tensile stress might cause cavitations inside the liquid droplet and might cause solid surface erosion (Sanada et al., 2008). It was proposed that LDE behavior could be predicted by the CE test or vice versa (Field, 1999, Preece and Brunton, 1980, Thruvengadam, 1970). In addition, slurry erosion (erosion by liquid-particle two-phase flow) can be used to simulate CE (liquid-gas two-phase flow) because the two erosions have the similar erosion properties including materials deformation due to high-velocity impact, and the erosion morphologies. Therefore, CE experiments in conjunction with jet impingement erosion (JIE) experiments were employed to evaluate the resistance of the deposited coatings to LDE. The same method was also used in our previous work (Hu et al., 2010).

The purpose of the present study was to investigate the possibility of the Ni–Al2O3 composite coating as in situ repair coating for thin bellows expansion joints and its damage mechanism under CE and JIE conditions. It will be of great significance to extend the application of this kind of coatings.

Section snippets

Materials and coating deposition

Commercial nickel and alumina powders were used as feedstock materials. The SEM morphologies of the two powders as-purchased are shown in Fig. 1a and b, respectively. The nickel powder was produced by water atomization process and exhibited spherical or ellipsoidal morphology in the size range of 10–50 μm. The final powders partly shows agglomerate and the boundaries can be clearly detected in high magnification (Fig. 1a). As seen in Fig. 1b, the α-Al2O3 powder used is angular with an average

Microstructure and hardness

The image of the cold sprayed plate with the sketch of the cross section of the coating is shown in Fig. 4. The coating was rough with triangles in cross section with a maximum thickness of approximately 1100 μm and a minimum thickness of 620 μm. The triangles were mainly caused by the mismatch of the spray gun travels. The microstructure did not change with the coating thickness and is not shown here.

The OM images of the polished and etched Ni–Al2O3 coating surface at different magnifications

Conclusions

The cold sprayed Ni–Al2O3 coating on Inconel 600 substrate with the ratio of Ni-60 wt.% and α-Al2O3-40 wt.% has a compact microstructure and low porosity. The bonding mechanism was dominated by deformation and interlocking of the inter-particles accompanied with local metallurgical bonding.

The resistance to CE of the coating was lower than that of the uncoated Inconel 600, which was attributed to the low cohesion between the Ni and Al2O3 particles. However, this study will help to the further

Acknowledgement

The authors acknowledge the Financial Support of the Special Funds for the Major State Basic Research Projects (2011CB610504).

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