Slurry erosion behaviour and mechanism of HVOF sprayed micro-nano structured WC-CoCr coatings in NaCl medium

Highlights

• Various structured WC-10Co4Cr coatings are deposited by two HVOF processes.

• The multiscale WC particles are evenly distributed in multimodal coating.

• The multimodal WC-10Co4Cr coating exhibits the best comprehensive performance.

• Crack growth is the dominate mechanism of coatings' slurry erosion behavior.

• High corrosion resistance is critical to reduce erosion rate in NaCl solution.

Abstract

In this work, we fabricate nanostructured, bimodal, and multimodal WC-10Co4Cr coatings via two high velocity oxy-fuel (HVOF) systems and investigate the coatings’ microstructure, mechanical, electrochemical properties and slurry erosion resistance in 3.5 wt% NaCl solution. The multimodal coating, with a dense microstructure, exhibits lowest decarburization and highest resistance to both corrosion and slurry erosion. The slurry erosion resistance of multimodal coating is enhanced approximately 18% and 11% than the nanostructured and bimodal coatings. However, the performance can easily drop to the worst by changing the HVOF spraying method. The outstanding performance can be attributed to uniform distribution of the multiscale WC grains in the multimodal coating, which effectively reduces the material loss rate during corrosion-accelerated erosion in NaCl solution.

Introduction

The slurry erosion, a major cause for the component failures in dredging engineering, is widely observed in various equipment such as hoppers, mud pumps, reamers and blades [1,2]. The problem becomes more complicated for the equipment operated in the ocean as the seawater is a corrosive medium, causing serious damage to the equipment due to corrosion-accelerated erosion [3]. Because slurry erosion only occurs on the surface, surface modification is an effective means to improve the slurry erosion resistance. Several surface engineering techniques have been developed to minimize the erosion rates. Among them, thermal spray technologies, especially air plasma spray (APS) and high velocity oxy-fuel (HVOF) spraying, have attracted an increasing interest from researchers since it is able to fabricate high-quality erosion resistant coatings, for instance, WC-based cermet coatings [4,5].

To date, a number of studies have demonstrated the high hardness, toughness, and anti-wear performance of the WC-Co cermet coatings sprayed by HVOF [6,7]. Moreover, by adding chromium, WC-CoCr coatings show better corrosion and wear resistance than WC-Co coatings [8,9]. Galileo et al. [10] founded that the slurry erosion resistance of HVOF sprayed conventional WC-10Co4Cr coating enhanced approximately 50% than martensitic stainless steel at 30° impact angles. Meanwhile, Cui et al. [11] investigated the effect of the rotational speeds on the slurry erosion behavior of WC-CoCr coating and found that cavitation and abrasive wear were the main failure mechanism of the coating at low and high erosion velocity, respectively.

The performance of WC-based coatings strongly depends on the deposition process [12,13]. HVOF can be mainly divided into high velocity oxygen gas fuel spraying (HVOGF) and high velocity oxygen liquid fuel spraying (HVOLF) [14,15]. The fuel in HVOLF is kerosene, while propane and hydrogen are often employed as fuel in HVOGF. Furthermore, the composition and structure of feedstock powders also play a critical role. Thakur et al. [16] added multi-walled carbon nanotubes (MWCNTs) into nanostructured WC-CoCr powder and demonstrated the addition of MWCNTs effectively improved the erosion resistance of nanostructured WC-CoCr coating. Liu et al. [17] added CeO₂ into WC-12Co feedstock powder and found that the HVOF sprayed WC-12Co/CeO₂ coating exhibited lower corrosion-accelerated erosion rate compared to the nanostructured WC-12Co coating.

Previous research has shown that the hardness and fracture toughness of WC-based coatings can be simultaneously enhanced by employing nano WC, but the properties often diminish due to severe oxidation and decarburization of the nano particles [18,19]. This leads to the innovation and research of bimodal coatings which contain both nano and micro WC particles [20]. Yuan et al. [21] deposited bimodal WC-12Co coatings by mixing submicron WC into conventional WC-12Co feedstock powder and observed even distribution of submicron WC particles at the interface of WC-Co splats, which effectively enhanced the coatings' sliding wear resistance. Ding et al. [22] prepared bimodal and conventional WC-10Co4Cr coatings by HVOLF and HVOGF and found the HVOLF sprayed bimodal coating had the least WC decarburization, lowest porosity, highest hardness, fracture toughness, and erosion resistance. Liu et al. [23] studied the effect of average particle size, slurry concentration and pH value on the slurry erosion behaviors of bimodal WC-CoCr coating, which showed better erosion resistance than the conventional coating under all conditions. In addition to bimodal coatings, recent study has demonstrated the improvement of wet abrasive resistance from HVOF sprayed multimodal WC-10Co4Cr coatings which contain micron, submicron, and nanometer WC grains [24]. To date, no report has been found on the multimodal WC-CoCr coatings’ slurry erosion resistance and mechanism in 3.5 wt% NaCl medium.

In this work, nanostructured, bimodal and multimodal WC-10Co4Cr coatings were prepared by different HVOF processes. The slurry erosion resistance of various structured coatings was investigated in 3.5 wt% NaCl solution. The effect of WC grain size and different HVOF processes on the coatings’ slurry erosion resistance were analyzed. These results can provide a valuable reference for the selection and application of anti-slurry erosion coatings for marine equipment.