Corrosion Resistance of HVOF-sprayed Nano- and Micon-structured WC-η Coatings against Molten Zinc

doi:10.15541/jim20160648

Abstract

Abstract:

WC-η (i.e. Co₆W₆C, Co₃W₃C, etc) composite powder was synthesized by in situ reduction and carbonization reactions using tungsten oxide, cobalt oxide and carbon black as raw materials. Mean particle size of the as-synthesized composite powder is about 155 nm. Thermal spraying powder with a high density and an excellent flowability was prepared using WC-η composite powder by agglomeration and heat-treatment. Cemented carbide coatings were then fabricated by high velocity oxy-fuel (HVOF) spraying technique using nanostructured and commercially available low-carbon WC-12Co thermal spraying powders as feedstock. The results show that equiaxed W₂C particles form at certain amount in the nanostructured coating while cracks propagate mainly along grain boundaries and phase interfaces. However, the micron-structured coating still contains W besides W₂C, which mainly distribute in outer surface of WC grains. The micron-structured coating has stronger tendency to get transgranular fracture. As a result, the cracks go forward along a relatively straight path. Due to higher density, finer grain size and much larger interface area, the nanostructured coating has simultaneously higher hardness and fracture toughness as compared to the micron-structured coating. After immersion into molten zinc for 200 h, the micron-structured coating suffers from more serious cracking along the directions perpendicular and parallel to the interface between coating and substrate, which causes the large-scale exfoliation of coating material and the corrosion of substrate. In contrast, zinc diffusion is not observed in the nanostructured coating and only a few cracks perpendicular to the coating/substrate interface are locally produced. Moreover, the inner surfaces of the crack are covered with tungsten and cobalt oxides, which inhibit corrosion of the nanostructured coating in molten zinc. Therefore, the nanostructured coating has a promising corrosion resistance against molten zinc.

Key words: WC-η composite powder, nanostructured coating, toughness, crack propagation path, corrosion resistance to molten zinc

CLC Number:

TG174

YANG Tao, WANG Hai-Bin, SONG Xiao-Yan, LIU Xue-Mei, HOU Chao, WANG Xue-Zheng. Corrosion Resistance of HVOF-sprayed Nano- and Micon-structured WC-η Coatings against Molten Zinc[J]. Journal of Inorganic Materials, 2017, 32(8): 806-812.

Figures/Tables 10

Table1 Spraying parameters of both coatings

Kerosene/ GPH	Oxygen/ SCFH	Distance/ mm	Carrier gas/ SCFH	Feed rate/ (r·min^-1)
6.0	2000	380	23	6.2

Fig. 1 Morphology (a) and particle size distribution (b) of the in situ synthesized WC-η composite powder

Fig. 2 XRD patterns of nano- and micron-structured WC-12Co thermal spray powders with low amount of carbon

Fig. 3 Morphologies of nanostructured low-carbon WC-12Co thermal spray powder with prepared under optimized granulation process and commercially available micron-structured counterpart(a) Nanostructured feedstock powder; (b) Cross-sectional microstructure of a single nanostructured spray particle with WC phase marked in white, η phase marked in green and pores marked in black; (c) Micron-structured feedstock powder; (d) Cross-sectional microstructure of a single micron-structured spray particle

Fig. 4 XRD patterns of HVOF sprayed nanostructured and micron-structured WC-based coating

Fig. 5 SEM microstructures of two kinds of coatings(a, b) Nanostructured coating; (c, d) Micron-structured coating

Fig. 6 Hardness and fracture toughness of two kinds of coatings

Fig. 7 Typical morphologies of indentations applied on both coatings and its corresponding crack propagation paths(a, b) Nanostructured coating; (c, d) Micron-structured coating

Fig. 8 Microstructures of the micron-structured cemented carbide coating after immersion into molten zinc for 200 h and EDS analyses at local areas of the coating(a) Interlayer fracture of the coating and Fe and O distributions at a local area of the crack, and (b) locally magnified image of (a)

Fig. 9 Microstructure of the nanostructured WC-η coating after immersion into molten zinc for 200 h and EDS analysis at local areas of the coating(a) Typical microstructure; (b) Locally generated crack and oxygen distribution in the center of the crack; (c) Elemental analysis of the rectangular region in (a); (d) Elemental analysis of the region marked with “+” in (b)

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