超音速火焰喷涂制备纳米和微米结构WC-η涂层的耐熔锌腐蚀性能

doi:10.15541/jim20160648

摘要/Abstract

摘要：

以钨氧化物、钴氧化物和炭黑为原料, 通过原位还原碳化反应制备纳米WC-η(η为Co₃W₃C、Co₆W₆C等缺碳相)复合粉, 粉末平均粒径为155 nm。该复合粉经团聚造粒制备得到具有高致密性和良好流动性的热喷涂粉末。以此纳米结构和商业化的微米结构低碳WC-12Co粉末作为喂料, 通过超音速火焰喷涂制备硬质合金涂层。结果表明, 纳米结构涂层中生成了一定量等轴状的W₂C相, 裂纹主要沿晶界或相界面扩展, 而微米结构涂层中除W₂C外还含有较多的W相, 主要包裹在WC颗粒表面, 穿晶断裂比例较高, 裂纹扩展路径较平滑。由于纳米结构涂层组织致密、晶粒细小、界面积大, 因此比微米结构涂层具有更高的硬度和断裂韧性。两种涂层在熔融锌液中浸泡200 h后, 微米结构涂层中产生了较多的横向和纵向裂纹, 导致材料的大面积剥落和基材腐蚀; 纳米结构涂层中没有发生锌的浸蚀, 在局部产生了少量纵向裂纹, 裂纹间隙被钨钴氧化物所填充, 反而抑制了熔锌对涂层的腐蚀, 因此纳米结构涂层表现出更高的耐熔锌腐蚀性能。

关键词: WC-η复合粉, 纳米结构涂层, 韧性, 裂纹扩展途径, 耐熔锌腐蚀性能

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

中图分类号:

TG174

杨涛, 王海滨, 宋晓艳, 刘雪梅, 侯超, 王学政. 超音速火焰喷涂制备纳米和微米结构WC-η涂层的耐熔锌腐蚀性能[J]. 无机材料学报, 2017, 32(8): 806-812.

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.

图/表 10

表1 涂层喷涂工艺参数

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

图1 原位合成的WC-η复合粉末的显微形貌(a)及其粒径分布(b)

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

图2 纳米和微米结构低碳WC-12Co热喷涂粉末的XRD图谱

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

图3 优化的团聚造粒工艺下制备的纳米结构低碳WC-12Co热喷涂粉末和商用微米结构喷涂粉末的显微形貌

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

图4 HVOF喷涂纳米和微米结构WC基涂层的XRD图谱

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

图5 两种涂层的SEM照片

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

图6 两种涂层的硬度和断裂韧性

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

图7 两种涂层上压痕的典型形貌及裂纹扩展路径

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

图8 微米结构硬质合金涂层在熔锌中浸泡200h后的显微组织及局部区域的能谱分析

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)

图9 纳米结构WC-η涂层在熔锌中浸泡200 h后的SEM照片及局部区域的能谱分析结果

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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