摘要
针对Ni60A/WC复合涂层硬质相分布不均、减摩性能不足等问题,利用碳纳米管(CNTs)的高熔点和优良的自润滑性能,采用激光熔覆技术在45钢基体表面制备了添加不同含量CNTs的镍基耐磨涂层。通过扫描电子显微镜、能谱仪和X射线衍射仪(XRD)分析了涂层的显微组织、元素组成和相组成。通过显微硬度计和摩擦磨损试验机测试了涂层的硬度和耐磨性能。XRD图谱表明:熔覆层主要由Ni-Cr-Fe固溶体和WC、W_(2)C、Cr_(3)C_(2)、Cr_(7)C_(3)、Cr_(23)C_(6)、B_(4)C等硬质相组成。显微组织结果表明:CNTs的添加促进了异质形核,有利于硬质相均匀分布,明显细化了熔覆层的显微组织。由于CNTs具有细化晶粒以及提升自润滑性能的作用,适量添加CNTs可提升熔覆层的显微硬度和耐磨性能。当CNTs的质量分数为0.5%时,熔覆层的显微硬度为1100 HV,摩擦系数为0.3,磨损体积为1.24×10^(-4)mm^(3)。相比于不添加CNTs的熔覆层,硬度提升了10%,磨损体积减少了35%。研究结果为制备性能优良的镍基复合涂层提供了参考。
Objective The 45 steel is widely used in the manufacture of gears, connecting rods, pistons, and other mechanical parts due to its good cold/hot workability, high strength, good toughness, and low price. However, the 45 steel cannot meet the requirements in the fields of mining machinery, petroleum exploration, and marine engineering because of the harsh working environment. Thus, the surface modification technologies are usually adopted to strengthen the surface of the 45 steel. Among them, laser cladding is a promising technology. The cermet coating prepared by laser cladding can significantly improve the properties of the surface. Among many cladded materials, the Ni60 A/WC composite coating has been widely used because of its high hardness and good wear resistance. However, it also has some problems such as the uneven distribution of hard phase and insufficient lubrication property. Therefore, in this study, based on the characteristics of high melting points and the excellent selflubricating property of carbon nanotubes(CNTs), the Ni-based wear-resistant coatings with different contents of CNTs are prepared on the surfaces of 45 steel by laser cladding, and the results show that the hardness and wear resistance of the Ni60 A/WC composite coating are significantly improved. This work provides a reference for the preparation of Ni-based composite coatings with excellent properties.Methods The 45 steel with dimension of 40 mm×20 mm×10 mm is used as the substrate on which Ni60 A, WC, and CNTs are used as cladding powders. As for the Ni60 A and WC powders, their particle sizes are 48-106 μm and 100 μm, respectively. Before laser cladding, CNTs are first dispersed by ultrasonic wave. Then the Ni60 A and WC powders are added into CNTs dispersions with different contents according to the designed ratio and subsequently stirred by magnetic force at the speed of 500 r/min for 30 min. After magnetic stirring, the ball milling is carried out with planetary ball mill at a milling speed of 275 r/min, a mass ratio of grinding ball to material of 3∶1, and milling time of 1 h. After ball milling, the mixed powder is dried, and the 5% mass fraction polyvinyl alcohol(PVA) binder is added and preplaced on the surface of the substrate. The thickness of the preplaced coatings is approximately 1 mm. Finally, the laser cladding experiment is carried out under the process parameters of 1500 W laser power, 5 mm/s scanning speed, and 10 mm defocusing distance, in which the microstructure of the cladding layer is observed by scanning electron microscope, the composition of the cladding layer is analyzed by energy dispersive spectrometry, the phase of the cladding layer is analyzed by X-ray diffraction, the microhardness from the top of the cladding layer to the substrate is measured by a microhardness tester, and the wear resistance of the coating layer is tested by tribometer under dry friction conditions.Results and Discussions The cladding layer has good metallurgical bonding with the substrate. The results show that the addition of CNTs improves the laser absorption characteristics of the powder, increases the dilution ratio,enhances the convection in the molten pool, and forms Marangoni flow from the center to the periphery of the molten pool under the effect of surface tension, which promotes the uniform distribution of hard phase in the cladding layer(Fig. 3). As the content of CNTs increases, the microhardness of the cladding layer first increases and then decreases. When an appropriate amount of CNTs is added, the CNTs possessing a high melting point, play the role of heterogeneous nucleation and improve the nucleation probability. Thus, the coarse dendrite structure in the cladding layer is obviously refined, the spacing between adjacent dendrites becomes smaller, and the dendrites are closer to each other(Fig. 7). The refinement of grains hinders the movement of dislocations and improves the microhardness of the cladding layer. When the mass fraction of CNTs is 0.5%, the microhardness of the cladding layer reaches 1100 HV, which is 10% higher than that of the cladding layer without CNTs(Fig. 8). With the increase of CNTs content, the friction coefficient and wear volume of the cladding layer first decrease and then increase. Due to the improvement of the hardness of the cladding layer and the excellent self-lubricating property of CNTs, the friction coefficient is 0.3 and the wear volume is 1.24×10^(-4)mm^(3)when the mass fraction of CNTs is 0.5%. Compared with that of the coating layer without CNTs, the wear volume here is reduced by 35%(Fig. 9).Conclusions In this study, the Ni60 A/WC composite coatings with different contents of CNTs are successfully prepared on the surface of 45 steel by laser cladding. The results show that the five coatings are composed of Ni-CrFe solid solution and hard phases such as WC, W_(2)C, Cr_(3)C_(2), Cr_(7)C_(3), Cr_(23)C_(6), and B_(4)C. Adding an appropriate amount of CNTs promotes the uniform distribution of hard phase, refines the microstructure of cladding layer, and greatly improves the microhardness of cladding layer. When 0.5% mass fraction CNTs are added, the microhardness reaches 1100 HV, the friction coefficient decreases to 0.3, and the wear volume decreases to 1.24×10^(-4)mm^(3), indicating CNTs have excellent self-lubricating properties and Ni-based composite coatings with excellent properties can be prepared by adding an appropriate amount of CNTs.
作者
张家诚
江吉彬
黄旭
练国富
陈昌荣
冯美艳
周梦宁
Zhang Jiacheng;Jiang Jibin;Huang Xu;Lian Guofu;Chen Changrong;Feng Meiyan;Zhou Mengning(School of Mechanical&Automotive Engineering,Fujian University of Technology,Fuzhou,Fujian 350118,China)
出处
《中国激光》
EI
CAS
CSCD
北大核心
2022年第2期224-233,共10页
Chinese Journal of Lasers
基金
第三批福建省特殊支持“双百计划”人才项目(闽委人才2018-5号)
福建工程学院科研启动基金(GY-Z18163)。
关键词
激光技术
激光熔覆
碳纳米管
镍基涂层
耐磨性能
laser technique
laser cladding
carbon nanotubes
Ni-based coating
wear resistance
