不锈钢表面冷喷涂铜涂层制备及性能研究

Preparation and Properties of Cold Sprayed Copper Coating on Stainless Steel Surface

目的通过工艺的匹配优化,采用冷喷涂技术在不锈钢表面制备高结合强度铜涂层,并研究热处理工艺对不锈钢表面冷喷涂铜涂层组织及性能的影响规律。方法分别以高纯氮气和氦气作为加速气体,通过冷喷涂技术,在1 mm厚的304不锈钢基体表面制备铜涂层。采用光学显微镜(OM)对涂层的孔隙率及微观组织结构进行表征。采用场发射扫描电子显微镜(FESEM)对涂层拉伸断面结构进行分析。借助维氏显微硬度仪、万能拉伸试验机和涡流导电仪测试分析退火热处理工艺对不锈钢基体表面冷喷涂铜涂层硬度、结合强度和电导率的影响规律。结果利用氮气作为加速气体,在薄304不锈钢基体上获得铜涂层困难,涂层形成后,易发生整体剥落。使用氦气作为加速气体,可在薄304不锈钢板表面成功制备结合强度高于81.7 MPa、硬度为99.6HV0.1、孔隙率小于0.1%的高致密铜涂层。退火热处理引起涂层组织再结晶,可显著消除冷喷涂过程中的加工硬化影响。随着热处理温度从300℃上升到500℃,涂层硬度由99.6HV0.1下降至63.7HV0.1。退火温度为400℃时,涂层导电率最优(93.94%IACS)。当热处理温度升高到500℃,涂层导电率异常下降(89.02%IACS),分析认为过高的热处理温度会造成涂层内部氧扩散偏聚,缺陷增加,导致涂层电导率下降。结论相较于氮气作为加速气体,采用氦气作为加速气体进行冷喷涂,可在薄不锈钢基体上制备的涂层具有高致密度、高结合强度的铜涂层。退火热处理对铜涂层硬度和导电性能的影响较大,涂层经过退火热处理后,电导率可以达到和铸态铜电导率相当的水平。

Cold spraying is a kind of low-temperature solid-state coating process in which spraying particles undergo extensive plastic deformation at extremely high strain-rates.As a result,cold sprayed coatings have excellent mechanical,electrical and thermal properties.Due to the large hardness difference between copper and stainless steel,it is difficult to prepare copper coating on stainless steel.Therefore,the work aims to prepare copper coatings with high interfacial bonding strength on thin 304 stainless steel substrates by optimizing the cold spraying process,and to study the effect of heat treatment process on the microstructure and properties of cold sprayed copper coatings.The 304 stainless steel plate was cut into 100 mm×100 mm×1 mm by wire cutting,and the base material was polished and cleaned.With high-purity nitrogen and helium as accelerating gases,copper coating was prepared on the surface of 1 mm thick 304 stainless steel substrate by cold spraying.Optical microscope(OM)was used to characterize the porosity and microstructure of the coating.The tensile cross-sectional structure of the coating was analyzed by field emission scanning electron microscopy(FESEM).The deposition appearance of single particle on the surface of stainless steel was observed by changing the powder feeding speed.Vickers microhardness tester,universal tensile testing machine and eddy current conductivity tester were used to test and analyze the effect of annealing heat treatment on hardness,bond strength and electrical conductivity of cold sprayed copper coating on the surface of 304 stainless steel substrate.It was difficult to obtain a copper coating on the 304 stainless steel substrate with nitrogen as the accelerating gas,and the coating peeled off during deposition and failed to form effective bonding.With helium as the accelerating gas,a high-density copper coating with a bonding strength higher than 81.7 MPa,a hardness of 99.6HV0.1,and a porosity of less than 0.1%could be successfully obtained on the surface of the 304 stainless steel plate.Compared with nitrogen,helium used as the accelerating gas caused a larger plastic deformation of the 304 stainless steel substrate by the accelerated particles.The recrystallization caused by annealing heat treatment could significantly eliminate the effect of work hardening during the cold spraying process.As the heat treatment temperature increased from 300℃to 500℃,the coating hardness decreased from 99.6HV0.1 to 63.74HV0.1.When the annealing temperature was 400℃,the conductivity of the coating was the best(93.94% IACS).When the heat treatment temperature increased to 500℃,the conductivity of the coating decreased abnormally(89.02%IACS).By analyzing the microstructure of the coating,the high heat treatment temperature caused the diffusion and segregation of oxygen inside the coating and resulted in the increase of the defects and the decrease of electrical conductivity.Compared with nitrogen,the helium can be used as the accelerating gas to prepare a coating with high density and high bonding strength on thin stainless steel substrate.Annealing heat treatment has a great effect on the hardness and electrical conductivity of the copper coating,and the conductivity of the coating after annealing heat treatment can reach a level comparable to that of as-cast copper.