摘要
针对如何降低NiCrCu涂层氧化物含量以改善涂层内粒子层间结合的问题,本研究设计了含B的NiCrCu粉末,以期通过B在高温熔滴飞行中的牺牲氧化并蒸发去除氧化物机制,在大气等离子喷涂条件下获得无氧化物的熔滴制备高致密的金属涂层。为此,采用粒度30~50μm的NiCrCu1.5B和NiCrCu4B喷涂粉末,研究了粉末成分与大气等离子喷涂工艺参数对涂层成分、组织结构及力学性能的影响。结果表明,等离子喷涂制备NiCrCuB涂层时,基体在等离子射流中的受热程度对涂层氧化物含量影响显著,随喷枪移动速率和喷涂距离的增加,等离子射流对基体加热效果减弱,涂层氧化物含量显著降低。采用NiCrCu4B粉末制备涂层时,涂层氧含量随喷涂距离的增加而显著减小,这与迄今等离子喷涂金属合金涂层的氧含量随距离的增加而增加的规律完全相反,该结果表明NiCrCuB熔融粒子中的硼在粒子飞行中具有优先氧化的去氧化效应。采用含硼量为1.2 wt.%的NiCrCu1.5B粉末喷涂涂层时,涂层内氧元素含量呈先降低后增加趋势,表明B含量低于约0.6 wt.%的临界值时,无法实现完全的氧化保护,致使其他元素随飞行距离的增加逐步氧化所致。针对等离子喷涂中金属熔滴飞行中的氧化与碰撞基体沉积后的氧化两种机制,采用较高B含量粒子可使飞行中氧化得到抑制,通过强化熔滴沉积后的冷却过程,可显著降低涂层氧含量,达到最低氧含量值,约0.6 wt.%。研究表明涂层的结合强度约为40MPa,受喷涂参数的影响有限。涂层中的B以硼化物弥散分布使得涂层的硬度显著增加,当B含量从0.2 wt.%增加至3.2 wt.%时,涂层的硬度呈线性相关,从280HV0.3增加至700HV0.3。
The oxidation during plasma spraying of metal coatings in the ambient atmosphere always occurs.The resultant oxide inclusion not only reduces the wettability of molten droplets to splat surface and subsequently inhibits the lamellar bonding formation,but also degrades the performance of the coatings.It is still great challenge to effectively reduce the oxide contents in atmospheric plasma sprayed metal coatings by exploring novel approaches.Accordingly,we revently proposed a novel approach to achieve oxide-free molten metal droplets in atmospheric plasma spraying by utilizing sacrificial oxidation of boron in the in-flight molten droplet following by subsequent evaporation for high performance metal coating deposition.Therefore,in this study,boron containing NiCrCu spray powders were designed to examine the effect of powder composition and spray parameters on the oxide inclusion in the NiCrCu coatings and to clarify the sacrifical oxidation effect of boron during in-flight of molten droplet.Two powders with nominal compositions of NiCrCu1.5B and NiCrCu4B were used for atmospheric plasma spraying(APS)in a particle size range of 30~50μm.The in-flight particle temperature during plasma spraying was measured by commercial particle velocity and temperature diagnosis system DPV-2000.The microstructure of the coating was characterized by scanning electron microscopy and X-ray diffraction.The contents of oxygen and boron in the coating were measured by chemical approach using Nitrogen and Oxygen Determinator and Ion Coupled Plasma Mass Spectropy,respectively.The porosity of the coatings was estimated by image analyzing.The microVickers hardness and adhesive strength of the coatings were measured.The measurement of spraying molten particle temperature yielded the values higher than 2000℃,which fulfilled the requirement to evaporate boron oxide.The results showed that the porosity of the coatings decreased with the increase of the arc power and the oxygen content in the coatings decreased with the increase of the torch traverse speed.However,the change of the oxygen content in the NiCrCu4B coatings plasma-sprayed with higher boron content in spray powder against the spray distance presented the trend that always decreased with the increase of the spray distance.This trend was opposite to those observed for thermal spraying of metal coatings.Taking the fact that the boron content in the coating always decreased with the increase of spray distance,the results confirmed that the oxidation of boron and subsequently the evaporation continuously occurred and the in-flight oxidation of alloying elements was suppressed by the preferential sacrificial oxidation of boron in the molten droplets.Thus,the oxide inclusion in the NiCrCu4B coatings was mainly introduced by post-deposition oxidation.On the other hand,the results showed that the oxygen content in the NiCrCu1.5B coatings decreased with the increase of the spray distance up to spray distance of 110 mm and then turned to increase with the further increase of the spray distance.Taking account of boron content in the in-flight NiCrCu particle,it could be concluded that there existed a critical boron content of about 0.6 wt.%in the in-flight particle below which the boron could not provide sufficient protection of other alloying elements from oxidation.When the oxide inclusion was mainly resulted from post-deposition oxidation,the oxygen content in the coating was dependent on cooling condition of coating during deposition.It was confirmed that the oxygen content in the NiCrCu4B coating samples was significantly reduced through enhancing the cooling effect from the back of the substrate.As a result,NiCrCuB coating with an oxygen content of 0.6 wt.%was deposited.The measurement yielded the adhesive strength of about 40 MPa for NiCrCu coating which was influenced little by spray conditions.It was also found that the boron content changed with spray conditions which influenced the consume of boron during in-flight.The residual boron was present in the coating as dispersed borides by which the NiCrCu coating is strengthened.As a result,it was found that with the increase of boron content in the coating from 0.2 wt.%to 3.5 wt.%the hardness of NiCrCu coating was increased linearly from 280 HV0.3 to 700 HV0.3.
作者
孙浩
董昕远
任媛
雒晓涛
李成新
Mahrukh M.
朱永胜
李长久
Sun Hao;Dong Xinyuan;Ren Yuan;Luo Xiaotao;Li Chengxin;Mahrukh M.;Zhu Yongsheng;Li Changjiu(State Key Laboratory for Strength of Metal Materials,School of Materials Science and Engineering Xi'an Jiaotong Univeristy,Xi'an 710049)
出处
《热喷涂技术》
2021年第1期1-12,共12页
Thermal Spray Technology
基金
国家自然科学基金重点项目(52031010,U1837201)。
关键词
喷涂参数
NiCrCuB涂层
成分
微观组织
力学性能
氧化机制
Spray parameters
NiCrCu coating
Composition
Microstructure
Mechanical property
Oxidation mechanism
