HVOF喷涂纳米结构WC-12Co涂层的组织结构分析

纳米结构WC-12Co涂层的研究目前已受到了广泛重视,对其组织结构及影响因素的研究有利于提高涂层性能.采用HVOF工艺制备了纳米结构、多峰结构及普通微米结构3种WC-12Co金属陶瓷复合涂层,并采用SEM、XRD等对粉末及涂层的显微形貌、组织结构进行了分析;探讨了粉末在喷涂过程中的氧化脱碳机理,并指出了与之相关的影响因素.结果表明:纳米结构WC-12Co涂层结构致密,孔隙率低,与基体结合状态良好;纳米粉末在喷涂过程中比微米粉末氧化失碳严重,并发生了不同的纳米晶粒的长大;纳米粉末在喷涂过程中的氧化脱碳程度不仅与喷涂工艺有关,还在很大程度上取决于粉末本身的结构特性.     

Effects of Rare Earth Elements on the Microstructure and Mechanical Properties of HVOF-Sprayed WC-Co Coatings

Rare earth has been widely used in materials manufacturing to improve hardness and toughness. In this paper, conventional, nanostructured, and rare earth CeO₂-doped WC-12Co powders were sprayed by using HVOF spraying technology. Microstructure, hardness, elastic modulus, and fracture toughness of the three coatings were investigated. The results showed that nanostructured WC-12Co coatings possessed the densest microstructure and excellent combination of strength and toughness. The WC particles with the size ranging from 50 to 500 nm distributed uniformly in the nanostructured WC-12Co coating. The average free path of Co matrix in rare earth-doped WC-12Co coating was shorter than that of conventional WC-12Co coating. XRD results showed no obvious decarburization in all three coatings. The addition of rare earth could improve the mechanical properties of the coating compared with that without rare earth. The hardness value of nanostructured WC-12Co coating (12.2 GPa) was similar to that of rare earth-doped WC-12Co coating (12.2 GPa), which was 15.1% higher than that of conventional WC-12Co coating. The elastic modulus and fracture toughness of nanostructured WC-12Co coating were the highest, and that of conventional WC-12Co coating was the lowest.     

Sliding wear evaluation of hot isostatically pressed thermal spray ceramet coatings

The principal aim of this study was to compare the sliding wear performance of as-sprayed and Hot Isostatically Pressed (HIPed) thermal spray cermet (WC-12Co) coatings. Results indicate that HIPing technique can be successfully applied to post-treat thermal spray cermet coatings for improved sliding wear performance, not only in terms of coating wear, but also in terms of the total volume loss for test couples. WC-12Co coatings sprayed by a HVOF system were deposited on SUJ-2 bearing steel substrate and then encapsulated and HIPed at 850 °C for one hour. A high frequency reciprocating ball on plate rig was used to measure the sliding wear resistance of these coatings in dry conditions under steel and ceramic contact configurations at two different loads. Results are discussed in terms of coating microstructure, microhardness, fracture toughness and residual stress evaluations. Microstructural investigations indicate fundamental changes in grain morphology, whereas x-ray diffraction revealed beneficial transformations in phase composition of these coatings during the HIPing post treatment. The effects of these microstructural changes on the physical properties and wear resistance are discussed.     

Sliding and abrasive wear resistance of thermal-sprayed WC-CO coatings

We studied the resistance of the coatings to abrasive and unlubricated sliding wear of 40 WC/Co coatings applied by high velocity oxygen fuel (HVOF), high-energy plasma spray (HEPS), and high velocity plasma spray (HVPS), using commercial and nanostructured experimental powders. The hardness of the coatings varies from 3 to 13 GPA, which is much lower than that of sintered samples (10 to 23 GPA) because of the porosity of the coatings. Phase analysis by x-ray diffraction revealed various amounts of decarburization in the coatings, some of which contain WC, W₂C, W, and η phase. The abrasive and sliding wear resistance is limited by the hardness of the samples. For a given hardness, the wear resistance is lowered by decarburization, which produces a hard but brittle phase. Nanocarb powders have the shape of thin-walled hollow spheres that heat up rapidly in the gun and are more prone to decarburization than commercial materials. The work shows that, in order to obtain the performance of nanostructured coatings, the powder and spray techniques must be modified.     

A Comparison of Mechanical and Tribological Behavior of Nanostructured and Conventional WC-12Co Detonation-Sprayed Coatings

In the present study, WC-12Co coatings were deposited by detonation-spraying technique using conventional and nanostructured WC-12Co feedstock at four different oxy/fuel ratios (OF ratio). The coatings exhibited the presence of phases like W₂C and W due to the decarburization of the WC phase, and the proportions of these phases were higher in the nano WC-12Co coatings compared with conventional WC-12Co coatings. Coating hardness and fracture toughness were measured. The tribological performance of coatings was examined under dry sand rubber wheel abrasion wear, and solid particle erosion wear conditions. The mechanical and wear properties of coatings were influenced by degree of decarburization and more so in the case of nanostructured WC-Co coatings. The results indicate that the extent of decarburization has a substantial influence on the elastic modulus of the coating which in turn is related to the extent of intersplat cracking of the coating.     

爆炸喷涂WC-12%Co涂层的滑动磨损性能

采用爆炸喷涂技术制备纳米和普通WC-12%Co涂层,用往复试验机对涂层的干滑动磨损性能进行了研究,分析了涂层磨损前后的形貌、结构及成分变化.结果表明:相同的喷涂条件下,WC-12%Co纳米涂层比普通涂层结构均匀、致密,但碳化物分解严重.尽管纳米涂层与普通涂层具有相近的硬度,但普通涂层的耐磨性优于纳米涂层,尤其是在重载条件下.普通涂层的磨损机制为微切削;纳米涂层在轻载(10 N)下,以塑性变形为主要磨损机制,随载荷增加至30 N,纳米WC粒子不能起到阻抗陶瓷球对磨副的磨削作用,而是随粘结相一起被去除,同时由于纳米涂层脱碳导致的层间结合薄弱,在滑动磨损中易发生成片剥落,耐磨性大幅下降.     

Influence of Powder Porous Structure on the Deposition Behavior of Cold-Sprayed WC-12Co Coatings

The limited deformation of hard cermet particles and impacted coating makes it difficult for conventional thermal spray powders to continuously build up on impact in cold spraying. In this study, three nanostructured WC-12Co powders with different porous structure and apparent hardness were employed to deposit WC-Co coatings on stainless steel substrate by cold spraying. The deposition characteristics of three powders of porosity from 44 to 5% were investigated. It was found that WC-Co coating is easily built-up using porous powders with WC particles bonded loosely and a low hardness. The microhardness of WC-12Co coatings varied from 400 to 1790 Hv with powders and spray conditions, which depends on the densification effects by impacting particles. With porous WC-Co powders, the fracture of particles on impact may occur and low deposition efficiency during cold spraying. The successful building up of coating at high deposition efficiency depends on the design of powder porous structure.     

超音速火焰喷涂WC-10Co4Cr涂层的耐滑动磨损行为

采用超音速火焰喷涂(HVOF)工艺制备微米结构WC-10Co4Cr涂层,分别采用金相显微镜、扫描电镜(SEM)、X射线衍射(XRD)和滑动磨损设备分析涂层的微观结构和滑动磨损行为。结果表明:采用液体煤油燃料HVOF喷涂的微米结构WC-10Co4Cr涂层的脱碳程度较低,涂层中仅出现WC和W2C相,而无η相(Co3W3C、Co6W6C)以及软相W。涂层微观结构致密,孔隙率约为1%,平均显微硬度为1 322HV0.3;在相同试验条件下,WC-10Co4Cr涂层的摩擦因数(约0.8)高于不锈钢(1Cr18Ni9Ti)的摩擦因数(约0.5),其滑动体积损失量仅为不锈钢涂层的1/146,具有优异的抗滑动磨损性能。涂层在滑动磨损过程中首先是粘结相的脱落,然后是WC颗粒的磨损。     

低温超音速火焰喷涂工艺制备轴承钢为基体的WC-12Co厚涂层

采用超音速火焰喷涂工艺(HVOF)制备的WC-12Co涂层能够显著提高系统的硬度和耐磨特性。然而,该工艺中的高温参数会使得涂层在制备过程中产生脱碳现象。本文尝试将WC-12Co涂层引入到滚动副中以提高界面的摩擦学性能和抗磨损特性,例如固体火箭发动机中用于推力矢量控制的滚动轴承,通过温度可控的超音速火焰喷涂工艺在轴承钢基体上制备涂层。详细研究了涂层的相分布、成分组成、微观结构、与基体的结合强度、弹性模量和微观硬度,验证了改进后工艺的可行性和先进性,并阐明了涂层与轴承钢基体之间的结合机制。在WC骨架假设和钴相均匀分布的假设下,根据硬度性能的测试结果,给出了WC-12Co涂层微观硬度的一个经验公式,可用于涂层硬度的理论预估和设计优化。     

Examination of the wear properties of HVOF sprayed nanostructured and conventional WC-Co cermets with different binder phase contents

There has been an increase in interest of late regarding the properties of thermally sprayed WC-Co cermets with nanograin carbide particles. These powders have shown interesting properties in sintered components, giving high values of hardness (2200–2300 VHN) and improved wear properties. The method used for the processing for these materials—solution formation, spray drying and chemical conversion, rather than introduction of WC as solid particles to a molten binder—allows the formation of sub-100 nm WC particles as a hard second phase. The work presented here examined the effect of composition on the microstructure and wear properties of some nanostructured WC-Co materials. WC-Co cermets with 8, 10, 12, and 15% Co binder phase were deposited using a Sulzer Metco hybrid DJ HVOF thermal spray system. Optimization of deposition conditions was necessary because of the unique morphology of the powders (thick-shelled hollow spheres) to produce dense consolidated deposits. There is a higher degree of decarburization of the WC phase in the nanostructured materials compared with the conventional WC-Co. This dissolution of the hard phase is also noted to increase on decreasing binder phase content. The nanostructured WC-Co coatings have a lower wear resistance compared with the conventional WC-Co for abrasive wear and small particle erosion. The abrasive wear resistance of these nanostructured materials was found to increase on decreasing cobalt binder content. This trend in abrasive wear resistance is consistent with studies on conventional sized cermets and is believed to be more dependent upon proportion of binder phase content than degree of decarburization for the materials studied. The small particle erosion resistance of the nanostructured coatings was found to increase on increasing cobalt content.     

Influence of Rare Earth on the High-Temperature Sliding Wear Behavior of WC-12Co Coating Prepared by HVOF Spraying

In this work, WC-12Co coatings were prepared by high-velocity oxygen fuel spraying (HVOF) technology. The high-temperature sliding wear tests at 450, 550 and 650 °C were conducted on a pin-on-disk tribometer, and effects of CeO₂ on the high-temperature wear behavior were investigated. The results showed that CeO₂-modified WC-12Co coating possessed better sliding wear resistance than that of conventional WC-12Co coating at the tested temperatures. The maximum microhardness value of 1333 ± 25HV_0.5 was available at the temperature of 550 °C for CeO₂-modified WC-12Co coating worn track. The oxides formed on the worn surface played a significant role on the wear behavior. W₂C, Co₃O₄ and ratio of CoWO₄/WO₃ dominated the wear behavior of the coating at 450, 550 and 650 °C, respectively.     

Wear,erosion and corrosion resistance of HVOF-sprayed WC and Cr3C2 based coatings for electrolytic hard chrome replacement

Thermally sprayed carbide-based coatings are nowadays extensively considered as an alternative to electrolytic hard chrome (EHC) coatings to reduce the environmental impact and the overall cost associated with EHC process. In this investigation, high-velocity oxy-fuel (HVOF) spray process was employed to prepare coatings using the traditional carbide powders namely the WC-10Co4Cr, the Cr₃C₂-25NiCr and a new type of mixed carbide powder WC-40Cr₃C₂-25NiCr. The Powder deposition rate, basic mechanical properties, abrasive wear, slurry erosion and corrosion resistance of the three coatings were then compared with the EHC coating. The results show that WC-10Co4Cr coating exhibited the highest hardness, abrasive wear and slurry erosion resistance followed by WC-40Cr₃C₂-25NiCr, EHC, and Cr₃C₂-25NiCr coating. The deposition efficiency of the powders as per hierarchy was found to be WC-40Cr₃C₂-25NiCr > WC-10Co4Cr > Cr₃C₂-25NiCr and all the HVOF sprayed coatings exhibited higher corrosion resistance than EHC coating. The highest powder deposition efficiency coupled with low density, acceptable tribo-corrosion performance, as well as low post processing cost makes the HVOF sprayed WC-40Cr₃C₂-25NiCr coating a potential candidate to replace the EHC coating.     

纳米稀土对热喷涂WC-12Co涂层的改性作用

利用超音速火焰喷涂技术在45钢基体上制备了不同纳米稀土含量的WC-12Co涂层。通过物相分析并测定材料的显微硬度、结合强度、磨损性能,研究纳米稀土对WC-12Co涂层的改性作用。结果表明:适量纳米稀土的加入使WC-12Co涂层的显微硬度和结合强度显著提高,并且纳米稀土的加入有效地抑制了WC颗粒的脱碳,使组织细化。当纳米稀土含量在1.5wt%时,涂层的耐磨性最好。     

Nanocrystalline WC-Co HVAF Coatings by Utilizing Novel Powder Manufacturing Route Using Water-Soluble Raw Materials

In this study, nanostructured WC-Co coatings were produced using experimental nanocrystalline WC-12Co and WC-24Co powders produced by a novel chemical synthesis route. Test coatings were produced using HVAF spraying keeping the temperature as low as possible during the deposition in order to avoid decomposition of the nanocarbides. In experimental powders, two different Co incorporation methods were used: a conventional way in which cobalt was incorporated as a metallic Co powder and a chemical synthesis way in which cobalt acetate was used as a cobalt source. When using cobalt acetate, it decomposes to metallic cobalt during the process. Experimental powders in which cobalt acetate has been used as cobalt source resulted poor deposition efficiency. With warmer parameters, powders resulted better DE, but significant WC decarburization and the dissolution into the matrix phase occurred. Powders in which Co has been introduced as Co powder showed enhanced DE enabling spraying with decreased temperature and higher particle velocity, resulting in coatings with less WC decomposition. Especially, an experimental powder in which Co has been incorporated both as Co powder and as Co-Ac results very fine nanocarbide structure with significantly less WC decomposition having a hardness value of 1201 HV0.3, even with 24% Co.     

Effect of flame conditions on abrasive wear performance of HVOF sprayed nanostructured WC-12Co coatings

Nanostructured WC-12Co coatings were deposited by high velocity oxy-fuel (HVOF) spraying with an agglomerated powder. The effect of flame conditions on the microstructure of the nanostructured coatings was investigated. The wear properties of the coatings were characterized using a dry rubber-wheel wear test. The results show that the nanostructured WC-Co coatings consist of WC, W2C, W and an amorphous binder phase. The microstructure of the coating is significantly influenced by the ratio of oxygen flow to fuel flow. Under the lower ratio of oxygen/fuel flow, the nanostructured coating presents a relative dense microstructure and severe decarburization of WC phase occurs during spraying. With increasing ratio of oxygen/fuel flow, the bonding of WC particles in the coating becomes loose resulting from the original structure of feedstock and the decarburization of WC becomes less owing to limited heating to the powder. Both the decarburization of WC particles in spraying and the bonding among WC particles in the coatings affect the wear performance. The examination of the worn surfaces of the nanostructured coatings reveals that the dominant wear mechanisms would be spalling from the interface of WCCo splats when spray particles undergo a limited melting. While the melting state of the spray particles is improved,the dominant wear mechanisms become the plastic deformation and plowing of the matrix and spalling of WC particles from the matrix.     

粉末原料粒径对WC-17Co涂层性能的影响

本文利用超音速火焰喷涂技术喷涂四种不同粒径的WC-17Co粉末,评价粉末粒径对涂层机械性能和抗磨粒磨损性能的影响。结果表明,粉末的粒径越小,在超音速焰流作用下获得的速度和温度越高,形成的涂层越致密,颗粒间的粘接强度越高,同时涂层的显微硬度也越高。WC-17Co粉末的粒径越小,获得涂层的孔隙直径越小,颗粒间的粘接缺陷越少,因此涂层的抗磨粒磨损性能越好。但是当WC-17Co粉末的粒径过于微小时,涂层的断裂韧性将受到影响。在本文研究的四种粒径分布的WC-17Co粉末中,中间粒径且分布范围集中的粉末制得的涂层兼具良好的机械性能和抗磨粒磨损性能。     

Influence of the Spray Angle on the Properties of HVOF Sprayed WC–Co Coatings Using (−10 + 2 μm) Fine Powders

The application of fine powders in thermal spray technology represents an innovative approach to apply dense and smooth near-net shape coatings on tools with complex geometry. However, this aim can only be achieved as long as the influence of the handling parameters of the spray process, such as the spray angle, is sufficiently understood. In this study, the influence of the spray angle on the deposition rate as well as on the coating properties (microhardness, roughness, and porosity) of HVOF-sprayed, fine-structured coatings are investigated. A fine, agglomerated, and sintered WC-12Co powder (agglomerate size: 2-10 μm, WC-particle Fisher sub-sieve size = 400 nm) was used as feedstock material. It has been shown that HVOF spraying of fine powders is less susceptible to an alteration of the spray angle than most other thermal spray processes such as plasma- or arc-spraying. The reduction of the spray angle results in a decrease in the deposition rate, while no significant degradation of the coating properties is found up to 30°. However, at spray angles below 30° the coating strength is negatively affected by the formation of pores and cracks.     

冲压模具热喷涂涂层的优化*

为了提高冲压模具的使用寿命,分别采用电弧喷涂工艺制备FTC-FeCSiMn耐磨涂层和高速火焰喷涂工艺制备WC-12Co耐磨涂层,并对其参数进行优化。由于电弧喷涂工艺受到较少参数的影响,而高速火焰喷涂工艺受到煤油流量、氢气流量和氧气比等十几个参数的影响,所以优化过程采用单次单因子法的试验设计法。对微硬度、孔隙率、表面粗糙度及沉积效率等涂层性质进行研究,取得较好效果。其中高速火焰喷涂的WC-12Co涂层经优化后,硬度1 547HV0.1,沉积效率34.5%,孔隙率1.0%,粗糙度1.84μm,与理论预期值非常接近。最后使用销盘试验测试涂层耐磨性,结果表明电弧喷涂的FTC-FeCSiMn涂层使工件的抗磨性提高2个数量级,而高速火焰喷涂的WC-12Co涂层更使工件的抗磨性提高4个数量级。     

Microstructure and sliding wear behavior of nanostructured Ni60-TiB2 composite coating sprayed by HVOF technique

A nanostructured Ni60-TiB₂ composite coating (Ni60 is a brand of Ni-based self-fluxing alloy with a hardness of HRC60) was sprayed on steel substrate by high velocity oxy-fuel (HVOF) process using high energy ball milled powders. Its sliding wear resistance at room-temperature was evaluated by ball-on-disc testing. For comparison, conventional Ni60-TiB₂ composite coating was prepared by HVOF using mechanically mixed Ni60 and TiB₂ powders and tested under the same conditions. The results show that the nanostructured composite coating has excellent mechanical properties and sliding wear resistance due to the microstructural homogenization and the well preserved nanostructure characteristic of the ball milled powders. Adhesive and abrasive wears are found to be responsible for the wear down mechanisms of the nanostructured Ni60-TiB₂ composite coating.     

爆炸喷涂WC-12Co/MoS_2复合涂层的摩擦磨损性能

为进一步提高爆炸喷涂WC-12Co涂层的耐磨性,在WC-12Co合金粉末中添加不同比例的MoS2粉末,利用爆炸喷涂技术在Q235钢表面制备了系列WC-12Co/MoS2复合涂层.采用金相显微镜、扫描电子显微镜、X射线衍射仪、显微硬度计及摩擦磨损试验机对WC-12Co/MoS2复合涂层的微观组织形貌、结构、显微硬度、摩擦磨损性能进行了研究.结果表明,MoS2均匀的分布于复合涂层中,当MoS2含量为2%时,复合涂层的硬度、致密度变化不大,但摩擦系数和磨损率大幅度下降,分别为WC-12Co涂层的50%和36%.随着MoS2含量的增加,复合涂层的摩擦系数和磨损率均呈上升趋势.