Hot and cold erosive wear of thermal sprayed NiCr-based coatings: Influence of porosity and oxidation

In this work, the hot and cold erosive wear mechanisms of HVOF and plasma-sprayed NiCr-based coatings are evaluated. Erosion tests were carried out in specially developed equipment, with possible variations in temperature and attack angle. The effects of erodent incrustation (SEM/EDS mapping) and oxidation of metallic coating (mass weight) were considered. The results were associated with the microstructural characteristics of the coatings (porosity, size of lamellae) and with the mechanical properties, such as microhardness and elasticity modulus. The obtained results showed that NiCr coatings are ductile, a property similar to monolithic metallic materials. The porosity influenced both the amount of incrusted alumina particles and oxidation. The increase in temperature caused higher oxidation and erodent incrustation.     

电弧喷涂镍基涂层腐蚀及磨损行为

针对WTE垃圾焚烧炉过热器热腐蚀及磨损问题,设计开发了新型镍基粉芯丝材,并采用电弧喷涂方法在SA213-T2基体表面制备相应涂层.利用XRD,SEM,磨粒磨损试验机及高温箱式电阻炉等对涂层组织结构、磨损及热腐蚀行为进行研究.结果表明,所制备涂层具有热喷涂典型的层状结构,组织较为均匀致密,孔隙率在3%左右,氧化物含量较低.涂层平均显微硬度约为351.27 MPa,相对耐磨性达基体7倍以上.涂层在800℃下的热腐蚀动力学曲线呈典型抛物线形式,表面生成NiCr2O4及Cr2O3氧化膜,能够有效阻止腐蚀介质向涂层内部的渗透,使其抗热腐蚀性能远优于基体材料.     

A comparative study of tribological behavior of plasma and D-gun sprayed coatings under different wear modes

In recent years, thermal sprayed protective coatings have gained widespread acceptance for a variety of industrial applications. A vast majority of these applications involve the use of thermal sprayed coatings to combat wear. While plasma spraying is the most versatile variant of all the thermal spray processes, the detonation gun (D-gun) coatings have been a novelty until recently because of their proprietary nature. The present study is aimed at comparing the tribological behavior of coatings deposited using the two above techniques by focusing on some popular coating materials that are widely adopted for wear resistant applications, namely, WC-12% Co, A1₂O₃, and Cr₃C₂-MCr. To enable a comprehensive comparison of the above indicated thermal spray techniques as well as coating materials, the deposited coatings were extensively characterized employing microstructural evaluation, microhardness measurements, and XRD analysis for phase constitution. The behavior of these coatings under different wear modes was also evaluated by determining their tribological performance when subjected to solid particle erosion tests, rubber wheel sand abrasion tests, and pin-on-disk sliding wear tests. The results from the above tests are discussed here. It is evident that the D-gun sprayed coatings consistently exhibit denser microstructures and higher hardness values than their plasma sprayed counterparts. The D-gun coatings are also found to unfailingly exhibit superior tribological performance superior to the corresponding plasma sprayed coatings in all wear tests. Among all the coating materials studied, D-gun sprayed WC-12%Co, in general, yields the best performance under different modes of wear, whereas plasma sprayed Al₂O₃ shows least wear resistance to every wear mode.     

Thermally sprayed wear resistant coatings with nanostructured hard phases

The aim of this investigation was the development of a new quality of thermally sprayed coatings with high resistance against wear and corrosion and to evaluate the application potential of nano-sized hard phases in thermally sprayed layers. The newly developed material consists of a highly corrosion-resistant matrix of stainless steel (even without nickel) combined with nano-structured hard phases of vanadium nitrides (VN). On the other hand, matrices consisting of cobalt-chromium (CoCr) with submicron hard phases of tungstencarbides (WC) were investigated with respect to microstructure and wear resistance compared with conventional ones.     

Ambient and high-temperature thermal conductivity of thermal sprayed coatings

Aside from its importance as a design parameter for thermal barrier coatings, measuring thermal conductivity of thermal sprayed coatings itself provides a unique method to critically characterize the nature, quantity, and anisotropy of the defect morphologies in these splat-based coatings. In this paper, the authors present a systematic assessment of thermal conductivity of wide range using the flash diffusivity technique. For the case of plasma sprayed yttria-stabilized zirconia (YSZ), coatings obtained from wide-ranging initial powder morphologies as well as those fabricated under different particle states were characterized. Both in-plane and through-thickness properties were obtained. Other material systems that were considered include: metallic alloys and semiconductors of interests. Issues such as reproducibility and reliability in measurements were also considered and assessed. Finally, work in collaboration with the Oak Ridge National Laboratory (ORNL) for alternate approaches to characterization of thermal conductivity as well as high-temperature measurements was performed. This article was originally published inBuilding on 100 Years of Success, Proceedings of the 2006 International Thermal Spray Conference (Seattle, WA), May 15–18, 2006, B.R. Marple, M.M. Hyland, Y.-Ch. Lau, R.S. Lima, and J. Voyer, Ed., ASM International, Materials Park, OH, 2006.     

FeNiBSiCe喷熔层微动磨损特性

利用氧-乙炔火焰喷熔方法在45号钢表面制取了FeNiBSiCe喷熔层,在SRV球盘摩擦磨损试验机上研究了FeNiBSiCe合金的摩擦磨损行为,用扫描电子显微镜(SEM)、场发射扫描电镜(FEGSEM)和能量色散谱仪(EDS)等技术分析了喷熔层的表面、截面及磨损面形貌.结果表明,FeNiBSiCe喷熔层的耐磨性优于SAE52100钢,喷熔层磨损率随滑动速度增加,先快速降低,随后趋于缓慢,最后上升.摩擦系数随滑动速度的增大而增加,然后趋于稳定.磨损机理是低载荷下为微观切削和疲劳脱层,在相对高载荷下和滑动速度较高时产生显微裂纹,且在涂层表面形成了一层氧化物磨屑层,缓解了微动磨损.     

Thermal cyclic behavior of air plasma sprayed thermal barrier coatings sprayed on stainless steel substrates

Thermal barrier coatings (TBCs) were deposited by an Air Plasma Spraying (APS) technique. The coating comprised of 93 wt.% ZrO₂ and 7 wt.% Y₂O₃ (YSZ); CoNiCrAlY bond coat; and AISI 316L stainless steels substrate. Thermal cyclic lives of the TBC were determined as a function of bond coat surface roughness, thickness of the coating and the final deposition temperature. Two types of thermal shock tests were performed over the specimens, firstly holding of specimens at 1020 °C for 5 min and then water quenching. The other test consisted of holding of specimens at the same temperature for 4 min and then forced air quenching. In both of the cases the samples were directly pushed into the furnace at 1020 °C. It was observed that the final deposition temperature has great impact over the thermal shock life. The results were more prominent in forced air quenching tests, where the lives of the TBCs were observed more than 500 cycles (at 10% spalling). It was noticed that with increase of TBC's thickness the thermal shock life of the specimens significantly decreased. Further, the bond coat surface roughness varied by employing intermediate grit blasting just after the bond coat spray. It was observed that with decrease in bond coat roughness, the thermal shock life decreased slightly. The results are discussed in terms of residual stresses, determined by hole drill method.     

Characterization and wear properties of nanoscaled TiN/TaN multilayer coatings

The structural, morphological, mechanical and tribological characterization of nanoscaled multilayer TiN/TaN coatings deposited by magnetron sputtering technology were investigated by low angle X-ray diffractometry, high angle X-ray diffractometry, atomic force microscopy, microhardness, pin-on-disc testing and 3-D surface profiler. The results show that the TiN/TaN coatings exhibit good modulation period and sharp interface between TiN and TaN layers. In mutilayered TiN/TaN coatings, TiN layers have cubic structure, but hexagonal structure emerged among TaN layers besides cubic structure as modulation period is beyond 8.5 nm. The mierohardness is affected by modulation period and the maximum hardness value of 31.5 GPa appears at a modulation period of 8.5 rim. The coefficient of friction is high and the wear resistance is improved for TiN/TaN coatings compared with those of TiN coating; the wear mechanism exhibits predominantly ploughing, material transfer and localized spallation.     

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。     

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.     

Tribological behavior and wear mechanisms of TiN/TiCN/TiN multilayer coatings

This work employs the PVD process to deposit coatings of single layer TiN, binary layer TiN/TiCN, multilayer TiN⇔⇔N, and sequenced TiN⇔CN⇔N multilayer coatings with variable individual TiN-layer and TiCN-layer thicknesses on tungsten carbide disks and inserts. Also investigated are the fracture mechanisms and the influence of sequence and thickness of these coatings on cylinder-on-disk, line-contact wear mode and ball-on-disk, point-contact wear mode through SRV reciprocating wear tests. Actual milling tests identify wear performance. Experimental results indicate that the coating with a total thickness of 7 Μm and layer sequence TiN/TiCN/TiN exhibits good wear resistance on SRV wear test and milling test. The thickest multilayer TiN/Ti/TiN coating, although having the highest hardness, has the worst wear resistance for all tests. No-tably zero-wear performance was observed for all coating disks under cutting fluid lubricated condition due to the transferred layers formed between the contact interface.     

Evaluation of thermally sprayed coatings under reciprocating lubricated wear conditions

To find an alternate coating to hard electroplated chrome in internal combustion engines, wear tests and metallurgical characterization have been performed on plasma-sprayed chromium oxide, metal-arc-sprayed martensitic stainless steel, and electroplated chromium coatings applied to steel base material. A wear test rig was fabricated that simulated the reciprocating sliding wear under lubrication encountered in internal combustion engines. The chromium oxide coating was found to perform equally well compared to the hard chrome coating that is conventionally used.     

Abrasive wear resistance of plasma-sprayed glass-composite coatings

A ball-milled mixture of glass and alumina powders has been plasma sprayed to produce alumina-glass composite coatings. The coatings have the unique advantage of a melted, ceramic secondary phase parallel to the surface in an aligned plateletlike-composite structure. The alumina raises the hardness from 300 HV for pure glass coatings to 900 HV for a 60 wt.% alumina-glass composite coating. The scratch resistance increases by a factor of 3, and the wear resistance increases by a factor of 5. The glass wears by the formation and intersection of cracks, while the alumina wears by fine abrasion and supports most of the sliding load. The wear resistance reaches a maximum at 40 to 50 vol.% alumina, above which there is little further improvement. This critical alumina content corresponds to the changeover from a glass to a ceramic matrix.     

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.     

Thermomechanical behavior of nanostructured plasma sprayed zirconia coatings

Retaining nonmelted nanoparticles of zirconia in nanostructured coatings has been a challenge in the past. Recently an air plasma spray process was developed to produce coatings that retain up to 30–35% by volume nonmelted particles, resulting in a unique structure. The creep/sintering behavior of such thermal barrier coatings deposited from nanostructured feedstock has been measured and compared with deposits produced from hot oven spherical particles (HOSP). Both feedstocks contain 6–8 wt.% Y₂O₃ as a stabilizer. Flexure and compression creep testing were conducted under several different loads and temperatures to obtain creep exponents and parameters. This article was originally published inBuilding on 100 Years of Success, Proceedings of the 2006 International Thermal Spray Conference (Seattle, WA), May 15–18, 2006, B.R. Marple, M.M. Hyland, Y.-Ch. Lau, R.S. Lima, and J. Voyer, Ed., ASM International, Materials Park, OH, 2006.     

电弧喷涂铁基非晶涂层摩擦磨损性能分析

采用电弧喷涂技术在低碳钢基体上制备Fe-（Cr,Ni）-（C,B）系非晶合金涂层。利用X射线衍射仪、扫描电子显微镜和差热分析仪对涂层的相组成、微观组织和热稳定性进行了分析,用MRH-3型高速环块磨损试验机研究涂层的干摩擦磨损行为.结果表明,涂层中含有一定量的非晶相,呈典型的热喷涂层状组织结构,孔隙率较低;涂层具有良好的热稳定性,在500℃以下使用不会发生晶化转变;涂层具有较高的显微硬度和优异的摩擦磨损性能,平均显微硬度为1 155 HV0.1,相同试验条件下,涂层的相对耐磨性能约为Q235钢的13.3倍,涂层的磨损机制主要以疲劳磨损为主.     

镍基-碳纳米管电化学复合镀层的摩擦磨损行为

镍基-碳纳米管复合镀层是将不同含量的碳纳米管和镍通过电化学沉积的方法共同沉积在中碳钢基质中。在没有添加任何润滑油的情况下,采用销-盘摩擦磨损试验机对复合镀层进行了摩擦磨损试验,结果显示,摩擦因数和磨损率都随着碳纳米管含量的升高而逐渐下降,耐磨性能显著提高。     

NiCrBSiWCe合金粉末喷熔层滑动磨损特性研究

采用热喷熔方法在45钢表面制备了NiCrBSiWCe合金粉末喷熔层,在SRV磨损试验机上进行小振幅滑动磨损试验研究.结果表明,NiCrBSiWCe合金喷熔层的摩擦磨损性能明显高于SAE52100钢.在较低载荷和滑动速度较低下,小振幅滑动磨损机理为磨损表面的划痕、裂纹和疲劳脱层.而在较高滑动速度下,小振幅滑动磨损机理为磨损表面的氧化磨屑层的形成,含稀土的磨屑层阻碍了喷熔层小振幅滑动磨损.     

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.