Comprehensive process maps to synthesize high density plasma sprayed aluminum oxide composite coatings with varying carbon nanotube content

Comprehensive process maps have been developed to synthesize “high density” plasma sprayed Aluminum oxide (Al₂O₃) composite coatings with 0, 4 and 8 wt.% carbon nanotube (CNT) reinforcement. These process maps correlate the key processing parameters such as plasma power, powder feed rate, primary gas flow rate, and stand-off distance to the temperature and velocity of the in-flight particles and finally, to the porosity and microhardness of coatings. Relative importance of each significant process parameters was also investigated by making “Pareto diagrams”. Stand-off distance followed by the plasma power were found to be most sensitive parameters which affects the porosity of the plasma sprayed Al₂O₃-CNT composite coatings. Process maps showed that CNTs have a significant effect on altering the temperature and velocity of in-flight particles. Higher thermal conductivity of CNTs alters the heat transfer phenomena during the coating build up which assists in densification.     

Carbon nanotube reinforced aluminum composite coating via cold spraying

Multiwalled carbon nanotube (CNT) reinforced aluminum nanocomposite coatings were prepared using cold gas kinetic spraying. Spray drying was used to obtain a good dispersion of the nanotubes in micron-sized gas atomized Al-Si eutectic powders. Spray dried powders containing 5 wt.% CNT were blended with pure aluminum powder to give overall nominal CNT compositions of 0.5 wt.% and 1 wt.% respectively. Cold spraying resulted in coatings of the order of 500 μm in thickness. Fracture surfaces of deposits show that the nanotubes were uniformly distributed in the matrix. Nanotubes were shorter in length as they fractured due to impact and shearing between Al-Si particles and the Al matrix during the deposition process. Nanoindentation shows a distribution in the elastic modulus values from 40-229 GPa which is attributed to microstructural heterogeneity of the coatings that comprise the following: pure Al, Al-Si eutectic, porosity and CNTs.     

High temperature tribological behavior of W-DLC against aluminum

Diamond-like carbon (DLC) coatings are well suited for applications that require minimum adhesion and low coefficient of friction (COF) against aluminum alloys. These properties however deteriorate rapidly at elevated temperatures, and coating wear occurs. In this study, tribological behavior of W containing DLC (W-DLC) were studied as a function of testing temperatures up to 500 °C, and the sliding-induced surface and subsurface damage at these temperatures was investigated. Pin-on-disk tests performed on W-DLC run against 319 Al showed a low COF of 0.2 at 25 °C, whereas between 100 °C and 300 °C, a high average steady-state COF of 0.60 was recorded. At 400 °C the COF decreased to 0.18, and this reduction in COF continued with increasing the temperature to 500 °C (0.12). It was observed that the formation of transferred material layers on 319Al was the governing mechanism for the low COF. The Raman analysis revealed that at room temperature these layers were rich in carbon, whereas at 400 °C the transfer layers consisted of tungsten oxide. According to transmission electron microscopy (TEM), and X-Ray photoelectron spectroscopy (XPS), of the coatings tested at 400 °C and 500 °C a thin (20 nm) tungsten oxide layer was formed on their top surface. This in turn led to the formation of tungsten oxide rich transfer layers that is believed to reduce the COF at temperatures above 400 °C.     

Process map for plasma sprayed aluminum oxide-carbon nanotube nanocomposite coatings

Process map has been developed for plasma sprayed aluminum oxide (Al₂O₃) ceramic nanocomposite coatings with carbon nanotube (CNT) reinforcement in varying content and spatial distribution. The process map was constructed using the temperature and velocity data of the in-flight powder particles exiting from the plasma plume. Process map elucidates the interdependence of powder feedstock pre-treatment, CNT content and dispersion behavior on the in-flight particle thermal history and subsequently evolving microstructure and coating properties. High thermal conductivity of CNTs alters the heat transfer characteristic during the splat formation. Microstructure of the coatings consists of fully melted zone (FM), partially melted or solid-state sintered zone (PM) and porosity. Process map provides a processing control tool for plasma spraying of Al₂O₃-CNT nanocomposite coatings.     

Influence of ceramic thickness on residual stress and bonding strength for plasma sprayed duplex thermal barrier coating on aluminum alloy

NiCoCrAlY/8wt.%Y₂O₃–ZrO₂ coating was plasma sprayed on aluminum alloy to evaluate the effect of ceramic thickness on residual stress and bonding strength. A new stress calculation method based on Stoney equation and substrate-removal technique was proposed. Stress in both bond coat and ceramic was studied. With the increase of ceramic thickness, the residual stress in both layers was firstly compressive then turned tensile. The large thermal expansion coefficient of the substrate played an important role in residual stress formation when the ceramic was thin. However, the intrinsic deposition stress took a dominant position when the ceramic coating turned thicker. The bonding strength decreased and the location of the fractured surface moved toward the ceramic surface. The moving of the surface was mainly resulted from the variation of stress gradient and the weakness of high porosity zone near the bond coat–ceramic interface.     

Tribological properties of nanoporous anodic aluminum oxide film

Friction and wear properties of nanostructured anodic aluminum oxide (AAO)) films were studied in relation to contact load and pore size (pore diameter). Uniformly arrayed nanoporous aluminum oxide films (pores of 28 nm, 45 nm, 95 nm, and 200 nm diameter and 60-100 μm thick) were synthesized by anodization. Reciprocating wear tests using 1 mm diameter steel balls as counterpart were carried out for a wide range of load (from 1 mN to 1 N) at ambient environment. The friction coefficient reduced with the increase of load. The friction coefficient decreased by approximately 30% when the load increased by 3 orders of magnitude. The pore density marginally affected the frictional properties of AAO films. The influence of pore size on the friction coefficient was significant at relatively high loads (0.1 N and 1 N) whereas it was negligible at low loads (1 mN and 10 mN). The worn surface of AAO films tested at low loads did not experience tribochemical reaction and exhibited only mild plastic deformation. Dispersed thick smooth films were formed on the worn surface of all samples at relatively high loads whereas only extremely thin smooth film patches were rarely formed at low loads. These thick smooth films were generated by combined influence of tribochemical reaction at the contact interface and plastic deformation of compacted debris particles as evidenced by energy-dispersive spectroscopy analysis. We suggest that these thick films mainly contributed to the decrease of friction regardless of the pore size.     

Influence of process parameters on electrolytic plasma discharging behaviour and aluminum oxide coating microstructure

In this study, a plasma electrolytic oxidation process (PEO) was used to produce oxide coatings on commercially pure aluminum (Al 1100) at two different current modes, pulsed unipolar and bipolar modes. Optical emission spectroscopy (OES) in the visible and near ultraviolet (NUV) band (285 nm-800 nm) was employed to investigate the PEO plasma. The emission spectra were recorded and plasma temperature profile versus processing time was constructed using line intensity ratios method. Scanning Electron Microscopy (SEM) with energy dispersive x-ray analysis (EDS) was used to study the coating microstructure and coating cross section. It was found that the plasma discharge behavior significantly influenced the microstructure and the morphology of the oxide coatings. The main effect came from the strongest discharges which were initiated at the interface between the substrate and the coating. Through manipulation of process parameters to control or reduce the strongest discharge, the density and quality of the coating layers could be modified. This work demonstrated that by adjusting the ratio of the positive to negative pulse currents as well as their timing in order to eliminate the strongest discharges, the quality of the coatings was considerably improved.     

Synthesis of manganese oxide/carbon nanotube nanocomposites using wet chemical method

Novel material with peculiar properties can be obtained by introducing foreign materials into the inner cavity of carbon nanotubes. It has been suggested that the materials encapsulated into the hollow regions of carbon nanotubes could result in a significant change of the properties of these small particles, forming new hybrid composites with extraordinary properties. In this short communication, filling of carbon nanotubes with manganese oxide by wet chemical method is demonstrated. Transmission electron microscopy (TEM) result showed the hollow structure of carbon nanotubes were filled with manganese oxide. Energy dispersive X-rays (EDX) spectra elucidate the presence of manganese oxide in the filled carbon nanotubes whereas SEM result showed that manganese oxide is not crystallized at the outer surface of carbon nanotubes.     

Influence of siliconizing on the oxidation behavior of plasma sprayed MoSi2 coating for niobium based alloy

Plasma spraying combined with halide activated pack cementation (HAPC) was used to deposit silicide coating on Nb-based alloy. X-ray diffraction (XRD) and energy disperse spectrum (EDS) indicate the formation of the siliconized NbSi₂ transition layer and the sprayed MoSi₂ outer layer. NbSi₂ layer prepared with HAPC exhibits relatively uneven surface which could promote the deposition of the sprayed MoSi₂. The coating specimen with 5 h siliconizing presented the best oxidation resistance with only 0.18% mass gain after 25 h oxidation at 1200 °C in air. The synergistic protection effect, depending on the continuous silica layer formed on the coating surface and the dispersal silica within the coating and interface, is responsible for the excellent oxidation resistance of the coating.     

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.     

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.     

激光辅助热喷涂NiCoCrAlYTa-Cr2O3-Cu-Mo涂层高温静态氧化行为

为探究高温润滑耐磨涂层抗高温氧化行为,采用激光辅助等离子喷涂技术(LPHS)在GH4065A镍基高温合金上制备NiCoCrAlYTa-Cr₂O₃-Cu-Mo涂层,研究了该涂层在(850～1000)℃×220 h的抗高温氧化行为。计算得出氧化激活能约为128.5 kJ·mol^-1,850、900、1000℃氧化速率常数分别为1.44×10^-2、3.61×10^-2、7.71×10^-2 mg²·cm^-4·h^-1。结果表明,850℃×220 h氧化后表面生成Al₂O₃为主的连续致密氧化膜,阻碍涂层内部的进一步氧化;1000℃×220 h氧化后表面生成疏松NiO为主,致密Cr₂O₃·NiO为辅的氧化膜。致密氧化膜的生成阻止了涂层及基体的进一步氧化。     

Tribological behavior of B4C reinforced Fe-base bulk metallic glass composite coating

In the present study, the tribological behavior of B₄C reinforced Fe-based bulk metallic glass (BMG) in the form of spray coatings was investigated. These coatings were successfully deposited on mild steel substrates using shrouded plasma spray techniques. The B₄C fraction and distribution in the deposited BMG/B₄C coatings were evaluated by image analysis and scanning electron microscopy. Friction and wear experiments were performed under dry conditions using a pin-on-disk sliding wear test against SUJ2 countermaterial for different B₄C fractions. It was observed that the wear resistance of composite coatings was greatly improved relative to the BMG coating. The results show that the friction coefficient of BMG/B₄C coatings is dependent on the fraction of B₄C in the BMG matrix. The wear behavior of Fe-based BMG is governed by plastic deformation and fracture of the wear surface. By embedding a harder material, B₄C, in a comparatively soft matrix, the hardness of the wear surface can be increased, and plastic flow propagation is inhibited. Moreover, the lower friction coefficient of B₄C can lead to reductions in wear loss.     

高温下TiN涂层的摩擦与磨损行为

采用阴极离子镀方法在YT14硬质合金刀具表面制备了Ti N涂层,用高温摩擦磨损试验机考察Ti N涂层在500℃高温下摩擦-磨损行为。通过扫描电镜观察涂层表面-界面形貌和高温磨损后表面形貌,用XRD分析了Ti N涂层物相变化,并用EDS能谱仪对结合界面进行线扫描分析和磨痕进行面扫描分析,同时用工具显微镜观察了表面犁沟形貌,对Ti N涂层500℃下磨损机理进行探讨。结果表明,Ti N涂层在500℃磨损后发生高温氧化,Ti N涂层表面磨痕处主要以Ti O2为主,这些氧化层起到了润滑减摩的作用,适合于高速切削与干式切削;在5 N载荷作用下,Ti N涂层的摩擦系数平均值为0.7116;在高温下Ti N涂层表现为氧化磨损,同时伴随着一定的磨粒磨损和黏着磨损。     

多壁纳米碳管／Cu基复合材料的摩擦磨损特性

利用销－盘式磨损试验机研究了粉末冶金法制备的多壁纳米碳管／Cu基复合材料的稳态摩擦磨损行为,并用扫描电镜分析了复合材料的磨损形貌。结果表明：多壁纳米碳管／Cu基复合材料具有较小的摩擦系数,并随纳米碳管质量分数的增加而逐渐降低;由于复合材料中纳米碳管的增强和减摩作用,在低载荷和中等载荷作用下,随着纳米碳管质量分数的增加,复合材料的磨损率减小;而在高载荷作用下,由于发生表面开裂和片状层剥落,含纳米碳管质量分数高的复合材料的磨损率增高。     

高温变形条件下5A02铝合金的塑性成形性能

随着工业技术的发展和能源问题的突出,铝合金以其质量轻、耐腐蚀性能好、成形性能和加工性能良好等优势成为轻型化首选的材料类型之一。以5A02铝合金冷轧板材为研究对象,通过单向拉伸试验和金相试验对不同变形温度、应变速率条件下5A02铝合金的塑性性能进行分析,并且借助试验数据和Zener-Hollomo参数模型,对高温条件下5A02铝合金的本构模型进行研究。研究结果表明：5A02铝合金在高温条件下变形时,应变速率和变形温度对延伸率的影响很大。在应变速率为0.01s-1、0.001 s-1、0.0005 s-1和0.0001 s-1条件下,当变形温度大于250℃时,5A02铝合金的延伸率大于100%。当变形温度为150℃~250℃时,5A02铝合金的真实应力-应变曲线属于动态回复型,而当变形温度大于250℃时,流变应力曲线存在明显的软化现象。     

The corrosion behavior of plasma sprayed Fe2Al5 coating in molten Zn

Aluminum coating was plasma sprayed on Fe-0.14-0.22 wt.% C steel substrate, and heat diffusion treatment at 923 °C for 4 h was preformed to the aluminum coating to form Fe₂Al₅ inter-metallic compound coating. The corrosion mechanism of the Fe₂Al₅ coating in molten zinc was investigated. SEM and EDS analysis results show that the corrosion process of the Fe₂Al₅ layer in molten zinc is as follows: Fe₂Al₅ → Fe₂Al₅Zn_x (η) → η + L(liquid phase) → L + η + δ(FeZn₇) → L + δ → L. The η phase and the eutectic structure (η + δ) prevent the diffusion of zinc atoms efficiently. Therefore the Fe₂Al₅ coating delays the reaction between the substrate and molten zinc, promoting the corrosion resistance of the substrate.     

Abrasive slurry wear behavior of stainless steel surface produced by plasma transferred arc hardfacing process

Abrasive slurry wear is generally defined as a mechanical interaction in which material is lost from a surface which is in contact with a moving particle-laden liquid. Slurry wear abrasion occurs in extruders, slurry pumps, and pipes carrying slurry of minerals and ores in mineral processing industries. The life of components used under slurry abrasion conditions is governed by the process parameters, properties of the abrasive particles in the slurry and the material properties. This paper analyzes in detail the effects of operating variables such as abrasive particle size, slurry concentration, speed of rotation and slurry bath temperature on the abrasive slurry wear behavior of a stainless steel surface produced by Plasma transferred arc (PTA) hardfacing process. Of the four variables considered in this investigation, it is found that the slurry concentration has a predominant effect on wear rate of hardfaced surfaces compared to other variables. Microstructural analyses of the worn surfaces were carried out using SEM. Both experimental and mathematical investigations show that the wear resistance of the PTA hardfaced stainless steel surface is four times better than that of the carbon steel substrate.