Tribological Evaluation of an Al2O3-SiO2 Ceramic Fiber Candidate for High Temperature Sliding Seals

A test program to determine the relative slitting durability of an alumina-silica candidate ceramic fiber for high temperature sliding seal applications is described. Pin-on-disk tests were used to evaluate the potential seal material by sliding a tow or bundle of the candidate ceramic fiber against a superalloy test disk. Friction was measured during the tests and fiber wear, indicated h the extent of fibers broken in the tow or bundle, was measured at the end of each test. Test variables studied included ambient temperature from 25° to 900°C, loads from 1.3 to 21.2 N, and sliding velocities from 0.025 to 0.25 m/sec. In addition, the effects of fiber diameter and elastic modulus on friction and wear were measured. Thin gold films deposited on the superalloy disk surface were evaluated in an effort to reduce friction and wear of the fibers.

In most cases, wear increased with test temperature. Friction ranged from 0.36 at 500°C and low velocity (0.025 miser) to over 1.1 at 900°C and high velocity (0.25 m/sec). The gold films resulted in satisfactory lubrication of the fibers at 25°C. At elevated temperatures diffusion of substrate elements degraded the films. These results indicate that the alumina-silica (Al₂O₃SiO₂) fiber is a good candidate material system for high temperature sliding seal applications. More work is needed to reduce friction.     

Lubrication of Solids at High Temperatures

This paper deals with recent work applicable to the problems of high-temperature lubrication carried out in the Laboratory of Physics and Chemistry of Solids, University of Cambridge, England. Three main aspects are considered (a) friction of lamellar solids, (b) lubrication from the gas phase (c) frictional properties of hard metals at high temperatures.     

Tribological Properties of TiAl Matrix Self-Lubricating Composites Containing Multilayer Graphene and Ti3SiC2 at High Temperatures

An investigation is conducted on the unexplored synergistic effects of multilayer graphene (MLG) and Ti₃SiC₂ in self-lubricating composites for use in high-temperature friction and wear applications. The tribological properties of TiAl matrix self-lubricating composites with different solid lubricant additions (Ti₃SiC₂-MLG, MLG) are investigated from room temperature to 800°C using a rotating ball-on-disk configuration. Tribological results suggest the evolution of lubrication properties of MLG and the excellent synergistic lubricating effect of MLG and Ti₃SiC₂ as the testing temperature changes. It can be deduced that MLG has great potential applications as a promising high-temperature solid lubricant within 400°C, and a combination of MLG and Ti₃SiC₂ is an effective way to achieve and maintain desired tribological properties over a wide temperature range.     

High Velocity Oxyfuel Deposition for Low Surface Roughness PS304 Self-Lubricating Composite Coatings

Efforts were made to achieve lower (R _q < 0.1 μm) initial RMS roughnesses of PS304 coatings so that they may be considered for foil bearings operating under increasingly severe conditions that result in smaller air film thicknesses. Attainable roughness of conventional plasma-sprayed PS304 coatings has been typically R _q > 0.25 μm, as limited by porosity in the deposited coating and surface irregularities correspondingly formed upon finishing. Initial attempts at achieving dense coatings by instead using a high-velocity oxyfuel (HVOF) flame-spraying process failed due to insufficient heating and softening of the NiCr and Cr ₂ O ₃ constituents of the PS304 feed powder, which rebounded from the steel target substrate and resulted in low deposition yield. Efficient HVOF deposition by a hydrogen-fueled system was achieved using NiCr and Cr ₂ O ₃ constituent particles of reduced size that were more effectively heated. The resultant dense coatings provided roughnesses as low as R _q = 0.05 μm upon polishing. Tribological performance of these HVOF coatings was evaluated against Inconel X-750 in thrust-washer tests at a sliding speed of 5.4 m/s, with contact pressures of 20 and 40 kPa, and ambient or 500°C temperatures. The wear and friction performances in direct sliding contact, as would exist upon loss of separating air-bearing film, of the PS304 coatings produced by HVOF are found to be similar to those deposited by the plasma spray process.     

Wear and Friction of TiAl Matrix Self-Lubricating Composites against Si3N4 in Air at Room and Elevated Temperatures

More durable, low-friction bearing materials over a wide temperature range are needed for turbine components and other high-temperature bearing applications. The current study reported the tribological properties of TiAl matrix self-lubricating composites (TMC) containing MoS₂ (a low-temperature lubricant, below 500°C), hBN (a medium-temperature lubricant, below 600°C), and Ti₃SiC₂ (a high-temperature lubricant, above 600°C) designated as MhT against an Si₃N₄ counterface at temperatures ranging from 25 to 800°C in air. The load was 10 N and the sliding speed was 0.2 m/s for all tests. Tribological studies indicated that TMC containing MhT showed a lower friction coefficient and wear rate in comparison to TiAl-based alloy at all test temperatures, which was attributed to the excellent synergetic lubricating effect of MoS₂, hBN, and Ti₃SiC₂. TMC containing 5 wt% MhT exhibited the best tribological properties over a wide temperature range.     

Formation of Adsorbed and Chemically Reacted Sulfur Films on Steel Surfaces During Sliding

Neat thiophene, dimethyl disulfide and dibenzyl disulfide environments were tested for friction and wear with a pin-on-disk apparatus on two steels, 1045 and 4142, the former not containing chromium and the latter containing chromium. In general, friction and wear were changed on introduction of the sulfur compound. Thiophene reduced wear and friction at room temperature while dimethyl disulfide was prowear under similar test conditions. Flow rate of the vaporous lubricant proved to be a dominant factor. Wear tracks were analyzed by SEM, AES, and EDAX. Significant sulfur could only only be found in the wear tracks and only once a surface layer was removed. Interestingly enough, the maximum sulfur concentration was located about 50 Å below the surface. This depth of 50 Å corresponds to the oxide layer. Hardly any sulfur was detectable above the oxide layer. The sulfur concentration decreased exponentially at depths greater than 50 Å.     

高温密封中可膨胀柔性石墨密封件的设计

本文着重探讨使用在高温工况下的柔性石墨密封件一种新的制造工艺方法,通过再膨胀控制高温工况下动静密封间隙,实现高可靠性。     

A High-Resolution TEM/EELS Study of the Effect of Doping Elements on the Sliding Mechanisms of Sputtered WS2 Coatings

It has been shown many times that cosputtering low-friction coatings of molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂) with other elements can improve the structural, mechanical, and tribological properties. To achieve the lowest friction, MoS₂ or WS₂ should be doped with element(s) improving the hardness and density of the coatings. On the other hand, such elements, or their compounds, should not be present in the outermost molecular layers at the sliding interface. This article suggests that there are important differences between how MoS₂ and WS₂ coatings respond to or react with doping elements, despite the almost identical structure and behavior of the undoped materials. Two systems have been investigated by high-resolution transmission electron microscopy (HRTEM) and scanning TEM (STEM) electron energy loss spectroscopy (EELS), W-S-C-Cr and W-S-C-Ti, and showed significant amounts of oxides, which typically formed a layer just underneath the crystalline WS₂ top layer. Further, carbon was almost completely absent in the tribofilms, despite the fact that the as-deposited coatings contained as much as 40–50 at% C. An interesting observation here is that WS₂ basal planes surround or embed Fe wear particles, suggesting a relatively strong adhesion or a Fe-S chemical bonding between iron/steel and WS₂. The result of this is that the wear particles become pacified and remain in the contact as low-friction material.     

Tribological Performance of Diamond and Diamondlike Carbon Films at Elevated Temperatures

In this study, the authors investigated the tribological performance of diamond and diamondlike carbon (DLC) films as a function of temperature. Both films were deposited on silicon carbide (SiC) by microwave plasma chemical vapor deposition and ion-beam deposition processes. Tribological tests were performed on a reciprocating wear machine in open air (20 to 30% relative humidity) and under a 10 N load using SiC pins. For the test conditions explored, the steady-state friction coefficients of test pairs without a diamond or DLC film were 0.7 to 0.9 and the average wear rates of pins were 10^?5 to 10^?7 mm³/N·m, depending on ambient temperature. DLC films reduced the steady-slate friction coefficients of the test pairs by factors of three to five and the wear rates of pins by two to three orders of magnitude. Low friction coefficients were also obtained with the diamond films, but wear rates of the counterface pins were high due to the very abrasive nature of these films. The wear of SiC disks coated with either diamond or DLC films was virtually unmeasurable while the wear of uncoated disks was substantial. Test results showed that the DLC films could afford low friction up to about 300° C. At higher temperatures, the DLC films graphitized and were removed from the surface. The diamond films could withstand much higher tempera-lures, but their tribological behavior degraded. Raman spectroscopy and scanning electron microscopy were used to elucidate the friction and wear mechanisms of both films at high temperatures.     

高接触压力下齿轮油润滑特性

在重载条件下工作的润滑剂常常会受到高接触压力的影响,有时会导致润滑剂本身性质发生改变从而造成摩擦副失效,但目前对高接触压力条件下润滑剂性能的研究还不多。利用自制的新型膜厚测量仪测量GL-5型齿轮油在不同卷吸速度和接触压力下的干涉图像,并计算得到膜厚曲线进行成膜性能和润滑特性分析。结果表明,在低速重载的情况下GL-5齿轮油有较好的成膜能力;在高接触压力下,油膜形状呈平坦状分布,有助于减小压力峰值;随着接触压力的增加,由Hamrock-Dowson理论算得的膜厚值和实测值逐渐开始偏离,理论公式中卷吸速度和载荷的指数需要调整。     

Thrust-Washer Evaluation of Self-Lubricating PS304 Composite Coatings in High Temperature Sliding Contact

PS304 self-lubricating composite coatings were successfully deposited on steel substrates at various plasma spray facilities using mixtures blended from commercially obtained constituent particles. Coatings were evaluated in thrust-washer tests against Inconel X-750 at low contact pressures to 40kPa, sliding speed of 5Amis, and either ambient temperature or 500 °C chosen to simulate conditions in airfoil bearings during startup and shutdown contact. Wear factors for all PS304 coatings tested, regardless of contact pressure and temperature, ranged from 1–3*10^?4 mm³/Nm while coefficients of friction of approximately μ =0.5 were measured in all cases. While wear and friction behavior of PS304 in air foil bearings appear to have been simulated, surface roughening was observed in these thrust-washer tests which used continuous sliding contact, as opposed to the evolution of smoother surfaces observed in high-temperature foil bearings experiencing cyclic startup/shutdown. Wear-induced surface smoothening of PS304 was additionally simulated in thrust-washer tests with sliding contact instead imposed intermittently.     

Wear Mechanisms at High Temperatures. Part 1: Wear Mechanisms of Different Fe-Based Alloys at Elevated Temperatures

To extend the lifetime of the sinter grate used to crush the sinter cake into smaller pieces for steel fabrication, a study was undertaken to investigate which wear processes are primarily responsible for limiting the lifetime of the sinter grate. Several wear processes could be identified. The sinter temperature which is up to 800 °C causes temperature-induced material ageing and oxidation. The falling of the sinter cake onto the sinter grate causes high impacts, erosion and abrasive wear. There is enormous economic pressure, which makes the most cost-efficient solution the most attractive one, not the technically “best” coating material; thus, Fe–Cr–C hardfacing alloys are mostly used. In view of the above, four different alloys which are promising for this application were studied with regard to their wear resistance. Each wear mechanism was investigated in a special test tribometer. Fatigue wear caused by multiple impacts and abrasion was tested in the high-temperature continuous impact abrasion test. Materials behaviour in heavy single impacts was evaluated in the single impact test. Characterisation of microstructure and wear behaviour was performed by optical microscopy and scanning electron microscopy. The results obtained with the help of the different measurement techniques were linked and set into comparison to calculate the volumetric wear of the specimen. Aim of this work was to investigate the influence of the material parameters such as macrohardness, hard phase content, microstructure coarseness on the wear resistance in impact loading and abrasive applications at high temperatures. Results also indicate that the matrix ability to bind carbides at high temperature as well as the matrix hardness at high temperatures strongly influence the wear resistance in the different tests. Those material parameters get correlated to the wear rates in different material demands. The test results indicate that at higher temperatures material fatigue becomes a major wear-determining factor which makes the matrix hardness and the matrix ability to bind carbides at high temperatures very important. Especially, in abrasive wear, a certain content of hard phases is also necessary to keep the wear to a lower level. It could also be shown that in impact loading applications, a coarse microstructure is a disadvantage.     

High resolution SE-I SEM study of enamel crystal morphology

Until recently high resolution TEM was the only imaging mode capable of probing the atomic lattice structure of crystals composing tooth enamel. Studies designed to determine the polyhedral shape of normal enamel crystals and initiation of carious lesions in enamel crystals were hampered and limited by interpretation of two-dimensional TEM images from thin section and freeze fracture replica specimens lacking depth of field. The newly developed SE-I signal mode for SEM (SE-I/SE-II ratio) can produce images of enamel crystals approaching beam diameter dimensions (0.7–2.0 nm), rivaling the resolution of the TEM technique and generating topographic contrasts for three dimensional imaging at very high magnification (≈?1,000,000 X). Ultrathin chromium (Cr) films generate enriched high resolution SE-I contrasts of enamel crystal surfaces and when imaged using an immersion lens field emission SEM operated at high voltage (20–30 KeV) produce unsurpassed topographic contrasts. Since the grain size of Cr is below the resolution of any SEM and is ultrathin (≈?1 nm), then SE-I images can provide a more accurate representation of enamel crystal structure than TEM methodologies. Our SE-I SEM observations of normal human enamel crystals reveal fractured spicules which contain angled flat surfaces delineated by a prominent 2 nm wide SE-I edge brightness contrast. Although microscopic observations often show crystals which are hexagonal in cross-section, in both SEM and TEM many other growth habits, including rectangular or irregular crystals (30–40 nm in width) which contain “notches,” are also observed. More detailed morphological studies are therefore required to determine the most likely habit planes and their relevance to the function of the enamel crystals. The granular appearing fine structural contrast imposed onto <100> lattice planes of sectioned enamel in TEM micrographs is also resolved with topographic contrasts in SE-I micrographs. These granules probably represent one or both of the enamel protein classes.     

电火花放电熔涂层在边界润滑条件下的磨损特性

用环块磨损试验机对铬电极电火花熔涂层及淬火低温回火态45^#钢的边界润滑滑动磨损特性进行了研究。结果表明,轻载低速条件下,熔涂层与45^#钢的边界润滑滑动磨损机制以犁沟划伤为主,熔涂层的磨损量与45^#钢的相当;重载高速条件下,熔涂层的磨损机理以粘着磨损和磨料磨损为主,而45^#钢的磨损机理以点蚀剥落为主：熔涂层的磨损量远比45^#钢的人。     

一线总线型温度传感器在多点测温中的编码优化

运用数字型传感器DS18X20按一线总线方式进行多点测温的关键技术是传感器的地址编码问题。使用少数几位十进制编码取代48位二进制序列号作地址编码容易在工程中实施。根据地址码与序列号的关系介绍了3种处理方式：手工编制关系表、逐次比较式建立关系表和内嵌式智能建表，并对3种处理方式的可行性和实用性进行了比较。根据器件结构特征重点分析了效率最高的了第3种方法，并阐述了编码器的设计和使用方法。     

含硫镍基合金与YJ2硬质合金对偶的高温摩擦磨损特征

研究了通过热压法制备得到的含硫镍基合金与 YJ2硬质合金对偶的摩擦磨损特征。结果表明 :在高温摩擦过程中 ,材料中的硫化物共晶体是主要的润滑组元 ,摩擦面的温度对其润滑效果有较大的影响 ;材料与 YJ2对偶时的摩擦系数随着温度变化有所不同 ,但是磨损率都是随着温度升高而增大的 ,其高温下磨损形式主要为磨粒磨损和氧化磨损 ;YJ2硬质合金表面生成的氧化物对材料减摩性能有很大影响。     

不同温度下多层结构TiCN/TiC/TiN镀层的摩擦学行为

用化学气相沉积法在高速钢基体上制备了多层结构的TiCN/TiC/TiN镀层,分别用压入法和球坑法测定了镀层的结合强度和厚度;用显微硬度计测定了镀层的努氏硬度;在不同温度条件下,用球盘磨损试验机研究了镀层摩擦因数的变化规律,分析了温度对镀层摩擦磨损性能的影响。结果表明:该镀层的结合强度优异,厚度约为5μm,硬度约为2 352HK;在高温(550℃)下,镀层的摩擦因数一开始就很高,随后继续上升直到最大值,接着开始逐渐下降;室温下,镀层的摩擦因数一开始很低,随后较快地上升到一定值,之后上升速度减缓,最后达到稳定阶段;室温下,镀层磨痕中间局部有块状剥离,边缘有大量磨屑;高温下镀层出现塑性变形。     

纯铁高温高应变率下的动态本构关系试验研究

利用带有加热装置和同步组装系统的分离式Hopkinson压杆装置对纯铁进行宽温度范围(293～1073K)、高应变速率(2000～8500s-1)下的动态力学性能测试试验,获得材料在不同温度和应变率耦合作用下的应力—应变曲线,从中探讨温度和应变率对纯铁塑性流动应力的影响机制。研究表明,纯铁具有明显的热软化效应、应变率强化效应和应变强化效应,流变应力随温度的降低和应变率的增加而提高;利用所测的应力—应变曲线拟合的Johnson-Cook本构模型可以较好地预测纯铁的塑性流变应力。     

Wear Mechanisms at High Temperatures: Part 2: Temperature Effect on Wear Mechanisms in the Erosion Test

In previous investigations on wear mechanisms at high temperatures made in High Temperature-Single Impact Test (HT-SIT) and High Temperature-Continuous Impact Abrasion Test (HT-CIAT), predominant wear mechanisms were identified and correlations to different material parameters could be presumed. In order to confirm these correlations, four different alloys which are promising to be used in high temperature applications like a sinter grate have been studied in the High Temperature-Erosion Test (HT-ET) by the use of different impact angles and different impact energies. Especially the change of wear mechanism caused by increasing testing temperature was analysed in detail. Characterisation of microstructure and wear behaviour has been done by optical microscopy (OM) and scanning electron microscopy (SEM). Results obtained by the use of the different measurement techniques were linked and set into comparison to calculate the volumetric wear of the specimen. Predominant wear mechanisms were determined using OM in the mode of cross-section images and SEM. The results indicate that material parameters such as hardness and hard phase content can be correlated to the erosion wear rates at different impact angles. The test results indicate that at higher temperatures, the material fatigue becomes a major wear-determining factor. The test results also confirmed that there is a critical impact energy for each material above where the wear rate increases significantly. Test results with thermally aged materials also show that a better heat-resistant matrix reduces the material fatigue thus resulting in lower wear rates.     

颗粒致密度对WC-10Co-4Cr涂层性能的影响

利用超声速火焰喷涂技术(High velocity oxygen fuel,HVOF)并采用两种不同颗粒致密度的WC- 10Co-4Cr合金粉末在300M钢表面制备涂层.采用扫描电镜、X射线衍射仪、显微硬度计等设备对两种涂层的微观组织结构及特性进行分析,重点研究碳化钨颗粒的沉积过程,并通过腐蚀电化学试验研究两种涂层的耐...