Influence of Crystal Structure on Friction Characteristics of Rare-Earth and Related Metals in Vacuum to 10−10 mm of Mercury

Friction, wear, and metal-transfer characteristics were determined for rare-earth and related metals in vacuum to 10^?10 mm of mercury. The metals studied were lanthanum, neodymium, praseodymium, cerium, holmium, erbium, gadolinium, dysprosium, samarium, yttrium, and thallium. Friction and wear experiments were conducted with the rare-earth or related metals generally sliding against 440-C stainless steel.

The results of the investigation indicate that crystal structure considerations and polymorphism can explain the friction, wear, and metal-transfer characteristics of the rare-earth and related metals in vacuum. Close-packed hexagonal crystal forms of the rare earths and thallium had much lower friction, wear, and metal-transfer characteristics than face-centered or body-centered cubic structures.     

The Tribological Behavior of Polycrystalline Cobalt as Related to Crystallographic Texture and Structure

The friction and wear behavior of polycrystalline hexagonal cobalt was determined under normal atmospheric conditions, using heavy loads and sliding speeds ranging from 0.92 cm/sec to 36.8 cm/sec. Very uniform wear was observed for most conditions together with a low coefficient of friction. X-ray diffraction measurements showed that this desirable behavior is caused by the formation of a preferred orientation during sliding. Under high-load high-speed conditions a different mode of wear was observed, best described as brittle disintegration. When cobalt was alloyed with 8 percent iron and thus rendered face-centered cubic, a deeply torn wear track, an increased initial wear rate and a high coefficient of friction were obtained.     

A setup for studying thermal characteristics of metals at high temperatures

A setup for studying the radiation characteristics of metals based on calorimetry of metals exposed to electron heating under high-vacuum conditions has been developed. A new design of the cathode-anode unit ensures a uniform distribution of the electron flow over the sample surface. The created decelerating field prevents secondary electrons from leaving the cathode-anode unit. The serviceability of the setup was tested via measurements of the integral hemispherical emissivity of extremely pure titanium (>99.91 mass %). The emissivity changes in the temperature range 1155–1165 K, associated with the restructuring of the crystal lattice from a close-packed hexagonal structure into a body-centered cubic structure, has been revealed.     

Friction and wear of ceramics

The adhesion, friction, wear and lubricated behaviors of both oxide and non-oxide ceramics are reviewed. Ceramics are examined in contact with themselves, other harder materials and metals. Elastic, plastic and fracture behavior of ceramics in solid state contact is discussed. The contact load necessary to initiate fracture in ceramics is shown to be appreciably reduced with tangential motion. Both friction and wear of ceramics are anisotropic and relate to crystal structure as with metals. Grit size effects in two- and three-body abrasive wear are observed for ceramics. Both free energy of oxide formation and the d valence bond character of metals are related to the friction and wear characteristics for metals in contact with ceramics. Surface contaminants affect friction and adhesive wear. For example, carbon on silicon carbide and chlorine on aluminum oxide reduce friction while oxygen on metal surfaces in contact with ceramics increases friction. Lubrication increases the critical load necessary to initiate fracture of ceramics both in indentation and with sliding or rubbing.     

Crystal plasticity and the delamination theory of wear

Some implications of crystal plasticity effects with respect to the delamination theory of wear are discussed. Critical experiments to clarify the soft versus hard surface question, of interest with respect both to wear and to deformation in general, are suggested. A specific dislocation model for hexagonal close-packed (h.c.p.) metals under wear conditions is proposed and a correlation with stacking fault energy for face-centred cubic (f.c.c.) and h.c.p. metals is discussed.     

Frictional Characteristics of Quasicrystals at High Temperatures

AlCuFe quasicrystal coatings were deposited by unbalanced magnetron sputtering for study of their friction and wear properties at high temperatures. It has been shown that growth and annealing conditions can be controlled to produce icosahedral quasicrystal or the approximant cubic phase. The comparison of friction and wear properties between quasicrystal and an approximant with nearly the same stoichiometry permits assessment of the unique quasicrystalline structure for tribological applications. The goals of this study are to determine how crystal structure influences tribological properties and to study the general friction and wear behavior of quasicrystals. Tribological properties were evaluated using a pin-on-disk tribometer and crystal structure was characterized using an X-ray diffractometer. The tests specimens were 10 m thick films deposited on a 1 diameter steel disk and a 1/4 diameter steel ball. Data was collected over a range of temperatures from room to 600°C. Friction coefficients and wear rates of quasicrystals and approximants were nearly identical for room temperature tests. The wear process generated Al, Cu, Fe, and oxide debris on the side of the track, but overall wear was mild. The friction coefficient of the icosahedral phase was 25% lower than the cubic phase at 150 thru 450°C. Generally, only moderate differences in the friction and wear properties were observed between the icosahedral quasicrystal phase and the cubic phase.     

Friction and Wear Behavior of Single-Crystal Silicon Carbide in Sliding Contact with Various Metals

Sliding friction experiments were conducted with single-crystal silicon carbide in contact with various metals. Results indicate the coefficient of friction is related to the relative chemical activity of the metals. The more active the metal, the higher the coefficient of friction. All the metals examined transferred to silicon carbide. The chemical activity of the metal and its shear modulus may play important roles in metal-transfer, the form of the wear debris and the surface roughness of the metal wear scar. The more active the metal, and the less resistance to shear, the greater the transfer to silicon carbide and the rougher the wear scar on the surface of the metal. Hexagon-shaped cracking and fracturing formed by cleavage of both prismatic and basal planes is observed on the silicon carbide surface.     

Radiation characteristics of getter materials in the region of a phase transformation

The results of studies of the radiation characteristics of samples of titanium and alloys thereof in the region of their polymorphous transformation and the temperature dependence of the emissivity in the β phase are presented. The calorimetric method of electron heating in high vacuum was used. It is shown that changes in the integral hemispherical emissivity ɛ _th for titanium iodide and industrial-grade titanium related to the transformation of a hexagonal close-packed lattice into a body-centered cubic lattice have different characters. Original Russian Text ? B.A. Shur, V.E. Peletskii, 2009, published in Pribory i Tekhnika Eksperimenta, 2009, No. 1, pp. 162–167.     

Texturing in metals as a result of sliding

Sliding friction experiments were conducted with copper, nickel, iron and cobalt sliding on themselves in air and argon. The resulting wear surfaces were examined with X-ray analysis to determine if surface texturing had occurred as a result of sliding. Results of the investigation indicate that, for the face-centered-cubic metals copper and nickel, a (111) texture develops with the (111) planes tilted 10° in the direction of sliding. The bodycentered-cubic metal iron exhibited a (110) texture with the (111) direction oriented in the direction of sliding. It also exhibited a 10° tilt in the direction of sliding. The environment influenced the results in that the degree of texture observed in argon was less than that seen in air for iron. No texturing was observed for the close-packed-hexagonal metal cobalt. Recrystallization was observed with copper as a result of sliding.     

Friction and Wear Characteristics of Single Crystal Ni-Based Superalloys at Elevated Temperatures

The purpose of this study was to investigate the friction and wear behavior of single crystal superalloys at elevated temperatures. Pin-on-plate experiments were conducted using a custom-built high-temperature fretting/wear apparatus. Measurements were performed on two single crystal Ni-based alloys and Waspaloy^® (used as a baseline material). The coefficient of friction for the single crystal materials (i.e., during running-in and steady state) was lower compared to the Waspaloy^®. In addition, the experiments showed that the friction coefficient of the single crystal is dependent on the crystallographic plane; the friction coefficient was lower for the tests on the {100} plane compared to the {111} plane. The wear behavior was aligned with the friction behavior, where the single crystal Ni-based alloys showed slightly higher wear resistance compared to the Waspaloy^®. Ex situ analysis by means of FIB/SEM and XPS analysis revealed the formation of Co-base metal oxide layer on the surface of the single crystal alloy. Similarly, a Co-base oxide layer is observed on the counterface providing a self-mated oxide-on-oxide contact and thus lower friction and wear compared to the Waspaloy^®.     

The effect of microstructure on the wear of cobalt-based alloys used in metal-on-metal hip implants

The influence of microstructure on the wear of cobalt-based alloys used in metal-on-metal hip implants was investigated in a boundary lubrication regime designed to represent the conditions that occurred some of the time in vivo. These cobalt-chromium-molybdenum alloys were either wrought, with a total carbon content of 0.05 or 0.23 wt %, cast with a solution-annealing procedure or simply as-cast but not solution annealed. Bars of these different alloy grades were subjected to various heat treatments to develop different microstructures. The wear was evaluated in a linear-tracking reciprocating pin-on-plate apparatus with a 25 per cent bovine serum lubricant. The wear was found to be strongly affected by the dissolved carbon content of the alloys and mostly independent of grain size or the carbide characteristics. The increased carbon in solid solution caused reductions in volumetric wear because carbon helped to stabilize a face-centred cubic crystal structure, thus limiting the amount of strain-induced transformation to a hexagonal close-packed crystal structure. Based on the observed surface twining in and around the contact zone and the potentially detrimental effect of the hexagonal close-packed phase, it was postulated that the wear of cobalt-based alloys in the present study was controlled by a deformation mechanism.     

Friction-induced structural modifications of Mg and Ti surfaces

High-purity magnesium and titanium have been examined in order to study modifications to the near-surface layer during friction. Upsetting tests were conducted under atmospheric and imposed hydrostatic pressure to assess the plasticity and deformability of these metals. It is shown that the deformability of magnesium and to a lesser extent that of titanium can be considerably enhanced by superimposed hydrostatic pressure. Both metals were subjected to uniaxial, constant load, dry friction, pin-on-plate tests in air at room temperature and atmospheric pressure and the near-surface layer examined destructively and non-destructively in order to characterize induced changes. High-resolution X-ray diffraction examination of the worn and unworn surfaces suggested that the deformability of the near-surface layer in magnesium is associated with an increase in 〈a〉 Burgers vector screw dislocations able to cross slip and which contribute to recovery and recrystallization in the deformed region. A similar effect may be present for titanium although no recrystallization was observed.     

Theoretical surface temperatures generated from sliding contact of pure metallic elements

Surface temperatures and thermal effects produced in tribological processes are important not only in influencing possible mechanisms of friction, wear, and lubricant film failure but also in initiating protective film-formation. As part of a continuing combined theoretical and experimental study of surface temperatures generated by friction, the fundamental Green's function approach has been applied to a number of pure metallic elements to compare and discuss their predicted behavior in A-on-A sliding contact. Assuming a single area of real contact, calculated ratios of surface temperature rise to coefficient of friction plotted against area of contact, velocity and load on a logarithmic scale are presented and summarized for several pure metallic elements in the first transition series of the Periodic Table (e.g., Ti, V, Cr, Mn, Fe, Co, Ni) as well as members in connecting groups, e.g., Cr, Mo, and W in Group VIa and Cu, Ag, and Au in Group Ib. These include metals which are tribologically difficult to machine and use (e.g., Ti), common elements in bearing steels (e.g., Fe, Cr), and metals useful in reducing friction or wear when applied as thin surface coatings (e.g., Ag, Au). The results of this comparison are interesting and surprising. They may add to our understanding of why some metals are very “difficult” in a tribological sense while others provide benefits in controlling friction and/or wear.     

Friction Characteristics in Vacuum of Single and Polycrystalline Aluminum Oxide in Contact with Themselves and with Various Metals

Friction experiments were conducted in vacuum with outgassed surfaces. Experiments were made with single and polycrystalline Al₂O₃, sliding on themselves and in contact with metals at a sliding velocity of 0.013 cm per second, at loads to 1500 gm, temperatures to 575 C and ambient pressures to 10^?10 mm Hg. These studies were made with a hemispherical or spherical rider sliding on the flat of a rotating disk. The results of the investigation indicate that (a) the friction characteristics of sapphire sliding on sapphire is highly anisotropic, (b) with metals sliding on sapphire, fracture in sapphire occurs while with polycrystalline Al₂O₃ shear occurs in the metal, and (c) hexagonal metals exhibited lower friction co-efficients than cubic metals in contact with polycrystalline Al₂O₃.     

Fretting wear of carburized titanium alloy against ZrO2 under serum lubrication

Fretting wear of carburized titanium alloys was investigated on the universal multifunctional tester (UMT) with the ball-on-flat fretting style under bovine serum lubrication. The tangential load and friction coefficient during the fretting process were analyzed, and the evolution of fretting log during the fretting process was investigated to understand the wear mechanism of the titanium alloy and carburized titanium alloy. Furthermore, the wear scar was examined using a SEM and three-dimension surface profiler. It was found that the friction coefficient of the titanium alloy increased faster than that of carburized titanium alloy in the first stage under serum lubrication, and then remained steady with a similar value in the second stage. The Ft-D curve indicated that there was wear mechanism transition from gross slip to mixed stick and slip. Finally, it was observed that there was a slight damage of the titanium alloy and carburized titanium alloy showed excellent performance during the fretting wear process under serum lubrication. All of the results suggested that carburized titanium alloy was a potential candidate for the stem material in artificial joints.     

海洋环境下典型金属材料腐蚀与磨损研究进展

海洋环境下金属材料的腐蚀与磨损及其复杂的交互作用是影响海洋装备可靠性和工作效率的关键因素。在概述海洋环境下金属材料腐蚀和磨损的现象及问题的基础上,分析海洋环境下典型金属腐蚀与磨损的交互作用研究进展及发展趋势,并对不锈钢、钛合金与铝合金等主要海洋环境用金属材料在不同介质下的腐蚀与磨损问题进行探讨,重点阐述海洋环境下金属腐蚀与磨损中相互促进的正交互作用和相互抑制的负交互作用机制以及它们之间的相互影响、过渡和转换等方面的研究进展。在腐蚀与磨损正、负交互作用转变的过程中,从微观角度讲,奥氏体和马氏体相的多次转变是主要影响因素;而从宏观角度看,材料的摩擦因数、载荷、运动形式等动摩擦因素和Cl-浓度、pH值、频率及外加电位等电化学因素是两类主要影响因素。对比分析多种腐蚀与磨损交互作用的计算模型,阐述利用表面技术改善金属材料防腐耐磨性能的研究进展,提出金属材料腐蚀与磨损的负交互作用机制、微生物对腐蚀与磨损交互作用的影响机制、微动磨损与腐蚀的交互作用研究以及腐蚀与磨损交互作用量的准确量化是未来的研究重点。     

The fretting wear of Ti-6Al-4v and aged inconel 718 at elevated temperatures

The fretting wear of Ti-6Al-4V and Inconel 718 was investigated with a sphere-on-flat configuration. The spherical surface was 100 mm in radius and in all tests was made of the same material as the flat. The normal load was 2.75 N and the frequency of the tangential movement was 50 Hz. Two amplitudes of slip were used, 10 and 40 μm. Tests were conducted in air at temperatures up to 600 °C for the titanium alloy and up to 540 °C for the nickel alloy. High temperature strain gauges enabled a continuous record of the tangential stress to be made and subsequent calculation of the coefficient of friction. Wear was assessed from measurement of the scar volume. At 280 and 540 °C at an amplitude of 40 μm the coefficient of friction and wear rate decreased to a low value on the nickel alloy. This only occurred at 540 °C for the lower amplitude of slip. Low friction and wear are associated with the formation of a “glaze” oxide, which requires a larger slip amplitude at lower temperatures for its formation.The titanium alloy generally exhibited higher coefficients of friction which continued to increase at 10⁶ cycles, although wear rates at 200 °C and above were comparable with those on the nickel alloy. “Glaze” oxide begins to form at 200 °C and is well developed at 400 °C. At 600 °C breakdown occurs owing to local creep of the substrate.     

SLM成形表面织构对TC4钛合金干摩擦磨损性能的影响

基于一步法思路,采用金属3D打印机基于激光选区熔化（SLM）技术制备表面带有凹坑织构的TC4钛合金试样,采用光学相机、超景深显微镜和扫描电镜观察织构成形情况,利用激光共聚焦位移测试仪和显微维氏硬度计分别测试表面粗糙度和表面硬度,在干摩擦条件下采用摩擦磨损试验仪考察不同载荷下织构密度对TC4钛合金试样摩擦学性能的影响,并使用扫描电镜对摩擦实验前后的表面形貌进行分析。研究结果表明：一步法SLM成形能够在TC4钛合金表面获得成形良好的直径500 μm的织构;随着织构密度的提高,钛合金试样表面粗糙度增大,表面硬度有所降低;干摩擦条件下,提高TC4钛合金试样织构密度有利于磨屑的收集从而减少试样的三体磨损,提高载荷有利于改善摩擦副接触状态;5 N载荷下40%织构密度试样的平均摩擦因数和磨痕宽度均最小,与无织构试样相比,平均摩擦因数和磨痕宽度分别降低12%和16%;40%织构密度下,载荷提高会引起摩擦因数的降低和磨损量增大,磨损表面犁沟和片状剥落增多。在干摩擦条件下,3D打印一步法制备的表面织构可以显著改善TC4钛合金的磨粒磨损和黏着磨损。     

The influence of atmospheric humidity on the abrasive wear of metals

A pin-on-disc apparatus has been used to obtain continuous simultaneous measurements of the wear and friction (sliding force) behaviour of metals on bonded silicon carbide abrasive paper under conditions of controlled humidity. Iron, mild steel and copper exhibit qualitatively similar wear behaviour: the wear rate decreases progressively with the number of passes over the same track. In contrast, the wear rate of titanium remains constant. Variation in atmospheric humidity has little effect on the wear rates of copper or titanium, although a slight effect was found in mild steel and iron. A stronger dependence on humidity was found in the friction behaviour of all four metals, as well as a corresponding relationship between humidity and the specific energy required for metal removal by abrasion. Preliminary results from single-particle scratch tests reveal changes in the contact between a single silicon carbide particle and a polished iron surface at different humidity levels. Although only tentative explanations can be made at this stage for these effects, it is evident that any proposed mechanism must account for the behaviour of both the metal and the abrasive together, rather than of one component of the system alone.     

The influence of temperature on the deformation and fracture of an lif single crystal subjected to friction and wear

Sliding friction experiments were conducted with an LiF single crystal in contact with a steel ball at temperatures of 25–300 °C in air. The influence of break-in conditions and the initial structural state on the temperature dependence of microhardness as well as friction and wear characteristics were investigated. A comparison of the obtained results with the data in an LiF stress—temperature map has been carried out. The region of normal friction is associated with dislocation glide (plasticity region on the map) and the region of galling and seizure is associated with dislocation creep (power creep region on the map). The results indicate that the temperature of transition to seizure and galling is comparable with the temperature of transition to high temperature creep. Creep transition temperatures estimated for a series of metals showed fine agreement with the temperature of adhesive interaction.