Spanen metallischer Konstruktionswerkstoffe geringer Dichte

Machining of light metals Magnesium, aluminium and titanium are the only light metals that are also used in construction. They offer a significant prerequisite for weight reduction of workpieces. Especially the automotive and the aerospace industry have an increasing interest in using these lightweight materials as well as their alloys. The machining of light metals however is accompanied with several problems. With increasing the cutting speed high adhesive and abrasive effects between the cutting tool material and the workpiece material can occur. These effects lead to unsteady processes and also have a negative influence on the quality of functional surfaces as well as their subsurface properties. The influence of cutting tool materials, tool coatings and cutting conditions affecting the process when cutting magnesium, aluminium and titanium alloys is described. Adhesion can be reduced when machining magnesium and aluminium alloys in particular by the application of diamond-coated tools and by PCD-inserts. Diamond tools, due to their low coefficient of friction and the high thermal conductivity, furthermore contribute to the decrease of the thermal load within the contact zone between workpiece and cutting tool. Subsequently the danger of magnesium chip ignition can be minimized. For the machining of titanium alloys modern coatings based on (Ti,Al)N and TiCTiN are applied to reduce the adhesive and abrasive wear.     

Relationships between the fretting wear behavior and mechanical properties of thin carbon films

Mechanical load can drastically affect the properties of orthopedic implant materials. Damage of these materials usually occurs in contact surfaces, caused by abrasion, adhesion, fretting, delamination, pitting and fatigue depending on friction, lubrication, contact area, surface finish and level of loads (stresses).Carbon-based films are biocompatible with good bearing capacity, wear resistance, corrosion resistance and have a low coefficient of friction. However, great intrinsic stress prevents their wider application, mainly as implant coatings. To reduce this undesirable effect special deposition procedures are under development and/or the films are doped with suitable elements. It must be emphasized that DLC is not a material but a group of materials with a variety of properties. The relationships between the fretting wear behavior and mechanical properties of films based on carbon deposited by DC using the pulsed arc discharge PVD nitrogen doped (a-C) and the filtered pulsed arc discharge deposition system (ta-C) were tested.The composition of carbon films (sp³, sp²) was determined by Raman spectroscopy. Mechanical properties of elastic modulus and hardness were determined by a NanoTest apparatus with diamond Berkovich tip using the Oliver-Pharr procedure and adhesion was measured by nanoscratch tests. Tribological behavior was analyzed by fretting tests with a corundum ball under dry sliding lubricated conditions.The good performance of the hard carbon coatings is often discussed. Results from this study of fretting and the associated lubrication (bovine serum) show that ta-C coatings, despite their high hardness, have very low friction coefficients and low volume losses.     

热处理对激光熔覆钛基复合涂层组织和微动磨损性能的影响

为研究不同温度热处理对激光熔覆钛基复合涂层组织和微动磨损性能的影响,采用激光熔覆技术在TA2钛合金表面制备40%Ti-25.2%TiC-34.8%WS_2(质量分数)复合涂层,将涂层分别置于300,500℃和700℃真空中保温1h,分析热处理前后涂层的显微组织和微动磨损耐磨性能。结果表明:未经过热处理涂层及经过不同温度热处理涂层的主要物相均为α-Ti,(Ti,W)C_(1-x),TiC,Ti_2SC和TiS。未热处理及经过300,500℃和700℃热处理1h涂层的显微硬度分别为1049.8,980.7,1143.3HV_(0.5)和1190.7HV_(0.5)。经过700℃热处理1h涂层表现出优异的微动磨损性能,磨损机理为黏着磨损和磨粒磨损。     

Failure analysis of a railway copper contact strip

The present work investigated the causes of premature wear of the contact strips of a railway line working under voltage of 1.500 Vcc, current of 1120 A, normal force of 70 N and presence of graphite in the wire/strip interface. In all investigated regions (without apparent wear, moderate wear and severe wear), the presence of cracks in the hardened tribo-surface of the Cu-strip – which is caused either by work hardening or thermal cycling is usually followed by material detachment and production of hard abrasive debris. The presence of hard abrasive particles (such as SiO₂ and Al₂O₃,) and wear debris (Cu₂O and hardened Cu) promotes a regime of severe abrasion. The debris showed preferentially a flake-like morphology, being composed of graphite and highly deformed copper, suggesting the dominant action of mechanical wear mechanisms. The presence of some raindrop-like debris featuring an as-cast microstructure confirmed the occurrence of incipient fusion on the copper strip tribo-interface, possibly caused by electrical discharge (electrical induced wear). The results indicated that the wear mechanism of the Cu strip is divided into different stages. First, there is a mixed wear regime (adhesive and abrasive wear) of the graphite layer associated with lubricated adhesive wear of the Cu strip. After, there is the occurrence of a mixed wear regime between the strip/wire tribo surfaces, with simultaneous action of unlubricated adhesive wear, third body abrasive wear and electrical induced wear (local fusion). Finally, once the graphite has been completely consumed, the wear grooves are parallel to the sliding direction and the centre of the strip shows a much more severe wear rate.     

Self-lubricating properties of PTFE/serpentine nanocomposite against steel at different loads and sliding velocities

The influences of normal contact pressure and sliding velocity on self-lubricating property of PTFE-based composite filled with nano-serpentine particles were mainly studied in the range of 0.12–0.96 m/s and 1.43–8.55 MPa. For comparison, the friction and wear experiments of pure PTFE also were carried out under the same conditions. A scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) were employed to examine the worn surfaces of PTFE/serpentine nanocomposite and transfer films. The experimental results showed that normal contact pressure and sliding velocity had significant effects on the tribological performances of PTFE/serpentine composite. Under different loads, the nanocomposite suffered from abrasive wear, fatigue wear and adhesive wear successively. And at different sliding velocity, the main wear mechanisms were abrasive wear and adhesive wear. SEM and EDS analyses indicated that the continuous and uniform transfer film generated on counterfaces acted as a solid lubricant, which was important to improve the self-lubricating property of PTFE/serpentine nanocomposite.     

Sliding wear resistance of Fe-, Ni- and Co-based alloys for plasma deposition

Performance of Fe-, Ni- and Co-based alloys was evaluated in sliding wear and in the impact wear tests to simulate wear of valves and valve inserts in internal combustion engines. From the SEM investigations adhesive wear was confirmed by metal transfer between rubbing surfaces while surface deformation testify to abrasive wear. It was found that oxides formed on the surface prevented direct metal-to-metal contact and reduced the coefficient of friction thereby reducing wear. A comparison of wear resistances of Fe, Ni- and Co-based alloys used as hardfacings were given. It is also shown that Ni–Co alloys with Ni-alloy content between 40.4 and 64.3 have a considerable potential, they reveal above 2 times higher wear resistance than plasma-deposited cobalt alloy.     

Structure and wear of electroless nickel coatings

Abstract

The structure and properties of electroless nickel are highly sensitive to heat treatment owing to its pronounced metastability. The heat treatment process induces crystallization of amorphous nickel to nickel phosphides and fcc nickel by a mechanism similar to that of the solidification of liquid alloys. The process produces a twofold increase in hardness, the elimination of severe adhesive wear, and a deterioration in the abrasive scratch resistance. The susceptibility of the as deposited coating to severe adhesive wear during unlubricated sliding against plain carbon steel is due to the low interfacial and high surface free energies of the two surfaces, resulting in a high compatibility. Heat treatment removes severe wear by changing the structure from amorphous nickel to Ni₃P, which presents a surface incompatible to steel. The abrasive scratch resistance of the as-deposited coating is a result of its relatively high fracture toughness, which is attributable to the absence of heterogeneities in the amorphous structure. The relevance of the data to engineering applications is discussed.

MST/539     

Wear characteristics of plasma-nitrided CrMo steel under mixed and boundary lubricated conditions

An investigation of the sliding wear mechanisms of a plasma-nitrided CrMo steel is undertaken using a running-in procedure that eliminates severe wear under mixed and boundary lubricated conditions. In the running-in procedure, using a pin-on-disc wear machine, smooth contact surfaces are obtained rather than the rough contact surfaces generally found in laboratory experiments. This enables the wear mechanisms to be investigated more clearly, particularly mild wear processes. The work shows no measurable nor visible wear under full fluid film lubricated conditions, the existence of polishing under mixed lubricated conditions and micro-pits under boundary lubricated conditions. The results presented in this paper indicate that a mild abrasive wear mechanism predominates under mixed lubricated conditions and a micro-fatigue wear mechanism under boundary lubricated conditions. Examination of the microstructure reveals the formation of white layer regimes on the contact surfaces after tests under boundary lubricated conditions, which suggests severe work-hardening of the contact surfaces. This revised version was published online in November 2006 with corrections to the Cover Date.     

电刷镀Ni-CNTs/PTFE纳米复合镀层的摩擦磨损与耐腐蚀性能

再制造工程中很多表面镀层要求具有优异的摩擦磨损与耐腐蚀性能,利用纳米电刷镀技术在45钢基材上制备NiCNTs、Ni-CNTs/PTFE、Ni-WC/PTFE-CNTs复合镀层。采用XRD和SEM观察电刷镀复合镀层表面相结构和微观形貌,采用球盘式摩擦磨损试验机测试其在干摩擦条件下的摩擦磨损性能,采用动电位极化曲线研究其在3.5%NaCl溶液中的电化学腐蚀行为。结果表明:Ni-WC/PTFE-CNTs复合镀层耐磨性能最优,其次为Ni-CNTs/PTFE、Ni-CNTs复合镀层,均强于纯镍镀层;当CNTs质量浓度分别为1.5g/L和1.0g/L时,Ni-CNTs复合镀层分别表现出最优的摩擦磨损性能和最佳的耐腐蚀性能,Ni-WC/PTFE-CNTs、Ni-CNTs/PTFE复合镀层次之。纯镍镀层和Ni-CNTs复合镀层的磨损机制是粘着磨损,Ni-CNTs/PTFE复合镀层的磨损机制主要是粘着磨损,其次为磨粒磨损,Ni-WC/PTFE-CNTs复合镀层的磨损机制主要是磨粒磨损和接触疲劳磨损。     

Abrasive wear behaviour of powder flame sprayed coatings on steel substrates

The main purpose of thermal spraying method is to produce wear resistant surfaces. Easy applicability, very low possibility of metallurgical changes and low distortion of coated parts due to low heat transfer to the substrate and salvation of worn parts are some of the practical advantages of this process. In this study, abrasive wear behaviour of powder flame sprayed coatings on steel substrates has been investigated. Coating was carried out onto both hot and cold substrates by using four types of powders. Prior to the wear tests, the coated specimens were machined on a lathe and surface roughness and hardness measurements were carried out on the machined surfaces. Heating the substrates prior to the coating led to the decrease in the hardness of the coating layers. Abrasive wear resistance of flame sprayed coatings was seen to be dependent on the chemical composition and characteristics of coating materials and coating condition.     

Fretting–fatigue behavior of steel wires in low cycle fatigue

The effect of strain amplitude on fretting–fatigue behavior of steel wires in low cycle fatigue was investigated using a fretting–fatigue test rig which was capable of applying a constant normal contact load. The fretting regime was identified based on the shape of the hysteresis loop of tangential force versus displacement amplitude. The variations of the normalized tangential force with increasing cycle numbers and fretting–fatigue lives at different strain amplitudes were explored. The morphologies of fretting contact scars after fretting–fatigue tests were observed by scanning electron microscopy and optical microscopy to examine the failure mechanisms of steel wires. The acoustic emission technique was used to characterize the fretting–fatigue damage in the fretting–fatigue test. The results show that the fretting regimes are all located in mixed fretting regimes at different strain amplitudes. The increase in strain amplitude increases the normalized tangential force and decreases the fretting fatigue life. The abrasive wear, adhesive wear and fatigue wear are main wear mechanisms for all fretting–fatigue tests at different strain amplitudes. The accumulative total acoustic emission events during fretting–fatigue until fracture of the tensile steel wire decrease with increasing strain amplitude. An increase of the strain amplitude results in the accelerated crack nucleation and propagation and thereby the decreased life.     

TC4钛合金表面冷喷涂制备CuNiIn涂层组织及其微动磨损性能研究

为推动冷喷涂技术在航空防护涂层领域的应用,采用氮气冷喷涂技术在TC4钛合金表面制备了CuNiIn涂层,使用光学显微镜、扫描电镜、显微硬度计和微动磨损试验机对涂层宏/微观组织、显微硬度及微动磨损性能进行了分析。结果表明:越靠近基体,涂层致密性越好,整体涂层孔隙率约为2.8%,涂层微观组织较粉末原始组织有一定细化;涂层硬度值自基体向表面沿厚度方向有增大的趋势,靠近基体一侧硬度平均值为274 HV_2N,靠近表面的涂层硬度稳定在300 HV_2N左右;微动位移较小时,涂层失效机制主要是黏着磨损,磨损形貌为椭圆形,微动位移较大时,以磨粒磨损为主,磨损形貌接近圆形。     

Plasmaelektrolytische Oxidation thermisch gespritzter Aluminiumschichten

Plasma electrolytic oxidation of arc sprayed aluminium coatings Up to now different post treatment methods are developed to improve the properties of thermally sprayed coatings. In this work, arc sprayed aluminium coatings on aluminium substrates are post‐treated by plasma electrolytic oxidation. To estimate the wear resistance of resulting oxide coatings, two abrasive wear tests (ASTM G65 and ASTM C1624) are carried out. Worn surfaces are examined by scanning electron microscopy in order to establish the wear mechanisms. These results of the abrasive wear tests are correlated with the parameters of the PEO process and the resulting micro structures of the coatings.     

新型填料对高摩复合材料摩擦磨损性能影响及机制

采用干法热压成型工艺制备高摩复合材料,研究了基体材料腰果壳油改性酚醛树脂(CPR)与丁晴橡胶(NR)的质量比和新型高性能填料(主要成分为石墨粉Al2O3、MoS2、Fe粉)含量对高摩复合材料摩擦磨损性能的影响规律。在摩擦磨损试验机上测试了高摩复合材料的摩擦磨损性能,利用激光共聚焦显微镜、扫描电镜对摩擦表面形貌、磨屑进行观察和分析,借助EDS测定摩擦表面成分的变化。结果表明,随着CPR与NR质量比的增加,高摩复合材料的耐热性能、结合性能大幅提高,且具有较好的摩擦磨损性能。当高性能填料含量较低时,磨损表面出现大量连续的真实接触面,磨损机制为磨粒磨损和黏着磨损;当高性能填料含量较高时,真实接触面积减少,磨损表面剥落严重,并出现较多的裂纹,其主要磨损形式转变为磨粒磨损和疲劳磨损。随着高性能填料含量的增加,摩擦表面的元素从均匀分布逐渐转变为局部富集,磨粒的尺寸逐渐变大。     

Cu-Ag-Fe合金的电接触滑动磨损的研究

为改善接触线的耐磨性能,在Cu-Ag合金中添加微量Fe元素制备了Cu-Ag-Fe合金接触线,研究了电流强度、滑动速度和载荷对Cu-Ag-Fe合金的磨损形貌及磨损率的影响.实验结果表明：随电流强度、滑动速度和载荷的增加,合金的质量磨损率明显增加.磨损形貌表明,在受电滑动条件下,磨损形式以粘着磨损、磨粒磨损和电侵蚀磨损为主...     

TA15合金表面Ni-SiC复合镀层的摩擦磨损性能

采用共沉积法在TA15合金表面制备了Ni-SiC复合镀层, 分析了Ni-SiC复合镀层对基体合金硬度和磨擦磨损性能的影响, 并对其摩擦磨损机制进行了讨论。结果表明: 所制备的Ni-SiC复合镀层组织致密且与基体结合紧密, 硬度明显高于TA15合金基体。摩擦磨损实验结果表明, Ni-SiC复合镀层能为TA15合金提供良好的摩擦磨损抗力, 在相同的摩擦条件下, Ni-SiC复合镀层的磨损率明显低于TA15合金。TA15合金与GCr15球和Al₂O₃球对磨的磨损机制均主要为犁削磨损、粘着磨损, 同时伴随有氧化磨损和轻微的磨粒磨损; Ni-SiC复合镀层与GCr15磨球对磨的磨损机制主要为镀层组织的拔出及GCr15钢球在其表面上的涂抹, 与Al₂O₃磨球对磨时的磨损机制主要为疲劳磨损和削层磨损。     

Fabrication of 6061 aluminum alloy reinforced with Si3N4/n-Gr and its wear performance optimization using integrated RSM-GA

In this research work the dry sliding wear behavior of a hybrid aluminum metal matrix composite is evaluated. Al 6061 is used as a matrix material while Si₃N₄ and nanographite powder (3–15 wt%) are used as reinforcements. These two reinforcements (50 wt% of each) were blended in a high-energy ball mill for homogeneous mixing to derive the sound aluminum matrix composite (AMC). The hybrid composite is made by the stir casting route and its wear rate was investigated against an EN32 steel disc surface, using a pin-on-disc tribometer. Integrated response surface methodology (RSM) and genetic algorithm (GA) are used to optimize the pin-on-disc process parameters. Analysis of variance (ANOVA) shows that sliding distance plays a major role on the dry sliding wear rate followed by load, sliding speed and reinforcements. Two-factor interactions and quadratic terms have also significant contributions. GA suggested a minimum wear rate value of 0.827 mg at optimized setting. Microstructural analysis by scanning electron microscopy (SEM) reveals that very fine grooves are obtained at optimized settings while at other settings severe ploughing is observed. Transition of wear mechanism takes place with the increase of speed (i.e., temperature between the two rubbing surfaces) from abrasive to adhesive.     

Wide temperature spectrum self-lubricating coatings prepared by plasma spraying

Self-lubricating, multicomponent coatings, which lubricate over a wide range of operating conditions, are described. The coatings have been successfully applied by plasma spraying mixed powders onto superalloy substrates. They have been evaluated in friction and wear experiments and in sliding contact bearing tests. These coatings are wear resistant by virtue of their self-lubricating characteristics rather than because of extreme hardness; a further benefit is low friction. Experiments with simple pin-on-disk sliding specimens and oscillating plain cylindrical bearing tests were performed to evaluate the tribological properties of the coatings. It was shown that coatings of Nichrome, glass and calcium fluoride are self-lubricating from about 500 to 900°C, but give high friction at the lower temperatures. The addition of silver to the coating composition improved the low temperature bearing properties and resulted in coatings which are self-lubricating from cryogenic temperatures to at least 870°C; they are therefore “wide temperature spectrum” self-lubricating compositions.     

Solid lubricants: a review

The fundamental mechanisms of solid lubrication are reviewed with examples from well-known solid lubricants like the transition metal dichalcogenides and diamond-like carbon families of coatings. Solid lubricants are applied either as surface coatings or as fillers in self-lubricating composites. Tribological (friction and wear) contacts with solid lubricant coatings typically result in transfer of a thin layer of material from the surface of the coating to the counterface, commonly known as a transfer film or tribofilm. The wear surfaces can exhibit different chemistry, microstructure, and crystallographic texture from those of the bulk coating due to surface chemical reactions with the surrounding environment. As a result, solid lubricant coatings that give extremely low friction and long wear life in one environment can fail to do so in a different environment. Most solid lubricants exhibit non-Amontonian friction behavior with friction coefficients decreasing with increasing contact stress. The main mechanism responsible for low friction is typically governed by interfacial sliding between the worn coating and the transfer film. Strategies are discussed for the design of novel coating architectures to adapt to varying environments.     

Tribological investigations of polyetherimide composite

Polyetherimide (PEI), commercially known as ULTEM and manufactured by GEC (USA), is one of the newest high-performance thermoplastics. Its graphite and short-glass-fibre (GF) filled composition was evaluated for friction and wear properties. Tribological studies of the material sliding against mild steel, under different loads, counterface roughnesses and sliding distances were performed on a pin and disc configuration. It was observed that this composite displayed very good wear resistance due to glass-fibre reinforcement and low friction due to the solid lubricant graphite. The wear mechanism was studied with scanning electron microscopy by observing the worn pin and disc surfaces. Fatigue was observed to be the main factor in wear, along with adhesive and abrasive modes.