On the fretting wear mechanism of Zr-alloys

Zirconium alloys are highly desirable in nuclear applications due to their transparency to thermal energy neutrons and for their high corrosion resistance. The main objective of this study is to investigate the fretting wear mechanism of Zr–2.5%Nb alloy. The experimental work was carried out in air at 265 °C, using a specially designed fretting wear tribometer. The transfer of material, the change in the wear volume and the maximum wear depth with the number of cycles were measured through 3D mapping of the topography of the fretted surface. SEM and Fourier Transform Infrared Interferometry methods were used to examine the microspall pits and to measure the distribution of the thickness of oxide layer in the fretting region. For relatively small slip amplitude, the results showed that the fretting wear mechanism is initially dominated by adhesion and abrasion actions and then by delamination and surface fatigue. The time variation of the wear losses was shown to be cyclic until a steady state value is reached. At high slip amplitudes, however, abrasion and delamination are the only dominant wear mechanisms. The volumetric wear losses were found to decrease monotonically with the number of cycles. A novel approach was introduced, whereby the thermal and electrical contact resistances of the fretting interface are simultaneously measured. The results demonstrated the potential use of this non-intrusive approach for real-time monitoring of the fretting wear mechanism.     

Friction and wear of Inconel 690 for steam generator tube in elevated temperature water under fretting condition

Recently, material of Inconel 690TT (thermal treatment) for the steam generator tubes in a nuclear power plant was substituted for the existing material of Inconel 600HTMA (high temperature mill-annealing). Inconel 690TT has more chromium than Inconel 600HTMA in order to improve the corrosion resistance. In this study, to evaluate the friction and wear characteristics of Inconel 690TT under fretting condition, the fretting tests as well as sliding tests were carried out in air and in elevated temperature water environment, respectively. Fretting tests of the cross-cylinder type were done under various applied normal loads, and sliding tests of pin-on-disk type were also carried out to compare with the results of the fretting test. In summary, the results of the fretting tests correlated with the results of the sliding tests. The wear mechanism of Inconel 690TT in air was delamination wear and the mechanism in water was affected by micro-pitting. Also, it was found that the fretting wear coefficients in water were increased with increase in the temperature of water.     

Fretting wear behaviors of steel wires under friction-increasing grease conditions

Fretting wear tests were performed on the self-made fretting wear rig to investigate fretting wear behaviors of steel wires under friction-increasing grease conditions. The results demonstrated that the fretting regimes were dependent on displacement amplitudes and normal loads. The friction coefficient exhibited different variation trends in different fretting regimes. Friction-increasing grease changed the fretting running behavior and had a very good wear resistance for steel wires. Wear was slight in partial slip regime. Mixed regime was characterized by plastic deformation, fatigue cracks and abrasive wear. Slip regime presented main damage mechanisms of abrasive wear, fatigue wear and oxidation.     

Influence of heat and thermochemical treatment on abrasion resistance of structural and tool steels

Abrasive wear is responsible for intensive degradation of machine parts or tools. This process starts as an interaction between hard, mostly mineral, particles and the working surface. Methods of increasing the lifetime are based on application of abrasion resistant materials or creation of hard, wear-resistant surface layers or coatings on the surfaces of machine parts or tools. Carbon and low-alloy steels with different types of thermochemical treatment (case hardening, nitriding) are used in cases of low abrasion. Another method of increasing lifetime is the application of ledeburitic steels. The wear resistance of these steels depends on their chemical composition and heat treatment. The results of laboratory tests of thermochemically treated steels, heat-treated ledeburitic chromium steels and high-speed steels show the effect of the microstructure of these steels on their abrasion resistance. Abrasion resistance of carburized low-alloy steels is on the same level as in high-carbon structural and tool steels. In ledeburitic chromium steel maximum abrasion resistance was achieved by quenching from 1100 °C whilst in ledeburitic chromium–vanadium steel the optimum quenching temperature was 1150 °C. Growing abrasion resistance was caused by increasing amounts of retained austenite.     

Accelerated Fretting Wear Tests for Contacts Exposed to Atmosphere

Engineering components can be subjected to normal and/or rotational fretting wear with contacts that are intermittently exposed to the atmosphere. Exposure to the environment may lead to the alteration at the contact due to the changing role of third body particles such as hard oxides which can result in abrasion. The abrasion due to hard oxide particles differs for the closed contact and intermittently opened contact. In the former scenario, the oxides are compacted into tribo-film, while in the latter case they remain loose, leading to bigger role of abrasion. Standard fretting test setup employed to estimate the fretting wear characteristic operates with a constant load such that the contact remains closed between the counter surfaces and does not simulate the opening and closing of the contacts as observed in certain applications. The forceful interruptions to the experiments to simulate open and close condition of the contact require considerable amount of time and effort. In this paper, an accelerated test procedure is proposed and investigated to capture the effect of intermittent opening of the contact without stopping the experiments. A test rig is designed to simulate the opening and closing conditions, and tests were performed with abrasive diamond-like particles. Friction and wear results are compared with those of intermittently contact opening conditions along with operating wear mechanisms. Scanning electron microscope analysis showed that the wear mechanism observed in the case of fretting with intermittent opening of contact is similar to that of fretting with diamond-like abrasives at the contact.     

Fretting Wear Behavior of Medium Carbon Steel Modified by Low Temperature Gas Multi-component Thermo-chemical Treatment

The introduction of surface engineering is expected to be an effective strategy against fretting damage. A large number of studies show that the low gas multi-component (such as carbon, nitrogen, sulphur and oxygen, etc) thermo-chemical treatment(LTGMTT) can overcome the brittleness of nitriding process, and upgrade the surface hardness and improve the wear resistance and fatigue properties of the work-pieces significantly. However, there are few reports on the anti-fretting properties of the LTGMTT modified layer up to now, which limits the applications of fretting. So this paper discusses the fretting wear behavior of modified layer on the surface of LZ50 (0.48%C) steel prepared by low temperature gas multi-component thermo-chemical treatment (LTGMTT) technology. The fretting wear tests of the modified layer flat specimens and its substrate (LZ50 steel) against 52100 steel balls with diameter of 40 mm are carried out under normal load of 150 N and displacement amplitudes varied from 2 μm to 40 μm. Characterization of the modified layer and dynamic analyses in combination with microscopic examinations were performed through the means of scanning electron microscope(SEM), optical microscope(OM), X-ray diffraction(XRD) and surface profilometer. The experimental results showed that the modified layer with a total thickness of 60 μm was consisted of three parts, i.e., loose layer, compound layer and diffusion layer. Compared with the substrate, the range of the mixed fretting regime(MFR) of the LTGMTT modified layer diminished, and the slip regime(SR) of the modified layer shifted to the direction of smaller displacement amplitude. The coefficient of friction(COF) of the modified layer was lower than that of the substrate in the initial stage. For the modified layer, the damage in partial slip regime(PSR) was very slight. The fretting wear mechanism of the modified layer both in MFR and SR was abrasive wear and delamination. The modified layer presented better wear resistance than the substrate in PSR and MFR; however, in SR, the wear resistance of the modified layer decreased with the increase of the displacement amplitudes. The experimental results can provide some experimental bases for promoting industrial application of LTGMTT modified layer in anti-fretting wear.     

Fretting wear behavior of TiB2-based materials against bearing steel under water and oil lubrication

Lubricated fretting tests in water and paraffin oil were performed with a monolithic TiB₂, a TiB₂-based cermet with 16 vol.% Ni₃(Al, Ti) binder, a sialon–TiB₂ (60/40) composite and a ZrO₂–TiB₂ (70/30) composite against ball bearing grade steel. Based on the measured friction and wear data, the ranking of the investigated fretting couples was evaluated. Furthermore, the morphological investigations of the worn surfaces and transfer layers are carried out and the wear mechanisms for the investigated friction couples are elucidated. While fretting in water, experiments revealed that tribochemical reactions, coupled with mild abrasion, played a major role in the wear behavior of the studied material combinations. ZrO₂–TiB₂ (70/30)/steel wear couple has been found to have the highest fretting wear resistance among the different tribocouples under water lubrication. Under oil lubrication, extensive cracking of the paraffin oil at the fretting contacts, caused by tribodegradation, leads to the deposition of a carbon-rich lubricating layer, which significantly reduced friction and wear of all the investigated tribosystems.     

High temperature fretting wear behavior of WC-25Co coatings prepared by D-gun spraying on Ti-Al-Zr titanium alloy

Detonation gun (D-gun) spraying is one of the most promising spraying techniques for producing wear-resistance coatings. A thick layer (about 0.3 mm thickness) of WC-25Co with high hardness was covered on Ti-Al-Zr titanium alloy by D-gun spraying and the fretting wear behavior of WC-25Co coatings was studied experimentally on a high precision hydraulic fretting wear test rig. An experimental layout was designed to perform fretting wear tests at elevated temperatures from room temperature (25 °C) to 400 °C in ambient air. In the tests, a sphere (Si₃N₄ ceramic ball) was designed to rub against a plane (Ti-Al-Zr titanium alloy with or without WC-25Co coatings). It was found that the fretting running regimes of WC-25Co coatings were obviously different from those of Ti-Al-Zr titanium alloy. The mixed fretting regime disappeared in WC-25Co coatings, and the boundaries in the running condition fretting map (RCFM) showed hardly any change as temperature increased. The worn scars were examined using a laser confocal scanning microscope (LCSM), scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The results showed that the coefficients of friction (COF) of WC-25Co coatings at elevated temperatures were nearly constant in the partial slip regime and very low in the steady state. The fretting damage of the coatings was very slight. In the slip regime, the WC-25Co coatings exhibited a good wear resistance, and the wear volume of the coatings obviously decreased with increasing tested temperature. The fretting wear mechanisms of WC-25Co coatings were delamination, abrasive wear and oxidation wear at elevated temperature. The oxide debris layer formed at higher temperature was denser and thicker on top of WC-25Co coatings, thus providing more surface protection against fretting wear, which played an important role in the low fretting wear of the coatings.     

Effect of different surface treatments and coatings on the scuffing performance of hardened steel discs at very high sliding speeds

The paper describes the results of scuffing experiments using a high speed two disc rig, in which a range of different surface treatments and coatings was investigated. The rig used is capable of simulating the operation of heavily loaded gear contacts in terms of maximum Hertzian contact pressure (up to 1.7 GPa), temperature (100 °C oil feed), and sliding speed (up to 25 m/s). The reference scuffing performance was that of axially ground case-carburised or nitrided steel discs having a surface finish of 0.4 μm Ra. The different surface conditions were as follows. The ground discs were superfinished to less than 0.1 μm Ra using a proprietary polishing method. Two different super-hard coatings were investigated. The main conclusions to be drawn from the work are: nitrided surfaces are superior to case-carburised surfaces, but it is essential to remove the compound layer (‘white layer’) to achieve durability at high sliding speeds. The benefits of superfinishing are clearly demonstrated; this improves the scuffing performance and also gives lower friction and bulk temperatures. Hard coatings show promise, and the triple combination of nitriding, superfinishing and hard coating gave particularly impressive scuffing resistance in these tests.     

船舶柴油机气缸套拉缸机理的试验研究

根据模拟试验的相似准则设计磨损试验,研究对船舶航行安全构成严重威胁的异常磨损现象,例如拉缸和咬缸。试验结果表明硬化相的性能和含量与试验所选用的材料耐磨性相一致,且随着载荷的增加,材料发生拉毛的时间呈指数关系下降,本文根据摩擦副三滑磨面接触模型,进而提出摩擦界面温度升高导致粘着以及摩擦副表面的强烈摩擦导致硬化相的碎裂甚至折断等因素是气缸套异常磨损的根本原因,为预防气缸套拉缸或者咬缸提供了理论依据。     

Effect of contact load on fretting fatigue behaviour of steel wires

Abstract

The tension–tension fretting fatigue tests of steel wires were performed on a self-made fretting fatigue test equipment under contact loads ranging from 40 to 70 N and a strain ratio of 0·8. The results showed that when the contact load increased, the fretting regime of steel wires transformed from gross slip regime to mixed fretting regime. The fretting fatigue life in the mixed fretting regime was significantly lower than that in the gross slip regime. The main fretting wear mechanisms in the gross slip regime, where there were serious fretting damage and a lot of wear debris, were abrasive wear and fatigue wear. Microcracks were observed in the fretting scar of the mixed fretting regime, and the main fretting wear mechanisms were adhesive and fatigue wears. The fretting wear scar was the fatigue source region, and the fatigue fracture surface could be divided into three regions.     

核用TP316H钢在不同介质环境下的微动磨损性能

采用自制的液态钠环境微动磨损试验机,研究了核用TP316H传热管在不同环境 (室温大气、室温水、450 ℃大气、450 ℃液态钠)下的微动磨损性能。试验采用管/柱正交接触方式,试验结束后,采用超景深显微镜、三维光学显微镜、SEM和EDS对磨痕形貌、摩擦化学反应及磨损量进行了表征,对比分析了4种介质环境对TP316H的微动磨损的影响,发现TP316H合金管在450 ℃液态钠中的磨损率最大,在室温水中的磨损率最小。4种介质环境下的磨损机理不同,室温大气环境中的磨损主要是剥层和氧化磨损,随着温度升高,氧化磨损加重;室温水环境中的磨损以磨粒磨损为主;高温液态钠环境中的磨损为磨损和腐蚀的耦合协同作用。     

Investigating Corrosion Performance and Corrosive Wear Behavior of Sol–gel/MAO-Coated Mg Alloy

In this study, a hydroxyapatite composite coating was prepared by a sol–gel technique on the micro-arc oxidation (MAO)-coated AZ31 Mg alloy to seal the micro-pores. The composite coating achieved a larger hardness value and two times thickness more than pure MAO coating. The corrosion and wear resistance of the sol–gel/MAO coating in simulated body fluid were investigated compared to MAO coating. It was found that the composite coating presented a positive corrosion potential and a lower corrosion current density than MAO coating. The sol–gel/MAO composite coating could provide more effective barrier against corrosive ions than single MAO coating for AZ31 alloy. In the wear tests, a ball-on-disk tribometer was used to study the effect of loads on the wear properties of the coatings at 37 °C. The wear resistance of sol–gel/MAO composite coatings was apparently superior to MAO coating. The wear mechanisms of abrasion and adhesion in composite coatings are investigated. Finally, two physical models for the corrosion and sliding wear mechanisms of sol–gel/MAO composite coatings are proposed, respectively.     

690合金材料的微动磨损特性研究

利用微动磨损试验机,在载荷50N以及位移幅值为60μm、100μm、150μm的工况下,研究了690合金材料在常温下的微动磨损行为及其动力学特性,采用激光共焦扫描显微镜(LCSM)和扫描电子显微镜(SEM)观察磨痕微观形貌。结果表明,载荷和位移幅值对微动特征有很大的影响,微动运行完全处于滑移状态。在滑移区,滑移磨损严重、磨痕面积大。690合金材料的磨损机制主要表现为磨粒磨损与剥层的共同作用。     

四种核电用包壳材料的微动磨损性能研究

在切向微动磨损试验机上对4种核电用包壳材料(Zr合金、Zr/Cr涂层、FeCrAl和ODS-FeCrAl)进行切向微动磨损试验,考察不同包壳材料的微动磨损特性。研究结果表明:不同包壳材料的摩擦因数、耗散能曲线和形变有显著差异;4种包壳材料在切向微动过程中均处于部分滑移区。通过分析磨痕微观形貌和磨痕轮廓,发现ODS-FeCrAl相比FeCrAl具有更好的耐磨性;在常温环境下,Zr/Cr表现出更加优异的抗磨损性能。     

Resistance to fretting corrosion of VVER-440 fuel elements

The methods and results of research to substantiate the wear resistance of VVER-440 fuel assemblies with smaller nominal fuel-element diameters are shown. The results of tests of the fuel elements of VER-1000 fuel assemblies with zirconium spacer grids for fretting corrosion were used for substantiation. The test bench, the test model of the fuel element, the conditions of similarity, and the transferability of the results of experimental tests to standard fuel elements are described. It is shown that spent fuel elements with smaller diameters have significant wear resistance in contact with spacer grids.     

中性腐蚀环境下钢丝的微动疲劳行为

在自制的微动疲劳试验机上开展中性腐蚀环境下单根钢丝的微动疲劳实验,考察在相同接触载荷下,不同振幅对钢丝的微动疲劳行为的影响,并用扫描电子显微镜观察疲劳钢丝的磨痕和断口形貌,研究钢丝微动疲劳断裂机制.结果表明:在较大的振幅下,钢丝的微动区均处于滑移状态,而在较小振幅下,钢丝的微动区从滑移状态逐渐转变为黏着状态;磨损机制主要为磨粒磨损、疲劳磨损、腐蚀磨损和塑性变形;钢丝疲劳寿命随着微动振幅的增大而减小;钢丝的疲劳断口可分为3个区域,即疲劳源区、裂纹扩展区及瞬间断裂区.     

The mechanism of material removal in fretting

The fretting of annealed 4130 steel in air at room temperature was studied to determine the mechanisms of material removal and debris formation in fretting. On the basis of information developed by examination of the fretted surfaces, cross sections of the fretted surfaces and the fretting debris it was found that metallic debris was generated by the break up of the fretted surfaces into metal particles with a flake-like morphology. The abrasion or adhesive metal transfer mechanisms proposed to explain metal removal in fretting do not account for the morphology and surface features of the metallic debris, while the delamination theory of wear proposed by Suh accounts for both. It is concluded that a delamination mechanism is the most suitable model for the metal removal process in fretting under the conditions studied.     

飞机钢表面修复新工艺研究

飞机发动机架和作动筒等许多零部件的材料为30CrMnSiA,即飞机钢。在飞机的使用过程中,其表面会出现划伤、磨损、腐蚀、压坑等现象,需要修复。为此,研究了综合利用摩擦电喷镀技术和n—Al2O3／Ni—Wu纳米复合刷镀技术对30CrMnSiA飞机钢进行表面修复的新工艺。介绍了修复工艺和流程,并对修复层性能进行测试,结果表明,表面修复层完全满足修复要求,该工艺提高了飞机作动筒等部件修复面的耐磨性能和镀层结合强度,而且耐蚀性能良好。同时也解决了氢脆问题。