The use of nickel graphite composite coatings for the mitigation of gross slip fretting wear on Ti6Al4V interfaces

In metallic contacts, surface oxides, adhesion, and material transfer play a primary role in the initial stages of fretting wear degradation. Given this behavior, the focus of this study was to mitigate fretting wear within Ti6Al4V contacts at room temperature and 450 °C with the use of thermally sprayed nickel graphite composite coatings with 5–20% graphite. The results show that the embedded graphite particles reduced the friction of the nickel thermal sprayed coatings during both low and high temperature fretting wear experiments. Friction and wear mechanisms are discussed with correlations of contact chemistry, morphology, and mechanical performance. Wear on the mated Ti6Al4V surfaces was reduced by the formation of uniform transfer films that were identified as graphitic based at room temperature and NiO based at 450 °C.     

Fretting corrosion of tin-plated contacts: Evaluation of surface characteristics

The surface characteristics of the contact zone of tin-plated copper alloy contacts subjected to fretting motion for 8000, 16,800 and 48,000 cycles under unlubricated conditions are presented. The nature of the contact zone, at the verge of wearing out of the tin coating as well as upon the coating is completely worn out, is assessed using scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) analysis and X-ray dot mapping, and the influence of these changes on the contact resistance is correlated. The study reveals that under unlubricated conditions, fretting caused significant damage at the contact zone. Occurrence of adhesive wear failure is observed at early stages whereas at latter stages, delamination wear is the predominant mode of failure. As the fretting cycle increases, the concentration of copper increases whereas the concentration of tin decreases; oxygen concentration though not appreciable at the early stages, starts to build with increase in fretting cycles, attributing to the increase in contact resistance due to the formation of oxides of copper and tin at the contact zone.     

A tentative explanation for the tribochemical effects in fretting wear

In many fretting investigations, tribochemical reactions have been reported to critically determine the wear and friction behavior, however, different and contradictory assessments of the importance of mechanical and thermal effects on these reactions have been suggested. Since fretting is characterized by relatively slow sliding speeds, high temperatures are not generated over the entire nominal contact area. However, evidence for phase transformations, which are typical of high temperatures, have been observed many times in fretting experiments. In other words, there exists a discrepancy between the macro- and micro-scale observations. In our previous experimental and theoretical work, the tribochemical transformations of steel and ceramics were extensively investigated and the presence of very high flash contact temperatures under gross slip fretting was confirmed. In this paper we present a tentative explanation of the mechanism for the observed tribochemical changes under selected fretting conditions, which can also explain the discrepancy in the results from macro- and micro-scale studies. The proposed wear mechanism considers the tribochemical transformations at the asperity spot-to-spot contacts due to high flash temperatures, while the heat generation and dissipation at apparent contact area remain significantly lower. The observed overall wear transition occurs due to gradual accumulation of the transformed material, which in “closed” fretting contacts remains in great part within the contact.     

The effect of temperature on gross slip fretting wear of cold-sprayed nickel coatings on Ti6Al4V interfaces

Fretting wear is an accumulation of damage that occurs at component interfaces that are subjected to high contact stresses coupled with low-amplitude oscillation. In metallic contacts, surface oxides, adhesion, and material transfer play a primary role in the initial stages of fretting wear degradation. Given these behaviors, the focus of this study was to determine the effect of temperature on inter-metallic fretting wear between Ti6Al4V (titanium, 6% aluminum, 4% vanadium) and cold-sprayed, commercially pure nickel coatings. The results presented herein show that increased temperature decreases friction through the formation of a uniform NiO layer, and by a reduction of Ni₂O₃ in contacts. In addition, it was found that a localized minimum friction coefficient is achieved at approximately 300 °C, above which friction increases slightly due to annealing of the cold-sprayed coatings.     

Fretting corrosion of lubricated tin plated copper alloy contacts: Effect of temperature

The fretting corrosion behaviour of lubricated tin plated copper alloy contacts at ambient and elevated temperatures is addressed in this paper. At 27 °C, lubrication is very effective and the contact resistance remains stable for several thousand fretting cycles whereas at elevated temperatures (155 °C) the performance of lubricated contact is not appreciable. Surface profile and surface roughness confirm that the lubricated contacts have a smoother profile and experience a lesser damage at the contact zone at ambient as well as at elevated temperatures. The mechanism of fretting corrosion of tin plated contacts appears to be similar with and without lubrication at all the temperatures studied. The difference in performance of the lubricated contacts at ambient and elevated temperatures is due to the faster wear rate of tin coating at elevated temperatures. Oxidation of the contact zone of the lubricated contacts is prevented at all temperatures studied. The study concludes that lubrication is effective in improving the life of the tin plated copper alloy contacts under fretting conditions at ambient temperatures whereas at elevated temperatures lubrication provides only a marginal improvement in performance. The decrease in performance of lubricated tin plated contacts at elevated temperatures is due to the higher wear rate of tin coating and not due to evaporation of the lubricant.     

Application of influence function method on the fretting wear of tube-to-plate contact

The structural integrity of steam generators in nuclear power plants is much dependent on the fretting wear characteristics of Inconel 690 U-tubes. The influence function method for the tube-to-plate contact model is demonstrated in this study to investigate the fretting wear problems on the secondary side of the steam generator, which are caused by flow induced vibrations between the U-tubes and supports. Two-dimensional numerical contact model is developed and formulated in terms of the Cauchy singular integral equation. The distributions of normal pressures, shear stresses and displacement fields are derived between two contact bodies which have similar elastic properties. Based on the algorithms for normal pressures and relative slip, a numerical approach is developed to simulate the fretting wear of tube-to-plate contacts. The work rate model is adopted in this study to find the wear amounts between two materials. The results are compared with the solutions by finite element analyses to validate the application of the present method to fretting wear problems.     

Evaluation of fretting wear based on the frictional work and cyclic saturation concepts

It is time consuming or even impossible to simulate the whole process of fretting wear, since it always involves millions of cyclic loadings. This paper focuses on the modeling and evaluation method of fretting wear for the typical bridge type fretting test with a flat pad. The frictional work on the contact interface is chosen as the parameter to evaluate the fretting wear. To verify the fretting wear model, the predicted wear profile is compared with that obtained by the experimental results. Fretting wear always includes plastic deformations due to the edge stress singularity. The effect of cumulative plastic deformation is also taken into account in the wear model. The role of the coefficient of friction at the contact interface on the fretting wear has also been discussed.     

Elevated Temperature Fretting Evaluations Using a Flat-on-Flat Configuration

Traditional fretting tests rely on the high Hertzian stress contacts between a cylinder or sphere and a flat surface to generate oxide particles and an eventual wear scar. However, this configuration does not always match the stresses and wear mechanism associated with parallel surfaces where fretting may only initiate in limited regions of contacting asperities. To simulate these conditions at 175°F, fretting wear tests were used to evaluate the performance of High Velocity Oxy-Fuel (HVOF) and Plasma-sprayed Cu Ni In coatings for the reduction of gross-slip fretting (relative displacements of 100 μm) experienced between mating beta-c titanium bosses and 4340 steel lugs. Scanning electron microscopy was then used to compare the frequency and severity of fretting wear on the titanium blocks. Results of the analysis indicated the viability of the lubricious coatings for eliminating the instances of fretting. Furthermore, the tests indicated the usefulness of the flat-on-flat testing configuration for illuminating the potentially random occurrences of fretting damage between parallel surfaces.     

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。     

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.     

The influence of current load on fretting of electrical contacts

The fretting corrosion behavior of tin coated brass contacts is studied at various current loads (1, 2 and 3 A). The typical characteristics of the change in contact resistance with fretting cycles are explained. The fretted surface is examined using scanning electron microscope, laser scanning electron microscope and energy dispersive analysis of X-rays to assess the surface morphology, extent of fretting damage, extent of oxidation, surface profile and elemental distribution across the contact zone. The degradation of contacts at high and low values of current is explained with reference to the thermal and electrical phenomena occurring at the contact interface. The results showed that irrespective of the current loads under study, the contact resistance is maintained at 1.0±0.02 Ω where the oxide debris formation and the electrical breakdown of oxide particles competed with each other maintaining the equilibrium. The number of cycles to failure of the contacts is delayed at lower current. The fretting corrosion degradation of tin coated contacts occurs much faster at higher currents as it generates more accumulation of oxide wear debris at the contact zone. The observed surface morphology and the tin profile of the fretted surface are in agreement with the experimental results.     

电连接器微动磨损的超声识别与性能退化模型

针对振动环境下电连接器易产生微动磨损而接触性能降低这一问题，采用超声检测方法监测微动磨损过程中电连接器接触件间磨屑的特征值，研究了不同振动条件下接触件的磨损程度及接触性能的退化规律。结果表明，振动频率、振动加速度和振动次数对接触面磨屑的堆积和接触电阻的波动都有正向累积效应；电连接器轴向振动时，磨屑累积效应最为明显。接触电阻和磨屑特征值总量在高振频及加速度下呈现极高的相关性。以磨屑特征值构建的麻雀搜索算法优化BP神经网络性能退化模型的平均绝对误差小于5%。     

Fretting damage of TiN coated zircaloy-4 tube

Fretting has been reported and investigated for over 50 years. However, it is still one of the modern plagues for several industrial machineries. Especially, fretting of fuel rod cladding material, zircaloy-4 tube, in pressurized water reactor (PWR) must be reduced and avoided. Thin hard coatings are employed to improve the tribological properties such as friction and wear of conventional engineering materials. Among these coatings, physical vapor deposition (PVD) TiN coating is probably one of the most frequently and successfully used PVD coatings for the mitigation of fretting wear. Therefore, in this study a fretting wear experiment was performed using TiN coated zircaloy-4 tube as the fuel rod cladding material and uncoated zircaloy-4 tube as one of the grids. The fretting tester was designed and manufactured for this experiment. The number of cycles, slip amplitude and normal load were selected as main factors of fretting. The type of contact was cylinder-to-cylinder contact. The worn surface was observed by optical microscope, 3-D surface measuring instrument and scanning electron microscope (SEM). The results of this research showed that the wear volume of TiN coated zircaloy-4 tube decreased about 1.2–3 times more than uncoated tube and wear mechanisms were brittle fracture, fatigue fracture, adhesion, abrasion and oxidation.     

The tribological performance of DLC coatings under oil-lubricated fretting conditions

Whether or not the process of fretting occurs is to a large extent dependent on the coefficient of friction, because the coefficient of friction directly affects the amount of shear stress. As a result, the key factor when it comes to reducing the amount of fretting damage is to reduce the coefficient of friction. Various surface coatings, and especially hard, diamond-like carbon (DLC) coatings, are known to be able to produce surfaces with a low level of friction. Despite some such attempts in the past, which did not result in major improvements, the developments and improvements in DLC coatings in recent years suggest the need for a re-evaluation of these coatings for fretting applications. Another way to reduce the amount of friction in mechanical components is to apply lubricants, and recent studies on the lubrication of DLC coatings suggest that this combination could be very successful in preventing failures under boundary-lubrication conditions. Therefore, in this work we present the results of friction and wear measurements from three types of fretting contacts: steel/steel, steel/DLC and DLC/DLC. Boundary oil-lubrication conditions were investigated and a wide range of displacement amplitudes, i.e., from 25 to 500 μm, were selected to assess the fretting and sliding behaviours. The results show a significant difference between the fretting and sliding regimes. In the fretting regime, the DLC-containing contacts, and especially the self-mated DLC/DLC contacts, performed much better than the steel/steel contacts, and significantly reduced both the wear (a 3–10 times reduction with steel/DLC and DLC/DLC) and the friction (a more-than-two-times reduction with DLC/DLC). In the sliding regime, the lubrication effects governed the tribological performance, making the results for all three material combinations very similar.     

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.     

The effect of slip amplitude on fretting

The effect of slip amplitude on the mechanism of fretting was investigated. Measurements of wear volume, frictional coefficient and of electrical contact resistance were carried out to clarify the wear mechanism. X-ray microdiffraction was used to observe the difference of wear behaviour, and scanning electron microscopic observations were made.At small slip amplitudes wear damage was small compared with that at larger amplitudes the transition being in the region of 70 μ.At smaller slip amplitudes fretting oxidation, a mild type of wear occurs. At larger slip amplitudes, adhesion and abrasion together with oxidation cause fretting wear. At much larger slip amplitudes, wear similar to reciprocating sliding wear occurs.     

Elevated Temperature Evaluation of Fretting and Metal Transfer between Coated Titanium Components

During fretting, small amplitude displacements and high normal surface loads combined with abrasive oxide particles cause surface damage that acts as initiation sites for fatigue cracks. Since these conditions are prevalent within the titanium dovetail joints of jet engines a wear mode analysis was performed on extended service jet engine disks and compressor blades. The results of the wear mode analysis indicated that titanium from the uncoated disk was transferred to the softer copper-nickel-indium coated dovetail surface of the blades. This transfer created titanium on titanium contact and eventually fretting wear. In order to simulate these conditions, a moderate displacement (125 μm), low cycle phase followed by a small displacement (25 μm), high cycle fretting phase utilizing a cylinder on flat configuration was developed. The analysis and test procedure developed during this study will ultimately aid in the selection and evaluation of a new coating capable of preventing fretting.     

Fretting of WC/a-C:H and Cr2N Coatings Under Grease-Lubricated and Unlubricated Conditions

The fretting phenomenon was investigated experimentally in contacts between coated and uncoated steel rod and ball specimens generating a circular Hertzian contact. A fretting wear test rig equipped with a video camera was used to observe the effects of fretting on coated steel surfaces in both grease-lubricated and unlubricated environments. Tungsten carbide reinforced amorphous hydrocarbon (WC/a-C:H) and chromium nitride (Cr₂N) coatings were tested and compared. Fretting wear volumes and surface profiles are presented for both grease-lubricated and unlubricated conditions. Videos of a coated ball fretting against a transparent sapphire flat were recorded and screen captures are presented. The role of normal load, lubrication, frequency, and amplitude of motion on the fretting wear of coatings is discussed. The lubricant released from the grease was observed to flow through channels in the stick zone of the fretting contacts. Both coatings were found to reduce fretting wear. WC/a-C:H was more effective at reducing wear under unlubricated conditions. WC/a-C:H decreased fretting wear more than Cr₂N when delamination was avoided in grease-lubricated contacts.     

Nano-wear mechanism of amorphous carbon thin films

Tribological performance of diamond-like carbon (DLC) overcoat on a magnetic hard disk was evaluated using a contact start stop wear tester. The analysis of worn surfaces suggests that wear of the carbon overcoat took place by a delamination type of wear mechanism at a nanoscale, resulting in the formation of wear particles of about 1.5 nm thick and 30–90 nm wide. This revised version was published online in July 2006 with corrections to the Cover Date.     

Influence of high-power diode laser heat treatment on wear resistance of a mold steel

The effect of hardness on wear loss and wear behavior during fretting was studied. A high-power diode laser was used to achieve the surface hardening of a mold steel (AISI P20-improved) at temperatures of 1000 and 1200 °C. A hardness increment was detected in laser heat-treated specimens, which may be attributed to phase transformation from ferrite to martensite, influencing wear loss and fretting wear behavior. In the fretting test results, smaller wear scars and less wear loss were observed for laser heat-treated specimens in comparison to those of base metal. Moreover, relatively more stable friction coefficient profiles were obtained for the laser heat-treated specimens due to uniform contact characteristics at two contacting surfaces. The effectiveness of the proposed technique was verified by the morphology of the wear scars of the treated specimens, which had a smooth appearance and minor abrasion grooves.