Sliding wear behavior of hardfacing alloys in a pressurized water environment

In this study, sliding wear behavior of newly developed Fe-base Co-free hardfacing alloy (Fe–Cr–C–Si) was investigated and compared to that of Stellite 6 and Fe-base NOREM 02 in the temperatures ranging from 300 to 575 K under a contact stress of 103 MPa (15 ksi) in pressurized water. The weight loss of Fe–Cr–C–Si was equivalent to that of Stellite 6 over all temperatures range in 100-cycle wear test. The weight loss of Fe–Cr–C–Si 1000-cycle wear test increased almost linearly with increasing temperature up to 575 K. The weight loss of NOREM 02 was nearly equivalent to that of Stellite 6 below 475 K, however, galling occurred above 475 K in 100-cycle wear test. It was also found that the lubrication effect of pressurized water on the sliding wear behavior of the alloys was negligible under the present test conditions.     

Progression of fretting fatigue damage in Ti–6Al–4V

An investigation was conducted to explore the nature of fretting fatigue damage in the stages prior to crack formation. In the unique experimental apparatus employed in this study, where total slip never occurs, several locations on each test specimen exist where cracks can develop due to local contact conditions. Under the test conditions used, not all of the sites had cracks upon test completion. This study evaluated the condition of non-cracked sites on several fretted specimens in an effort to identify differences between these and sites where small cracks were observed.A single test condition of 620 MPa average applied static clamping stress and 250 MPa applied axial fatigue stress for R=0.5 was selected, which corresponds to a fretting fatigue life of 10⁷ cycles based on prior work. For specimens tested to 10⁶ cycles, or 10% of life, several destructive and non-destructive characterization methods were chosen: scanning electron microscopy (SEM), residual stress measurement and transmission electron microscopy (TEM). Each site at which crack nucleation could be expected was inspected in the SEM and was then characterized using surface X-ray diffraction to quantify the residual stresses field near that location. Then TEM foils were cut from one area on a specimen with tiny cracks and dislocation densities were observed. A novel technique was used which permitted TEM samples to be obtained from regions in close proximity on the original specimen.Comparisons were made between as-received (AR) and stress-relief annealed (SRA) specimens, on which the stress-relief was applied prior to fretting fatigue testing. SEM inspection was useful for qualitative analysis of wear debris and identification of cracks as small as 20 μm, but was unable to provide quantitative data on the level of fretting fatigue damage beyond crack size. Although differences were noted in the residual stresses for the SRA versus the AR specimens, no residual stress peaks were noted in the edge of contact regions where cracks would eventually develop. TEM observations in the vicinity of the crack nucleation region showed that the dislocation structure decayed rapidly into the specimen thickness. The cause of the dislocations was attributed to plastic deformation caused by the clamping stresses.     

A proposal for a fretting wear criterion for coated systems with complete contact based on accumulated friction energy density

This paper presents the results of basic studies that investigate fretting behaviour in different steel-pairing coatings. The experiments enabled the identification of the variation in the temporal and tribological behaviour of each pairing as a result of the chosen coating. The experiments were performed on a test bench for the determination of friction coefficients with a standardised torsion test. Using flat annular contact surfaces that were pressed together under a pressure of 25 or 50 MPa, the tested samples were alternatingly loaded with twisting angle amplitudes of 0.23° and 0.5° (or 46 and 100 μm slip, respectively). This technique enabled the recording of optical damage (fretting, etc.) and strength-affecting mechanisms (damage of the substrate material). The results were evaluated using an appropriate damage criterion, thereby identifying the accumulated dissipated friction energy as a fail-related physical quantity.     

Effect of high temperature on the characterization of fretting wear regimes at Ti6Al4V interfaces

Fretting wear is a destructive phenomenon that can accelerate crack initiation in vibrating components, leading to premature catastrophic failures. Among the most important characteristics of fretting is the transition from mixed fretting wear to gross slip because of its detrimental effects on fatigue life. In this paper, a study was conducted that characterized the critical normal loads and displacements corresponding to this transition in Ti6Al4V interfaces at room temperature and 450 °C (842 °F). Experimental testing shows that there is a relationship between critical load and displacement that is temperature dependant. Tests conducted at or near the critical values of load and displacement exhibited extreme galling and evidence of cold welding. In the gross slip tests, the measured coefficient of friction also varied with applied load, which correlates with the Hertzian contact model.     

Influence of temperature on the fretting response between AISI 301 stainless steel and AISI 52100 steel

Fretting of AISI 301 stainless steel sheet in contact with AISI 52100 steel from 20 °C to 550 °C in air and argon has been studied. Transitions in the mechanical properties of 301SS and oxidative behavior of this pair have been identified as a function of temperature. Strength and ductility of 301SS is reduced from 20 °C to 250 °C, increasing susceptibility to fretting damage. Steady state friction decreases as temperature increases, reducing cyclic stresses. Wear resistance increases in this temperature range, increasing fatigue damage due to the increase in fatigue life associated with increased wear. This study aims to identify the causes of the transitions in behavior and determine the net outcome of the competing effects with regard to fatigue damage.     

The effect of grain flow orientation on the fretting fatigue characteristics of forged AMS6415 steel

The effect of grain flow orientation on the fretting fatigue response of AMS6415 steel specimens in contact with fretting pads of the same material obtained from a shaft-propeller flange forging was investigated statistically. Coupon specimens were machined from one forging. The specimens were obtained at two different orientations (radial and tangential) with respect to the forging axis. S-N (maximum stress versus the number of stress cycles to complete fracture) fretting fatigue tests were performed with a randomized paired comparison test matrix. The nominal maximum fatigue stress was +400 MPa, with an R ratio of +0.05, and the nominal normal contact pressure was ?40 MPa.Resultant test data were studied statistically. Student t tests, analysis of variance tests and Weibull analyses were performed. Test results suggested strongly, but not conclusively, with 90% confidence, that fretting fatigue lives of tangential specimens had a larger mean and a greater variance than those of the radial specimens. Metallographic and fractographic observations also were performed and are presented in this paper.     

Rolling contact fatigue life of AISI 52100 steel balls with mineral and synthetic polyester lubricants with PTFE nanoparticle powder as an additive

This work evaluates the rolling contact fatigue life of AISI 52100 steel bearing balls with mineral and synthetic oil, with and without additive, using a four-ball tester. IP 300/87 was applied with a total load of 600 kg (corresponding to a maximum Hertz stress of 8.709 MPa). SN-350 (a neutral mineral solvent) and TMP-05 (an environment friendly synthetic polyester) of the same viscosity were additivated with PTFE nanoparticle powder in different percentages. The total test time for each specimen was recorded and processed on Weibull probability plots. L₁₀ and L₅₀ are given for pure base oils and oils additivated with 1, 3 and 5 wt.% of PTFE powder. The results point to an increase in the fatigue life when additive-containing oils were used. In order to provide some understanding of the protection mechanism of the additive, the contact angle and the influence of the surface wettability on the lubricant unctuosity were analysed. Scanning electron microscopy and LINK techniques were used to study pitting, crack propagation, the chemical influence of additive and fluorine content.     

Fretting fatigue behaviour of Ti–6Al–4V alloy under plane bending stress and contact stress

Fretting fatigue tests for Ti–6Al–4 V alloy were conducted by use of the plate fatigue specimen with bolt-tightened shoe on both sides of the plate. It was clarified that the repeated bending stress at the contact area where fretting fatigue failure starts linearly decreased as stress over the contact area increased. Fretting fatigue crack starts from the pit where stress concentrate. The pit initiates when fretting debris were removed from the surface striation formed due to the contact slip movement. The fretting fatigue crack initiation mode was transgranular, while the fretting fatigue crack propagation mode was striation.     

Fretting Fatigue Behavior of Cavitation Shotless Peened Ti–6Al–4V

Fretting fatigue behavior of cavitation shotless peened (CSP) titanium alloy, Ti–6Al–4V was investigated. Constant amplitude fretting fatigue tests were conducted at several maximum stress levels, σ_max, ranging from 400 to 555 MPa with a stress ratio of 0.1. Test results showed that the fretting fatigue life was enhanced by CSP treatment as compared to the unpeened specimen, but the enhancement was not as large as that from the shot-peening treatment. Residual stress measurements by X-ray diffraction method before and after fretting test showed that residual compressive stress was relaxed during fretting fatigue. Before fretting, CSP specimen had higher compressive residual stress on the surface than the shot-peened specimen. However, greater residual stress relaxation occurred in CSP specimen such that the relaxed compressive residual stress profile near the contact surface of CSP specimen was lower than that of shot-peened specimen. This lower compressive residual stress from fretting fatigue was the reason for shorter fretting fatigue life of CSP specimen as compared to shot-peened specimen at the applied stress level.     

Studies on wear behavior of nano-intermetallic reinforced Al-base amorphous/nanocrystalline matrix in situ composite

Resistance to wear is an important factor in design and selection of structural components in relative motion against a mating surface. The present work deals with studies on fretting wear behavior of in situ nano-Al₃Ti reinforced Al–Ti–Si amorphous/nanocrystalline matrix composite, processed by high pressure (8 GPa) sintering at room temperature, 350, 400 or 450 °C. The wear experiments were carried out in gross slip fretting regime to investigate the performance of this composite against Al₂O₃ at ambient temperature (22–25 °C) and humidity (50–55%). The highest resistance to fretting wear has been observed in the composites sintered at 400 °C. The fretting wear involves oxidation of Al₃Ti particles in the composite. A continuous, smooth and protective tribolayer is formed on the worn surface of the composite sintered at 400 °C, while fragmentation and spallation leads to a rougher surface and greater wear in the composite sintered at 450 °C.     

Effect of silicon nitride coating thickness on fatigue of silicon microbeams

In this paper, two silicon nitride layers with thickness, 0.2 and 0.4 μm, are coated onto single crystal silicon (SCS) in order to achieve Si₃N₄/Si cantilever microbeams. The effect of LPCVD silicon nitride surface coatings on fatigue properties of SCS cantilever microbeams is investigated. Fatigue testing is conducted at both 40 Hz and 100 Hz. Typical S–N (strain amplitude–fatigue cycle) curves of the beams are achieved and correlated fatigue failure modes are investigated. It is found that thinner Si₃N₄ coating of 0.2 μm results in better fatigue lives of Si₃N₄/Si beams than thicker Si₃N₄ coating of 0.4 μm. Both thinner and thicker coated beams have major fatigue crack planes along {1 1 1} planes; however, thicker coated beams possess specific failure mode of delamination, which is not found in thinner coated beams. Delamination reduces the reinforcing effect of thicker Si₃N₄ coating and leads to its shorter fatigue life. For thicker coated beams, fatigue life at 100 Hz is longer than that at 40 Hz. The mechanism for delamination and the effect of cyclic frequency is investigated, and factors for better fatigue life are proposed.     

Rolling contact fatigue using solid thin film lubrication

This paper addresses rolling contact fatigue (RCF) testing in ultra high vacuum (UHV) under high speed conditions. A ball–rod RCF test platform has been adapted for testing in UHV conditions that allows rapid accumulation of stress cycles, over 10 million cycles within 5 h of testing at 130 Hz rotation. The UHV environment and solid lubrication enables good vibration detection for the onset of spall. In this paper, approximately 0.2 μm of silver is applied to the balls and provides sufficient lubrication for up to 25 h of testing, or 50 million stress cycles in high vacuum at 130 Hz. Seventy-nine RCF tests using thin-film silver lubrication have been completed covering two ball sizes, and two rod and ball materials. 9.53 mm diameter Rex 20 steel and silicon-nitride (Si₃N₄) rods were tested against 7.94 mm diameter Rex 20 and 12.7 mm diameter M50 steel balls. It was found that ball size and material hardness did not affect the stress cycle life over a Hertzian contact stress range of 2.1–4.2 GPa and Rockwell C hardness range of 62–77. Rather, the key limiter to test length is lubrication depletion based on 79 tests and an average silver thickness of 0.2 μm. One of the two failure modes were observed for all tests: (i) early life spall of the silver coating, and (ii) depletion of silver lubrication followed by spall failure of both the ball and rod surfaces. A third-body storage model along with the Control Volume Fraction Coverage (CVFC) assumption and analysis was used to predict lubrication availability between asperities on the third body. There is good agreement between calculated and measured post-test lubrication thickness using the third-body storage model.     

Fretting fatigue crack initiation lifetime predictor tool: Using damage mechanics approach

Fretting fatigue is a combination of two complex mechanical phenomena. Fretting appears between components that are subjected to small relative oscillatory motions. Once these connected components undergo cyclic fatigue load, fretting fatigue occurs. In general, fretting fatigue failure process can be divided into two main portions, namely crack initiation and crack propagation. Fretting fatigue crack initiation characteristics are very difficult to detect because damages such as micro-cracks are always hidden between two contact surfaces.In this paper Continuum Damage Mechanics (CDM) approach in conjunction with Finite Element Analyses (FEA) is used to find a predictor tool for fretting fatigue crack initiation lifetime. For this purpose an uncoupled damage evolution law is developed to model fretting fatigue crack initiation lifetime at various fretting condition such as contact geometry, axial stress, normal load and tangential load. The predicted results are validated with published experimental data from literature.     

Tribological characterisation of siliconcarbonitride ceramics derived from preceramic polymers

Amorphous SiCN ceramics were prepared in a laboratory scale as disk shaped specimens with 10 mm diameter and 0.3 mm thickness. The friction and wear behaviour was characterised in gross slip fretting tests under unlubricated conditions at room temperature against steel (100Cr6) and ceramic (Al₂O₃). Tests with a ball-on-disk contact were performed in laboratory air with different content of water vapour. The results show clearly that the relative humidity has a significant effect on friction and wear behaviour. All tests in dry air lead to higher friction and higher wear rate than in normal air. Improved friction and wear behaviour was observed with increasing pyrolysis temperature up to 1100 °C of the SiCN specimens. This is attributed to increasingly better mechanical properties and higher stiffness of the amorphous network due to the evaporation of gaseous organic species and the formation of free graphite like carbon.     

TC4合金微动疲劳损伤研究

研究了TC4合金在柱面-平面接触务件下的微动疲劳行为，分析了其微动疲劳损伤机制。结果表明：在试验务件下，微动区边缘的损伤特征以粘着磨损为主，而微动区中部则以磨粒磨损和接触疲劳为主。疲劳裂纹易于在微动区．特别是在蚀坑处萌生和扩展。促使微动疲劳裂纹萌生的因素：一是法向应力和切向摩擦力引起的材料表层塑性变形，二是微动磨损破坏了材料的表面完整性，造成了缺口应力集中效应。     

Friction and wear characterization of sintered low alloyed chromium steels for structural components

Sintered and sintered/gas nitrided cylinders made of low alloyed chromium steel Astaloy CrL + 0.45 C at 7.25 g/cm³ density, have been tested for scuffing resistance and wear rate in a crossed cylinders test setup lubricated with a commercial SAE 10W40 engine oil at 90 °C. The results show large potential of 1 h gas nitriding of the sintered chromium steel cylinders. The nitrided cylinders experienced safe wear at 1000 MPa and scuffing at 1100 MPa at 2.5 m/s. At 0.5 and 0.1 m/s at least up to 800 MPa the wear was mild, as sintered chromium cylinders showed scuffing at pressure lower than 320 MPa and limited wear at 0.5 and 0.1 m/s.     

Low wear metal sliding electrical contacts at high current density

Operation of a low wear (2 × 10^?5 mm³/(N-m)), low contact resistance copper sliding electrical contact was demonstrated. The wear rate of a lightly loaded copper–beryllium metal fiber sliding on a polished copper counterface was insensitive to (DC) current density values as great as 440 A/cm² (in a brush positive or anodic configuration). Low wear and relatively low friction (μ ～ 0.2 to 0.3) was achieved by operating the contact immersed in a liquid medium consisting of a hydrofluoroether with helium cover gas, inhibitingoxidationand providing cooling of the contact. Similar experiments performed in liquid mediums of ultrapure water and dilute (3%) hydrogen peroxide show an order of magnitude increase in wear rate and provide further insight on the role of electrochemically enhanced oxidation and the degraded contact resistance and tribological behavior of non-noble sliding electrical contacts in general. In contrast to high current density slidingin hydrofluoroether, an order of magnitude greater wear rate was observed for similar sliding conditionsin hydrogen peroxide or water without the aid of externally supplied electric potential. A conceptual model is proposed correlatingthe rate of brush wear to fatigue strength and electrochemically enhanced oxidation as a result of high current density transport through the contact. A mathematical expression was derived to calculate the approximate wear volume of a single fiber laterally contacting a slip-ring, based on direct measurement of the wear scar geometry.     

Influence of plasma nitriding on fretting wear behaviour of Ti–6Al–4V

Plasma nitriding was performed on Ti–6Al–4V samples at 520 °C in two environments (pure nitrogen and a mixture of nitrogen and hydrogen in the ratio of 3:1) for two different time periods (4 and 18 h). Fretting wear tests were conducted on unnitrided and nitrided samples for 50,000 cycles using alumina ball counterbody. Plasma nitriding reduced the tangential force coefficient of Ti–6Al–4V. The samples nitrided for 4 h exhibited higher hardness and lower tangential force coefficient compared to the specimens nitrided for 18 h. The samples nitrided in nitrogen–hydrogen mixture environment exhibited higher hardness and lower tangential force coefficient compared to the specimens nitrided in pure nitrogen. The samples plasma nitrided in nitrogen–hydrogen mixture for 4 h exhibited the highest hardness and the lowest tangential force coefficient. The wear volume and specific wear rate of the plasma nitrided samples were lower than those of the unnitrided samples. A consistent trend was not observed regarding which nitriding condition would result in lower wear volume and specific wear rate at different loads.     

New results for the fretting-induced stress concentration on Hertzian and flat rounded contacts

Recent work on fretting fatigue has emphasized the role of stress concentration on fretting damage, while previous work had concentrated on empirical parameters to assess influence of fretting on fatigue life. In particular, analogies with fatigue in the presence of a crack or a notch have been noticed, suggesting that the stress field induced by frictional contact per se may explain the reduction of fatigue life due to fretting.In the paper, new analytical and numerical solutions are produced for the stress concentration induced in typical fretting contacts involving the Hertzian geometry or the flat punch with rounded corners in view of application to the dovetail joints. Normal and tangential load (in the Cattaneo–Mindlin sense) is considered with “moderate” or “large” bulk stresses.