A novel surface texture for magnetic fluid lubrication

Magnetic fluid has many advantages when serving as lubricant. With an appropriate magnetic field this lubricant can prevent leakage and increase the load capacity of lubricant film. It can also be fixed at the friction zone by applying an external magnetic field. A novel design of magnetic fluid lubrication with magnetic surface texture was proposed in this paper. A micro-scale dimple pattern was firstly fabricated on the surface of tribo-pair and then a permanent magnet material was electrodeposited into these dimples, so that there are both geometric surface texture and periodic distribution of magnetic field on the surface (magnetic surface texture). In this paper, CoNiMnP permanent magnetic film (about 25 μm thickness) was electrodeposited into micro-dimples (500 μm in diameter) on the surface of 316 stainless steel. The impact of magnetic surface texture on lubrication was investigated using a pin-on-disk test rig. The test results showed that magnetic surface texture was conducive to form effective lubrication at low sliding velocity when lubricated by magnetic fluids.     

Control of lubricant transport by a CrN diffusion barrier layer during high-temperature sliding of a CrN-Ag composite coating

CrN-Ag composite coatings, 2 and 5 μm thick and containing 22 at.% Ag solid lubricant, were grown on Si(001) and 440C stainless steel substrates by reactive co-sputtering at T_s = 500 °C, and were covered with 200 nm thick pure CrN diffusion barrier cap layers. Annealing experiments at T_a = 625 °C, followed by quantitative scanning electron microscopy, energy dispersive x-ray spectroscopy, and Auger depth profile analyses indicate considerable Ag transport to the top surface for a barrier layer deposited at a substrate floating potential of −30 V, but negligible Ag diffusion when deposited with a substrate bias potential of −150 V. This is attributed to ion-irradiation induced densification which makes the cap layer an effective diffusion barrier. High temperature tribological sliding tests of this coating system against alumina balls at T_t = 550 °C indicate an initial friction coefficient μ = 0.43 ± 0.04 which decreases monotonically to 0.23 ± 0.03. This is attributed to the development of wear mediated openings in the barrier layer which allow Ag lubricant to diffuse to the sliding top surface. In contrast, pure CrN exhibits a constant μ = 0.41 ± 0.02 while CrN-Ag composite coatings without cap layer show a low transient μ = 0.16 ± 0.03, attributed to Ag transport to the surface, that however increases to μ = 0.39 ± 0.04 after ~ 6000 cycles as the Ag reservoir in the coating is depleted. That is, the dense CrN cap layer reduces the Ag lubricant flow rate and therefore prolongs the time when the coating provides effective lubrication. This results in a cumulative wear rate over 10,000 cycles of 3.1 × 10⁻⁶ mm³/Nm, which is 3.3 × lower than without diffusion barrier layer.     

Evolution of through-thickness texture gradients in various steel sheets

In order to investigate the evolution of through-thickness texture gradients in various ferritic steel sheets, samples of a deep drawing steel, a ferritic stainless steel and a low carbon steel were deformed by cold rolling with and without application of a lubricant during the deformation process. The hot band texture of all samples examined showed notable through-thickness texture gradients. The evolution of the cold rolling texture in the ferritic stainless steel and the low carbon steel in a solution-treated state was nearly independent of the lubrication during rolling. However, in the deep drawing steel and the as-received low carbon steel, the lubrication played an important role in the formation of the through-thickness texture gradients. Whereas the samples rolled with lubrication showed uniform textures throughout the sheet thickness, in the samples rolled without lubrication pronounced shear textures were observed on the sheet surface. The evolution of these through-thickness texture gradients is discussed in terms of inhomogeneous strain states computed by an FEM model. This article is based on a presentation made in the symposium “ ’99 International Symposium on Textures of Materials”, held at Sunchun National University, Sunchun, April 21–22, 1999 under the auspices of The Korean Institute of Metals and Materials and The Research and Development Center for Automobile’s Parts and Materials.     

Tribological coatings for lubrication over multiple thermal cycles

Nanocomposite materials demonstrating multiple temperature-adaptive mechanisms including diffusion, oxidation and/or catalysis mechanisms to yield low friction coefficients of < 0.2 from room temperature to 700 °C were combined with diffusion barrier layers in coatings with different architectures (e.g., layer thicknesses, number of layers, etc.) to examine adaptation of contact surface chemistry and morphology over multiple thermal cycles. Multilayered coatings consisting of ceramic-metal nanocomposite adaptive lubricant layers separated by diffusion barriers allowed adaptation to occur only upon exposure of the lubricant layer by wear, which resulted in prolonged wear life at static and cycled temperatures. It was also observed that a relationship between the number of adaptive lubricant layers and the number of thermal cycles existed, where one thermal cycle consumed two adaptive lubricant layers. The thickness of the adaptive coating layers was also important because diffusion- and oxidation-based adaptation in these particular coatings required a minimum volume of solid lubricant material. The surface roughness of the adaptive coating materials played a significant role in their performance within multilayered coatings, where rough coatings (> 100 nm R_a) failed after relatively few sliding cycles. The utility and application of adaptive coatings materials providing lubrication over multiple thermal cycles is discussed.     

FEM analysis of contact mechanism in press-forming of lubricant pre-coated steel sheet

This paper describes the contact situation between the die and the lubricant pre-coated steel sheet in the press-forming by using FEM simulation. The FEM simulation is carried out by supposing that the lubricant pre-coated steel sheet consists of the lubrication layer and thick galvanized layer. Both the lubrication layer and galvanized layer are assumed as rigid–plastic material. The variations in the contact situation between the die and the lubricant pre-coated steel sheet are investigated by changing the friction coefficient between the die and lubrication layer, the thickness and hardness of lubrication layer, the velocity ratio of the relative sliding velocity to pressing velocity, etc. The simulated results show that the contact area ratio is influenced largely by the average contact pressure, the velocity ratio, the thickness of lubrication layer, and friction coefficient.     

Mechanisms of self-lubrication in patterned TiN coatings containing solid lubricant microreservoirs

The tribological mechanisms of friction and lubrication have been investigated in TiN coatings patterned to contain microscopic reservoirs for solid lubricant entrapment. Photo-lithography was used to fabricate three sets of samples on silicon wafers, varying the reservoir size (4 and 9 μm) and spacing (11 and 25 μm), which resulted in samples with a nominal reservoir area of either 2 or 10%. Pin-on-disk tests were run using lubricants of graphite and indium and counterfaces of alumina and steel (440C). In most cases, the samples with the 9 μm holes spaced 25 μm apart gave the lowest friction coefficients and longest wear life. Analysis of the wear tracks by SEM/EDS methods showed carbon to be present in the holes of the graphite/steel counterface samples, but TiO₂ was found in the holes of the graphite/alumina counterface samples. For the indium/steel counterface samples indium was detected within the microreservoirs, but iron was also found, transferred from the ball. These experiments highlight a variety of tribological mechanisms that can operate in microreservoir-patterned coatings.     

Corrosion behavior of ferritic-martensitic steel T91 in supercritical water

The corrosion behavior of a ferritic-martensitic steel T91 exposed to supercritical water at 500 °C and at two different dissolved oxygen concentrations, 25 ppb and 2 ppm, for exposure times up to 505 h was characterized. In this corrosion experiment, all the exposed samples formed a stable oxide layer, but of varying thicknesses. The dissolved oxygen concentration played a significant role in the oxidation behavior of this alloy. For lower oxygen concentration exposure, phase distribution analysis showed a duplex oxide structure, consisting of layered magnetite and spinel phases. Higher oxygen concentration resulted in an additional hematite enriched layer in the outer oxide region. Furthermore, samples exposed to high oxygen content supercritical water exhibited a more porous oxide scale with weaker adhesion to the substrate. Samples of T91 steel whose surface was modified by oxygen ion implantation were tested in the lower oxygen supercritical water environment and were noted to exhibit a lower oxide layer thickness compared to the untreated alloy. The formation of nanometer-sized oxides precipitates at the surface due to oxygen ion implantation appears to influence the nucleation and growth texture of the oxide scale, which may play a role in reducing the oxide layer thickness.     

A new lubricant carrier for metal forming

A lubricant carrier for metal forming processes is developed. Surfaces with pores of micrometer size for entrapping lubricant are generated by electrochemical deposition of an alloy, consisting of two immiscible metals, of which one metal subsequently is etched away leaving 5 μm layers with a sponge-like structure. The pores will act as lubricant reservoirs during severe forming processes. The deposited microporous layer is evaluated by friction tests in the form of ring compression tests and double cup extrusion tests. Furthermore the anti-seizure properties are investigated by single cup extrusion at high reduction and excessive stroke comparing with conventionally lubrication using phosphate coating and soap.     

Investigation of wear behavior of titanium oxide films, produced by anodic oxidation, on commercially pure titanium in vacuum conditions

The wear performances of titanium oxide films, produced by anodic oxidation, in vacuum conditions were investigated. Anodic oxidation treatments were carried out at − 3 and 40 °C temperatures using H₂SO₄(1.5 M)-H₃PO₄(0.3 M) solution and voltage of 200 V. Wear tests were performed at ambient air, pressures of 10^− 3 mbar and 10^− 6 mbar. Anodizing process produced a porous oxide layer on the surface. Although the pore size decreased with increasing process temperature, surface roughness decreased. Hardness results showed that anodic oxidation at lower temperatures produced oxide film with higher hardness. Wear rate of CP-Ti significantly decreased with anodic oxidation treatment in all wear conditions since oxide film acted like a solid lubricant. The best wear resistance was obtained from the hard-anodized samples both in ambient air and vacuum conditions.     

Computer controlled detonation spraying of WC/Co coatings containing MoS2 solid lubricant

Composite WC/Co + MoS₂ coatings were deposited onto steel substrates by Computer Controlled Detonation Spraying using three spraying modes: very cold, cold and normal. Maximal content of MoS₂ in a sprayed powder was 10 wt.%. Characterization of coatings was made with chemical and phase analyses, microhardness measurement, morphology and microstructure investigation. X-ray diffraction study shows that residual MoS₂ exists only in coatings obtained at very cold and cold spraying modes. At normal spraying mode complete decomposition of the solid lubricant occurs during spraying. From the engineering point of view, the coating applied at the cold mode using a powder containing 10 wt.% MoS₂ is the most promising. Such a coating has microhardness of 650 HV_0.2 and a porosity of 10%.     

Improvement of the cavitation erosion resistance of UNS S31803 stainless steel by duplex treatment

A duplex surface treatment consisting of High Temperature Gas Nitriding (HTGN) followed by Low Temperature Plasma Nitriding (LTPN) was carried out in an UNS S31803 duplex stainless steel. The HTGN treatment was intended to produce a relatively thick and hard fully austenitic layer giving mechanical support to the thinner and much harder expanded austenite layer. HTGN was performed at 1200 °C for 3 h, in a 0.1 MPa N₂ atmosphere while LTPN, was carried out in a 75% N₂ + 25% H₂ atmosphere, at 400 °C for 12 h, under a 250 Pa pressure, and 450 V. An expanded austenite γ_N layer, 2.3 μm thick, 1500 HV0.025 hard, was formed on top of a 100 μm thick, 330 HV 0.1 hard, fully austenitic layer, containing 0.9 wt% N. For comparison purposes LTPN was carried out with UNS S30403 stainless steel specimens obtaining a 4.0 μm thick, 1500 HV 0.025 hard, expanded austenite layer formed on top of a fully austenitic matrix having 190 HV 0.1. The nitrided specimens were tested in a 20 kHz vibratory cavitation-erosion testing equipment. Comparison between the duplex treated UNS S31803 steel and the low temperature plasma nitrided UNS S30403 steel, resulted in incubation times almost 9 times greater. The maximum cavitation wear rate of the LTPN UNS S30403 was 5.5 g/m²h, 180 times greater than the one measured for the duplex treated UNS S31803 steel. The greater cavitation wear resistance of the duplex treated UNS S31803 steel, compared to the LTPN treated UNS S30403 steel was explained by the greater mechanical support the fully austenitic, 330 HV 0.1 hard, 100 μm layer gives to the expanded austenite layer formed on top of the specimen after LTPN. A strong crystallographic textured surface, inherited from the fully austenitic layer formed during HTGN, with the expanded austenite layer showing {101} crystallographic planes//surface contributed also to improve the cavitation resistance o f the duplex treated steel.     

Cold metal transfer joining aluminum alloys-to-galvanized mild steel

The cold metal transfer (CMT) welding–brazing process provides a potential method to join dissimilar metals. In this research, various 1 mm thick aluminum alloys were joined to 1 mm thick mild steel (Q235) by the CMT welding technology. It was found that it is feasible to join aluminum alloys-to-galvanized mild steel using the cold metal transfer method. The optimum process variables for weldability of aluminum-galvanized steel of dimension 200 mm × 50 mm × 1 mm could be obtained with a wire of Al4043, 100% argon shielding gas, a welding voltage range of 12–14 V, a deviation distance range of 2–3.5 mm, a welding speed range of 6–8 mm/s and a wire-feed speed range of 4–6 m/min. The joint strength depended primarily on the thickness of the intermetallics and softening of the Al heat-affected-zone. By properly controlling the heat input (100–200 J/mm), the degradation of the HAZ property and thickness of the intermetallic reaction layer can be minimized, and consequently produced a hybrid aluminum-to-steel joint which had comparable strength to that of CMT weld-brazed Al-to-Al joint.     

Selective single pulse femtosecond laser removal of alumina (Al2O3) from a bilayered Al2O3/TiAlN/steel coating

We studied surface modification of a double layer protective coating on steel induced by single fs laser pulse irradiation in ambient air. The outer alumina (Al₂O₃) layer, which protects against aggressive environments, was 1.7 μm thick and the titanium aluminum nitride (TiAlN) layer in contact with the steel surface had a thickness of 1.9 μm. The pulses (λ = 775 nm, τ = 200 fs) were generated by a Ti:sapphire laser source. The pulse energy was varied from 0.32 μJ to 50 μJ, corresponding to an incident laser fluence of 0.11 J cm^− 2 to 16.47 J cm^− 2. The surface damage threshold was found to be 0.20 J cm^− 2 and the alumina layer removal was initiated at 0.56 J cm^− 2. This selective ablation of alumina was possible in a wide range of fluences, up to the maximum applied, without ablating the TiAlN layer beneath.     

Corrosion properties of plasma nitrided AISI 410 martensitic stainless steel in 3.5% NaCl and 1% HCl aqueous solutions

Samples of an AISI 410 martensitic stainless steel were plasma nitrided at a temperature of 420 °C, 460 °C or 500 °C for 20 h. The composition, microstructure and hardness of the nitrided samples were characterised using a variety of analytical techniques. In particular, the corrosion properties of the untreated and plasma nitrided samples were evaluated using anodic polarisation tests in 3.5% NaCl solution and immersion tests in 1% HCl acidic water solution. The results showed that plasma nitriding produced a relatively thick nitrided case consisting of a compound layer and a nitrogen diffusion layer on the 410 stainless steel surface. Plasma nitriding not only increased the surface hardness but also improved the corrosion resistance of the martensitic stainless steel. In the immersion test, nitrided samples showed lower weight loss and lower corrosion rate than untreated one. In the electrochemical corrosion tests, the nitrided samples showed higher corrosion potentials, higher pitting potentials and greatly reduced current densities. The improved corrosion resistance was believed to be related to the iron nitride compound layer formed on the martensitic stainless steel surface during plasma nitriding, which protected the underlying metal from corrosive attack under the testing conditions.     

Austenite modification of AISI 316L SS by pulsed nitrogen ion beams generated in dense plasma focus discharges

We present the results of a surface modification of AISI 316L stainless steel by surface irradiation with high energy, pulsed nitrogen ion beams generated with 0.8 kJ dense plasma focus. The surface characterization was done using GAXRD, Auger electrons spectroscopy, TEM and optical microscopy. After the irradiation, we found a modification of a 1 μm thick surface layer, on which a gradual lattice expansion of the austenite with the number of irradiation pulses, i.e. with the total nitrogen ion fluence, was observed.In addition, ~ 40 nm close to the surface layer, a disordered lattice structure had been observed through TEM analysis. Those results can be explained in terms of the extreme thermal effect induced on the surface through the fast high energy release during the pulsed ion interaction with the steel surface, followed by an also rapid cooling down process which limits the nitrogen diffusion to the bulk.     

Influence of time on the microstructure of AISI 321 austenitic stainless steel in salt bath nitriding

Influence of nitriding time on the microstructure and microhardness of AISI 321 austenite stainless steel was investigated, using a complex salt bath heat-treatment at low temperature, 430 °C. Experimental results revealed that after salt bath nitriding, a modified layer was formed on the surface of substrate with the thickness ranging from 2 μm to 30 μm with changing treating time. The nitrided layer depth thickened extensively with increasing nitriding time. The growth of the nitrided layer takes place mainly by nitrogen diffusion according to the expected parabolic rate law. Scanning electron microscopy and X-ray diffraction showed that in 321 stainless steel subjected to complex salt bathing nitrided at such temperature for less than 8 hours, the main phase of the nitrided layer was expanded austenite (S phase) by large. When the treatment time is prolonged up to 8 hours and more, S phase is formed and subsequently transforms partially into CrN, and then the secondary CrN phase precipitated. With treating time prolonged, more CrN precipitates formed along the grain boundaries in the outer part. In the inside part between the some CrN and the substrate, there is still a broad single S phase layer. All treatments can effectively improve the surface hardness.     

基于Deform复合润滑剂镦粗试验和摩擦磨损研究

以聚四氟乙烯（PTFE）乳液为主要润滑成分,通过辅助添加剂制备了新型复合高分子水基润滑剂,并进行均匀设计试验获得最优的成分配比。基于Deform有限元模拟镦粗试验探究润滑剂的摩擦行为,通过SEM和EDS表征方法研究润滑机理,发现润滑性能的提高是由于PTFE在基体表面形成了一层转移膜,h-BN吸附在基底表面保护了PTFE膜而有效降低了基体表面阻力,提高了润滑剂的润滑性能,此润滑剂可用于冷挤压成形的润滑。     

Analysis of wear mechanisms in low-friction AlMgB14-TiB2 coatings

Recent developments in coating science and technology offer new opportunities to enhance the energy-efficiency and performance of industrial machinery such as hydraulic fluid pumps and motors. The lubricated friction and wear characteristics of two wear-resistant coatings, diamond-like carbon and a nanocomposite material based on AlMgB₁₄-50 vol.% TiB₂, were compared in pin-on-disk tribotests using Mobil DTE-24™ oil as the lubricant. In each case, the pins were fixed 9.53 mm diameter spheres of AISI 52100 steel, the load was 10 N, and the speed 0.5 m/s in all tests. Average steady-state friction coefficient values of 0.10 and 0.08 were measured for the DLC and nanocomposite, respectively. The coatings and their 52100 steel counterfaces were analyzed after the tests by X-ray photoelectron and Auger spectroscopy for evidence of material transfer or tribo-chemical reactions. The low-friction behavior of the boride nanocomposite coating is due to the formation of lubricative boric acid, B(OH)₃. In contrast, the low-friction behavior of the DLC coating is related to the relatively low dielectric constant of the oil-based lubricant, leading to desorption of surface hydrogen from the coating.     

Effects of cooling air temperature on cryogenic machining of Ti-6Al-4V alloy

This paper presents experimental investigations on influence of different coolant strategies such as dry, wet, minimum quantity lubrication (MQL) and MQL with cooling air on performance in milling of the Ti-6Al-4V alloy with uncoated cemented carbide inserts. Cutting force, tool wear, surface roughness and chip morphology are experimentally studied to compare the effects of different cooling air temperatures. The results showed that minimum quantity lubrication (MQL) with cooling air significantly reduces cutting force, tool wear and surface roughness. Unfortunately, MQL (without cooling air) condition cannot produce evident effect on cutting performance, and flaking wear on the flank surface of the insert has been found under this condition. Four different cooling air temperatures are used to investigate the effects of cooling air temperature on the machinability characteristics of Ti-6Al-4V alloy. Based on the experimental results, MQL with cooling air of −15 °C provides more favourable effects compared to other cooling air temperatures (0 °C, −30 °C,−45 °C). Short chips are produced under MQL with cooling air conditions due to the high velocity of cooling air enhances the chip brittleness for easy chip breaking, and the effective penetration of lubricant to the chip-tool interface results in lower friction. However, due to the dramatic increase in chip hardness at lower temperature, MQL with cooling air environments cannot promote chip curl to some extent.     

铝箔冷轧工艺润滑状态与润滑效果研究

铝箔冷轧工艺润滑中,采用综合考虑变形区膜厚比、摩擦因数、轧后表面质量与轧制油添加剂影响性相结合的方法,综合评价润滑状态。结果表明:入口处的油膜厚度明显小于综合表面粗糙度,获得的轧后铝箔表面质量较好,轧后铝箔表面粗糙度都比仅加入基础油时的有所降低,添加剂已经发挥作用,润滑状态以边界润滑为主。