Unlubricated Conditions Research Articles

Effect of flow softening on ring test calibration curves

Abstract The application of the ring test to assess die–workpiece interface friction during forging of a flow softening material was investigated. Calibration curves were generated by conducting finite element method (FEM) simulations of ring forging with different friction factors. The method was validated by conducting isothermal, hot compression tests on rings of an α 2 titanium aluminide alloy for both lubricated and unlubricated interface conditions. The FEM results for the flow softening material were compared to those generated assuming no flow softening. In all cases, the differences between the curves predicted for the flow softening and non-softening behaviors were quite small. Furthermore, for given values of the interface friction factor, the calibration curves were essentially independent of the strain rate sensitivity ( m ) and flow softening.

Tribological behaviour of line hardening of steel U13A with Nd: YAG laser

To diminish wear in tribological systems is frequently to harden locally the load carrying areas, which are subjected to wear. A Nd:YAG laser was used for the improvement of hardness and wear resistance of steel U13A. The friction and wear characteristics of steel U13A in sliding contact against steel 65MN4 under unlubricated conditions were evaluated for conventional treatments and after laser irradiation. In addition the transformations occurring during laser treatments and the influence of laser parameters for quenching on tribological characteristics are presented. The experimental work indicates that wear resistance of steel U13A (AISI Wl 12) is several times higher then that for conventional heat treatments.

Preparation and fretting wear behavior of ion-beam-enhanced-deposition CrN films

Hard chromium nitride films were deposited by ion-beam-enhanced-deposition (IBED) technique. The effects of ion beam energy and ion beam current on microstructure and mechanical properties of IBED CrN films were studied, and the optimized parameters were found to prepare the films to improve the fretting wear resistance of Ti–6Al–4V. The fretting wear behavior of the IBED CrN film was evaluated using a ball-on-flat system under unlubricated conditions, and the results were compared with those of PVD CrN films. Results showed that both the bombardment energy and ion beam current had significant effects on the microstructure, preferred orientation, hardness and adhesion strength of the IBED CrN thin films. The Fretting wear resistance of IBED CrN films was better than those of PVD CrN films due to a dense and fine microstructure, good adhesion strength, deep hardening depth, etc. Fretting parameters (normal load and amplitude) played an important role in the transition of the fretting regime (from partial slip to gross slip regime). Oxidation and abrasive wear were the main fretting wear mechanisms for IBED CrN film.

Tribological behaviour of line hardening of steel U13A with Nd:YAG laser

To diminish wear in tribological systems is frequently to harden locally the load carrying areas, which are subjected to wear. A Nd:YAG laser was used for the improvement of hardness and wear resistance of steel U13A. The friction and wear characteristics of steel U13A in sliding contact against steel 65MN4 under unlubricated conditions were evaluated for conventional treatments and after laser irradiation. In addition, the transformations occurring during a laser treatments and the influence of laser parameters for quenching on tribological characteristics are presented. The experimental work indicates that wear resistance of steel U13A (AISI W112) is higher at several times when compared to conventional heat treatments.

Tribological behaviour of alumina sliding against Ti6Al4V in unlubricated contact

Tribological behaviour of alumina balls (99.5%) sliding against a Ti6Al4V disc over a range of loads (5–80 N) and speeds (0.0625–1 m s −1) has been investigated using a pin-on-disc tribometer under unlubricated conditions. The maximum wear coefficient was observed to be several orders of magnitude higher than the reported value for alumina against alumina or alumina against steel counterfaces. When the load or speed increased, the wear rate of the alumina ball increased initially and then decreased, showing typical transition features. On the other hand, the friction coefficients for the Ti6Al4V/alumina tribosystem were found to increase inversely with the applied loads or the sliding speeds. The wear mechanisms and the wear transition were investigated based on examinations of worn surfaces as well as debris using SEM, XRD and XPS, and it was revealed that a tribochemical mechanism accounted for the observed high wear rate of alumina sliding against the titanium alloy.

Mechanisms of friction and wear of metals against ceramics in vacuum

For the combinations of five kinds of metallic pin and five kinds of ceramic disk (Al 2O 3, ZrO 2, SiC, Si 3N 4, TiC), a friction and wear test was carried out under unlubricated condition in a vacuum, at a sliding speed of 0.136–0.226 m/s, and under a constant load of 5.0 N. A topographical analysis was also performed on the micro-asperities of the wear surfaces to estimate the behavior of the wear particles, the degree of surface damage and the wear rate. As a result, the following observations were found, (1) a WC pin had the smallest wear rate for TiC, ZrO 2 and Al 2O 3 disks. For SiC and Si 3N 4 disks, the wear rates of each mating Ag and SUS303 pins became small. (2) When the wear particles of pin are transferred to the disks, the greater the amount of the transferred particles, the greater the wear rate of pin becomes. (3) The greater the mean depth of micro-grooves on both pin and disk surfaces, the greater the wear rate becomes. The wear rates of a pin and a disk also depend on the depth of mean line and the tip radius of asperities on a counterface. (4) The mean coefficient of friction increases with an increase in the equivalent depth of mean line.

Strengthening of Sheath-Rolled Aluminum Based MMC by the ARB Process

The Accumulative-Roll Bonding (ARB) process was applied to an aluminum based MMC fabricated by a sheath rolling method, in order to improve the mechanical properties. The 8 cycles of ARB were performed at ambient temperature under unlubricated conditions. The ARB process was also performed on the unreinforced material, for comparison with the composite sample. The tensile strength of the composite increases with the number of cycles up to 5 cycles of ARB, above which it decreases slightly. The tensile strength of the unreinforced material increases up to 2 cycles of ARB, above which it does not change. The strengthening in the composite by the ARB process is caused by the improvement in homogeneity of distribution of reinforcement particles with the number of cycles as well as work hardening of the matrix material, while the strengthening in the unreinforced material is only caused by work hardening. The drop in strength for both materials is caused by recovery or recrystallization. On the other hand, the elongation tends to greatly decrease after I cycle, but from 2 cycles it increases with the number of cycles for both materials. The unreinforced material shows an inhomogeneous microstructure along the thickness direction at lower cycles of ARB; having a finer grained structure near surface and bonded interfaces than the other area. However, the inhomogenity of microstructure vanishes gradually as the number of cycles increases. As a result, the microstructure after 8 cycles consists almost entirely of ultra-fine grains under 1 μm in diameter along the thickness direction. The ultra-fine grain initiate to develop at lower ARB cycle in composite than in unreinforced material. This is probably due to enhanced strain induced by locally accumulated deformation around reinforcement particles.

The wear of unlubricated metallic spur gears

Published work on metallic gear wear focuses, almost exclusively, on gears operating under normal, lubricated conditions. There are instances however, when it is desirable to run gears in a dry, unlubricated state. In these cases it is usual to employ non-metallic, polymer gears. This study was made to evaluate the performance of metal involute spur gears running under dry, unlubricated conditions. The stock (off the shelf) gears examined in this paper comprised mainly of carbon steel and spheroidal graphite iron but also included cast iron and aluminium bronze. A number of gears specially coated, inter alia, with molybdenum disulphide, electroless nickel, copper and lead/tin and various other specialised coatings by electro, chemical and physical vapour deposition were also tested for the purposes of comparison.

The Ring Test for P/M Materials

The application of the ring test to assess die-workpiece interface friction during forging of powder metallurgical materials has been investigated. As for fully dense materials, the change in internal diameter as a function of height reduction was used to quantify friction. To this end, calibration curves were generated by conducting finite element method (FEM) simulations of ring forging with different friction factors. The method was validated by conducting isothermal, hot compression tests on rings of an alpha-two titanium aluminide alloy with starting densities of 75, 85, or 100 percent of full theoretical density. Comparison of the test data to the calibration curves revealed somewhat higher friction factors for the porous materials (as compared to the fully dense material) for both lubricated and unlubricated interface conditions. Furthermore, the accuracy of the FEM simulations of the ring test was confirmed by good agreement between measured and predicted density distributions and load-stroke curves for tests on the porous titanium aluminide material.

Revealing the hidden world of fretting wear processes of surface coatings by X-ray imaging

Detailed wear and friction phenomena are usually hidden between the two contacted surfaces. Direct or in-situ observation of wear process is supposed to obtain some time-dependent changes and shed more light on the real wear and friction mechanisms. In this paper, X-ray imaging, one of the advanced tools for modern non-contact and non-destructive inspection, was applied to observe in-situ the fretting wear processes of a laser-alloyed coatings and plasma-sprayed hydroxyapatite (HA) coatings against stainless steel under unlubricated conditions. X-ray images clearly showed the contact nature, the wear process, the formation and compaction of wear debris, spallation of the coatings, large-scale removal of materials, crack formation and propagation, etc. According to the analysis of X-ray images, the fretting wear process of laser-treated specimens was a combination of adhesive, abrasive, oxidative and delamination wear; and the fretting wear mechanisms for the plasma-sprayed HA coating were mainly delamination and abrasive wear. It can be concluded that X-ray imaging is a promising method for the in-situ observation of wear processes and deterioration mechanisms of coatings under fretting conditions.

Study on fretting wear behavior of laser treated coatings by X-ray imaging

The probability of encountering fretting in machines and engineering structures is extremely high, and it is well known that aluminum alloys have poor fretting wear resistance. In this investigation, effect of laser surface alloying of Ni, Cr on the fretting resistance of 6061 aluminum alloys under unlubricated conditions was studied. The fretting wear performance of the laser treated alloys was characterized by evaluating the wear volume loss, the coefficient of friction and fretting mechanisms. It was found that the fretting wear resistance of 6061 aluminum alloy was improved by a factor of 3 after laser alloying while the coefficient of friction was decreased. In situ observation of fretting wear was performed by means of X-ray imaging (real time radiography) in this investigation. The obtained X-ray images and SEM post-observation of the fretting scar indicated that the fretting wear process of laser treated specimens was a combination of adhesive, abrasive, oxidation and delamination wear. The mechanisms during the running-in period are adhesive and abrasive wear. During prolonged running, delamination and oxidation were the main wear mechanisms for both laser treated and untreated specimen.

Role of chromium carbide in tungsten carbide based hard composites

The surface wear characteristics of the nitrocarburized JIS SKD61 tool steel were investigated. The ferritic substrate (FS) JIS SKD61 tool steel as received was pretreated in quenching and tempering processes to obtain the tempered martensitic microstructure. Specimens were then nitrocarburized for 1, 3, and 5 h. The chemistry, microstructure, and hardness of the nitrocarburized coatings were analyzed. Wear behavior was examined on a block-on-roller wear tester under unlubricated conditions. The results have shown that white layers were predominantly composed of the ϵ-phase iron nitrides dispersed with γ′-phase, and a large amount of nitrogen and carbon trapped in the ferrite phase was observed in the diffusion layer for FS. The nitrocarburizing treatment increased the surface roughness, hardness and the friction coefficients of the specimens. Wear contact pressures and nitrocarburized holding time also affected the friction coefficients of the nitrocarburized specimens. In this study, both the FS and the tempered martensitic substrate (TMS) specimens treated with subsequent nitrocarburizing process have shown significant improvement of the wear resistance. Wear behaviors of the specimens nitrocarburized for 1, 3, and 5 h did not vary significantly.

Fretting wear behaviors of thermal sprayed hydroxyapatite (HA) coating under unlubricated conditions

Fretting damage (fretting wear and fretting fatigue) is one of the modern plagues for the orthopedic implants because the oscillatory micromovement often often occurs at the interface between the implant and bone. In this investigation, the fretting wear behaviors of plasma sprayed hydroxyapatite (HA) bioceramic coatings on titanium alloy substrate were investigated under unlubricated conditions as a function of number of oscillatory cycles, normal load and amplitude. Coefficient of friction, wear volume and wear mechanisms were studied. For the different test conditions in this study, the fretting regime is gross-slip. Results showed that the fretting wear mechanisms of plasma sprayed bioceramic coating were mainly delamination and abrasive wear, whereas the wear mechanisms of the substrate Ti alloys were oxidation, delamination as well as abrassive wear. Under unlubricated condition, the fretting wear resistance of bioceramic coatings was not as good as that of Ti6Al4V substrate due to the porous, lamellar and loose structure of HA coating.

Laser alloying of aluminum alloy AA 6061 with Ni and Cr. Part II. The effect of laser alloying on the fretting wear resistance

The probability of encountering fretting in machines and engineering structures is extremely high, and it is well known that aluminum alloys have poor fretting wear resistance. In this investigation, the effect of laser surface alloying of Ni and Cr on the fretting resistance of 6061 aluminum alloys under unlubricated conditions was studied. The fretting wear performance of the laser-treated alloys was characterized by evaluating the wear volume loss, the coefficient of friction and fretting mechanisms. It was found that the fretting wear resistance of 6061 aluminum alloy was improved by a factor of three after laser alloying, while the coefficient of friction was decreased. The effects of fretting conditions such as load and amplitude on the fretting wear behavior were evaluated. The fretting wear mechanisms of both untreated and laser-treated specimens were adhesive, abrasive, oxidation and delamination wear. At the beginning of fretting cycles, adhesive and abrasive wear were the main wear mechanisms. During long-time running, delamination and oxidation were the main wear mechanisms for both laser-treated and untreated specimens.

The Entry Mechanism of Solid Particle to the Interface between Sliding Surfaces.

The entry mechanism of solid particle to the interface between sliding surfaces in unlubricated condition was experimentally investigated by the in-situ observations of entry processes with optical microscope. The number of solid particles which entered to the interfaces between sliding surfaces immediatelly decreased at a certain entrance angle of pin specimen. This entrance angie was named critical entrance angle. The critical entrance angles were changed with the surface roughness of sliding surfaces, the materials of specimens and the load. It was found that the roughness, the materials of specimens and the load affected the friction coefficient between particle and sliding surface and the deformation of specimens respectively. And we discussed the results with the simple model which neglected the deformation of specimens. Then we compared expeimental values of critical entrance angle and theoritical values based on the model.

Evolution of recrystallization texture from aluminum sheet cold rolled under unlubricated condition

The texture of cold-rolled aluminum sheet has been known to vary through thickness due to inhomogeneous deformation, which can be caused by a characteristic deformation zone geometry and friction between materials and rolls during rolling. The copper texture is obtained in the center layer, which is plane strain compressed, while the shear texture is in the surface layer, which is approximated by major {001}t〈110t〉 and minor {111}t〈112t〉 and {111}t〈110t〉 components. The recrystallization texture of the surface layer is approximated by {225}t〈10 5 2t〉. The evolution of the recrystallization texture has been explained by the maximum energy release theory, in which the absolute maximum normal stress direction in the deformed state becomes parallel to the minimum elastic modulus direction of the recrystallized grains.

Rolling contact fatigue failure mechanisms in plasma and HVOF sprayed WC-Co coatings

The rolling contact fatigue (RCF) behaviour of thermally sprayed WC-Co coatings with nominal compositions of WC-12%Co, WC-10%Co-4%Cr and WC-17%Co was studied with a two-roll configuration roll-against-roll testing apparatus under 420–600 MPa Hertzian contact stresses in unlubricated pure rolling conditions. The coatings were prepared by atmospheric plasma spray (APS) and two high-velocity oxyfuel (HVOF) spray processes. In the APS sprayed WC-12%Co coating the RCF damage was dominated by an increased surface roughness due to spallation of flakes and a formation of a network of cracks within the coating layer. HVOF sprayed WC-12%Co and WC-10%Co-4%Cr coatings were damaged either by the formation of vertical, linear cracks or pitting of the contact surface. The formation of pits in the HVOF sprayed coatings was significantly less than that found in the APS sprayed coatings. The HVOF sprayed WC-17%Co coating showed the best RCF behaviour among the studied coatings with unchanged surface roughness, no formation of cracks and only a few pits were found on the contact surface. The good resistance of this coating against formation of failure in the RCF testing is caused by its higher ductility and fracture toughness due to a higher metallic binder content in comparison with the other coatings. Other characteristics such as a low amount of brittle Co-W-C carbides and a dense microstructure are also believed to be beneficial for a RCF resistant coating.

Tribological characterization of composite powder metallurgy valve train components for heavy duty diesel engines under starved or unlubricated conditions

A valve train system which could work in the absence of lubricants could reduce engine oil consumption. Routes to produce such a valve system have been explored, and composites, mostly powder metallurgy (PM) containing solid lubricants, have been tried. The expectation from utilizing these materials, keeping, at least, unchanged the life of engine components, is a reduction in oil consumption and oil fog in the cylinder head and a big environmental benefit. Various sintered materials for three different engine components ‐ valve cap, valve seat and valve guide ‐ have been developed and tested by disk‐on‐disk and fretting tests, in cold and hot conditions, respectively. Prototype components were produced, and the successful ones were then evaluated by an engine endurance test. The sintered valve cap and guide gave very interesting results in terms of friction coefficient and reduction of environmental pollution. Discusses the tribological characterization and the evaluation of these sintered materials.

Wear prediction for metals

In spite of the large number of wear models found in the literature, no model can predict metal wear a priori based only on materials property data and contact information. The complexity of wear and the large number of parameters affecting the outcome are the primary reasons for this situation. This paper summarizes the current understanding of wear modelling for metals. Several recent approaches such as wear mapping and wear transition diagrams have suggested some future possible directions for improvement. Some success has been achieved in describing severe wear of steels under unlubricated conditions using thermomechanical approaches. However, modelling of mild wear remains problematic, especially under lubricated conditions. In mild wear, asperity contact events dominate the wear processes. A single asperity collision simulation apparatus has been used to study asperity-asperity contact phenomena. Shear strain and strain accumulation were found to be the dominant underlying causes for wear. It is proposed that future research in wear prediction for metals incorporate the following aspects: wear mapping, temperature, shear strain response, boundary lubricating film strength, and surface roughness.

Microstructural dependence of friction and wear behaviours in low purity alumina ceramics

The friction and wear behaviours of four low purity commercial alumina ceramics coupled with the same high purity alumina were investigated. Tribological tests were carried out using a pin-on-disc apparatus in unlubricated conditions, in air, at room temperature, at different sliding speeds, on the same sliding distance and under the same applied load. The dependence of the tribological behaviour of the sliding alumina ceramics is discussed in terms of microstructural features such as grain size and content of secondary phase. The objective of the present research was to obtain a better understanding of the role of the microstructural parameters and how they can influence the friction and wear behaviours of low purity alumina ceramics. The contradictory behaviour of the lowest purity alumina is attributed to the microstructural characteristics of the material and in particular the presence of the secondary phase.