Mating Materials Research Articles

Sliding wear behavior of WC–12% Co coatings at elevated temperatures

WC–Co coatings with low degree of decomposition were thermally sprayed by a high velocity oxy-fuel system from three agglomerated WC–12 mass% Co powders with different carbide grain size distributions. Dry sliding friction and wear behavior of the WC–12% Co coatings was investigated by using sintered alumina (Al2O3) as the mating material at 200 °C, 300 °C and 400 °C. The friction coefficient of these WC–Co/alumina pairs was nearly constant regardless of the test temperature and the carbide grain size of the coatings. The specific wear rate of the coatings increased with increasing the carbide grain size at a given testing temperature. It decreased with increasing the temperature for a given carbide grain size. The specific wear rate was reduced by more than one order of magnitude when the test temperature was increased from room temperature to 400 °C. The microscopic analyses of the worn tracks showed that the tribofilm formed at higher temperature is denser and more adhesive to the underlying surface, thus providing more surface protection against wear. The results show that the formation of dense and adhered tribofilms plays an important role on the low sliding wear rate of the coatings at elevated temperatures.

Tribological characteristics of diamond-like carbon films based on hardness of mating materials

Tribological testing of DLC films was conducted using a rotating type ball on a disk friction tester in a dry chamber. This study made use of four kinds of mating balls that were made with stainless steel but subjected to diverse annealing conditions in order to achieve different levels of hardness. In all load conditions using martensite mating balls, the test results demonstrated that the friction coefficient was lower when the mating materials were harder. The high friction coefficient found in soft martensite balls appeared to be caused by the larger contact areas. Measuring the wear track of both the DLC films and the mating balls revealed a similar tendency compared to the results of friction coefficients. The wear track on the mating balls indicated that a certain amount of material transfer occurs from the DLC film to the mating ball during the high friction process. Raman spectra analysis showed that the transferred materials were a kind of graphite and that the contact surface of the DLC film seemed to undergo a phase transition from carbon to graphite during the high friction process.

Some Observations on Frictional Force During Fretting Fatigue

Frictional force behavior during fretting fatigue and its interdependence on other fretting variables are investigated. Both coefficient of static friction and the normalized frictional force (i.e., the ratio of frictional force and normal contact load) increase during the earlier part of a fretting fatigue test and then both reach to a stabilized value. The variation of temperature in the contact region and normalized frictional force with increasing cycle numbers and bulk stress show similar trend implying that normalized frictional force represents the average friction in the contact region during a fretting fatigue. An increase in bulk stress, relative slip, and hardness of pad material results in an increase of the normalized frictional force, while an increase in contact load, frequency and temperature decreases the normalized frictional force. The normalized frictional force is also affected by the contact geometry. On the other hand, coefficient of static friction increases with an increase in the hardness of mating material, temperature and roughness from shot-peening treatment, but is not affected by contact geometry and displacement rate. Further, the normalized frictional force is not affected by the contact geometry, roughness and applied bulk stress level when fretting fatigue test is conducted under slip controlled mode, however it increases with increasing applied relative slip and decreasing contact load in this case.

Tribological properties of DLC films deposited on steel substrate with various surface roughness

Tribological properties of diamond-like carbon (DLC) films in water were investigated concerning with the influence of surface roughness and various mating materials. The DLC films were deposited by pulsed-bias CVD method on AISI630 stainless steel. The substrate roughness ( R a) is in the range of 1.4–740 nm. AISI 440C, AISI 304 stainless steel and brass balls were used as a mating ball. The friction coefficients of DLC films against with AISI 440C stainless steel ball indicated under 0.1 irrespective of the roughness. The film having smooth surface ( R a=1.4 nm) had severe damage at a load of 9.4 N. However, the film having rough surface ( R a=263 nm) had no damage at the same load. The specific wear rate of the steel ball increased with increase of roughness of the surface. In the case of AISI 304 stainless steel ball, the specific wear rate of the ball showed similar tendency. The friction with brass ball showed relatively high friction coefficient in the range of 0.12–0.25. However, the damage on the films could not be observed after friction test. It is considered that the roughness of the surface is important factor for the rupture of the film in water environment.

Wear characteristics of injection-moulded unfilled and glass-filled nylon 6 spur gears

The tribological behaviour of polymer and polymer composite gears is reported in this paper. Gear tests were conducted using a power absorption-type gear test rig. Injection-moulded nylon 6 and glass-filled nylon 6 gears were tested at different speeds and loads. Gear rotational speed influences the tooth wear of unfilled and glass-filled nylon 6 injection-moulded gears due to the change in contact period of teeth in mesh. When the polymer-based gear drives a steel gear, the difference in the elastic moduli of the mating materials causes rotational lag during gear running. When the polymer gears transmit a higher torque, a higher amount of rotational lag causes an excessive wear in the dedendum region of the polymer gear. In the cases of both unfilled and glass-filled nylon 6 gears, at higher-torque test conditions, non-uniform wear on the gear tooth surface was observed. Glass-filled nylon 6 gears showed a better wear resistance than the unfilled nylon 6 gears due to the improved compressive strength, creep resistance, and elastic modulus. A detailed wear particle analysis was carried out to understand the gear wear mechanism. The contact stress and gear material hardness affect the gear wear mechanism.

Frictional Coatings in Powertrain Applications

Load transmission in friction joints is limited by the coefficient of static friction of the respective mating materials. ESK Ceramics GmbH & Co. KG produces nickel-diamond coated foils that can increase the load transfer capability of friction joints subjected to torsional or shear loads by up to 300 %.

Density Functional Study of the Aluminum—Graphite Interface

Our group has previously performed a series of ab initio calculations to study the atomic structure, ideal work of adhesion, Wad, and bond character of several aluminum—ceramic interfaces. Periodic interfaces were used where the thicknesses of both the Al and ceramic slabs were previously converged with respect to surface energy. Using a plane-wave density functional code with the local density and generalized gradient approximation, our past calculations find the Wad is proportional to the surface energies of the mating materials, and the degree of polarity in the ceramic surface. Our calculations compare favorably with available experimental work of adhesion values of liquid Al on graphite, and Al on Al2O3. We investigated interfacial geometries after atomic relaxation, and interfacial bonding via plots of charge density, charge density difference, the electron localization function, and the projected density of states. In this paper we focus on our preliminary results for the aluminum—graphite interface.

水環境下におけるDLC膜と各種金属材料との摩擦摩耗

水環境下におけるDLC膜と各種金属材料との摩擦摩耗

Fretting wear of Al–Si alloy matrix composites

Ball-against-disk type fretting wear tests for Al–Si alloy matrix composites in contact with bearing steel were conducted in wet air to investigate the effects of relative slip amplitude on friction and wear of the composites. In the larger range of relative slip amplitude, the Al–Si alloy-impregnated graphite composite (ALGR-MMC) shows lower friction coefficients than those of alumina short fiber-reinforced composite (ASFR-MMC) and hollow silica particle-reinforced composite (HSPR-MMC). Although the wear rate of the ALGR-MMC is higher than that of the ASFR-MMC and HSPR-MMC, the composite hardly causes damage to the mating material due to adhesion of compacted films of graphite powder and Al–Si alloy wear particles.

Fretting fatigue behavior of Ti–6Al–4V with dissimilar mating materials

The effects of dissimilar mating materials on fretting fatigue behavior of titanium alloy, Ti–6Al–4V were investigated. Fretting fatigue configuration involved cylindrical pads on the flat specimen. Specimens were made of Ti–6Al–4V and cylindrical pads were made of Ti–6Al–4V, aluminum alloy 2024 and Inconel 718 alloy. All fretting fatigue tests were conducted at 200 Hz with the maximum axial stress ranging from 400 to 700 MPa, and stress ratio, R ranging from nominal value of 0.0–0.7. Applied normal loads for each material of pads were set to provide a constant Hertzian peak pressure of 292 MPa. Finite element analysis of fretting fatigue specimens was also conducted. Results showed that there was no significant effect of different pad materials on fretting fatigue life of Ti–6Al–4V at the same applied effective stress. W- or V-shaped surface profiles of the fretting scar were observed with three pad materials, but the specimen tested with aluminum 2024 pad material showed the narrowest scar due to the lowest hardness of aluminum 2024 pad material and the smallest slip region, which was verified from finite element analysis. Microscopic analysis showed that cracks originated at the contact surface and crack orientations were approximately ±45° from a perpendicular to the loading direction. Four critical plane based parameters: Findley, Smith–Watson–Topper, shear stress range (SSR), and modified shear stress range (MSSR) parameters, were evaluated in terms of ability to correlate/predict number of cycles for crack initiation (nucleation and growth to a certain size), crack location and crack angle. All parameters were effective to correlate with number of cycles to crack initiation regardless of pad materials, but only SSR and MSSR parameters could predict both location and orientation of the crack correctly.

Novel silver/mica multilayer compressive seals for solid-oxide fuel cells: The effect of thermal cycling and material degradation on leak behavior

A novel Ag/mica compressive seal was thermally cycled between 100 °C and 800 °C in air to evaluate its stability. The novel Ag/mica compressive seal was composed of a naturally cleaved Muscovite mica sheet and two thin silver layers, and was reported in a previous study to have very low leak rates at 800 °C. In the present study, we examined the thermal cycle stability of the Ag/mica-based compressive seals pressed between mating couples with large and small mismatch in thermal expansion. For comparison, thermal cycling also was conducted on plain mica as well as plain silver only. In addition, the results were compared with published data of a similar mica seal using glass instead of Ag as the interlayers. For mating materials of large mismatch in thermal expansion coefficient (CTE; Inconel/alumina), the Ag/mica seal showed lower leak rates than the plain mica. For mating materials of small mismatch in CTE (SS430/alumina), the leak rates were similar for both the Ag/mica and the plain mica seal. Scanning electron microscopy was used to characterize the microstructure of the mica after thermal cycling. Microcracks, fragmentation, and wear-particle formation were observed on the mica and were correlated to the leak behavior. Overall, the novel Ag/mica seals present good thermal cycle stability for solid-oxide fuel cells, although the leak rates were greater than the corresponding mica seals with glass interlayers.

Effect of dissimilar mating materials and contact force on fretting fatigue behavior of Ti–6Al–4V

Fretting fatigue behavior of a titanium alloy, Ti–6Al–4V, in contact with two pad materials having quite differing values of hardness and elastic modulus (aluminum alloy 2024 and Inconel 718) using “cylinder-on-flat” configuration was investigated at different applied stress levels and contact forces. Applied contact forces for both pad materials were selected to provide two Hertzian peak pressures of 292 and 441 MPa. Finite element analyses of all tests were also conducted which showed that an increase in contact force resulted in a smaller relative slip amplitude and a larger width of stick zone. These two factors, along with the lower coefficient of friction during fretting, resulted in less fretting damage on the contact surface of specimen subjected to higher contact force relative to that at lower contact force regardless of the hardness difference of mating materials. Also, an increase in hardness resulted in greater fretting damage on the contact surface of specimens only at higher contact force. Further, the fretting fatigue life decreased with an increase of applied contact force at higher applied effective stress, while it increased at lower applied effective stress with both pad materials. These observations suggest that there is complex interaction among hardness difference between mating surfaces, relative slip amplitude, and stress state in the contact region during fretting fatigue of dissimilar materials.

ロータリエンジン用アペックスシール部材としてのＦｅ‐Ｐ‐Ｃ系焼結摺動材料の開発

The effect of phosphorous and carbon contents and sintering conditions on the wear resistant properties of ferrous sintered materials were investigated. Wear test was conducted using the sintered material as the pin and hard Cr-plated Al-alloy disk as the mating material. By choosing compacting pressure of 490MPa and sintering conditions of 1203 K and 0.3 ks for Fe-5.6mass%P-1.6mass%C alloyed powder, the porosity of 5.5%, the average pore size of 23μm and the average grain size of 15μm were obtained. Proper anti-wear characteristics are obtained after making the fine-grained microstructure. Minimum wear amount was achieved on the Fe-5.5 mass%P-2.0mass%aC-3.0mass%Ni-3.0mass%Mo alloyed powder specimen pressed at 294 MPa, sintered in vacuum at 1203 K for 0.3 ks and heat-treated. The specimen shows 0.7% porosity, the average pore size of 8μm, the average grain size of 15μm, and the macro-hardness of Hv700 with a hardened matrix.

ロータリエンジン用サイドシール部材としての高炭素鉄系焼結摺動材料の開発

The effect of the carbon content and sintering conditions on the wear resistant properties of ferrous sintered materials were investigated in order to develop rotary engine side seal. Wear test was conducted using cast iron as the mating material. The specimens were made by pressing the mixture of reduced iron powder and natural graphite powder. The compacts were sintered in H2 at 1393 K for 1.2 ks. In the wear test, the wear amount of the couple 3mass%C specimen and cast iron disk was minimum.The specimen of 3 mass% C showed the brittleness, which have a Fe3C network structure and nodular α phase iron. The brittleness of this specimens was overcome changing the structures into spheroidal Fe3C and redusing a phase iron by special heat treatment. The heat treatment is that specimens were hold or slow cooling around the spherodizing of Fe3C and the carbon potential was kept in the range from 0.7% to 0.9%. The specimens sintered on these conditions were rolled into side seals of the rotary engines with 1.0 mm in thickness and 186 mm in radius of curvature.

Evaluation of Wear Characteristics of Artificial Joint Materials in the Multi-Directional Pin-on-Plate Tester

For the tribological evaluation of artificial joint materials, it is important that in vitro wear characteristics should correspond to the wear in vivo. As the first screening test of new materials, current simple test methods such as unidirectional pin-on-disk and reciprocating pin-on-plate tests are imperfect to simulate the physiological motion. Therefore, we developed a new type of multi-directional pin-on-plate tester to reflect the three-dimensional motion in human joint. ln this tester, the friction path is controlled by the movement of pin specimen with the programmable data. Value of UHMWPE wear rate can be 1-5×10-7mm3/N·m, for two arcs paths in this tester, while that is 1-5×10-8mm3/N·m in traditional tester which is significantly underestimated in comparison with clinical wear rate. The superiority of ceramics for wear rate to metallic materials was comfirmed as indicated for clinical results. Additionally, it was shown that new type of mating materials can improve UHMWPE wear rate compared with traditional materials.

Reciprocating sliding wear of Inconel 600 tubing in room temperature air

The sliding wear behavior of the material of a steam generator in a nuclear power station (Inconel 600) was investigated at room temperature. Effects of the wear parameters such as material combination, sliding distance and contact stress were examined with various mating materials including 304 austenitic stainless steel, Inconel 600 and Al–Cu alloy 2011. In the prediction of the wear volume by Archard’s wear equation, the standard error range was calculated to be ±4.04×10 −9 m 3 and the reliability to be 71.9% for the combination of Inconel 600 and 304 stainless steel. The error range was considered to be relatively broad because the wear coefficient in Archard’s equation was assumed to be a constant, regardless of the changes in the mechanical properties during the wear. In the present study, the sliding wear behavior turned out to be influenced by the material combination; the wear volume of 304 stainless steel did not linearly increase with the sliding distance, while that of other material combinations exhibited linear increases. Based on the experimental results, the wear coefficient was modified as a function of the sliding distance. The calculation with the modified wear equation showed that the error range narrowed down to ±2.60×10 −9 m 3 and the reliability increased to 75.3%, compared to Archard’s original equation.

Effect of carbide grain size on microstructure and sliding wear behavior of HVOF-sprayed WC–12% Co coatings

In order to examine the effect of carbide grain size on the wear behavior of WC–Co coatings, coatings with low degree of decomposition of WC were thermally sprayed by a high-velocity oxy-fuel (HVOF) system from three agglomerated WC–12% Co powders with various carbide size distributions. Characterization of the coating showed that the average carbide grain sizes of the coatings were 0.8, 1.4 and 2.8 μm and that a decrease in carbide grain size led to slightly higher degree of decomposition of WC. Dry sliding friction and wear tests using sintered alumina (Al 2O 3) as the mating material were performed. The coefficient of friction of the coatings was nearly constant regardless of the test conditions and carbide grain sizes. The specific wear rate of the coatings was very low ∼10 −6 mm 3/(Nm) and increased with increasing carbide grain size. The microscopic analyses of the worn tracks have shown that binder extrusion followed by carbide removal or carbide fracture are the dominant material removal mechanisms. The extruded cobalt acts as binder to form a ductile, dense and well cohered tribofilm on the worn surface to protect the surface from further wear, decreasing the wear rate of the coatings. Because a pull-out of single carbide particle provides less damage to the finer coating and also because the debris consisting of finer carbides are less effective as the third-body abrasions, the wear rate decreases with decreasing carbide size in the coatings.

Friction mechanisms of amorphous carbon nitride films under variable environments: a triboscopic study

In this work, a carbon nitride film prepared by ion-beam-assisted deposition onto silicon was investigated using reciprocating sliding tests under different surrounding atmospheres (ambient air, dry air, dry nitrogen (DN), and ultra-high vacuum) and testing conditions (contact pressure, nature of the counterface). The carbon–nitrogen (CN x ) coating provided a ‘superlow’ friction coefficient of 0.007–0.008 in DN atmosphere independently of the mating material (steel or sapphire balls). The build-up of a smooth transfer film onto the ball surface was detected by optical microscopy. The change in the friction coefficient was spatially recorded along the track as sliding progressed. This technique, called ‘triboscopy’, combined with variable amplitude tests performed on the same wear track, revealed that the nature of both counterfaces (ball and film) has to be modified during a short running-in period before achievement of the superlow friction regime. When other surrounding atmospheres are used, the superlow friction coefficient is not attained even if the counterfaces are previously rubbed in DN. This assembly of tools allowed the characterization of environmental influence on the tribological behavior of CN x coatings in terms of friction coefficient, material transfer, and chemistry of the sliding interface, and the comparison with that observed for diamond-like coatings.

상대 재료의 경도를 고려한 DLC필름의 트라이볼로지 특성

DLC films were deposited on Si wafer by RF plasma assisted CVD using CH4 gas. Tribological tests were conducted using rotating type ball on disk friction tester in dry air. Four kinds of mating balls were used. The mating balls were made with stainless steel but apply different annealing conditions to achieve different hardness conditions. Testing results in all load conditions showed that the harder the mating materials, the lower the friction coefficient among the three kind of martensite mating balls. In case of austenite balls, the friction coefficients were lower than fully annealed martensite ball. The high friction coefficient in soft martensite balls seems to be caused by the larger contact area between DLC film and ball. The wear tracks of DLC films and mating balls could have proven that effect. Measuring the wear track of both DLC films and mating balls have similar tendency comparing to the results of friction coefficients. Wear rate of austenite balls were also smaller than that of fully annealed martensite ball. The results of effect of applying load showed, the friction coefficients were become decrease when the applying loads exceed critical load conditions. The wear track of mating balls showed that some material transfer occurs from DLC film to mating ball during the high friction process. Raman spectra analysis showed that transferred material was a kind of graphite and contact surface of DLC film seems to undergo phase transition from carbon to graphite during the high friction process.

Study on the tribological characteristics of solid lubricants embedded tin‐bronze bearings

AbstractThe friction and wear characteristics of graphite, MoS2, and PTFE embedded tin‐bronze bearings were studied using a pin‐on‐disc tester. The results indicated that solid lubricants decreased and stabilized the friction coefficient, and decreased the wear rate by two to three orders of magnitude. When the content of solid lubricants, PTFE mixed with graphite, was 20–40%, the performance of the solid lubricants embedded bearing (SLEB) was the best. Wear scar was analyzed by means of X‐ray diffraction (XRD), Auger electron spectroscopy (AES), and scanning electron microscopy (SEM). The results show that the transfer films of solid lubricants reduce adhesion between the SLEBs and the mating material, and the wear mechanism of SLEBs changes to fatigue and adhesive wear. The main reason for fatigue wear is microcracks expanding at Pb points in SLEBs. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 2394–2399, 2001