Key Engineering Materials Vols. 353-358

Abstract: In this study, microstructure and wear behavior of thermally sprayed Fe-based alloy coatings were investigated. Fe-based alloy coatings were formed on a carbon steel substrate and then heat-treated at temperature of 700 oC for 20 minutes. Dry sliding wear tests were performed using the sliding speeds of 0.4 and 0.8 m/s, the applied loads of 3 and 6 N. Microstructure and wear behavior of as-sprayed and heat-treated Fe-based amorphous coatings were studied using a scanning electron microscope(SEM), transmission electron microscopy(TEM) and X-ray diffraction(XRD).

Abstract: The friction and wear properties of phosphor bronze discs sliding against SAE52100 steel balls were evaluated using an Optimol SRV reciprocating friction and wear tester under the lubrication of pure poly-alpha-olefin (PAO) and PAO containing commercial phosphite ester, zinc dialkyldithiophosphate (ZDTP) and oleic acid additives. The morphologies of the worn surfaces of the phosphor bronze discs were observed using a scanning electron microscope (SEM). In addition, the elemental compositions and chemical states of several typical elements on the worn surfaces of the phosphor bronze discs were examined by means of X-ray photoelectron spectroscopy (XPS). The findings indicated that the phosphor bronze discs exhibited higher wear resistance under the lubrication of PAO containing phosphite ester than pure PAO, PAO containing ZDTP, and PAO containing oleic acid. Also, the phosphor bronze discs exhibited stable friction coefficients as low as 0.09-0.11 when lubricated with PAO containing phosphite ester. According to the XPS analyses, it was considered that the formation of CuO and phosphate led to lower friction coefficient and high wear resistance of phosphor bronze specimens lubricated with PAO containing phosphite ester. In the case of the phosphor bronze specimens lubricated with PAO containing ZDTP and oleic acid, the formation of organic compounds seemed to lead to lower friction coefficient, but did not seem to affect the wear resistance.

Abstract: The objective of this study is to evaluate the effect of stress relief groove on fretting fatigue strength. Fretting fatigue tests and finite element analyses were done. The shape of groove was controlled by groove radius R and tangential angle θ. The depth of groove was specified by R and θ. Fretting fatigue strength was increased with an increase of θ and then it turned into a decrease. The decrease was caused by the transition of failure mode from fretting fatigue at the contact part to plain fatigue at the groove root. The transition was caused by an increase of stress concentration at the groove root with an increase of the groove depth. Therefore, the maximum improvement of fatigue strength was achieved by the largest θ limited by fatigue strength of the groove root. In the analysis, the groove generates high compressive stress field at the contact edge, where small cracks never propagate. Therefore, assumptions to relieve the contact pressure concentration at the contact edge were taken into the analysis model. The values of stress intensity factor ranges for small cracks introduced near the contact edge were almost the same between grooved and non-groove specimens.

Abstract: The fatigue wear behavior of PA1010 composites filled with metal oxide particles was experimentally investigated. The nylon composites were prepared by compression molding method with different contents of PA1010 and CuO, Fe3O4, Al2O3 particles. The rolling contact counterpart for nylon composite samples was 1045 carbon steel ring. The rolling contact tests were performed on M-2000 wear test machine in dry friction condition. It was found that the fatigue wear measured in mass loss of nylon composites increases to rolling contact cycles. Most of the samples show an approximately linear relationship between wear mass loss and rolling cycles. Three kinds of metal oxide particles have different effects on the fatigue wear behavior of nylon composites. The contents of metal oxide particles have important influence on the fatigue wear behavior. The nylon composites filled with CuO (30 wt.%) or Fe2O3 (30 wt.%) will result in the comparatively highest wear resistance, the wear mass loss reduces to about 70% of pure nylon. Al2O3 particles is not a good choice for increasing the rolling wear resistance of nylon composites. It is found that the contact fatigue controls the wear mechanism of nylon composites under rolling contact.

Abstract: In the subsurface damage observations on the wear behavior of brittle dental porcelains, it still remains unsolved that how to distinguish the cracks that develops accidentally during the preparing sample process from those actually produced in the tests. In the sliding friction tests, the bonded-interface technique (BIT) was successfully used for subsurface damage evaluations. The profile of wear scar was easily observed on the cemented section of the blocks without inducing any cracks. The stresses on the surface and internal of dental porcelains were calculated with the contact element method (CEM) by ABAQUS software. The model of rectangular dental porcelain and Si3N4 ball were developed based on sliding friction tests. The Si3N4 ball modeled in this study was of the diameter of 4mm and 12 mm. The size of Vita VMK 95 porcelain blocks was 5 mm wide, 2 mm thick and 15 mm long. The glue layer thickness values are 10 .m, 20 .m and 30 .m. The loading of the Si3N4 ball was modeled as a constant distributed vertical load applied across the bonding line of the porcelain. Stresses and displacements of all nodes of the model, especially at the bonding interface, were analyzed and compared. The results of the study indicated that stress values correlated strongly to the applied loads. Stress distribution was symmetric about the bonded-interface plane. The maximum occurred in surface layer and the minimum on the bottom line in the porcelain blocks. Along the interface, stresses decreased with the distance from the surface of porcelain. On the other hand, high stresses focus on the surface part and go sharply down in the internal region along the depth direction. Among the parameters of sliding test on the stresses, the load effect is prominent. The BIT model experiences a different, non-axisymmetric stress field than that of real work-piece. So the BIT should not be considered as a reliable source of quantitative analysis but as a qualitative method for observing the form of the damage modes with more visible information than the conventional tools.

Abstract: A complex relative motion of fretting combined by dual motions of radial and tangential fretting was achieved on a modified fretting tester. The composite fretting motion was induced by the action of an oscillating normal force in a sphere-on-inclined flat contact (52100 steel ball against 2091 aluminum alloy). Two types of inclined angles (45° and 60°) were used in the tests. Variations of veridical force vs displacement have been recorded and analyzed as a function of cycles. Effects of the cyclic normal force and the inclined angle were discussed. The test results showed that wear, cracking and plastic deformation accumulation with a strong dissymmetry in damage morphology was observed. A transformation of fretting mode from composite to radial fretting mode occurred due to a strong modification at local contact configuration. As a conclusion, a physical model for wear mechanism of composite fretting was presented.

Abstract: Under imitating human physiological solution, the fretting behaviors of flat cortical bone specimen from fresh human femur in the axial orientation against TA2 titanium ball with diameter of 40 mm had been investigated. The coefficients of friction (COF) and fretting logs were calculated automatically. The worn surfaces of the cortical bone were analyzed by means of laser confocal scanning microscopy. The results showed that the fretting logs for the cortical bone varied with the increase of the cycles from the elastic partial slip (D=5 (m), the elastoplastic partial slip (D=15 (m) to gross slip (D=30 (m), respectively. The fretting regime transformed from the partial slip regime directly to the gross slip regime without the mixed fretting regime. And the evolution of the COF in each fretting condition was also compared statistically. The fretting damage in the elastic partial slip state was slight. But the abrasive wear and micro-fracture were observed in the plastoelastic partial slip state and gross slip regime. The wear depth increased with cycles and well related to the COF of the cortical bone in each fretting condition. To reduce the fretting damages of bone, it is advisable to control the displacement amplitude at bone-implant interface.

Abstract: Nickel base Ni60 and cobalt base Co-Cr-W coatings were prepared on substrate of refractory alloy steel 4Cr14NiW2Mo by laser-cladding technology. A 2 kW transverse-flowing CO2 laser device was used for cladding alloy powder. Another Co-Cr-W coating was formed by bead welding. Under room temperature, 250 oC.and 420 oC, reciprocate sliding wear tests with a ball-on-flat contact for three coatings have been carried out. For the laser cladding coating, a good metallurgical bonding with the substrate was obtained. Different tribological behaviors of three coatings were observed. For the two Co-Cr-W coatings, the friction coefficients decreased with the increase of test temperature, and larger spot-like stripping pit and wider ploughing trenches in the wear scars were presented. The wear mechanism was the combination of abrasive wear, delamination and oxidation wear. For the Ni60 laser cladding layer, the friction coefficient hardly changed with the elevated temperature, and the wear mechanism mainly performed as abrasive wear due to its high oxidation resistance. The wear tests showed that the Ni60 clad coating had provided excellent wear resistance than Co-Cr-W layers under high temperature reciprocating sliding wear condition.

Abstract: In this paper we present the estimation methods of fretting wear process and fretting fatigue life using this wear process. Firstly the fretting-wear process was estimated using contact pressure and relative slippage. And then the stress intensity factor for cracking due to fretting fatigue was calculated by using contact pressure and frictional stress distributions, which were analyzed by the finite element method. The S-N curves of fretting fatigue were predicted by using the relationship between the calculated stress intensity factor range ( #K) with the threshold stress intensity factor range ( #Kth) and the crack propagation rate (da/dN) obtained using CT specimens of the material. Finaly fretting fatigue tests were conducted on Ni-Mo-V steel specimens. The S-N curves of our experimental results were in good agreement with the analytical results obtained by considering fretting wear process. Using these estimation methods we can explain many fretting troubles in industrial fields.

Abstract: CP-Nb-Cr/Cu-Cd electrical contact material, which contains 1.7wt.% of Cd, 0.5wt.% of CP (man-made diamond particles), 2.0 wt.% of Nb and 0.7 wt.% of Cr, was fabricated by powder metallurgy process. The contact material was installed in different ac contactors covering current range from 40 to 630A. While tested under AC-4 electrical load, the electrical contact material was worn out gradually by repeated electrical arc, and microstructure of the arc-affected surface layer changed dramatically. Mechanism of arc erosion process and contact resistance fluctuation was discussed by corresponding microstructure observation and chemical composition analysis of the surface layer. CP-Nb-Cr/Cu-Cd electrical contact material is capable to be used in 63~250A ac contactors under AC-4 working conditions.