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Research Papers: Friction and Wear

J. Tribol. 2018;141(2):021601-021601-12. doi:10.1115/1.4041246.

A corrosive wear model is considered at the asperity-scale of a tribocorrosive wear system as well as the traditional Archard-type mechanical wear model. The geometry of the surface asperities is modified in a contact mechanics model with respect to both corrosive and mechanical wear calculations. This model was presented and validated for prediction of the electrochemistry in the first part of this work. The material used in the experimental part of this work was CoCrMo plate working electrode (WE) and Si3N4 ball as the counter body in a reciprocating configuration. Experiments were conducted at loads of 5, 7.5, and 10 N and the contributions of total mechanical wear and corrosion were measured. The model is then tuned to predict the chemical and mechanical components of the total wear of the system. The synergistic effect of corrosion on mechanical wear and mechanical wear on corrosion are modeled numerically in this work. The values are then used to explain different components of mechanistic tribocorrosive wear models present in the literature. This deterministic model, for the first time, calculates the corrosion-enhanced wear in a tribocorrosive wear environment and proposes that changes in the topography are responsible for this synergistic effect. The results show a linear dependence of the corrosion enhanced wear, wear-enhanced corrosion, and the pure mechanical wear on the applied load. Results also suggest that the wear enhanced corrosion has a significant contribution in the overall degradation of the material.

Topics: Wear , Corrosion , Stress
Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(2):021602-021602-12. doi:10.1115/1.4041019.

Acrylonitrile butadiene styrene (ABS) polymer is cost-effective and also possesses high toughness and resistance to corrosive chemicals. However, pure ABS does not show significant wear resistance and also it has a high friction coefficient. Incorporation of a solid lubricant and nanofiller in a polymer matrix improves its tribological properties significantly. The addition of solid lubricant makes it suitable for application where self-lubrication is desirable (sliding bearings, gears). This paper deals with the study of tribological behavior of ABS hybrid composites reinforced with nano zirconia and polytetrafluoroethylene (PTFE). ABS hybrid composites with varying proportions of nano zirconia and PTFE were prepared using melt blending. Dispersion of reinforcement in the polymer matrix has been studied with the help of transmission electron micrographs. Influence of reinforcements on the mechanical behavior is studied by tensile testing according to the ASTM standard. The tribological behavior of composites was determined in a pin-on-disk tribometer according to the ASTM G99 standard. Worn surfaces were analyzed using scanning electron microscope (SEM) in order to identify the different types of wear and various wear mechanisms. Transfer film formation was studied by analyzing the counterbody surface. A wear mechanism map has been developed, which helps in identifying various wear mechanisms involved under given loading conditions. The results reveal that the addition of PTFE reduces the wear rate and coefficient of friction (COF) significantly. Nano zirconia effectively transfers the load, thereby improving wear resistance, and the addition of PTFE results in continuous transfer film formation thereby reducing the COF. Also from the wear map, it has been identified that abrasion, adhesion, plowing, plastic deformation, melting, and delamination are the dominant wear mechanisms involved.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(2):021603-021603-7. doi:10.1115/1.4041126.

This study aims to investigate the effect of volume fraction of commercially pure titanium (CP-Ti) on microstructural, mechanical, and tribological features of A356 aluminum matrix composites. Vacuum-assisted melt infiltration casting was performed to produce composites with 50%, 65%, 75%, and 80% CP-Ti contents. CP-Ti sawdusts were assembled under mechanical pressure in order to attain porous one-piece CP-Ti preforms which were infiltrated by A356 melt at 730 °C under 10−5 Pa vacuum atmosphere. TiAl3 layer was formed at the interface between A356 and CP-Ti phases. Owing to increased diffusion time through decreased diffusion path length, both thickness and hardness of TiAl3 phase were increased with increasing CP-Ti ratio, whereas the best wear resistance was obtained at 65% CP-Ti ratio. The main reason for decrease in wear resistance of 75% and 80% CP-Ti reinforced composites was fragmentation of TiAl3 layer during wear process due to its excessively increased brittleness. Strongly bonded TiAl3 phase at the interface provided better wear resistance, while weakly bonded ones caused to multiply wear rate.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(2):021604-021604-14. doi:10.1115/1.4041072.

Inconel 100 (IN100) aerospace superalloy is used in manufacturing aero-engine components that operate at intermediate temperatures. It is considered to be a hard-to-cut material. Chipping of the tool edge is one of the major failure mechanisms of ceramic tools in finish cutting of superalloys, which causes a sudden breakage of the cutting edge during machining. Cutting temperature significantly depends on cutting speed. Varying the cutting speed will affect the frictional action during the machining operations. However, proper selection of the cutting variables, especially the cutting speed, can prevent chipping occurrence. In this work, the influence of controlling the cutting speed on the chipping formation in dry finish turning of IN100 aerospace superalloy using SiAlON ceramic tool has been investigated. Scanning electron microscope (SEM)/energy dispersing spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and three-dimensional wear measurements were used to make the investigations of the worn tool edges. It was found that variations of the cutting speeds in a certain range resulted in the generation of different lubricious and protective tribo-films. The presence of these tribo-films at the cutting region proved essential to prevent chipping of the cutting tool edge and to improve its wear resistance during finish turning of age-hardened IN 100 using SiAlON ceramic tools. Chip compression ratio and calculated values of the coefficient of friction at the tool–chip interface confirmed these results.

Commentary by Dr. Valentin Fuster

Research Papers: Hydrodynamic Lubrication

J. Tribol. 2018;141(2):021701-021701-14. doi:10.1115/1.4041211.

The quality of predictions for the operating behavior of high-speed journal bearings strongly depends on realistic boundary conditions within the inlet region supplying a mixture of hot oil from the upstream pad and fresh lubricant from the inlet device to the downstream located pad. Therefore, an appropriate modeling of fundamental phenomena within the inlet region is essential for a reliable simulation of fluid and heat flow in the entire bearing. A theoretical model including hydraulic, mechanical, and energetic effects and the procedure of its numerical implementation in typical bearing codes for thermo-hydrodynamic lubrication is described and validated. Convective and conductive heat transfer as well as dissipation due to internal friction in the lubricant is considered for the space between pads or the pocket where the inlet is located. In contrast to most other models, the region between the physical inlet and the lubricant film is part of the solution domain and not only represented by boundary conditions. The model provides flow rate and temperature boundary conditions for extended Reynolds equation and a three-dimensional (3D) energy equation of film and inlet region, respectively. The impact of backflow from the inlet region to the outer supply channel possibly occurring in sealed pockets is taken into account. Moreover, the model considers the influence of turbulent flow in the inlet region.

Commentary by Dr. Valentin Fuster

Research Papers: Lubricants

J. Tribol. 2018;141(2):021801-021801-7. doi:10.1115/1.4041364.

Soft solids, such as rubbers, elastomers, and gels, are the important polymeric materials. A better understanding of their interfacial properties such as friction and adhesion is critical for variety of technological applications. Motivated by the experimental observation that interfacial properties can be modified even without changing the content of a soft solid, the effect of specimen thickness on the energy release rate $(G)$ of a soft gelatin hydrogel is investigated in direct shear test. Slide-hold-slide (SHS) experiments have shown that shear strength decreases, while corresponding crack length increases, with increase in thickness of gel specimens. However, $G$ at static, dynamic and residual strengths increase with specimen thickness. At the end, these observations are explained in light of mixed mode I/II fracture and shear rate effects at the sliding interface.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(2):021802-021802-7. doi:10.1115/1.4041124.

Understanding the pressure and shear dependence of viscosity is essential to an understanding of the mechanisms of film forming and friction in concentrated contacts. The blending of different molecular mass polyalphaolefins (PAOs) may permit the formulator to arrive at a desired combination of film thickness and friction. The viscosities of PAO base oils and their blends were measured versus temperature, pressure, and shear stress to 1 GPa in pressure. The Grunberg–Nissan mixing rule, with effective mole fractions, provides an excellent mixing rule for the temperature and pressure-dependent low-shear viscosity. This work provides the first look at a possible mixing rule for the non-Newtonian response of mixtures of base oils.

Commentary by Dr. Valentin Fuster

Research Papers: Micro-Nano Tribology

J. Tribol. 2018;141(2):022001-022001-9. doi:10.1115/1.4041074.

During the last decades, there has been an increased interest in the use of lignin-based composites following the ideas of developing green materials for fossil-based raw materials substitution. The biopolymer Arboform is a mixture of lignin, plant fibers, and additives, which is nowadays successfully used in many applications. As a thermoplastic, it can be molded and is therefore also called “liquid wood.” In this paper, we report a study comparing the nanomechanical and tribological properties of Arboform (AR), and Aramid-reinforced Arboform (AR-AF) composite biopolymers. The samples were produced in an industrial-scale injection molding machine. Nanoindentation experiments have revealed that, in both series of biopolymer samples, an increase in temperature or a change in the injection direction from 0 deg to 90 deg produces an increase in hardness. On the other hand, Young's modulus is slightly affected by the increase in temperature, and not affected by the injection angle. Tribological characterization has shown that all samples, except the AR-AF injected at 175 °C, present noticeable wear and have a similar friction coefficients μ ∼ 0.44–0.49 at Hertzian contact pressures p0 between 90 and 130 MPa. Interestingly, the reinforced polymer produced at 175 °C shows no wear and low friction of μ ∼ 0.19 at p0 = 90 MPa. Our results show that the reinforced Arboform biopolymers are a good candidate to replace other polymers in many mechanical and tribological applications.

Commentary by Dr. Valentin Fuster

Research Papers: Other (Seals, Manufacturing)

J. Tribol. 2018;141(2):022201-022201-12. doi:10.1115/1.4040574.

Numerical and experimental analyses of the static and dynamic characteristics of the liquid annular seals with axially partial helical grooves were conducted to investigate the effects of the axial length gal of a helically grooved section in a seal stator. The numerical solution and experimental procedures were applied in the same manner as in previous studies on through-helically grooved seals, wherein the grooves extend across the seal length. The numerical results qualitatively agreed with the experimental results, demonstrating the validity of the numerical analysis. The leakage flow rate Q was lower in the partially helically grooved seals than that of conventional through-helically grooved seals across a small range of rotor spinning velocities. In contrast, the reduction in Q due to the pumping effect caused by the spin of the rotor diminished with the decrease in gal. For a small concentric whirling motion of the rotor, the radial dynamic reaction force Fr and magnitude of variation in the tangential dynamic reaction force Ft with the whirling angular velocity increased with the decrease in gal, and their values approached the corresponding values for the smooth-surface seal. Under the same rotor whirling velocity, the Ft for the partially helically grooved seals became lower than that for the smooth-surface seal (similar to the case for the through-helically grooved seal), although decreasing gal tended to increase Ft. These results suggest that partially helically grooved seals can improve the efficiency and stability margin of the pumps because of the reduction in leakage flow rate and suppression of the rotor forward whirling motion (with large radial and tangential dynamic reaction forces) as compared to conventional through-helically grooved seals.

Commentary by Dr. Valentin Fuster