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### Research Papers: Applications

J. Tribol. 2018;141(3):031101-031101-8. doi:10.1115/1.4041670.

Charged particles are emitted when materials undergo tribological interactions, plastic deformation, and failure. In machining, plastic deformation and shearing of work piece material takes place continuously along with intense tool-chip rubbing contact interactions; hence, the emission of charged particles can be expected. In this work, an in-situ sensor has been developed to capture the emitted positive (positive ion) and negative (electron and negative ion) charged particles in real-time in an orthogonal machining process at atmospheric conditions without the use of coolant. The sensor consists of a Faraday plate, mounted on the flank face of the cutting tool, to collect the emitted ions and the intensity of emissions is measured with an electrometer. Positively and negatively charged particles are measured separately by providing suitable bias voltage supply to the Faraday plate. Ion emissions are measured during machining of three different work piece materials (mild steel, copper, and stainless steel) using a carbide cutting tool. The experimental results show a strong correlation between the emission intensity and the variation in machining parameters and material properties. Increasing material removal rate increases the intensity of charged particle emissions because of the increase in volume of material undergoing shear, fracture, and deformation. It is found that emission intensity is directly proportional to the resistivity and strength of workpiece material. Charged particles emission intensity is found to be sensitive to the machining conditions which enables the use of this sensor as an alternate method of condition monitoring.

Commentary by Dr. Valentin Fuster

### Research Papers: Contact Mechanics

J. Tribol. 2018;141(3):031401-031401-11. doi:10.1115/1.4041535.

This work employs a three-dimensional (3D) finite element analysis (FEA) to investigate the fretting metallic contact between a deformable hemisphere and a deformable flat block. Fretting is governed by displacement-controlled action where the materials of the two contacting bodies are set to have identical properties; studied first is steel-on-steel and then copper-on-copper. At contact onset, a normal interference (indentation) is applied, which is then followed by transverse cyclic oscillations. A large range of coefficients of friction (COFs) is imposed at the interface. The results show that the maximum von Mises stress is confined under the contacting surface for small COFs; however, that maximum reaches the contacting surface when the COFs are sufficiently large. It is also shown that fretting under sufficiently large COFs forms large plastic strains in “ring” like patterns at the contacting surfaces. Junction growth is found where the contacting region is being stretched in the direction of the fretting motion. At large COFs, pileups show up at the edges of the contact. The fretting loops of the initial cycles are found along with the total work invested into the system. At certain interference, there exists a certain COF, which results in the largest work consumption. The magnitude of the COF is found to produce either partial slip (prone for fretting fatigue) or gross slip (prone for fretting wear). A scheme of normalization is proposed, and it is shown to be effective for the two said materials that have vastly different material properties. Hence, the normalized results may well characterize a range of contact scales (from micro to macro) of various ductile material pairs that behave in an elastic–plastic manner with strain hardening.

Commentary by Dr. Valentin Fuster

### Research Papers: Elastohydrodynamic Lubrication

J. Tribol. 2018;141(3):031501-031501-11. doi:10.1115/1.4041590.

Friction is usually induced when the contacts are in relative motion, leading to mechanical vibration and consequently heat generation. The reduction of these undesirable parameters is possible by the application of greases, which intends to increase the service life of the bearings. The present work incorporates the frictional and vibration behaviors of concentrated point contact lubricated with bare and nanocomposite greases. The nanocomposite greases were formulated by dispersing different categories of nano-additives like reduced graphene oxide (rGO), calcium carbonate (CaCO3), and alumina (α-Al2O3) in bare grease (BG). The formulated nanocomposite greases are tested for film formation, frictional and vibrational response under a limited supply of greases. The use of transparent glass disk better analyses the profile of film thickness to understand the lubrication mechanism of the point contact. The microstructure of nano-additives and the formulated nanocomposite greases were characterized using high-resolution transmission electron microscopy (HRTEM). The presence of different functional groups in nano-additives and the formulated nanocomposite greases were characterized using Raman spectroscopy. The tribological contact operates under 3% and 30% slide-roll-ratio (SRR) for varying rolling speed (0.001–1 m/s) at a load of 30 N (Hertzian pressure, pH = 0.9 GPa). Film thickness, friction and vibration behavior were recorded to focus the tribo-performance, degree of starvation and dynamics of the tribological contact with slip varying from 3% to 30% SRR. The vibration level was refined to 32% with the addition of rGO nanosheets in BG. The incompatibility of α-Al2O3 with the grease structure results in disruption of tribo-dynamics behavior of the point contact.

Commentary by Dr. Valentin Fuster

### Research Papers: Friction and Wear

J. Tribol. 2018;141(3):031601-031601-9. doi:10.1115/1.4041536.

The mechanical properties of crystalline phase of glass ceramics are critical. This study aimed to evaluate wear resistance of different crystalline-reinforced dental chairside computer-aided design/computer-aided manufacturing (CAD/CAM) glass ceramics. Materials of feldspar (Vita Mark II, VM), leucite (IPS Empress CAD, EC), lithium disilicate (IPS e.max CAD, EX), lithium disilicate enriched with zirconia (Vita Suprinity, VS), and enamel were embedded, grounded, and polished, respectively. Samples were indented with a Vickers hardness tester to test the fracture resistance (KIC). Two-body wear tests were performed in a reciprocal ball-on-flat configuration under artificial saliva. The parameters of load force (50 N), reciprocating amplitude (500 μm), frequency (2 Hz), and the test cycle (10,000 cycles) were selected. Specimen microstructure, indentation morphology, and wear scars were observed by scanning electron microscope (SEM), optical microscopy, and three-dimensional profile microscopy. EX, VS, and EC demonstrated significantly higher KIC values than the enamel, while ceramic materials showed smaller wear depth results. Cracks, massive delamination, and shallow plow were seen on the enamel worn scar. Long deep plow, delamination, and brittle cracks are more common for VM and EC, and short shallow plow and smooth subsurface are the characteristics of EX and VS. Greater fracture toughness values indicated higher wear resistances of the materials for the test glass ceramics. The CAD/CAM glass ceramics performed greater wear resistance than enamel. Feldspar- and leucite-reinforced glass ceramics illustrated better wear resistance similar to enamel than lithium disilicate glass ceramics, providing amicable matching with the opposite teeth.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(3):031602-031602-6. doi:10.1115/1.4041644.

The interaction observed between two surfaces in contact with one another is part of a number of physical processes, such as wear. In this paper, we present a numerical study of the asperities between two surfaces in contact with each other. The real contact area between two surfaces varies due to the multiple roughness scales caused by the stochastic nature of asperities. In our research, we employ a tribological system comprising two partitions: C1 is the contact set (CS), where the two surfaces are in direct contact with each other, and C2 is the noncontact set, where the two surfaces are not in contact with each other. Here, we have developed a new numerical model to describe the CS using ε-entropy to prove the existence of a minimum value for entropy in sliding contact scenarios. In this system, the lower and upper bounds of entropy are determined through the Kolmogorov approach using the aforementioned model. Using this model, we conclude that the ε-entropy value is bound between ln 2 and 2·ln 2 for a tribological system comprising two partitions. Additionally, we conclude that a correlation between the stochastic tribological contact behavior and the rate of entropy change is the key parameter in thermal nonequilibrium scenarios.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(3):031603-031603-13. doi:10.1115/1.4041587.

A new wear prediction method of tooth surfaces of involute gears based on a real tooth surface model and a modified fractal method is developed. The real tooth surface model of an involute gear pair is introduced, and microgeometry feature detection of tooth surfaces is achieved by monitoring variations of normal vectors of each discrete data point of the real tooth surface model. To predict wear progression of tooth surfaces of a gear pair, an abrasive wear analysis model and the modified fractal method are used to analyze contact performance and its changes with accumulation of surface wear. The abrasive wear analysis model can analyze wear depths of gear tooth surfaces with sliding distances, local contact pressure, and directions of wear progression based on Archard's model. The modified fractal method is proposed to calculate instantaneous contact stiffness and estimate elastic and plastic deformation regions based on an asperity contact model. Microgeometry features of tooth surface asperities can be described as the basis of an asperity contact model and allow tooth contact analysis of real tooth surface models with their local microgeometry feature changes due to plastic deformations. Feasibility and effectiveness of this wear prediction method were verified by comparing predicted results of gear surface wear progression with gear wear test results.

Topics: Wear , Stress , Gears , Fractals
Commentary by Dr. Valentin Fuster

### Research Papers: Hydrodynamic Lubrication

J. Tribol. 2018;141(3):031701-031701-9. doi:10.1115/1.4041460.

A rotor supported by gas bearings vibrates within the clearance. If the static imbalance of the rotor is large, even if the rotation speed is low, large amplitude vibration is generated by the centrifugal force. This is a serious problem because the risk of bearing damage increases. In order to solve this problem, an externally pressurized gas journal bearing with asymmetrically arranged gas supply holes has been developed. This type of bearing has a large load capacity as compared with the conventional symmetric gas supply bearing because pressurized gases are supplied to the loaded and counter-loaded side bearing surfaces via asymmetrically arranged gas supply holes. The bearing has a new gas supply mechanism in which gas is supplied from the rotor through inherent orifices. The characteristics of the developed bearing are beneficial from the viewpoint of using the bearing in rotational-type vibration exciters. In other words, this rotor has a large static imbalance. Numerical calculations of the characteristics of this bearing were performed, and the resulting characteristics were compared with those of a conventional symmetric gas supply journal bearing. The bearing load capacity of the developed bearing is considerably larger than that of conventional symmetric type bearings. The load capacity increases owing to the asymmetry of the gas supply holes. In the controlled gas supply pressure condition, rotor radial vibration during rotation can theoretically be zero. A test rig and gas control system to realize vibration reduction was constructed. A rotational test under the gas pressure control condition was conducted using a large unbalanced rotor taking advantage of this property. The control program was constructed using matlab and simulink. The devices were driven by a digital signal processor. The magnitude of the unbalance of the rotor is 13.5 × 10−3 kg m. The bearing diameter and length were 60 and 120 mm, respectively. The rotational vibration amplitude decreased at a high rotational frequency under the proposed bearing configuration, although the amplitude increases monotonically with the frequency in the conventional bearing. When the gas supply pressure was controlled synchronously with the rotation frequency modulation of the large unbalanced rotor, the amplitude of the vibration amplitude was greatly reduced. The rotor of the test rig was safely supported by this bearing, and effective data for practical operation were obtained.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(3):031702-031702-9. doi:10.1115/1.4041511.

The ultrasonic technique is very effective in measuring lubricant film thickness in a noninvasive manner. To estimate the film thickness with reflection signals, two main ultrasonic models are often applied in cases of different film thicknesses; they are the spring model for thin films and the resonant model for thick films. However, when measuring oil film thicknesses distributed in a wide range, there is an inherent blind zone between these two models. This problem is especially prominent in online monitoring because the abrupt variation of film thickness is highly correlated with the occurrence of abnormal conditions. To address this issue, we further proposed a method using the phase spectrum of reflection coefficient which can cover a wide range of film thicknesses. The slight variation of reflection signal in the blind zone can then be identified and bridged the measurement gap between those two traditional models. A calibration rig was used to verify the theoretical analysis and the results indicated that the developed model is capable of providing reliable ultrasonic measurement of lubricant film thicknesses in a wide range.

Commentary by Dr. Valentin Fuster

### Research Papers: Lubricants

J. Tribol. 2018;141(3):031801-031801-6. doi:10.1115/1.4041538.

Grease plays important roles in reducing frictional loss and providing protection of rubbing surfaces. In this research, we investigated the effects of $α$-zirconium phosphate nanoparticles as additives in grease on the galling behavior of a pair of steels (4130 against P530). The results showed that the addition of 0.5 wt% of nanoparticles in petroleum jelly could reduce the friction for 10% and the area being galled for 80%. In terms of particle sizes, the 1 $μm$ sized particles have profound influence in galling reduction. This is due to the increased contribution of van der Waals forces in the stacked layers of those particles. Under shear, those particles are exfoliated, resulting in low friction and more surface coverage to protect surfaces from galling.

Commentary by Dr. Valentin Fuster

### Research Papers: Other (Seals, Manufacturing)

J. Tribol. 2018;141(3):032201-032201-12. doi:10.1115/1.4041461.

A drawback of polymer materials is their low thermal conductivity which affects the operating temperature of polymer gears. The mechanical properties of a polymer gear are critically dependent on the maximum operating temperature. In order to improve thermal behavior of polymer gears, a hybrid polymer gear concept is suggested which consists of a polymer gear tooth with a metallic insert to promote heat evacuation from the meshing surface. The material selection based on finite element method (FEM) simulations showed that an aluminum insert performed better than copper and steel for a hybrid polymer gear. The results show that an aluminum insert increases the mass by 9% in comparison with a standard polymer gear but it decreases the maximum operating temperature by 28%. Insert geometries of different complexity were studied and their influence on operating temperature assessed.

Commentary by Dr. Valentin Fuster