Particle Size Effects of Silicon Carbide on Wear Behavior of SiC<Subscript>p</Subscript>-Reinforced Magnesium Matrix Composites

This article investigated the particle size effect of micro-sized SiC on the tribological behavior of SiC_p-reinforced AZ91D Mg-based metal–matrix composites (MMCs). The Mg MMCs were prepared by the melt-stirring technique for wear tests. The hardness and coefficient of friction of Mg MMCs increases as particle size of SiC particle in MMCs increases, except for the hardness tendency at the region between particle size of 11 and 15 μm. The SiC_p/AZ91D MMCs exhibit superior wear resistance under lower and moderate sliding condition. However, the effects of the SiC particle reinforcements on wear resistance are not as conclusive under severe sliding condition, since the matrix of MMCs were softened at elevated temperature under such a severe condition.     

Nanoscratch Characterization of GaN Epilayers on c- and a-Axis Sapphire Substrates

In this study, we used metal organic chemical vapor deposition to form gallium nitride (GaN) epilayers on c- and a-axis sapphire substrates and then used the nanoscratch technique and atomic force microscopy (AFM) to determine the nanotribological behavior and deformation characteristics of the GaN epilayers, respectively. The AFM morphological studies revealed that pile-up phenomena occurred on both sides of the scratches formed on the GaN epilayers. It is suggested that cracking dominates in the case of GaN epilayers while ploughing during the process of scratching; the appearances of the scratched surfaces were significantly different for the GaN epilayers on the c- and a-axis sapphire substrates. In addition, compared to the c-axis substrate, we obtained higher values of the coefficient of friction (μ) and deeper penetration of the scratches on the GaN a-axis sapphire sample when we set the ramped force at 4,000 μN. This discrepancy suggests that GaN epilayers grown on c-axis sapphire have higher shear resistances than those formed on a-axis sapphire. The occurrence of pile-up events indicates that the generation and motion of individual dislocation, which we measured under the sites of critical brittle transitions of the scratch track, resulted in ductile and/or brittle properties as a result of the deformed and strain-hardened lattice structure.     

Influence of pressure dependence of the yield criterion on the strain-rate-intensity factor

The strain-rate-intensity factor is the coefficient of the principal singular term in an expansion of the equivalent strain rate in a series in the vicinity of maximum-friction surfaces. Its value controls the physical processes in a thin material layer near the frictional interfaces. The objective of the present paper is to study the effects of the pressure dependence of the yield criterion on the strain-rate-intensity factor. The results can be used to develop new methods for predicting the evolution of material properties in the vicinity of surfaces with high friction in metal-forming processes.     

Piezoviscous Effects in Nonconformal Contacts Lubricated Hydrodynamically

The analysis in this Paper is concerned with the piezoviscous-rigid regime of lubrication for the general case of elliptical contacts. In this regime, several, formulas of the lubricant film thickness have been proposed. However, either the load parameter W is not included, which has a strong effect on film thickness, or the film thickness is overestimated by using the Barus formula for pressure viscosity characteristics. In the current study, the Roelands formula has been used for the pressure-viscosity relationship. The effects of the dimensionless load, speed, and materials parameters, the radius ratio, and the lubricant entrainment direction have been investigated. Forty-one cases were used in obtaining the minimum film thickness formula: H₀ = 178G^0.386U^1.266W^?0.880 (1 ? e^?0.0387α Contour Plots indicate in detail the pressure developed between the contacting solids.     

A Molecular Dynamics Study of Lubrication Rheology of Polymer Fluids

This study investigated the characteristics of lubrication rheology for polymer fluids using the molecular dynamics method. The confining potential and FENE potential were used to describe the bonding force in a polymer chain. Both Poiseuille and Couette flows were simulated and the results are presented for the density distribution, velocity profiles, slip ratio, viscosity, and relation between shear stress and shear rate. The effects of the number of molecular layers and film thickness on the rheology characteristics were also investigated.     

A Legendre collocation method for thermohydrodynamic journal-bearing problems with Elrod's cavitation algorithm

A Legendre collocation method is proposed for journal-bearing problems. The model combines the Legendre collocation method, the bulk-flow model and Elrod's cavitation algorithm. The Legendre collocation method is used to solve the momentum equations and energy equation to determine the velocity and temperature distributions while the pressure is determined by solving the bulk-flow equations using the SIMPLER scheme. This model can analyze a wide range of problems from laminar to turbulent flow with or without thermohydrodynamic (THD) effects, and resolve the detailed flow structure such as the temperature distribution near the wall region in the turbulent THD case.     

Tribological Performance of Oil-Based Lubricants with Carbon-Fe Nanocapsules Additive

The present study investigates the tribological properties of carbon-Fe nanocapsules (CFNCs) under high contact loads. Block-on-ring wear tests are performed using mineral oil lubricants containing CFNC particles with concentrations ranging from 0.01 to 0.1 wt%. In addition, high-resolution electron transmission microscopy (HR-TEM) and scanning electron microscopy (SEM) analysis were conducted on the test samples. The results show that for a contact load of 650 N, a CFNC concentration of 0.07 wt% and a sliding velocity of 1.65 m/s enhance the surface permeability, fill-up properties, and microbearing lubrication mechanism and promote effective reduction in the wear at the surface contact interface.     

Enlargement of a circular hole in a disc of plastically compressible material

A semi-analytic solution for the elastic/plastic distribution stress and strain in a thin annular disc subject to pressure over its inner radius is presented. It is assumed that a pressure-dependent yield criterion and its associated flow rule are valid in the plastic zone. Thus, the material is plastically compressible, which is a distinguished feature of the solution. Also, in contrast to most studies on elastic/plastic deformation of thin plates and discs under plane stress conditions, the flow theory of plasticity is adopted in conjunction with a smooth yield surface. Numerical methods are only necessary to evaluate ordinary integrals and to solve simple transcendental equations. It is shown that the stress path is not proportional and, therefore, the application of deformation theories of plasticity widely used to calculate the distribution of stresses and strains in thin plates and discs is not justified.     

Theoretical and Experimental Study of a Thermal Contact Conductance Model for Elastic,Elastoplastic and Plastic Deformation of Rough Surfaces

This study developed a thermal contact conductance model that takes into account surface asperities with elastic, elastoplastic and plastic deformation. The surface asperity model considers the continuity and smoothness of variables across different modes of deformation. Experiments were also conducted to measure thermal contact conductance for samples with different surface roughness when the contact pressure was increased or decreased. The trend of the test data supports the theoretical results.     

Theoretical Analysis of Piston-Ring Lubrication Part I—Fully Flooded Lubrication

A one-dimensional analysis for lubrication between the piston ring and cylinder wall has been developed. A fully flooded inlet condition and axisymmetric geometry are considered. The piston ring is treated as a reciprocating, dynamically-loaded bearing with combined sliding and squeeze motion. A system of two nonlinear differential equations is used to model the lubrication including the Reynolds cavitation boundary condition. A numerical procedure is then developed to obtain the cyclic variations of film thickness, frictional force, power loss, and oil flow across the ring.

Results are presented for a typical automotive engine. The effects of ring profile, ring tension, and engine speed are examined. It is shown that this analysis can be used to study the influence of ring design parameters in order to improve the design of the ring pack in reciprocating engines.