Ryan D. Evans

Transmission Electron Microscopy of Boundary-Lubricated Bearing Surfaces. Part II: Mineral Oil Lubricant with Sulfur-and Phosphorus-Containing Gear Oil Additives

Transmission electron microscopy (TEM) was performed on cross-sectional samples of tapered roller bearing cone surfaces that were tested at two levels of local boundary lubrication severity, Λ ∼ 1.1 and 0.3. Unlike our previously reported work in which a base mineral oil was used, the bearing tests were conducted in mineral oil with sulfur- and phosphorus-containing gear oil additives. Structural and compositional characterization of undetached antiwear surface layers on the base steel (cone raceway) revealed that the films contained crystalline and amorphous regions. A sharp interface (<∼10 nm) that separated the surface layer and base steel was imaged. The surface layer for the cone tested at Λ ∼ 1.1 consisted of Fe, O, and P, whereas that for the cone tested at Λ ∼ 0.3 consisted of Fe, O, P, C, Ca, and S. Various TEM analytical techniques were used to study the segregation of these elements throughout the antiwear surface layer volume.

Microstructural Alterations in Bearing Steels under Rolling Contact Fatigue Part 1—Historical Overview

Microstructural alterations in bearing steels during rolling contact cycling have been reported in the literature for more than half a century. These structural changes are primarily caused by the decay of parent martensite and have been designated as white and dark etching regions due to their preferential etching characteristics. One of the most striking features of the white etching bands is their repeatable directionality, which has puzzled investigators for decades. Despite numerous attempts, a satisfactory explanation for the orientation of these bands is still not available. In this article (Part 1), an overview of the phenomenon is presented with detailed discussion of various experimental observations from the literature. The article also examines the previous approaches adopted to explain white etching bands and address their limitations. In Part II of the article, a J2-based elastic–plastic finite element model coupled with a carbon diffusion model is developed that directionally predicts the occurrence and orientation of the white etching bands.

The effects of structure, composition, and chemical bonding on the mechanical properties of Si-aC:H thin films

Abstract The objective of this study was to correlate mechanical properties with the structure and chemistry of silicon-incorporated amorphous hydrocarbon (Si-aC:H) films deposited by reactive sputtering. Hardness and elastic modulus were measured via microindentation, and intrinsic compressive stress was determined from radius-of-curvature measurements using surface mapping microscopy. Film chemistry was investigated with X-ray photoelectron spectroscopy, electron energy-loss spectroscopy, Raman spectroscopy, and attenuated total reflection Fourier transform infrared spectroscopy. Conventional- and high-resolution transmission electron microscopy revealed that the Si-aC:H phase is amorphous and TiC exists at the Si-aC:H/Ti phase boundary. Mechanical properties such as hardness, modulus, and intrinsic stress decreased with increasing Si and H content in the films, for Si/C⩾0.04. Measurements show that this is most likely due to a decrease in Cue5f8C sp3 bonds, accompanied by an increase in Cue5f8Si and Cue5f8H bonds. In addition, the Si-aC:H film with Si/C=0.04 is fundamentally different from the other Si-aC:H films with higher Si and H contents. It is concluded that a film with diamond-like carbon characteristics can be deposited using a low tetramethyl silane (TMS) flow rate, such that Si/C=0.04 in the film. However, films deposited with higher TMS flow rates (such that Si/C⩾0.06 in the films) are more characteristic of amorphous hydrogenated silicon carbide.

Influence of Plasticity-Induced Residual Stresses on Rolling Contact Fatigue

Preloading the rolling elements of a rolling element bearing beyond their yield limit will result in subsurface plastic strains in the deformed material. These plastic strains are manifested in the form of a three-dimensional state of residual stresses. In this analysis, a two-dimensional plane–strain J2-based plasticity model with two different hardening laws—that is, (1) linear kinematic hardening and (2) nonlinear kinematic hardening—is utilized. Using the J2 plasticity model, the effects of hardening laws, material properties, and residual stress on rolling contact fatigue are investigated. Due to the presence of initial residual stresses, the equivalent von Mises stress decreases, which consequently leads to improved rolling contact fatigue life. However, due the presence of the residual stresses, the local yield point of the material also decreases. Due to these competing mechanisms, there is an optimum level up to which beneficial effects of the residual stresses are observed. For each of the hardening models, three different yield limits typical of bearing materials are studied and the effect of residual stress is evaluated. It is observed that the optimum pattern of residual stress is a function of the hardening response of the material, yield limit, and service load of operation. Based on the numerical results, a generalized equation for both of the hardening laws is proposed in order to estimate the optimum preload required to achieve maximum enhancement in rolling contact fatigue life.

Microstructural Alterations in Bearing Steels under Rolling Contact Fatigue: Part 2—Diffusion-Based Modeling Approach

The microstructure of rolling element bearings can experience significant transformation when subjected to repetitive contact cycling. In Part 1 of this article, a detailed overview of the known microstructure alteration phenomena was presented and the different mechanisms for describing them proposed by various investigators were critically reviewed. It is agreed generally that the primary path of these structural changes is the decay of martensite due to carbon diffusion, leading to the formation of ferrite and lenticular carbides. Furthermore, these altered regions in the material microstructure can be stress concentration regions leading to crack initiation and propagation. In this article, a J2-based elastic–plastic Voronoi finite element model (previously developed by the authors) is coupled with a carbon diffusion–based model. Using Ficks law for stress-assisted diffusion, the dispersion of carbon in the bearing microstructure is evaluated. A backward Euler finite difference scheme is employed to solve the partial differential equations. The model can accurately predict the onset of martensitic decay and formation of the white etching bands along with their distinctive orientation. A comparison of the numerical results shows good corroboration with experimental observations.

Radial Distribution Function Analyses of Amorphous Carbon Films Containing Silicon and Hydrogen by Energy-Filtered Diffraction and EXELFS

Short-range order in amorphous materials is most commonly characterized with the use of radial distribution functions (RDFs). Two analytical electron microscopy methods were used in this study to measure RDFs from amorphous carbon films containing different levels of silicon and hydrogen (Si-aC:H): energy-filtered convergent-beam electron diffraction (EFCBED) and extended electron energy-loss fine structure (EXELFS) analyses. The films were deposited in an industrial-scale system onto a thin adhesive titanium layer on silicon substrates by reactive sputtering of carbon with a feed gas of tetramethyl silane (TMS) and argon, to produce a series of films with different Si and H contents (Si/C = 0, 0.04, 0.10, and 0.18).