Patrick S. M. Dougherty

Orientational order controls crystalline and amorphous thermal transport in superatomic crystals

In the search for rationally assembled functional materials, superatomic crystals (SACs) have recently emerged as a unique class of compounds that combine programmable nanoscale building blocks and atomic precision. As such, they bridge traditional semiconductors, molecular solids, and nanocrystal arrays by combining their most attractive features. Here, we report the first study of thermal transport in SACs, a critical step towards their deployment as electronic, thermoelectric, and phononic materials. Using frequency domain thermoreflectance (FDTR), we measure thermal conductivity in two series of SACs: the unary compounds Co6E8(PEt3)6 (E = S, Se, Te) and the binary compounds [Co6E8(PEt3)6][C60]2. We find that phonons that emerge from the periodicity of the superstructures contribute to thermal transport. We also demonstrate a transformation from amorphous to crystalline thermal transport behaviour through manipulation of the vibrational landscape and orientational order of the superatoms. The structural control of orientational order enabled by the atomic precision of SACs expands the conceptual design space for thermal science.

Lubrication Enhancement for UHMWPE Sliding Contacts through Surface Texturing

Considered a self-lubricating, abrasion-resistant biomaterial, ultra-high-molecular-weight polyethylene (UHMWPE) has become the most common acetabular cup material used in artificial hip joints. Artificial hip joints exist primarily in the boundary and mixed regimes, which can lead to premature failure due to surface wear, debris generation, and subsequent osteolysis. The tendency of failure due to the generation of wear debris has turned the hip problem into a complex tribological challenge with a focus on abrasion-resistant surfaces. Surface texturing in the form of microdimples has been shown in other applications to provide enhanced wear resistance through the trapping of wear debris and through the enhancement of fluid pressures to levels that promote mixed- to full-film lubrication even at low speeds. The objective of this investigation is to explore the potential for surface texturing of UHMWPE sliding contacts in enhancing the lubrication regimes present in total hip replacements. Surface textures were created using a mechanical micromachining process, and experiments were conducted on a pin-on-disk tribometer under lubricated conditions present in the hip for textured and untextured samples. Results show that the surface textures were able to reduce friction by up to 50% over that observed with untextured UHMWPE samples, with evidence that suggests a regime shift from boundary lubrication to mixed and full-film lubrication as the primary mechanism for friction reduction.

On the Role of Bit Cutter-on-Rock Tribometry to Aid the Drilling Process for New Energy Resources

In this study, techniques for studying the Tribology of rock cutting are being developed using bit cutter-on-rock tribometry (BCORT). Tribo-testing is carried out on a slider pad-on-disk tribometer, retrofitted to include water-jet fabricated rock disks, a variable radius cutter assembly, and a system for the capture of in situ depth of cut. Preliminary findings for BCORT testing of O1 tool steel on Carthage marble are presented, in addition to verification of depth of cut measurements with a scanning white light optical interferometer. The interdependence of interfacial cutting friction, depth of cut rate, and disk speed are discussed in conjunction with experiments.Copyright

Adaptive Solid Lubricant Transfer Films for Conductivity and Oxidation Control

The success of solid lubricants to exhibit ultra-low friction and wear behaviors in oil-prohibiting environments, has led to a major effort to optimize their performance and enhance their applicability. Depending on the operating conditions, solid lubricants may take on a plethora of forms including fabricated composite coatings, thick-film powder lubricants, nano-particle additives for hard surfaces or liquid lubricants, and self-replenishing transfer films. One of the benefits of transfer films are their freedom from the complex deposition techniques required for most other solid lubricant systems. In this work, the potential for adaptive self-replenishing transfer films was explored by creating composite powders of well-known powder lubricants and electrically conductive or anti-oxidation materials. MoS2, WS2, and Graphite Powders were mixed in varying composition with Cu, Sb2O3, and BO3 additives and compacted to form “tuned” or adaptive powder pellets. Relationships between friction, wear, electrical resistance, tribo-induced oxidation and powder composition, will be presented in order to investigate the potential of composite property optimization for lubricious, highly conductive, and oxidation resistant transfer films.Copyright

The Influence of W-DLC and CrxN Thin Film Coatings on Impact Damage between Bearing Materials

ABSTRACT In this work, the effect of hard tribological coatings was studied in terms of mitigating impact damage between tungsten carbide spherical elements and two different types of steel substrates. The coatings included a hard, highly elastic Tungsten-incorporated diamond-like carbon (W-DLC) coating at two different thicknesses and a harder, less elastic CrxN coating. Impacts were created using a drop-test rig described herein and characterized in three ways: a measure of the coefficient of restitution during impact, investigation of the impact site using an optical interferometer, and fixed ion beam cross sections of select impacts for observation of subsurface damage within the coating and substrate. It was found that hard coatings on softer substrates such as 440C steel were able to mitigate surface damage up to a certain impact speed, depending on the coating, but were unable to influence the coefficient of restitution. On harder substrates like 52100 alloy steel, the coatings were found to increase the coefficient of restitution, indicating a reduction in energy loss due to plastic deformation, and to reduce damage at each tested speed. These effects and their potential influence on bearing performance are discussed in regard to impact mechanics, surface metrology, and the material properties of the coating and substrate acquired by nanoindentation.

Single Particle Interaction Properties: Investigations on the Coefficient of Restitution and Coefficient of Friction

Understanding granular flows has always been important for predicting natural phenomena such as rockslides and soil erosion, as well as industrial processes such as coal-based fossil fuel systems and solids processing. As such, it becomes important to understand granular flows from both a classical granular flow and tribological perspective. Inherently important in the study of granular flows is the study of the individual particle level interactions, which define the global behavior of the flow. The current work examines both the coefficient of restitution (COR) and coefficient of friction (COF) for various material combinations. COR and tribological experiments are performed on various sphere and plate (disk) materials, such as low carbon steel, tungsten carbide (WC), and NITINOL 60.Copyright

An Investigation of Wear Mechanisms for Transfer Films in an In Situ Self-Replenishing Tribosystem

While continuum-based approximations have had success modeling tribosystems with thick film powder flows, they do not predict the behavior of thin powder transfer films which can be worn away from the asperity region during mixed lubrication. This is especially the case for in situ self-replenishing transfer films in the form of compacted solids which are set up to undergo deposition and depletion simultaneously. Although control volume models have been developed to describe this type of tribosystem, very little is known about the wear mechanisms that govern the transfer film in sliding contact. The problem is highlighted through the sudden breakdown of the lubricant film which inevitably leads to interfacial starvation. This study examines evidence that will lead to a fundamental description of the deposition and depletion of powder transfer films by leveraging abrasive wear theory. Justification for this theory is provided by examining the relationship between the coefficient of friction (COF), wear rate, and atomic force microscope (AFM) images of wear interfaces; so that a more accurate model of an in situ self-replenishing solid lubrication process may be constructed.Copyright