Rachel S. Colbert

Bio: Rachel S. Colbert is an academic researcher from University of Florida. The author has contributed to research in topics: Dry lubricant & Tribometer. The author has an hindex of 4, co-authored 4 publications receiving 122 citations.

A Possible Link Between Macroscopic Wear and Temperature Dependent Friction Behaviors of MoS2 Coatings

Abstract: Studies to explore the nature of friction, and in particular thermally activated friction in macroscopic tribology, have lead to a series of experiments on thin coatings of molybdenum disulfide. Coatings of predominately molybdenum disulfide were selected for these experiments; five different coatings were used: MoS2/Ni, MoS2/Ti, MoS2/Sb2O3, MoS2/C/Sb2O3, and MoS2/Au/Sb2O3. The temperatures were varied over a range from −80 °C to 180 °C. The friction coefficients tended to increase with decreasing temperature. Activation energies were estimated to be between 2 and 10 kJ/mol from data fitting with an Arrhenius function. Subsequent room temperature wear rate measurements of these films under dry nitrogen conditions at ambient temperature demonstrated that the steady-state wear behavior of these coatings varied dramatically over a range of K = 7 × 10−6 to 2 × 10−8 mm3/(Nm). It was further shown that an inverse relationship between wear rate and the sensitivity of friction coefficient with temperature exists. The highest wear-rate coatings showed nearly athermal friction behavior, while the most wear resistant coatings showed thermally activated behavior. Finally, it is hypothesized that thermally activated behavior in macroscopic tribology is reserved for systems with stable interfaces and ultra-low wear, and athermal behavior is characteristic to systems experiencing gross wear.

Uncertainty in Pin-on-Disk Wear Volume Measurements Using Surface Scanning Techniques

Abstract: The uncertainty of wear volumes measured using surface scanning techniques is often neglected or assumed to be equivalent to the instrument error. A method is proposed that accounts for the number of wear volume scans, the variations in those scans, and the geometry of the experimental system as an improved measure of uncertainty. It demonstrates that the uncertainty in volume is directly correlated to the number of scans taken. A non-uniform wear track was used to validate the method, and the minimal and optimal number of scans was found.

Solid Lubrication with MoS2: A Review

Abstract: Molybdenum disulfide (MoS2) is one of the most broadly utilized solid lubricants with a wide range of applications, including but not limited to those in the aerospace/space industry. Here we present a focused review of solid lubrication with MoS2 by highlighting its structure, synthesis, applications and the fundamental mechanisms underlying its lubricative properties, together with a discussion of their environmental and temperature dependence. The review also includes an extensive overview of the structure and tribological properties of doped MoS2, followed by a discussion of potential future research directions.

Edge-dependent structural, electronic and magnetic properties of MoS2 nanoribbons

Abstract: Two-dimensional materials have various applications in next-generation nanodevices because of their easy fabrication and particular properties. In this work, we studied the effects of edge structures on the edge stability, and electronic and magnetic properties of MoS2 nanoribbons by first-principles calculations. We predicted that S-terminated zigzag nanoribbons are the most stable even without hydrogen saturation because of their low and negative edge energies, although hydrogen saturation of the edge states can stabilize other nanoribbons with different edge structures. MoS2 zigzag nanoribbons are metallic and ferromagnetic. Importantly, their conductivity may be semiconducting (n- or p-type) or half metallic by controlling the edge structures saturated with H atoms. The magnetic states of the MoS2 zigzag nanoribbons are enhanced by H-saturation and are much stronger than those of graphene zigzag nanoribbons. The armchair nanoribbons are semiconducting, with bandgaps increased by the hydrogen saturation of their edge states, and are nonmagnetic. These MoS2 nanoribbons with versatile functions may have applications in spintronics, nanodevices, and energy harvesting.

Tuning the Electronic and Magnetic Properties of MoS2 Nanoribbons by Strain Engineering

Abstract: First-principles calculations are carried out to study the effects of strain on the electronic and magnetic properties of MoS2 nanoribbons. We predict that MoS2 nanoribbons are stretchable up to a strain of 10%. The band structures of the nonmagnetic armchair MoS2 nanoribbons change from direct character to indirect with the increase of strain due to the shift of the energy states near the Fermi level. The ferromagnetic states of metallic zigzag MoS2 nanoribbons are greatly improved because the energy difference between the nonmagnetic and magnetic states is increased up to 4.9 times, and the magnetic moments are increased up to 2 times under a strain up to 10%. Our calculations show that the electronic and magnetic properties of MoS2 nanoribbons can be controlled by applying strain, indicating their potential applications to spintronics and photovoltaic cells.

Hard coatings with high temperature adaptive lubrication and contact thermal management: review

Abstract: Progress in the design and exploration of hard coatings with high temperature adaptive behavior in tribological contacts is reviewed. When coupled with most recent surface engineering strategies for high temperature contact thermal management, this progress opens a huge opportunity for adaptive coating applications on machine parts, where oils and coolants are commonly used. The adaptive mechanisms discussed here include metal diffusion and formation of lubricant phases at worn surfaces, thermally- and mechanically-induced phase transitions in hexagonal solids, contact surface tribo-chemical evolutions to form phases with low melting point, formation of easy to shear solid oxides, and others. All of these adaptive mechanisms are combined in nanocomposite coatings with synergistic self-adaptation of surface structure and chemistry to lubricate from ambient temperatures to 1000 °C and provide surface chemical and structural reversibility during temperature cycling to maintain low friction coefficients. The review also highlights emerging surface adaptive concepts, where advances with ab initio modeling of intrinsically layered solids point to new compositions for thermally stable, easy to shear ceramic coatings, load- and temperature-adaptive surfaces with arrays of compliant carbon and boron nitride nanotubes as well as low friction two-dimensional structures. Approaches for self-regulation of coating thermal conductivity, heat flow, and thermal spike mitigations are discussed in the context of surface structure evolution and phase transitions. Future progress is linked to the development of in situ exploration techniques, capable of identifying adaptive surface chemistry and structural evolutions in broad temperature regimes. When combined with predictive modeling, such approaches drastically accelerate adaptive coating developments. The review identifies opportunities, strategies, and challenges for designs and applications of hard coatings with high temperature adaptive lubrication and contact thermal management.

Chameleon Coatings: Adaptive Surfaces to Reduce Friction and Wear in Extreme Environments

Abstract: Adaptive nanocomposite coating materials that automatically and reversibly adjust their surface composition and morphology via multiple mechanisms are a promising development for the reduction of friction and wear over broad ranges of ambient conditions encountered in aerospace applications, such as cycling of temperature and atmospheric composition. Materials selection for these composites is based on extensive study of interactions occurring between solid lubricants and their surroundings, especially with novel in situ surface characterization techniques used to identify adaptive behavior on size scales ranging from 10 −10 to 10 −4 m. Recent insights on operative solid-lubricant mechanisms and their dependency upon the ambient environment are reviewed as a basis for a discussion of the state of the art in solid-lubricant materials.