Hard Coatings with High Temperature Adaptive Lubrication and Contact Thermal Management: Review

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

Embedded Phase Change Material Microinclusions for Thermal Control of Surfaces

The performance and lifetime of sensors, microelectronic devices, mini air vehicle components and other systems can benefit from maintenance of a constant temperature profile or a local temperature that does not exceed a pre-selected, critical value during short-term transient loading events. In this work, the utility of surfaces featuring phase change materials (PCMs) encapsulated within micro-reservoirs was evaluated as a passive thermal management system for mitigation of transient temperature spikes. Patterned silicon substrates with 40 μm diameter, 10 μm deep trenches were prepared by reactive ion etching in an Ar/CF4 plasma. The features were filled with an organic phase change material possessing a high latent heat and known to undergo melting at a target operating temperature of approximately 60 °C. An infrared microscope was used to produce temporally and spatially resolved temperature maps of the surface during heating. When a constant heat flux of approximately 200 W m− 2 was used for uniform heating of the sample area from 50 to 75 °C the PCM encapsulated materials demonstrated an isothermal plateau period lasting 5–8 s near the PCM melting point. A similar plateau was also observed during cooling below the PCM melting point. From the isothermal time during heating the areal thermal energy storage density was estimated to be approximately 800 J m− 2, close to that predicted by the calculated volume of PCM contained within the sample. The effects of PCM inclusions on surface temperature gradients during pulsed laser heating were also investigated. An infrared (980 nm) laser at a fixed power and repetition rate was focused into a 1 mm diameter beam on surfaces with and without PCM encapsulation for rapid heating (analogous to heating from resistive contacts in microelectronic circuits) of different duration from 0.5 to 5.0 s. The surface temperature stayed below the desired temperature limit for 100 mW laser pulses lasting up to 2 s in duration. Transient temperature profiles of Si/PCM surfaces showed that micro-scale volumes of embedded phase change material stored sufficient quantities of heat to stabilize and otherwise control surface temperatures for potentially useful periods of time in selected applications

In-Situ Techniques to Understand Changes in Surface Chemistry During Ablation

Ablation is an effective and reliable method largely used in aerospace structures to protect the payload from damaging effects of external high temperatures. Substantial research is required to develop basic knowledge that is required to characterize the response of a high temperature thermal protection system to extreme hypersonic environment. This presentation will provide an overview of experimental techniques that are currently being used to understand the degradation behavior of composite materials used for thermal protection. Advantages and disadvantages of each method will be discussed. In addition, novel in situ quantitative methods of material degradation during high temperature ablation events will be identified. Specific techniques developed by the authors for hypersonic applications will be discussed. For example, in situ Raman spectroscopy during high temperature wear testing of chameleon coatings was employed by the lead author to correlate surface chemistry to measured changes in friction coefficients simultaneously. Chameleon coatings are adaptive coatings that reduce friction coefficient from 25 to 1000 °C for moving assemblies for next-generation hypersonic aircraft and missiles

In situ Raman Spectroscopy for Examination of High Temperature Tribological Processes

Raman spectroscopy of solid lubricant coatings during high temperature (300–700 °C) wear testing was employed for real-time correlation of sliding contact surface chemistry to the measured friction coefficient. Two coatings were investigated in this work – MoS2and VN-Ag. Immediately prior to failure of the MoS2 coating at 350 °C, molybdenum trioxide was detected in the wear track, and an increase in friction coefficient and ultimate failure of the coating was associated with buildup of the abrasive oxide compound. For the VN-Ag nanocomposite coating, in situ Raman analysis of the contact surface during heating revealed the appearance of silver vanadate compounds at a temperature of 375 °C. At higher temperatures, competitive evolution of different silver vanadate phases (i.e., Ag3VO4, AgVO3) was observed. For the conditions examined in this work, the wear process at 700 °C inhibited formation of AgVO3 in the sliding contact, as determined by comparison of the composition of the wear track to that of the adjacent, unworn coating surface. Additionally, the composition of the wear track was significantly different after the sample had cooled sufficiently to allow handling for post-test surface characterization with conventional Raman, XRD, and SEM techniques, further illustrating the utility of in situ diagnostics for identification of active lubricant phases during wear tests. This ability to characterize surfaces during wear tests at elevated temperatures fills an important gap left by current in situ tribology techniques that are currently used to provide insight on mechanisms governing the performance of solid lubricant film materials

Lubricious Oxide Coatings for Extreme Temperature Applications: A Review

This article provides an overview of the latest research developments on binary and ternary oxide coatings that have the potential to be used as solid lubricants at elevated temperatures. The review focuses on understanding the major mechanisms that lead to a reduction in friction and/or wear in high temperature lubricious oxides. Changes in the structural, chemical, and electronic properties of these oxides as a function of temperature will be correlated to their mechanical and tribological performance using a range of experimental tools in addition to modeling based on ab initio calculations and molecular dynamics simulation methods. This review also includes a discussion of the industrial applications of these coatings as well as of potential improvements to the coating design and other anticipated future developments

Progress in the Development of Adaptive Nitride-Based Coatings for High Temperature Tribological Applications

Adaptive tribological coatings were recently developed as a new class of smart materials that were designed to adjust their surface chemical composition and structure as a function of changes in the working environment to minimize friction coefficient and wear between contact surfaces. This paper provides an overview of the current research developments in this field, including: (1) Chameleon nanocomposite coatings which are produced by depositing a multi-phase structure whereby some of the phases provide mechanical strength and others are lubricious; (2) Micro- and nano-textured coatings which consist of hard nitride films with highly ordered micropores and nanopores that are subsequently filled with solid lubricants using various techniques such as lithography, reactive ion etching, laser texturing, pulsed air arc treatment, and ceramic beads as placeholders for sputter deposition; and, (3) Carbon and nitride nanotubes that are filled electrochemically with solid lubricants. The frictional and wear properties of the above three classes of newly developed adaptive structures, tested in various controlled environmental conditions (temperature, humidity), will be discussed in detail

Chemical basis of the tribological properties of AgTaO3 crystal surfaces

The chemical properties of a surface determine the friction and wear behavior of a material during sliding. In this article, we study the mechanisms underlying the sliding behavior of the AgTaO3 perovskite material, a promising high-temperature solid lubricant that presents excellent friction properties and is chemically inert. In particular, by employing a combination of molecular dynamics simulations and density-functional theory calculations, we show that the low friction of AgTaO3 at high temperature is explained by silver aggregation on the surface, which is enabled by the low energy barriers associated with silver migration. Two different surface terminations (AgO and TaO2) are studied, and we show that the migration barrier on the AgO surface is smaller, favoring silver aggregation, which affects both friction and wear. Regardless of the termination, the formation of soft silver clusters dominates the sliding behavior when enough energy (mechanical or thermal) is imparted to the surface.Peer reviewed: YesNRC publication: Ye

Tribological Investigation of Adaptive Mo2N/MoS2/Ag Coatings with High Sulfur Content

Adaptive nanocomposite Mo2N/MoS2/Ag coatings were deposited on Inconel and silicon substrates by magnetron sputtering with individual targets of Mo, MoS2 and Ag. The tetragonal β-Mo2N structure in addition to Ag and MoS2 phases were detected using X-ray diffraction. The elemental composition of the coatings was investigated using Auger electron spectroscopy. The tribological properties of the coatings were studied at room temperature (RT), 350, and 600 °C against Si3N4 balls. The lowest friction coefficients that were obtained were 0.4, 0.3, and 0.1 at RT, 350 °C, and 600 °C, respectively. The average friction coefficient was maintained at 0.1 for more than 300,000 cycles at 600 °C due to the formation of lubricious silver molybdate phases at the contact surfaces. Three types of silver molybdate phases were detected by both X-ray diffraction and micro-Raman spectroscopy in the wear tracks, namely, Ag2Mo4O13, Ag2Mo2O7 and Ag2MoO4depending on the Mo and Ag contents in the coatings. The superior performance of all three compounds is due to their layered structure with weaker Ag–O bridging bonds. These relatively weak bonds may shear or even break easily at high temperatures to account for the observed friction reduction

Adaptive NbN/Ag Coatings for High Temperature Tribological Applications

Nanocomposite films that consist of niobium nitride with silver nanoinclusions were created using unbalanced magnetron sputtering to investigate their potential as adaptive, friction reducing coatings. The coatings were tribotested against a Si3N4counterface in the 25 to 1000 °C temperature range. The coatings displayed coefficients of friction in the 0.15 to 0.30 range at T \u3e 700 °C. Post-wear testing structural and chemical characterization revealed that, in the low to mid-temperature range, silver migrated to the surface to reduce friction. At higher temperatures, oxygen, silver and the transition metal reacted to form lubricious binary metal oxide phases (silver niobate) in addition to pure silver. In situ Raman spectroscopy measurements were taken during heating and wear testing at 750 °C to identify the evolution of phases in the coatings surface and in the wear track. The analysis of the in situ Raman spectroscopy data revealed the various stages of formation of these binary metal oxides. The coatings were subsequently doped with MoS2 to investigate the effect of the introduction of a low temperature lubricant. The addition of MoS2 did not appreciably reduce the room temperature coefficient of friction, likely due to the miscibility of this compound with the transition metal nitride. However, the coefficient of friction was significantly reduced at high temperatures because of the synergistic lubricious effect of silver niobates and molybdates