A duplex architecture MoS2/WC-Co coating was developed as a wear protective coating for AISI 52100 steel surfaces in mechanical contacts of fuel-handling systems that operate with low-viscosity hydrocarbon fuels. The duplex coating was prepared by a subsequent application of thermal sprayed WC17 wt% Co (WC-17Co) layer and solution sprayed MoS2 layer. The coating surface morphology and architecture were investigated with scanning electron microscopy and a focused ion beam cross-section of the duplex coating. Ethanol and dodecane were used in wear tests as representatives of fuel surrogates with low viscosity. Reciprocating sliding friction and wear tests were performed in unlubricated, ethanol-lubricated, and dodecane-lubricated conditions in a dry nitrogen atmosphere. Significant differences in tribological behavior were observed between MoS2 coatings in ethanol versus dodecane. In dodecane, the coefficient of friction values were considerably lower for both WC17Co and steel coated with MoS2 compared to uncoated baseline tests. The wear of the MoS2/WC-17Co and bare WC-17Co was much lower than that of bare steel and MoS2/steel when sliding in dodecane. In ethanol, removal of the MoS2 top layer and increased wear of the steel counterbody were observed under small contact loads, which was associated with the formation of abrasive MoO3 particles in the sliding contact. However, when testing in ethanol at higher contact loads, friction decreased substantially and a transfer of MoS2 to the steel counterface was observed and mapped with Raman spectroscopy. A tribological mechanism was suggested that at higher contact pressures the fluid film thickness of ethanol decreased, allowing for an easier transfer of solid lubricant to the sliding counterpart and the subsequent formation of a thick and lubricious tribolayer which remained for the duration of the test. This study highlights the benefits of combining MoS2 and a hard WC-Co underlayer in a duplex architecture for use as a protective coating on steel parts in low-viscosity fuel environments. The study also contributes to the understanding of tribological processes that occur at MoS2/WC-17Co sliding surfaces operated in both an alcohol-based and an alkane-based low viscosity hydrocarbon environment.

Hard nitride and diamond-like carbon (DLC) protective coatings currently used in fuel-lubricated mechanical assemblies will have to accommodate greater temperature, load, and pressure requirements to prevent scuffing wear enabling future engines to operate efficiently with various lower lubricity fuels. In this study, load progression tribological experiments were conducted to investigate the scuffing resistance of three coatings, a chromium nitride (CrN) coating, DLC coating, and DLC coating with a CrN underlayer in decane and ethanol fuel environments. The results were compared to the baseline material, hardened AISI 52100 steel. Multiple characterization techniques, including progressive load scratch testing, optical microscopy, optical profilometry, nanoindentation, scanning electron spectroscopy, and energy dispersive spectroscopy were used to uncover the mechanisms responsible for their performance. The DLC coating delaminated in both fuel environments, exposing the steel substrate underneath which subsequently scuffed in the decane fuel environment. In contrast, the CrN coating remained adhered in both fuel environments, effectively shielding the steel substrate from scuffing and wear. The multi-layer coating, composed of the DLC coating with the CrN underlayer, maintained protection of the steel substrate in both fuel environments while also reducing the friction and wear of the sliding surfaces. In the ethanol environment, however, the DLC layer was worn away, leaving the CrN underlayer to protect the steel substrate. These results indicate that CrN is a promising candidate for protecting fuel-lubricated components. Moreover, incorporating a CrN underlayer with DLC coatings significantly enhances their friction and wear-reducing characteristics, making them suitable for applications involving ethanol-based fuels.

To enhance the durability and reliability of high-pressure fuel system components operating with low-viscosity fuels, an analysis of the tribological performance of potential coating material candidates was conducted. This study focuses on evaluating the friction and wear characteristics of various coatings widely used by industry for surface protection, including carbides, nitrides, diamond-like carbon (DLC), and solid lubricants, under accelerated reciprocating ball on flat conditions intended to provide information on longer term operation in fuel pumps. The tribological performance of these coatings in fuels of varying chemistry and viscosity, including ethanol, decane, dodecane, and aviation fuel (F-24), was compared to the mechanical properties of materials, such as hardness and elastic modulus. Results indicate that carbides show the lowest friction and wear values across different fuel environments. CrN-based coatings demonstrate a decrease in friction and wear in comparison to other nitrides. This comprehensive investigation lays the groundwork for informed design decisions in developing high-performance coatings tailored to withstand the challenges of low-viscosity fuel environments.

Effective thermal insulation materials rely on the fabrication of dense, ultra-high-temperature ceramics that can withstand harsh environments. Hafnium carbide-based ceramics are one of the leading materials that have the potential to perform relatively well in high-temperature oxidizing environments under mechanical loading due to their thermal and mechanical stability. In this study, we explore the effects of processing conditions to create dense, ablation-resistant HfC-SiC composites with a fixed composition of HfC-20 wt.% SiC using a hot-pressing method. Sintering pressure, temperature, and time were varied and the material's relative density, phase composition, morphology, microstructure, and hardness were investigated. Composite ceramics with 99% relative density relative to a theoretical value were created by hot pressing at 2100 & DEG;C for 1 h at 50 MPa and displayed a fine microstructure with an average grain size of & SIM;5 & mu;m and a Vickers hardness of 22.9 & PLUSMN; .8 GPa. The mass loss was determined using an oxyacetylene torch with cross-section investigations of oxide surface formations and subsurface microstructural changes. These HfC-SiC samples developed a 410 nm hafnium oxide layer on the surface upon torch exposure and had an average calculated recession rate of .028 & mu;m/s.

The performance and durability of high-pressure fuel systems in combustion engines are critical for consistent operation under extreme conditions. High-pressure fuel systems are traditionally lubricated with fuel that is compressed and delivered to the combustion chamber. However, lubrication with fuel presents significant challenges in these systems when used with low-viscosity fuels, leading to increased wear rates, especially in reciprocating contacts. This study delved into the tribological performance of steels of varying alloy content (annealed and hardened variants of AISI-52100, CF2, and D2) against alumina and hard 52100 counterbody materials in ethanol and decane environments. Friction and wear behaviors were evaluated, highlighting the influence of material interactions and environmental factors. Elastohydrodynamic lubrication analysis of the tested systems indicated that ethanol and decane form lubricating films of nanometer-scale thickness, confirming the boundary lubrication regimes of the performed tests. In summary, the tribological behavior trends were similar for alumina and 52100 counterbodies. Even though soft 52100 steel demonstrated low friction, its wear was the largest for both tested environments and counterface materials. Among all the tested materials, hard D2 experienced the lowest wear. 52100 and D2 steels showed opposite friction change behavior when comparing hard and soft samples, with lower friction observed for softer 52100 steel and harder D2 steel. Meanwhile, the wear was lower for harder candidates than for softer ones independent of the environment and counterbody material. Raman spectroscopy analysis of the formed wear tracks indicated the formation of carbon films with larger intensities of characteristic carbon peaks observed for more wear-resistant materials. These results suggest the synergistic effect of hardness and tribochemical activity in reducing the wear of materials.

Refractory complex concentrated alloys (RCCAs) are a unique group of materials defined by the shared principality between all alloying elements and the tailorable properties at high temperatures (>1000 degrees C). RCCA systems can be optimized using high-throughput processing which allows for simultaneous characterization and testing over a range of compositions. A compositionally-graded Hf-Al-Si coating was compositionally mapped using scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) to identify relative compositions as a function of position on the wafer. High temperature CALPHAD phase diagrams and thermodynamic predictions for a Hf50Al50-xSix alloy and its (Hf50Al50-xSix)(1-y)O-y oxide were coupled with experimental results. The RCCA wafer was heat treated at 1200 degrees C to observe any phase changes and oxidation behavior at elevated temperatures. Characterization of the RCCA was performed before and after heating using synchrotron radiation X-ray diffraction (SR-XRD) to identify the phases and oxidation states as a function of the compositional gradient and to determine an alloy composition range with the least oxidation. SR-XRD pre- and post-treating revealed an inherent amorphous Hf20Al7Si13 phase across the as-deposited coating, similarities in spectra pre- and post-treating for compositions containing less Hf and more Al and Si relative to the entire gradient film, and the presence of significant HfO2 and Al6Si2O13 oxide for compositions consisting of more Hf and less Al and Si relative to the entire gradient film. The SR-XRD and CALPHAD modeling suggests that compositions of 31-48 at.% Hf, 21-48 at.% Al, and 9-27 at.% Si display the greatest resistance to detrimental oxidation upon heating to 1200 degrees

Inhibiting the tribological failure of mechanical assemblies which rely on fuels for lubrication is an obstacle to maintaining the lifetime of these systems with low-viscosity and low-lubricity fuels. In the present study, a MoVN-Cu nanocomposite coating was tribologically evaluated for durability in high- and low-viscosity fuels as a function of temperature, load, and sliding velocity conditions. The results indicate that the MoVN-Cu coating is effective in decreasing wear and friction relative to an uncoated steel surface. Raman spectroscopy, transmission electron microscopy, and electron-dispersive spectroscopy analysis of the MoVN-Cu worn surfaces confirmed the presence of an amorphous carbon-rich tribofilm which provides easy shearing and low friction during sliding. Further, the characterization of the formed tribofilm revealed the presence of nanoscale copper clusters overlapping with the carbon peak intensities supporting the tribocatalytic origin of the surface protection. The tribological assessment of the MoVN-Cu coating reveals that the coefficient of friction decreased with increasing material wear and initial contact pressure. These findings suggest that MoVN-Cu is a promising protective coating for fuel-lubricated assemblies due to its adaptive ability to replenish lubricious tribofilms from hydrocarbon environments.

Magnetic field-assisted freeze-casting was used to create porous B4C ceramic preforms. An optimum slurry consisted of a mixture of B4C powders and 6-wt

The tribological behavior of MoS2 and WS2 transition metal dichalcogenide (TMD) spray coatings in low-viscosity hydrocarbon environments was investigated. Sliding tests were performed in two hydrocarbons – hydrophilic ethanol and hydrophobic dodecane and were compared to tests in humid air and dry nitrogen conditions. Coating steel surfaces decreased friction across all test conditions, with the largest and most sustainable decreases being in dry nitrogen and dodecane environments, where friction coefficients of less than 0.1 were sustained. Analysis was performed using scanning electron microscopy, Raman spectroscopy, and x-ray photoelectron spectroscopy. Friction reductions in dodecane were explained by reduced oxidation of the coating, due to protection from the hydrophobic hydrocarbon film, allowing basal plane alignment and uninhibited sliding of TMD sheets.

The incorporation of traditional steels in combustion engines poses challenges in adapting to evolving manufacturing designs, repair requirements, and performance demands.However, the emergence of additive manufacturing has provided a promising solution by enabling the integration of new steel compositions for mechanical systems.This study focuses on evaluating the tribological performance of three additively manufactured steels containing 8-20 wt% carbides and carbonitrides dispersed in a steel matrix to determine their suitability for diverse fuel environments, specifically resembling conditions encountered in high-pressure diesel fuel pumps.The selection of these steels was based on their properties.Through a comprehensive tribological analysis, we investigated the behavior of these additively manufactured steel candidates in terms of friction and wear across different fuel environments.Our results unveiled distinct performance variability among the investigated additively manufactured steel candidates, depending on the specific fuel environment.We summarize the additively manufactured candidate steel behavior in ethanol and decane fuel surrogates to support informed decisionmaking regarding implementation of different fuel chemistries in practical scenarios.This research contributes to the ongoing efforts to advance additive manufacturing techniques and novel steel compositions to enhance the adaptability and performance of mechanical systems within fuel systems for combustion engines.

Anisotropic porous boron carbide (B4C) structures were successfully produced, for the first time, using the magnetic field-assisted freeze casting method. The effect of the magnetic field on the structure and mechanical strength of the formed porous B4C was compared for two different magnetic field directions that were either aligned with ice growth (vertical), or perpendicular to the ice growth direction (horizontal). It was shown that applying even a weak horizontal magnetic field of 0.1-0.3 T noticeably affected the alignment of mineral bridges between lamellar walls. Both the porosity and the channel widths decreased with increasing horizontal magnetic field strength. In the case of a vertical magnetic field, a larger strength of 0.4 T was required for highly aligned lamellar walls and larger channel widths. Compression strength tests indicated that the application of magnetic fields led to more homogeneously aligned channels, which resulted in increased compression strength in the longitudinal (parallel to the ice growth) direction. Applying a vertical magnetic field of 0.4 T with a cooling rate of 2 degrees C/min during the freezing step of the magnetic field-assisted freeze-casting method was found to result in the best conditions for producing highly anisotropic structures with large channel widths and fewer mineral bridges, which led to an increase in the mechanical strength.

Amphiphobic surfaces are found in many natural organisms for protection from external contamination because they can repel both water and oil. The development of an amphiphobic surface is based on the construction of hierarchical structures and chemical modification. In this study, thermal imprinting and sputter etching were conducted to create a micropillar array and nano-protrusions to form a hierarchical structure. Different hierarchical structures were developed by using different nickel stamps and upon varying the etching time. These structures were modified using an environment-friendly drysurf (G993G3, Harves) solution through a spraying method. The surface presented high repellency toward various liquid droplets with surface tensions ranging from 72 to 22 mN m−1, with contact angles of 158° for water and 120° for hexadecane. The surface showed good self-cleaning performance without any traces of water droplets or glass bead particles. In addition, various tests including sand abrasion, bending, and immersion tests were conducted to evaluate the mechanical and chemical robustness of the hierarchical structure, which exhibited good performance. Hierarchical structures with these excellent properties are expected to be widely applied in surface waterproofing, antifouling, and self-cleaning treatments.

Al/Al2O3 metal matrix composites (MMCs) were produced by metal infiltration of porous ceramic preforms. The porous ceramic preforms were fabricated using the magnetic field-assisted freeze-casting method, resulting in vertically aligned porous channels. Preforms were prepared by freezing an Al2O3/Fe3O4-containing slurry within an applied magnetic field. Vertical alignment of the channels was facilitated by the magnetic response of the Fe3O4 in the slurry during the freezing process. After freezing and sublimation, the ceramic preforms were sintered and then infiltrated with molten A356 Al-based alloy. The mechanical properties of the resulting Al2O3/A356 MMCs were compared to those of bulk Al2O3, bulk Al-based alloy (A356), and porous Al2O3 preforms using micro-indentation testing. The indentation hardness and elastic moduli values of Al2O3/A356 MMCs showed good agreement with the predicted theoretical calculations. This study provides a new approach for the design of MMCs with controlled composition and improved mechanical characteristics.

This paper provides an overview of the latest research developments in the design and exploration of ceramic coatings with high temperature adaptive behavior. The adaptive behavior, triggered by thermal or thermomechanical stimulus, may be used to create smart surfaces that are able to change their chemistry and structure to achieve the desired functionality. The initial focus of the paper will be to provide an overview on the basics of self-repairing materials. This will be followed by a brief outline of the work that has been reported on self-healing/adaptive mechanisms in bulk ceramics. We will then focus on providing a thorough review on self-healing ceramics with a focus on adaptation/healing in tribology as well as thermal barrier, anticorrosion, and oxidation resistant coatings. Advantages and disadvantages of using hybrid polymer-ceramic coatings will also be discussed toward the end of the article. This overview will provide a fundamental understanding of the changes in the structural and chemical properties of these materials and how that correlates to their performance. This review also includes a discussion on anticipated future developments in this important and upcoming area of research.

Composite coatings whereby soft and lubricious phases were embedded in a hard matrix displayed considerable enhancement in their mechanical and tribological properties that include frictional response, wear resistance, and material toughness. In the current study, composite coatings of a yttria-stabilized zirconia (YSZ) matrix embedded with different contents of AgTaO3 particles (0-30 wt%) were prepared by a sol-gel process and subsequently deposited on Inconel 718 substrates using the spin coating technique. X-ray diffraction (XRD) measurements indicated the presence of YSZ in the cubic phase and of AgTaO3 The tribological properties of the coatings were evaluated at 25 and 650 degrees C against an alumina (Al2O3) ball using a 2 N load. AgTaO3 was found to significantly enhance the coatings' wear and friction properties especially at 650 degrees C. The addition of this high temperature solid lubricant was found to decrease the steady state friction coefficient from 0.65 to 0.18 and the wear rate from 7 x 10(-5) mm(3)/nm to 6 x 10(-6) mm(3)/nm for YSZ-30 wt%AgTaO3. Scanning electron microscopy (SEM) provided information about the surface topography and the elemental composition of the coatings before and after wear testing. SEM images suggested that an increase in AgTaO3 content resulted in the formation of a continuous film of this solid lubricant over an underlying YSZ-based coating. This continuous film provided better wear protection to the composite coating. These observations provide insights useful for developing composite coatings using the sol gel method and that display tunable tribological properties depending on the intended applications.

Surface roughness is an important factor in improving the bone-implant contact area to enhance bone regeneration, yet this aspect has not been applied to absorbable metals. Textured zinc surfaces with varying degrees of surface roughness were produced using a salt-preform method with fine- and coarse-grained salts and compared with a polished control sample. The resulting surfaces were characterized by scanning electron microscopy, surface roughness, corrosion rates, and in vitro cytotoxicity. The resulting textured surfaces exhibit micron-sized cavities and increased roughness consistent with the initial salt particle size. The corrosion rate was shown to accelerate significantly compared with the polished control sample, and pre-osteoblasts displayed healthy morphologies on the textures. The results confirm textured zinc surfaces support cell adhesion and can be used to control the corrosion rate. This study represents an important intermediate step that can be applied to porous absorbable metal scaffolds for bone-implant applications.

Hybrid dual-phase coatings composed of an A356 aluminum alloy modified by plasma electrolytic oxidation (PEO) and burnished with graphite-MoS2-Sb2O3 chameleon solid lubricant powders have been produced. The PEO layer provides high hardness and load support while the solid lubricant powders reduce friction. These hybrid coatings were tribotested against steel and silicon nitride counterparts in air from 25 degrees C to 300 degrees C and using variable contact loads. The open porosity and surface roughness of the PEO layers were reduced considerably by the burnishing process. Polishing of the PEO surfaces prior to burnishing significantly reduced the resulting coefficient of friction (COF) of the hybrid coating. In-situ Raman spectroscopy revealed the chemical stability of the dual phase coatings at temperatures up to 300 degrees C with no signs of oxidation or reduction of the chameleon components. COF values ranged from 0.2 at room temperature down to 0.02 at 300 degrees C. The observed low friction values were attributed to the synergism between PEO and chameleon layers that promote defect healing and adaptive behavior of the coating. Top view scanning electron microscopy (SEM) and cross-sectional transmission electron microscopy (TEM) confirmed that the thermo-mechanical stimulus caused the chameleon coating to fill the voids in the PEO layer. In-situ Raman spectroscopy revealed that the lubricating phases, i.e. MoS2 and graphite, were protected from oxidation by the porous PEO structure. These lubricious phases formed a transfer film in the wear tracks and the counterpart bodies as a result of the contact pressure (up to 1.4 GPa) and thermal energy, which led to an order of magnitude reduction in the COF at high temperatures. The low shear strength of MoS2 and graphite and the good adhesion and integration of the chameleon coating with the PEO sublayer due to high contact pressures during sliding were responsible for the ultra-low friction behavior of the composite coating.

In the present work, the effect of annealing temperature on the microstructure, mechanical and tribological properties of NiCr–WC–Co coatings produced by the high-velocity oxy-fuel (HVOF) technique has been investigated. X-ray diffraction and scanning electron microscopy revealed the dissolution of WC into the NiCr matrix to form W2C and Cr3C2 with the annealing process. This dissolution became complete at 800 °C. The mechanical properties of the coatings were investigated using nano-indentation and Vickers fracture toughness measurements. These measurements suggested that the hardness, Young’s modulus, and fracture toughness values increased because of the newly formed carbide phases as a result of the dissolution of the WC particles. The overall properties of the coatings were found to be optimum for annealing temperatures of 800 °C. The wear mechanism appears to be abrasive in the as-sprayed coating, and it becomes a combination of an abrasive and oxidative wear with increasing the annealing temperature.

The mitigation of CMAS (calcium–magnesium–aluminum–silicon oxide) infiltration is a major requirement for the stability of thermal barrier coatings. In this study, yttria-stabilized zirconia (YSZ)–Al2O3–SiC, YSZ–Al2O3–Ta2O5, and YSZ–Al2O3–Nb2O5 self-healing composites produced by uniaxially pressing powders were investigated as an alternative to YSZ. CMAS infiltration in these materials was tested at 1250 °C for 10 h. Comparing the depth of CMAS infiltration using scanning electron microscope (SEM) in tandem with electron-dispersive X-ray spectroscopy (EDS), all self-healing materials were found to perform better than the reference materials. While standard YSZ shows massive CMAS infiltration, SEM micrographs and EDS maps revealed a 33-fold improvement in CMAS resistance for the YSZ–Al2O3–Nb2O5 system, which exhibited the best performance among the selected self-repairing materials. X-ray diffraction and high-resolution SEM micrographs taken 10 μm below the surface revealed that CMAS only infiltrated pores in the topmost region of the samples. Both YSZ–Al2O3–Ta2O5 and YSZ–Al2O3–Nb2O5 systems showed no signs of chemical reaction with CMAS.