Recent work suggests that thermally stable nanocrystallinity in metals is achievable in several binary alloys by modifying grain boundary energies via solute segregation. The remarkable thermal ...stability of these alloys has been demonstrated in recent reports, with many alloys exhibiting negligible grain growth during prolonged exposure to near‐melting temperatures. Pt–Au, a proposed stable alloy consisting of two noble metals, is shown to exhibit extraordinary resistance to wear. Ultralow wear rates, less than a monolayer of material removed per sliding pass, are measured for Pt–Au thin films at a maximum Hertz contact stress of up to 1.1 GPa. This is the first instance of an all‐metallic material exhibiting a specific wear rate on the order of 10−9 mm3 N−1 m−1, comparable to diamond‐like carbon (DLC) and sapphire. Remarkably, the wear rate of sapphire and silicon nitride probes used in wear experiments are either higher or comparable to that of the Pt–Au alloy, despite the substantially higher hardness of the ceramic probe materials. High‐resolution microscopy shows negligible surface microstructural evolution in the wear tracks after 100k sliding passes. Mitigation of fatigue‐driven delamination enables a transition to wear by atomic attrition, a regime previously limited to highly wear‐resistant materials such as DLC.
A stable nanocrystalline alloy of Pt and Au is shown to be extremely resistant to mechanical abrasion and fatigue, having volumetric or specific wear rates comparable to diamond‐like carbon. This is the first report of a metal having such wear resistance.
Molybdenum disulfide (MoS2) is a 2D material widely used as a dry lubricant. However, exposure to water and oxygen is known to reduce its effectiveness, and therefore an understanding of the uptake ...of water is important information for mitigating these effects. Here we use grand canonical Monte Carlo simulations to rigorously study water adsorption on MoS2 surfaces and edges with different concentrations of defects under realistic atmospheric conditions (i.e. various temperatures and humidity levels). We find that the amount of water adsorbed depends strongly on the number of defects. Simulations indicate that defect sites are generally saturated with water even at low ppm levels of humidity. Water binds strongly to S vacancies on interlamellar surfaces, but generally only one water molecule can fit on each of these sites. Defects on surfaces or edges of lamellae also strongly attract water molecules that then nucleate small clusters of water bonded via hydrogen bonding. We demonstrate that water preferentially binds to surface defects, but once those are saturated at a critical humidity level of about 500–1000 ppm water, water binds to edge sites where it negatively impacts the tribological performance of MoS2.
Molybdenum disulfide (MoS
) is a 2D material widely used as a dry lubricant. However, exposure to water and oxygen is known to reduce its effectiveness, and therefore an understanding of the uptake ...of water is important information for mitigating these effects. Here we use grand canonical Monte Carlo simulations to rigorously study water adsorption on MoS
surfaces and edges with different concentrations of defects under realistic atmospheric conditions (
various temperatures and humidity levels). We find that the amount of water adsorbed depends strongly on the number of defects. Simulations indicate that defect sites are generally saturated with water even at low ppm levels of humidity. Water binds strongly to S vacancies on interlamellar surfaces, but generally only one water molecule can fit on each of these sites. Defects on surfaces or edges of lamellae also strongly attract water molecules that then nucleate small clusters of water bonded
hydrogen bonding. We demonstrate that water preferentially binds to surface defects, but once those are saturated at a critical humidity level of about 500-1000 ppm water, water binds to edge sites where it negatively impacts the tribological performance of MoS
.
Laser beam directed energy deposition has become an increasingly popular advanced manufacturing technique for materials discovery as a result of the in situ alloying capability. In this study, we ...leverage an additive manufacturing enabled high throughput materials discovery approach to explore the composition space of a graded Wx(CoCrFeMnNi)100−x sample spanning 0 ≤ x ≤ 21 at%. In addition to microstructural and mechanical characterization, synchrotron high speed x-ray computer aided tomography was conducted on a W20(CoCrFeMnNi)80 composition to visualize melting dynamics, powder-laser interactions, and remelting effects of previously consolidated material. Results reveal the formation of the Fe7W6 intermetallic phase at W concentrations> 6 at%, despite the high configurational entropy. Unincorporated W particles also occurred at W concentrations> 10 at% accompanied by a dissolution band of Fe7W6 at the W/matrix interface and hardness values greater than 400 HV. The primary strengthening mechanism is attributed to the reinforcement of the Fe7W6 and W phases as a metal matrix composite. The in situ high speed x-ray imaging during remelting showed that an additional laser pass did not promote further mixing of the Fe7W6 or W phases suggesting that, despite the dissolution of the W into the Fe7W6 phase being thermodynamically favored, it is kinetically limited by the thickness/diffusivity of the intermetallic phase, and the rapid solidification of the laser-based process.
•Additive Manufacturing of Wx(CoCrFeMnNi)100−x High-Entropy Alloys with site-specific microstructure-properties relationships.•In situ imaging of melt pool dynamics during additive manufacturing via high speed synchrotron x-ray tomography.•Comparison of phase stability and mechanical performance with established theories/models for High-Entropy Alloys.
We report an investigation of the friction mechanisms of MoS
2
thin films under changing environments and contact conditions using a variety of computational and experimental techniques. Molecular ...dynamics simulations were used to study the effects of water and molecular oxygen on friction and bonding of MoS
2
lamellae during initial sliding. Characterization via photoelectron emission microscopy (PEEM) and Kelvin probe force microscopy (KPFM) were used to determine work function changes in shear modified material within the top few nanometers of MoS
2
wear scars. The work function was shown to change with contact conditions and environment, and shown by density functional theory (DFT) calculations and literature reports to be correlated with lamellae size and thickness of the basally oriented surface layer. Results from nanoscale simulations and macroscale experiments suggest that the evolution of the friction behavior of MoS
2
is linked primarily to the formation or inhibition of a basally oriented, molecularly thin surface film with long-range order.
Pure molybdenum disulfide (MoS
2
) solid lubricant coatings could attain densities comparable to doped films (and the associated benefits to wear rate and environmental stability) through ...manipulation of the microstructure via deposition parameters. Unfortunately, pure films can exhibit highly variable microstructures and mechanical properties due to processes that are not controlled during deposition (i.e., batch-to-batch variation). This work focuses on developing a relationship between density, hardness, friction, and wear for pure sputtered MoS
2
coatings. Results show that dense films (
ρ
= 4.5 g/cm
3
) exhibit a 100 × lower wear rate compared to porous coatings (
ρ
= 3.04–3.55 g/cm
3
). The tribological performance of high density pure MoS
2
coatings is shown to surpass that of established composite coatings, achieving a wear rate 2 × (
k
= 5.74 × 10
–8
mm
3
/Nm) lower than composite MoS
2
/Sb
2
O
3
/Au in inert environments.
For the first time, we demonstrate that PTFE filled with iron–cobalt (FeCo) microparticles is an ultralow wear, magnetic, multifunctional tribological material. PTFE filled with 5 wt% of equiatomic, ...pre-alloyed FeCo powder resulted in steady-state wear rates of 2.8 × 10
–7
mm
3
/Nm, approaching that of PTFE-filled alumina. Comparable wear rates were not observed for PTFE filled separately with elemental iron (Fe) or cobalt (Co) microparticles. PTFE filled with either Fe or Co microparticles exhibited only incremental improvements in steady-state wear behavior when compared to unfilled PTFE (1 order of magnitude or less improvement). Particle size analysis and morphology indicate that the Fe and Co microparticles are strongly fused agglomerates (5–20 µm) made of smaller primary particles or features, while the FeCo microparticles are large (~ 40 µm), spherical, dense particles. IR spectroscopy shows that PTFE-FeCo composites form more tribochemical species than elemental Fe- or Co-filled composites, leading to the observed improvements in wear rate. The FeCo particles are surprisingly large as a filler for ultralow wear PTFE. From these results, we conclude that the fully dense, metallic, microscale, and intrinsically brittle FeCo particles may be friable and break down during sliding to reinforce and promote stable tribofilms, akin to the previously reported alumina particles in ultralow wear PTFE-alumina composites.
Steady-state fluctuations in the friction force of molybdenum disulfide (MoS
2
), a prototypical lamellar solid, were analyzed experimentally for newton-scale forces and computationally via molecular ...dynamics simulations for nanonewton-scale forces. A mean field model links the statics and the dynamics of the friction behavior across these eight orders of magnitude in friction force and six orders of magnitude in friction force fluctuations (i.e., avalanches). Both the statistics and dynamics of the avalanches match model predictions, indicating that friction can be characterized as a series of avalanches with properties that are predictable over a wide range of scales.
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