This work investigates the role of water and oxygen on the shear-induced structural modifications of molybdenum disulfide (MoS2) coatings for space applications and the impact on friction due to oxidation from aging. We observed from transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) that sliding in both an inert environment (i.e., dry N2) or humid lab air forms basally oriented (002) running films of varying thickness and structure. Tribological testing of the basally oriented surfaces created in dry N2 and air showed lower initial friction than a coating with an amorphous or nanocrystalline microstructure. Aging of coatings with basally oriented surfaces was performed by heating samples at 250 °C for 24 h. Post aging tribological testing of the as-deposited coating showed increased initial friction and a longer transition from higher friction to lower friction (i.e., run-in) due to oxidation of the surface. Tribological testing of raster patches formed in dry N2 and air both showed an improved resistance to oxidation and reduced initial friction after aging. The results from this study have implications for the use of MoS2-coated mechanisms in aerospace and space applications and highlight the importance of preflight testing. Preflight cycling of components in inert or air environments provides an oriented surface microstructure with fewer interaction sites for oxidation and a lower shear strength, reducing the initial friction coefficient and oxidation due to aging or exposure to reactive species (i.e., atomic oxygen).
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.
The mechanical performance of an Fe-Co intermetallic alloy processed by laser powder bed fusion (L-PBF) and laser directed energy deposition (L-DED) additive manufacturing is compared. L-PBF material was characterized by high strength (500–550 MPa) and high ductility (35%) in tension, corresponding to a ~250% increase in strength and an order-of-magnitude improvement in ductility relative to conventional material. Conversely, L-DED material exhibited similarly poor tensile properties to the conventional wrought alloy, with low strength (200–300 MPa) and low ductility (0–2.7%). The disparity in properties between L-PBF and L-DED material is discussed in the context of the fundamental differences between manufacturing methods.
Additive Manufacturing (AM) presents unprecedented opportunities to enable design freedom in parts that are unachievable via conventional manufacturing. However, AM-processed components generally lack the necessary performance metrics for widespread commercial adoption. We present a novel AM processing and design approach using removable heat sink artifacts to tailor the mechanical properties of traditionally low strength and low ductility alloys. The design approach is demonstrated with the Fe-50 at.% Co alloy, as a model material of interest for electromagnetic applications. AM-processed components exhibited unprecedented performance, with a 300 % increase in strength and an order-of-magnitude improvement in ductility relative to conventional wrought material. These results are discussed in the context of product performance, production yield, and manufacturing implications toward enabling the design and processing of high-performance, next-generation components, and alloys.
Intermetallic alloys possess exceptional soft magnetic properties, including high permeability, low coercivity, and high saturation induction, but exhibit poor mechanical properties that make them impractical to bulk process and use at ideal compositions. We used laser-based Additive Manufacturing to process traditionally brittle Fe–Co and Fe–Si alloys in bulk form without macroscopic defects and at near-ideal compositions for electromagnetic applications. The binary Fe–50Co, as a model material, demonstrated simultaneous high strength (600–700 MPa) and high ductility (35%) in tension, corresponding to a ∼300% increase in strength and an order-of-magnitude improvement in ductility relative to conventionally processed material. Atomic-scale toughening and strengthening mechanisms, based on engineered multiscale microstructures, are proposed to explain the unusual combination of mechanical properties. This work presents an instance in which metal Additive Manufacturing processes are enabling, rather than limiting, the development of higher-performance alloys.
Diamond-like carbon (DLC) films were tribochemically formed from ambient hydrocarbons on the surface of a highly stable nanocrystalline Pt-Au alloy. A sliding contact between an alumina sphere and Pt-Au coated steel exhibited friction coefficients as low as μ = 0.01 after dry sliding in environments containing trace (ppb) organics. Ex situ analysis indicated that the change in friction coefficient was due to the formation of amorphous carbon films, and Raman spectroscopy and elastic recoil analysis showed that these films consist of sp2/sp3 amorphous carbon with as much as 20% hydrogen. Transmission electron microscopy indicated these films had thicknesses exceeding 100 nm, and were enhanced by the incorporation of worn Pt-Au nanoparticles. The result was highly wear-resistant, low-friction DLC/Pt-Au nanocomposites. Atomistic simulations of hydrocarbons under shear between rigid Pt slabs using a reactive force field showed stress-induced changes in bonding through chain scission, a likely route towards the formation of these coatings. This novel demonstration of in situ tribochemical formation of self-lubricating films has significant impact potential in a wide range of engineering applications.
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.
We demonstrate inverse Hall-Petch behavior (softening) in pure copper sliding contacts at cryogenic temperatures. By kinetically limiting grain growth, it is possible to generate a quasi-stable ultra-nanocrystalline surface layer with reduced strength. In situ electrical contact resistance measurements were used to determine grain size evolution at the interface, in agreement with reports of softening in highly nanotwinned copper. We also show evidence of a direct correlation between surface grain size and friction coefficient, validating a model linking friction in pure metals and the transition from dislocation mediated plasticity to grain boundary sliding.
An investigation of the temperature-dependent friction behavior of PTFE, MoS2, and PTFE-on-MoS2 is presented. Friction behavior was measured while continuously varying contact temperature in the range −150 to 175 °C while sliding in dry nitrogen, as well as for self-mated PTFE immersed in liquid nitrogen. These results contrast with previous reports of high-friction transitions and plateaus for pure and composite MoS2 at temperatures below about −20 °C; instead, we have found persistently weak thermal behavior between 0 and −196 °C, providing new insight about the molecular mechanisms of macroscale friction. The temperature-dependent friction behavior characteristic of self-mated PTFE was found also for PTFE-on-MoS2 sliding contacts, suggesting that PTFE friction was defined by subsurface deformation mechanisms and internal friction even when sliding against a lamellar lubricant with extremely low friction coefficient (µ ~ 0.02). The various relaxation temperatures of PTFE were found in the temperature-dependent friction behavior, showing excellent agreement with reported values acquired using rheological techniques measuring energy dissipation through internal friction. Additionally, hysteresis in friction behavior suggests an increase in near-surface crystallinity upon exceeding the high-temperature relaxation, Tα ~ 116 °C.