Stephen V. Didziulis

Structural and tribological studies of MoS2 solid lubricant films having tailored metal-multilayer nanostructures

Abstract Molybdenum disulfide (MoS 2 ) solid lubricant films were prepared by r.f. magnetron sputtering on 440C steel, 52100 steel, and silicon substrates. This study concentrated on films that were multilayer coatings of MoS 2 with either nickel or Au-(20%)Pd metal interlayers. Multilayer thicknesses ranged from 0.2 nm to 1.0 nm while the multilayer periodic spacing ranged from 3 to 10 nm. Scanning electron microscopy and X-ray diffraction revealed that the multilayer films had dense microstructures that, in some cases, exhibited significant orientation of their basal planes parallel to the substrate. Film endurance was assessed in sliding contact using thrust washer tests and in rolling contact using thrust bearing tests. Some film microstructures exhibited excellent endurance. Brale indentation indicated that the metal layers can improve film fracture toughness. Friction in air and ultrahigh vacuum (UHV) was investigated using a UHV-compatible test apparatus. Friction coefficients between 0.05 and 0.08 were measured in UHV.

Chemical and tribological studies of MoS2 films on SiC substrates

Abstract The lubrication of ceramic surfaces is vital to permit the use of ceramics in many types of moving mechanical assembly. This paper reports the results of pin-on-flat friction and wear studies conducted in vacuum on lubricating MoS2 films that were r.f. sputter deposited onto hot-pressed SiC coupons pretreated to alter available surface bonding sites. Pretreatment consisted of chemical etching with HF and HNO3, which removed surface active sites for MoS2 adhesion. Films grown on etched surfaces have shorter wear lives and lower initial friction than similar films grown on non-etched surfaces. The surface chemical interactions of SiC with molybdenum, sulfur and oxygen were also briefly examined using X-ray photoelectron spectroscopy. SiC was observed to form strong bonds with all three species; molybdenum had an interfacial reaction, while sulfur and oxygen competed for strong surface bonding sites.

Frictional properties of titanium carbide, titanium nitride, and vanadium carbide: Measurement of a compositional dependence with atomic force microscopy

The frictional properties of TiC(100), TiN(100), and VC(100) surfaces have been investigated on the microscopic scale by atomic force microscopy. In this work, friction measurements were performed under controlled ambient conditions to emulate the use of these materials as hard coatings. A variety of tip materials, silicon nitride, titanium carbide, titanium nitride, and tungsten carbide, were used to investigate the correlation between surface composition and frictional properties of the carbide and nitride substrates. The surface compositions of both clean and air-exposed samples were characterized by x-ray photoelectron spectroscopy to complete the composition/friction correlation. In these studies, the TiC(100) substrate exhibited the lowest frictional response of the substrate samples, regardless of tip composition. The friction measurements on TiN(100) and VC(100) exhibited a strong dependence on counterface composition and were consistently higher than those of TiC. In addition to the compositional...

Soft X-Ray Photoelectron Spectroscopy Study of the Interaction of Cr with MoS2(0001)

Abstract The interaction of Cr with the MoS 2 (0001) surface was studied with core-level and valence-band soft X-ray photoelectron spectroscopy employing synchrotron radiation ( hv = 150−400 eV). Cr vapor-deposited at room temperature formed relatively flat films on MoS 2 (0001). The Cr underwent a limited chemical reaction with the MoS 2 substrate to form a chromium sulfide and Mo metal. The Mo metal formed a thin layer between the Cr film and the MoS 2 substrate. In contrast, reacted S was incorporated throughout the Cr film, with its concentration enhanced at the surface. Annealing the Cr-covered sample drove the reaction between Cr and MoS 2 , forming additional Mo metal that alloyed with Cr, and several adsorbed S species on the Cr film surface. Annealing also caused partial coalescence of the Cr-containing portion of the film, uncovering parts of the Cr-Mo alloy. The reactivity of Cr with respect to MoS 2 (0001) is greater than that of Fe and somewhat less than that of Mn.

Reactions of water and ethanol with polycrystalline TiC surfaces

The adsorption and reaction of water and ethanol with polycrystalline TiC coatings have been investigated and compared with those of the nonpolar (100) face of single crystal TiC. This work is pursued to develop a fundamental understanding of the surface bonding and reaction properties, thus enabling the use of TiC as a tribological coating material. Temperature-programmed desorption has been used to characterize the desorption behavior of these model adsorbates as well as that of products resulting from their reaction with the TiC surface. Following adsorption at 100K, molecular desorption as well as desorption of reaction products is evident for both water and ethanol. Approximately 70% of the water in the monolayer is judged to irreversibly react with the TiC surface, producing a surface oxide and gaseous hydrogen and carbon monoxide. Approximately 92% of the ethanol in an adsorbed monolayer reacts, predominantly producing gaseous ethene. These results are similar to those on TiC(100) surfaces, althoug...

Spectroscopic and scanning probe studies of oxygen and water on metal carbide surfaces

Spectroscopic and scanning probe techniques were applied to the study of clean, (100) single-crystal surfaces of titanium carbide (TiC) and vanadium carbide (VC), and their interactions with oxygen and water. We find that oxygen adsorbs dissociatively on both surfaces, but discriminates between them by reacting with TiC to form an oxide while forming a metastable overlayer on VC. Water bonds molecularly and dissociatively at low temperatures and behaves similarly on both materials. These results are interpreted in the context of surface electronic structure.

The effect of hydrochloric acid pretreatments on 440C steel surface composition and the adhesion and endurance of sputter-deposited MoS2 solid lubricant films

Abstract The effects of substrate pretreatment on substrate surface chemistry and on film-substrate adhesion, and film endurance (wear life) of sputter-deposited molybdenum disulfide (MoS 2 ) on 440C bearing steel were investigated. Specifically, 20% hydrochloric acid-ethanol mixtures were used to etch the steel surface prior to deposition or X-ray photoelectron spectroscopy (XPS). Acid etching inhibited fractures at the film-substrate interface generated by brale indentation testing. XPS indicated that the acid etching removed an iron-rich oxide surface layer, exposing a chromium-rich oxide underlayer on the steel matrix. Acid etching did not significantly affect thrust washer sliding wear life.

Photoelectron spectroscopy of MoS2 at the sulfur 2p absorption edge

Abstract The sulfur 2p absorption edge and valence-band resonant photoelectron spectra of molybdenum disulfide (MoS2), obtained using synchrotron radiation, are compared to the Mo4p edge results from previous work. A pre-edge feature in the S2p data shows transitions to unoccupied, dominantly Mo4d anti-bonding levels, indicating that significant covalent interactions exist between Mo and S valence orbitals. Resonant enhancement of valence-band photoelectron shake-up peaks is observed at the S2p → Mo4d excitation energies, while the sulfur (L2,3VV) Auger peak dominates the spectrum when the S2p ionization threshold is reached. Anti-resonant intensity modulation of main valence band peaks is observed at photon energies corresponding to the S2p → Mo4d pre-edge feature. The results show that the principal decay channel after the S2p → Mo4d excitation leads to the enhancement of final states having S(3p)4 Mo(4d)3 electron configurations.

Assessment of Particle Deposition inside Payload Fairing from Launch Vehicle Plume Contribution

Concerns were raised for potential payload contamination inside payload faring (PLF) contributed from the soot particles in the launch vehicle ignition plume. Soot particles, once ingested into PLF through vents, can pose potential payload contamination risks due to their light absorbing characteristics. To gain insights into the extent of soot particle contamination inside the PLF, analytical calculations and laboratory experiments were performed using a PLF simulator to determine the rate of soot particle deposition onto surfaces. The analysis assumed a non-venting setting as the worst case scenario, in which particles were trapped inside the PLF simulator and allowed to deposit onto available surfaces. Soot particles were briefly introduced inside a PLF mockup and after the soot generation source ceased, particle deposition rates were examined by measuring the particle concentration decay as a function of time. Based on the experimentally determined particle deposition rates and other parameters including the venting scenarios, the impact of soot particle deposition for the full scale PLF and payload was evaluated. The effects of soot particles contamination were also studied, and pronounced transmission degradation toward the UV region on a fused silica substrate was observed.