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Ultrathin carbon nitride (CN/sub x/) overcoats were grown using pulsed dc magnetron sputtering. Substrates were mounted on a holder that allowed 45/spl deg/ tilt angle and rotation at different speeds. Effects of sputter gas composition, target power, substrate bias, substrate tilt, and rotation on film growth and properties were explored. Atomic force microscopy scans over large sampling areas show that thin CN/sub x/ films obtained at -100 V substrate bias with 45/spl deg/ substrate tilt and 20 r/min rotation have root-mean-square roughness almost four times lower than those prepared without substrate tilt and rotation. Several percent of helium in an Ar-5%N/sub 2/ sputter gas was found to improve surface roughness of these coatings. These 1-nm-thick ultrasmooth coatings reduced corrosion damage compared with coatings of the same thickness grown without substrate tilt and rotation. This improved performance appears to be directly related to the growth of smoother and denser coatings synthesized under the present conditions.

“Superhard” coating materials are defined by hardness values that exceed 40 GPa. In this issue of MRS Bulletin, we focus on noncarbon-based superhard coatings, with the exception of a review of carbon nitride (CN) materials. Nanometer-scale multilayered nitride coatings were the first to show the superhard property, and these coatings have quickly made their way into industry as protective coatings for cutting-tool operations. Nanocomposite thin films also exhibit superhardness, and some of these materials have hardnesses approaching that of diamond. Cubic boron nitride (c-BN), which is naturally superhard, has proven very difficult to deposit at thicknesses exceeding 0.1 μm, but it is now reported that chemical vapor deposition techniques based on fluorine chemistries can produce c-BN films up to 20 μm thick. The search to produce cubic β-CN has led to the development of noncubic, fullerene-like forms of CN that are both hard and elastic, a very interesting combination of properties that has already been put to use in the hard-disk industry. Overall, the development of hard and superhard coatings during the past 20 years has been remarkable. We have progressed from trying (and often failing) to deposit hard coatings to now designing new nanometer-scale multilayered and nanocomposite coatings that exhibit excellent hardness properties and other high-performancecharacteristics.

Coating properties and performance can be tailor-designed by proper control of nano structure andprocess conditions. This paper presents two examples: first, the synthesis and optimization of 1 nm thick nitrogenated carbon (CNx) nano-layers as protective overcoats in extremely highdensity harddisksystems, and, second, the development of multilayer coatings for wear protection at elevated temperatures. Synthesis of these coatings was done by magnetron sputtering. These studies demonstrate that one canproduce atomically smooth 1 nmthickCNxovercoatswith acceptable corrosionperformance. In addition, this paper shows that, with the proper choice of nanolayer thickness and process conditions, superhard coatings based on TiN/SiNx and TiB2/TiC multilayers with high thermal stability, low internal stress, and high wear resistance can be synthesized.

Single and multilayer boron nitride and boron carbide films were grown using d.c. magnetron sputtering in a single cathode deposition system. High frequency pulsed bias voltages were applied to both the target and the substrate during deposition. An external solenoid was utilized to increase the ion flux during deposition. Momentum of incident ions was found to play a key role for cubic BN:C formation. Multilayer film growth was investigated as a way to obtain thick and adherent cubic boron nitride containing coatings. Boron carbide films were found to have a hardness exceeding 40 GPa and a smooth surface morphology (r.m.s. roughness 0.1–0.15 nm), high elasticity, and high wear resistance.

Theoretical studies of β-C3N4 and other crystalline carbon nitride structures indicate that several crystalline phases are metastable, with high bulk modulus values. There are many reports of successful synthesis of some crystalline carbon nitride phases. In spite of questionable data analysis in a significant number of such reports, there is sufficient evidence in the literature to indicate that some nanocrystalline carbon nitride phases have been synthesised. Usually, such crystalline phases are embedded in an amorphous carbon nitride matrix. Amorphous carbon nitride films can be synthesised readily using many different techniques and are dominated by sp2 carbon bonding, along with strong evidence of carbon-nitrogen bonding. Above a certain deposition temperature, fullerene-like structures can be formed with high hardness and excellent elastic recovery in nanoindentation testing. These thin films display excellent wear performance compared with carbon and hydrogenated carbon films under a wide ...

We reported in an earlier study the use of amorphous SiNx to periodically interrupt the growth of TiN in order to suppress the columnar structure. X-Ray diffraction and transmission electron microscopy studies confirmed that the columnar growth was effectively suppressed. In this paper, we showed that these TiN/SiNx multilayer coatings attain lower dry friction coefficient (half of TiN) and better nanoscratch resistance than TiN coatings. Block-on-ring wear tests showed that the wear rate of optimized TiN/SiNx multilayer coatings is three times better than that of TiN coatings. Most important, the rolling-contact-fatigue life of the M50 bearing rod, coated with a 0.75 μm thick TiN/SiNx multilayer coating, is more than 10 times (test stopped manually before failure) better than that of uncoated ones and five times better than rods coated with conventional TiN coatings. The optimum TiN/SiNx coating thickness appears to be approximately 0.75 μm. This remarkable fatigue performance appears to be due to a combination of improved microstructure, adhesion, hardness and surface topography.

Hard TiN/SiN x multilayer coatings with an equiaxed microstructure were prepared using a dual-cathode unbalanced reactive-magnetron sputtering system. These multilayer coatings were then annealed at 1000 °C for 1 h in vacuum. These TiN/SiN x multilayer coatings before and after annealing were characterized at room temperature and compared in terms of microstructure and mechanical properties. X-Ray diffraction and cross-sectional transmission electron microscopy studies showed that the layer structure of these coatings is preserved after annealing at 1000 °C when the SiN x layer thickness is 0.8 nm or greater. The high hardness of these multilayer coatings is either maintained or improved after annealing. Therefore, with proper control of the SiN x thickness, TiN/SiN x multilayer coatings demonstrate excellent thermal stability and are potential candidates for high-temperature tribological applications.

A series of light- and air-stable silver(I) pyrazolylmethylpyridine complexes [Ag(L(R))]n(BF4)n (L = pyrazolylmethylpyridine; R = H, 1; R = Me, 2; R = i-Pr, 3) and [Ag(L(R))(NO3)]2 (L = pyrazolylmethylpyridine; R = H, 4; R = Me, 5; R = i-Pr, 6) has been synthesized and structurally and spectroscopically characterized. In all of the molecular structures, the pyrazolylmethylpyridine ligands bridge two metal centers, thus giving rise to dinuclear (2, 4, 5, and 6) or polynuclear structures (1 and 3). The role played by the counteranions is also of relevance, because dimeric structures are invariably obtained with NO3(-) (4, 5, and 6), whereas the less-coordinating BF4(-) counteranion affords polymeric structures (1 and 3). Also, through atoms-in-molecules (AIM) analysis of the electron density, an argentophilic Ag···Ag interaction is found in complexes 2 and 4. Thermogravimetric analysis (TGA) shows that the thermolytic properties of the present complexes can be significantly modified by altering the ligand structure and counteranion. These complexes were further investigated as thin silver film precursors by spin-coating solutions, followed by annealing at 310 °C on 52100 steel substrates. The resulting polycrystalline cubic-phase Ag films of ∼55 nm thickness exhibit low levels of extraneous element contamination by X-ray photoelectron spectroscopy (XPS). Atomic force microscopy (AFM) and scanning electron microscopy (SEM) indicate that film growth proceeds primarily via an island growth (Volmer-Weber) mechanism. Complex 4 was also evaluated as a lubricant additive in ball-on-disk tribological tests. The results of the friction evaluation and wear measurements indicate a significant reduction in wear (∼ 88%) at optimized Ag complex concentrations with little change in friction. The enhanced wear performance is attributed to facile shearing of Ag metal in the contact region, resulting from thermolysis of the silver complexes, and is confirmed by energy-dispersive X-ray analysis of the resulting wear scars.

Amorphous and crystalline alumina thin films were grown by reactive magnetron sputtering. A pulsed d.c. bias was applied to both the aluminum target and the substrate to maintain a stable deposition process and high deposition rate. An external solenoid coil was placed underneath the substrate to form a magnetic trap between the substrate and the target, thus enhancing ion bombardment of the growing film. Effects of substrate bias and magnetic trap on film properties were investigated. Under optimum conditions, crystalline alumina films were obtained at substrate temperatures of ∼340°C, with hardness ∼25 GPa and refractive index 1.65–1.85 in the visible region. Friction and wear properties of crystalline and amorphous alumina thin films were compared.

TiN coatings have been widely used in various tribological applications. However, TiN coatings predominantly grow with a columnar grain structure, and these columnar grain boundaries become the sites for crack initiation, resulting in premature failure of TiN coatings. In this paper, we report the use of amorphous SiN x to periodically interrupt the growth of TiN in order to suppress the columnar structure. The growth was performed in a dual-cathode unbalanced reactive magnetron sputtering system. The resulting TiN/SiN x multilayer coatings, when deposited under optimum conditions (TiN layer thickness=2.0 nm, SiN x layer thickness=0.5 nm and substrate bias=−80 to –90 V) are smooth and exhibit an equiaxed grain structure with no evidence of columnar growth. The highest hardness is in excess of 45 GPa.

The interaction between water vapor (D2O) and boron-modified Ni3(Al,Ti)(110) was investigated using temperature-programmed desorption and X-ray photoelectron spectroscopy. Boron dosing was performed using a solid-state boron ion source. Results showed that boron reacts with water to form hydroxyls at temperatures as low as 130 K. D2 desorption occurs at ∼950 K from boron-modified Ni3(Al,Ti)(110), indicating strong B–D bonding. Surface diffusion coefficients of atomic D, obtained from dissociation of water, on clean and boron-modified Ni3(Al,Ti)(110) surfaces were measured at 270 K with electron-stimulated desorption. The surface diffusion coefficient of atomic D on 0.05 monolayer boron-modified surface is about 10 times smaller than that on the boron-free surface. This slower diffusion of atomic hydrogen may explain why boron improves the ductility of polycrystalline Ni3Al in moist environments.

Ultrathin films of carbon nitride were grown using d.c. magnetron sputtering in a single cathode deposition system. A high frequency pulsed bias was applied to both the target and the substrate during deposition to help sustain a stable plasma and enhance ion bombardment of the growing film. These films exhibit excellent corrosion resistance at film thickness down to 1 nm. Atomic force microscopy measurements reveal an atomically smooth surface. The role of pulsed bias and bombardment by ionized species in obtaining low defect-density coatings is discussed.

Possible reasons for the high ductility of Ni3Fe and its insensitivity toward the testing environment have been investigated. Thermal desorption experiments have shown that water dissociates on clean Ni3Fe surfaces to produce atomic hydrogen. Ductility measurements of cast and cold-rolled polycrystalline Ni3Fe demonstrated that the reduced ductility was obtained only when the testing was performed with oxygen carefully removed. X-ray photoemission studies indicate that oxygen interacts with water to form hydroxyls, thereby suppressing the production of atomic hydrogen. Hydrogen desorption from the Ni3Fe surface requires a lower activation energy, resulting in a smaller surface hydrogen concentration at a given temperature. Hence it is possible that there is insufficient hydrogen to cause the nucleation and growth of brittle cracks for severe embrittlement.

Highly pure, ultralong, and uniform-sized semiconductor nanowires in bulk quantity were synthesized by thermal evaporation or laser ablation of semiconductor powders mixed with oxides. Transmission electron microscopy study shows that decomposition of semiconductor suboxides and defect structures play important roles in enhancing the formation and growth of high-quality nanowires. A new growth mechanism is proposed on the basis of microstructure and different morphologies of the nanowires observed.

Titanium nitride normally grows with a columnar grain structure. In this study, we prepared TiN/CNx multilayered coatings with the intent of using the CNx layers to interrupt the grain growth of TiN to attain an equiaxed grain structure. The TiN/CNx coatings were synthesized by direct current magnetron sputtering. Our investigations showed that three important process conditions are required to obtain superhard and smooth TiN/CNx coatings with low internal stress: (i) a TiN buffer layer with strong (111) texture; (ii) proper bias during the growth of the TiN buffer and TiN/CNx coating; and (iii) small thickness for the CNx layers.

A laboratory apparatus for surface science and in situ x-ray powder diffraction studies of highly dispersed particles has been constructed. The chamber incorporates standard sample preparation and characterization facilities into a high-flux x-ray diffraction setup, capable of two-circle goniometry with 0.1 nm−1 Q resolution from samples as thin as 1 nm average thickness (1 μg), at an elevated temperature under ultrahigh vacuum, in minutes.

As a result of the prediction by Liu and Cohen that crystalline β‐C3N4 has mechanical properties similar to diamond, many research groups have attempted to synthesize carbon nitride using various deposition techniques. Most groups succeeded in producing amorphous carbon nitride coatings. These coatings, though softer than diamond, display excellent tribological properties and are now used as protective overcoats for computer hard drive systems. The lack of success to make crystalline coatings suggests that β‐C3N4 is metastable. Using TiN(111) and ZrN(111) as seeding layers (which have similar lattice symmetry and commensurate unit cell dimensions as β‐C3N4(001)), we were able to synthesize fully crystalline carbon nitride composite coatings with hardness in the 50 GPa range, which is in the low end of diamond films.

The size-dependent melting and surface melting of Pb nanocrystals is demonstrated by x-ray powder diffraction in ultrahigh vacuum. Whereas some prior studies have measured the size-dependent melting temperature via the diffraction intensity, it is shown here that crystallite reorientation makes the diffraction intensity an unreliable indicator of melting. Instead of the diffraction intensity, the diffraction peak shape reveals the size-dependent melting via changes in the crystallite size distribution. Measurements showed that the melting temperature varies inversely with the crystallite size and quantitatively favors the liquid-skin melting model over the homogeneous melting model. Surface melting is demonstrated via the reversible growth of a 0.5 nm liquid skin on 50 nm crystallites just below the size-dependent melting temperature.

In this study, we explored the properties of amorphous nitrogenated carbon films grown in a standard magnetron sputter-deposition system without a magnetic trap. Silicon wafers and hard disk substrates were used. To simplify process control, we operated the graphite target at constant voltage and used 3% nitrogen in argon from a premixed feed. Under these conditions, most nitrogenated carbon films contain about 6–8 a/o of nitrogen. Films with the highest hardness values (28 GPa) and lowest root-mean-square surface roughness (0.2 nm on hard disk substrates) were obtained at low sputtering pressure (5 mTorr) and large substrate pulse bias (−300 V). These observations demonstrate the importance of ion bombardment in controlling film morphology and properties. The harder films appear to have better resistance against scratch damage, as is demonstrated by the nano-scratch tests using a two-dimensional nanoindentor.

Single- and dual-cathode DC magnetron sputtering was used to produce TiB 2 coatings and CN x /ZrN multilayers, respectively, with hardness exceeding 40 GPa. The composition, structure, topography, and mechanical properties were determined by various techniques, including Auger electron spectroscopy, X-ray diffraction, high-resolution electron microscopy, atomic force microscopy, and nanoindentation. An optimum combination of the sputtering pressure and substrate bias results in the production of ultrasmooth TiB 2 coatings with hardness up to 50 GPa and excellent wetting properties. The rationale for studying the CN x /ZrN system is that ZrN(111) provides excellent lattice match to the hypothetical β-C 3 N 4 (0001) face (β-C 3 N 4 was predicted to have mechanical properties comparable to diamond). Using a dual-cathode sputtering system, we produced crystalline multilayers of CN x /ZrN with bilayer thickness of 1-2 nm. Using various combinations of nitrogen partial pressure, target powers, and substrate bias, we found that the hardness of these coatings correlates very strongly with the occurrence of (111) texture of ZrN, consistent with the lattice-match strategy. Even with a ZrN volume fraction of 70 percent, such multilayer coatings have been synthesized with hardness in the 50 GPa regime.

Titanium diboride has many interesting physical and chemical properties that make it attractive as a tribological coating material. We focus our study on the relationship between hardness, crystal structure and processing parameters for TiB2 thin films produced by conventional and ionized magnetron sputtering. When synthesized by conventional magnetron sputtering, TiB2 films with the highest degree of crystallinity have the highest hardness and are obtainable at an optimum combination of argon pressure and substrate bias. The films also show strong (0001) texture. Such high degree of crystallinity can be obtained without substrate bias by ionized magnetron sputtering, in which enhanced ionization of the plasma is obtained by inductively coupling RF power in the region between the magnetron target and the substrate.

Two common tribo-failure mechanisms are discussed in this paper.The first mechanism deals with the role of asperity flashtemperature in scuffing (severe adhesive wear) failure. Thesecond mechanism is concerned with the generation andaccumulation of wear particles in the contact zone in initiatingtribo-failure. These failure mechanisms can be suppressed bycontrolling the surface geometry, incorporating the properlubricant package and surface modification.

Siloxane-based polymers (polysiloxanes) exhibit a range of volume, viscosity, and pressure-viscosity behaviors that are strongly influenced by the macromolecular structure. In this report, a combination of extant rheological models is applied to develop a molecular-rheological modeling formalism that predicts polysiloxane rheological properties, such as specific volume, which means density, viscosity, and pressure-viscosity index variations with macromolecular structure, pressure, and temperature. Polysiloxane molecular features are described in terms of alkyl branch length L, pendant type J, density of branch functional monomers Q, and degree of polymerization DP. Both new and published data are used for model parameter determination and validation. Several siloxane-based polymers with alkyl, aryl, alkyl-aryl, cycloalkyl, and halogenated branches were synthesized to examine the modeled relationship between their molecular structures and rheological behaviors.

Automotive and Aerospace Tribology.- Bearing Design Procedures.- Bio-Related Tribology.- Boundary Lubrication.- Contact Mechanics, Isothermal.- Contact Mechanics, Thermal.- Corrosion.- Contact Fatigue.- Conformal-Contact Interface Mechanics.- Economic aspects.- Electrical Contact.- Failure of Tribological Components and Systems and Condition-Based Monitoring.- Friction Science.- Gas Lubrication and Vapor-Phase Lubrication.- Health, Safety and Environmental Issues.- Heat Transfer.- Lubricants.- Lubrication Fundamentals.- Magnetic Storage Tribology.- Manufacturing Tribology.- Materials Science.- Micro and nanotribology.- Reliability.- Surface Forces and Characterization.- Seals.- Surface Topography, Mathematical Analysis.- Surface and Thin-Film Characterization.- Surface Engineering- Coatings.- Surface Engineering- Heat Treatment and Other Enhancement.- Surface Engineering- Machine Finishing and Texturing.- Test Methodologies and Standards.-Tribochemistry.- Vibration.-Wear.

A single-cathode magnetron sputtering system was used to deposit CN x and CN x /TiN coatings onto Si (100) and polished M2 substrates. It was shown that optimum choice of substrate bias and pressure is required to obtain smooth films with maximum hardness and wear performance for carbon nitride. This work that it is possible to obtain CN x /TiN composite films using the same single-cathode approach, although the C/Ti ratio required to achieve the best hardness and wear properties has yet to be optimized.

Crystalline β- C 3 N 4 was predicted to have mechanical properties similar to those of diamond. Until recently, only amorphous carbon nitride coatings were made in spite of numerous attempts. This lack of success in making crystalline coatings suggests that β- C 3 N 4 is metastable. Using TiN as seeding layers and under proper processing conditions, we were able to synthesize fully crystalline carbon nitride composite coatings with hardness in the 45–55 GPa range, which is at the low end of diamond. Suggestions for further improvements and implementation with other deposition techniques will be made.

The interaction of water vapor and oxygen with polycrystalline Ni3Fe has been studied by Auger electron spectroscopy, X-ray photoelectron spectroscopy (XPS) and thermal desorption. Water adsorbs molecularly on the surface at or below 150 K. Between 150 and 200 K, some adsorbed water molecules desorb in vacuum, while others dissociate into hydroxyls, atomic oxygen, and hydrogen. XPS studies suggest iron to be the active species in controlling the dissociation of water on polycrystalline Ni3Fe surface. Water dissociation into atomic hydrogen is greatly suppressed by the presence of small concentrations of oxygen in water vapor.

Lubricant effects in the friction transition from boundary to microelastohydrodynamic lubrication were investigated by using a ballon-flat tribotester at sliding speeds from 0.02 to 0.88 mm/sec. Three lubricants—cyclophosphazine (X-IP), poly-alpha-olefin (PAO) and Z-DOL—were used, in this investigation. When X-IP was used at room temperature, a drop in friction coefficient from 0.22 to 0.12 at sliding speeds ≥0.10 mm/sec. (an unusually low speed) was observed, accompanied by a rise in the contact electrical resistance across the ball-fiat interface. The friction drop did not occur at temperatures ≥100°C. The friction transition was achieved at lower speeds when sliding perpendicular to the surface roughness texture. No transition occurred when PAO and Z-DOL were substituted as the lubricant. The latter him lubricants were working in the boundary lubrication regime as indicated by the contact resistance measurement. Presented at the 51st Annual Meeting in Cincinnati, Ohio May 19–23, 1996

Effects of the thermally grown wustite on the fatigue crack initiation and growth in HSLA steel are evaluated as a function of oxide thickness, strain amplitude, and gaseous environment in the push-pull plastic strain control mode, with special attention being given to the early stage of microcrack initiation. Specimens with a wustite surface layer thermally grown to 0.2 and 0.6 μm thicknesses show predominantly intergranular cracking at plastic strain amplitudes of 5×10−4 and 1×10−3 both in air and in ultrahigh vacuum (UHV), in contrast to the as-polished specimens where slip band cracking is the favoured mode. The cracking mode in the oxide layer is discussed in terms of the strain amplitude and the dislocation behaviour near the oxide/metal interface. The features of microcrack initiation in the oxide layer is not affected by the gaseous environment. Once, however, the surface oxide fractures, the rate of crack growth through the base metal is greatly reduced in UHV.

Nanoindentation hardness and compressive stress in amorphous carbon nitride thin films prepared by unbalanced magnetron sputter-deposition were studied. The coating hardness and compressive stress were found to be strongly dependent on processing parameters such as substrate bias and nitrogen partial pressure. Under optimized deposition conditions, carbon nitride thin films with nanoindentation hardness about 25 GPa have been coated onto Si wafers and M2 steels. A strong correlation between coating hardness and compressive stress in the coating was observed.

Carbon nitride thin films were prepared using an ionized magnetron sputtering system. An inductively coupled rf plasma was generated in the region between the sputtering source (high purity graphite) and the substrate table. An argon–nitrogen mixture was used as the sputtering gas. Sputtered atoms which pass through the radio frequency (rf) plasma may be ionized, and the degree of ionization depends on several processing variables, such as the gas total pressure, reactive gas partial pressure, and applied rf power. Pulsed substrate bias voltage was varied up to −500 V. Chemical bonding and composition of the deposited films were studied by infrared and Auger electron spectroscopy, respectively. An ultramicroindentation system was used to measure the hardness. Nitrogen was found to be bonded to carbon in various configurations. The dependence of the nitrogen‐to‐carbon ratio and film hardness on the deposition conditions was studied. Under optimum conditions, films with nitrogen‐to‐carbon ratio ∼0.3–0.4 and...

Dry sliding experiments of 3004 aluminum pins against H13 steel disks were performed at a load of 98 mN, sliding speed of 1 m/s and fixed sliding distance of 600 meters under vacuum and air with different relative humidities. Friction, microscopy and energy-dispersive X-ray studies showed that friction and wear increase with relative humidity and that material transfer occurs from the pin to the disk and vice-versa. The increase of friction and wear with relative humidity is believed to be due to the formation of oxide particles acting as hard third bodies ploughing into both contacting surfaces. Since oxidation is suppressed in vacuum, the relatively low wear in vacuum (compared with regular air ambients) is explained by the absence of such hard oxide particles. Presented as a Society of Tribologists and Lubrication Engineers paper at the STLE/ASME Tribology Conference in New Orleans, Louisiana, October 24–27, 1993

The interaction of water vapor with single crystal Ni 3 (Al, Ti) has been studied with Auger electron spectroscopy, photoemission spectroscopy and thermal desorption. Water adsorbs molecularly onto the (111) and (100) surfaces at 140 K. When warmed up to room temperature, water remains as an intact molecule on (111), but dissociates to surface hydroxyl and atomic oxygen on (100). The latter process is accompanied by the evolution of hydrogen and oxidation of Al. It appears that the dissociation of water is structure-sensitive and that Al is the active species in controlling the dissociation of water on Ni 3 Al surfaces.

Ultrathin nitrogenated carbon (CNx) films were synthesized using pulsed dc magnetron sputtering. The influence of substrate tilt angle and rotation speed on surface roughness was explored. Atomic force microscopy studies showed that the smoothest films were obtained at substrate tilt of 45° and rotation speed 20–25 rpm, corresponding to 2–3 rotations per deposited atomic layer. The root-mean-square surface roughness under these optimum conditions is ∼0.3 nm when sampled over 20×20 μm2 areas, increasing to ∼0.4 nm when sampled over ∼0.05×3 cm2 using x-ray reflectivity measurements. In addition, x-ray reflectivity measurements showed that the mass density of these CNx films is ∼2.0 gm/cc, independent of film thickness from ∼1 to 10 nm, consistent with ion beam analysis.

This study investigates the rheological properties, elastohydrodynamic film thickness, and friction coefficients of several commercially available polyalphaolefin (PAO) and polydimethylsiloxane (PDMS)-based lubricants to assess relationships between molecular structure and lubricant performance. Molecular structures and masses were determined by nuclear magnetic resonance spectroscopy and gel permeation chromatography, respectively. Density and viscosity are measured from 303 to 398 K, while elastohydrodynamic lubricant film thickness and friction measurements were made at temperatures, loads, and speeds that are representative of boundary, mixed, and full-film lubrication regimes. The results show that PDMS-based lubricants are thermally and oxidatively more stable than PAOs, while the viscosity of PDMS-based lubricants is generally less temperature sensitive than PAOs, except for highly branched polysiloxanes. In particular, this study provides quantitative insight into the use of PDMS-based lubricants to obtain low friction through the entire lubrication regime (boundary to full film) by optimal tuning of the molecular mass and chain branching.

Carbon nitride thin films were synthesized using d.c. unbalanced magnetron sputtering of a high-purity graphite target in a nitrogen-containing plasma onto various substrates held at ambient temperatures. The N/C ratio was found to vary from zero to 0.8 depending on deposition conditions. There was evidence of multiple bonding states for carbon and nitrogen (sp, sp2 and sp3), and the bulk of the film was found to be amorphous. Nanoindentation studies showed that under appropriate substrate bias conditions the carbon nitride coating hardness can be enhanced to levels well above that of conventional amorphous hard carbon coatings. CNx coatings replicate the substrate topography, giving rise to surface roughness equal to or better than the original substrates. Lubricated tribotesting showed the relationship between wear performance and deposition conditions. Under pure sliding in air, the CNx coating provided a 30-fold reduction in wear rate for M2 steels (compared with uncoated M2) under our testing conditions.

Low cycle fatigue behaviors in Nb-bearing high strength, low alloy (HSLA) steel have been studied at plastic strain amplitudes Δϵ p /2 ranging from 1 × 10 −5 to 1 × 10 −2 in air and in ultrahigh vacuum (UHV). Microcracks are initiated along slip lines both in air and in UHV, and grain boundaries act as effective barriers to the propagation of microcracks. Coffin-Manson relations are reasonably satisfied for the microcrack initiation life N i and the fatigue life N f both in air and in UHV. The accumulated total cyclic plastic strain energy W i required for fatigue crack initiation and the accumulated total cyclic plastic strain energy W f required for fatigue failure are expressed mathematically in the form of K ( Δϵ p /2) N . The computed values of W i and W f are in good agreement with the values determined experimentally. The vacuum environment enhances N i , N f , W i and W f . The environmental effect becomes greater as the plastic strain amplitude decreases.

STM lithography by pulsing the tip-sample bias was used to deposit nanometer-sized clusters on a copper surface using gold tips. Prior to writing each cluster, we extracted the local tip radius of curvature from STM images of slip band steps previously formed on the copper surface. This novel technique permitted in-situ observation of changes in both cluster size and local tip structure while using a single tip. It was found that the cluster diameter increases with the square root of tip radius. Using a model in which we assume that deposition occurs only from that region of the tip within a certain distance from the sample surface, we were able to derive an expression consistent with the experimental data.

Carbon nitride thin films were prepared by dc magnetron sputtering of a graphite target in a nitrogen containing gas mixture onto Si(100) substrates held at ambient temperatures. All CNx coatings grown to a thickness of 1.5 μm are adherent and smooth. Infrared absorption and isotropic nitrogen labeling indicate that carbon and nitrogen are chemically bonded together. Both solid‐state 13C and 15N magic angle spinning nuclear magnetic resonance (NMR) analyses were performed. NMR measurements indicate the presence of sp2 bonded C and N.

Carbon nitride (CNX) thin films were prepared by DC magnetron sputtering of a graphite target in a nitrogen ambient onto Si(100) and M2 steel substrates held at ambient temperatures. All CNx coatings grown to a thickness of 1.5 μm are adherent and smooth. Nanoindentation studies showed that amorphous CNx thin films have hardness and effective Young's modulus comparable to those of amorphous hard carbon. In dry sliding against 52100 steel, CNx-coated M2 substrates give initial friction coefficients ˜0.1, rising to around 0.5 in the steady state. The wear life of coated M2 steels varies inversely with normal load, rising to about two orders of magnitude higher than that of uncoated M2 at a normal load of 1.3N and a sliding speed of 0.12 m/s. This improvement is attributed to the high yield strength of carbon nitride coatings. Presented as a Society of Tribologists and Lubrication Engineers paper at the STLE/ASME Tribology Conference in New Orleans, Louisiana, October 24–27, 1993

A method for predicting scuffing failure based on the competitive kinetics of oxide formation and removal has been developed and applied to the sliding of AISI 52100 steel on steel with poly-α-olefin as the lubricant. Oxide formation rates were determined using static oxidation tests on coupons of 52100 steel covered with poly-α-olefin at temperatures of 140°C to 250°C. Oxide removal rates were determined at different combinations of initial average nominal contact pressures (950 MP a to 1578 MP a) and sliding velocities (0.4 m/s to 1.8 m/s) using a ball-on-disk vacuum tribotester. The nominal asperity flash temperatures generated during the wear tests were calculated and the temperatures corresponding to the intersection of the Arrhenius plots of oxide formation and removal rates were determined and taken as the critical failure temperatures. The pressure-velocity failure transition diagram was constructed by plotting the critical failure temperatures along isotherms of average nominal asperity flash tem...

Carbon nitride films were grown on silicon and hard disk substrates using pulsed dc magnetron sputtering in a single-cathode deposition system. Substrates were mounted on a specially designed holder that allowed 45° tilt angle and substrate rotation about the surface normal up to 20 rpm. The influence of substrate bias, substrate tilt, and rotation on film growth and properties was studied. Films with the lowest rms surface roughness and corrosion performance were obtained at −100 V substrate bias with substrate tilt and rotation. Atomic force microscope scans over 10×10 μm2 sampling areas showed that 50 nm thick CNx films prepared under these conditions have roughness almost four times lower than those prepared without substrate tilt and rotation. We observed a twofold reduction in corrosion damage for hard disk substrates with 1 nm thick CNx overcoats deposited with substrate tilt and rotation. This improved performance is likely a result of more efficient and uniform momentum transfer parallel to the surface during deposition in this configuration.

Introduction, Yip-Wah Chung Macroscale to Microscale Tribology: Bridging the Gap, Kathryn J. Wahl A Continuum Overview of Contact and Lubrication, Q. Jane Wang Surface Energy and Surface Forces, Robert W. Carpick Nanoscale Friction: Measurement and Analysis, Rachel J. Cannara Effects of Micro- and Nanoscale Texturing on Surface Adhesion and Friction, Min Zou Environmental Effects in Tribology, Seong H. Kim Molecular Dynamics Simulation of Nanotribology, Ashlie Martini Index

Silver single crystals oriented in the single glide orientation were fatigued in air and vacuum. The surface morphology was observed using a scanning tunneling microscope. A gold film deposited on the silver surface was used as a reference marker so that the same region can be observed upon repeated fatigue cycling. Pit-shaped features were observed to form within a narrow range of cycles and to eventually develop into micro-sized cracks, lending support to the notion that cracks are formed by a nucleation process. In addition, by measuring the slip band height and spacing as a function of cycling, we were able to determine the slip irreversibility and the Coffin-Manson exponents in both environments. With these two sets of data, we were able to verify a new scaling relationship for fatigue life based on our earlier theoretical work.