Thomas W. Scharf

Structural and tribological characterization of protective amorphous diamond-like carbon and amorphous CNx overcoats for next generation hard disks

Further insight into processing-structure-property relationships have been carried out for existing and candidate carbon-based protective overcoats used in the magnetic recording industry. Specifically, 5 nm thick amorphous diamond-like carbon (a:C) and nitrogenated diamond-like carbon (a:CNx) overcoats were deposited by low deposition rate sputtering onto a thin film disk consisting of either CoCrPt/CrV/NiP/AlMg or CoCrPt/CrV/glass. The wear durability and frictional behavior of these hard disks were ascertained using a recently developed depth sensing reciprocating nanoscratch test. It was determined that the CN0.14/CoCrPt/CrV/glass disk exhibited the most wear resistance, least amount of plastic deformation, and lowest kinetic friction coefficient after the last wear event. Core level x-ray photoelectron spectroscopy (XPS) results of sputter cleaned overcoats indicated that nitrogen up to 14 at. % incorporated into the amorphous network resulted in these improvements near the overcoat/magnetic layer in...

Solid lubricants: a review

The fundamental mechanisms of solid lubrication are reviewed with examples from well-known solid lubricants like the transition metal dichalcogenides and diamond-like carbon families of coatings. Solid lubricants are applied either as surface coatings or as fillers in self-lubricating composites. Tribological (friction and wear) contacts with solid lubricant coatings typically result in transfer of a thin layer of material from the surface of the coating to the counterface, commonly known as a transfer film or tribofilm. The wear surfaces can exhibit different chemistry, microstructure, and crystallographic texture from those of the bulk coating due to surface chemical reactions with the surrounding environment. As a result, solid lubricant coatings that give extremely low friction and long wear life in one environment can fail to do so in a different environment. Most solid lubricants exhibit non-Amontonian friction behavior with friction coefficients decreasing with increasing contact stress. The main mechanism responsible for low friction is typically governed by interfacial sliding between the worn coating and the transfer film. Strategies are discussed for the design of novel coating architectures to adapt to varying environments.

Role of third bodies in friction and wear of protective coatings

The literature on protective tribological coatings often focuses on correlations with measurable coating properties (composition, structure, and mechanical) but ignores the mechanisms of friction and wear. In fact, long-lived coatings often survive because of third bodies that form inside the moving contact. This article reviews earlier studies of third body processes carried out by mainly ex situ methods and reports more recent studies investigating third body processes using in situ techniques. Direct evidence that third bodies control friction and wear processes has been obtained with a tribometer incorporating in situ optical microscopy and Raman spectroscopy. Videotapes and Raman spectra of the sliding contact were recorded during reciprocating sliding tests performed in both dry and humid air with transparent hemispheres (glass or sapphire). Third body processes were correlated directly to friction and wear behavior of three low friction coatings: amorphous Pb–Mo–S; diamond-like carbon (DLC); and an...

Nanostructured magnetic networks

Nanostructured contiguous networks of Fe well suited to fundamental studies of the effects of confinement are described. The Fe networks are prepared by sputter deposition onto the surface of nanochannel alumina. One of the most obvious consequences of confining the magnetic material to a nanostructured network is a dramatic enhancement in the coercivity when compared to a continuous Fe film of the same thickness.

Adhesion assessment of silicon carbide, carbon, and carbon nitride ultrathin overcoats by nanoscratch techniques

This paper presents experimental results for adhesion assessment of 20 nm ultrathin SiC, amorphous carbon and carbon nitride films deposited on silicon (111) substrates by sputtering. In the experiment, the ultrathin overcoats were scratched by a face-forward Berkovich diamond indenter with a continuous depth sensing nanoscratch system. Experimental results indicate that the nanoscratch system has very good sensitivity for detecting ultrathin overcoat cracking, delamination, and brittle fracture caused by scratching. A well-defined critical load for each film was determined by the nanoscratch techniques. The highest critical load (800 μN) is found for the CN film and the lowest (500 μN) is for SiC. Scanning electron microscopy (SEM) investigations to the surface damage of the samples reveal that the SiC overcoat was damaged more severely than other films due to its brittle fracture. The adhesion properties of the three films tested in this work are best in carbon nitride, followed by amorphous carbon and ...

Self-lubricating carbon nanotube reinforced nickel matrix composites

Nickel (Ni)—multiwalled carbon nanotube (CNT) composites have been processed in a monolithic form using the laser-engineered net shape (LENS™) processing technique. Auger electron spectroscopy maps determined that the nanotubes were well dispersed and bonded in the nickel matrix and no interfacial chemical reaction products were determined in the as-synthesized composites. Mechanisms of solid lubrication have been investigated by micro-Raman spectroscopy spatial mapping of the worn surfaces to determine the formation of tribochemical products. The Ni-CNT composites exhibit a self-lubricating behavior, forming an in situ, low interfacial shear strength graphitic film during sliding, resulting in a decrease in friction coefficient compared to pure Ni.

Mechanisms of friction in diamondlike nanocomposite coatings

Diamondlike nanocomposite (DLN) coatings (C:H:Si:O) processed from siloxane precursors by plasma enhanced chemical vapor deposition are well known for their low friction and wear behaviors. In the current study, we have investigated the fundamental mechanisms of friction and interfacial shear strength in DLN coatings and the roles of contact stress and environment on their tribological behavior. Friction and wear measurements were performed from 0.25to0.6GPa contact pressures in three environments: dry (<1% RH) nitrogen, dry (<1% RH) air, and humid (50% RH) air, with precise control of dew point and oxygen content. At 0.3GPa contact stress, the coefficient of friction (COF) in dry nitrogen was extremely low, ∼0.02, whereas in humid air it increased to ∼0.2, with minimal amount of wear in both environments. The coatings also exhibited non-Amontonian friction behavior, with COF decreasing with an increase in Hertzian contact stress. The main mechanism responsible for low friction and wear under varying cont...

Nanotribology of ultrathin a:SiC/SiC-N overcoats using a depth sensing nanoindentation multiple sliding technique

Abstract Conventional and reactive (Ar+N 2 atmosphere) magnetron sputtering were used to deposit amorphous SiC and SiC-N thin films, respectively. The ultrathin films (25 nm) were grown on Si(111), single crystal sapphire, and barium borosilicate glass substrates. A commercial nanoindentation system equipped with a continuous depth-sensing scratch option was utilized to assess wear and frictional behavior. Tangential or frictional forces were monitored via two proximity probes oriented in the plane of the film surface in the x - and y -directions, while the normal forces were concurrently generated. In situ surface wear displacement profiles of the films on different substrates were strongly dependent on their intrinsic elastic/plastic properties. For a constant load of 150 μ N under 31 multiple sliding scans, delamination only occurred in the SiC/sapphire system (27th scan) while the SiC/glass and Si(111) systems remained adhered. Interestingly there was no debonding in the SiC-N/sapphire system, which also reflects how the technique can qualitatively describe the adherence behavior of different films on the same substrate. This improvement in the wear resistance for SiC-N films resulted from an increase in compressive residual stress.

Atomic layer deposition of tungsten disulphide solid lubricant thin films

The synthesis and characterization of crystalline tungsten disulphide (WS 2 ) solid lubricant thin films grown by atomic layer deposition (ALD) using WF 6 and H 2 S gas precursors was studied. A new catalytic route was established to promote nucleation and growth of WS 2 films on silicon surfaces with native oxide. Scanning electron microscopy with energy dispersive spectroscopy and Raman spectroscopy were used to determine the film morphology, composition, and crystallinity. The films exhibited solid lubricating behavior with a steady-state friction coefficient of 0.04 in a dry nitrogen environment.

Nanotribology studies of Cr, Cr2N and CrN thin films using constant and ramped load nanoscratch techniques

Abstract The nanotribological behavior of DC magnetron sputtered Cr, Cr 2 N and CrN thin films has been studied using nanoscratch techniques. Constant and ramped load (5 mN) scratches were made using a Nano Indenter II system on 500-nm-thick Cr, Cr 2 N and CrN thin films. Ramped-load scratch studies have been carried out on 10-nm-thick Cr, Cr 2 N and CrN. Wear tracks were imaged by atomic force microscopy. The dependence of the displacement, residual wear depth, percent elastic recovery, and friction coefficient on load is compared in constant load and ramped load tests. Under the same (maximum) load, constant load tests exhibit higher displacements, residual depths and friction coefficients, but lower percent elastic recoveries. The chromium nitride films (Cr 2 N and CrN) show less permanent damage, lower coefficients of friction and higher percent elastic recovery than pure chromium under the same (maximum) load. However, the adhesive strength of Cr 2 N needs to be improved.