B. K. Gupta

Micromechanical properties of amorphous carbon coatings deposited by different deposition techniques

Abstract Amorphous carbon coatings about 20 nm thick are commonly used as an overcoat on magnetic thin-film rigid disks and tape and disk head surfaces to improve their wear performance. In this study, we deposited amorphous carbon coatings with thicknesses ranging from 20 to 400 nm on single-crystal silicon substrates by four deposition processes: cathodic arc, ion beam deposition, r.f.-plasma-enhanced chemical vapor deposition, and r.f. sputtering. R.f.-sputtered SiC coatings were also deposited for comparison. The hardness, elastic modulus, and scratch resistance of these coatings were measured by nanoindentation and microscratching using a nanoindenter. The cathodic arc carbon coatings followed by sputtered SiC coatings exhibited the highest hardness, elastic modulus, scratch resistance/adhesion, and residual compressive stresses. The critical load, a measure of the scratch resistance/adhesion of the coating, increases with thickness. The cathodic arc coatings of lower thicknesses (~ 30 nm ) exhibited instant damage when the normal load exceeded the critical load, whereas thick coatings (greater than or equal to 100 nm) exhibited gradual damage through the formation of tensile cracks. The sputtered carbon coatings exhibited damage to the coating at very low loads and ploughing of the tip into the coating occurred right from the beginning of the scratch.

Nanoindentation, microscratch, friction and wear studies of coatings for contact recording applications

Abstract A nanoscale monolithic slider-suspension produced by photolithography is used for contact recording. The contact pad consists of a multilayered structure consisting of SiC, amorphous hydrogenated carbon (a-C:H), Al203, Si, and CoNbZr films. In this study, we have compared hardness, Youngs modulus of elasticity, and scratch resistance or adhesion of various coatings deposited on a single-crystal silicon wafer by nanoindentation and microscratch techniques and friction and wear performance by sliding against a diamond tip and sapphire ball in reciprocating mode. SiC coatings exhibit the highest hardness, about 27 GPa, and the highest elastic modulus, about 255 GPa. Microscratch data indicate that SiC and a-C:H coating exhibit the highest resistance to scratching or debonding from the substrate. During scratching, an Al203 coating deforms like a ductile metal rather than like a ceramic. Si and CoNbZr coatings exhibit ploughing of the tip into the sample surface and debris generation right in the beginning of the scratch. SiC coatings exhibit the best wear performance against a diamond tip as well as a sapphire ball. For comparisons, we also made mechanical property measurements on bulk materials used in conventional recording: NiZn ferrite, Al203-TiC, and SiC (under development). The bulk Ni-Zn ferrite sample was found to be damaged by grain pull-out during scratching even at a low load of 3 mN. Bulk Al203-TiC exhibits unexpected ploughing of the sample right from the beginning of the scratch. Bulk SiC did not exhibit any signs of significant damage up to a normal load of about 15 mN. Overall comparison of mechanical properties of bulk materials and coatings suggest that SiC is the most desirable coating in the contact pad for low wear. An SiC coating is also recommended as an overcoat for thin film magnetic disks.

Mechanical and tribological properties of hard carbon coatings for magnetic recording heads

Abstract Thin film read-write magnetic heads are commonly used in data processing tape and rigid disk drives. The head body is made of magnetic ferrites or nonmagnetic A1203-TiC and the head construction includes coatings of soft magnetic alloys, soft oxides and adhesives. Pole tip/ gap recession (relative wear of the pole tip and gap materials with respect to air bearing surface or ABS) in the inductive head and scratching/smearing, electrical short, electrostatic charge build up, and corrosion of the MR stripe in the MR heads are major problems. Wear of head structure can be minimized by the application of a wear resistant coating over the entire ABS including the head structure. In this study, we have deposited amorphous carbon by several deposition processes: cathodic arc deposition, (direct) ion beam deposition, plasma-enhanced chemical vapor deposition (PECVD), and DC magnetron sputtering and ion beam sputtered A1203 and RF sputtered SiC. These coatings were deposited on Ni-Zn ferrite and A1203-TiC substrates and were characterized for mechanical and tribological properties. The ion beam carbon coatings on Ni-Zn ferrite and cathodic arc carbon coating on A1203-TiC exhibited the highest resistance to scratch and wear among all carbon coatings followed by the sputtered SiC coatings on A1203-TiC. Hardness and modulus of elasticity were measured for thick coatings. Cathodic arc carbon, ion beam carbon, and SiC coatings (400 nm thick) on silicon exhibited hardnesses of about 38, 19, and 27 GPa and elastic moduli of 350, 150, and 240 GPa, respectively. Propensity to produce wear debris were also lowest for cathodic arc carbon, and ion beam carbon coatings.

Tribological properties of polished diamond films

Despite the rapid progress being made in the synthesis of diamond films and recent interest in polishing of diamond films, no systematic measurements of friction and wear on polished diamond films have been reported. In the present study, chemomechanical and laser polishing techniques are used, and friction and wear data on the chemomechanically polished diamond films are presented. With the chemomechanical polishing technique used in this study, the rms roughness of hot filament chemical vapor deposited diamond films can be reduced from about 657 to about 170 nm with rounding off of sharp asperities with no change in the diamond structure. The polished films exhibit coefficient of friction (∼0.1) and wear rates much lower than that of unpolished films. Friction and wear properties of the polished films are comparable to that of single‐crystal natural diamond. Based on this study, it is concluded that polished films are potential candidates for tribological applications.

Fullerene (C60) Films for Solid Lubrication

The advent of techniques for producing gram quantities of a new form of stable, pure, solid carbon, designated as fullerene, opens a profusion of possibilities to be explored in many disciplines including tribology. Fullerenes take the form of hollow, geodesic domes, which are formed from a network of pentagons and hexagons with covalently bonded carbon atoms. The C60 molecule has the highest possible symmetry (icosaliedral) and assumes the shape of a soccer ball. At room temperature, fullerene molecules pack in a face centered cubic (fee) lattice bonded with weak van der Waals attractions. Fullerenes can be dissolved in solvents such as toluene and benzene and are easily sublimed. The low surface energy, high chemical stability, spherical shape, weak intermolecular bonding, and high load bearing capacity of C60 molecules offer potential for various mechanical and tribological applications. This paper describes the crystal structure and properties of fullerenes and proposes a mechanism for self-lubricatin...

Sublimed C60 films for tribology

Fullerenes take the form of hollow, geodesic domes, which are formed from a network of pentagons and hexagons. The C60 molecule has the highest possible symmetry (icosahedral) and assumes the shape of a soccer ball. At room temperature, fullerene molecules pack in a face‐centered‐cubic lattice bonded with weak van der Waals attractions. Fullerenes can be dissolved in solvents such as toluene and benzene and easily sublimed. The resilience, high load bearing capacity, low surface energy, high chemical stability, and spherical shape of C60 molecules and weak intermolecular bonding offer great potential for various mechanical and tribological applications. Sublimed films of C60 have been produced and friction and wear performance of these films in various operating environments are the subject of this letter.

Tribology of ion bombarded silicon for micromechanical applications

Silicon is used in the fabrication of microelectromechanical systems (MEMS). The friction and wear characteristics are of major design concern for any mechanical device requiring relative motion. In the present investigations we have studied the influence of ion bombardment on the microstructure, crystallinity, composition, microhardness, friction, and wear behavior. The ion bombardment modifies the elastic/plastic deformation characteristics and crack nucleation that occurs during the indentation. C+ bombarded monocrystalline and polycrystalline Si exhibit very low coefficient of friction (0.025–0.05) and wear factors (10−7 mm3 /N m) while slid against 52100 steel and alumina in dry and moist air and dry nitrogen atmospheres. Ion bombardment resulted in the formation of an amorphized layer that consists of SiC, C, and Si. We have shown that the improvements in friction and wear are because of the formation of SiC and not because of amorphization alone.

Materials characterization and effect of purity and ion implantation on the friction and wear of sublimed fullerene films

In previous studies, sublimed C 60 -rich fullerene films on silicon, when slid against a 52100 steel ball under dry conditions, have exhibited low coefficient of friction (∼0.12). Films with different purities can be produced by sublimation at different substrate temperatures. In this paper, effects of purity of fullerene films and ion implantation of the films with Ar ions on the friction and wear properties of sublimed fullerene films are reported. C 60 -rich films (called here films with high purity) exhibit low macroscale friction. An increased amount of C 70 and impurities in the fullerene film determined using Raman and Fourier transform infrared (FTIR), increases its coefficient of friction. Microscale friction measurements using friction force microscopy also exhibited similar trends. Low coefficient of friction of sublimed C 60 -rich films on silicon is probably due to the formation of a tenacious transfer film of C 60 molecules on the mating 52100 steel ball surface. Based on scanning tunneling microscopy (STM), transmission electron microscopy (TEM), and high resolution TEM (HRTEM), we found that fullerene films primarily consisted of C 60 molecules in a fcc lattice structure. Nanoindenter was used to measure hardness and elastic modulus of the as-deposited films. Ion-implantation with 1 × 10 16 Ar + cm −2 reduced macroscale friction down to about 0.10 from 0.12 with an increase in wear life by a factor of 4; however, doses of 5 × 10 16 ions cm −2 gave three times higher friction and poorer wear life; higher doses disintegrated the C 60 molecules. Based on STM, TEM, Raman, FTIR, and laser desorption Fourier-transform ion cyclotron resonance mass spectrometer (LD/FT/ICR) studies, we found that the ion implantation with a dose of 1 × 10 16 Ar + cm −2 resulted in smoothening of the fullerene film surface probably by compacting clusters, but without disintegrating the C 60 molecules. However, a high dose of 5 × 10 16 Ar + cm −2 damaged the C 60 molecules, converting it to an amorphous carbon. Nanoindentation studies show that ion implantation with a dose of 1 × 10 16 Ar + cm −2 resulted in an increase in the hardness from about 1.2 to 4.0 GPa and in elastic modulus from about 70 to 75 GPa and modified the elastic-plastic deformation behavior.

Modification of Tribological Properties of Silicon by Boron Ion Implantation

Friction and wear properties of silicon used in the fabrication of microelectromechanical systems (MEMS) are important for their long-term reliability. In the present study, the authors have implanted single-crystal and polycrystalline silicon wafers with boron ions to improve their mechanical and tribological properties. The authors have studied the effects of ion implantation on the crystallinity, microstructure, nanohardness, and friction and wear properties and have found that silicon remains crystalline after ion bombardment at doses up to 2 × 1017 ions.cm−2 but with a large amount of defects. The ion bombardment modifies elastic/plastic deformation characteristics and crack nucleation that occurs during indentation. There is a minor increase, ˜ 10-15 percent, in the nanohardness as a result of boron-ion implantation. Ion bombarded single-crystal silicon exhibits very low friction (0.05) and low wear factor (10−6 mm3·N−1m−1) while slid against a 52100 steel ball. The coefficient of friction of bombar...

Nanoindentation studies of ion implanted silicon

Abstract Cyclic nanoindentation studies were conducted on (111) single-crystal silicon modified by implanting O + , N + 2 , P + , As + , and Ar + ions with different doses ranging from 1 × 10 16 to 5 × 10 17 ions cm -2 at 200 keV. Hysteresis in cyclic indentation and discontinuity kinks during unloading are considerably reduced by the ion implantation. Ion implantation of compound-forming species O + and N + 2 into silicon improves the resistance to deformation, which is defined as the ratio of hardness to the square of Youngs modulus. By contrast, the implantation of P + , As + , and Ar + species that do not form compounds reduced the resistance to deformation to almost half. Based on this study, it is concluded that the ion implantation with an appropriate ion species can be used to modify resistance to deformation.