Vilas N. Koinkar

Nanoindentation hardness measurements using atomic force microscopy

An atomic force microscope (AFM), with a specially prepared diamond tip, has been modified to measure indentation hardness with an indentation depth as low as 1 nm. This indentation depth is much smaller than the depth of more than 20 nm that have been reported to date. The AFM indentation technique allows the hardness measurements of surface monolayers and ultrathin films in multilayered structures at very shallow depths and low loads. The nanoindentation hardness of single crystal silicon is measured using this technique. A subtraction technique is also described which allows the actual hardness measurements of rough surfaces such as magnetic thin film rigid disks.

Microtribological studies of unlubricated and lubricated surfaces using atomic force/friction force microscopy

Microtribological studies were performed on unlubricated and lubricated (Z‐15 and Z‐DOL with hydroxyl end groups) silicon, double grafted C18 layer, Langmuir–Blodgett (LB) film, gold film and mica samples in ambient, dry nitrogen and dry air environments using atomic force/friction force microscopy (AFM/FFM). A sharp tip of AFM/FFM sliding on a surface simulates a single asperity contact. The silicon samples lubricated with Z‐15 and Z‐DOL lubricant films exhibit lower coefficient of microfriction than that of the unlubricated silicon sample. Microfriction values are lower than that of the macrofriction. Ploughing contribution is responsible for the higher macrofriction values. The microfriction experiments on bare silicon and lubricated with Z‐15 and Z‐DOL carried out in dry nitrogen and dry air exhibit lower coefficient of microfriction values than those in ambient atmosphere measurements. Dewetting of the liquid film in a humid environment is believed to be responsible for higher microfriction. The dependence of scanning velocity on microfriction was studied and it was found that the friction force decreases with an increase in the scanning velocity for the liquid‐lubricated samples in the humid environment and it is relatively insensitive to scanning velocity in dry environments. Alignment of liquid molecules (shear thinning) is believed to be responsible for the drop in microfriction at high scanning velocities. Micro‐ and nanowear tests reveal that the chemically bonded Z‐DOL lubricant is more durable than the Z‐15, double grafted C18, LB and gold films.

Tribological studies of silicon for magnetic recording applications (invited)

In the present study our objective is to investigate whether the friction and wear performance of bare silicon is adequate for disk drive application or whether certain coatings/treatments are necessary for low friction and wear. Macrotribological experiments have been performed with various pin/slider materials and magnetic disks in a modified disk drive. Microtribological studies have also been conducted on silicon using a friction force microscope. Based on macrotests, we found that the friction and wear performance of bare silicon is not adequate. With single and polycrystalline silicon, transfer of amorphous carbon from the disk to the pin/slider and oxidation‐enhanced fracture of pin/slider material followed by oxidation of the transfer coating (tribochemical oxidation) is responsible for degradation of the sliding interface and consequent friction increase in ambient air. With dry‐oxidized or plasma‐enhanced chemical‐vapor deposition‐SiO2‐coated silicon, no significant friction increase or interfac...

Effect of scan size and surface roughness on microscale friction measurements

The effect of scan size (scan length) and surface roughness on microscale friction was studied using atomic force/friction force microscopy. Three silicon specimens with different surface roughnesses were studied. Surface height and friction force plots were obtained simultaneously and friction mechanisms for the correlation between them were sought. The standard deviation of surface heights increases with the scan size initially and approaches a constant value at a scan length greater than the long wavelength limit of the roughness structure. Change in the value of the coefficient of friction at different scan lengths is random. In these measurements, the sampling interval is always lower than the correlation length which is believed to be responsible for the random variation in microscale friction with the scan size. The good correlation observed between the local change in friction force and the surface slope can be explained by a ratchet mechanism.

Micro/nanoscale studies of boundary layers of liquid lubricants for magnetic disks

The atomic force/friction force microscope is used to study the micro/nanotribiological properties of perfluoropolyether lubricants. Single‐crystal silicon wafers were lubricated with nonpolar (Z‐15) and polar (Z‐DOL and Demnum S‐100) lubricants. The nanowear tests show that the nonpolar (Z‐15) lubricant depleted from the wear track within a few cycles, whereas polar (Z‐DOL) lubricant exhibits excellent nanowear resistance with no degradation. The polar lubricant results in a lower value of microfriction as compared to the nonpolar lubricant and unlubricated silicon sample. The effect of thickness of polar lubricant is studied for the thermally bonded Z‐DOL lubricant before and after wash. Unwashed polar lubricant film with unbonded fraction exhibited better resistance to wear than that of washed lubricant film. Thicker films are also more durable. Wear experiments with magnetic disks show that lubricant films on a super smooth disk is more effective in reduction of friction and wear than a smooth disk. C...

Microtribology of Magnetic Media

Scanning tunnelling microscopy (STM), atomic force microscopy (AFM) and the modifications of AFM [such as friction force microscopy (FFM)] are becoming increasingly important in the understanding of fundamental mechanisms of friction, wear and lubrication and in studying the interfacial phenomena in micro- and nanostructures used in magnetic storage devices and microelectromechanical systems (MEMS). This paper describes modified AFM and FFM techniques and presents data on microtribological studies of magnetic media-magnetic tapes and disks. Local variation in microscale friction is found to correspond to the local slope, suggesting that a ratchet mechanism is responsible for this variation. Wear rates for magnetic tapes are approximately constant for various loads and test duration. However, for magnetic disks, the wear of the diamond-like carbon overcoat is catastrophic. Evolution of the wear has also been studied using AFM. AFM has been modified for nanoindentation hardness measurements. It has been shown that hardness of ultra-thin films can be measured using AFM. AFM has also been shown to be useful for nanofabrication.

Scanning and transmission electron microscopies of single-crystal silicon microworn/machined using atomic force microscopy

Atomic force microscopy (AFM) is commonly used for microwear/machining studies of materials at very light loads. To understand material removal mechanism on the microscale, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies were conducted on the microworn/machined single-crystal silicon. SEM studies of micromachined single-crystal silicon indicate that at light loads material is removed by ploughing. Fine particulate debris is observed at light loads. At higher loads, cutting type and ribbon-like debris were observed. This debris is loose and can be easily removed by scanning with an AFM tip. TEM images of a wear mark generated at 40 μN show bend contours in and around the wear mark, suggesting that there are residual stresses. Dislocations, cracks, or any special features were not observed inside or outside wear marks using plan-view TEM. Therefore, material is mostly removed in a brittle manner or by chipping without major dislocation activity, crack formation, and phase transformation at the surface. However, presence of ribbon-like debris suggests some plastic deformation as well.

Microtribological studies of doped single-crystal silicon and polysilicon films for MEMS devices

Abstract Microelectromechanical systems (MEMS) devices are made of doped single-crystal silicon, LPCVD polysilicon films and other ceramic films. Very little is understood about tribology and mechanical characterization of these materials on micro- to nanoscales. Atomic force microscopy/friction force microscopy (AFM/FFM) have been used to study surface roughness, friction, scratching/wear and indentation hardness, and Youngs modulus of elasticity of p-type (lightly boron-doped) single-crystal silicon (referred to as ‘undoped’), p + -type (boron-doped) single-crystal silicon, polysilicon, and n + -type (phosphorus-doped) LPCVD polysilicon films. Undoped silicon samples are very smooth (r.m.s. = 0.08 nm). Surface-roughness values for p + -type silicon and lapped polysilicon bulk samples are comparable. As-deposited polysilicon films are rougher than undoped and doped single-crystal silicon, unlapped and lapped polysilicon bulk samples. Microscale friction measurements show that the friction values for undoped silicon, p + -type silicon and lapped polysilicon bulk samples are comparable. Polysilicon films exhibit the highest coefficient of friction as compared to other samples, due to the roughness effect. Macroscale friction values for undoped silicon and polysilicon bulk are comparable with those of p + -type silicon sample having the highest value and polysilicon films having the lowest values. The microscale friction values for all samples are lower than the macroscale friction values as there is less ploughing contribution in microscale friction measurements. Local variation in microscale friction is found generally to correspond to the local surface slope. Directionality in the friction is generally observed on the microscale which results from the surface preparation and anisotropy in surface roughness. Microscratching/wear studies indicate that scratch and wear resistance of p + -type silicon is poor as compared to other samples used in this study. Nanoindentation studies indicate that the p + -type silicon sample exhibits lowest hardness and Youngs modulus as compared to the other samples, which is responsible for high macrofriction and poor microscratch and microwear behavior of the p + -type silicon sample. Microtribological behavior and nanohardness and Youngs modulus values for polysilicon bulk and n + -type polysilicon film are comparable.

Microtribology of PET Polymeric Films

Atomic force microscopy (AFM) and friction force microscopy (FFM) are used to conduct microtribological studies of thin polyethylene terephthalate (PET) films which are commonly used in many industrial applications including as a magnetic tape substrate. In the PET films, microscale friction values are found to be smaller than macrofriction values. Local variation in microscale friction is found to correspond to the local slope which suggests that a ratchet mechanism is responsible for this variation. There is also a directionality effect in the local variation of microfriction. Micro-wear tests are also reported. The polymeric material tears in micro-wear tests. Ceramic particles added to the PET films as an anti-slip agent, affect the microfriction values, scratch depth and wear depth. For the polymeric material, scratch depth and wear depth increase approximately linearly with an increase in the normal load and test duration. Nanoindenlation behavior and nanohardness values of the films vary from one l...

Microtribological studies of Al2O3, Al2O3TiC, polycrystalline and single-crystal MnZn ferrite, and SiC head slider materials

Abstract Surface topography, friction, scratching, wear and nanoindentation behavior of head slider materials were studied using atomic force/ friction force microscopy (AFM/FFM). Materials studied include polycrystalline Al 2 O 3 , Al 2 O 3 TiC (70-30 wt.%), polycrystalline MnZn ferrite, single-crystal MnZn ferrite and polycrystalline SiC. Al 2 O 3 is not used for construction of head slider but it was studied for comparisons with Al 2 O 3 TiC. SiC is a candidate slider material. The Al 2 O 3 TiC specimen exhibits the highest coefficient of microscale friction followed by polycrystalline MnZn ferrite, Al 2 O 3 , single-crystal Mn ferrite, Al 2 O 3 , single-crystal MnZn ferrite and SiC. Two phase Al 2 O 3 TiC exhibited higher microscale friction than a single phase Al 2 O 3 and polycrystalline MnZn ferrite exhibits microscale friction than a single-crystal MnZn ferrite. The local variation in friction force for Al 2 O 3 , polycrystalline and single-crystal MnZn ferrite was found to correspond to the lcoal surface slope (first derivative of the surface roughness profile). For Al 2 O 3 TiC and SiC specimen, the local variation in friction force at the scratches corresponds to the local surface slope. However, local variation over entire region corresponds to the different phases present on the specimen surface. Thus friction force imaging can be used for structural mapping of the surfaces. Directionality effects in the friction were studied on a microscale for the Al 2 O 3 specimen. Macroscale friction values for all samples are higher than microscale friction values as there is less ploughing contribution in microscale friction measurements. Microscratching/wear studies indicate that scratch and wear resistance of SiC and Al 2 O 3 are higher than that of the other specimens. However, Al 2 O 3 specimen shows the presence of porous holes on the surface that affect the wear performance. Al 2 O 3 specimen exhibits superior microscratch and microwear resistance than Al 2 O 3 TiC. Wear evolution for single and two phase materials was studied in detail. Nonoindentation hardness values found to be generally higher than that of microhardness data. Based on these measurements, coefficient of friction, microscratch and microwear resistance of singe phase materials are superior to that of two phase materials.