Tomoko Hirayama

A Theoretical Analysis Considering Cavitation Occurrence in Oil-Lubricated Spiral-Grooved Journal Bearings With Experimental Verification

Performances of an oil-lubricated spiral-grooved journal bearing are investigated in this paper with special attention paid to cavitation occurrence. The equivalent flow model, which is a theoretical scheme for taking the cavitation occurrence into hydrodynamic lubrication theory, is applied to the analyses by a finite difference treatment of the Reynolds equation that deals with the geometry of a finite number of grooves. The calculated results are compared with experimental results under eccentric states, and verified in terms of cavitation map and pressure distribution. The cavitated area ratio, load capacity and bearing stiffness are also theoretically calculated. The difference between the theoretical results with and without consideration of the cavitation occurrence is considerable, and thus the influence of cavitation occurrence should not be ignored in theoretical studies on bearing characteristics.

Fatty Acid Adsorption on Several DLC Coatings Studied by Neutron Reflectometry

The application of diamond-like carbon (DLC) coatings on the contacts of mechanical systems is becoming widespread thanks to their excellent tribological properties. Numerous studies of DLC coatings have been reported over the past decade and, as a result, the understanding of their lubrication has improved. The tribological properties of boundary-lubricated contacts are drastically affected by adsorbed layers; however, due to the variety of lubricant additives and coating structures, no general adsorption mechanisms for DLC coatings have been put forward until now. This has, unfortunately, hindered improvements in their lubrication performance. Many of the essential physical properties of the adsorbed layers also remain undefined. In this work, we used neutron reflectometry to determine the thickness and the density of the adsorbed layers of fatty acid molecules on coatings of a-C, a-C:H, a-C:H:F and a-C:H:Si. The results showed that a 0.9-nm-thick layer adsorbed onto the a-C and a-C:H coatings. In contrast, both doped coatings, i.e. the a-C:H:F and a-C:H:Si, showed a poorer adsorption ability towards the fatty acid molecules than the a-C and a-C:H. Continuous adsorption layers were not detected on the a-C:H:F and a-C:H:Si; however, the possibility of adsorption in lower quantities cannot be ruled out.

Effect of Nanotexturing on Increase in Elastohydrodynamic Lubrication Oil Film Thickness

The effects of nanotexturing on oil film thickness and shape under pointcontact elasto-hydrodynamic lubrication (EHL) conditions were experimentally investigated. A disk-onball friction tester with an optical interferometer was used to measure oil film thickness and to observe the oil film shape. Periodic groove structures with a spiral, perpendicular, or parallel shape and with various groove depths and distances were formed by irradiation of a femtosecond laser onto the surface of steel balls. These nanotextured balls were tested under a load of 20 N and at rotational speeds from 1.0 to 3.0 m/s. Most of the balls with nanotexturing had a thicker oil film than those without texturing. The groove depth and angle were the key parameters determining the thickness of the oil film as they controlled the amount of side leakage of oil from the contact point. Optimization of these parameters resulted in an oil film that was almost twice as thick as that on the ball without texturing.

Cross-Sectional Imaging of Boundary Lubrication Layer Formed by Fatty Acid by Means of Frequency-Modulation Atomic Force Microscopy

To observe in situ the adsorption of fatty acid onto metal surfaces, cross-sectional images of the adsorption layer were acquired by frequency-modulation atomic force microscopy (FM-AFM). Hexadecane and palmitic acid were used as the base oil and typical fatty acid, respectively. A Cu-coated silicon wafer was prepared as the target substrate. The solvation structure formed by hexadecane molecules at the interface between the Cu substrate and the hexadecane was observed, and the layer pitch was found to be about 0.6 nm, which corresponds to the height of hexadecane molecules. This demonstrates that hexadecane molecules physically adsorbed onto the surface due to van der Waals forces with lying orientation because hexadecane is a nonpolar hydrocarbon. When hexadecane with palmitic acid was put on the Cu substrate instead of pure hexadecane, an adsorption layer of palmitic acid was observed at the interface. The layer pitch was about 2.5-2.8 nm, which matches the chain length of palmitic acid molecules well. This indicates that the original adsorption layer was monolayer or single bilayer in the local area. In addition, a cross-sectional image captured 1 h after observation started to reveal that the adsorbed additive layer gradually grew up to be thicker than about 20 nm due to an external stimulus, such as cantilever oscillation. This is the first report of in situ observation of an adsorbed layer by FM-AFM in the tribology field and demonstrates that FM-AFM is useful for clarifying the actual boundary lubrication mechanism.

Nano-Ordered Wear Property of Magnesium Obtaining Nanoindentation

The nano-scale wear properties of single-crystalline and polycrystalline magnesium were investigated using the nanoindentation technique. The results for single crystals revealed that the crystal orientation affected the wear rate in the case that no deformation twinning was formed, e.g., under conditions of low applied loads. However, when deformation twinning formed during scratch testing, the wear properties became worse, i.e., the wear rate increased. One reason was that twin boundaries did not play a role as dislocation sources and sinks; the dislocations at twin boundaries brought about the expansion and growth for deformation twinning. As for the impact of grain boundaries on the wear properties, the wear rate for fine-grained magnesium was similar to that for single crystals. This result indicated that the existence of grain boundaries did not effectively improve the wear properties of magnesium, as in large-scale wear testing, such as the pin-on-disk configuration.

Development of ultraprecise positioning X–Y stage composed of pneumatic servo bearing actuators:

A prototype X–Y stage for ultraprecise positioning composed of six pneumatic servo bearing actuators was proposed and developed. First, the basic performance of the pneumatic servo bearing actuator with a spherical aerostatic bearing was investigated before assembling it to the X–Y stage. In the actuator, pressurized air controlled by a servo valve moves an actuated spool ultraprecisely. Even under open-loop control, the actuator provided accurate replication and positioning resolution on a nanometer order. Second, the specifications of the constructed X–Y stage were evaluated, especially focusing on its minimum resolution. The minimum positioning resolution of the X–Y stage was 3 nm under the best conditions, and the positioning resolution as a practical specification was approximately 10 nm for all conditions, even without feedback controls.

Analysis and experimental study on oil leakage from fluid bearing

The fundamental characteristics of the taper seal used to prevent oil leakage from fluid bearings were analysed theoretically and investigated experimentally. The three-dimensional flow field was theoretically analysed using numerical fluid dynamics, and the profile of the oil meniscus was calculated using the balance between the pressure field and surface tension. The oil meniscus was experimentally investigated using a micro-focus laser microscope. Three triggers for oil leakage were identified: the oil contact angle on the sleeve surface becoming excessive, the convex portion of the meniscus profile extending beyond the edge of the sleeve, and cavitation occurring in the oil.

Effect of Alloying Elements on Nano-ordered Wear Property of Magnesium Alloys

The effect of alloying elements on nano-ordered wear properties was investigated using fine-grained pure magnesium and several types of 0.3 at. pct X (X = Ag, Al, Ca, Li, Mn, Y, and Zn) binary alloys. They had an average grain size of 3 to 5 μm and a basal texture due to their production by the extrusion process. The specific wear rate was influenced by the alloying element; the Mg-Ca and Mg-Mn alloys showed the best and worst wear property, respectively, among the present alloying elements, which was the same trend as that for indentation hardness. Deformed microstructural observations revealed no formation of deformation twins, because of the high activation of grain boundary-induced plasticity. On the contrary, according to scratched surface observations, when grain boundary sliding partially contributed to deformation, these alloys had large specific wear rates. These results revealed that the wear property of magnesium alloys was closely related to the plastic deformation mechanism. The prevention of grain boundary sliding is important to improve the wear property, which is the same as that of a large-scale wearing configuration. One of the influential factors is the change in the lattice parameter with the chemical composition, i.e., ∂(c/a)/∂C. An alloying element that has a large value of ∂(c/a)/∂C effectively enhances the wear property.

Hydrodynamic Performance Produced by Nanotexturing in Submicrometer Clearance With Surface Roughness

Surface texturing is a promising way to expand the hydrodynamic lubrication regime and thereby modify the tribological properties of sliding surfaces. Spiral-groove textures in particular have attracted much attention over the past several decades because they produce a thicker lubrication film in the gap. However, no research has been reported on the effect of periodic texturing with a several 100 nm depth on hydrodynamic performance in submicrometer clearance with surface roughness. The purpose of the study reported here was to investigate the effect of such nanotexturing on hydrodynamic performance. This was done by conducting ring-on-disk friction tests, focusing on the existence of surface roughness in the narrow clearance. The samples were rings with various degrees of surface roughness and disks with spiral-groove textures produced by femtosecond laser processing. The friction coefficients experimentally obtained were plotted as a Stribeck curve and compared with a theoretical one calculated using a Reynolds equation formulated from two physical models, the Patir–Cheng average flow model and a sinusoidal wave model. The results showed that surface roughness did not affect the friction coefficient in the hydrodynamic lubrication regime. However, the hydrodynamic lubrication regime gradually shrank with an increase in surface roughness, and mild transitions to the mixed lubrication regime were observed at higher rotational speeds. The minimum clearances reached at the transition speed were almost the same, about 200–300 nm, for all experiments regardless of surface roughness.

Hydrodynamic bearing with non-uniform spiral grooves for high-speed HDD spindle

A novel hydrodynamic bearing with non-uniform spiral grooves was proposed for high-speed HDD spindle in this paper. Theoretical approach indicated that the non-uniform grooved bearing expanded the critical bearing number about 123% larger than the classic bearing with uniform grooves. The utility of the proposed bearing for high-speed spindle was experimentally confirmed.