Surface texturing has emerged in the last decade as a viable option of surface engineering resulting in significant improvement in load capacity, wear resistance, friction coefficient etc. of tribological mechanical components. Various techniques can be employed for surface texturing but Laser Surface Texturing (LST) is probably the most advanced so far. LST produces a very large number of micro-dimples on the surface and each of these micro-dimples can serve either as a micro-hydrodynamic bearing in cases of full or mixed lubrication, a micro-reservoir for lubricant in cases of starved lubrication conditions, or a micro-trap for wear debris in either lubricated or dry sliding. The present paper reviews the current effort being made world wide on surface texturing in general and on laser surface texturing in particular. It presents the state of the art in LST and the potential of this technology in various lubricated applications like mechanical seals, piston rings and thrust bearings. The paper also describes some fundamental on going research around the world with LST.

State of the art in Laser Surface Texturing

Surface texturing has emerged in the last decade as a viable option of surface engineering resulting in significant improvement in load capacity, wear resistance, friction coefficient etc. of tribological mechanical components. Various techniques can be employed for surface texturing but Laser Surface Texturing (LST) is probably the most advanced so far. LST produces a very large number of micro-dimples on the surface and each of these micro- dimples can serve either as a micro-hydrodynamic bearing in cases of full or mixed lubrication, a micro-reservoir for lubricant in cases of starved lubrication conditions, or a micro-trap for wear debris in either lubricated or dry sliding. The paper reviews the current effort being made world wide on laser surface texturing in particular. It presents the state of the art in LST and the potential of this technology in various lubricated applications like mechanical seals, piston rings, thrust bearings, magnetic recording etc. The paper also describes some fundamental on-going research around the world with LST.

State of the Art in Laser Surface Texturing

https://cyberleninka.org/article/n/544115.pdf

Hydrodynamic lubrication of textured surfaces: A review of modeling techniques and key findings

A critical assessment of surface texturing for friction and wear improvement

We introduced some of the basic principles, techniques, and key design issues common to ISPs used in many diverse scientific, military, and commercial applications, and we touched on some of the less intuitive effects that must be dealt with. Many of these effects can influence the initial configuration of the system as well as design details later in the design process. The successful design of an ISP usually requires a multidisciplinary design team. The design of an ISP must often be closely coordinated with that of other major subsystems such as the primary sensor and the optics. The role of the systems engineer in the design process is perhaps the most critical because other members of the team may not be aware of the consequences of many of the effects discussed above. Inertially stabilized platforms (ISPs) are used to stabilize and point a broad array of sensors, cameras, telescopes, and weapon systems.

Inertially stabilized platform technology Concepts and principles

This paper presents a numerical study of the effects of different shapes of deterministic microasperities in sliding surface lubrication when hydrodynamic films are found. Positive (protruding) and negative (recessed) asperities of constant height (depth) are considered with circular, square, diamond, hexagonal and triangular cross-sections. Of particular interest is the impact of asperity/cavity cross-sectional geometry on friction and leakage, which has importance in sealing applications. The results indicate that the friction coefficient is insensitive to asperity/cavity shape, but quite sensitive to the size of the cross-section. By contrast, leakage rates are found to be quite sensitive to both cross-sectional shape and size, with triangular asperities giving the smallest leakage rate and square asperities giving a largest leakage rate. The minimum coefficient of friction for all shapes is found to occur at an asperity area fraction of 0.2 for positive asperities and 0.7 for negative asperities. Finally, the results indicate the existence of a critical asperity area fraction where the performance curves for positive and negative asperities cross over These cross-over points are identified for friction coefficient and leakage rate.

Effect of Deterministic Asperity Geometry on Hydrodynamic Lubrication

Deterministic micro asperities show potential for enhancement of lubrication in conformal contacts as found in many bearing and seal designs. Several manufacturing methods have been proposed for deterministic micro asperities. Of these, laser texturing has emerged as the most viable option. This paper proposes the LIGA MEMs manufacturing method as an alternative. Using LIGA, surfaces with patterned micron sized surface features of arbitrary cross section (cylindrical, hex, triangular, etc.) can be fabricated from electroplated nickel, gel-cast silicon nitride, or plastic. The resulting asperities can be positive (protuberances) or negative (recesses) and can have heights (depths) from 1-1000 microns and be patterned over surface areas up to about 150 mm X150 mm. In this paper, the LIGA method is used to fabricate a sample thrust bearing surface with a hexagonal array of positive asperities. The resulting asperities are 550 μm in average diameter, 165 μm in edge-to-edge spacing and have heights of 3-100 μm. Surface metrology indicates submicron accuracy of form and 13 nm Ra roughness on the asperity tops (land). Tribology testing in a nonpressurized oil bath indicates full film conditions and shows a 14-22% reduction in friction coefficient for a thrust surface covered with the micro asperities. A model confirms the experimental trends and indicates the potential to further reduce the friction coefficient by about 60% through optimization of the asperity geometry and layout.

Deterministic Micro Asperities on Bearings and Seals Using a Modified LIGA Process

A numerical analysis is conducted to investigate the elastohydrodynamic effect of deterministic microasperities on the shaft of a lip seal. Various geometries of microasperities (triangular, square, hexagonal, and circular) are put into a 100-by-100-μm unit cell and are investigated using Reynold’s Equation. For each shape, the area fraction of the microasperity is varied between 0.2 and 0.8, and the asperity height is varied between 0.3 and 5 micron. The calculation for load capacity and friction coefficient indicates that there are values for asperity height where load capacity and friction coefficient are optimized. These optimum heights were reached at 1–3 μm. Although the lip seal surface is considered to be smooth, reverse pumping can still be obtained using an oriented triangular design. The Couette flow rate for this asperity showed lubricant is reverted back toward the seal side 2.6 times more than using a conventional lip seal. The presence of all designs in the lip seal shows significant improvement on lubrication characteristics, i.e. increasing load support and reducing friction coefficient.Copyright © 2006 by ASME

Soft Elastohydrodynamic Analysis of Radial Lip Seals With Deterministic Microasperities on the Shaft

A new type of self-bearing motor is presented that uses Lorentz-type forces to produce both bearing force and motoring torque. The design eliminates the tradeoff between motoring torque and bearing force with respect to permanent-magnet thickness found in many previous designs. The self-bearing motor layout and servo control are described including alternative winding schemes. Permeance and flux models are presented and used to derive expressions for torque and force production. As with conventional magnetic bearings, these expressions are quadratic with both rotor position and control current. A linearized force-current- displacement relationship is then derived for a general operating point. Experimental results are presented for a prototype servomotor. These results show good agreement with theory and, hence, validate the permeance and flux models, and the linearized force-current relationship. The prototype servomotor, 152 mm in outer diameter and 25.4 mm, in length, was a peak hearing force capacity of 213.6 N and a peak torque capacity of 24 N/spl middot/m, Which is limited by power amplifier saturation.

Force and torque characteristics for a slotless Lorentz self-bearing servomotor

This work presents experimental results for a heat-sink mechanical seal installed in a 1 x 1.5 x 6 in. ANSI centrifugal water pump. The heat-sink seal is constructed of a stainless steel substrate with an electrodeposited pin fin micro-heat sink located within 3 mm of the end face. Each pin has a ten-sided polygon cross section with a flat-to-flat diameter of 675 μm, a height of 856 μm, and a 300-μm edge-to-edge spacing. The end face is coated with a WC thin film that forms the wear surface for the rotating ring. The heat-sink seal's effectiveness is demonstrated in a significant reduction of both the seal interface and seal chamber temperature when compared to a similar industry standard seal. The heat-sink seal removes 750 W of heat and reduces the stationary ring temperature by 80° C under dead-head conditions using only 1.6 W of coolant pumping power.

Lyndon Scott Stephens

Papers

Effect of Deterministic Asperity Geometry on Hydrodynamic Lubrication

Deterministic Micro Asperities on Bearings and Seals Using a Modified LIGA Process

Soft Elastohydrodynamic Analysis of Radial Lip Seals With Deterministic Microasperities on the Shaft

Force and torque characteristics for a slotless Lorentz self-bearing servomotor

Experimental results for a heat-sink mechanical seal