Annex IV: Cooperative Program on Integrated Engineered Surface Technology

Chairman                                                                    

Dr. Stephen M. Hsu
Email: stephen.hsu@erols.com

Background

Friction arises from two interacting surfaces sliding over one another under load. The nature of the surface properties (topography, hardness, elasticity, etc.) and the operating conditions control influences the magnitude of the frictional force.  Surface topography or roughness in contact has been known to affect friction in engines, therefore, attention has been paid to how surface finishing techniques such as grinding, lapping, polishing, etc. influence engine performance. Recently, laser dimpled surfaces have demonstrated that surface texturing can provide control of friction under certain conditions. In engines, cross-hatching texturing of diesel cylinder liners was introduced in the 1940’s to prevent scuffing and increase durability. Additionally, coatings and thin films have been used to increase wear life of engine components for decades. Yet the fundamental understanding of how to design a surface using surface textures, thin films, coatings, together to achieve synergistic surface technology to control friction has eluded the research community.

In the mid 1990s, dimples and other forms of surface textures have been examined as a means to increase durability and reduce friction. In the magnetic hard disk area, laser bumps have been introduced to reduce stiction. In the orthopaedic joint replacement, patterned circular holes have been tried to trap wear particles. Meanwhile, we have also witnessed the tremendous progress made in lubricating thin films, functional gradient multilayer films, and nano-composite films. The potential of a vertically integrated engineered surface combining surface texturing, thin films, and lubrication to The use of surface modification technology can have significantly impact on fuel efficiency in the transportation technologies, especially in the heavy duty trucks engine area which is evolving rapidly to meet the pending emission standards. Successful application of engineered surface technology to a host of engine technologies can lead to significant benefits in terms of fuel economy, scuffing resistance, and longer durability. However, the benefits have to be weighted against the added cost of fabricating the surfaces.

Objective

The objective of this Annex is to conduct cooperative research and development on an integrated engineered surface technology, leading to the standardization of testing and characterization methods for textured surfaces made from various materials. Through this collaboration, participating countries may avoid unnecessary duplication of research efforts, thereby accelerating the achievement of higher fuel economy.

Subtask 1: Technical Information Exchange

The objective of this Subtask is to achieve a balanced exchange of technical information among the participating countries on engineered surface technology applicable to engines and engine components.

Subtask 2: Characterization of Engineered Surfaces

When a surface is textured, coated, and lubricated, the local properties and the global properties must be fully characterized to quantify the benefits. This characterization must include physical, mechanical, and chemical characterization. Many of the characterization methods have not been commonly accepted or agreed upon by the scientific community.

Subtask 3: Evaluation of Engineered Surfaces

There are no standard test methods or internationally agreed upon standard practices for the determination of the effectiveness of a textured, coated, and lubricated surface fabricated from various materials. The goal of this Subtask is to compare the currently available test techniques used to evaluate such surfaces and to develop new test methods where needed.

Current participants

Technical highlights

The basic idea of controlling the surface topography to improve friction and wear performance has been recognized since the early 1900s and has been practiced in specific instances through trial and error for improved performance and durability. Commercialization of such concept is limited by cost-effectiveness ratio between the increased production costs versus benefits gained. The development of the technology has also been limited by the lack of understanding of the fundamental processes involved in the friction reduction mechanisms and its interplay with various regimes of lubrication.

As technology evolves, advanced surface processing techniques such as controlled grinding, chemo-mechanical polishing, and single diamond turning techniques have become available to precisely control surface topography. Laser texturing using tightly focused advanced lasers has also been developed (1-3). Photolithography combined with chemical etching has become increasingly common and they have been successfully used to texture surfaces (4). With the availability of nanoindenters and scratch testers, nanomechanical means to carve textural features on surfaces have become feasible to produce precisely controlled geometric sizes and shapes at micrometer scales. Focused ion beam techniques and reactive ion etching (RIE) have also been used to texture surfaces (5). Thin films and coatings deposition techniques are sufficiently controlled that intrinsic surface textures can be designed into the coatings. The availability of all these tools has enabled a new wave of surface texturing engineering developments.

Cross-hatching of diesel cylinder liner to reduce scuffing and seizure was introduced in the 1940s and they are still in-use today (6). Dimples have been introduced on the surface of golf balls to reduce the aerodynamic drag so that they can fly longer and straighter. Modern tires use intricate surface textural designs to control traction and friction with road surfaces, and this has spawned a whole new area of technological innovations (7). As fabrication methods and materials continue to improve, surface engineering and textural control are increasingly being recognized as potential tool to overcome specific performance challenges. For example, the magnetic hard disk technology uses laser textured bumps to reduce contact area in the landing zone to minimize stiction (8). Pits have been introduced in metal forming operations to prevent adhesion and seizure due to lubricant starvation (9). Recently, laser ablation has been used successfully to create dimples on mechanical seal surfaces to reduce friction and increase durability (10).

These examples highlight the increasing use of surface texturing as part of surface engineering technology to achieve desirable outcomes through empirical trial and error. Our understanding of the textures, patterns, shapes, and orientation, however, remains rudimentary. One of the issues is the interplay between operating conditions and surface texture size, shape, and patterns. The fundamental understanding of how surface textures influence the contact mechanics and the friction phenomenon under different load and sliding speed conditions is poorly understood.

The effects of geometric shapes were evaluated with a conventional pin-on-disk wear tester. Load and sliding speed were varied to control the contact conditions. Tests were conducted using mineral oil without additives having a kinematic viscosity of 2.73E-5 m^2/s at 40 degrees Celsius. Pin and disk specimens were fabricated from cold rolled 1017/1018 steel and 304 stainless steel. Friction force was monitored and recorded during the test.

Two methods could be used to produce surface textures on specimens. One is a photolithography coupled with chemical etching technique. A mask with the desirable feature size and shape was fabricated. A photoresist was applied to the polished (Ra ≈ 0.01 micro-meter) test surface and exposed to light through the mask. After development the specimen was electrolytically etched to produce the desirable textural features. The other method was by mechanical scribing. A scribe (Rockwell “C” indenter) was held without rotation in the spindle of a small milling machine and pressed against the specimen mounted on the milling machine table. By controlling the table motion and application of the scribe, either indentations or grooves could be produced. After scribing, the specimen was polished flat to remove the raised lips around the scribed features.

Various features consisting circular dimples, triangles, and ellipses were fabricated at the same area coverage. The effects of orientation of the features with respect to the sliding directions were also studied. Test results for the patterns are shown in the figure below and compared to a polished surface without texture. The coefficient of friction is plotted as a function of the Sommerfeld number (viscosity × speed/load). This figure shows the Stribeck curves of untextured and textured specimens with the surface features oriented parallel and perpendicular to the sliding direction. All of the curves show clear transitions from hydrodynamic to mixed lubrication. Overall, the untextured surface has the highest friction with circular dimples show lower mixed lubrication friction but higher transition friction. The ellipses and triangles show similar trends and slightly better than the untextured surface.

The effect of geometrical shapes on coefficient of friction under pin-on-disk experiments ran under paraffin lubrication in a steel-on-steel contact geometry.

When the textures are oriented perpendicular to the sliding direction, both the reversed triangles and ellipses (become horizontal slits) all show dramatic improvement with the elliptical surface features having the lowest friction and also the lowest transition point in the Stribeck curve shown in the figure above. 

References

Technical Workshops and Symposiums held under this Annex:

For further information contact Stephen Hsu at stephen.hsu@erols.com.