TLT: What is laser surface texturing?
Etsion: LST is a surface engineering process used to improve tribological characteristics of materials. Using a laser to create patterned microstructures on the surface of the materials can improve load capacity, wear resistance and lubrication lifetime and can reduce friction.

The use of surface irregularities to improve tribological properties was first discussed in the 1960s and is implemented in several manufacturing techniques, including machining, ion beam texturing, etching techniques and laser texturing. While surface texture engineering has been studied for many years, the use of laser patterned surface microstructures for tribological improvements emerged in the 1990s and continues to undergo significant technological advancements.

Lasers offer unparallel control of the surface microstructure and a low environmental impact compared to other surface texturing processes. This is because the laser is extremely fast, allows short processing times, is clean to the environment and provides excellent control of the shape and size of the texture, which allows realization of optimum designs.

By controlling energy density, the laser can safely process hardened steels, ceramics and polymers as well as crystalline structures. Given that friction and wear create unavoidable losses in countless processes and devices, the opportunities for improving efficiency and lifespan with the LST technology are extensive. Indeed, LST is starting to gain more and more attention in the tribology community, as is evident from the growing number of publications on this subject (see Figure 1).


Figure 1. The growing number of publications on surface texturing over the years before and after 1996.

TLT: Please describe how laser surface texturing works in real-world applications.
Etsion: LST produces a very large number of microdimples on a surface (see Figure 2) , and each of these microdimples can serve as a microhydrodynamic bearing in cases of full or mixed lubrication, a microreservoir for lubricant in cases of starved lubrication conditions or as a microtrap for wear debris in either lubricated or dry sliding.


Figure 2. LST regular microsurface structure in the form of microdimples. These particular dimples are spherical cavities with a diameter in the order of 100 μm, depth of a few μm and area density of about 20%.

LST has three main functions:

1. Microhydrodynamic bearing. Each microcavity, whether it is a groove or dimple, acts as a miniature hydrodynamic bearing during relative motion between the two contact surfaces. This hydrodynamic effect is a result of the pressure gradient that forms in each cavity, which can be modeled using the Reynolds equation. With the relative motion between the two contact surfaces and the shear forces acting on the lubricant at the surfaces, a pressure profile is created due to the wedge effect. Microhydrodynamic bearings reduce friction and wear between sliding surfaces. Lubrication is necessary for the microhydrodynamic bearing effect to become influential.

2. Lubricant reservoirs. If an area of surface contact loses lubrication, the microcavities can provide additional lubricant that is drawn to the starved area, possibly through capillary action. The patterned geometry allows for countless miniature lubricant reservoirs, providing direct and immediate lubricant relief for starved areas. In order for these reservoirs to exist, the geometries of the pattern microstructure must be closed to prevent lubricant from being forced out through channels.

3. Debris traps. Microcavities provide a sink for debris particles to fit into and reduce the associated additional abrasive wear of debris in the contact zone. The function of the cavities as debris traps is found in both lubricated and non-lubricated applications and is the main positive effect of surface texturing for non-lubricated applications.

TLT: What are some of the latest advances in surface texturing?
Etsion: In the earlier days, laser surface texturing was applied mainly to mechanical seals, providing about 60% reduction in friction torque and up to a three-fold increase in seal life.

The wear debris trapping function of LST was tested on electrical contacts of vibrating radio devices like cell phones. Vibration of such devices causes fretting wear of the electrical contacts and generates small oxidized wear debris, which have high electrical resistance. Accumulation of this oxidized debris over time eventually causes failure in the device due to loss of good connection with the battery. Laser textured electrical contacts had substantially increased the time between failures compared to non-textured contacts.

In a recent experimental study on a firing 2.5-liter diesel engine, a reduction of up to 4% in fuel consumption was obtained with laser surface textured top piston rings. The number of studied applications that can benefit from surface Texturing increases every year. The list includes tappet shims in valvetrains, journal bearings, cutting tools, adaptive solid lubrication, sliding guideways, elastohydrodynamic lubrication (EHL), soft EHL in elastomers and nanoscale texturing.

Newer laser machines are reasonably priced, and a growing interest in energy conservation bring more and more industries to explore the potential use of surface texturing for their products.


Figure 3. Specially designed test rig for evaluating the effect of LST on mechanical seals and thrust bearings. Tests can be run at speeds of up to 3,000 rpm and fluid pressures up to 25 bars. The rig is capable of measuring friction torque and fluid film thickness for a range of normal loads up to 500 N.

TLT: What are the main challenges of surface texturing?
Etsion: One is the integration of surface texturing in mass production of mechanical components. This is now feasible with fiber lasers that can be assembled on CNC machines with fast scanners. Other challenges include research studies of the microreservoirs function of LST to understand the exact mechanism by which lubricant is being emitted from textured cavities in case of starved lubrication. Also, the exact mechanism by which cavities are getting filled with wear debris is not completely understood. Resolving these two issues will help in optimizing the geometry of textured cavities for best performance in starvation and in removing wear debris from the contacting surfaces.

Another challenge is developing an efficient solution of the Reynolds equation in cases where a very large number of cavities must be considered. This happens where the mating surfaces in relative sliding are not parallel as, for example, in eccentric journal bearings and in tilted mechanical seals or thrust bearings. An approach similar to that of Patir and Chang for solving lubricated rough surfaces might be helpful.

TLT: What is your vision of the future of surface texturing?
Etsion: I believe that surface texturing will become a common surface engineering technology similar to the widespread hard coating technology. Both technologies can be used to reduce friction and wear. With coatings that goal is achieved by adding material to a surface, while texturing achieves the same goal by removing material from the surface.

Hence, in a way surface texturing can be described as an “inverse coating.” Perhaps the main difference between those two technologies is that hard coating is more beneficial under dry sliding conditions while surface texturing works better with lubricated sliding. It therefore seems reasonable to combine these two technologies to obtain low friction and wear over the entire spectrum of sliding conditions.


Figure 4. Reciprocating test rig for studying friction reduction in LST piston rings. Authentic piston rings and cylinder liners can be tested with stroke of up to 100 mm and speeds of up to 1,200 rpm. Lubricant flow rate can be controlled to study both full and starved lubrication conditions. Ambient temperature can be adjusted to simulate lubricant viscosity in a firing engine.

TLT: How did you end up in tribology?
Etsion: I first heard about tribology in 1970 when working as a young aeronautical engineer in a company that manufactured mechanical components. I had an idea of developing a small hovercraft for agricultural applications and found a course on hydrodynamic lubrication that was given at Technion by professor Oscar Pinkus. The syllabus of that course included lectures on hydrostatic gas bearings, which seemed to me similar to a small hovercraft. I decided to attend the course and was attracted by the new horizons it opened to me. I quit my job in industry and went back to Technion to pursue my doctorate in tribology.


Figure 5. A 6-pads stator of a LST thrust bearing and two smaller rings of mechanical seals. The mate areas on the thrust bearing are the textured zones for a clockwise rotation of the mating rotor. The inset at the upper left corner shows a magnified area on the wider seal face showing the dimples produced by the laser.

TLT: Name some of the individuals in your profession or in general who inspired you.
Etsion: The first name that comes to mind is Oscar Pinkus, my doctorate supervisor and mentor from 1971 until 1974. Oscar, who in the early 1960s pioneered the 2-D solution of the Reynolds equation for finite hydrodynamic bearings by using finite differences numerical analysis, was an unusual person and a bright scientist. In addition to co-authoring with Beno Strenlicht the classical textbook Theory of Hydrodynamic Lubrication, Oscar also was a novelist and a painter. We used to spend hours almost every day discussing hydrodynamic lubrication problems and many other global issues. From Oscar I have learned perhaps the most important lesson for my future academic career: When facing a specific problem, you better make every effort to come up with a broader general solution of which the original specific problem is just a single special case.

The second person that inspired me was the late Bill Anderson, chief of the tribology division at then NASA Lewis Research Center in Cleveland, where I did my post-doctorate work in 1975-76 and later my first sabbatical in 1979-80. The tribology group at NASA LeRC, under the leadership of Bill, was perhaps the best one in the world in those days. Working in that group and closely interacting with Bill tremendously enriched my knowledge and experience in tribology research.

Bill introduced me to Larry Ludwig who was the head of the seals section at NASA LeRC. From Larry I have learned about the interesting problem of mechanical seals, which at that time were described as “the most non-predictable machine element.” In the years to come, I studied extensively the dynamics of mechanical seals, trying to understand what could be the cause for their unpredictable behavior. It was the vast experience and insight that I gained through my research on mechanical seals that in 1996 lead me to try laser surface texturing on seals to improve their performance.

All photos courtesy of Y. Itovitch.

You can reach Izhak Etsion at etsion@technion.ac.il.