Humans recognized the phenomenon of friction including its side effects early on – and learned how to put it to use. For instance, to generate heat for starting a fire. Often, though, friction proved to be a hindrance, so the desire to minimize it became the mother of invention even in the days of our ancient forebears: To reduce the physical exertion involved in pulling heavy loads, early tinkerers as far back as in the Neolithic Age 5000 BC came up with rolling logs and using sleds as precursors to the wheel and axle combination. The first machines that generated friction were bow drills and potter’s wheels. Pre-Christian finds prove that lubricants were used to minimize friction as far back as in Ancient Egypt and China.

In the Late Middle Ages, the precision of observations relating to friction clearly increased. In the 15th century, Leonardo da Vinci gathered initial scientific findings about static friction, in other words the force that keeps two contacting objects from sliding against each other. For his observations, the universal genius measured the angles of various inclined planes on which solid objects began to slide. In addition, he determined the force that’s needed to cause an object to slide on a horizontal plane: dynamic friction. To minimize the force that needs to be exerted, da Vinci placed balls between two plates and so came to be regarded as the inventor of the ball bearing, which he went on to optimize with a cage guide.

Scientists subsequently described various physical laws around friction. Sir Isaac Newton (1643–1727), for instance, investigated the adhesive force that’s generated by molecular interactions of the interfacial layer of surfaces. This adhesive force causes a protective film to stick to the display of a cellphone, for example. The Swiss Leonhard Euler (1707–1783) with the coefficient of friction µ defined a key characteristic that describes the ratio between frictional force and contact force. During the same era as well, Charles Augustin de Coulomb described the frictional resistance of rolling wheels in proportion to the load and inversely proportional to the radius: The higher the load the higher the frictional resistance and the larger the wheel the lower the friction. In addition, he elaborated the differences between static (excitation of the sliding action) and dynamic (preservation of the sliding action) friction. In this context, Coulomb observed that in the case of sliding the frictional force is nearly independent of the relative speed of the friction pairing, unlike, for example, in the case of air resistance.

Friction as a science in its own right

Even though the research of friction began more than 500 years ago, the scientific term for it, tribology, wasn’t coined until the nineteen-sixties. Tribology derives from “tribein,” an Old Greek word for rubbing and wearing. The term tribological system describes the interaction of surfaces in relative motion to each other, which serves to perform specific technical functions. That sounds complicated – and is, in fact, a highly complex subject. “As the study of friction, lubrication and wear, tribology is a cross-sectional technology of macroeconomic importance. It enables energy efficiency and sparing use of resources by reducing friction and wear,” says Prof. Dr.-Ing. Tim Hosenfeldt, Senior Vice President, Corporate Research and Innovation & Central Technology at Schaeffler and Honorary Professor of Surface Technology and Tribology at University of Erlangen-Nuremberg.

Engineers today distinguish between the following terms for friction, depending on the state of contact: solid friction, boundary friction, mixed friction, fluid friction and gas friction. In this context, scientists not only look at the tribological objects rubbing against each other, namely the base body and the mating body, but also investigate the elements acting between the surfaces such as lubricants, particles and even air. The ambient medium, be it liquid or gaseous, is decisive as well. The ambient air, for instance, produces chemical effects that promote lubricant aging. To be considered as well are aspects such as viscosity and aging of a lubricant that evaporates under excessive loads. Furthermore, service parameters affect tribological behavior: the type of motion, load, speed, temperature and duration of friction. Plus, there are some other factors. Scientists investigate tribological phenomena on macro-, micro- and even on nano-scale levels.

Enormous efficiency potential

What purpose does all this research ambition serve? Even in a single component assembly – for instance in a passenger car, a commercial vehicle or a train – significant friction losses can be measured. That raises the question of how massive the total effect of all friction losses must be, considering that such products are used day in day out in hundreds of millions or even billions of units worldwide. A scientific engineering study calculated these effects in 2017. The figures are impressive: Tribological contacts account for 23 percent of the world energy consumption of which 20 percent serves to overcome friction and 3 percent to recondition components and spare parts that are worn in the process. In terms of absolute numbers, this 23 percent corresponds to 119 exajoules (EJ). That’s 33,055 billion kilowatt hours (kWh), in ­other words nearly ten times the primary energy consumption of Germany.

This incredible amount of energy requires high capital expenditures, is produced in complex ways, has adverse effects on the environment and climate – and is ultimately lost to friction. And the smallest part of that is intentional, for instance due to applying the brakes. Kenneth Holmberg and Ali Erdemir, the academic authors of the aforementioned study, performed further calculations showing that friction and wear could be reduced in the long term by up to 40 percent by means of new surfaces, materials and lubrication solutions, which roughly equates to potentially improving the global primary energy requirement by 8.7 percent. The transportation sector accounts for a fourth of these savings and energy production for a fifth, according to the study. Both of these are areas in which Schaeffler is intensively involved with its Automotive and Industrial divisions. The following figures are impressive as well: With forward-thinking tribological technologies, the study found, 3,140 metric megatons (3,460 short megatons) of CO₂ and and 970 billion euros could be saved in the long term, according to Holmberg and Erdemir.

11,522.4 Vickers hardness (HV)

is achieved by a carbon-based glass recently developed by researchers at Yanshan University in China. The material called AM-III is 30 times harder than stainless steel and can even scratch diamonds (10,000 HV). In addition, AM-III is semi-conductive and therefore suitable as a viable silicon alternative in the field of photovoltaics.

“Surface technology is a key enabling technology”

Not least in view of these figures, Schaeffler, as a global automotive and industrial supplier, intends to use its tribological know-how to minimize friction with the objective of enhancing efficiency as well as increasing the service life of products through wear protection and corrosion protection. As early as in 2007, the Group opened its “Surface Technology” competence center at its headquarters in Herzogenaurach and has successively been extending the development capacities at the site. The researchers and developers there are highly motivated, telling us that surface technology is no less than one of the key enabling technologies in industrial countries. That assumption certainly sounds plausible if for no other reason than the aforementioned figures.

Due to its expertise in coating technologies and in the fundamentals of tribology and nanotechnology, Schaeffler has acquired a position of worldwide leadership in functional surfaces and coatings. This field has long ceased to be strictly about optimizing the surfaces of heavy-duty rolling bearings, for instance in wind turbines or aircraft engines, with innovative coating technologies. In fact, coatings from Schaeffler have already made it into automotive powertrains, for instance in the engine’s valve train. And the performance of bipolar plates in fuel cells and electrolyzers for the utilization and production of green hydrogen applications is raised to new levels by the tribological know-how of the automotive and industrial supplier as well. Many other fields of application are emerging and will be addressed later.

Diversity is an advantage: Schaeffler has developed a Coating Toolbox enabling the company to offer its customers tailored solutions. The toolbox covers these five main requirements: corrosion protection, wear protection, friction reduction, current insulation and sensors, as well as any intersections between them. “These surface modifications have to be designed for the relevant application and should be considered as early as in the system’s design phase. Consequently, the coating should by all means be seen as a valuable design element that is necessary for achieving the best possible success,” says Professor Tim Hosenfeldt, who is in charge of the competence center.

© Schaeffler

There is a clear trend toward custom-developed multifunctional and sensorial coatings

Tribology expert Prof. Dr.-Ing. Tim Hosenfeldt,
Senior Vice President, Corporate Research and Innovation & Central Technology at Schaeffler

One of Schaeffler’s fortes is the transformation of complex niche applications into mass production with corresponding requirements such as first-rate reproducibility to meet exacting quality standards and, of course, in consideration of strict cost requirements. The extremely thin application of the heavy-duty layers is an art in itself, considering that we’re talking about ultra-fine layer thicknesses, often in the nano range. All the more astonishing is their robustness: Schaeffler offers so-called Triondur coatings that easily withstand heat of up to 600 degrees centigrade. In terms of hardness, peak values of more than 4,000 Vickers hardness (HV) are currently achieved. For comparison: standard bearing steel peaks at 700 HV and diamonds as the hardest natural mineral reach 10,000 HV.

The future: plasma coatings and multifunctional surfaces

In addition to electrochemical techniques (galvanic), painting and thermal spraying, plasma-assisted chemical vapor deposition (PACVD) and physical vapor deposition (PVD) are playing increasingly important roles also at Schaeffler, especially with multifunctional thin-film coatings. The plasma surface technology is truly a multi-talent with a performance range far exceeding the areas of friction reduction and wear protection. This technology, for instance, can be used to clean and activate surfaces for enhanced adhesion of paints and adhesives or for icing prevention. Furthermore, it enables the provision of surfaces with additional optical, electrical or chemical functions.

5 key areas of tribological research and development in the field of thin-film technology

• Innovative coating methods and processes
• Tribological and corrosion-­protection coatings for mobility and energy
• Coatings for sensorial and bionic applications
• Coatings for optical and ­electronic applications
• Coatings for energy storage ­devices and converters

© Getty

“There is a clear trend toward using precise layer systems as design elements with multifunctional and mechatronic properties,” says Prof. Tim ­Hosenfeldt. Here’s another number that underscores the importance and diversity of tribological plasma technology: plasma is utilized in 14 of the 17 “Future Fields” identified by the German Federal Ministry of Education and Research, ranging from the hydrogen economy to medicine to biotechnology.

That’s because PVD and PACVD have a crucial advantage: The technologies are suitable not only for use with metals but with most polymers and even with many organic substances as well. Highly effective membranes or individual fibers can be treated with plasma technology too, for instance to produce specialty yarns. Fraunhofer Institute for Manufacturing Technology and Advanced Materials IFAM, for instance, has developed a plasma-polymeric coating for sealing elements made of rubber. The coating reduces friction and wear of such seals that serve to prevent lubricant oil leakage. ­Schaeff­ler was involved as a partner in this project called “Poseidon.” In tests on a model test bench, the scientists were able to reduce friction by 23 to 55 percent – depending on the oil used and its additives. With fully additive-enhanced production lubricants in wheel bearings, the coefficient of friction in laboratory tests was reduced even by up to 71 percent.

Coatings with sensory properties enable experts to determine and analyze the in-service wear behavior of components to improve products or better yet: to directly and proactively intervene to prevent wear. In addition, such data make it possible to calculate residual service life precisely, thereby optimizing lifecycles and maintenance intervals, which saves resources. At the same time, unplanned downtimes can be minimized. Ideally, surfaces regenerate their friction-induced substance loss automatically and tribologists are already working on such wear- and friction-reducing “healing processes” as well. So, the subjects of surfaces and friction will continue to inspire our ingenuity – just like they did with our forebears thousands of years ago.