The epoch from 1750 to 1850 was characterized by improved designs of machine elements. It was the era of Watt and his steam engine. New gears, journal bearings (also for railway), roller bearings and lubricants (mineral oils, lubricant formulations and solid lubricants) were developed. Very important studies on friction were made by Coulomb (1782) that confirmed the laws of Leonardo da Vinci and Amontons. The word viscosity was coined by Navier that defined with Stokes the well-known equations. Poiseuille performed his studies on fluid flow in a pipe.

About a century (from 1850 to 1940) of technical progress follows the previous epoch, with several further developments in bearings and lubrication. The relationship between friction, wear and lubrication was investigated. The hydrodynamic pressure development in journal bearings was discovered by Tower, the hydrodynamic friction was investigated by Hirn and the rolling friction by Hertz. Reynolds developed his well-known equation for the hydrodynamic pressure that is the basis for the calculation of bearings (Reynolds, 1886). Stribeck determined his curve showing the relationship between friction, load, speed and viscosity; Sommerfeld proposed an analytical solution of Reynolds' equation and the Sommerfeld number. Rolling bearings were developed by Wingquist, founder of SKF. The first additives were put in oils. Many scientists and engineers were involved in theoretical aspects of journal and thrust hydrodynamic bearings at the beginning of the 20th century, as Michell, Stribeck, Kingsbury, Rayleigh, Gümbel. Particular types of bearings were also invented, as the tilting pad (Michell, Kingsbury) and the step (Rayleigh) bearings, and hydrostatic lubrication was also introduced. During the 1930s further analytical and approximate solutions of the Reynolds equation were proposed by Swift, Stieber, and Kingsbury. The relationship between scuffing and the total transient temperature that includes the flash temperature was proposed by Blok.

The time span from the 1940s to the 1960s can be considered the starting epoch of the Elastohydrodynamic lubrication (EHL) theory, the fundamental kind of lubrication for rolling bearings and gears that takes into account, in addition to the hydrodynamic effect, the effects of elastic deformation of the contacting bodies and the dependence of the viscosity of the lubricant on pressure. Starting from the so called Grubin's formula published in 1949 for the evaluation of the minimum film thickness, that more correctly should be mentioned as Ertel-Grubin formula (Popova and Popov, 2015; Morales-Espejel and Wemekamp, 2018), numerical solutions were obtained in the late 1950s by Dowson and Higginson (1966). Further developments were also produced in other tribological fields. The approximate solution for short journal bearings was also proposed by Ocvirk and Du Bois. Starting from the 1950s, the first numerical solutions were proposed by Cameron and Wood and then by Pinkus, Raymondi, and Boyd. Friction studies progressed with the development of the adhesion theories. Bowden and Tabor proposed their simple relationship for the friction coefficient as the ratio of the shear stress and the hardness of the asperity junctions. Archard investigated wear problems producing the classical equation for the volume of worn material.

The epoch from the late 1960s to the late 1990s is characterized by the birth of the “official” tribology, commonly dated in 1966 with the Jost Report (Jost, 1966) that highlighted the economic importance of tribology for a bigger reliability and efficiency of the mechanical systems thanks to the possible reductions of friction and wear. Tremendous progresses in both understanding of fundamental aspects of tribology as well in its industrial applications occurred some of which are reported in the following. Elastohydrodynamic studies continued with numerical extension to point contacts thanks also to the availability of the new powerful computers (Hamrock and Dowson, 1981). Non-Newtonian rheological models (for instance the one of Bair and Winer) were developed that allowed more realistic evaluation of friction and therefore of power losses. Both computational and experimental developments (optical interferometry for film thickness evaluation) and several experimental studies allow investigations up to mixed and boundary lubrication conditions, including the presence of asperity contacts (micro-EHL), very thin film lubrication and non-steady state conditions (for instance Spikes and Kaneta). Further developments in very low friction bearings, as air lubricated (Gross, 1962) and magnetic (Moon, 1993) bearings, and in hydrostatic lubrication (Bassani and Piccigallo, 1985). Surface engineering aspects were further investigated: it's worth mentioning the contact mechanics studies of Greenwood with the introduction of the plasticity index, and the studies on friction up to the atomic scale of molecular dynamics and experimentally with the surface force apparatus (SFA) and atomic force microscope (AFM). Tribology of magnetic storage systems also developed (Bhushan, 1999). Several nondimensional groups useful for lubrication and contact mechanics studies were developed by Blok, Greenwood, Johnson, Dowson, Higginson, Hamrock, Moes. New fundamental studies on wear were performed (Kimura, Ludema, and Kato, just to mention a few). Progress in reducing wear was also performed with new materials and coatings, particularly with physical or chemical vapor deposition (PVD or CVD), as diamond like carbon (DLC) coatings. The third body concept was also introduced (Godet, 1984). Advances in non-metallic bearings occurred; the PV (pressure times velocity) factor became a well-established criterium for material selection of bearings. Lubricant technology also developed with the use of new solid lubricants and synthetic lubricants and additives. Condition monitoring started to be applied particular in large-scale production where the downtime (machine-stops) produce big economical losses. Temperature and vibrations are the most common monitored quantities, but also checks of the lubricant conditions through techniques such as ferrography was recognized to be very useful. Studies on lubrication and wear of synovial joints started and the term “Biotribology” was officially introduced by Dowson in 1970, including other topics as wear of dental tissues and replacement heart valves, tribology of skin and friction of hair.

From 2000 to today is the present tribology epoch. Further developments occur in all previously mentioned fields. Tribology involves more and more scientific, environmental and socioeconomic aspects. Tribological studies regard several aspects of transports, as road, rail and engines, machine components, machining processes (grinding, turning, …), cold/hot forming processes, atomic scale investigations, industrial applications of micro and nano mechanics, information systems (storage technology), bio-technology (new biomaterials and biomedical applications), new materials (ceramics), surface texturing and coatings, lubricants (ionic liquids) and additives. There is a significant development of the environmental friendly tribology, or ecotribology; the term “Green Tribology” is introduced by Zhang in 2001. Superlubricity and hydration lubrication concepts are introduced by Klein. Space tribology is also developed for aerospace applications. The term “Tribotronics,” or active tribology, integrating tribology and electronics, is coined at Luleå University of Technology (2008). Smart tribological systems, including models. sensors and actuators, need to be developed to improve performances of industrial machinery. The tribological researches of this epoch more connected to industry 4.0 are reported in more detail below.

The tribology evolutions are schematically represented in Figure 2. Some important tribological achievements are shown connected to the beginning of each industrial revolution, as highlighted in the next section. A logarithmic scale is used for time on the abscissa with the full scale coincident with the present year.

Cyber physical systems relate the concepts of internet of things that enables real-time data sharing among all parts of the industrial systems, information and communication technology, internet of services, connectivity, wireless network, cloud computing and services, machine-to-machine communication. Internet of things has brought to have sensors of every kind (biosensors, smart sensors, photonic sensors) used almost everywhere today (Winters, 2018).

Advanced monitoring, diagnostics and maintenance are connected with the use of sensors, wireless sensor network, real-time data acquisition and analysis, signal processing, remote access and control, remote maintenance.

Advanced technologies for energy saving include environment-friendly or green technologies, alternative energy conversion (solar, wind and hydrogen energy production), energy recovery.

Advanced manufacturing solutions with digital-to-physical conversion include flexible manufacturing lines, supervisory control and data acquisition (SCADA), computer-aided design and manufacturing (CAD, CAM), and additive manufacturing, also known as fast prototyping or 3-D printing, that can produce reduction in waste and costs and simplification in assembly (Brown, 2018a).

Advanced engineering technologies and materials include laser, plasma, ultrasound, radiation and optical technologies, microtechnologies for microelectromechanical systems (MEMS), nanotechnologies, new polymers, composite and ceramic materials and coatings, new efficient, and intelligent materials.

Machine learning indicates the capability of a software to analyze big amount of data and to learn how to solve problems automatically. A related concept is adaptive control (the capability of a system to identify the problem and to decide how to modify its operation to achieve the best possible mode of operation). Big data is connected for instance to the use of costumer data to optimize product strategies and the analysis of big amount of data to optimize products and processes.

Human machine interaction includes augmented and virtual reality, wearable technologies, human-machine interface (HMI) or man-machine interface (MMI) (speech-recognition interfaces, touch screens, 3D scanner). Robotics includes autonomous robots, interconnected robots, co-robots (or collaborative robots, robots working together with people) allowing the human labor to be addressed toward more value-added activities.

It is worth noting that a keyword frequently used in industry 4.0 publications and conferences is the word “smart.” It is mainly used for defining something related with interconnectivity and automation. Some samples are: smart applications, smart grid, smart manufacturing, smart systems, smart technologies, smart objects, and also smartifying industry as reported in Gerdin (2016).

The connection of tribology with electronics (sensors) and informatics (computers, internet) is today extremely strong. The need of reliable and affordable on-line sensors providing information on both lubricant and machinery wear condition in order also to reduce manpower needs was already stated in Murphy et al. (2005). The connection between tribology and electronics is well-highlighted by the term “Tribotronics” (Glavatskih and Höglund, 2008). The smart machines are intelligent devices able to monitor and change their own conditions thanks to the use of sensors and actuators. Smart tribological systems, including models, sensors and actuators, can improve the performances of industrial machinery. They should be self-adjusting including on-line tuning of tribological systems to optimize performances. Live data related particularly to wear, friction, vibrations and temperatures can be recorded and used to improve the performances of the tribological systems so that efficiency and reliability of the entire machine are increased. For instance sensors can monitor the conditions of the surfaces and of the lubricant in the machines and through communication systems the information is transferred for control and possible consequent actions. In this way the machines are connected into internet of things establishing typical cyber-physical systems. Possible actuation mechanisms could modify the geometry of tribological components. For instance active controlled bearings can be used, as the actuated tilting pad journal bearings reported in Salazar and Santos (2017a,b). The use of a feedback-controlled or active lubrication allows further vibration reductions with suppression of resonant vibrations thus improving the whole dynamic performance of turbomachinery. The properties of the lubricant can be controlled by using smart materials as magneto-rheological and electro-rheological fluids (Phulé, 2001) and active control of friction can be realized by electronic control of fluid viscosity (Nakano et al., 1995) or by transverse oscillations (Benad et al., 2019). Progresses on bearings can be directly pushed by the internet of things (Brown, 2018b).

The fundamental role of tribology in maintenance engineering is well-known (Kimura, 1997; Tung et al., 2005). When a machine is working, it is of fundamental importance to detect possible problems as soon as possible in order to reduce its downtime and the related costs. Tribology is involved in the improvements in life prediction of components, as rolling bearings (Ioannides, 1997), in the investigation on the failure mechanisms and in the diagnostic analysis and prognostic predictions (Holmberg, 2001), in maintenance of operating machinery (Roylance, 2003). Rolling bearings with integrated sensors, measuring torque, force, vibrations and temperatures, have been developed, as for instance the sensor bearing with encoder unit reported in Mathé and Grellier (2016), or even a combination of sensors, power, and diagnostics and communications within the bearings for a complete condition monitoring system (Howieson, 2014). Online monitoring of lubricants in machinery is essential for increasing the lifetime of the lubricated components. Real time measurement of wear debris in the lubricant are performed (Iwai et al., 2010), but often vibration analysis is used, in combination with wear debris analysis or not, for fault diagnosis of machinery containing rolling bearings and gears (Peng and Kessissoglou, 2003; Tandon et al., 2007; Randall, 2016). Several examples of techniques used for tribological condition monitoring are reported in Marklund (2017). Maintenance-free mechanical assemblies are required in space applications; the study of tribological interactions is important for industrially relevant applications in the space environment (Krick et al., 2013).

The importance of tribology for energy saving is well-evidenced in Holmberg and Erdemir (2017) where the impact of friction and wear on energy losses also from economical and emissions points of view is presented. It is shown in the paper that about 23% of the world's energy consumption, the energy used by people for their activities, mainly related to transportation, manufacturing, power generation, and residential, originates from tribological contacts. The contribute of tribology is fundamental both at the design stage of industrial components or systems and during their working (monitoring, corrective actions based on friction-wear-lubrication problems). Efficiency, performance, maintainability and lifetime of machine elements as seals, bearings and gears can be dramatically increased by a correct tribological design. There is a high industrial demand nowadays toward higher power density machines with higher efficiency. Reduced dimensions to have lighter machine components are consequently requested. To achieve the same or even higher power than in the former machines higher speeds are necessary that are normally connected to hotter operating conditions. Environmental aspects and costs are also taken into account which brings to the employment of minor quantities of lubricant, related again to higher temperatures. In these ways more severe conditions for the lubricated contacts are reached so that their design becomes extremely complex and tests on real machine components are necessary (Ciulli, 2019). Tribological studies related to transports (cars, airplanes, helicopters, trains) can have a big impact on the environment due to the enormous number of means of transport around the world. Automotive tribology is still an open issue; applications of tribological results to the world of cars is probably one of the most visible example of how tribology can have a great impact on energy and environmental issues (Masuko and Sugimura, 2017). Studies on engines are reported for instance in Woidt (1997) and Erdemir et al. (2017). Particle emissions is another important problem related to vehicles. One of the main source of emission of small polymer particles (microplastics) in the environment is tire wear (Kole et al., 2017; Sommer et al., 2018). According to Kole et al. (2017), pollution produced by wear and tear from tires is as important as the one produced by plastic bottles and bags and could therefore produce some effects on human health. Tribological studies can be very useful for a reduction in particle emission from tires (Foitzik et al., 2018) and also from brakes (Perricone et al., 2018; Joo et al., 2019).

Bearings and gears, mainly for automotive and aeronautical applications, are surely among the most investigated components. New bearings and gears are under development for a completely new aeronautical engine, the LEAP (Leading Edge Aviation Propulsion) that represents one of the major recent technological advances (Halimi, 2018). Rolling bearings constitute one of the classical example of components that have reduced their size during the years. The improvements in their design has taken during 100 years of bearing evolution to bearings with reduced size and friction, high carrying capacity, higher speed, and longer life (Weidinger, 2009; Grellier and Re, 2014; Landerl, 2018). The minimization of the use of lubricants for bearings is reported in Schmidt and Schwartz (2014). The hybrid bearings, with steel rings and silicon nitride rolling elements, show good performance in severe conditions such as poor lubrication and contaminated conditions (Vieillard et al., 2017). Studies on gears are performed to eliminate (or at least to reduce) noise and vibrations, to increase power density (to use smaller and lighter gearboxes than before), to realize low-loss gears by minimizing the quantity of lubricant necessary. Gears, with reduced quantities of oil are for instance tested in Ebner et al. (2018). A final target could be to arrive to oil-free powertrain, as well as to oil-free engines (Woidt, 1997). All these tribological studies are very important for a sustainable development that meets the present human needs and the integrity and stability of the environment. More generally, tribology can have influence, directly or indirectly, on several of the 17 United Nations sustainable development goals (United Nations, 2015), such as the goal 3, good health and well-being for people (think to biotribology), goal 7, affordable and clean energy (improving energy efficiency by friction reduction), goal 12, responsible consumption and production (reducing material waste by reducing wear), and goal 13, climate action (reducing emissions by friction losses reduction). A big effort has been done toward new ecological lubricants and sustainability of tribology. The importance of increasing environmental compatibility minimizing health and water hazards is reported in Bartz (1997). Novel and environmentally friendly lubrication concepts and additives are under development. Several aspects of green tribology are treated in Nosonovsky and Bhushan (2012) and Bhushan (2013).

Alternative energies can pose new tribological problems. Wind turbines constitute a significant example because their maintenance problems. New bearings, surface treatments as the black oxidation and lubrication systems are developed for wind turbines (Stadler et al., 2015; Bittorf and Slembeck, 2016; Hofmann, 2017). Condition monitoring of wind turbines helps to control operation and maintenance costs (Drommi, 2017).

Tribology in manufacturing is another important tribological research field. Hot metal forming (friction and heat transfer) is for instance investigated in Beynon (1997), while the importance of the reduction of fluids is reported in Shareef and Natarajan (2005). The Minimum Quantity Lubrication (MQL), an efficient method of applying cutting fluids by minimizing their amount, is today important in machining processes, being environmentally friendly and reducing costs (Emami et al., 2014; Li et al., 2017).

Tribology can contribute to the development of additive manufacturing with tribological investigations on the new printed products in order to investigate their surface mechanical characteristics (friction, wear, lubricant interaction). Additive manufacturing can be used to integrate sensors and actuators directly into machine components.

Polymer and ceramic tribology, micro- and nanotribology, and nanotechnology, investigating for instance nanoparticles to increase material resistance, nanocoatings and nano-reinforced materials, and the development of 2D materials like graphene are surely related to development of new advanced techniques and materials. Attention should be payed to the potential toxicity of the new materials and their structures. Biomimetics can be helpful for this, learning from living nature which materials and nanostructures are not toxic.

Big data is a concept that can be related to tribology in several ways. Tribology networks and databases of tribological data (for instance friction coefficient or wear data) already exist. Big quantity of data from disparate instruments and sensors are captured particularly in transient tribological experimental tests (Ciulli, 2019). Data are often recorded on magnetic storage systems, with their tribological problems.

Human machine interaction and wearables are related to tribological aspects at the interface, as friction and wear problems of the skin.

Friction, wear and lubrication problems of joints in robots have to be solved, particularly to avoid vibrations and allow precise positioning.