Phil. Trans. R. Soc. A (2010) 368, 4677–4694doi:10.1098/rsta.2010.0200

INTRODUCTION

Green tribology: principles, research areasand challenges

BY MICHAEL NOSONOVSKY1 AND BHARAT BHUSHAN2,*1College of Engineering and Applied Science, University of Wisconsin,

Milwaukee, WI 53201, USA2Nanoprobe Laboratory for Bio- and Nanotechnology and Biomimetics (NLBB),

Ohio State University, 201 West 19th Avenue, Columbus,OH 43210-1142, USA

In this introductory paper for the Theme Issue on green tribology, we discuss theconcept of green tribology and its relation to other areas of tribology as well as other‘green’ disciplines, namely, green engineering and green chemistry. We formulate the12 principles of green tribology: the minimization of (i) friction and (ii) wear, (iii) thereduction or complete elimination of lubrication, including self-lubrication, (iv) naturaland (v) biodegradable lubrication, (vi) using sustainable chemistry and engineeringprinciples, (vii) biomimetic approaches, (viii) surface texturing, (ix) environmentalimplications of coatings, (x) real-time monitoring, (xi) design for degradation, and(xii) sustainable energy applications. We further define three areas of green tribology:(i) biomimetics for tribological applications, (ii) environment-friendly lubrication, and(iii) the tribology of renewable-energy application. The integration of these areas remainsa primary challenge for this novel area of research. We also discuss the challenges of greentribology and future directions of research.

Keywords: tribology; alternative energy; nanotechnology; biomimetics

1. Introduction

Tribology (from the Greek word trı́bu ‘tribo’ meaning ‘to rub’) is defined bythe Oxford dictionary as ‘the branch of science and technology concerned withinteracting surfaces in relative motion and with associated matters (as friction,wear, lubrication and the design of bearings)’ (Oxford English Dictionary;http://grove.ufl.edu/∼wgsawyer/). The term was introduced and defined for thefirst time in 1966 by Prof. H. Peter Jost, then the chairman of a working group oflubrication engineers, in his published report for the UK Department of Educationand Science (Jost 1966). It was reported that huge sums of money have been lost

*Author for correspondence (bhushan.2@osu.edu).

One contribution of 11 to a Theme Issue ‘Green tribology’.

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in the UK annually owing to the consequences of friction, wear and corrosion.As a result, several centres for tribology were created in many countries. Sincethen, the term has diffused into the international engineering field, and manyspecialists now claim to be tribologists.

Typical tribological studies cover friction, wear, lubrication and adhesion,and involve the efforts of mechanical engineers, material scientists, chemistsand physicists (Bhushan 1999, 2001, 2002). Since the emergence of the wordtribology almost 50 years ago, many new areas of tribological studies havedeveloped that are at the interface of various scientific disciplines, and variousaspects of interacting surfaces in relative motion have been the focus oftribology. These areas include, for example, nanotribology, biotribology, thetribology of magnetic storage devices and microelectromechanical systems(MEMS)/nanoelectromechanical systems and adhesive contact (Bhushan &Gupta 1991; Bhushan 1996, 1999, 2000, 2001, 2002, 2008, 2010). The researchin these areas is driven mostly by the advent of new technologies and newexperimental techniques for surface characterization.

Recently, the new concept of ‘green tribology’ has been defined as ‘thescience and technology of the tribological aspects of ecological balance andof environmental and biological impacts’. H. P. Jost (2009, unpublished data)elaborated on the need for green tribology and has mentioned that ‘the influenceof economic, market and financial triumphalisms have retarded tribology andcould retard “green tribology” from being accepted as a not-unimportant factor inits field. . .. Therefore, by highlighting the economic benefits of tribology, tribologysocieties, groups and committees are likely to have a far greater impact on themakers of policies and the providers of funding than by only preaching thescientific logic. . . Tribology societies should highlight to the utmost the economicadvantage of tribology. It is the language financial oriented policy makers andmarkets, as well as governments, understand’.

The specific field of green or environment-friendly tribology emphasizes theaspects of interacting surfaces in relative motion, which are of importancefor energy or environmental sustainability or which have impact upon today’senvironment. This includes tribological technology that mimics living nature(biomimetic surfaces) and thus is expected to be environment friendly, thecontrol of friction and wear, which is of importance for energy conservationand conversion, environmental aspects of lubrication and surface-modificationtechniques and tribological aspects of green applications, such as wind-powerturbines, tidal turbines or solar panels (figure 1). It is clear that a number oftribological problems could be put under the umbrella of green tribology and areof mutual benefit to one another.

Green tribology can be viewed in the broader context of two other ‘green’areas: green engineering and green chemistry. The US Environmental ProtectionAgency defines green engineering as ‘the design, commercialization and useof processes and products that are technically and economically feasiblewhile minimizing (i) generation of pollution at the source and (ii) risk tohuman health and the environment’ (US Environmental Protection Agency2010). The three tiers of green engineering assessment in design involve:(i) process research and development, (ii) conceptual/preliminary design, and(iii) detailed design pollution prevention, process heat/energy integration andprocess mass integration (Allen & Shonnard 2001).

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(a) (b) (c)

Figure 1. The paradigm of green tribology: (a) renewable energy (represented by a wind turbine),(b) biomimetic surfaces (represented by a gecko foot) and (c) biodegradable lubrication (representedby natural vegetable oil).

Another related area is green chemistry, also known as sustainable chemistry,which is defined as the design of chemical products and processes that reduceor eliminate the use or generation of hazardous substances (US EnvironmentalProtection Agency 2010). Green chemistry technologies provide a number ofbenefits, including reduced waste, eliminating costly end-of-the-pipe treatments,safer products, reduced use of energy and resources and improved competitivenessof chemical manufacturers and their customers. Green chemistry consists ofchemicals and chemical processes designed to reduce or eliminate negativeenvironmental impacts. The use and production of these chemicals mayinvolve reduced waste products, non-toxic components and improved efficiency.Anastas & Warner (1998) formulated the 12 principles of green chemistry thatprovided a road map for chemists to implement green chemistry:

(1) Prevention of waste is better than cleaning up.(2) Maximum incorporation into the final product of all materials used in the

process.(3) Chemical synthesis should incorporate less hazardous or toxic materials,

when possible.(4) Chemical products should be designed to reduce toxicity.(5) Auxiliary substances, such as solvents, should be safe whenever used.(6) Energy efficiency requirements should be recognized. Synthetic methods

should be conducted at ambient temperature and pressure, wheneverpossible.

(7) A raw material or feedstock should be renewable, whenever possible.(8) Reduce unnecessary derivatives.(9) Catalytic reagents are superior to stoichiometric reagents.

(10) Chemical products should be degradable at the end of their function.(11) Real-time analysis, monitoring and control should be implemented to

prevent the formation of hazardous substances.(12) Substances and their use in the chemical process should be chosen to

minimize the risk of accidents and prevent fires, explosions, spills, etc.

A number of green chemistry metrics have been suggested to quantifythe environmental efficiency of a chemical process. These metrics include theenvironmental factor (‘E-factor’), which is equal to the total mass of waste divided

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by the mass of product (Sheldon 1992), the atom economy (Trost 1991), theeffective mass yield (Hudlicky et al. 1999), the carbon efficiency and reactionmass efficiency (Constable et al. 2001), etc.

Since tribology is an interdisciplinary area that involves, among other fields,chemical engineering and materials science, the principles of green chemistry areapplicable to green tribology as well. However, since tribology involves, besidesthe chemistry of surfaces, other aspects related to the mechanics and physicsof surfaces, there is a need to modify these principles. The principles of greentribology will be formulated in the following section.

2. Twelve principles of green tribology

Below, we formulate the principles of green tribology, which belong to the threeareas, suggested in the preceding section. Some principles are related to the designand manufacturing of tribological applications (iii–x), while others belong to theiroperation (i, ii, xi and xii). We followed tradition and limited the number ofprinciples to twelve.

(i) Minimization of heat and energy dissipation. Friction is the primary sourceof energy dissipation. According to some estimates, about one-third of theenergy consumption in the USA is spent to overcome friction. Most energydissipated by friction is converted into heat and leads to the heat pollutionof atmosphere and the environment. The control of friction and frictionminimization, which leads to both energy conservation and prevention ofdamage to the environment owing to heat pollution, is a primary task oftribology. It is recognized that for certain tribological applications (e.g. carbrakes and clutches), high friction is required; however, ways of effectiveuse of energy for these applications should be sought as well.

(ii) Minimization of wear is the second most important task of tribologythat has relevance to green tribology. In most industrial applications,wear is undesirable. It limits the lifetime of components and thereforecreates the problem of their recycling. Wear can lead also to catastrophicfailure. In addition, wear creates debris and particles that contaminate theenvironment and can be hazardous for humans in certain situations. Forexample, wear debris generated after human joint-replacement surgery isthe primary source of long-term complications in patients.

(iii) Reduction or complete elimination of lubrication and self-lubrication.Lubrication is a focus of tribology since it leads to the reduction of frictionand wear. However, lubrication can also lead to environmental hazards. Itis desirable to reduce lubrication or achieve the self-lubricating regime,when no external supply of lubrication is required. Tribological systemsin living nature often operate in the self-lubricating regime. For example,joints form essentially a closed self-sustainable system.

(iv) Natural lubrication (e.g. vegetable-oil-based) should be used in cases whenpossible, since it is usually environmentally friendly.

(v) Biodegradable lubrication should also be used when possible to avoidenvironmental contamination.

(vi) Sustainable chemistry and green engineering principles should be usedfor the manufacturing of new components for tribological applications,coatings and lubricants.

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(vii) Biomimetic approaches should be used whenever possible. These includebiomimetic surfaces, materials and other biomimetic and bioinspiredapproaches, since they tend to be more ecologically friendly.

(viii) Surface texturing should be applied to control surface properties.Conventional engineered surfaces have random roughness, and therandomness is the factor that makes it extremely difficult to overcomefriction and wear. On the other hand, many biological functional surfaceshave complex structures with hierarchical roughness, which defines theirproperties. Surface texturing provides a way to control many surfaceproperties relevant to making tribo-systems more ecologically friendly.

(ix) Environmental implications of coatings and other methods of surfacemodification (texturing, depositions, etc.) should be investigated and takeninto consideration.

(x) Design for degradation of surfaces, coatings and tribological components.Similar to green chemistry applications, the ultimate degradation/utilization should be taken into consideration during design.

(xi) Real-time monitoring, analysis and control of tribological systems duringtheir operation should be implemented to prevent the formation ofhazardous substances.

(xii) Sustainable energy applications should become the priority of thetribological design as well as engineering design in general.

3. Areas of green tribology

The following three focus areas of tribology have the greatest impact onenvironmental issues, and, therefore, they are of importance for green tribology:(i) biomimetic and self-lubricating materials/surfaces, (ii) biodegradable andenvironment-friendly lubricants, and (iii) tribology of renewable and/orsustainable sources of energy. Below, we briefly discuss the current state of theseareas and their relevance for the novel field of green tribology.

(a) Biomimetic surfaces

Biomimetics (also referred to as bionics or biomimicry) is the applicationof biological methods and systems found in nature to the study and design ofengineering systems and modern technology. It is estimated that the 100 largestbiomimetic products generated approximately US $1.5 billion over the years2005–2008. The annual sales are expected to continue to increase dramatically(Bhushan 2009). Many biological materials have remarkable properties thatcan hardly be achieved by conventional engineering methods. For example, aspider can produce huge amounts (comparing with the linear size of his body)of silk fibre, which is stronger than steel, without any access to the hightemperatures and pressures that would be required to produce such materialsas steel using conventional human technology. These properties of biomimeticmaterials are achieved owing to their composite structure and hierarchical multi-scale organization (Fratzl & Weinkamer 2007). The hierarchical organizationprovides biological systems with the flexibility needed to adapt to the changingenvironment. As opposed to the traditional engineering approach, biologicalmaterials are grown without the final design specifications, but by using the

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recipes and recursive algorithms contained in their genetic code. The differenceof natural versus engineering design is the difference of growth versus fabrication(Fratzl 2007; Nosonovsky & Bhushan 2008, 2009a). Hierarchical organizationand the ability of biological systems to grow and adapt also provide a naturalmechanism for the repair or healing of minor damage in the material.

The remarkable properties of the biological materials serve as a sourceof inspiration for materials scientists and engineers, indicating that suchperformance can be achieved if the paradigm of materials design is changed.While, in most cases, it is not possible to directly borrow solutions from livingnature and to apply them in engineering, it is often possible to take biologicalsystems as a starting point and a source of inspiration for engineering design.Molecular-scale devices, superhydrophobicity, self-cleaning, drag reduction influid flow, energy conversion and conservation, high adhesion, reversible adhesion,aerodynamic lift, materials and fibres with high mechanical strength, biologicalself-assembly, antireflection, structural coloration, thermal insulation, self-healingand sensory-aid mechanisms are some of the examples found in nature that areof commercial interest.

Biomimetic materials are also usually environmentally friendly in a naturalway, since they are a natural part of the ecosystem. For this reason, thebiomimetic approach in tribology is particularly promising. In the area ofbiomimetic surfaces, a number of ideas have been suggested (Scherge & Gorb2001; Bar-Cohen 2006; Gorb 2006; Nosonovsky & Bhushan 2008; Favret &Fuentes 2009).

(i) The Lotus-effect-based non-adhesive surfaces. The term ‘Lotus effect’stands for surface-roughness-induced superhydrophobicity and self-cleaning.Superhydrophobicity is defined as the ability to have a large (more than 150◦)water contact angle and, at the same time, low contact angle hysteresis. Thelotus flower is famous for its ability to emerge clean from dirty water and to repelwater from its leaves. This is due to a special structure of the leaf surface (multi-scale roughness) combined with hydrophobic coatings (figure 2) (Nosonovsky &Bhushan 2007a,b). These surfaces have been fabricated in the laboratory withcomparable performance (Bhushan et al. 2009).

Adhesion is a general term for several types of attractive forces thatact between solid surfaces, including the van der Waals force, electrostaticforce, chemical bonding and the capillary force, owing to the condensationof water at the surface. Adhesion is a relatively short-range force, and itseffect (which is often undesirable) is significant for microsystems that havecontacting surfaces. The adhesion force strongly affects friction, mechanicalcontact and tribological performance of such system surfaces, leading, forexample, to ‘stiction’ (combination of adhesion and static friction; Bhushan1996, 2008), which precludes microelectromechanical switches and actuators fromproper functioning. It is therefore desirable to produce non-adhesive surfaces,and applying surface microstructure mimicking the Lotus effect (as well as itsmodifications, e.g. the so-called ‘petal effect’; figure 3) has been successfully usedfor the design of non-adhesive surfaces, which are important for many tribologicalapplications.

(ii) The Gecko effect, which stands for the ability of specially structuredhierarchical surfaces to exhibit controlled adhesion. Geckos are known for theirability to climb vertical walls owing to a strong adhesion between their toes and

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(a)

(b)

(i) (ii) (iii)

Figure 2. (a) Scanning electron microscope (SEM) micrographs (shown at three magnifications)of lotus (Nelumbo nucifera) leaf surface, which consists of microstructure formed by papilloseepidermal cells covered with epicuticular wax tubules on the surface, which create nanostructureand (b) image of a water droplet sitting on the lotus leaf (Bhushan et al. 2009). Scale bars, (a)(i)10 mm, (ii) 2 mm and (iii) 0.4 mm.

(a) (b)

Figure 3. Optical micrographs of water droplets on Rosa cv. Bairage at (a) 0◦ and (b) 180◦ tiltangles. A droplet is still suspended when the petal is turned upside down (Bhushan & Her 2010).Scale bars, (a,b) 500 mm.

a number of various surfaces. They can also detach easily from a surface whenneeded (figure 4). This is due to a complex hierarchical structure of gecko footsurface. The Gecko effect is used for applications when strong adhesion is needed(e.g. adhesive tapes). The Gecko effect can be combined with the self-cleaningabilities (Bhushan 2007; Nosonovsky & Bhushan 2008).

(iii) Microstructured surfaces for underwater applications, including easy flowowing to boundary slip, the suppression of turbulence (the shark-skin effect;figure 5) and antibiofouling (the fish-scale effect). Biofouling and biofilming arethe undesirable accumulation of micro-organisms, plants and algae on structuresthat are immersed in water. Conventional antifouling coatings for ship hulls are

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Figure 4. The Tokay gecko has the ability to climb walls and detach from surfaces easily at will.

Figure 5. Scale structure on a Galapagos shark (Carcharhinus galapagensis; Reif 1985).Scale bar, 0.5 mm.

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often toxic and environmentally hazardous. On the other hand, in living nature,there are ecological coatings (e.g. fish scale), so a biomimetic approach is sought(Chambers et al. 2006; Genzer & Efimenko 2006; Nosonovsky & Bhushan 2009b;Dean & Bhushan 2010; Jung & Bhushan 2010).

(iv) Oleophobic surfaces capable of repelling organic liquids. The principlecan be similar to superhydrophobicity, but it is much more difficult to producean oleophobic surface because surface energies of organic liquids are low, andthey tend to wet most surfaces (Nosonovsky 2007; Tuteja et al. 2007, 2008;Nosonovsky & Bhushan 2009b). Underwater oleophobicity can also be used todesign self-cleaning and antifouling surfaces (figure 6; Jung & Bhushan 2009).

(v) Microstructured surfaces for various optical applications, including non-reflective (the Moth-eye effect), highly reflective, coloured (in some cases,including the ability to dynamically control coloration) and transparent surfaces.Optical surfaces are sensitive to contamination, so the self-cleaning abilityshould often be combined with optical properties (Nosonovsky & Bhushan 2008;Gombert & Blasi 2009).

(vi) Microtextured surfaces for de-icing and anti-icing (figure 7). De-icing(the removal of frozen contaminant from a surface) and anti-icing (protectingagainst the formation of frozen contaminant) are significant problems for manyapplications that have to operate below the water-freezing temperature: aircrafts,machinery, road and runway pavements, traffic signs and traffic lights, etc.The traditional approaches to de-icing include mechanical methods, heatingand the deposition of dry or liquid chemicals that lower the freezing point ofwater. Anti-icing is accomplished by applying a protective layer of a viscousanti-ice fluid. All anti-ice fluids offer only limited protection, dependent uponfrozen-contaminant type and precipitation rate, and they fail when they nolonger can absorb the contaminant. In addition to limited efficiency, thesede-icing fluids, such as propylene glycol or ethylene glycol, can be toxicand raise environmental concerns. Anti-icing on roadways is used to preventice and snow from adhering to the pavement, allowing easier removal bymechanical methods.

Ice formation occurs owing to condensation of vapour-phase water and furtherfreezing of liquid water. For example, droplets of supercooled water that exist instratiform and cumulus clouds crystallize into ice when they are struck by thewings of passing airplanes. Ice formation on other surfaces, such as pavementsor traffic signs, also occurs via the liquid phase. It is therefore suggested that awater-repellent surface can also have de-icing properties (Cao et al. 2009). Whena superhydrophobic surface is wetted by water, an air layer or air pockets areusually kept between the solid and the water droplets. After freezing, ice will notadhere to the solid owing to the presence of air pockets and will be easily washedor blown away.

(vii) MEMS-based dynamically tunable surfaces for the control ofliquid/matter flow and/or coloration (for example, mimicking the colorationcontrol in cephalopods), used for displays and other applications, the so-called‘origami’ (Sidorenko et al. 2007; Bucaro et al. 2009).

(viii) Various biomimetic microtextured surfaces to control friction, wear andlubrication (Varenberg & Gorb 2009; Bhushan 2010).

(ix) Self-lubricating surfaces, using various principles, including the ability forfriction-induced self-organization (Nosonovsky & Bhushan 2009c).

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oil droplet water

microsyringe

specimen

(a)

water droplet (89°)

water droplet (142°) oil droplet (115°)

oil droplet (approx. 0°) oil droplet in water (109°)

oil droplet in water (approx. 0°)

(b) (i)

(ii)

Figure 6. (a) Schematics of a solid–water–oil interface system. A specimen is first immersed inwater, and then an oil droplet is gently deposited using a microsyringe, and the static contactangle is measured. (b) Optical micrographs of droplets at three different phase interfaces on amicropatterned surface (shark-skin replica): (i) without and (ii) with C20F42 (Jung & Bhushan2009). Scale bar, (b) 0.5 mm.

(x) Self-repairing surfaces and materials, which are able to heal minor damage(cracks, voids; Nosonovsky & Bhushan 2009c, 2010).

(xi) Various surfaces with alternate (and dynamically controlled) wettingproperties for micro-/nanofluidic applications, including the Darkling beetleeffect, e.g. the ability of a desert beetle to collect water on its back using the

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smooth surfacewater

ice

ice blown away

freezing

water

superhydrophobic surface superhydrophobic surface

smooth surface smooth surface

Figure 7. The principle of applying of surface microstructure for de-icing.

(a) (b)

Figure 8. The water-capturing surface of the fused overwings (elytra) of the desert beetle Stenocarasp. (a) Adult female, dorsal view; peaks and valleys are evident on the surface of the elytra and(b) SEM image of the textured surface of the depressed areas (Parker & Lawrence 2001). Scalebars, (a) 10 mm and (b) 10 mm.

hydrophilic spots on an otherwise hydrophobic surface of its back (Parker &Lawrence 2001; Rechenberg & El Khyeri 2007; Nosonovsky & Bhushan 2008)(figure 8).

(xii) Water-strider effect mimicking the ability of insects to walk on waterusing the capillary forces. The hierarchical organization of the water-strider legsurface plays a role in its ability to remain dry on the water surface (figure 9;Gao & Jiang 2004).

(xiii) The ‘sand fish’ lizard effect, able to dive and ‘swim’ in loose sand owingto special electromechanical properties of its scale (Rechenberg & El Khyeri 2007;Nosonovsky & Bhushan 2008).

Environmental engineers have only just started paying attention to biomimeticsurfaces. Raibeck et al. (2009) investigated the potential environmental benefitsand burdens associated with using the Lotus-effect-based self-cleaning surfaces.They found that while the use-phase benefits are apparent, productionburdens can outweigh them when compared with other cleaning methods,

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(i) (ii)

(a)

(b)

Figure 9. (a) Water strider (pond skater, Gerris remigis) walking on water and (b) SEM imagesof a pond-skater leg showing (i) numerous oriented microscale setae and (ii) nanoscale groovedstructures on a seta (Gao & Jiang 2004). Scale bars, (b) (i) 20 mm and (ii) 200 nm.

so a more thoughtful and deliberate use of bioinspiration in sustainableengineering is needed. Clearly, more studies are likely to emerge in thenear future.

(b) Biodegradable lubrication

In the area of environment-friendly and biodegradable lubrication, several ideashave been suggested:

— The use of natural (e.g. vegetable-oil-based or animal-fat-based)biodegradable lubricants. This involves oils that are used for engines,hydraulic applications and metal-cutting applications. In particular,corn, soybean and coconut oils have been used so far (the latteris of particular interest in tropical countries such as India). Theselubricants are potentially biodegradable, although in some cases, chemicalmodification or additives for best performance are required. Vegetableoils can have excellent lubricity, far superior than that of mineral oil.In addition, they have a very high viscosity index and high flash/firepoints. However, natural oils often lack sufficient oxidative stability, whichmeans that the oil will oxidize rather quickly during use, becoming thickand polymerizing to a plastic-like consistency. Chemical modification ofvegetable oils and/or the use of antioxidants can address this problem(table 1; Mannekote & Kailas 2009).

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Table 1. The content of CO and CO2 in exhaust gas lubricated with regular and vegetable oil(Mannekote & Kailas 2009).

sample CO2 (%) CO (%)

engine oil 4.5 0.92coconut oil 2.9 0.67palm oil 3.4 0.73

— Powder lubricants and, in particular, boric acid lubricants. In general,these tend to be much more ecologically friendly than the traditional liquidlubricants (Kabir et al. 2008). Boric acid and MoS2 powder can also be usedas an additive to the natural oil. Friction and wear experiments show thatthe nanoscale (20 nm) particle boric acid additive lubricants significantlyoutperformed all of the other lubricants with respect to frictional and wearperformance. In fact, the nanoscale boric acid powder-based lubricantsexhibited a wear rate more than an order of magnitude lower than theMoS2 and larger sized boric acid additive-based lubricants (M. R. Lovell,M. A. Kabir, P. L. Menezes & C. F. Higgs 2010, personal communication).

— Self-replenishing lubrication that uses oil-free environmentally benignpowders for lubrication of critical components such as bearings used infuel cell compressors and expanders (Wornyoh et al. 2007).

(c) Renewable energy

The tribology of renewable sources of energy is a relatively new field oftribology. Today, there are meetings and sections devoted to the tribology ofwind turbines at almost every tribology conference, and they cover certain issuesspecific for these applications. Unlike in the case of the biomimetic approachand environment-friendly lubrication, it is not the manufacturing or operation,but the very application of the tribological system that involves green issues,namely, environmentally friendly energy production. The following issues canbe mentioned.

(i) Wind-power turbines have a number of specific problems related totheir tribology, and constitute a well-established area of tribological research.These issues include water contamination, electric arcing on generator bearings,issues related to the wear of the mainshaft and gearbox bearings and gears,the erosion of blades (solid particles, cavitation, rain, hail stones), etc.(Kotzalas & Lucas 2007).

(ii) Tidal-power turbines are another important way of producing renewableenergy, which involves certain tribiological problems. Tidal-power turbines areespecially popular in Europe (particularly, in the UK), which remains theleader in this area, although several potential sites in North America have beensuggested. There are several specific tribological issues related to the tidal-power turbines, such as their lubrication (seawater, oils and greases), erosion,corrosion and biofouling, as well as the interaction between these modes of damage(Batten et al. 2008).

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Figure 10. Double-helix Gorlov turbine for underwater installation (Gorban’ et al. 2001).

Besides tidal, the ocean-water flow energy and river flow energy (without dams)can be used with the application of special turbines, such as the Gorlov helicalturbine (figure 10; Gorban et al. 2001), which provides the same direction ofrotation independent of the direction of the current flow. These applications alsoinvolve specific tribological issues.

(iii) Geothermal energy plants are used in the USA (in particular, at the Pacificcoast and Alaska); however, their use is limited to the geographical areas at theedges of tectonic plates (Rybach 2007). In 2007, they produced 2.7 GW of energyin the USA, with Philippines (2.0 GW) and Indonesia (1.0 GW) in second andthird places (Bertani 2007). There are several issues related to the tribology ofgeothermal energy sources that are discussed in the literature.

4. Challenges

In the preceding sections, we have outlined the need for green tribology, itsprinciples and primary areas of research. As a new field, green tribology hasa number of challenges. One apparent challenge is the development of the above-mentioned fields in such a manner that they could benefit from each other.Only in the case where such a synthesis is performed is it possible to seegreen tribology as a coherent and self-sustained field of science and technology,

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rather than a collection of several topics of research in tribology and surfaceengineering. There is apparently potential synergy in the use of the biomimeticapproach, microstructuring, biodegradable lubrication, self-lubrication and othernovel approaches, as well as in developing methods of their applications tosustainable engineering and energy production. Clearly, more research should bedone for the integration of these fields. Some ideas could be borrowed from therelated field of green chemistry, for example, developing quantitative metrics toassess the environmental impact of tribological technologies.

Green tribology should be integrated into world science and make its impacton the solutions for worldwide problems, such as the change of climate andthe shortage of food and drinking water. H. P. Jost (2009, unpublished data)mentioned the economical potential of the new discipline: ‘The application oftribological principles alone will, of course, not solve these world-wide problems.Only major scientific achievements are likely to be the key to their solution,of which I rate energy as one of the most important ones. For such tasks tobe achieved, the application of tribology, and especially of green tribology canprovide a breathing space which would enable scientists and technologists tofind solutions to these, mankind’s crucial problems and allow time for them tobe implemented by governments, organizations and indeed everyone operatingin this important field. Consequently, this important—albeit limited—breathingspace may be extremely valuable to all working for the survival of life as we knowit. However, the ultimate key is science and its application. Tribology—especiallygreen tribology can and—I am confident—will play its part to assist and give timefor science to achieve the required solutions and for policy makers to implementthem’.

5. Conclusions

Green tribology is a novel area of science and technology. It is related to otherareas of tribology as well as other ‘green’ disciplines, namely, green engineeringand green chemistry. We have formulated the 12 principles of green tribology anddefined three areas of tribological studies most relevant to green tribology. Theintegration of these areas remains the primary challenge of green tribology anddefines future directions of research.

References

Allen, D. T. & Shonnard, D. R. 2001 Green engineering: environmentally conscious design ofchemical processes. Upper Saddle River, NJ: Prentice-Hall.

Anastas, P. T. & Warner, J. C. 1998 Green chemistry: theory and practice. New York, NY: OxfordUniversity Press.

Bar-Cohen, Y. 2006 Biomimetics: biologically inspired technologies. Boca Raton, FL: Taylor &Francis.

Batten, W. M. J., Bahaj, A. S., Molland, A. F. & Chaplin, J. R. 2008 The prediction ofthe hydrodynamic performance of marine current turbines. Renew. Energy 33, 1085–1096.(doi:10.1016/j.renene.2007.05.043)

Bertani, E. R. 2007 World geothermal generation in 2007. Geo-Heat Cent. Q. Bull. 28, 8–19.

Phil. Trans. R. Soc. A (2010)

4692 M. Nosonovsky and B. Bhushan

Bhushan, B. 1996 Tribology and mechanics of magnetic storage devices, 2nd edn. New York, NY:Springer.

Bhushan, B. 1999 Principles and applications of tribology. New York, NY: Wiley.Bhushan, B. 2000 Mechanics and reliability of flexible magnetic media, 2nd edn. New York, NY:

Springer.Bhushan, B. 2001 Modern tribology handbook, vol. 1—Principles of tribology; vol. 2—Materials,

coatings, and industrial applications. Boca Raton, FL: CRC Press Inc.Bhushan, B. 2002 Introduction to tribology. New York, NY: Wiley.Bhushan, B. 2007 Adhesion of multi-level hierarchical attachment systems in gecko feet. J. Adhes.

Sci. Technol. 21, 1213–1258. (doi:10.1163/156856107782328353)Bhushan, B. (ed.) 2008 Nanotribology and nanomechanics: an introduction, 2nd edn. Heidelberg,

Germany: Springer.Bhushan, B. 2009 Biomimetics: lessons from nature – an overview. Phil. Trans. R. Soc. A 367,

1445–1486. (doi:10.1098/rsta.2009.0011)Bhushan, B. (ed.) 2010 Springer handbook of nanotechnology, 3rd edn. Heidelberg, Germany:

Springer.Bhushan, B. & Gupta, B. K. 1991 Handbook of tribology: materials, coatings, and surface treatments.

New York, NY: McGraw-Hill Book Company.Bhushan, B. & Her, E. K. 2010 Fabrication of superhydrophobic surfaces with high and low

adhesion inspired from rose petal. Langmuir 26, 8207–8217. (doi:10.1021/la904585j)Bhushan, B., Jung, Y. C. & Koch, K. 2009 Micro-, nano- and hierarchical structures for

superhydrophobicity, self-cleaning and low adhesion. Phil. Trans. R. Soc. A 367, 1631–1672.(doi:10.1098/rsta.2009.0014)

Bucaro, M. A., Kolodner, P. R., Taylor, A., Sidorenko, A., Aizenberg, J. & Krupenkin, T. N. 2009Tunable liquid optics: electrowetting-controlled liquid mirrors based on self-assembled janustiles. Langmuir 25, 3876–3879. (doi:10.1021/la803537v)

Cao, L., Jones, A. K., Sikka, V. K., Wu, J. & Gao, D. 2009 Anti-icing superhydrophobic coatings.Langmuir 25, 12 444–12 448. (doi:10.1021/la902882b)

Chambers, L. D., Stokes, K. R., Walsh, F. C. & Wood, R. J. K. 2006 Modern approaches tomarine antifouling coatings. Surf. Coat. Tech. 201, 3642–3652. (doi:10.1016/j.surfcoat.2006.08.129)

Constable, D. J. C. et al. 2001 Green chemistry measures for process research and development.Green Chem. 3, 7–9. (doi:10.1039/b007875l)

Dean, B. & Bhushan, B. 2010 Shark-skin surfaces for fluid-drag reduction in turbulent flow: areview. Phil. Trans. R. Soc. A 368, 4775–4806. (doi:10.1098/rsta.2010.0201)

Favret, E. & Fuentas, N. O. (eds.) 2009 Functional properties of bio-inspired surfaces:characterization and technological applications. Hackensack, NJ: World Scientific Publishing Co.

Fratzl, P. 2007 Biomimetic materials research: what can we really learn from nature’s structuralmaterials? J. R. Soc. Interface 4, 637–642. (doi:10.1098/rsif.2007.0218)

Fratzl, P. & Weinkamer, R. 2007 Nature’s hierarchical materials. Prog. Mater. Sci. 52, 1263–1334.(doi:10.1016/j.pmatsci.2007.06.001)

Gao, X. F. & Jiang, L. 2004 Biophysics: water-repellent legs of water striders. Nature 432, 36.(doi:10.1038/432036a)

Genzer, J. & Efimenko, K. 2006 Recent developments in superhydrophobic surfaces and theirrelevance to marine fouling: a review. Biofouling 22, 339–360. (doi:10.1080/08927010600980223)

Gombert, A. & Blasi, B. 2009 The moth-eye-effect—from fundamentals to commercial exploitation.In Functional properties of bio-inspired surfaces: characterization and technological applications(eds E. Favret & N. O. Fuentas), pp. 79–102. Hackensack, NJ: World Scientific Publishing Co.(doi:10.1142/9789812837028_0004)

Gorb, S. 2006 Functional surfaces in biology: mechanisms and applications. In Biomimetics:biologically inspired technologies (ed. Y. Bar-Cohen), pp. 381–397. Boca Raton, FL: Taylor &Francis.

Gorban’, A. N., Gorlov, A. M. & Silantyev, V. M. 2001 Limits of the turbine efficiency for freefluid flow. ASME J. Energy Resour. Technol. 123, 311–317. (doi:10.1115/1.1414137)

Phil. Trans. R. Soc. A (2010)

Introduction. Green tribology 4693

Hudlicky, T., Frey, D. A., Koroniak, L., Claeboe, C. D. & Brammer, L. E. 1999 Toward a reagent-free synthesis. Green Chem. 1, 57–59. (doi:10.1039/a901397k)

Jost, H. P. 1966 Lubrication (tribology): a report on the present position and industry’s needs.Report to the UK Department of Education and Science.

Jung, Y. C. & Bhushan, B. 2009 Wetting behavior of water and oil droplets in three-phaseinterfaces for hydrophobicity/philicity and oleophobicity/philicity. Langmuir 25, 14 165–14 173.(doi:10.1021/la901906h)

Jung, Y. C. & Bhushan, B. 2010 Biomimetic structures for fluid drag reduction in laminarand turbulent flows. J. Phys. Condens. Matter 22, 035104. (doi:10.1088/0953-8984/22/3/035104)

Kabir, M. A., Higgs III, C. F. & Lovell, M. R. 2008 A pin-on disk experimental study on a greenparticulate-fluid lubricant. ASME J. Tribol. 130, 041801. (doi:10.1115/1.2908913)

Kotzalas, M. & Lucas, D. 2007 Comparison of bearing fatigue life predictions with test data. InProc. AWEA Wind Power 2007, 3–6 June, Los Angeles, CA. [CD-ROM].

Mannekote, J. K. & Kailas, S. V. 2009 Performance evaluation of vegetable oils as lubricant ina four stroke engine. In Proc. 4th World Tribology Congress, Kyoto, Japan, 6–11 September,p. 331.

Nosonovsky, M. 2007 Multiscale roughness and stability of superhydrophobic biomimetic interfaces.Langmuir 23, 3157–3161. (doi:10.1021/la062301d)

Nosonovsky, M. & Bhushan, B. 2007a Biomimetic superhydrophobic surfaces: multiscale approach.Nano Lett. 7, 2633–2637. (doi:10.1021/nl071023f)

Nosonovsky, M. & Bhushan, B. 2007b Multiscale friction mechanisms and hierarchical surfacesin nano- and bio-tribology. Mater. Sci. Eng. R 58, 162–193. (doi:10.1016/j.mser.2007.09.001)

Nosonovsky, M. & Bhushan, B. 2008 Multiscale dissipative mechanisms and hierarchical surfaces:friction, superhydrophobicity, and biomimetics. Heidelberg, Germany: Springer.

Nosonovsky, M. & Bhushan, B. 2009a Multiscale effects and capillary interactions in functionalbiomimetic surfaces for energy conversion and green engineering. Phil. Trans. R. Soc. A 367,1511–1539. (doi:10.1098/rsta.2009.0008)

Nosonovsky, M. & Bhushan, B. 2009b Superhydrophobic surfaces and emerging applications:non-adhesion, energy, green engineering. Curr. Opin. Colloid Interface Sci. 14, 270–280.(doi:10.1016/j.cocis.2009.05.004)

Nosonovsky, M. & Bhushan, B. 2009c Thermodynamics of surface degradation, self-organization, and self-healing for biomimetic surfaces. Phil. Trans. R. Soc. A 367, 1607–1627.(doi:10.1098/rsta.2009.0009)

Nosonovsky, M. & Bhushan, B. 2010 Surface self-organization: from wear to self-healing inbiological and technical surfaces. Appl. Surf. Sci. 256, 3982–3987. (doi:10.1016/j.apsusc.2010.01.061)

Parker, A. R. & Lawrence, C. R. 2001 Water capture by a desert beetle. Nature 414, 33–34.(doi:10.1038/35102108)

Raibeck, L., Reap, J. & Bras, B. 2009 Investigating environmental burdens and benefitsof biologically inspired self-cleaning surfaces. CIRP J. Manuf. Sci. Technol. 1, 230–236.(doi:10.1016/j.cirpj.2009.05.004)

Rechenberg, I. & El Khyeri, A. R. 2007 The sandfish of the Sahara. A model for friction and wearreduction. Department of Bionics and Evolution Techniques, Technical University of Berlin. Seehttp://www.bionik.tu-berlin.de/institut/safiengl.htm.

Reif, W. E. 1985 Squamation and ecology of sharks. Courier Forschungsinstitut Senckenberg 78,1–255.

Rybach, L. 2007 Geothermal sustainability. Geo-Heat Cent. Q. Bull. 28, 2–7.Scherge, M. & Gorb, S. 2001 Biological micro- and nanotribology: nature’s solutions. Heidelberg,

Germany: Springer.Sheldon, R. A. 1992 Organic synthesis: past, present and future. Chem. Ind. 23, 903–906.Sidorenko, A., Krupenkin, T., Taylor, A., Fratzl, P. & Aizenberg, J. 2007 Reversible switching

of hydrogel-sctuated nanostructures into complex micropatterns. Science 315, 487–490.(doi:10.1126/science.1135516)

Phil. Trans. R. Soc. A (2010)

4694 M. Nosonovsky and B. Bhushan

Trost, B. M. 1991 The atom economy—a search for synthetic efficiency. Science 254, 1471–1477.(doi:10.1126/science.1962206)

Tuteja, A., Choi, W., Ma, M., Mabry, J. M., Mazzella, S. A., Rutledge, G. C., McKinley,G. H. & Cohen, R. E. 2007 Designing superoleophobic surfaces. Science 318, 1618–1622.(doi:10.1126/science.1148326)

Tuteja, A., Choi, W., Mabri, J. M., McKinley, G. H. & Cohen, R. E. 2008 Robustomniphobic surfaces. Proc. Natl Acad. Sci. USA 105, 18 200–18 205. (doi:10.1073/pnas.0804872105)

US Environmental Protection Agency. 2010 Green engineering. See http://www.epa.gov/oppt/greenengineering/.

Varenberg, M. & Gorb, S. 2009 Hexagonal surface micropattern for dry and wet friction. Adv.Mater. 21, 483–486. (doi:10.1002/adma.200802734)

Wornyoh, E. Y. A., Jasti, V. K. & Higgs III, C. F. 2007 A review of dry particulate lubrication:powder and granular materials. ASME J. Tribol. 129, 438–449. (doi:10.1115/1.2647859)

Phil. Trans. R. Soc. A (2010)