**2. Green tribology: concept, goal and role**

It has been noted that the concepts in the main development direction of tribology are historically changed as follows (**Figure 1**).

Nowadays, the term "green tribology" has become part of the engineering dictionary. Green tribology is an emerging and actual area in tribological science with more focus on energy saving and environmental protection. Although green tribology is a fairly new concept; however, it already plays an important role in ensuring that all industrial systems can be able to function in an environmentally friendly manner. Green tribology is especially tuned to sustaining an ecological balance and biological effects on contact between surface systems from different materials. Green tribology ensures that any process of friction and wear is as environmentally friendly as possible. Thus, green tribology can be defined as an interdisciplinary field attributed to the broad induction of various concepts such as energy, materials science, green lubrication, and environmental science [8–11].

We have known the concept of green engineering for a long time. The United States Environmental Protection Agency (USEPA) defines green engineering as

**47**

economic costs.

*Green Tribology*

researchers today [12, 13].

systems in the course of a lifecycle".

overall sustainability and safety in human life [15].

while the wear resistance and longevity are greatly increased.

*DOI: http://dx.doi.org/10.5772/intechopen.94510*

"the design, commercialization and application of processes and products that technically and economically reduce sources of pollution and risks that adversely affect human health and the environment". Speaking of green tribology, one cannot ignore the terms "Green Technology", "Green Engineering", "Green Metalworking", etc., but the first association and historically, the first green science, as applied to science naturally, was "Green Chemistry". Green engineering and green chemistry are two closely related fields of green tribology that are actively engaged by

Specifically, green tribology has been identified as an area of engineering that could go beyond its original remit of improving efficiency by minimizing wear and friction in tribological processes to save energy and resources, minimize noise pollution, develop new bio-lubricants. In general, green tribology gives a positive contribution in reducing environmental harm. Inevitably, the term "green tribology" is spoken of in the context of quality of life. According to professor's Zhang opinion [2]: "Thus, the concepts and objectives of green tribology might be summarized into 3L + 1H, namely, low energy consumption, low discharge (CO2), low environmental cost, and high quality of life. The mission of green tribology is researching and developing tribological technologies to reach the main objectives, thus making the sustained artificial ecosystems of the tribological parts and tribo-

Emilia Assenova and her colleagues in their work "Green tribology and quality of life" [14] reported: *"*Nowadays, losses resulting from ignorance of tribology amount to about 6% of the gross national product (GNP) in the United States alone. This figure is around USD 900 milliard annually. As far as China is concerned, they could save above USD 40 milliard per year by the application of green tribology or more than 1.5% of the GNP". It is clear that the basic goals of green tribology are "friction control, wear reduction and improved lubrication". Nevertheless, from a socio-economic point of view, it is possible to extend and confirm that the goal and essence of research works in the field of green tribology is to save material resources, improve energy efficiency, decrease emissions, shock absorption, investigate and apply novel natural bio- and eco-lubricants as well as to reduce the harmful effects of technical systems on the environment, and consequently, improve the quality and welfare of society. All advances in green tribology will lead to a high economic efficiency due to reduced waste and increased equipment service life, improved technological and environmental balance, decreased carbon footprint of mechanical systems, as a result, mitigate climate changes, and improve

Green tribology will play an irreplaceable role in saving energy, material resources and environment. Trusted researches reveal that about 23% of energy consumption in the world today is the result of inefficient performance of tribological systems (**Figure 2**) [16]. In this case, approximately 18–20% of the energy is consumed to solve friction problems, and the remaining 3–5% is used to rebuild, repair and replace parts worn out due to wear and other failures associated with wear. The researchers estimated that by applying advances in green tribology in terms of new surfaces, materials and lubrication technologies, the total global energy loss in tribological systems could be decreased by 18% in the next 8 years and up to 40% in the next 15 years. An additional advantage of environmentally friendly green tribology is a significant reduction in carbon dioxide emissions and

In works [17–23] the researchers applied green tribology concept using new class of eco-friendly lubricants and materials for manufacturing anti-friction contact surfaces, as a result of which their coefficient of friction is significantly reduced

**Figure 1.**

*Tribology concepts in the new main direction of development [1, 2].*

#### *Green Tribology DOI: http://dx.doi.org/10.5772/intechopen.94510*

*Tribology in Materials and Manufacturing - Wear, Friction and Lubrication*

It is noticeable that tribology has continuously developed into new phases. After opening in 1956, the effect of selective transfer (ST) ("zero-wear" effect) during friction, in tribology for the first time, was a basis to build the whole new paradigm of "friction without wear with minimal energy consumption", which was denied by all previous practical experience in operating movable coupling and by theoretical constructions of science of the friction and wear solids. At the same time, by the beginning of this century, the concept of "green tribology" was formed, which actually lists all the achievements in the study of the mechanisms of wearlessness (zero-wear) and super-anti-friction, as well as in the development of lubricants for their implementation in practice [3, 4]. In the 21st century and beyond, green tribology is expected to play an increasingly important role and become the key and strategies for solving a series of global problems in energy, environment and

In recent years, there has been a rapid growth in research activities in green tribology field. A fairly large number of articles, world conference reports and academic books in related to this area have been published [2, 5–15]. However, there are still few publications that expounded the concepts, technological connotations, principles and disciplinary features of green tribology in precise, comprehensive definition and in an all-round way. The first scientific work completely devoted to green tribology, which emphasized the scientific rather than the economic and social aspects was published by M. Nosonovsky and B. Bhushan

Being a new field of tribology still in its infancy, an accurate understanding of the fundamentals of green tribology is important from both a scientific and practical point of view. In this regard, the aim of this work is to clarify the fundamental scientific and technological foundations of green tribology based on the analysis and generalization

It has been noted that the concepts in the main development direction of

Nowadays, the term "green tribology" has become part of the engineering dictionary. Green tribology is an emerging and actual area in tribological science with more focus on energy saving and environmental protection. Although green tribology is a fairly new concept; however, it already plays an important role in ensuring that all industrial systems can be able to function in an environmentally friendly manner. Green tribology is especially tuned to sustaining an ecological balance and biological effects on contact between surface systems from different materials. Green tribology ensures that any process of friction and wear is as environmentally friendly as possible. Thus, green tribology can be defined as an interdisciplinary field attributed to the broad induction of various concepts such as energy, materials

We have known the concept of green engineering for a long time. The United States Environmental Protection Agency (USEPA) defines green engineering as

**46**

**Figure 1.**

resources.

in 2010 [5].

of the research achievements of green tribology.

**2. Green tribology: concept, goal and role**

tribology are historically changed as follows (**Figure 1**).

science, green lubrication, and environmental science [8–11].

*Tribology concepts in the new main direction of development [1, 2].*

"the design, commercialization and application of processes and products that technically and economically reduce sources of pollution and risks that adversely affect human health and the environment". Speaking of green tribology, one cannot ignore the terms "Green Technology", "Green Engineering", "Green Metalworking", etc., but the first association and historically, the first green science, as applied to science naturally, was "Green Chemistry". Green engineering and green chemistry are two closely related fields of green tribology that are actively engaged by researchers today [12, 13].

Specifically, green tribology has been identified as an area of engineering that could go beyond its original remit of improving efficiency by minimizing wear and friction in tribological processes to save energy and resources, minimize noise pollution, develop new bio-lubricants. In general, green tribology gives a positive contribution in reducing environmental harm. Inevitably, the term "green tribology" is spoken of in the context of quality of life. According to professor's Zhang opinion [2]: "Thus, the concepts and objectives of green tribology might be summarized into 3L + 1H, namely, low energy consumption, low discharge (CO2), low environmental cost, and high quality of life. The mission of green tribology is researching and developing tribological technologies to reach the main objectives, thus making the sustained artificial ecosystems of the tribological parts and tribosystems in the course of a lifecycle".

Emilia Assenova and her colleagues in their work "Green tribology and quality of life" [14] reported: *"*Nowadays, losses resulting from ignorance of tribology amount to about 6% of the gross national product (GNP) in the United States alone. This figure is around USD 900 milliard annually. As far as China is concerned, they could save above USD 40 milliard per year by the application of green tribology or more than 1.5% of the GNP". It is clear that the basic goals of green tribology are "friction control, wear reduction and improved lubrication". Nevertheless, from a socio-economic point of view, it is possible to extend and confirm that the goal and essence of research works in the field of green tribology is to save material resources, improve energy efficiency, decrease emissions, shock absorption, investigate and apply novel natural bio- and eco-lubricants as well as to reduce the harmful effects of technical systems on the environment, and consequently, improve the quality and welfare of society. All advances in green tribology will lead to a high economic efficiency due to reduced waste and increased equipment service life, improved technological and environmental balance, decreased carbon footprint of mechanical systems, as a result, mitigate climate changes, and improve overall sustainability and safety in human life [15].

Green tribology will play an irreplaceable role in saving energy, material resources and environment. Trusted researches reveal that about 23% of energy consumption in the world today is the result of inefficient performance of tribological systems (**Figure 2**) [16]. In this case, approximately 18–20% of the energy is consumed to solve friction problems, and the remaining 3–5% is used to rebuild, repair and replace parts worn out due to wear and other failures associated with wear. The researchers estimated that by applying advances in green tribology in terms of new surfaces, materials and lubrication technologies, the total global energy loss in tribological systems could be decreased by 18% in the next 8 years and up to 40% in the next 15 years. An additional advantage of environmentally friendly green tribology is a significant reduction in carbon dioxide emissions and economic costs.

In works [17–23] the researchers applied green tribology concept using new class of eco-friendly lubricants and materials for manufacturing anti-friction contact surfaces, as a result of which their coefficient of friction is significantly reduced while the wear resistance and longevity are greatly increased.

#### **Figure 2.**

*Energy consumption, costs and CO2 emissions due to inefficient performance of tribological systems globally [16].*

A survey of gross energy consumption in the United States in four main areas: transportation, turbomachinery, power generation and industrial applications showed that savings of about 11% are achieved thanks to recent developments in lubrication and green tribology [24]. Chinese estimated that they could save more than \$ 40 billion per year by applying advances in green tribology [25].

If we look at the share of wind energy in the total installed electricity capacity in Europe over the last decade, according to the European Wind Energy Association, it has increased more than quadrupled from 2.2% in 2000 to 10.5% in 2011 thanks to new developments in tribology, in particular as a result of the application of green tribology [26].

Many tribological problems can be put under the umbrella of "green tribology" and are mutually beneficial to each other. These problems are primary focus point of researchers and engineers, which include tribological technology that mimics living nature (biomimetic surfaces) and thus is expected to be environment-friendly, the friction and wear control that is important for energy conservation and conversion, environmental aspects of lubrication and surface modification techniques. These problems and aspects will be clarified in more detail in the next section.

### **3. Green tribology: principles, focus areas, and challenges**

#### **3.1 Principles of green tribology**

As noted above, the interdisciplinary nature of green tribology often integrates aspects of chemical engineering and materials science in order to completely understand both chemistry and mechanics of surface. Since tribology is an interdisciplinary field, the principles of green engineering and green chemistry should also apply to green tribology. However, tribology includes not only chemistry of surfaces, but also other aspects related to the mechanics and physics of surfaces, there is a need to modify these principles.

Formulated by Paul Anastas in 1991, the 12 principles of green chemistry into a constant amount (12) upgraded to the 12 principles of green engineering [27, 28], and later, in the 12 principles of green tribology [1] mapped in **Table 1**.

These principles of green tribology can be assorted into 5 following groups: Friction, Wear, Lubrication, Material and surface production and treatment, and Tribology in the renewable energy sources.

**49**

*Green Tribology*

• Prevention. • Atom Economy.

Syntheses. • Designing Safer Chemicals. • Safer Solvents and Auxiliaries. • Design for Energy Efficiency. • Reduce Derivatives.

• Catalysis.

**Table 1.**

• Design for Degradation. • Real-time analysis for Pollution. • Prevention. • Inherently Safer. • Chemistry for Accident Prevention.

• Less Hazardous Chemical

*DOI: http://dx.doi.org/10.5772/intechopen.94510*

**Green chemistry Green engineering Green tribology**

• Inherent rather than circumstantial. • Prevention instead of treatment. • Design for separation. • Maximize mass, energy, space, and time efficiency. • Output-pulled versus input-pushed. • Conserve complexity. • Durability rather than immortality.

• Minimization of heat and energy

• Environmental implications of

• Sustainable energy applications.

• Design for degradation. • Real-time monitoring.

dissipation. • Minimization of wear. • Reduction or complete elimination of lubrication and

self-lubrication. • Natural lubrication. • Biodegradable lubrication. • Sustainable chemistry and green engineering principles. • Biomimetic approach. • Surface texturing.

coatings.

**Friction** *(minimization of heat and energy dissipation)*. Friction is the main source of energy dissipation, most of which is converted to heat. Controlling and minimizing friction, which results in both energy savings and the prevention of damage to the environment owing to heat pollution, is a top priority for green tribology. In addition, the friction in mechanical systems that operate on friction,

• Meet need, minimize excess. • Minimize material diversity. • Integrate local material and

energy flows. • Design for commercial "afterlife".

• Renewable rather than depleting.

**Wear** *(minimization of wear)*. This is the second most important task of green tribology. In most technological processes, wear is undesirable, it decreases the lifetime of elements/machine and creates the problems of their recycling/replacements which in turn leads to environmental damage by way of the emission. Wear can also lead to a large waste of material resources. In addition, due to wear, debris in the form of particles is generated, which pollutes the environment and in certain

**Lubrication**. *Reduction or complete elimination of lubrication and self-lubrication*. Lubrication is at the forefront of tribology as it reduces friction and wear. However, lubrication is also hazardous to the environment. It is desirable to reduce the use of lubricants or achieve a self-lubrication regime when no external lubrication is required. Tribological systems in living nature often operate in the selflubricating mode. For example, the joints form a closed, self-sufficient system. Green tribology prompted researchers to think about self-lubricating materials,

*Natural lubrication.* In green tribology Natural lubricants such as vegetable oils

*Biodegradable lubrication.* Biodegradable lubricants should also be used when possible to avoid environmental pollution. In particular, water lubrication is an area that has attracted the attention of tribologists in recent years. Lubrication with

such as clutches and brakes, also has to be well optimized.

*12 principles of green chemistry, green machine building and green tribology.*

which also eliminated the external supply of lubricants.

should be used in cases when possible, since they are eco-friendly.

situations can be dangerous to humans.

natural oils is another good option.


#### **Table 1.**

*Tribology in Materials and Manufacturing - Wear, Friction and Lubrication*

A survey of gross energy consumption in the United States in four main areas: transportation, turbomachinery, power generation and industrial applications showed that savings of about 11% are achieved thanks to recent developments in lubrication and green tribology [24]. Chinese estimated that they could save more

If we look at the share of wind energy in the total installed electricity capacity in Europe over the last decade, according to the European Wind Energy Association, it has increased more than quadrupled from 2.2% in 2000 to 10.5% in 2011 thanks to new developments in tribology, in particular as a result of the application of green

Many tribological problems can be put under the umbrella of "green tribology" and are mutually beneficial to each other. These problems are primary focus point of researchers and engineers, which include tribological technology that mimics living nature (biomimetic surfaces) and thus is expected to be environment-friendly, the friction and wear control that is important for energy conservation and conversion, environmental aspects of lubrication and surface modification techniques. These problems and aspects will be clarified in more

As noted above, the interdisciplinary nature of green tribology often integrates

Formulated by Paul Anastas in 1991, the 12 principles of green chemistry into a constant amount (12) upgraded to the 12 principles of green engineering [27, 28],

These principles of green tribology can be assorted into 5 following groups: Friction, Wear, Lubrication, Material and surface production and treatment, and

aspects of chemical engineering and materials science in order to completely understand both chemistry and mechanics of surface. Since tribology is an interdisciplinary field, the principles of green engineering and green chemistry should also apply to green tribology. However, tribology includes not only chemistry of surfaces, but also other aspects related to the mechanics and physics of surfaces,

and later, in the 12 principles of green tribology [1] mapped in **Table 1**.

than \$ 40 billion per year by applying advances in green tribology [25].

*Energy consumption, costs and CO2 emissions due to inefficient performance of tribological systems* 

**3. Green tribology: principles, focus areas, and challenges**

**48**

tribology [26].

**Figure 2.**

*globally [16].*

detail in the next section.

**3.1 Principles of green tribology**

there is a need to modify these principles.

Tribology in the renewable energy sources.

*12 principles of green chemistry, green machine building and green tribology.*

**Friction** *(minimization of heat and energy dissipation)*. Friction is the main source of energy dissipation, most of which is converted to heat. Controlling and minimizing friction, which results in both energy savings and the prevention of damage to the environment owing to heat pollution, is a top priority for green tribology. In addition, the friction in mechanical systems that operate on friction, such as clutches and brakes, also has to be well optimized.

**Wear** *(minimization of wear)*. This is the second most important task of green tribology. In most technological processes, wear is undesirable, it decreases the lifetime of elements/machine and creates the problems of their recycling/replacements which in turn leads to environmental damage by way of the emission. Wear can also lead to a large waste of material resources. In addition, due to wear, debris in the form of particles is generated, which pollutes the environment and in certain situations can be dangerous to humans.

**Lubrication**. *Reduction or complete elimination of lubrication and self-lubrication*. Lubrication is at the forefront of tribology as it reduces friction and wear. However, lubrication is also hazardous to the environment. It is desirable to reduce the use of lubricants or achieve a self-lubrication regime when no external lubrication is required. Tribological systems in living nature often operate in the selflubricating mode. For example, the joints form a closed, self-sufficient system. Green tribology prompted researchers to think about self-lubricating materials, which also eliminated the external supply of lubricants.

*Natural lubrication.* In green tribology Natural lubricants such as vegetable oils should be used in cases when possible, since they are eco-friendly.

*Biodegradable lubrication.* Biodegradable lubricants should also be used when possible to avoid environmental pollution. In particular, water lubrication is an area that has attracted the attention of tribologists in recent years. Lubrication with natural oils is another good option.

**Material and surface production and treatment.** *Sustainable chemistry and green engineering principles*. These principles should be observed in the production of new materials, elements, parts, machines for tribological applications, coatings and lubricants.

*Biomimetic approach*. Wherever possible, biomimetic surfaces and materials, as well as other biomimetic and biological approaches, should be applied as they tend to be more environmentally friendly. Common engineered surfaces have occasional roughness, which makes friction and wear extremely difficult to overcome. On the other hand, many biological functional surfaces have complex structures with hierarchical roughness that determines their good properties for tribological systems.

*Surface texturing*. This technology should be used to provides a way to control many surface properties relevant to making tribo-systems more ecologically friendly.

*Environmental implications of coatings.* Environmental implications of coatings and other methods of surface modification (texturing, depositions, etc.) should be studied and taken into consideration.

*Design for degradation.* The ultimate degradation and utilization of contact surfaces, coatings, and tribological components should be considered during design.

*Real-time monitoring*. Tribological systems should be analyzed and monitored during operation to prevent the formation of hazardous substances.

**Renewable energy sources** (*Sustainable energy applications).* Sustainable energy applications should be a priority direction for tribological design, as well as engineering design in general.

Correct observation of discussed above principles of green tribology can greatly reduce the environmental impact of tribological process's products, assist economic development and, consequently, improve respectively the quality of life.

#### **3.2 Focus areas of green tribology**

Green tribology includes 3 main areas [1, 2, 5, 14], these are (1) Biomimetics (imitating living nature in order to solve complex human problems) and selflubricating materials/surfaces; (2) Biodegradable and environmentally friendly lubrication and materials; and (3) Renewable and/or sustainable sources of energy. These 3 focus areas of green tribology aim to ensure a limited impact of tribological processes on the environment and human health. Below is a brief description and discussion about the features, contents, aspects of these areas and their relevance to green tribology.

*Biomimetic and self-lubricating materials/surfaces.* This is an important area of green tribology, the main task of which is the development and application of tribological technologies that mimic living nature (biomimetic surfaces). Many biological materials have amazing properties (superhydrophobicity, self-cleaning, self-healing, high adhesion, reversible adhesion, high mechanical strength, antireflection, etc.) that can hardly be achieved by conventional engineering methods. These properties of biological and biomimetic materials are reached due to their composite structure and hierarchical multiscale organization. It is noted that hierarchical organization and the ability of biological systems to grow and adapt also ensure a natural mechanism for the repair or healing of insignificant damage in the material. Biomimetic materials are also usually environmentally friendly in a natural way, since they are a natural part of the ecosystem. For this reason, the biomimetic approach in green tribology is especially promising.

In the field of biomimetic surfaces, a number of typical ideas have been proposed: (1) *The lotus effect based non-adhesive surfaces; (2) The Gecko effect based materials with the ability of specially structured hierarchical surfaces to* 

**51**

*Green Tribology*

*DOI: http://dx.doi.org/10.5772/intechopen.94510*

*as self-cleaning, self-lubrication, and self-healing.*

special micro-textures such as bumps and grooves.

etc., which can be prospectively applied in green tribology field.

which is ensured by the presence of many dissolved biomolecules.

water while reducing their interaction with each other.

only become effective in recent decades.

*exhibit controlled adhesion; (3) Fish-scale effect based micro-structured surfaces for underwater applications, including easy flow due to boundary slip, the suppression of turbulence and anti-biofouling; (4) Oleophobic surfaces capable of repelling organic liquids; (5) Microtextured surfaces for de-icing and anti-icing; (6) Various biomimetic microtextured surfaces to control friction, wear and lubrication; (7) Self-lubricating surfaces, using various principles, including the ability for friction-induced selforganization; (8) Self-repairing surfaces and materials, which are able to heal minor damage (cracks, voids); (9) The "sand fish" lizard effect, able to dive and "swim" in loose sand due to special electromechanical properties of its scale; (10) Nanocomposite materials tailored in such way that they can produce required surface properties, such* 

**Figure 3** shows typical biological and biomimetic surfaces with hair or pillar like surface structures for various functions (**Figure 3**) [29]. Recently, the mechanisms of sand erosion resistance of the desert scorpion were studied to improve the erosion resistance of components in tribo-systems [30]. It was found that the biological surfaces used for sand erosion resistance of the desert scorpion were built by the

In works [31–33], the authors presented overview and studies of various biomimetic microtextured surfaces to control friction, wear and lubrication. Generally, biomimetic techniques have provided the different surface structures with strong adhesion, high hydrophobic properties, high coefficient of friction, self-lubrication,

*Biodegradable and environmentally friendly lubrication and materials.* Advanced biomimetics is biomimicry used to identify best practices from nature on key tribological issues, such as finding improved lubrication solutions [1, 14, 34]. Natural lubrication is very effective at providing low coefficients of friction even at low speeds, and relies entirely on water as the base component, the effectiveness of

Imitating such constructs of molecules, understanding their tribological performance is helpful. An example is the process of imitating natural lubricants, e.g. glycoproteins in synovial fluid [32]. By imitating this mechanism in the laboratory, molecules were synthesized that spontaneously produce polymer brushes on the surface. Brushes are formed on surfaces in an aqueous medium when end-grafted, water-soluble polymers are located at distance about one radius of gyration (Rg) from each other (**Figure 4**) [34] and stretch to maximize their interaction with

The use of lubricants in machine components poses a serious threat to the environment, since they released into the environment not only contain harmful toxic waste but also contain the wear debris from machine parts. Development of environmentally acceptable lubricant products is one of priority direction in green tribology. Vegetable oils and animal fats have been used as lubricants for a very long time throughout human history. However, following the industrial revolution and the advent of lubricants made from mineral oils, bio-based lubricants have again come to be seen as an environmentally alternative for lubricant production and have

Researchers confirmed that properly formulated bio-lubricants are comparable

with mineral based lubricants, so they could be used as an adequate substitution in appropriate cases. Vegetable-oil-based or animal-fat-based lubricants are potentially biodegradable that can be used for engines, hydraulic and metal-cutting applications. Vegetable oils i.e. corn, soybean and coconut oil, can have excellent lubricity, far superior than that of mineral oil [12, 14]. In general, the advantages of using bio-lubricants are non-toxic, biodegradable, renewable resources, good lubricity and high viscosity indices (**Table 2**) [35], while disadvantages are:

#### *Green Tribology DOI: http://dx.doi.org/10.5772/intechopen.94510*

*Tribology in Materials and Manufacturing - Wear, Friction and Lubrication*

and lubricants.

friendly.

studied and taken into consideration.

neering design in general.

green tribology.

**3.2 Focus areas of green tribology**

**Material and surface production and treatment.** *Sustainable chemistry and green engineering principles*. These principles should be observed in the production of new materials, elements, parts, machines for tribological applications, coatings

*Biomimetic approach*. Wherever possible, biomimetic surfaces and materials, as well as other biomimetic and biological approaches, should be applied as they tend to be more environmentally friendly. Common engineered surfaces have occasional roughness, which makes friction and wear extremely difficult to overcome. On the other hand, many biological functional surfaces have complex structures with hierarchical roughness that determines their good properties for tribological systems. *Surface texturing*. This technology should be used to provides a way to control

many surface properties relevant to making tribo-systems more ecologically

during operation to prevent the formation of hazardous substances.

development and, consequently, improve respectively the quality of life.

*Environmental implications of coatings.* Environmental implications of coatings and other methods of surface modification (texturing, depositions, etc.) should be

*Design for degradation.* The ultimate degradation and utilization of contact surfaces, coatings, and tribological components should be considered during design. *Real-time monitoring*. Tribological systems should be analyzed and monitored

**Renewable energy sources** (*Sustainable energy applications).* Sustainable energy applications should be a priority direction for tribological design, as well as engi-

Correct observation of discussed above principles of green tribology can greatly reduce the environmental impact of tribological process's products, assist economic

Green tribology includes 3 main areas [1, 2, 5, 14], these are (1) Biomimetics (imitating living nature in order to solve complex human problems) and selflubricating materials/surfaces; (2) Biodegradable and environmentally friendly lubrication and materials; and (3) Renewable and/or sustainable sources of energy. These 3 focus areas of green tribology aim to ensure a limited impact of tribological processes on the environment and human health. Below is a brief description and discussion about the features, contents, aspects of these areas and their relevance to

*Biomimetic and self-lubricating materials/surfaces.* This is an important area of green tribology, the main task of which is the development and application of tribological technologies that mimic living nature (biomimetic surfaces). Many biological materials have amazing properties (superhydrophobicity, self-cleaning, self-healing, high adhesion, reversible adhesion, high mechanical strength, antireflection, etc.) that can hardly be achieved by conventional engineering methods. These properties of biological and biomimetic materials are reached due to their composite structure and hierarchical multiscale organization. It is noted that hierarchical organization and the ability of biological systems to grow and adapt also ensure a natural mechanism for the repair or healing of insignificant damage in the material. Biomimetic materials are also usually environmentally friendly in a natural way, since they are a natural part of the ecosystem. For this reason, the

biomimetic approach in green tribology is especially promising.

In the field of biomimetic surfaces, a number of typical ideas have been proposed: (1) *The lotus effect based non-adhesive surfaces; (2) The Gecko effect based materials with the ability of specially structured hierarchical surfaces to* 

**50**

*exhibit controlled adhesion; (3) Fish-scale effect based micro-structured surfaces for underwater applications, including easy flow due to boundary slip, the suppression of turbulence and anti-biofouling; (4) Oleophobic surfaces capable of repelling organic liquids; (5) Microtextured surfaces for de-icing and anti-icing; (6) Various biomimetic microtextured surfaces to control friction, wear and lubrication; (7) Self-lubricating surfaces, using various principles, including the ability for friction-induced selforganization; (8) Self-repairing surfaces and materials, which are able to heal minor damage (cracks, voids); (9) The "sand fish" lizard effect, able to dive and "swim" in loose sand due to special electromechanical properties of its scale; (10) Nanocomposite materials tailored in such way that they can produce required surface properties, such as self-cleaning, self-lubrication, and self-healing.*

**Figure 3** shows typical biological and biomimetic surfaces with hair or pillar like surface structures for various functions (**Figure 3**) [29]. Recently, the mechanisms of sand erosion resistance of the desert scorpion were studied to improve the erosion resistance of components in tribo-systems [30]. It was found that the biological surfaces used for sand erosion resistance of the desert scorpion were built by the special micro-textures such as bumps and grooves.

In works [31–33], the authors presented overview and studies of various biomimetic microtextured surfaces to control friction, wear and lubrication. Generally, biomimetic techniques have provided the different surface structures with strong adhesion, high hydrophobic properties, high coefficient of friction, self-lubrication, etc., which can be prospectively applied in green tribology field.

*Biodegradable and environmentally friendly lubrication and materials.* Advanced biomimetics is biomimicry used to identify best practices from nature on key tribological issues, such as finding improved lubrication solutions [1, 14, 34]. Natural lubrication is very effective at providing low coefficients of friction even at low speeds, and relies entirely on water as the base component, the effectiveness of which is ensured by the presence of many dissolved biomolecules.

Imitating such constructs of molecules, understanding their tribological performance is helpful. An example is the process of imitating natural lubricants, e.g. glycoproteins in synovial fluid [32]. By imitating this mechanism in the laboratory, molecules were synthesized that spontaneously produce polymer brushes on the surface. Brushes are formed on surfaces in an aqueous medium when end-grafted, water-soluble polymers are located at distance about one radius of gyration (Rg) from each other (**Figure 4**) [34] and stretch to maximize their interaction with water while reducing their interaction with each other.

The use of lubricants in machine components poses a serious threat to the environment, since they released into the environment not only contain harmful toxic waste but also contain the wear debris from machine parts. Development of environmentally acceptable lubricant products is one of priority direction in green tribology. Vegetable oils and animal fats have been used as lubricants for a very long time throughout human history. However, following the industrial revolution and the advent of lubricants made from mineral oils, bio-based lubricants have again come to be seen as an environmentally alternative for lubricant production and have only become effective in recent decades.

Researchers confirmed that properly formulated bio-lubricants are comparable with mineral based lubricants, so they could be used as an adequate substitution in appropriate cases. Vegetable-oil-based or animal-fat-based lubricants are potentially biodegradable that can be used for engines, hydraulic and metal-cutting applications. Vegetable oils i.e. corn, soybean and coconut oil, can have excellent lubricity, far superior than that of mineral oil [12, 14]. In general, the advantages of using bio-lubricants are non-toxic, biodegradable, renewable resources, good lubricity and high viscosity indices (**Table 2**) [35], while disadvantages are:

#### **Figure 3.**

*Typical biological and biomimetic surfaces with hair or pillar like surface structures for various functions. (a) The nano to micro hierarchical hair-like surface structure of geckos' feet for strong adhesion; (b) The nano to micro hierarchical structure of plant leaves for superhydrophobic dewetting properties. (c) The microfabricated polyimide biomimetic hairs bunching together under the van der Waals interaction [29].*

#### **Figure 4.** *The formation of polymer brushes on surfaces [34].*

oxidative instability, poor low temperature properties, and hydrolytic instability. Applying chemical modification or additives can address these problems of bio-lubricants.

In the area of eco-friendly and biodegradable lubrication and materials we should also notice other following interesting ideas:

*Hyrdo-lubrication*. These are homogeneous lubricants containing water as a functional component. Tribological study and case analyses of the elastomeric bearings lubricated with seawater for marine propeller shaft systems were conducted [36].

*Ionic liquids for green molecular lubrication.* Ionic liquids (ILs) have been explored as lubricants for various device applications due to their excellent electrical conductivity as well as good thermal conductivity, where the latter allows frictional heating dissipation [37].

**53**

*Green Tribology*

**Table 2.**

*DOI: http://dx.doi.org/10.5772/intechopen.94510*

lubricity characteristics are significantly improved [38].

ment, they are environmentally friendly [1, 2, 6].

energy, geothermal energy, and so on.

turbines and wave devices.

*Powder lubrication*. Generally, these tend to be much more eco-friendly than the traditional liquid lubricants. Recent researches show that when using some nanoscale additives, such as boric acid and MoS2 nanopowders to natural oils, their

*The percentage content of CO and CO2 in exhaust gas lubricated with regular mineral and vegetable oils [35].*

**Oil type Engine oil Coconut oil Palm oil** CO2 (%) 4.5 2.9 3.4 CO (%) 0.92 0.67 0.73

*New eco-friendly coating materials for tribological applications*. Recently, special attention has been paid to the development of "green" coatings in tribo-systems, which have improved tribological properties (low friction coefficient, high wear resistance), and therefore, not releasing a lot of worn-out waste into the environ-

*Tribology in the Renewable Energy Sources (RES).* Controlling and minimizing of friction and wear in tribology is important for energy and resources conservation. Sustainable energy applications have become priority of the tribological design, as well as an important area of green tribology. In contrast to the biomimetic approach and environmentally friendly lubrication, RES is not about manufacturing or operation, but about the application of the tribological system in production of renewable eco-friendly energies such as wind energy, marine energy, solar

In work [39] Wood et al. carried out the tribological studies on renewable sources of energy, namely three green energy systems: wind, tidal and wave machines. The authors also highlighted the role of design and durability for such large scale engineering systems from sustainability point of view. These systems are sensitive to operation and maintenance costs and thus depend on functioning tribological parts and lubrication. It was noted that weight reduction to reduce tribological and gravity loads would be beneficial for machines designs. Attention should also be paid to the knowing of dynamic loads to predict fatigue life and tribological loads on wind, tidal and wave machines. Structures and properties of tribological components must be considered for the inherent lack of stiffness of the

Wind turbines have fairly many specific problems related to their tribology, which involve water contamination, electric arcing on generator bearings, wear of the main shaft and gearbox bearings and gears, the erosion of blades due to solid particles, cavitation, rain, hail stones, etc. The most commonly observed and discussed tribological problems in wind turbines are in the transmission system, in the gearbox. They are mainly the result of insufficient lubrication and/or lack of regular maintenance under extreme operating conditions. The solution to this problem is the use of lubricants and/or materials with improved tribological characteristics [40]. REWITEC nano-coatings is a metal treatment that can be applied to gearboxes and bearings during regular operation for restoration of its efficiency and economy. When examining certain micro-pitting areas on the metal surfaces of a wind turbine gear before applying REWITEC and after 6 months of treatment, it was found that the surface damage was filled and the asperities were smoothed out, and thus the surfaces became smoother with higher surface contact area (**Figure 5**) [2].

Tidal power turbines are another important way of producing renewable energy. Besides tidal, the ocean water flow and wave energy and river flow energy (without dams) can be used with the application of special turbines, which provides the same


**Table 2.**

*Tribology in Materials and Manufacturing - Wear, Friction and Lubrication*

**52**

**Figure 4.**

**Figure 3.**

bio-lubricants.

ducted [36].

dissipation [37].

*The formation of polymer brushes on surfaces [34].*

should also notice other following interesting ideas:

oxidative instability, poor low temperature properties, and hydrolytic instability. Applying chemical modification or additives can address these problems of

*Typical biological and biomimetic surfaces with hair or pillar like surface structures for various functions. (a) The nano to micro hierarchical hair-like surface structure of geckos' feet for strong adhesion; (b) The nano to micro hierarchical structure of plant leaves for superhydrophobic dewetting properties. (c) The microfabricated polyimide biomimetic hairs bunching together under the van der Waals interaction [29].*

In the area of eco-friendly and biodegradable lubrication and materials we

*Hyrdo-lubrication*. These are homogeneous lubricants containing water as a functional component. Tribological study and case analyses of the elastomeric bearings lubricated with seawater for marine propeller shaft systems were con-

*Ionic liquids for green molecular lubrication.* Ionic liquids (ILs) have been explored as lubricants for various device applications due to their excellent electrical conductivity as well as good thermal conductivity, where the latter allows frictional heating *The percentage content of CO and CO2 in exhaust gas lubricated with regular mineral and vegetable oils [35].*

*Powder lubrication*. Generally, these tend to be much more eco-friendly than the traditional liquid lubricants. Recent researches show that when using some nanoscale additives, such as boric acid and MoS2 nanopowders to natural oils, their lubricity characteristics are significantly improved [38].

*New eco-friendly coating materials for tribological applications*. Recently, special attention has been paid to the development of "green" coatings in tribo-systems, which have improved tribological properties (low friction coefficient, high wear resistance), and therefore, not releasing a lot of worn-out waste into the environment, they are environmentally friendly [1, 2, 6].

*Tribology in the Renewable Energy Sources (RES).* Controlling and minimizing of friction and wear in tribology is important for energy and resources conservation. Sustainable energy applications have become priority of the tribological design, as well as an important area of green tribology. In contrast to the biomimetic approach and environmentally friendly lubrication, RES is not about manufacturing or operation, but about the application of the tribological system in production of renewable eco-friendly energies such as wind energy, marine energy, solar energy, geothermal energy, and so on.

In work [39] Wood et al. carried out the tribological studies on renewable sources of energy, namely three green energy systems: wind, tidal and wave machines. The authors also highlighted the role of design and durability for such large scale engineering systems from sustainability point of view. These systems are sensitive to operation and maintenance costs and thus depend on functioning tribological parts and lubrication. It was noted that weight reduction to reduce tribological and gravity loads would be beneficial for machines designs. Attention should also be paid to the knowing of dynamic loads to predict fatigue life and tribological loads on wind, tidal and wave machines. Structures and properties of tribological components must be considered for the inherent lack of stiffness of the turbines and wave devices.

Wind turbines have fairly many specific problems related to their tribology, which involve water contamination, electric arcing on generator bearings, wear of the main shaft and gearbox bearings and gears, the erosion of blades due to solid particles, cavitation, rain, hail stones, etc. The most commonly observed and discussed tribological problems in wind turbines are in the transmission system, in the gearbox. They are mainly the result of insufficient lubrication and/or lack of regular maintenance under extreme operating conditions. The solution to this problem is the use of lubricants and/or materials with improved tribological characteristics [40]. REWITEC nano-coatings is a metal treatment that can be applied to gearboxes and bearings during regular operation for restoration of its efficiency and economy. When examining certain micro-pitting areas on the metal surfaces of a wind turbine gear before applying REWITEC and after 6 months of treatment, it was found that the surface damage was filled and the asperities were smoothed out, and thus the surfaces became smoother with higher surface contact area (**Figure 5**) [2].

Tidal power turbines are another important way of producing renewable energy. Besides tidal, the ocean water flow and wave energy and river flow energy (without dams) can be used with the application of special turbines, which provides the same

**Figure 5.** *3D-images of the metal surface before and after treatment with REWITEC 6 months [2].*

direction of rotation independent of the direction of the current flow. Production processes of tidal, water flow and wave energy involve certain specific tribological problems such as lubrication of machine components (by seawater, oils, and greases), their erosion, corrosion, and biofouling, as well as the interaction between these modes of damage [1, 39].

Geothermal energy plants are widely used now, however, their application is limited to the geographical areas at the edges of tectonic plates. There are several specific tribological issues related to the geothermal energy sources which are discussed in the literature [1, 5, 15, 39].
