Green Tribology

In this chapter the concept of green tribology and its relation to other areas of tribology is discussed as well as other “green” disciplines, namely, green engineering and green chemistry. The twelve principles of green tribology are formulated: the minimization of (1) friction and (2) wear, (3) the reduction or complete elimination of lubrication, including self-lubrication, (4) natural and (5) biodegradable lubrication, (6) using sustainable chemistry and engineering principles, (7) biomimetic approaches, (8) surface texturing, (9) environmental implications of coatings, (10) real-time monitoring, (11) design for degradation, and (12) sustainable energy applications. Three areas of green tribology are further defined: (1) biomimetics for tribological applications, (2) environment-friendly lubrication, and (3) the tribology of renewable energy application. The integration of these areas remains a primary challenge for this novel area of research. The challenges of green tribology and future directions of research are also discussed.

The Lotus and rose petal effects have become a subject of active investigation by scientists, as they involve different modes of the interaction of wetting with roughness. The contact angle (CA) and CA hysteresis are two parameters, which characterize the hydrophobicity/philicity of a solid surface. Lotus-effect surfaces have a high CA and low CA hysteresis. However, it was found recently that a high CA can coexist with strong adhesion between water and a solid surface (and high CA hysteresis) in the case of the so-called “rose petal effect.” It is clear now that wetting cannot be characterized by only the CA, since several modes or regimes of wetting of a rough surface can exist, including the Wenzel, Cassie, Lotus, and Petal regimes. This is due to the hierarchical structure of rough surfaces built of micro- and nanoscale roughness, so that a composite interface can exist at the microscale, while a homogeneous interface can exist at the nanoscale or vice versa. The understanding of the wetting of rough surfaces is important in order to design non-adhesive surfaces for various applications, including environmental.

Despite the fact that self-organization during friction has received relatively little attention of the tribologists so far, it has a potential for the creation of self-healing and self-lubricating materials, which are of importance for the green or environment-friendly tribology. The principles of the thermodynamics of irreversible processes and of the nonlinear theory of dynamical systems are used to investigate the formation of spatial and temporal structures during friction. The transition to the self-organized state with low friction and wear occurs through the destabilization of the steady-state (stationary) sliding. The criterion for the destabilization is discussed and examples like formation of a protective film and slip waves are discussed. Some cases like running-in stage, elastic structures, and Turing pattern formation as evidences of self-organization are studied. A special self-healing mechanism may be embedded into material by coupling corresponding required forces. The analysis provides a structure–property relationship which can be applied for the design optimization of composite self-lubricating and self-healing materials for various ecologically friendly applications and green tribology.

In marine environments or industrial water systems, microorganisms are likely to adhere onto surfaces and form biofilms. Such biofouling creates significant adverse effects, e.g., increases flow friction by roughening surfaces. Previous studies demonstrated the effectiveness of surface microstructures on the prevention of biofouling, which is also closely associated with the surface energy and wettability. Unfortunately, the study of the anti-biofouling property of the micro- and nanostructured surfaces with regulated surface wettability is underperformed at present. In this paper, we report on the bio-adhesions of various cell types on nanoengineered surfaces with dense-array nanostructures whose physical and chemical properties are systematically controlled for the prevention of biofouling. Two nanopatterns (pillar and grating) with varying three-dimensionalities (e.g., structural heights are varied from 50 to 500 nm while the pattern periodicity is fixed at 230 nm) are tested in both hydrophilic and hydrophobic surface conditions. The structural tips are especially sharpened (<10 nm in tip radius) to minimize the cell contact to the substrate and potentially biofouling. The experimental results show that cells were much smaller and their proliferation significantly lower on taller nanostructures in both hydrophilic and hydrophobic surface conditions. Cells were found levitated by sharp tips and easily peeled off, i.e., their adherence to the sharp-tip tall nanostructures was relatively weak regardless of the surface wettability. The ability to control adherence and growth of cells by nanoscale surface topographies can empower the micro- and nanotechnology-based materials, devices, and systems for anti-biofouling and anti-microbial applications. The knowledge obtained through this investigation will also be useful in engineering problems that involve contact with biological substances and in the development of energy efficient surfaces for green tribology.

This chapter deals with green and sustainable nanotribology. It highlights the challenges, development and opportunities of these new, emerging fields of science and embeds them in the major frame of the most serious problems we currently face on our planet. Fifteen global challenges are annually identified by the Millennium Project, a major undertaking that was started in 1996 and that incorporates organizations of the United Nations, governments, corporations, non-governmental organizations, universities and individuals from more than 50 countries from around the world.

Biomimetic hierarchical surfaces demonstrate a potential for a variety of green technologies, including energy conversion and conservation, due to their remarkable water repellence. The design of such surfaces allowing emerging green applications remains a challenging scientific and technological task. Understanding the physical mechanism of wetting transitions is crucial for design of highly stable superhydrophobic materials. The main experimental and theoretical approaches to wetting transitions are reviewed. General scaling arguments shedding light on the complicated physics of wetting transitions are supplied. Reducing the micro-structural scales is the most efficient measure needed to enlarge the threshold pressure of a wetting transition. The trends of future investigations are envisaged.

The Lotus effect involving roughness-induced superhydrophobicity is a way to design biomimetic non-wetting, non-sticky, self-cleaning, omniphobic, icephobic, and anti-fouling surfaces, which can be applied for various purposes related to green tribology. However, such surfaces require micropatterning, which is extremely vulnerable to even small wear rates. This limits the applicability of the Lotus effects to situations, when wear is practically non-present. To design sustainable superhydrophobic surfaces, we suggest using metal matrix composites (MMC) with hydrophobic reinforcement in the bulk of the material, rather than at its surface. Such surfaces provide roughness and heterogeneity needed for superhydrophobicity. In addition, they are sustainable since when surface layer is deteriorated and removed due to wear, hydrophobic reinforcement and roughness remains. We present a model and experimental data on wetting of MMCs. We also conduct experiments with graphite-reinforced MMCs and show that the contact angle can be determined from the model. In order to decouple the effects of reinforcement and roughness, the experiments were conducted for initially smooth and etched matrix and composite materials. Micropatterned surfaces can be used for underwater oleophobicity and self-cleaning, in a manner, similar to the Lotus effect. However, wetting of a rough surface by oil (or any non-polar organic liquid) can follow more complex scenarios than just wetting of a rough surface by water, since a four-phase solid–oil–water–air interface can be involved.

Adhesive properties of polymeric materials and modern techniques of surface modification make polymers appropriate for Green Tribology applications, which require functional surfaces and the ability to control, and modify and surface properties, such as adhesion and wetting. Polymers, along with polymer composites, are appropriate materials for coating and various biomimetic applications, such as those utilizing the Lotus and gecko effects. In this chapter, we review polymer properties relevant to adhesion and wetting, modern methods and techniques of surface modification which are used to synthesize and produce superhydrophobic biomimetic materials as well as the methods of surface characterization.

This chapter addresses ice friction from a biomimicry perspective. Ice and its liquid form water are integral parts of the natural life cycle and therewith stand at the center of our existence, which makes them interesting targets for biomimetic engineering. A historic overview of friction and ice friction introduces the matter. The relevant tribological background is established and ice as a matter is discussed. On this basis the different parameters that influence ice friction are discussed. Biomimetic approaches to adapt the influence of material-related parameters to the desired amount are outlined. Furthermore, experimental methods to measure ice friction are addressed and finally the most important ice friction models are briefly introduced.

The depletion of the world’s crude oil reserve, increased oil prices, and the demand to protect the environment against pollution exerted by hydraulic and gear oils have brought about renewed interest in the development and use of green lubricants. In this light, many automotive and manufacturing industries are actively seeking out new green lubricants. Although, many green lubricants have exhibited excellent properties, more improvement in their friction and wear performance are still needed for them to become mainstream. Consequently, environmental friendly additive materials are being included in green lubricant formulations to improve their friction and wear properties. In this study, a review of the tribological behavior of the green lubricants presently available has been performed. Overall, the review indicates that green lubricants can significantly outperform conventional lubricants with respect to frictional and wear performance. In addition, the review shows that the size and composition of the lubricant additives play an important role in determining a green lubricant’s overall performance.

Lubricants and lubrication have been inherent in a machine ever since man invented machines. It was water and natural esters like vegetable oils and animal fats that were used during the early era of machines. During the late 1800s, the development of the petrochemical industry put aside the application of natural lubricants for reasons including its stability and economics. The growing awareness of the lower biodegradability and higher toxicity of petrochemical-based lubricants created the requirements of the best possible protection of nature. The recent research on the adverse effects of mineral oil-based lubricants on the environment has reconfirmed its role in polluting groundwater for up to 100 years and its effects on reducing the growth of trees and the life span of aquatic life [1]. This awareness, of the use of ecofriendly processes and materials, increases interest in Tribology for the use of natural esters in lubrication processes [2]. The development of the retro parade attitude in the lubricant industry and its customers with more environmental awareness, keen to prefer products which do not diminish the world’s finite resource of mineral hydrocarbons and which have a minimal adverse effect on the environment, created an opportunity to use naturally available ecofriendly lubricants [3]. The potential candidates for ecofriendly lubricants include vegetable oils, animal fats and synthetic esters. Although animal fats are also considered biodegradable the most common mineral oil substitutes consist of vegetable oils and synthetic esters [4]. The economical concerns and price stability edge the potential use of vegetable oils as lubricants over synthetic esters. With the notion that we live on a planet with finite resources, we have to think about the coming generations and work for sustainable development in the field of Tribology. This chapter has key concepts like the advantageous and inherent limitations of vegetable oils over mineral oils, possible application of vegetable oil in the field of Tribology, composition and structure of vegetable oils and use of different vegetable oils as bioderived lubricants with their properties and functions.

The gradual development of asbestos in automotive friction materials in many parts of the world has sparked the onset of extensive research and development into safer alternatives. The development of green friction products for automotive application is important to minimize the environmental impacts caused by asbestos-based products. Natural fibers have been used to reinforce materials for over thousand years. More recently, they have been employed in combination with plastics. Natural fibers are environmentally friendly, fully biodegradable, abundantly available, renewable and cheap and have low density. Natural fiber reinforced polymer composites have emerged as a potential environmentally friendly and cost-effective option to synthetic fiber reinforced composites. The availability of natural fibers and ease of manufacturing have tempted researchers to study their feasibility of reinforcement and to what extent they satisfy the required specifications for good reinforced polymer composite for tribological applications. In this study, a review on the tribological behavior of natural fiber reinforced composites was made to understand their usability for various automotive applications.

The increasing ecological awareness and stringent requirements for environmental protection have led to the development of water lubricated bearings in many applications where oil was used as the lubricant. The chapter details the theoretical analysis to determine both the static and dynamic characteristics, including the stability (using both the linearised perturbation method and the nonlinear transient analysis) of multiple axial groove water lubricated bearings. Experimental measurements and computational fluid dynamics (CFD) simulations by the Tribology research group at Queensland University of Technology, Australia and Manipal Institute of Technology, India, have highlighted a significant gap in the understanding of the flow phenomena and pressure conditions within the lubricating fluid.

An attempt has been made to present a CFD approach to model fluid flow in the bearing with three equi-spaced axial grooves and supplied with water from one end of the bearing. Details of the experimental method used to measure the film pressure in the bearing are outlined. The lubricant is subjected to a velocity induced flow (as the shaft rotates) and a pressure-induced flow (as the water is forced from one end of the bearing to the other). Results are presented for the circumferential and axial pressure distribution in the bearing clearance for different loads, speeds and supply pressures. The axial pressure profile along the axial groove located in the loaded part of the bearing is measured. The theoretical analysis shows that smaller the groove angle better will be the load-carrying capacity and stability of these bearings. Results are compared with experimentally measured pressure distributions.

General characteristics of waxes, adhesives and lubricants as well as the recent fundamental investigations on their physical and mechanical behavior are introduced. The current R & D status for new type/generation of waxes, adhesives, and lubricants from natural products is reviewed, with an emphasis on their tribological applications. In particular, some crucial issues and challenges on the technological improvement and the materials development are discussed. Based on the current predicted shortage of energy resources and environmental concerns, prospective research on the development of green waxes, adhesives, and lubricants is suggested.

Due to growing global demand for cement, the production of cement worldwide has significantly increased in the past 15 years, and this trend is the most significant factor affecting the technological and manufacturing advancements in the cement industry. While the increase in demand for cement reflects the growth of national economies, the production of cement clinker is ecologically harmful because it consumes considerable energy and natural resources, and it emits many pollutants into the atmosphere. Therefore, new ways to produce high volumes of cement clinker with less energy and less impact on the environment is greatly needed. One such approach is the production of tribo-chemically activated, high-volume mineral additive (HVMA) cement, which helps to improve the ecological compatibility of cements. This “green” technology is based on the intergrinding of portland cement clinker, gypsum, mineral additives, and a special complex admixture. Tribo-chemical activation increases the compressive strength of ordinary portland cements, improves the durability of cement-based materials, can be processed with a high volume of inexpensive indigenous mineral additives or industrial by-products, and which reduces energy consumption per unit of the cement produced. Additional ecological advantages for green HVMA cements include higher strength, better durability, less pollution at the clinker production stage, and fewer industrial by-products placed in landfills.

Growing concern for the environment coupled with the increasing cost of petro-based resources and advancements in the fields of biotechnology, nanotechnology and materials science, and engineering has led to the development of green materials for various applications. Fly ash is a particulate waste by-product formed as a result of coal combustion in power plants. Worldwide, more than 65% of fly ash produced from coal power stations is disposed off in landfills and ash ponds. The recycling of fly ash has become an increasing concern in recent years due to increasing landfill costs and the current interest in sustainable development. The use of fly ash as a filler or reinforcement for composites is desirable from an environmental standpoint. Recently, fly ash was successfully used in metal matrix composites to reduce overall weight and these composites are successfully used in automotive and aerospace applications. The polymer matrix composites developed using fly ash can be used as low cost green friction materials. This study is a review on the tribological behavior of fly ash-based green friction composites to understand their usability for various automotive applications.

Lubricants are extensively used between the contacting surfaces to reduce friction and wear. These lubricants are usually toxic and not readily biodegradable and thus these lubricants can cause considerable damage to the environment. The use of external lubricants can be eliminated by designing self-lubricating composite materials. These composite materials have the ability to achieve low friction and wear at the contact surfaces without any external supply of lubrication during sliding. The metal matrix composites reinforced with various self-lubricating particles such as graphite, molybdenum disulfide are being used as self-lubricating materials for various engineering applications. In this paper, the tribological behavior of metal matrix composites reinforced with graphite particles has been reviewed. More specifically, aluminum-graphite, magnesium-graphite, copper-graphite, silver-graphite and nickel-graphite composites have been studied. The influence of various variables on the friction coefficient and wear rate is discussed. It was found that the amount of graphite released on the wear surface forms a tribo film on the contact surfaces. This reduces the overall friction coefficient and wear rate. The formation and retention of this tribo layer on the sliding surface as well as its composition, area fraction, thickness and hardness are important factors in controlling the friction and wear behavior of the material and depend on the nature of the sliding surface, the test condition, environment and the graphite content in the composite. The presence of graphite tribo layer also increases the seizure resistance and enables to run under boundary lubrication without galling.

Wind power is of increasing interest in society due to its prospects as an environmentally friendly source of renewable energy. The use of wind turbines to extract electrical energy from wind can be dated back to the late-1800s, with the 12 kW windmill generator by Charles Brush, as well as the mid-1900s, with the 1250 kW Smith-Putnam wind turbine. Developments in the wind industry were encouraged by the oil crisis in 1973.

The lack of fresh clean water is an economical and ecological problem which affects half of humanity. More than 97.5% of all water on the Earth is seawater, so the ability to harvest even a small fraction as fresh water would have a huge impact on water scarcity. Reverse osmosis (RO) is currently the main technique of seawater desalination. During RO, salt water under pressure exceeding the fluids osmotic pressure is forced through a semipermeable membrane. RO requires significant energy inputs and affects the environment due the greenhouse gas emissions (usually associated with an external power source), the output of brine with high salt concentration, and other negative effects. Improving the efficiency and environmental impact of RO plants involves several challenges, some of which are related to surface science and tribology. This involves mimicking water filtration by cell membranes, as well as creating biomimetic antifouling coatings on membranes. We present a comprehensive review of RO and other desalination techniques and suggest how a composite material can improve permeability and antifouling properties of RO membranes.

As a developing country, South Africa is in a unique position with regard to establishing, maintaining and expanding infrastructure to ensure compliance with international trends with respect to environmental regulations, while at the same time establishing the means to provide access to affordable energy to all its citizens to share the potential of its resources. In many respects, tribology plays an important role in saving of energy as well as ensuring that requirements with regard to protecting the environment are complied with. Green tribology can rightly be regarded as an approach which is timely and which has an impact on many activities like electricity generation, production of synthetic fuels and lubricants, mining operations and protection of the environment and its resources. Focusing on the interface between Tribology and Biorefining, several interesting possibilities open up. With the constant rise in the price of oil, alternatives to crude oil as primary energy source and as basic feedstock for fuels and chemicals are becoming more and more attainable. In this chapter an overview is provided of the above, from a South African perspective. A number of case-study examples are given which indicate that a “green” approach in finding engineering solutions to tribological problems which could have a far-reaching impact on the environment. Three examples are used, namely how proper selection of tailor-made lubricants could decrease energy usage in gear-driven systems. The focus here is on the power industry, where coal-based power plants are the only economically feasible solution to the increasing demand for electricity in a developing economy with virtually no crude oil reserves. The success atttained in this endeavour should stimulate similar projects in the mining sector of the country. In the second instance, ingenious application of tribology with respect to application of specialised lubricants from a renewable source, namely plant oils, can decrease cost of lubrication and, in addition, can resolve difficult issues with regard to disposal of contaminated waste in metal cutting operations, indicating the value of a “green tribology approach”. Thirdly, combining the concept of biorefineries, tribology and the ability to synthesise products to suit specific requirements, including formulation of lubricants and fuels, can lead to substantially improved products, impacting in a positive way on the environment.

Creating new sources of renewable and sustainable energy is a critical problem in the world today and will continue to be so in the future. A major sector of renewable energy is the hydro energy, and in particular, harvesting wave energy of seas and lakes. Various types of designs have been suggested for marine energy sources. These new applications bring new problems for mechanical engineers and tribologists. Operating in marine conditions is difficult due to the biofouling, water contamination, particulate contamination, and other tribological issues. Efficiency and optimization are major issues in the energy generation industry are. In this chapter, we review offshore marine and lake wave energy systems and discuss in more detail wave energy collector (WEC) that is being developed and optimized at the University of Wisconsin-Milwaukee (UWM).