**5. Conclusion**

Green tribology is a novel area of science and technology. It is related to other areas of tribology as well as other "green" disciplines, namely, green engineering and green chemistry. In this chapter the main scientific and technological aspects of green tribology such as the concept, role and goal, principles, focus areas, challenges, "zero-wear" effect were considered in details.

The concept, role and goal of green tribology were clarified. 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. The goal and essence 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 of human life.

The twelve principles and three areas of green tribology were analyzed. Observation of these principles can greatly reduce the environmental impact of tribological processes, assist economic development and, consequently, improve the quality of life. The integration of these areas remains the major challenge of green tribology and defines the future directions of research in this field.

Within the framework of this work, one of the most important tribological effects, which is exclusively the basis for green tribology - the "zero-wear" effect in friction (selective transfer effect) was discussed.

As a result, this work allows us to conclude that green tribology is an environmentally friendly and energy-saving concept and, moreover, there are many opportunities for its inclusion in a sustainable society on a global scale. Furthermore, there is a need for tribologists to collaborate towards the development and application of Green tribology.
