In tribology, friction control is the most important issue. Surface texturing plays an important role in friction control. In recent years, friction reduction technology, which aims to reduce energy loss in mechanical systems for solving global environmental problems, has received much attention. On the other hand, control of constant high frictional forces such as power transmission mechanisms and brakes is also an important technical issue. Looking to the future, technology for controlling friction at the interface between living organisms and artifacts will be indispensable for realizing a collaborative society in which humans and robots coexist. This article reviews the role and the key findings of surface texture for friction control, and outlines the possibility and future trends.
This article introduces a method combining surface texturing and hard diamond-like carbon coating, as a candidate of wear resistant surface treatment against hard dust like sands. Visualization of dusts reveals behavior of dusts during sliding test. Dusts are crushed to small particles due to large normal force and frictional force. Particles are trapped into grooves of texture. Wear resistance of textured stainless steel substrate is approximately 19% higher than that of plane stainless. Moreover, 200 nm thick diamond-like carbon was prepared on textured stainless steel substrate using filtered cathodic vacuum arc method. Nanoindentation hardness of diamond-like carbon is approximately 68 GPa. Specific wear ratio of diamond-like carbon coated textured stainless steel substrate shows approximately twentyfold smaller than that of plane stainless steel. It is important to design appropriate curvature radius, width and depth of each groove, when we aim to realize wear resistant surface against hard dusts.
Surface texturing has been used extensively to improve the tribological properties of machine elements. There are so many reports on surface texturing not only from the basic scientific view but also from practical applications. In this article, we will present an overview of research on surface textures from the perspective of nano-tribology. We treated atomic or molecular arrangement, modulation of surface chemical, electronic and phononic properties as a broad-sense of textures, the effect of atomic/molecular arrangements and of surface chemical property on their frictional property will introduce on the basis of our experimental results and previous reports. Reducing the size of the materials or textures should lead to show the fascinating properties differing from macro-scale. This would provide a stimulus for technological development leading to novel approach for controlling the friction.
An applicability of interrupted micro cutting process as surface texturing is reviewed. The surface texturing is fabricated using developed tools having asymmetrically aligned cutting edges with a conventional CNC machining center. The resulted surface morphology consists of precisely aligned micro sized dimples. It is also found that various surface textures including dimple shape and size are possible to fabricate to actual mechanical components such as a piston skirt surface in a short time. Combined effects of the proposed micro cutting process and the surface modification technique to penetrate solid lubricant into dimples using a shot peening and a roller burnishing are also described and a possibility of the texture of the material structure is discussed.
Picosecond and femtosecond lasers have grown up as a powerful and reliable means to make micro-drilling, precise shearing and micro-texturing. In particular, the laser microtexturing has been widely utilized to form the regularly aligned micro-dimples even in the inner surface of cylinder block liner as well as the piston skirt for significant reduction of friction and wear. This technology was also available to improve the machining tool life by trapping the machined debris into micro-pockets. In addition, the laser micro-/nano-texturing worked to control the surface properties; e.g., the hydrophilic stainless steel surface was changed to be super-hydrophobic. Furthermore, with aid of the low temperature plasma nitriding, the micro-/nano-textured tool and part life could be prolonged in practical applications.
There are biological textures which have unique functions to survive, such as a microtexture of the lotus leaf for superhydrophobic surfaces, a huge number of submicron-size spatula-shaped structures on gecko's foots for climbing vertical smooth surfaces and moth eye structures for anti-reflective. These biological textures basically require hydrophobicity and dryness to have their superior performances. Nepenthes have pitchers to capture insects. For dropping insects into the pitchers, rims of the pitchers (peristomes) have directional low friction surfaces which require hydrophilicity and wetness. A microtexture on the peristomes would assist to drop insects into the pitchers. In addition, the hydrophilic microtexture on the peristomes generate continuous directional water transport. In this article, the special functions and applications of the Nepenthes will be discussed.
This article introduces applications of textured or micro-structured surfaces in various fields. Typical example is frictional properties of the floors because a decrease in dynamic friction from static friction causes falling accidents. The decrease in friction due to hydrodynamic pressure can be avoided by textures. Such friction change in outdoor becomes complicated because sand particles work as rolling element and decrease friction in addition to hydrodynamic pressure. Next, the effect of bridging or sandwiched fluid between two surfaces is introduced. Depending on the wettability, the fluid generates attractive or repulsive force between the surfaces without any relative motion. Surface wettability can also be changed with surface modification as well as texture, and some examples are introduced. Finally, other applications are introduced referring biomimetics, because various functionalities are realized in nature. Typical example is self-cleaning surface referring the surface of lotus or nepenthes leaves. Many of these will have impact on the future application though they are still under development.