• We discovered friction-generated micro-scale plasma or simply micro-plasma and achieved a world-first success in total micro-plasma imaging. (Fig.1)
• The discovery will open a new page in the science and technology of friction (tribology).
• A micro-plasma is generated even under such low-friction conditions as writing on a pad or rubbing a finger over a surface and may throw light on unexplained phenomena around us such as the causal mechanisms of frictional luminescence and seemingly spontaneous ignition.
• Micro-plasma technology may also hold the key to the development of next-generation ultra-compact precision machines.

Fig.1 Planar Image of Comet-Shaped Plasma

The discovery of micro-scale plasma or micro-plasma was made at the National Institute of Advanced Industrial Science and Technology (AIST; President: Dr. Hiroyuki Yoshikawa), by its Nanotechnology Research Institute's (Director: Dr. Hiroshi Yokoyama) Research Team of Mesotechnology (Leader: Dr. Keiji Nakayama), which also achieved a world-first success in taking a total image of the micro-plasma. The discovery may open up new avenues in our understanding of mysterious and hitherto unexplained phenomena around us and lead the way to finding solutions to the intractable problems of friction and lubrication that have defied industry. It may also mark the beginning of a new chapter in the science and technology of friction (tribology).

• Dr. Nakayama, the Research Team Leader at AIST, has proposed a new theory to account for the occurrence of a micro-plasma. Until the present, it has not been possible to explain the many intractable problems of tribology solely on the basis of frictional heat. The discovery of micro-plasma may well revolutionize our understanding of tribology. The reality of the micro-plasma phenomenon has been substantiated without the shadow of a doubt.
• Based on the discovery, a completely novel concept has been proposed to resolve the difficult problems of tribology. The micro-plasma phenomenon marks a new turn in the history of tribology.
• The micro-plasma was found to occur even at a low frictional speed of only 2cm/s. These speeds correspond to the rate at which we move a finger over or scratch a surface. The importance of the discovery therefore lies in the fact that we are constantly surrounded by a micro-plasma in our ordinary life situations without being aware of it. The occurrence of a micro-plasma might even explain the seemingly spontaneous ignition of a flammable fluid (gas) due to static electricity as may happen when we open the cap of our car's gasoline tank. No doubt, the micro-plasma will be a major theme for future research.
• The occurrence of a micro-plasma was also observed at extremely light load conditions of as low as 3g which is roughly the load applied to the head of a computer's hard disk. In view of these findings, the micro-plasma will be a significant factor in the technical development of the next-generation ultra-compact precision machines in which friction plays an important role such as copying and micro-machines and magnetic recording systems.
• The micro-plasma phenomenon was discovered in the course of friction experiments carried out as part of the Ministry of Education, Culture, Sports, Science and Technology's Millennium Project for the "Development of Large-Capacity High-Reliability Magnetic Recording Systems" (Head of Research: Dr. Keiji Nakayama; Project Period: 2000-2002). Their purpose was to investigate the friction and lubrication occurring between the head of the computer hard disk drive and the magnetic disk.

Details of the micro-plasma discovery are presented in the following paper, which was published on June 21, 2002.

"Plasma generation in a gap of sliding contact," Journal of Physics D. Applied Physics, K. Nakayama and R.A. Nevshupa, 35 (2002) L53-L56.

Since the discovery of the light flash generated on a friction surface and the measurement of the flash temperature at University of Cambridge, U.K., in 1947, practically all friction phenomena have been interpreted and explained in terms of the temperature rise associated with friction. There are many mysterious phenomena, however, that can not be interpreted in terms of the frictional temperature rise alone. It seemed that some other unknown high-energy state quite separate from the temperature rise occurred at or near the frictional contact area.

In an attempt to shed light on these questions, AIST has developed a measuring system capable of detecting the emission of energetic particles such as electrons, ions and photons (light) from the friction surface. The study of the energetic particle emission phenomena has led the AIST to conclude that "a micro-plasma is generated in the vicinity of the sliding contact gap." To explain the mechanism by which the micro-plasma is generated, the electric gas discharge micro-plasma model shown in Fig.2 was proposed (in 1995).

Fig.2 Electric Discharge Micro-Plasma Model

Fig.3 shows an overall view and the principle of the measuring system with which we succeeded in observing the micro-plasma image quantitatively for the first time in the world. The figure makes clear the principle on micro-plasma generation. With this measuring system it was possible to measure from the rear through the rotating disk the planar two-dimensional distribution image (planar plasma image) of the photons (light) emitted from the micro-plasma. The small image of the weak light emitted from the plasma was magnified using an optical microscope and focused on a high-sensitivity CCD camera for computer processing to display the plasma image.

		Fig.3 Micro-Plasma Image Measuring Deviceb

Fig.1 gives a planar view of the micro-plasma measured from the rear through the disk. The plasma is generated in the frictional contact gap between a hemispherical diamond pin with a tip radius of 300μm and a sapphire disk with a thickness of 1mm. The plasma propagating at the rear of the sliding contact has a comet-like appearance. Its length to the rear of the contact gap is over 100μm in major axis and its tail length is also over 100μm. It was found that a strong light was emitted on the outer side of the contact point. This means that the plasma is generated outside the contact . This discovery upturns the conventional theory that light is emitted from the contact as a result of frictional heat.

Fig.4 shows the plasma image as seen in UV light through an optical filter passing only UV light. It can be seen that the plasma has a horseshoe-like shape. Spectral analysis of the UV light has validated our electric discharge micro-plasma model.

Fig. 4UV Image of Micro-Plasma

Fig.5 is a side view of the plasma measured from the sides of the contact point, with the optical axis of the microscope being the horizontal. Because the optical axis of the microscope was slightly tilted from the horizontal, the reflection image of the plasma from the disk surface (image below the horizontal line) also appears in the image. It can be seen very clearly, however, that the plasma is generated in the frictional contact gap.

Fig.5 Lateral Plasma Image

We were successful in obtaining a sequence of moving pictures of the plasma. This makes it possible to understand how the shape and the distribution of the plasma changes with time. As stated above, this plasma was found to occur as a low speed of only 2cm/s and a light load of only 3g. It has also been established that a plasma occurs as a results of friction with virtually any material, including insulating materials, semiconductors and metal oxide films. These observations substantiate the view that "micro-plasma is always present when friction occurs," a fact that applies to everyday life situations as well as the many activities and processes of industry