Rubbing extremely damages shallow regions of friction surfaces and sometimes leads to transformation. Unidirectional, discontinuous dry rubbing, however, often damages relatively deep regions of friction surfaces. Although this mode of rubbing had been considered as a kind of usual repeated rubbing, the observed state and/or the structure of the damaged region by the rubbing mentioned above is not quietly the same as that of usual repeated rubbing. We may, therefore, expect to fabricate new materials using this mode of rubbing.
The hardness of the damaged region mentioned above is much higher than that of the same metals or alloys hardened by usual repeated rubbing, e.g., the highest and average hardness of pure iron after 25,000 traversals is 1431 Hv and 1146 Hv [1] and that of 18-8 stainless steel is 1332 Hv and 708Hv[2], respectively. These materials are highly wear-resistant. By covering metals and alloys with thick layer of these materials, we may put them to practical use.
We can also make fundamental studies of the extremely damaged state of metals, alloys and ceramics by the rubbing mentioned above.
As seen from the facts mentioned above, it is important to focus attention on phenomena that tribologists have paid little attention to, from different points of view.
In the case of discontinuous rubbing, a slider discontinuously separates a specimen, whereas a slider always touches a specimen in cases of usual repeated rubbing. We see the former rubbing in the flanks of gear teeth (rolling friction accompanies sliding).
In the process of discontinuous rubbing, the slider to which debris adhered comes off the specimen and again it touches the specimen and the rubbing again. During the rubbing, the adhered debris is again severely sheared. Then the slider again comes off the specimen. Some debris adheres again to the slider, some to the specimen. Newly-formed debris joins this process. This phenomenon is usually repeated many times. In the course of the repetition, debris piles up on the friction surface and under some conditions the thickness of the piled up layer exceeds 100 mm [1].
Friction and wear occur in the shallow region of the friction surface. To study their mechanisms, the Beilby layer cannot be overlooked. Considering this fact, discontinuous rubbing is appropriate for this study because of the thick layer from which the polish pattern appears.
On the friction surface of a slider and that of a specimen, minute asperties exist. During rubbing, high pressure and high temperature arise at the minute eruptions. After the slider leaves the specimen, the asperties on both friction surfaces become cold and harden till the slider begins rubbing against the specimen. The sequence of rubbing and leaving of the slider that causes cooling and hardening of the asperties generates the difference between unidirectional discontinuous rubbing and usual repeated rubbing. When the cooling period is too short and the asperties do not cool enough, the special features of unidirectional, discontinuous rubbing, therefore, do not appear.
Although high-pressure phase usually exist only at high pressures, they remain at ambient conditions when sheared at high pressure [2].
[1] H. Furuichi et al., Wear, 184 (1955)99.
[2] H. Furuichi et al., High Pressure Res., 24 (2004) 385.