Kyuichiro Tanaka

The mechanism of wear of polytetrafluoroethylene

Abstract Experiments to study the effects of heat treatment, speed and temperature on the friction and wear properties of polytetrafluoroethylene (PTFE) have been carried out. The mechanism of wear is discussed on the basis of the results of these experiments and electron microscopy of the friction surfaces. The wear rate is affected by the width of bands in the fine structure rather than the crystallinity, while the friction is little affected by both the factors. The effect of speed and temperature on the wear rate is expressed by a master curve. From the temperature dependence of the shift factor, the activation energy of the slippage between the crystalline slices in bands is estimated to be about 7 kcal/mol. As the result of easy slipping of crystalline slices, the destruction of banded structure occurs easily without any melting of the sliding surface and a film of about 300 A in thickness is produced on the surface. Since the film detaches easily from the surface, PTFE reveals a very high rate of wear. A mechanism of formation of the film is also proposed on the basis of electron microscopy of the worn surfaces.

Effect of various fillers on the friction and wear of polytetrafluoroethylene-based composites

Abstract The friction and wear behaviour of polytetrafluoroethylene (PTFE)-based composites incorporating various fillers was determined under a constant load and at various sliding speeds when composite pins were rubbed against a steel disk. Factors influencing the wear-reducing mechanism of the fillers were studied. The friction of PTFE-based composites was generally independent of the type of filler. Fibre and particle fillers of suitable size were more effective than lamellar solid lubricants and very small hard particles. The load-supporting action and the prevention of large-scale destruction of the banded structure of the PTFE matrix at frictional surfaces contribute to the wear-reducing action of the fillers. The effects of the material and the shape and size of the fillers on the wear of composites are discussed. The load-supporting action of the fillers is discussed theoretically.

Chapter 5 - Effects of Various Fillers on the Friction and Wear of PTFE-Based Composites

Abstract In this chapter, the fundamental behavior on the transfer, friction and wear of unfilled PTFE are first briefly surveyed and discussed on the basis of the molecular and morphological structures of PTFE. Secondly, the fundamental behavior on the friction, wear and abrasiveness of PTFE incorporating various fillers are surveyed in detail, mainly on the basis of the work which has been carried out in the authors laboratory, and the effects of various fillers on the friction and wear of PTFE-based composites are described. Thirdly, the effect of fillers are discussed on the basis of the properties of unfilled PTFE and microscopic examinations carried out on the worn surfaces of composites and also on the counterface. Thus, the mechanism of the wear-reducing action of fillers in PTFE are explained. Furthermore, the effects of water lubrication on the friction and wear of PTFE incorporating various fillers are presented and discussed.

Transfer of semicrystalline polymers sliding against a smooth steel surface

Abstract The interrelationships between transfer and wear in polymers were studied using a pin-disk-type wear testing apparatus. The wear rates of polymers except polytetrafluoroethylene (PTFE) were high for up to about the first 100 revolutions of the disk and decreased gradually until the steady low wear rates which generally occurred after about 2000 revolutions. However, PTFE exhibited an almost constant high wear rate throughout the wear process. The thickness of transferred polymer increased rapidly with increasing number of revolutions in the initial wear stage but after about several hundred revolutions remained constant. A coherent transfer film was formed in most parts of the friction track after about 100 revolutions. It was found that polymer wear could occur in polymers sliding on a transferred polymer layer. All polymers except PTFE exhibited smaller wear rates when sliding on the transferred layer. The load dependence of the thickness was very small compared with that of the wear rate. PTFE produced a very dense and coherent transferred layer compared with that of other polymers. However, there was no clear relationship between friction and the thickness of the transferred polymer layer.

Studies on the friction and transfer of semicrystalline polymers

Abstract The friction and transfer of various semi-crystalline polymers were studied in several experiments. The cylindrical surfaces of polymers were slid over glass plates at low speed and under constant load. The kinetic friction of PTFE in repeated traverses did not vary with the number of traverses and the transfer of PTFE occurred successively on previously transferred PTFE films. The film transferred at each traverse was extremely thin (

Fundamental studies on the brake friction of resin-based friction materials

Abstract The effects of various factors on brake friction were studied on the braking of a cast iron disk by small brake specimens made of resin, resin-asbestos composites and three-component composites. When the disk temperature rises, the resin-based composites containing asbestos and high melting point additive exhibit considerably lower friction during braking following high friction in the initial braking stage. The initial high friction is attributed to the deformation resistance of the resin in the composite. Low friction is attributed to the lubricating action of resin decomposition products in the neighbourhood of the filler raised to a high temperature by frictional heating. Increase of friction toward the end of braking is mainly due to increase of the true contact area as the frictional speed decreases. Variation of the average frictional coefficient during braking with load and the initial speed is generally small. There is no correlation between the mechanical properties and the brake frictional behaviour of resin-based composites.

Wear laws for polytetrafluoroethylene

Abstract Experiments were made to determine the effect of sliding speed, contact pressure and rubbing surface temperature on the wear of a polytetrafluoroethylene (PTFE) pin rubbed against a chromium-plated brass disk. An automatic temperature control type wear apparatus to control the rubbing surface temperature was constructed and used to minimize the influence of frictional heat. The linear wear rate is proportional to contact pressure to the nth power (n > 1). Wear curves of PTFE with a characteristic variation of the linear wear rate with sliding speed were obtained. Increase of contact pressure shifts the curves towards a higher wear rate. When the temperature was increased the curves were shifted towards a higher speed and higher wear rate. A master curve obtained by translation of each wear curve vertically or horizontally to fit the reference curve can be used to determine the wear rate.

Friction and damage of coatings formed by sputtering polytetrafluoroethylene and polyimide

Abstract To investigate the tribological performance of coating films formed by sputtering, polytetrafluoroethylene (PTFE) and polymide (PI) films were prepared on glass substrates. A sapphire ball 5 mm in diameter was slid on the films at a slow speed. The friction coefficients of these films did not depend on the load and film thickness. When the sliding was repeated on the same track, the friction of PTFE films increased as the number of passes increased. A large fluctuation in the friction was observed as the film wore away from the track. With PI films the friction coefficient was almost constant during the experiment. Using a pin-on-disk apparatus, the life of the films was estimated as the number of passes when the friction coefficient exceeded 0.5. Although almost all films were worn away from the track in relatively small passes, thin films left on the track were effective in suppressing the friction increase. PI film exhibited a much longer life than PTFE, although PI film exhibited a brittle fracture in contrast to PTFE film. Heat treating the films effectively increased the PTFE film life but decreased the PI film life.

Effect of the degree of crystallinity on the friction and wear of poly(ethylene terephthalate) under water lubrication

Abstract By means of a pin-on-disk type of wear-testing apparatus, coefficients of friction and wear depths of various poly(ethylene terephthalate) (PET) specimens with different degrees of crystallinity were measured at sliding speeds of 0.01 and 0.1 m s −1 under a constant load of 10 N with water lubrication. The coefficients of friction with lubrication were lower than those for unlubricated conditions and there was little dependence on the degree of crystallinity. However, the wear rates with lubrication were higher than those obtained under dry conditions and decreased with increasing degree of crystallinity, in contrast with the unlubricated wear rates. These results are discussed on the basis of the plasticization of worn polymer surfaces by water. The worn surfaces and frictional tracks were observed by optical and electron microscopy. The frictional tracks for low crystallinity PET were smooth with many fine scratches and characteristic of transfer wear. For high crystallinity, frictional tracks were somewhat different from those for low crystallinity PET.

Wear of magnetic materials and audio heads sliding against magnetic tapes

Abstract The friction and wear of various materials and audio heads sliding against magnetic tapes were studied. Magnetic materials such as Permalloy, Sendust and hot-pressed ferrite (HPF) and some other non-magnetic materials were used as material specimens. Three types of audio heads for cassette tape recorders were used as the head specimens. Their magnetic cores were made from ordinary Permalloy, hard Permalloy and HPF. An experiment using a conical diamond slider was carried out to study the wear resistance of various materials. The specific wear rates of the various material specimens and components of the heads such as the core, shield plate and epoxy resin were measured using the Knoop indentation technique. Wear did not occur uniformly over the surfaces of the Permalloy heads. This irregular wear was examined in detail and its origin is discussed. The specific wear rates of Permalloy and HPF are of the order of 10−5 mm3 N−1 m−1 and 10−6 mm3 N−1 m−1 respectively and that of epoxy resin is of the order of 10−4 – 10−5 mm3 N−1 m−1. The coefficient of friction of an HPF head is about 0.3, while that of both types of Permalloy head is about 0.7. It is concluded that the mechanism of wear by magnetic tape is not entirely due to the abrasive action of the magnetic powder in the tape but is also partially adhesive in nature.