Measurement of Friction Coefficients Between Diamond-Like Carbon Coated VCR Head Drum and VCR Tapes
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ABSTRACT In order to characterize friction and wear behavior between cylindrical specimens and flexible counterface materials such as magnetic tape, a tribometer is suggested. The tribometer is used to investigate the behavior between video cassette recorder (VCR) head drums and VCR tapes. Friction coefficients of uncoated and diamond-like carbon (DLC) film coated head drums of Al alloy are compared before and after a wear test. Before the wear test, an uncoated head drum shows friction coefficients ranging from 0.22 to 0.3 as the applied tension increases from 200 to 600 gf at the sliding speed of 0.16 m/s. The friction coefficients of DLC coated head drum are 14 to 30 % lower than those of uncoated head drums. At the sliding speed of 4.9 m/s, the friction coefficients of both uncoated and DLC coated drums are reduced to less than 0.1. After the wear test, however, the friction coefficients of uncoated head drums increase to the range from 0.3 and 0.47 depending on the applied tensions. Furthermore, the friction coefficients are not reduced even at the high sliding speed of 4.9 m/s. DLC coated head drums, on the other hand, exhibit similar friction coefficients to those before the wear test. Degradation of frictional property in the uncoated head drum is discussed in terms of the surface damages mainly due to magnetic metal oxide particles transferred from VCR tapes. This result shows that the DLC coating of VCR head drum can improve the friction and wear properties. It is also observed that the DLC coating can reduce the damage of VCR tapes.
INTRODUCTION As video cassette recorder (VCR) becomes smaller and lighter, the thickness of air gap between VCR head drum and tape decreases. Furthermore, sudden change in relative speed of the VCR tape with respect to the head drum can occur when using a function such as 'jog-shuttle'. Physical contact between the tape and the head drum thus occurs more frequently, which results in damages of both surfaces. This problem would be more important in digital VCR where the drum rotates much faster and metal evaporated tape is used for high magnetic recording capability [1]. Hence, the lubrication and anti-abrasion properties of VCR head drum surface become necessary to protect both the head drums and the tapes. Diamond-like carbon films, also called hydrogenated amorphous carbon films, have distinguished tribological properties of low friction coefficients (especially in low relative humidity) and high wear resistance [2]. Combined with surface smoothness and low deposition temperature, this film has been considered as a candidate for protective layer of VCR components and tapes [3, 4]. Nakaue et al and Meunier et al have reported that DLC coating on the VCR components and VCR tape improves the life time of the components by reducing the damages caused by contacting with each other. However, few investigations have been reported on the details of friction and wear behaviors. Most conventional tribometers such as ball on disk or pin on disk type one were designed for flat specime
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