Internal DLC coating of narrow metal tubes using high-density near plasma sustained by microwaves propagating along plasma–sheath interfaces

Recently, internal plasma processing of narrow halls and tubes is strongly desired in industry. Uniform processing of such a 3-dimensional metal surface by plasma is typically achieved by RF and DC glow discharge where negative voltage is applied to a metal substrate against a grounded chamber at low gas pressures. However, the plasma electron density, ne of RF and DC plasmas is typically no more than 109–1010 cm− 3 at low pressure, and thus plasma cannot be steadily sustained inside narrow halls and tubes less than 1 cm in width. For successfully processing the internal surface of such narrow halls and tubes with plasmas, it is essential to employ higher-density (ne > 1011 cm− 3) plasma. Therefore, we proposed a new method to generate high-density plasma along 3-dimensional metal surfaces by using microwave propagation along plasma-sheath interface, and further demonstrated that high-density plasma column can be steadily sustained inside a narrow metal tube whose inner diameter is in the range of millimeters and the length is more than 10 times of it. In this work, we tried to deposit diamond-like carbon (DLC) film onto the inner surface of a stainless-steel tube 4.4 mm in inner diameter and 50 mm in length by applying the new method. The tribological properties of the DLC film were then investigated, by conducting a rotational friction test of the DLC coated tube against a cast iron shaft under dry condition; as a result, the minimum friction coefficient of 0.17 was obtained at the position 40 mm distant from the bottom end of the tube where microwaves are injected during deposition. However, the friction coefficient was 0.4 at the rest positions (10 and 25 mm from the bottom end), where the films were peeled off the substrate during the friction test.