DLC coating of interior surfaces of steel tubes by low energy plasma source ion implantation and deposition
Introduction
The treatment of the inner surface of a tube with ions for purposes of surface modification is usually difficult because the access to the inside is limited for directed ion beams. Physical vapor deposition is also intricate in cases where uniform coatings are required [1], [2]. In most cases there will be a concentration gradient starting from the opening of the tube that was oriented toward the deposition source. Additionally, shadowing effects can lead to inhomogeneous treatments. One solution for this problem is based on the movement of a conical sputter target inside the tube along its axis while it is irradiated with a collimated ion beam [3]. Although this setup could be applied successfully to a number of problems [4], [5], [6], [7], the instrumental effort is quite large.
By using a non-line-of-sight technique, complex shaped objects can be treated from all sides simultaneously. One of those techniques is plasma source ion implantation (PSII) [8] which in its most basic setup only requires a pulsed high voltage generator. The sample is held in a lower pressure environment (ca. 1 Pa) and is connected to the pulse generator. By applying the pulses a plasma is generated from the surrounding gas. The ions are directed toward the substrate and implanted into its surface, provided that the voltage is of negative polarity. Since the plasma is also generated inside of the tube, the inner surface can be treated. Depending on the type of gas, a coating can also be applied there. Utilizing hydrocarbon gases, diamond-like carbon (DLC) can be deposited by PSII [9].
This technique has been applied to tubes of different shapes, from very small diameters of sub mm [10], [11] to longer tubes of 1 or 2 m length [12], [13] and also to bend tubes [13], [14]. Typically, the process is done with higher ion energies of 10 keV or more. One reason for this is the loss of the full acceleration potential near the middle of the tube, since – depending on the ratio of length and diameter of the tube – the central axis is not at ground potential. A remedy for this is the use of an auxiliary electrode along the central axis [15] which lowers the deposition rate, however. Without the auxiliary electrode, the resulting film thickness and the properties of the DLC film usually show a dependence on position, i.e. the distance from the edge of the tube [13], [16], [17]. In order to investigate whether this effect is also present with lower energies (and if so, to what extent), deposition was carried out with negative pulse voltages of −5 kV.
Section snippets
Experimental
Commercial grade SUS304 austenitic type stainless steel tubes with inner diameters of 20 mm and lengths of 100 mm and 200 mm, respectively, were used as the substrate tubes. Small strips cut from Si (1 0 0) wafers and from SUS304 plates were positioned inside the tubes (see Fig. 1) and were used as substrates for the subsequent analysis after the PSII treatment. The plasma was generated by the application of a pulsed voltage to the substrate tube. The base pressure in the chamber was 10−4 Pa.
Results and discussion
By using acetylene ion implantation as a plasma source, a DLC film was formed on the inner surface of the tubes. The adhesive strength was high enough that no exfoliation of the film was observed. Since the aim of the coating is usually a uniform film formation on the inner surface of the tube, the thickness distribution of the DLC films was determined. The cross sections of the silicon strips that were placed inside the tube were measured by SEM. The thickness of the DLC film which was
Summary/Conclusion
Inner surfaces of tubes can be treated successfully with low energy PSII. DLC films that exhibit constant composition with depth were deposited throughout the tube. Although higher gas pressures have to be used to be able to strike a plasma, there is an advantage in the much higher deposition rates that can be achieved. There is some variation of the film thickness, hydrogen content and film structure with position inside the tube. The extent of the variation depends on the one hand on the
References (22)
- et al.
Surf. Coat. Technol.
(2004) - et al.
Surf. Coat. Technol.
(2008) Surf. Coat. Technol.
(1996)Surf. Coat. Technol.
(1996)- et al.
Surf. Coat. Technol.
(2002) - et al.
Surf. Coat. Technol.
(1988) - et al.
Surf. Coat. Technol.
(2002) - et al.
Nucl. Instrum. Methods Phys. Res. B
(2003) - et al.
Surf. Coat. Technol.
(2000) - et al.
Surf. Coat. Technol.
(2002)