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A novel technique for depositing thick DLC based films on the inside of cylindrical substrates, like pipes, tubes and valves, has been developed. A plasma enhanced chemical vapour deposition (PECVD) technique has been used to engineer and optimize the above mentioned films for maximum coating performance. Of particular importance is the corrosion and wear resistant qualities of these films. Changes in film chemistry, structure and thickness are attributed to the improved corrosion and wear resistance. Details will be given of the corrosion testing which has taken place, such as exposure to HCL (hot and ambient temperature), NaCl and H2S environments. One such test is a very aggressive sour autoclave test where the film is exposed to an aqueous, organic and gas phase over a 30 day period and no damage to the film was found. In depth details of this sour autoclave test will be shown including photographs of the film before and after testing. Wear testing has also been carried out in dry and wet sand slurry environments where very low coefficient of friction (COF) and wear rates were found. It is believed that this thick DLC based film can increase the component life in applications where internal surfaces are exposed to highly corrosive and abrasive media, in particular the oil and gas industry. Examples of such applications are mud pump sleeves, deep well components, directional drilling, abrasive flow spools, pump barrels and in sour fields (H2S).

Careful consideration of corrosion management during component manufacture is critical to protect assets over their useful field life. Recently, a new technology was introduced that can delay or mitigate the onset of corrosion on oil country tubular goods (OCTG) used in oil and gas production operations. The innovative method produces and applies a layered diamond- like-carbon (DLC) to the internal surface of OCTG tubulars by using a plasma enhanced chemical vapor deposition (PECVD) process. The technique makes use of the hollow cathode effect (HCE) to achieve a high deposition rate and a high ion bombardment rate. Initially developed for the coating of internal surfaces of piping with length:diameter aspect ratios less than 20, the process has recently been extended to high aspect ratio piping including OCTG. Its ion bombardment produces a dense barrier of a hard, wear and corrosion resistant DLC surface coating. The improvement in deposition rate and increase in thickness compared to earlier plasma coatings has enabled new applications such as corrosion and abrasion protection. Corrosion resistance is demonstrated based on exposure to HCl and NaCl environments. Mechanical properties include high hardness, high adhesion, and good wear resistance. The new technology enables a widespread use of DLC based coating and has significant potential to increase component life in applications where the internal surface of pipes are exposed to corrosive and abrasive environments especially in the oil and gas industry. The described surface treatment provides a new alternative to traditional polymer type coatings and/or increased alloy contents.

A novel technology developed for coating the internal surfaces of parts from small components to production pipe will be described. The coating is appropriate for use in many environments for corrosion, erosion and wear reduction. The process technology that enables the coating of interior surfaces with a hard, wear and corrosion resistant DLC layer will be described. Additionally the development of improved hardness of the DLC will be described, based on the selection of precursor and process parameters hardness of > 25GPa has been achieved while maintaining a high deposition rate. Introduction The new process technology uses a Plasma Enhanced Chemical Vapor Deposition (PECVD) process that has been modified to use a high density, hollow cathode plasma that enables a high deposition rate that is generated based on the diameter of the pipe and the pressure, this in conjunction with DC plasma pulsing enables very high deposition rates of >0.3 micron/min. Coating properties down the length of a steel pipe are controlled by adjusting process parameters. Ion bombardment is used to improve film quality by biasing the part negative. The process doesn't require a traditional vacuum chamber, but uses the pipe or part itself as the vacuum chamber. In addition to improving corrosion resistance the films are hard, pure, amorphous, dense and wear resistant. Environmentally friendly precursors such as acetylene or other hydrocarbons are used to deposit inert corrosion resistant DLC based films with the potential to replace environmentally damaging precursors such as hexavalent chromium. Adhesion is improved by adding silicon to the DLC layer at the steel interface, and wear resistance and corrosion is improved with a pure DLC cap layer. This paper reports the results of a study evaluating the use of a range of hydrocarbon precursors with respect to hardness, adhesion and deposition rate. With the optimization of process parameters the hardness was improved from an average of 15GPa using acetylene to 25 GPa using butene, while maintaining a high deposition rate. Several authors have reported the effect of ion energy per carbon atom on the hardness of DLC coatings. Optimum ion energy per carbon atom of approximately 70eV is reported for high hardness and high sp 3 content with softer films reported at lower or higher energy per carbon atom. (Roberston, 2002) The size of the hydrocarbon precursor molecule will effect this energy as the molecule will break apart on impact with the substrate, for example as an acetylene molecule ion is accelerated across the plasma sheath with an energy of 1000 eV compared to a butene molecule with the same bias, and assuming a collisionless sheath, the acetylene will have an energy of 500eV per carbon atom and the butane will have an energy of 250eV per atom. Higher hydrogen content has also been reported to reduce hardness of DLC coatings. Since the process used in this report operates at a higher pressure of ~70mtorr, there are some collisions in the plasma sheath so the correlation to precursor size is not straightforward; additionally there is significant radical based chemical film growth at these pressures that will also be affected by the precursor used.

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