A Novel Corrosion-Resistant Internal-Coating Method Provides Improved Hardness
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 114020, "A Novel Corrosion-Resistant Internal-Coating Method Using Hollow-Cathode PECVD Technology With Improved Hardness," by Bill Boardman, Karthik Bonapolly, Deepak Upadhayaya, and Tom Casserly, Sub-One Technology, prepared for the 2008 Western Regional and Pacific Section of AAPG Joint Meeting, Bakersfield, California, 31 March-2 April. The paper has not been peer reviewed.
A novel technology has been developed for coating the internal surfaces of parts ranging from small components to production pipe. 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 silicon-containing diamond-like carbon (DLC-Si) layer is described. Hardness greater than 25 GPa has been achieved while maintaining a high deposition rate.

Introduction
A new process technology uses a plasma-enhanced chemical-vapor deposition process that has been modified to use a high-density, hollow-cathode plasma that enables a high deposition rate that is generated based on pipe diameter and pressure. In conjunction with DC plasma pulsing, this enables very high deposition rates greater than 0.3 μm/min. Coating properties along the length of a steel pipe are controlled by adjusting process parameters. Ion bombardment improves film quality by biasing the part negative. The process does not require a traditional vacuum chamber, but uses the pipe or part 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 are improved with a pure-DLC cap layer.
The full-length paper reports the results of a study evaluating 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 15 GPa 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 car-bon atom of approximately 70 eV is reported for high hardness and high sp3 content, with softer films reported at lower or higher energy per carbon atom. The size of the hydrocarbon precursor molecule affects this energy because the molecule will break apart on impact with the substrate. For example, as an acetylene ion is accelerated across the plasma sheath with an energy of 1,000 eV compared to a butene molecule with the same bias, and assuming a collisionless sheath, the acetylene will have an energy of 500 eV per carbon atom and the butene will have an energy of 250 eV per atom.