Eliminating Galling of High-Alloy Tubular Threads by High-Energy Ion Deposition Process
Authors: White G.W. 
Published: December 2007

White, G.W., SPE, White Engineering Corp.

Galling is a form of adhesive wear that typically occurs in the presence of relatively high stresses. The worst-case result is actual seizure and cold welding of mating parts. This may occur very early in the life of parts, in parts. This may occur very early in the life of parts, in many cases at original assembly. Threaded components have been the traditional sites for galling failure. With the increased use of high-alloy materials to combat corrosive effects of sour service, the tendency of threads to gall has become severe. A method of high-energy metal implanting has been developed to protect threads of all the various alloys from galling by disrupting the basic mechanism that leads to galling. This method of ion plating has been applied successfully to sliding surfaces plating has been applied successfully to sliding surfaces in general.

Introduction

The background, methodology, and application of the subject matter of this paper draws heavily from the fields of thin-film technology, plasma physics, and tribology, which is the field of lubrication, friction, and wear. The potential for thin films in enhancing the performance of potential for thin films in enhancing the performance of oilfield equipment is an easy one to overlook in view of the traditional attitude of "the bigger, the better." In the more subtle area of surface physics when one surface slides across another, quite the opposite is true because some of the very fundamental processes occur at the thin-film level. As used in this text, thin films are those whose thicknesses are given in angstroms (A) or millionths of an inch. Vacuum technology has developed along the lines of modern thin-film technology and is used as the predominant source for the thin films that have made such predominant source for the thin films that have made such innovations as the integrated circuit possible. As normally used, vacuum deposition of thin films is done by simply evaporating the material to be deposited and allowing it to re-form by condensation on the desired substrate. Over the years, this technique has been improved by further development of the "sputtering" process whereby positive ions of argon are used to provide a bit more positive ions of argon are used to provide a bit more energy to the evaporant and enhance film adhesion. This still left a lot to be desired when applying thin films for heavy-industry applications. The ion plating process came about through the use of argon gas partial pressure in a vacuum system, causing ionization of evaporant atoms by collision as they proceeded from the vapor source on their way to the substrate. Once this is accomplished, a positive charge on the substrate will cause the ion of evaporant to be accelerated to the oppositely charged substrate. This phenomenon is illustrated in Fig. 1. The region adjacent to the negatively charged substrate, called the "Crooke's dark space" in honor of its discoverer, is vital to the ion plating process. The full voltage drop to the cathode may be applied directly to an ion in transit across this region of high vacuum, resulting in an extremely high velocity of the ion as it arrives at the surface. The result is penetration of the substrate surface and diffusion of the ion into the substrate lattice structure. As we shall see, this is essential to breaking up the mechanism of galling. Early problems with argon contamination of the deposited films leading to inconsistent properties, not the least of which was embrittlement, made it necessary to develop a high-energy process whereby pure films of the deposited material could be employed. This required a totally different ionization mechanism and is the subject of U.S. Patent No. 4,420,386 described in Ref. 1. In the new approach, the ionization of evaporant atoms was accomplished by passing them through an intense radio frequency field. This resulted in a higher percentage of ionization as well as eliminating the need for the percentage of ionization as well as eliminating the need for the argon gas. A higher percentage of ionization meant improved three-dimensional coverage and made possible the application to irregular surfaces such as thread forms with excellent profiling. Applications of ion plating to oil industry problems is not new. Examples of successes would include a dry lubricant film for the working surface of one manufacturer's subsurface safety valve. Thousands of brazed assemblies where an ion-plated nickle interface was used to provide enhanced brazing of tungsten carbide to various alloys of stainless steel are in service. Quite a number of the turbine splines used in commercial jet airliners have seen ion-plated films result in a ten-fold increase in service life. With these successes in the background, it seemed only logical to extend the application to the problem of thread galling.

Theoretical Aspects of Galling

The subject of galling is closely related to lubricity since drilling occurs after a breakdown of lubricity. Lubricants are commonly used to prevent galling. Therefore, it is worthwhile to consider the role of boundary-layer films in providing lubrication.

JPT

p. 1345