Traditional lubricants form a fluid or mixed lubrication film at the friction interface, whereas solid lubricants rely on the material’s low shear properties or transfer film to enhance wear resistance and reduce friction. Solid lubricants are typically applied as powders or coatings to the material surface, providing self-lubricating properties. They are widely used in mechanical transmission, machining, electronics, and various other industries. Over the years, a variety of solid lubricants have been developed, including soft metals (gold, silver, and lead), metal compounds (sulfides, and halides), inorganic materials (graphite, talc, and mica), and organic compounds (paraffin, polyethylene, and PTFE).
Nano copper exhibits excellent ductility, thermal conductivity, and wear resistance. Recently, it has been utilized as a friction-reducing agent in composite coatings. In these nanomaterials, nano copper adheres to the surface, forming a robust, smooth protective layer that fills micro-scratches. This significantly reduces wear and friction, improves lubrication performance, and enables self-repair of material defects. Ticai He developed a nano-composite anti-friction coating using nano copper as the base material. Zhao Meng et al. and Xuefeng Zhang et al. incorporated nano copper as the lubricating matrix to create a copper-based nano-composite anti-friction coating, which was applied to phosphated N80S, P110S, and TP9STS oil casing couplings for performance testing. The results showed favorable assembly and disassembly performance. Yun Yang et al. used nano copper as the matrix and epoxy resin as the binder to examine the tribological properties of coatings with varying epoxy resin concentrations, comparing them to thread grease lubrication performance. The study found that the coating with 40% epoxy resin (by weight) exhibited the best anti-friction performance, surpassing thread grease. During operational tests, no sticking occurred after 10 cycles, demonstrating the potential of this coating to replace conventional thread grease.
In addition to nano copper, other widely used solid lubricants in thread surface coatings include molybdenum disulfide and polytetrafluoroethylene (PTFE). Zhao Meng et al. reported a nano PTFE anti-friction (NPAF) coating, produced by combining nano polytetrafluoroethylene, polyacrylic acid resin, and high-concentration nano copper in an ethanol colloidal solution. This coating significantly enhances the surface properties of oil casing threads and reduces torque fluctuations. Li Wang et al. applied a self-lubricating coating to a P110 special-threaded joint. The coating mainly used molybdenum disulfide and polytetrafluoroethylene as lubricants, providing excellent adhesion, friction resistance, and wear properties. Shilong Zhao et al. used a nano copper suspension, nano PTFE, nano TiO2, and epoxy resin to prepare an anti-friction (AF) wear-resistant coating, which was applied to P110S oil casing threads. The coating formed a uniform nano copper protective layer on the phosphated surface, effectively reducing the friction coefficient. The results from five sets of casing couplings in the test showed satisfactory performance. Yin Qishuai et al. incorporated nano-scale Cu powder into a MoS₂/C coating to create a MoS₂/Cu/C composite coating using airless spraying and high-temperature curing. The coating was applied to offshore pipe threads, significantly improving their anti-sticking performance. Sandblasting initially applied, followed by airless spraying of the MoS₂/Cu/C composite coating. The coating thickness was 176.2 µm and was tightly bonded to the substrate, with no significant change in the substrate's hardness. Its average friction coefficient was lower than that of the traditional high-temperature manganese phosphating process, as well as the actual casing and drill pipe unbuckling tests without sandblasting. After more than 10 well operations in offshore oil and gas fields, no sticking was observed, and the thread parameters remained stable.
Sandblasting is a mechanical process that plastically deforms the material’s surface or removes surface imperfections. The purpose of sandblasting is to achieve specific surface roughness and morphology, eliminate contaminants, and improve surface adhesion. Common sandblasting materials include quartz sand, glass, and ceramics. The appropriate mesh size and sandblasting pressure are selected based on the desired surface roughness. Surface roughness after sandblasting depends on the base material’s characteristics, as well as the type, size, impact rate, and density of the sandblasting materials. Studies have shown that sandblasting the threads of oil pipes and casings significantly affects anti-galling performance. Surfaces treated with spherical sand particles exhibit more uniformity than those treated with angular diamond particles, and glass beads of the same size produce lower surface roughness than diamond sand. Excessive sandblasting pressure can lead to excessive deformation of the thread surface, negatively impacting anti-galling performance, while insufficient pressure may clog the nozzle. The sandblasting angle is also critical. An improper angle can result in uneven treatment along the thread profile, affecting stress distribution. In severe cases, this can reduce the thread’s fatigue life and cause partial fracture at the root. By optimizing the choice of sandblasting materials and process parameters, a smooth, uniform thread surface can be achieved, ensuring effective anti-galling for oil casings. Rolling strengthening involves using a rolling wheel to impact the material’s surface at high frequency, generating compressive stress in partial areas, refining the grain structure, increasing dislocation density, improving surface roughness, and inducing cold plastic deformation to enhance surface strength and hardness (Figure 5).
Figure 5 Thread rolling
Zou Jie et al. performed cold rolling of J55 steel-grade oil pipe threads, using finite element analysis to select the optimal rolling pressure and angle. This resulted in elastic-plastic deformation at the thread root, optimized the thread profile, and reduced surface roughness at the root fillet. The grain structure was significantly refined, with hardness increasing from approximately 200 HV to 280 HV, and fatigue life improving by a factor of 1.34. Suying Zhang et al. applied micro-rolling to the root of the NC31 drill pipe joint thread. After rolling, the root hardness increased by approximately 33%, the hardened layer reached 0.66 mm, surface roughness was reduced, and both corrosion and fatigue resistance were enhanced. Fibrous microstructures formed on the thread flanks, and material density was increased.
This paper discusses the application of solid lubricating coatings in the surface treatment of oil well pipe threads, focusing on their role in enhancing anti-galling and wear resistance. Solid lubricating coatings, especially composite coatings based on nano copper, molybdenum disulfide, polytetrafluoroethylene, and other materials, effectively reduce the friction coefficient and improve lubrication performance of the thread surfaces, thereby minimizing thread grease usage and enhancing operational efficiency. These coatings not only simplify the construction process but also extend the service life and improve operational safety of oil well pipes. With the continued advancement of tribology, materials science, and chemistry, the types and properties of solid lubricating coatings are expected to be further refined and optimized, broadening their potential applications. Emerging surface treatment technologies, including laser heat treatment, nitriding, and plating, also offer innovative possibilities for thread surface treatment. However, current research on the anti-galling effects of various surface treatment technologies for oil well pipe threads lacks clear statistical evidence, limiting their widespread adoption in the oil pipeline industry. Therefore, extensive statistical studies are needed to further assess the applicability of different oil well pipe types and service conditions, providing stronger technical support for real-world industrial applications.
