Friction and wear behaviour of boron based surface treatment and nano-particle lubricant additives for wind turbine gearbox applications

Abstract

Gears and other mechanical assemblies are some of the key components for conversion of wind energy to electrical energy in wind turbines, but their durability and efficiency are severely impaired by some tribological issues like micro-pitting, wear, scuffing, and spalling. To address these issues, in this study, a combinational approach is proposed to incorporate surface treatment (electrochemical boriding) in coordination with the use of nano-colloidal lubricant additives. Boron nitride based solid lubricants were manufactured and flat gear steel samples were borided using a novel electrochemical boriding process. Combinations of nano-colloidal lubricant additives and borided surfaces are tested for their tribological performance, mainly friction and wear, over a wide range of contact conditions using a sliding contact linear reciprocating rig. Post-test surface analyses were carried out to investigate tribochemical interactions of colloidal lubricants with the steel surface and the chemical characterization of the tribofilms was investigated using XPS (X-ray Photoelectron Spectroscopy). Wear was also characterized and measured using optical profilometry. The borided surfaces enhanced the mechanical properties of the surface layer, leading to improved wear resistance. Moreover, it was observed that boron nitride stayed well dispersed within the oil and formed a stable tribofilm which was important to achieve improved tribological performance. The results of this current investigation are expected to aid in ongoing research efforts aimed at prolonging durability and efficiency of drivetrain components in advanced wind turbines.

Introduction

The wind energy industry has seen rapid growth within the last decade or so; however, it has been continuously plagued by high gearbox failure rates [1], [2]. Considering the high cost of replacing a gearbox, improving the reliability of the wind turbine drivetrain is a critical aspect of lowering the cost of wind energy and achieving the US Department of Energy (DOE) scenario of having wind energy provide 20% US electricity needs by 2030 [3], [4], [5]. Wind turbine drivetrains are subject to severe operating conditions such as high loading, unsteady/interrupted operation, system vibration, system misalignment, and exposure to extreme environmental conditions. These conditions are not common with other types of machinery and are the driving factors for accelerated component failure. The failures manifest as surface originated damage on contacting components, such as micropitting, spalling, scuffing, excessive abrasive wear, and corrosive wear. Blau et al. [6] conducted a detailed study of failed bearing components from a wind turbine drivetrain, which pointed to the severity and characteristics of such surface originated failures. It is clear that, in addition to improving the overall gearbox design, the design of the materials used for the contacting gearbox components as well as the lubricating oil, also need to be considered.

Steel used in the manufacture of gears for such an application typically have a core that is relatively soft and tough, to accommodate bending (i.e. at the root of the gear tooth). According to the newly developed American Gear Manufacturers Association (AGMA) standard for wind turbine gearbox design [7], the core hardness should be 290-450 HV (28-45 HRC) for external gears. It is also stipulated that these gear components are surface hardened to enhance the load carrying capacity and durability to a point that is appropriate for the particular design. Carburizing is common and effective thermochemical method and achieves better wear properties than through hardened gears. Carburization is typically used to achieve a case hardness of 900 HV by enhancing the carbon content at the surface through a thermal diffusion process. In comparison, boron can be diffused at the surface to increase hardness up to a much higher level of 2000 HV [8]. Boronizing processes include: powder pack, molten salt, vacuum, laser, electrochemical, etc. [9]. Electrochemical boriding has been shown to produce a very thick and dense layer of iron borides [FeB and Fe₂B] in a very short time [10]. The process time of electrochemical boriding is much shorter than thermochemical processes, 0.5 h compared to more than 5 h respectfully. The electrochemical boride layer also significantly enhances the corrosion and wear resistance of the material [11]. The enhanced surface protection and accelerated process time of electrochemical boriding makes it a feasible surface treatment for the wind turbine gearbox industry.

Careful consideration is also necessary for the engineering of advanced lubricants for wind turbine gearbox applications. Surface damage common in this application is likely to be affected by the type and formulation of the lubricating oil in the system [12]. Nano-lubricants have recently been considered as a potential lubricant additive mainly due to advantages, such as, likeable size to enter contact asperities, thermal stability, variety of particle chemistries, and reaction rate with the surface without induction period (required for conventional lubricant additives) [13], [14], [15], [16], [17]. When fluid and boundary films fail or are broken down the use of solid lubricant may provide improved tribological properties of sliding contact interfaces, carry the load and act as back-up lubricating film under boundary lubricated sliding conditions. MoS₂, WS₂ graphite, and other carbon and boron based micro-particles have been used largely in industrial applications. Recently, it was presented that h-BN nano-particles can generate boron compounds by tribochemical reaction under boundary lubrication regime [18]. Under the impact of tribo-electrons produced by rubbing, the borate molecules are broken down, in the presence of nitrogenous additives, and recombine to form a h-BN tribofilm.

In consideration of the major failure mechanisms present in wind turbine gearboxes, the current objective for this work is to investigate the potential tribological benefits of electrochemical boron based surface treatment, in combination with, nano-colloidal boron based lubricant additives. Specifically, the sliding wear and friction behaviour of these treatments are currently evaluated for severe operating conditions. Given the advantages of the fast processing time demonstrated for electrochemical boriding and the scalability of nano-colloidal boron based lubricant additives, these technologies have the potential for industrial application in advanced wind turbine gearboxes as a way to mitigate early system failures.