PSI - Issue 42

Mushfiq Hasan et al. / Procedia Structural Integrity 42 (2022) 1169–1176 Mushfiq Hasan et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Failure in gears may occur due to insufficient lubrication and excessive applied load (1). Using low viscosity lubricants has become a trend to enhance energy efficiency in machine elements. Therefore, friction pairs are more exposed to boundary or mixed lubrication regimes instead of running in full film EHL, especially at higher load and low speed conditions. This enhances the degree of asperity to asperity contact and can initiate fatigue after a certain number of cycles. For rolling/sliding applications like bearings and gears, rolling contact fatigue (RCF) can be a dominant failure mode besides wear. Tallian described this damaging process in several phases (2). Initially, the surface modification occurs by asperity deformation under shear stress and cyclic loading. Thus, alternation in material structure evolves deformation bands, and cracks start to initiate. The crack initiation occurs in between the contact surface and maximum Hertzian shear stress depth. Then it propagates until maximum shear stress depth is attained and creates a crater by removing a volume of material. This damage process is known as ‘pitting’ due to rolling contact fatigue. Several factors affect micropitting in gears and bearings application. Oila and Bull (3) found that high contact pressure (2.2 GPa) accelerates the formation of micropitting at a lower contact cycle. However, micropitting can be delayed by a very smooth surface even at a higher contact pressure. The hardness of the material can also influence the process by prolonging the crack initiation. Slide-to-roll ratios (SRR) are another important parameter where the direction of rolling and sliding has a massive effect on micropitting (4). The crack grows opposite to the sliding direction. The impact of SRR direction on micropitting has been studied in (5). They concluded that negative SRR leads to higher micropitting damage than the equivalent positive SRR. Moreover, the damage correlates with the overall sliding distance rather than the magnitude of SRR. However, fewer micropits could be seen at higher SRR if the wear mechanism dominates (6,7) stated that. Surface roughness and texture also play a vital role in micropitting. Generally, higher roughness leads to lower film thickness which accelerates asperity contacts. Liu and others (7) concluded that the risk of fatigue increases with a higher RMS roughness. On the other hand, Winkelmann et al. (8) mentioned that super finished surface can stop micropitting formation. Several literatures focus on micropitting prevention in steel material using coatings like diamond-like carbon (DLC), black oxide and hydrogenated tungsten carbide (9,10). In most cases, the researchers concluded better fatigue life compared to the uncoated steel up to a certain extent. Despite much research regarding micropitting, some areas are still unexplored. Particularly wear and pitting behaviour in boundary lubrication under high SRR values. This phenomenon can be found in the transmission gearbox when it is in a state where maximum torque is required. This work conducted a series of experiments under severe gear test conditions mimicking a gear contact in a twin disk gear rig. Moreover, industry standard roughness and hardness were considered to apply the outcomes directly for industrial application. Based on the data, we have recommended a suitable running condition to reduce pitting and wear risk. Also, the pitting progression mechanism under boundary lubrication is explained and visualised through surface characterisation techniques. 2. Methodology Gear tooth engagement can be approximated by two cylinders with a line of contact. Therefore, circular disks were used instead of gears to mimic the contact points with a similar pressure that happens in a real gear tooth engagement. 2.1. Twin disk rig To induce fatigue in a contact surface under different SRRs, a Wazau UTM 2000 twin disk machine was used, Fig.1. The speed can be set up to 3000 RPM for varying entrainment speeds as well as SRRS. To generate the required contact pressure the radial applied load can be up to 2000N.

2.2. Surface Characterisation

A white light optical interferometer (Zygo 7300) was used for capturing surface topography. Also, different surface parameters information like waviness, texture, roughness, flatness and step heights can be obtained from the measured surface profiles. Later, a Scanning Electron Microscope (JEOL JSM IT300) was used to better understand the surface