PSI - Issue 75

D. Jbily et al. / Procedia Structural Integrity 75 (2025) 158–175 Author name / Structural Integrity Procedia (2025)

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Keywords: Spur gears ; micropitting ; macropitting ; contact fatigue ; shot peening ; roughness ; residual stress; EBSD; friction coefficient

1. Introduction Micropitting is a surface fatigue phenomenon that frequently occurs in case-hardened gears operating under mixed elastohydrodynamic or boundary lubrication conditions (NF ISO 10825-1&2). It typically manifests as numerous microscopic craters on the gear tooth flanks, resulting from repeated rolling – sliding contact and inadequate lubricant film thickness. While initially minor, micropits can propagate and merge, leading to macropitting, tooth profile degradation, increased noise and vibration levels, and potentially catastrophic gear failure (Mallipeddi (2018)). As early as the 1990s, Höhn et al. (1996) identified micropitting as a primary limitation on gear performance and durability. Over the last decade, significant progress has been made in understanding the mechanisms and contributing factors of micropitting. Key parameters include contact pressure, slide-to-roll ratio, surface roughness, lubrication regime, and material properties (Olia et al., 2005; Liu et al., 2019). For instance, the load condition has been identified as a primary factor influencing micropitting initiation, while the slide-to-roll ratio more strongly influences its propagation (Olia et al.,2005). Studies reveal a competitive relationship between wear and micropitting during contact fatigue (Rongxin Guan1, 2024.) Olver (2005) explained that contact fatigue results from the combined effects of asperity-level fatigue and mild wear, under high loads and inadequate lubrication, surface fatigue initiates through asperity interactions, while wear removes sharp peaks, promoting detachment of fatigued material. The balance between these two mechanisms determines the onset and severity of micropitting. Morales Espejel et al. (2011) were the first to explicitly propose the failure competition mechanism between wear and micropitting. Shot peening has become a widely adopted surface treatment technique aimed at improving the fatigue resistance of gears. By bombarding the surface with small spherical media, shot peening induces compressive residual stresses and modifies surface topography. While shot peening has proven effective to enhance the bending fatigue strength and pitting load-carrying capacity of gears (Inoue et al. (1989), Lambert at al. (2018), Kobayashi et al. (1990), Townsend (1992), Güntner et al. (2017)), its relationship with micropitting remains complex and not fully understood. More recently, Jbily et al. (2024) examined the effect of optimized shot peening on case hardened gears and found that while the treatment delayed the onset of macropitting, it can led to more pronounced micropitting and greater profile loss compared to non-peened flanks. To address micropitting, alternative surface engineering techniques such as superfinishing has been explored. Winkelmann et al. (2008) demonstrated that superfinishing reduces micropitting by improving surface finish and increasing load-carrying capacity. When combined with shot peening, further benefits are realized especially when gear teeth incorporate proper tip relief for optimized load distribution. Carranza et al. (2025) confirmed that the integration of superfinishing and shot peening significantly extends gear life. Meanwhile, Geitner et al. (2023) investigated the effect of nitriding on micropitting and wear behavior under varying loads and speeds, emphasizing the critical role of the compound layer's characteristics in enhancing wear and micropitting resistance. Furthermore, several studies have investigated the use of coatings on steel gears to prevent micropitting. Moorthy and Shaw (2012) showed that Nb – S and BALINIT® C coatings significantly enhance contact fatigue resistance, with Nb – S offering the best overall contact fatigue performance and BALINIT® C effectively minimizing micropitting. Their follow up study (Moorthy & Shaw, 2013) confirmed that both coatings improve micropitting resistance by reducing localized stress concentrations associated with ground surfaces. Recent advances in modeling and simulation have contributed to the understanding of micropitting evolution in gear systems. Brandão et al. (2015) proposed a coupled wear and fatigue model for spur gears to predict surface degradation due to micropitting and wear. Morales-Espejel et al. (2018) developed a numerical model incorporating roughness, lubrication, and stress history effects, combining Archard’s wear model with contact fatigue predictions. However, the two effects were analyzed sequentially rather than in a coupled manner. More recently, Portron et al. (2025) developed a model to simulate the influence of micropitting on the dynamic behaviour of a gear system. The model accounts for roughness-dependent friction and represents micropitting as a localized increase in composite