Scuffing resistance mechanism for coating strengthened gear transmission

Scuffing is one of the most typical damage forms in the gear transmission of agricultural machinery. The fast bursting speed and serious damage degree should be strictly avoided in the conditions of the low lubrication and extreme loading during the gear service. This study aims to improve the scuffing resistance load carrying performance in the transmission gears of agricultural machinery under severe lubrication, variable speed, and heavy-duty conditions. The surface coating strengthening was applied to the meshed tooth surface. A mathematical model was established for the relationship between the contact properties of coatings and gears, as well as the scuffing load capacity. The gear meshing, tribology, and thermodynamics were also utilized in this case. The finite element model of plane strain was selected to explore the influence of elastic modulus ratio between coating and substrate on the stress field distribution. Anti-scuffing gear coatings were achieved to clarify the effect of tooth surface frictional coefficient on oil film thickness and transient contact temperature, according to the calculation in ISO/TS 6336-22. Then, the carbon films were prepared by low-temperature plasma enhanced chemical vapor deposition technology. Surfaces of standard steel balls were coated with the tungsten containing ta-C and a-C:H coatings. Tribological properties of coating materials were evaluated using the four-ball method. A series of experiments of scuffing resistance were performed on the two coated gears in the FZG (Forschungsstelle für Zahnräder und Getriebesysteme) transmission test rig. The results showed that the carbon film friction pairs had the better running-in performance, compared with the uncoated components. The a-C:H coating presented the higher sp² C-C bond content than ta-C, resulting in a lower frictional coefficient. Also, the scuffing resistance capacity increased by 2 FZG loading stages in the ta-C coating, while at least 4 loading stages were found in the a-C:H coating one. In the process of engagement, the ta-C coating was peeled off, and then the peeled coating particles were pressed into, adhered to the contact surface, or discharged from the scratched surface, thus forming abrasive wear between the tooth surfaces, and finally the gear steel substrate was completely exposed. Furthermore, the failure mode was changed from abrasive wear to adhesive wear with the increase of frictional temperature. The surface material was subject to adhesive tearing. Moreover, both uncoated and ta-C coated tooth surfaces showed the significant competitive relationship between the thermal scuffing and micro pitting damage, which was mainly determined by the oil film thickness and contact temperature. In addition, the concave plastic deformation near the driving wheel pitch line was found on both surfaces. The a-C:H coating showed the conventional fatigue wear with the smooth and flat wear trace, where the tooth surface coating had the high integrity without outstanding damage. Theoretical analysis and experimental data were combined to enhance the gear scuffing resistance capacity. Although the frictional coefficient was low, the ta-C coating was more prone to the coating peeling and interface damage, due to the high elastic modulus ratio between coating and substrate. By contrast, a-C:H coating shared the relatively low elastic modulus ratio. The interface stress was smaller and more difficult to peeling, and the frictional coefficient was smaller, resulting in a relatively thicker oil film thickness and lower transient contact temperature on the tooth surface. The achievements demonstrated that the a-C:H coating exhibited the excellent scuffing resistance load carrying performance suitable for gear transmission. The finding can lay the foundation for the application of coating strengthening technology in the high-performance agricultural machinery gears, even the other transmission systems with harsh service conditions.