Surface Integrity of Case-hardened Gears - with Particular Reference to Running-in and Micropitting

Doktorsavhandling, 2018

A gearbox with gears of different sizes is part of a vehicle transmission system and plays an important part in transmitting the engine power to the wheels. The efficient energy transmission highly relies on the performance of gears. Together, the mesh efficiency and durability determines the performance of gears.

The hard finishing of gear surfaces by means of different methods; grinding, honing and superfinishing etc., produces unique characteristics in terms of surface roughness, microstructure and residual stresses. These characteristics of tooth affect the gear performance. Running-in process is known to alter them along with surface chemistry and presets the gear for service. This fact creates an interest to understand the initial running-in with the purpose to improve the performance of gears. Thus, this study addressed, the influence of running-in on the evolution of surface characteristics generated by the mentioned methods, and how they developed further during initial usage, represented by efficiency test. Gears tested in a FZG back-back test rig were characterized by combining different analytical techniques. These included scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Surface roughness was found to be the most influential factor and virtually all changes were confined to ~5 μm below the surface. The running-in process smoothened the surface asperities through plastic deformation and the severity of deformation increased with load. Micropitting was also associated with asperity deformation and hence only seen in ground and honed gears, while being absent for superfinished gears. Micropitting was promoted by higher running-in load and this trend continued for subsequent efficiency testing. The running-in load also promoted the deformation bands frequently found in connection with the cracks. Compressive residual stresses beneficial for fatigue life varied between finishing methods, highest stresses recorded for honed gears. The stresses differed between profile and axial direction after manufacturing and, reached similar levels after efficiency testing, but remained compressive throughout the test. The initial increase in compressive residual stresses was linked to retained austenite transformation and its later decrease to crack formation. The indicated tribofilm formation was connected to the surface roughness and promoted by running-in load.

Micropitting is a surface contact fatigue failure that occurs in all types of gears. This failure mechanism was also investigated from material perspective. Gears were tested in a sequence from 200 to 2.2 x 107 cycles. The micropitting initiated due to the deformation of asperities and associated microstructural changes; plastically deformed regions (PDR) and deformation bands (thin martensite lath with epsilon carbides precipitated at boundaries). These structural changes started already within 200 cycles and cracks occurred after 2000 cycles, signifying that micropitting can initiate already after short period of operation. Thus, the running-in of gears from materials perspective can be as short as 2000 cycles. The findings presented are expected to contribute to the technical and industrial aims for optimized gear preparation.