Surface Integrity of Case-hardened Gears - with Particular Reference to Running-in and Micropitting
Doctoral thesis, 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.

surface chemistry

deformation bands


residual stresses

surface asperities

plastic deformed regions (PDR)



Virtual Development Laboratory (VDL), Hörsalsvägen 7A
Opponent: Prof. Brian Shaw, Newcastle University, UK


Dinesh Mallipeddi

Chalmers, Industrial and Materials Science, Materials and manufacture

Stress distribution over gear teeth after grinding, running-in and efficiency testing

VDI-Berichte,; Vol. 2(2015)p. 973-984

Conference contribution

Influence of running-in on surface characteristics of efficiency tested ground gears

Tribology International,; Vol. 115(2017)p. 45-58

Journal article

Effect of running-in - load and speed - on surface characteristics of honed gears

World Tribology Conference 2017,; (2017)

Conference contribution

"D. Mallipeddi, M. Norell, M. Sosa, L. Nyborg". The effect of manufacturing method and running-in load on the surface integrity characteristics of efficiency tested ground, honed and superfinished gears

"D. Mallipeddi, X. Zhang, H. Alimadadi, L. Nyborg, M. Norell". Localized deformation in nano-sized martensite laths associated with micropitting in case carburized gears

"D. Mallipeddi, M. Norell, V. M. Subbaramaiah Naidu, X. Zhang, M. Näslund, L. Nyborg". Micropitting and microstructural evolution during gear testing - from initial cycles to failure

Gears are an integral part of modern life, necessary for both production and transport. The
compact and efficient transmission offered by gears made their usage predominant compared
to other drives. Recent development have increased both the efficiency and durability of gears,
especially in the automotive industry. Still, enhanced performance is required due to global
demands on sustainability and energy consumption. Actually, one billion cars are rolling on the
streets around the globe, without counting trucks and busses. This means even small increase
in efficiency could significantly reduce the energy usage.

Efficiency and durability determines the gear performance, which highly relies on the surface
integrity characteristics topography, residual stresses and microstructure. These are generated
by hard finishing in the last manufacturing step using one of the methods grinding, honing or
superfinishing. Running-in (initial cycles of operation) further alter the surface characteristics
and presets the gear for service. Nevertheless, every finishing process produces unique
characteristics, which responds differently to the operating conditions, and in turn influences
the surface integrity of gears accordingly. In this context, a detailed investigation has been
performed to understand the surface characteristics generated by the different hard finishing
methods and their evolution during usage. The role of microstructure in the failure mechanism
micropitting was also studied. The overall aim is to contribute to improved gear performance.

To summarize, surface roughness was found to be the most influential factor among all the
characteristics. During running-in the peaks on the rougher surface for ground and honed gears
were locally deformed. This induced localized microstructutal changes that led to crack
initiation and ultimately micropitting. Clearly, higher running-in load enhanced this pattern.
Instead, for smoother superfinished gears no cracks or structural changes occurred even after
200.000 cycles. The microstructural changes (PDR and localized deformation of martensite
lath) associated with micropitting started already within 200 cycles and cracks within 2000
cycles. Thus, from a materials perspective the running-in of gears can be as short as 2000 cycles.
Compressive residual stresses, beneficial for fatigue life, varied between finishing methods,
with the highest stresses for honed gears. Mostly for the rougher gears, stresses evolved during
testing but always remaining compressive. The initial increase in compressive residual stresses
was linked to retained austenite transformation and its later decrease to crack formation. The
formation of lubricating tribofilm from oil additives was promoted by roughness and runningin
load. All in all this study showed how the surface roughness influences gear performance
and can initiate gear failure after 200 cycles even for load conditions when failure occurs after
22 million cycles.

Subject Categories


Manufacturing, Surface and Joining Technology

Other Materials Engineering



Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: nr 4477


Chalmers University of Technology

Virtual Development Laboratory (VDL), Hörsalsvägen 7A

Opponent: Prof. Brian Shaw, Newcastle University, UK

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