For next-generation automobiles, we must develop powertrain components that can respond to the further evolution of internal combustion engines and the higher performance of electric vehicles against the backdrop of tightening regulations on global environmental protection. Sliding parts such as powertrain gears are should not only further increase their tooth root, tooth surface strengths to meet high-speed rotation and high torque and to reduce size and weight but also have functions such as high transmission efficiency and low noise due to high precision and low friction. This report describes the needs and background for next-generation automobiles, and outlines the research and development trends of surface modification technology mainly for materials and heat treatment of powertrain gears.

Regarding the functional improvement of gears by using steel materials, we have reviewed the factors affecting the fatigue strength in terms of materials and introduced the developed steels. As the surface hardening heat treatment applied to gears, carburizing, nitriding and induction hardening are also targeted. For carburizing, bending fatigue, roller pitting fatigue, and impact fatigue are described separately. The factors of bending fatigue were inclusions, grain boundary oxidation, austenite grain size, retained austenite, roller pitting fatigue of temper softening resistance, and impact fatigue of grain boundary properties. Since gears undergo complicated processes (working and heat treatment) before they become finished parts, an example of steel material development that takes into account workability and heat treatment during the process was presented.

Automobile transmissions gears have been required to have high strength, high accuracy, and low cost. Further, in the production line of gears, stable quality, high productivity, energy saving, low environmental load, and improved safety have been demanded. In recent years, the environment has changed greatly due to CASE, and the demand for gears will become more sophisticated and complex in the future. Therefore, we will discuss the evolution and the problems of conventional surface-modified gears, and explain mild carburized gears that simultaneously achieved some difficult problems.

Induction heating and quenching is a useful surface modification process. This process hardens only the surface layer of several mechanical parts and gives higher compressive residual stress at the surface and finer grain size microstructure. In this paper, I will introduce technological improvement of high-performance gears by characteristic induction heating and quenching.

With the aim of providing long service life and noise reduction of drivetrain components such as gears, various materials have been developed and various type of heat treatment and surface modification has been applied. Fine Particle Bombarding, which is one of the surface modification methods, has been widely used due to excellent fatigue properties by introducing compressive residual stresses, and due to low friction properties by creating surface profile. As for automotive drivetrain components etc., besides material-related development, lubricants and additives have a huge effect on properties above, and therefore, it is becoming important to investigate regarding the combination of surface modification methods, lubricant and additive technology. On the other hand, concerning food machinery, there are many regulations that enable securing food safety. Above all, in Japan it was mandated for food-related business operator to introduce HACCP (Hazard Analysis and Critical Control Point) systems-compliant sanitary management system from June 2020. And powertrain components of the food machinery such as gears are made of stainless steel for rust prevention. Furthermore, as lubricants and additives for them, only the NSF H1-certified lubricants and the FDA (U.S. Food and Drug Administration)-approved additives are allowed to be use. This paper describes a new hybrid technique utilizing Fine Particle Bombardment process and DLC (Diamond-Like Carbon) coating process, which improves sliding properties and anti-seizing properties under the restricted conditions of food machinery noted above.

In recent years, oil-less technologies have been attracted much attention in research and industry because of increasing awareness of environmental destruction. In this problem, hydrogenated amorphous carbon (a-C:H) films is one of the key technologies to accomplish the development of oil-less sliding parts. In our previous studies, we focused on improvement technique to reduce friction of a-C:H films. In this manuscript, we introduce friction reducing methods of a-C:H films in oil-less conditions, such as pre-heat treatment, surface profile optimization, and chlorine-doping.

Influence of alkyl group of primary zinc dialkyldithiophosphate (ZDDP) on the friction and wear properties was studied using two different kinds of ZDDPs under the boundary lubrication condition with a pin-on-disk type tribotester. A coefficient of friction (COF) was determined under the following three operating conditions: the constant condition, the step-load condition and the variable-speed condition. COF of linear-alkyl ZDDP showed lower value than that of branched-alkyl ZDDP under all these conditions. Nano-friction force of the tribofilm obtained in the constant condition test was measured by means of an atomic force microscope. Meanwhile, nano-friction force ratio of the branched type to the linear type was almost same as COF ratio. It was suggested that macro-scale frictional behaviour was governed by nano-scale mutual interaction of tribofilms. On the other hand, the wear scar diameter decreased with increasing ZDDP concentration. From the energy dispersive X-ray spectroscopy analysis, the wear scar diameter was inversely correlated with phosphorus concentration on the worn surface. These results suggested that COF depended on mechanical properties of top layer of tribofilm while wear resistance depended on the thickness of base layer of tribofilm containing polyphosphate.