Coatings for gear wheels

• Published in: Vakuum in Forschung und Praxis : Zeitschrift für Vakuumtechnologie, Oberflèachen und Dünne Schichten Vol. 20; no. 4; pp. 6 - 12
• Main Authors: Petrik, Marko, Wittdorf, Ralf, Thomsen, Helge, Kaestner, Peter, Kropp, Jan Peter
• Format: Journal Article
• Language: English; German
• Published: 01.08.2008
• ISSN: 0947-076X; 1522-2454
• DOI: 10.1002/vipr.200800361

In order to optimize the goal, steel gear wheels regarding load-carrying capacity and wear, thin film coatings were tested. Different coating systems were examined numerically with the software ELASTICA(R) for their suitability. The characteristics of the coating systems were determined dependent on the material, its surface treatment as well as the diameters of relevant rolling partners. Differences were made between macroscopic rolling contacts between the teeth profiles and microscopic contacts with surface roughness and abrasion particles. First the four best suitable coating systems were deposited on simplified rollers and examined under different conditions. Two coating systems were determined, which show special suitability for the coating of the gear wheels. The first system is an a:C-H coating with an CrN interlayer. The second system is an a:C-H coating wtih an CrN interlayer on a plasma-nitrided substrat (Duplex-process). In order to protect the coatings on the teeth, their involute profile was provided with a tip relief. As gear wheel materials 16MnCr5, 42CrMo4 as well as the special steel ETG(R)88 were used. Two kinds of flow fats were used as lubricants and additional the unlubricated operation was examined. The gear wheels were tested at three different speeds on a test machine especially built for it. The testing routine was carried out in so-called power-stages. Each stage means a defined number of contacts and a certain Hertzian stress on the teeth profiles. With each stage the Hertzian stress was increased. The end of operation time is the beginning of cavitation pitting. The test results showed that in particular with the steel 16MnCr5 and 42CrMo4 the used Duplex-systems leads to considerable increases of the load-carrying capacity of the tooth flanks and the wear resistance. This increase was observed both with fat-lubricated and with unlubricated gear wheels. The results for the special steel ETG(R)88 were less promising. An unlubricated operation could be made possible by the described coatings, which would not have been possible without the coatings. However the results are not as good as the results of uncoated gear wheels wih use of minimum fat lubrication. The reason could be an insufficient nitriding-depth. For the selected steel the plasma-nitriding parameters were investigated with the goal of optimized nitriding-depth. Unfortunately up to the end of this project the gear wheels could not be treated with the optimized Duplex-Process. At present the attempts with optimally plasma-nitrided gear wheels are carried out. After that it would be possible to give recommendations for the plasma-nitriding process and the necessary coating as well as the selection of the fat.