Control of Micropitting in Carburised Gears
Micropitting ("grey staining") is increasingly being observed by industry in surface hardened (case carburised) gears. In the past there was a tendency to regard it as a secondary wear problem and much more attention was focused on more macroscopic pitting which occurs due to contact fatigue. However, the introduction of modern clean steels in engineering applications and the use of more highly formulated lubricants to prevent scuffing has changed this view. Macroscopic pitting is still observed in clean steels but the evidence indicates that it is nucleated from surface micropits or similar defects. The use of high performance gearbox lubricants with EP additives dramatically reduced scuffing wear in gears, but some of these additives increase the propensity for such micropitting. A considerable amount of test data on the micropitting performance of different oils has been accumulated using accelerated gear-life tests (e.g. FZG micropitting test, BGA micropitting test, etc.). However, the reasons for the greatly differing micropitting performance of different oils and additive packages are not understood.
Figure 1: Micropitting in carburised steels; (a) initiation of micropitting on asperities left from grinding process, (b) section through a region close to the pitch line showing a greater depth of crack growth prior to micropitting.
Currently the mechanisms for micropitting are also not well understood. Metallographic sections and other methods of examination suggest that the micropitting cracks and the cracks producing macro-scale pitting have similar morphology. The main difference seems to be that of scale. The cracks usually grow on planes that are inclined to the surface plane at angles, typically between 30 and 70° , depending on the position of the micropit on the gear tooth, the details of lubrication and the gear design. After some growth (5-10mm for micropits, much larger for macropits) the crack plane turns to be parallel to the surface. For micropits the crack may branch at this point with one part of the branched crack continuing to propagate into the bulk - this may be the origin of a subsequent macropit. The main micropit crack grows parallel to the surface for some distance before it turns back to the surface making a pit. This mechanism assumes that both micropits and macropits develop in the same way which may not be true in the early stages of growth which are dominated by surface effects. The lubricant may interact with the surface chemically causing stress corrosion cracking, etching, or the formation of a chemical reaction layer ("plating") which modifies surface tractions or it may interact in other ways (e.g. oil-pressure induced opening of cracks). In order to improve gear life it will be necessary to determine the micropitting mechanism in more detail in order to know all the factors which influence performance and need to be controlled.
It has been recently shown that thin vapour deposited coatings (such as WC/C, or B-DLC) can improve the resistance of gears against abrasive wear and macropitting in standard FZG gear tests and if the similarity of mechanism between macro and micropitting is correct there should also be an improvement in micropitting performance. Both these coatings produce a hard, wear and corrosion resistant layer on the surface of the gear which also has a low friction which would be an advantage under conditions of poor lubrication, reducing surface traction at highly loaded asperity tips. In this project it is planned to perform gear tests to see if such coatings do indeed reduce micropitting and determine the mechanism by which this improvement comes about using a range of laboratory wear and mechanical tests and careful microscopy of the test surfaces.
The primary aim of this project is thus to develop a good scientific understanding of the mechanism of micropitting by investigating and quantifying any reduction in micropitting produced by novel vapour deposited coatings. The project would suit a materials scientist, mechanical engineer or physicist with an interest in mechanics and offers the prospect of developing skills in microscopy (including high resolution scanning electron microscopy and atomic force microscopy), spatially resolved mechanical assessment techniques (Nanoindentation and scratch testing) and tribological testing.
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on 18th January 2002.
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