Colloquia

Summary

In the offshore industry, ships are equipped with open gear systems to ensure the rotation and translation required during operation. Current systems use grease to reduce friction and wear. This mainly happens in open systems, of which an example is a rack-pinion system.  Out of these open systems, grease can spill into the sea resulting in harm to nature. A solution to this problem would be a greaseless open gear that still includes a friction- and wear-reducing layer.  A proposed solution is to coat the pinion with a laser-clad added cobalt-based superalloy that has these properties and a high bond strength can be reached. 

Wear and friction are system properties so to determine the quality of the coatings, it should be experimented on, mimicking the real system. To do this, the real contact conditions are matched based on an analysis of the real system. 

A combination of rolling and sliding is present in a real gear system. In these experiments, pure sliding is experimented. A disc-on-disc experiment on the UMT Tribolab is used for this. The influence of different parameters is assessed.  After the wear experiments are carried out, the height profile of the wear track is measured with a profilometer. This is used to calculate the specific wear rate. To determine the type of wear, the morphology of the wear track surface is also observed and together with the wear, this can tell the type of wear. Friction is analysed to further increase understanding of the phenomena happening, and this value must be low as the coatings replace the grease.

The proposed laser cladded cobalt-based superalloy coatings do reduce wear compared to the uncoated material. Stellite 190 is found to be the best wear-reducing. Unlubricated wear is undesirable as severe adhesive wear is present. (Sea)water lubrication shows better performance. However, the uncoated pinion with grease lubrication which is the current situation in the observed applications performs better because boundary lubrication is present.

It is found that a higher velocity leads to less wear. The reason could be corrosive wear and build-up of an oxide layer. A higher load leads to more plastic deformation, however, the specific wear rate will be a lower value due to normalisation over the load. A hardness increase in the coatings decreases the wear only with similar material composition. The base material in the same conditions shows more wear but has a higher hardness. 

It is still to be determined if the wear reduction by the coatings is enough to be feasible as a replacement for grease in offshore applications.