Colloquia

Summary

Hard chrome coatings are thick coatings from chromium, and ubiquitous in many industries. Hard chrome exhibits a unique combination of properties that it excels in, such as hardness, wear resistance and corrosion resistance.

As result of these superior properties, VDL ETG applies hard chrome coatings in one of their products, the XT4 base module. The hard chrome is applied in the linear air bearing system in the base module, specifically, on the bearing guide surface. Air bearings are bearings that enable high precision movement by separating two surfaces by a thin layer of pressurised air, under normal operation.

Problems are arising due to increasing restriction on use of chromium trioxide, one of the main substances required for deposition of hard chrome coatings. Therefore, VDL ETG is interested in replacing the hard chrome layer. The aim of this research was to find and research alternatives for the hard chrome.

Therefore, the air bearing system was investigated and characterised to determine the highest loads upon the coating, which occur during so-called crashes. This resulted in defining the tribological system for a crash, which comprises all relevant parameters to describe a wear situation. After this analysis, abrasive wear was expected to be the main wear mechanism during a crash situation. This was supported by findings in literature.

Hard chrome alternatives were researched in literature, which resulted in selecting six alternatives to test. Scratch experiments were designed to replicate the tribological system during a crash. The test materials consisted of pins, and plates coated with hard chrome and the alternative coatings. The goal of these scratch experiments was to simulate a crash on a small scale, and therefore subjecting coatings to the loads found during a crash.

All possible contact situations for the bearings in the base module have been replicated in the scratch experiments. After the experiments, the material wear surfaces were captured and statistically analysed. Because of the hypothesised abrasive wear, the influence of hardness on the statistical differences was researched. A correlation was found between the mean scratch depth and coating surface hardness.

From the results was concluded that abrasive wear is indeed the failure mechanism during a crash, and therefore, the alternatives for the bearing guide surface must be harder than the opposing bearing surface, with a difference of at least 20%. Therefore, of the tested materials, the suited alternatives are the tungsten carbide HVOF coating, chromium nitride PVD coating, and titanium nitride PVD coating.