Surface coatings for tribological contacts in factories have always been used to solve problems, increase efficiency or increase reliability.The most common of these focused on corrosion protection of non-contacting surfaces, such as electrocoating or powdercoating housings. But lately the focus has shifted more toward applications inside bearings and gear contacts.They experience heavy loads and deformations in machines, and prior generations of coatings did not work effectively.
One example of a new application iszinc alloy coatings that can be applied to ferrous surfaces to prevent corrosion. Originally developed for high strength structural components, the coatings can now be found in process equipment that needs protection due to the operating environment. This is evidenced by the less than 0.03 percent surface rust after 1,000 hours in the ASTM B117 salt spray test.
"The current coating technologies are quickly moving into areas where factory problems can be solved to increase uptime and efficiency"
The most common use of this coating in the industry is in primary metal rolling mills. The typical practice to maximize bearing performance is to mark the bearing outer ring into quadrants, and then rotate the outer ring through each of the four positions to spread out the load and increase the operating life of the component. However, when the rolling fluid penetratesan idle bearing the water can collect at the roller-raceway contact areas and corrosively etch the bearing components. Spalling can occur at these etched areas and limit the useful life of the bearing.By using zinc alloy coatings, the non-loaded portion of the outer ring does not etch during the idle times and that allows the mill to use all four quadrants, which increases equipment uptime. With the coating the outer ring did not fret in the housing and it was easier and therefore faster, to remove it during planned maintenance inspections.
Another coating that is becoming more common in industry is diamond-like coatings (DLCs). Specifically, the DLCs for tribological contacts are doped with metallic carbides to further enhance their hardness and make them better at standing up to the very high pressures in bearings and gears. For reference, the nano-hardness of the DLCs is ~13 GPa, which is double that of the base steel.
DLCs have been used where wear is the life-limiting damage mode of the tribological components in the factory.
One good example is in paper machines on the suction roll of the press section end where loads are light relative to the size of bearings needed due to other physical constraints in the system.In one specific case the bearings on the suction roll were changed out yearly due to wear as a preventative maintenance practice.By applying a 1- to 2-micron thick DLC that was doped with tungsten carbide nano particles, the mill stretched the maintenance cycle to three years, saving a significant amount of money in replacement bearing costs and shorteningtheir yearly planned maintenance downtime.
The current coating technologies are quickly moving into areas where factory problems can be solved to increase uptime and efficiency.