What is Black Oxide?
By Cory Mittleider on 10/06/20
Cylindrical Roller Bearings with Black Oxide
What is Black Oxide?
Black oxide coating (a.k.a. BO or BOC) is a chemical conversion of the bearing steel. This is a special form of oxidation achieved with a controlled process involving several dip tanks and very precise temperature controls. When completed the bearing will have a 1-2µm thick layer of this special oxide that leaves the bearing with a black appearance.
Why use Black Oxide coated bearings?
There are two primary benefits of a black oxidized bearing:
Improved corrosion and chemical resistance
Improved oil adhesion
Wind turbine gearboxes are a very demanding application with a wide variety of speed, load and environment leading to premature bearing failures. Both of these core features of a black oxidized bearing are useful in Wind
Improved corrsion and chemical resistance:
Bearing steel is chemically active and will readily corrode if exposed for long periods of time. This is why bearings are carefully protected during shipment and storage in multiple ways including plastic bags, lubricant coating, and VCI (vapor corrosion inhibitor) wrapping.
Wind turbine gearboxes have been known to have water suspended in the oil and oil additive packages that are overly aggressive to steel. Both of these things will lead to a shorter bearing life in your gearbox as the bearing steel is chemically attacked.
The process of black oxidizing essentially pre-corrodes the bearing in a controlled process. By doing so the surface of the bearing steel is more chemically stable and less vulnerable to corrosion and chemical attack.
Improved oil adhesion
Figure 1 below is from SKF's presentation at the NREL DRC in February of 2018. The graph shows the theoretical cage speed vs the actual measured cage speed of the 2 HSS cylindrical roller bearings. This data was taken at a point of low load and high speed that would occur while bringing the turbine online.
Fig 1. Cage slip of WTG gearbox HSS under low load / high speed condition
The actual measured cage speed was at less than 30% of the theoretical cage speed. This happens because the load applied to the bearing was too low. With too low of a force the rolling element is allowed to slide rather than roll. This sliding that happens in this scenario can lead to the lubricating oil being wiped away allowing bearing surface asperities to contact leading to smearing damage.
The black oxide process increases oil adhesion by making the surface asperities "fuzzier". This increases surface area of the coated parts and therefore a more robust oil adhesion. The improved adhesion will reduce the possibility of this smearing damage occurring.
Conclusions for Wind Applications
With gearboxes 80m up in the air in extreme enviornments, frequent starts and stops, wide variations in loads and only physically inspected 1 or 2 times per year, Wind turinbe gearboxes demand more considerations than a typical industrial gearbox. The industry has found that Black Oxide applied to most bearing positions in a Wind turbine gearbox offers benefits that are well worth the additional minimal cost of this upgrade.
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If you have any questions please give me a call at 605-357-1076
-Cory Mittleider