Specialized PVD coatings and nitriding significantly increase the durability and lifespan of wind turbine components like roller bearings and planetary gears under metal-on-metal, high load conditions

With the increasing demand for carbon-free renewable energy, more companies are turning to wind power as a source for their energy needs. Although this presents a good deal of potential for the manufacturers of wind turbines and their components, the technological challenges of providing a reliable product that operates with minimal maintenance or repair is increasingly daunting.

That's because enormous forces are at work on the individual internal components. 

Turbine shaft bearings, planetary and sun gears, and rotating shafts, for example, operate under high load conditions that involve direct metal-on-metal contact, often in poor lubricating conditions. With components increasing in size, these forces only become more extreme. When this occurs, even components made of hardened steel or metal alloys break down from excessive wear, scuffing, surface fatigue, pitting and galling.

Wind farm operators bear significant costs for overhauls, which often involves using cranes for on-site repairs. When you have to bring in a crane to change the main shaft bearing, for example, the cost includes not only the new bearing but also the labor involved.

At $100,000 or $200,000 per overhaul, all of a sudden, the green energy becomes quite expensive, which is why wind farm operators want to extend the longevity of the components as much as possible.

Coatings and surface treatments can address this issue by significantly extending the service life of wind turbine components. Today, this is being accomplished through the application of specialized physical vapor deposition (PVD) coatings, and nitriding treatments; both of these methods increase surface hardness and durability.

By applying coatings optimized for punishing environments, components benefit from increased surface hardness and a much lower coefficient of friction. Therefore, these critical parts do not have to be replaced as frequently, if at all, reducing maintenance and unplanned downtime, while improving wind turbine performance.

PVD Coatings

Physical vapor deposition encompasses a wide range of vacuum deposition methods. It essentially covers components with thin coatings to increase their surface hardness and durability, lower the friction coefficient, and resist corrosion. 

By applying components with PVD coatings designed for such demanding conditions, not only is the surface hardness and durability increased, but essential parts are far less likely to fail, if at all. As a result, maintenance and unexpected downtime is drastically reduced. 

A particularly effective PVD coating can be applied in thicknesses of 0.5 to 4 micrometers on roller bearings and gear parts. The WC/C ductile carbide carbon coating has a high load-bearing capacity, even when used with insufficient lubrication or dry contact.  Due to its low-friction coefficient, the coating can drastically reduce fretting corrosion and pitting. By forming an effective barrier between metal-on-metal contacts, it reduces metal structural damages such as white etch cracks and fatigue failure.

Alternatives to PVD coatings include black oxide, which is produced by a chemical reaction between the iron on the surface of a ferrous metal, and oxidizing salts.  After a post-treatment with oil, the surface provides protection against corrosion, improved lubricity, and prevents galling during metal-to-metal interactions. However, black oxide is not very durable; it can be worn away quickly in repetitive, high load applications.

Nitriding

There are limitations to the size of products that can be coated with PVD, such as the ring gears (in newer wind turbines) that can measure up to 2-3 meters in diameter. For these large gears, a nitriding process can be used. Nitriding is a heat treating process that diffuses nitrogen deep into the surface of a metal to create a case-hardened surface. Because it is not a coating, it does not affect the overall dimensions of the component. Although traditional gas nitriding costs less, plasma nitriding has the advantage of making the treatment more precise, by minimizing warping and distortion, while achieving a higher load-bearing capacity. 

In an FZG pitting test, one plasma nitriding process exhibited five times less roundness deviation, and seven times better planarity than gas nitriding on a 2-meter diameter ring gear. Another potential application for plasma nitriding is for treating the surfaces of large bearing cages used with wind turbine bearings. This increases the sliding wear resistance against the rollers, and can be used on components up to 3 meters in diameter, 10 meters in length, and weighing up to 40 tons. Compared to gas nitriding, the tolerances for roundness, planarity and parallelism can be adhered to much better, even with such large parts as the ring gears - vital for the service life of a system in which enormous forces are at work simply because of its size.

Dr. Rovere is the Director, Precision Components for North America for Oerlikon Belzers Coating USA where the company develops high performance PVD & PACVD coating solutions and surface technologies to improve the performance and durability of precision components and tools for the metal and plastics processing industries.

Oerlikon Balzers | www.oerlikon.com/balzers