«
»

DLC: a coating for heavy duty

Original article date: March 1999

ANNA HULTMAN AND HANS SJOSTROM fof SKF Nova AB in Gothenburg Sweden explain how diamond-like coatings combine hardness with low friction helping to eliminate wear and increase bearing service life.

Hardness combined with low friction bearing surfaces can help eliminate smearing and other wear-related problems. This contributes to a significant increase in bearing service life. Diamond is of course the hardest material known to man and has these desirable qualities but it is too expensive to make a full diamond bearing commercially viable. The answer is to apply a hard thin coating onto the existing bearing steel: one such coating is called diamond-like carbon (DLC).

DLC is a generic name for an amorphous mixture of graphite and diamond-type bonded carbon. Today this coating is applied in extremely thin layers of only a few microns. It makes it possible to combine the endurance and toughness of a conventional steel body and the exceptionally low friction and considerable hardness of the DLC coating. The result is a bearing that has both high surface hardness and a very low coefficient of friction.

To a certain extent it is possible through application of a DLC coating to tailor the future properties of the bearing. Surface hardness depends slightly on the process used. Typical values are of the order of 1200HV compared with values of between 650 and 850HV for bearing steel. Matching the mechanical properties of the coating and the steel is crucial for the performance of the coated component.

The preferred coefficient of friction for such coated bearings depends on the environment in which they are to be used. Since rings or rolling elements (or both) may be coated several different coefficients of friction are offered. The lowest values represent the types of bearing where both the ring and rolling elements are DLC-coated. Under dry conditions however the wear life in a sliding test is greatly prolonged by coating only one surface with DLC although further enhancements are possible by coating both surfaces.

The reason for the increased bearing service life with DLC-coated surfaces is the very low coefficient of friction for such coatings combined with the very high surface hardness. The friction values are actually very similar to values for low friction materials such as the solid lubricants PTFE and MoS2. But there is one important difference: DLC does not wear away at high pressures. Another feature of DLC is the running-in characteristics. Although it is very hard a small portion of the surface transforms into a low friction contact zone. This effect acts to protect the counter surfaces of the steel that otherwise would be damaged by the harder coating. Combined with the hardness this effect results in an ideal solid lubricant condition.

So far both laboratory and application evaluation of DLC-coated bearings demonstrate lower running temperatures and increased bearing life by typically five to ten times. This is particularly relevant in cases involving zero and low load which normally cause failure due to smearing damage.

The low friction also prevents wear and bearing seizure and allows running at lower temperatures in cases with poor lubrication and/or high speeds. This feature can be used to downsize lubrication systems and results in more environmentally friendly solutions.

The high surface hardness coupled with the chemical nature of the carbon results in a higher resistance to abrasive and adhesive wear which extends life in harsh and dirty environments.

In addition the solid lubrication effect of DLC-coated bearings make them ideal for applications with low viscosity lubricant and in cases involving vibration or small movements that cause “false brinelling” or fretting corrosion. In most cases coating a single surface will provide a satisfactory solution.

DLC-coated bearings have been used successfully in applications such as compressors papermaking machines and hydraulic moors. In these applications the coating helps prevent smearing damage in the bearings. In soft calendars which are giant rolls in the final stages of a paper mill DLC coatings have proven very successful and superior to thin dense chrome (TDC).

Most hydraulic motors are lubricated with low viscosity oil which in some cases can cause friction and wear. The solid lubrication effect of DLC-coated bearings has been shown to extend the bearing service life in certain hydraulic applications by up to 10 times. In addition other hydraulic applications require expensive sealed and expensive lubrication systems to provide the necessary operating conditions. DLC-coated bearings remove the need for them.

March 1999

Added on 17 April 2009 at 1:32 pm | Read more Power Transmission articles