Lubricating and maintaining such lubrication is critical to the life of the bearing. A bearing is designed to carry tremendous loads while also being soft enough to allow small particles to "embed" in the bearing material. The various designs of bearings all address these primary needs. In addition, a bearing is a sacrificial part; in that it is easier and less expensive to replace bearings due to wear, than to replace the crankshaft.
Coatings come in two basic formulations; Solid Film and Dry Film. While the two terms can be and are to a great extent interchangeable, when it comes to bearings, the differences are important.
A Solid Film lubricant (SFL) derives its name from the resin film formed during curing. This film has greater internal bond strength than the bond to the substrate, in this case the bearing.
A Dry Film (DFL), on the other hand, has a weaker internal bond than the bond to the substrate. When these features are applied to a bearing, the Dry Film is superior. Since no coating is a 100% friction-free material, some pressure will be exerted against the coatings. In a rotational application, such as we are discussing, the coating needs to be able to minimally flake away when such pressure are reached. Otherwise, the coating can delaminate. DFL functions in this manner. Generally the particle size that can be removed is under 1 micron.
SFL are like a very thin film of tape. As the coatings delaminate it comes off in thin sheets.
DFL are capable of withstanding pressures in excess of 350,000 psi while lubricating. This is well beyond the strength of the bearing itself. As pressure increases the coating actually becomes more lubricious. In addition, DFL is a "fluid retaining" material that actually holds oil in place on the bearing. Under a microscope the surface looks like a very fine piece of sandpaper (2000 Grit). These peaks and valleys are what hold the oil.
One of the most important features of DFL is its ability to maintain its full lubrication characteristics even in extremely thin films. DFL typically will be applied in a film thickness of .0003". During running, the coating will burnish to a near "0" dimension. This characteristic allows the bearing to be run with its normal installed clearance. Coatings that form a "harder" film (SFL) will also reduce friction and provide a protective layer to the bearing; however, as these coatings begin to wear, clearances will open up. DFL do not have this problem, due to the above features. DFL can extend the life of a bearing while reducing friction, particularly in instances where normal oil film failure could lead to bearing failure.
Integral bearings in OHC cylinder heads (Head is the Bearing) have become a whole new problem within the Hi-Perf arena. With the increase in cam lift and engine RPM range. Increased spring tension is required. The load on the aluminum bearing goes up. These aluminum bores can be coated with a DFL to increase lubricity and oil retention and the life of your head.
Insert Bearings can be considered a sacrificial part. It’s easier and less expensive to replace bearings than it is to replace a crankshaft. Overhead cam (OHC) cylinder heads are a different story. The bearings are an integral part of the head. With rice rockets becoming so prevalent in the Hi-Perf arena, a new problem has arisen. The cam bearings were originally designed to only cope with a predetermined load. With Hi-Perf springs the load in many cases has doubled even tripled. The bearing cannot carry this load.