This top ring groove coating...
This top ring groove coating creates a hard mating surface which virtually eliminates micro-welding while decreasing ring groove wear. It has proven to increase power output by allowing for extremely tight ring clearances. However, buildup is .00025-inch per surface and clearance must be added during manufacturing to accommodate the change. Courtesy of JE Pistons
Further, because the distribution of heat in a valve spring tends to not be uniform, coatings (particularly of the dry film variety) can both reduce overall spring temperature and create more uniformity in how the heat is distributed in the spring. The latter of these two benefits goes to the issue of eliminating "hot spots" within spring material, likened in a sense to what you might find from stress risers in other engine components.
In addition, valve springs don't necessarily wear in a uniform fashion. Consequently, coating them with a thermally dispersive material can aid in creating a more uniform distribution of heat within the spring. What you need to avoid are coatings that tend to insulate oil from the spring, thereby preventing the lubricant from absorbing unwanted heat. This can be a particular problem in multiple spring sets where the additional friction surfaces tend to increase temperature at a greater rate and cause the cooling benefits of the oil to be even more critical.
Cylinder Heads - There are multiple areas that can benefit from coating portions of cylinder heads. In particular, coated combustion chambers can increase heat rejection, thereby maintaining more heat in the form of piston-delivered pressure. A more uniform thermal distribution on combustion surfaces has a tendency to suppress detonation, even allowing an increase in mechanical compression ratio. Plus, as you would expect, corresponding changes to ignition spark timing and fuel enrichment can provide additional power.
Increasing heat retention in the combustion space can also lead to less cylinder head distortion from thermal loading, plus reduced coolant temperature. By coating cylinder heads in the valve spring area, some additional heat control is possible. Intake and exhaust ports can also be coated for net gains in power, particularly on the intake side where an air/fuel charge temperature reduction is beneficial in helping reduce tendency toward detonation. Especially at higher engine speeds, coated exhaust ports will aid net flow and reduce heat transfer into the cooling system.
The thermal barrier crown...
The thermal barrier crown coating is applied to the top of the piston and is designed to reflect heat into the combustion chamber, thereby increasing exhaust gas velocity and greatly improving scavenging potential. The .0015-inch thick coating can also assist in extending piston life by decreasing the rate of thermal transfer. Courtesy of JE Pistons
You may also find it interesting that properly coated combustion chambers can lead to an increased burn rate, thereby requiring somewhat less spark timing. From previous discussions, you will recall that when it's possible to reduce spark timing (from a quicker burn) without an initial loss in power, a net gain in torque can result from a reduction in negative torque (pre-TDC pressure on the ascending piston). In effect, more of the burn occurs near TDC than further afterward. Coating intake manifold surfaces can help retard cylinder head heat into the manifold and, therefore, provide some temperature reduction for inlet air/fuel charges. The benefits of cooler charge mixtures are well known to the Circle Track readership.
Bearings - Conventional wisdom suggests dry film coatings are the best for coating bearings. Properly compounded, these coatings can withstand loadings (pressure) typically in excess of the bearing's pressure limits. According to Tech Line, it's important to make certain the applied film thickness of the coating (using their dry film material) not exceed 0.0003-inch. This dimension usually requires some degree of burnishing to obtain since the as-coated thickness trends more to around 0.001-inch. The latter of these could affect bearing clearance, once the engine sees service and the film thickness is reduced. It's these type of nuances that suggest you should discuss coating needs with your choice of material suppliers, or else application mistakes could negate any benefits from the process, even to the point of parts damage.
Crankshafts - Essentially, there are two areas of most frequent coatings application. The obvious one is bearing surfaces for improved lubrication and additional temperature control of the oil. The other less obvious is coating of non-bearing surfaces with a material that prevents or reduces the tendency of oil to cling to the crank. Particularly at high rpm (intermittent or sustained), oil escaping past the connecting rod and main bearings tends to "knot" or "rope" around the crankshaft, creating a braking effect and reducing net torque at the flywheel. Properly selected and applied coating on all non-bearing crankshaft surfaces will help reduce or prevent such action on the crankshaft.
Once again, whether you plan to purchase precoated parts or do the coating yourself, it's critical to discuss materials and application steps with your chosen supplier. Bearing fit and clearance issues should be on your discussion agenda, at the minimum.