Wear-Resistant Peripheral Coatings
Thermal Spray Coatings
Spray coatings on piston rings in the top groove are preferred to hard chromium plated rings above all when engines - chiefly diesel engines - are subject to increased risk of scuffing. Materials deposited by a spray process can better withstand this adhesive wear caused by direct metal-to-metal frictional contact, as in some cases they have a very high ceramic content.
The materials for piston ring wear coatings range from all-molybdenum, which has a low adhesion tendency due to its proneness to form hard (ceramic) oxides during spraying and its high melting temperature, to metal-ceramic composite coatings and all-ceramic coating systems.
Metal-ceramic composite coatings today predominate in most applications as armouring for the running face of piston rings in diesel engines, while molybdenum coatings are now only used occasionally because of their comparatively low wear resistance. Coatings based on molybdenum and self-fluxing hard alloys, such as MP43, are nowadays practically only used in gasoline engines. The all-ceramic spray coatings will play a more significant role in the future if the brittleness typical of ceramics can be successfully reduced.
For the manufacture of metal-ceramic composite coatings the plasma spraying process is still largely used. The coating known by the name MKP 81A is used particularly successfully in passenger car and truck diesel engines. Owing to its high melting efficiency, plasma spraying is a very economical universal process  but is not suitable for producing the hard metal-like structures that are of advantage. Therefore, the HVOF (High Velocity Oxy-Fuel) spraying process was developed .
In a controlled process operating at relatively moderate temperatures the powdery materials are shot onto the piston ring surface and sintered in an ultrasonic flame. In the GOETZE range of ring coatings such coatings are identifiable by the name MKJet® .
Among the outstanding advantages of the MKJet® coatings over plasma coatings are the finer coating structure (Fig. 26), easily controllable residual compressive stresses, an extremely low and fine porosity and exceptional adhesive strength. This reduces wear compared to plasma coatings by a factor of 3 to 4 (Fig. 27) and increases spalling resistance on all ring designs. The wear characteristics are comparable to those of CKS®36 coated piston rings.
MKJet® coatings exhibit a directional hard metal-like structure which originates chiefly through powder compaction. They have a ceramic content of up to 80%.
MKP coatings have a lamellar structure typical of plasma spraying which originates through powder melting. The ceramic content is up to 40%.
As there is a wide range of materials available for spraying, coatings can be specially matched to specific engine operating requirements. Consequently, spray coatings are offered in a larger selection of alloy compositions than chromium coatings.
Fig. 26: Thermal Spray Coating Structures
Fig. 27: Wear in Engine Operation