What is the purpose of alu and cermet coating on Main Engine piston rings?
The cermet coating is a composite coating, partly ceramic partly metallic, applied on the base metal of piston rings. It enhances the resistance against scuffing and reduces the wear rate of the piston rings. It combines the properties of a ceramic (hardness, high temperature resistance) and a metal (high elastic durability). The cermet coating was introduced as a counter measure against scuffing and is now a standard feature in long stroke, large bore, slow speed diesel engines (conventional and gas-fueled).
The alu coating (aluminum bronze) is applied on top of the cermet coating to ensure a smooth surface finish on the piston ring and cylinder liner surfaces during the running-in process. It is normally worn off between the first 1,000 - 3,000 running hours.