Abnormal wear at the ring groove not only decreases output power but also increases oil consumption. At worst, a seizure can take place. Groove wear is strongly influenced by piston thermal deformation under sustained working conditions. Ideally, the lateral faces of both the ring and the groove should be parallel and perpendicular to the cylinder bore wall, so that the contact and the resulting pressures can be distributed evenly, minimizing wear. However, the thermal distortion generates an inclination in the groove to give localized contact between the lateral faces of the groove and the ring. Of course, too high a temperature in the ring groove softens aluminum which accelerates wear, but much wear is the result of distortion.
During piston operation, the area particularly around the top-ring groove is liable to overheat. The aluminum softens and sometimes fuses with the top ring. This fusing is also termed micro-welding.
Remedy: Hard anodizing (hard anodize oxide coating) is often used to avoid this fusing. Anodizing is a surface treatment widely used for aluminum appliances. When a piece of aluminum is anodized, a thin oxide layer is formed on its surface. Hard anodizing produces a particularly hard layer. To obtain a suitable hardness, a piston is anodized in a dilute sulfuric acid electrolyte at a low temperature. Pure aluminum generates a homogeneous anodized layer. The layer of the piston alloy contains Si particles and inter-metallic compounds which come from the substrate alloy.
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