Research on Pb-free Electroless Ni Plating for Pistons of General-purpose Engines 2009-32-0065

Ni-P-B plating applied to the piston of a general-purpose engine with an aluminum alloy cylinder contains some thousands ppm of Pb, which is used as the treating liquid stabilizer, in its plating. But, Pb is considered an environmental hazardous and it is necessary to reduce its amount in the plating.

Our previous researches, however, reveal that Pb plays a major role in wear and seizure resistance. In order to obtain the sliding properties equivalent to Ni-P-B plating that contains Pb, therefore, it is necessary to review the composition of Ni-P-B plating.

In this research, therefore, we specifically focus on the plating hardness of Ni-2%P plating subject to the temperature environment in which the piston operates, and discuss whether the same sliding effects as that of the Pb containing Ni-P-B plating is obtainable by restraining the hardness decrease.

The hardness of Ni-P-B plating is reduced by approximately 20% around 240°C when compared to that of the lower range of room temperature.

On the other hand, by adding approximately 20wt% of Co to Ni-2% P plating, the reduction of coating hardness around 240°C was reduced to one third of that of Ni-P-B plating. While observing decreased hardness in the lower range of temperature by added Co, it was confirmed that the coating hardness could exceed that of the conventional plating even in the lower temperature by adding W by 1% or so.

We applied this plating to a piston, and operated an engine to test plating durability, and successfully obtained the equivalent seizure and wear resistance to those of conventional plating.