Not a coating for a piston, but an alloy where silicon is added to that aluminum, copper, magnesium as part of the alloy essentially to limit expansion not to increase it.
Correct term is a eutectic piston, hypereutectic means over eutectic that is no good as it makes the piston too brittle and would crack like a wine glass if a bit too much pressure is applied. Typically only about 10% of silicon is considered safe.
By controlling expansion, the piston will last a lot longer, is an American invention, but the stockholders and marketing wouldn't let us use it, back in the 70's would add a whopping 15 cents to the cost of each piston.
And engines that lasted longer, owners would keep vehicles longer reducing future sales. No BS here, did you work in automotive the most of your life? Were you in these meetings?
In the 70's, the Japanese vehicles were the laughing stock in the automotive world and quickly learned people wanted quality, they did not invent the eutectic piston, we did, but they were the first to use it for the so-called 200K mile engines. Really cut into the domestic market so forced domestics to use eutectic pistons as well, and this is the only reason.
But their quality really went down when they devalued the buck, the Chinese sure learned from this. But still getting by with terrible quality.