Norvel use a form of hard anodizing on their bores which creates a layer of sapphire (aluminum oxide), a type of ceramic which is an extremely hard and wear-resistant substance. Even better, hard anodizing is slightly porus in nature, thus it retains a small amount of oil that helps with lubrication.

One of the problems with hard-anodizing is that it becomes increasingly difficult to get a thick layer of oxide built up because the oxide itself is an insulator. In effect, the anodizing current (and therefore the rate of anodizing) decreases rapidly as the oxide builds up.

Plasma-sprayed ceramics achieve the same result, albeit usually imparting a thicker layer of the oxide more quickly by virtue of being sprayed on in a superheated stream of plasma (10,000 deg C) rather than being built up slowly in an electrochemical reaction. In theory, you can deposit almost any thickness of oxide through this spray deposition process.

Norvel obviously think that the level of deposition offered by anodizing is adequate for their engines, larger engines however probably need the kind of layers that only plasma spray deposition can provide.

If you use a ceramic-coated bore it becomes more important to have a softer piston material in order to allow for some degree of "running in" to occur. Unlike steel, or even chrome, ceramic won't show any appreciable wear for a long time so the use of a hard piston/ring material will mean very long break-in times. It's probably for this reason that Norvel nickel-plate their pistons -- nickel being a somewhat softer material than chrome. The nickel has a very low coefficient of friction (much lower than aluminum) yet is soft enough (unlike chrome) to allow sufficient wear for a good fit.