The sprocket is a plastic cog, and the chain is stainless steel. Why not cast an alloy cog? Kinda make sense...yes?
Hobie used stainless sprockets for years.
They have some good points, but some serious flaws as well. The current sprockets engage the chains better and are not prone to skipping out if proper adjustment is neglected.
This damage is consistent with an incorrectly mounted chain rather than normal wear. In this next picture, one of these sprockets has over 800 logged (pedaled) miles and the other is new? Any guesses which is which?
I can only tell by looking because the used sprocket (left) still has the sprocket guard attached. This glass filled nylon is pretty tough stuff.
So how can the chain be installed incorrectly? the next pic shows a chain seemingly installed correctly (sorry about the bad flash angle).
But if you look from the other side, it's a half link off -- not that easy to spot for the casual observer. This will rip up the cogs
The chain must be pressed into the master (extra wide) cog FIRST, before any other links are applied. It is not easy to press in all the way (when new) and can not be done correctly at any other time. When properly installed, it stays in place, even if inverted as in the case with this well used chain chain and sprocket:
Subsequently the other links should be pressed in individually and inspected for any gaps or high riding tendencies before adjusting cable tension.
Needless to say, the chain/cable should be properly adjusted -- not so much for sprocket wear, but excess (power robbing) friction on one extreme, and drive slack on the other.
This does not preclude other possible causes for sprocket damage. Nevertheless, this would be my best guess by far.
PS If you still want stainless sprockets, I believe Hobie still has some available. You'll also need to order bushings different sprocket guards and clew outhauls (as the top picture shows). I have both types and consider the current Nylon sprockets to be superior.