Posted 05 April 2013 - 23:17
in the early days, before the PS3 and before BD was really public and there wasn't really any BD movies. but you could buy insanely overpriced BD readers and players. 3-4+ thousand dollar range. The BD disks didn't have the ceramic layer, and the BD disks didn't handle scratches AT ALL due to the extremely thin protection over the data layer. it was only marginally better than CD's which have the data foil unprotected on top of the plastic.
and of course they have error correction built in. error correction doesn't do you much good when you get a scratch in the data layer. a hair thing scratch on a BD disks, would cover hundreds of tracks with of it's scratched circularly. That means you can't read from advacent tracks, and depending on the length of the scratch, anythign from tens to hundreds megabytes of data could be gone. and if you scratch the data layer, there's not buffing or polishing, there's no data under the scratch, you scratched the data away. it's not there anymore.
So, yes, the coating was added to protect the data layer. and it's very effective at it's job, it's very hard to scratch or damage a bluray disk. DVD have a 50/50 sandwich with the data layer in the middle, BD is more like 90-95/10-5. And this was a big problem with the early prototypes. and one of the big concerns in the BD vs HD-DVD wars until the ceramic was added. granted there was also a lot more concerns like the fact BD wasn't finished, was fare more expensive to make and so on... but Sony had to much power.