GT, in my limited experience with plating, I am not aware that it would effect hardness to any extent that could be measured by a hardness tester. The plating is most often used to prevent oxidation and occasionally to improve "slipperiness" The later friction reduction often involves chrome, chome nickle or chrome-nickle-molybdenum. The identification of these plating compounds is usually done by chemical testing that will discolor the plating.
You might find what you want on this website
http://www.finishing.com/1200-1399/1269.shtml
I linked into a likely topic, you will have to surf a bit.