I know, I know, It's a fine balance between realism/believability and creative freedom. But I *personally* would argue throwing some existing material into some other existing material to create some non-existent material without a plausible explanation how it works is stretching artistic freedom just a little too much. But that's jsut the opinion on this one guy here, so go ahead if you think this is what would be best
I can assure you putting diamond dust in some steel is not going to help in any way towards making the steel stronger. Throwing strong stuff into weaker stuff is not always going to result in something that was stronger than before. If anything, I'd probably make a grouty mess of the steel... though using magic to roll sheets of graphite into carbon nanotubes and creating a sort of mesh, almost like composite plastics, would help. In fact, nanotubes have been found in damascus steel and it is hypothesized that this is the reason why the material is so damn tough.
Could you make a sideview of the helmet? Currently, it seems to me like it is some kind of hybrid between a barbute and an armet? Both of which were made in the 15th century BTW, so no problem. It might be nice if the cheekguards were on hinges and were tightened around the lower face like with an armet. Also, the poor man might need a little throat protection. It would be a shame if such a heavily armored dude died because some peasant poked him in the throat with a sharp stick.
I don't wanna whine, but the upper-left piece of rope doesn't seem to go to the upper piece of rope in the rightmost loop, if you see what I mean... Now, I dont know who made this when, but I just thought I'd point it out...
So in short, NO leather was not used as armor. However, rawhide was used as armor after it was treated somehow to make it harder. This could be done by boiling it, treating it with chemicals or lacquering it. Now, we know fantasy doesn't care about this at all, and most players don't know about this and as such don't care. Do we care though?