hardness of DLC coatings depend, among other things, on its hydrogen content. With hydrogen in the range of 15-20at% hardness can be in the range of 15-30GPa as already described by Gedvidas. With lower hydrogen content of 5 ... 2at% hardness can even increase upto 50GPa. While high hydrogen content of 30-40at% will decrease hardness to be below hardness of quartz.