Strength refers to the resistance to cutting and scratching, khuan of material. The material is rigid, it is resistant to scratches and when it will not scratch surface.
Hardness refers to the resistance to cuts and scratches of the material. The material is very solid, it is not very resistant to abrasion and scratch marks are not born.
Hardness, mean tolerance to cut and scratch (call of the material. The material with the hardness will withstand scratching as and when scratch is no trace.