Elastic modulus as a measure of diamond likeness and hardness of amorphous carbon films

Abstract

The elastic modulus directly reflects the bonding strength and structure of a solid state. Usually this factor can only be modified slightly for a given material, but in amorphous carbon films large variations in Young's modulus (from less than 100 GPa up to more than 500 GPa) are found. This can be attributed mainly to the varying fraction of sp 3 and sp 2 bonds. Even for hard amorphous carbon films of diamond-like nature, the modulus achieves only 30%–50% of the value of crystalline diamond. Young's modulus was measured by a method based on the determination of the frequency-dependent propagation velocity of ultrasonic surface waves and its mathematical evaluation. The potential of this non-destructive method has been demonstrated for amorphous carbon films down to a thickness of 100 nm. In addition to reference films deposited by various methods, films prepared by pulsed arc processes were investigated systematically. The influence of the technological key parameters (ion flux, substrate temperature and hydrogen content) has been stressed. The changes in elastic modulus reflect directly the diamond likeness of the carbon-carbon network. Hence, Young's modulus represents a suitable parameter for the quantitative characterization and classification of amorphous carbon materials. Apart from certain contributions by compressive internal stresses, the hardness can be estimated to be about one-tenth of the elastic modulus.