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

The elastics 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 sp3 and sp2 bonds. Even for hard amorphous carbon films of diamond-like nature, the modulus achieves only 30 per cent-50 per cent 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 ist mathematical evaluation. The potential of this non-destructive method has been demonstrated for amorphous carbon films down to a thickness of 10Onm. 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.