288 



THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



the dependence of lattice constants on crystal size.^^-^^ The difference be- 

 tween the measured density and that computed from lattice constants in- 

 dicates that pyrolytic carbon is slightly porous, and this porosity obscures 

 the correlation otherwise to be expected between density and crystal size. 



5.2 Hardness 



The scratch hardness or micro-hardness of carbon films deposited on fused 

 silica plates was determined with the Bierbaum Micro-character^^. Fused 

 siUca, with a Moh's hardness of 7, has a microhardness of 1980, while silicon 

 carbide with a Moh's hardness of 9-\- gave an average value of 7000. Re- 



20,000 



18,000 



16,000 



14,000 



tn 



^ 12,000 



Q 



cr 

 < 



I 

 o 



o 



10,000 



4000 



925 



950 975 1000 1025 1050 1075 



TEMPERATURE OF PYROLYSIS IN DEGREES CENTIGRADE 



Fig. 11 — Dependence of the hardness of pyrolytic carbon films on the temperature of 

 formation. 



peated measurements of thick films of carbon produced at 1000 deg C and a 

 37 per cent methane concentration gave values for the micro-hardness 

 ranging from 19000 to 19300, practically equivalent to diamond, or about 

 9.8 on Moh's scale. 



The hardness of pyrolytic carbon is dependent on the pyrolyzing con- 

 ditions. Figure 1 1 shows the dependence of hardness on pyrolyzing tempera- 

 ture for a 37 per cent methane concentration and illustrates the distinct 

 maximum between 1000 deg C and 1025 deg C. When the temperature of 

 the furnace is held fixed and specimens are prepared at progressively higher 

 hydrocarbon concentrations, the micro-hardness increases monotonically. 

 Values as high as 50,000 have been observed, and, in some instances, no 

 perceptible mark was produced by the diamond point. The hardness was 



