302 THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



6.3 Protection of the Carbon Film 



The conducting film of pyrolytic carbon is extremely thin and, unpro- 

 tected, it is subject to change or damage from several causes. Principal 

 among these are increases in resistance due to gas adsorption, oxidation, and 

 physical damage as a result of unintentional abrasion or other rough hand- 

 ling. To lessen or eliminate these causes of change, protection is given the 

 film in various ways. 



The simplest and most generally accepted method of providing this pro- 

 tection consists in the application of one or more coats of baking varnish 

 over the carbon film. The application of the varnish causes an increase in 

 resistance of the film which must be compensated for in adjusting the re- 

 sistance to tolerance. This increase, which is generally less than one per 

 cent, corresponds roughly to that observed over long periods of time in 

 free air and is probably due to satisfaction by the varnish or its solvent of 

 the adsorptive forces previously discussed. 



While an organic protective film over the carbon surface inhibits time 

 aging of the resistance, it also introduces a complexity in resistor behavior. 

 The varnish film is strongly adherent to the carbon and it has a thermal 

 coefficient of expansion greater than either the carbon or the ceramic core. 

 Further, thermal expansion of the varnish is subject to a form of hysteresis 

 in that stresses introduced by a large temperature change relax only slowly 

 with time after return to the original temperature. These properties have 

 an important bearing on the change of resistance, with time and tempera- 

 ture, of varnish-protected resistors, particularly when the carbon films are 

 thin. 



As noted earlier, stresses set up in the carbon films due to the greater 

 thermal expansion of the core produce changes in the resistance of the films. 

 The stresses set up in the films by expansion or contraction of the protective 

 layer do likewise. Figure 19 illustrates the fact that the stresses set up during 

 curing of the varnish change subsequently with time in such a way that the 

 resistance decreases, approaching an asymptotic limiting value. If the re- 

 sistors are cycled in temperature, the immediate result is an increase in 

 resistance followed by a slow decrease towards the initial value. Cycling of 

 unvarnished resistor units sealed in vacuo or in helium, however, produces 

 no change in resistance nor does shelf aging, thus leaving little doubt that 

 the observed changes are due to the protective varnish finish. 



The carbon films are most stable and reproducible in properties when no 

 solid material is in contact with their exposed surfaces. However, as 

 discussed earlier, the resistance of a carbon film in free air increases with 

 time, due to adsorption of atmospheric constituents. When resistor blanks 

 are hermetically sealed in suitable enclosures filled with air at atmospheric 

 pressure, the magnitude of the resistance increase with time due to the sorp- 



