Sputtering endurance

The best coating performance is often obtained with hard substrates. This is consistent with the discussion of friction of thin films in Chapter 5, but with sputtered coatings there may be an additional factor involved. The endurance of sputtered films has been shown to depend critically on the strength of adhesion to the substrate, and if adhesion involves intermingling of sputtered atoms with those of the substrate, it seems likely that the strength of adhesion will be inherently greater with hard substrates. 

The oxidation problem restricts the performance of sputtered coatings even in dry nitrogen at elevated temperatures. Anderson and Roberts tested a sputtered molybdenum disulphide coating in nitrogen containing less than 15 ppm of oxygen, and found a marked deterioration in both friction and endurance at 400 C, which they ascribed to oxidation. 

There is very little information available about corrosion risks with the other lubricating dichalcogenides. Spalvins mentioned that the friction and endurance of a sputtered tungsten disulphide film deteriorated considerably when tested in the atmosphere instead of in vacuum, and the deterioration was associated with formation of sulphuric acid and corrosion of the substrate. These results are not very different from those obtained with molybdenum disulphide in the same period, and there seems to be no reason to expect the problem to be any greater or any less with the other dichalcogenides. 

Spalvins, T., Bearing Endurance Tests in Vacuum for Sputtered Molybdenum Disulfide Films, NASA TM S-3193, (Jan. 1975). 

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