INDEX with enhanced degradation

Forbes and Silver [40] published data directly comparing the alkyl ester tri-n-butyl phosphate and the aryl ester tricresyl phosphate. Table 11-13 shows the details of this comparison as well as wear data for the acid ester di-n-butyl phosphate. The wear/load index and the initial seizure load show substantially no discrimination between tributyl phosphate and tricresyl phosphate and very little advantage of the compounded oil over the base oil. The low-load wear test distinctly shows better performance with tricresyl phosphate. The data for di-n-butyl phosphate are at variance with the hypothesis that hydrolytic degradation to the acid ester is the first step in the antiwear action of neutral phosphate esters. On the other hand, Bieber, Klaus and Tewksbury [41] separated acidic constituents from commercial tricresyl phosphate by preparative chromatography, and on blending these constituents back into the original tricresyl phosphate at various concentrations they observed enhancement of antiwear action in the four-ball test, as shown in Fig. 11-7. It should be noted that Bieber et at. worked with only 0.051% phosphorus in the lubricant, which may explain the sensitivity they observed to acid impurities. 

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