Thermal Decomposition of Tricresylphosphate Isomers on Fe

Triresylphosphate (TCP) [(CH₃-C₆H₄O)₃P=O] is the most commonly tested vapor phase lubricant and is readily available as a mixture containing all three isomers (ortho-, meta-, and para-) of the cresyl groups. The surface chemistry of ortho-, meta-, and para-TCP on Fe foil was studied in order to examine the possible differences in decomposition mechanisms among the isomers. All three TCP isomers decomposed by the same reaction mechanisms and with roughly the same kinetics on Fe. Upon heating, they decomposed on the Fe surface to deposit carbon and phosphorous and produce gas phase H₂, CO, toluene and cresol. The amounts of carbon and phosphorous deposited onto the Fe surface by TCP (arylphosphate) decomposition were compared to those deposited by decomposition of tributylphosphate (TBP), an alkylphosphate. Thermal decomposition of all three TCP isomers deposits substantial amounts of carbon ono the Fe surface while TBP decomposition does not. We suggest that it is the lack of -CH bonds in the methylphenoxy intermediates and the high reactivity of the tolyl intermediates generated during TCP decomposition are the primary reasons for the differences in behavior of TCP and TBP and the root cause of the differences in behavior of arylphosphates and alkylphosphates as vapor phase lubricants.     

The surface chemistry of vapor phase lubricants: tricresylphosphate on Ni(1 0 0)

Tricresylphosphate (TCP) is known to serve as an excellent high temperature vapor phase lubricant with some metals such as Fe but not with others such as Ni. The surface chemistry of m-TCP has been studied on clean and phosphorous covered Ni(1 0 0) surfaces in order to understand the differences between its reactivity on Fe and Ni. Our results show that upon heating to 800 K m-TCP decomposes on the clean Ni(1 0 0) surface to deposit carbon and phosphorous with the evolution of H₂, CO, benzene, and toluene into the gas phase. During further heating to 1000 K, all the carbon on the surface dissolves into the Ni bulk leaving only phosphorous. The adsorption and heating of m-TCP on the phosphorous-modified Ni surface does not result in significant decomposition. The clean Ni substrate is able to activate TCP decomposition in much the same way that it decomposes on the clean Fe surface. The lack of significant differences in the chemistry of TCP on the clean metal surfaces cannot explain the fact that TCP vapor will form thick lubricating films on Fe but not on Ni.     

Activation of the SiC surface for vapor phase lubrication by Fe chemical vapor deposition from Fe(CO)5

The feasibility is demonstrated of a new approach to the vapor phase lubrication of ceramics using organophosphorus compounds. The surface of SiC is shown to be unreactive for the decomposition of trimethylphosphite, (CH₃O)₃P, a simple model for organophosphorus vapor phase lubricants such as tricresylphosphate. In order to activate the surface of SiC it has been exposed to Fe(CO)₅ at a temperature of 600 K. Chemical vapor deposition serves as a means of depositing Fe on the SiC surface. The Fe-modified SiC surface is then shown to induce the decomposition of adsorbed (CH₃O)₃P. The mechanism of (CH₃O)₃P decomposition is similar to that observed on Fe(110) surfaces modified by the presence of oxygen. It is initiated by P–O bond cleavage to produce adsorbed methoxy groups, CH₃O_(ad), which then decompose by -hydride elimination resulting in H₂, CO, H₂CO, and CH₃OH desorption. It is suggested that chemical vapor deposition of metals using high vapor pressure metal-containing compounds such as Fe(CO)₅ can serve as a mechanism for continuous, in situ activation of ceramic surfaces for vapor phase lubrication in high temperature engines.