Solution decomposition of zinc dialkyl dithiophosphate and its effect on antiwear and thermal film formation studied by X-ray absorption spectroscopy

A detailed study was undertaken to investigate the effect of ZDDP oil solut ion chemistry changes due to thermal decomposition, on antiwear and thermal film chemistries, film thickness and wear. P and S K- and L-edge X-ray abs orption near edge structure (XANES) spectroscopies were used to characteriz e film chemistry, and 31-P NMR spectroscopy was used to monitor the ZDDP oi l solution chemistry. P L-edge XANES results of antiwear films prepared fro m ZDDP oil solutions preheated at 150 degrees C for various lengths of time , showed a decrease in polyphosphate chain length as ZDDP thermal solution decomposition progressed. Film thickness and wear increased with increasing ZDDP oil solution preheating time (decomposition). Antiwear films formed f rom ZDDP oil solutions preheated at a higher temperature (200 degrees C) fo r 1 and 3 h, yielded thinner films and showed catastrophic wear. 31-P NMR s pectra showed that no oil soluble P containing species were left in solutio n after heating at 200 degrees C for Ih and yet the 200 degrees C, 6 h anti wear film was found to be as thick as that generated from previously unheat ed solution. Wear was comparable to that obtained by using base oil alone. These films were found to be of short chain polyphosphate structure. ZDDP o il solution chemistry was also shown to have an effect on the chemistry of thermally generated films. Film chemistry changed with ZDDP oil solution he ating time. A linkage isomer of ZDDP is proposed as an important precursor for film formation after analysis and comparison of an oil insoluble ZDDP d ecomposition product with the thermal and antiwear film chemistries. As wit h the related antiwear films, thermal film thickness was also shown to incr ease dramatically when ZDDP decomposition in solution increased. An overall mechanism for film formation, taking into account the ZDDP linkage isomer and the deposition of colloidal polyphosphate material, is proposed. (C) 19 99 Elsevier Science Ltd. All rights reserved.