Providing Wear Protection for Flat-Tappet (Sliding) Camshaft Followers in Rebuilt Engines

Rebuilt engine ＂break - in＂ is the most critical period for long - term durability for a fiat - tappet valvetrain. Zinc dialkyl dithio phosphate （ZDDP） has proven to be a cost - effective engine oil additive for anti - wear protection and oxidation resistance. But, tightening exhaust emission standards are requiring modem lubricants to have reduced levels of ZDDP additives. If the valvetrain break- in is successfully completed, then modem low- phosphorus lubricants can be used with confidence at subsequent oil change service intervals.     

Evaluating thermal and thermooxidation stability for lubricating oils via microscale methods

Differential thermal analysis (DTA) and thermogravimetry (TG) methods have been developed to evaluate the thermal and thermooxidation stability of different lubricating oils. Measurements are carried out either by heating the oil sample at a constant rate (direct programme) or by keeping it at a fixed high temperature (isothermal programme). Oxygen at a constant flow rate is used for thermooxidation measurements. Tested samples included different viscosity grades: neutral base oils, unadditivated lubricating oil blends, and industrial oils. The mechanism of thermal decomposition for ZDDP and the estimation of its optimum concentration was also considered.     

Tribological,electrochemical, and spectral analysis of zinc dialkyldithiophosphates (ZDDP) of different thermal stabilities

The antiwear properties of five zinc dialkyldithiophosphates, which differed in thermal stability, were evaluated in a 4-ball machine by a preliminary wear track formation method. After testing the additives, the wear resistance of the films formed was measured by the increase in preliminary wear scar diameter over that of the additive-free oil. IR spectroscopy was then used to study the transformation products of additives after thermostatic testing at between 100 and 240°C on steel surfaces, where the interaction products were evaluated by electrode potential method. The interaction of the decomposition products of dithi-ophosphates with metal surfaces was found to be followed by a change in electrode potential. For the tested additives, a relationship is shown between antiwear effectiveness of the film on a friction track during tribological tests and the electrode potential value of the steel sample. Different parts of the relationship between the electrode potential and temperature relate to different stages of dithiophosphate decomposition measured by IR spectroscopy. It can be said that chemical modification of the steel surface which produces an improvement of antiwear properties, starts at the decomposition temperature of the dithiophosphate neutral salt.     

The influence of base oil properties on the efficiency of action of ZDDP-type additives

This paper deals with the analysis of the influence of base oil viscosity on the ability of ZDDP-containing lubricants to form protective layers (especially antiwear layers). The chemical structure of additives used was characterised according to their thermal stability, determined by differential scanning calorimetry (DSC). The additives were added to base oils of various viscosities. The prepared lubricants were tested using a four-ball machine, and analysed by DSC. The relationships between base oil viscosity and seizure load (P_t), weld load (P_w) and DSC test results are presented and discussed. The mechanism of interaction between ZDDP and the base oils was analysed by the use of a mathematical model.     

Tribochemistry and the development of AW and EP oil additives — a review

The antiwear (AW) and extreme pressure (EP) properties of chlorine, sulphur, phosphorus-containing organic compounds and zinc dialkyldithiophosphate (ZDDP) oil additives are affected greatly by their reactive ability to the metal surface, as well as by the chemical composition, chemical state, physical and mechanical properties of the protective films formed. Over four decades, research has taken place on boundary lubrication, and a much better understanding of the AW and EP action mechanism of one additive by itself in base oil has been obtained, and much more knowledge on the relationship between wear and decomposition of additives, adsorption and reaction of the additive or its decomposed products with metal, has been gained. A series of analytical methods and a great number of modern surface analytical tools have been set up and used for research in this area. The problems and some suggestions for the future study on boundary lubrication of oil additive are proposed.     

A new approach to the Noack volatility test

This paper reports on the method and preliminary results of a significant modification of the Noack volatility test. This modification permits collection and examination of the volatile products as well as of the residue. The analysis of such volatiles by chromatography and spectroscopy revealed surprising information that is especially relevant in the face of increasingly difficult automotive and heavy duty engine emission limitations. The data also showed very close agreement with the Unified Test Method for the Noack Volatility Test now covered by CEC L-40-T-87. Important to operators, the authors' modified Noack apparatus avoids the use of the toxic Wood's Metal of the earlier Noack apparatus.     

X-Ray Absorption Spectroscopy and Morphology Study on Antiwear Films Derived from ZDDP Under Different Sliding Frequencies

X-ray absorption near-edge structure (XANES) spectroscopy has been used to characterize the chemistry of antiwear (AW) films generated from mineral base oil containing a zinc dialkyl dithiophosphate additive. These films were formed on rubbed steel surfaces with a reciprocating boundary contact using different sliding frequencies. The phosphorus L-edge XANES spectra show that these films have slightly different chemical natures. Longer chain polyphosphates were present on the steel surface prepared at the higher sliding frequencies. The surface morphology of these films was investigated using atomic force microscopy. These images show that the surface morphology of the AW films changes with the sliding frequency. Round and bigger antiwear pads were formed at a lower frequency while higher frequencies resulted in thinner films and flattened surfaces. Nanomechanical properties of these antiwear films were investigated by nanoindentation measurement and the elastic moduli extracted from force–displacement (f–d) curves are similar for all antiwear films, 100 ± 10 GPa.     

Tribochemistry of ZDDP in molecular orbital calculations

The molecular orbital parameters of zinc dialkyldithiophosphate (ZDDP) and several metal-atom-cluster models were calculated. The nature and the strength of the interactions between the ZDDP molecules and different metal surfaces are analysed and discussed with the use of frontier orbital theory. By comparing the highest occupied molecular orbital energy (E_HOMO) and the lowest unoccupied molecular orbital energy (E_LUMO) of the ZDDP and the atoms cluster models of Al₆, Cu₆, and Fe₅, it is concluded that ZDDP behaves as an excellent boundary lubricant additive at the interface with iron. The derived molecular orbital parameters illustrate the advantages for tribochemistry studies.     

X-ray absorption near-edge structure analysis of the chemical environment of zinc in the tribological film formed by zinc dialkyl dithiophosphate decomposition on steel

Calculation of X-ray absorption near-edge structure (XANES) has been used to investigate the chemical nature of zinc in thermal films formed on the surfaces of steel coupons by the additive zinc dialkyl dithiophosphate (ZDDP) in oil solution. Carbon K, oxygen K, and zinc L edge XANES spectra were used to characterize the thermal films and a model film formed by addition of cumene hydroperoxide (CHP) to the ZDDP. Thermal films formed by heating at up to 100 °C for 5 h showed little change from unreacted ZDDP. At 160 °C for 5 h, the ZDDP underwent thermal solution decomposition with similarities to the oxidation induced by CHP. The Zn L edge XANES spectra were compared to calculated spectra of the model surface species ZnO and ZnS in the wurtzite structure and ZnS in the zinc blende structure. Agreement between experiment and simulation is found for the 160 °C thermal film with a roughly two monolayer film of ZnO and for oxidation by CHP with a three to four monolayer film of ZnS.