Tribological Properties and Lubrication Mechanism of Lithium Greases Containing ZDDP Additives

The lubrication mechanism of ZDDPs (zinc dialkyldithiophosphates) as a kind of common extreme pressure anti-wear agents in greases has been the focus and difficulty of research. Four types of ZDDPs (zinc dibutyl dithiophosphate, zinc diheptyldithiophosphate, zinc dioctyl dithiophosphate and zinc dibutyl octyl dithiophosphate) with different carbon chain lengths were selected as grease additives. By using SRV friction and wear testing machine, SEM and XPS, their tribological properties under different operating conditions (temperature, load and frequency) and the film formation mechanism of ZDDP in greases under reciprocating sliding conditions were systematically investigated. The upper test ball was made of AISI 52100 steel, φ10 mm, with a hardness of about 730 HV, and the lower test disc was made of AISI 52100 steel, φ24×7.5 mm, with the same hardness of about 730 HV and a roughness less than 0.084 μm. The three-dimensional profile instrument was utilized to observe the wear morphology of the test discs and to calculate the wear volume. The base grease used in this experiment was the lithium grease, and the base oil was PAO8 (the base oil had a kinetic viscosity of 8 mm²/s at 100 ℃). The results showed that the additive of ZDDPs could improve the friction reduction and anti-wear properties of the base grease. Frequency, load and temperature had an effect on the friction reduction and anti-wear performances of greases containing ZDDPs, and the influence of temperature was particularly. The harsher the operating conditions (high-frequency, high-load, high-temperature) were the better friction reduction and anti-wear abilities of greases with containing ZDDPs were. The reason for this may be that the more intense friction produces more frictional heat, the more conducive to the growth of the friction tribofilm. For ZDDP greases with different carbon chain lengths, the extreme pressure performance of ZDDP decreased with the increasing of alkyl carbon chain length under grease lubrication. Their tribological properties showed difference under most operating conditions. Based on these above results, the improvement of the anti-friction and anti-wear properties of the greases containing ZDDPs was mainly due to the growth and formation of friction chemical films. During the reciprocating sliding conditions, the friction film growth process had been observed by SEM, and the abrasion surfaces from the upper test ball at different test times were analyzed by XPS and EDS. The test results showed that the friction film morphologies on the ball surface varied with the friction time. In the reciprocating sliding conditions of the friction film growth process, the initial friction film was thin and uniform, then followed by the emergence of a number of small solid particles. With the increasing of the operating time, the accumulation of small particles evolved into a loose friction film, which would be transformed to the dense friction film after a long time. Combined with the elemental analysis results of EDS, it was assumed that the deposition or reaction of sulfur elements to form lubricant film mainly occurred in the early stage, while the deposition or reaction of phosphorus and zinc elements continued throughout the entire operation process. This paper was of reference significance for the tribological researches of ZDDP-containing greases under reciprocating sliding conditions, and provided theoretical support for ZDDP to improve the friction reduction and anti-wear performance of greases.