Synergistic And Antagonistic Effect Of Sulfur-based Additives On The Performance Of Greases With ZDDP And PTFE
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Lithium-base greases constitute about 50% of the grease market. The current trend is towards developing greases with longer working life and wider application flexibility. To address this concern, greases are blended with antiwear and extreme pressure (EP) additives that serve to reduce wear and increase the operating life of the machinery by creating a low strength, shearable film on the moving surfaces. In the current study, greases were developed to address the need for both performance as prescribed by the ASTM standards D2266 and D2596, respectively. The actual bearing conditions vary significantly with load, rpm and duration of test. These three factors have important consequences such that the load dictates the activation of chemistries, the rpm controls the entry of grease into the contact point and the durability of grease is governed by the test duration. Accordingly, greases with good antiwear and extreme pressure properties were developed and their behavior studied under both standard ASTM conditions as well as by varying test conditions of load, rpm and test duration. For achieving both wear and load-bearing capacity, a multitude of additives have been investigated with varying degrees of success. The major among these are MoS₂ and graphite to impart extreme-pressure properties to greases, polytetrafluoroethylene (PTFE) for friction reduction and ZDDP for wear reduction. With the intent to replace MoS₂in greases, the synergistic combination of ZDDP and PTFE has proven to provide superior wear and EP performance and appear to be more durable than their MoS₂counterparts. The interactions among these additives have been analyzed through design of experiments using the factorial and response surface design approaches. This approach would give the grease-maker preliminary, yet critical, information about suitable chemistry to employ under specific conditions of load, rpm and duration of test.To develop greases that provide both wear and weld protection, the sulfur additives probably the earliest known EP compounds in lubricants, were tested for interactions with ZDDP and PTFE combination. Since the wear or EP action involves the interposition of a film between mating metal surfaces in order to prevent metal-to-metal contact, the film was analyzed for compositional and morphological features through extensive SEM/EDS experiments. It was deduced that formation of sulfides was important for wear and phosphate pads for load-bearing characteristics. Through this work, four different aspects of the greases have been identified; (1) the design of experiments approach is used to develop an understanding of the interaction of the role played by various additives singly or in conjunction with other additives, (2) the role played by test conditions on the evaluation of grease properties is dealt with, (3) the role played by sulfur chemistries when used with solid lubricants like PTFE is addressed and (4) an understanding on the interaction of various chemistries is established with the goal of developing a Universal grease.