International Colloquium Tribology
ict
expert verlag Tübingen
125
2022
231
Comparisons of Boundary Lubricant Additive Screening Strategies, Including DOE Methos, Utilizing Twist Compression Tests (TCT), Early in the Lubricant Formulation Process
125
2022
Ted G. McClure
Alexes Morgan
Robert Stubbs
ict2310053
23rd International Colloquium Tribology - January 2022 53 Comparisons of Boundary Lubricant Additive Screening Strategies, Including DOE Methods, Utilizing Twist Compression Tests (TCT), Early in the Lubricant Formulation Process Ted G. McClure Sea-Land Chemical Co./ SLC Testing Services, Westlake, Ohio, USA Alexes Morgan Sea-Land Chemical Co./ SLC Testing Services, Westlake, Ohio, USA Robert Stubbs Sea-Land Chemical, Europe, Ltd., Manchester, United Kingdom 1. Introduction Lightweighting is requiring changes in manufacturing methods and materials. Lubricant additive availability is being altered. Lubricant formulations must be developed quickly in this dynamic environment. Rapid and flexible friction test methods, including twist compression tests (TCT), are being used by formulators, along with statistical mixture design of experiments (DOE) techniques. Mixture DOE methods are used to quantify the relative contributions of ingredients to a response in mixtures, predict the response for untested mixtures of the ingredients, and optimize combinations for the response(s) [1]. However, when beginning a project, formulators have numerous additives available, many, with little or no comparative friction test data available with the surface materials of interest. The purpose of this work is to compare boundary lubricant additive screening strategies using TCT tests, while providing useful test data for an austenitic stainless steel 304, (ISO X5CrNi18-9). 1.1 Additive Screening Strategies Formulators have several options for screening additives. Any of these strategies may be used to select additives for further work. One factor at a time (OFAT) is the most basic. It involves testing one variable at a time. Pairs of additives tested individually and together to identify binary synergies is another common screening approach. Mixture DOE screening methods are used to estimate the relative contributions of ingredients to a response, in mixtures, and predict the response for untested mixtures of the ingredients, using linear models. [1] 1.2 Twist Compression Test (TCT) The TCT is a bench test used to rank lubricant performance under boundary and E.P. conditions. TCT creates lubricant starvation conditions, under high pressure, and sliding contact: a condition common to operations in areas where lubricant film failure is likely to occur [2]. In this test COF vs. time graphs are generated. TCT has been used to compare E.P. additive responses in different base stocks, and wear tracks have been used for to study lubrication mechanisms [3]. Figure 1: Twist Compression Test Schematic 1.3 Experimental A D-optimal non-simplex mixture screening DOE was developed, using Design Expert 12 software, to evaluate a series of nine lubricating additives. This resulted in a 23-test matrix. Fig. 1: Screened additives and design space constraints 54 23rd International Colloquium Tribology - January 2022 Comparisons of Boundary Lubricant Additive Screening Strategies, Including DOE Methods, Utilizing Twist Compression Tests (TCT) TCT was used to evaluate this 23 sample DOE at 13.8MPa interface pressure and 10rpm, 1.2cm/ s velocity. Test materials were 304 stainless steel (ISO X5CrNi18-9) flat sheet and D2 tool steel annular specimens, hardened to approximately 62Rc. Specimens were cleaned with odourless mineral spirits, followed by n-heptane. An excess of lubricant was applied with disposable pipettes immediately before testing. All tests included four repeats. Data was collected at 50Hz. The TCT response used in this analysis was the time until lubricant film breakdown (TBD). The TBD was set at the time when the coefficient of friction (COF) first reached 0.18. This COF threshold was determined by observing annular specimens after testing for adhesion. TBD is used to compare lubricants in their ability to survive the test conditions and prevent adhesive wear. 2. Results and Conclusions A reference blend, containing all nine additives is included in the DOE. The resulting linear model equation in real % units is shown below, with terms in order of decreasing effect on TBD. Screening DOE is limited to linear models, giving little to no information on interactions. The model does provide information useful for specifying the design space to investigate in further work with additives selected. TBD = 0.49*SO(A) + 0.46*SE(E) + 0.33*PEL(B) + 0.32*FA(H) + 0.09*ND(J) + 0.08*Poly(G) + 0.008*VO(C) + 0.02*EHS(D) - 0.01*PEH(F) This is represented visually below in Figure 2. Fig. 2: Trace plot of effect on TBD of increasing or decreasing additive levels relative to reference blend. Based on this analysis, the best candidates for further study and optimization are the sulfurized olefin (SO), Sulfurized ester/ olefin (SE), C-18 ethoxylated phosphate ester, and the polymerized fatty acid (FA). One factor at a time (OFAT) is the most basic and involves testing one variable at a time to identify promising candidates for further work. The factor varied can be different additives, various concentrations of a specific additive, or additives in various base stocks. A variation on this is when one is modifying an existing product. Individual additives may be added to a finished product of interest and tested. Figure 3 is an example of an OFAT screening with 304 SS for TBD. This previous work ranks TBD for, SO and PEL in the same order as the screening DOE. The OFAT approach does not give information on percentage ranges for additives in further work. Fig. 3: OFAT TCT TBD Ranking with 304 SS [4] Screening pairs of additives is another screening strategy used to identify synergies. The additives are tested individually and in combination. This includes the data that is obtained with the OFAT approach, along with information on a single interaction. Figure 4 below is an example of synergy on 304 SS between SO and a 400 TBN calcium sulfonate. Vertical lines show TBD. Fig. 4: TCT COF vs time graphs: TBD synergy [4] Each of these screening strategies have advantages and disadvantages, and may be selected based on what is known about the project status, application and additives 23rd International Colloquium Tribology - January 2022 55 Comparisons of Boundary Lubricant Additive Screening Strategies, Including DOE Methods, Utilizing Twist Compression Tests (TCT) along with the budget, and time limits. Figure 5 summarizes typical effort involved for each strategy. Fig. 5: Testing required summary References [1] Cornell, J.: Experiments with Mixtures, 3 rd Ed. Ch. 1-2, 5. (2002) [2] Schey, J.: Trib. in Metalworking, (1983) 211-212 [3] J. Baltrus, T. McClure, G Bikulčius, S. Asadauskas: Formation of Carbonaceous Nano-Layers under High Interfacial Pressures during Lubrication with Mineral and Bio-Based Oils, Chemija, vol 25 (2014), 3 161-170 [4] T. McClure, Presented STLE Annual Mtg (2017)
