1 KINETICS OF - EXTREME-PRESSURE LUBRICANT ADDITIVES. by Andrew Allan Bayles A thesis submitted for the degree of DOCTOR OF PHILOSOPHY of the University of London and also for the DIPLOMA OF MEMBERSHIP of the IMPERIAL COLLEGE SEPTEMBER 1975 Lubrication Laboratory, Department of Mechanical Engineering, Imperial College of Science and Technology, London, SW7 2BX. ii ABSTRACT This thesis has investigated the kinetics of the reaction between steel surfaces and a simple E.P. oil, Dibenzyl Disulphide in cetane, by examining the friction-temperature characteristics of the metal-lubricant system on a variable, slow-speed, ball-on-triplane machine. A theoretically derived relationship linking sliding speed with transition temperature has been confirmed experimentally and values for the Activation Energy of the reaction and the Heat of Adsorption of the additive to the surface have been obtained. A mechanism is proposed to explain the observed kinetic behaviour. The function of the E.P. film in preventing scuffing failure at high speed has been examined from a kinetic stand point. It is suggested that lubricant failure is governed by a balance between the rate of reaction of the additive with the surface and the rate of removal of the E.P. film from the surface. iii ACENOWT7DGEMENTS The author is greatly indebted to Professor Cameron for his guidance and sunervision, throughout the course of this nroject; to Mr. Reg Dobson for his encouragement and invaluable technical assistance; and to the members of the laboratory, who have assisted in numerous ways. Sincere thanks are also due to Ruston, Paxman Diesels Ltd., and to the Lubrication Engineers Ltd., for providing the funds which made this study possible. iv CONTENTS Page Title page Abstract ii Acknowledgements iii Contents iv List of Figures vii Nomenclature xi Chapter 1 Literature Review 1 1.1 Introduction 1 1.2 Extreme-Pressure Lubrication 2 1.3 Composition of the E.P. film 4 1.4 Investigations into E.P. Activity 6 1.5 Effects of Load, Speed and Temperature on E.P. film formation 8 1.6 Mechanism of E.P. Action 11 1.7 Kinetics of the Reaction between E.P. Additives and Iron Surfaces 15 1.8 Concept of the Research Programme 19 Chanter 2 Annaratus and Preliminary Investigations 20 2.10 Introduction 20 2.20 Apparatus 20 2.21 Materials 28 2.22 Experimental Procedure 29 2.30 Accuracy 30 Page 2.40 Results and Discussion - I 39 2.41 E.P.M.A. Introduction 39 2.42 Procedure . 41 2.43 Limitations of Results 42 2.44 Results 42 2.50 Results and Discussion - II 49 2.51 Powder Analysis 53 2.60 Conclusions 56 3 Chapter The Effects of Sliding Sneed on the Lubrication of EN 31 Steel by Dibenzyl Disulphide 57 3.10 Introduction 57 3.20 aperimental 57 3.21 Speed Range 57 3.22 Heating Rate 58 3.23 Materials and Lubricant 58 3.30 Accuracy 65 3.40 Results 65 3.50 Discussion 68 3.60 Conclusions 74 4 Chapter The Effects of Additive Concentration on the Lubrication of EN 31 Steel 75 4.10 Introduction 75 4.20 Results 84 4.30 Discussion - I 85 4.40 Discussion - II 89 4.50 Discussion - III 92 vi Page 4.60 Conclusions 95 Chapter 5 Investigations into High-Speed Failure 97 5.10 Introduction 97 5.20 Lubricant Failure 97 5.30 Apparatus 103 5.31 Materials 104 5.32 Procedure 104 5.33 Accuracy 107 5.40 Results 107 5.50 Discussion 108 5.51 The Mechanism of Scuffing Failure 115 Chapter 6 Conclusions and Suggestions for Future Work 118 6.10 Conclusions 118 6.11 Slow Speed Work (Chapters 2,3,and 4) 118 6.12 High Speed Tests (Chapter 5) 119 6.20 Future Work 120 Appendix 122 References 124 vii LIST OF FIGURES Chapter 2 Page 2.1 Ball-on-triplane machine. 21 2.2 Ball-on-triplane machine, schematic diagram. 22 2.3 Ball-on-Triplane Machine: Peg Positions. 23 2.4 Peg-cup. 25 2.5 Heater. 26 2.6 Drive System. 26 2.7 Typical Pen Recorder Trace.DBDS on EN 31 steel. 27 2.8 Friction vs Temperature. DBDS on EN 58 steel, 31 1 r.p.m. 2.9 Friction vs Temperature. DBDS on EN 31 steel, 32 1 r.p.m. 2.10 Friction vs Temperature. DBDS on EN 31 steel, 33 1 r.p.m. Heat rate dependence. 2.11 Friction vs Time. Pure Cetane on EN 31 steel, 34 1 r.p. 2.12 Friction vs Temperature. Unlubricated EN 31 35 steel,1 r.p.m. (16.7N load). 2.13 Friction vs Temperature. Unlubricated EN 31 36 steel,1 r.p.m. (62.3N load). 2.14 Unlubricated EN 31 steel,1 r.p.m. Reversibility 37 Trace. 2.15 Friction vs Temperature. DBDS on EN 58 steel 38 1 r.p.m. Reversibility of friction with temperature. viii Page 2.16 Electron Probe Microanalyser. 40 2.17 E.P.M.A. Count across track rubbed in DBDS solution. 2.18 Back-Scattered Electron Image. (B.E.I.). 44 2.19 Absorbed Electron Image. (A.E.I.). 44 2.20 X-ray Image. (X.R.I.). 44 2.21 E.P.M.A. Count outside track, rubbed in DBDS 45 solution. 2.22 E.P.M.A. Count across track rubbed in pure 46 cetane. 2.23 Debye-Scherrer Powder Method: 52 (a ) Relation of film to specimen and incident beam. (b ) Appearance of film after exposure. 2.24 Friction vs Film Thickness Reproduced from (73). 55 Chapter 3 3.1 Friction vs Temperature. 1% DBDS solution on 1 EN 31 steel ; /50 r.p.m. 59 3.2 Friction vs Temperature. 1% DBDS solution on 1 EN 31 steel, /10 r.p.m. 59 3.3 Friction vs Temperature. 1% DBDS solution on 1 EN 31 steel /3 r.p.m. 60 3.4 Friction vs Temperature. 1% DBDS solution on EN 31 steel, 1 r.p.m. 60 3.5 Friction vs Temperature. 1% DBDS solution on EN 31 steel, 3 r.p.m. 61 3.6 Friction vs Temperature. 1% DBDS solution on 1 EN 31 steel; 3 /3 r.p.m. 62 ix Page 3.7 Friction vs Temperature. 1% DBDS solution on 1 En 31 steel , 6 /2 r.p.m. 63 3.8 Friction vs Temperature. 1% DBDS solution on EN 31 steel , 10 r.p.m. 64 3.9 Logi° Sliding Speed vs 1/TT EN 31 steel. 69 3.10 Significance of Activation Energy, 70 1 3.11a Logi() Reaction vs /T, Mild steel. 72 1 b Logio Reaction vs /T,Stainless steel. 72 Chapter 4 4.1 Dependence of Friction Transition on Sliding Speed. 2% DBDS. 76 4.2 Dependence of Friction Transition on Sliding Speed. 0.5% DBDS, 77 4.3 Dependence of Friction Transition on Sliding Speed. 0.2% DBDS, 78 4.4 Logi° Sliding Speed vs 1/TT, 2% DBDS. 80 4.5 Logic) Sliding Speed vs 1/TT, 0.5% DBDS. 81 4.6 Logic) Sliding Speed vs 1/TT, 0.2% DBDS. 82 1 4.7 Combined Logic) Sliding Speed vs /TT.Effect of Concentration. 83 1 4.8 Logi° Rate Constant vs IT (reproduced from (34)). 86 4.9 Friction vs Temperature. 1% DBDS + 1% Toluene on 1 EN 31 steel. /10 r.p.m. 94 Chapter 5 Page 5.1 Four-Ball Apparatus. 102 5.2 Dependence of Scuff load on Additive Concentration. 105 5.3 Dependence of Scuff load on Sliding Speed. 106 5.4 Rate vs Concentration, Dyphenyl Disulphide in White Oil (reproduced from (38)), 113 5.5. Film thickness vs Sliding Speed, DBDS in White Oil (reproduced from (38)), 113