THE EFFECTS OF ZDDP AND ASHLESS ANTIWEAR ADDITIVES ON THE FRICTION AND WEAR CHARACTERISTICS OF TRIBOLOGICAL COATINGS ON STEEL EDWARD NG SOO YONG NATIONAL UNIVERSITY OF SINGAPORE 2014 THE EFFECTS OF ZDDP AND ASHLESS ANTIWEAR ADDITIVES ON THE FRICTION AND WEAR CHARACTERISTICS OF TRIBOLOGICAL COATINGS ON STEEL EDWARD NG SOO YONG (B. Eng. (Hons.), NUS) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF MECHANICAL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2014 Declaration I hereby declare that the thesis is my original work and it has been written by me in its entirety. I have duly acknowledged all the sources of information which have been used in the thesis. This thesis has also not been submitted for any degree in any university previously. _________________________ Edward Ng Soo Yong 18 July 2014 i Preface This thesis is submitted for the Degree of Doctor of Philosophy in the Department of Mechanical Engineering, National University of Singapore, under the supervision of Dr Christina Lim and Dr Sujeet Kumar Sinha (Indian Institute of Technology Kanpur, India). All the work in this thesis is to the best of my knowledge original unless reference is made to other work. No part of this thesis has been submitted for any degree or qualification at any other Universities or Institutions. Part of this thesis has been published/ accepted and/or under review for publication as listed below: Journal Papers: 1. Ng E, Sinha SK. Effects of Antiwear Additives in the Base Oil on the Tribological Performance of Hydrogen-Free DLC Coating. Manuscript accepted by Industrial Lubrication and Tribology on 31 January 2013. (DOI:10.1108/ilt-04-2012-0037.R1). 2. Ng E, Sinha SK, Narayan A, Satyanarayana N, Lim C. Tribological Performances of ZDDP and Ashless Triphenyl Phosphorothionate (TPPT) Additives in Base Oil for Cr-N Coated Steel. Manuscript accepted by Tribology - Materials, Surfaces & Interfaces on 26 February 2014. (DOI: 10.1179/1751584X14Y.0000000072). 3. Ng E, Lim C, Sinha SK, Satyanarayana, N, Zhang Z. Tribological Performances of ZDDP and Ashless Triphenyl Phosphorothionate (TPPT) as Lubricant Additives on Ti-N and ii Ti-Al-N Coated Steel Surfaces. Manuscript accepted by Tribology - Materials, Surfaces & Interfaces on 15 July 2014. Other Research/Conference Papers and Presentations (as employee of BASF South East Asia Pte Ltd from 2007 to 2011): 1. Ng E, Watanabe T, Huang R, Sharma A. Ashless, Hydrolytically Stable, Load Carrying and Antiwear Agent. International Tribology Conference, Hiroshima, Japan. Oct 30 – Nov 3, 2011. 2. Ng E, Egiziaco M, Chasan D, Fasano P. Handling the Impact of Biodiesel Fuel on Lubricants. The 17th Annual Fuels & Lubes Asia Conference, Singapore, Mar - 11, 2011. 3. Ng E, Huang R, Zhou J. Ashless Multi-Functional Friction Modifier for Modern Engine Oils. Lubricant Technique & Economy Forum, Dalian, China, Sep 15 - 17, 2010 / Lubricating Oil 2011; 26 : 25-30. 4. Chasan D, Ng E. Phenothiazine Derivatives as Antioxidants for Lubricants. World Tribology Congress IV, Kyoto, Japan, Sep 11, 2009 / Tribology Online 2010; : 220-224. 5. Choudhary A, Kumar T, Ng E. Multi-Metal Corrosion Inhibitor for Aqueous Media. 7th International Symposium on Fuels and Lubricants (ISFL), New Delhi, India, Mar - 12, 2010. 6. Ng E, Nehls E. Additive Technology for EU Ecolabel Formulations. The 15th Annual Fuels & Lubes Asia Conference, Hanoi, Vietnam, Mar - 6, 2009. iii Summary Over the years, various types of surface coatings have been developed to protect substrates or base materials from wear and corrosion. Examples of such advanced coatings include diamond-like carbon (DLC), chromium nitride (Cr-N), titanium nitride (Ti-N) and titanium aluminium nitride (Ti-Al-N). At the same time, there has been a growing trend towards ‘‘greener’’ lubricant additives, driven by environmental legislation. For decades, zinc dialkyl dithiophosphates (ZDDP) have been extensively used in engine oil and industrial lubricants as antiwear agents, antioxidants and corrosion inhibitors. However, the pressure to reduce sulphated ash, phosphorus and sulfur (SAPS) content in engine oils is increasing as SAPS-containing additives have a detrimental effect on exhaust after-treatment devices fitted in modern vehicles. For hydraulic applications, the use of a zincfree fluid is required in many cases. It has been reported that heavy metals like zinc can be hazardous to human health. As a result, zinc– containing lubricants are not considered safe to be used in the food and agricultural industries. As environmental regulations become more stringent, it is increasingly important and urgent to find a substitute that is more environmentally friendly (i.e. with zero or acceptably low SAPS content). It is also recognized by equipment manufacturers, additive and lubricant companies as well as research institutes that there is a need to review which materials and lubricants are being used in partnership in engineering systems to capitalize on the synergies existing between iv surfaces and lubricants. Similarly, there are some compatibility issues that need to be identified and an appreciation of such challenges can help engineers select an optimal lubrication system and avoid counterproductive results. Various investigations have been carried out in the area of tribological coatings with regard to antiwear additives but they have not always come to the same conclusions. On many occasions, the evaluation included only ZDDP but not greener alternatives like ashless triphenyl phosphorothionate (TPPT). The objectives of this study are to investigate the influence of ZDDP as well as ashless TPPT - a more environmentally friendly antiwear additive, on the durability of state-ofthe-art tribological coatings (i.e. hydrogen-free DLC, Cr-N, Ti-N and TiAl-N); and to postulate the likely wear protection mechanisms based on experimental evidences and supporting analytical information. The investigation for hydrogen-free DLC coatings was carried out using a disk-on-cylinder tribometer (with a line contact). The disks and cylinders were made of AISI 52100 bearing steel, and the normal load was 30 N. The base oil used was API Group II mineral base oil. The lubricants evaluated included the base oil with no additive, base oil with wt% of ZDDP, and base oil with wt% of TPPT. It was found that both ZDDP and TPPT exhibited a negative impact on the friction behaviour of the coating. Also, it was demonstrated that ZDDP had a negative influence on the antiwear property, whereas TPPT helped to increase wear resistance of the DLC coatings. v For Cr-N coatings, experiments were performed using the same disk-on-cylinder tribometer, with a slightly higher normal load of 40 N. It was shown that both ZDDP and TPPT helped to lower the friction on surfaces. Between the two antiwear additives, ZDDP exhibited better friction reduction benefit than TPPT. Experimental results also indicated that the wear resistance property of the Cr-N surface could be enhanced by both ZDDP and TPPT (to a lesser extent). It is proposed that friction is influenced substantially by the shear strength of the film formed from the additives. The higher coefficient of friction obtained for TPPT compared to that of ZDDP was likely due to higher shear strength of the film derived from TPPT. As the Ti-N and Ti-Al-N coatings (2300 HV and 3300HV respectively) were harder than DLC and Cr-N coatings (2000 HV and 2100 HV respectively), it was recognized that the investigation needed to be performed under more severe test conditions. Therefore, a new pin-on-cylinder tribometer (with a point contact) was specially designed and fabricated to evaluate the influence of the antiwear additives on the friction and wear properties of Ti-N and Ti-Al-N coatings. Also, the normal load was increased to 150 N. It was observed that both ZDDP and TPPT (to a lesser extent) increased the friction coefficient on the Ti-N and Ti-Al-N surfaces. It was also demonstrated that ZDDP (to a greater extent) and TPPT helped to reduce wear on the Ti-N and Ti-AlN surfaces. It is proposed that the relatively higher coefficient of friction measured for ZDDP compared to that for TPPT was potentially caused by higher shear strength of the ZDDP-derived film. It was also vi found that the presence of aluminium in the Ti-Al-N coating had reduced the formation of Ti2O3 while increasing the content of TiON, thereby improving its oxidation resistance and antiwear property. In this regard, no significant impact from ZDDP or TPPT was observed. Based on the overall findings, it is concluded that TPPT can perform adequately well as a suitable and greener substitute for ZDDP for enhancing wear protection of hydrogen-free DLC, Cr-N, Ti-N and TiAl-N coatings. However, it is suggested that lubricants developed for equipment with hydrogen-free DLC, Ti-N or Ti-Al-N coated parts should contain suitable friction modifiers to compensate for the negative impact on friction reduction caused by the use of ZDDP and TPPT. vii Acknowledgements First of all, I would like to express my immense gratitude to my supervisors Dr Christina Lim and Dr Sujeet K. Sinha for their dedicated supervision, guidance, and advice without which I would not have made any progress in this PhD course. Next, it is a tremendous blessing to have Dr Nalam Satyanarayana as a close mentor. His numerous insights and suggestions have helped me significantly to improve my research and analytical work. The support and assistance rendered by laboratory colleagues Mr Thomas Tan, Mr Abdul Khalim Bin Abdul, Mr Ng Hong Wei and Mr Abdul Malik Bin Baba has been nothing short of excellent and is therefore greatly appreciated. Co-workers like Jonathan Leong, Sandar Myo Myint, Keldren Loy have been very warm, friendly and helpful, and I appreciate each and every one of them for their wonderful friendship and encouragement. The opportunity to work with Dr Zhang Zheng from the Institute of Materials Research and Engineering (IMRE) on surface analysis has been a highly rewarding experience and I am truly grateful to him for giving his time and lending his expertise. viii 93. 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Applied Surface Science 2013; 286 : 137-141. 182 [...]... coatings namely DLC, Cr-N, Ti-N and Ti-Al-N coatings, and antiwear additives i.e ZDDP and TPPT The discussion also includes the impact of these lubricant additives produced on the friction and wear properties of the coatings and attempts to understand the antiwear mechanism and tribofilm formation, as well as technological challenges in this area 16 2.1 State -of -the- Art Surface Coatings 2.1.1 Diamond-Like... investigation of the effects of antiwear additives, namely ZDDP and ashless TPPT, on the tribological characteristics of state -of -the- art surface coatings Chapter 3 provides details of the experimental methods and materials in the investigations It also describes the coating deposition techniques and the lubricant additives used to formulate the lubricants evaluated in the study Chapter 4 looks into the influence... friction results 3 from the friction between lubricant molecules The curve goes through a minimum coefficient of friction value and then increases, largely as a result of internal friction The lubricant film thickness depends on the friction and lubrication conditions including the surface roughness In hydrodynamic lubrication, the lubricant is pulled into the converging clearance by the rotation of the. .. phosphorothionates (TPPT) These ashless antiwear additives are considered to be more environmentally friendly than ZDDP There is now increasing attention on the use of such additives, as well as the application of state -of -the- 12 art surface coatings, to optimize wear resistance (and friction performance) without compromising environmental protection and human safety Both extreme pressure and antiwear additives have... boundary lubrication conditions Therefore, this study focuses on hydrogen-free DLC coatings Chapter 5 investigates the impact of primary ZDDP and TPPT on the friction and wear properties of Cr-N coatings Since Cr-N can be considered as one of the least complicated systems among the commonly used hard coatings (Van Stappen et al 1995), it is important to understand how each of the two types of additive chemistry... general concepts of tribology and lubrication; provides an overview of advanced surfaced coatings and lubricant additives, with references to a combination of the two disciplines; and concludes with a brief description of the scope of the thesis 1 1.1 Introduction to Tribology and Lubrication A tribological system (commonly referred to as a tribosystem) consists of four main elements: the two contacting... 74 4.2.1 Friction Analysis 74 4.2.2 Wear Analysis based on SEM 76 4.2.3 Surface Roughness .80 4.2.4 Wear protection Mechanism 80 4.3 Conclusions .84 Chapter 5 Effects of Primary ZDDP and Ashless TPPT as Antiwear Additives on the Friction and Wear Behaviour of Cr-N Coatings 86 5.1 Experimental Preparations 87 5.2 Results and Discussion 88 5.2.1... applications The objectives of this study are to investigate the influence of ZDDP and ashless TPPT as lubricant additives on the wear and friction properties of state -of -the- art surface coatings, namely diamond-like carbon (DLC), chromium nitride (Cr-N), titanium nitride (Ti-N), and titanium aluminium nitride (Ti-Al-N) coatings; and to postulate the likely 13 wear preventive mechanisms based on experimental... at the contact region It initially causes sporadic mixed friction but nevertheless significantly reduces the coefficient of friction As the speed continues to increase, a full and uninterrupted film is formed over the entire bearing faces This drastically reduces the coefficient of friction Also, as the speed increases, internal friction in the lubricating film adds to external friction Internal friction. .. Experiments 88 5.2.2 Friction Analysis 90 5.2.3 Surface Analysis based on FESEM and EDX 93 xi 5.2.3 Surface Analysis based on XPS 99 5.3 Conclusions .108 Chapter 6 Impact of Primary ZDDP and Ashless TPPT as Antiwear Additives on the Friction and Wear Behaviour of Cr-N Coatings 110 6.1 Experimental Preparations 111 6.2 Results and Discussion 112 6.2.1 Initial . THE EFFECTS OF ZDDP AND ASHLESS ANTIWEAR ADDITIVES ON THE FRICTION AND WEAR CHARACTERISTICS OF TRIBOLOGICAL COATINGS ON STEEL EDWARD NG SOO YONG NATIONAL UNIVERSITY. UNIVERSITY OF SINGAPORE 2014 THE EFFECTS OF ZDDP AND ASHLESS ANTIWEAR ADDITIVES ON THE FRICTION AND WEAR CHARACTERISTICS OF TRIBOLOGICAL COATINGS ON STEEL EDWARD NG SOO YONG (B test conditions. Therefore, a new pin -on- cylinder tribometer (with a point contact) was specially designed and fabricated to evaluate the influence of the antiwear additives on the friction and