Characterisation of coated lightweight brake rotors by Abdulwahab Ali Alnaqi Submitted in accordance with the requirements for the degree of Doctor of Philosophy The University of Leeds School of Mechanical Engineering July 2014 The candidate confirms that the work submitted is his own and that appropriate credit has been given where reference has been made to the work of others. This copy has been supplied on the understanding that it is copyright material and that no quotation from the thesis may be published without proper acknowledgement. ii __________________________________________________________________________________________________________ Intellectual Property and Publication Statements The candidate confirms that the work submitted is his own, except where work which has formed part of jointly-authored publications has been included. The contribution of the candidate and the other authors to this work has been explicitly indicated below. The candidate confirms that appropriate credit has been given within the thesis where reference has been made to the work of others. Paper Chapter G. Bozic, A. A. Alnaqi, P. C. Brooks, and D. C. Barton, "Thermal Ch. 1, 2, 4 Characterisation of Lightweight Brake Rotors for Passenger Car and 5 Application Using A small Scale Brake Dynamometer and One Dimensional Model," Eurobrake 2012, Germany, 2012. A. A. Alnaqi, D. C. Barton, and P. C. Brooks, Thermal Performance of Ch. 1, 2, 4 Monolithic and Coated Disc Brakes Using Abaqus and Matlab Software, and 5 Vienna, Austria, SIMULIA Community Conference, 2013. A. A. Alnaqi, S. Shrestha, P. C. Brooks, and D. C. Barton, "Thermal Ch. 1, 2, 6 Performance of PEO Coated Lightweight Brake Rotors Compared with and 7 Grey Cast Iron," Eurobrake 2014, France, 2014. A. A. Alnaqi, S. Shrestha, P. C. Brooks, and D. C. Barton, "Optimisation of Ch. 1, 2, 7 alumina coated lightweight brake rotor," SAE 2014, USA, 2014. and 9 A. A. Alnaqi, D. C. Barton, and P. C. Brooks, "Reduced scale thermal Ch. 1, 2, 6 characterization of automotive disc brake," Applied thermal engineering, and 7 2014. A. A. Alnaqi, S. Kosarieh, D. C. Barton, P. C. Brooks, and S. Shrestha, Ch. 1, 2, 3 “Material characterisation of lightweight disc brake rotors," 2014. and 8 The candidate undertook major tasks of the work presented in these published papers, such as undertaking experimental works, design, analysing and presenting the results. The co-authors reviewed and guided the candidate and provided valuable discussion. This copy has been supplied on the understanding that it is copyright material and that no quotation from the thesis may be published without proper acknowledgement. The right of Abdulwahab A. A. Alnaqi to be identified as Author of this work has been asserted by him in accordance with the Copyright, Designs and Patents Act 1988. © 2014 The University of Leeds and Abdulwahab A. A. Alnaqi iii __________________________________________________________________________________________________________ To my wife, Hussah’, who has been patient, supportive, and shared many sacrifices during the completion of this thesis. Also, to my little sons, Mohammad and Ali, who are a constant source of happiness. To my parents for their endless support. iv __________________________________________________________________________________________________________ Acknowledgement First and foremost I would like to offer my sincerest gratitude to my supervisors, Professor David Barton and Dr. Peter Brooks, for their support and encouragement during my PhD research. Your advice on both my career as well as on my research has been priceless. One simply could not wish for better or friendlier supervisors. I also owe my deepest thanks to Tony Wiese and David Readman for their help and support in the laboratory during the experimental work. Special thanks to Tony Wiese for his support in the research and for being a great friend. Moreover, I would like to thank Professor Suman Shrestha from Keronite International Ltd for his support and valuable advice throughout this research. Also, special thanks to my colleagues Abdulaziz Alghatani, Ranvir kalare and Shahriar Kosarieh for sharing experiences, knowledge and advice. I would also like to thank the Public Authority for Applied Education and Training and the Kuwait National Government for their financial support throughout my study. Finally, I would like to express my love and appreciation to my family in Kuwait for their infinite love and support. Furthermore, I would like to express my special thanks for my beloved wife, Hussah, and my lovely sons, Mohammad and Ali, for sharing good and hard times during my PhD. v __________________________________________________________________________________________________________ Abstract Numerical and experimental studies were undertaken using lightweight brake rotors to reduce vehicle weight and thereby improve fuel efficiency and vehicle emissions. Abaqus finite element and Matlab software were used to construct one dimensional (1D), two dimensional (2D) and three dimensional (3D) thermal models to investigate the general thermal performance of disc brakes to develop a valid method of reduced scale testing. Five small scale solid brake rotors were investigated experimentally: grey cast iron, wrought aluminium alloy (6082), the same 6082 alloy with an alumina surface layer applied by plasma electrolytic oxidation (PEO), cast aluminium MMC (AMC640XA) and the same MMC with a PEO alumina surface layer. The disc and pad temperatures, brake pressure, coefficient of friction and brake torque were monitored during the tests for each material. Surface morphology, microstructure and micro hardness of the coatings and substrate were evaluated before and after the tests. Numerical simulations confirmed the equivalence between the full and small scale disc thermal performance using the proposed scaling methodology and also provide a good agreement with the experimental results. The coated 6082 alloy rotor was shown to give good thermal and friction performance up to relatively high rubbing surface temperatures of around 500oC. This rotor failed at a surface temperature of about 550oC due to brittle fracture of the wrought aluminium substrate. The proposed scaling methodology was shown to be a valid method of investigating a rotor design concept in the laboratory at low cost and reduced operating time. The PEO coating on aluminium alloy was denser and more uniform compared to the PEO coating on aluminium MMC. In addition, the PEO coating improved the hardness and thermal resistance of both the aluminium alloy and aluminium MMC. A sensitivity analysis based on the Taguchi approach was carried out on the PEO coated aluminium alloy rotor to investigate the effect of various parameters on thermal performance. Optimisation of the structure was carried out using a genetic algorithm to design coated aluminium alloy discs that are potentially technically viable on small- medium passenger cars. Keywords: lightweight rotor, finite element model, plasma electrolytic oxidation, optimisation, material characterisation, brake dynamometer and scaling methodology. vi __________________________________________________________________________________________________________ List of Contents Acknowledgement ...................................................................................... iv Abstract ........................................................................................................ v List of Contents .......................................................................................... vi Nomenclature ............................................................................................ xvi List of Abbreviations................................................................................ xxi List of Figures .......................................................................................... xxii List of Tables ......................................................................................... xxxii Chapter 1: Introduction ............................................................................. 1 1.1 Research overview and background.................................................................. 1 1.2 Research aim and objectives ............................................................................. 7 1.2.1 Aim ................................................................................................................ 7 1.2.2 Objectives ...................................................................................................... 7 1.3 Structure of remainder of thesis ........................................................................ 7 Chapter 2: Literature Review .................................................................. 10 2.1 Overview ......................................................................................................... 10 2.2 Disc brake system overview ........................................................................... 11 2.3 Thermal analysis of rotors ............................................................................... 14 2.3.1 Simple one-dimensional analysis ................................................................ 15 2.3.2 Finite element analysis ................................................................................ 19 2.3.3 Experimental analysis ................................................................................. 22 vii __________________________________________________________________________________________________________ 2.4 Scaling methodologies .................................................................................... 23 2.5 Investigation of disc brake rotor material ....................................................... 25 2.6 Aluminium alloys ............................................................................................ 27 2.6.1 General background .................................................................................... 27 2.6.2 Aluminium alloys classifications and properties ........................................ 28 2.6.3 Aluminium alloys applications ................................................................... 30 2.6.4 Surface engineering on aluminium alloys and aluminium metal matrix composite ................................................................................................................ 31 2.7 Aluminium metal matrix composite................................................................ 31 2.7.1 General background .................................................................................... 31 2.7.2 Aluminium metal matrix composite classifications and properties ............ 32 2.7.3 Aluminium metal matrix composite applications ....................................... 32 2.7.4 Surface engineering of aluminium metal matrix composite ....................... 33 2.8 Coating technologies ....................................................................................... 33 2.8.1 Keronite PEO coating technology ............................................................... 36 2.9 Design of experiment (DoE) and optimisation analysis ................................. 40 2.10 Summary of the findings and implications for the current research ............... 42 Chapter 3: Material and characterisation method ................................ 45 3.1 Introduction ..................................................................................................... 45 3.2 Materials .......................................................................................................... 45 3.2.1 Grey cast iron .............................................................................................. 46 viii __________________________________________________________________________________________________________ 3.2.2 Aluminium alloy ......................................................................................... 46 3.2.3 Aluminium metal matrix composite............................................................ 47 3.2.4 PEO coating ................................................................................................ 47 3.2.5 Brake pad material ...................................................................................... 48 3.3 Sample preparation and metallographic preparation of samples .................... 49 3.4 Experimental techniques ................................................................................. 52 3.4.1 Optical microscopy of materials ................................................................. 53 3.4.2 Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray analysis (EDX) ........................................................................................................ 53 3.4.2.1 SEM and EDX fundamentals .............................................................. 53 3.4.2.2 SEM and EDX sample preparation ..................................................... 55 3.4.3 Micro-Hardness tests ................................................................................... 58 3.4.4 Roughness analysis ..................................................................................... 59 3.4.5 Geometry measurement .............................................................................. 60 3.5 Summary ......................................................................................................... 62 Chapter 4: One-dimensional thermal models ........................................ 63 4.1 Introduction ..................................................................................................... 63 4.2 Background and theories ................................................................................. 64 4.2.1 Fourier‟s law for heat conduction ............................................................... 64 4.2.2 Newton‟s law of cooling ............................................................................. 65 4.2.3 Radiation heat transfer ................................................................................ 66 ix __________________________________________________________________________________________________________ 4.2.4 Thermal contact resistance .......................................................................... 66 4.2.5 Braking energy and power of the vehicle ................................................... 66 4.3 One-dimensional thermal model theory .......................................................... 68 4.3.1 Assumption ................................................................................................. 68 4.3.2 Formulation ................................................................................................. 69 4.3.3 Initial condition parameters and initialization............................................. 74 4.3.4 Solution procedure ...................................................................................... 77 4.3.5 Matlab model structure ............................................................................... 79 4.3.6 Sensitivity analysis ...................................................................................... 81 4.3.7 Limitations .................................................................................................. 82 4.4 Abaqus One Dimensional Model .................................................................... 83 4.5 Validation of the One-dimensional Matlab model .......................................... 86 4.5.1 Validation of the 1D model against literature ............................................. 86 4.5.2 Validation of the 1D Matlab model with 1D Abaqus model ...................... 88 4.6 Coating layer study ......................................................................................... 88 4.6.1 Thermal resistance modelling ..................................................................... 89 4.6.2 Mesh density analysis ................................................................................. 92 4.6.3 Sub-modelling analysis ............................................................................... 94 4.7 Controlling the Abaqus model through Matlab code ...................................... 99 4.7.1 Problem definition ....................................................................................... 99 4.7.2 Solution procedure ...................................................................................... 99 x __________________________________________________________________________________________________________ 4.7.3 Benefits ..................................................................................................... 101 4.8 Parametric study ............................................................................................ 101 4.8.1 Effects of disc materials ............................................................................ 101 4.8.2 Effects of convective heat loss .................................................................. 102 4.8.3 Effects of disc thickness ............................................................................ 103 4.8.4 Effects of pad material .............................................................................. 104 4.8.5 Effects of initial vehicle velocity .............................................................. 105 4.8.6 Effects of vehicle mass.............................................................................. 106 4.8.7 Effect of rotating heat source .................................................................... 107 4.8.8 Effect of coating thickness ........................................................................ 108 4.9 Summary ....................................................................................................... 109 Chapter 5: Scaling methodologies and test rig design ......................... 111 5.1 Introduction ................................................................................................... 111 5.2 Scaling methodologies .................................................................................. 112 5.2.1 Scaling method .......................................................................................... 112 5.2.2 Scaling principle........................................................................................ 112 5.2.2.1 Rotational inertia ............................................................................... 112 5.2.2.2 Disc mass .......................................................................................... 114 5.2.2.3 Brake torque ...................................................................................... 115 5.2.2.4 Functional parameter ......................................................................... 116 5.2.3 Selection of vehicle for scaling ................................................................. 119