THE DURABILITY OF AN ADHESIVELY BONDED ALUMINIUM ALLOY by Raymond John Davies, B.Sc. A thesis submited for the degree of Doctor of Philosophy in the Faculty of Engineering, University of London and the Diploma of Imperial College. Department of Mechanical Engineering, Imperial College of Science, Technology and Medicine, London SW7 2BX December 1989 This Thesis is dedicated to the memory of my mom, whose inspiration and support will never leave me. Abstract The adhesive bonding of aluminium alloys is of considerable interest to the aerospace industry and the satisfactory performance of such adhesive joints throughout the required service life of the bonded component is of major importance. To achieve good resistance to environmental attack, especially by water, it is well established that some form of surface pretreatment of the aluminium alloy is essential. Such pretreatments as commonly used by industry have been developed empirically. These treatments are based upon complex etching and anodising treatments using various acid solutions. However, the empirical approach to the development of surface pretreatments has led to a lack of Turn understanding as to why some treatments impart superior durability to the bonded joint than others. A main aim of the present work is to improve our understanding of the fundamental material science involved in this important area of adhesives technology. The present work investigates the detailed nature of the oxide formed on an aluminium-magnesium alloy which has been pretreated using the common methods employed by the defence and aerospace industries. The surface characterisation of the pretreated surface and the interactions between the surface oxide, adhesive and primer employed are assessed by the use of scanning electron microscopy (SEM), transmission electron microscopy (TEM), x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS) and Auger spectroscopy. The results from these studies are correlated to durability tests in hot water, including the use of single lap shear tests and a novel accelerated ageing test based upon a fracture mechanics approach. The nature and locus of failure of these joints are assessed by use of scanning electron microscopy and x-ray photoelectron spectroscopy, allowing the possible mechanisms of environmental attack to be outlined. l CONTENTS PAGE ABSTRACT (i) CONTENTS (ii) ACKNOWLEDGEMENTS (x) LIST OF SYMBOLS (xi) CHAPTER 1 : INTRODUCTION AND AIMS 1 CHAPTER 2 : LITERATURE SURVEY 2.1 Introduction 3 2.2 Aspects of Adhesion : Establishing Interfacial Contact 2.2.1 Introduction 6 2.2.2 Wetting equilibria 6 2.2.3 Kinetics of wetting 8 2.2.4 The initial bonding environment 12 2.3 Aspects of Adhesion : Mechanisms of Adhesion 2.3.1 Introduction 13 2.3.2 The mechanical interlocking theory 13 2.3.3 The diffusion theory 15 2.3.4 The electronic theory 15 2.3.5 The adsorption theory 16 2.3.6 Weak boundary layers 19 2.4 Aspects of Adhesion : The Strength of Adhesive Joints 2.4.1 Introduction 21 2.4.2 Single lap joints 22 2.4.3 Double lap joints 23 ii 2.4.4 Modified lap joints 24 2.4.5 Failure criteria 25 2.4.6 Fracture mechanics of adhesive joints 26 2.4.6.1 Introduction 26 2.4.6.2 The energy balance approach 27 2.4.7 Experimental adhesive joints 29 2.5 Surface Treatment of Aluminium for Adhesive Bonding 2.5.1 Introduction 32 2.5.2 The oxides and hydroxides of aluminium 35 2.5.3 The etching of aluminium 38 2.5.4 The anodising of aluminium 41 2.5.4.1 Introduction 41 2.5.4.2 Porous film growth on aluminium 41 2.6 Aerospace Adhesives and Primers 2.6.1 Introduction 46 2.6.2 Phenolic resin adhesives 46 2.63 Epoxy resin adhesives 48 2.6.4 Aerospace primer systems 49 2.7 Mechanisms of Environmental Attack 2.7.1 Introduction 50 2.7.2 Stability of the adhesive 50 2.7.3 Stability of the interface 51 2.7.4 Stability of the substrate 53 2.7.5 Kinetics of failure 56 2.7.6 Effects of stress 58 2.7.7 Effects of pH in the bond 59 2.8 Summary 61 iii CHAPTER 3 : EXPERIMENTAL METHODS 3.1 Materials 3.1.1 Aluminium alloy used 63 3.1.2 Adhesive 63 3.1.3 Primer 63 3.2 Substrate Surface Pretreatments 3.2.1 Pretreatment procedure 64 3.2.2 The pretreatment baths 67 3.2.2.1 Tap water rinse tank 67 3.2.2.2 Distilled water rinse tank 67 3.2.2.3 Sodium hydroxide etch 67 3.2.2.4 Nitric acid desmutt 67 3.2.2.5 Chromic acid etch (CAE) bath 68 3.2.2.6 Phosphoric acid anodising (PAA) bath 68 3.2.2.7 Chromic acid anodising (CAA) bath 69 3.2.2.8 Sulphuric acid anodising (SAA) bath 70 3.2.2.9 Phosphoric acid dip (PAD) bath 71 3.2.2.10 Jigging of specimens 71 3.3 Characterisation of Substrate Surfaces and Locus of Joint Failure Studies 3.3.1 Introduction 73 3.3.2 Scanning electron microscopy 73 3.3.3 Transmission electron microscopy 74 3.3.4 X-ray diffraction 77 IV 3.3.5 X-ray photoelectron spectroscopy 78 3.3.6 Auger electron spectroscopy 80 3.4 Joint Durability Studies 3.4.1 Introduction 82 3.4.2 Single lap shear tests 82 3.4.3 Long term double cantilever beam (DCB) tests 83 3.4.4 pH measurements within the DCB joints 84 3.5 Mechanical Testing of Oxides 85 CHAPTER 4 : SURFACE CHARACTERISATION : RESULTS AND DISCUSSION 4.1 Introduction 87 4.2 Surface Characterisation of Pretreated Surfaces 4.2.1 Introduction 87 4.2.2 Scanning electron microscopy studies 88 4.2.3 X-ray diffraction studies 101 4.2.4 X-ray photoelectron spectroscopy 104 4.2.5 Comparision of surface pretreatments 108 4.3 Characterisation of Pretreated and Primer/Adhesive Coated Surfaces 4.3.1 Introduction 110 4.3.2 Scanning and transmission electron microscopy studies 110 4.3.3 Scanning Auger spectroscopy 118 4.3.4 Comparision of surface pretreatments 122 4.3.5 Theoretical studies 124 4.4 Characterisation of the Hydration Resistance of the Pretreated Surfaces 4.4.1 Introduction 126 v 4.4.2 Scanning electron microscopy studies 126 4.4.3 Effect of primer/adhesive coating 135 4.4.4 Transmission electron microscopy studies 138 4.4.5 Comparision of surface pretreatments 138 4.5 Summary 141 CHAPTER 5 : THE MECHANICAL TESTING AND DURABILITY OF THE ADHESIVE JOINTS : RESULTS AND DISCUSSION 5.1 Introduction 142 5.2 Single Lap Shear Joints 5.2.1 Introduction 142 5.2.2 Unprimed joints 142 5.2.3 Primed joints 145 5.2.4 Comparision between unprimed and primed joints 147 5.3 Double Cantilever Beam Joints 5.3.1 Introduction 148 5.3.2 Unprimed joints 149 5.3.3 Primed joints 151 5.3.4 Comparision between unprimed and primed joints 153 5.4 Summary 154 CHAPTER 6 : POST FAILURE ANALYSIS OF THE ADHESIVE JOINTS : RESULTS AND DISCUSSION 6.1 Introduction 156 6.2 Loci of Failure for Single Lap Shear Joints 6.2.1 Introduction 156 6.2.2 Visual study 157 6.2.3 Scanning electron microscopy studies 161 vi 6.2.3.1 Chromic acid etched (CAE) joints 161 6.2.3.2 Phosphoric acid anodised (PAA) joints 162 6.2.3.3 Chromic acid anodised (CAA) joints 162 6.2.3.4 Sulphuric acid anodised (SAA) joints 162 6.2.3.5 SAA followed by phosphoric acid treatment (SAA/PAD) joints 163 6.2.4 X-ray photoelectron spectroscopy studies 170 6.2.5 Interpretation of the locus of failure studies 175 6.2.5.1 Chromic acid etched (CAE) joints 175 6.2.5.2 Phosphoric acid anodised (PAA) joints 176 6.2.5.3 Chromic acid anodised (CAA) joints 177 6.2.5.4 Sulphuric acid anodised (SAA) joints 177 6.2.5.5 SAA followed by phosphoric acid treatment (SAA/PAD) joints 178 6.3 Loci of Failure Study for the Double Cantilever Beam Joints 6.3.1 Introduction 180 6.3.2 Visual assessments 180 6.3.3 Scanning electron microscopy studies 185 6.3.3.1 Chromic acid etched (CAE) joints 185 6.3.3.2 Phosphoric acid anodised (PAA) joints 186 6.3.3.3 Chromic acid anodised (CAA) joints 186 6.3.3.4 Sulphuric acid anodised (SAA) joints 187 6.2.3.5 SAA followed by phosphoric acid treatment (SAA/PAD) joints 187 6.3.4 X-ray photoelectron spectroscopy studies 196 6.3.5 Intrepretation of the loci of failure studies 199 6.3.5.1 The critical regions of interfacial failure 199 vii 6.3.3.1 Chromic acid etched (CAE) joints 200 6.3.3.2 Phosphoric acid anodised (PAA) joints 201 6.3.3.3 Chromic acid anodised (CAA) joints 202 6.3.3.4 Sulphuric acid anodised (SAA) joints 203 6.2.3.5 SAA followed by phosphoric acid treatment (S AA/PAD) joints 204 6.4 Summary 206 CHAPTER 7 : THE MECHANICS AND MECHANISMS OF ENVIRONMENTAL FAILURE 7.1 Introduction 208 7.2 Displacement Mechanism 7.2.1 Secondary bonding 209 7.2.2 Chemical bonding 211 7.3 Hydration of Oxide 212 7.4 Weakening of boundary layer of adhesive 214 7.5 Role of 'micro-composite' interphase 7.5.1 Introduction 215 7.5.2 Mathematical model of the interfacial region of the joint 215 7.5.2.1 The case of the "micro-composite" joints 217 7.5.3 The single lap shear joints 218 7.5.4 The double cantilever beam joints 7.5.4.1 Introduction 220 7.5.4.2 Mechanics and mechanisms of environmental failure 221 7.6 Summary 223 viii