Abstract
The use of adhesive bonding as a joining method has been the subject of interest to the aerospace industry over the last 50 years. It is now common place to use adhesive bonding in non-structural applications, however due to the increase in use of composite/metal and composite/composite aero-structures the identification and use of suitable adhesive bonding regimes has become a high priority. Currently one of the most important materials in the aircraft industry is aluminium and its alloys. To obtain strong durable joints some form of surface treatment of the aluminium is necessary. Pre-treatments that are commonly used today involve combinations of etching and anodising. In recent years there has been growing concern over the environmental impact of some of these treatments, particularly those that contain chromates. There is also the increasing pressure to undertake in-situ repair of aircraft structures using techniques not involving toxic, harmful materials or difficult anodising processes. To this end, there has been growing interest in the so-called "green", pre-treatments offered by silane adhesion promoters. The Boeing wedge test was used to identify the optimum durability characteristics for a number of silane process and solution variables. The experimental results indicated that optimum adhesive joint durability was obtained using a silane concentration of 1.0% in distilled water at an acid/alkali adjusted pH of 5.0 and hydrolysed for one hour at 23°C. This coating is then cured at 93°C for 1 hour. Work contained in this thesis has been able to identify the true fracture path associated with joint exposure to hot wet conditions. Joint failure with optimum silane conditions was identified as fracture through the silane inter-phase region, with some indication of microscopic stress assisted hydration of the oxide near the crack. The use of a pre-bonding hydration process in conjunction with the optimum silane pretreatment procedure has shown a marked improvement in the durability characteristics and, has in-fact, bettered the durability characteristics measured on the industry standard PAA and CAA pre-treated joints.