The Importance of Surface Treatment of Aluminium Surface in the Context of Cost-Effective Fibre Reinforced Polymer/Metal Joint Structural Frameworks

Saniee, Ardeshir
Schaefer, Marcel
Chen, Zhan
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Doctor of Philosophy
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Auckland University of Technology

The joining of dissimilar materials, including metals and carbon fiber reinforced polymers (CFRPs), has emerged as a critical area of research in various advanced industries, including aerospace, automotive, electronics, and construction. This is driven by the growing demand for lightweight structures, improved performance, and enhanced functionality. The combination of aluminium and CFRP offers a unique opportunity for achieving lightweight and high-strength structures. Among the various joining techniques, adhesive bonding had garnered significant attention due to its unique advantages which can enables the formation of durable and resilient joints between aluminium and CFRP, providing good load transfer, fatigue resistance, and overall joint integrity. However, the full realisation of this potential is hindered by certain limitations and dependencies associated with adhesive bonding, specifically related to surface treatment and the challenges posed by the aluminium surface. Surface treatment plays a crucial role in enhancing the adhesion strength and durability of bonded joints. Yet, the effectiveness of surface treatments in achieving optimal bonding performance is still a subject of ongoing research. Additionally, the high level of dependency on surface characteristics, such as surface roughness and chemistry, further complicates the adhesive bonding process. In the case of aluminium and CFRP bonding, the aluminium surface presents particular challenges due to its oxide layer and modified chemical composition. These factors require careful consideration and specialised surface preparation techniques to ensure successful adhesive bonding. Addressing the limitations and dependencies of adhesive bonding for surface treatment, particularly in aluminium and CFRP bonding, is crucial to fully exploit the potential of adhesive bonding and realise its benefits in creating durable and high-performance structures. In light of these limitation and significance of surface treatment for aluminium surfaces in adhesive bonding, this study aims to provide further insights and advancement in this field. By delving into the intricate details of surface treatment techniques, including mechanical abrasion, chemical etching, and surface modification methods such as deposition of coupling agents, a comprehensive understanding of their impact on the adhesion performance both initial and exposed of the adhesively aluminium adhesively bonded joints can be gained. As a result, to investigate the influence of different set of surface treatments, both individually and in combination to each other, on the adhesion performance of the adhesively bonded joints different approaches, analytical and experimental, were conducted. For this purpose, sandpapering with mesh sizes of 240, 120 and 80, etching in diluted NaOH solution and deposition of silane, dipodal silane and zirconate were selected as the mechanical, chemical and modification methods of surface treatment, respectively, Initially, the influence of abovementioned surface treatment on different surface characteristics of the aluminium alloy, such as surface roughness parameters as well as total surface free energy and its polar and dispersive components, were investigated. Then the single lap joint and Boeing wedge test were conducted to investigate the influence of the selected surface treatment on initial and exposed performance of the adhesively bonded joints, respectively.

A total of 80 treated specimens were fabricated to investigate the influence of selected surface treatment on five different surface roughness parameters, surface free energy and its polar and dispersive components and elemental composition. The polished condition of aluminium surface was selected as a benchmark in this study. The result form this study shows that etching aluminium alloy in NaOH solution can significantly alter the surface roughness parameters such as Sa and Sq of the polished aluminium surface, which was misjudged in the literature due to the lack of systematic comparison. In addition to this it was argued that the rough surface texture developed by sandpapering can enhance the effectiveness of NaOH treatment and depositing the coupling agents. First, it provides a larger surface area for NaOH treatment to react with the aluminium surface. Secondly, it can facilitate mechanical interlocking, which can favour higher adhesion between adhesive and adherend in the case of adhesive bonding.

On the other hand, a total of 180 aluminium-to-aluminium bonded specimens were fabricated to study the influence of the investigated surface treatment and identify the correlation between different surface parameters and initial and exposed performance of the adhesively bonded joints. The results reveal that the NaOH treatment can significantly enhance the ultimate shear strength of adhesively bonded joints. This enhancement can be accurately predicted by the combination of surface roughness parameters Sa and Sq, as a direct relationship was observed. Furthermore, it was found that surface roughening of the aluminium alloy can indirectly affect the inital adhesion performance of the bonded joint. Specifically, roughening the aluminium surface can increase the reaction zone for NaOH treatment, thereby leading to an increase in the shear strength of the bonded joint. It was also found that the surface roughening of aluminium surface prior to NaOH treatment and coupling agent application can enhance initial adhesion performance compared to polished specimens, as observed in this study. The most significant improvement, with a 22.33% increase of ultimate shear strength, was achieved by roughening the aluminium surface with #120 grit prior to NaOH treatment, followed by zirconate deposition. This finding highlights the importance of surface preparation in achieving optimal adhesion performance in aluminium-to-aluminium bonding. Furthermore, in order to achieve the best exposed performance of the PUR adhesively bonded joint, it was found that the optimal surface roughness should be in the range of 2-2.5µm, which can be achieved through a combination of sandpapering prior to NaOH treatment and dipodal silane. It has been observed that having a high fracture toughness in dry conditions does not necessarily ensure better exposed performance of the PUR adhesively bonded joint. The fracture toughness of the PUR adhesively bonded joint decreased to 30% of its original value within the first three days of exposure time when only surface roughness was altered through manual sandpapering and NaOH treatment. This phenomenon was also observed when coupling agents were used. The study revealed that silane and dipodal silane coupling agents had better exposed performance in chamber environmental conditions compared to a zirconate coupling agent, despite having lower initial fracture toughness values.

Adhesive bonding , Combination of surface treatment , Correlation of surface characteristics and adhesion performance , Mechanical Abrasion , Etching , Coupling agent , Initial strength , Durability
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