The effects of water aging on the interphase region and interlaminar fracture toughness in polymer–glass composites A. Hodzic a, * , J.K. Kim b , A.E. Lowe a , Z.H. Stachurski a a Department of Engineering, The Australian National University, ACT 0200, Canberra, Australia b Department of Mechanical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, People’s Republic of China Received 12 April 2001; received in revised form 14 January 2004; accepted 26 March 2004 Available online 1 June 2004 Abstract Three different unidirectional polymer–glass composite systems involving phenolic and polyester resins were aged for 6 and 11 weeks in tap water and tested in the mode I double cantilever beam (DCB) test. The results showed a dramatic increase in water absorption and a decrease in fracture toughness for phenolic/glass systems. Fractographic analysis revealed interfacial debonding to be dominant failure mechanism, indicating a strong influence of water degradation on fracture toughness results. The interphase region of each system was investigated using the nano-indentation and the nano-scratch techniques before and after aging in water. The nano-indentation test produced a series of indents as small as 30 nm in depth, to detect water degradation of the material properties at the interphase region between the fibre and the matrix. The nano-hardness results indicated interdiffusion in water aged interphase regions. The nano-scratch test was used in conjuction with the nano-indentation test, in order to detect the width of the interphase regions before and after water degradation. It was shown, from the coefficient of friction and the scratch profile depth, that the interphase region width increased and the material properties degraded during water aging. Qualitative links between water degradation of the glass–polymer interphase on a nanometer level and interlaminar fracture toughness are discussed. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Nanoindentation; B. Fracture toughness; B. Interface 1. Introduction There have been two important reasons for investi- gating the effects of water aging of polymer/glass sys- tems in this work. The first reason was of a practical value, as the materials used in this work were designed primarily for fire-resistant components (phenolics/glass) and construction parts (polyester/glass) in marine structures. Long time exposure to high humidity envi- ronment would have a similar effect on these materials as aging in water, as experimentally simulated in this work. The other objective was to investigate the influ- ence of degradation of the fibre–matrix bond and sur- rounding interphase region on the fracture toughness of the bulk material. The interphase is defined as a region which is formed as a result of bonding and reactions between the fibre and the matrix. Designed to enhance the bond between fibres and matrix in polymer/glass composite materials, silane coupling agents [1] react to varying degrees with different matrix polymers extending interphase regions deeper in the matrix. It is not yet clearly understood how the matrix properties are affected in the interphase re- gion, where the silane physically and chemically inter- acts with the matrix polymer. This region is the site of synergy in composite materials, as the stress redistribu- tion from the matrix to the fibres takes place through their bond/interphase. Therefore, although the inter- phase region appears to have insignificant volume frac- tion, its influence to overall material properties is significant [2,3]. * Corresponding author. Present address: School of Engineering, James Cook University, Townsville, Qld 4811, Australia. Tel.: +61-7- 4781-5082; fax: +61-7-4781-4660. E-mail address: alma.hodzic@jcu.edu.au (A. Hodzic). 0266-3538/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2004.03.011 Composites Science and Technology 64 (2004) 2185–2195 COMPOSITES SCIENCE AND TECHNOLOGY www.elsevier.com/locate/compscitech