Mechanical properties and characterisation of substrates for flexible displays

This project is concerned with the electro-mechanical reliability and characterisation of ITO coated polyester substrates for use in touch screen and flexible display applications. Flexible display anode components such as ITO coated polyesters are common in almost all flexible display technologies. However, these hybrid thin systems are unusual in mechanical terms. There is a mismatch between the mechanical properties of the inorganic coating and the organic substrate. It is therefore important to investigate the electromechanical response of such flexible anodes under various stress states and deformation modes. It is also important to develop new mechanical testing techniques for flexible displays. Numerous experimental techniques were used in order to characterise and test the available ITO coated/uncoated PET and PEN substrates. Also, the development of new experimental mechanical testing methods, such as the biaxial 'bulge' apparatus, was undertaken. During this work, various ITO coated polyester substrates were mechanically tested under uniaxial tension, controlled buckling and biaxial tension. In-situ electrical resistance monitoring and ex-situ atomic force microscopy, were used in order to detect and characterise ITO failure mechanisms. Tribological investigation of bare polyester substrates was undertaken. Preliminary nanoscratch and nanoindentation studies were also conducted on coated and uncoated systems. Overall it was shown that ITO coated polyester flexible display electrodes can properly function up to relatively low strains. Electrical resistance generally does not recover during unloading in cyclic experiments. These factors currently limit the use of such components to slightly curbed displays. Various ITO failure modes were observed, depending on the applied deformation mode. It was also shown, that the ITO adhesive failure is as critical as cohesive failure