Colloids and Surfaces A: Physicochemical and Engineering Aspects 146 (1999) 123 – 137 Interaction forces between  -alumina fibres in aqueous electrolyte measured with an atomic force microscope Laurence Meagher a, *, George V. Franks b , Michelle L. Gee a , Peter J. Scales a,b a School of Chemistry, Uniersity of Melbourne, Parkille VIC 3052, Australia b School of Chemical Engineering, Uniersity of Melbourne, Parkille VIC 3052, Australia Received 5 June 1998; accepted 31 August 1998 Abstract The surface charging properties of polycrystalline  -alumina fibres in aqueous electrolyte solutions have been investigated by direct force and streaming potential measurements. The presence of both Al and Si on the surface of the fibres resulted in a chemically heterogeneous surface. The heterogeneous distribution of Si resulted in large attractive forces between the fibres at moderate to low pH values and a pzc/iep at a pH value of approximately 5.5. The origin of this force was electrostatic in nature as the force profiles were well described by the DLVO theory of colloid stability. The agreement between the direct force and streaming potential measurements was good both in terms of the magnitude of the potentials and the position of the pzc/iep. By acid washing the fibres the chemical heterogeneity of the surface was reduced and the attractive force profiles at lower pH values were not observed. Instead repulsive forces were observed which were well described by DLVO theory at all separation distances greater than 8 nm. At smaller separation distances an additional repulsive force was measured which was attributed to the presence of a Al(OH) 3 like layer on the surface of the alumina. The acid washing treatment also resulted in a shift in the pH at which the pzc/iep occurred to a value of 6.5, presumably due to a lower surface silica concentration. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Surface forces; Atomic force microscope;  -Alumina; Streaming potential; Chemical heterogeneity 1. Introduction Alumina fibres are important reinforcements in ceramic matrix composites (CMCs) [1] as well as metal matrix composites (MMCs) [2]. One method of processing CMCs involves infiltrating suspensions of ceramic powders into ceramic fibre preforms [3]. In this methodology the fibres should be dispersed from each other to allow the powder to easily fill the void spaces between the fibres. When powder cannot be uniformly packed between the fibres, voids and inhomogeneous ar- eas lead to flaws in the composite, reducing its strength and reliability. To obtain dispersed and therefore uniformly packed particles, a large re- pulsive interaction energy between the particles is necessary. An obvious way of achieving this goal * Corresponding author. Tel.: +61-3-93445180; fax: +61- 3-93475180; e-mail: l.meagher@chemistry.unimelb.edu.au. 0927-7757/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII:S0927-7757(98)00779-1