In Situ Transmission Electron Microscopy (TEM) Investigation of Fracture Mechanisms in a Calcium Aluminate MDF Cement.
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In Situ Transmission Electron Microscopy (TEM) Investigation of Fracture Mechanisms in a Calcium Aluminate MDF Cement.
0. 0. Popoola, W. M. Kriven and J. F. Young Department of Materials Science and Engineering, University of Illinois at Urbana-
Champaign, Urbana, IL 61801. ABSTRACT Fracture mechanisms in calcium aluminate macro-defect-free cement were studied by in situ deformation in the TEM. Fracture occurred predominantly at the cement / polymer interface. The toughening mechanism involved crack bridging at the interphase / polymer interface. XPS studies of the chemistry of the interphase revealed it to be made up of an A1OC complex in addition to the amorphous hydration product phase. INTRODUCTION Macro-defect-free cements have improved strength and toughness due to reduced porosity, pore size and chemical interaction between the polymer and the ceramic grains [1-3]. The mechanisms that operate during fracture have been discussed in some detail [4,5]. It was generally accepted that the classical Griffith equation (even when modified to incorporate porosity terms) did not apply to MDF because it neglected the effects of chemical reactions between the constituents. Eden and Bailey [5] proposed a "tied crack" model and extensive polymer deformation as the main toughening and strengthening mechanisms but they provided no experimental proof for these claims. Crack resistance curve (R-curve) behavior in calcium silicate and calcium aluminate MDF cements has been observed [6-8] and the main toughening mechanism has been identified by scanning electron microscopy (SEM) as crack bridging. To date there has been no report on the role of constituents (ceramic grains, polymer, interfaces etc.) during fracture. Previous microstructural studies revealed that the microstructure consisted of CaA120 4 and CaA140 7 grains randomly distributed in the amorphous polymer matrix. These phases were separated by an amorphous interphase layer inside of which were distributed fine crystallites of CaA120 5 . 8H20 [9]. The aim of this communication is to report the results of in situ deformation TEM studies of calcium aluminate MDF cements. It was found that fracture occurred predominantly at the ceramic / polymer interface through crack bridges at the interphase/polymer boundary. The chemistry of the interphase layer was studied on a CaA120 4 / polyvinyl alcohol couple by X-ray photoelectron spectroscopy (XPS). The interphase consisted of the amorphous hydration product of calcium aluminate and an AlOC complex formed by the reaction of the ceramic grains and the polymer during processing. These results are used to explain the microstructural and microchemical aspects of fracture in MDF cements. EXPERIMENTAL PROCEDURES The TEM double tilt holder built for use on the AEI/Kratos EM-7 electron microscope was designed to use a common tensile specimen configuration shown in Figure 1. Typically, rectangular specimens lmm thick were made by cutting and machining to the dimensions shown. The lmm holes were required for attaching the specimens t
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