A petrographic thin sectioning technique for evaluating composite materials

The development of tough high-peformance composites often employs a variety of analytical methods that correlate morphological, yield and fracture characteristics to the corresponding properties of a composite. Most of these techniques can be severely limited due to the high concentration of fibres used in high performance composites (nominally 60 wt %). Several light microscopy techniques can be used to identify relationships between morphological features and composite properties, for example fracture surface studies using reflected light microscopy, microtomed sections, cast films and etched surfaces. These techniques are limited for the following reasons: reflected light microscopy is restricted to surfaces, cast films do not necessarily reflect properties of the bulk material, microtomed sections are very difficult to produce with such a high concentration of fibres, and etched surfaces can be difficult to interpret because artefacts may be created by the use of the etchant. Petrographic thin sectioning has been used in conjunction with transmission polarized light microscopy for microstructural and deformation mechanism characterizations in a variety of polymer systems such as: identifying the failure modes in glass-filled polycarbonate and toughened nylon [1], evaluating the toughening mechanisms in rubber-toughened epoxy [2], and observing the development of the plastic zone in front of a crack tip in toughened polycarbonates and epoxy resins [3]. The technique used for preparing petrographic thin sections is described by Holik et al. [1]. This paper will focus on adaptations of this technique to facilitate the examination of high performance composites. We have adapted a transmitted light microscopy thin sectioning technique for use in studying high performance composites which overcomes several of the limitations associated with the more standard techniques. Thin sectioning of composites is extremely useful for identifying the presence of spherulites within a composite matrix and relating the effect of thermal treatment on spherulite morphology. This technique is also applicable to subsurface deformation analysis for identifying the plastic zone within a high performance composite. In addition, thin sectioning provides insight into the types of energy absorption mechanisms which occur during fracture. Three types of thermoplastic composite were studied for illustrative purposes: a poly(ether ether ketone) (PEEK)-based, a BPA polycarbonate (PC)-based and a rubber-modified PC-based composite. Before we discuss the results, a description of our thin sectioning technique for composite materials is in order. First, a composite sample is sectioned perpendicular to the fibre direction using a Buehler Isomet low speed diamond saw. Attempts were made to produce thin sections by sectioning parallel to the fibres; however, once the thin section approached the final thickness, portions of the fibres pulled out of the matrix and damaged the section. This resulted in a surface which could not be successfully polished and therefore was discontinued. Perhaps sectioning slightly off-axis to the fibre direction would provide a successful thin section method; however, this was not investigated. After the sample is sectioned, it is mounted using a two-part epoxy (epoxide resin and hardener, part nos CM-161-R and CM-161-H; Mager Scientific Inc., Dexter, Michigan). Once the mount has cured, the specimen is rough-ground using a standard waterrinse metallographic grinding wheel and silicon carbide abrasives. Alumina powder abrasive was used for fine polishing starting with 5/tm and followed by 1.0, 0.3 and 0.05/~m. Due to the difference in hardness between the fibres and the matrix, one must be careful to avoid relief polishing. A short nap polishing cloth as well as a minimum polishing time reduces the amount of refief that may occur. The next step involves mounting the sample on to a glass slide. First the slide is cleaned with isopropyl alcohol and allowed to dry. The slide is then coated with a thin layer of an epoxy adhesive. Adhesives that bond well to both the glass and the sample surface may be used as long as they are transparent and do not alter the specimen's structure. We have found that a two-part adhesive used for bonding fibre optics works very well for this application (TRA-CON epoxy resin TRA-BOND BA-Fll3; TRA CON Inc, Medford, Massachusetts). The key to adhering the sample to the glass slide is to obtain a very thin adhesive layer that is free from entrapped air. The entrapped air is removed by pressing the sample (polished side down) on to the glass slide. This also aligns the sample surface parallel to the glass slide. The specimen should be lightly clamped while the adhesive cures. Once the specimen has been mounted to a glass slide, a Buehler Petro-Thin was used to rough-grind the sample to approximately 901,m. The Petro-Thin has a glass slide vacuum attachment which maintains the slide parallel to a sectioning and grinding wheel. This equipment is normally used for producing thin sections of petrographic samples, but we found that it works very well for polymeric materials and produces a thinned specimen with parallel surfaces. Using a