Effect of the Substrate on Microindentation Behavior
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EFFECT OF THE SUBSTRATE ON MICROINDENTATION BEHAVIOR
T.J. IARDNER, J.E. RITTER, AND H.B. KARAMUSTAFA University of Massachusetts at Amherst
ABSTRACT A continuous microindentation apparatus was constructed with a maximum load capacity of 45 N (resolution of 0.1 N) and with a maximum displacement of 500 pm (resolution of 2.0 pm) and used to test bulk materials (brass, aluminum, steel, soda-lime glass and PMMA) and epoxy coatings on substrates of aluminum, steel, soda-lime glass and PMMA. The hardness (H) was determined with a Vickers indenter and compared to that obtained with a Tukon hardness tester. The elastic modulus of the coated systems was also estimated from the unloading curves. In general, we found that the indentation behavior of the epoxy coated system was influenced by the substrate with softer substrates (as with PMMA) being deformed before the indenter penetrates the coating. With all coated systems, the on-load value of H was independent of the penetration depth until the indenter penetrated the substrate. For the aluminum, steel and soda-lime glass substrates, the value of H after penetration increased slowly due to the pile up of the epoxy coating underneath the indenter supporting more of the load. For the PMMA substrate, hardness decreased after penetration because the hardness of PMMA was less than the epoxy coating.
INTRODUCTION Continuous indentation instruments are often used to measure the elastic and plastic properties of thin coatings and to assess the quality of the adhesion between these coatings and the substrate,[l,2]. Thicknesses of coatings can vary from less than 1 nm, e.g., diamond films on cutting tools, to more than 100 pm, e.g., compliant polymer coatings on optical glass fibers. For the case of very thin films, nano-indentation instruments can measure the load-depth behavior with a force range up to 1 N and a displacement range up to about 4 pm. Microindentation instruments operate in the load range of 0.1 to 50 N with impression sizes on the materials of the order 10 - 100 pm. These microindentation instruments are ideally suited for the determination of surface properties of systems with relatively thick polymer coatings [3]. The purpose of this paper is to report on results from microindentation of epoxy coatings of different thicknesses on different substrate materials using a Vickers microindentation instrument. Of particular interest is the effect of the substrate on the load-penetration depth behavior and the use of the data to determine the hardness of the coated systems as a function of depth and the elastic modulus of the coating.
Mat. Res. Soc. Symp. Proc. Vol. 308. ©1993 Materials Research Society
190
EXPERIMENTAL The microindentation instrument constructed in our lab is similar in design and configuration to that developed by Pharr and Cook [3]. Load was measured continuously during indentation by a load cell while the displacement of the Vickers indenter was monitored by two non-contact capacitance displacement gauges. The load was applied through a stepper
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