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Research Papers

# Effect of Shape of the Tip in Determining Interphase Properties in Fiber Reinforced Plastic Composites Using Nanoindentation

[+] Author and Article Information
S. B. Yedla, M. Kalukanimuttam, R. M. Winter

Department of Chemistry and Chemical Engineering, South Dakota School of Mines and Technology, Rapid City, SD 57701

Sanjeev K. Khanna1

Department of Mechanical and Aerospace Engineering, University of Missouri, Columbia, MO 65211

1

Corresponding author.

J. Eng. Mater. Technol 130(4), 041010 (Sep 19, 2008) (15 pages) doi:10.1115/1.2975234 History: Received October 19, 2006; Revised June 16, 2008; Published September 19, 2008

## Abstract

Fiber reinforced polymer composites are two component material systems in which fibers are embedded in a polymer matrix. Such a system inherently has an interface where the two components meet. Adjacent to the interface extending beyond the fiber surface is the “interphase region.” Properties within the interphase vary due to variations in the chemistry. The study of mechanical property variations with changing chemistry will help in better understanding and tailoring of the composite properties. The present work concentrates on the investigation of nanomechanical properties within the interphase of a glass fiber embedded in polyester matrix system. The glass fibers were coated with two types of silanes to produce a strong and a weak bond at the fiber-matrix interface. Nanoindentation techniques coupled with atomic force microscopy imaging capabilities have been used for this investigation. Two different tips were employed for indenting, one being a Berkovich diamond tip supplied by Hysitron, Inc., Minneapolis, MN and another being a parabolic tungsten tip, which was made in the laboratory. Indentations were performed within the interphase region, also in the bulk matrix, and on the glass fiber. The variation in mechanical properties such as modulus, stiffness, hardness, and penetration depth were obtained within the interphase by indenting at the fiber surface outward. Variations of the elastic modulus in the interphase region and its relation to the chemistry are presented. The results obtained using two different tip shapes have been compared. Phase imaging was performed using tapping mode atomic force microscopy to qualitatively identify the presence of an interphase near the glass fiber-polyester interface. These experiments show that when no coupling agent is used the interphase thickness is less than $0.1 μm$, and its exact determination is limited by the spatial resolution of the tips employed and the process of indentation. Phase imaging results with composite samples made of coated glass fibers corroborate the results obtained from nanoindentation experiments.

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## Figures

Figure 3

AFM surface plot showing the circular indent made by the parabolic tungsten tip in the sample P16f and section analysis of the symmetrical indent showing pile-up on the sides of the indent

Figure 4

AFM section analysis of a fiber-reinforced polyester composite sample

Figure 5

Elastic modulus of fused quartz as a function of contact depth for (a) Berkovich indenter and (b) parabolic indenter

Figure 6

(a) SEM image of parabolic tip before indentation at a magnification of 5000×; (b) SEM image of tip after indentation at a magnification of 5000×

Figure 7

Elastic modulus of bulk E-glass fiber (a) versus contact depth for Berkovich indenter and (b) versus contact depth for a parabolic indenter

Figure 8

Elastic modulus of bulk polyester matrix (a) versus contact depth for Berkovich indenter and (b) versus maximum depth for parabolic indenter

Figure 9

Normalized elastic modulus variation as a function of distance from the fiber/matrix interface for an uncoated embedded glass fiber: (a) Berkovich indenter and (b) parabolic indenter

Figure 10

Normalized elastic modulus variation as a function of distance from the fiber/matrix interface for an MPS-silane coated embedded glass fiber: (a) Berkovich indenter and (b) parabolic indenter

Figure 11

Normalized elastic modulus variation as a function of distance from the fiber/matrix interface for an OTS-silane coated embedded glass fiber: (a) Berkovich indenter and (b) parabolic indenter

Figure 12

((1a) and (2a)) Phase images of the MPS and OTS samples. ((1b) and (2b)) Average section analysis of the MPS and OTS samples. ((1c) and (2c)) Average phase shift of the bulk fiber, interphase, and the bulk polyester matrix of the MPS and OTS samples.

Figure 1

AFM height and current images of indentations made in the interphase region of a sample

Figure 2

SEM images of parabolic tungsten tip: (a) a rough tip at 10,000× and (b) smooth tip at 5000×

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