Chapter 7 Chapter 7 Conclusions and Future Scopes
2.6 Nanoindentation Test
2.6.1 Principle and Procedure
Nanoindentation tests are conducted on CETR Nanoindenter (UMT-2, Figure 2.16) using a Berkovich indenter tip (tip radius 50 nm) on a polished composite samples. A total of two indentations are performed on each specimen using the continuous stiffness measurement (CSM) technique and the average values of indentations are presented in this work.
Chapter 2 Fabrication and Characterizations…
Figure 2.16 CETR Nanoindenter (UMT-2)
Figure 2.17 shows the representative load vs. displacement curves for pure polypropylene matrix and for the 5, 10 and 15% cement filled composites. Continuous nanoindentation is performed following six steps: indenter approach to the surface, loading to peak load for 24 s, holding the indenter at peak load i.e. creep for 10 s and allowing thermal drift for 5 s, unloading 90% of peak load for 20 s, holding the indenter after approximate 90% unloading for 15 s, and finally, unloading completely. This curve of quasi-static loading vs. indentation depth comprises of both elastic and plastic deformations. This is, from the starting of the unloading slope, the conventional elastic indentation stiffness is calculated (Wong et al., 2006). The indentation is monitored and applied on the polished surface of the composites and as well as pure polypropylene samples also. The instrumented indenter operating under continuous stiffness measurement (CSM) technique can sensitively assess local stiffness as the indenter tip traverses the polished surface of the composite medium (Wong et al., 2006;
Thome´ et al., 2009).
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0 2 4 6 8 10 12
0 0.5 1 1.5 2 2.5 3
Load (mN)
Depth (micron)
0%
5%
10%
15%
Figure 2.17 Load vs. depth of the composite materials for nanoindentation
An unremitting stiffness measurement that implicated a progressive series of loading and unloading cycle is conducted until the final indentation depth is achieved, generating a series of hardness and modulus values as a function of indentation depth (Fang and Chang, 2004;
Fang et al., 2005).
Hardness (H) and elastic modulus (E) are calculated from the load–displacement results.
When, the indenter penetrates into the specimen, both elastic and plastic deformation takes place out of which only the elastic segment of the displacement is retrieved during unloading condition. According to Oliver and Pharr (1992), hardness of nanoindentation can be defined as:
Pmax
H A (2.10) where Pmax is the load measured at a maximum depth of penetration in an indentation cycle; A is the projected contact area of the hardness impression. The elastic modulus of the sample can be derived from the initial unloading contact stiffness (S). This contact stiffness, expressed as dP
dh, is the slope of the initial part of the unlading curve (Oliver and Pharr 1992; Lee et al., 2007). A relation among contact stiffness, contact area, and elastic modulus can be expressed as:
c r
dP A
S 2 E
dh
(2.11)
Chapter 2 Fabrication and Characterizations…
Where, β is a constant which depends on the geometry of the indenter (β = 1.034 for Berkovich tip) and Er is reduced elastic modulus, which attributes that elastic deformation occurs in both the sample and the indenter.
The sample elastic modulus (E) can then be calculated as (Oliver and Pharr 1992; Lee et al., 2007):
2 1
2 i
r i
1 1
E 1
E E
(2.12)
where ν and νi are the Poisson’s ratios of the specimen and indenter, respectively, while Ei is the modulus of the indenter Er is the reduced modulus. Table 2.4 shows a series of hardness, indentation Young’s modulus and reduced modulus values of polypropylene matrix and its composites obtained by continuous nanoindentation with 1 mN/s indentation rate. The variation of the values may be due to the roughness of the samples and its different compositions (Odegard et al., 2005).
Table 2.4 Nanoindentation properties of the composite materials
Figure 2.18(a)-(c) illustrate the indentation stiffness, tensile stiffness and bending stiffness as a function of filler percentages are calculated through indentation, tensile and bending tests.
The indentation stiffness is compared to the stiffnesses obtained from (i) standard tensile and (ii) 3-point bending tests using the specimens discussed in the chapter 3. Figure 2.18 shows the stiffnesses of unreinforced polypropylene and 5, 10, 15% cement filled polypropylene composites obtained from the nanoindenter and from the uniaxial tensile and 3-point bending tests. As shown in Figure 2.18 (a), it is observed that indentation stiffness increases when cement content increases except pure polypropylene; the probable reason could be the high variation of surface roughness of polypropylene sample. It can be attributed from the results that higher cement contents provide significantly higher resistance against materials
Filler content Indentation Young's modulus (GPa)
Reduced modulus (GPa)
Hardness (GPa)
0% 3.36 3.45 0.048
5% 3.97 3.05 0.062
10% 3.03 3.10 0.061
15% 3.75 3.83 0.046
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deformation under both tensile and bending. Similarly, indentation stiffness of the material indicates that higher percentage of cement fillers provide considerably greater resistance along with the puncture deformation of the materials. This finding can be streamlined by the fact that the cement possesses inherent stiffness higher than the matrix material.
0 15 30 45 60
0 5 10 15
Indentation stiffness (mN/micron)
Cement content (wt%) (a)
0 2000 4000 6000 8000 10000
0 5 10 15
Tensile stiffness (mN/micron)
Cement content (wt%) (b)
Chapter 2 Fabrication and Characterizations…
0 15 30 45 60
0 5 10 15
Bending stiffness (mNm2 )
Cement content (wt%) (c)
Figure 2.18 Variation of different stiffness (a) continuous indentation stiffness, (b) tensile stiffness and (c) bending stiffness with respect to filler content
Further, nanoindentation gives measure of the work done by loading and unloading cycle and are evaluated. Figure 2.19 shows the work done for the elastic and plastic deformation of the materials. It is observed that both the cases work done for elastic and plastic deformation decreases as cement contents increases. This finding can be rationalized by the fact that the cement particles enhanced inherent energy absorption during plastic deformation higher than the elastic deformation for pure polypropylene as well as its composites. At the same time, it is observed that work done decreases owing to the increase of cement contents, results in increase of brittleness as well as stiffness of the composites.
0 1 2 3 4 5 6 7
0 5 10 15
Work done (nJ)
Cement content (wt%)
Plastic deformation Elastic deformation
Figure 2.19 Work done due to plastic and elastic deformation subjected to nanoindentation test
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