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3.4 Evaluation of Mechanical Properties
Elastic modulus, hardness and fracture toughness of HA-nanotube composites and coatings have been studied. Nanoindentation and microindentation techniques were used to measure mechanical properties at multiple length scales.
3.4.1 Nano-Indentation: Elastic Modulus and Hardness
Hysitron Triboindenter TI-900 (Hysitron Inc., Minneapolis, MN, USA) with 100 nm Berkovich pyramidal tip, was used in quasi-static indentation mode to measure the elastic modulus and hardness of the sintered pellets and coatings. Tip-area calibration was done using a standard fused quartz substrate of known modulus (69.6 GPa).
Indentation was performed with a constant loading/unloading rate for 10 s and 3 s hold at the peak load of 2500 µN. Elastic modulus (E) was calculated from the unloading segment of the load-displacement curves using Oliver-Pharr method [6]. Following procedure has been adopted for E and H calculation:
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(i) The selected portion of the unloading curve (upper 95% to lower 20%) is fit to the power law relation:
……… (3.1)
where, P is the applied load, h is the depth of penetration and α and m are constants.
(ii) The derivative of the power law relation with respect to h is evaluated at the maximum load to calculate the contact stiffness, S.
(iii) The contact depth, hc, is calculated using S as:
0.75. ……. .(3.2) (iv) The hardness is calculated as:
……… (3.3)
where, A(hc) is the area as a function of contact depth, obtained from the tip area calibration function.
(v) The reduced modulus is calculated as:
√ . ……… (3.4) (vi) The elastic modulus of sample E is calculated as:
1
where, υ is Poisson’s ratio. The indenter used is diamond with E = 1140 GPa and υ = 0.07.
Nanoindentation provides localized mechanical properties. In order to get an impression of the bulk mechanical properties of the composites, more than 100 indents were made at randomly chosen regions throughout the polished cross-section of each of the composites. In each region, the indents were made at a distance of 9 µm from each
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other. Total area covered by the indents was > 5000 µm2 in each sample. The statistical distribution of elastic modulus, measured from individual indents, thus provides the mechanical properties of the composite at macro-scale length.
3.4.2 Micro-Indentation: Fracture Toughness and Hardness
Microhardness was measured using a microhardness tester (Shanghai Taiming Optical Instrument Co. Ltd., model HXD-1000 TMC, Shanghai) with Vickers probe and application of 1 kg load for 15 seconds of dwell time. Microindents in the consolidated composites were performed to determine the fracture toughness by initiating the cracks.
Fracture toughness was evaluated using Anstis’ equation [7] expressed as:
32 12
) ( 016 . 0
c P H
KIC = E ………(3.6)
where, P is the applied load, E is the elastic modulus, H is the Vickers hardness and c is the radial crack length (measured from the center of the indent). For an accurate measurement of radial crack length, the indents were also observed under SEM. Elastic modulus values for the composites were estimated from nanoindentation. The microindents on the polished cross-section and the radial cracks generated were observed through high resolution SEM imaging to understand the role of nanotubes in toughening of the composite.
122 3.5 Evaluation of Tribological Behavior
Tribological behavior of the composite is analyzed at multiple length scales by estimating the wear volume and the coefficient of friction. Wear volume is indicative and inversely related to the wear resistance. Wear at macro scale was performed using ball-on-disk method. Nano-scale tribological studies were performed using nano-scratch.
3.5.1 Tribology: Macro-scale Wear
Ball-on disk tribometer (Nanovea, Micro Photonics Inc., CA) was used to evaluate the macro-scale wear resistance and coefficient of friction (CoF) of sintered pellets and coatings. Samples were polished to a roughness (Ra) of 0.5 µm or less.
Macro-wear studies are performed at 50 RPM speed with a circular track of 2 mm radius and a total travel distance of 100 m. The linear speed of wear probe on wear track was ~ 10.5 mms-1. An alumina ball of 3 mm diameter is used as the counter surface (probe).
The lateral force between the alumina ball and the composite surface and depth of wear track is measured by the linear variable differential transformer (LVDT) sensor. The coefficient of friction data is acquired at a frequency of 16.67 Hz. In case of HA-CNT and HA-BNNT sintered pellets, the wear volume is measured by considering the depth of wear track from LVDT and the geometry of the wear probe. In case of plasma sprayed HA/HA-CNT coatings, the wear track profiles across the tracks are obtained using Nanovea ST400 Optical Profiler. Wear volume is computed using the depth profile from the wear tracks. The depth from optical profile is in good agreement with the ones measured by LVDT in this case, proving the accuracy of the LVDT data used in other cases. Macro-wear tracks are observed closely along with the wear debris to understand
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the wear mechanism in HA-nanotube composite. Elemental analysis of the samples, as required is some cases, is performed using energy dispersive spectroscope (EDS) attached with a JEOL JSM 5910LV scanning electron microscope.
Wear study in physiological condition, is performed by immersing the sample in simulated body fluid (SBF), while carrying out the ball-on-disk wear using the same testing condition and wear probe as dry wear. The SBF is prepared using Kokubo’s recipe [8] with the chemical composition as presented in table 3.2.
Table 3.2: Chemical composition of Simulated Body Fluid (SBF)
3.5.2 Nano-Scratch: Micro/Nano-scale Wear
Hysitron Triboindenter TI-900 (Hysitron Inc., Minneapolis, MN, USA) with 100 nm Berkovich pyramidal tip, is used in 2D scratch mode for nano-wear studies. The scratches of 10 µm length are made with constant normal loads of 3500 µN and 4500 µN
Ingredient Amount (g/l)
NaCl 7.996
NaHCO3 0.350
KCl 0.224
K2HPO4 . 3H2O 0.228
MgCl2 . 6H2O 0.305
CaCl2 0.278
Na2SO4 0.071
(CH2OH)3CNH2 6.057
1 kmol/m3 HCl To adjust the pH – 7.25 Ultrapure water To make volume upto 1 litre
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on the metallographically polished SPS HA-CNT composite cross sections. Lateral force and scratch depth along the scratch length is recorded by two different piezoelectric sensors working in directions perpendicular to each other. Scratch profiles are obtained by scanning probe microscopy (SPM) with the same tip at a set point load of 2 µN. The topography image processing is performed using Scanning Probe Image Processor (SPIP) version 4.5.1 (Image Metrology, Denmark) [9]. Scratch volume calculation has been performed from the geometry of the scratches obtained through 2D profiles of scratch along the length and width, using the following expression:
=
l
dl h
V
0
2
tan θ .
………(3.7)where, V is the volume of the scratched groove, h is the height of the groove (obtained from 2D SPM profile of scratch along the length), θ is the average angle of the groove measured at five points along the scratch length using 2D SPM profile and l is the length of the scratch. The detailed procedure of this volume calculation is available in our earlier publication [10].