Mechanical Property Testing 1. Vickers Microhardness

Increase in the hardness of the composites reflects the resistance to plastic deformation and provides an idea about the strengthening due to addition of CNTs.

Microhardness measurements were made on metallographically polished coating cross sections. Microhardness measurements on cold sprayed coatings were made using a LECO M-400 microhardness tester. A load of 200 g and a dwell time of 15 sec were chosen so that the indent formed was of sufficiently large size so that hardness values represented the average values. Microhardness of the plasma sprayed coatings was measured using a Vickers microhardness Tester (Shanghai Taiming Optical Instrument Co. Ltd., model HXD-1000 TMC, Shanghai, China). A load of 200 g was applied for a dwell time of 15 seconds for the purpose. The average value of at least 6 indents was reported.

3.5.2. Nanoindentation

Nanoindentation technique is a relatively new technique which has been brought about by the advancements in precision measurement of forces and displacements in the range of few micro-Newtons and nanometers respectively. In nanoindentation, the load and depth of penetration are recorded during loading and unloading. Figure 3.4 shows the indent formed underneath a Berkovich tip and the typical load displacement curve. Oliver and Pharr have proposed a simple method to for calculating the reduced elastic modulus

Fig. 3.4: a) Schematic of the indent formed by a Berkovich tip, and b) typical load depth curve obtained in nanoindentation

We have

and hardness from the initial slope of the unloading portion of the curve [230].

S h P

h h

h_c  _max  _s  _max  ^max Equation 3.1

where  is is a constant that depends on geometry ( = 0.72 for conical, 0.75 for paraboloid of a revolution and 1 for flat punch) and S is the slope of the unloading portion of the curve at maximum load. The reduced modulus and hardness are given by the following equations:

A

H  P^max Equation 3.2

A E

S _r

 ²

 Equation 3.3

where H and Er refer to hardness and elastic modulus and  is a correction factor close to unity.

The nano-mechanical properties were measured by carrying out nanoindentation using a Hysitron Triboindenter TI 900 (Hysitron Inc., Minneapolis, MN, USA). A Berkovich type diamond indenter having tip radius of 100 nm was used. Nanoindentation was carried out on polished cross section of both coatings. For cold sprayed coatings, indentations were carried out at a load of 600 N. The load function comprised a linear increase in load up to 600 N in 10 s followed by a 10 s halt at maximum load and followed by a linear decrease in load to zero in 10 s. A matrix of 7 x 7 indentations (49 indents) was made for cold sprayed for Al-0.5CNT coating. Each indent was 10 m apart. Hence these values are obtained from an area of 70 m x 70 m. A matrix of 5 x 5 indentations (25 indents) representing an area of 50 m x 50 m was made for Al-1CNT coating. Fewer indents were made for the Al-1CNT coating because there was a lower spread in the values. For the plasma sprayed coatings, indentations were carried out at loads of 2000 N, 3000 N and 4000 N. The load was applied linearly up to the

maximum load in 5 s followed by a halt of 2 s at the maximum load followed by unloading in 5 s. Nine indents were made for each load on the matrix part of the nanocomposite coating, which makes it a total of 27 values of hardness and elastic modulus per sample. It was found that the results were consistent and nine values at each load were sufficient to generate an average value for the properties.

Scanning probe microscopy (SPM) images of the indent were obtained using the same Berkovich tip by rastering over the surface with a contact load of 2 N. The resultant scanning probe microscopy (SPM) images were analyzed using the SPM image processing software SPIP^TM (Image Metrology A/S, Horsholm, Denmark).

3.5.3. Nanoscratch Testing

Nanoscratch testing was also carried out on the polished cross sections using Hysitron Triboindenter TI 900 (Hysitron Inc., Minneapolis, USA). It has a horizontal capacitive transducer for applying normal load and two vertical capacitive transducers for measuring the lateral force experienced by the indenter during scratching. From plasma sprayed coatings, scratches of length 20 m were made at loads of 1000, 2000 and 3000

N using a Berkovich tip. During the loading cycle, the indenter moves 10 m to one side of the mean position after which the load is applied. During this movement the indenter records the surface profile from which the tilt of the sample is measured. The correct instantaneous depth is obtained by subtracting the tilt from the measured values.

After the load has reached the set value, the indenter starts scratching at a speed of 0.67

m s^-1. When the scratch length has reached 20 m, the load is released. The Berkovich tip is in the form of a triangular pyramid with total included angle of 142.3^o and has a tip

of radius of curvature equal to 100 nm. After the scratching has been performed, the same tip was used to image the surface by applying a contact load of 2 N. The resultant scanning probe microscopy (SPM) images were analyzed using the SPM image processing software SPIP^TM (Image Metrology A/S, Horsholm, Denmark). Depth profiles were taken along lines parallel and perpendicular to the scratch using SPIP^TM. The scratches were also examined using a JEOL JSM 630F scanning electron microscope employing a field emission electron gun in the secondary electron imaging mode. For cold sprayed samples, 10 m scratches were made using a Berkovich tip at a load of 1000

N.

3.5.4. Bulk Tensile Testing

Samples for tensile testing were machined out from the bulk cylindrical samples fabricated by plasma spray forming. Tensile specimens (all dimensions in mm) were machined along the axis as shown in Fig. 3.5 using wire electro-discharge machining (EDM). The spray direction was perpendicular to the axis of the specimens. Tensile samples were obtained from plasma-sprayed Al-Si, Al-5CNT and Al-10CNT cylinders.

Tensile tests were carried out using an MTS model 858, servo-hydraulic test system.

Hydraulic wedge grips were used to clamp the sample. The tests were run at a constant crosshead rate of 0.0085 mm.s^-1. The engineering stress in the sample was calculated by dividing the load by the original area of cross section. To measure the strain in the sample, a strain gage was attached to the center of the specimens using glue. Minimum amount of glue was used in order to not add to the strength of the composite. A total of 4 samples were tested for each composition for getting an average value and to check

reproducibility. The fracture surface of the tensile samples was examined under SEM to study the mechanism of failure under tension.

Fig. 3.5: Schematic of the tensile specimen prepared from the bulk spray formed cylinder (all dimensions are in mm)

3.5.5. Compression Testing

Compression testing was carried out cube shaped samples of edge 4mm cut from the bulk cylinder as shown in Fig. 3.6. Tests were carried out on Si, 5CNT and Al-10CNT Samples. Tests were carried out on three samples each and the best results were reported. The loading direction was along the cylinder axis so that load was applied parallel to the splats.

Fig. 3.6: Picture of the compression sample. The loading direction is parallel to the axis of the hollow cylinders.

As the specimen is loaded, the load varies linearly with displacement. When failure occurs, there is sudden drop in load. The engineering stress was calculated by the dividing the load by original area of cross section. The strain was calculated by dividing the distance moved by the crossheads by the original height of the cube. The proportional limit and the fracture strength of the materials under compression were used to compare the strength of the three materials. The fractured pieces were obtained and observed under SEM to study the deformation and failure mechanisms in compression.

4. RESULTS AND DISCUSSION