Advanced Surface Mechanical Testing
Vickers Hardness (by indentation)
Instrumented indentation technique can also be used to calculate Vickers hardness automatically without the optical measurement of indentation impression. As the nanoindenter system records indentation depth, the contact area of Vickers indentation is calculated based on the tip geometry and the final indent depth. There is a direct mathematical correlation between the instrumented hardness (in Pa unit) and the Vickers hardness (in Vickers unit). Of course if the optical method is still preferred to obtain the Vickers values, our software can measure the diagonal length of the indentation impression under the microscope, and the Hv can be derived from the following equation, where d is the average diagonal length (in mm) of the impression, F is the load (in N).
Fracture Toughness (by indentation)
When indenting on brittle materials, the sharp corner of the indenter tip will cause strong localized stress concentration and fracture the materials as shown in the picture below. The length of these corner radial cracks is related to the materials fracture toughness (Kc). By measuring the crack length from center of indentation (c), as well as the materials modulus (E) & hardness (H), Kc can be calculated mathematically. The typical indenter tips used for fracture toughness measurements are Cube Corner, Berkovich, and Vickers, Cube Corner is usually preferred as it is sharper than the other two, which makes the fracture failure more prominent. It should be noted that not all the materials show such radial cracks at the corner of indentation imprints. Ductile metals like gold and silver will never behave like that. This technique is limited to brittle materials such as ceramic and semiconductor materials.
Stress-Strain Curve & Yield Stress (Compressive)
Compressive stress-strain curve can be obtained via a multiple-cycle nanoindentation test. The multi-cycle indentation continuously hits the same spot on the sample surface, but each time with higher load and deeper penetration depth. From each cycle, the stress and strain values are calculated, so each data point in the stress-strain curve represents one cycle of nanoindentation. One of the main functions of stress-strain curve is to capture the Yield Stress (onset of material plastic deformation). So the multi-cycle nanoindentation usually starts with a very small load to make sure the first few indents are still introducing elastic nanoindentation only. Also to further reduce the contact pressure, the multi-cycle nanoindentation employs sphero-conical tip so that the contact area is large enough to prevent plastic deformation from happening too early. The following figures shows how the contact area radius (a) is calculated from indentation contact depth (hc). The stress and strain are both functions of contact area radius (a). In a typical stress-strain curve, the first linear session of the curve represents the elastic deformation zone of nanoindentation. The slope of the linear curve represents the elastic modulus of the material. When plastic deformation (permanent damage) starts to kick in, the curve loses it linear trend and start to follow the non-linear fashion. The stress at the onset of non-linear curve is the Yield Stress.
Rather than targeting specific unique sites, it is often useful to look at the distribution of hardness and modulus across a large area of interest. This can highlight areas of non-uniformity due to structural anomalies, variation in surface treatments or simply changes in properties at joints and boundaries. The mechanical mapping is usually achieved by performing an array of indentation ( 10x10, 20x20, etc.) with a separation distance of at least 3 times the indentation depth between each two adjacent impressions. In this case, a good area of sample surface is covered by indentation, and the modulus and hardness properties can be mapped within this area. The mechanical mapping can provide a visual clarification that which area of the sample surface is harder or softer than the rest of the sample surface. It can also be used to locate the surface defects in very small (micro or nano) scales.