Nanoindentation is a technique for measuring the mechanical properties of materials. Conventional indentation hardness tests involve measuring the size (area) of a residual plastic impression in the specimen as a function of the indenter load. This provides a measure of the hardness of a sample. In nanoindentation, the size of the residual impression is often only a few microns making it very difficult to obtain a direct measure using optical techniques. Nanoindentation testing is performed using a sharp indenter made from a hard material (commonly diamond). This indenter is pressed into the specimen to extract the hardness and the elastic modulus from load-displacement curves. It has become one of the most widely used techniques for measuring the mechanical properties of films and soft structures. Other advantages of nanoindentation stem from the ease with which a wide variety of mechanical properties can be measured. Both bulk specimens and thin films can be measured, as well as the ability to probe a surface at specific points creating a spatial mechanical property map. [4]
In nanoindentation testing, the initial response of the material is elastic but plastic deformation can occur at higher loads. Typically, the depth of penetration beneath the specimen surface is measured as the load is applied to the indenter. The known geometry
of the indenter allows the size of the area of contact to be determined. Different indenter tip geometries (including spherical, Vickers, Berkovich and conical) can be responsible for the change of mechanical properties of the material. [8] Nanoindentation hardness tests are generally made with spherical or pyramidal indenters (Berkovich) using load versus penetration depth curves. [8] Details of the plastic deformation regime can be studied from the load-unload measurements via discontinuity events such as pop-in or pop-outs or departure of the unloading curve from that of loading. The nature of the plastic deformation obtained from the details of load versus penetration depth curves reveal much information about the pathway of deformation as well as the stresses involved in the residual impressions.
Indentation tests on materials can probe both the elastic and plastic deformation behaviour of the material. In brittle materials, plastic deformation is the most easily probed with sharp indenters (Vickers or Berkovich) since high loads are needed under conditions that minimize brittle fracture. In ductile material plastic deformation more readily occurs using blunt indenters (spherical) at low loads. Indentation testing is not only a very useful technique to measure the hardness of the material but can be used to measure other mechanical properties like material strength, fracture and residual stresses in the material, and hence has been an important tool in the study of materials for many years. [5]
The indentation hardness is defined as:
H = Pmax ∕ A
Where H – Hardness, Pmax – maximum applied load and A - (projected) area of the
residual impression. [6] As previously stated, the conventional method to measure hardness requires imaging of the residual impression by optical microscopy to obtain the area of the plastic zone. However, this method is not accurate for the micron sized residual impressions associated with nanoindentation. Thus other methods are necessary. In nanoindentation testing, the projected area of the residual impression for an indentation subjected to a maximum load can be calculated from the indenter geometry and the P-h curves according to the methods of Oliver and Pharr [7] or Field and Swain [8] as indicated later. The geometry of the indenter plays an important role in nanoindentation testing and in this work spherical indenters with a radius of ~ 4.3 μm or ~ 20 μm are used.
2.3.1 UMIS (ultra micro indentation system)
Figure 2.2: Schematic of the ultra-micro indentation system (UMIS).
Fig. 2.2 shows a schematic of the UMIS-2000. During nanoindentation, a piezoelectric actuator applies load to the main carriage. Leaf springs transfer force from the carriage to the indenter shaft. A linear variable differential transformer (LVDT) measures the displacement of the indenter shaft relative to the carriage: from this, using the spring constant of the leaf springs, the force on the indenter tip can be calculated. Another LVDT attached to the frame of the indenter measures the depth.
The force and the depth measuring systems are basically the same, differing only in the gain of the final amplifier. They are based on high linearity LVDTs. The upper unit measures depth and the lower unit measures the displacement of the force generating
springs. The associated electronics are complex in order to achieve the very high sensitivity and stability required by the system. [5]
The depth LVDT continuously monitors spring deflection as the indenter approaches the specimen; it is the role of the “depth offset” circuit to reference the moment of contact as being the zero penetration voltage and subtracting that voltage from all subsequent readings. This enables the full range of the amplifiers and the A/D converter to be utilised. The “force offset” circuit is activated during the zeroing phase to cancel any voltage resulting from the force LVDT prior to a measurement cycle. The depth signal is monitored by the analog interface and the various depth readings are measured at that point. [5]
UMIS is used to perform indents in the range of 50 mN to 1000 mN in this work. The UMIS nanoindenter can be operated in two modes, closed loop and open loop. In this study we have used the closed loop mode. In the closed loop mode, a feedback loop is used to obtain a precise value of load (or depth) at each measured increment on the loading cycle. In open loop mode, no feedback is used; the load signal is simply ramped up at a fixed rate. The closed loop mode gives more control over the loading cycle; the open loop mode allows a higher rate of data collection, and allows higher maximum loading rates. [9]
In this study spherical indenters are especially useful because of their smooth transition from elastic to elastic-plastic contact. Berkovich indenters are also used for to measure the hardness and other mechanical properties of the material but have been used to a lesser degree in this thesis since the onset of phase transformation is more difficult to determine. A Berkovich is a three sided pyramid which is geometrically self-similar. It has a very flat profile, with a total included angle of 142.3 degrees and a half angle of 65.31 degrees. [2] The Berkovich tip has the same projected area to depth ratio as a Vickers indenter but, because it is designed to be sharper than the Vickers geometry, it ensures the more precise control over the indentation process. [9] Oliver et al. study has reported nanoindentation on Ge using both spherical and Berkovich indenters. Observations have clearly shown phase transformation can be induced in Ge using both indenter types.