Nanoindentation

The trend towards miniaturization of components has created a corresponding need for methods of determining the mechanical load capacity of thin films and coatings. Included in this are adhesive strength and wear - altogether nanoindentation.

Principle of the QCSM method

Depth-dependent measurements with the QCSM module

During depth sensing nanoindentation measurements for the determination if the indentation hardness HIT according to DIN EN ISO 14577, the load-displacement curve F(h) is measured with a certain maximum load. The hardness can only be given for the maximum depth which is reached in this measurement. A hardness profile along the depth can only be determined by many measurements with different loads on different places of the sample. This is a time-consuming procedure and requires much effort for the analysis of the data.

With the CSM or QCSM method, the contact stiffness is measured already during
loading as quotient of force amplitude and displacement amplitude of a small oscillation.

Amplitudes of all single oscillations during a dwell time of 1.4 s and for a frequency of 40 Hz. The first 12 of 56 oscillations are not used for averaging because creep effects are largest immediately after reaching the target force.

The Continuous Stiffness Measurement (CSM) method adds a continuous small oscillation to the force signal. The ratio of force and displacement amplitude delivers the contact stiffness between indenter and sample after some corrections which consider moving mass, frequency and damping coefficient. In the CSM method the static force during loading is for every oscillation slightly different. This complicates the averaging of several oscillations and the feedback control.

In contrast during the QCSM method the force is increased in small steps and the oscillation is only switched on during a short dwell time between about 0.5 s and 3 s (see principle of QCSM method). This allows an easy averaging of several oscillations and also the feedback control is more accurate. For example there are amplitudes from 56 oscillations measured at a frequency of 40 Hz and a dwell time of 1.4 s. In the QCSM method the first 20 % of the measured amplitudes are not considered for averaging to reduce the creep influence on the results. This is especially important for viscous materials.

Nanoindentation: Micro wear investigations of coatings

Diamond like Carbon coatings (DLC) are widely used in industrial applications due to their high hardness, low friction and high corrosion resistance. There are still discrepancies between test results of industrial wear tests under application conditions and standard laboratory wear tests.

To investigate and understand the dominating wear mechanisms it is necessary to investigate single asperity contacts with contact radii between about 0.1 μm - 20 μm with high resolution. There exist scarcely wear measuring techniques in the load range below 1 N with nanometer resolution in the displacement measurement.

Nanoindentation technique in combination with high resolution lateral force-displacement measurements can now be used for such investigations.

For the micro wear tests the Universal Nanomechanical Tester ZHN with Lateral Force Unit LFU is used.

  • Test condtitions:
  • 500 cycles oscillating sliding
  • 80 μm amplitude, constant speed
  • 6 s per cycle → speed 26.7 μm/s
  • 3024 s measurement time
  • 8 Hz data rate

Samples, indenters, normal forces

Parameter

Film Material

Film thickness µm

Hardness GPa

Young's modulus GPa

Yield strength GPa

Poisson's ratio

Sample 1

a-C:H

4

14.5

120

10.9

0.2

Sample 2

a-C (high sp3)

5

50.0

542

30.1

0.2

Sample 3

a-C

3

15.0

170

8.8

0.2

Sample 4

a-C:W (17%)

3

14.5

140

9.5

0.2

Sample 5

a-C:H (structured)

4

12.2

103

9.0

0.2

  • Indenter 1: Diamond, 67 μm initial radius, 5 forces 50 mN – 1000 mN
  • Indenter 2: Diamond, 6 μm initial radius, 7 forces 5 mN – 200 mN
  • Indenter 3: Hard metal, 100 μm initial radius, 4 forces 100 mN – 1000 mN

Summary of wear rate results

  • Wear during slow oscillatory motion and about 50 % humidity is starting for DLC coatings when the contact pressure is about 10 % - 30 % of yield strength.
  • The wear mechanism is changing when the contact pressure reaches the yield strength
  • The wear rate is approximately proportional to contact pressure for smooth surfaces. The depth increase per sliding movement is smaller than 0.15 nm and therefore only 0 - 2 atomic layers. Wear is a continuous process without quarrying out particles.
  • Wear starts at lower normalized contact pressure for hydrogen containing DLC.
  • Higher hardness is no advantage for this type of wear. At equal load the (absolute) wear rate for hard coatings is approximately the same than for soft coatings.
  • Counterparts made from hard metal induce about 2.5 times higher wear rate of the coatings than diamond tips.
  • For the investigated type of wear there is no correlation between wear rate and friction.

Fig. 4: Contour plot of the force amplitude

Several indents have been done into a fused silica sample with a spherical indenter of about 10 µm radius. The same tip was also used for scanning the sample. Fig. 2 shows the glass surface with indents of 800 mN (upper left) and 2x 500 mN. Additional indents at lower forces have been fully elastic. One indent at 200 mN is hard to recognize optically, however, a small plastic deformation of some nanometer can be measured.

  • The scans have been carried out with a contact force of 15 mN. This is a relatively large scanning force but the contact is fully elastic and it allows a better measurement of the lateral force for the expected small friction coefficient. The scan range corresponds to the image size of 97 µm x 77.5 µm for the optical image with the highest magnification of about 3350 on the screen. The other test parameters have been as follows:
  • 45 lines
  • Scan time per line: 25 s (for high resolution)
  • Data rate: 8 Hz
  • Offset 10 % (additional scan length at both sides outside the analyzed range to exclude start-stop effects)
  • Vibration frequency 40 Hz
  • Amplitude 0.1 V (corresponds to about 5 nm displacement and 0.8 mN force amplitude)

The mapping of the normal force signal allows a clear detection of the indent positions because the force becomes lower when the indenter slides into the impression and becomes bigger when it goes out. The force control is not fast enough to cancel out this effect.

Fig. 5: Young’s modulus mapping. The results at the indent positions are incorrect.

In fig. 3 a slight distortion is also visible at the position of the 200 mN indent. A similar result is available when only the force amplitude of the oscillation is presented (fig. 4).

For the determination of the Young’s modulus not only the contact stiffness is necessary – which can easily be obtained from the measured force and displacement amplitudes – but also the correct indentation depth. Therefore a zero point correction for the displacement measurement is necessary. It can be done in the same analysis window using the button „Zero point correction“. The result of the modulus mapping of fused silica is shown in fig. 5. The expected value of 72 GPa is well attained over the complete area with the exception of the indent positions. There the analysis model is incorrect which assumes a flat surface and therefore the results are too big.

The friction coefficient between diamond tip and glass is obtained from the ratio of lateral and normal force. It is shown in fig. 6 and 7. At the indent positions the friction is first going down in direction of movement according to fig. 3 and is increasing when the tip moves out of the impression.

In the flat area the friction coefficient is between 0.7 - 0.8. Only in the front part of the sample it is a little lower. The reason for that is not clear.
The measurements of all the presented properties have been done during one scan which was with about 2000s relatively long. A considerable reduction of the scan time is possible; however, this may result in an increase of the scatter. The best parameters have to be checked for every single sample.

Further nanoindentation applications

  • Evolución de capas de blandas (polímeros) a duras (capas tipo diamante)
  • Determinación de las tensiones críticas para formación de grietas o deformación plástica
  • Capas de materiales duros para herramientas y protectores de rayaduras
  • Capas protectoras sobre vidrio
  • Barnices y capas sol-gel
  • Medición automática del gradiente de dureza en la sección transversal
  • Nanocapas para sensores y MEMS/NEMS
  • Materiales biológicos
  • Efectos matriciales en aleaciones (mapping)
  • Materiales cerámicos y composites
  • Implantación iónica en superficies
  • Análisis de daños en microelectrónica
  • Determinación de la capacidad de carga en superficie (ELASTICA)
Nanoindentador ZHN con LFU en su aplicación

Brazilian university relies on ZwickRoell testing technology

The comprehensive mechanical characterization of thin films or small surface areas with the necessary force resolution and displacement resolution is a key area of research at universities around the world. To this end a University in Brazil has recently begun using a ZHN universal nanomechanical tester by Zwick Roell.
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