By- D.R. Kartikayan

Topics to be covered

Initial

• Introduction to some characteristic material properties

• Why Nanoindentation (NI)?

Intermediate

• The requirements and the procedure of NI

• Analysis of Nanoindentation results

Conclusion

• Factors affecting Nanoindentation

• Scope of Finite Element Method (FEM) in interpreting the results

Characteristic Material Properties

Hardness

(On the Basis of Load and Depth of Penetration)

Macro Hardness

Micro Hardness

Elastic Modulus

It is a measure of the stiffness of the Material

For Metallurgical Purposes – Determined through Indentation Experiments

“ Resistance to the penetration of a hard indenter upon application of load”

Nanoindentation

What is Nanoindentation

• Such an indentation testing in which both load and the depth of penetration are in the micro scale or lower.

• Gives real time measurement of Load and Depth

Why Nanoindention?

• Can be readily used in the measurement of mechanical properties of thin films and surface layers.

• Less error prone and less time consuming

Conventional Hardness testing Vs

Nanoindentation

Conventional Hardness Testing

Nanoindentation Testing

Requirements of

NanoindentationUnlike Conventional hardness

testing Nanoindentation requires

some conditions to be fulfilled:

High accuracy, precision

equipments for recording the

small load and displacement

Analytical modeling for

utilizing the load

displacement curve data for

determining the various

mechanical properties.

Total included angle: 142.3˚ with

an half angle of 65.35˚

Average radius of curvature

between 100 and 200 nm

Procedure

Loading and then Unloading is done on the material surface

Loading considered to have elastic plastic deformations

while unloading is considered fully elastic

Maximum load is so selected that a fully developed plastic

zone is formed in the material

The maximum value

of mean contact

pressure obtained

during loading is

called HARDNESS

The data obtained

during unloading

forms the basis for

contact area

during Maximum

load

Stiffness

obtained from

the unloading

curve

Load Displacement curve analysis

Load Displacement curve consists

of two parts:

Loading part

Unloading Part

Analysis of the curve can give us:

1) Hardness

2) Elastic Modulus

3) Strain rate sensitivity

4) Activation volume

Contd….

Analysis of the unloading curve:

The slope of the unloading curve is a measure of the

stiffness of contact

Such analysis can be done by two methods

a)Doerner-Nix Method: Less complicated,Linear curve

fit

b)Oliver-Pharr method: More complicated, non-linear

curve fit, more accurate results

Such analysis has:

a) Deformation upon unloading to be purely elastic

b) Contact modeled as rigid indenter of defined shape with homogenous elastic

half space.

c) The compliance of sample and indenter tip can be thought of as combination of

springs in series.

Indentation Mechanics

Specimen Surface

Indenter Tip

Nanoindentation Result

Some Models to explain the Indenter-

Specimen interaction

Elastic

Model

Rigid

Plastic

Model

Spherical

cavity

model

Elastic and

perfectly

plastic

model

Factors affecting Nanoindentation

Thermal Drift

Instrument Compliance

Indenter Geometry

Indentation Size effect

Piling up and Sinking In

Initial Penetration Depth

Surface Roughness

Tip Rounding etc

Indentation size effect

Increase in Hardness

(ISE)

Decrease in Hardness

(RISE)

Decrease in

Load

Hinders the Use of Hardness as a

Characteristic material property

Various Models proposed to explain ISE

•Minimum resistance model

•Proportional specimen resistance

•Energy balance approach

•Combined approach

•Elastic plastic deformation model

•Indentation induced cracking

•Strain gradient plasticity

•Dislocation mechanics

Piling up

Raised Surface at Edge of the

indentation

Support for IndenterIndenter

penetrates lesser

Hardness and Elastic Moduli

are Overestimated

Finite element Analysis of Load

indentation data

Factors affecting NI not considered More error prone results

Deformed Indenter Tip Less

penetration depth

Calculated contact region different from the actual contact region

The results are error prone

ISE can be accounted for

Empirical and analytical

methodologies

Higher accuracy of results as well

as other advantages

FEM Analysis

FEM applied for contact between the indenter

tip and material surface.

FEM applied to study the deformation of

Indenter tip upon indentation.

FEM applied to the surface under the indenter

to account for piling up or sinking down.

All of the above makes FEM a very

efficient tool for analyzing NI Results

References1) M.F. Doerner and W.D.Nix, ‘A method for interpreting the data from depth-sensing

indentation instruments’, J. Mater. Res 1 (1986)2) W.C. Oliver and G.M. Pharr, ‘An improved technique for determining hardness and elastic

modulus using load and displacement sensing indentation experiments’, J.Mater. Res 7(1992)

3) W.C. Oliver, R Hutchings and J.B. Pethica ,(American society for testing and materials) ASTMSTP (1986)

4) N.K.Mukhopadhyay and P.Paufler, Micro and Nanoindentation techniques for mechanicalcharacterisation of materials, IMR 453.3d (2005)