Dr. A. Subramania
Centre for Nanoscience and Technology
Vickers hardness test
The Vickers hardness test was developed in 1921 by Robert L. Smith and
George E. Sandland at Vickers Ltd as an alternative to the Brinell method
to measure the hardness of materials.
The Vickers test is often easier to use than other hardness tests since the
required calculations are independent of the size of the indenter, and the
indenter can be used for all materials irrespective of hardness.
The basic principle, as with all common measures of hardness,
is to observe the questioned material's ability to resist plastic
deformation from a standard source. The Vickers test can be
used for all metals and has one of the widest scales among
The unit of hardness given by the test is known as the Vickers
Pyramid Number (HV) or Diamond Pyramid Hardness (DPH).
The hardness number can be converted into units of pascals.
The hardness number is determined by the load over the
surface area of the indentation and not the area normal to
the force, and is therefore not a pressure.
To calculate Vickers hardness number using SI units one needs to convert the
force applied from kilogram-force to newtons by multiplying by 9.806 65
(standard gravity ) and convert mm to m
Today’s mechanical systems are in need of continuous
improvements in enhancement of performance,
durability and efficiency of the components.
So, Super hard nanocomposites have gained attention
inorder to satisfy the needs.
Highly sophisticated surface related properties such as
mechanical, chemical and tribological properties of
super hard nanocomposites provide a best solution for
the improved efficiency of today’s mechanical systems.
On the basis of its hardness, nanocomposites
are classified into 3 categories
1) Hard materials – Hardness greater than
2) Super hard materials – Hardness greater than
3) Ultra hard materials – Hardness greater than
Super Hard Nanocomposite
Nanocomposites are materials that comprises of
two different materials in a standard proportion,
whose properties are better than the individual
Super hard nanocomposites (SHN) are those
which posses a vicker’s hardness greater than 40
Such materials are widely applied as coatings
over the mechanical devices.
Classification of Super Hard MaterialsMaterials
Generally, Super hard materials can be broadly classified
into 2 types
Intrinsically super hard materials
Hardness arises due to the atomic arrangement
itself. (E.g) Diamond, c- Boron Nitride.
Extrinsically super hard materials
Hardness arises due to the external processes
such as ion bombardment, production of
(E.g) nc-MN/a–Si3 N4 (M = Ti, W, V,etc.)
Intrinsically Super-Hard Materials
Diamond and cubic BN generally exhibits this
super hard capacity.
This is due to the arrangement of the atoms
present in the structure.
For example, in
diamond, the super hard property is due to the
diamond cubic structure of the crystal.
Hence, such materials found applications in
mechanical systems, where high load and heat
bearing capacity is inevitable.
Extrinsically Super- Hard Materials
In extrinsically super hard materials,
hardness is induced/generated in two ways
1. Ion bombardment
2. Formation of composites
The hardness of materials can be enhanced
by bombarding the deposited film using high
The hardness enhancement is due to a
complex effect involving
o decrease of the crystallite size
o densification of grain boundaries
o formation of frenkel pairs and other point defects
o built in biaxial compressive stress.
Hardness (vs) Compressive stress
• First, very high enhancement
of the hardness of TiN (up to
80GPa) and (TiAlV)N (up to
100 GPa) during deposition
by means of unbalanced
magnetron sputtering at
negative substrate bias is
• Later it was found that there
a correlation exist
enhancement and the biaxial
compressive stress induced
in the films.
Reason for Hardness
• The highest hardness enhancement upon energetic ion
bombardment is obtained in refractory hard ceramic coatings
deposited at a relatively low temperature of about <300oC.
• At a higher temperature, the hardness enhancement decreases and
completely vanishes above 600–700 oC
• Reason: The ion-induced effects anneal out during the film growth
within the deeper regions that are not accessible to the ions with
typical energy of a few 100 eV and corresponding projected ranges
of <10 nm.
• When the compressive (or tensile) stress is induced in a bulk
specimen by bending it, such an enhancement (or decrease)
doesn’t corresponds only to the amount of that stress.
• Therefore, a compressive stress alone can never enhance the
hardness to 60–100 Gpa , then ??????
The hardness enhancement
results from a complex
synergistic effect of the
– decrease of crystallite size
– built in compressive stress
– Formation of radiation
damage (Frenkel pairs,
etc.) upon energetic ion
Super Hard Composites
Depending on the crystallite size, the above said factors may
hinder the dislocation activity
Dislocation activity is absent in the superhard thermally highly
stable nanocomposites that consist of a few-nanometer small
crystallites of a hard transition metal nitride (or carbide,
boride,...) glued together by about one-monolayer-thin layer
of nonmetallic, covalent nitride such as Si3N4, BN (or in the
case of carbides by excess carbon, CNx, and others)
• These coatings, when correctly prepared, posses an
unusual combination of mechanical properties, such as
• high hardness of 40 to 100 Gpa
• high elastic recovery of 80% to 94%,
• elastic strain limit of 10%, and
• high tensile strength of 10 to 40 Gpa
• Moreover, the nanostructure and the superhardness
(measured at room temperature after each annealing
step) remain stable up to 1100oC
• Superhard “Ti–Si–N” coatings were produced
by means of plasma induced CVD (P CVD) using
chlorides as a source of Ti and Si.
• It is attributed the hardness enhancement to
the precipitation of small Si3N4 particles within
• The maximum hardness of 60–70 Gpa was
probably due to ternary nc-TiN/a-Si3N4/a-TiSi2
nature of this coatings.
Hardness - Composite formation
• In the majority of coatings deposited by PVD techniques at low
pressure of the order of <10-3 mbar and negative substrate bias,
there is a large biaxial compressive stress of 5 – 8 GPa due to
the energetic ion bombardment during their deposition.
• To check “if the measured hardness is not only enhanced by
• When the coatings deposited were annealed to 600–800oC, the
hardness increased to about 40 Gpa and the originally
amorphous films showed nanocrystalline XRD patterns.
• Thus, although the TiB2.2and TiN coatings deposited have a high
hardness enhanced by energetic ion bombardment during their
deposition , the hardness of the “Ti–B–N” coatings from the
middle of the nitrogen range in is due predominantly to the
formation of the nanocomposite structure
• The hardness maximum at about 27% of nitrogen is less
pronounced than usually found in our nanocomposites deposited
by P CVD.
Ion bombardment (vs)Composites
Stability of the hardness upon annealing
• The hardness that has been enhanced by energetic
ion bombardment strongly decreases with
annealing temperature to the ordinary bulk value
upon annealing to 400–600oC
• Superhard nanocomposites remains unchanged
upon annealing up to 1100oC .
• This softening upon annealing of the superhard
coatings hardened by energetic ion bombardment
is a general phenomena associated with the
relaxation of ion-induced defects in the films that
causes the hardness enhancement during
Ion bombardment (vs)Composites
Dependence of hardness with composition.
• TiN1-xCx forms a solid solution and therefore the
hardness follows the rule-of-mixtures
• In the case of the so-called nanocomposites,
consisting of a hard transition metal nitride and
ductile metal, the maximum hardness is achieved
with the pure nitride without that metal.
• The superhard nanocomposites prepared
according to our design principle show a
maximum hardness at a percolation threshold.
Intrinsic vs Extrinsic
Extrinsic materials are far better than the intrinsic materials.
Hardness (vs) Grain size
• With a decrease in the grain size, the hardness of the
• Hall-petch relationship
H(d) = H0 + Kd-1/2
• Dislocation movement, which determines the hardness
and strength in bulk materials, has little effect when the
grain size is less than approximately 10nm.
• At this grain size, further reduction in grain size brings
about a decrease in strength because of grain boundary
• Good mechanical properties of a coating require
•High adhesion strength on substrate
•Good load support capability and
•Chemical and thermal stability, etc.
• At present, nanoindentation is regarded as a good
method in hardness determination.
• In nanoindentation test, a diamond indenter is forced
into the coating surface. The load and depth of
penetration is recorded from which the hardness and
other elastic properties are calculated.