2. nanocomposite coating
• Comprises of at least two phases
Nanocomposite coatings can be
• hard (hardness > 20 Gpa),
• Superhard(above 40 Gpa)
• Ultra-hard(above 80 Gpa)
Application
• To achieve Highly sophisticated surface properties
(i.e. optical, magnetic, electronic, catalytic,
mechanical, chemical and tribological properties)
3. Design methodology for nanocomposite coating
• Reduction in grain size(<10nm) leads to decrease in
strength because of grain boundary sliding
• Softening by grain boundary sliding attributed to
large amount of defects in grain boundaries
To increase hardness
• Requires hindering of grain boundary sliding by
increasing the complexity and strength of grain
boundaries
5. Ways to achieve hardness and toughness
• Multiphase structures
• deflection, meandering and termination of
nanocracks
• Some degree of grain boundary diffusion and grain
boundary sliding
6. Possible design methods
• A combination of two or more nanocrystalline
phases
• hard nanocrystalline phases within a metal matrix
-Yttrium; to improve thermal stability
-modify the interface complexity using a ternary system
• embed nanocrystalline phases in an amorphous
phase matrix
7. Possible design methods
• Include two or more nanocrystalline phases-
provides complex boundaries to accommodate
coherent strain
• Segregation of nanocrystalline phases to grain
boundaries- generate the grain boundaries
strengthening effect, and stops grain growth
8. Possible design methods
• The above composite design could significantly
increase hardness and elastic modulus.
• to increase toughness-sufficient cohesive strength of
the interface to withstand the local tensile stress at
the crack tip
9. Hard nanocrystalline phases within a metal
matrix
• Like TiN in Ni, ZrN in Ni.
• The hardness achieved- 35-60 GPa.
• Show a wide miscibility gap in the solid state and a
certain chemical affinity to each other to form high
strength grain boundaries.
• Both the dislocation mechanism and the grain
boundary mechanism contribute to the hardness
10. Thermal stability
• Diamond-like carbon (DLC) based or metal matrix
nanocomposites coatings undergo structural change
at elevated temperatures
Hardness decreases due to –
• Relaxation of compressive stress and
• Rapid diffusion
Ways to achieve
• Include high thermal stability elements in the coating
such as yttrium
• Modify the interface complexity
11. Embed nanocrystalline phases in
an amorphous phase matrix
• Matrix with high hardness and elastic modulus eg
DLC, carbon nitride
• Strengthening phases- nano-sized refractory nitrides
eg TiN, Si3N4, AlN
• the size, volume percentage and distribution of the
nanocrystals need to be optimized
• The distance between two nanocrystals should be
within a few nanometers. Else, when too close will
cause the interaction of atomic planes in the adjacent
nanocrystalline grains.
12. To design a nanocomposite coating
• Possess both high hardness and high toughness,
• maximize interfaces and form well-defined spinodal
structure at interfaces
• Thrmal stability of structure at or above 1000 C
• Use ternary, quaternary or even more complex
systems
• Matrix- amorphous phase and
• Nanocrystalline phase- transition metal-nitride
nanocrystals (such as TiN, W2N, BN, etc.) as to
increase grain boundary complexity and strength.
13. Synthesis methods
• Magnetron sputtering- ionisation of the sputtered
metal and molecular gas dissociation to yield a high
density of deposited films.
• Chemical vapor deposition (CVD)- the wafer
(substrate) is exposed to volatile precursors, to react
and/or decompose on the substrate surface to
produce the desired deposit
14. Chemical vapor deposition (CVD)
Advantages compared to sputtering
• High deposition rate and
• Uniform deposition (for complicated geometries).
Limitation(s)
• A low deposition temperature is difficult to achieve
required to prevent substrate distortion and loss of
mechanical properties
the main concern
• corrosive nature and danger of fire hazard of
precursor gases (TiCl4, SiCl4)
15. Magnetron sputtering
• can operate at low temperatures to deposit films with
controlled texture and crystallite size.
process parameters affecting the grain size of the coatings
substrate temperature,
• substrate ion current density
• bias voltage,
• partial pressure of reactive gas (e.g. nitrogen for nitrides)
and
• post-annealing temperature.
• A minimum temperature is required to promote growth of
crystalline phase to the required diameter and or to allow a
• sufficient diffusion within the segregation
16. Evaluation of mechanical properties
Nanoindentation- A diamond indenter is forced into the
coating surface.
hardness of coating depends on
• Load
• depth of penetration
Measuring method
• measure with a low stress (-1 GPa) in coating
• Measure after stress-relief annealing above 400–500
C.
•Evaluation method
•Residual stress
17. Fracture toughness
• The ability of a material to resist the growth of a pre-
existing crack or flaw.
• Method- Use ultra-low load indentation.
• After the indentation, when no cracking occurs, the
coating is said to have good toughness.
18. Fig. various stages in
nanoindentation fracture
for the coating substrate
systems
Based on energy release in through thickness cracking
19. Fracture toughness
• Area under the indentation profile- work done by the
indenter during deformation
• Fig. below illustrates the indentation profile in such a
process.
• OACD is the loading curve and DE is the unloading
curve.
• The energy difference before and after the crack
generation is the area ABC..
• This energy will be released as strain energy to create
the ring-like through-thickness crack.
20. Fig. a load–displacement curve, showing a step during
the loading cycle and associated energy release
OACD - loading curve
DE- unloading curve.
21. Adhesion of coating
• Scratch adhesion- to evaluate the coating adhesion
strength.
• however, this only reveals load bearing capacity of
the coating
To improve coating adhesion- Add a bonding layer in
between
22. References
• Sam Zhang, Deen Sun, Yongqing Fu, Hejun Du
A review on Recent advances of superhard
nanocomposite coatings, Surface and Coatings
Technology 167 (2003),113–119
