Superhard nanocomposite coatings

Presented by Nand Kishore Kumar (13MT92P02)Pintu Kumar (13MT60R30)

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)

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 grainboundaries

Fig. Hardness of a material as a function of grain size

Ways to achieve hardness and toughness

• Multiphase structures• deflection, meandering and termination of

nanocracks• Some degree of grain boundary diffusion and grain

boundary sliding

Possible design methods

• A combination of two or more nanocrystallinephases• 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

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

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

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

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

Embed nanocrystalline phases inan 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.

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.

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

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)

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

Evaluation of mechanical properties

Nanoindentation- A diamond indenter is forced into the coating surface.

hardness of coating depends on• Load • depth of penetrationMeasuring method• measure with a low stress (-1 GPa) in coating • Measure after stress-relief annealing above 400–500

C.

•Evaluation method•Residual stress

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.

Fig. various stages in nanoindentation fracture for the coating substrate

systems

Based on energy release in through thickness cracking

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.

Fig. a load–displacement curve, showing a step during the loading cycle and associated energy release

OACD - loading curve DE- unloading curve.

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

References

• Sam Zhang, Deen Sun, Yongqing Fu, Hejun DuA review on Recent advances of superhard

nanocomposite coatings, Surface and Coatings Technology 167 (2003),113–119

Thank you