Recent advances of superhard nanocomposite coatings

PRESENTED BY

D. RATHIRAM NAIK, P. VIJAYA DURGA

15MT60R35, 15MT60R46

I IT KHARAGPUR

Sam Zhang, Deen Sun, Yongqing Fu, Hejun Du

ContentsIntroduction

Classification of coatings on the basis of hardness

Design methodology for nanocomposite coatings

Possible design methods

• Results

Synthesis methods

• Chemical Vapor Deposition (CVD)

• Magnetron sputtering

Evaluation of mechanical properties

Conclusion

References

Why coatings?• To protect from physical and chemical degradation.• Wear-, corrosion-, and fatigue

resistance

• To improve the service life of cutting tools

• To enhance the overall performance

• To improve the productivity

Introduction•Highly sophisticated surface related properties obtained by advanced nanostructured coatings : optical

magnetic

Electronic

Mechanical

chemical and tribological properties

•Industries prefer coatings because of their special properties listed above due to size effect

•Properties depends on material selection, deposition methods and process parameters

Applications

Automotive body partsJewelleryMirrorsMedical fieldMechanical fieldsOxidation resistance

• On cutting tools• Chemical factories

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 attributes to large amount of defects in grain boundaries

To increase hardness and toughness

• Requires hindering of grain boundary sliding by increasing the complexity and strength of grain boundariesMultiphase structures gives complexity at grain boundaries

By termination of Nano cracks

• With decreasing the grain size according to “Hall-Petch” relationship

Hardness as a function of grain size

Possible design methods• A combination of two or more nanocrystallinephases:Example: TiN-TiB2, Ti-B-N

Results in increasing coating hardness

• Segregation of nanocrystalline phases to grainboundariesAdvantage:

o increase in hardness and elastic modulus

Disadvantage:

o Lack of toughness

•

Design methodology•Hard nanocrystalline phases within a metal matrix:TiN in Ni

ZrN in Ni

Zr-Y-N, ZrN in Cu

CrN in Cu

•Resulted 35 – 60 Gpa hardness

•Contribution to hardness:

Dislocation mechanism

Grain boundary mechanism

•Metal Matrix improves toughness

Design methodology (Contd..)•Disadvantage:• Poor toughness

• Lack of thermal stability

•To improve thermal stability Addition of high thermal stability element

Example: Yittrium

Modify the interface complexity by ternary system

Embed nanocrystalline phases in an amorphous phase matrix

Enhancement of toughness along with hardness• Embed nanocrystalline phases in an amorphous phase matrix

•Amorphous matrix

•Possess high structural flexibility to accommodate

coherent strains

•Helps to deflect and terminate nanocracks

•Grain boundary sliding

•Improved coating toughness

•Matrix with high hardness and elastic modulus • Eg. DLC, carbon nitride

Continued•Strength phases: Nano-sized refractory nitride• Should be random orientation

oMinimizes incoherent strains

oEasy slide in amorphous matrix to release strain and obtains high toughness

• Eg. TiN, Si3N4, AlN, BN

•The size, volume percentage and distribution of the nanocrystals need to be optimized

• The distance between two Nano crystals should be within a few nanometres• Too large distance cause crack in matrix

• Too close to each other will cause interaction of planes

Results

Nano crystalline phase

Matrix Hardness(GPa)

TiC DLC 32

TiN Si3N4 50 - 70

TiCrCN DLC 40

TiN, TiS2 Si3N4 105 GPa

These Coatings are having thermal stability also

Synthesis methods

CHEMICAL VAPOR

DEPOSITION

MAGNETRON SPUTTERING

Chemical Vapor Deposition 1. Transport of reactants deposition

chamber

2. Diffusion of reactants towards substrate

3. Adsorption of reactants on the substrate

4. Surface reaction

5. Desorption of by-products away from substrate

6. Transport of by-products by diffusion

7. Transport of by-products away from chamber

CVD Continued•Advantages compared sputtering:High deposition rates

Uniform deposition for complicated geometries

•Disadvantages: Low deposition temperature difficult to maintain

oSubstrate distortion

•Main problem:• Precursor gases(TiCl4, SiCl4) creates problem due to their corrosive nature

• Chloride induces interface corrosion

Magnetron SputteringBasic Sputtering process:

atoms are ejected from the surface of amaterial when that surface is stuck bysufficiency energetic particles.

1. Ions are generated and directed at a target.

2. The ions sputter targets atoms.

3. The ejected atoms are transported to the substrate.

4. Atoms condense and form a thin film.

ContinuedLimitation of basic sputtering process

• Low deposition rates

•Low ionisation efficiencies in the plasma

•High substrate heating

Above limitations are overcome by magnetron sputtering

Magnetron Sputtering(MS)Magnets are used to increase thepercentage of electrons that take part inionization events, increase probability ofelectrons striking

Another reasons to use magnets:◦ Lower voltage needed to strike

plasma.◦ Controls uniformity.◦ Reduce wafer heating from electron

bombardment.◦ Increased deposition rate

Continued • Trapping the electrons increases theprobability of an ionising electron atomcollision occurring.

• The increased ionisation results in adense plasma

•This leads to increased ion bombardmentof the target, giving higher sputteringrates

•Therefore, higher deposition rates at thesubstrate.

ContinuedMS can operate at low temperatures to deposit films with controlled texture and crystallite size

Process parameters affecting the grain size of coatings:• Substrate temperature

• Substrate ion Current density

• Bias voltage

• Partial pressure of reactive gas (e.g. Nitrogen for nitrides)

• Post annealing temperature

Evaluation of mechanical properties Nanoindentation: Vickers hardness test. A diamond indenter is forced into the coating surface

Hardness of coating depends:• Load

• Evaluation method

• Depth of penetration

• Residual stress

Fracture toughness The ability of a material to resist the growth of pre-existing crack

Evaluation method-1: ultra- low load indentation• If no crack is forming then coating is having good toughness

Evaluation method -2: based on the energy released in through – thickness cracking• Area under the indentation profile

ContinuedFracture process follows three steps• Stage-1:

ofirst ring like crack form around the indenter

• Stage -2:

o delamination and buckling at coating – substrate interface

• Stage -3:

oSecond ring like crack due to high stress

Fracture toughness

Area under the indentation profile OACD is loading and DE is unloading Energy difference is ABC Strain energy is released to create crack Fracture toughness given by

Adhesion of coating Big concern for industrial coatings

Adhesion of coating gives good load bearing capacity

To improve adhesion : Add a bonding layer in between

Evaluation method: Scratch adhesion test

ConclusionsVarious deposition techniques have been used to prepare nanocompositie

Advantageous methods magnetron sputtering and CVD

Attention is paid to increase hardness , toughness and thermal stability

Optimum design of parameters gives good hardness and toughness

Superhard nanocompositie coatings needs vigorous theoretical andexperimental verification

References[1] Sam Zhang, Deen Sun, Yongqing Fu, Hejun Du, a riview on recent advances of nano composite coatins,

Surface and coating Technology 167 (2003), 113-119

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(2001) 167

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