Nanocomposite coating materials offer strength toughness based on a combination of:

1
RVE (Representative Volume Element)
N C S TATE U N I V ERS I TY
Micromechanical Modeling of Nanotribological
Failure of Crystalline-Amorphous Nanocomposites
in Extreme Environments J. D. Pearson, W. M.
Ashmawi, and M.A. Zikry North Carolina State
University
Results
Computational Approach
Background
Case II 40 Au
Case III 40 DLC

• Nanocomposite coating materials offer strength
  toughness based on a combination of
• Crystalline phases
• Amorphous phases
• Extreme changes in
• Temperature
• Strain Rate
• Environment
• Require one system over ranges
• Optimal Coating design
• - Low coefficient of friction - Low wear rate

• Finite Element Thermal and Mechanical model
  commensurate with prediction of failure
• Brittle
• Ductile
• Material wear and failure criterion in Au and
  MoS2 phases
• Gold material treated as J2 plasticity with
  kinematic hardening
• YSZ, DLC, MoS2 elastic
• Quasi-static, Contact elements, Periodic B.C.,
  Thermal and Mechanical loadings

• Homogenized modulus, Aggregate model
• Random material distributions
• Equal size of each material in each distribution

Uniform Pressure
Case I 25 All Materials
ROM (Rule of Mixtures)
Uniform Pressure
Challenge
Horizontal Displacement
For an optimal coating How and What to combine?
6.8E-7
5.3E-7
3.8E-7
2.3E-7
0.8E-7
6.1E-7
4.6E-7
3.0E-7
1.5E-7
0
Element Wear and Failure

• D.O.F x,y 0

• Periodic BC

DLC
YSZ
GOLD
MoS2

• Plastic deformation in Au and MoS2 at free
  surface based on above criterion after indentor
  travel across and return

• Grain Size
• Grain Spacing
• Concentration Gradients
• Residual Stress Effects
• Thermal Annealing
• Deposition conditions

• Crystalline
• Amorphous
• Constituent Elements (Gold, Carbon, MoS2,
  ZrO2-Y2O3)
• Tungsten, Titanium
• Carbides, Nitrides, Oxides
• HDLC

COATING MODEL - MECHANICAL
Case III 40 DLC
Case II 40 Au
Formation of Transfer film from
elements exceeding yield criterion
Steel
Case i Aggregate
MoS2
Case IIIi
Case IIi
YSZ
Case iiDistributed
GOLD
Case Ii
Case Iii
Case IIIii
Case IIii
DLC

• Sliding indentation of Rigid indentor
• Travel across and return over surface at 1m/s
  with linear increase until final indentation of
  .0012 m
• Random distribution of material into identical
  grain size and spacing (i) or varying grain size
  and spacing (ii)

• Large gradientsbetween grains
• Maximum stressdependant upon material beneath
  indentor

Objectives
-.51E6
-.141E9
-.11E9
-.71E8
-.36E8
-.12E9
-.89E8
-.53E8
-.18E8
.17E8

• Major Goal
• Determine metal alloy coating response with known
  composition and microstructure under
• ? Temperature ? Atmosphere/Humidity
• ? Pressure ? Strain Rate
• Understand wear mechanisms and failure modes
  inherent to each coating based on composition and
  loading
• Simulate and predict wear and friction
  coefficients
• Model both microstructure composition
  corresponding changes (chameleon adaptive
  behavior) with environment
• At Low Temp, In Vacuum or Dry Air, MoS2 changes
  from amorphous to hexagonal
• At Low Temp, In Humid Air, Diamond Like Carbon
  (DLC matrix, sp3) changes from Graphite Like
  (sp2)
• At High Temp, Amorphous/poorly crystalline gold
  on the surface

Conclusions

• Crystalline and ductile phases needed for
  toughness
• Distributions can be optimized by grading the
  material through determining effective local
  properties
• Design guidelines can be tailored for optimized
  coating behavior
• ROM consistently estimates a higher homogenized
  modulus compared to FEA RVE
• Large gradients in stress between different
  materials
• Thermal strains and residual stresses have a
  significant role on coating behavior under
  loading
• GB sliding and GB deformations need to be
  delineated for ductile phases

-.33E9
-.26E9
-.18E9
-.10E9
-.27E8
-.30E9
-.22E9
-.14E9
-.65E8
.11E8
COATING MODEL - THERMAL
Effective Stress, ?Eff , under 0.0006 m final
indentation
Case I i
Case II i
Case III i
Case III ii
Case I ii
Case II ii
0 lt ? lt .5 Mpa
5 lt ? lt 10
60 lt ? lt 119
263 lt ? lt 307
.5 lt ? lt 5
10 lt ? lt 60
119 lt ? lt 263
307 lt ? lt 351