An Examination of Coating Architecture in the Development of an Optimized Die Coating System for Aluminum Pressure Die Casting

1
An Examination of Coating Architecture in the
Development of an Optimized Die Coating System
for Aluminum Pressure Die Casting

• Jianliang Lin, John J Moore, S, Myers, F. Wang,
  B. Mishra
• Advanced Coatings and Surface Engineering
  Laboratory (ACSEL)
• Colorado School of Mines, Golden, Colorado

• Peter Ried,
• Ried and Associates, LLC, Portage, Michigan

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Why the Toughness is Critical for a Die Coating

• Premature failure of the die
• Erosion
• Wear
• Thermal fatigue

• General considerations
• Adherent to and compatible with the die material.
  
• Satisfies a range of specific mechanical,
  chemical and physical properties required by the
  forming process (High hardness and low
  coefficient of friction)
• Thermally stable at die casting operation
  temperature (oxidation resistance)
• Chemical inertness (non-wetting) with liquid
  alloy, e.g. aluminum (corrosion resistance)
• Able to accommodate the thermal residual stresses
  induced by shot cycling (temperature and
  pressure) during the pressure die casting
  process. (Need High toughness and low residual
  stress)

Can be effectively minimized by coating protection
Need high toughness, low residual stress coating
Substrate
Coating
Pitting area in the die formed under the
TiAlN/CrC coating after 12000 shot cycles in the
in-plant trial
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The Concept of an Optimized Die Coating System
US Patent PCT/US2005/17818------Designed on the
philosophy of integrating the best properties
from individual coatings into a coating system to
extend die life by minimizing premature die
failure
Designed Coating Architecture
An Example Coating Architecture
Non-wetting with molten Al, Good mechanical
strength
Working Layer
(Cr,Al)2O3
High toughness, accommodation of thermal stress,
and crack propagation resistance
CrN-CrAlN
Intermediate Layer
Provide good adhesion to the die material
Cr
Adhesion Layer
Ferritic Nitrocarburized H13 substrate
Ferritic Nitrocarburized H13 substrate
Increase the substrate strength to provide good
support to the top layers

• Different architectures of the intermediate layer
  (CrAlN) will have different microstructure and
  properties (mechanical, tribological, toughness,
  etc.)
• The purpose of our recent work is to investigate
  the effect of the intermediate layer
  architecture on the coating structure and
  properties, especially the toughness and
  plasticity.

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Three Different Intermediate CrAlN Layer
Architectures
Different Approaches for the Intermediate Layer
The composition of the CrAlN coating is
consistent through the coating thickness. The
Al/(CrAl) atomic ratio in Cr1-xAlxN coating was
maintained constantly in the range of 55-60 at.
(optimized from our previous work)
CrAlN
Homogeneous
Graded CrN
The Al concentration was increased from bottom to
the top in CrAlN coating according to the Power
Law with the exponent P0.2 (the black line)
(optimized from our previous work)
Cr1-xAlxN
Al Rich compositionally graded
CrxNy
Graded CrN
CrN and AlN layers were alternately deposited
with the bilayer thickness of 2-10 nm
CrN/AlN Superlattice
Will be focused in the current research
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Coating Deposition System

• Deposition system
• Pulsed closed field unbalanced magnetron
  sputtering (P-CFUBMS)
• Depositions of three CrAlN intermediate layer
  architectures
• For the homogeneous coating, the power densities
  and other deposition parameters were kept
  constantly during deposition period
• For the Al rich graded coating, the power density
  on the Al target was increased in accordance with
  the P0.2 power law, while maintaining other
  deposition parameters constant.
• For the CrN/AlN superlattice, the substrate was
  rotated back and forth between Al and Cr target
  at different power densities and settle times
  using a planetary rotation system.

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Superlattice CrAlN Intermediate Structure
Both homogeneous and graded CrAlN coatings
exhibit a typical columnar structure, the
columnar grain boundaries were clearly observed
Superlattice CrAlN coatings exhibit a bilayered
structure, with extremely fine grain size and
further improved dense structure
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Calculation of the Bilayer Period in CrN/AlN
Superlattice Coatings
Low angle XRD Confirming a layered structure,
the bilayer thickness can be calculated from
modified Braggs law
Where m is the order of the reflection, ? is the
X-ray wavelength (?cu1.54056), ? is the bilayer
thickness, and ? is the real part of the average
refractive index of the film, which is of the
order of 1x10-5, By plotting vs. m2 into a line,
the bilayer thickness ? can be calculated from
the slope of the line (about 5 nm for this
CrN/AlN coating)
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Crystal Phase in CrN/AlN Superlattice Coatings
High angle XRD showing the coating was
crystallized in the cubic NaCl B1 structure
(fcc), in which the (111), (200) and (220)
diffraction peaks were observed. There is no
hexagonal wurtzite-type AlN phase observed in the
XRD pattern, therefore the AlN layers in CrN/AlN
coatings exhibit an isostructural structure with
CrN layer
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The Plasticity of CrAlN Coatings with Different
Architectures
The plasticity of CrAlN coatings with different
structures was calculated from the ratio of the
plastic deformation over the total displacement
in the load-displacement curve
The plasticity of three different CrAlN coating
architectures were 1) For Homogeneous 50 2)
For Al rich graded CrAlN coating 60 3) For
CrN/AlN superlattice coating 63 (?3.8 nm)
Load-displacement curves obtained from
Nano-indentation tests
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Rockwell-C Indentation Test and Indent
Morphologies
A HF adhesion strength quality as standardized in
the VDI guidelines 3198, (1991)
Load 150 kg
Similar to HF2
HF1-HF4 define a sufficient adhesion
Better than HF1
HF5 and HF6 represent insufficient adhesion
Better than HF1
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Wear Resistance of Graded and Superlattice CrAlN
Layers
Test conditions - CETR microtribometer - 3 N
normal load - 100 m sliding length
Decreased wear depth
Decreased wear debris
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Summary of the Properties of the properties of
Graded and Superlattice CrAlN layers

Hardness GPa 36.38?3.98 34.61?3.22 41.3?2.89 (?3.8 nm)
Youngs Modulus (E) GPa 369.9?29.3 378.47?24.72 377.653?14.21(?3.8 nm)
H/E 0.0984 0.091 0.109
Plasticity 50 60 63
Residual Stress GPa -4.8 -2.25 Characterization in progress
Lc N 28 42 Characterization in progress
Coefficient of Friction 0.38 0.45 0.35 (?5.4 nm)
Wear Rate (WN) 10-6mm3N-1m-1 2.87 3.12 0.95 (?5.4 nm)
Homogeneous
CrN/AlN superlattice
Al rich graded
Super hardness
Good toughness
Decreased residual stress
Increased adhesion
Good wear resistance
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Summary

• The approaches to design and deposit an example
  surface engineered coating system for aluminum
  pressure die casting applications have been
  introduced.
• The microstructure, mechanical and tribological
  properties of the CrN/AlN superlattice coatings
  were investigated and compared with the
  homogeneous Cr0.42Al0.58N single layer coating
  and an Al rich graded CrAlN coating.
• The superlattice CrN/AlN coating architecture
  produced a super hard (41 GPa), high toughness
  (63 plasticity, no crack observed in the
  Rockwell-C indentation tests), and high wear
  resistance (low wear rate of 0.95x10-6 mm3N-1m-1)
  with a bilayer period of 3.85.4 nm.
• It is expect that the superlattice CrN/AlN and Al
  rich graded CrAlN coatings are very promising
  coating candidates for the aluminum high pressure
  die casting dies.

Future work Systematic investigate the effect of
the CrN, AlN nanolayer thickness on the
superlattice coating structure and properties