Jianliang Lin, John J Moore, S, Myers, F. Wang, B. Mishra

transcript

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

Aluminum Pressure Die Casting

Advanced Coatings and Surface Engineering Laboratory (ACSEL)

Colorado School of Mines, Golden, Colorado

Peter Ried,

Ried and Associates, LLC, Portage, Michigan

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

Pitting area in the die formed under the

TiAlN/CrC coating after 12000 shot cycles in the

in-plant trial

Need high toughness, low residual stress coating

Coating cracks

Fe-Cr-Al-Si intermetallics

Coating

Substrate

The Concept of an Optimized Die Coating System

– 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.

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

Working Layer

Intermediate Layer

Adhesion Layer

Ferritic NitrocarburizedH13 substrate

(Cr,Al)2O3

CrN-CrAlN

Ferritic NitrocarburizedH13 substrate

Designed Coating Architecture

An Example Coating Architecture

Non-wetting with molten Al,

Good mechanical strength

High toughness, accommodation

of thermal stress, and crack

propagation resistance

Provide good adhesion to the die material

Increase the substrate strength to provide good support to the top layers

Three Different Intermediate CrAlN Layer Architectures

CrAlN Homogeneous

Al Rich compositionally graded

Cr1-xAlxN

CrN/AlNSuperlattice

Different Approaches for the Intermediate Layer

The composition of the CrAlN coating is consistent The composition of the CrAlN coating is consistent

through the coating thickness. The Al/(Cr+Al) atomic through the coating thickness. The Al/(Cr+Al) atomic

ratio in Crratio in Cr1-x1-xAlAlxxN coating was maintained constantly in N coating was maintained constantly in

the range of 55-60 at.% (optimized from our previous the range of 55-60 at.% (optimized from our previous

work)work)

The Al concentration was increased from The Al concentration was increased from

bottom to the top in CrAlN coating according bottom to the top in CrAlN coating according

to the Power Law with the exponent P=0.2 to the Power Law with the exponent P=0.2

(the black line) (optimized from our previous (the black line) (optimized from our previous

work)work)

CrN and AlN layers were alternately CrN and AlN layers were alternately

depositeddeposited with the bilayer thickness

of 2-10 nm

Graded CrN

Graded CrNLCr1-xAlxN

Will be focused in the current research

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 P=0.2

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.

Closed magnetic field lines

EQP orificeEQP detector

Quadrupole lens and energy filter

Substrate

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

CrN AlN

Bilayer thickness of 7-8 nm

CrN/AlN superlattice

Dense structure with super fine grains

Homogeneous Graded (Al rich graded)

Dense columnar structure with fine grains

Dense columnar structure

Calculation of the Bilayer Period in CrN/AlN Superlattice Coatings

Where m is the order of the reflection, λ is the X-ray wavelength (λcu=1.54056), 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)

2 3 4 5 6 7 8

m=5m=4m=3

Diffraction Angle [2-Theta]

Low angle XRD: Confirming a layered structure, the bilayer thickness can be calculated from modified

Bragg’s law:

Crystal Phase in CrN/AlN Superlattice Coatings

30 35 40 45 50 55 60 65 700

ity [C

Diffraction Angle [2-Theta]

c-CrAlN(200)

c-CrAlN(111)

c-CrAlN(220)

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

The Plasticity of CrAlN Coatings with Different Architectures

0 50 100 150 200 250 300 350 400 450 5000

CrN/AlN superlattice(Bilayer period=3.8 nm)

Single Layer CrAlN

Elastic deformation

Displacement into the surface [nm]

Plastic deformation

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:

Load-displacement curves obtained from Nano-

indentation tests

ntDisplaceme Totalndeformatio PlasticPlasticity

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)

Rockwell-C Indentation Test and Indent MorphologiesHomogeneous

Al rich graded

CrN/AlN superlattice

A HF adhesion strength quality as standardized in the

VDI guidelines 3198, (1991)

Load: 150 kg

HF1-HF4 define a

sufficient adhesion

HF5 and HF6 represent

insufficient adhesion

Similar to HF2

Better than HF1

Wear Resistance of Graded and Superlattice CrAlN Layers

Test conditions:

- CETR microtribometer

- 3 N normal load

- 100 m sliding length

Graded p=0.2Al rich graded

Homogeneous

Superlattice

Decreased wear

debris

Summary of the Properties of the properties of Graded and Superlattice CrAlN layers

Hardness [GPa] 36.383.98 34.613.22 41.32.89 (=3.8 nm)

Young’s Modulus (E) [GPa]

369.929.3 378.4724.72 377.65314.21(=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)

HomogeneousHomogeneous Al rich gradedAl rich graded CrN/AlN superlatticeCrN/AlN superlattice

Super hardness

Increased adhesion

Good wear resistance

Decreased residual stress

Good toughness

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 3.8~5.4 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

Jianliang Lin, John J Moore, S, Myers, F. Wang, B. Mishra

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