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

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

2. 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) Coating cracks Fe-Cr-Al-Si intermetallics Why the Toughness is Critical for a Die Coating 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

3. 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 • 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. 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

4. 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/(Cr+Al) 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 P=0.2 (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

5. 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.

6. Superlattice CrAlN Intermediate Structure Dense columnar structure with fine grains Dense structure with super fine grains Dense columnar structure Homogeneous Graded (Al rich graded) CrN/AlN superlattice CrN AlN Bilayer thickness of 7-8 nm 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

7. 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 Bragg’s law: 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)

8. 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

9. 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

10. Rockwell-C Indentation Test and Indent Morphologies A HF adhesion strength quality as standardized in the VDI guidelines 3198, (1991) Homogeneous Load: 150 kg Similar to HF2 HF1-HF4 define a sufficient adhesion Al rich graded Better than HF1 HF5 and HF6 represent insufficient adhesion CrN/AlN superlattice Better than HF1

11. Wear Resistance of Graded and Superlattice CrAlN Layers Test conditions: - CETR microtribometer - 3 N normal load - 100 m sliding length Graded p=0.2 Al rich graded Decreased wear depth Decreased wear debris Homogeneous Superlattice

12. Homogeneous CrN/AlNsuperlattice Al rich graded Super hardness Good toughness Summary of the Properties of the properties of Graded and Superlattice CrAlN layers Decreased residual stress Increased adhesion Good wear resistance

13. 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