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ISSN : 1225-0562(Print)
ISSN : 2287-7258(Online)
Korean Journal of Materials Research Vol.31 No.6 pp.331-338
DOI : https://doi.org/10.3740/MRSK.2021.31.6.331

Failure and Phase Transformation Mechanism of Multi-Layered Nitride Coating for Liquid Metal Injection Casting Mold

Changwoo Jeon1, Juho Lee2, Eun Soo Park2
1School of Engineering, Brown University, Providence, Rhode Island 02912, USA
2Eloi Materials Laboratory, Suwon 16229, Republic of Korea
Corresponding author E-Mail : changwoo_jeon@brown.edu (C. Jeon, Brown Univ.)
April 30, 2021 May 18, 2021 May 18, 2021

Abstract


Ti-Al-Si target and Cr-Si target are sputtered alternately to develop a multi-layered nitride coating on a steel mold to improve die-casting lifetime. Prior to the multi-layer deposition, a CrN layer is developed as a buffer layer on the mold to suppress the diffusion of reactive elements and enhance the cohesive strength of the multi-layer deposition. Approximately 50 nm CrSiN and TiAlSiN layers are deposited layer by layer, and form about three μm-thickness of multi-layered coating. From the observation of the uncoated and coated steel molds after the acceleration experiment of liquid metal injection casting, the uncoated mold is severely eroded by the adhesion of molten metallic glass. On the other hand, the multi-layer coating on the mold prevents element diffusion from the metallic glass and mold erosion during the experiment. The multi-layer structure of the coating transforms the nano-composite structured coating during the acceleration test. Since the nano-composite structure disrupts element diffusion to molten metallic glass, despite microstructure changes, the coating is not eroded by the 1,050 °C molten metallic glass.



초록


    © Materials Research Society of Korea. All rights reserved.

    This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

    1. Introduction

    As the automobile, aircraft, heavy equipment, and electronics industries have rapidly advanced, the improvement of metallic materials’ properties is essential to overcome existing material limits. Since the 1990s, due to these requirements, bulk amorphous alloys have been investigated continuously to achieve excellent strength, hardness, stiffness, and corrosion resistance.1-3) Besides, a lot of forming processes have been developed to fabricate bulk amorphous alloys with excellent properties.4-11) A superplastic forming process, including press forming and blow molding, is a well-known process which is forms and produce the metallic glass components at the glass transition temperature.6,10,11) A couple of industries have also developed liquid injection casting producing the bulk metallic glass above its melting temperature. During the liquid injection casting, a master alloy is melted under vacuum conditions, poured into a mold, squeezed by a punch to make a specific shape for production, and rapidly cooled down to room temperature. Due to fast and continuous mass-production, liquid injection casting is the most promising method for commercializing bulk amorphous alloys (Fig. 1).

    However, the short lifetime of mold material, achieved through high operating temperature, has been discussed as a significant problem of liquid injection casting. To fabricate the bulk metallic glass with a high melting point, the master alloy should be melted at 1,000 ~ 1,100 °C and injected into the mold. This high operating temperature has been reported as a critical parameter to damage the mold.12-14) The injected bulk metallic glass containing zirconium (Zr) and titanium (Ti), high reactive elements with the mold material, easily adheres to the mold surface and causes the reaction layer erosion. Due to the shortcoming, utilizing nitride as a surface coating layer was suggested to enhance the lifetime of mold material and prevent the reaction layer formation on the mold surface. In particular, physical vapor deposition (PVD) has been considered an excellent method to concretely produce the nitride coating on the mold surface.15,16) At the initial stage of PVD development, CrN and TiN film were deposited on the mold surface as a protective coating.17,18) Subsequently, it was reported that the addition of Al to CrN and TiN film significantly increases the hardness and thermal stability at high temperatures.17)

    Furthermore, the coating with complex nanostructure was developed by simultaneously alloying Cr, Ti, and Al.19,20) According to Yang et al., the nanostructured layer is formed with NaCl-type structured Cr0.44Ti0.07Al0.09N0.39 and exhibits high hardness and oxidation resistance.19) Besides, Danek et al. investigated the Cr rich and depleted- CrTiAlN multi-layered structure showing excellent hardness and oxidation resistance [20]. However, the oxidation resistance of coated layers rapidly dropped above 900 °C due to the reactive diffusion of Ti to the surface. Therefore, the coating’s stability against reactive elements at high temperature should be increased to deposit the nitride coating on the mold material for bulk metallic glass fabrication.

    In the present study, a Si-alloyed multi-layer coating was investigated to obtain excellent hardness along with erosion and oxidation resistance at 1,000 ~ 1,100 °C. The addition of Si would be expected to increase the hardness and oxidation resistance resulting from grain refinement of coated layer microstructure.21,22) Prior to the multilayer deposition, a CrN layer was developed as an inhibition layer on the mold, which suppresses the reactive elements diffusion and enhances the cohesive strength of the multi-layer deposition. About 50 nm of the CrSiN and TiAlSiN layers are deposited layer by layer and form about three μm-thickness of the multi-layered coating. An acceleration experiment was performed by soaking the coated mold material into the molten metallic glass to measure the degree of reactive adhesion of the molten metallic glass alloy and erosion of the multi-layer coated mold at the operating temperature of the liquid injection casting. The coated mold microstructure was analyzed before and after the acceleration experiment, and the multi-layer coating film's phase transformation mechanism at the operating temperature was investigated.

    2. Experimental Procedure

    2.1 Multi-layered Coating

    A tool steel block (SKD 61) was supplied from Hyundai Special Steel Co. and used for injection casting acceleration tests. Its nominal chemical composition is listed in Table 1. The 20 × 20 × 150 mm sized blocks were machined from the steel block, ground by sandpapers, and washed using ethyl alcohol in an ultrasonic bath as substrates for multi-layered nitride coating. Multi-layered nitride coating was developed on the cleaned substrate by a multi-target Radio Frequency (R.F.) magnetron sputtering coating machine, of which a schematic diagram is shown in Fig. 2.18,23) Two types of nano-thickness nitrides with different compositions were stacked one upon another to develop a multi-layered coating. Three hot pressed targets of Al-Ti-Si, Cr-Si, and Cr of 50 mm in diameter, fabricated by sintering each alloy powder, were sputtered in the high vacuum chamber. Cr target was sputtered to develop about 1μm thickness of a CrN layer on the substrate to improve cohesive strength between the substrate and the multi-layered nitrides and prevent iron diffusion from the substrate into the coating. After the process, the Ti-Al-Si target and Cr-Si target were sputtered alternately to develop multi-layered nitride coating (Ti, Al, Si)N and (Cr, Si)N layers on the CrN bottom layer. The sputtering was conducted in an argon/nitrogen atmosphere, and detailed sputtering parameters for the multi-layered nitride coating was listed in Table 2.

    The developed multi-layered nitride coating was sectioned in the thickness direction and polished in diamond pastes (size; 0.25 mm). Nano-indentation tests then were carried out on the polished nitride coating by utilizing a nanoindenter (model; Nanoindenter XP, MTS, USA) equipped with a triangular Berkovich diamond indenter. Tests were repeatedly conducted 15 times at the different locations with a constant loading rate of 5 mN/s and the 0.6 μm depth. The hardness values of the coating were obtained. Scratch tests were also performed to measure the cohesive strength of the coating. A diamond stylus with a 0.2 mm radius was drawn across the coating surface under an increasing loading condition with a 1 N/s rate and a 0.1 mm/s moving speed until the load reached 80 N. A focused ion beam (FIB, Quanta 3D FEG, FEI Company, USA) was applied to the coated steel mold through the coating thickness direction to make a thin foil for the transmission electron microscope (TEM, JEM-2100F, Jeol, Japan) investigation. The microstructure of the multilayered coating was investigated by the TEM, and the chemical composition of each layer was examined by an energy dispersive spectroscopy (EDS) attached to the TEM.

    2.2 Injection Casting Acceleration Experiment

    Injection casting acceleration equipment was developed to measure the resistance and durability to reactive adhesion and erosion and investigate changes in the microstructure of coating at high temperatures. The schematic diagram of the equipment is shown in Fig. 3. A graphite crucible was installed in the high vacuum chamber filled with chunks of Zr-Ti-Ni-Cu-Al bulk metallic glass master alloys. The chunks were melted by an induction coil in the vacuum chamber to prevent the metallic glass oxidation, and the temperature of molten metallic glass was controlled by a thermometer to maintain its temperature at 1,050 °C. Then, half the height of uncoated and coated molds was submerged into molten metallic glass periodically by a controlled hydraulic pump. The molds were submerged for three seconds and taken out for three seconds with 1,000 cycles. Thin foils of two submerged molds were additionally prepared by a FIB for TEM investigation.

    3. Result and Discussion

    3.1 Microstructure of multi-layered coating.

    The hardness and cohesive strength of the coating were measured by nano-indentation tests and scratch tests. The minimum, maximum, and average hardnesses of the coating obtained from the 15 times nano-indentation tests were 32.4, 38.4, and 35.8 GPa, respectively. The surface regions of the coating, where the scratch tests were performed, are shown in Fig. 4. As the load continuously increases with the rate of 1 N/s, the width of scratches also widens. When the load reached 80 N, the width of the scratch also increased to 0.25 mm. Chipped regions adjacent to the scratch are traces of coating failure. The failure on the coating begins to occur at the location of 26 N and continuously occurs above 28 N.

    Fig. 5(a) shows the TEM micrograph of the deposited layer consisting of three different parts, and multi-layered coating parts were magnified in Fig. 5(b) and (c). The chemical compositions of the CrN layer on the substrate, the dark grey colored layer, and the bright grey colored layer were examined by EDS and were listed in Table 3. The EDS results identified that the dark and bright grey layers are (Cr, Si)N and (Ti, Al, Si)N layer, respectively. The entire thickness of the deposited coating is 5.3 μm. The CrN layer was deposited to a thickness of 1.3 μm on the substrate before the multi-layer coating. Then, (Cr, Si)N layer and (Ti, Al, Si)N layer were alternately stacked to a thickness of 4 μm. The thickness of each layer in the multi-layered nitride was varied depending on the height of the coating. The thicknesses of (Cr, Si)N and (Ti, Al, Si)N layers were 8.7 nm and 12.6 nm until 1.5 μm inside from the top of the coating, and the thicknesses of each layer were changed to 10.3 nm and 23.0 nm from 1.5 μm inside to CrN layer.

    3.2 Acceleration test

    Fig. 6, the SEM images, shows the surface region of the non-coated mold after the acceleration test. The region of mold submerged into the liquid metallic glass was eroded approximately 700 μm thickness from the surface, and approximately 500 μm thickness of the solidified metallic glass remained on the eroded surface [Fig. 6(a)]. The high magnification image in Fig. 6(b) shows that dark grey colored particles are presented in the remaining metallic glass. The chemical composition of particles was examined by the EDS, and the particles were identified as Zr-rich intermetallic compounds. Contrary to the die-casting machine, the acceleration equipment slowly cooled down the tested specimen to room temperature, which influenced the generation of particles in the metallic glass. The size of particles was different in the region, which was 10 μm in the inside of the metallic glass and 1 ~ 3 μm in the interface region between the steel mold and the metallic glass. Since Zr is a reactive element and carbon (C) in the steel mold diffuses into the metallic glass at the test temperature, Zr and C were combined into intermetallic compounds in the interface region during the acceleration test. Due to the intermetallic compounds along the interface region, metallic glass adhered to the uncoated mold. When the mold was periodically taken out of the molten liquid glass and cooled to room temperature, thermal contraction occurred in the steel mold and solidified glass. Due to the thermal contraction difference of mold and glass, erosion has occurred on the mold [24]. The particle size difference in the inside region and interface region of remained metallic glass is also related to carbon diffusion. The interface region is a site where the carbon diffuses, and the intermetallic compound is easily generated, compared to the area not adjacent to the interface. Fig. 7 illustrates the process of erosion that occurred during the acceleration test. Failure of die-casting dies for aluminum die-casting was reportedly caused by various mechanisms including corrosion, erosion, soldering, and thermal fatigue.12,25,26) Corrosion is caused by dissolvable alloying elements in aluminum liquid. Erosion and soldering to the steel surface take place during solidification, which causes a sticking problem.26) The acceleration experiment designed for liquid metal injection casting was conducted at 1,050 °C, which is a higher temperature than the temperature for aluminum die-casting, and liquid metal has reactive elements of Ti and Zr. Stress cracks by thermal fatigue are nucleated as a result of stress concentration.12) Generally, thermal fatigue cracks in the steel mold for aluminum die-casting appear after thousands of cycles, but stress crack may occur after 1,000 cycles of the acceleration experiment due to the high temperature and solidified liquid metal. The failure mechanism of the steel mold during the acceleration test can be explained as follow. When the uncoated specimen is periodically submerged into the molten glass, solidified metallic glass adheres to the mold surface [Fig. 7(a)]. In the interface region between the metallic glass and the mold, Intermetallic compounds are generated by atomic diffusion and particle coarsening takes place. Subsequently, stress is concentrated in the interface region due to the mismatch of thermal expansion and contraction of two materials [Fig. 7(b)]. The diffused surface of the mold is eroded along with the intermetallic compounds, and the solidified metallic glass adheres to the eroded mold surface.

    Fig. 8(a) shows a cross-sectioned coated mold after 1,000 cycles of acceleration experiments, and the solidified metallic glass remained on the coated mold after the acceleration experiment, which was similar to the uncoated mold after the experiment. The grey colored particles in the non-coated mold were not present in the interface region between the coating and solidified metallic glass, which implies that the nitride coating efficiently prevents the diffusion of carbon and nucleation intermetallic com-pounds. Nevertheless, solidified metallic glass remained on the coated mold surface because of its viscosity and adhesivity in the liquid state. From the observation of the multi-layered coating region, the structure of nano-sized stacked layers was broken by periodic exposure to the 1,050 °C molten metallic glass, and the layered microstructure transformed into black and dark grey colored nano-sized particles [Fig. 8(b)].

    The chemical compositions of each particle were measured by EDS, which identified that the black particle is (Ti, Al, Cr, Si)N and the grey particle is (Ti, Al, Si)N. (Ti, Al, Si)N layer changed its morphology, but the chemical composition of it remained similar. In the case of (Cr, Si)N layer, however, Ti and Al in the (Ti, Al, Si)N layer diffused in to (Cr, Si)N, resulting in the transformation of (Cr, Si)N into the (Ti, Al, Cr, Si)N. Those two types of particles were well distributed like a nanocomposite structure in the entire region of a multi-layered structure. Even though the multi-layered structure disappeared, elements in the structure did not diffuse into the CrN layer and remained metallic glass (Fig. 9), which implied that the nanocomposite structure with two types of particles still played a role in preventing erosion by the molten metallic glass. Huang et al. have investigated structural changes of multilayer coating composed of tungsten (W) and boron carbide (B4C) nanolayers with average thicknesses of 10.5 nm and 15.8 nm, respectively, under an annealing condition at 600 °C for 30 min.27) Due to the interdiffusion near the interface of each layer, the thickness of W layers increases, and crystallization in each layer occurs. Then, crystallized particles accumulate, which leads to irregular interfaces during the annealing process. After the annealing process, the lamellar structure disappeared and large irregular grains of particles are present in each layer. The multilayer coating (Cr, Si)N and (Ti, Al, Si)N layer was exposed to a temperature of 1,050 °C which is a more severe condition compared to the experiment mentioned above, which accelerates interdiffusion, crystallization, and grain coarsening. Eventually, the lamellar structure of the coating is destroyed as shown in Fig. 8. Despite the crystallization and grain coarsening, both generated (Ti, Al, Cr, Si)N and (Ti, Al, Si)N are well distributed. Unlike the multilayer coating, the CrN layer retains its structure observed prior to the acceleration test. The CrN layer has a thickness of about 1 μm, which is sufficient to maintain the layer structure, even though the interdiffusion, crystallization, and grain coarsening occurred.

    The multi-layered coating, with nano-sized thickness layer stacks, has been used for working tools in the machining industries.28-30) To enhance the lifetime of working tools, the coating materials have been developed to improve their hardness, oxidation resistance, and hightemperature stability. When the working temperature of tools with single-layer coatings, such as TiN (max. hardness; 21.5 GPa), CrN (max. hardness; 19.6 GPa), and (Al, Ti)N (hardness; 16.6-31.3 GPa), exceeds 500 ~ 700 °C, oxide particles were nucleated in the coating, which can initiate a crack of the coating during working.31-33) On the other hand, the multi-layered coating with two or more kinds of nitride layers such as (Ti, Cr, Al)N/(Al, Si)N (max. hardness; 36.2 GPa) showed higher hardness and oxidation resistance. This enhancement is explained by the effect of elastic modulus difference and the prevention of element diffusion by each other layer.28) Interestingly, the essential properties of the coating in the tool industries are well matched to those in the die-casting mold industries, especially for high-temperature material such as bulk metallic glass. Injection casting molds for bulk metallic glass also require high hardness, oxidation resistance, and high-temperature stability.

    Additionally, in this study, to enhance the resistance to oxidation and elements diffusion at the operating temperature of injection casting, Si was added to each coating materials of (Ti, Al)N and (Cr)N, which produced the multi-layer coating with (Ti, Al, Si)N and (Cr, Si)N (max. hardness; 32.8 GPa). The effect of Si on the nitride coating has been researched and determined that nitride particles presented in the amorphous Si3N4 form the nanocomposite structure in the entire coating layer.21,22) The nano-composite structure reduces the grain size and increases hardness, related to the Hall-Petch equation. Moreover, the nitride coating with a nano-composite structure still disrupted element diffusion to molted metallic glass and the CrN buffer layer as shown in Fig. 8. The multi-layer coated mold, which was soaked in 1,050 °C molten glass for 1,000 cycles, was not eroded at all, even though the solidified metallic glass adhered to the coating surface. The nano-composite structure with (Ti, Al, Si) N and (Ti, Al, Cr, Si) N efficiently prevents diffusion of reactive elements at a high temperature of 1,050 °C. This result provides an opportunity to develop a new structured coating by adding an annealing process to the coated material. By controlling annealing time, temperature, and the chemical composition of the multilayered coating, the structure of nanocomposite coatings is modified and their properties could be improved. This annealing method can also be applied to the working tool industries to improve the coating properties concerning hardness, wear resistance, and oxidation resistance.

    4. Conclusion

    Multi-layered nitride coating with (Ti, Al, Si)N and (Cr, Si)N was deposited on the steel mold, and resistance to erosion and adhesion was investigated by the acceleration experiment of liquid metal injection casting. Change in microstructure after the acceleration experiment was also investigated.

    • 1) (Cr, Si)N layer and (Ti, Al, Si)N layer were alternately stacked to a thickness of 4 μm. The thicknesses of (Cr, Si)N and (Ti, Al, Si)N layers were 8.7 nm and 12.6 nm until 1.5 μm inside from the top of the coating, and the thicknesses of each layer were changed to 10.3 nm and 23.0 nm from 1.5 μm inside to CrN buffer layer.

    • 2) From the observation of cross-sectioned uncoated molds, the solidified metallic glass remained on the mold was eroded, caused by the interdiffusion, intermetallic compound formation, and stress concentration during the acceleration tests. On the other hand, the coating thickness of the coated mold was kept intact even though the solidified metallic glass remained on the coating.

    • 3) The structure of nano-sized stacked nitride layers was broken by periodic exposure to the 1,050 °C molten metallic glass, and the layered microstructure transformed into the nanostructured composite coating. This nanocomposite structure with (Ti, Al, Si) N and (Ti, Al, Cr, Si) N efficiently prevents diffusion of reactive elements at a high temperature of 1,050 °C.

    Acknowledgment

    This work was supported by the Center for Advanced Materials Processing (CAMP) of the 21st Century Frontier R&D Program (No. F00030492007-311006000115) funded by the Ministry of Knowledge Economy, Korea, and by the National Research Foundation of Korea (NRF) grant (No. 2010-0026981) funded by the Ministry of Education, Science, and Technology, Korea.

    Figure

    MRSK-31-6-331_F1.gif

    (a) Schematic diagram of liquid injection casting equipment for the production of bulk metallic glass component.

    MRSK-31-6-331_F2.gif

    Schematic diagram of multi-target Radio Frequency (R.F.) magnetron sputtering coating machine.

    MRSK-31-6-331_F3.gif

    Schematic diagram of the injection casting acceleration equipment put in and takes out (a) uncoated steel and (b) coated steel, periodically.

    MRSK-31-6-331_F4.gif

    The sectioned and polished coating regions, where the scratch tests were performed, show failure at the 26N load.

    MRSK-31-6-331_F5.gif

    The microstructure of multi-layered nitride coating obtained from a TEM. (a) The entire thickness of the coating is about 5.3 μm. (b) The thicknesses of dark color and bright color layers are 8.7 nm and 12.6 nm until 1.5 μm inside from the top of the coating, and (c) the thicknesses of each layer are 10.3 nm and 23.0 nm from 1.5 μm inside to CrN layer.

    MRSK-31-6-331_F6.gif

    SEM images showing (a) surface region of steel mold fell out by bonding and erosion of melted metallic glass and (b) remained solidified metallic glass on the surface of steel mold including intermetallic particles.

    MRSK-31-6-331_F7.gif

    Illustrations of the process of failure mechanism of the steel mold during the acceleration test. (a) Solidified metallic glass adheres to the mold surface. (b) Intermetallic compounds are generated in the interface region between the metallic glass and the mold by atomic diffusion. Subsequently, stress is concentrated in the interface region due to the mismatch of thermal expansion and contraction of two materials. (c) The diffused surface of the mold is eroded along with the intermetallic compounds, and the solidified metallic glass adheres to the eroded mold surface.

    MRSK-31-6-331_F8.gif

    (a) A TEM image showing the microstructure of steel mold, coating, and remained metallic glass after the injection casting acceleration experiment and (b) an high magnification TEM image showing the microstructure of transformed coating layer including nano-sized particles of TiAlCrSiN and TiAlSiN.

    MRSK-31-6-331_F9.gif

    EDS mapping result showing the distribution of Al, Cr, Fe, and Ti elements in the coated mold after 1,000 cycles of acceleration experiment.

    Table

    The nominal chemical composition of SKD 61 die-casting steel. (wt%)

    Sputtering conditions of multi-layer nitride coating.

    Chemical compositions of each layer in the 5.3 μm thickness coating. (at%)

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