42 JOM • September 2002

Nanomaterials

The Spray Forming of Nanostructured Aluminum Oxide Arvind Agarwal, Tim McKechnie, and Sudipta Seal

Research Summary

Nanostructured ceramics and their composites possess improved proper-ties such as tensile strength, fatigue strength, hardness, and wear resistance. Freestanding, near-net shape, nano-structured Al2O3 components can be synthesized via plasma-spray forming. In this study, plasma-spray parameters were optimized and an innovative sub-strate cooling technique was developed to retain nanosize Al2O3 in the spray deposit. Nanosize Al2O3 particles were partially melted and trapped between the fully melted coarser, micrometer-size Al2O3 grains. Densification of the spray-deposited Al2O3 occurred via solidification and sintering. A similar processing approach can be adopted for fabrication of near-net shapes of a variety of nanostructured materials (metals, ceramics, and intermetallics) and their combinations by selecting suitable powder-treatment and plasma-spray parameters.

INTRODUCTION

Nanostructured materials provide a significant reduction in grain size over conventional materials, improving properties such as tensile strength,

PowderFeeder (2)

InertGas (5)

PowderFeeder (3)

TungstenCathode (4)

Plasma Gun (1)Arc (7)

Copper Anode (8)

Spray Deposit

Rotating Mandrel(+)

(-)

Plasma Flame with High Velocity Molten Particles (10)

Transfer ArcPower

Supply (9)

+

-Main Power Supply (6)

hardness, fatigue strength, and wear resistance. The last decade has seen significant research efforts in develop-ing novel processing techniques to synthesize nanostructured materials for a variety of applications, including powder production, electronics and semiconductors, biological applica-tions, and structural applications. For example, plasma spraying has been investigated by several researchers to deposit nanostructured ceramic coatings such as WC, Cr3C2, Al2O3-TiO2, and ZrO2.

1–6 However, the use of plasma spraying to fabricate a bulk, freestanding nanostructured Al2O3 component has rarely been reported in the literature. In this study, freestanding, net-shape nanostructured Al2O3 structures were fabricated using plasma-spray forming. This article summarizes the process and the microstructure of the plasma-spray-formed Al2O3 structures.

NEAR-NET SHAPE FORMING

Near-net shape forming using the plasma-spray technique involves the simultaneous melting of powder and accelerating the molten particles for deposition on a rotating mandrel or

substrate.7–10 Figure 1 shows a schematic of the plasma-spray-forming technique. As shown in the figure, the nozzle creates an arc that ionizes a gas stream, forming the plasma with extremely high temperatures. Powders are fed into the plasma, where they melt and are accelerated to supersonic speeds. The molten particles are directed toward a rotating mandrel, where they deposit and rapidly cool, forming the desired shape. The spray-deposited structure is built up on the surface of the mandrel, which has the negative figure of the desired shape, and is then removed from the mandrel, usually by making use of the large difference in thermal expansion between the two components. A wide variety of complex shapes and configurations can be made using this technique.7–10 The process lends itself especially to ceramics and refractory metals, which are brittle at room temperature and hard to machine. Since there is minimal waste and parts are sprayed to near-net shape, the process effectively uses precursor materials. The density of the deposited materials depends directly on the plasma velocity. The velocity controls the time that the

Figure 1. A plasma-spray forming illustration.

100 µm

Figure 2. An SEM micrograph showing a homogenous mixture of micrometer-size and nanosize alumina powder.

432002 September • JOM

100 µm

Figure 4. An SEM micrograph of the cross-section of the spray-deposited Al2O3.

particles are exposed to the heating zone and the kinetic energy with which they impact the rotating mandrel. The plasma gun and the mandrel are computer-controlled, allowing fabrication of complex shapes. The key to develop-ing freestanding nanostructured materi-als via plasma-spray forming lies in modifications to the processing technique such that nanostructure is retained without significant grain growth. That goal is achieved in this study by optimizing powder feedstock and controlling plasma parameters and the substrate temperature. Commercially available aluminum-oxide powder (99.8% pure) in the particle size range of 15–45 µm was mixed in a ball mill with nanosize aluminum-oxide agglomerates (99.99% pure) in a 60:40 wt.%, respectively, ratio. Figure 2 shows a homogenous blend of micrometer-size, angular alumina particles and spherical agglomerates (<20 µm) of nanosize alumina particles. It was anticipated that a mixture of nanosize and micrometer-size powder would enable an easier flow of feedstock powder in the plasma flame, as it is extremely difficult to flow nanosize powder only.3 Moreover, in an earlier study, the addition of nanosize alumina dispersions was found to increase the fracture toughness and thermal shock of the conventional alumina ceramics.11 The dual-size mixed alumina powder was plasma sprayed using a Praxair Surface Technologies SG 100 plasma-spray system operating at 800 amperes and 35 volts. Argon was used as the primary gas (75 SLPM) whereas

helium was the secondary gas. Plasma parameters were critically controlled to minimize the melting of nanosize powders.12 Aluminum-oxide coatings were spray deposited on 6061 aluminum mandrels to a coating thickness of 300–500 µm. The powder particles exiting from the plasma were deposited on the rotating mandrel and acquired the same shape as the mandrel. An innovative, proprietary technique was developed to actively cool the mandrel, increasing the nucleation rate and minimizing the coarsening of the nanosize grains in the spray deposit. After spray deposition, mandrels were cryogenically cooled with liquid nitrogen to facilitate the release of the spray-formed alumina shell due to a mismatch between coefficients of thermal expansion (CTE). The CTE value for 6061 aluminum is 25 (10–6/K) whereas alumina has CTE of 7(10–6/K). The mandrel design, selection criterion, and plasma processing conditions have been detailed elsewhere.10,12

Figure 3 shows the spray-formed nanostructured alumina tapered ring and cylindrical shells of varying sizes. These structures were plasma-spray formed and released from the 6061 aluminum mandrels. Tapered cylindrical alumina shells have a wall thickness of 0.4–0.6 mm and height varying between 45–100 mm. The spray-formed ring has a diameter of 500 mm and a wall thickness of 0.5 mm. The fabrication of such shapes and configurations was guided by the need for space-based, lightweight x-ray mirror shells with a smooth inside finish.10

MICROSTRUCTURALANALYSIS

Figure 4 is the cross-sectional view of the spray-formed nanostructured alumina shell. The spray-deposited Al2O3 is homogenous and free from cracks. There are few unmelted, spheri-cal nanosize powder agglomerates present in the deposit. The porosity in the spray deposit was estimated to be 9 vol.% using quantitative microscopy. The microhardness of the spray-deposited alumina was 1,065 ± 73 Vickers hard-ness number, which is higher than conventional alumina coatings in as-sprayed condition.3 The higher microhardness value could be attributed to the presence of nanosize grains in the structure. Transmission-electron microscopy (TEM) revealed the physical phenom-enon occurring during spray forming of Al2O3 structures. Figure 5 is a bright-field TEM image of the overall structure of the spray-deposited nanostructured alumina. It shows fully melted coarse Al2O3 grains surrounded by ultrafine grain structure (marked with arrows). Figure 6 shows a high-magnification view of the ultrafine structure trapped between fully melted coarse grains. It shows Al2O3 fine grains ≤ 100 nm, which confirms the retention of nanostructure after plasma-spray forming. The evolu-tion of bimodal grain-size distribution in the spray-formed Al2O3 microstructure is due to the powder-flow behavior through the plasma and associated densification mechanism.12 Micrometer-size powder particles flow coherently

Figure 3. The plasma-spray formed nano-structured Al2O3 shells and ring with a wall thickness of 0.4–0.6 mm. The diameter of the tapered shell is between 38–62 mm, whereas diameter of the ring is 500 mm.

200 nm

Figure 5. A bright-field TEM micrograph showing the overall microstructure of the spray-formed Al2O3 shells.

Unmelted Particles

Spray Deposited Alumina

44 JOM • September 2002

through the hot zone of the plasma, whereas a larger fraction of nanosize powder does not flow through the hot core of the plasma due to their smaller size. Hence, a larger degree of melting occurs in coarse powder particles, whereas nanosize powder particles are partially melted or unmelted and get trapped as overspray between the molten coarser particles, as evident from Figures 5 and 6. Though a fraction of the 40 vol.% nanosize agglomerates in the feedstock powder must have also melted in the plasma flame, it appears that a larger fraction of nano-size particles has been retained as a partially melted or sintered structure trapped between the coarser grains. The hypothesis is further validated by Fig-ure 7, which shows a scanning-electron micrograph of the top surface of the500 µm thick spray-formed alumina shells. It clearly shows fully melted coarse alumina particles and the partially melted/unmelted, retained spherical nanosize agglomerates. Some of these nanosize agglomerates have coalesced at their edges, indicating densification through solid-state sintering. Hence, densification in the plasma-sprayed nanostructured Al2O3 occurs via solidi-fication in coarser particles and sintering in nanosize particles. Shaw et al. performed a study on plasma-sprayed nanostructured Al2O3-13 wt.% TiO2 coatings and

concluded that densification depends on the plasma-processing parameter (IV/Ar) where IV is the electrical power (watts) and Ar is the primary gas flow rate.3 An IV/Ar ratio >310 suggests a higher degree of melting and densification by solidification whereas a ratio <240 indicates relatively lower melting and densification by sintering.3 In this study, the plasma-spray parameter IV/Ar ratio was 180.12 The densification of spray-deposited nanostructured alumina occurred by solidification and sintering, which is in contradiction to the hypothesis of Shaw et al. A higher degree of melting in the present research (IV/Ar ratio = 180) could be attributed to the difference in thermal conductivity values of Al2O3 and TiO2. Al2O3 has a thermal conductivity of 5.9 W/mK at 1,000°C, whereas TiO2 has a thermal conductivity of 3.3 W/mK.13 Due to the higher thermal conductivity of Al2O3, the outside of the particle gets superheated and effectively conducts the heat to the core to cause melting. Hence, it could be concluded that in the absence of TiO2, a lower value of IV/Ar ratio would result in a relatively higher degree of melting in pure Al2O3.

ACKNOWLEDGEMENT

The authors express their apprecia-tion to Z. Rahman at the UCF-CIRENT MCF, University of Central Florida, Orlando for his assistance with TEM

work. The authors acknowledge financial support from the NASA Goddard Space Flight Center (Contract No. NAS5-0008) for this work.

References

1. R.S. Lima et al., “Properties and Microstructures of Nanostructured Partially Stabilized Zirconia Coatings,” J. Ther. Spray Tech., 10 (1) (2001), pp. 150–152.2. J. He et al., “Thermal Stability of Nanostructured Cr3C2-NiCr Coatings,” J. Ther. Spray Tech., 10 (2) (2001), pp. 293–300.3. L.L. Shaw et al., “The Dependency of Microstructure and Properties of Nanostructured Coatings on Plasma Spray Conditions,” Surf. Coat. Tech., 130 (2000), pp. 1–8.4. Y. Zhu et al., “Tribological Properties of Nanostructured and Conventional WC-Co coatings Deposited by Plasma Spraying,” Thin Solid Films, 388 (2001), pp. 277–282.5. Y. Wang et al., “Abrasive Wear Characteristics of Plasma Sprayed Nanostructured Alumina/Titania Coatings,” Wear, 237 (2000), pp. 176–185.6. H. Chen and C.X. Ding, “Nanostructured Zirconia Coating Prepared by Atmospheric Plasma Spraying,” Surf. Coat. Tech., 150 (2002), pp. 31–367. A. Agarwal and T. McKechnie, “Spray Forming Aluminum Structures,” Advanced Mater. Process, 159 (5) (2001), pp. 44–46.8. A. Agarwal et al., “Advances in Near Net Shape Forming and Coating of Erosion Resistant Ultra High Temperature Materials” (Paper presented at the Tri-Service Sponsored Symposium on Advancements in Heatshield Technology, Huntsville, Alabama, May 2000, U.S. Army Aviation & Missile Command, SR-RD-PS-00-01).9. R. Hickman, T. McKechnie, and A. Agarwal, “Net Shape Fabrication of High Temperature Materials for Rocket Engine Components” (Paper presented at the 37th AIAA/ASME/SAE/ASEE/Joint Propulsion Conference, Salt Lake City, Utah, 8–11 July 2001, AIAA-2001-3435).10. A. Agarwal and T. McKechnie, Low Cost Fabrication of Lightweight Optics, Mirrors and Benches, NASA Goddard Space Flight Center, Technical Report NAS5-0008 (November 2001).11. G. Li, A. Jiang, and L. Zhang, “Mechanical and Fracture Properties of Nano Al2O3,” J. Mater. Sci. Lett. 15 (19) (1996), pp. 1713–1715.12. A. Agarwal, T. McKechnie, and S. Seal, “Net Shape Nanostructured Aluminum Oxide Structures Fabricated by Plasma Spray Forming”, communicated to J. Thermal. Spray Tech., accepted (July 2002).13. TAPP, A Database of Thermochemical and Physical Properties, v. 2.2 (Hamilton, OH: ESM Software Inc., 1998).

Arvind Agarwal and Tim McKechnie are with Plasma Process. Sudipta Seal is an associate professor at Advanced Materials Processing & Analysis Center and Department of Mechanical, Materials and Aerospace Engineering, University of Central Florida, Orlando, Florida 32816.

For more information, contact A. Agarwal at Plasma Process, 4914 Moores Mill Road, Huntsville, AL 35811, (256) 851-7653; fax (256) 859-4134; e-mail Arvind@plasmapros.com.

100 nm 10 µm

Figure 7. An SEM micrograph of the top surface of the plasma-sprayed Al2O3 shell showing melting and sintering.

Figure 6. A bright-field TEM micrograph showing nanosize grains in the plasma-sprayed nanostructured Al2O3.