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MICROWAVE ASSISTED (MASS) PROCESSING OF METAL-CERAMIC AND REACTION-BONDED COMPOSITES P. G. Karandikar and M. K. Aghajanian M Cubed Technologies, Inc. 1 Tralee Industrial Park Newark, DE 19711 D. Agrawal and J. Cheng Materials Research Laboratory Pennsylvania State University University Park, PA, 16802 ABSTRACT

Composites offer tailorable physical, thermal and mechanical properties for a variety of applications. M Cubed manufactures metal-ceramic composites and reaction bonded ceramic composites via pressureless infiltration techniques. Conventional manufacturing of these composites involves use of the resistance heated furnaces and retorts. In this work the ability to produce infiltrated composites using microwave heating has been demonstrated. Identical composite compositions were also fabricated using conventional heating for comparison. Properties of the composites made by microwave assisted (MASS) processes were comparable to the properties of the composites made by the conventional methods. Substantial process time reductions were achieved for all the infiltration processes using microwave assistance. Several prototype components were fabricated to demonstrate process scale up. Thus, MASS processing will lead to lower process time, higher productivity, lower energy consumption and overall lower component cost.

ADVANTAGES OF METAL CERAMIC AND REACTION BONDED COMPOSITES

The materials currently used for fabricating various engineering components can be broadly classified into polymers, metals and ceramics. Each class of these materials has different ranges of physical, thermal, mechanical and chemical properties based on the composition and atomic or molecular structure. For example, polymers (polyethylene, epoxy) have very low density, low modulus (E), high toughness (K), low strength, low thermal conductivity (TC) and high coefficient of thermal expansion (CTE). Metals (e.g. aluminum, magnesium, copper, steel) have higher density, medium modulus, high toughness, high strength, high thermal conductivity and medium CTE. Ceramics (e.g. SiC, Al2O3, B4C, AlN) have low density, high modulus, low toughness, medium to low strength, low to medium thermal conductivity and low CTE.

By combining these materials or in other words by making composite materials, desired properties of materials from different classes can be combined in a single composite material. Figure 1 shows how E, K, CTE and TC can be varied by adding different volume fractions of SiC particles in aluminum (Al) or silicon (Si) matrices. M Cubed makes these composites for a range of applications including personnel, vehicle and aircraft armor; semiconductor capital equipment components; thermal management components; wear and erosion resistant components; and aerospace mirrors and structural components.

A variety of processes have been developed for manufacturing these composites, e.g. sintering, pressure infiltration, squeeze casting, hot pressing. Many of these require an external

Ceramic Engineering and Science Proceedings Vol. 27, No. 2 , 435-446 (2007) In Mechanical Properties and Performance of Engineering Ceramics and Composites II, Editor R. Tandon, American Ceramic Society

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force to combine the two components of the composite and produce a fully dense body. This can limit the tooling that can be used and in turn the shape and size of the component that can be produced. To overcome these limitations, M Cubed Technologies (MCT) has focused on pressureless infiltration techniques to make composites. These involve infiltration of a “matrix” component into a preform of a “reinforcement” component. Patented techniques are used to modify the surfaces of the reinforcements to allow good wetting and spontaneous infiltration by the matrix without application of external pressure.

Examples of composites that have been processed by these approaches include ZrBC, SiCp,f/Al, SiCp,f/Mg, Al2O3p,f/Al, Al2O3p,f/Mg, SiCf/Al2O3, Al2O3p,f/Al2O3 , Cf/SiC, SiCf/SiC, SiCp/Si, B4Cp/Si etc. Here, subscript p refers to particles and f refers to fibers. A variety of components are routinely made out of these materials including personnel armor (SAPI, ESAPI), stages for wire bonding, optics boxes and other components for lithography machines, automotive components (pistons, connecting rods, brake rotors), rocket nozzles for THAAD missiles, F/A-18 avionics substrates, vehicle and aircraft armor (marine LAV, C-130) etc.

Figure 1. Tailoring of properties of M Cubed’s Si (∆), Al (○) and hybrid Si-Al (□) matrix

composites by varying the SiC particle content. E – elastic modulus, K – fracture toughness, TC – thermal conductivity, and CTE – coefficient of thermal expansion.

MCT’S PRESSURELESS INFILTRATION AND REACTION BONDING PROCESSES

The following subsections briefly describe MCT’s conventionally practiced pressureless metal infiltration (PRIMEX), directed metal oxidation (DIMOX) and reaction bonding (RB) processes. These process are compared in Table I. Figure 2 (Top) shows a schematic of these processes as they are practiced in the “conventional” mode.

Table I. Comparison of M Cubed’s infiltration processes

Process Matrices Mechanism Temperature PRIMEX Metals: Al, Al

alloys Mg dopant, Mg3N2 coating formation and spontaneous wetting / wicking by Al

700-800ºC

DIMOX Ceramic: oxides, nitrides

Mg, Si, Fe dopants, Reactive atmospheres: air, nitrogen

900-1150ºC

RB SiC, Si, Si alloys Si wetting/ reaction with carbon, wicking 1200-1550ºC

050

100150200

250300350400

0 10 20 30 40 50 60 70 80 90 100Volume % SiC Particles

E (G

Pa)

0

5

10

15

20K

(MPa

m1/

2)

Al matrix

0

50

100

150

200

250

0 10 20 30 40 50 60 70 80 90 100Volume % SiC Particles

TC (W

/mK

)

0

5

10

15

20

25

CTE

(ppm

/K)

Al matrix

Si matrix

Si matrix

Al matrix

Al matrix

Si matrix

Si matrix

3

PRIMEX

The PRIMEX process1 is used for making reinforced metals. First, a preform of fibers, whiskers or particulates (SiC, Al2O3, AlN etc.) is made by using one of many preforming processes such as cold pressing, injection molding, slip casting, fiber weaving, or sediment casting. Next, the preform is brought in contact with the molten alloy in a controlled atmosphere. For Al matrix composites, the key feature of the process is the use of Mg dopants or alloying elements. Mg is either added to the preform in the form of powders, or it is added as an alloying element to the melt. The process is carried out in a flowing N2 environment. In the case of Al alloys, the process temperature is about 800ºC.

Figure 2. Comparison of M Cubed’s composite manufacturing processes: TOP: conventional,

and BOTTOM: microwave assisted (MASS).

Microwave Generator T-H

Tuner

Vacuum Pump

Gas Source

Pyrometer

PreformAlloy

MicrowaveTransparentInsulation

Furnace

Retort/Chamber

Preform (SiC, B4C, Al2O3, …)

Metal/Alloy

Heating Element

Vacuum / N2 /air; 800-1550OC

Alloy Container

Microwave Assisted (MASS) Processing

Conventional Processing

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Under the processing conditions, Mg vaporizes and reacts on the preform surface to form

Mg3N2. When molten Al comes in contact with Mg3N2, it spontaneously wets it and wicks into the pore network in the preform. Molten Al also reacts with Mg3N2 forming AlN and releasing Mg. The newly released Mg then reacts with N2 and coats the reinforcement ahead of the infiltration front with Mg3N2. The Al wetting, wicking and reaction processes are repeated through the thickness, advancing the infiltration front. When the alloy comes in contact with the barrier layer applied to the preform outer surface, the reaction stops, retaining the part shape. DIMOX

The DIMOX process2 is used for making reinforced ceramics. The term oxidation can mean formation of not only an oxide but also a nitride, a carbides etc. Similar to the PRIMEX process, the first step is the fabrication of a net-shape preform with the desired reinforcement and a binder. For alumina matrix composites, the preform is placed in contact with Al containing special alloying elements (e.g. Mg, Si) and is heated to 900 to 1150°C in a furnace. The alloying elements cause the liquid alloy to react with the vapor phase to grow a ceramic material in a directed fashion through the porosity in the preform. The dopants alter the metal surface properties allowing it to wet the reinforcement and the oxidation product and cause it to travel through the reaction product and bring it continuously to the growth front. Magnesium forms a protective layer on the metal surface without passivating it. Silicon helps in initiation of oxygen dissolution in the melt which then precipitates as aluminum oxide. When the growth front reaches the outer surface of the preform, a gas permeable barrier applied to the surface (e.g. calcium sulfate) terminates the reaction locally, allowing the retention of the part shape. REACTION BONDING (RB)

While the PRIMEX and DIMOX processes were licensed by M Cubed from the Lanxide Corporation, the reaction bonding or melt infiltration process has been developed by M Cubed for making SiCp/Si, SiCp/Al-Si, B4Cp/Si, Cf/SiC and other composites. In this process, a preform of fibers or particulates is made. Carbon is added in a controlled manner to this preform. A variety of precursors can be used to introduce this carbon into the preform such as pitch, phenolics, sugars, etc.

In the reaction bonding process (silicon-based matrices), good wetting and the highly exothermic reaction between liquid silicon and carbon are utilized to achieve pressureless infiltration of a reinforcement preform. This process has been reported in the literature since the 1940s3-5 and is given many names such as reaction-bonding, reaction-sintering, self-bonding, and melt infiltration.

M Cubed significantly refined this process6-14 to obtain composites with fine microstructures, higher toughness compared to traditional reaction bonded ceramics, and higher machinability. In addition, the process was optimized to produce complex, net-shape components and to allow cost-effective high-volume manufacturing. The first key element of the process refinement was the optimization of the carbon content in the preforms to minimize shrinkage in the final infiltration process to less than 0.5%. Low shrinkage allows inexpensive machining of the components to net shape in the green (preform) state. The second key element of the process is the ability to cast near-net shape preforms with high (70-80%) reinforcement loading using inexpensive tooling. The third key element of the process is the ability to bond various preforms to create complex structures12, followed by infiltration. The metal content and the particle size in

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the composite are controlled to obtain an electrically conductive product that allows electric discharge machining (EDM) of precision features. Nominal properties of MCT materials can be found in the Materials Datasheet on MCT’s web page15.

The SSC (SiCp/Si) series composites offer an excellent CTE match with Si-wafers and are ideal for Si–wafer handling applications. The HSC (SiCp/Al-Si) series offers higher toughness, higher thermal conductivity, and better CTE match with AlN and Al2O3 ceramics. Due to the complex shape capability, HSC and SSC series materials are ideal for large, complex, precision structures. MCT manufactures SSC and HSC structures weighing in excess of 250 kg and larger than 1 x 0.75 x 0.3 m using reaction bonding. The Cf/SiC composites offer low CTE (<1 ppm/ºC). The ballistic performance of personnel armor packages containing SSC and SBC (B4Cp/Si) composite tiles are equivalent to the performances of the respective packages containing hot pressed SiC and B4C on specific weight basis7. M Cubed makes armor tiles for small arms protective inserts (SAPI and ESAPI) for the US Army and Marines at a rate of over 25,000/month. Until recently, most SiC and B4C based armor tiles were made by hot pressing. As is well known, hot pressing is limited to simple shapes, requires expensive, finer raw materials, requires a higher process temperature than RB, and is expensive to scale up. MASS PROCESSING

Recently, microwave assisted processing has been demonstrated for carrying out sintering of ceramic powders as well as powdered metals16-22. Several advantages of microwave assisted processing have been reported in these instances: lower process times, lower process temperatures, enhanced process kinetics, enhanced microstructure, and enhanced properties. Both PRIMEX and especially the DIMOX processes are very slow. The infiltration rates could be as slow as 1.5 inch /day in the case of DIMOX. Thus, a significant potential exists for accelerating this process via the use of microwave energy. It is possible that in addition to the volumetric (faster) heating effects, the rapid diffusion observed in the sintering of ceramics may help accelerate the PRIMEX and DIMOX processes. Although the RB process is very rapid, for large, complex parts the heating and cooling cycles have to be slow to prevent thermal stress- induced cracking. Microwave assisted processing may reduce these cycle times due to the volumetric heating phenomenon where conventional radiation and diffusion of heat are not required. Based on these potential benefits, M Cubed is developing methods to conduct PRIMEX, DIMOX and RB in the microwave assisted (MASS) mode. Figure 2 (bottom) shows a schematic of how these processes can be conducted in the MASS mode. Due to different interactions of different materials with microwaves at different temperatures, innovative, proprietary process modifications were required to successfully conduct these processes in the MASS mode.

The goals of this study were: (1) Demonstrate that these processes can be carried out in the MASS mode, (2) Assess if MASS processes can produce composites with properties equivalent to the properties of composites made by conventional processes, (3) Assess if any process enhancement or benefits can be obtained by MASS processing, and (4) Demonstrate that the MASS processes can be scaled up to manufacture real-life parts.

The first three objectives were met by processing 2 x 2 inch specimens by conventional as well as MASS processing followed by characterization. Specimen thicknesses were varied from 0.125 inch to about 1 inch. The infiltration rates were monitored by stopping the processes at various times and measuring the infiltration distance. Test specimens were machined from the infiltrated specimens for microstructural, physical (density) and mechanical (modulus, flexural

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strength and fracture toughness) characterization. The densities of the test samples were determined using the Archimedes principle per ASTM C373. Elastic moduli were measured by the ultrasonic pulse echo technique (ASTM E494-95). The flexural strengths were determined per ASTM standard C1161. Fracture toughness measurements were made on selected specimens by the Chevron notch method. Fractured specimens were observed via scanning electron microscope to determine failure modes. Figures 3 through 6 show the microstructures of various composites made by MASS processing.

Figure 3. Microstructures of composites made by MASS PRIMEX: (a) 45% SiCp /Al, (b) 55%

SiCp /Al, (c) 70% SiCp /Al, and (d) 50% SiCp /Mg

a

dc

b

SiC SiC

SiC

SiC

7

Figure 4. Comparison of microstructure of 55% Al2O3p /Al composite made by conventional

and MASS PRIMEX. The conventionally processed composite shows voids that are absent in the MASS PRIMEX composite.

Figure 5. Microstructures of Al2O3p/Al2O3 composite made by microwave-assisted DIMOX

processing (MASS DIMOX) In all the cases, fully dense composites were successfully produced by MASS processing. In addition, in the case of Al2O3p/Al composites conventionally processed composite showed porosity. This porosity was eliminated in the sample processed by MASS PRIMEX. Thus, microstructural enhancement was obtained in this instance. Figure 7 compares the infiltration rates obtained with conventional PRIMEX process with those obtained by the MASS PRIMEX process for two reinforcements: SiC and Al2O3. As the chart shows, the MASS PRIMEX process was substantially faster. For example, for SiC preforms, 15 mm infiltration was obtained in 13 hours by conventional process and in 5.5 hours in the MASS process. Thus, the process time was reduced by more than 50%.

Conventional MASS

Al2O3

Reaction formed Al2O3

8

Figure 6. Microstructures of composites made by MASS RB: (a) 70%SiCp/Si and (b) 70%SiCp

/Si-Al. By adding a small amount of aluminum to the silicon matrix, the fracture toughness can be increased from 4 to 5.5 MPa m1/2.

Figure 7. Comparison of aluminum infiltration rates for conventional and MASS PRIMEX

processing.

0.00

5.00

10.00

15.00

20.00

25.00

30.00

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Infiltration Time (hr)

Infil

trat

ion

Dis

tanc

e (m

m)

SiC Preforms MASS

Alumina Preforms MASS

Alumina Preforms Conventional

SiC Preforms Conventional

Si SiC

Si

SiC

Al

a b

9

PROPERTY COMPARISON

The properties of composites made by MASS processes are summarized in Table II. Properties of similar composites made by conventional processes are also listed for comparison. It can be seen that comparable properties were obtained by MASS processing. Table II. Comparison of properties of composites made by MASS and conventional processes.

Composite ID Rein-

force-ment

Vol. %

Matrix Process Den-sity (g/cc)

Modu-lus (GPa)

Flexural Strength (MPa)

Fracture Tough- ness (MPam ½)

111303MW-1 SiC 70 Al-20Si-4Mg MASS-PRIMEX 3.02 251 216 -- 062804DAS-1 SiC 70 Al-20Si-4Mg Conv. PRIMEX 3.04 280 272 -- 011704MW-1 SiC 70 Al-20Si-4Mg MASS-PRIMEX -- -- 284 -- 052004MW-1-2 SiC 70 Al-20Si-4Mg MASS-PRIMEX 3.04 -- 232 -- 062204DAS-1 SiC 70 Al-20Si-4Mg Conv. PRIMEX 3.01 279 229 -- 100404MCTMW-1 SiC 55 Al-20Si-4Mg MASS-PRIMEX 2.98 235 235 -- 122904DAR-1 SiC 55 Al-20Si-4Mg Conv.-PRIMEX 2.91 -- -- 5.8 011105MCTMW-1 SiC 55 Al-20Si-4Mg MASS-PRIMEX 2.93 -- -- 5.3 021105MW-1 SiC 55 Al-20Si-4Mg MASS-PRIMEX 2.95 -- 221 7.4 033105MCTMW-1 SiC 55 Al-10Si-1.5Mg MASS-PRIMEX 2.93 205 318 9.5 031005MCTMW-1 SiC 50 Al-20Si-4Mg MASS-PRIMEX 2.91 196 262 6.6 031705MCTMW-1 SiC 50 Al-10Si-1.5Mg MASS-PRIMEX 2.91 186 274 8.3 040105MCTMW-1 SiC 50 Al-10Si-1.5Mg MASS-PRIMEX 2.90 185 292 9.4 032805MCTMW-1 SiC 45 Al-10Si-1.5Mg MASS-PRIMEX 2.90 181 310 9.3 032905MCTMW-1 SiC 45 Al-10Si-1.5Mg MASS-PRIMEX 2.88 173 326 11.4 061305MCTMW1-1 SiC 50 Mg-10Al MASS-PRIMEX 2.56 148 361 -- 080205MCTMW1-1 SiC 50 Mg-10Al Conv.-PRIMEX 2.56 149 227 -- 083004MCTMW-1 Al2O3 55 Al-7Mg MASS-PRIMEX 3.28 170 454 -- 012904MW-1 Al2O3 55 Al-7Mg MASS-PRIMEX 3.32 178 446 -- 061704DAS-1 Al2O3 55 Al-7Mg Conv. PRIMEX 3.33 173 472 -- 100902MW-1 Al2O3 55 Al-7Mg MASS-DIMOX 3.58 239 283 -- 081804MW-2 SiC 70 Si-20Al MASS-RB 2.99 336 244 -- Standard HSC702 SiC 70 Si-20Al Conv. RB 3.04 345 270 -- 051005MCTMW2 SiC 70 Si MASS-RB 3.00 359 236 -- Standard SSC702 SiC 70 Si Conv.-RB 3.02 359 288 -- All reinforcements are in particulate form. MASS: Microwave Assisted, Conv. – conventional

Figures 8 and 9 show SEM photos of fracture surfaces of composites made by MASS processes. In all the cases, trans-granular fracture was observed. Thus, the matrix-particle bond is so strong that the particles have cracked within themselves rather than being pulled out from the matrix

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due to interfacial failure. Thus, a strong particle-matrix bond was obtained by MASS processing – similar to what is observed in the conventional processes. Figure 8. An SEM photo of fracture surface of composites made by MASS PRIMEX

processing. (a) Al/SiCp (b) Al/Al2O3p. Trans-granular failure was seen in the reinforcing particle indicating a strong particle-matrix bond.

Figure 9. An SEM photo of fracture surface of the Si/SiCp composites made by MASS RB

processing. Trans-granular failure was seen in the reinforcing particle indicating a strong particle-matrix bond.

PROCESS SCALE UP AND COMPONENT MANUFACTURING USING MASS PROCESSES Larger plates and several components were fabricated by MASS PRIMEX and MASS RB to demonstrate the ability to scale up the MASS processes. Figure 10a shows a photo of a 7 x 7 x 0.5 inch billet of 45% SiCp/ Al Figures 10b and 10c show photos of components made by MASS PRIMEX and MASS RB, respectively. Densities and moduli of these components were

a b

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comparable to the those measured on respective coupons made by conventional and MASS processes. Thus, process scale-up and shape capability were achieved without affecting the material quality.

Figure 10. Photos of components made by MASS processes (a) 7 x 7 x 0.5 inch 45% SiCp/Al

MMC billet, (b) 5-inch diameter 55%SiCp/Al chuck and (c) 3-inch diameter 70%SiCp/Si mirror substrate.

SUMMARY

1. PRIMEX, DIMOX and reaction bonding processes were successfully conducted in the microwave assisted (MASS) mode.

2. The process times for PRIMEX (~50%) and reaction bonding (~60%) were substantially reduced using the MASS mode.

3. Physical, microstructural, chemical and mechanical properties of MASS processed composites were comparable to the respective properties of conventionally made composites.

4. Porosity observed in conventionally processed Al2O3p/Al composites was eliminated with microwave assisted processing, demonstrating enhanced microstructure.

5. Microwave assisted processing set ups were built to scale up the processes. 6. Several prototype components were successfully fabricated using the MASS PRIMEX

and MASS RB processes. ACKNOWLEDGEMENTS This work was supported by the Missile Defense Agency (MDA) through contract number DASG60-03-C-0076. The authors wish to acknowledge significant helpful discussions with the program technical monitor Dr. Doug Deason. Technical input from Dr. Daniel Vukobratovich and Walter Wrigglesworth from Raytheon Missile Systems is also acknowledged. REFERENCES

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