TECH SPOTLIGHT

SiAlON Ceramics

Vladimir D. Krstic new generation of silicon-aluminum-oxygen-nitrogen (SiAlON) ceramics ex- Functional Materials A hibits a unique combination of fracture toughness, hardness, and strength that Manufacturing Inc. provide the potential for applications in many industries. These SiAlON ce- Kingston, Ontario, Canada ramics provide fracture toughness of over 10 MPa-m1/2 and flexural strengths of over 1150 MPa, resulting in exceptionally high resistance to thermal shock. Efforts to further increase the reliability of these ceramics have led to process im- provements such as sintering aids and densification methods involving both pressure and pressureless sintering. The goal of these de- velopments has been to impart high fracture toughness and, at the same time, retain high hard- ness and strength. The unique feature in this new class of SiAlON ceramics is the microstructure, which consists of elongated grains of circular cross-section that re- semble fibers rather than plates (Fig. 1), and the inter-granular phase that encourages toughening by crack bridging. Researchers managed to change the morph- Fig. 1 — Transformation of - to -Si3N4 during sintering at high temperature. ology of growth of elongated grains by alloying with ions that change the valence state at high temperature and strain the SiAlON lattice, creating the conditions for growth of fiber-like grains. The key process step that controls the strength and toughness of SiAlON ceram- ics is the alpha to beta phase transforma- tion during densification at high temperature. This transformation is ac- companied by a growth of elongated β- phase at the expense of equiaxed α-phase, as shown in Fig. 2. It turns out that to achieve high strength and toughness, the shape of the transformed β-Si3N4 grains must remain small in diameter and long in length, with a circular cross-section rather Fig. 2 — SEM micrograph of a whisker-like structure of a new generation of SiAlON than a rectangular cross-section. ceramic produced by Functional Materials With this microstructure, it is possible to Manufacturing Inc. fabricate ceramics with an outstanding com- bination of fracture toughness and hardness not found in other ceramics (Table 1). The structure is shown in Fig. 2 for samples sintered to high density. Instead of the platelet- shaped grain commonly observed in this type of ceramics, the new SiAlON ceramics contain rod or whisker type β-SiAlON grains with diameters from one to two microns, and an aspect ratio ranging from 6 to 12.

Applications The benefits of this new SiAlON have been proven in racing car seals and in molten metal handling applications where the life of seals, crucibles, and tubes has

TABLE 1 — COMPARISON OF MECHANICAL PROPERTIES* Fracture toughness 1/2 Cutting tool insert material KIC, MPa-m Hardness, GPa Based on SiAlON 8.76 16.87 Based on alumina-silicon 7.47 17.36 carbide whiskers *Cutting tool inserts made of pressureless- sintered SiAlON vs. composites of hot-pressed alumina/silicon carbide whiskers

ADVANCED MATERIALS & PROCESSES • JUNE 2010 29 TABLE 2 — DENSITY, HARDNESS AND FRACTURE TOUGHNESS* FMM 1 FMM 2 FMM 3 FMM 4 FMM 5 Sample SiAlON Silicon nitride4 Silicon nitride4 Silicon nitride4 SiAlON Theoretical Density, % 99.7 99.4 99.7 98.8 98.4 Hardness HV, GPa 15.42 14.94 14.83 13.2 17.79

1/2 KIC, MPa.m 8.01 8.27 8.55 10.2 7.36 *Various grades of SiAlON and silicon nitride ceramics manufactured by Functional Materials Manufacturing Inc.

been increased by 50%. High fracture toughness and high the tool during machining of steel has been the largest im- hardness make this class of ceramics suitable for applica- pediment to the wider application of silicon nitride as a tions previously limited to metals and composites. These tool material in machining steel and nonferrous alloys. include cutting tools for machining steel and nonferrous The new generation of SiAlON ceramics has been shown alloys, engine components, and parts for smelting to significantly improve the shock resistance and decrease equipment. wetting by molten aluminum, making it highly attractive Tests have shown this ceramic to exhibit excellent serv- for the fabrication of crucibles, tubes, and fixtures in ice in cutting operations against cast iron and steel. It could metal smelting operations. also serve as a substitute for alumina-silicon carbide whiskers cutting tools, with the added advantage of in- creased inertness in contact with iron during high-speed cutting. For more information: High reactivity with the workpiece has also been a Vladimir Krstic is president of Functional Materials Manufac- major problem for existing silicon nitride-based ceramics turing Inc., 55 Gore Road, Kingston, Ontario, Canada K7L 5H6; in cutting operations. In fact, reaction-assisted wear of [email protected]; www.fmmceramics.com. Don’t be Left out in the Cold. TSS Cold Spray Conference Plan today for this intensive two-day meeting featuring invited talks by cold spray experts from September 27-28, 2010 around the world. Keep current with today’s cold Akron, Ohio spray technology. Along with targeted presentations on applications for specific industries, there are several networking Don’t miss an opportunity to join the growing list opportunities as well as a poster session, Q&A panel of exhibitors and sponsors. For more information, discussion and table top exhibit area planned for please contact Kelly Thomas, National Account information sharing, connecting with colleagues and Manager, [email protected] just for fun. or 440.338.1733. • Gain basic understanding of the cold spray process • Follow global R&D programs on cold spray technology See you in September in Akron! • Receive first-hand information on industrial Advance your industry and your career. applications • Network with international experts Plan and register today. www.asminternational.org/coldspray

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30 ADVANCED MATERIALS & PROCESSES • JUNE 2010