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Advanced Ceramics: At the Cutting Edge

April 5, 2000
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While ceramic cutting tools have been in use for over 60 years, it is only within the past two decades that they have found major applications, principally in the turning and milling of cast iron, nickel based superalloys and the finishing of hardened steels. In these areas, ceramics based on aluminum oxide and silicon nitride are significantly outperforming cemented carbides and coated carbides. While the latter two materials contain significant amounts of ceramic powder, they are metal bonded and thus are cermets, not true ceramics. Similarly, sintered diamond and polycrystalline CBN can be considered ceramics, but within the cutting tool industry only the alumina and Si3N4 based materials are referred to as ceramics.

High speed cutting tool tips can encounter temperatures of 1000·C or higher, so a key property for an efficient cutting tool is hot hardness. Both the alumina and Si3N4 families of materials retain a higher hardness at temperatures between 600-1000·C than either tool steels or cobalt bonded WC cermets. The ceramics are also more chemically inert. The combination of hot hardness and chemical inertness means that the ceramics can run hotter and longer with less wear than the competing materials.

Historic concerns with ceramic cutting tools have focused on low toughness, susceptibility to thermal shock and unpredictable failure times. Improvements in processing, together with microstructural modifications to increase fracture toughness, have greatly increased the reliability of the ceramics over the past decade.

Alumina Based Tools

Alumina based inserts include alumina-zirconia, alumina-TiC (or TiN), and alumina reinforced with silicon carbide whiskers. Alumina-ZrO2, also called "white ceramic," contains up to 10% ZrO2, which toughens the ceramic via a matrinsitic transformation. These inserts are particularly effective in the finishing of low to medium alloy steels. Alumina-TiC, "black ceramic," is typically 30-40% TiC and is used for machining chilled cast iron and hardened steels. This material is particularly abrasion resistant. Alumina-SiCw reinforced composite (typically 25-50% whiskers) is the toughest and most thermal shock resistant of the alumina based ceramic inserts, and in contrast to black or white ceramic inserts, it can be run with a coolant. The high speed finishing of Ni-based superalloys is typically carried out using cooled alumina-SiCw.

Silicon Nitride Based Tools

Silicon nitride based inserts include fully dense Si3N4 (typically with yttria and alumina densification aids), and SiAlONs, which are solid solutions of alumina in Si3N4. Fully dense Si3N4 can have a fracture toughness of 6-7 MPa m1Ú2, almost as high as cemented carbides (~9 MPa m1Ú2), a high strength (greater than 1,000 MPa) and a low thermal expansion that yields excellent thermal shock behavior. Silicon nitride is the most efficient insert for the turning of gray cast iron, and is also used for milling and other interrupted cut operations on gray iron. Because of its thermal shock resistance, coolant may be used with silicon nitride for turning applications. SiAlONs are typically more chemically stable than the Si3N4s, but not quite as tough or thermal shock resistant. They are mainly used in rough turning of Ni-based superalloys.

Benefits

Ceramic inserts are generally more costly than carbides (11Ú2-2 times more) but their metal removal rates are ~3-4 times greater. Ceramic inserts also demonstrate reduced wear rates. The combination of lower wear and faster metal removal means more many more parts can be produced before tools have to be indexed or replaced. In some cases, this enhanced productivity is truly astonishing.

For example, a SiAlON tool was substituted for a coated carbide tool in the interrupted single point turning of the outer diameter counterweights on a gray cast iron crankshaft. This change resulted in the metal removal rate increasing by 150% and the tool life increasing by a factor of 10. Each tool produced 10 times as many parts, and in much less time.

By changing to SiC whisker reinforced alumina inserts, a gas turbine manufacturer reduced its machining time for two operations from five hours to only 20 minutes. This yielded a direct savings of $250,000 per year, freed up 3,000 hours of machine time per year, and avoided the need to purchase a second machine tool.

Future Directions and Markets

Even with significant performance advantages, ceramic cutting tools have to struggle to maintain market share. Major markets for ceramic tools include automotive and aerospace manufacturers. As aluminum engine blocks and other components replace gray cast iron, there is less gray cast iron to machine. To compensate, alumina and TiN coated Si3N4s have been developed that outperform coated carbides in turning and milling of ductile iron, thus opening new markets. Similarly, nanosized additions of ZrO2 into "white" ceramic tools and submicron TiC into "black" ceramic tools are aimed at expanding the markets for the alumina based tools.

Presently ceramics command about 5% of the global ~$3 to $4 billion tool insert market. This ~$150 to $200 million market for ceramic cutting tools is expected to remain at 5% of the total market for the near future. Further ahead, as environmental regulations increase the disposal costs for cutting fluids, the dry machining ability of the ceramics may provide an opportunity to increase its percentage of the market.

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