Ceramic Steel – Another Great Aussie Invention

Just like Wi-Fi (wireless internet), the origins of Partially Stabilised Zirconia (PSZ) is a commercial success story that was born from research carried out at Australia’s CSIRO. The original patent on PSZ technology dates back to 1978 (US Patent 4,067,745), awarded to researchers Garvie, Hannink and Pascoe.

One of the most significant events in the history of PSZ was the paper published in the journal Nature in 1975 by Garvie, Hannink and Pascoe, titled “Ceramic Steel”.

This paper outlined the transformation toughening process that gives rise to PSZ’s exceptional mechanical properties, in particular strength (Modulus of Rupture) and fracture toughness. Citing some physico-chemical similarities between alloy systems in the zirconia and iron systems, they proposed that PSZ could be considered the ceramic analogue of TRIP steel.

Australian company Nilsen Sintered Products originally acquired exclusive rights to the technology in 1979. Manufacture of PSZ under the tradename Nilcra® Zirconia continues in Melbourne to this day with the company acquired by Morgan Advanced Materials plc (UK) in 2008.

The History of Nilcra® Zirconia in Australia              

• 1975     CSIRO advertises license for PSZ
• 1979     Nilsen Sintered Products Pty Ltd, Separation Street, Northcote, acquired the license for exclusive worldwide production of PSZ
• 1980     First production furnace delivered
• 1981     Nilsen (USA) Inc. formed in Elmhurst, Chicago to promote and market PSZ in North America
• 1984     Nilcra^® Ceramics Pty Ltd was established by joint venture between Nilsen and CRA Limited
• 1984     Construction of a new dedicated plant was completed including a new laboratory and development facilities at the NSP Northcote site
• 1988     ICI Australia Pty Ltd acquired Nilcra^® and also purchased alumina components business R&W in Auburn, California, USA which became ICI Ceramics Inc. new North American sales and marketing office
• 1990     ICI Advanced Ceramics relocated to a dedicated facilities in Redwood Drive, Notting Hill
• 1991     European sales office opened in Bracknell, UK
• 1997     Carpenter Technology Inc., USA acquired ICI Advanced Ceramics and established ‘Carpenter Advanced Ceramics Pty Ltd’ as a wholly owned subsidiary
• 2008     The Morgan Crucible Company plc, UK acquired company and established ‘Morgan Technical Ceramics Australia Pty Ltd’ as a wholly owned subsidiary

Zirconia Engineering Ceramics

Zirconia is an engineering ceramic that exists in three primary crystalline forms depending on temperature.

Of particular interest is the disruptive phase transformation from tetragonal to monoclinic that occurs during cooling. This phase change is accompanied by a 3-5% volume expansion which is detrimental when producing high purity zirconia ceramics as it induces cracking that significantly compromises the mechanical integrity of the final product.

It is possible to produce a fully stabilised zirconia ceramic material that consists wholly of the cubic phase. However, extensive research into zirconia ceramics has revealed that optimal mechanical properties are generated when the multiphase PSZ material is produced. This material consists of the cubic phase with a fine dispersion of nano-sized, tetragonal precipitates.

The Nilcra^® Zirconia PSZ composition can be achieved through the addition of stabilising agents and suitable heat treatment. The tetragonal phase is metastable and will transform to monoclinic when subjected to external stresses e.g. diamond grinding.

High levels of stress induced at stress concentrations, or micro-flaws in the material, cause the tetragonal phase to transform locally into the monoclinic phase. As mentioned, there is an associated volumetric expansion that exerts a compressional closing stress on any crack fronts that halt crack propagation. This ultimately produces a “transformation toughening” effect that results in very high strength (MOR) and fracture toughness.

Typical stabilising agents include cubic oxides such as CaO, MgO, Y₂O₃ and CeO₂. While commercial formulations are optimised and kept secret, the additions of stabilising agents are quite small. In the case of MgO, CSIRO data indicates an additional level of 3mol.% to produces the maximum rupture strength.

Properties of Nilcra^® Zirconia PSZ

Imagine a material that was extremely hard and wear resistant, able to operate at high temperatures and in corrosive atmospheres. Numerous engineering ceramics fit the bill. However, if they were required to remain unscathed after repeated blows with a sledge hammer, then PSZ would likely be the prime candidate. If you don’t believe it, check out the video here. It is an engineering ceramic like no other before it.

PSZ is characterised by the following properties:

• Exceptional strength and toughness
• High fracture toughness
• High hardness
• Excellent corrosion resistance
• Chemically inert, making it suitable for use in harsh environments
• Excellent abrasion, erosion and wear resistance
• Non-magnetic
• Electrically insulating
• Low thermal conductivity
• Thermal expansion co-efficient similar to steel

This combination of properties has seen PSZ replace more traditional materials such as metal alloys, hardened steels and tungsten carbides, especially in applications demanding high levels of wear and corrosion resistance.

Applications

Nilcra^® Zirconia’s excellent mechanical and chemical properties see it perform successfully in a broad range of severe service industries as diverse as mining and minerals processing, pulp and paper, cement, power generation, food and battery production, and metal forming. It is also commonly used in a variety of process valve and pump components, including ball and butterfly valves, seats, liners, discs, plugs, plungers, sleeves and cages. It is suited to aggressive bearing applications, including bearings for screw conveyors, pumps and chemical digestion tanks.

Nilcra^® Zirconia tooling is also utilised in the steel canning industry for seaming and spin flange rollers and in other metal forming applications, such as, calendaring rollers for copper wire drawing and extrusion dies. The material also finds applications in directional drilling tool components used in oil and gas exploration and in fuel injectors for diesel engines. Components made from PSZ can minimise wear and corrosion and provide the benefits of longer life, less downtime and lower maintenance costs.

Summary

While the term “Ceramic Steel” may seem contradictory, Nilcra^® Zirconia has rightfully earned the title. With its unique combination of properties largely facilitated by the transformation toughening mechanism, Nilcra^® Zirconia is a high performance engineering ceramic that can operate in severe service conditions. It will continue to find applications where it outperforms specialty steels and alloys. For wear and corrosion challenges, conversion from metal to Nilcra^® Zirconia will benefit users with significant gains in product performance, reduced downtime and extended service life.