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.
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.
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:
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.
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.
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.