Zirconium oxide, ZrO2, is a white crystalline powder with a specific gravity of 5.7, hardness 6.5, and refractive index 2.2. When pure, its melting point is about 2760°C, and it is one of the most refractory of the ceramics. It is produced by reacting zircon sand and dolomite at 1371°C and leaching out the silicates. The material is used as fused or sintered ceramics and for crucibles and furnace bricks. From 4.5 to 6% of CaO or other oxide is added to convert the unstable monoclinic crystal to the stable cubic form with a lowered melting point.
Zirconia is produced from the zirconium ores known as zircon and baddeleyite. The latter is a natural zirconium oxide. It is also called zirkite and Brazilite. Zircon is zirconium silicate, ZrO2 SiO2, and comes chiefly from beach sands. The sands are also called zirkelite and zirconite, or merely zircon sand. The white zircon sand has a zirconia content of 62%, and contains less than 1% iron.
Uses
Fused zirconia, used as a refractory ceramic, has a melting point of 2549°C and a usable temperature to 2454°C. The Zinnorite fused zir-conia is a powder that contains less than 0.8% silica and has a melting point of 2704°C. A sintered zirconia can have a density of 5.4, a tensile strength of 82 MPa, compressive strength of 1378 MPa, and Knoop hardness of 1100. Zircoa B is stabilized cubic zirconia used for making ceramics. Zircoa A is the pure mon-oclinic zirconia used as a pigment, as a catalyst, in glass, and as an opacifier in ceramic coatings.
As an opacifier, zirconium compounds are used in glazes and porcelain enamels. Zirconium dioxide is an important constituent of ceramic colors and an important component of lead-zirconate-titanate electronic ceramics.
Pure zirconia also is used as an additive to enhance the properties of other oxide refractories. It is particularly advantageous when added to high-fired magnesia bodies and alumina bodies. It promotes sinterability and, with alumina, contributes to abrasive characteristics.
Zirconia brick for lining electric furnaces has no more than 94% zirconia, with up to 5% calcium oxide as a stabilizer, and some silica. It melts at about 2371 °C, but softens at about 1982°C. The IBC 4200 brick is zirconia with calcium and hafnium oxides for stabilizing. It withstands temperatures to 2316°C in oxidizing atmospheres and to 1849°C in reducing atmospheres. Zirconia foam is marketed in bricks and shapes for thermal insulation. With a porosity of 75% it has a flexural strength above 3 MPa and a compressive strength above 0.7 MPa. For use in crucibles, zirconia is insoluble in most metals except the alkali metals and titanium. It is resistant to most oxides, but with silica it forms ZrSiO4, and with titania it forms ZrTiO4. Because structural disintegration of zir-conia refractories comes from crystal alteration, the phase changes are important considerations. The monoclinic material, with a specific gravity of 5.7, is stable to 1010°C and then inverts to the tetragonal crystal with a specific gravity of 6.1 and volume change of 7%. It reverts when the temperature again drops below 1010°C. The cubic material, with a specific gravity of 5.55, is stable at all temperatures to the melting point, which is not above 2649°C because of the contained stabilizers. A lime-stabilized zirconia refractory with a tensile strength of 138 MPa has a tensile strength of 68 MPa at 1299°C.
Stabilized zirconia has a very low coefficient of expansion, and white-hot parts can be plunged into cold water without breaking. The thermal conductivity is only about one third that of magnesia. It is also resistant to acids and alkalies, and is a good electrical insulator.
To prepare useful formed products from zirconium oxide, stabilizing agents such as lime, yttrium, or magnesia must be added to the zirconia, preferably during fusion, to convert the zirconia to the cubic form. Most commercial stabilized zirconia powders or products contain calcium oxide as the stabilizing agent.
The stabilized cubic form of zirconia undergoes no inversion during heating and cooling.
Stabilized zirconia refractories are used where extremely high temperatures are required. Above 1649°C, in contact with carbon, zirconia is converted to zirconium carbide.
Zirconia is of much interest as a construction material for nuclear energy applications because of its refractoriness, corrosion resistance, and low nuclear cross section. However, zirconia normally contains about 2% hafnia, which has a high nuclear cross section. The hafnia must be removed before the zirconia can be used in nuclear applications.
Forms
Zirconia is available in several distinct types. The most widely used form is stabilized in cubic crystal form by a small lime addition. This variety is essential to the fabrication of shapes because the so-called unstabilized, mon-oclinic zirconia undergoes a crystalline inversion on heating, which is accompanied by a disruptive volume change.
Zirconia is not wetted by many metals and is therefore an excellent crucible material when slag is absent. It has been used very successfully for melting alloy steels and the noble metals. Zirconia refractories are rapidly finding application as setter plates for ferrite and titanate manufacture, and as matrix elements and wind tunnel liners for the aerospace industry.
Other Types
Toughening mechanisms, by which a crack in a ceramic can be arrested, complement processing techniques that seek to eliminate crack-initiating imperfections. Transformation toughening relies on a change in crystal structure (from tetragonal to monoclinic) that zirconia or zirconium dioxide (ZrO2) grains undergo when they are subjected to stresses at a crack tip. Because the monoclinic grains have a slightly larger volume, they can "squeeze" a crack shut as they expand in the course of transformation. Because of the transformation toughening abilities of ZrO2, which impart higher fracture toughness, research interest in engine applications has been high. In order for ZrO2 to be used in high-temperature, structural applications, it must be stabilized or partially stabilized to prevent a monoclinic-tetragonal phase change. Stabilization involves the addition of calcia, magnesia, or yttria followed by some form of heat treatment. PSZ ceramic, the toughest known ceramic, has been investigated for diesel-engine applications.
PSZ is a transformation toughened material consisting of a cubic zirconia matrix with 20 to 50 vol% free tetragonal zirconia added in the matrix. The material is converted into the stabilized cubic crystal structure using oxide stabilizers (magnesia, calcia, yttria). The conversion is accomplished by sintering the doped zirconia at 1700°C. Magnesia-stabilized zirco-nia exhibits serrated plastic flow during compression at room temperature. The flow stress is strain rate sensitive. Several different grades are available for commercial use, and the properties of the material can be tailored to fit many applications.
One typical PSZ used for applications requiring maximum thermal shock resistance has a four-point bend strength of 600 MPa; PSZ is used experimentally as heat engine components, such as cylinder liners, piston caps, and valve seats. Vanadium impurities from fuel oil can cause zirconia destabilization, and sodium, magnesium, and sulfur impurities can cause yttria to dissociate from yttria-stabilized zirco-nia. Another area of interest for PSZ is in bio-ceramics, where it has use in surgical implants.
A new zirconia ceramic being developed, tetragonal zirconia polycrystal (TZP) doped with Y2O3, has the most impressive room-temperature mechanical properties of any zirconia ceramic. The commercial applications of TZP zirconia include scissors with TZP blades suitable for industrial use for cutting tough fiber fabrics, e.g., Kevlar, cables, and ceramic scalpels for surgical applications. One unique application is fish knives. The knife blades are Y-TZP and can be used when the delicate taste of raw fish would be tainted by slicing with knives with metal blades.
Another zirconia ceramic-developed material is zirconia-toughened alumina (ZTA). ZTA zirconia is a composite polycrystalline ceramic containing ZrO2, as a dispersed phase (typically ~15 vol%). Close control of initial starting powder sizes and sintering schedules is thus necessary to attain the desired ZrO2 particle dimensions in the finished ceramic. Hence, the mechanical properties of the composite ZTA ceramics limit current commercial applications to cutting tools and ceramic scissors.
PSZ is also finding application in the transformation toughening of metals used in the glass industry as orifices for glass fiber drawing. This material is termed zirconia grain-stabilized (ZGS) platinum.
Clear zircon crystals are valued as gem-stones because the high refractive index gives great brilliance.
Zirconia fiber, used for high-temperature textiles, is produced from zirconia with about 5% lime for stabilization. The fiber is polycrys-talline, has a melting point of 2593°C, and will withstand continuous temperatures above 1649°C. These fibers are as small as 3 to 10 |im and are made into fabrics for filter and fuel cell use. Zirconia fabrics are woven, knitted, or felted of short-length fibers and are flexible. Ultratemp adhesive, for high-heat applications, is zirconia powder in solution. At 593°C, it adheres strongly to metals and will withstand temperatures to 2427°C. Zircar is zirconia fiber compressed into sheets to a density of 320 kg/m3. It will withstand temperatures up to 2482°C and has low thermal conductivity. It is used for insulation and for high-temperature filtering.
