1-1.3 Zirconia
Zirconium dioxide (ZrO2, zirconia) is principally derived from zircon, ZrSiO4,
which occurs in igneous rocks such as granites and pegmatites. Decomposed
pegmatites have been worked for zircon in Madagascar and Brazil. Zircon is
also a constituent of some metamorphic rocks and also occurs as secondary
deposits in beach sands in Australia, Brazil, India, and Florida. In these
secondary deposits, which have been worked commercially, the zircon occurs
together with other minerals such as ilmentite, rutile, and monazite.
Zirconia is an oxide with very high melting temperature (T ≈ 2,880°C), which
solidifies in cubic phase (ZrO2-c, group space Fm 3m), then transforms (T < ≈
2,370°C) to tetragonal phase (ZrO2-t, P42/nmc) and finally, below ≈ 1,170°C,
becomes mono-clinical (ZrO2-m, P21/c). This last transition t→m is
accompanied by considerable dimensional variations (shear strains of ≈ 0.16 and
increase in volume of ≈ 4%), which largely exceed the maximum stress limit,
resulting in a fragmentation of the material. A zirconia part sintered at
temperatures that the refractarity of this oxide requires – let us say sintered at
about 1,500°C – breaks up and is destroyed during cooling, during the t→m
transition. This means that “pure” zirconia can be used only in powder form (for
example, as starting product for the manufacture of ceramic enamels), and
therefore for uses that do not require consolidation into a massive part. There are
a number of commercial approaches to producing pure zirconia from zircon.
Zircon dissociates above 1750oC into ZrO2 and SiO2. Injection of zircon sand
into a plasma (at temperatures >6000°C) results in dissociation and melting. The
zirconia solidifies first, in the form of dendrites, and the silica solidifies as a
glassy coating on the zirconia. The silica may be removed by leaching in boiling
sodium hydroxide solution. The residue is washed and the zirconia is removed
by centrifuging. The main production method for zirconium oxide is electric arc
melting of zircon between 2100 and 2300°C.
Dissociation still occurs at these lower temperatures, but solid zirconia is
produced along with liquid silica. The purity of the ZrO2 produced is about 99%.
Another, although commercially less significant, source of zirconia is
baddeleyite (impure monoclinic ZrO2). Baddeleyite is found in small deposits
and usually contains contaminants such as silica, iron oxide, and Titania.
Baddeleyite deposits are mined commercially in Brazil and South Africa.
Zirconium ores all contain varying amounts of hafnium, typically 1.5 – 3 wt% of
the Zr content. As a result of the chemical similarity of Hf to Zr, separation
techniques are expensive. Unless specifically required separation is not
performed and technical grade zirconia is sold containing up to 3 wt% Hf.
To produce zirconia sintered pieces, ZrO2 must be combined with other oxides
known as “stabilizers” (MxOy= primarily CaO, MgO or Y2O3): the ZrO2 MxOy
phase diagram is then modified favorably, which helps preserve (at the stable
state or metastable state) a “stabilized zirconia”, free from transitions in the
entire useful temperature range – in practice from the sintering temperature to
room temperature. In the ZrO2-CaO diagram, for example, it is observed that for
20 mol% CaO, the material remains in cubic phase from room temperature to
practically the melting temperature. Among the uses of stabilized zirconia,
denoted “SZ”, we can mention four main fields:
– The production of monocrystals for jewelry, because the optical properties of
zirconia is not very different from those of a diamond, at an incomparably lower
cost.
– The manufacture of crucibles and other refractory parts, because of the high
melting temperature and good resistance to corrosive mediums, including
molten glass (refractories of A-Z-S system: Al2O3-ZrO2-SiO2);
– The manufacture of thermal barriers, for example deposited by plasma
spraying using plasma torches for the internal protection of the combustion
chamber of jet engines, because the thermal conductivity of zirconia is one of
lowest ever known among non-metallic inorganic solids (k ≈ 1 W.m-1.K-1, i.e. 30
times lower than alumina).
– The manufacture of ionic conductors. We must mention it here because it is
related to the same mechanism of “stabilization”. ZrO2-c has a fluorite structure
(like, for example, UO2), with the zirconium atoms at the nodes of a facecentered cubic lattice and the oxygen atoms occupying all the tetrahedral
interstices of this lattice (four nodes for a CFC array and eight tetrahedral sites
per array give the stoichiometry ZrO2). The unit is very compact and we can say
that the transitions that occur when we cool ZrO2-c come from the need to
decompress the structure. However, the introduction of bivalent (CaO and MgO)
or trivalent (Y2O3) metal oxides that are used as stabilizers requires, for the
charges to be balanced, that the substitution of Zr4+ by M2+ or M3+ is
compensated by the presence either of interstitial cations or anionic vacancies. It
is the second case that occurs here: a usual SZ, 20% CaO-80% ZrO2 (in moles),
therefore contains a 20% deficit of oxygen atoms: Zr1-xCaxO2-xVOx, in KrgerVink notation, where VO represents oxygen vacancies and x is equal to 0.2. The
anionic sublattice is thus decompressed and this considerable concentration in
vacancies – more than ten orders of magnitude higher than the normal
concentration of defects in thermodynamic equilibrium – allows a considerable
mobility of residual oxygen ions, since we know that the diffusion proceeds
primarily by a vacancy mechanism, like a puzzle. By virtue of these
constitutional oxygen vacancies, the stabilized zirconia offers properties of ionic
conduction that allow its application as solid electrolyte, particularly in oxygen
sensors and in solid oxide fuel cells.
●Ceramic steel?
SZ has rather modest mechanical properties, significantly less remarkable
compared to alumina which, associated with higher density, and higher thermal
expansion (consequently greater sensitivity to thermal shocks) and markedly
increased costs explain why these stabilized zirconias a priori do not have a
mechanical application.
It is partially stabilized zirconia (PSZ) that have justified the resounding article
(“Ceramic steel?”) published in 1975 by Garvie et al. The title suggests that a
ceramic can exhibit the high mechanical performances associated with steel, but
also that toughening mechanisms recall those used by steel manufacturers. The
t→m transformation of zirconia is a martensitic transformation, in analogy with
the transformation used to obtain martensite in tempered steels, and the role of
microstructural parameters in ZrO2 is similar to what is observed in metals.
The mechanical properties of zirconia with high mechanical performances – of
which there are multiple varieties – constitute one of the themes that have
inspired the greatest number of publications in the field of ceramics. [HAN 00]
constitutes an excellent study on the subject.
Figure 1.4: ZrO2-MgO and ZrO2-Y2O3 diagrams; the hatched zones show
Compositions normally chosen for ceramics with TT effect [HAN 00].
Let us start by analyzing what it is about, by reasoning at room temperature.
Anon-stabilized zirconia (Z) is subjected, during its cooling from its processing
temperature, to destructive phase transition t→m: its mechanical performances
are therefore almost zero. A stabilized zirconia (SZ) is free from these
transitions; it remains in general in cubic phase: its mechanical performances are
modest. Lastly a partially stabilized zirconia (PSZ) can be made up of a matrix
rich in stabilizers, therefore in the form of SZ, within which there is a dispersion
of small precipitates of zirconia poor in stabilizers, which should be in
monoclinic form but can subsist in a metastable state in tetragonal form. The
propagation of a microscopic crack relaxes the stresses applied by the matrix on
these precipitates, which enables them to switch to the monoclinic, stable state.
Hence, local swelling (increase in volume of ≈ 4% associated with the t→m
transformation) which “clamps” the crack and stops its propagation: we can thus
considerably increase mechanical strength and toughness. However simplistic
this explanation might be, it highlights the two essential points:
i) The t→m transformation, previously considered as a drawback, can become
an advantage;
ii) The control of microstructural characteristics (for instance, grain size) is
essential. Figure 1-4 shows the ZrO2-CaO and ZrO2-Y2O3 phase diagrams,
where the main PSZ materials are located.
1-1.4 Rutile
Rutile (TiO2, Titania) occurs as a constituent of igneous rocks such as granites
and also in metamorphic derivatives such as gneiss. It also occurs as fine needles
in slates, biotite mica, quartz, and feldspar. Economically the most important
deposits are segregations in igneous rocks as found in Virginia, Canada, and
Norway. Rutile is also an important constituent of beach sands resulting from
denudation of rutile-bearing rocks, as in Australia, Florida, and India. Titania is
also produced by reacting ilmenite FeTiO3 with sulfuric acid at 150–180°C to
form titanyl sulfate, TiOSO4:
FeTiO3 (s) + 2H2SO4 (aq) + 5H2O (l) → FeSO4 ・ 7H2O (s) + TiOSO4 (aq)
Titanyl sulfate is soluble in water and can be separated from undissolved
impurities and the precipitated iron sulfate by filtration. Hydrolyzing at 90°C
causes the hydroxide TiO (OH)2 to precipitate:
TiOSO4 (aq) + 2H2O (l) → TiO (OH) 2 (s) + H2SO4 (aq)
The titanyl hydroxide is calcined at about 1000oC to produce Titania TiO2.