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C -H. Chao, H.Y. Lu/Materials Science and Engineering 4328(2002)267-276 4. Discussion lized B-phase. The indications are:(a) the codoping of Na,O+Al,O3) enables the stabilization of the high 4.1. Chemical and mechanical terms in the temperature cubic phase to room temperature, and(b) B-a-cristobalite phase transformation particle size [18, 25] is also a determining factor in the metastable retention, as discussed in the literature [6,7 The volume contraction upon the B-a phase trans- The stabilization mechanism is therefore likely to asso- formation of cristobalite and quartz results in dusting ciate with both the chemical and mechanical terms [7] of fused silica prepared from quartz sand as in the Again, it is similar to the stabilization of c-and tetrag Osram process [24]. For samples with y2 19.5, at-cristo- onal(t)-ZrO2 to room temperature. A critical mechani- balite is the predominant crystalline phase in the mix- cal constraint imposed by the matrix of either glass or ture(Fig. 4(a)). Although large cracks may have other (a+ B)-cristobalite grains would have existed as already existed in the as-sintered microstructure(as in the partially stabilized zro2(PSz)[8], ZrO2-tough arrowed in Fig. 5(a)), extensive microcracking only ened Al,O3(ZTA)[26] and t-ZrO2 polycrystals(TZP occur after sample grinding(e.g y=19.5 in Fig. 5(b)). [27]. The relief of the constrained stresses by surface Large cracks generated thermally are attributed to the nding has resulted in the transformation of the em- 5 vol. volume contraction upon the B-a-cristo- bedded t-ZrO2 particles to the monoclinic(m)-phase by balite phase transformation since no quartz was found a stress-induced martensitic phase transformation in these samples. Microcracks, however, are more likely mechanism [28]. Similarly, the B-+a-cristobalite pha to be of the stress-induced nature. They are originated transformation also occurs when the matrix constraint from the B-+a-cristobalite phase transformation, trig- has been removed by external stresses. Those particles gered off by the stress applied in grinding. It is evi- in the(Na++Al+)-codoped SiO2 ceramics containing denced from the increased amount of a-cristobalite solute cations below the critical concentration for full after sintered samples have been re-ground to fine stabilization then transform to a-cristobalite during powders(of 38-45 and <38 um in size), as shown by surface grinding and or pulverizing to powder C-l(a) and C-1(b)in Fig. 2 for y= 24.6. The y= 24.6 The B-cristobalite(e.g. Fig. 2)that sustained pulver samples initially contain a mixture of a-and B-phas izing or polishing can therefore be perceived as being (C-l in Fig. 2)(although not immediately discernible equivalent to the fully stabilized c-ZrO2(FSZ). It still from the XRD trace, it has been demonstrated for retains metastably to room temperature is therefore samples with y= 19.5 by deconvulsion(Fig. 1(b)). It is chemically ' non-transformable. The full stabilization of plausible that some of the initially metastable p-cristo- the cubic symmetry in cristobalite may require a lower balite confined in the matrix has transformed to the limit of solute content similar to that at 13.2 mol% a-phase because the mechanical constraint is relieved by Mgo for the fully stabilized c-ZrO2 [29]. Those trans- grinding and polishing. In fact, phase transformation to formed to a-cristobalite upon surface grinding(Fig a-cristobalite was also attributed [13] to the tensile 1(c) and 2) are only ' partially' stabilized when the stresses induced upon cooling due to differential ther- modification of chemical free energy(AGchem term)[26] mal expansion. The actual stress state involving AE alone by solute cations(e.g. Mg+ for Zro2 and(Na (differential elastic moduli), Az(differential thermal +Al+)for SiO, )is not sufficient to render full stabi pansion) and the sample grinding technique at lization. The p-crystals containing less solute content present can not be assessed easily. The B-phase found in can only become metastable if augmented by the me- Breuckener's samples [ 13] was probably due to the chanical constraint from the matrix. It is necessary for stabilization by the chemical impurities associated with these crystals to remain confined to a matrix(of glass the initial colloidal silica(Vitresil containing impuri- and (B+a)-cristobalite mixture) in order to fulfil the ties of Al+, Na+ and OH-) thermodynamic requirement of AGrotal>0. Since the The B-cristobalite survived from pulverizing to pow- inhomogeneity of solute oxides exists in the initial der, as represented by (111B in C-1(a)of Fig. I(c), must mixture of Na2O-containing colloidal silica sols doped have been fully stabilized to the extent that they have with additive salts, some of the cristobalite crystallized become 'non-transformable' meaning its resistance of with higher solute contents therefore become the fully the phase transformation to a-cristobalite even under stabilized B-phase. The lower limit of Na,O and Al2O3 grinding stresses. In fact, particles larger than 300-500 solute oxides would be an overall concentration of 6.29 nm, and not completely detached from the cracked mol% since samples with y= 13.9 contain only B-cristo matrix(Fig. 5(b)) after surface polishing, are probably balite as detected by XRD(Fig. 4(a). For the Na2O the survived non-transformable B-cristobalite. The ma- and Al2O3-additive content of 4.65 mol% (=19.5), trix transformed to a-phase and experienced the accom- below the critical solute level, crystallization(probably panied volume contraction has resulted in extensive to the spherical particles indicated in Fig. 5(b)may microcracking. It is also clear from Fig 4(a) that higher have also occurred from regions of higher solute con- doping level (with y= 13.9) has led to the fully stabi- tent than the overall concentration of the initial mix-274 C.-H. Chao, H.-Y. Lu / Materials Science and Engineering A328 (2002) 267–276 4. Discussion 4.1. Chemical and mechanical terms in the -cristobalite phase transformation The volume contraction upon the phase transformation of cristobalite and quartz results in dusting of fused silica prepared from quartz sand as in the Osram process [24]. For samples with y19.5, -cristobalite is the predominant crystalline phase in the mixture (Fig. 4(a)). Although large cracks may have already existed in the as-sintered microstructure (as arrowed in Fig. 5(a)), extensive microcracking only occur after sample grinding (e.g. y=19.5 in Fig. 5(b)). Large cracks generated thermally are attributed to the 5 vol.% volume contraction upon the -cristobalite phase transformation since no quartz was found in these samples. Microcracks, however, are more likely to be of the stress-induced nature. They are originated from the -cristobalite phase transformation, triggered off by the stress applied in grinding. It is evidenced from the increased amount of -cristobalite after sintered samples have been re-ground to fine powders (of 38–45 and 38 m in size), as shown by C-1(a) and C-1(b) in Fig. 2 for y=24.6. The y=24.6 samples initially contain a mixture of - and -phase (C-1 in Fig. 2) (although not immediately discernible from the XRD trace, it has been demonstrated for samples with y=19.5 by deconvulsion (Fig. 1(b)). It is plausible that some of the initially metastable -cristobalite confined in the matrix has transformed to the -phase because the mechanical constraint is relieved by grinding and polishing. In fact, phase transformation to -cristobalite was also attributed [13] to the tensile stresses induced upon cooling due to differential thermal expansion. The actual stress state involving E (differential elastic moduli), (differential thermal expansion) and the sample grinding technique at present can not be assessed easily. The -phase found in Breuckener’s samples [13] was probably due to the stabilization by the chemical impurities associated with the initial colloidal silica (Vitresil® containing impurities of Al3+, Na+ and OH−). The -cristobalite survived from pulverizing to powder, as represented by (111) in C-1(a) of Fig. 1(c), must have been fully stabilized to the extent that they have become ‘non-transformable’ meaning its resistance of the phase transformation to -cristobalite even under grinding stresses. In fact, particles larger than 300–500 nm, and not completely detached from the cracked matrix (Fig. 5(b)) after surface polishing, are probably the survived non-transformable -cristobalite. The matrix transformed to -phase and experienced the accompanied volume contraction has resulted in extensive microcracking. It is also clear from Fig. 4(a) that higher doping level (with y=13.9) has led to the fully stabilized -phase. The indications are: (a) the codoping of (Na2O+Al2O3) enables the stabilization of the hightemperature cubic phase to room temperature, and (b) particle size [18,25] is also a determining factor in the metastable retention, as discussed in the literature [6,7]. The stabilization mechanism is therefore likely to associate with both the chemical and mechanical terms [7]. Again, it is similar to the stabilization of c- and tetragonal (t)-ZrO2 to room temperature. A critical mechanical constraint imposed by the matrix of either glass or other (+)-cristobalite grains would have existed as in the partially stabilized ZrO2 (PSZ) [8], ZrO2-toughened Al2O3 (ZTA) [26] and t-ZrO2 polycrystals (TZP) [27]. The relief of the constrained stresses by surface grinding has resulted in the transformation of the embedded t-ZrO2 particles to the monoclinic (m)-phase by a stress-induced martensitic phase transformation mechanism [28]. Similarly, the -cristobalite phase transformation also occurs when the matrix constraint has been removed by external stresses. Those particles in the (Na++Al3+)-codoped SiO2 ceramics containing solute cations below the critical concentration for full stabilization then transform to -cristobalite during surface grinding and/or pulverizing to powder. The -cristobalite (e.g. Fig. 2) that sustained pulverizing or polishing can therefore be perceived as being equivalent to the fully stabilized c-ZrO2 (FSZ). It still retains metastably to room temperature is therefore chemically ‘non-transformable’. The full stabilization of the cubic symmetry in cristobalite may require a lower limit of solute content similar to that at 13.2 mol% MgO for the fully stabilized c-ZrO2 [29]. Those transformed to -cristobalite upon surface grinding (Fig. 1(c) and 2) are only ‘partially’ stabilized when the modification of chemical free energy (Gchem term) [26] alone by solute cations (e.g. Mg2+ for ZrO2 and (Na+ +Al3+) for SiO2) is not sufficient to render full stabilization. The -crystals containing less solute content can only become metastable if augmented by the mechanical constraint from the matrix. It is necessary for these crystals to remain confined to a matrix (of glass and (+)-cristobalite mixture) in order to fulfil the thermodynamic requirement of Gtotal0. Since the inhomogeneity of solute oxides exists in the initial mixture of Na2O-containing colloidal silica sols doped with additive salts, some of the cristobalite crystallized with higher solute contents therefore become the fully stabilized -phase. The lower limit of Na2O and Al2O3 solute oxides would be an overall concentration of 6.29 mol% since samples with y=13.9 contain only -cristobalite as detected by XRD (Fig. 4(a)). For the Na2Oand Al2O3-additive content of 4.65 mol% (y=19.5), below the critical solute level, crystallization (probably to the spherical particles indicated in Fig. 5(b)) may have also occurred from regions of higher solute content than the overall concentration of the initial mix-