High Pressure Sintered ZrO 2 – 6 % wt REO

The sintering conditions employed in this work are innovative, due to the use of an alternative technology to process ZrO2-REO (rare earth oxide mixture), so called high temperature – high pressure (HPHT). A pressure of 5GPa was used, temperatures of 1100, 1200, and 1300C, for times of 2 and 5 minutes. The best results were obtained for samples sintered at 5GPa/1300C/5min., where a micro-hardness of 4.8GPa, fracture toughness of 5.3MPa.m, density of 97.9%, and 88% in volume of a tetragonal phase retained at room temperature were achieved.


Introduction
The most promising applications of zirconia is as structural ceramicс (combustion engine parts, turbine blades, cutting tools, prosthetics, etc) and as solid electrolytes (oxygen and gas sensors, fuel cells, oxygen pump, etc) [1].
Pure ZrO 2 presents 3 polymorphs at atmospheric pressure: monoclinic -m -from room temperature to 1170 o C, tetragonal -t -from 1170 to 2370 o C, and cubic -c -from 2370 to 2680 o C [2].Due to the large volume change associated with the tetragonal to monoclinic phase transformation, pure ZrO 2 has no practical applications for engineering components.Zirconia can be stabilized with various additives, among which yttria and ceria are the most useful stabilizers.In particular, yttria-stabilized zirconia is known to be hard and tough at room temperature, that enables the use of this material, in a tetragonal phase, as an advanced ceramic [3], and prominently in applications as cutting tools.
The ZrO 2 polymorph stability depends upon the type and amount of oxide that is used to diminish the t-m transformation temperature.Feder et al [4] and Piconi and Maccauro [5] affirmed that the t-m transformation is of a martensitic nature, which is accompanied by a volumetric expansion (∆V) of 4%.Normally, when such a transformation takes place, a tension field is generated at the cracks' surrounds, due to ∆V, which in turn leads to a fracture toughness (K Ic ) increase in ZrO 2 .
In his very concise study, Kuranaga [6], and more recently Nono and Freitas [7] have shown that the use of the rare earth oxide mixtures (REO) instead of pure yttria, as a stabilizer agent for the tetragonal phase of ZrO 2 at room temperature is a very interesting alternative to reduce the powder processing costs, with maintenance of the mechanical properties.In this case, the co-precipitation route to obtain ZrO 2 -REO powders was used.The results obtained have shown that it is possible to obtain totally tetragonal ZrO 2 , via addition of 6% in weight REO.Kuranaga [6] also proved that the best properties for this mixture are achieved when the material is sintered at the temperature of 1400 o C, during 3 hours.
During the 60' and 70' of the last century, Weber [8], Vahldiek et al [9], Bendeliani et al [10], and Liu [11], and more recently Haines et al [12] directed efforts towards studying the behavior of the polymorphism of pure ZrO 2 under high pressures and high temperatures (HPHT).They was interested only in the orthorhombic phase, as can be seen in fig. 1.In this context, this work aims at obtaining dense bodies of ZrO 2 -6%wtREO, through an alternative processing route, the above mentioned HPHT technique, and posterior characterization of some mechanical properties such as micro-hardness and fracture toughness, as well as determination of the amounts of each polymorph phase present and micro-structural analysis.It is worth to mention that this work deals with the achievement of the tetragonal polymorph as the main ZrO 2 phase, to ensure the technical possibility of the use of the HPHT technology in manufacturing of ZrO 2 based cutting tools, in very short sintering times.

Methodology
The route to process the ZrO 2 -6%wtREO powders was well described by Kuranaga [6], and Nono and Freitas [7].The particle size ranged from 0.2 to 6µm, and the mean particle size was 1.25µm.Table 1 shows the chemical analysis of the REO fraction, where 44.56%wt yttria in its composition is observed.The ZrO 2 -6%wtREO powder was cold compacted at 125MPa in a DAN-PRESSE press -30 tons capacity.A calculated mass of 0.425g per sample was used.Green bodies were HPHT sintered in a special hydraulic hot-press -Ryazantyashpressmash -2,500tons capacity.The high pressure device (HPD) scheme is shown in fig. 2. Heating was promoted by direct current passage through the samples.The micro-structural aspect of the sintered bodies was observed by scanning electronic microscopy -SEM Zeiss DSM 962.The sintered samples' densities were measured according to the Archimedes' method -water displacement, by using a balance -Scaltec SBC 31-0.0001gresolution.Vickers micro-hardness indentation was carried out in a proper device coupled to an optical microscope -Zeiss Jenavert 32.A load of 50gf during 15s was used.Five repetitions for each sample were carried out.Micro-hardness values were calculated using eq.1 [13]: Where: Hv -Vickers micro-hardness, kgf/mm 2 ; c -applied load, kgf; d -mean diagonal length of the indentation, mm.The mode I fracture toughness, K Ic , was determined by eq.2 [14], because it is the most suitable equation for this material.
A X-Ray diffractometer -Seifert URD 65 -Cukα radiation was used to define the phases present in the samples.These results were also used to calculate the tetragonal and monoclinic phase fractions for each sample group, according to eq.3 [15]: Where: X m -monoclinic phase fraction; I m and I t are, respectively, peaks intensities corresponding to the monoclinic and tetragonal phases; Miller's indexes (hkl), are referred as the planes considered for the X-Ray intensity measurements.

Results and discussion
According to fig. 1 and tab.II, it is seen that all samples were HPHT sintered into the tetragonal phase stability region.Group A samples have shown a high level of fragility, that restrained their characterization.This is attributed to the large amount of cracks in these samples -known as the main characteristic of the t-m transformation.Fig. 3 shows the X-ray diffraction patterns of samples B to F. It can be observed that all patterns are quite similar for each sample group, although group F presented a larger amount and intensity of peaks related to the tetragonal phase.
According to the X-ray diffraction results, the graph of fig. 4 was built using eq.3, which gives the amounts of the t and m phases, for each group of samples.It can be observed that the t phase amount increases with temperature and/or time of sintering, for the pressure of 5GPa.It is explained by the fact that the t-m transformation occurs at 1170 o C at atmospheric pressure -see ref. [2], thus, sintering at lower temperatures favors appearing of the m phase.It must also be noted that the frontier line of the t-m transition is not completely inside the thermo-dynamic equilibrium, leading to the appearance of some uncertainty region.It indicates that for group B, the sintering condition (the 5GPa pressure) does not appear to be an influencing parameter to maintain t phase stability, for T=1100 o C.
Samples of groups C and D have shown the same amount of t and m phases, indicating that for temperatures at the same order of magnitude in which t-m transition takes place -1200 o C, the time influence on ZrO 2 -6%wtREO HPHT sintering is inexpressive.When increasing the temperature to 1300 o C, an increment in the t phase amount is noted.Samples of group E showed 60% in volume of the t phase.Samples F presented 88% in volume of the t phase, evidencing that for temperatures up to the t-m transition, the HPHT sintering time becomes an influencing parameter, for the pressure of 5GPa.Tab.III gives the results of densification, micro-hardness, and fracture toughness of the HPHT sintered samples.It is seen that a tendency of the density to increase with time and/or temperature of sintering exists.It is also observed that group F samples presented the highest density values.These results are in fair agreement with the above mentioned volumetric expansion, verified in the studies of refs.[4] and [5], because the t-m transformation strongly diminishes the density of the ZrO 2 sintered samples.The results of micro-hardness can be explained according to the same above mentioned reason, by considering that t phase hardness is considerably higher than the m phase one.This ∆V promoted by t-m transition can be observed mainly in the samples which showed a larger amount of the m phase, as fig. 5   In relation to the K Ic results, its evolution with the t phase retention is very clear.However, it is also seen that K Ic results for groups C and D are of the same order of magnitude, when the deviation measured for these results are taken into account.It can be explained by the fact that for both groups the same amount of t phase is present -see fig. 4. The samples of group F showed K Ic =5.3MPa.m½ -the highest value for the processed samples, due to the large amount of t phase retained.Figure 6 presents an example of indentation and crack formation for the Hv and K Ic calculations -it refers to a sample of group F. Tab.IV shows some literature results of fracture toughness for several ZrO 2 based materials.It is worth to say that the result of K Ic =5.3MPa.m½ obtained in the present work is in good agreement with the literature values.It must be said that the in refs.[1] and [18] using exactly the same material as the present work -ZrO 2 -6%wtREO -reached the maximum value of K Ic =4,3 MPa.m ½ and 4.25 MPa.m ½ for samples conventionally sintered at 1400 o C/3 hours -a little bit lower than the above mentioned value.This is a strong indicative that the use of high pressures can be a technically suitable route to sinter ZrO 2 based materials for cutting tools.

Conclusions
In this exploratory work, in which the the main goal was reached -processing of dense samples of ZrO 2 -6%wtREO by HPHT sintering, with a large amount of the tetragonal phase fraction.The concluding remarks are listed below: 1. Larger amounts of t phase were achieved by employing the most elevated temperature and time of sintering, for 5GPa pressure; 2.
For sintering temperatures lower than the t-m transformation one (1170 o C), the applied pressure had no influence on the sintered properties; 3.
The use of high pressure enabled processing of dense bodies in very short sintering times; 4.
Even under high pressure, the samples have shown micro-cracks, due to the t-m transformation; 5.
K Ic results obtained in the present work were slightly higher than the ones cited in literature, using exactly the same material -conventionally sintered.

Fig. 3 X
Fig.3X-ray difratograms of the high pressure sintered samples.

Fig. 4
Fig. 4 Monoclinic and tetragonal phase percentage for each sample.

Fig. 6
Fig. 6 Group F sample micrograph after micro-hardness indentation.See details of indentation and cracks -black arrows.
Table 2 gives the HPHT sintering conditions for 6 groups of 7 samples each.
Fracture toughness for various zirconia based materials.