Investigation of the sintering of zirconia

Conclusions We investigated the sintering of zirconium dioxide as a function of the activity of the starting material. It was shown that preliminary firing or stabilization of zirconium dioxide reduces its capacity for densification at temperatures of up to 1500°C, while stabilizing it directly during sintering intensifies this capacity. The prestabilized zirconia has the lowest sintering rate.Introducing monoclinic unfired zirconia into the prestabilized material intensifies sintering; the optimal addition is 30%.It is established that the zirconia is sintered by volume diffusion of vacancies.We investigated the sintering of active zirconia obtained by decomposing zirconium nitrate. The greatest degree of densification is obtained with a preliminary short heat processing of the nitrate at 400°C. Activation is connected with the formation of metastable tetragonal and cubic modifications with defect crystal lattices. Increasing the heat-process temperature of the nitrate or prolonging the soak at 400°C, leading to the formation of stable monoclinic ZrO₂ with an ordered crystalline lattice, impairs sintering.Incorporating small additions of active ZrO₂ in the industrial material, and providing rapid firing in an oxidizing atmosphere, greatly increases the degree of sintering. Introducing stabilizing additives intensifies sintering. The maximum densification is obtained by the formation of 60–70% solid solution. Further increase in these additions reduces the shrinkage and densification of the specimens.Translated from Ogneupory, No. 6, pp. 33–40, June, 1968.     

Effect of stabilizing additions on the thermal shock resistance of zirconia products

Conclusions To obtain thermal-shock resistant products from zirconia the amount of monoclinic phase in the fired specimens prepared on the basis of granular bodies should be not less than 15%.It is found that with an increase in the content of CaO from 7.0 to 20 mole %, the thermal-shock resistance of the articles made from ZrO₂ is diminished. Introduction of up to 20% monoclinic ZrO₂ in the batch increases the thermal-shock resistance, but less so the higher the content of CaO in the stabilized part of the material. This produces additional stabilization of the zirconia as a result of the migration of the calcium oxide from the stabilized grains into the monoclinic ZrO₂. Additional stabilization of monoclinic zirconia is observed also during cyclic heating in the range 20–1600–20°C.Specimens of zirconia stabilized with CaO possess higher thermal-shock resistance than those made from ZrO₂ stabilized with MgO with the same contents of monoclinic phase.Translated from Ogneupory, No.1, pp.50–55, January, 1967.     

Preparation and properties of zirconium refractories stabilized by cerium dioxide

Conclusions We studied the reaction of zirconium dioxide with cerium dioxide in mixtures with CeO₂ contents of 6, 8, 10, 12, 15, 18, 20, 33, and 50%. The properties of the samples of these compositions were determined.On the addition of amounts of CeO₂ from 6–12% complete stabilization of ZrO₂ was not achieved by a single sintering at 1750°C. The samples with the composition 88% ZrO₂+12% CeO₂, sintered twice at 1750°C with an intermediate grinding, stabilized almost completely as a solid solution of tetragonal structure.The thermal stress resistance of dense, completely stabilized samples with CeO₂ contents of 15–18%, prepared from finely ground raw oxides, was 3–4 thermal cyclings. It improved when the CeO₂ content was decreased, or when more monoclinic ZrO₂ was added.It was found that the onset and the inversion temperature interval depend on the CeO₂ content, the granular composition of the original oxides, the temperature, and the gaseous sintering medium. We studied the properties of synthesized compositions and their dependence on the reducing or oxidizing conditions of sintering due to a change in the valency of cerium. In order to obtain zirconium-cerium refractories with definite properties it is necessary to have strict control of the gaseous medium during sintering.Translated from Ogneupory, No. 3, pp. 37–44, March, 1969.     

Densification of “active” zirconia powders during hot pressing

Conclusions We studied the influence of temperature and heat process time for zirconium nitrate on the phase composition, dispersion of grains and crystalline lattice defects in the resulting zirconia, and also on its capacity for subsequent compaction during hot pressing. This capacity is connected with the formation, during a short heating period up to 400°C, of metastable tetragonal and cubic modifications having a highly defective crystalline lattice.We also studied the densification during hot pressing of active zirconia obtained by decomposing zirconium nitrate. During decomposition of zirconium nitrate at 400°C with a short soak we noted the formation of zirconium dioxide which was actively compacted at 1100°C and a pressure of 200 kg/cm². We studied the influence of additions of CaO and the methods of adding it on the densification of active zirconia. An increase in the content of CaO leads to a reduction in the capacity of ZrO₂ for compaction during hot pressing. It is necessary to introduce the additive to the nitrate before it decomposes. During hot pressing of active zirconia containing additions of CaO and Y₂O₃ at low temperatures (1100–1300°C) cubic solid solutions are not formed owing to the short process time, and the zirconia in the specimens consists of monoclinic form. Subsequent heating of the pressed specimens forms cubic solid solutions.It was found that the introduction of 10–20% additive of zirconia obtained by heat processing the nitrate to industrial ultrapure zirconium dioxide yields dense specimens with a reduction in the optimum pressing temperature of 200°C.Translated from Ogneupory, No.7, pp. 39–45, July, 1967.     

Formation and properties of solid solutions of zirconium dioxide with oxides of rare earth elements

Conclusions Interaction of zirconium dioxide with oxides of cerium, yttrium and lanthanum in solid phases occurs at 1400°C with the formation of solid solutions with the cubic structure.Sintering of the specimens may result at 1700–1750°C with a 3-h soak. At 1400°C and a 6-h soak the porosity of the specimens was 30–40%.Complete stabilization of the zirconia is attained by heating to 1700–1750°C with additions of 20 mol.% CeO₂, 15% Y₂O₃ or 25% La₂O₃. An addition of ceria and yttria displaces the effects of polymorphic inversion of the zirconia to the lower temperature region.New highly refractory materials may be obtained from solid solutions of ZrO₂-20% CeO₂, ZrO₂-80% CeO₂, ZrO₂-15% Y₂O₃, ZrO₂-80% Y₂O₃ and ZrO₂-25% La₂O₃ and firing to 1750°C. Some of them have a low coefficient of thermal expansion compared with ZrO₂, stabilized with calcium oxide and magnesium oxide, and apparently better thermal-shock resistance. The advantage in regard to resistance during prolonged heating at 1200°C is possessed by the solution ZrO₂-Y₂O₃. The region of the most effective use of goods made from solid solutions of ZrO₂ with CeO₂, Y₂O₃ and La₂O₃ as highly refractory materials should be determined by extra studies.The possibility of reducing CeO₂ (fusing temperature about 2700°C) to Ce₂O₃ (fusing temperature about 1700°C) limits the use of cerium-containing materials as refractories in chiefly oxidizing conditions.     

Processing of ultrafine ZrO2 toughened WC composites

The interrelationships between the dispersion of the secondary ZrO₂ phase and the material properties of WC-based composites with up to 10 vol% of ZrO₂ are investigated. The homogeneity of the ultrafine WC–nanometric ZrO₂ powder mixtures was optimized by means of multidirectional milling and bead milling. In an alternative route, zirconium butoxide was used as a ZrO₂ source. The composites were fully densified by means of pulsed electric current sintering (PECS), also known as spark plasma sintering, within a few minutes at 1700 °C allowing to maintain an ultrafine grained microstructure combining a hardness of 2600 kg/mm² with an indentation toughness of 6 MPa m^1/2. The ZrO₂ content and Y₂O₃ stabilization were found to strongly influence the mechanical properties and especially strength of the WC–ZrO₂ composites.     

Change in the phase composition and properties of zirconia refractories under the prolonged effect of high temperatures

Conclusions A systematic study has been made of the behavior of ZrO₂ stabilized with various oxides under the prolonged action of temperatures up to 2300°C.The optimal concentrations of Y₂O₃, Nd₂O₃, CaO, and MgO in the zirconia refractories have been established; this ensures the successful service life of the articles under conditions of multiple prolonged action of high temperatures.The ultimately permissible temperature at which is it possible to operate the zirconia articles has been determined: with the stabilization of ZrO₂ by MgO it is 1900°C; with CaO, 2000°C; and with oxides of rare-earth elements, more than 2300°C.Translated from Ogneupory, No. 2, pp. 13–19, February, 1985.     

Heat-treatment effect on the properties of a zirconia ceramic with a grainy structure

Conclusions The heat treatment at 2000°C of zirconia ceramic with a grainy structure causes the destabilization of the ZrO₂-CaO solid solutions but this is not complete after a 200-h dwell; the electrosmelted ZrO₂ ceramic stabilized with CaO is more stable under these conditions. The ZrO₂-Y₂O₃ solid solutions have greater stability on heating.When the grainy-structure zirconia ceramic is subjected to an isothermal dwell at 2000°C there is a consequent reduction in the strength properties caused either by the decomposition if the solid solutions (CaO-stabilized ceramic) or by the growth of crystals (Y₂O₃-stabilized ceramic).Deceased.Translated from Ogneupory, No, 7, pp. 51–55, July, 1979.     

Development of Ce‐PSZ‐/Y‐PSZ‐Toughened Alumina Inserts for High‐Speed Machining Steel

The nanocomposite CeO₂/Y₂O₃ partially stabilized zirconia (Ce‐PSZ/Y‐PSZ)‐toughened alumina was prepared by wet chemical simultaneous coprecipitation process. The thermal stability of phases and morphology of powders were characterized by TG‐DTA, FTIR, and FESEM. The microstructure, stabilization of phases and compositional analysis with different mol% CeO₂/Y₂O₃‐doped zirconia in alumina are characterized by FESEM, XRD, and EDAX spectra. Significant improvement in fracture toughness and flexural strength has been observed in 10 vol% of partially stabilized zirconia (2.5 mol% Y₂O₃ in ZrO₂/9 mol% CeO₂ in ZrO₂)‐toughened alumina, which is suitable for high‐speed machining applications.     

Preparation and properties of ceramics in the ZrO2-CeO2-CaO system

The effect of bivalent calcium cations introduced in the stage of homogenous precipitation on the phase and morphological compositions of a tetragonal zirconia powder and the properties of a polycrystalline material based on it is described. The experiment was conducted for a base system containing 12% CeO₂, (0.5–3%) CaO, and the remainder ZrO₂. The use of calcium oxide as a modifier in the ZrO₂–12% CeO₂ system changes the crystallization temperature of the amorphous hydroxide residue. In the presence of 0.8–1% calcium oxide the structure of the sintered ceramic material mostly contains fine round crystallites 0.3 µm in size. Simultaneously, the fracture toughness increases considerably (K _Ic=15 MPa · m^1/2), which seems to be caused by the changed mechanism of destabilization of tetragonal zirconia.Translated from Ogneupory, No. 11, pp. 14–17, November, 1995.     

The question of the stabilization and sintering of high purity zirconium dioxide

Conclusions The results of an investigation of the stabilization and sintering of test bodies based on zirconia of high purity (with 99.5% base oxide) and with the addition of 4–15 mol.% CaO or MgO established the following.In conditions of oxidizing fire at 1710°C with a 5-h soak, complete sintering occurs in the body containing 10 mol.% of stabilizing oxide (zero open porosity), there is adequate stabilization, the material assumes great strength and spalling resistance compared with the other bodies investigated.With an increase in the amount of stabilizing oxide to 12–15% although a sintered and fully stabilized product results there is a reduction in density which is especially strong with CaO additions. Furthermore, there is a sharp reduction in the strength and spalling resistance.The relatively low density of the fired bodies containing 10 mol.% stabilizing addition (for CaO 5.20, for MgO 5.28 g/cm³) is determined chiefly by the presence of pores both inside and on the boundaries of the crystals in the material.An increase in the firing temperature at 2200°C only slightly affects the density of the material. An increase in the density of the material containing 10 mol.% CaO is attained by:changing the form of the anion added with the stabilizer from CO₃ to F with this there is a sharp deterioration in spalling resistance of the material;by precalcining the original ZrO₂;by sintering the preliminary stabilized product as a result of which specimens are obtained with a bulk density of 5.54 g/cm³.     

Electrically conducting high-refractory ceramic for magnetohydrodynamic generator electrodes

Conclusions The studies of the stabilization of ZrO₂ by rare-earth oxides alone and in combination with MgO, CaO, and Y₂O₃ have made it possible to select compositions suitable for preparing a grainy-structure ceramic.The study of three different methods showed that the most thermally stable and thermal-shock resistant is the grainy-structure ceramic based on fused materials.Tests of the ceramic as the electrodes for the test channel of the MHD generators have confirmed its excellent thermal stability and thermal-shock resistance.Translated from Ogneupory, No. 5, pp. 53–56, May, 1981.     

Enhanced phase stability in hydroxylapatite/zirconia composites with hot isostatic pressing

Hydroxylapatite (HA) composites with pure zirconia (ZrO₂), and 3 and 8% Y₂O₃ doped ZrO₂ were pressure-less sintered in air and hot isostatically pressed (under 120 MPa gas pressure) at 1100 °C for 2 h. The reactions and phase transformations were monitored by X-ray diffraction, thermal analysis, and Raman spectroscopy. HA/pure ZrO₂ composites were not thermally stable in air sintering; HA dissociated into α and β tricalcium phosphate while monoclinic ZrO₂ was transformed into tetragonal and cubic phases. No decomposition in HA or phase transformation in ZrO₂ were observed in hydroxylapatite/3% Y₂O₃ doped ZrO₂ or HA/8% Y₂O₃ doped ZrO₂ composites. On the other hand, HA and ZrO₂ phases in hot isostatically pressed composites remained stable. The highest densification was found in a composite initially containing 10% monoclinic ZrO₂ among the composites sintered in air. The densification of the composites decreased at lower sintering temperatures and higher ZrO₂ contents upon air-sintering. The HIPped composites were densified to about 99.5% of theoretical densities in all mixing ratios. The reactivity between ZrO₂ and HA was dependent on the amount of air in the sintering environment. Hot isostatic pressing with very limited retained air was proved to be a very convenient method to insure both phase stability and full densification during the production of hydroxylapatite zirconia composites.     

Second-phase evolution and densification behavior of AlN with CaO–Y2O3–C multicomponent additive system

AlN compacts with different CaO–Y₂O₃–C mixtures were sintered between 1100 and 1850 °C to understand the effects of the in situ formed reducing atmosphere on the densification behavior and evolution of the second-phases. AlN with Y₂O₃ densified at 1750 °C, but the addition of C changed the second-phases evolution towards Y-rich phases that delayed the densification. For AlN containing CaO, the second-phases were little influenced by the reducing atmosphere, but the addition of C increased the evaporation of the second-phase compounds during sintering, limiting the densification due to the reduction of the liquid-phase fraction and the gas trapping inside the pores. AlN with CaO–Y₂O₃ mixtures could be completely densified at 1650 °C, but the addition of C inhibited the densification below this sintering temperature because liquid-phase had poor wetting and spreading characteristics and the second-phase a high melting point (>1800 °C).     

Zirconia refractories

Results on the thermal conductivity of powders of monoclinic zirconia as a function of the grain size are presented. It is established that of all the investigated sintered powders of monoclinic ZrO₂ the 1 – 0.5-mm and 2 – 1-mm fractions have the lowest thermal conductivity and the least apparent density (2.190 and 2.262 g/cm³). Fused powders of monoclinic and stabilized ZrO₂ have a higher thermal conductivity than their sintered counterparts. Powders prepared from monoclinic ZrO₂ with a burning-off addition of polystyrene have larger pores in the grains, which increases the share of the radiation component in the heat transfer, and hence their thermal conductivity is higher than in powders obtained by sintering pure monoclinic ZrO₂. For the same reason, powders from void granules of the 3 – 0.5-mm fraction have a higher thermal conductivity than sintered monoclinic powders of ZrO₂.Translated from Ogneupory, No. 1, pp. 26 – 29, January, 1995.     

Controlled sintering and phase transformation of yttria-doped tetragonal zirconia polycrystal material

In this paper, 3 mol% yttria-doped tetragonal zirconia polycrystal material (3 mol% Y₂O₃–ZrO₂) was prepared using an optimised pressureless sintering process. The phase change and particle size distribution of Y₂O₃–ZrO₂ during sintering were studied, and the effect of sintering temperature on the properties of Y₂O₃–ZrO₂ was analysed. The raw materials and prepared samples were analysed using XRD, Raman spectroscopy, SEM, and Gaussian mathematical fitting. The results show that sintering encourages the transformation of the monoclinic phase into the tetragonal phase, thus improving the crystallinity of the sample. The relative content of the tetragonal phase in the sample increased from 57.43% to 99.80% after sintering at 1200 °C for 1 h. In the range of sintering temperatures studied in this paper (800–1200 °C), the zirconia material sintered at 1000 °C presented the lowest porosity and the best density.     

Some thermomechanical properties of pure oxide ceramics

Conclusions The proposed vacuum furnace makes it possible to determine the deformation temperature under load and the coefficient of thermal expansion to high temperatures.Ceramics of pure oxides, Al₂O₃, ZrO₂, MgO, and BeO, have high temperatures of softening under load. The temperature of initial softening of Al₂O₃ ceramics containing additives lies in the range 1860–1930°C. The magnesia and beryllia specimens show high softening temperatures under load, but in vacuum at high temperatures they are very volatile. The initial softening temperature of ZrO₂ is about 2250°C.The linear expansion of pure-oxide ceramics reaches 2–3% at 1800–2000°C. Values obtained for the average coefficients of expansion for Al₂,O₃, ZrO₂, MgO, and BeO are little different from those in the literature.The compressive strength and bending strength of pure oxides at high temperatures are relatively low. The highest o_bend at high temperatures is possessed by specially pure zirconia, stabilized with MgO. Magnesia and beryllia in compressive strength at high temperatures exceed the other oxides.The highest spalling resistance is shown by beryllia ceramics. The combined addition to alumina of 1% TiO₂ and 5% ZrO₂ leads to a reduction in sintering temperature and an increase in thermal shock resistance. Ceramics based on specially pure zirconia stabilized with an optimum amount of CaO and MgO show a high thermal shock resistance.     

Sintering high-purity fusions of MgO and MgO·Al2O3

Conclusions The porosity and strength of specimens of fused MgO obtained by induction smelting in cold crucibles and having 1% impurities is significantly increased by the addition of 1% CeO₂, 5% ZrO₂, and 20% of the vibromilled component plus 5% Y₂O₃. An addition of 5% Y₂O₃ has the greatest effect on the sintering characteristics of the MgO·Al₂O₃ specimens. A significant improvement in the characteristics of the materials is observed with an increase in the pressing pressure of the specimens from 100 to 200 MPa.Translated from Ogneupory, No. 3, pp. 54–56, March, 1981.     

Sintering of Corundum Mixtures Modified by a Vitreous Phase of a Eutectoid Composition

The effect of a vitreous phase of a eutectoid composition on sintering of corundum mixtures is considered. It is established that the additive corresponding to a eutectic composition of the CaO – Al₂O₃ system melting at a temperature of 1395°C improves the service properties of corundum concrete through activating its sintering.     

Modern high-density oxide ceramics with a controlled microstructure. Part V. Dense chemically stable ceramics based on yttrium,scandium, and aluminum oxides

Data on the technology of dense Y₂O₃ ceramics are presented. Special features of the structure of powders and their effect on shaping of specimens and their sintering are described. Methods for leveling the characteristics of Y₂O₃ powders with different properties and decreasing the sintering temperature are developed.Translated from Ogneupory i Tekhnicheskaya Keramika, No. 12, pp. 2–6, December, 1996.For the beginning of the article see No. 1 of 1996, for the continuation see Nos. 2, 4–7, and 9.