DE19938752A1 - Production of a ceramic material based on zirconium dioxide comprises adding an inorganic oxide through a stabilizer to a partially and completely stabilized zirconium oxide powder, molding into a workpiece and sintering - Google Patents

Abstract

Production of a ceramic material based on zirconium dioxide comprises adding an inorganic oxide through a stabilizer to a partially and completely stabilized zirconium oxide powder containing less than 0.5 wt.% SiO2, molding into a workpiece and sintering. The inorganic oxide is added in such an amount that if forms a spinel phase with the stabilizer on or during sintering, the stabilizer is then removed from the zirconium dioxide lattice, the monoclinic zirconium dioxide amount raised to up to 80 vol.% with the partially stabilized zirconium oxide powder, and the cubic zirconium dioxide amount reduced to 30 vol.% with the completely stabilized zirconium oxide powder.

Description

The invention relates to a process for the production of a ceramic material based on zirconium dioxide, in which at least one inorganic oxide is added to the zirconium dioxide powder, partially or fully stabilized by a stabilizer, with an SiO ₂ content of less than 0.5% by weight is identical to the stabilizer, the mixture thus obtained being shaped into a workpiece and sintered. Such ceramic materials can be used as ge shaped and unshaped refractory products in iron, steel and non-ferrous metallurgy, in the glass, cement and chemical industries, and as engineering ceramic components, ion conductors and sensors.

Zirconia materials have excellent erosi on- and corrosion resistance to steel / slag Attacks. These two properties are based on the pronounced ion binding. Because of the often inadequate thermal shock behavior become zirconia plants So far, substances have only been subjected to predominant corrosion stress for slide closures, distributor nozzles and for Protection components in the impact area of the distributor during of continuous casting (Jeschke, Elstner and Leistner, "Zirconium oxide as refractory material for steel casting systems ", VDEh training, 1983, 84-108; Klein, Münchberg, Prange and Stradtmann, "Delivery concepts of ladles in secondary metallurgy due to speci properties of refractory materials ", Unitecr, 1991, 93-110). In the case of horizontal continuous casting, one encounters off cast iron pipes and tear-off rings and in ladle metallurgy special flushing heads on the lances. For billet systems either through a free-wheel nozzle or a slide poured closure. The freewheel nozzle must be a special have high wear resistance so that the casting channel remains almost constant even with long casting times Diameter retains to an unacceptable increase in the Avoid pouring speed (2-3 m / s). By the ge target the use of materials containing zirconium dioxide (Zirconium silicates in combination with partially stabilized Zirconium oxides) higher casting times are achieved. The one set of a slide closure system in the same type layer enables longer casting times and closing individual strands. Similar locking systems are also used at Bloom and slab plants are used, especially for  Casting very aggressive steels. Another, since Years of application are encountered in the Dipping spouts of the distributor. Zirconia plates or layers are in carbon-bonded aluminum oxide Immersion spouts integrated so that the wear in the casting powder area can be reduced. In simple out The carbon-bound immersion nozzle is included in the guided tours Zirconium dioxide powder targeted at the wear zones strengthens.

Partially stabilized materials with approx. 20% porosity and with hot bending strengths of approx. 8 to 30 MPa are generally used in the refractory industry. The modification change from tetragonal to monoclinic is used to create a dense network of microcracks, which should improve the thermal shock behavior. At room temperature, the zirconia matrix has all three phases (monoclinic, tetragonal and cubic) and a dense crack network from the phase change tetragonal to monoclinic (crack configuration I). Other thermal shock-improved zirconia modifications have a high metastable tetragonal phase. Only with thermal shock stresses does the metastable tetragonal phase change to monoclinic, and a dense crack network is created (crack configuration II). In addition, targeted temporary additives and special sintering profiles introduce closed micropores into the structure, which have a fracture-related meaning similar to that of the cracks, but which are not directly accessible to the corrosive attack. Fully stabilized zirconium dioxides are not used in steel applications due to the greater linear thermal expansion and the low thermal conductivity (overall poor thermal shock behavior). In the case of high-purity steel production, it must be taken into account that the otherwise usual addition of SiO _{2 is} undesirable when optimizing the thermal shock properties of dense zirconium dioxide materials. On the one hand, the SiO _{2 of} the ceramic lining reacts with the Al of the calmed steel and produces Al ₂ O ₃ inclusions, on the other hand, the SiO _{2 dissolves} in the steel and causes increased oxygen concentrations.

With the help of suitable stabilizers, which are installed in the lattice of zirconium dioxide, the tetra gonal high-temperature phase can be shifted metastably to room temperature in order to completely or partially prevent the volume expansion associated with the phase change. Alkaline earth and rare earths, e.g. B. MgO, Y ₂ O ₃ , CeO ₂ and CaO used. With the same degree of stabilization, the stabilizers have a different influence on the structure and properties, in particular on thermal shock behavior, corrosion resistance and ion conductivity. Furthermore, the powder preparation and the way in which the stabilizer is added can have a considerable impact on the properties and in particular on the corrosion resistance. A distinction is made between melt-cast and chemically precipitated zirconium dioxide powders. Components made from melt-cast powder have better corrosion resistance to steel / slag attacks.

With MgO stabilization, increasing destabilization is observed with higher proportions of TiO ₂ and SiO ₂ . These phase components can be present as impurities in the powder or can be introduced via slag components. They remove the stabilizer from the zirconium dioxide, form new mixed phases (no spinels) with the MgO and settle primarily at the grain boundaries. The destabilizing effect of SiO _{2 is} significantly higher than that of TiO ₂ . The Ti ⁴⁺ cations can also be built into the lattice as a stabilizer and can substitute the MgO (Endres, "Influence of the sintering behavior and microstructure on the expansion and thermal shock behavior of zirconium dioxide materials", dissertation RWTH Aachen, 1986, 60-110).

The three-phase system ZrO ₂ -TiO ₂ -Al ₂ O ₃ offers an option for controlling the thermal shock parameters of materials containing zirconium dioxide by modifying the zirconium dioxide structure and by creating secondary phases such as Al ₂ TiO ₅ or ZrTiO ₄ . Oxidic additives (68% by weight of Al ₂ O ₃ , 26% by weight of ZrO ₂ and 6% by weight of TiO ₂ ) in carbon-bound immersion outlets led to very good thermal shock properties (Ichikama, Noburo, "Development of Alumina, Titania and Zirconia raw materials and its application for continuous casting of steel ", Symposium of refractories in China, 1988, 267-279). It was found that the formation of Al ₂ TiO ₅ gives the composite significant advantages over thermal shock attacks. In the case of Y ₂ O ₃ stabilized zirconia with TiO ₂ additives, it has been found that an increased TiO ₂ content leads to grain growth and destabilization of the zirconia grains (Kountouros, Petzow "Defect chemistry, phase stability and properties of zirconia polycrystals", Science and Technology of Zirconia V, Technomic Publis hing Com, USA, 1993, 30-48).

In our own research work on slip cast samples it was found that approx. 10% by weight of TiO ₂ in MgO partially stabilized zirconium dioxide materials increases the mono-clinical proportion from approx. 10% by volume to approx. 80% (Aneziris, Pfaff and Maier, "Designing principles of Zir cona-Titania-Alumina-Magnesia based ceramics for refrac tory applications in the ferrous and non ferrous metall urgy ", Irefcon 1998, Calcutta India, 2, 1998, 339-444). Subcritical cracks occur which lead to a considerable loss in strength without the desired reduction in the elasticity module and the coefficient of thermal expansion.

Menschke and Claussen use pre-synthesized spinel (MgAl ₂ O ₄ ) as an additional phase in Y ₂ O ₃ / MgO partially stabilized zirconium dioxide ("Preparation and characterization of fine grained (Mg, Y) -PSZ ceramics with spinel additi ons", Science and Technology of Zirconia V, Technomic Pu blishing Com, USA, 1993, 378-385). The aim was to increase the fracture toughness of MgO and yttrium stabilized materials at room temperature and to increase the stability against the effects of destabilization caused by moisture. With targeted heat treatments, convertible tetragonal precipitates are generated (crack configuration II), which increase the fracture toughness of the material at the seam temperature. The spinel does not decompose during the heat treatment and primarily fulfills the role of a grain growth inhibitor for the cubic phase.

According to DE 37 87 981 T2, a precursor powder is provided which can be converted into zirconium oxide spinel ceramic composite materials. The spinel-forming metals Mg and Al are precipitated in a soluble salt form, preferably with a pH of 4-12. The resulting precipitate (precursor powder) is washed, dried and calcined to produce a powder that is suitable for pressing. The spinel-forming metals are present in molar ratios to form the spinel structure as the main component of the composite powder. The addition of the spinel-forming precursor powder to zirconium dioxide powder creates a ceramic composite body which is said to have improved thermal shock properties. Spinel generation is based on the addition of Al ₂ O ₃ and MgO. The effect of the destabilization of the zirconium dioxide cannot be caused by this. With the pre-synthesized spinel, the monoclinic zirconia content cannot be increased in partially stabilized zirconia.

A precursor material is also used in the same document wrote in one of the spinel-forming metal salts is present in an amount in excess of that for Bil stoichiometric spinel. That mate rial decomposes into its oxide form when heated if there is a need for a zirconia stabilizer fulfills manageable requirements, act as such (cf. Page 7, 2nd section of DE 37 87 981 T2). Even in the In this case there is no destabilization of the zirconium dioxide material and can the monoclinic portion at partially stable zirconia are not increased or at full stabilized zirconium dioxide the cubic fraction redu be decorated.

The state-of-the-art zirconia materials do not meet the requirements for corrosion and themock resistance to a sufficient extent. The object of the invention is therefore to significantly improve the level of corrosion resistance and thermal shock resistance with limited SiO ₂ contents and at Sin ter temperatures less than 1700 ° C.

This object is achieved in that the organic oxide is of such kind and in such quantity admitted that it with the stabilizer during sintering  or during application of the ceramic material in high temperatures Temperature range spinel phases with a share up to 5 wt .-% forms in the zirconia matrix and the Stabilizer partially removed from the zirconia lattice gene and with partially stabilized zirconium dioxide the monoclinic Zirconia content increased to up to 80 vol.% And at fully stabilized zirconium dioxide the cubic zirconium dioxide danteil is reduced by up to 30 vol.%.

With the withdrawal of the stabilizer, a phase change is caused and a high monoclinic proportion is generated, which is favorable for the corrosion resistance. The tetragonal to monoclinic phase change causes a volume expansion that leads to crack formation (crack configuration I). The spinel formation according to the invention creates an additional crack configuration III. When free Al ₂ O ₃ and MgO react to form MgAl ₂ O ₄ , a volume increase of approx. 5% is caused. This reaction starts at approx. 1000 ° C and is responsible for the additional micro-crack initiation. According to the invention, the two crack configurations I and III are superimposed, so that zirconium dioxide composites are formed which have an improved combination of thermal shock and corrosion properties. The cracks, which are based on the formation of spindles, partially dampen the cracks from the zirconia stabilization. A component with an increased monoclinic content can thus be produced from the destabilization process without a drastic loss of strength being observed. In addition, the porosity, which is based on the crack formation and is partly responsible for the corrosion resistance, is kept within limits. The microcracks arising from the destabilization and spinel formation strongly influence the thermal and mechanical properties. In particular, they reduce the coefficient of thermal expansion and the modulus of elasticity and lead to an increase in fracture toughness. According to the invention, TiO ₂ and / or Fe ₂ O ₃ can additionally be added as a destabilization-promoting inorganic additive and promote the formation of spinel. The Tio ₂ accumulates near the grille. Due to the increased vibration between the doping and the defects, the Ti ⁴⁺ cations force Mg ²⁺ out of the ZrO ₂ crystal and lead to the thermodynamically preferred spinel reaction with the free Al ₂ O ₃ . The TiO ₂ does not act as a stabilizer.

The detection of the effects according to the invention is based on finely granular MgO-stabilized zirconium dioxide produced by the melting process with an SiO ₂ content of <0.3% by weight. From the prior art, materials I and II are used as a reference, material II usually being used in the refractory area. Material III is produced by the process according to the invention. Material IV was manufactured in accordance with the Claussen publication.

Powder for material I

Melted zirconium dioxide, MgO stabilized (3.5% by weight), SiO ₂ content 0.12% by weight, phase proportion cubic + tetagonal approx. 90%, grain size 0 to 12 µm, d ₅₀ = 2 to 3.5 µm.

Powder for material II

Melt-cast zirconium dioxide, MgO stabilized (3.5% by weight), SiO ₂ content 0.12% by weight, phase proportion cubic + tetagonal approx. 90%, grain size 0 to 12 µm, d ₅₀ = 7 to 9 µm.

Powder for material III

98.0% by weight of melted zirconium dioxide, MgO stabilized (3.5% by weight), SiO ₂ content 0.12% by weight, phase fraction cubic + tetagonal approx. 90%, grain size 0 to 12 µm, d ₅₀ = 7 to 9 µm
Additions: 1.0% by weight Al ₂ O ₃ with d ₅₀ = 1 µm and 1.0% by weight TiO ₂ with d ₅₀ = 0.2 µm.

Powder for material IV (according to Claussen's publication, but with addition of TiO ₂ and without heat treatment)

98.0% by weight melt-cast zirconium dioxide, MgO stabilized (3.5% by weight), SiO ₂ content 0.12% by weight, phase fraction cubic + tetagonal approx. 90%, grain size 0 to 12 µm, d ₅₀ = 7 to 9 µm
Additions: 1.0% by weight MgAl ₂ O ₄ with d ₅₀ = 3-4 µm and 1.0% by weight TiO ₂ with d ₅₀ = 0.2 µm.

Materials I to IV have the same tempera Tur / time sintering profile (1600 ° C / 2 h) subjected. The cha Characterization features are geo on identical sample metrics determined under comparable conditions.

The slip casting method is selected for the preparation of the samples, since the powder components can easily be varied with good homogenization via the suspension. After a homogenization time of approx. 180 min on a rolling bench, plates with dimensions of ∅ = 100 mm, wall thickness = 5 mm are formed using the hollow casting process. After a drying time of 24 h at room temperature, the plates are fired at 1600 ° C for 120 min in an electric furnace. A rapid cooling rate of approx. 250 K / h is selected during the cooling phase in order to suppress the thermal destabilization. The microstructures created are assessed using SEM images and EDX analyzes. The XRD measurements are used to identify the phase components monoclinic, tetragonal and cubic and the spinel phase (see Tab. 1). The materials are examined with regard to their porosity and their characteristic values σ _b4 , E, ν, α, λ which determine the thermal shock resistance (see Tables 2 and 3). For thermal shock assessment, residual strengths are determined on samples that have previously been quenched in water at 600 ° C and 1000 ° C.

Tab. 1

Phase composition using XRD analysis

Tab. 2

Offset, grain size and porosity

Tab. 3

Characteristic values of ZrO ₂ materials up to 1450 ° C

As a final test, corrosion tests on egg ner rotary and oscillation finger test device and on Crucibles with steel (ST 37) and thin slab casting powder (AV 1X) and the corresponding wear rates determined (see Tab. 4).

Tab. 4

Wear rates of materials containing zirconium dioxide

The materials III and IV with the Al ₂ O ₃ / TiO ₂ and MgAl ₂ O ₄ / TiO ₂ additives achieve smaller open porosities compared to the reference material II. The material I with the smaller initial grain size has the smallest porosity and accordingly the greatest corrosion resistance.

With regard to the XRD evaluation, materials I, II and IV have similar monoclinic zirconium oxide components after the fire. With material III, the Al ₂ O ₃ / TiO ₂ addition changes almost the entire tetragonal phase, and the monoclinic proportion increases dramatically. Only spinel is identified and there is no free Al ₂ O ₃ or TiO ₂ . Al ₂ TiO ₅ is not registered using XRD. TiO ₂ is embedded in the vicinity of the crystal lattice. Part of the MgO stabilizer is removed and reacts with the Al ₂ O ₃ to form MgAl ₂ O ₄ . The loss of the MgO stabilizer causes a martensitic transformation. The Ti ⁴⁺ cations are not bound directly to specific lattice sites, but move towards energy-preferred locations. Due to the increased vibration between the doping and the defects, the Tia ⁴⁺ cations force Mg ²⁺ out of the ZrO ₂ crystal and lead to the thermodynamically preferred spinel reaction with the free Al ₂ O ₃ . TiO ₂ contributes to the destabilization of the zirconium dioxide and thus the spinel generation is also promoted.

By adding pre-synthesized MgAl ₂ O ₄ (material IV) instead of Al ₂ O ₃ (material III), no significant change in the monoclinic content is observed. In this case, no free MgO is identified at the zirconium dioxide limits by means of EDX and XRD analyzes, and the MgAl ₂ O _{4 also has} the same MgO content (approx. 78% by weight Al ₂ O ₃ and approx. 22% by weight MgO). The material IV has an increased tetragonal component compared to the reference materials I and II. It is believed that because of the dispersed spinel phase (smaller coefficient of thermal expansion), a mechanical transformation with tetragonal precipitates from the cubic phase is caused.

With material III, the zirconium dioxide is destabilized by withdrawing the stabilizer. The tetragonal phase is drastically reduced, and long, sharp monoclinic twin grains can be seen. The tetragonal to monoclinic phase change causes a volume expansion that leads to crack formation. In addition, cracks are observed in the region of the spinel phase. When free Al ₂ O ₃ and MgO react to form MgAl ₂ O ₄ , a volume increase of approx. 5% is caused. This reaction starts at approx. 1000 ° C and is responsible for the additional micro crack initiation. The cracks, which are based on the spinel formation, partially dampen the cracks from the zirconia stabilization. A component with an increased monoclinic content can thus be generated from the destabilization process without a drastic loss of strength being observed. In addition, the porosity, which is based on the crack formation and is partly responsible for the corrosion resistance, is kept within limits. The microcracks arising from the destabilization and spinel formation strongly influence the thermal and mechanical properties. In particular, they reduce the coefficient of thermal expansion and the modulus of elasticity and lead to an increase in fracture toughness.

With material IV, tetragonal excretions and Spinel grains in the cubic phase or on the grain borders observed. In addition to less common tetragonal Single grains are predominant in the cubic matrix Clusters of 20-40 units found.

With regard to the material properties, material III at 1450 ° C has a thermal expansion coefficient reduced by approx. 50% of 5.5 10 ^-6 K ^-1 compared to reference materials I and II due to the higher monoclinic content. In addition, material III achieves the highest residual strength after the thermal shock tests and has the highest fracture toughness in comparison to materials I, II and IV.

Both materials III and IV have spinel as a disperser sionsphase possibly also as "crack diversion phase". there the material of the induced Ther mosquito tears tetragonally due to the phase change monoclinic dismantled (crack configuration II) and at the factory fabric III with the existing micro cracks (from the Zir Condioxide destabilization and spinel formation, crack confguration I and III) and the rerouting to the mono disintegrated grains.  

For subject shock loads above 1170 ° C (phase change change from monoclinic to tetragonal) heal the cracks and no longer contribute to thermal shock resistance (Material IV). In contrast, remains for material III the positive effect of the cracks from the spinel reaction on received. Material III is therefore still advantageous lower thermal shock properties than material IV in particular at higher application temperatures such. B. at the Steel production (1500-1650 ° C).

Material III shows a higher wear resistance than the material IV, although both are from the same The main phases are spinel and zirconium dioxide. The German material III gives a much higher monoclinic content better corrosion resistance.

According to claim 8, pre-synthesized spinels or tanates and combinations of both can be added before the fire with the inorganic additives according to claims 1 to 7. It can e.g. B. zirconium titanates (ZrTiO ₄ ) can be added, which have a negative thermal expansion coefficient in certain temperature ranges (AE Mchale and RR Roth, "Low temperature phase relationships in the system ZrO ₂ -TiO ₂ ", J. Am. Ceram. Soc. 69 (11), (1986), 5.827-832 and FH Brown and P. Duwez, "The zirconia-titania system", J. Am. Ceram. Soc., 37 (3) (1953), pp. 129-133 ). This allows certain areas of the zirconia composite material to be pressurized. With the addition of pre-synthesized spi nelles, both primary and secondary spines with special zirconium dioxide phase ratios and crack nets can be present after the fire or during use.

The invention enables within the quantified Limits the setting of the necessary thermo shock and corrosion resistance for different temperature / environment application profiles. Be next to it also advantages in fracture toughness at room temperatures achieved. The effects are not just on the mud energy limited, but with the usual granulation method drive on all primary shaping processes (cold isostati pressing, extrusion, injection molding, etc.) transfer bar.

Claims (9)

1. A process for producing a ceramic material based on zirconium dioxide, in which at least one inorganic oxide is added to a zirconium oxide powder which is partially or fully stabilized by a stabilizer and has an SiO ₂ content of less than 0.5% by weight, which is not compatible with the stabilizer is identical, the mixture thus obtained being shaped into a workpiece and sintered, characterized in that the organic oxide is of such a type and is added in such an amount that it is used with the stabilizer during sintering or during use of the ceramic material in the high temperature range Spinel phases with a proportion of up to 5% by weight in the zirconium dioxide matrix form and the stabilizer is partially removed from the zirconium dioxide lattice and in the case of partially stabilized zirconium dioxide the monoclinic zirconium dioxide component is increased to up to 80% by volume and in the case of fully stabilized zirconium dioxide the cubic zirconium dioxide component is reduced by up to 30% by volume. 2. The method according to claim 1, characterized in that the zirconium dioxide powder is stabilized with MgO, Y ₂ O ₃ CeO ₂ or mixtures thereof and that as inorganic oxide Al ₂ O ₃ , Cr ₂ O ₃ , Fe ₂ O ₃ , CO ₃ O ₄ or mixtures thereof can be added. 3. The method according to claim 2, characterized in that the inorganic additive, in particular as Al ₂ O ₃ , is added in a range of 0.5 to 5 wt .-%. 4. The method according to any one of claims 1 to 3, characterized in that the mixture TiO ₂ or Fe ₂ O ₃ or a mixture thereof is added from destabilization-promoting additive. 5. The method according to claim 4, characterized in that TiO _{2 is added} in an amount of 0.5 to 5 wt .-% ben. 6. The method according to any one of claims 1 to 5, characterized characterized in that the inorganic oxide of such Is kind and is added in such an amount that with partially stabilized zirconia a monocli a share of over 40 vol.% is achieved.   7. The method according to claim 6, characterized in that the zirconia powder 3.5 wt .-% is partially stabilized by means of MgO and is added as inorganic oxide Al ₂ O _{3 to} the extent of at least 1 wt .-% and additionally as a destabilization additive at least 1 % By weight of TiO _{2 is} added. 8. The method according to any one of claims 1 to 7, characterized characterized in that additionally pre-synthesized Spinels or titanates alone or in mixtures of up to 5% by weight can be added. 9. Use of the ceramic material, manufactured according to one of claims 1 to 8, as a molded and unsung shaped refractory products in the iron, steel and non-ferrous metallurgy, in glass, cement and chemical industry as well as engineering ceramics cal component, ion conductors and sensors.