DE102012200654B4 - Slips, granules and ceramics, process for their preparation and use - Google Patents

Abstract

A slurry comprising a liquid medium having dispersed therein solid ZrO 2 particles doped with from 5 mole% to 15 mole% MgO comprising at least the monoclinic modification, wherein the ZrO 2 particles have an average agglomerate size in the range of 20 nm to 500 nm and a ratio of the peak intensities (I) between the monoclinic and the cubic modification of greater than 0.3, and at least one hydrogen-containing compound attached to the surface of the ZrO 2 particles, wherein the hydrogen content of the hydrogen-containing compounds is 0.06 wt% to 1.1 wt .-% based on the ZrO2 particles, so that the ZrO2 particles can form a ceramic structure with a sintering density greater than 95% at a maximum sintering temperature of less than 1500 ° C.

Description

The present invention relates to a ZrO ₂ -based ceramic material and its precursors in the form of powders, slips, granules or presintered moldings and to processes for their preparation, wherein the ZrO ₂ precursors have a high sintering activity and form a finely divided ceramic structure.

Moreover, the invention relates to the use of the ZrO ₂ -based ceramic material according to the invention in the medical and technical field, in particular for the production or coating of medical implants and of technical machine parts.

Ceramics have proven themselves in a variety of applications, combining high hardness, high wear resistance, high corrosion resistance, good high temperature stability and low specific weight. Since the mid-1980s, zirconia (ZrO ₂ ) has a firm place among high-performance ceramics. ZrO ₂ occurs as a function of temperature in monoclinic, tetragonal and cubic crystal modification. From room temperature to 1173 ° C, the monoclinic phase is stable. At temperatures above 1173 ° C to 2370 ° C the tetragonal phase is present and above 2370 ° C the cubic phase is stable. Since the respective phase transition is associated with a change in volume, pure ZrO _{2 is} mixed with stabilizers (also additives or doping elements) in order to avoid destruction of the structure in the sintering process by the change in volume during the phase transition. These additives are incorporated into the crystal structure of the ZrO ₂ (→ doping), thus achieving complete or at least partial stabilization of the tetragonal or cubic phase. Densely sintered components can be produced in the cubic and / or tetragonal crystal modification.

The stabilization of the tetragonal phase can be achieved, for example, by addition of rare earth metals (such as yttrium, cerium) as doping elements. In particular, the doping with yttrium oxide (Y ₂ O ₃ ) has proved to be favorable, since the thus stabilized ZrO ₂ ceramic have a high mechanical strength. Depending on the type and content of the dopants, the ZrO ₂ ceramics can be subdivided further. For fully stabilized zirconia (FSZ: fully stabilized zirconia), the concentration of extraneous oxides is chosen so high that the cubic high-temperature phase is stabilized to room temperature. With the addition of Y ₂ O ₃ , this is the case at a concentration of 8 mol%. Compared to fully stabilized zirconia, the partially stabilized zirconia (PSZ) has a certain volume fraction of transformable tetragonal phase embedded in a matrix of cubic zirconia.

The partially stabilized zirconium oxide (PSZ) has great technical significance. At room temperature, the cubic phase with tetragonal areas is present, which is maintained metastable in its state by suitable process control. This prevented conversion to the monoclinic phase and "pre-stressed" the microstructure, which is associated with an increase in strength and toughness. As partial stabilizers (doping elements) z. B. CaO and MgO (about 8-10 mol%) serve.

MgO-PSZ is a magnesium-stabilized zirconium oxide with a largely cubic-tetragonal structure. Partial stabilization gives MgO-PSZ excellent toughness and thus reliability for ceramic materials. MgO-PSZ has particularly good tribological properties, high corrosion resistance and similar thermal expansion to cast iron.

From the DE 39 37 640 A1 It is known that in hitherto practiced processes for the production of zirconia powder, the use of aqueous solutions and the separation of precipitates are significant disadvantages. In addition, the process products have due to an inhomogeneous distribution of the stabilizer and the presence of large. Crystallites and hard agglomerates often do not have the desired flow, pressing and sintering properties. By dissolving zirconyl chloride and optionally a stabilizer or its precursor in concentrated formic acid at elevated temperature, drying the solution by evaporation of water, hydrogen chloride and formic acid, calcining the residue at elevated temperature and optionally grinding the calcined product can be an optionally stabilized zirconia powder with good Create properties. The optionally stabilized zirconium oxide powder is suitable for the production of sintered bodies which can be exposed to high mechanical and / or thermal stresses or which are to be used for optical purposes.

• (a) ein erstes Metalloxid-Pulver (mit und ohne Oberflächenmodifikation); und
• (b) ein zweites, nanoskaliges Metalloxid-Pulver (mit und ohne Oberflächenmodifikation). Das erste Metalloxid-Pulver (a) weist einen d50-Wert von 0,2–12 μm auf. Das zweite, nanoskalige Metalloxid-Pulver (b) weist einen d50-Wert von 10–300 nm auf. Das Grössenverhältnis der d50-Werte von (a) zu (b) liegt bei max. 40:1 und das Mengenverhältnis von 0,1:99,9 bis zu 99,9:0,1, wobei bimodale Metalloxid-Pulver aus y-Al₂O₃, bestehend aus einem ersten y-Al₂O₃-Pulver mit einer durchschnittlichen Korngrösse von 1 um und einem zweiten y-Al₂O₃-Pulver mit einer durchschnittlichen Korngrösse von 70 bis 120 nm ausgenommen sind.

From the DE 102 61 720 A1 are metal oxide powders with a bimodal particle size distribution, ceramics which can be produced from these metal oxide powders, in particular milling ceramics for use in dental technology, processes for the preparation of metal oxide powders and ceramics, the use of nanoscale metal oxide Powder for the preparation of metal oxide powders and ceramics and dental ceramic products known. The bimodal metal oxide powder or a bimodal metal oxide binder composite comprises

• (a) a first metal oxide powder (with and without surface modification); and
• (b) a second nanoscale metal oxide powder (with and without surface modification). The first metal oxide powder (a) has a d50 value of 0.2-12 μm. The second nanoscale metal oxide powder (b) has a d50 value of 10-300 nm. The size ratio of d50 values from (a) to (b) is max. 40: 1 and the ratio of 0.1: 99.9 to 99.9: 0.1, wherein bimodal metal oxide powder of y-Al ₂ O ₃ , consisting of a first y-Al ₂ O ₃ powder with an average grain size of 1 .mu.m and a second y-Al ₂ O ₃ powder having an average grain size of 70 to 120 nm are excluded.

From the US 4,639,356 For example, a process for producing a fully or partially stabilized zirconia is known. The process comprises the preparation of an aqueous solution of zirconium sulfate mixed with an inorganic or organic, water-soluble salt of one or more metals from the group consisting of Be, Mg, Ca, Sr, Ba, Ra, Sc, Y, La, Ac, Ce, Hf, Th and Al. The aqueous solution is sprayed into an at least partially water-soluble solvent, the solvent being stirred during the addition of the solution. The co-precipitated metal salt formed is separated from the solvent, washed with inert solvent or inert solvents, dried and calcined.

The invention is therefore based on the object to provide a MgO-PSZ material (ZrO ₂ doped with MgO), with which even at less than 1500 ° C, a relative sintering density of greater than 99%, a homogeneous ceramic structure with narrow particle size distribution and particularly high Strength receives.

The object is achieved in that a combination of raw materials is converted by means of a defined thermal process control to a MgO-PSZ raw powder material, the agglomerated / aggregated particles are then dispersed / separated and thus a slip with defined properties is generated, this slip then into a powder , a pressable granules and / or in a pressed, optionally pre-sintered shaped body can be transferred. The resulting MgO-PSZ materials (powder, slurry, granules, pre-sintered moldings) have the property that a relative sintering density of greater than 99% of the theoretical density is achieved at a maximum sintering temperature of less than 1500 ° C, a particularly homogeneous structure forms and the ceramic has a high strength, particularly good tribological properties and high corrosion resistance.

In contrast to conventional MgO-PSZ (ZrO ₂ doped with 10 mol% MgO), which in the production of almost densely sintered ceramic moldings (relative sintered density greater than 95%) sintering temperatures in the sintering process of well over 1600 ° C, sometimes greater than 1700 ° C, this can be achieved at less than 1500 ° C in the MgO-PSZ materials according to the invention. Compared with the conventional MgO-PSZ this requires less equipment, the lower process temperatures wear the sintering furnace less than conventional MgO-PSZ. In the conventional MgO-PSZ, the time required for sintering is particularly long due to the necessary high process temperatures. A shortening of the sintering time by a higher gradient during heating or cooling is very limited and largely dictated by the requirements of the microstructure. The high sintering temperatures mean a high energy consumption during sintering. The long sintering times result in high production costs and ultimately a limitation of the production capacity. These disadvantages are overcome by the MgO-PSZ materials according to the invention.

Due to inhomogeneous powder materials with a broad particle size distribution, the use of commercially available MgO-PSZ powder also often leads to inhomogeneous ceramic structures with a broad particle size distribution. The consequence of this is, for example, a low strength.

The core of the invention is that a ZrO ₂ powder doped with MgO having defined particle properties is converted into a slurry such that a defined particle or agglomerate size and a predetermined phase composition or lattice structure are established. As a result, according to the invention, particularly reactive ZrO ₂ particles are obtained which have a particularly high sintering activity. The particularly high sintering activity means that densely sintered ceramics are already achieved at maximum sintering temperatures of less than 1500 ° C.

The phase transition between monoclinic and tetragonal or cubic is associated with a volume change known to cause cracking in the ceramic. For this reason, ₂ doping elements are added to the ZrO _{2 to} at least partially increase the cubic or tetragonal phase stabilize. ZrO ₂ powders are generally suitable for the production of ceramics when essentially the cubic or tetragonal phase is present and the monoclinic phase has only a small proportion.

Surprisingly, it has now been found in ZrO ₂ doped with MgO that densely sintered, crack-free ceramics can also be produced at a significantly higher proportion of monoclinic phase, if a maximum sintering temperature of less than 1500 ° C. is sufficient and the particles contain the doping element distributed very homogeneously.

A maximum sintering temperature of less than 1500 ° C is sufficient if the MgO-doped ZrO _{2 has} a particularly high reactivity. This high reactivity is achieved by the particle production process and the preparation according to the invention of the ZrO ₂ particles for the formation of the slip or granules according to the invention.

The theoretical density is the pure density (also called skeletal density, absolute or true density). This density can obtain a ceramic body with complete freedom from pores. In the literature, theoretical densities in the range from 5.70 to 5.74 g / cm ³ can be found for MgO-PSZ (ZrO ₂ doped with 10 mol% MgO). This document assumes a theoretical density of 5.74 g / cm ³ . Sintered density is the density that a ceramic molding achieves after sintering and indicates the ratio of mass to the total volume of the molding. Open and closed pores can be included. The relative sintering density indicates the ratio between sintered density and theoretical (maximum achievable) density and is expressed in percent. In the case that the sintering density corresponds to the theoretical density, the result is a relative sintering density of 100%.

All percentages by weight refer hereinafter to weight percent (wt%, i.e.,% w / w) unless otherwise specified.

Sintering involves the complex process of densification of a ceramic material under the influence of temperature. Significant features of this process are, macroscopically, the increase in strength and volume loss. Inside the ceramic, the redistribution of material increases the density. Targeted process management attempts to achieve technologically suitable phase and microstructure training. Essential parameters are the heating rate (also heating gradient), with which the necessary process temperatures (maximum sintering temperature) are set and the holding time, in which this temperature acts on the sintered body.

Further reaction-determining parameters during sintering are the pressure and the composition of the sintering atmosphere. The combination of the thermal and mechanical compression method, as is the case, for example, with hot isostatic pressing, often proves to be a suitable method for achieving the best possible compaction effect in view of the structural features. Hot isostatic pressing (HIP), that is, a heat treatment with simultaneous application of pressure, enables the reduction (expulsion) of pores and thus an increase in the sintering density and consequently the strength. Unless explicitly stated otherwise, sintering is understood to mean the purely thermal treatment under an air atmosphere. With the MgO-PSZ materials according to the invention (powder, slurry, granules, shaped bodies), relative sintering densities of greater than 99% at a maximum sintering temperature of less than 1500 ° C. can already be achieved without hot isostatic pressing.

A slurry according to the invention comprises a liquid medium having solid ZrO ₂ particles dispersed therein doped with 5 mol% to 15 mol% MgO, comprising at least the monoclinic modification of the ZrO ₂ , the ZrO ₂ particles having a mean agglomerate size in the Range of 20 nm to 500 nm and a ratio of the peak intensities between the monoclinic and the cubic modification of greater than 0.3, wherein the ZrO ₂ particles, a ceramic structure with a sintering density greater than 95% at a maximum sintering temperature of less than 1500 ° C. can form.

A slip is a special form of suspension in which ceramic particles are dispersed as a solid phase in a liquid phase. As the dispersing medium (liquid medium), for example, water can be used.

The peak intensities are determined as follows: The phase composition is determined by means of X-ray powder diffractometry, for example using the diffractometer Broker AXS (Siemens) D5000. The basis for this is that each crystallographic phase has a unique pattern of the diffraction diagram (X-ray diffractogram); unique in terms of the position of the reflections, determined by the grating metric and with respect to the intensity distribution, which is essentially determined by the cell content of the crystal structure. The X-ray diffractogram shows typical maxima, so-called peaks. Their amplitudes, the peak intensities are given in counts per second (cps). The diffraction patterns can be assigned to the crystallographic phases using the ICDD / PDF2 (International Center for Diffraction Data, Database Set 1-45, 1995 version 2.16) database.

By definition, the ratio of the peak intensities is determined by the intensity value of a main peak of the monoclinic phase at a 28 value of 30.3 ° (theta, Bragg angle) and the intensity value of the main peak of the cubic phase at a 28 value of 28.2 ° (theta , Braggwinkel) formed as follows: Peak monoclinic ZrO ₂ (28 value = 28.2 °): peak cubic ZrO ₂ (28 value = 30.3 °) = V

The main peaks of the cubic (28 value = 30.3 °) and the tetragonal ZrO ₂ phase (28 value = 30.2 °) are close together. If both modifications are present, the intensities add up accordingly. By definition, the following applies: The intensity of the main peak at 28 value = 30.3 ° is completely assigned to the terms "peak or peak intensity for the cubic phase". The main peak of the monoclinic phase at 28 value = 28.2 ° is referred to as the "peak or peak intensity for the monoclinic phase". The peak levels as well as the theoretical peak intensities can be found in the ICDD / PDF2 (International Center for Diffraction Data, Database Set 1-45, 1995 Version 2.16). Here, the reference was used for the cubic phase "27-997" (ZrO ₂ zirconium oxide), the tetragonal phase "17-923" (ZrO ₂ zirconium oxide) and "37-1484" (Baddeleyite, syn).

The agglomerate size of the slurry indicates the size of the particle aggregates in the slurry. Agglomerates are composed of primary particles. The agglomerate sizes given below are to be understood as mean agglomerate sizes (d ₅₀ ), unless explicitly stated otherwise. The dispersing process is intended to dissolve the agglomerates as far as possible. If this succeeds predominantly, the agglomerate size in the slurry approaches the primary particle size of the starting powder.

Surprisingly, it was found that for the further processing to dense sintered ceramic bodies with particularly high strengths slip are particularly suitable if they have a monoclinic content of greater than 30 wt .-% based on the ZrO ₂ particles, a mean agglomerate size in the range of 20 nm to 500 nm and a ratio of the peak intensities of greater than 0.3. From this, densely sintered ceramic bodies can be produced at a maximum sintering temperature below 1500 ° C.

In a preferred embodiment, the average agglomerate size is 50 nm to 400 nm, the proportion of monoclinic phase greater than 40 wt .-% based on the ZrO ₂ particles and the ratio of the peak intensities of greater than 0.5. As a result, sintered ceramic bodies with a relative density of more than 98% based on the theoretical density with a maximum sintering temperature of less than 1500 ° C can be produced.

In a further preferred embodiment, the average agglomerate size is 100 nm to 250 nm, the monoclinic fraction is greater than 50% by weight, based on the ZrO ₂ particles, and the ratio of the peak intensities is greater than 1. This makes it possible to sinter sintered ceramic bodies having a relative density of more be prepared as 99% based on the theoretical density with a maximum sintering temperature of less than 1500 ° C, preferably less than 1450 ° C, more preferably less than 1400 ° C.

The ZrO ₂ particles in the slip have at least the monoclinic modification of ZrO ₂ with a proportion of greater than 30% by weight, preferably greater than 40% by weight and particularly preferably greater than 50% by weight. In addition, a proportion of cubic modification with less than 70 wt .-%, preferably less than 55 wt .-%, more preferably less than 40 wt .-% and a tetragonal portion of preferably less than 40 wt .-%, preferably less than 25 wt .-% , more preferably less than 15 wt .-% are present. The Zro ₂ particles in the slurry can be doped with 5 mol% to 15 mol% MgO.

In one embodiment, the peak intensity for the cubic phase of the ZrO ₂ particles in the slurry is set to less than 600 cps, preferably to less than 300 cps and particularly preferably to less than 50 cps, the agglomerate size being in the range from 20 nm to 500 nm, preferably Range of 50 nm to 400 nm, more preferably in the range of 100 nm to 250 nm. The adjustment of the peak intensity for the cubic phase to less than 50 cps means that the cubic modification is barely present, that is, at or below the detection limit. This may result in a ratio of peak intensities greater than 20 or even greater than 100. The slips thus adjusted have a monoclinic proportion relative to the solid ZrO ₂ particles of greater than 50 wt .-%, preferably greater than 70 wt .-%, more preferably greater than 90 wt .-% and are particularly suitable for further processing into ceramic moldings and in particular lead to high strength and good corrosion resistance ceramics.

In a preferred embodiment, the slip has a concentration of more than 40% by weight, preferably in the range between 50% by weight to 80% by weight, particularly preferably in the range between 60% by weight to 75% by weight. ZrO ₂ particles on. These slip are particularly suitable for further processing such as slip casting or similar processes.

According to the invention, at least one hydrogen-containing compound is attached to the surface of the ZrO ₂ particles in the slurry or granules, the hydrogen content of the hydrogen-containing compounds being from 0.06% by weight to 1.1% by weight, based on the ZrO ₂ particles.

A granule according to the invention comprises solid ZrO ₂ particles doped with 5 mol% to 15 mol% MgO comprising at least the monoclinic modification, the ZrO ₂ particles having a ratio of the peak intensities between the monoclinic and the cubic modification of greater 0.3, wherein the ZrO ₂ particles can form a ceramic structure with a sintering density greater than 95% at a maximum sintering temperature of less than 1500 ° C.

The granules may have a moisture content of 0.1 wt .-% to 3 wt .-% and in addition compounds of hydrogen and oxygen without carbon with a content of 0.5 wt .-% to 10 wt .-% and in addition a share of carbon, bound in carbonaceous compounds, of less than 2%.

Granules that can be used for compression molding to form bodies are distinguished in the context of the present description of agglomerates, as compared to powder and non-dispersed Schlickeragglomeraten fundamental differences in the structure arise. In powder agglomerates or unbroken slip agglomerates, there is a bond between the ZrO ₂ particles. However, if the particles are dispersed in the slurry, the particles are largely coated with a dispersing additive. Subsequent reagglomeration (eg in the granules) results in no binding between ZrO ₂ particles, but between the particles is the dispersant component. It was found that this results in a significantly changed sintering behavior, whereby the sintering behavior is strongly influenced by the agglomerate size of the ZrO ₂ in the slurry. It is true that with a smaller agglomerate size, the sintering activity increases.

If the agglomerate size in the slurry decreases, a tendency for a higher sintering density is to be expected for ceramic shaped bodies produced therefrom. This results from the fact that in larger agglomerates usually more pores are included, which must be removed or closed in the sintering process. This type of pores within the agglomerates of the specific size and design can often be incompletely expelled or closed. If these agglomerates no longer exist or are significantly smaller, these pores need no longer be expelled or closed during the sintering process and higher sintering densities and thus higher strengths are achieved.

Surprisingly, it was found that the sintering densities obtained initially increase with decreasing agglomerate size (otherwise identical slips, identical procedure for the production of the ceramic). When falling below a defined agglomerate size in the slip, however, this dependence is reversed, that is, with then further decreasing agglomerate size lower sintering densities are achieved.

This surprising finding is based on the fact that in order to achieve lower agglomerate sizes, a higher energy input must be made in the slurry. The investigation of the phase composition by means of X-ray phase analysis shows that with increasing energy input, the crystal structure is increasingly damaged (amorphization) or undergoes a transformation (phase transformation). For example, the cubic or tetragonal phase is converted to monoclinic. However, the lattice structure of the monoclinic phase is also damaged as the energy input progresses. Surprisingly, it has now been found that cracks in the ceramic occur from a certain proportion of damaged grid structure, these cracks increase with a higher proportion of damaged grid structure. The cracks reduce the sintering density and significantly reduce the strength of the ceramic. According to the invention, the phase composition or the lattice structure is adjusted so that densely sintered ceramics can be produced at a maximum sintering temperature of less than 1500 ° C.

The moldable granules can be made from the slips. It has surprisingly been found that the hydrogen-containing compounds bound to the particle surface during the dispersion have a particularly favorable effect on the pressing behavior of granules produced therefrom. Thus, for example, the amount of otherwise necessary pressing and lubricants can be reduced.

A pre-sintered molded article according to the invention can be formed from the slip according to any one of claims 1 to 4 and / or from the granules according to any one of claims 5 to 8, wherein ZrO ₂ in the monoclinic modification is present in a proportion of greater than 30 wt .-%. In addition, a proportion of cubic modification with less than 70 wt .-% and a tetragonal portion of preferably less than 40 wt .-% may be present. The ZrO ₂ is doped with 5 mol% to 15 mol% MgO. The ratio of the peak intensities of the monoclinic and the cubic phase is greater than 0.3. In a preferred embodiment, the ZrO _{2 is present} in the presintered moldings as a monoclinic modification with a proportion of greater than 50 wt .-%; These are particularly suitable for further processing into ceramic moldings and in particular lead to high strength and good corrosion resistance of the ceramic.

A ceramic according to the invention is formed from the slurry according to any one of claims 1 to 4 and / or from the granules according to any one of claims 5 to 8, wherein the ceramic structure has an average grain size in the range of less than 1000 nm and a sintering density of greater than 95%. having.

In a method according to the invention for producing the slip, at least one ZrO ₂ powder doped with 5 mol% to 15 mol% MgO, comprising at least the cubic or monoclinic modifications, is ground and / or dispersed in at least one liquid dispersing medium, wherein an energy input is controlled and / or regulated during the dispersion and / or grinding so that the resulting solid ZrO ₂ particles in the slurry after dispersion and / or grinding have a mean agglomerate size in the range between 20 nm and 500 nm, a monoclinic phase fraction greater than 30 Wt .-% and an intensity ratio between the monoclinic and the cubic modification of greater than 0.3.

Milling is understood to mean comminution of particles or separation of hard agglomerates by mechanical energy. In both cases, chemical binding forces are broken up. Aggregates are corresponding structures made of chemically combined (intergrown) particles. Agglomerates, on the other hand, are formed by particles, whereby the bonding force here is purely physical, for example due to surface energy. Typically, mixtures of agglomerates and aggregates occur, so that the term agglomerates also includes a small proportion of aggregated structures. Dispersion is understood to mean the fine distribution of solid particles in a liquid phase, but no particle comminution must take place, and only particle agglomerations can be dissolved.

A significant advantage when using finely divided particles or agglomerates resulting from the grinding or dispersion for the production of ceramics is that their sintering activity is significantly increased and thereby the necessary maximum sintering temperature can be selected lower. Due to a lower sintering temperature, the grain growth during sintering is lower, the sintered structure accordingly has a finer grain structure and thus offers a higher flexural and abrasion resistance.

Due to the controlled and / or regulated energy input during grinding or dispersing, a defined agglomerate size in the slurry is achieved, but the crystal lattice of the ZrO ₂ particles is damaged only to a limited extent. In conventional processes, either the agglomerates are not sufficiently dispersed or the crystal lattices damaged (for example, amorphization) such that the volume change, for example, in phase-to-phase conversion into cubic modification, leads to cracks. In the method according to the invention only a limited conversion into the monoclinic phase and a limited damage to the lattice structure, so that it is prevented that it comes in subsequent sintering to undesirable cracking in the ceramic structure. This slip is suitable for the further processing and production of ceramics with particularly high strengths.

The energy input can be influenced for example by the rotational speed of a grinder and / or the grinding or dispersion time.

In addition, a dispersant, preferably in the range of 0.1 to 3.0 wt .-% (active substance of the dispersant based on the ZrO ₂ content) may be added to the slurry. In this concentration range, a particularly good dispersion and a sufficiently good stabilization of the slurry are achieved.

These slips can be prepared in such a way that the liquid medium is initially charged, optionally the dispersant fed, the components are well mixed and then the ZrO ₂ powder is supplied, with good mixing by, for example, stirrers, Disolver, Ultratorax or ultrasound is advantageous. Alternatively, the slurry preparation may also be carried out directly in the apparatus in which the dispersion and / or grinding takes place, for example in a bead mill. For this purpose, for example, water and the dispersant are initially introduced and gradually fed to the ZrO ₂ powder. Other sequences are also possible.

The slips produced in this way initially have an agglomerate size above 500 nm, usually well above 1000 nm. In order to reduce the agglomerate size below 500 nm, it is usually necessary to introduce a high energy, for example mechanical shear forces, into the slurry. This can be done for example by mills, for example by bead mills. The energy input, in particular when using bead mills, the lattice structure and / or the modification can be influenced. Amorphization, lattice mismatches, and / or modification changes can occur. According to the invention, the energy input is controlled such that a defined order of the crystal structure results or a defined proportion of monoclinic ZrO ₂ is retained.

The energy input in the dispersion is controlled in particular so that the conversion of the crystalline ZrO ₂ phase does not exceed a defined level, in particular the monoclinic phase is not less than 30 wt .-% and that the ratio between the peak intensities of the monoclinic and cubic ZrO ₂ phase in the diffraction diagram of the X-ray diffraction powder is greater than 0.3.

In one embodiment of the process, the ZrO ₂ powders are predispersed or pre-ground in dry processes. Under predispersion or premilling is to be understood that the agglomerates are at least partially broken or destroyed. Dry process means that no dispersion or grinding takes place in a liquid dispersion medium, but liquid components can be added separately to the ZrO ₂ or separately in the process, but which at least partially serve to coat the particles and thus promote the dispersion process or stabilize the dissolved / broken agglomerates. Such dry processes can be carried out, for example, in jet mills, fluid bed jet mills, spiral mills, centrifugal mills, impact mills or the like. For example, the dry methods may be based on the principle of particle fracture by collision in a pressurized medium and at high speeds. An example of this is the RINA-JET turbo air jet mill by Riera Nadeu.

From the thus pre-dispersed or pre-ground ZrO ₂ powders slip are prepared according to the invention. It has surprisingly been found that the combination of dry predispersion or premilling followed by dispersion into a liquid medium results in slips in which the lattice structure of the ZrO ₂ particles is less changed or damaged than the wet milling with the same agglomerate size, ie ZrO ₂ particles in these slips have a higher peak intensity for the crystalline phases, ie a higher crystallinity.

When dispersing the ZrO ₂ particles, hydrogen-containing groups and / or compounds (hydrogen-containing compounds consisting of hydrogen and oxygen without carbon), such as, for example, hydroxyl groups or hydroxide compounds, can accumulate on their surfaces. The concentration of the hydrogen-containing groups and / or compounds increases with the length and intensity of the dispersion. Therefore, the proportion of hydrogen-containing groups and / or compounds is a measure of the intensity of the dispersion. The proportion of these phases can be determined by drying the slips or the granules prepared therefrom at 50 ° C. to constant weight (thus separating the physically bound adhesive water (moisture)) and then determining the weight loss at 200 ° C. Alternatively, the proportion of these phases can be determined by determining the H ⁺ proportion (for example by means of a carbon-hydrogen analyzer) in the slurry dried at 50 ° C., which is in the range between 50 ° C. and 200 ° C is released. Typically, these values are converted into H ₂ O ⁺ (water of crystallization). Thus, a uniform spelling is taken into account, even if no deduction can be made to the present bond. The ZrO ₂ particles may have a hydrogen content bound in the hydrogen-containing phases and attached to the ZrO ₂ particles of from 0.06 wt.% To 1.1 wt.% (Corresponding to about 0.5 wt 10 wt.% H ₂ O ⁺ ), preferably from 0.11 wt.% To 0.89 wt.% (Corresponding to about 1 wt.% To 8 wt.% H ₂ O ⁺ ), especially preferably from 0.33 wt .-% to 0.67 wt .-% (corresponding to about 3 wt .-% to 6 wt .-% H ₂ O ⁺ ) have.

The slip can be further components, in particular pressing, sliding and sintering aids are added. The addition of these additives can take place before, during or after the dispersion.

For the production of the ZrO ₂ powder materials used for the production of the slurry, fundamentally different thermal processes may be suitable, such as, for example, flame or spray pyrolysis, production in plasma or pulsation reactors, but also wet-chemical processes such as precipitation reactions or sol-gel processes are suitable. Such powders are produced, for example, by a production process in the pulsating hot gas stream, as disclosed in the published patent applications DE 10 2006 046 803 A1 . DE 10 2007 003 744 A1 or DE 10 2008 006 607 A1 is known received.

Alternatively, such ZrO ₂ powders can also be obtained by optionally drying a suitable raw material mixture and thermally treating this raw material mixture in the range of 500 ° C to 1300 ° C.

Likewise, zirconium-containing raw material components such as salts can be converted into a solution, to this zirconium-containing raw material solution, the doping elements are added, so that the doping elements are present dissolved.

According to the invention, the ZrO ₂ particles of the ZrO ₂ powder have a very homogeneous distribution of the doping element. This is achieved, for example, by using raw material solutions in which at least the zirconium and the doping element are dissolved in order to produce the doped ZrO ₂ particles. This creates an almost ideal solution (ideal element distribution), which is almost ideally mixed homogeneously.

Such raw material solutions can be introduced in a hot gas stream at a hot gas temperature at the feed location of 50 ° C. to 500 ° C., preferably in the range from 80 ° C. to 150 ° C., the solid particles forming from the raw material solution. This can be done for example by spray drying. The powder thus produced is then thermally treated at 500 ° C to 1300 ° C, preferably at 700 ° C to 1200 ° C, more preferably at 900 ° C to 1100 ° C, since in this temperature range experience shows a particularly good ratio between low aggregation and sufficient crystallinity prevails. As a result, these powders can be dispersed in the slurry with a comparatively low energy input up to the agglomerate size according to the invention.

Alternatively, to prepare the ZrO ₂ powder, the prepared raw material solution containing doping elements may directly form a pulsating or non-pulsating hot gas stream having a hot gas temperature at the feed location of 800 ° C to 1300 ° C, preferably in the range of 900 ° C to 1200 ° C , Particularly preferably be supplied in the range 1000 ° C to 1200 ° C.

Particularly suitable are those ZrO ₂ powder from which slips with a low agglomerate size can be produced even at low energy input. Powders of the invention have a structure with a low primary particle size and the lowest possible aggregation. Aggregation is understood here to mean the aggregation of primary particles, with strong binding forces, in particular chemical bonds, being present due to sintering phenomena between the particles.

The powders used have a carbon content of less than 0.2 wt .-%, preferably less than 0.1 wt .-%, particularly preferably less than 0.05 wt .-% to. According to experience, low carbon contents lead to better compaction properties during sintering and thus to higher densities. In addition, elemental carbon often can not be completely removed during sintering, which is accompanied by an undesirable discoloration of the ceramic.

In a preferred embodiment, the specific surface area (according to BET) is in the range from 1 m ² / g to 15 m ² / g, preferably in the range from 1.5 m ² / g to 10 m ² / g, particularly preferably 2 m ² / g to 6 m ² / g. Powders with higher specific surface areas are more difficult to handle and often cause difficulties in dispersing. Lower specific surface areas are an indicator of larger primary particle diameters and / or hard aggregation. In these powders, the agglomerate sizes according to the invention can be achieved in the slurry only with high energy input.

The ZrO ₂ powders according to the invention are doped with 5 mol% to 15 mol% MgO. The doping of ZrO _{2 is} intended to completely or partially stabilize certain modifications. This is predominantly necessary in the field of ceramic production, since a phase change in particular between monoclinic and tetragonal or cubic associated with volume changes, which can lead to cracks. Stabilizer additives in the form of other metal oxides, such as, for example, calcium oxide, magnesium oxide, yttrium oxide or cerium oxide, are added to the ZrO ₂ , individually or in combinations. Depending on the proportion of stabilizer additives, for example, the cubic high-temperature phase of the zirconium dioxide can be fully or partially stabilized up to room temperature. The type of stabilizer determines which modification is stabilized or partially stabilized by the ZrO ₂ .

Optionally, further doping elements may be added in addition to the MgO. By combining with other doping elements, the ceramic properties, such as bending tensile strength or toughness, can be adjusted specifically.

Surprisingly, it was found that the ZrO ₂ powder does not necessarily have to have the cubic modification as the main phase and possibly a low monoclinic fraction. It has even been found that the cubic modification in the ZrO ₂ powder is not necessary at all and that the monoclinic phase can be the main component. Provided that the doping element is distributed very homogeneously in the particle and the particles show a high sintering activity, a densely sintered ceramic can be obtained therefrom, but only if a low maximum sintering temperature (below 1500 ° C) is sufficient for it. At higher sintering temperatures, the ceramics increasingly show cracks because here the phase transformation processes are different and accordingly the associated volume change leads to cracking. It is therefore crucial that the homogeneous distribution of the doping element in the sintering with maximum sintering temperatures below 1500 ° C in combination with a suitable sintering profile (heating and cooling) then leads to sufficient stabilization of the cubic phase in the ceramic.

It has been found that the phase composition of the ZrO ₂ powders can be influenced and specifically adjusted over the residence time in the thermal production process of the powders. Thus, with otherwise identical process parameters (identical raw material mixture, same process temperature, etc.) with a long residence time, the monoclinic modification is preferably formed, with shorter residence times, the cubic or proportionate tetragonal modification. Surprisingly, it has been found that especially with a combination of a defined particle size and a defined phase composition, sinter-active ZrO ₂ particles result. In a preferred embodiment, the proportion of monoclinic phase is greater than 50 wt .-%, preferably greater than 70 wt .-%, more preferably greater than 90 wt .-%.

In order to obtain densely sintered ZrO ₂ ceramics doped with MgO at maximum sintering temperatures below 1500 ° C., the ZrO ₂ particles must have a particularly high sintering reactivity. The high sintering reactivity according to the invention is based, on the one hand, on the selection of suitable ZrO ₂ powders and, on the other hand, on the activation by the dispersing process according to the invention.

Surprisingly, ZrO ₂ powders prepared from raw material solutions show a significantly higher sintering activity than conventionally produced ZrO ₂ powders in which at least one raw material component (zirconium and / or doping element) was present in solid form. If the components are in solution, a specific lattice structure with a homogeneous distribution of the doping element forms during particle production. Particularly reactive powders result from manufacturing processes where the thermal process of forming the particles occurs within a very short time. Powder prepared via the Pulsationsreaktor process (shown for example in the publications DE 10 2006 046 803 A1 . DE 10 2007 003 744 A1 or DE 10 2008 006 607 A1 , are particularly suitable.

The ZrO ₂ powder according to the invention may contain further constituents, which typically result from the paragenes of the raw materials, for example, hafnium z. B. in oxidic form with ZrO ₂ . By definition, hafnium is not considered to be a doping agent in the following since it is present as a paragenesis in the raw material and is not deliberately added. The proportion of these components, which is entered by the Zr raw materials, depends on the type of Zr raw material. In addition, traces of contamination may occur, on the one hand due to the raw materials, on the other hand due to the thermal production process of the powder materials. The sum of the acceptable impurities depends on the use and necessary properties of the ceramic. Typically, impurities less than 1%, preferably less than 0.5%, more preferably less than 0.1% and most preferably less than 0.05%, based on the powder material, are acceptable.

Frequently, ZrO ₂ raw materials are used for the production process, which are highly chlorine- and / or sulfur-containing, for example in the form of salts. Through the manufacturing process, these raw materials are largely converted, but usually not completely. Accordingly, shares of Cl and / or S or their compounds in ZrO ₂ . For the production of ceramics, S and Cl are considered to be particularly disturbing, since in particular the strength properties are strongly negatively influenced. In addition, Cl can be incorporated into the lattice structure of the ZrO ₂ , which in addition to undesirable changes in the properties of the ZrO ₂ also be associated in addition a change in color, which is also undesirable. The ZrO ₂ powders according to the invention have a particularly low content of Cl (chlorine) and S (sulfur) or of their compounds. The proportion of chlorine is less than 1000 ppm, preferably less than 500 ppm, more preferably less than 200 ppm and the sulfur concentration is less than 800 ppm, preferably less than 200 ppm, more preferably less than 100 ppm.

In a preferred embodiment, the MgO-doped ZrO ₂ powder according to the invention is present, at least proportionally, in the cubic modification, preferably in a proportion of greater than 30% by weight, particularly preferably in a proportion greater than 60% by weight. In addition, the monoclinic modification of ZrO ₂ may also be present, preferably in the range of less than 50% by weight, particularly preferably in the range of less than 30% by weight. In addition, other phases are possible, in particular tetragonal ZrO ₂ or X-ray amorphous phases, which can therefore be identified in the X-ray phase analysis only as amorphous. The ZrO ₂ powders have a specific surface area in the range from 1 m ² / g to 5 m ² / g. In a further preferred embodiment, the ZrO ₂ powder doped with MgO is present essentially in the monoclinic modification, preferably in a proportion of greater than 70% by weight, particularly preferably in a proportion greater than 90% by weight. In addition, the cubic modification of ZrO ₂ may also be present, preferably in the range of less than 30% by weight, more preferably in the range of less than 10% by weight. In addition, other phases are possible, in particular tetragonal ZrO ₂ or X-ray amorphous phases. The ZrO ₂ powders have a specific surface area in the range from 1 m ² / g to 5 m ² / g. ZrO ₂ powder from these two preferred embodiments are suitable for dispersing according to the invention, wherein the dispersing the ZrO ₂ particles can be activated so that they then have a particularly high sintering activity and densely sintered ceramics at maximum sintering temperatures below 1500 ° C. can be formed.

As a result of the energy input during dispersion or grinding, the cubic phase is progressively converted into the monoclinic modification. In addition, the monoclinic phase is simultaneously damaged so that the peak intensity decreases in the X-ray phase analysis, that is, the crystallinity decreases. The percentage of monoclinic phase and the crystallinity (peak intensity) of the ZrO ₂ particles after dispersion is therefore a measure of the intensity of the energy input. Sintered MgO-doped ZrO ₂ ceramics can be made by setting the monoclinic portion and crystallinity within a defined range. By contrast, lower crystallinity, for example, leads to cracking. Since the phase formation or phase transformation reactions are also significantly dependent on the sintering activity and thus on the homogeneous distribution of the doping element and the particle or agglomerate size, in addition to the adjustment of the phase composition according to the invention, the agglomerate size in the slurry must also be adjusted in the range according to the invention. With smaller agglomerate sizes, cracking may occur during ceramic formation, whereas lower sintering density with a higher pore content results in coarser agglomerates.

The dispersion according to the invention is controlled accordingly so that the ZrO ₂ particles doped with MgO are present in the slip at least proportionally in the monoclinic modification, preferably with a proportion of greater than 30% by weight, particularly preferably with a proportion greater than 60% by weight. , In addition, the cubic modification of ZrO ₂ may also be present, preferably in the range of less than 50% by weight, more preferably in the range of less than 30% by weight. In addition, other phases are possible, in particular tetragonal ZrO ₂ or X-ray amorphous phases. In a preferred embodiment, the peak intensity of the monoclinic phase is set in the range of 200 to 2000 cps, preferably in the range of 300 to 1000 cps, and more preferably in the range of 400 to 800 cps. Slurry containing ZrO ₂ particles are particularly suitable for the production of densely sintered ceramics at maximum sintering temperatures below 1500 ° C.

Forming processes serve the purpose of reshaping a ceramic heap by external forces in such a way that both the desired geometric shape and a permanent cohesion with optimal homogeneous compression is achieved. In principle, three ceramic molding processes can be distinguished: casting, plastic molding and pressing, wherein in principle all of the ZrO ₂ particles can be used to produce moldings which form a ceramic structure during a sintering process.

The design for high-performance ceramics can generally be divided into wet and dry processes. Here, the conventional slip casting, the pressure slip casting and injection molding as Wet shaping established. Axial and isostatic pressing as a molding process are used predominantly for the manufacture of simple geometry ceramic components. These pressing methods are often used commercially because of their automation.

In one embodiment, the slips are first converted into moldable granules, for example by means of spray granulation, which are particularly suitable for axial and isostatic pressing as a molding process. Spray granulation is the drying of liquids (solutions, suspensions, melts) with the simultaneous construction of granules. Germs for the granules can be submitted (foreign nuclei) or formed by abrasion and breakage (intrinsic germs). The sprayed liquid coats the seeds and is dried. By varying the type and concentration of additives added, such as dispersants, binders, pressing aids and lubricants, as well as by targeted adjustment of the process parameters in the granulation process, such as process temperature, residence time or drop size, can be selectively influenced the strength, shape and granule size distribution. Preference is given to granules consisting of solid granules with an ideal spherical shape (for example, because of better flowability) and an average granule size of the granules in the range of 5 .mu.m to 500 .mu.m, preferably 10 .mu.m to 200 .mu.m, particularly preferably 20 .mu.m to 150 .mu.m.

The shaping is then carried out by pressing, whereby the axial and the isostatic pressing (CIP = cold isostatic pressing) differs. The latter, in turn, can be subdivided into wet and dry die technology. In addition, the pressing process can also be combined with a temperature cycle. In this case we speak of hot or hot isostatic pressing (HP or HIP).

The granules have a phase composition, wherein at least a proportion of the monoclinic modification of ZrO _{2 is} present, preferably with a proportion of greater than 30% by weight, particularly preferably greater than 60% by weight. In addition, the cubic modification of ZrO ₂ may also be present, preferably in the range of less than 50% by weight, more preferably in the range of less than 30% by weight. In addition, other phases are possible, in particular tetragonal ZrO ₂ or X-ray amorphous phases.

The granules may contain pressing agents, lubricants or other auxiliaries (eg binders). Such additives are used, for example, to adjust the strength of the granules so that they promote the formation of a compact, particularly homogeneous microstructure during pressing. The organic part of these additives (carbon-containing compounds, the additives may be purely organic compounds), is removed during debindering and / or the sintering process. These components initially leave behind pores, which must then be closed by sintering. The same applies to physically bound water, z. As adhesive water or referred to as moisture and the hydrogen-containing compounds on the ZrO ₂ particle surface, which are deposited by the dispersing and grinding process. These components are thus also part of the green compact and are separated in the debinding and sintering process, removed and leave first pores. If the proportion of organic components, water (moisture) or hydrogen-containing compounds is very large, in particular the sum value, the many pores can not be completely closed. In addition, cracks can increasingly form due to the then greater shrinkage. Both are associated with the loss of strength. The granules therefore preferably have a moisture content (= weight loss on drying at 50 ° C. to constant weight) of from 0.1% by weight to 3% by weight, preferably from 0.3% by weight to 1% by weight. and a proportion of hydrogen-containing components (determined on samples dried at 50 ° C, the weight loss is determined at 200 ° C), ie compounds of hydrogen and oxygen without carbon in the range of 0.5 wt .-% to 10 wt. -% H ₂ O ⁺ , preferably from 1 wt .-% to 8 wt .-% H ₂ O ⁺ , more preferably 3 wt .-% to 6 wt .-% H ₂ O ⁺ on. The loss on ignition (= weight loss on annealing at 1000 ° C. for 2 hours of samples dried at 50 ° C.) is in the range from 0.2% by weight to 20% by weight, preferably in the range from 1% by weight to 10 Wt .-%, particularly preferably in the range 5 wt .-% to 10 wt .-% and forms the sum of organic components and hydrogen-containing compounds (H ₂ O ⁺ ). The granules may have a carbon content bound in the carbonaceous compounds in the range of less than 2% by weight, preferably less than 1.5% by weight, more preferably less than 1% by weight. The granules produced in this way can be used to produce pressed moldings which are suitable for a subsequent sintering process.

Alternatively, the slurry may be further processed through other molding processes such as slip casting, die casting, ceramic injection molding, hot casting, gel casting, or freeze casting + Quickset ^™ . Both hot-casting and injection molding processes are suitable for the production of small, complicated-shaped components, with injection molding being more suitable for series production. Very high surface qualities can be achieved with sintered hot-cast or injection-molded parts, so that reworking by means of grinding or polishing is often unnecessary. A particularly advantageous Process is the so-called DCC process (Direct Coagulation Casting). This method is based on the destabilization of high-solids suspensions by means of in situ enzyme-catalyzed reactions. This allows the pH of the suspension to be shifted from its stable, low-viscosity region to its isoelectric point, whereupon the suspension coagulates and solidifies. Thus, highly flowable suspensions can be poured into molds and demolded as wet, solid green bodies. Alternatively, the shaping can also take place via plastic shaping, for example by means of extrusion.

The moldings may already correspond to the desired shape of the ceramic, taking into account the shrinkage occurring during sintering. Alternatively, the shaped bodies can also be reworked. In a preferred variant, the shaped bodies are pre-sintered at temperatures between 600 ° C. to 1300 ° C., preferably between 800 ° C. and 1200 ° C., so that the shaped bodies achieve a higher strength and / or debind the shaped bodies, ie the organic constituents are at least partially burned out become. These moldings thus obtained are referred to as "presintered moldings". Subsequently, the shape of the presintered molded body can be processed and / or sintered.

A shaped body according to the invention is formed from the slip according to the invention and / or from the granules according to the invention. The ZrO ₂ is doped with 5 mol% to 15 mol% MgO and comprises at least the monoclinic modification, wherein the ZrO ₂ particles have a ratio of the peak intensities between the monoclinic and the cubic modification of greater than 0.3, wherein the ZrO _{2 can form} a ceramic structure with a sintering density greater than 95% at a maximum sintering temperature of less than 1500 ° C.

These pre-sintered moldings are particularly suitable for further processing to densely sintered ceramic bodies at maximum sintering temperatures below 1500 ° C with particularly high strengths, when the ZrO ₂ in the monoclinic modification with a proportion of greater than 30 wt .-%, preferably greater than 40 wt. -% and more preferably greater than 50 wt .-% is present. In addition, a proportion of cubic modification with less than 70 wt .-%, preferably less than 55 wt .-%, more preferably less than 40 wt .-% and a tetragonal portion of preferably less than 40 wt .-%, preferably less than 25 wt .-% , more preferably less than 15 wt .-% be present. The ZrO ₂ is doped with 5 mol% to 15 mol% MgO. The ratio of the peak intensities of the monoclinic and the cubic phase is greater than 0.3, preferably greater than 0.5, particularly preferably greater than 1.

In a preferred embodiment, the ZrO _{2 is present} in the presintered moldings as a monoclinic modification with a proportion of greater than 50 wt .-%, preferably greater than 70 wt .-%, more preferably greater than 90 wt .-% before; These are particularly suitable for further processing into ceramic moldings and in particular lead to high strength and good corrosion resistance of the ceramic.

The ZrO ₂ particles after dispersion, in the present case in the form of a slip, bound in the granules or bonded as presintered shaped bodies, have a particularly high sintering activity. The sintering activity is measured in a dilatometer on defined test specimens, the linear thermal expansion of a test specimen being determined as a function of the temperature. The thermal expansion is a measure of how the entropy of a body reacts to changes in volume. For this purpose, first test specimens are produced. The preparation of the test specimens with dimensions of 25 mm by 9 mm by 5 mm takes place here by definition by uniaxial compression of the granules at 100 MPa and subsequent isostatic recompression at 330 MPa. The debindering (= dissolving out of the organic additives that were required for dispersion or shaping of the workpiece) takes place at a heating rate of 1 K / min to 1000 ° C. The dilatometric examination of the moldings is then carried out to 1500 ° C at a heating rate of 5 K / min. The ZrO ₂ particles or the molded articles produced therefrom have a sintering beginning (thermodynamic fixed point at which the shrinkage of the test specimen begins by incipient sintering, the detection is carried out by the dilatometric examination) of less than 1100 ° C., preferably less than 1000 ° C on. The temperature for the sintering end (shrinkage of the test specimen is completed, the ceramic bodies are largely densely sintered) is less than 1500 ° C, preferably less than 1450 ° C, more preferably less than 1400 ° C.

The slip and / or granules according to the invention are suitable for the production of high-performance ceramics, ie ceramics with, for example, particularly high strengths, low abrasion and high corrosion resistance. For this example, the granules are pressed into shaped bodies (so-called green body or green compacts), for example by uniaxial and / or isostatic pressing method, then optionally debinded and then sintered.

After sintering, a ceramic structure is obtained, which has a sintering density of greater than 95%, preferably greater than 98% and very particularly preferably greater than 99% in relation to the theoretical density. The ceramic structure has an average particle size of less than 1000 nm, preferably less than 600 nm, more preferably less than 450 nm. The particle size distribution is narrow, so that the d ₉₉ value is less than 3 times d ₅₀ value (mean particle size). The ZrO ₂ structure of this type has a high strength. The bending tensile strength according to DIN-EN-843-1 is greater than 400 MPa, preferably greater than 500 MPa and particularly preferably greater than 600 MPa.

The ceramic thus obtained can be further densified by subsequent process steps, for example by hot isostatic pressing (HIP), preferably sintering densities of 99%, particularly preferably more than 99.5% and particularly preferably more than 99.8% of the theoretical density of Materials are achieved. Higher densities and thus a lower percentage of pores are known to result in higher strengths.

The slip, granules, presintered shaped bodies and / or the ceramics can be used for the production or coating of technical machine components, in particular of bearing parts, engine or turbine parts, sealing parts, nozzle parts or probe parts. Other typical applications of these ceramics are in the technical field for the production or coating of various components that are exposed to particularly high mechanical and / or thermal loads, such as extrusion dies, wear-resistant machine parts (dies, seals, gaskets bearing parts), sliding and counter rings for Mechanical seals, engine parts, such as rocker arms, cylinder liners, piston crowns. The ceramic can also be used for coatings of other materials, for example for turbine coating or lining. Also suitable is the ceramic in the range of functional components, such as oxygen probes (lambda probe).

Embodiments of the invention are explained in more detail below with reference to drawings. Show:

1 superimposed diffractograms of a ZrO ₂ material before a grinding process and at various stages of the milling process, and

2 superimposed diffractograms of a ZrO ₂ material before a grinding operation, during the grinding process and after sintering,

3 superimposed diffractograms of a ZrO ₂ material before a grinding operation, during the grinding process, during the production of a presintered shaped body and after a sintering,

4 superimposed diffractograms of ZrO ₂ materials before the grinding process, produced at different process parameters, and

5 a dilatometer curve of a ZrO ₂ material compared to conventional MgO-PSZ.

Corresponding parts are provided in all figures with the same reference numerals.

The following measurement conditions and preparation conditions for determining the peak intensity apply to all the following information: Recorder: Diffractometer Broker-AXS (Siemens) D5000 with Cu tube (CuKα λ = 1.5418 Å), the X-ray tube is operated with a voltage of 40 kV and a current of 20 mA (generator parameter), aperture and divergence diaphragm V20, Conditions for registration: Increment = 0.04 ° 20 and count time = 2 sec. Sample preparation: The sample to be examined is carefully ground by hand in an agate mortar.

• – Auswertesoftware Diffrac-AT (V3.2).
• – Zunächst wird der Untergrund abgezogen (Standradeinstellung und anschließend die Kurve geklättet (Faktor 0,300)
• – Daran wird die Höhe des Peaks (= [cps] = Peakintensität) ermittelt.

Evaluation:

• - Evaluation software Diffrac-AT (V3.2).
• - First, the ground is subtracted (Standradestellung and then the curve smoothed (factor 0.300)
• - This determines the height of the peak (= [cps] = peak intensity).

1 shows superimposed diffractograms of a ZrO ₂ material prior to a dispersing and at various stages of the milling / dispersing process.

A phase composition of the ZrO ₂ material is determined by means of X-ray powder diffractometry, for example with the diffractometer Bruker-AXS (Siemens) D5000. The basis for this is that each crystallographic phase has a unique pattern of the diffraction pattern (X-ray diffractograms); unique in terms of the position of the reflections, determined by the grating metric and with respect to the intensity distribution, which is essentially determined by the cell content of the crystal structure. The X-ray diffractogram shows typical maxima, so-called peaks. Their amplitudes, the peak intensities I are given in counts per second (cps) and mapped in the diffractogram over a Bragg angle 2θ.

A peak intensity I _{kub_A} at a Bragg angle 2θ _cubic of 30.3 ° is assigned to a cubic modification of ZrO ₂ . A peak intensity I _{mono_A} , I _{mono_B} , I _{mono_C} at a Bragg angle 2θ _mono of 28.2 ° is assigned to a monoclinic modification of ZrO ₂ .

The peak intensities I _{kub_A} and I _{mono_A} for intensities of ZrO ₂ particles are in front of a dispersion.

The peak intensities I _{mono_B} represent the intensities of the ZrO ₂ particles after four hours of milling in a bead mill.

The peak intensities I _{mono_C} represent the intensities of the ZrO ₂ particles after eight hours of milling in the bead mill.

With increasing energy input through the grinding, the crystal structure is increasingly damaged or undergoes a transformation (phase transformation). The ZrO ₂ particles were intensively dispersed or ground in a bead mill (Netzsch Labstar, 0.2 mm grinding ball size, 2000 rpm, 0.7 kW power input). The investigation of the phase composition by means of X-ray phase analysis shows that with increasing dispersing or grinding time, the peak intensity of the cubic ZrO ₂ modification decreases. The cubic modification is first converted into the monoclinic modification, in which case the monoclinic modification is also damaged, that is, the peak intensity and thus the crystallinity decreases.

By a subsequent thermal treatment, for example in the sintering, a recrystallization or (re) conversion takes place in the cubic modification. 2 shows by way of example that the ZrO ₂ particles before the dispersion / grinding have the cubic and the monoclinic phase, by the dispersion / grinding, the cubic is completely converted, whereby the monoclinic phase is damaged by the dispersion / grinding so that the Intensity decreases, after the sintering, the cubic modification is present as the main constituent in addition to the significantly inferior monoclinic phase. Proportionally, the tetragonal phase may also be present after the sintering, the main peak being superimposed at 2θ of 30.2 ° from the main cubic phase.

A peak intensity I _{kub_A} , I _{kub_D} at a Bragg angle 2θ _cubic of 30.3 ° is assigned to a cubic modification of ZrO ₂ . A peak intensity I _{mono_A} , I _{mono_B} , I _{mono_E} , at a Bragg angle 2θ _mono of 28.2 ° is assigned to a monoclinic modification of ZrO ₂ .

The peak intensities I _{kub_A} and I _{mono_A represent} intensities of the ZrO ₂ particles before the dispersion.

The peak intensities I _{kub_B} and I _{mono_B} represent the intensities of the ZrO ₂ particles after six hours of milling in the bead mill.

The peak intensities I _{kub_D} and I _{mono_D} represent the intensities of the ZrO ₂ particles in the sintered ceramic.

3 shows, by way of example, superimposed diffractograms of a ZrO ₂ material, for example a ZrO ₂ powder, before dispersion, in which case substantially the monoclinic modification is present. By dispersing / grinding the monoclinic phase is damaged, that is, the peak intensity decreases. From this, a pre-sintered molded article can be produced, for example as in 3 shown at a maximum temperature of 1000 ° C. The pre-sintered shaped body essentially contains here also the monoclinic phase, wherein the intensity is greater than after the dispersion. As a result of the temperature treatment, recrystallization or rearrangement of the lattice structure already begins. In the sintering, the conversion / formation of the cubic modification takes place, with a minor proportion of monoclinic phase still present. Proportionally, the tetragonal phase may also be present after the sintering, the main peak being superimposed at 2θ of 30.2 ° from the main cubic phase.

The peak intensity I _{mono_E stands} for the intensity of the ZrO ₂ particles before the dispersion.

The peak intensity I _{mono_F} represents the intensity of the ZrO ₂ particles after six hours milling in the bead mill.

The peak intensity I _{mono_G} stands for the intensity of the ZrO ₂ particles in the presintered molded body, produced at a maximum firing temperature of 1000 ° C.

The peak intensities I _{kub_H} and I _{mono_H} represent the intensities of the ZrO ₂ particles in the sintered ceramic.

4 shows by way of example the influence of the residence time in the thermal production process of ZrO ₂ powder. At otherwise identical process conditions, for example, a residence time of 2 hours essentially leads to the monoclinic modification. If this residence time is shortened to 1 hour, in addition to the monoclinic phase, at least a proportionate amount of cubic modification is present. Shortening the residence time further, here for example to 30 minutes, the monoclinic portion continues to decrease and the cubic modification increases.

The peak intensities I _{mono_E} for intensities of the ZrO ₂ particles are at a residence time of 2 hours.

The peak intensities I _{kub_I} and I _{mono_I} represent the intensities of the ZrO ₂ particles at a residence time of 1 hour.

The peak intensities I _{kub_J} and I _{mono_J} represent the intensities of the ZrO ₂ particles with a residence time of 30 min.

In 5 are dilatometer curves (also called sintering curves) SK _A , SK _{B of} a ZrO ₂ material according to the invention and compared to a conventional MgO-PSZ shown. The dilatometer curves SK _A , SK _B describe the sintering process and are measured in a dilatometer on defined test specimens, the linear thermal expansion d1 of a test specimen being determined as a function of the temperature T. The thermal expansion d1 is a measure of how the entropy of a body reacts to changes in volume. For this purpose, specimens with the dimensions of 25 mm by 9 mm by 5 mm were produced in each case from the granules by uniaxial compression of the granules at 100 MPa and subsequent isostatic recompression at 330 MPa. Debinding was carried out at a heating rate of 1 K / min to 1000 ° C. The dilatometric examination of the moldings was then carried out to 1500 ° C at a heating rate of 5 K / min.

5 shows that the ZrO ₂ particles according to the invention or the moldings produced therefrom have a sintering start SB _A of 931 ° C. and a sintering end SE _A (shrinkage of the test specimen is complete, the ceramic bodies are densely sintered as far as possible) at 1281 ° C. The measurement on the test specimens after the test showed a sintering density of 99.2% (max 1500 ° C in the dilatometer). The conventional MgO-PSZ comparative sample showed sintering on SB _B at 969 ° C. The shrinkage is much slower, indicating the lower sintering activity. At the maximum temperature in the dilatometer (1500 ° C), the sintering (shrinkage) is not yet completed, meaning that the sintering end has not yet been reached. On this test specimen (maximum temperature = 1500 ° C), a sintered density of 88.3 ° and an open porosity of 8.78% was determined.

It stands in 5 the sintering curve SK _A , the sintering beginning SB _A and the sintering end SE _A for the course of the ZrO 2 according to the invention.

It stands in 5 the sintering curve SK _B and the sintering start SB _B for the course of the conventional MgO-PSZ comparison pattern.

Example 1)

First, a solution of 45.95 kg of sixty percent acetic acid and 45.95 kg of Zr carbonate is prepared under heating. For this purpose, 0.70 kg of MgO are added and dissolved accordingly. This raw material mixture thus formed is at a throughput of 40 kg / h using a peristaltic pump in a pulsating hot gas flow of a pulsation reactor (known from DE 10 2006 046 803 A1 ) At the beginning of the resonance tube with respect to the hot gas stream directly behind the combustion chamber via a 1.8 mm titanium nozzle finely dusted and thermally abandoned. The temperature at the job site is 850 ° C.

Before entering the separation device, the hot gas flow HGS containing the particles formed is cooled to approximately 160 ° C. by supplying cooling air. As a separator for separating the finely divided particles from the hot gas stream, a bag filter is used. The resulting ZrO ₂ powder is then thermally treated in a rotary kiln at 900 ° C thermally.

The powder material produced in this way has MgO-doped ZrO ₂ particles. The doping of the ZrO ₂ particles is 10 mol%, based on the ZrO ₂ content. The X-ray phase analysis revealed monoclinic, cubic and tetragonal ZrO ₂ as the crystalline phase. The specific surface area is 4 m ² / g. The carbon content is 0.06 wt%. The determined chlorine content is 187 ppm, the sulfur content is 85 ppm. In the quantitative phase analysis by means of Rietveld refinement, 68% by weight of tetragonal ZrO ₂ , 18% by weight of monoclinic ZrO ₂ and 13% by weight of cubic ZrO ₂ and 1% further phases were determined.

Quantitative analysis is performed in all examples by Rietveld refinement. The software package AutoQuant ^® from Rirma Seifert is used for this purpose. (Measurement Conditions: Diffractometer TT3003 (GE Inspection Technologies Ahrensburg GmbH & Co. KG); geometry reflection, Theta / Theat; tube: Cu fine focus, maximum power 2200 W; Filter Ni; Detector: Scintillation counter; Radiation CuK; Tube voltage: 40 KV; tube current 30 mA; measuring range 280 mm; increment: 0.03 °; integration time 3 s; angular range 15 ° -80 ° 2θ). The quantification of amorphous phases is critical and strongly error prone by Rietveld refinement. By definition, quantification of the amorphous phases is completely omitted here, and only the crystalline phases are included in the evaluation. Since the automatic evaluation normalizes the result to 100%, therefore, increases the percentage of crystalline phases in the presence of amorphous portions. However, the ratio of the crystalline phases is retained.

From the produced ZrO ₂ powder material is an aqueous slurry having the composition 40 wt .-% ZrO ₂ powder material, 58.5 wt .-% water, 1.5 wt .-% Dolapix CE 64 (dispersing and liquefying agent, Zschimmer & Schwarz GmbH & Co.KG). The dispersion / grinding of this slip takes place in a bead mill (LMZ ZETA RS System, NETZSCH-Feinmahltechnik GmbH). As grinding balls ZrO ₂ grinding balls were used with the size of 0.2 mm to 0.3 mm and a degree of filling of 85% and set a rotor speed of 1000 U / min and a flow rate through grinding chamber of 380 kg / h.

After dispersion / milling, the slurry has a mean agglomerate size of 221 nm. Using X-ray phase analysis, monoclinic, tetragonal and cubic ZrO _{2 are} determined on the dried slurry (dried at 100 ° C.). The main peak of the monoclinic phase (2θ value = 28.2 °) is 286 cps, the main peak of the cubic phase (2θ value = 30.3 °) is 441 cps. Accordingly, there results an intensity ratio V between cubic and monoclinic phase of 0.64. In the quantitative phase analysis by means of Rietveld refinement, 27% by weight of tetragonal ZrO ₂ , 65% by weight of monoclinic ZrO ₂ and 7% by weight of cubic ZrO ₂ and 1% further phases were determined.

1.5% by weight of polyethylene glycol (PEG 4000) as lubricant and 0.5% by weight of Optapix PAF II (Zschimmer & Schwarz GmbH & Co. KG) are added to this slurry, the concentration being based in each case on the ZrO ₂ - Particle is. This slip is converted to granules in a spray granulation at 105 ° C. The resulting granules have a nearly spherical shape, hollow sphere structure and shell formation is not significantly detectable, the average granule size is 50 microns. The phase composition of the inorganic components corresponds to that of the abovementioned dried-up slip. The ZrO ₂ granules produced in this way are suitable for the production of ceramic bodies.

The ZrO ₂ particles may have a hydrogen content bound in the hydrogen-containing phases and attached to the ZrO ₂ particles of 0.09 wt% (corresponding to about 0.82 wt% H ₂ O ⁺ ).

By dilatometric investigation on separately produced from this granules compacts (25 × 7 × 5 mm), a sintering end of 1395 ° C was determined.

The granules produced in this way have a particularly good pressing behavior. From the granules square green compacts (size about 2.7 × 1.2 × 0.6 cm) are prepared by uniaxial compression at 100 MPa and subsequent isostatic recompression at 330 MPa. The green compacts have a density of 50.78% (based on the theoretical density at 0% porosity). No significant cracks were visually detected on the green compacts.

Debinding of these green compacts takes place at a heating rate of 1 K / min to 1000 ° C. For the subsequent sintering, a heating rate of 5 K / min and a maximum sintering temperature of 1450 ° C. were selected. A crack-free ceramic having a relative density of 99.10% and an open porosity of less than 0.1% (based on the theoretically achievable density at 0% porosity) was obtained. On the ceramic structure, a mean grain size of 470 nm was determined by means of a line-cutting method. The bending tensile strength according to DIN-EN-843-1 was 510 MPa for this ceramic.

Example 2)

Ceramics are produced as in Example 1. Here, however, the resulting ceramics are densified by hot isostatic pressing (HIP) at a heating rate of 2 k / min to 1450 ° C and a holding time of one hour there. On the cooled ceramics, a sealant value of 99.82% of the theoretical density and a flexural strength according to DIN-EN-843-1 of 560 MPa are determined.

Example 3)

Analogously to Example 1, a raw material mixture is produced. This raw material mixture in the form of a raw material suspension thus formed is introduced at a throughput of 20 kg / h by means of a peristaltic pump in a non - pulsating hot gas stream of a Sprühgranulierers over a 1.8 mm titanium die and thermally treated, the temperature at the job site is 105 ° C. ,

The powder thus obtained is then thermally treated in a rotary kiln at 900 ° C. In this case, ZrO ₂ particles according to the invention form. The powder material produced in this way has MgO-doped ZrO ₂ particles. The doping of the ZrO ₂ particles is 10 mol% based on the ZrO ₂ - share. With the X-ray phase analysis, in addition to traces (at the detection limit, <2% by weight) of cubic ZrO ₂ as crystalline phase, only the monoclinic ZrO ₂ modification could be determined. The peak intensity for the monoclinic phase (diffraction reflection at 2θ value = 28.2 °) was 1889 cps. The specific surface area is 2 m ² / g. The carbon content is 0.09 wt%. The determined chlorine content is 175 ppm, the sulfur content is 97 ppm. In the quantitative phase analysis by means of Rietveld refinement, 95% by weight of monoclinic ZrO ₂ , 5% by weight of cubic ZrO ₂ and 1% further phases were determined.

The slurry preparation and dispersion / grinding of this slurry is carried out analogously to Example 1. After dispersion / milling, the slurry has an average agglomerate size of 205 nm. The X-ray phase analysis is used to determine monoclinic ZrO ₂ on the dried slurry (dried at 100 ° C.) and cubic ZrO ₂ in traces. The main peak of the monoclinic phase (2θ value = 28.2 °) is 434 cps and the main peak of the cubic phase (2θ value = 30.3 °) is 22 cps. Accordingly, there results an intensity ratio V between tetragonal and monoclinic phase of 19.7. In the quantitative phase analysis by means of Rietveld refinement, 98% by weight of monoclinic ZrO ₂ , 1% by weight of cubic ZrO ₂ and 1% further phases were determined.

The granulation of this slurry is carried out analogously to Example 1. The resulting granules have a nearly spherical shape, hollow sphere structure and shell formation is not significantly detectable, the average granule size is 45 microns. The ZrO2 granules produced in this way are suitable for the production of ceramic bodies.

This granulate has a particularly good pressing behavior. From the granules square green compacts (size about 2.7 × 1.2 × 0.6 cm) were prepared by uniaxial compression at 100 MPa and subsequent isostatic recompression at 330 MPa. The green compacts have a density of 51.25% (based on the theoretical density at 0% porosity). No cracks were detected on the green compacts.

Debinding and sintering were carried out as in Example 1. A crack-free ceramic having a sintered density of 99.19% and an open porosity of less than 0.1% (based on the theoretically achievable density at 0% porosity) was obtained. On the ceramic structure, a mean grain size of 550 nm was determined by means of a line-cutting method. The bending tensile strength according to DIN-EN-843-1 was 489 MPa for this ceramic.

LIST OF REFERENCE NUMBERS

• 2θ

  Bragg angle

  2θ _cub

  Bragg angle of the cubic modification

  2θ _mono

  Bragg angle of the monoclinic modification

  d1

  linear thermal expansion

  I

  peak intensity

  I _{kub_A} , I _{kub_B} , I _{kub_D} , I _{kub_H}

  Peak intensity of cubic modification

  I _{kub_I} , I _{kub_J}

  Peak intensity of cubic modification

  I _{mono_A} , I _{mono_B} , I _{mono_C} , I _{mono_D}

  Peak intensity of the monoclinic modification

  I _{mono_E} , I _{mono_F} , I _{mono_G} , I _{mono_H}

  Peak intensity of the monoclinic modification

  I _{mono_I} , I _{mono_J}

  Peak intensity of the monoclinic modification

  SB _A , SB _B

  sintered beginning

  SE _A

  sintered end

  SK _A , SK _B

  sintering curve

  T

  temperature

Claims (11)

A slurry comprising a liquid medium having dispersed therein solid ZrO ₂ particles doped with from 5 mole% to 15 mole% MgO comprising at least the monoclinic modification, wherein the ZrO ₂ particles have an average agglomerate size in the range of 20 nm to 500 nm and a ratio of the peak intensities (I) between the monoclinic and the cubic modification of greater than 0.3 and at the surface of the ZrO ₂ particles at least one hydrogen-containing compound is attached, wherein the hydrogen content of the hydrogen-containing compounds 0.06 wt. -% to 1.1 wt .-% based on the ZrO ₂ particles, so that the ZrO ₂ particles can form a ceramic structure with a sintering density greater than 95% at a maximum sintering temperature of less than 1500 ° C. A slurry according to claim 1, wherein the ZrO ₂ particles have the monoclinic modification in a proportion of greater than 30 wt .-%. A slurry according to claim 1, wherein the ZrO ₂ particles have a monoclinic content of greater than 50% by weight and a cubic proportion of less than 50% by weight. Granules comprising solid ZrO ₂ particles doped with 5 mol% to 15 mol% MgO comprising at least the monoclinic modification, the ZrO ₂ particles having a ratio of the peak intensities (I) between the monoclinic and the cubic modification have greater than 0.3 and at least one hydrogen-containing compound is attached to the surface of the ZrO ₂ particles, wherein the hydrogen content of the hydrogen-containing compounds from 0.06 wt .-% to 1.1 wt .-% based on the ZrO ₂ particles is such that the ZrO ₂ particles can form a ceramic structure with a sintering density greater than 95% at a maximum sintering temperature of less than 1500 ° C. Granules according to claim 4, wherein the ZrO ₂ particles have the monoclinic modification in a proportion of greater than 30 wt .-%. Granules according to claim 4, wherein the ZrO ₂ particles have a monoclinic proportion of greater than 50 wt .-% and a cubic proportion of less than 50 wt .-%. Granules according to one or more of claims 4 to 6, having a moisture content of 0.1 wt .-% to 3 wt .-%, in addition compounds of hydrogen and oxygen without carbon with a content of 0.5 wt .-% to 10 Wt .-% and a content of carbon, bound in carbon-containing compounds, of less than 2%. Pre-sintered shaped body formed from the slip according to one of claims 1 to 3 and / or from the granules according to one of claims 4 to 7. Ceramic, formed from the slip according to one of claims 1 to 3 and / or from the granules according to one of claims 4 to 7, characterized in that the ceramic structure has a mean grain size in the range of less than 1000 nm and a sintering density of greater than 95% , A process for producing a slurry according to any one of claims 1 to 3, wherein at least one ZrO ₂ powder doped with 5 mol% to 15 mol% of MgO and comprising at least the cubic or monoclinic modification is ground and / or in at least one liquid dispersing medium is dispersed, wherein an energy input during the dispersion and / or grinding is controlled and / or regulated so that the resulting solid ZrO ₂ particles in the slurry after dispersion and / or grinding have a mean agglomerate size in the range between 20 nm and 500 nm, have a monoclinic phase fraction greater than 30 wt .-% and an intensity ratio between the monoclinic and the cubic modification of greater than 0.3 and at the surface of the ZrO ₂ particles at least one hydrogen-containing compound is attached, wherein the hydrogen content of the hydrogen-containing compounds 0, 06 wt .-% to 1.1 wt .-% based on the ZrO ₂ particles. Use of the slip according to one of Claims 1 to 3 and / or the granules according to one of Claims 4 to 7 and / or the presintered shaped body according to one of Claims 8 and / or the ceramic according to Claim 9 for the production or coating of a) medical implants, in particular in dental or joint prosthetics or of b) technical machine components, in particular of bearing parts, engine or turbine parts, sealing parts, nozzle parts or probe parts.

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