PROCESS FOR PRODUCING A NON-DENSE SINTERED CERAMIC MOLDED BODY HAVING AT LEAST TWO LAYERS

Abstract

A process for producing a non-dense sintered ceramic molded body having at least two layers, wherein a first powdery ceramic material forming a layer is contacted with at least a second powdery material forming at least a second layer; said first powdery material has a presintering temperature T.sub.1 that is higher than the presintering temperature T.sub.S of said at least second powdery ceramic material; the course of a curve of shrinkage S.sub.1 of said at least first powdery ceramic material differs from the course of a curve of shrinkage S.sub.2 of said at least second powdery material, wherein curve of shrinkage S.sub.1 is shifted towards higher temperatures as compared to curve of shrinkage S.sub.2; and the layers are subjected to a common temperature treatment at a presintering temperature T.sub.S that is lower than the presintering temperature T.sub.1 and at least equal to T.sub.3 to cause sintering that remains in a stage of sintering that has not proceeded to the theoretical density; wherein the curve of shrinkage S.sub.1 is modified by admixing at least one component having a curve of shrinkage S.sub.3 which material is compatible with said powdery ceramic material into said first powdery ceramic material, i.e. has a grain size smaller than the first powdery ceramic material, to equalize the curves of shrinkage S.sub.1 and S.sub.2 in the region of the presintering temperature T.sub.S.

Claims

1. A non-dense sintered ceramic molded body having at least two layers, obtainable by a process, wherein a first powdery ceramic material forming a layer is contacted with at least a second powdery material forming at least a second layer; said first powdery material has a presintering temperature T.sub.1 that is higher than the presintering temperature T.sub.S of said at least second powdery ceramic material; the course of a curve of shrinkage S.sub.1 of said at least first powdery ceramic material differs from the course of a curve of shrinkage S.sub.2 of said at least second powdery material, wherein curve of shrinkage S.sub.1 is shifted towards higher temperatures as compared to curve of shrinkage S.sub.2; and the layers are subjected to a common temperature treatment at a presintering temperature T.sub.S that is lower than the presintering temperature T.sub.1 and at least equal to T.sub.3 to cause sintering that remains in a stage of sintering that has not proceeded to the theoretical density; wherein the curve of shrinkage S.sub.1 is modified by admixing at least one component having a curve of shrinkage S.sub.3 which material is compatible with said powdery ceramic material into said first powdery ceramic material, i.e. has a grain size smaller than the first powdery ceramic material, to equalize the curves of shrinkage S.sub.1 and S.sub.2 in the region of the presintering temperature T.sub.S.

2. The molded body according to claim 1, wherein said first powdery ceramic material contains yttrium-stabilized zirconia.

3. The molded body according to claim 1, wherein said at least second powdery ceramic material contains colored yttrium-stabilized zirconia.

4. The molded body according to claim 1, wherein said component compatible with said first powdery ceramic material, which is admixed with said first powdery ceramic material, has a higher sintering activity and optionally a higher BET surface area than those of the first powdery material.

5. The molded body according to claim 1, wherein T.sub.1 is within a range of from 900 C. to 1300 C.

6. The molded body according to any claim 1, wherein T.sub.S is within a range of from 850 C. to 1250 C.

7. The molded body according to claim 1, wherein T.sub.3 is within a range of from 850 C. to 1200 C.

8. The molded body according to claim 1, wherein the porous sintered molded body obtained is further processed by forming processes.

9. The molded body according to claim 1 comprising two or more layers, wherein all the layers are dimensionally stable in the porous sintering.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] FIG. 1 shows schematically the shrinking behavior for 3 different materials during sintering (in a representation of three curves of shrinkage).

[0026] FIG. 2A schematically shows a first embodiment of a three-layer block according to the invention;

[0027] FIG. 2B schematically shows a second embodiment of a three-layer block according to the invention;

[0028] FIG. 2C schematically shows a second embodiment of a three-layer block according to the invention; and

[0029] FIG. 2D schematically shows a second embodiment of a three-layer block according to the invention.

DETAILED DESCRIPTION

[0030] The invention is explained in more detail in the following. FIG. 1 schematically shows the course of the curves S.sub.1, S.sub.2 during sintering of the first and second powdery ceramic materials. The presintering temperatures of the first and second powdery ceramic materials are designated as T.sub.1 and T.sub.S. Presintering temperature means the temperature that is lower than the corresponding sintering temperature of the respective material at which dense sintering to the theoretical density or at least to very close to the theoretical density occurs. The curve of shrinkage of the component compatible with the first powdery ceramic material is designated as S.sub.1 in FIG. 1. T.sub.S is the presintering temperature at which the layers forming the molded body are subjected to the common temperature treatment to form a porous sintered molded body.

[0031] For example, the first powdery ceramic material is white or only weakly colored, and the second powdery ceramic material represents a more intensely couloured powdery ceramic material having a grain size in the range of the first powdery ceramic material. The third curve S.sub.3 has a smaller grain size than the first powdery ceramic material. The curves of shrinkage S.sub.1 and S.sub.2 of the two powdery ceramic ground materials are so much different that the disadvantageous distortions of the first and second layers occur.

[0032] Surprisingly, admixing a component with a curve of shrinkage S.sub.3 compatible with the first powdery ceramic material to the first powdery ceramic material can equalize its curve of shrinkage S.sub.1 to the curve of shrinkage S.sub.2 of the second powdery ceramic material within the range of the selected presintering temperature, or even achieve identical curves of shrinkage. The shifting of the curve of shrinkage S.sub.1 depends on the level of admixing the component compatible with the first powdery ceramic material. The compatibility of the component with curve S.sub.3 is related with its smaller grain size compared with the first powdery ceramic material. In particular the difference in grain size of the two materials is at least 5%, more particularly 10% or 20% or even 30% or more such as 40%.

[0033] Said component compatible with the first powdery ceramic material may be, for example, a zirconia material containing, in addition to yttria in the percent range, especially 6-7%: less than one percent of alumina, traces of silica, and iron oxide as a pigment, wherein less alumina is present as compared to the first powdery ceramic material. The particle size of the component is smaller than that of the first powdery ceramic material, and typically the size of the particles of the component is about half the size of the particles of the first powdery ceramic material, whose particle size is within a range of 80-100 nm. The BET value of the component to be admixed is about double the BET value of the first powdery ceramic material. Typical BET values for the component to be admixed are 11-15 m.sup.2/g. Such materials are commercially available and are distributed under the designation TZ-PX-242A or Zpex from TOSOH, Japan, for the component to be admixed, and under the designation TZ-3YSB-C, TOSOH, Japan, for the first powdery ceramic material. The material forming component 2 is the at least second powdery ceramic material according to the present invention, being provided with a higher pigmentation, typically in the form of iron oxide, as compared to the first powdery ceramic material. Iron oxide has a yellowish color.

[0034] The following table shows properties of a special first and second powdery ceramic material.

TABLE-US-00001 TZ-PX-242A TZ-3YSB-C Crystallite Size [nm] 32 36 Particle size D50 [m] 0.43 0.60 Granule Size [m] 52 60 Bulk Density [g/cm.sup.3] 1.06 1.20 Specific surface area [m.sup.2/g] ca. 12 ca. 6

[0035] The component compatible with the first powdery ceramic material is admixed with the first powdery ceramic material in proportions within a range of 25 percent by weight to 50 percent by weight (especially from 30 to 40% by weight) in order to achieve the effect of equalizing the curves of shrinkage as desired according to the invention.

[0036] The molded bodies according to the invention, which can be prepared by applying the process according to the invention, advantageously show no distortions that are due to sintering distortion after completion of the presintering, so that the molded body in this porous sintered state can be processed by material removal, and does not experience any deformation after dense sintering.

[0037] In a three-layer stacking in FIGS. 2A-2D, the molded body according to the invention is schematically represented together with information about the layer composition and the color values of the individual layers according to the CIE-Lab color system. Component 1 according to FIG. 2A is a powdery ceramic material forming the first layer 1 and being a mixture consisting of white granules, for example TZ-3YSB-C and TZ-PX-242A, commercially available from TOSOH, Japan. The middle layer 2 is formed from a mixture of component 1 and a component 2. Component 2 is more intensely yellow/brown pigmented granules, for example, TZ-Yellow-SBC. In the embodiment according to FIG. 2A, layer 2 consists of a mixture of 80% component 1 and 20% component 2. The middle layer 3 is formed from a mixture of components 1 and 2 in a proportion of 60%/40%. The further embodiments shown in FIGS. 2B-2D are each characterized by a more intense pigmentation.

[0038] The molded body according to the invention is advantageously prepared by intimately mixing the first powdery ceramic material with the component compatible with this material. The mixed material is filled into a mold, the forming layer is smoothed and then covered with a second layer of a mixture of component 1 and component 2. This step is repeated to form the third layer. The material is compacted under pressure (typically 100 to 200 MPa) and then released from the mold.

[0039] This is followed by a sintering process in which the temperature is raised. The temperature raise is stopped at a temperature selected in such a way that the molded body obtained can be readily machined, and subjected to dense sintering after processing. The presintering temperature is within a range of from 800 C. to 1200 C., especially from 950 C. to 1150 C., 1000-1100 C., or 1040-1080 C.

[0040] For facilitating the machining, the molded body according to the invention is provided with relevant elements known to the skilled person that are suitable for fixing the block within a CAD/CAM machine.

[0041] In a further embodiment of the process according to the invention, the porous sintered molded body obtained is further processed by forming processes.

[0042] The present invention also relates to a molded body obtainable by the process according to the invention. Advantageously, the layers are dimensionally stable in the porous sintering.

Example 1

[0043] Component 1 is obtained by intimately mixing 650 g of TZ-3YSB-C from TOSOH, Japan (T.sub.11076 C.), with 350 g of TZ-PX-242A from TOSOH, Japan (T.sub.31038 C.), in a mixer from the company Bachofen, type DynaMix CM200 (or CM100, CM500, CM1000). For forming by means of an axial compression method, component 1 is filled into the stamper by means of a filling unit known to the skilled person. The layer is uniformly spread in the mold by pulling away the filling unit. Typically, the layer thickness is higher than the desired layer thickness of the final product because of the bulk density being lower than the compressed density, and depends on the raw materials employed. Typically, the filled-in layer thickness of the powder is 2.318 times the corresponding layer thickness of the compressed component and is 14.88 mm. Onto this layer, 22.29 g of a mixture of 80% by weight component 1 and 20% by weight component 2, which consists of TZ-Yellow-SBC from TOSOH, Japan, is applied. After the second layer has been filled in, it is also smoothed and thereafter covered by the third layer consisting of the intimately mixed components 1 and 2 in a mixing ratio of 60% by weight to 40% by weight. Optionally, smoothing is again performed. Thereafter, the layered structure in the mold is compacted under a pressure of 200-165 MPa to a density of 3.13 g/cm.sup.3.

[0044] After the compressed molded body has been released, it is subjected to sintering until a temperature (T.sub.S) of 1060 C. is reached. The molded body can be processed into multi-unit bridges by means of CAD/CAM methods. The bridges obtained are dense sintered at temperatures of 1530 C.

The corresponding colors of the individual layers can be seen from FIG. 2A.

[0045] The sintered block is provided with a holding member. This is realized by adhesive-bonding the block and holding member. The porous ceramic component is placed by means of a centering device onto the holding member fixed in a base plate and provided with adhesive, and is thus fixed.

Example 2

[0046] The blocks shown in FIGS. 2B to 2D were prepared by analogy with Example 1, the following mixing ratios of components 1 and 2 being employed:

TABLE-US-00002 Layer 1 Layer 2 Layer 3 Block Comp. 1 to Comp. 1 to Comp. 1 to according comp. 2 comp. 2 comp. 2 to [% by weight] [% by weight] [% by weight] FIG. 2B 80 to 20 60 to 40 40 to 60 FIG. 2C 60 to 40 40 to 60 20 to 80 FIG. 2D 40 to 60 20 to 80 0 to 100

[0047] The following Table illustrates the quantitative ranges in which the component compatible with the first powdery ceramic material can be mixed with the first powdery ceramic material in order to achieve the desired presintering temperature in the production process and to arrive at molded bodies that are according to the invention.

TABLE-US-00003 Proportion of Proportion of Proportion of TZ-3YSB-C TZ-PX-242A TZ-Yellow-SB-C in layer 1 in layer 1 in layer 2 Presintering [% by [% by [% by temperature Example weight] weight] weight] [ C.] 1 70 30 100 1066 2 60 40 100 1066 3 50 50 100 1066