CA2020575A1

CA2020575A1 - Sintered material based on aluminum oxide, process for its production and process for its use

Info

Publication number: CA2020575A1
Application number: CA002020575A
Authority: CA
Inventors: Reiner Kunz; Konrad Kampfer
Original assignee: Lonza AG
Current assignee: Lonza AG
Priority date: 1989-07-07
Filing date: 1990-07-06
Publication date: 1991-01-08
Also published as: DD297387A5; DE59004979D1; ATE102907T1; AU640150B2; PT94624A; DK0406847T3; EP0406847A1; US5114891A; FI903437A0; ES2053021T3; EP0406847B1; NO903024D0; JPH0397661A; NO903024L; HUT64931A; US5076815A; RU2021225C1; HU904117D0; HU210296B; AU5866790A

Abstract

ABSTRACT OF THE DISCLOSURE

By adding very-finely-divided or highly-dispersed titanium dioxide or aluminum titanate, optionally in a mixture with aluminum oxide, sintered products of great hardness, whose fracture toughness can be adjusted over a wide range, are obtained from aluminum hydroxide according to a sol-gel process depending on the selection of the sintering conditions. The materials are especially suitable as abrasives or for the production of ceramic powder or components.

Description

r ~ 2V2~r~

INT}:RED MA~ERIA~ BP.SED ON I_~_C) IDE~
P~OS~Eg~ FOR ITS PRODUCTION AND PROCESS FOR ITS USE
Bac:k~round Of The ~nvention 1, Field Of The Inv~ntion The invention relates to a sintered material based on ~luminum oxide, a sol-gel process for its production, and a process for its use as an abrasive, ceramic powder or molded article.

2. Background Art oxidic abrasives, such as, corundum or zirconi~ -~L~ R
~e~Y~W~J usually are produced by melting the oxide or oxide mixture, allowing the melt to solidify, crushing it, and grading it. Drawbacks of the process are its high energy expense and the limited possibility of influencing the mechanical properties of the product, which essentially are determined by the composition and the cooling-of~ conditions.
Sintered materials based on aluminum oxide also are known which are suitable for use as an abrasive. The first o~ these sintered products, as they axe described, ~or example, in U.S.
Patent No. Z,278,442, were produced from pure aluminum oxide or aluminum hydroxide or from bauxite by a fine grinding, optionally adding sintering auxiliary agents or glass phase makers, compactiny and sintering or hot pressing. However, it was shown that the materials thus obtained also exhibit no optimal grinding properties, since their crystallite size is, ~or example, in the range o~ 5 to 20 microns, while substantially smaller crystallites are necessary to reach a maximum hardness and toughness. In West German OS 3,604,848, it was proposed to grind alumina with additives up to a particle size of less than 0.1 micron and then to subject it to a multistage heat and sintering treatment. Although the pro~ess starts from reasonably priced raw materials, the grinding process is very expensive and its feasibility on a arge scale is questionable.
c~ utio~
The problem of ~h~ Ghin~ is avoided by the so-called sol-gel processes, which, for example, are described in U.S.
Patent No. 3,144,827 and West German OS 3,219,607. The previously known embodiments of the sol-gel process generally require the addition of magnesium oxide or a precursor of it or another compound of a spinel-forming metal.
Another known possibility is the addition of up to 60 percent by weight (calculated as ZrO2 and in relation to the sintered product) of zirconium compounds. In this connection, the additives preferably are introduced in the fo~n of soluble salts or alcoholates, which leads to a considerable amounk of organic substance or inorganic acid components, such as, a nitrate, in the sols and gels formed. These compounds must be completely volatilized or decomposed in the heat treatment before the sintering. On the other hand, it is desirable to bring in the additives in the smallest possible amount, and, 2 ~ 7~

if possible, as an oxide or a hydroxide, so that just water must be removed before the sintering.
Further, it is desirable to be able to influence the properties o~ the sintered product, especially its fracture toughness and hardness, ~ Yvrl ~_ ~ ~ in a comparatively simple way, without using numerous different additives.

Broad Descriptio~ of The Invention The main object of the invention is to provide a process for the production of ceramic materials based on aluminum oxide, that can be per~ormed in a simple way and yields products with the use of comparatively small amounts of preferably oxidic additives, which are suitable as abrasives and can be influenced simply and reproducibly in their mechanical properties. Other advantages and objects of the invention are set out herein or obvious herefrom to one skilled in the art.
The objects and advantages of the invention are achieved by the processes and products of the invention.
The process of the invention involves a process for the production of sintered ceramic materials on the basis of alpha-aluminum oxide, that. contains the steps of sol production, gel formation, drying, optionally crushing and grading, as well as sintering. A homogeneous sol of 90 to 99.9 percent by weight, calculated as ~1203 and in relation to ~2~

the sintered product, of an aluminum hydroxide and/or at least a precursor of alpha-aluminum oxide is produced with adding altogether 0.1 t~ lO percent by weight, calculated as Tio2 and in relation to the sintered product, (a) of very finely divided or highly dispersed titanium dioxide, and/or (b) very finely divided or highly dispersed aluminum titanate, and/or (c) a mixture of very finely divided or highly dispersed titanium dioxide and very finely divided or highly dispersed aluminum oxide, and/or (d~ at least one precursor ~orming titanium dioxide and/or aluminum titanate, and optionally of one or more sintering auxiliary agents, grain growth inhibitors, nucleating agents or glass phase makers. The sintering is performed, after a preheatîng phase at 300 to 700~C, at a temperature of 1250 to 1500C~
It surprisingly was ~ound that, by an addition o~
titanium oxide or aluminum titanate in finely distributed form or as a mixture o~ both compvunds or o~ suitable precursors aluminum oxide or aluminum hydroxide~ sols produced in a way known in the art, dense bodies o~ great hardness and toughness and with very ~ine texture can be produced according to the usual working method even without other additives provided that suitable sintering conditions are maintained.

2 ~ 7 ~

It was indeed known (T~ Woiqnier et alO, J. Non Cryst.
Solids, 100, (1988), 325-329) that dense sintering bodies can be obtained from Al203 and TiO2 without further additives, but these consisted of equimolar amounts of Al203 and Tio2 or of e~it~ s a~;h~
Al2TiO5 and ~ only by hot pressing.
The titanium dioxide added according to the invention suitably has an average particle size, expressed as d50 value, of less than 5 microns, preferably less than 1 micron.
Especially preferred is a so-called highly dispersed product, with an average primary particle size of 10 to 50 nm, produced by flame hydrolysis from anhydrous titanium tetrachloride.
Such a product is, for example, obtainable from the Degussa Company under the designation Titandioxid P25 (i.e., titanium dioxide P25~.
Another embodiment of the process according to the invention uses very-finely-divided aluminum titanate with an average particle size, expressed as d50 value, of less than 5 microns, preferably less than 1 micron. Especially preferred i5 again a highly dispersed product with an average primary particle size of 10 to 50 nm.
Very~finely-divided titanium dioxide, such as, from hydrolyzed tetraalkoxytitaniums, obtained from other sources can be used, or a mixture of titanium dioxide and aluminum oxide (instead of aluminum titanate) can be used.
As an aluminum oxide in this case, preferably a highly dispersed aluminum oxide with an average particle size of 5 to 2020~7~

50 nm, which mostly consists of th~ gamma-modi~ication, is used. Such highly dispersed aluminum oxides are produced by flame hydrolysis from anhydrous aluminum chloride and sold, e.g., by the Degussa Company under the designation Aluminiumoxid C (i.e., aluminum oxide C).
The titanium dioxide or the other titanium-containing additives suitably are added to aluminum oxide or aluminum hydroxide sol in such an amount that the titanium oxide content in the finished sintered product is 0.1 to 10 percent by weight. Here, the elementary analytical composition and not the phase composition is meant by the titanium dioxide ontent, i.e., it does not make any difference whether the pr~s~t titanium is ~a~lablq in a finished product as Tio2 or A12TiO5 or in another form. The Tio2 content in the finished sintered product preferably is l to 5 percent by weight.
The titanium dioxide or the other titanium-containing additives suitably are dispersed intensively in the sol, to achieve as homogeneous a distribution as possible. This can be achieved by stirring with a high-speed stirrer or, for example, by treatment in a ball mill or vibration grinding mill. In sols with high solid content, the homogeneous distribution of the additives advantageously is achieved with the help of a pressure mixer, a kneader, an extruder or a similar device.
In addition to titanium dioxide or its precursors or aluminum titanate, of course, still other sintering auxiliary 2~2~

agent~, grain growth inhibitors, nucleating agents or glass phase makers can be added. Such other additives are known from the prior art and comprise, for example, SiO2, MgO or ZrO2 or precursors of these oxides, such as, silicic acid esters, magnesium salts, zirconyl salts or zirconium alcoholates, further MgAl2O4, MgTiO3, alpha-Fe2O3, FeAl2O4, Niol NiTio3l NiAl2O4, alpha-Al2O3, alpha-Cr2O3, CeO2, ZnTiO3, ZnAl204 or Y2O3.
These additional additives preferably are ~e~ ¦added~ in ¦
amounts of 0.001 to 5 percent by weight, in relation to the sintered product, and optionally can modify the properties of the sintered material according to the invention.
The sol then is converted to a gel in a way known in the (pre~r~b~
art and dried at a temperature ~ than 100C. The gel formation preferably takes place by the slow addition of nitric acid, hydrochloric acid or acetic acid. Preferably it is performed in ~lat dishes in a layer thickness of a few centimeters. The drying preferably is performed in a drying oven and, for example, takes several days at 75C. A~ter the drying, the gel can be crushed in a way known in the art, and, provided that the end pxoduct is to be used as an abrasive~
can be graded according to the desired grain size, in which the shrinking during the sintering process suitably is taken into consideration.
The sintering process suitably is performed in several stages. First, the dried gel is heated to a temperature of 300 to 700C for at least 1 hour to remove physically- and æo20~7~

chemically-bound water and other components volatile at this temperature and to convert the hydroxides and hydrated oxides into the oxides. Then, the ma~erial thus calcined is heated further to the sintering temperature, and optionally it can be advantageous to insert a holding time before the actual sintering at ~ temperature somewhat below the sintering temperature, for example, of 1100C. The sintering suitably takes place at a temperature of 1250 to 1500C, and depending on the temperature, a holding time of a few minutes up to several hours is necessary. The optimal holding time also depends significantly on the rates of heating-up and cooling-off and must be determined by tests, as this i5 customary in ceramics and familiar to the expert and one skilled in the art. The sintered material thus obtained consists of crystallites, at least 95 percent of which exhibit a size of less than 4 microns, preferably less than 2 microns. It is penetrated by numerous micropores, which mainly exhibit a size of 10 to 500 nm and which are not interconnected. These micropores partly are found between the crystallites, partly in the individual cryskallikes. Their number can be determined based on scanning electron microscopic photos in ,t~lreo~ r f ~eS
~s~re~l. In general, the material exhibits at least one pore per crystallite.
Fra~ture toughness KlC of this material is high because of the numerous micropores and preferably is at least ~.5 jt~

MPa-m1~2. The microhardne~s o~ this material pref~rably is at least 16 GPa.
In the X-ray diffraction diagram of this material, the presence of a rutile phase can be seen in Tio2 contents starting from about 2 pexcent by weight.
After the sintering, by a h~at treatment at a temperature which is below the sintering temperature, preferably about lOO~C lower, and preferably lasting 1 to 8 hours, the number of micropores can be greatly reduced. It is even possible to make them largely disappear. The crystallite size increases in this process and, however, one can obtain a product with an average crystallite size of 2 microns, for example, in which at least 95 percent of the crystallites are smaller than 4 microns, and which is essentially free of micropores. By the additional heat treatment, the microhardness increases to preferably at least 18 GPa, while the fracture toughness returns to values of 2 to 2.5 MPa-m1/2.
After the heat treatment, no more diffraction lines of rutile are recognizable in the X-ray diffraction diagram.
Independently of the micropores, the sintered products produced accord.ing to the invention still exhibit closed, sizable pores. The densities determined by pycnometry therefore are influenced by the degree of crushing of the samples. But for the preferred uses as abrasive or for the production of ceramic powders, this means no significant drawback.

The sintsred materials according to the invention preferably are used as abrasives, and there~ore both in loose blAst ;n~ orm, depending on grain size, for exampla, as ~ grain or i~ co~t~ o~bYa~
lapping grain, and ~ abrasiveS ~ such as in ;ve paper or ~ cloth or in ceramic bonded or synthetic resin bonded abrasive wheels, such as, grinding wheels, cutting o~f wheels or ~ wheels. Another preferred use of the sintered materials according to the invention is for the production of ceramic powders, which are processed further into ceramic components in a way known in the art or are used as ceramic plasma spray powder. Other preferred uses of the sintered materials according to the invention are ~or the production of ceramic cutting tools for machining, especially of metals, as well as the production of grinding media, for example, for ball mills.

Brief Descriptlon Of The Drawing~
In the drawings:
Figures 1 to 12 are electron microscope photographs o~
materials in the examples.

Detailed De cription Of The Invention The following examples illustrate the invention.

2 ~ 7 ~

ExampleS 1 to_2 General Instructions: .
(Cohd~c~ tyJ
In a mixing vat, 20 1 of demineralized water (smaller than or equal to 5 microS) was introduced and 5 kg of Pural~
SB (Condea Chemie GmbH) was stirred in with a propeller mixer (n = 1000 min 1). Then, the appropriate amount vf highly dispersed titanium dioxide P25 (Degussa AG) was added and the low-viscosity mixture thus obtained was intensively dispersed for at least 24 hours. Then, by adding about 0.75 1 of concentrated acetic acid, the pH was adjusted to 3.5, and the mixture was poured into flat dishes of a layer thickness of 2 to 3 cm. At room temperature, a punctureproof gel formed within 45 to 60 minutes, which was dried at 75C within 2 to 3 days in a drying oven. The dried gel was sintered in large ~sf f ;r~ k~h pieces in a ~ m~h~ a~rnaa~ under various conditions.
The characterizing of the sintered product took place by determining the following properties:
a) Density, pycnometrically in xylene on crushed samples of grain size 0 to 0.5 mm.
b) Microhardness according to Vickers (indentation force 1 N) (on polished surfaces).
c~ Macrohardness according to Vickers (indentation force 100 N) (on polished surfaces).
d) Fracture toughness KlC according to Anstis and Niihara (on polished surfaces).

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e) Crystallite and ~ore size by scanning electron . ro~ct~r~ol s~tt~e~
microscope photographs of ~ (Fiyures 1 to 7~.

Sinterinq Conditions and_Results-Examples l_to 3 The amount ~ titanium dioxide P ~ s 0.1 percent by weight (Example 1), 1 percent by weight ~Example 2) and 2 percent by weight (Example 3), each in relation to the Pural0 used. The dried gel was heated within 1 hour to 400C, kept at such temperature for 1 hour, then heated within 30 minutes to 1100C, kept at such temperature for 3 hours, heated within 30 minutes to the end temperature of 1300C, kept at such temperature for 15 minutes and finally cooled off. Ths properties of the materials thus obtained are compiled in Table 1:

202~ 7~

Tab~le 1 Example 1 Example 2 Exa~ple 3 Amount of Tio ~ % by weight~
added in re~ation to Pural~ SB 0.1 ~ 2 Density tg/cm3] 3.4 3.61 3.69 Microhardness [GPa] 4.5 17.9 18.4 Macrohardness [GPa] nd 15.1 15.7 K~c [MPa m1/2] nd 4.6 5.1 Average crystallite size [microns] wormlike 0.7 0.7 d = 0.4 NOTE: nd = not determined ~ro.t,~ S~ C~ S
Scanning electron microscope photos of ~ of the products according to Examples 1 and 3 (Figures 1 and 2) illustrate the structure of the sintered materials.

Examples 4 to 6 af ( o~
The amount ~dcd-t~ titanium dioxide P25' was 2 percent hy weight (Example 4), 3.5 percent by weight (Example 5) and 5 percent by weight (Example 6), each in relation to the Pural~
used. The dried yel was heated with 1 hour to 400C and kept at such temperature for 1 hour. Then, it was heated within 30 minutes to 1330C, kept at such temperature for 5 minutes and cooled off. The properties of the materials thus ohtained are compiled in Table 2:

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~k~
Example 4 _ample 5 Example,6 Amount of Tio~ t% by wei~ht]
added in re ation to Pural~ SB 2 3.5 5 Density [g/cm3] 3.65a 3.85 3.79 Microhardness [GPa] 18.~ 19.0 18.5 ¦ ~.0 Macrohardness [GPa] 16.5 15.0 16.5 Klc [MPa-m1/2] 5.7 4.9 5.2 Average crystallite size [microns~ 1.0 1.0 1.5 Average micropore size [microns] 0.15 0.15 0.15 f Examp~l,es 7 to_s ~dd~
The amount b~e~ titanium dioxide P2 ~was 2 percent by weight (Example 7), 3.5 percent by weight (Example 8), and 5 percent by weight (Example 9), each in relation to the Pural~
used. The procedure was as in Examples 4 to 6; however, a~ter the holding time of 5 minutes at 13301C, the material was not immediately cooled of~ to room temperature, but kept at 1250~C
~or another 5 hours. The micropores in the sintered product thus substanti.ally disappeared. The other properties of the materials thus obtained are compiled in Table 3:

~0~ J~

~k~
~lm~ xample 8 Exampls 9 Amount of Tio [~ by weight]
added in re~ation to PuralX SB 2 3.5 5 Density [g/cm3] 3.66 3.84 3.80 Microhardness [GPa] 19.6 20.0 l9.4 Macrohardness [GPa] 16.4 15.2 16.5 K~c [MPa m1/2] 2.1 2.0 2.5 Average crystallite size [microns] 2.0 2.0 2.5 Average micropore size [microns] none none none Examples 10 and 11 The production of gel took place as described in Examples 1 to 9; however, instead of the highly dispersed titanium dioxide, an equimolar mixture of highly dispersed aluminum oxide C (Degussa AG) and titanium dioxide P25 (Deyussa AG) corresponding to a composition of 56 percent by weight of Al203 and 44 percent by weight of Tio2, was used. The total amount of this additive mixture was 2 percent by weight, in relation to the weighted sample of Pural~ SB.

Sinterinq Conditions and Results:

Example lO
The dried gel was heated within l hour to 400C, kept at this temperature for 1 hour, heated to 730C within lO minutes and kept at this temperature for 30 minu~es. Within 30 ~100 minutes, the temperature was increased to ~qoc a~d kept at this level for 1 hour, after that, the end temperature of 1310C was reached within 30 minutes and kept for another 10 minutes and finally cooled off to room temperatureO Figure 3 frRct~rQd ~ fqe~
shows a scanning electron microscope photograph o~ a in the product. The properties of the product thus obtained are compiled in Table 4:

Table_4 Density [g/cm3~ 3.66 Microhardness [GPa] 19.0 Macrohardness [GPa] 15.2 KlC [MPa-m1/2] 5.1 Average crystallite size [microns~ 0.7 Average micropore size [microns] 0.15 Example- 11 The control of the temperature took place as described in Example 10; however, the material before the cooling off was kept at a temperature of 1250C for another 5 hours. The micropores substantially disappeared in this subsequent treatment. A scanning electron microscope photograph of a frq~t\~ S~r~C~
~suPY~of the material i5 shown in Figure 4. The other 5 ~ ~

properties o~ the mat~rials thus obtained are ~ound in ~able 5:

Table 5 Density [g/cm3] 3.66 Microhardness tGPa] 20.2 Macrohardness [GPa] 15.4 K~c [MPa ml/2] 2.

Average crystallite size [microns] 2.5 Average micropore size [microns] no micropores Examples 12 and 13 The production of gel took place as described in Examples 10 and 11; instead of the dispersing of the sol with a -fi~C 9r;hCtih'~, X
stirrer, a ~ ~ was performed in a vibration grinding mill with aluminum oxide balls ~ 97% Al2O3, d~l.5 to 2 cm). The grinding duration was 30 minutes; the weiyht ratio of the grinding balls to the Pural0 used was about 10:1. The sintering conditions were the same as in Examples 10 and ~l.
Pigures 5 and 6 show scanning electron microscope photographs ~ `tqct~Q~ Sur~lteS
of ~issure~ of the materials. The propertiPs of the products thus obtained are compiled in Table 6:

Table 6 Example 12 Example 13 Without subsequent With subseouent treatment at 1250C trea~ment at 1250C
Density [g/cm3] 3.7 3.7 Microhardness [GPa] 19.2 20.7 Macrohardness [GPa] 16.0 16.1 Xlc [MPa-ml~2~ 4.9 2.2 Average crystallite size 0.9 2.5 [microns]

Average micropore size 0.15 none [microns]

Examples 14-15 The production of gel took place as described in Examples 1 to 9; instead of the highly dispersed titanium dioxide, 2 percent by weight (in relation to the amount used on Pural~) of aluminum titanate powder (H~ C. Starck, grain size 0 to 10 microns, d50 = 4.5 microns) was used. The dried gel was heated to 400C within 1 hour, kept at such temperature for 1 hour, then heated to 1450C within 3U minutes, kept at such temperature for 10 minutes and .finally immediately (Example 14) or after 5 hours at 1250C (Example 15) cooled off to room temperature. A scanning electron microscope photograph of a .fr~c~r~d ~ of the product of Example 14 is shown in Figure 7.

The properties of the products thus obtained are compiled in Table 7:

2 f3 ~

Table 7 Exam~le 14 Example_15 Without__ubsequent Wîth sub_equent treatment at 1250C treatment at 1250C
Density [g~cm3~ 3.76 3.75 Microhardness [GPa] 19.1 20.0 Macrohardness [GPa] 15.0 15.0 KlC tMPa m ] 6.1 2.4 Average crystallite size 2.0 3.0 ~microns~

Average micropore size 0.2 none [microns]

Example 16 In a mixing vat, 300 ml o~ demineralized water, 100 g of Pural~ SB, 3 g of titanium dioxide P25, 0.53 g of magnesium acetate (tetrahydrate) and 3 ml of glycerin were intensively dispersed within 20 hours. Then, the low-viscosity mixture with about 12 ml of concentrated acetic acid was adjusted to a pH of about 3.5 and poured on a drying ~ Wit}l a layer thickness of about 3 cm. The gelling took place within about 45 minutes at room temperature. The gel was dried in the drying oven at 80C. The dried gel was heated in a ~ -~eeq f~r-h~ k-l~
~om~tion-f~ ~ to 400C within 1 hour, kept at such temperature for 1 hour, then heated to 700C within 1 hour and kept at such temperature for 30 minutes and finally heated to 1310C within 1 hour, kept at such temperature for 10 minutes c~o~eD~ o~.

~02V~7~`
(Q~rOh h~lcr~S~o~ p~O~o~
q ~r~tu~ed ~) of ~issurc~ of the product of Example 16 is shown in Figure 8.
The properties of the product are compiled in Table 8:

Table 8 Density [g/cm3] 3.72 Microhardness [GPa] 21.4 Macrohardness [GPa] 15.0 Klc [MPa m~/2] 5.5 Average crystallite size S
[microns] 2.~ ¦

Average micropore size [microns] 0.2 ExamPles 17 to 20 General Instructions:
In a mixing vat, 400 ml of diluted hydrochloric acid (0.74 percent by weight) was introduced, 100 g of Disperal~
and optionally the additives were added, and intensively stirred for about 50 minutes. The gel formation took place at room temperature after about 2 hours. The gel was dried at 80 C within about 72 hours and then heated to 200C in a drying oven for about another 4 hours. The dried gel was heated to 400C within 1 hour, kept at such temperature for 1 hour, then heated to 700C within 1 hour and kept at such temperature for 30 minutes and finally heated to 1350C within 1 hour, kept at such temperature for 15 minutes and cooled off.
Example 17 (comparison example): Disperal~ without additives.

~02057~

Exam~le_18: Disperal0 with 2 percent by weight o~
titanium dioxide P25.
Example 19: Disperal~ with 1.12 percent by weight of aluminum oxide C and 0.88 percent by weight of titanium dioxide P25.
Example 20: Disperal0 with 2 percent by weight of aluminum titanate ~H. C. Starck). ~r~ttuY~d S~t~S
Scanning electron microscopic photographs of ~ r~ o~ ¦
the materials are shown in Figure 9 tExample 17), Figure lO
(Example 18), Figure 11 (Example 19~ and Figure 12 (Example 20). The properties of the sintered products thus obtained are compiled in Table 9:

Table 9 ~
Example Example Example Example Density [g/cm3] 3.75 3.813.693.69 Microhardness [GPaJ nd 19.218.720.2 (porous) Macrohardness [GPa] nd 15.715.015.0 (porous) K~c [MPa~m1/2] nd 4.7 4.84.2' ' (porous) Average crystallite size about 1 2.5 [microns] wormlike Average micropore size -- 0.250.2 0.2 tmicronSJ
NOTE: nd = ~ R
not ~tt~ ihe~l ~

2~2~

Example 21 From the dried gel produced according to Example 4 with 2 percent by weight of Tio2~ the grain fraction smaller than 200 microns was screened out and calcined at 1200 to 1300C for 4 hours, The calcined product, which was over S0 percent o~
alpha-Al203, was attrited in an attrition mill with alpha-Al203 grinding media (d = 2 mm~ with adding 0.2 percent by weight of magnesium oxide in water ~or about 2 hours, so that a suspension with about 35 percent solid content resulted. The average particle size (d50 value) was about 0.4 micron. The suspension was dehydrated in a filter press; the filter cake was dried. With the sinterable powder thus obtained, a slip was produced according to the ~ollowing formulation:
80 parts by weight of powder and 20 parts by weight of demineralized water were dispersed with 0.7 parts by weight of Dolapix0 ~-anufacturer Zschimmer & Schwarz Company) and 0.2 parts by weight o~ polyvinyl alcohol in a drum mill with Al203 grinding balls ~d = 15 mm) for 3 hours. The weight ratio of the powder:grinding balls was about 1:2. The 51ip was subjected to a wet sifting (screen under~low smaller than 53 microns), to separate a coarse agglomerate, and was poured into plaster molds for crucibles with about a 50 mm diameter and about a 50 mm height and a wall thickness of 2 to 3 mm.
The green compacts were dried at 100C and sintered at 1350C

for 15 minutes. The overall linear shrinkage during drying and sintexing was about 26 percent.
The crucible thus obtained showed the ~ollowing properties:
sintering density: larger than 99 percent of the theoretical density microhardness: 20 GPa K}c 5-6 MPa-m1~2 According to the same process, sample rods for the measurement of the bending strength were produced and g~ound to a cross section of 4x4 mm. The measured bending strength (4-point bending test, distance of the supports 40 mm, distance of the stress points 20 mm) was 700 to 750 M~a.

_ample 22 The procedure was as described in Example 21, however, a spray dryer was used instead o~ a filter press. The properties of the products thus obtained were identical (within the margin of error) to those oE Example 21.

Claims

1. Process for the production of sintered ceramic materials on the basis of alpha-aluminum oxide, that contains the steps of sol production, gel formation, drying, optionally crushing and grading, as well as sintering, characterized in that a homogeneous sol of 90 to 99.9 percent by weight, calculated as Al2O3 and in relation to the sintered product, of an aluminum hydroxide and/or at least one precursor of alpha-aluminum oxide is produced with adding altogether 0.1 to 10 percent by weight, calculated as TiO2 and in relation to the sintered product, (a) of very finely divided or highly dispersed titanium dioxide, and/or (b) very finely divided or highly dispersed aluminum titanate, and/or (c) a mixture of very finely divided or highly dispersed titanium dioxide and very finely divided or highly dispersed aluminum oxide, and/or (d) at least one precursor forming titanium dioxide and/or aluminum titanate, and optionally of one or more sintering auxiliary agents, grain growth inhibitors, nucleating agents or glass phase makers, and in that the sintering is performed, after a preheating phase at 300° to 700°C, at a temperature of 1250°
to 1500°C.

2. The process according to Claim 1 wherein, after the sintering, the product is subsequently treated while at a temperature of 800° to 1300°C, but at least 20°C below the sintering temperature until at least one part of the micropores present after the sintering has disappeared.

3. The process according to Claim 2 wherein the sintering is performed at atmospheric pressure.

4. The process according to Claim 3 wherein the sol is produced with adding 0.1 to 10 percent by weight, in relation to the sintered product, of titanium dioxide of a particle size of less than 5 microns in d50 value.

5. The process according to Claim 4 wherein the titanium dioxide is used in highly dispersed form with an average primary particle size of 10 to 50 nm.

6. The process according to Claim 5 wherein the titanium dioxide is used in a mixture with aluminum oxide of a particle size of less than 5 microns in d50 value.

7. The process according to Claim 6 wherein the titanium dioxide is used in a mixture with highly dispersed aluminum oxide of an average primary particle size of 10 to 50 nm.

8. The process according to Claim 3 wherein the sol is produced with adding 0.1 to 10 percent by weight, calculated as TiO2 and in relation to the sintered product, of aluminum titanate of a particle size of less than 5 microns in d50 value.

9. The process according to Claim 8 wherein the aluminum titanate is used in highly dispersed form with an average primary particle size of 10 to 50 nm.

10. The process according to Claim 9 wherein the sol is produced with adding 0.001 to 5.0 percent by weight, in relation to the sintered product, of one or more sintering auxiliary agents, grain growth inhibitors, nucleating agents or glass phase makers from the group consisting of SiO2 or precursors, MgO or precursors, ZrO2 or precursors, MgAl2O4, MgTiO3, alpha-Fe2O3, FeAl2O4, NiO, NiTiO3, NiAl2O4, alpha-Al2O3, alpha-Cr2O3, CeO2, Y2O3, ZnTiO3 and ZnAl2O4.

11. The process according to Claim 10 wherein a very finely divided boehmite or alpha-aluminum hydroxide is used as aluminum hydroxide or precursor of alpha-Al2O3.

12. The process according to Claim 1 wherein the sintering is performed at atmospheric pressure.

13. The process according to Claim 1 wherein the sol is produced with the adding 0.1 to 10 percent by weight, in relation to the sintered product, of titanium dioxide of a particle size of less than 5 microns in d50 value.

14. The process according to Claim 1 wherein the sol is produced with the adding 0.1 to 10 percent by weight, calculated as TiO2 and in relation to the sintered product, of aluminum titanate of a particle size of less than 5 microns in d50 value.

15. The process according to Claim 1 wherein the sol is produced with the adding 0.001 to 5.0 percent by weight, in relation to the sintered product, of one or more sintering auxiliary agents, grain growth inhibitors, nucleating agents or glass phase makers from the group consisting of SiO2 or precursors, MgO or precursors, ZrO2 or precursors, MgAl2O4, MgTiO3, alpha-Fe2O3, FeAl2O4, NiO, NiTiO3, NiAl2O4, alpha-Al2O3, alpha-Cr2O3, CeO2, Y2O3, ZnTiO3 and ZnAl2O4.

16. The process according to Claim 1 wherein a very finely divided boehmite or alpha-aluminum hydroxide is used as aluminum hydroxide or precursor of alpha-Al2O3.

17. Sintered material based on alpha-aluminum oxide and optionally sintering auxiliary agents, grain growth inhibitors, nucleating agents or glass phase makers, characterized by a content of 90 to 99.9 percent by weight of Al2O3 and 0.1 to 10 percent by weight of TiO2, which optionally is present in the form of aluminum titanate, and by a crystal size of at least 95 percent of all crystallites of less than 4 microns.

18. The sintered material according to Claim 17 characterized by a content of 95 to 99.0 percent by weight of Al2O3 and 1 to 5 percent by weight of TiO2, which optionally is present in the form of aluminum titanate.

19. The sintered material according to Claim 18 characterized by a crystallite size of at least 95 percent of all crystallites of less than 4 microns.

20. The sintered material according to Claim 19 characterized by a microhardness of at least 18 GPa and a fracture toughness of at least 2 MPa?m1/2.

21. The sintered material according to Claim 18 characterized by a crystallite size of at least 95 percent of all crystallites of less than 2 microns and micropores with a diameter of 10 to 500 nm.

22. The sintered material according to Claim 21 characterized by a microhardness of at least 16 GPa and a fracture toughness of at least 4.5 MPa?m1/2.

23. The sintered material according to Claim 22 wherein it contains 0.001 to 5.0 percent by weight of one or more sintering auxiliary agents, grain growth inhibitor, nucleating agent or glass phase maker from the group consisting of SiO2, MgO, ZrO2, MgAl2O4, MgTiO3, alpha-Fe2O3, FeAl2O4, NiO, NiTiO3, NiAl2O4, alpha-Al2O3, alpha-Cr2O3, CeO2, ZnTiO3, ZnAl2O4 and Y2O3,

24. The sintered material according to Claim 17 characterized by a crystallite size of at least 95 percent of all crystallites of less than 4 microns.

25. The sintered material according to Claim 17 characterized by a crystallite size of at least 95 percent of all crystallites of less than 2 microns and micropores with a diameter of 10 to 500 nm.

26. The sintered material according to Claim 17 wherein it contains 0.001 to 5.0 percent by weight of one or more sintering auxiliary agents, grain growth inhibitor, nucleating agent or glass phase maker from the group consisting of SiO2, MgO, ZrO2, MgAl2O4, MgTiO3, alpha-Fe2O3, FeAl2O4, NiO, NiTiO3, NiAl2O4, alpha-Al2O3, alpha-Cr2O3, CeO2, ZnTiO3, ZnAl2O4 and Y2O3.

27. Process comprising using the sintered material according to Claim 17 as loose abrasive.

28. Process comprising using the sintered material according to Claim 17 for the production of bonded abrasives

29. Process comprising using the sintered material according to Claim 17 for the production of ceramic powders.

30. Process comprising using the sintered material according to Claim 17 for the production of ceramic components.

31. Process comprising using the sintered material according to Claim 17 for the production of tools for machining.

32. Process comprising using the sintered material according to Claim 17 for the production of grinding media.