DE2708211A1 - CERAMIC BODY AND ITS PRODUCTION - Google Patents

Description

BEETZ-LAMPRECHT-BEETZ PATENTANWÄLTE

8OOO Munich 22 - Stelnsdorfstr. IO oipi.-ing. r.Beetz aen.

TELEPHONE (O88) aa 78 OI - 39 73 44-as Ββ 1O Dlpl.-lng. K. LAMPRECHT Telex BaaO48 telegram Allpatent MDnOhWi 270821 1 Dr.-Ing. R. BEETZ Jr. Dlpl.-Phy ·. U. HEIDRICH

3 Dr.-Ing. W. Tl M PE

Dipl.-Ing. J. SIEaFRIED

293-26.568P February 25, 1977

National Research Development Corporation, LONDON (Great Britain)

Ceramic body and its manufacture

The invention relates to improved reactively, ie by reaction, bonded ceramic bodies made of silicon nitride, in which the influence of oxidation on the strength of the ceramic body is reduced.

Ceramic bodies made from silicon nitride are currently manufactured either by compression or reactive bonding. Hot-pressed silicon nitride ( 'hot-pressed silioon nitride ^1, HPSN) is conventionally prepared by compressing Siliciumnltridpulver, containing a small amount of magnesium oxide or other suitable flux in carbons at temperatures of about 1750 ⁰ C. This procedure produces a ceramic body of excellent strength, strengths of over 900 MN / m ^{2 being} achievable

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However, driving the "" '"is for application to more complex ones Forming ceramic bodies is very complex in that it is a stage of painstaking processing for the production of precision products by diamond grinding is required.

Reactively bonded silicon nitride (so-called "reaction bonding silicon nitride, RBSN.) Can be prepared by reaction of compacts of silicon powder with nitrogen at 1250-1450 ⁰ C are prepared. Reactively bonded silicon nitride (hereinafter referred to as RBSN for short) is of particular interest mainly because the compact used can easily be produced from silicon powder in the required precise shape when the material is still soft and easy to process, since the dimensional change in the course of the Binding reaction is only very low (<0.05 %)

The oxidation resistance of silicon nitride is generally good because the initially high reaction rate drops rapidly to a low value according to a quadratic law. However, the oxidation appears to have a marked influence on the strength of ceramic bodies made of silicon nitride of the reactively bonded type. In experimental investigations by the inventor, reductions in strength at room temperature of up to 54 % following an oxidation period of 100 hours at 1250 ^° C. were observed. This loss of strength at room temperature as a result of oxidation, i.e. the oxidative loss, can appear in the case of RBSN materials after a certain period of use at normal working temperatures for these materials.

It can be assumed that the

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Formed silicon dioxide crystallized into ß-Cristoballt. This product is thermally unstable; When the material cools after a period of time under oxidizing temperature conditions, it is subject to ^{a strong volume contraction in the temperature range of 280 200 0} C with the formation of ot cristobalite, which represents the low temperature door form. This change in crystal shape, as it is associated with a corresponding change in volume, leads to the formation or intensification of cracks and cracks due to fracture or microstress mechanisms and is responsible for the oxidative-induced loss of strength at room temperature.

A previously known method for reducing the loss of strength caused by oxidation at room temperature of RBSN consists in the removal of the friable white surface deposit by superficial grinding, the normally found on such materials after manufacture. This leads to a favorable reduction the loss of strength due to oxidation, however, is only applicable to bodies with a fairly simple shape. About that In addition, any machining after the nitriding step runs counter to the main benefit of RBSN, which is it is based on the fact that no further processing is required for shaping.

Various additives have also already been specified which influence the strength of RBSN at room temperature. For example, aluminum oxide was specified as an additive to reduce oxidation; from carried out within the scope of the invention Experimental studies show, however, that the reduction in oxidation achieved thereby does not lead to a significant reduction in the strength loss caused by oxidation

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, lust leads when the oxidation temperatures will be more than 1000 ⁰ C at room temperature. Furthermore, chromium oxide was added to ceramic bodies to fill open pores, for example by the American so-called K-ramic process, but the results of the experimental investigations show that the addition of chromium oxide only slightly reduces the strong oxidative loss of strength at room temperature.

The invention accordingly outputs / ceramic body

reactively bonded silicon nitride, which are impregnated at least in the surface zone with an additive that the loss of strength of the silicon nitride caused by oxidation and that of the metal oxides calcium oxide, strontium oxide, barium oxide, mixtures of these oxides with iron oxide and a mixture of magnesium oxide and aluminum oxide corresponding in the proportions to the spinel MgAlpOfc is selected.

The invention also provides a method of manufacture of the ceramic body, which is characterized in that reactively bonded silicon nitride with an in the Metal oxide transferable compound is impregnated; the compound is preferably a salt of the metal. The salt is preferably a nitrate, since these compounds produce only nitrogen oxides and oxygen-containing gaseous compounds Products are easily thermally decomposed; Sulphates are less suitable for this, as they are less like chlorides are easily decomposable and can form compounds with silicon.

The impregnation of the reactively bonded silicon nitride

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is preferably carried out, if it is an anhydrous salt, by incorporation into the molten salt of the corresponding metal or in the case of a water-containing salt into a boiling solution of the corresponding metal salt in its own crystal water.

After the impregnation, the reactively bonded silicon nitride can be dried by heating to remove crystal water and for calcination with decomposition of the metal salt.

The preferred oxidi see additives include Calcium oxide, strontium oxide and mixtures of calcium oxide and iron oxide. The stoichiometric mixture of MgO and AIgO is also very effective in reducing the loss of strength of RBSN caused by oxidation.

The amount of oxide used can be varied within a wide range and is not particularly important decisive influence on the property improvement achieved according to the invention. Since the impregnated oxide is located within a superficial zone of the ceramic body, the amount of oxide is most favorable to the surface of the ceramic body are related. As a general reference value, the inventors determined that oxide amounts in the range of 1 to 10 mg / cm surface of the ceramic body are effective according to the invention.

The invention is described below with reference to experiments relating to the implementation of the invention, explained in more detail.

To determine the influence of the impregnation of RBSN with various additives on the oxidative

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ixust of the material were test samples from each Approach divided into three groups. These groups are (a) the untreated (i.e. only nitrided) control samples, which have been tested to determine the natural strength of each batch for comparison purposes, (b) the untreated (i.e. only nitrided and uncalcined) control samples that were used for determination before the test the normal oxidation-induced reduction in strength of the material, as well as (c) samples that have been oxidized before the Oxidation and the test of one of the impregnation treatments (including calcination).

The RBSN samples were made as 30.5 x 4.6 χ 4.6 mm rods made of silicon powder with a mean effective particle diameter of 25.5 μm by isostatic pressing at 185 MPa, argon sintering, form grinding and nitriding to produce the RBSN Ceramic body made. The calcination of the samples was carried out at 850 ⁰ C.

The samples of group (c) intended for impregnation were given one of the following impregnation treatments prior to oxidation subjected:

Impregnation with calcium oxide:

Calcium nitrate hydrate of the formula Ca (NO-Z) ₂ ^ H ₂ O was heated until it dissolved in its own crystal water. Then RBSN rods were immersed in the solution, which was heated to boiling for 30 min. The rods were then removed by wiping übersohtisalger liquid, 2 h at 65 ⁰ C and dried in air to convert

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of calcium nitrate to calcium oxide at 850 ⁰ C calcined.

Impregnation with strontium oxide:

Rod-shaped samples of RBSN were immersed in molten anhydrous stronitol nitrate of the formula Sr (NO,) ₂ , which was then heated to boiling for 30 minutes. The rods were then wiped, dried and calcined as described above.

Impregnation with barium oxide;

Rod-shaped samples of RBSN were immersed in molten anhydrous barium nitrate of the formula Ba (N0,) ₂ , which was then heated to boiling for 30 minutes. The rods were then wiped, dried and calcined as described above.

Impregnation with a calcium oxide iron (lli) oxide

Mixture:

10 g calcium nitrate hydrate of the formula Ca (NOO ₂ ^ H ₂ O and 24.4 g iron (ni) nitrate hydrate of the formula Fe (NO ₅ ) y 9H ₂ O were heated until the solids were dissolved in their own crystal water Rod-shaped samples of RBSN were immersed in the liquid, which was then heated to boiling for 30 minutes, after which the rods were wiped, dried and calcined as described above.

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Impregnation with a magnesium oxide-aluminum oxide mixture:

10 g of magnesium nitrate hydrate of the formula Mg (NO,) ₂ and 21.2 g of aluminum nitrate hydrate of the formula Al (NO,) were heated until the solids had dissolved in their own crystal water. Rod-shaped RBSN samples were immersed in the liquid, which was then heated to boiling for 30 minutes. Magnesium nitrate hydrate and aluminum nitrate hydrate were used in this impregnation test in stoichiometric proportions corresponding _{to the spinel MgAl 2 O ^.} The rods were then wiped, dried and calcined as described above.

Impregnation with other materials;

Additional rod samples of RBSN were obtained using procedures similar to the other impregnation treatments described above subjected.

In the experiments, the samples were oxidized by exposure to air either in a refractory brick combustion chamber heated with silicon carbide rods or in a mullite tube heated externally via a bundle of silicon carbide rods. In the case of the chamber furnace, the ends of the samples rested on thin pieces of RBSN on porous MI28 aluminum oxide bricks. In the tube furnace, the ends of the samples rested on three rows of RBSN bricks inserted into a thick RBSN tube. ^{The oven temperature was kept at 1250 °} C. for the duration of the oxidation, which lasted 100 h in all experiments.

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The difficulties associated with precisely adjusting the very strong, stiff and non-ductile materials make accurate measurements of tensile strength extremely difficult. For this reason, all samples were subjected to a three-point bending test investigated, with the samples a deformation rate of 0.2 mm / min over hardened steel rollers of 8 mm in diameter were exposed, which were provided at a distance of 19.05 mm in square grooves.

The test results obtained are shown in Tables 1 to 3.

Table 1 shows the flexural strengths at room temperature and the data on the rate of oxidation for RBSN samples which contain additives according to the invention; Similar data are given in Table 2 for RBSN containing other additives for comparison purposes. Table 3 relates to the flexural strength data of different RBSN rods at 1250 ⁰ C, which are impregnated with additives of this invention.

Most of the impregnation attempts given in the tables refer to the impregnation of RBSN with oxides. The aim was that at least some of the investigated oxides react with the silicon oxide formed by oxidation of the RBSN without accelerating this oxidation would. As can be seen from the results in Table 4, this is actually the case. In Table 4 are those by X-ray analysis of the phases determined percentage compositions of various RBSN samples that were used in the oxidation tests. In the case of those impregnated with CaO and SrO RBSN samples show that the additive reacts with the cristobalite in each case and the formation is more significant Prevents amounts of this material from creating a harmful one

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Volume changes due to heating, which is caused by its formation, can be avoided. With spinel as an additive, the mode of action is less clear, but it reacts Spinel apparently with at least part of the cristobalite formed in the RBSN.

The underlying experimental investigations also show that calcium oxide and strontium oxide give significantly better results than barium oxide, while magnesium oxide alone gives practically no improvement. Because of the toxicity risks, beryllium oxide was not included in the experiments. Although magnesium oxide alone shows little effect and aluminum oxide alone was either ineffective or had an unfavorable influence, surprisingly, magnesium oxide and aluminum oxide in the stoichiometric proportions corresponding to the spinel MgAlgO ^ proved to be very effective. The properties of mixtures of calcium oxide and strontium oxide with iron oxide are comparable to the properties of calcium or strontium oxide alone. The addition of iron oxide thus leads in these cases to a certain improvement in strength at room temperature per se , it being found that the impregnation of RBSN with iron oxide alone generally gives the ceramic body greater strength at room temperature.

Some other impregnation treatments that can help increased strength of RBSN at room temperature measured after the oxidation (see Table 2), but were found to be unsatisfactory insofar as they were one caused oxidative weight loss. For example, silver oxide gave a favorably improved strength

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after oxidation, but with considerable weight loss occurred by oxidation; the same applies to a lesser extent to copper (n) oxide. The occurrence of such Weight loss suggests that the RBSN is being attacked in a significant way, although the Loss of strength during oxidation is less than for untreated RBSN, which is perhaps due to the rounding of Break points or corners in pores is caused.

The fracture patterns of impregnated RBSN samples with good Properties after oxidation were determined using a scanning electron microscope (SEM) as well as by energy dispersive X-ray analysis (EDXA) examined. A sample treated with calcium oxide showed a 20-30 Aun thick surface layer with a coarse crystalline appearance, containing calcium, but with calcium except in the silicon nitride microstructure largely lacking one or two large cavities. The surface layer occasionally showed fine, parallel to the Surface cracks where they were adjacent to the silicon nitride. The fracture surface of a strontium oxide treated Stabs similarly showed a strontium-containing surface layer and demonstrated the practical absence of strontium in the rest of the fracture surface. These plague positions are very encouraging in that calcium and strontium did not penetrate the grain boundaries in sufficient quantities could be to cause the well-known decrease in resistance to slip and subcritical crack growth, otherwise usually with calcium impurities is connected in ceramic bodies made of silicon nitride. Examination of the samples impregnated with spinel revealed one Number of discrete areas of the fracture surface that contained magnesium, aluminum, and silicon, as well as clues for

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a superficial occurrence of these elements (as well as some calcium).

The cited experiments show that the impregnation of RBSN with metal oxide additives according to the invention increases leads to a favorable improvement in the strength properties present after the oxidation. The inventive Impregnation procedures as well as the corresponding tests are not limited to the data given, since corresponding modifications are also included in the concept of the invention.

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Tabel

2708?

approach
No.

Impregnation agent

Weight gain after impregnation and calcining

ren

(mg) Flexural strength at room temperature

before

Oxidation (MPa) after oxidation (MPa)

Reduction through oxidation (*)

Weight gain due to oxidation (mg / m ² .h)

1913

1929

1941

nothing BaO CaO

nothing CaO

CaO SrO

SrO

nothing CaO

CaO MgAl ₂ O ₁₁

CaPe ₂ O ₁₁

22.4 40.4

44.5

41.7 48.2

47.6

43.2

40.7 22.2

22.8 35.2

33.2

244 *

238 * 112 165 197

135 203

195 199

199

200

235 115 l80l

V I85j 2Ο3Ί

V 2O8J

V 214J

54 32 20

43

51

254

88

147

483 160

169 210

207

428 154

165 291

284 279

250

nothing

ti

Il

CaO

CaO SrO

SrO MgAl ₂ O ₁₁ MgAl ₂ O ₁₁

Weight it

2.56

2.35 2.79

3.10 1.85

2.17

241 + 8

233) V242

251J

213 * 1 V.

199J

246)

V241 235J 138 * 153

126 210

215 170

165 209

136

139 +

213

168

173

43

Weight, -

18 28

0.02

0.04 2.17

2.32 3.06

3.49 1.94

2.18

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Table 1 »continued

Impreg Weight gain Flexural strength with space 234 temperature 134 Ί36 132Ί 206 Decrease
tion through
oxidation J > 7 I. Weight • 545 approach
No. national . ^ gnie- i ¹² according to Oxi
dation
(MPa) ♦ 8 » 143J ¹³⁸⁺ (*) I.
i 2k
I. increase
by + 143 middle tion and Cal
cinematic
(mg) before
Oxides
tion Oxi
dation
(mg / m ^z «h) (MPa) IN THE? 1965 nothing - 233 2H77
Γ258
270J "129 208 "] l | 2 573 ' Il η _ 2H3 P. I8i)
V183
185J 567 577Ί
) 553
529J 231]
V222
213J 205J 31 Il Il - 241 199 ")
1-194
I89J 495 ^ 300 ")
y 296
292J ti • ι - 218J 47 Il Il 0 275 * 1
> 267
259J ti Il 0.01 It η 0 261]
V256
250J 169Ί
J- 169
169J It MgAl ₂ O ₁ , 2.114 ti η I.9I •1 • ι 2.03 2O4 "l
V 174
1H3J It η I.92 Il CaO, "2.1 * 7 then Pe 0 2.81 2.63
0
. _fc 2.5 ¹ » It SrO, • 1.02 then
Fe _o 0_ J 0.83 2 3 1.06

Not calcined (i.e. results before oxidation, i.e. only after nitration)

+ Calcined at 850 ⁰ C (all impregnated samples were calcined at 85Ο ⁰ C).

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ORIGINAL INSPECTED

Tabel

approach No.

Impregnating

national

middle

Weight gain after impregnation and calcining

ren

(mg) Flexural strength at room temperature

before

Oxidation (MPa) after oxidation (MPa)

Weight gain through

Reduction ¹ ? ΪίΤ _ΛΜ through ^datio 5 oxidation (mg / m η) (*)

1913

1929

nothing
Co ₃ O ₁₁

MgO

CuO LiCl NaPO ₄ NaWO ₃ nothing

CuO

CuO NOK

NOK ZnO

ZnO ZrO ₂

ZrO ₂ Al ₂ O ₃

Al ₂ O ₃ SnO ₂

SnO ₂ MnO ₂

MnO ₂ UO ₂

uo ₂

Al ₂ O ₃

B ₂ O ₃ B ₂ O ₃

^B 2 ° 3 nothing

25.7 25.5 20.5 37.1 57.1 79.7

44.3

42.5 43.4

40.7 61.8

65.0 28.0

29.8 32.9

31.4 31.0

26.2 35.3

38.6 80.2

91.7 28.8

21.3 8.0

241

238 112 106 136 158 108 137 89 135 160

161

151 134

138 156

118 135

139 131Ί

161

153

136

137

150 150

152 130

131

116

137

141

235 138 131

147 115

151

131

127

139

54 57 47 35 53 44 64 43

33 36

43 42 42 41 37 45 47

42 51

257

137

509

- 8th

-196

-134

-375

471

-79

-205 606

558 47

86 221

215 269

170 367

368 362

363 162

164 256

193 405

406 428

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At

Table 2, continued

1941
Il MnO ₂
MnO ₂ 32.6
30.4 (235 *) 145 |
132J } ■■ 241 408
363 Il
Il AgO ₂
AgO ₂ 151.2
165.1 196Ί
174J Ί -1320
r ²¹
J -1429 60
26th Il
It TiO ₂
TiO ₂ 10.3
8.0 861
y 95
104J y 4o 382
306 Il
Il Cr ₂ O ₃
Cr ₂ O ₃ 72.3
70.5 102 ")
[ill
12 IJ } " 263
264 Il
Il MgO
MgO 26.0
24.8 158Ί
H43
12 QJ } ³⁹ Il
Il Co ₃ O ₄ 48.9
51.3 142Ί
133
125J

Tabel

approach
No.

Impregnation
middle

Weight gain after
Impregnation and
Calcining
(mg)

Flexural strength at 1250 ^° C

before oxidation (MPa)

after oxidation
(MPa)

Weight gain due to oxy

Decrease

tion by ^ ^Ä ^. ,

Oxidation ^da ^ ^on

nothing

It
Il

CaO

CaO
SrO

SrO
MgAl ₂ O ₄

0.02 +

0.04 +
2.20

2.26
3.10

3.11
1.95

1.93

218]

T237 256J

153 ")

Vl40 + 126Y

216Ί

V213 210J

180Ί

V187 194J

200 "j

U93

187J

2331

V232
24lJ

1931

V197
201J

185 "!

171
193

201

-66

O.O5 2.34

2.24 3.12

3.23 I.83

I.94

not calcined (i.e. use as nitrided)

+ calcined at 85Ο ⁰ C (all impregnated samples were ^{calcined at 850 0} C)

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Table 4

Composition of various RBSN samples before and after oxidation

(Wt .- ^)

approach
No. State of
analyzed
sample 85
79
76
52 ß-Si ^ N ^ Cristobalite
SiO ₂
34 Siliciurr
Si other
Phases 1929
I929
I941
1941
I941 Surface ana
lysis before oxidation
Analysis of the inside
ren from oxidation
Surface analysis
from oxidation
Analysis of the inside
ren from oxidation
Analysis of the inside
after oxidation 64 * 13th
15th
21
25th
14th nothing
small
lot
18 * I I I - <I I I I I I I929
1929
1941 Surface analysis
of the impregnated with CaO
ned material
after oxidation
Surface analysis
of the impregnated with SrO
ned material
after oxidation
Surface analysis
the with MgAIpOi,
impregnated Ma
terials according to Oxi
dation 18 * CaSiO ₃
CapSiOjj '
SrSiO,
SrSiO ₂
MgAIpOj.
as well as a
further Pha
se, poss
sually
Chlorine! T

Percentages of total weight of OUSi ₅ N ₁₁ , ß-Siyfy and cristobalite alone (i.e. without other phases)

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Claims (9)

Expectations Ceramic body made of reactively bonded silicon nitride, characterized in that it is impregnated at least in the surface zone with an additive which reduces the oxidative loss of strength of the silicon nitride and which among the metal oxides calcium oxide, strontium oxide, barium oxide, mixtures of these oxides with iron oxide and one in the Ratios of the spinel MgAl ₂ O ₂ , appropriate mixture of magnesium oxide and aluminum oxide is selected. 2. Ceramic body according to claim 1, characterized in that the additive calcium oxide, strontium oxide, a mixture of calcium oxide or strontium oxide with iron oxide or a mixture of magnesium oxide and aluminum oxide in the Spinel MgAIgO ^ is appropriate proportions. 3. Process for the production of the ceramic body according to Claim 1 or 2, characterized in that reactively bonded silicon nitride is impregnated with at least one compound which is in the corresponding metal oxide or a mixture of the corresponding metal oxides can be converted, and the compound (s) by calcining into the corresponding oxide or oxide mixture are transferred. 4. The method according to claim 3, characterized in that a metal salt is used as the compound which can be converted into the metal oxide. 709846/0677 INSPECTED 5. The method according to claim 4, characterized in that a nitrate is used as salt. 6. The method according to claim 4 or 5 *, characterized in that the reactively bonded silicon nitride is impregnated by introducing it into a molten salt. 7. The method according to claim 4 or 5 *, characterized in that that the reactively bound silicon nitride is impregnated by introducing it into a boiling solution of a salt in its own crystal water. 8. The method according to claim 7, characterized in that the reactively bound silicon nitride is dried to remove remaining water. 9. The method according to any one of claims 3 to 8, characterized in that the compound (s) is converted into the corresponding oxide or oxides ^{by calcining at, for example, 850 ° C.} 709846/0677