DE3029831C2 - Dense, yttrium oxide-containing molded body made of silicon nitride and process for its manufacture - Google Patents

Description

The invention relates to a dense, ytrium oxide-containing shaped body made of sintered and / or silicon nitride pressed at high temperatures and is also directed to a method for producing such dense shaped bodies. _{J (1}

Molded bodies of silicon nitride are characterized in particular by high strength at room temperature up to 1400 ° C from, have excellent creep resistance, and have in each temperature range, namely up to 1400 ⁰ C, j- an excellent oxidation-, resistance and TempcraturwechsJbeständigkeii on. This makes them suitable as structural parts that are exposed to high temperatures and a corrosive atmosphere. Silicon nitride is also of particular interest as a material for _Jo parts in gas turbines that are subject to high dynamic loads.

For the sintering or pressing at high temperatures of molded bodies made of dense silicon nitride, the addition of sintering-promoting additives is necessary in order to achieve high densities that are close to the theoretical density. approach _n.

From US-PS 4102 698 it is known to add as a sintering promoting agent Y ₂ O ₃ in amounts between 1 and 25 wt .-%, wherein, to avoid instability in the middle temperature range between - _{connected) 0} 800 1100 ⁰ C and strong increases in volume and weight as well as cracking, a very pure silicon nitride powder must be used, with cationic impurities of less than 0.1% to a maximum of 1% in the form of oxides as well as a defined y, oxygen content and precise composition. Since fluctuating raw material compositions are practically unavoidable, reproducible material production using this process is only possible on a laboratory scale. The same applies to the _{teachings known from DE-OS b0} 27 18 729 and 29 37 740.

The invention aims to provide a remedy here. The invention as characterized in the claims is. solves the problem of creating dense silicon nitride bodies with a proportion of yttrium oxide, and a ,, =; To develop processes for their production, in which the material properties both at room temperature and at higher temperatures in the 79.95 to 98.95 wt% Si ₃ N ₄ 1.00 to 20.00 _{wt% Y 2} O ₃ 0.05 to 3.00 wt% Li ₂ O

or a compound which forms lithium oxide when the temperature is increased is used.

3. The method according to claim 2, characterized in that the silicon powder used to produce the silicon nitride powder, the yttrium oxide and the lithium oxide or before nitriding the respective forming compounds are added in powder form.

Short-term as well as long-term behavior due to oxidation did not experience any drastic deterioration and reproducibility is guaranteed.

The particular advantage achieved by the invention is that by adding another Component to the misalignment is possible to come to material properties due to fluctuating Raw material composition and analysis fluctuations are no longer significantly influenced.

In the following, the invention is explained in more detail with the aid of exemplary embodiments.

The starting material can be essentially completely crystalline silicon nitride powder, but also amorphous material that has been partially crystallized by annealing, or a mixture of amorphous and crystalline silicon nitride. As a rule, a powder is selected as the crystalline silicon nitride powder, of which> 90% is in the «phase and whose CaO, MgO, Na ₂ O impurities are as low as possible, ie CaO <0.1%, MgO <O , l _{%, Na 2} O <0.05%. The selected Si ₁ N ₄ -Ma (CrJaI is mixed with I to 20% Y ₂ O ₁ and 0.05 to 3 wt .-% Li ₂ O or compounds that convert into the corresponding oxides, and mixed . to obtain the aid of a Mahlfiüssigkeit such. as methanol, toluene, cyclohexane and the like., is ground to a reactive powder blend It is desirable while a specific surface area by the BET Melhode of> 3 m ² / g, with a Uniform grain distribution with a maximum grain size of 20 μm proves to be advantageous. The ground slurry is loaded into the corresponding graphite die, e.g. during hot pressing after drying and sieving, prepressed with preferably 10 to 20 N / mm ² and then at a pressure of preferably 20 hot-pressed to 60 N / mm ² in a protective gas atmosphere, usually nitrogen, or vacuum, at temperatures of preferably 1700 to 1800 ⁰ C. the hold time at pressure and temperature stability I amounts to 2 Sunden. These are sufficient conditions to r Production of essentially completely compressed SijN ₄ bodies. Surprisingly, these materials have the previously known yttrium oxide-containing silicon nitride

materials clearly outperformed in terms of properties.

In various materials produced in this way, flexural strengths were obtained from the SijN ^ -body worked out bending samples with Dimensions of 3.5 4.5 45 mm determined and in a 4-point bending arrangement with the support spacings 10 - 20 - 10 mm between room temperature and 1400 ° C with a test speed of 0.1 mm per Minute, determined.

Table 1

High temperature strength and creep speed of dense S i ₃ N ₄ shaped bodies (8% YiO ₃ ) with and without Li ₁ O-ZuSaIz

Temperature [ ⁰ C]

Flexural strength [N / mm ³ ]

with Li ₂ O without Li ₂ O

20th 813 800 784 900 797 1000 751 1100 723 1200 681 1300 637 1400 573 Crawl speed ε at 1250 ° C, 180 10 ⁶ Ii N / mm ²

Temperature [ ⁰ C]

Weight gain [g / m ² ]

with Li ₂ O without LhO

500 0.114 1.33 600 0.118 1.34 700 0.121 1.75 800 0.128 2.16 900 0.136 3.72 1000 0.131 4.58 IKXl 0.142 2.63 1200 0.140 1.44

In comparison, Table 2 shows the extremely good oxidation resistance that is achieved when Y ₂ O ₁ Li ₂ O is used as an additive to promote sintering.

Table 3

Effects of respectively 24/75 Stundenglühung on the average room temperature strength, wherein the output resistance of a Si ₃ N ₄ -QUaIiUU with 8 wt .-% Yio ₃ i "and Li ₂ O at 813 N / mm ² without Li ₂ O at 793 N / mm ² was

793 76! 734 592 587 666 595 533

7.5 · 10 ~ ⁶ rT

Table 1 makes it clear that when L12O is used in addition to Y ₂ O ₃ as a sinter-promoting additive at higher temperatures, as is generally the case, a drop in flexural strength is noticeable, but in the critical temperature range there is no jump in strength, 4η as occurs with dense S13N4, which only contains Y2O3 as an additive to promote sintering, but not Li ₂ O.

Table 1 also shows that the flexural strength is always at a higher level _{when Li 2 O is added}

The improvement in creep speed can also be seen from this table.

Table 2

Influence of the oxidation on the weight increase in the range from 5 to 11% Y ₂ O ₁ with and without Li ₃ after in each case 100 hours at the specified temperature

temperature with Li ₂ O 75 h without Li ₂ O 75 h [ ⁰ C] [N / mm ³ ] 794 [N / mm ² ] 679 after 786 after 593 24 hours 791 24 hours 441 800 805 762 685 532 900 775 761 655 629 1000 800 510 1100 751 621 1200 750 630

Table 3 shows that when 8% Y2O3 and Li ₂ O are added, the annealing temperature and duration have practically no effect on the room temperature stability. Above all, in the long-term behavior there is practically no discernible drop.

Table 4

Resistance to temperature changes of Li ₂ O doped Y ₂ O ₃ -containing material compared to a material without Li ₂ O, whereby 1 cycle includes annealing at 3 x 1260 C (see also the drawing)

Initial strength at room temperature [N / mm ² ]

with
Li-, 0

without Li, O

813

Room temperature resistance X 1260 ⁰ C 804 [N / mm ² ] X 1260 ⁰ C 793 after 1 X I26O ° C 767 2 X 1260 ⁰ C and 3 X 900 ⁰ C 744 3 Total cycles 715 1 after 5

For the thermal shock resistance test was a test cycle (see drawing) used, which fü? the Investigation of conventional gas turbine components is common. The determined room temperature strength (Table 4) also shows that in the critical temperature range as well as in the long-term behavior Material according to the invention in turn is superior to that of the prior art.

Due to the high availability of the usable Si3N ₄ powder and the insensitivity of the offset during production, this high-strength, creep-resistant, temperature and oxidation-resistant material can be produced economically and reproducibly in a technically relevant quantity and quality.

Table 5

Comparison of the middle height wedge of an L-shaped body. made of Si ₁ N, powder with a MeO content of 2.46 Cicw .-% and SiO ₁ -AnIeM of only 1.30 wt. ... as well as a / usal /. of 11 wt. ¹ '"YO, and 0.6 wt. ¹ '" Li ₂ O with another l -. ^ m body. which was hot-tested with one of the most common pressing aids

IN /

learn -, r ilur H, e, cl Vstigkat | N /! 'Inv | rc ι Vt> ■ I i.l) MuO 20th 810 Μ} 800 7 ¹ M) 57ft I) IM) 803 54 ') 1000 741 41) ι I 100 75 ') 4M 1200 656 3W 111 ci / 11 I HI.lit

(1 mill

562

Table 5 shows that the significantly lower SiOi content when using 11% Y2O1 as an additive through the use of Li> O - in this case 0.6% - even when using an Si, N ₁ powder with a: iohai Proportion of metal oxide compounds (MeO) '"Ii'! Isgesaint 2.46% and an SiO proportion of only 1.30% of the offset, i.e. a composition outside the stable area in the three-substance system. A silicon nitride body that is stable in all temperature ranges can be obtained.

It has thus been shown that only through the orfinduni £ S appropriate composition of Y_> O, and 1.i> O a silicon nitride material can be obtained, which is superior to conventional silicon nitride materials in all respects.

Claims (2)

Patent claims: ! Dense, yttrium oxide-containing molded body made of sintered and / or pressed silicon nitride at high temperatures, characterized in that it contains 0.05 to 3.00% by weight, _{based on the total amount of Si 3} N ₄ , Y ₃ O ₃ and Li _{2 O} Contains Li _{2 O} 2. Process for the production of dense molded bodies from silicon nitride by sintering and / or Pressing at high temperatures with the addition of yttrium oxide or one with an increase in temperature Yttrium oxide-forming yttrium compound, characterized in that an offset consists of 10