CA1268488A - Silicon nitride with improved high temperature strength - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
The flexural strength of isostatically hot pressed silicon nitride containing between 1 and 12 weight per cent of rare earth oxide and not more than 0.5 weight per cent alumina is substantially increased by treating green bodies in flowing nitrogen at a temperature be-tween 1000 and 1500C before degassing for the isostatic hot pressing. The iron content of the bodies is also reduced by this heat treatment, and this is believed to eliminate sources of fracture failure. Silicon nitride bodies with a flexural strength in excess of 525 MPa at 1370°C can be prepared in this way.

Description

Docket P-2123 SILICON NITRIDE WITH IMPROVED HIGH TEMPERATURE STRENGTH

R~SSELL L. YECKLEY
Box 232, Barre Road Oakham, MA 0106$
BACKGROUND OF THE_ NVENTION
Field of the Invention This invention relates to the field of materials requiring strength in high temperature environments.
More particularly it relates to a material composed pri-marily of silicon nitride, also containing a rare earth metal oxide, but having no more than 0.5% alumina. This material is particularly suited for use as components of turbines and engines which are exposed to combustion temperatures.
Technical Background Polycrystalline silicon nitride ceramics are a well known class of materials. They are commonIy made by compressing either silicon or silicon nitride powder to 15 give a coherent green body in the general shape of the ~
final ceramic article desired~ Depending on the method ~n used for forming the green body, a fugitive binder may or may not be needed to give coherence to the green body, and a second compression step may or may not be advantageous. After adequate compression, the body is debinderized if necessary and then is finally converted into a form ready to use by a process called densifica-tion. If the body before densification consists primar- ~
ily of elemental silicon, it may be converted to silicon `
25 nitride by exposure to nitrogen gas at an appropriate ``~
temperature, a process known as reaction bonding. If the body before densification is already primarily sili-con nitride, densification is usually accompIished by a combination of heat and pressure.

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~X6~348~3 Most metals and their oxides have lower melting points and are considerably weaker at high temperatures than silicon nitride. However, it has been found in practice that the presence of some lower melting compo-nent, called a densification or sintering aid, is neces-sary to allow densification of silicon nitride bodies under practicall~ attainable conditions of temperature and pressure. The amount of sintering aid must be con-trolled carefully, because too much will weaken the product and too little will lead to inadequate densifi-cation. Some metals and a wide variety of oxides and mixtures of oxides, including yttria and other oxides of the rare earth metals, have been reported by others to be suitable densifying aids for silicon nitride to be used at high temperatures.
One of the most effective densification techniques is that generally known in the art as hot isostatic pressing (often abbreviated hereinafter as "HIP"). The technique of HIP best suited to manufacture of silicon nitride articles is that described in U. S. Patent 4,339,271 of July 13, 1982 to Isaksson et al. Addition-al variations and improvements of this process, some of them particularly applicable to silicon nitride, are de-scribed in U. S. Patents 4,081,272 of Mar. 28, 1978;
25 4,112,143 of Sep. 5, 1978; 4,256,688 of Mar. 17, 1981; '!
4,446,100 of May 1, 1984; and 4,455,275 of June 19, 1984; all to Adlerborn, either alone or with various co-workers. All these patents teach that a silicon nitride body should be degassed at a temperature of about 950C
before being encapsulated in the glass envelope in which HIP actually occurs.
U. S. Patent 4,457,958 of July 3, 1984 to Lange et al. teaches the use of diffusion techniques after densi-fication of silicon nitride bodies to improve the creep resistance and strength by reducing the amount of in-ter-granular phase. While this technique is not at all closely related to that of the present -invention, it did -.~ :. ~ , ., ~ . . :

~2684~8 achieve a reported value of 82,000 psi or 565 MPa at 1400C for the flexural strength of silicon nitride, one of the higher value known to the applicant from the pri-or art. The type of silicon nitride with which this high value for flexural strength was achieved contained deliberately added magnesia and almost certainly some silica as its primary densifying additive; it did not contain any significant amount of rare earth oxide. Al-though the technique taught by Lange was applied to some silicon nitride bodies which did have yttria as the pri-mary glass forming densification aid, the flexural strength values for these samples were not reported; on-ly improvements in creep strength were reported for these yttria-containing samples.
D. C. Larsen et al., Ceramic Materials for Advanced Heat Engines (1985), reviews the effect of various den-sifying aids on the high temperature properties of sili-con nitride. This reference reports one material, con-taining 4% yttria and 3% alumina, which achieved flexur-al strengths of as much as 100,000 psi or nearly 700 MPa at about 1370C (see graphs on pages 121 and 127.) How-ever, it is also noted that this material "appears to be oxidation limited at 1500C. This is thought to be due to the A1203 additive." (page 120). It is also be-lieved by the present applicant that the use of aluminaas a densifying aid in silicon nitride is likely to re-sult in relatively poorer high temperature strength at low strain rates than at high strain rates, when com-pared with-silicon nitride containing rare earth oxides such as yttria, substantially free from alumina, as the densifying aid.
The Larsen reference also notes (pages 120-24), "The success of Y203 as a densification aid for HP-Si3N4 lies in the fact that the resulting yttrium silicate intergranular phase can be crystallized. If more than 4% Y203 is used (i.e., 8 % or more), we have found that there is a strong tendency to be in that part of the " : ~
.. :: :, , !L~68488 Si3~4-Y203-SiO2 phase triangle that results in oxyni-tride phases that are unstable in oxidizing environ-ments." In a later passage (page 221), the same refer-ence notes that Si3Y203N4, YSiO2N, and Ylo 7 23 4 es are not desirable intergranular constituents because they are susceptible to rapid oxidation, which can lead to catastrophic failure of the silicon nitride bodies with such intergranular phases. However, an intergranu-lar phases of Y2Si207 is recommended as free from this difficulty.
Japanese Patent Application No. 56-185122 of Novem-ber 17, 1981, published May 26, 1983 under No. 58-88171, describes a method of preparing dense silicon nitride bodies by preparing green bodies, heating them in a ni-trogen atmosphere, and then finally densifying the bod-ies by HIP. However, the heating recommended by this reference is at temperatures above 1600C and the specif-ic microstructural effect intended to be accomplished by the heating is transformation of the crystal form of the silicon nitride from alpha to beta. Flexural strengths for the products made according to this reference are giv-en only at room temperature and 1200C. No indication of the units intended for the flexural strength values could be found, but it is likely that units of kg/mm2 were in-tended. The highest value reported at 1200C is 7~.SUMMARY OF THE INVENTION
It has been found that the strength a-t high temper-atures of silicon nitride bodies containing between 1 and 5 % of`rare earth oxide sintering aids and less than 0.5 ~ alumina can be increased substantially by treating the green bodies before HIP with nitrogen gas at a tem-perature between 1000 and 1500C for a time sufficient to reduce below the X-ray diffraction (XRD) detection limit the SiO2 and Y2Si207 phase content of the bodies af-ter HIP. Normally a time of 20 to 60 minutes of heating is sufficient. The heat treatment also reduces the content of iron in the bodies and should thus increase the ser--. , ~ .

,. ,.., . : , ~684a8 vice reliability o~ the bodies made according to this invention, because inclusions of elemental iron present in high temperature silicon nitride articles have been observed to be associated with failure cracks and are believed to contribute to failure initiation.
By this invention silicon nitride articles with a flexural strength of more than 525 megapascals (MPa) at 1370C can be produced.
DESCRIPTION OF THE PREFERRED E,MBODIMENTS
The invention is applicable to any composition of silicon nitride containing a sufficient amount of a rare earth sintering aid to densify under conditions suitable for HIP. A combination of rare earths, such as yttria and ceria, may be used, and additional oxides may be present. Yttria in an amount between 1 and 5 per cent by weight is preferred, with an amount from 2-5% partic-ularly preferred.
~ ny conventional source of silicon nitride powder and of appropriate rare earth oxides may be used. Suit-able materials are commercially available silicon ni-tride powder with a surface area of 6-16 square meters per gram (m2/g), an oxygen content of about 1.5 %, and an iron content of about 0.03 %, along with an yttria powder of 99.99 % purity available from Molycorp, Inc.
of White Plains, New York.
The silicon nitride and rare earth oxide are pref-erably milled together until the mixed powders have de-veloped a surface area of at least 10 square meters per gram (m2/g)- as measured by conventional techniques.
Satisfactory results are obtained by simple ball milling in a suitable organic solvent such as 2-propanol with siiicon nitride balls, but the method of milling is not ~-believed to be important to the invention so long as the proper particle size and intimate mixture of the materi-als are achieved and the introduction of deleterious im-purities from the milling media is avoided.

After milling, the powder should be dried, prefera-bly under a partial vacuum, and then formed into a green body by any suitable conventional technique, such as cold pressing in a die at about 22 MPa followed by con-ventional cold isostatic pressing (CIP) at 200-400 MPa.
The green body from CIP is then degassed at a tempera-ture between 750-950C and subjected to heat treatment according to this invention at a temperature above lOOOC
in flowing nitrogen gas at normal atmospheric pressure.
The heat treatment should be continued for a time suffi-cient to reduce the content of the silica phase to less than 1% and the final content of the Y2Si207 phase to an amount undetectable by X-ray diffraction after HIP.
Generally a time between 20 and 60 minutes is preferred for heat treatment. After the heat treatment, the sam-ple is again degassed and subjected to conventional HIP
as taught by the patents already noted.
While the invention is not limited by any particu-lar theory, the applicant believes that the improved high temperature strength achieved results from control of the type and amount of rare earth metal silicates in the final ceramic product after HIP. Both elemental silicon and silicon nitride spontaneously form silica on their surfaces when exposed to air or other sources of oxygen at reasonable pressures, and the silica tends to concentrate in the intergranular phase. This phase also contains the deliberately added sintering aids such as the rare earth oxides, and when these are present in sufficient-quantity, as they are in the preferred compo-sitions of products according to this invention, reac-tions to form silicates are likely.
The compound Y2Si2o7 is a silicate especially like-ly to form in materials containing the preferred yttria component- Y2Si207 has been reported (by K- Liddell and D. P. Thompson, 85 British Ceramic Society Transac-tions and Journal 17-22 {1986}) to be capable of three phase transitions at atmospheric pressure within a range .. ~ :.. .. ..

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~X6~348~3 of temperatures tllat is at least partially within the range used for HIP and could be encountered in practical service conditions: alpha to beta at 1225C, beta to gamma at 1445C, and gamma to delta at 1525C. Volume changes are associated with these phase transitions, and such volume changes would be expected to result in microcrac~ing of or introduction of stresses into the silicon nitride bodies formed by HIP according to the prior art. The present invention results in the ab-sence of XRD-detectable amounts of SiO2 and alpha Y2Si207 phases in the bodies formed, although the ele-ments of these phases are still present according to chemical analysis. The silicon and yttrium atoms pres-ent may be in glasses, other crystal phases of Y2Si207, or other complex oxynitrides; the specific phases pres-ent have not been identified.
The scope and variety of the invention may be fur-ther appreciated from the following examples. For all of them, silicon nitride and yttria powders as already described above were mixed together in appropriate amounts to give 4 wt % yttria in the total, slurried with 57 parts by weight of isopropyl alcohol to 43 parts of powder, and tumbled together in a ball mill with sil-icon nitride balls of 9.5 mm diameter until the powder had been sufficiently finely divided to have a specific surface area of 10-12 m2/g. The powder was then dried in a rotating vessel at about 70C at a partial vacuum of ~`
about 50 kPa for 2 hrs. The dried powder was pressed without any binder in a steel die at room temperature and about 25 MPa pressure to form a coherent body in the shape of a thin parallelepiped or "tile". This tile was encapsulated in a conventional polyurethane rubber mem-brane and subjected to CIP using water as the pressuring fluid at 200-400 MPa, then degassed as taught in U. S.
Patent 4,446,100 to produce a green body ready for heat treatrnent according to this invention.
Heat treatment was performed in a furnace supplied .

12~8~38 with a Elow of nitrogen of 99.999% chemical purity.
Other conditions of treatment are shown in Table 1.
After the heat treatment the tiles were degassed and subjected to HIP as taught in U. S. Patent 4,446, 100. A mechanical test specimen with dimensions 3 x 4 x 55 mm was machined from the densified tile and used in a four point, quarter point bend test with a 40 mm center span and a plunger rate of 5 mm/min. The bend test was performed in air at 1370C. Results are shown in Table 1. Fracture toughness measurements were made according to a conventional indentation direct crack measurement technique on other samples prepared from the densified tiles. These results are also shown in Table 1.
The XRD peak at d = 0.301 nm shown in Table 1 is lS one characteristic of the alpha Y2Si207 phase. The XRD
results were obtained on still other samples, from the same ceramic bodies densified by HIP as described above, using a Philips Model AP 3720 Automated Powder Diffrac-tometer. This is a diffracted beam crystal monochroma-tor using Cu K-alpha X-radiation. Powdered samples of the ceramics were examined with this machine, using elec~rical settings of 45 kilovolts and 40 milliamps.
The sample was rotated at 1.75 degrees per second through a total rotational angle of 70 degrees. The values shown in Table 1 for Relative Intensity are per cent relative to the strongest peak for beta silicon nitride in the same sample. The detection limit of the technique is believed to correspond to about 0.5 % by weight of alpha Y2Si207.
In Table 1, Example Numbers shown with no prefix indicate examples according to this invention, while those with a prefix P were prepared from identical mate-rials and identically processed, except that they were not heat treated according to this invention.
The total of silicon and silica determined by chem-ical analysis in sample 1 was about 2 ~, but neither of these phases was detectable by XRD, with a detection . . - , , -: ' :
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1268~38 Table 1 Heat Treatment Time and Temperature and Resultant ~lpha Y~Si 07 Content Example Heat Treatment Fracture Flexural Relative Number Conditions Toughness, Strength Intensity Deg. C Min. MPa/m 5 at 1370C, of XRD
MPa Peak at d=0.301 nm 1 1450 30 4.10 575 0 10 Pl nonenone 3.15 474 8.11

2 1450 30 3.72 642 0 P2 nonenone 3.10 516 7.78 ~ limit of about 1%, or even by electron spectroscopic analysis, which should have detected as little as 0.2 atomic %. No simple yttria phase was detectable either, indicating that substantially all the yttrium and sili-con in the product are present as complex phases, but not as alpha Y2Si207. ``
The iron content of ceramic samples made similarly 20 to samples 1 and Pl as noted above was also measured. -"
Iron, believed to be present as the elemental phase, amounted to 0.22 wt % of t'ne sample like Pl but only 0.04 wt % of the sample like 1. Scanning electron microscope fractography and electron dispersive spec-troscopy of fractured samples of silicon nitride bodies made according to the methods described above showed that all failures originated at iron inclus;ons. Thus -`
reduction of the iron content in dense silicon nitride bodies is believed to be at least one factor contribut-ing to reliably attaining a high fracture strength for densified silicon nitride.
While the examples have been concerned primarily with yttria as the rare earth sintering aid, the well 9 ~:, ~848~3 known similarity of the chemical properties and ionic radii of all the rare earth metals indicates that other rare earth metals could be substituted for yttrium.
Silicon nitride objects made according to this invention are excellently suited for use as turbine blades, vanes, rotors, combustion liners, flameholders, struts, and other hot section components in gas turbines and for valves, cylinder liners, valve seats, tappets, and other hot section components in reciprocating piston engines. Products made according to this invention are also suitable for all the established uses for prior art silicon nitride objects, including but not limited to:
thermocouple sheats, riser stalks for low pressure die casting, crucibles, and furnace tapping seals and plugs for foundries for non-ferrous metals, particularly alum-inum; degassing tubes and lining plates for primary aluminum smelters; precison jigs and fixtures for sol-dering, brazing, and heat treatment processes in the manufacture of electronic and semiconductor goods, jew-elry, or any other metal or glass object requiring heattreating; wear resistant fixtures for optical devices, nose guides and electrode holders for electrodischarge machining, or guides and templates for electrochemical machining; welding nozzles and insulators, components of pumps or valves for handling or containing corrosive chemicals and abrasive mixtures; artificial teeth and dental bridges; and metal cutting tools.
The greater strength and toughness of silicon nitride bodies made according to this invention will 30 also make them useful in additional applications previ- `
ously avoided for silicon nitride because of inadequate high temperature strength.

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Claims (20)

What is claimed is:

1. A polycrystalline ceramic material comprising at least three-fifths atomic fraction of silicon ni-tride, between 1 and 5 weight percent of a rare earth oxide, not more than 1 weight percent silica as deter-mined by X-ray diffraction, and not more than 0.5 weight percent alumina as determined by chemical analysis, said ceramic material having a flexural strength of at least 525 megapascals when measured at 1370 C.

2. A material according to claim 1, wherein said amount of rare earth oxide is between 2 and 4 weight percent.

3. A material according to claim 2, wherein said rare earth oxide comprises at least 2 % yttria.

4. A material according to claim 1, wherein said rare earth oxide comprises at least 2 % yttria.

5. A material according to claim 4, comprising no amount of alpha Y2Si2O7 detectable by X-ray diffraction.

6. A material according to claim 3, comprising no amount of alpha Y2Si2O7 detectable by X-ray diffraction.

7. A material according to claim 6 comprising no more than 0.05 weight percent iron.

8. A material according to claim 5 comprising no more than 0.05 weight percent iron.

9. A material according to claim 4 comprising no more than 0.05 weight percent iron.

10. A material according to claim 3 comprising no more than 0.05 weight percent iron.

11. A material according to claim 2 comprising no more than 0.05 weight percent iron.

12. A material according to claim 1 comprising no more than 0.05 weight percent iron.

13. In a process comprising (a) preparing a sili-con nitride green body, said green body comprising at least three fifths atomic fraction of silicon nitride, from 1 to 12 weight percent of rare earth metal oxide, and not more than 0.5% alumina, (b) degassing said green body, (c) encapsulating said green body with a flexible fluid-impermeable membrane, and (d) subjecting said fluid-impermeable membrane to uniform fluid pres-sure while simultaneously maintaining said green body at elevated temperature, whereby said green body is densi-fied, the improvement wherein said green body is heated in an atmosphere of nitrogen at a temperature between 1000C and 1500C prior to step (c), for a time sufficient to reduce below the X-ray diffraction detection limit the content of each of the SiO2 and alpha E2Si2O7 phases, where E represents any rare earth metal, within the body after step (d).

14. A process according to claim 13, wherein said temperature between 1000C and 1500C is at least 1350C.

15. A process according to claim 14, wherein said green body is heated prior to step (c) for a time of at least twenty minutes.

16. A process according to claim 15, wherein said rare earth metal oxide comprises yttria to the extent of at least 2 weight percent of said green body.

17. A process according to claim 14, wherein said rare earth metal oxide comprises yttria to the extent of at least 2 weight percent of said green body.

18. A process according to claim 13, wherein said rare earth metal oxide comprises yttria to the extent of at least 2 weight percent of said green body.

19. A ceramic product produced by a process ac-cording to Claim 13.

20. A ceramic product produced by a process ac-cording to Claim 16.