FR2605625A1 - SILICON NITRIDE MATERIAL HAVING IMPROVED RESISTANCE AT HIGH TEMPERATURES - Google Patents

Abstract

THE MATERIAL IS SHAPED FOR AT LEAST THREE-FIFTHS OF AN ATOMIC FRACTION OF SILICON NITRIDE, BETWEEN 1 AND 5 OF A RARE EARTH OXIDE, AT MAXIMUM 1 OF SILICA AS DETERMINED BY X-RAY DIFFRACTION, AND AT MAXIMUM 0, 5 ALUMINA, AS DETERMINED BY CHEMICAL ANALYSIS. THE BENDING STRENGTH IS AT LEAST 525 MPA, AS MEASURED AT 1370 C.

Description

The present invention relates to the field of materials in front of

  withstand exposure to high ambient temperatures. More particularly, it relates to a material consisting essentially of silicon nitride, also containing a rare earth metal oxide, but containing not more than 0.5% of alumina. This material is particularly suitable for producing turbine and engine components that are subjected to combustion temperatures. Ceramics formed of polycrystalline silicon nitride constitute a well-known class of materials. They are commonly prepared by compressing silicon or silicon nitride powder to obtain a coherent green body having the general shape of the desired final ceramic article. Depending on the method used to form the green body, a fugitive binder may or may not be necessary to give the green body the desired consistency, and it may or may not be advantageous to proceed with a second compression step. After a suitable compression, the body is freed of the binder if necessary, then it is finally transformed into a ready-to-use form by a so-called densification process. If the body, prior to densification, consists essentially of elemental silicon, it can be converted to silicon nitride by exposing it to nitrogen gas at an appropriate temperature, which process is known as a reactive bond. If the body, before densification, is already basically made of silicon nitride, the densification is usually

  accomplished by a combination of heat and pressure.

  Most metals and their oxides have melting points lower than that of silicon nitride and they are

  considerably lower at high temperatures.

  However, it has been found in practice that the presence of a certain amount of lower melting component, referred to as densification or sintering aids, is necessary to allow the densification of silicon nitride bodies in suitable conditions, in practice, of temperature and pressure. The amount of sintering aid must be carefully controlled because too much will weaken the product while too little will lead to inadequate densification. Some metals and a wide variety of oxides and oxide mixtures, including yttrium oxide and other rare earth metal oxides, have been described by others as suitable as auxiliaries for densification for the silicon nitride to be

  used at high temperatures.

  One of the most efficient densification techniques is one which is generally known in the art as hot isostatic pressure (frequently abbreviated hereinafter as "PIC"). The PIC technique which is most suitable for the manufacture of silicon nitride articles is that described in US Pat. No. 4,339,271 of July 13, 1982 to Isaksson et al. Other variants and improvements of this process, some of which are particularly applicable to silicon nitride, are described in US Pat. Nos. 4,081,272 of March 28, 1978; 4,112,143 of September 5, 1978; 4,256,688 of March 17, 1981; 4,446,100 from May 1, 1984; and 4,455,275 dated June 19, 1984; all in the name of Adlerborn, either alone or with different collaborators. All these patents teach that a silicon nitride body must be degassed at a temperature of about 950 C before being encapsulated in the glass envelope in which is made

practice PIC.

  US Patent 4,457,958 of July 3, 1984 to Lange et al teaches the use of diffusion technique after densification of silicon nitride bodies to improve creep resistance and body resistance in general by reducing the amount of intergranular phases. Although this technique is not at all close to that concerned by the present invention, it reports a value of 565 MPa at 1400 C for the flexural strength of silicon nitride, which is one of the highest known values. of the Applicant according to the prior art. The type of silicon nitride with which this high flexural strength value was obtained contained deliberately added magnesium oxide and almost certainly some silica as the primary densification additive; it did not contain a significant amount of rare earth oxide. Although the technique taught by Lange has been applied to some silicon nitride bodies which contained yttrium oxide as a primary densification aid for glass formation, the flexural strength values of these samples did not been reported; only improvements in creep resistance have been reported for these

  samples containing yttrium oxide.

  D.C. Larsen et al. Have reviewed in Ceramic Materials for Advanced Heat Engines (1985) the effect of different densification aids on the high temperature properties of silicon nitride. This reference describes a material containing 4% yttrium oxide and 3% alumina which provided flexural strengths of up to about 700 MPa at about 1370 ° C (see graphs on pages 121 and 127). However, it has also been noted that this material "appears to be limited to 15,000 C due to oxidation, it is believed that this is due to the additive AO1203.") Pages 120). The present Applicant also believes that the use of alumina as a densification aid in silicon nitrides is likely to result in higher temperature resistance relatively poorer at low strain rates than at high strain rates, when they are compared with silicon nitride containing rare earth oxides such as yttrium oxide, substantially free of alumina, as a densification aid. The article by Larsen also states (pages 120-24), "the success of Y203 as a densification aid for HPSi3N4 lies in the fact that the intergranular phase of the resulting yttrium silicate can be crystallized. 4% Y203 (ie 8% or more), it was found to have a strong tendency to be in the part of the Si3aN4-Y203Si02 phase triangle that leads to unstable oxynitride phases in an oxidizing environment. " In a subsequent passage (page 221), the same reference indicates that the Si3Y203N4, YSiO2N and YIeSi7O23N4 phases constitute undesirable intergranular constituents because they are likely to undergo rapid oxidation, which can lead to a catastrophic rupture of the silicon nitride bodies with such intergranular phases. However, intergranular phases of Y2Si2O7 are recommended as far as

  o they do not present this difficulty.

  Japanese Patent Application No. 56-185122 of November 17, 1981, published on May 26, 1983, under number 58-88171, describes a process for the preparation of bodies of dense silicon nitride which consists in preparing green bodies, heating them under a nitrogen atmosphere, and finally to densify the bodies by PIC. However, the heating recommended by this reference is at temperatures above 1600 C and the specific microstructural effect that is sought to obtain by heating is the transformation of the crystalline form of silicon nitride from alpha to beta. The bending strengths of the products prepared according to this reference are only given at ambient temperature and at 1200 ° C. No indication is given of the units used for the flexural strength values, but it is likely that they are expressed in kg / mm2. The highest value that is

reported at 1200 C is 74.

  It has now been found that the high temperature resistance of silicon nitride bodies containing between 1 and 5% of rare earth oxide processing aids and less than 0.5% of alumina can be substantially increased. treating the green bodies before the CIP with nitrogen gas at a temperature of between 1000 and 1500 C for a time sufficient to reduce the SiO2 and Y2Si2O7 phase content of the bodies after PIC below the detection threshold of a X-ray diffraction (XRD). Normally a heating time of 20 to minutes is sufficient. The heat treatment also reduces the iron content of the bodies and must therefore increase the reliability in service of the bodies manufactured according to the invention, since it has been found that the inclusions of elemental iron present in the silicon nitride articles for high temperatures are associated with the appearance of cracks and it is thought that this helps to initiate a

breaking process.

  More specifically, the invention provides a polycrystalline ceramic material formed for at least three fifths of an atomic fraction of silicon nitride, between 1 and 5% of a rare earth oxide, at most 1% of silica as determined by X-ray diffraction, and at most 0.5% alumina, as determined by chemical analysis. The implementation of the invention makes it possible to produce silicon nitride articles having a resistance to

  bending greater than 525 megapascals (MPa) at 13700C.

  The invention is applicable to any silicon nitride composition containing a sufficient amount of a rare earth sintering aid to perform densification under conditions suitable for PIC. A rare earth combination, such as yttrium oxide and cerium oxide, may be used, and additional oxides may be present. It is preferred that the yttrium oxide be used in an amount of from 1 to 5% by weight, and more preferably from 2 to 5% by weight. Any suitable source of suitable silicon nitride and rare earth oxide powder may be used. Suitable materials are a commercially available silicon nitride powder having a surface area of 6-16 square meters per gram (m2 / g), an oxygen content of about 1.5% and an iron content about 0.03%, with 99.99% purity of yttrium oxide powder available from

  Molycorp, Inc. of White Plains, New York.

  Preferably, the silicon nitride and the rare earth oxide are kneaded together until the blended powders have a surface area of at least 10 square meters per gram (m 2 / g) as measured by conventional techniques. . Satisfactory results are obtained in a simple ball mill in a suitable organic solvent, such as 2-propanol with silicon nitride balls, but it is not believed that the kneading process is important for the invention as long as the appropriate particle size and the homogeneous mixture of materials are obtained and the introduction of harmful impurities from the mixing medium is avoided. After mixing, the powder must be dried, preferably under a partial vacuum, and then shaped into a green body by any suitable conventional technique, such as cold pressing in a matrix at about 22 MPa followed by conventional cold isostatic pressing ( PIF) at 200-400 MPa. The green body from the PIF is then degassed at a temperature of between 750 and 950 ° C. and then subjected to a heat treatment according to the invention at a temperature above 1000 ° C. in a stream of nitrogen gas at atmospheric pressure. normal. The heat treatment must be continued for a time sufficient to reduce the content of the silica phase to less than 1% and the final content of the Y2Si207 or E2Si2O7 phase to a non-detectable amount by X-ray diffraction after PIC, where E represents any rare earth metal. In general, a time of between 20 and 60 minutes is preferred for the heat treatment. After the heat treatment, the sample is then degassed and subjected to a conventional PIC as taught

by the aforementioned patents.

  Although the invention is not limited to any theory, we believe that the improved high temperature resistance that is obtained according to the invention stems from the control of the type and amount of rare earth metal silicate in the final ceramic product after PIC. Both elemental silicon and silicon nitride spontaneously form silica at the surface when exposed to air or other oxygen sources at reasonable pressures, and silica tends to concentrate in the intergranular phase. This phase also contains deliberately added sintering aids such as rare earth oxides and when these are present in sufficient quantity, as they are in the preferred compositions of products according to the invention, it is likely that produces reactions

forming silicates.

  The compound Y2Si207 is a silicate which is particularly likely to originate in materials containing the preferred yttrium oxide component. It has been reported (by K. Liddell and DP Thompson, 85 British Ceramic Society Transactions and Journal 17-22 (1986)) that Y2Si2O7 is capable of having three phase transitions at atmospheric pressure within a temperature range that is at least partially within the range used for PIC and that could be found in practice under the conditions of use: alpha to beta at 1225 C, gamma beta at 1445 C, and gamma at delta at 1525 C. Volume changes are associated with these phase transitions, and it is to be expected that these volume changes will cause microcracks or the introduction of voltages into the silicon nitride bodies formed by PIC. according to the prior art. The present invention results in the absence of a detectable amount by XRD of SiO2 and alpha Y2Si2O7 phases in the formed bodies, although the elements of these phases are always present according to the chemical analysis. The silicon and yttrium atoms present may be in the glasses, other crystalline phases of Y2Si2O7, or other complex oxynitrides, the specific phases

  present have not been identified.

  The scope and variety of the invention may be further appreciated from the following examples. In all these examples, the silicon nitride and yttrium oxide powders, as described above, were mixed together in appropriate amounts to give a proportion of 4% by weight of yttrium oxide in together, slurried with 57 parts by weight of isopropyl alcohol per 43 parts of powder, and blended with a drum in a ball mill with 9.5 mm diameter silicon nitride pellets until the powder was sufficiently divided to have a specific surface area of 10-12 m 2 / g. The powder was then dried in a rotating drum at about 70 ° C under a partial vacuum of about 50 kPa for 2 hours. The dried powder was pressed without any binder into a steel matrix at room temperature and pressure of about 25 MPa to form a coherent body in the form of a thin parallelepiped plate or tile. This tile was encapsulated in a conventional polyurethane rubber membrane and subjected to PIF using water as pressurizing fluid at 200-400 MPa, then degassed as taught in US Patent No. 4,446,100 to produce a green body ready for the heat treatment according to the invention. The heat treatment was carried out in a furnace fed with a stream of nitrogen having a chemical purity of 99.999%. Other processing conditions are shown in Table I. After the heat treatment, the tiles were degassed and subjected to a PIC as taught in US Pat. No. 4,446,100. A mechanical test specimen having dimensions of 3 x 4 x 55 mm was machined from the densified tile, and used in a four-point, quarter-point bending test, with a center 40 mm gap and a plunger speed of 5 mm / minute . The bending test was performed in air at 13700C. The results are shown in Table I. The fracture toughness measurements were made according to the standard indentation direct crack measurement technique on other samples prepared from the densified tiles. These results are also given in Table I. The peak DRX at d = 0.301 nm shown in Table I is one of the characteristics of the alpha phase Y2Si2O7. The DRX results were obtained on still other samples from the same PIC-densified ceramic bodies as described above, using a Philips model AP 3720 automatic powder diffractometer. This is a crystal monochromator with diffracted beam using X-ray Cu K-alpha radiation. The powdery samples of the ceramics were examined with this machine using electrical settings of 45 kilovolts and 40 milliamps. The sample was rotated 1.75 per second at a total rotation angle of 700. The values shown in Table I for the relative intensity are as a percentage of the highest peak found for the nitride. beta silicon in the same sample. It is believed that the detection limit of this technique is approximately

0.5% by weight of the alpha Y2Si2O7.

  In Table I, the numbers of the non-prefix examples indicate the examples according to the invention, whereas those preceded by the prefix P were prepared from identical materials and treated in the same way, except that they did not not undergone heat treatment according to

the invention.

  The total silicon and silica determined by chemical analysis of Sample 1 was about 2%, but none of these phases were detectable by XRD, with a detection limit of about 1%, or even an analysis. by electron spectroscopy which should have detected quantities as low as 0.2 atomic%. Nor was it able to detect single yttrium oxide phase, indicating that substantially all the yttrium and silicon in the product are present in phase form

  complex but not in the form of alpha Y2Si2O7.

TABLE I

  DURATION AND TEMPERATURE OF THERMAL TREATMENT AND CONTENT

  RESULTING ALPHA Y2Si2O7 EXAMPLE Conditions Resistance Resistance Intensity Number of Treat- ment at Relatively Bending Break at Thermal Peak MPa / me, 5 1370 C, DRX at C min MPa d = 0.301 nm

1 1450 30 4.10 575 0

P1 none none 3.15 474 8.11

2 1450 30 3.72 642 0

  P2 Nil Nil 3.10 516 7.78 The iron content of the ceramic samples made in a manner similar to Samples 1 and P1 as noted above was also measured. Iron, which is thought to be present in the elemental phase, represented 0.22% by weight of the sample identical to P1 but only 0.04% by weight of the sample identical to sample 1. Fractography Scanning electron microscope and electron dispersive spectroscopy of broken samples of silicon nitride bodies made by the methods described above, showed that all breaks originated in iron inclusions. Thus, it is believed that the reduction of iron content in dense silicon nitride bodies is at least one of the factors reliably allowing for high case-resistance for

densified silicon nitride.

  Although the examples given above have basically concerned yttrium oxide as a rare earth sintering aid, the well-known similarity of chemical and ionic properties of all rare earth metals indicates that other earth metals

  rare could be substituted for yttrium.

  The silicon nitride articles manufactured according to the invention are excellently suited as turbine blades, vanes, rotor, combustion liner, burner, spacer and other components of hot parts in gas turbines as well as for valves, cylinder liners, valve seats, pushers and other components of hot parts in reciprocating piston engines. The products manufactured according to the invention are also suitable for all established uses of previously known silicon nitride articles, including, but not limited to, thermocouple sleeves, low pressure die-casting rods, crucibles, furnace seals and pistons for non-ferrous foundries, in particular aluminum; degassing tubes and lining plates for primary aluminum smelters; Precision jigs and accessories for welding, soldering, and other heat treatments in the manufacture of electronic and semiconductor products, jewelery, and other metal or glass objects requiring heat treatment, wear-resistant accessories, optical devices, guide devices and electrode holders for electro-discharge machining, or guiding devices and electrochemical machining jigs; welding nozzles and insulators, pump or valve components for the handling or storage of corrosive chemicals and abrasive mixtures; artificial teeth and dental bridges;

  as well as metal cutting tools.

  The greater strength and greater tenacity of the silicon nitride bodies manufactured according to the invention make them useful in other applications that silicon nitride could not fulfill until now because of its

  insufficient resistance to high temperatures.

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Claims (8)

  1-polycrystalline ceramic material formed for at least three fifths of an atomic fraction of silicon nitride, between 1 and 5% of a rare earth oxide, at most 1% of silica as determined by X-ray diffraction, and not more than 0,5% of alumina, as determined by chemical analysis, characterized by resistance to   bending of at least 525 MPa, as measured at 13700C.   2- Material according to claim 1, characterized in that said amount of rare earth oxide is between 2 and 4%.   3- material according to claim 1 or 2, characterized in that said amount of rare earth is composed of minus 2% yttrium oxide.   4- Material according to any one of claims 1   a 3, further characterized in that it does not contain an alpha phase of Y2Si2O7 detectable by X-ray diffraction.   Material according to any one of claims 1   at 4, further characterized in that it does not contain more 0.05% by weight of iron.   A process for preparing a ceramic material formed of polycrystalline silicon nitride, comprising (a) preparing a green body formed for at least three fifths of atomic fractions of pulverulent silicon nitride, from 1 to 12% by weight of a rare earth metal oxide, and at most 0.5% alumina; (b) degassing said green body; (c) encapsulating said degassed green body in a fluid impermeable flexible membrane; and (d) subjecting said fluid impermeable membrane to uniform fluid pressure while simultaneously maintaining said green body at elevated temperatures, such that said green body is densified to provide a strong and consistent ceramic, characterized in that said green body is heated under a nitrogen atmosphere at a temperature between 10000 C and 1500 C before step (c), for a time sufficient to reduce the content of each of the silica and alpha E2Si2O7 phases below the X-ray diffraction detection threshold, with E representing any rare earth metal, in the material after step (d).  7- Process according to claim 6, characterized in that said temperature between 1000oC and 1500 C is at least 1350 C.   8- Process according to claim 6 or 7, characterized in that the heating prior to step (c) is carried out   for a period of at least 20 minutes.   9- Method according to any one of claims 6   at 8, characterized in that said rare earth metal oxide is formed of yttrium oxide in an amount of at least   2% by weight with respect to said green body.