FR2568865A1 - PROCESS FOR OBTAINING SINTERED BODIES OF HIGH DENSITY SILICON NITRIDE - Google Patents

Abstract

THE PRESENT INVENTION RELATES TO A PROCESS FOR OBTAINING A HIGH DENSITY SRIED BODY OF SILICON NITRIDE. ACCORDING TO THE INVENTION, IT CONSISTS OF PREPARING A BODY INCLUDING SI PARTICLES AND A COMPOUND LIKELY TO REMAIN AS SIC OR C IN THE FORM OF A FILM ON THE SURFACE OF SAID SI PARTICLES UNDER A NON-OXIDIZING ATMOSPHERE AT A TEMPERATURE VARYING BETWEEN 900 ET 1400C, TO SUBMIT THEIT BODY TO SINTERING WITH A CONTRACTION GREATER THAN 1 UNDER AN INTERESTED GAS ATMOSPHERE AT A TEMPERATURE EXCEEDING 900C AND NOT EXCEEDING THE SOFTENING POINT OF SI, AND SUBSEQUENTLY SUBMIT THE RESULTING BODY TO THE REACTION OF NITRURATION UNDER A NITRURATION ATMOSPHERE AT A PRESSURE EXCEEDING THE ATMOSPHERIC PRESSURE AND AT A TEMPERATURE VARIING BETWEEN 1200 AND 1500C. THE PRESENT INVENTION FOUND IN PARTICULAR AN APPLICATION FOR THE MAKING OF CERAMICS.

Description

The present invention relates to a process for obtaining ceramics

of silicon nitride

  high density by the process of the sintering reaction.

  Silicon nitride, similar to silicon carbide, is excellent in mechanical strength, thermal shock resistance, wear resistance, and chemical stability at elevated temperatures and, therefore, finds extensive utility as material

structural high temperature.

  Among the processes known hitherto for obtaining silicon nitride ceramics are the process of the sintering reaction which allows this to be obtained by subjecting a Si powder body to the nitriding reaction in an atmosphere of

nitriding.

  This process has the following merits: a) the formed body can be molded by

various methods.

  b) Since the dimensional contraction after the nitriding reaction is extremely small, complicated and extensive shape products can be obtained relatively easily with accuracy

high dimensional.

  c) Since the nitriding reaction does not generally require the use of a sintering additive, the formed body thus produced retains its strength

  and other properties intact even at high temperatures.

  Despite the merits described above, the sintered body obtained by the conventional sintering process acquires a high density with difficulty compared to the sintered body obtained by the reduced pressure sintering process or the hot press process. . This body is, therefore, deficient in mechanical properties such as strength and wear resistance and finds a useful utility.

  limited as high temperature structural material.

  When this process of the sintering reaction is improved for the manufacture of products with a very high density, it is expected that the formed body thus obtained will acquire improved mechanical properties, that it will exhibit the above-mentioned characteristics in the most efficient way. complete, and shows its usefulness as a material

structural high temperature.

  As a means of increasing the density of the silicon nitride sintered body, there is provided a method of adjusting the size of the Si particles to increase the molding pressure thereby increasing the resulting body density to Si particles, and at

  subject the formed body to the nitriding reaction.

  This process is not fully satisfactory,

  since the final density obtained has its limits.

  Another method developed analogously for the purpose of increasing the density is to sinter Si under an inert gas atmosphere such as argon before

  subject the formed body to the nitriding reaction.

  This method, however, does not lead to an effective sintering contraction. This is because Si migration results predominantly from vaporization and surface diffusion and the sintering contraction is hindered by volume expansion. It has been reported that boron or a compound thereof has an ability to suppress the vaporization and surface diffusion of Si which occurs at the expense of Sintering contraction of Si and, therefore, effectively serves the increase in density. The addition of boron, however, leads to a

  degradation of the resistance to oxidation.

  An object of the present invention is to eliminate the aforementioned drawbacks of the traditional process of the sintering reaction and to make it possible to obtain a

  sintered body of high density silicon nitride.

  As a means of decreasing the vaporization and surface diffusion of Si which are detrimental to Si-sintering contraction under an inert gas atmosphere, the present invention utilizes a shaped body comprising Si particles and a compound remaining under the form a SiC or C film on the surface of the Si particles under a non-oxidizing atmosphere at a temperature in the range of 900 to 1400Co It is desirable that the above-mentioned compound be of a nature such that most of it is persists in SiC or C form. This is due to the fact that the compound, while most of its particles may accumulate, has only a small amount of residual SiC or C actually covering the surface of the Si particles and the departure of the major parts. piled up leaves behind these pores on the surface. When this compound is of a type which is in a resinous state at room temperature, it easily covers the surface of the Si particles during its mixing with the Si particles. The coating of this resinous compound thus formed is susceptible, when the mixture The resultant is heated under a non-oxidizing atmosphere to maintain, in the form of a film on the surface of Si particles, SiC or C. Subsequently during sintering of Si, the persistent coating plays a role of suppression of vaporization and the surface diffusion of Si which are detrimental to Si sintering contraction and cause shrinkage by Si sintering as a result of

volume diffusion.

  Examples of resinous compounds usable advantageously herein include large molecule silicon organic compounds such as polycarbosilane and polysiloxane which are intended to survive as SiC and phenol resin, furan resin, epoxy resin, xylene resin, tar and pitch which are intended to survive as C. A compound or a mixture of two or more compounds

  selected from the compounds listed above is used.

  When the resinous compound is of the type intended to survive as C, most of the compound reacts with Si and forms SiC. Generally, it does not cause any degradation in chemical stability or oxidation resistance. The amount of resinous compound to add is

  determined by the proportion of the compound to remain

  SiC or C. It is desired to add this

  placed in an amount such that the proportion of from 1 to 10% by weight of this compound eventually survives as SiC or C. If the added amount is too small, of course the sintering contraction of Si due to the aforementioned mechanism does not occur as expected . If it is too large, the added compound may interfere with sintering contraction and subsequent nitriding of Si and result in degradation of the mechanical properties and

chemical.

  The sintering contraction of Si must be carried out by carrying out most of the aforementioned mechanism at a point where the nitriding reaction will not be obstructed by other possible encumbrances of the paths.

  for the infiltration of nitrogen during nitriding.

  This sintering contraction of Si is carried out under an inert gas atmosphere such as argon at a temperature exceeding 900 ° C. and not exceeding the softening point of Si, preferably at a temperature of

  temperature in the range of 1,100 to 1,200 C.

  The degree of contraction by sintering is controlled by the temperature and the duration of the treatment. Although the treatment can be carried out in a completely open chamber, it is preferably carried out under the aforementioned atmosphere of inert gas for consideration of

the vaporization of Si.

  The striking advantages of complete sintering contraction of interstices between Si particles prior to the nitriding reaction include improved mechanical properties due to increased density and improved reliability. In other words, the defects which are veiled during the preparation of a body of Si particles and which are liable to persist after the completion of the nitriding reaction are reduced by allowing the sintering contraction to proceed completely in the

interstices between Si particles.

  The body, after completion of the sintering contraction of Si, is subjected to the nitriding reaction under a nitrogen atmosphere at a temperature in a range of 1 2000 to 1 5000C. The rate of nitriding increases in proportion in the same way as increases the partial pressure of nitrogen. A pressure exceeding the atmospheric pressure is applied to the reaction system in order to accelerate the permeation of the nitrogen in the sintered mass of the Si particles and consequently to increase the nitriding rate during the aforementioned sintering contraction treatment. If, the paths for the infiltration of nitrogen into the inner parts of the sintered mass of the Si particles are tightened. The application of a high partial pressure of nitrogen serves to promote the permeation of the nitrogen inside the sintered Si mass and accelerates the nitriding of Si and decreases the reaction time of the Si

nitriding.

  The invention will be better understood, and other objects, features, and advantages thereof will become more clearly apparent in the light of non-limiting examples.

limitations given below.

  Example 1. A solution of polycarbosilane in hexane and Si particles having a particle size adjusted in advance to a value of less than 44 μm were mixed by stirring and, at the same time, evaporated to remove hexane. A mixed powder consisting of 90% by weight of Si particles thus obtained and 10% by weight of a high molecular weight organic silicon compound was molded to produce a sample

  witness measuring 50 mm x 30 mm x 10 mm.

  This control sample was maintained in an argon gas atmosphere at 1100 for 20 hours and then thermally synthesized under a nitrogen gas atmosphere of 4 kg / cm 2 pressure at a temperature possibly raised to 4 ° C. 5000C, to obtain a sintered body of silicon nitride. This sintered body exhibited highly desirable results of 6.3% contraction, 2.92 g / cm 3 in density, and 63 kg / mm 2 in

flexural strength.

Example 2

  A solution of phenolic resin in the alcohol and Si particles having a predetermined particle size of less than 44 microns were mixed by stirring and, at the same time, evaporated to remove the alcohol. A mixed powder consisting of 90% by weight of Si particles thus obtained and 10% of phenolic resin was molded to produce a similar control sample in the form of the sample.

witness of Example 1.

  This control sample was subjected to the nitriding reaction under the same conditions as those of Example 1. The resultant sintered body had highly desirable results of 6.8% contraction, 2.90 g / cm 2.

  density and 65 kg / mm in flexural strength.

  Comparative Experiment 1: An identical control sample shaped to the control sample of Example 1 was produced following the procedure of Example 1, except that the ratio of polycarbonate was changed to 3% by weight . This control sample was subjected to the nitriding reaction under the same conditions as those given in Example 1. The sintered body obtained consequently had only unsatisfactory results of 098% contraction, 2.35 g / cm 3 in density, and 13 kg / mm2 in

flexural strength.

  Comparative Example 2: A control sample identical in shape to the control sample of Example 1 was obtained by following the procedure of Example 1 except that the

  Polycarbosilane ratio was changed to 20% by weight.

  When this control sample was subjected to the nitriding reaction under the same conditions as those of Example 1, finally obtained nitriding was defective and a large amount of Si particles remained in unaltered form. Comparative Example 3: L The control sample of Example 1 was subjected to the nitriding reaction under the same conditions as in Example 1, except that the nitrogen gas pressure was maintained at atmospheric pressure in the resulting sintered body. thus, was a part

  unaltered due to insufficient nitriding.

  Comparative Example 4 The nitriding reaction was carried out following the same procedure as in Example 1, except that a solution of polyvinyl butyral alcohol (PVB), a substance likely to produce virtually no C residual, was used in place of the resin likely to significantly produce residual SiC and C. The sintered body consequently produced only unsatisfactory results of 0.1% contraction, 2.38 g / cm 3 in density, and 7 kg / mm 2 in

flexural strength.

  It is clear by comparison of the examples of this invention and comparison experiments that the invention is noticeably effective in allowing the sintered body to acquire a sufficiently high sintering contraction and density and to acquire

  remarkable mechanical properties.

  The silicon nitride sintered body obtained by the process of this invention has a high density and high strength which can not be

  achieved by the process of the traditional sintering reaction

  and, therefore, finds utility in part

  as high-temperature structures in applications that can not be used

  sintered body by the conventional reaction.

Claims (1)

CLAIM   Process for obtaining a sintered body of high density silicon nitride, characterized in that   it consists in preparing a body comprising   of Si and a compound capable of remaining as SiC or C as a film on the surface of said Si particles under a non-oxidizing atmosphere at a temperature of between 900 and 1400 C, of subjecting said body to sintering with a contraction greater than 1% under an inert gas atmosphere at a temperature   exceeding 900 ° C and not exceeding the softening point   Si, and subsequently subject the resultant sintered body to the nitriding reaction under an atmosphere.   nitriding at a pressure exceeding atmospheric pressure   at a temperature between 1200 and 1500 C.