Heat resistive and reinforced composite material
CROSS-REFERENCE TO RELATED APPLICATION 
     This invention is a continuation-in-part of our copending patent application Ser. No. 169,821 filed Aug. 6, 1971, now abandoned, and assigned to the same assignee as the present invention. 
    
    
     This invention relates to a heat resistive and reinforced articles, and more particularly to a heat resistive and reinforced articles made of composite material prepared by sintering mixed and molded raw materials containing the major component if trisilicon tetranitride. 
     It is already known that a sintered material consisting of trisilicon tetranitride (Si 3  N 4 ) has many superior characteristics, such as high heat resistivity to temperatures up to 1900°C, good mechanical strength at high temperatures, low thermal expansion coefficient, and a resultant high resistivity to heat shocks. It is now being developed to utilize this material as a substitute of cermet for machine parts which require good heat resistivity and mechanical strength at high temperatures. Further, this material is now utilized for making molten metal vessels because of its excellent corrosion resistivity to molten metals. 
     The densification property of trisilicon tetranitride sintered under atmospheric pressure is very inferior. Therefore, it must be sintered under high pressure when a product of high density and good strength is desired. However, such pressurized sintering is not practical because of its troublesome operation. 
     A modified method of sintering under atmospheric pressure is known to replace the hot-press sintering of trisilicon tetranitride. In this modified method, a press-molded silicon powder, which may be more or less densely formed, is sintered under atmospheric pressure in an atmosphere of nitrogen which reacts with the silicon. The density of the sintered trisilicon tetranitride, however, is no more than 85 percent of the theoretical value, and no sintered article of greatly high density can be obtained. 
     An object of this invention is to obtain a heat resistive and reinforced article made of composite materials having high density and good bending strength by ordinary sintering. 
     The above object can be attained in accordance with the present invention by press-molding a finely powdered mixture of trisilicon tetranitride, an oxide of yttrium group elements and aluminum oxide, and sintering said press-molded article under atmospheric pressure. 
     The features of the invention which are believed to be novel are set forth with particularity in the appended claims. The invention itself, however, as to its organization may best be understood by reference to the following description. 
     The heat resistive and reinforced article made of composite materials in accordance with this invention is prepared by sintering under atmospheric pressure a press-molded mixture of finely powdered raw materials of trisilicon tetranitride, an oxide of yttrium, scandium, lanthanum or cerium and aluminum oxide. 
     In the sintering process of this invention, a chemical compound of Si-Y-Al-O-N series is formed on the basis of the mutual reaction between trisilicon tetranitride, an oxide of the afore-mentioned yttrium, scandium, lanthanum or cerium or a mixture thereof and aluminum oxide, and the sintered substance mainly contains trisilicon tetranitride and a small amount of above-mentioned chemical compound. The microscopic structure of this sintered substance is fibrous, needle-like or columnar (hereinafter referred to merely as &#34;fibrous&#34;). 
     The preferable range of mixing ratio of the three kinds of raw material is 70 to 90 weight percent of trisilicon tetranitride, 5 to 20 weight percent of an oxide of yttrium, scandium, lanthanum or cerium or any of mixtures thereof, and 1 to 20 weight percent of aluminum oxide, based respectively on the weight of mixed raw materials. Mixing outside this range will result in no favorable composite materials. In addition, about 5 weight percent, based on the weight of mixed raw materials, of a binding material such as stearic acid may be added to the mixed raw materials to form a molded article to be sintered. This binder will vaporize and disappear from the article when heated during the sintering process. 
     The particle size of each raw material should preferably be from 0.1 to 2.0 microns. 
     The preferable sintering temperature is from 1400° to 1900 °C. At a temperature lower than 1400 °C, there is scarcely recognized a progress of sintering, and at a temperature of over 1900 °C, a decomposition of the fibrous structure depending on the sublimation of trisilicon tetranitride occurs actively, so that in these cases a favorable sintered composite material can not be obtained. 
     Sintering should preferably be carried out in a non-oxidizing atmosphere. The presence of a large amount of oxygen will oxidize trisilicon tetranitride at a high temperature and change it into silicon dioxide, so that the inherent nature of the material will be lost. The presence in the atmosphere of oxygen in the amount less than 10 volume percent will not function to change the nature of the sintered substance since only a small part of trisilicon tetranitride changes into harmless silicon oxynitride (Si 2  ON 2 ), with the result that the sintered substance can be utilized for any purpose without practical disadvantages. 
     It is one feature of this invention to employ a method of ordinary sintering, that is, sintering at atmospheric pressure, and also a method of pressurized sintering. In the latter case, the structure and characteristics of the product may differ from one obtained in the former case to some extent. That is to say, though the products by both methods have a fibrous structure respectively, the method of atmospheric pressure sintering produces a fibrous structure arranged in three dimensional directions, while the method of pressurized sintering brings on two dimensional directions parallel to the pressing plane. Therefore, it is necessary to select either one of the methods in compliance with a particular application of the product. 
     As a substitute of trisilicon tetranitride, silicon or a mixture of silicon and trisilicon tetranitride may be used. In these cases, it is necessary to sinter the mixed raw material in a nitrogenous atmosphere, formed for example of ammonia gas or a mixed gas of nitrogen and hydrogen, in order to change the silicon into trisilicon tetranitride during sintering. 
     As a substitute of yttrium oxide, yttrium carbonate [Y 2  (CO 3 ) 3  ] or yttrium hydroxide (Y(OH) 3 ) may be used by reason of the fact that such substitutes are changed by heating into Y 2  O 3  during sintering. By the same reason Al 2  (SO 4 ) 3  may be used as a substitute of Al 2  O 3 . 
     Yttrium oxide can be entirely or partly replaced by scandium oxide, lanthanum oxide or cerium oxide, with substantially the same results. 
     A suitable heat resistive substance such as any of the oxides, nitrides, carbides, silicides, sulfides or boron compounds may be included into the raw materials in order to add its specific character thereto. 
     The invention will be more clearly understood from the following examples, given by way of illustration and not limitation. All parts are by weight. 
    
    
     EXAMPLE 1 
     A mixture consisting of 80 parts of trisilicon tetranitride, 10 parts of yttrium oxide, 10 parts of aluminum oxide, the mean particle sizes thereof being 1.5, 1.6 and 1.1 microns respectively, and 5 parts of stearic acid as a binder was press-molded into small rods, the size thereof being 8 mmφ × 15 mm, under a pressure of 5000 kg/cm 2 . These rods were filled into a muffle, using aluminum nitride powder as a filler, and heated by the following rates of temperature rise in nitrogen gas atmosphere which was prepared by passing the gas through the muffle at a rate of 800 l per hour. 
     Room temperature to 400 °C . . . . for 4 hours, and 
     400 °C to 1600 °C . . . . for 1 hour. 
     After sintering for 2 hours at 1600 °C under atmospheric pressure, the sintered substance was permitted to be self-cooled. 
     The density of the obtained article was 3.40 g/cc. For reference, the theoretical density of Si 3  N 4 , Y 2  O 3  and Al 2  O 3  are 3.20 g/cc, 4.8 g/cc and 3.9 g/cc respectively. The mechanical bending strength of the obtained article was 45.7 kg/mm 2 . 
     In comparison with the density and bending strength of similarly sintered simple trisilicon tetranitride, those were 2.4 g/cc and 2 kg/mm 2  respectively. 
     The obtained composite material had a three dimensional fibrous structure under microscopic observation. In this structure, the fibrous crystals consisted of Si 3  N 4 , and the matrix thereof was made of Si-Y-Al-O-N compounds. 
     EXAMPLES 2 TO 11 
     Raw materials of trisilicon tetranitride, yttrium oxide or its substitute and aluminum oxide, together with 5 parts of stearic acid as a binder, were mixed in various proportions as shown in Table 1. Each mixture was shaped and heated up similarly to Example 1 in argon gas atmosphere, and thereafter sintered under the condition as shown in Table 1 respectively in the same atmosphere. The bending strength of each sintered substance is shown in Table 1. All sintered substances had a three dimensional fibrous structure. 
     Comparatively, when Al 2  O 3  was not used in the process of Example 2, the bending strength of the sintered substance obtained was 32.0 kg/mm 2 . 
     
                       Table 1                                                     
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Results of the experiments                                                
     Proportion of raw                                                    
No.  materials (wt. %    Condition of                                     
                                    Bending                               
of   based on the weight sintering  strength                              
Ex.  of mixed raw mate-  Temp.   Time (kg/mm.sup.2)                       
     rials)              (°C)                                      
                                 (hr)                                     
______________________________________                                    
     Si.sub.3 N.sub.4                                                     
              Y.sub.2 O.sub.3                                             
                        Al.sub.2 O.sub.3                                  
2                              1600  1    34.5                            
     89.0     10.0      1.0                                               
     Si.sub.3 N.sub.4                                                     
              Y.sub.2 O.sub.3                                             
                        Al.sub.2 O.sub.3                                  
3                              1600  1    42.0                            
     85.0     10.0      5.0                                               
     Si.sub.3 N.sub.4                                                     
              Y.sub.2 O.sub.3                                             
                        Al.sub.2 O.sub.3                                  
4                              1600  1    48.5                            
     70.0     10.0      20.0                                              
     Si.sub.3 N.sub.4                                                     
              Y.sub.2 O.sub.3                                             
                        Al.sub.2 O.sub.3                                  
5                              1600  1    35.5                            
     90.0     5.0       5.0                                               
     Si.sub.3 N.sub.4                                                     
              Y.sub.2 O.sub.3                                             
                        Al.sub.2 O.sub.3                                  
6                              1600  1    43.8                            
     75.0     20.0      5.0                                               
     Si.sub.3 N.sub.4                                                     
              Y.sub.2 O.sub.3                                             
                        Al.sub.2 O.sub.3                                  
7                              1500  1    36.5                            
     85.0     10.0      5.0                                               
     Si.sub.3 N.sub.4                                                     
              La.sub.2 O.sub.3                                            
                        Al.sub.2 O.sub.3                                  
8                              1600  1    42.1                            
     85.0     10.0      5.0                                               
     Si.sub.3 N.sub.4                                                     
              Ce.sub.2 O.sub.3                                            
                        Al.sub.2 O.sub.3                                  
9                              1600  1    45.5                            
     85.0     10.0      5.0                                               
     Si.sub.3 N.sub.4                                                     
              Y.sub.2 (CO.sub.3).sub.3                                    
                        Al.sub.2 O.sub.3                                  
10                             1600  1    36.2                            
     85.0     10.0      5.0                                               
     Si.sub.3 N.sub.4                                                     
              Y.sub.2 O.sub.3                                             
                        Al.sub.2 O.sub.3                                  
11                             1600  1    45.0                            
     85.0     5.0       5.0                                               
              Sc.sub.2 O.sub.3                                            
              5.0                                                         
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     EXAMPLE 12 
     A mixture consisting of 88 parts of trisilicon tetranitride, 7 parts of yttrium oxide, 5 parts of aluminum oxide, the mean particle sizes thereof being 0.4, 1.6 and 1.3 microns respectively, and 5 parts of stearic acid as a binder was press-molded into small rods, the size thereof being 8 mmφ × 15 mm, at a pressure of 5000 kg/cm 2 . These rods were gradually heated up to 400 °C during 4 hours to remove the binder by its own vaporization. Then they were filled into a carbon mold, 30 mmφ × 8 mmφ × 70 mm size, and hot-pressed at a pressure of 400 kg/cm 2  and a temperature of 1700 °C in nitrogen gas atmosphere for 30 minutes to obtain a sintered article. The obtained article was highly densified and had high bending strength of 105 kg/mm 2 , and it was specifically strengthened in two dimensional directions parallel to the pressed surface thereof.