Silicon nitride sintered bodies
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a silicon nitride sintered body having an improved oxidation resistance or wear resistance. 
     2. Related Art Statement 
     Silicon nitride sintered bodies are often used at their as-sintered or worked surfaces for gas turbine members and high temperature structural members. On the other hand, as disclosed in Japanese Patent laid open Nos. 60-161,383 and 60-200,882, the surface of the silicon nitride sintered body is covered with a coating of silicon nitride by a chemical vapor deposition (CVD) process to eliminate the surface defects and enhance the strength prior to the use. 
     In these silicon nitride sintered bodies constituting the above member at their as-sintered or worked surfaces, however, when used at high temperature, the oxidation resistance and the wear resistance are deteriorated as compared with the case of using at room temperatures. Therefore, when they are used as a gas turbine member or a high temperature structural member, the service life is considerably shortened. 
     Further, in the silicon nitride sintered body covered with the coating of silicon nitride by the CVD process, the coating of silicon nitride is destroyed by reacting with the sintered body thereby losing the effect of the coating formation. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to overcome the above-mentioned drawbacks and provide a silicon nitride sintered body which is particularly excellent in oxidation resistance or wear resistance at high temperatures. 
     According to the invention, there is the provision of a silicon nitride sintered body having on its surface a coating of silicon carbide. In particular, the coating of silicon carbide is formed of a CVD process. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     According to the invention, the surface of the silicon nitride sintered body having high strength at high temperatures but low oxidation resistance and wear resistance is covered with a crystalline silicon carbide coating having high oxidation resistance and wear resistance and a high coexistent property with the silicon nitride sintered body to provide a silicon nitride sintered body having particularly improved oxidation resistance or wear resistance at high temperatures. 
     The term &#34;silicon nitride sintered body&#34; used herein means to include a pressureless sintered body, hot pressed or hot isostatic pressed sintered body and the like, which may occasionally contain various additives. However, the additives contained in the silicon nitride sintered body to be covered according to the invention are insignificant because the oxidation resistance or the wear resistance is improved by coating, so that it is possible to use any kind of silicon nitride sintered bodies. Further, the thickness of the silicon carbide coating is dependent on the producing method, but it is preferably within a range of 10-300 μm. This is due to the fact that when the thickness is less than 10 μm, it is difficult to obtain a uniform coating, while when it exceeds 300 μm, there remarkably appears a poor adhesion such as separation or the like resulting from the difference in thermal expansion coefficients between the silicon carbide coating and the silicon nitride sintered body. Furthermore, it is favorable that the thermal expansion coefficient of the silicon nitride sintered body to be covered be close to that of the silicon carbide as a coating. 
     The production of the silicon nitride sintered body according to the invention will be described below. 
     Although the silicon nitride sintered body generally has an as-sintered surface or a worked surface, either of these surfaces may be covered with the coating of silicon carbide. 
     The following processes can be utilized for forming the silicon carbide coating a CVD process in a which SiCl  4  gas or the like is reacted with C 3  H 8  gas or the like in vapor phase to form a coating on the sintered body surface, a reaction sintering coating; process in which carbon is applied to the sintered body surface and then immersed in molten Si and reacted therewith at a temperature of 1,600°-1,700° C. to form a coating, a surface reacting carbonization process in which carbon is applied to the sintered body surface and heated at a temperature of 1,700° C. to conduct the formation of a coating through the following reaction; 
     
         Si.sub.3 N.sub.4 +3C→3SiC+2N.sub.2. 
    
     Incidentally, according to any of the above-mentioned processes, the formed silicon carbide is α type or β type crystalline. 
     According to the invention, it is desirable that the thermal expansion coefficient of the silicon nitride sintered body is increased to be made close to that of silicon carbide as a coating. For this purpose, it is preferable that on the grain boundary of the silicon nitride sintered body is precipitated a crystalline phase having a high thermal expansion such as forsterite (2MgO·SiO 2 ), enstatite (MgO·SiO 2 ), yttrium silicate (Y 2  Si 2  O 7 ), yttrium-silicon-oxygen-nitrogen (Y--Si--O--N) or the like, or a solid solution thereof. 
     The following examples are given in the illustration of the invention and are not intended as limitations thereof. 
     EXAMPLE 1 
     On an as-sintered or worked surface of a silicon nitride sintered body having a composition and a thermal expansion coefficient as shown in the following Table 1 was formed a coating of silicon carbide having a crystalline structure as shown in Table 1 by a CVD process using raw gases and temperatures shown in Table 1, to thereby obtain silicon nitride sintered bodies (Nos. 1-10) according to the invention. 
     Samples of the silicon nitride sintered bodies (Nos. 1-10) having a diameter of 30 mm and a thickness of 10 mm, the surfaces of which are covered with a silicon carbide coating having a thickness of 10-150 μm, and samples of silicon nitride sintered bodies (Nos. 11-20) as Comparative Examples having a diameter of 30 mm and a thickness of 10 mm and no silicon carbide coating, were oxidized at a temperature of 1,400° C. for 100 hours to measure a weight gain by oxidation, from which a weight gain by oxidation per unit area was calculated. The obtained results are shown in Table 1. 
     Further, samples were also prepared wherein the curved surface of each of the silicon nitride sintered bodies (Nos. 1-10) having a diameter of 35 mm and a thickness of 8 mm was covered with a silicon carbide coating having a thickness of 150-300 μm, samples wherein one surface of 6 mm×16 mm of rectangular parallelpiped silicon nitride sintered bodies having a block form of 6 mm×16 mm×10 mm was covered with the silicon carbide coating, and samples of Comparative Examples wherein silicon nitride sintered bodies (Nos. 11, 13, 15, 17 and 19) having the same form as the above block form and no silicon carbide coating. With respect to these samples, the surface covered with the coating was mirror-finished to conduct a wear test at a temperature of 600° C. through a block on ring type machine. In this test, a load was 2 kg, a rotating speed was 1,000 rpm and a test period was 10 hours. From the wear volume of the rectangular parallelpiped samples was calculated a specific wear rate. The obtained results are shown in Table 1. 
     
                                           TABLE 1                                 
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                   Thermal                                                
                   expansion                                              
                   coeffi-                Weight                          
                                               Specific                   
       Composi-                                                           
              Surface                                                     
                   ceint of          Crystal-                             
                                          gain by                         
                                               wear                       
       tion of                                                            
              to be                                                       
                   sintered                                               
                         CVD process                                      
                                Temper-                                   
                                     line oxidation                       
                                               rate                       
No.    additive                                                           
              treated                                                     
                   body (/°C.)                                     
                         raw gas                                          
                                ature                                     
                                     type (mg/cm.sup.2)                   
                                               (mm.sup.2 /N)              
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Invention 1                                                               
       MgO    worked                                                      
                   4.0 × 10.sup.-6                                  
                         SiCl.sub.4, C.sub.3 H.sub.8                      
                                1,350                                     
                                     β                               
                                          0.6  3 × 10.sup.-9        
2      &#34;      sintered                                                    
                   &#34;     &#34;      1,350                                     
                                     β                               
                                          0.7  5 × 10.sup.-9        
3      MgO, Y.sub.2 O.sub.3                                               
              worked                                                      
                   3.9 × 10.sup.-6                                  
                         Si(CH.sub.3).sub.4                               
                                1,400                                     
                                     α,β                       
                                          0.5  2 × 10.sup.-9        
4      &#34;      sintered                                                    
                   &#34;     &#34;      1,400                                     
                                     α,β                       
                                          0.6  2 × 10.sup.-9        
5      Y.sub.2 O.sub.3                                                    
              worked                                                      
                   4.1 × 10.sup.-6                                  
                         SiCl.sub.4, C.sub.3 H.sub.8                      
                                1,400                                     
                                     β                               
                                          0.6  3 ×  10.sup.-9       
6      &#34;      sintered                                                    
                   &#34;     &#34;      1,400                                     
                                     β                               
                                          0.4   6 × 10.sup.-10      
7      MgO, Al.sub.2 O.sub.3                                              
              worked                                                      
                   4.3 × 10.sup.-6                                  
                         SiCl.sub.2, (CH.sub.3).sub.2                     
                                1,350                                     
                                     α,β                       
                                          0.4   5 × 10.sup.-10      
8      &#34;      sintered                                                    
                   &#34;     &#34;      1,350                                     
                                     α,β                       
                                          0.5   8 × 10.sup.-10      
9      Al.sub.2 O.sub.3, Y.sub.2 O.sub.3                                  
              worked                                                      
                   3.7 × 10.sup.-6                                  
                         SiCl.sub.4, C.sub.3 H.sub.8                      
                                1,400                                     
                                     β                               
                                          1.2  7 × 10-9             
10     &#34;      sintered                                                    
                   &#34;     &#34;      1,400                                     
                                     β                               
                                          1.4  6 × 10.sup.-9        
Comparative                                                               
Example 11                                                                
       MgO    worked                                                      
                   4.0 × 10.sup.-6  &gt;5   4 × 10.sup.-7        
12     &#34;      sintered                                                    
                   &#34;                      &gt;5   &#34;                          
13     MgO, Y.sub.2 O.sub.3                                               
              worked                                                      
                   3.9 × 10.sup.-6  &gt;5   9 × 10.sup.-8        
14     &#34;      sintered                                                    
                   &#34;                      2.3  &#34;                          
15     Y.sub.2 O.sub.3                                                    
              worked                                                      
                   4.1 × 10.sup.-6  4.2  6 × 10.sup.-8        
16     &#34;      sintered                                                    
                   &#34;                      &gt;5   &#34;                          
17     MgO, Al.sub.2 O.sub.3                                              
              worked                                                      
                   4.3 × 10.sup.-6  &gt;5   3 × 10.sup.-7        
18     &#34;      sintered                                                    
                   &#34;                      &gt;5   &#34;                          
19     Al.sub.2 O.sub.3, Y.sub.2 O.sub.3                                  
              worked                                                      
                   3.7 × 10.sup.-6  &gt;5   8 × 10.sup.-8        
20     &#34;      sintered                                                    
                   &#34;                      &gt;5   &#34;                          
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     As seen from the results of Table 1, the silicon nitride sintered bodies covered with the silicon carbide coating by CVD process according to the invention exhibit excellent oxidation resistance and wear resistance as compared with the silicon nitride sintered bodies having no coating. 
     EXAMPLE 2 
     Carbon was applied to a surface of a silicon nitride sintered body having a composition and a thermal expansion coefficient as shown in the following Table 2 and then reacted with molten Si at a temperature as shown in Table 2 to form a coating of β-type crystalline silicon carbide on the surface of the sintered body, which was polished to obtain a silicon nitride sintered body covered with a coating of silicon carbide (Nos. 21-23) according to the invention. These silicon nitride sintered bodies were subjected to the oxidation test and the wear test in the same manner as in Example 1, together with Comparative Examples of silicon nitride sintered bodies having no coating of silicon carbide (Nos. 24-26). The measured results of weight gain by oxidation and specific wear rate are shown in Table 2. 
     
                                           TABLE 2                                 
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                   Thermal                                                
                   expansion                                              
                         Reaction                                         
                              Weight                                      
                                   Specific                               
       Composi-                                                           
              Surface                                                     
                   coefficient                                            
                         Temper-                                          
                              grain by                                    
                                   wear                                   
       tion of                                                            
              to be                                                       
                   of sintered                                            
                         ature                                            
                              oxidation                                   
                                   rate                                   
No.    additive                                                           
              treated                                                     
                   body (/°C.)                                     
                         (°C.)                                     
                              (mg/cm.sup.2)                               
                                   (mm.sup.2 /N)                          
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Invention 21                                                              
       MgO    worked                                                      
                   4.0 × 10.sup.-6                                  
                         1,600                                            
                              0.9  8 × 10.sup.-7                    
22     MgO, Y.sub.2 O.sub.3                                               
              sintered                                                    
                   3.9 × 10.sup.-6                                  
                         1,650                                            
                              0.8  9 × 10.sup.-8                    
23     Al.sub.2 O.sub.3, Y.sub.2 O.sub.3                                  
              worked                                                      
                   3.7 × 10.sup.-6                                  
                         1,700                                            
                              1.3  5 × 10.sup.-7                    
Comparative                                                               
Example 24                                                                
       MgO    worked                                                      
                   4.0 × 10.sup.-6                                  
                              &gt;5   4 × 10.sup.-7                    
25     MgO,Y.sub.2 O.sub.3                                                
              sintered                                                    
                   3.9 × 10.sup.-6                                  
                              &gt;5   9 × 10.sup.-8                    
26     Al.sub.2 O.sub.3, Y.sub.2 O.sub.3                                  
              worked                                                      
                   3.7 × 10.sup.-6                                  
                              &gt;5   8 × 10.sup.-8                    
__________________________________________________________________________
 
    
     As apparent from the results of Table 2, the silicon nitride sintered bodies covered with the silicon carbide coating according to the invention, which were prepared by the surface reaction process, have considerably improved oxidation resistance as compared with the silicon nitride sintered bodies having no silicon carbide coating, but the improvement of wear resistance is not observed. 
     EXAMPLE 3 
     Carbon was applied to a surface of a silicon nitride sintered body having a composition and a thermal expansion coefficient as shown in the following Table 3 and then reacted with the silicon nitride sintered body at a temperature as shown in Table 3 to obtain a silicon nitride sintered body having a coating of β-type crystalline silicon carbide (Nos. 27-29). These sintered bodies were subjected to the oxidation test and the wear test in the same manner as in Example 1, together with silicon nitride sintered bodies having no coating of silicon carbide (Nos. 30-32) as Comparative Examples. The measured results of weight gain by oxidation and specific wear rate are shown in Table 3. 
     
                                           TABLE 3                                 
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                  Thermal-                                                
                  expansion                                               
                        Reaction                                          
                             Weight                                       
                                  Specific                                
       Composi-                                                           
             Surface                                                      
                  coefficient                                             
                        Temper-                                           
                             grain by                                     
                                  wear                                    
       tion of                                                            
             to be                                                        
                  of sintered                                             
                        ature                                             
                             oxidation                                    
                                  rate                                    
No.    additive                                                           
             treated                                                      
                  body (/°C.)                                      
                        (°C.)                                      
                             (mg/cm.sup.2)                                
                                  (mm.sup.2 /N)                           
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Invention 27                                                              
       MgO   worked                                                       
                  4.0 × 10.sup.-6                                   
                        1,750                                             
                             0.9  7 × 10.sup.-8                     
28     MgO,Y.sub.2 O.sub.3                                                
             sintered                                                     
                  3.9 × 10.sup.-6                                   
                        1,700                                             
                             0.9  8 × 10.sup.-8                     
29     Y.sub.2 O.sub.3                                                    
             worked                                                       
                  4.1 × 10.sup.-6                                   
                        1,800                                             
                             1.1  6 × 10.sup. -8                    
Comparative                                                               
Example 30                                                                
       MgO   worked                                                       
                  4.0 × 10.sup.-6                                   
                             &gt;5   4 × 10.sup.-7                     
31     MgO, Y.sub.2 O.sub.3                                               
             sintered                                                     
                  3.9 × 10.sup.-6                                   
                             &gt;5   9 × 10.sup.-8                     
Example 32                                                                
       Y.sub.2 O.sub.3                                                    
             worked                                                       
                  4.1 × 10.sup.-6                                   
                             2.3  6 × 10.sup.-8                     
__________________________________________________________________________
 
    
     As apparent from the results of Table 3, the silicon nitride sintered bodies having the silicon carbide coating according to the invention, which were prepared by the surface reaction process, have a considerably improved oxidation resistance as compared with the silicon nitride sintered bodies having no silicon carbide coating, but the improvement of wear resistance is not observed. 
     As mentioned above, the silicon nitride sintered body covered with the silicon carbide coating according to the invention possesses not only a high-temperature strength resulting from the silicon nitride sintered body itself but also high oxidation resistance and wear resistance resulting from the silicon carbide, which is applied to high temperature structural members such as engine parts, for example, a gas turbine engine rotor, a turbocharger rotor and the like to thereby improve the oxidation resistance or wear resistance thereof. 
     Further, according to the invention, the silicon carbide coating is particularly formed on the silicon nitride sintered body by CVD process, whereby the oxidation resistance and the wear resistance can simultaneously be improved.