Boride-based refractory materials
BACKGROUND OF THE INVENTION 
     The present invention relates to a novel class of boride-based refractory, abrasive materials or, more particularly, to a class of refractory and abrasive materials based on certain metal diborides as sintered with high density and high mechanical strengths even at elevated temperatures. 
     With their extremely high melting points and hardness as well as excellent mechanical strengths at elevated temperatures, metal diborides such as titanium diboride are the most promising materials for construction of rockets, cutting tools, parts of thermal engines and the like. When metal diborides are to be sintered for use in these applications, however, difficulties are encountered because the sintering temperature is extremely high and the sintered bodies obtained are poor in the bending strength and relatively brittle regardless of whether the metal diborides are used either singly or as a combination of two or more. 
     The inventors conducted extensive investigations to solve the above problems in the sintered bodies of metal diborides and discovered that titanium diboride could give excellent sintered bodies when admixed with an alloy of nickel and phosphorus (see, for example, Japanese Patent Publication No. 51-23625). 
     These titanium diboride-based sintered bodies are developed by the inventors possess a bending strength reaching 100 to 150 kg/mm 2  but, needless to say, even higher bending strengths are sometimes required in the practical use of these kinds of refractory materials. In addition, the heat resistance of the above developed titanium diboride-based refractory materials is not always sufficient due to the use of a nickel-phosphorus alloy with a relatively low melting point down to 890° C. 
     Another problem of the metal diboride-based sintered bodies for refractory and abrasive uses is that they are prepared by sintering at an undesirably high sintering temperature of 1800° C. or higher. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a novel sintered body for refractory and abrasive uses based on metal diborides with a bending strength as high as 150 to 200 kg/mm 2  and an excellent heat resistance as evaluated by the hardness at elevated temperatures. 
     Another object of the present invention is to provide a novel sintered body for refractory and abrasive uses based on metal diborides, which has a high bending strength and an excellent heat resistance and yet can be prepared by sintering at a relatively low sintering temperature. 
     Thus the sintered body according to the present invention comprises 
     (a) from 99.99 to 70% by weight of a combination of at least two of the metal diborides selected from the group consisting of TiB 2 , CrB 2 , TaB 2 , MnB 2 , MoB 2 , YB 2 , VB 2 , HfB 2 , NbB 2 , AlB 2  and ZrB 2 , and 
     (b) from 0.01 to 30% by weight of a metal boride or a combination of metal borides selected from the group consisting of borides of nickel, iron and cobalt. 
     When properly formulated and sintered at about 1800° C., the sintered body with the above composition may have a sufficiently high bending strength of 150 kg/mm 2  or larger and a heat resistance as evaluated by the Vickers hardness at 1000° C. of 2000 kg/mm 2  or higher. 
     It is also proposed that the above given sintering temperature can be lowered to about 1600° C. without substantially affecting the properties of the sintered body by adding, in addition to the above given ingredients (a) and (b), from 0.001 to 15% by weight, based on the weight of the sintered body, of a mixture or an alloy composed of from 97 to 80% by weight of nickel and from 3 to 20% by weight of phosphorus. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As is described above, the component (a) of the inventive sintered body is a combination of two kinds or more of metal diborides selected from the group consisting of diborides of titanium, chromium, tantalum, manganese, molybdenum, yttrium, vanadium, hafnium, niobium, aluminum and zirconium. In order to fully obtain the beneficial effect, e.g. high heat resistance, given by combining two kinds or more of the metal diborides, it is recommended that the content of each of the metal diborides in the sintered body is at least 0.5% by weight. It is also desirable that one of the metal diborides as combined is titanium diboride TiB 2  or, rather, substantial portion, say, 20% by weight or more, in the sintered body is titanium diboride in consideration of the high melting point and relatively low cost. 
     The second component of the inventive sintered body which serves as a binder in the sintered body is a metal boride or a combination of metal borides selected from borides of nickel, iron and cobalt. 
     The borides of nickel, iron and cobalt each comprise several kinds of the borides according to the molar proportion of boron and the metallic element. For example, the nickel borides include NiB, Ni 4  B 3 , Ni 2  B and Ni 3  B, the iron borides include FeB and Fe 2  B and the cobalt borides include CoB, Co 2  B and Co 3  B. These borides may be used as a mixture of two or more. Preferred borides are NiB, Ni 4  B 3 , FeB and CoB. 
     The amount of the metal boride or borides in the sintered body should be in the range from 0.01 to 30% by weight or, preferably, from 0.05 to 15% by weight, especially, when the sintered body is required to withstand an extremely high temperature as in cutting tools, the balance being the combination of the metal diborides as the component (a), since smaller amounts than 0.01% by weight cannot give sufficient mechanical strengths to the sintered body which excessive amounts over 30% by weight result in marked decrease in the heat resistance of the sintered body. 
     It should be noted that the metal boride or borides as the second component in an amount in excess over 15% by weight may be disadvantageous if the mechanical strengths of the sintered body at an elevated temperature are of utmost importance because the Vickers hardness of the sintered body at 1000° C. is decreased to about 1200 kg/mm 2  or below when the amount of the metal boride or borides exceeds 15% by weight in contrast to the extremely high Vickers hardness at 1000° C. of 2000 to 2400 kg/mm 2  or even higher with smaller amounts of the metal boride or borides. The above disadvantage is, however, not significant when the sintered body is directed to the use such as parts for thermal engines where no such an extremely high heat resistance is required. On the contrary, advantages are obtained by increasing the amount of the metal borides over 15% by weight in the possibility of lowering the sintering temperature down to, say, 1600° C. in contrast to 1800° C. necessitated for sintering when the amount of the metal boride is larger as well as in obtaining a somewhat increased bending strength of the sintered body. 
     The metal diborides and the borides of nickel, iron and cobalt to be used in the present invention have desirably a particle size distribution such that the average particle diameter is 5 μm of smaller. 
     It is noted that substantially the same effect is obtained by the replacement of the borides of nickel, iron or cobalt with an ingredient or ingredients which can form the respective boride in the course of sintering at an elevated temperature. For example, a mixture of a powder of elementary boron and a metal powder of nickel, iron or cobalt in a proper molar mixing ratio can substitute the boride of nickel, iron or cobalt, respectively. 
     The boride-based sintered body for refractory and abrasive use in accordance with the present invention is prepared in a manner as follows. The sintering is performed by the techniques of hot pressing, by sintering of a green body prepared in advance by compression molding of the powder mixture at room temperature or by the techniques of so-called infiltration. Typically, a powder mixture obtained by intimate mixing of the ingredients is packed in a mold made of, for example, graphite and subjected to sintering by heating at a temperature of 1300° C. or higher under a pressure of 50 to 300 kg/cm 2  for 10 to 60 minutes in vacuum or in an atmosphere of a neutral or reducing gas such as argon or hydrogen. Alternatively, the powder mixture as above is shaped in advance into a green body by compression molding under a pressure in the same range as above at room temperature and the green body is subjected to sintering similarly. 
     The atmospheric condition in the sintering step is also of some significance. When the sintering is carried out in vacuum, for example, trace amounts of remaining oxygen may adversely affect the mechanical properties of the sintered bodies, especially, when the powder mixture under sintering contains a component susceptible to oxidation such as AlB 2  and ZrB 2 . In this regard, the degree of vacuum should be as good as possible or, desirably, 1×10 -2  mmHg or better. Alternatively, it is recommended to carry out the sintering in an atmosphere of a reducing gas such as hydrogen rather than in a vacuum of 0.1 mmHg or higher in pressure when the powder mixture under sintering contains AlB 2  or ZrB 2 . 
     As is mentioned earlier, an additional advantage is obtained by admixing the above described components (a) and (b) with a third component (c) which is a mixture or an alloy of nickel and phosphorus. The alloy of nickel and phosphorus here mentioned is an alloy composed of 97 to 80% by weight of nickel and 3 to 20% by weight of phosphorus and the mixture of metallic nickel and elementary phosphorus may be composed of the same weight ratio of nickel and phosphorus both in powdery forms to give an equivalent effect to the use of the nickel-phosphorus alloy. 
     The amount of the mixture or alloy of nickel and phosphorus to be contained in the sintered body is in the range from 0.001 to 15% by weight based on the amount of the sintered body because any smaller amounts than 0.001% by weight cannot give a desired effect of lowering the sintering temperature while an excessive amount over 15% by weight results in an undesirable lowering of the heat resistance of the sintered body. 
     The procedure for the preparation of a sintered body with admixture of the mixture or alloy of nickel and phosphorus is substantially the same as in the case without the addition of the mixture or alloy of nickel and phosphorus excepting the possibility of further lowering the sintering temperature down to about 1000° C. 
     The sintered bodies prepared as described above in accordance with the present invention are suitable for use as a refractory and abrasive material with their high mechanical strengths, e.g. bending strength and hardness, especially, at elevated temperatures so that they can find wide applications in the fields of cutting tools, materials for rocket construction, parts for thermal engines, anti-abrasive materials and the like. 
    
    
     Following are the examples to illustrate the present invention in further detail, in which percentages are all given by % by weight. 
     EXAMPLE 1. (EXPERIMENTS NO. 1 TO NO. 86) 
     Powdery mixtures were prepared each by admixing and intimately blending titanium diboride TiB 2  with a second metal diboride which was one of TaB 2 , NbB 2 , VB 2 , CrB 2 , MoB 2 , MnB 2 , HfB 2 , AlB 2 , ZrB 2  and YB 2  and one or a combination of the binder borides NiB, Ni 4  B 3 , FeB and CoB in amounts from 2.2 to 50% and from 0.01 to 30%, respectively, the balance being the titanium diboride, and each of the powdery mixtures was subjected to sintering by the techniques of hot pressing for 30 minutes at a temperature of 1600° to 1800° C. in a graphite mold under a pressure of 50 to 300 kg/cm 2  in vacuum of 0.1 to 0.5 mmHg or in an atmosphere of hydrogen or argon. The formulations and the sintering conditions are summarized in Tables 1--1 to 1-3. In Experiments No. 62 to No.  65 and No. 71 to No. 74 where the second metal diboride was AlB 2  or ZrB 2 , the pressure of the vacuum atmosphere was maintained at 1×10 -3  mmHg or lower. In some of the experiments, the powder mixture was shaped by compression molding at room temperature under a pressure as indicated in the tables into green bodies which were then subjected to sintering for 60 minutes at a temperature and in an atmosphere also indicated in the tables. 
     The thus obtained sintered bodies were subjected to the measurements of the relative density, bending strength, and Vickers hardness either at room temperature or at 1000° C. The determination of the bending strength was carried out substantially according to the procedure specified in JIS H 5501 though with somewhat modified dimensions of the test specimens. The results of the measurements are set out in Tables 2-1 to 2-3. 
     For comparison, the second metal diboride in the above formulations was omitted and titanium diboride was admixed with either one of NiB, CoB and FeB in an amount of 1% or 10%, the balance being the titanium diboride. Each of the powder mixtures was subjected to sintering by hot pressing at 1600° C. for 30 minutes in vacuum under a pressure of 200 kg/cm 2  or 165 kg/cm 2 . The bending strength of the thus sintered bodies was determined at room temperature and the results are set out in Table 3 together with the relative density of the sintered bodies as well as with the formulation and the pressure under which sintering was carried out. 
     EXAMPLE 2. (EXPERIMENTS NO. 87 TO NO. 104) 
     The experimental procedures were substantially the same as in Example 1 except that the metal diboride as the base component was TaB 2 , NbB 2  or VB 2  instead of TiB 2 . Thus, the base diboride, i.e. TaB 2 , NbB 2  or VB 2 , was admixed with titanium diboride and either one of NiB, Ni 4  B 3 , FeB and CoB in amounts of 
     from 1 to 40% and from 0.5 to 1.0%, respectively, the balance being the base diboride, and sintering of each of the powder mixtures was carried out by the techniques of hot pressing as in Example 1 at a temperature from 1600° to 1800° C. under a pressure of 230 kg/cm 2  in vacuum. 
     The formulation, sintering temperature and the results of the measurements of the mechanical strengths of the sintered bodies are summarized in Table 4. The relative density of the sintered bodies was 100% in each of the experiments. 
     
                                           TABLE 1-1                               
__________________________________________________________________________
                         Sintering conditions                             
Exp.                                                                      
   Second diboride,                                                       
            Binder boride, %   Pressure,                                  
No.                                                                       
   %        NiB                                                           
               Ni.sub.4 B.sub.3                                           
                   FeB                                                    
                      CoB                                                 
                         Temp., ° C.                               
                               kg/cm.sup.2                                
                                    Atmosphere                            
__________________________________________________________________________
1       5   -- --  -- 1  1800  230  Vacuum                                
2       5   1  --  -- -- 1800  230  Vacuum                                
3       5   1  --  -- -- 1800  --*  Vacuum                                
4  TaB.sub.2                                                              
        5   -- 0.5 -- -- 1800  230  Hydrogen                              
5       5   -- --  1  -- 1800  230  Vacuum                                
6       5   0.5                                                           
               --  0.5                                                    
                      0.5                                                 
                         1800  230  Vacuum                                
7       5   -- --  0.5                                                    
                      -- 1800  --*  Argon                                 
8       40  -- --  -- 0.1                                                 
                         1800  300  Vacuum                                
9       20  30 --  -- -- 1600   50  Vacuum                                
10      5   1  --  -- -- 1800  230  Vacuum                                
11      5   1  --  -- -- 1800  230  Hydrogen                              
12      5   1  --  -- -- 1800  230  Argon                                 
13      5   1  --  -- -- 1800  --*  Vacuum                                
14 NbB.sub.2                                                              
        5   -- --  -- 1  1800  230  Vacuum                                
15      5   -- --  1  -- 1800  230  Vacuum                                
16      50  -- --  3  -- 1800  230  Vaccum                                
17      10  0.1                                                           
               --  -- -- 1800  300  Vacuum                                
18      5   -- --  30 -- 1700   50  Vacuum                                
19      5    0.05                                                         
                0.05                                                      
                    0.05                                                  
                       0.05                                               
                         1800  300  Vacuum                                
20      5   1  --  -- -- 1800  230  Vacuum                                
21      5   -- 1   -- -- 1800  230  Vacuum                                
22      5   -- --  -- 1  1800  230  Vacuum                                
23      5   -- --  1  -- 1800  230  Vacuum                                
24 VB.sub.2                                                               
        5   0.5                                                           
               --  0.5                                                    
                      0.5                                                 
                         1800  230  Vacuum                                
25      5   -- --  -- 1  1600  --*  Vacuum                                
26      50  -- --  -- 1  1800  230  Vacuum                                
27      10  -- --  -- 0.1                                                 
                         1800  300  Vacuum                                
28      5   -- --  0.1                                                    
                      -- 1800  300  Hydrogen                              
29      50  30 --  -- -- 1600   50  Vacuum                                
__________________________________________________________________________
 *Green bodies were shaped by compression molding at room temperature     
 followed by sintering at the temperature and in the atmosphere indicated.
 
    
     
                                           TABLE 1-2                               
__________________________________________________________________________
                         Sintering conditions                             
Exp.                                                                      
   Second diboride,                                                       
            Binder boride, %   Pressure                                   
No.                                                                       
   %        NiB                                                           
               Ni.sub.4 B.sub.3                                           
                   FeB                                                    
                      CoB                                                 
                         Temp., ° C.                               
                               kg/cm.sup.2                                
                                    Atmosphere                            
__________________________________________________________________________
30      2.2 1  --  -- -- 1800  230  Vacuum                                
31      2.2 -- 1   -- -- 1800  230  Vacuum                                
32      2.2 -- --  -- 1  1800  230  Vacuum                                
33      2.2 -- --  1  -- 1800  230  Vacuum                                
34      2.2 -- --  -- 1  1800  230  Argon                                 
35 CrB.sub.2                                                              
        2.2 -- --  -- 1  1800  230  Hydrogen                              
36      2.2 -- --  1  -- 1800  --*  Vacuum                                
37      2.2 1  --  -- -- 1600  --*  Hydrogen                              
38      2.2 -- --  -- 0.1                                                 
                         1800  300  Vacuum                                
39      3.9 -- --  -- 20 1600   50  Vacuum                                
40      2.2 0.1                                                           
               --  0.1                                                    
                      0.1                                                 
                         1800  300  Vacuum                                
41      5   1  --  -- -- 1800  230  Vacuum                                
42      5   -- 1   -- -- 1800  230  Vacuum                                
43      5   -- --  -- 1  1800  230  Vacuum                                
44      5   -- --  1  -- 1800  230  Vacuum                                
45 MoB.sub.2                                                              
        5   -- --  -- 1  1800  --*  Vacuum                                
46      5   -- --  1  -- 1800  230  Hydrogen                              
47      50  -- --  -- 1  1800  230  Vacuum                                
48      5   -- --  -- 0.1                                                 
                         1800  300  Vacuum                                
49      5   -- --  30 -- 1600   50  Vacuum                                
50      5   0.5                                                           
               --  0.5                                                    
                      0.5                                                 
                         1800  230  Vacuum                                
51      5   1  --  -- -- 1800  230  Vacuum                                
52      5   -- 1   -- -- 1800  230  Vacuum                                
53 MnB.sub.2                                                              
        5   -- --  -- 1  1800  230  Vacuum                                
54      5   -- --  1  -- 1800  230  Vacuum                                
55      5   0.5                                                           
               --  0.5                                                    
                      0.5                                                 
                         1800  230  Vacuum                                
56      5   0.5                                                           
               --  0.5                                                    
                      0.5                                                 
                         1800  --*  Vacuum                                
__________________________________________________________________________
 
    
     
                                           TABLE 1-3                               
__________________________________________________________________________
                         Sintering conditions                             
Exp.                                                                      
   Second diboride                                                        
            Binder boride, %   Pressure                                   
No.                                                                       
   %        NiB                                                           
               Ni.sub.4 B.sub.3                                           
                   FeB                                                    
                      CoB                                                 
                         Temp., ° C.                               
                               kg/cm.sup.2                                
                                    Atmosphere                            
__________________________________________________________________________
57      5   1  --  -- -- 1800  230  Vacuum                                
58      5   -- 1   -- -- 1800  230  Vacuum                                
59 HfB.sub.2                                                              
        5   -- --  -- 1  1800  230  Vacuum                                
60      5   -- --  1  -- 1800  230  Vacuum                                
61      5   0.5                                                           
               --  0.5                                                    
                      0.5                                                 
                         1800  230  Vacuum                                
62      5   1  --  -- -- 1300  300  Vacuum                                
63      5   -- 1   -- -- 1500  230  Vacuum                                
64      5   -- --  -- 1  1500  230  Vacuum                                
65      5   -- --  1  -- 1600  230  Vacuum                                
66 AlB.sub.2                                                              
        5   1  --  -- -- 1300  300  Hydrogen                              
67      5   -- 1   -- -- 1500  230  Hydrogen                              
68      5   -- --  -- 1  1500  230  Hydrogen                              
69      5   -- --  1  -- 1600  230  Hydrogen                              
70      5   0.5                                                           
               --  0.5                                                    
                      0.5                                                 
                         1600  230  Hydrogen                              
71      5   1  --  -- -- 1800  230  Vacuum                                
72      5   -- 1   -- -- 1800  230  Vacuum                                
73      5   -- --  -- 1  1800  230  Vacuum                                
74      5   -- --  1  -- 1800  230  Vacuum                                
75 ZrB.sub.2                                                              
        5   0.5                                                           
               --  -- -- 1800  230  Vacuum                                
76      5   -- 0.5 -- -- 1800  230  Hydrogen                              
77      5   -- --  -- 0.5                                                 
                         1800  230  Hydrogen                              
78      5   -- --  0.5                                                    
                      -- 1800  230  Hydrogen                              
79      5   0.5                                                           
               --  0.5                                                    
                      0.5                                                 
                         1800  230  Hydrogen                              
80 YB.sub.2                                                               
        5   -- --  -- 0.5                                                 
                         1800  230  Vacuum                                
__________________________________________________________________________
 
    
     
                       TABLE 2-1                                                   
______________________________________                                    
     Bending                                                              
Exp. strength, Vickers hardness, kg/mm.sup.2, at                          
                                  Relative                                
No.  kg/mm.sup.2                                                          
               room temp.  1000° C.                                
                                    density, %                            
______________________________________                                    
1    180       3300        2200     100                                   
2    150       3300        2300     100                                   
3    150       3200        2200     99.9                                  
4    140       2900        2100     100                                   
5    170       3300        2400     100                                   
6    180       3300        2200     100                                   
7    140       3000        2100     99.9                                  
8    160       3000        2200     100                                   
9    180       2000        1000     100                                   
10   170       3200        2300     100                                   
11   170       3000        2200     100                                   
12   150       2900        2000     99.9                                  
13   160       3000        2100     99.9                                  
14   180       3200        2300     100                                   
15   160       3300        2400     100                                   
16   170       3000        2200     100                                   
17   150       3300        2300     100                                   
18   160       2000        1100     100                                   
19   170       3200        2400     100                                   
20   150       3300        2300     100                                   
21   140       3200        2300     100                                   
22   160       3300        2400     100                                   
23   150       3300        2400     100                                   
24   170       3200        2300     100                                   
25   160       3200        2300     99.9                                  
26   140       3000        2000     100                                   
27   150       3300        2400     100                                   
28   150       3300        2300     99.9                                  
29   140       2000        1100     100                                   
______________________________________                                    
 
    
     
                       TABLE 2-2                                                   
______________________________________                                    
     Bending                                                              
Exp. strength, Vickers hardess, kg/mm.sup.2, at                           
                                  Relative                                
No.  kg/mm.sup.2                                                          
               room temp.  1000° C.                                
                                    density, %                            
______________________________________                                    
30   150       3200        2300     100                                   
31   140       3100        2200     100                                   
32   180       3300        2300     100                                   
33   170       3200        2300     100                                   
34   160       3200        2100     99.9                                  
35   170       3300        2300     100                                   
36   160       3100        2200     100                                   
37   150       3100        2200     100                                   
38   160       3300        2400     100                                   
39   150       2100        1200     100                                   
40   180       3300        2400     100                                   
41   160       3200        2200     100                                   
42   150       3100        2100     100                                   
43   180       3300        2400     100                                   
44   170       3200        2200     100                                   
45   160       3100        2100     100                                   
46   160       3200        2200     100                                   
47   160       3100        2100     100                                   
48   150       3000        2300     99.9                                  
49   180       2200        1100     100                                   
50   180       3300        2200     100                                   
51   180       3300        2400     100                                   
52   160       3100        2200     100                                   
53   180       3300        2300     100                                   
54   170       3200        2300     100                                   
55   180       3300        2200     100                                   
56   160       3000        2000     100                                   
______________________________________                                    
 
    
     
                       TABLE 2-3                                                   
______________________________________                                    
     Bending                                                              
Exp. strength, Vickers hardness, kg/mm.sup.2, at                          
                                  Relative                                
No.  kg/mm.sup.2                                                          
               room temp.  1000° C.                                
                                    density, %                            
______________________________________                                    
57   160       3200        2200     100                                   
58   150       3100        2100     100                                   
59   180       3300        2300     100                                   
60   170       3300        2400     100                                   
61   180       3300        2300     100                                   
62   120       3000        2100     100                                   
63   110       2900        2000     100                                   
64   130       3200        2100     100                                   
65   120       3200        2200     100                                   
66   150       3100        2200     100                                   
67   140       3100        2100     100                                   
68   160       3200        2200     100                                   
69   150       3300        2300     100                                   
70   170       3200        2100     100                                   
71   110       3000        2000     100                                   
72   100       2900        1800     100                                   
73   120       3100        2200     100                                   
74   120       3200        2200     100                                   
75   150       3200        2200     100                                   
76   130       3100        2000     100                                   
77   160       3300        2300     100                                   
78   160       3300        2300     100                                   
79   170       3200        2000     100                                   
80   120       3000        2200     100                                   
______________________________________                                    
 
    
     
                       TABLE 3                                                     
______________________________________                                    
                            Bending                                       
     Binder boride,                                                       
                  Sintering stength at                                    
                                      Relative                            
Exp. %            pressure, room temp.,                                   
                                      density,                            
No.  NiB    CoB    FeB  kg/cm.sup.2                                       
                                kg/mm.sup.2                               
                                          %                               
______________________________________                                    
81    1     --     --   200     45        99.3                            
82   --      1     --   200     42        99.8                            
83   --     --      1   200     42        99.8                            
84   10     --     --   165     110       --                              
85   --     10     --   165     80        --                              
86   --     --     10   165     140       --                              
______________________________________                                    
 
    
     EXAMPLE 3. (EXPERIMENTS NO. 105 TO NO. 117) 
     The experimental procedures were substantially the same as in Example 1 except that the number of the metal diborides was increased to at least three including TiB 2  and TaB 2  in all of the experiments. Thus, the amounts of the metal diborides combined with TiB 2  including TaB 2  were each from 5 to 15% and the amounts of the binder borides, i.e. NiB, Ni 4  B 3 , FeB and CoB, were each 1%, the balance being TiB 2 . Sintering of each of the powder mixtures was carried out by the techniques of hot pressing in a graphite mold for 30 minutes at 1800° C. under a pressure of 230 kg/cm 2  in vacuum. 
     The formulation and the results of the measurements of the mechanical strengths of the sintered bodies are summarized in Table 5. The relative density of the sintered bodies was 100% in each of the experiments. 
     EXAMPLE 4. (EXPERIMENTS NO. 118 TO NO. 123) 
     Powder mixtures were prepared each by intimately blending titanium diboride with one or all of the metal diborides TaB 2 , VB 2 , NbB 2 , MoB 2  and CrB 2 , one or all of the binder borides NiB, FeB and CoB and an alloy composed of 92% of nickel and 8% of phosphorus. The amounts of the metal diborides other than titanium diboride, the binder borides and the nickel-phosphorus alloy were each from 1 to 5%, from 0.5 to 1.0% and from 0.01 to 0.5%, respectively, the balance being titanium diboride. 
     
                                           TABLE 4                                 
__________________________________________________________________________
            Binder    Bending                                             
                           Vickers hardness,                              
Exp.                                                                      
   Base TiB.sub.2,                                                        
            boride                                                        
                 Sintering                                                
                      strength,                                           
                           kg/mm.sup.2, at                                
No.                                                                       
   diboride                                                               
        %   (%)  temp., ° C.                                       
                      kg/mm.sup.2                                         
                           room temp.                                     
                                 1000° C.                          
__________________________________________________________________________
87      1   CoB (1)                                                       
                 1800 160  2700  1800                                     
88      20  CoB (1)                                                       
                 1800 170  2800  2000                                     
89 TaB.sub.2                                                              
        40  CoB (1)                                                       
                 1800 170  3000  2100                                     
90      10  FeB (1)                                                       
                 1800 170  2700  1900                                     
91      10  NiB (1)                                                       
                 1800 160  2700  1900                                     
92      10  Ni.sub.4 B.sub.3 (1)                                          
                 1800 140  2600  1700                                     
93      1   NiB (0.5)                                                     
                 1800 130  2300  1200                                     
94      10  NiB (0.5)                                                     
                 1800 140  2500  1300                                     
95 NbB.sub.2                                                              
        40  NiB (0.5)                                                     
                 1800 160  3000  1900                                     
96      5   Ni.sub.4 B.sub.3 (1)                                          
                 1800 120  2200  1200                                     
97      5   CoB (1)                                                       
                 1800 140  2300  1300                                     
98      5   FeB (1)                                                       
                 1800 140  2300  1500                                     
99      1   CoB (1)                                                       
                 1600 100  2000  1300                                     
100     10  CoB (1)                                                       
                 1600 110  2100  1400                                     
101                                                                       
   VB.sub.2                                                               
        40  CoB (1)                                                       
                 1600 120  2300  1600                                     
102     5   NiB (1)                                                       
                 1600 100  2000  1200                                     
103     5   Ni.sub.4 B.sub.3 (1)                                          
                 1600  90  2000  1100                                     
104     5   FeB (1)                                                       
                 1800 120  2100  1300                                     
__________________________________________________________________________
 
    
     
                                           TABLE 5                                 
__________________________________________________________________________
                                      Bending                             
                                           Vickers hardness,              
Exp.                                                                      
   Combined metal diboride, %                                             
                         Binder boride, %                                 
                                      strength                            
                                           kg/mm.sup.2, at                
No.                                                                       
   Ta                                                                     
     Nb                                                                   
       V Cr                                                               
           Mo Mn Hf                                                       
                   Al                                                     
                     Zr                                                   
                       Y NiB                                              
                            Ni.sub.4 B.sub.3                              
                                FeB                                       
                                   CoB                                    
                                      kg/mm.sup.2                         
                                           room temp.                     
                                                 1000° C.          
__________________________________________________________________________
105                                                                       
   5 5 --                                                                 
         --                                                               
           -- -- --                                                       
                   --                                                     
                     --                                                   
                       --                                                 
                         -- --  -- 1  180  3100  2300                     
106                                                                       
   5 --                                                                   
       5 --                                                               
           -- -- --                                                       
                   --                                                     
                     --                                                   
                       --                                                 
                         -- --  -- 1  170  3200  2400                     
107                                                                       
   5 --                                                                   
       --                                                                 
         2 -- -- --                                                       
                   --                                                     
                     --                                                   
                       --                                                 
                         -- --  -- 1  160  3200  2300                     
108                                                                       
   5 --                                                                   
       --                                                                 
         --                                                               
           5  -- --                                                       
                   --                                                     
                     --                                                   
                       --                                                 
                         1  --  -- -- 170  3200  2300                     
109                                                                       
   5 --                                                                   
       --                                                                 
         --                                                               
           -- 5  --                                                       
                   --                                                     
                     --                                                   
                       --                                                 
                         -- --  1  -- 180  3200  2200                     
110                                                                       
   5 --                                                                   
       --                                                                 
         --                                                               
           -- -- 5 --                                                     
                     --                                                   
                       --                                                 
                         -- --  -- 1  170  3300  2400                     
111                                                                       
   5 5 5 --                                                               
           -- -- --                                                       
                   --                                                     
                     --                                                   
                       --                                                 
                         1  --  1  1  180  3300  2400                     
112                                                                       
   10                                                                     
     10                                                                   
       10                                                                 
         --                                                               
           -- -- --                                                       
                   --                                                     
                     --                                                   
                       --                                                 
                         1  --  -- -- 180  3300  2400                     
113                                                                       
   15                                                                     
     15                                                                   
       15                                                                 
         --                                                               
           -- -- --                                                       
                   --                                                     
                     --                                                   
                       --                                                 
                         -- 1   -- -- 170  3200  2400                     
114                                                                       
   5 5 5 --                                                               
           -- -- --                                                       
                   --                                                     
                     --                                                   
                       --                                                 
                         -- --  -- 1  180  3300  2400                     
115                                                                       
   5 5 5 --                                                               
           -- -- --                                                       
                   --                                                     
                     --                                                   
                       --                                                 
                         -- --  1  -- 180  3300  2400                     
116                                                                       
   5 5 5 5 5  -- --                                                       
                   --                                                     
                     --                                                   
                       --                                                 
                         -- --  -- 1  170  3000  2200                     
117                                                                       
   5 5 5 5 5  5  5 5 5 5 1  --  1  1  170  3000  2200                     
__________________________________________________________________________
 
    
     The powder mixtures were subjected to sintering by the techniques of hot pressing for 30 minutes in a graphite mold at a temperature of 1600° or 1700° C. under a pressure of 230 kg/cm 2  in vacuum. 
     The formulation, sintering temperature and the results of the measurements of the mechanical strengths are summarized in Table 6. In one of the experiments, the nickel-phosphorus alloy was replaced with a powder mixture of 80% by weight of nickel and 20% by weight of phosphorus. The relative density of the sintered bodies was 100% in each of the experiments. 
     
                                           TABLE 6                                 
__________________________________________________________________________
              Binder boride,       Bending                                
                                        Vickers hardness,                 
Exp.                                                                      
   Combined diboride, %                                                   
              %        Ni-P alloy,                                        
                             Sintering                                    
                                   strength,                              
                                        kg/mm.sup.2, at                   
No.                                                                       
   Ta                                                                     
     V Nb                                                                 
         Mo Cr                                                            
              NiB                                                         
                 FeB                                                      
                    CoB                                                   
                       %     temp., ° C.                           
                                   kg/mm.sup.2                            
                                        room temp.                        
                                              1000° C.             
__________________________________________________________________________
118                                                                       
   5 --                                                                   
       --                                                                 
         -- --                                                            
              -- -- 1  0.5   1600  190  3200  2000                        
119                                                                       
   --                                                                     
     3 --                                                                 
         -- --                                                            
              1  -- -- 0.01  1600  150  3200  2000                        
120                                                                       
   --                                                                     
     --                                                                   
       5 -- --                                                            
              -- 1  -- 0.005 1700  160  3200  2000                        
121                                                                       
   --                                                                     
     --                                                                   
       --                                                                 
         3  --                                                            
              -- -- 1  0.1** 1600  180  3000  2000                        
122                                                                       
   --                                                                     
     --                                                                   
       --                                                                 
         -- 2 -- -- 1  0.2   1600  160  3000  1900                        
123                                                                       
   1 1 1 1  1 0.5                                                         
                 0.5                                                      
                    0.5                                                   
                       0.1   1600  170  3000  2100                        
__________________________________________________________________________
 **A mixture of 80% by weight of nickel and 20% by weight of phosphorus