Abrasive material based on zirconium corundum a process for its production and its use
This is a continuation of application Ser. No. 08/138,401, filed Oct. 18, 1993 now abandoned. 
    
    
     This invention relates to an abrasive material based on α-Al 2  O 3  and ZrO 2  with a content of titanium compounds in the form of suboxides, carbides and/or oxycarbides, to a process for its production and to its use. 
     BACKGROUND OF THE INVENTION 
     Zirconium corundums, which are produced by extremely rapid cooling of a melt, are distinguished by particularly advantageous abrasive properties. 
     Thus, German Patent Application DE-A 2 519 569 describes an abrasive material based on Al 2  O 3  and ZrO 2 , optionally with accompanying substances or additions consisting of chromium, iron, titanium, vanadium, magnesium and/or rare earths in the form of oxides, carbides, carbonitrides and nitrides for use in precision grinding or semi-precision grinding. The additions are not used to increase grinding performance, but are supposed to be kept as small as possible to avoid reductions in quality. 
     German Patent Application DE-A 2 160 705 describes an abrasive material produced by melting a mixture of Al 2  O 3  with an addition of 5 to 45% by weight ZrO 2 , preferably 25% by weight ZrO 2  and 0 to 4% by weight TiO 2 . 
     German Patent Application DE-A 2 227 462 describes a zirconium corundum which is cooled extremely rapidly by pouring a melt of Al 2  O 3  and ZrO 2  prepared under reducing conditions between metal plates and which is eminently suitable for use in abrasives by virtue of the crystallite structure thus stabilized and the content of tetragonal high-temperature modification of ZrO 2 . 
     The phase transition of the ZrO 2  during the grinding process from the tetragonal high-temperature modification into the monoclinic form stable at room temperature causes the appearance of transition-induced microcracks through the resulting increase in volume so that the material has to be strengthened by energy dissipation. 
     German Patent Application DE-A 3 040 992 describes a zirconium corundum containing 27 to 35% by weight ZrO 2  and--expressed as TiO 2  --1 to 10% by weight oxycarbides, carbides or suboxides of titanium obtained by reduction of TiO 2 . The performance of this abrasive material is characterized as equal to or better than that of the material described in DE-A 2 227 642. However, the advantage of this abrasive material is seen above all in the reduction in manufacturing costs for the same abrasive performance which is achieved by reduction of the ZrO 2  content. 
     The mechanism on which this improvement in the properties of the abrasive material by the titanium compounds is based remains largely unresolved. It is assumed in DE-A 3 040 992 that the grinding process is directly influenced by the titanium compounds which are said to participate actively in the grinding process. 
     Both DE-A 3 040 992 and DE-A 2 227 642 disclose abrasive materials which contain at least 25% by weight, based on the ZrO 2  content, of tetragonal ZrO 2  modification. 
     U.S. Pat. No. 5,143,522 describes an abrasive grain with a content of 20-50% by weight of ZrO 2 , more than 25% of which is in the tetragonal form, 1-10% by weight of reduced titanium oxide in the form of suboxides, carbides or oxycarbides, 0.03-0.5% by weight of carbon, less than 3% by weight of impurities and Al 2  O 2  as the main constituent. The structure of the grain is described as consisting of primary Al 2  O 3  crystals embedded in a eutectic alumina-zirconia matrix. The size of the primary crystals is between 5 and 50 microns. 
     Japanese patent 1 614 974 (Appl. No. Sho 58-102289) describes zirconium corundums containing titanium dioxide in which up to 100% of the ZrO 2  is present in the tetragonal modification. It is apparent from this document that the Y 2  O 3  also added is solely responsible for the stabilization and enrichment of the tetragonal phase. Thus, the Examples in the specification only show an increase over the undoped standard in the case of the samples doped with Y 2  O 3 . 
     The addition of Y 2  O 3  adds to the production costs of the abrasive material. The abrasive performance improved by the high percentage contents of tetragonal ZrO 2  modification is also limited by the fact that Y 2  O 3  or Y 2  O 3  -containing compounds do not themselves have any properties beneficial to the grinding process. 
     The object of the present invention is to provide a zirconium corundum abrasive material which would show improved performance compared with hitherto known zirconium corundums without any increase in production costs by comparison with conventional zirconium corundums so that, where the abrasive material according to the invention is used, an even more favorable price-to-performance ratio could be achieved for a number of grinding operations than was possible in the prior art. 
     SUMMARY OF THE INVENTION 
     It has now surprisingly been found that the percentage contents of the tetragonal ZrO 2  modification can be drastically increased by addition of small quantities of TiO 2  to an Al 2  O 3  /ZrO 2  mixture, melting of the mixture in the presence of reducing agents and sudden quenching of the melt. It has been found that there is linear dependence between the quantity of TiO 2  and the proportion of the tetragonal phase. 
     It is possible in this way to obtain an abrasive material based on α-Al 2  O 3  and ZrO 2  with a content of titanium compounds in the form of suboxides, carbides and/or oxycarbides, in which more than 80% by weight of the ZrO 2  is present in the tetragonal crystal form. The abrasive material according to the invention is distinguished by extremely high abrasive performance levels. 
     In one particularly preferred embodiment of the abrasive material according to the invention, more than 90% by weight of the ZrO 2  is present in the tetragonal crystal form. Particularly high abrasive performance levels are achieved when the ZrO 2  content is between 35 and 50% by weight. 
     A particularly preferred mixing range of the Al 2  O 3  melt is in the vicinity of the eutectic (between 37 and 45% by weight ZrO 2 , based on the sum of ZrO 2  and Al 2  O 3 ). It is possible in this range to stabilize particularly high percentage contents of tetragonal ZrO 2  modification. 
     TiO 2  or other Ti compounds are not described in the literature as stabilizers for the high-temperature modification of ZrO 2 . CaO, MgO, Y 2  O 3  or other oxides of the rare earths are normally used for the purpose. 
     Comparative tests with these stabilizers have surprisingly shown that, in the Al 2  O 3  /ZrO 2  system, the stabilizing effect of titanium compounds in the form of suboxides, carbides and/or oxycarbides is more effective than that of the known stabilizers MgO and CaO. Only Y 2  O 3  surpasses the reduced form of TiO 2  in its stabilizing effect on ZrO 2 . 
     Particularly good properties are obtained when the quantity of titanium compounds is between 0.5 and 10% by weight, preferably between 1 and 5% by weight, expressed as TiO 2 . 
     It has surprisingly been found that the most successful grinding results are obtained with a grain which has a highest possible content of tetragonal ZrO 2  phase but not too high a content of Ti compounds. 
     A high content of Ti compounds apparently changes the viscosity of the melt and makes it difficult to quench the melt uniformly and quickly, so that ideally completely homogeneous eutectic structures of ZrO 2  and Al 2  O 3  crystals with a crystallite size in the submicron range are increasingly impaired by segregation effects due to the unsatisfactory quenching process. 
     Such segregation effects change the properties of the grain and reduce the grinding performance of the abrasive materials produced therefrom. 
     An optimum structure is one which is present in a completely eutectic form. Experience has however shown that a small degree of segregation cannot be avoided even under optimum quenching conditions, since zirconium corundum, which is an excellent insulating material, does not allow completely simultaneous solidification of the entire melt. 
     The content of impurities, expressed as oxides, should advantageously be at most 3% by weight. The content of rare earths, expressed as oxides, is preferably at most 0.1% by weight. 
     The surprisingly good grinding results which can be obtained with the abrasive materials according to the invention are far superior to those of known zirconium corundums with comparable percentage contents of tetragonal ZrO 2  phase. This suggests that the titanium compounds in the form of carbides, suboxides and/or oxycarbides actively participate in the grinding process. 
     The present invention also relates to a process for the production of the abrasive material according to the invention, characterized in that a mixture of Al 2  O 3 , ZrO 2  and TiO 2  and/or raw materials containing these substances are melted in the presence of reducing agents and quenched. Quenching should preferably be carried out in such a manner that the melt solidifies completely in no more than 10 seconds. Thereafter the material should not be exposed to temperatures above about 700° C. to prevent phase transition of the ZrO 2  present in the tetragonal phase into the monoclinic modification. 
     The process can be carried out particularly economically by using natural raw materials to form the melt, including in particular alumina, bauxite, baddeleyite, zirconium sand, rutile and ilmenite. 
     Carbon in the form of graphite or coke is preferably used as reducing agent in the process according to the invention. 
     The present invention also relates to the use of the abrasive material according to the invention in bonded abrasives and abrasives on substrates. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The following examples are intended to illustrate the invention without limiting it in way. 
     EXAMPLE 1 
     Quantities of 0, 0.5, 1, 2.5, 5 and 10% by weight TiO 2  (rutile), based on the sum of the individual components, were added to a mixture of 240 kg alumina, 170 kg baddeleyite and 12 kg petroleum coke which was then melted in an electric arc furnace. The furnace chamber had a diameter of 2 m, the voltage was 110 V and the selected load 1100 kWh. The liquid melt was quenched to temperatures below 600° C. by pouring into an approx. 5 mm wide gap between metal plates with full solidification achieved in less than 20 seconds oxygen was substantially excluded during quenching. 
     The products thus obtained are characterized in Table 1: 
     
                       TABLE 1                                                     
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                      T (%) = percen-                                     
                      tage contents of                                    
             Analytical tetragonal phase                                  
        TiO.sub.2 (%)                                                     
               data (%)     based on                                      
Test No.  as rutile                                                       
                   TiO.sub.2                                              
                            ZrO.sub.2                                     
                                  total ZrO.sub.2                         
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1 (Comparison)                                                            
          --       0.15     41.6  46                                      
2         0.5      0.63     42.4  55                                      
3         1.0      1.08     43.1  78                                      
4         2.5      2.72     41.2  97                                      
5         5.0      4.98     40.8  100                                     
6         10.0     9.82     39.5  100                                     
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     The percentage contents of tetragonal phase were calculated in accordance with the following equation: ##EQU1## where t=intensity of the tetragonal peak at theta=30.3 
     m1=intensity of the monoclinic peak at theta=28.3 
     m2=intensity of the monoglinic peak at theta=31.5 
     The equation is based on the X-ray diffraction analysis of zirconium corundum powders (X-ray diffractograms). 
     The various materials were tested in P 36 grit in abrasive belts against 42CrMo4 under a contact pressure of 70N. The grinding time was 12 minutes. 
     
                       TABLE 2                                                     
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                 Grinding performance (%) compared                        
Test             with the prior art                                       
No.     Abrasion (test No. 1)                                             
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1       937.6    100                                                      
2       965.1    103                                                      
3       1105.6   118                                                      
4       1386.7   148                                                      
5       1236.8   132                                                      
6       1189.9   127                                                      
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     EXAMPLE 2 
     Quantities of 1% TiO 2 , Y 2  O 3 , CaO or MgO were added to a mixture of 240 kg alumina, 170 kg baddeleyite and 12 kg petroleum coke. The melting and cooling conditions were as in Example 1. 
     The products thus obtained are characterized in Table 3: 
     
                       TABLE 3                                                     
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         Stabilizers used in %, based                                     
         on the sum of the individual                                     
         components                                                       
Test No.   TiO.sub.2                                                      
                    Y.sub.2 O.sub.3                                       
                           CaO    MgO  T (%)                              
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 7         1        --     --     --   76                                 
 8 (Comparison)                                                           
           --       1      --     --   96                                 
 9 (Comparison)                                                           
           --       --     1      --   56                                 
10 (Comparison)                                                           
           --       --     --     1    42                                 
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     The grinding tests were carried out under the same conditions as in Example 1. To allow direct comparison, a sample according to test No. 1 (undoped) was also ground as standard. 
     
                       TABLE 4                                                     
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            Grinding test                                                 
              Abrasion Grinding performance                               
Test No.      (%)      (%)                                                
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 1 (Comparison)                                                           
              953.7    100                                                
 7            1200.8   126                                                
 8            1115.8   117                                                
 9            638.5     67                                                
10            933.9     98                                                
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     EXAMPLE 3 
     A mixture of 240 kg alumina, 170 kg baddeleyite and 11 kg rutile was melted and quenched under the conditions of Examples 1 and 2 without the addition of carbon as reducing agent (test 11). 
     In test 12, 12 kg petroleum coke were added as usual; the melting conditions were as in the other Examples except that the melt was not quenched, but instead was poured into an open crucible. 
     For direct comparison, test 4 of Example 1 is included in Table 5. 
     
                       TABLE 5                                                     
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         Additions                                                        
         used                                                             
Test No.   TiO.sub.2                                                      
                   C      Cooling conditions                              
                                      T (%)                               
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 4         2.5%    2.8%   Quenching   96                                  
11 (Comparison)                                                           
           2.5%    --     Quenching   36                                  
12 (Comparison)                                                           
           2.5%    2.8%   Slow cooling                                    
                                      42                                  
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     No grinding tests were carried out with the test materials of Example 3.