Method of cutting with ceramic cutting tool having improved toughness behavior
This application is a divisional of application Ser. No. 201,931, filed June 3, 1988, U. S. Pat. No. 4,849,381. 
    
    
     The present invention relates to a cutting tool material. More specicially, the present invention relates to a cutting tool material exhibiting excellent toughness through the utilisation of different toughening additives. 
     A metal cutting tool is subjected to fluctuating stresses and temperatures of dynamic nature. The conditions to which a cutting edge is subjected in operation are severe, the temperature of the tool surface often exceeding 1000° C. as the chip contacts the rake face of the tool at a pressure close of 150 MPa and at a relative speed in the range 0-1000 mmin -1 . Steep temperature and stress gradients also exist in the cutting tool inserts. A consequence of the dynamic conditions described above is that generally several different failure mechanisms operate simultaneously for given cutting conditions. However, for a specific set of cutting data, one mechanism is dominating and limiting of the tool life. 
     One objective of the present invention is to obtain a cutting tool material having a unique combination of good resistance against several failure mechanisms. The result is a tool material showing improved tool life and wider application area. 
     Known toughening mechanisms in cutting tool materials include i) transformation toughening, which utilises a phaes transformation of ZrO 2  -particles dispersed in a ceramic matrix, this type of material is described in U.S. Pat. No. 4,218,253; ii) whisher pull out which utilises the bridging effect of small diameter (typically 0.06 /  um), high aspect ratio and high strength single crystal whiskers in a ceramic matrix as described in U.S. Pat. No. 4,543,345, and the combined effect of ZrO 2  and small diameter SiC-whisker as described in U.S. Pat. No. 4,657,877. 
     Both of these effects have led to substantial improvements of the toughness behaviour in certain metal cutting operations. However, the search for greater improvements in the properties of cutting tools is continuously ongoing. 
     The primary objective of the present invention is to provide a material with an improved toughness behaviour and a wider application area than has heretofore been known. This objective is achieved through the simultaneous action of several toughening mechanisms. In different practical machining tests, it has been found that the maximum tool life is dependent on signle crystal size and geometry of the reinforcing additives for a specified operation. Therefore, a particular prior art material has a relative narrow optimum application area. It has now surprisingly been found that by adding several different classes of reinforcing single crystals, with respect to geometry and size, not only a widening of the application area is obtained but also an improvement in tool life is realised. 
     Generally, the present invention is directed to the fabrication of whisker-reinforced ceramic cutting tool materials characterised by increased tool life and a wider application area that prior art materials. The composites comprises a matrix based on Al 2  O 3  or Si 3  N 4  and up to 35 weight-% single crystals or whiskers with different geometries and sizes. Preferably an adequate concentration (in the alumina based matrix preferably 3-20, most preferably 5-15 weight% and in the silicon nitride based matrix up to 10 weight-%) of unstablized and/or partially stabilized zirconia increases the toughness behaviour of the composite in a wider range of metal cutting applications that obtainable with only one of the additives mentioned above. The alumina based matrix may further comprise chromium in amounts corresponding to a total of 1-20 weight-% as Cr 2  O 3 . The whiskers, preferably of SiC, used in the present invention are of a monocrystalline structure and are divided into three types characterized by different diameters and aspect (length/diameter or e.g., diameter/thickness) ratios at least 5 weight-% of each of at least two of the three types being present in the matrix: 
     (i) Single crystral fibers with a diameter less than 1  /  um, typically 0.6  /  um, and an aspect ratio of 15-150; 
     (ii) Single crystal fibers with a diameter of 1-6  /  um, typically 3-4  /  um, and an aspect ratio of 5-100; and 
     (iii) Single crystal discs with an equivalent diameter (hypothetical diameter of a circle with the same area as the disc) of 5-50  /  um, typically 20  /  um and an aspect ratio (aspect ratio being defined as ratio of equivalent diameter through thickness of the disc) of 5-50, typically 10-20. 
     The composite may further comprise refractory nitrides or carbides to increase hot hardness and thermal conductivity which is advantageous in certain metal cutting applications. 
    
    
     EXAMPLE 1 
     Cutting tool materials are prepared from the following starting materials: 
     A. Alumina with a grain size ≦1 /  um; 
     B. SiC-single crystal with average diameter of 0.6  /  um and average aspect ratio 60; 
     C. SiC-single crystal with average diameter of 4  /  um and average aspect ratio 10; 
     D. SiC-single crystal with average equivalent diameter of 20 /  um and aspect ratio 10; 
     E. ZrO 2  with a grain size &lt;2 /  um. 
     The SiC-single crystals are dispersed and wet milled with alumina powder. After drying, the mixture is hot pressed at 1725° C. for 60 min., and then with ZrO 2  at 1650° C. for 60 min. to 99.6 % of theoretical density. The composition variants are shown in Table 1. 
     
                       TABLE 1                                                     
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Composition weight percent                                                
Var.   A       B     C     D   E                                          
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1      bal     24    0     0    0    prior art                            
2      bal     24    0     0   10    prior art                            
3      bal      0    24    0   10    own application                      
4      bal      8    8     8   10    invention                            
5      bal      8    8     8    0    invention                            
6      bal     12    12    0   10    invention                            
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     EXAMPLE 2 
     The materials from Example 1 are tested as inserts SNGN 120412 in cast iron SS 0125 in an interrupted facing operation with high toughness demands. The following tool life ranking is obtained for different cutting conditions (Table 2). 
     
                       TABLE 2                                                     
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Tool Life Ranking for Various Cutting Conditions                          
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Cutting speed,                                                            
          400      400    700    700  Total                               
mmin.sup.-1                                                               
Feed rate,                                                                
          0.3      0.5    0.3    0.5  Ranking                             
mmrev.sup.-1                                                              
Variant                                                                   
1         6        6      6      6    24                                  
2         5        5      4      5    19                                  
3         4        4      5      4    17                                  
4         1        2      2      1     6                                  
5         3        3      1      3    10                                  
6         2        1      2      2     7                                  
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     The results shows that variants 4, 5 and 6 according to the present invention utilise multiple toughening mechanisms and are superior to the prior art variants. 
     EXAMPLE 3 
     Inserts SNGN 120412 made according to Example 2 are tested in a continuous turning operation of cast iron SS 0125 with high demands of wear resistance (at a cutting speed of 700 mmin -1  and a feed rate of 0.3 mmrev -1 ). The relative tool life ranking is shown in Table 3. 
     
                       TABLE 3                                                     
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Variant       1     2        3    4     5   6                             
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Relative Ranking                                                          
              2     not tested  4     1   3                               
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     The results shows that improved toughening with ZrO 2  additives results in a reduction of wear resistance.