Abstract:
A process for manufacturing large, fully dense, high purity TiB 2  articles by pressing powders with a sintering aid at relatively low temperatures to reduce grain growth. The process requires stringent temperature and pressure applications in the hot-pressing step to ensure maximum removal of sintering aid and to avoid damage to the fabricated article or the die.

Description:
It was developed in research sponsored by Army Materials and Mechanics Research Center, Watertown, Mass. under Interagency Agreement MIPR 83-D071-05, under Union Carbide Corporation contract DE-AC05-840R21400 with the United States Department of Energy. 
     BACKGROUND OF THE INVENTION 
     This invention relates to a process for fabrication of large, fully dense, high strength titanium diboride ceramic bodies. 
     The ceramic titanium diboride (TiB.sub. 2) has numerous exceptional properties including high hardness, high melting temperature, high electrical conductivity, and nonreactivity with various liquid metals that make it an attractive candidate for technological applications such as cutting tools, valve trim for erosive environments, cathodes in Hall-Heroult cells for aluminum smelting and for military armor. In general, the various shapes and sizes of such articles require a capability of manufacturing large, high density, polycrystalline ceramic structures. However densification of TiB 2  ceramics is complicated by two characteristics of this compound, its high melting point and its hexagonal structure. In the first instance, the high melting point of 2980° C. requires sintering temperatures on the order of 2000° C. for pressureless sintering to produce a ceramic having greater than 95% theoretical density; however, maintenance of such temperatures is costly and requires special equipment. Secondly, the hexagonal structure of TiB 2  results in marked thermal expansion anisotropy, i.e., expansion along the c-axis is considerably greater than that along the a-axis. The expansion difference is about 42% between 25° and 930° C., and this difference increases as the temperature exceeds 930° C. The expansion anisotropy produces considerable internal stress during specimen cooling that generates microcracking when the grain size is above a critical value. The microcracking occurs in the grains and at grain boundaries, with a resultant degradation of macroscopic mechanical properties. 
     At conventional sintering temperatures grain growth readily occurs, so that even if fine grained ceramic powder is used, a relatively large grained ceramic body is produced. To correct these problems a process of hot pressing in a vacuum at a temperature of 1800° C. was developed. However, such high temperatures and large vacuum systems are very costly, die lives are short, and even this lower temperature still promoted unacceptable grain growth. 
     Therefore, there is a need to provide a method for preparing TiB 2  articles in non-vacuum environments, at relatively low sintering temperatures and with minimum grain growth. 
     SUMMARY OF THE INVENTION 
     In view of the need it is an object of this invention to provide fully dense, high strength TiB 2  ceramic bodies in various shapes and sizes. 
     It is another object of this invention to provide a method of preparing TiB 2  ceramic bodies with minimum grain growth at relatively low sintering temperatures in non-vacuum environments. 
     Another object of this invention is to minimize strength-limiting microcracks and internal stresses in TiB 2  ceramic bodies upon cooling after sintering. 
     It is also an object of this invention to provide a process for fabrication of large, dense, strong TiB 2  ceramic bodies that have a minimum of retained sintering aid. Other objects and advantages of the invention will be apparent to those skilled in the art upon study of the specification an appended claims. 
     The invention is generally described as a process for forming large TiB 2  ceramic bodies by mixing TiB 2  powder with a sintering aid and in a nonoxidizing atmosphere gradually applying and properly maintaining sufficient pressure and temperature to achieve a fully dense, fine grained body. The sintering aid can be nickel or a mixture of nickel and aluminum. The preferred amount of sintering aid is from 1 to 15 wt. percent nickel of a mixture of nickel and aluminum having the nominal composition Ni 3  Al, the preferred amount of the latter also being 1 to 15 wt. percent. Also, the invention is the above described process wherein 0.5 to 2.0 wt. percent carbon is added to the TiB 2  powders and sintering aid. In addition, the invention is a composition of matter of TiB 2  containing from 0.5 to 2.0 wt. percent of carbon. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Titanium diboride ceramics have several important characteristics that make them attractive candidate materials for such diverse applications as cutting tools, military armor and cathodes in Hall-Heroult cells for aluminum smelting. In general, such applications require fabrication of TiB 2  in various shapes and with material properties of high density, strength and toughness. Since hot pressing TiB 2  powder in a vacuum atmosphere at a temperature of 1800° C. has many disadvantages, a preferred approach is to use one or more sintering aids that form a liquid film on the TiB 2  grains during sintering. This liquid allows grain rearrangements into a more closely packed structure, and also dissolves some of the TiB 2  to promote rapid diffusive transport of both Ti and B throughout the microstructure. The liquid phase sintering process of this invention enables high density ceramic bodies to be produced at temperatures of 1450° to 1600° C. such that grain growth and the damaging microcracking, characteristic of conventionally fabricated TiB 2 , are avoided. It utilizes a nonoxidizing gaseous atmosphere to greatly reduce fabrication costs relative to vacuum processing, and a carefully controlled time-temperature-pressure cycle to promote exudation of the sintering aid as densification progresses. Following this procedure precisely enhances removal of sintering aid and provides high purity TiB 2 . 
     EXAMPLE I 
     In this process, compacts were uniaxially hot pressed with graphite dies and punches in a press equipped for either vacuum or inert atmosphere. The die cavity was about 2.7 cm ID and was lined with 0.25-mm thick Graphoil, a graphite product made by Union Carbide Corporation, but other suitable compositions capable of absorbing the extruded sintering aid and protecting the die can be substituted. The ends of the punches were also separated from the powder charge by Graphoil. The principal steps in the procedure were as follows: 
     1. Weigh powders of TiB 2  and a proportionate amount of Ni or other sintering aid to make 1 to 15 wt. percent of the total, typically 38 g. 
     2. Blend powders about 16 hours in an oblique blender using a blending container having a volume at least three times the volume of the powder. 
     3. Line die and punch faces with Graphoil and heat in air for about 16 hours at about 110° C. to reduce moisture content. 
     4. Load powder into cool die in an argon filled enclosure and prepress powder at approximately 0.5 MPa. 
     5. Install die in press and introduce the desired atmosphere at a pressure of 90 kPa. 
     6. Heat to about 1550° C. with an applied pressure of approximately 0.6 MPa which is 5 percent of the ultimate pressing pressure of 12 MPa. 
     7. Increase pressure continuously or in approximately 6 equal increments over a 30 minute period to 12 MPa. 
     8. Hold at temperature and pressure for 2 hours. 
     9. After 2 hours release pressure and cool to ambient temperature. 
     This procedure resulted in compacts measuring approximately 2.7 cm in diameter by 1.2 cm thick. 
     RESULTS 
     The hot pressing conditions and properties of compacts prepared in this investigation are shown in the following tables. 
     SUBSTITUTION OF GASEOUS ATMOSPHERE FOR VACUUM 
     The compacts of Table 1 revealed the effects of various gaseous atmospheres used during densification on properties of dense compacts. In these experiments the powder blend consisted of TiB 2  and 10 wt. percent Ni. Nitrogen atmosphere yielded poor results. At a pressing temperature of 1450° C., very poor sintering resulted in a density of only 3.08 g/cm 3  (the theoretical density of TiB 2  is 4.5 g/cm 3 ), and there was no evidence of Ni removal during pressure. Another compact hot pressed in N 2  at 1550° C. had a density of 5.22 g/cm 3 , which indicated significant retention of Ni even though some exudation was observed. In addition, microstructural examination indicated that N 2  reacted with TiB 2 , possible forming nitrides. 
     Subsequently, compacts hot pressed in an Ar atmosphere (samples 3, 4, 5, and 6 in Table 1) had desnities near the theoretical density of TiB 2 . Microstructoral examination of sample 3 in a plane parallel to the pressing direction revealed a predominantly equiaxed grain structure with a second phase at grain boundaries. An average grain size of 13 μm was measured by the line intercept method. 
     Single samples were also hot pressed in Ar-4% H 2  and in vacuum. The conclusion deduced from these experiments was that Ar and Ar-4% H 2 , but not N 2 , are suitable substitutes for vacuum in hot pressing TiB 2 . We would also expect that the inert gas helium would be acceptable. 
     
                                           TABLE 1__________________________________________________________________________Hot pressing conditions and properties of TiB.sub.2Principal Variable: Atmosphere Composition, wt %: 90 TiB.sub.2 --10 NiTemperature.sup.b : 1550° C. Pressing Conditions: Hot pressingpressurewas 12 MPa. Time at pressure and temperature was 2 hoursexcept sample 6, which was 6 hours.__________________________________________________________________________Identity             Sonic Velocity                           Shear                                Young&#39;s    of TiB.sub.2           Density                Linear                     Transverse                           modulus                                modulusSample    powder.sup.a    Atmosphere           9/cm.sup.3                km/s km/s  GPa  GPa__________________________________________________________________________1   C030 N.sub.2           3.082   C030 N.sub.2           5.223   C030 Ar     4.598                11.09                     7.363 249  5524   B190-1    Ar     4.459                10.82                     7.233 233  5105   B190-1    Ar     4.473                10.89                     7.371 243  5246   B190-1    Ar     4.482                10.89                     7.404 246  52612  B190-1    Ar--4% H.sub.2           4.434                10.504                     7.129 225  48415  B190-2    Vacuum 4.468                10.767                     7.318 239  512__________________________________________________________________________                                NominalLame&#39;s        Fracture    Fracture   grainConstant Poisson&#39;s         Strength              Hardness                     Toughness, MPa · m.sup.1/2                                sizeSample    GPa  ratio         MPa  kg/mm.sup.2                     A.sup.c                           B.sup.d                                μm__________________________________________________________________________23   67.3 0.106         400 ± 25              2701 ± 33                     5.1 ± 0.4                           4.4 ± 0.3                                134   55.4 0.096         470 ± 80              2277 ± 95                     6.0 ± 0.6                           8.5 ± 0.9                                65   44.5 0.077     2207 ± 16                     6.7 ± 1.1                                66   40.0 0.070         425 ± 25               2284 ± 625                     6.4 ± 1.5                           8.2 ± 0.8                                812  38.5 0.073      2333 ± 325                     7.5 ± 1.1                                615  39.4 0.071     2300 ± 98                     5.9 ± 0.9                                6__________________________________________________________________________ .sup.a Source  C030, B1901, B1902: Kawecki Berylco Industries, Inc., Boyertown, Penn. .sup.b Sample 1 was at 1450° C. .sup.c Indentation method. .sup.d Double cantilever beam method. 
    
     EFFECT OF Ni CONTENT 
     The effect of initial Ni content of the blended TiB 2  and Ni powders on compact properties was investigated. The compacts of Table 2, which were arranged in order of increasing initial Ni content, do not show a strong dependence within the range 1 to 10 wt. percent Ni. Sample 9, which had only 1 wt. percent Ni in the starting powder, had the lowest density in the group, while sample 32, which had 3 wt. percent Ni, had the highest density. the densities of samples 8, 9, 10, 11, and 13, each prepared from the same powder, exhibit a small increase with increasing Ni content. 
     Microstructures of compacts prepared from powder mixtures containing 1, 3, 5 and 10 wt. percent Ni were very similar in terms of size and distribution of the grain boundary phase. Isolated high concentrations of Ni and O were found in the grain boundary phase by electron microprobe analysis. Similar distributions of Ni and O were found in the microstructure in regions near the surface. 
     The weight loss during hot pressing exceeds the amount of Ni in the initial powder mixture. The difference between predicted and actual weight losses indicates handling losses and/or dissolution of TiB 2  and ejection with the Ni. Loss of some TiB 2  probably occurs, and a small amount of Ni was retained in the grain boundary phase. Most of the Ni exudes from the compacts during hot pressing of powder mixtures initially containing 1 to 10 wt. percent Ni. 
     
                                           TABLE 2__________________________________________________________________________Hot pressing conditions and properties of TiB.sub.2Principal Variable: Ni Content Temperature: 1500° C. Atmosphere:ArPressing Conditions: Hot pressing pressure was 12 MPa exceptsample 10 pressed at 24 MPa. Time at pressure was 2 hours.__________________________________________________________________________    Initial    composition ofIdentity blended                Shear                                Young&#39;sof TiB.sub.2    powders, wt %                 Sonic Velocity                           modulus                                modulusSample    powder.sup.a    TiB.sub.2        Ni  Density                 Linear                     Transverse                           GPa  GPa__________________________________________________________________________ 9  B190-1    99   1  4.407                 10.56                     7.129 224  48332  B190-2    97   3  4.472                 10.450                     7.067 223  482 8  B190-1    95   5  4.413                 10.49                     7.102 223  48010  B190-1    90  10  4.421                 10.51                     7.144 224  47811  B190-1    90  10  4.429                 10.417                     7.097 223  47613  B190-1    90  10  4.428                 10.385                     7.054 220  472__________________________________________________________________________                                NominalLame&#39;s        Fracture    Fracture   grainConstant Poisson&#39;s         Strength               Hardness                     Toughness, MPa · m.sup.1/2                                sizeSample    GPa  ratio         MPa   kg/mm.sup.2                     A.sup.b                           B.sup.c                                μm__________________________________________________________________________ 9  43.8 0.082         393 ± 25               1883 ± 17                     5.7 ± 0.2                           6.8 ± 0.4                                732  41.7 0.079      3175 ± 330                     5.8 ± 1.0                                6 8  40.5 0.077         480 ± 15               2138 ± 3                     5.3 ± 0.1                           6.7 ± 0.1                                810  36.4 0.070         525 ± 45               2120 ± 18                     4.9 ± 0.1                           7.2  811  34.5 0.067      2152 ± 272                     6.0 ± 0.5                                613  36.9 0.072            3.4 ± 0.7                                7__________________________________________________________________________ .sup.a Source  B1901, B1902: Kawecki Berylco Industries, Inc., Boyertown, Penn. .sup.b Identation method. .sup.c Double cantilever beam method. 
    
     EFFECT OF TiB 2  POWDER 
     The compacts of Table 3 were hot pressed under identical conditions with different TiB 2  powders. All powders yielded compacts having densities greater than 98 percent theoretical density except the ART powder. Both compacts pressed with UCC powder, however, contained cracks. The UCC and ART powders had substantially larger average particle sizes than the other powders. These results indicate that the average particle size of the TiB 2  powder should be substantially less than 10 micrometers to ensure highly dense, crack-free compacts. 
     
                                           TABLE 3__________________________________________________________________________Hot pressing conditions and properties of TiB.sub.2Principal Variable: TiB.sub.2 Powder Composition wt %: 90 TiB.sub.2 --10NiTemperature: 1550° C. Atmosphere: Ar Pressing Conditions: Hotpressingpressure was 12 MPa. Time at pressure and temperature was 2 hours.                           Lame&#39;s                    NominalIdentity   Den-       Sonic velocity                 Shear                      Young&#39;s                           Con-           Fracture   grainSam-   of TiB.sub.2   sity       Linear           Transverse                 modulus                      modulus                           stant                               Poisson&#39;s                                    Hardness                                          Toughness, MPa ·                                          m.sup.1/2  sizeple   powder.sup.a   9/cm.sup.3       km/s           km/s  GPa  GPa  GPa ratio                                    kg/mm.sup.2                                          A.sup.b    μm__________________________________________________________________________24 B190-2   4.462       10.780           7.310 238  512  41.7                               0.074                                    2610 ± 71                                          5.1 ± 0.2                                                     625 B190-2   4.458       10.704           7.238 234  504  43.7                               0.07938 Starck   4.460       10.589           7.071 223  490  54.1                               0.098                                    2100 ± 20                                          6.1 ± 0.4                                                     639 UCC  4.525       9.793           6.738 208  433  17.6                               0.039                                    1713 ±  20                                          5.6 ± 0.3                                                     741 UCC  4.52940 Cotronics   4.410       10.174           6.830 206  448  45.0                               0.089                                    2376 ± 30     542 C310 4.417       10.455           7.053 220  478  43.4                               0.08243 ART  4.243       10.420           6.889 201  448  57.9                               0.112__________________________________________________________________________ .sup.a Sample  B1902, C310: Kawecki Berylco Industries, Inc., Boyertown, Penn. Starck: Hermann C. Starck, West Germany UCC: Union Carbide Corporation Cotronics: Cotronics Corporation, Brooklyn, N.Y. ART: Advanced Refractories Technologies, Inc., Buffalo, N.Y. .sup.b Indentation method. 
    
     EFFECT OF TEMPERATURE 
     A group of four samples is arranged in Table 4 to show the effect of hot pressing temperature on properties particularly density in relation to temperature. Nickel (m.p. 1453° C.) is either nearly or actually molten at each pressing temperature in the range 1450° to 1600° C. Apparently Ni aids densification more or less equally at these temperatures. The temperature of 1550° C. used primarily in this work was considered to be sufficient to produce compacts of high density while still retaining the advantage of minimizing detrimental grain growth. There are also obvious economic advantages associated with lower processing temperatures. 
     
                                           TABLE 4__________________________________________________________________________Hot pressing conditions and properties of TiB.sub.2Principal Variable: Temperature Composition wt %: 90 TiB.sub.2 --10 NiAtmosphere: Ar Pressing Conditions:Hot pressing pressure was 12 MPa. Time at pressure and temperature was 2hours.__________________________________________________________________________Identity                       Sonic Velocity  ShearSample    of TiB.sub.2 powder.sup.a        Temperature (C.°)                  Density 9/cm.sup.3                          Linear km/s                                 Transverse km/s                                          Modulus GPa__________________________________________________________________________ 7  B190-1   1450      4.405   10.43  7.026    21828  B190-2   1500      4.407   10.453 7.017    21733  C310     1550      4.415   10.441 7.035    21929  B190-2   1600      4.450   10.470 7.172    229__________________________________________________________________________                              FractureYoung&#39;s    Lame&#39;s                  Toughness, MPa · m.sup.1/2                                         Nominal grainSample    modulus GPa      Constant GPa             Poisson&#39;s ratio                     Hardness kg/mm.sup.2                              A.sup.b                                    B.sup.c                                         size μm__________________________________________________________________________ 7  474    44.5   0.085   2551 ± 26                              5.9 ± 0.4                                    2.8 ± 0                                         628  473    47.6   0.0899  2314 ± 65                              5.7 ± 0.5                                         633  474    44.3   0.0843  3246 ± 304                              5.7 ± 0.8                                         529  484    30.0   0.0580  2512 ± 166                              5.4 ± 0.5                                         6__________________________________________________________________________ .sup.a Source  Kawecki Berylco Industries, Inc., Boyertown, Penn. .sup.b Indentation method. .sup.c Double cantilever beam method. 
    
     EFFECT OF CARBON CONTENT 
     Based on observations of modified microstructure in the portion of experimental TiB 2  compacts in contact with the graphite die liners, the deliverate addition of carbon was investigated to determine if (1) the microstructure as modified throughout the compact by addition of carbon and (2) if properties of the compacts were significantly affected. Carbon additions of 0.5 to 2 wt. percent were made as elemental carbon or phenolic resin. Various mixing methods were used including simple dry blending, slurry mixing, ball milling, coating with resin, etc. Resin coating followed by thermal decomposition of the resin to C was less effective because of the high volatility (and resultant loss) of the resin. Ball milling of elementary carbon with the powder was the preferred technique for producing a homogeneous distribution of carbon. 
     We found that the amount of grain boundary phase decreased with increasing C content and its shape changed from angular to rounded. Both changes would be expected to be beneficial to mechanical properties. 
     
                                           TABLE 5__________________________________________________________________________Hot pressing conditions and properties of TiB.sub.2Principal Variable: C Content Temperature: 1550° C. Atmosphere:ArPressing Conditions: Hot pressing pressure was 12 MPa. Time atpressure and temperature was 2 hours.__________________________________________________________________________    Initial compositionIdentity of blended     Sonic velocityof TiB.sub.2    powders, wt %              Density                   Linear                       Transverse                             ShearSample    powder.sup.a    TiB.sub.2       Ni  C  9/cm.sup.3                   m/s km/s  modulus GPa__________________________________________________________________________16  B190-2    89.5       10  0.5              4.492                   10.978                       7.417 24717  B190-2    89.5       10  0.5              4.481                   11.090                       7.395 24518  B190-2    89.5       10  0.5              4.492                   10.963                       7.427 24819  B190-2    89.5       10  0.5              4.395                   10.435                       7.071 22020  B190-2    89 10  1.0              4.505                   11.118                       7.524 25521  B190-2    89 10  1.0              4.500                   11.109                       7.494 25322  B190-2    89 10  1.0              4.477                   11.061                       7.403 24523  B190-2    89 10  1.0              4.38226  B190-2    89 10  1.0              4.497                   11.033                       7.540 25627  B190-2    89 10  1.0              4.490                   11.089                       7.450 24930  B190-2    94  5  1.0              4.490                   11.007                       7.492 25231  B190-2    96   3 1.0              4.487                   11.113                       7.530 25414  B190-2    88 10  2.0              4.556                   9.920                       6.821 213__________________________________________________________________________    Lame&#39;s          Fracture   NominalYoung&#39;s  Constant         Poisson&#39;s              Hardness                    Toughness, MPa · m.sup.1/2                               grain sizeSample    modulus    GPa  ratio              kg/mm.sup.2                    A.sup.b                          B.sup.c                               μm__________________________________________________________________________16  534  47.1 0.081              2285 ± 184                    5.9 ± 0.3                          3.2 ± 0.8                               817  539  61.0 0.0997              2216 ± 104                    5.8 ± 0.8                               618  533  44.3 0.0759              2148 ± 87                    5.3 ± 0.7                               719  473  39.1 0.0755              2027 ± 160                    5.8 ± 1.2                               820  550  46.8 0.0775              2223 ± 57                    4.5 ± 0.5                          3.5 ± 0.2                               821  547  49.9 0.0825              2245 ± 53                    4.4 ± 0.3                          3.6 ± 1.0                               622  537  57.0 0.0943              2206 ± 121                    5.2 ± 0.4                               62326  543  36.1 0.0618              2408 ± 142                    6.0 ± 0.2                               827  543  53.7 0.0865              2818 ± 17                    5.7 ± 0.7                               1030  539  39.9 0.0684              2402 ± 36                    5.8 ± 0.8                               631  547  45.3 0.0756              2420 ± 81                    6.0 ± 0.1                               514  448  24.5 0.052              1924 ± 154                    5.2 ± 0.5                          10__________________________________________________________________________ .sup.a Source  Kawecki Berylco Industries, Inc., Boyertown, Penn. .sup.b Indentation method. .sup.c Double cantilever beam method. 
    
     OTHER SINTERING AIDS 
     The samples of Table 6 were blended and pressed with the compounds CoB, Tab 2 , W 2  B 5  and Ni 3  Al as densification aids rather than Ni. Other investigators determined that TiB 2  containing additions of CoB, or CoB plus TaB 2  or W 2  B 5 , minimized grain growth and improved mechanical properties of TiB 2  ceramics. Our results show that the compact densified with Ni 3  Al had slightly larger grains but was harder and about as tough as the ceramics densified with CoB or CoB plus TaB 2  or W 2  B 5 . the advantage of the Ni 3  Al as densification aid over CoB, TaB 2  or W 2  B 5  is in cost, and based on the hardness values we would predict a stronger ceramic. 
     
                                           TABLE 6__________________________________________________________________________Hot pressure conditions and properties of TiB.sub.2Principal Variable: Hot Pressing Aid Temperature: 1550° C.Atmosphere: Ar Pressing Conditions: Hot pressingpressure was 12 MPa. Time at pressure and temperature was 2__________________________________________________________________________hours.Identity     Initial composition of blended powders, wt                                      Sonic velocitySample    of TiB.sub.2 powder.sup.a        TiB.sub.2               Ni      C.sup.b                              Density 9/cm.sup.3                                      km/s                                          km/s__________________________________________________________________________34  B190-2   99     1 CoB          4.396   10.557                                          7.13735  B190-2   94     1 CoB   5 TaB.sub.2                              4.536   10.522                                          6.99736  B190-2   94     1 CoB   5 W.sub.2 B.sub.5                              4.455   10.579                                          6.69337  B190-2   91.8   8.2 Ni.sub.3 Al                              4.438   10.948                                          7.302__________________________________________________________________________                                         Nominal         Young&#39;s                   Hardness                                         grain sizeSample    Shear modulus GPa         modulus GPa                Lame&#39;s Constant GPa                           Poisson&#39;s ratio                                   kg/mm.sup.2                                         μm__________________________________________________________________________34  224       483    42.1       0.079         635  222       490    58.0       0.104   2100 ± 71                                         436  200       466    99.5       0.166   2233 ± 20                                         437  237       520    58.7       0.099   2580 ± 92                                         10__________________________________________________________________________ .sup.a Source  B1902: Kawecki Berylco Industries, Inc., Boyertown, Penn. .sup.b Except as noted. 
    
     EXAMPLE II 
     Scale-up of Fabrication Procedure 
     Having demonstrated that a gaseous atmosphere (Ar or Ar-H 2 ) could be substituted for vacuum during hot pressing of TiB 2  with Ni (or Ni 3  Al) as a densification aid, the next goal was to fabricate nominal 15 cm×15 cm×2.5 and 25×25×5 cm TiB 2  ceramic tiles. Six of the smaller and three of the larger tiles were produced using an argon atmosphere, with duplicate runs made under vacuum. Essentially the same procedure was used as described above except that a pressure of 24 MPa was used (instead of 12 MPa) to ensure high density and low residual Ni content in these large bodies. We found that if this process were rigorously followed that fine-grained, essentially fully dense TiB 2  ceramics were produced, and that the material s were the same for either vacuum or inert-gas processing. The ceramic bodies also had excellent strength (&gt;400 MPa) and fracture toughness (&gt;6 MPa·m 1/2 ). The procedure is specific and slight deviations can result in an unacceptable product or die damage. 
     For example, increasing the ram pressure over a shorter period of time than the 30 minutes ramp time as specified in this processing procedure caused the failure of very large filament-wound graphite dies apparently as the result of hydraulic forces exerted by the liquid nickel on the side wall of the die cavity. when the ram pressure is slowly applied, the pressure on the side-wall is minimized and the compacting body behaves much like a densifying ceramic with no liquid present. 
     In another instance two 25.4×25.4×5 cm tiles were densified by hot-pressing in vacuum. In both cases the conditions were identical except for one case the time-temperature-load procedure was deviated, resulting in a product with 7.1 wt. percent retained nickel and a fracture toughness of 5.8 MPa·m 1/2 . In the second case the procedure was followed as previously specified and a ceramic body was produced having only 1.9 wt. percent retained nickel and corresponding improved toughness of 6.7 MPa·m 1/2 . 
     When five 15×15×2.5 cm tiles were densified by hot-pressing following the process of this invention they had an average retained nickel content of only 1.1 wt. percent indicating the effectiveness of the specific procedures. 
     These examples demonstrate that the hot pressing process in a nonoxidizing environment under pressure at lower temperatures using a sintering aid results in a large TiB 2  article with small grain size, free of cracks and with good strength. The economy of this process makes it attractive for industrial fabrication of large TiB 2  ceramic bodies.