Abstract:
A method of making an intermetallic-bonded ceramic composite involves combining a particulate brittle intermetallic precursor with a particulate reactant metal and a particulate ceramic to form a mixture and heating the mixture in a non-oxidizing atmosphere at a sufficient temperature and for a sufficient time to react the brittle intermetallic precursor and the reactant metal to form a ductile intermetallic and sinter the mixture to form a ductile intermetallic-bonded ceramic composite.

Description:
The United States Government has rights in this invention pursuant to contract no. DE-AC05-96OR22464 between the United States Department of Energy and Lockheed Martin Energy Research Corporation. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to intermetallic-bonded ceramic composites and methods of making the same, and more particularly to those compositions wherein NiAl and Ni are included as starting materials in a method of making Ni 3  Al/ceramic composites. 
     BACKGROUND OF THE INVENTION 
     Conventional methods of making ductile intermetallic-bonded ceramic composites involve hot-pressing of powder mixtures in graphite dies. Complex shaped articles cannot generally be made via such methods because of the axial nature thereof. 
     For such materials to become viable commercial products, development of sintering techniques not involving the application of mechanical pressure was required. A method was developed to sinter these types of composites to densities to about 90% theoretical density (T.D.) without the application of mechanical pressure. 
     Attempts to sinter intermetallic-bonded ceramic composites utilizing the same types of powders used for the hot-pressing of these materials were unsuccessful. The relative particle size of the Ni 3  Al powders was significantly larger than the ceramic powders. In addition, the Ni 3  Al powders employed were pre-alloyed with boron and were characterized by ductility and high strength. Normally, powder mixtures are dispersed by milling together, however, because of the large size difference and the inherent ductility of the Ni 3  Al powders, milling was not sufficiently effective in producing a homogeneous mixture. The poor densification (&lt;90% T. D.) is attributed to the insubstantial wetting behavior between the large Ni 3  Al particles and the smaller WC and TiC. In addition, the sintered composites revealed large pores believed to be due to the void left by the original Ni 3  Al particle after it was `wicked` into the surrounding carbide particles. 
     For further information, please refer to the following: 
     1. U.S. Pat. No. 4,762,558 issued to German, et al. teaches the use of elemental Ni and Al powders in a reaction sintering method of making Ni 3  Al at low temperatures (&lt;1000° C.). No additions of ceramic powders are used; Ni 3  Al content is 100%. 
     2. U.S. Pat. No. 4,919,718 issued to Tiegs, et al. teaches the use of large-particle-size (&gt;20 μm) pre-alloyed ductile Ni 3  Al powders to make composites that are densified by hot-pressing. The content of Ni 3  Al ranges from 5-20 wt. %. 
     3. U.S. Pat. No. 5,271,758 issued to Buljan, et al. teaches the use of elemental Ni and Al powders with Al 2  O 3  --TiC mixtures that are densified by hot-pressing. The content of Ni 3  Al ranges from 5-20 wt. %. 
     4. Mei, et al, &#34;Investigation of Ni 3  Al-Matrix Composites Strengthened by TiC&#34;, J. Mater. Res., Vol. 8, No. 11, Mater. res, Soc. (1993) teaches the use of an in-situ reaction of Ni, Al, C and Ti powders to produce materials with 35 wt. % (45 vol. %) TiC in a Ni 3  Al matrix (55 vol. %). 
     OBJECTS OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide 
     It is another object of the present invention to provide 
     Further and other objects of the present invention will become apparent from the description contained herein. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, the foregoing and other objects are achieved by a method of making an intermetallic-bonded ceramic composite including the steps of: 
     a. combining a particulate brittle intermetallic precursor with a particulate reactant metal and a particulate ceramic to form a mixture; and 
     b. heating the mixture in a non-oxidizing atmosphere at a sufficient temperature and for a sufficient time to: 
     (1) react the brittle intermetallic precursor and the reactant metal to form a ductile intermetallic; and 
     (2) sinter the mixture to form a ductile intermetallic-bonded ceramic composite. 
     In accordance with another aspect of the present invention, a method of making a Ni 3  Al-bonded ceramic composite includes the steps of: 
     a. combining particulate NiAl with particulate Ni and a particulate ceramic to form a mixture; and 
     b. heating the mixture in a non-oxidizing atmosphere at a sufficient temperature and for a sufficient time to: 
     1. react the NiAl and the Ni to form Ni 3  Al; and 
     2. sinter the mixture to form a Ni 3  Al-bonded ceramic composite. 
     In accordance with a further aspect of the present invention, an intermetallic-bonded ceramic composite comprising a body of ceramic material sintered with a intermetallic having a ductility of at least 10% elongation, said body having a density of at least 90% theoretical density. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     In the drawing: 
     FIG. 1 is a graph showing a typical sintering schedule used in methods of making intermetallic-bonded ceramic composites in accordance with the present invention. 
     FIG. 2 is a graph showing the effects of sintering temperature upon sintered density of intermetallic-bonded ceramic composites made in accordance with the present invention. 
     For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the above-described drawings. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A major problem associated with the use of a pre-alloyed Ni 3  Al powder is the inherent ductility and high strength thereof. Upon subjection to conventional milling processes, Ni 3  Al particles do not reduce in size as intended, but rather tended to flatten. 
     In contrast to Ni 3  Al, other nickel aluminide intermetallics such as NiAl, NiAl 3 , Ni 2  Al 3 , and Ni 5  Al 3  are inherently brittle and the particle size thereof can be reduced by conventional milling processes. 
     In accordance with the present invention, a brittle nickel aluminide is reduced to small-particle-size and used as a precursor along with a sufficient amount of fine Ni powders and optionally other alloying additives, for example, B, Fe, Cr, Zr, Ti, W, Hf, Mo, to produce a final composition of ductile Ni 3  Al via the reaction: 
     
         NiAl(s)+2Ni(s)&gt;Ni.sub.3 Al(s) 
    
     These method steps overcome the problems described hereinabove relating to the use of large-particle-size pre-alloyed ductile, Ni 3  Al powders. The optional additives are generally those typically used in ductile Ni 3  Al materials. 
     The present invention thus involves the use of a brittle intermetallic precursor that can be reduced in size (&lt;20 μm, preferably &lt;10 μm) to produce a homogeneous mixture with fine (&lt;20 μm, preferably &lt;10 μm) ceramic powders and a reactant which converts the brittle intermetallic precursor to a ductile intermetallic. Moreover, the homogeneous mixture can be sintered to densities of at lease 90% T.D. without the application of mechanical pressure. The volume content of Ni 3  Al generally ranges from 5 to 50 vol. %. 
     The present invention includes the use of other aluminide systems such as, for example, FeAl, FeAl 2 , Fe 2  Al 5 , FeAl 3 , TiAl, TiAl 2 , TiAl 3 , and Ti 3  Al. Moreover, the present invention includes the use of other aluminide bondable ceramic systems such as, for example, TiN, Ti(C,N), VC, Mo 2  C, TaC, ZrC, HfC, TiB 2 , ZrB 2 , HfB 2 , chromium carbides, etc. The benefits of the present invention are best realized where brittleness of the intermetallic precursor is no more than 5% elongation, and the ductility of the intermetallic product is at least 10% elongation. 
     EXAMPLE I 
     59.16 g TiC powder (1.3 μm average particle size) and a stoichiometric amount (9.378) of NiAl powder (-325 mesh, &lt;44 μm) were ball milled in iso-propyl alcohol for 20 hours using ZrO 2  milling media (Y-TZP). 12.851 g of a fine Ni powder (&lt;10 μm) was added (an amount sufficient to form Ni 3  Al via reaction with the NiAl component in a subsequent reaction step), and the resulting mixture was ball milled for a further 4 hours. After milling the mixture was dried and sieved to -200 mesh (&lt;75, um). Samples thereof were compacted into 32 mm diameter discs by uniaxial cold pressing at .sup.˜ 42 MPa. The samples were sintered under vacuum according to the following sintering schedule, illustrated in FIG. 1: 
     a. a first heating rate of 10° C./min. 
     b. a first holding time of 60 min. at a first holding temperature of 1200° C. 
     c. a second heating rate of 5° C./min. 
     d. a second holding time of 60 min. at a second holding temperature (sintering temperature) 1500° C. 
     The resultant ductile intermetallic-bonded ceramic composite were characterized at &gt;93% T.D. The effects of sintering temperature upon sintered density is shown in FIG. 2. 
     The sintering schedule can vary widely, depending on composition of the composite and the size and shape of the article being made. For example, typical sintering schedules suitable for many applications on the present invention are suggested: 
     a. A first heating rate in the range of about 3° C./min. to about 25° C./min. 
     b. A first holding time in the range of about 30 min. to about 90 min. at a first holding temperature in the range of about 800° C. to about 1300° C. 
     c. A second heating rate in the range of about 3° C./min. to about 10° C./min. 
     d. A second holding time in the range of about 30 min. to about 90 min. at a second holding temperature in the range of about 900° C. to about 1600° C. 
     The above described type of sintering schedule is especially suitable for allowing sufficient out-gassing of the sample during heating in order to prevent any internal gas pressure build-up from the formation of CO within the article. CO is a product of a reaction of surface oxide on any of the powder constituents with a carbide constituent. 
     EXAMPLE II 
     Various intermetallic-bonded ceramic composites were made. Selected amounts of powder starting materials were ball milled together to form Ni 3  Al hardmetal mixtures. The powder physical characteristics of selected starting materials are shown in Table I. All of the compositions were made with 0.1 wt. % boron addition, and some composites were made with alloying additives, such as Fe, W or Ti. The milling was carried out in isopropanol for 16 hours using conventional powder processing techniques. The mixtures were then dried and screened to -100 mesh. Specimens were uniaxially pressed in 25 mm steel dies at .sup.˜ 70 MPa (10 ksi) and iso-pressed at 350 MPa (50 ksi). Sintering was carried out under vacuum in a tungsten element furnace at temperatures of 1550° C. to 1600° C. Compositions with binder contents ranging from 20 to 30 vol. % were made in this fashion. The discs were then heat treated at 1450° C. and 1.7 MPa gas pressure to increase the densities thereof. The compositions and sintered densities after each step are shown in Table 2. As indicated, high densities were achieved for most of the samples. 
     
                       TABLE I______________________________________Starting Material        Average Particle Diameter (μm)______________________________________WC           2.5TiC          1.3Ni           5NiAl         10.9Fe           5W            1Ti           7B            0.3______________________________________ 
    
     
                       TABLE II______________________________________                      % Theoreti-                      cal Density          % Theoretical Density                      After Heat          After Sintering                      TreatmentComposition      1550° C.                     1600° C.                              1550°                                   1600°______________________________________WC-20 vol. % Ni.sub.3 Al            96.8     96.9     98.6 98.3WC-20 vol. % Ni.sub.3 Al + 5% Fe            96.0     98.3     98.9 99.5WC-20 vol. % Ni.sub.3 Al + 5% W            96.0     97.1     98.2 98.3WC-20 vol. % Ni.sub.3 Al + 5% Ti            92.0     92.9     95.7 95.4WC-5 wt. % TiC-20 vol. % Ni.sub.3 Al            90.0     92.1     93.0 94.4WC-30 vol. % Ni.sub.3 Al            98.4     97.9     98.7 98.1______________________________________ 
    
     Intermetallic bonded composites have been shown to have mechanical properties appropriate for structural applications such as cutting tools and wear parts. In addition, they have been shown to have significant improvement in corrosion resistance compared to comparable materials such as WC--Co. Various properties of these materials include: high strength; high toughness; high hardness; high corrosion resistance; electrical conductivity; non-magnetic; strength retention to elevated temperatures (for example, 800° C.) and high reflectivity when polished. 
     Applications for these types of materials include, but are not limited to: wear parts and pads; cutting tools; forming dies; pump seals; valves, including stems and seats; washers; thread guides; wire drawing dies; can forming dies, especially with synthetic lubricants; plastic drawing dies; thermal spray coatings; sour gas (natural gas with hydrogen sulfide) applications; non-magnetic applications such as guidance gyroscopes, dies for ceramic magnets, and tape player heads; and gage blocks. 
     While there has been shown and described what are at present considered the preferred embodiments of the present invention, it will be obvious to those skilled in the art that various changes and modifications can be made therein without departing from the scope of the inventions defined by the appended claims.