Silicon carbide whisker reinforced ceramic composites and method for making same

Abstract

The present invention is directed to the fabrication of ceramic composites which possess improved mechanical properties especially increased fracture toughness. In the formation of these ceramic composites, the single crystal SiC whiskers are mixed with fine ceramic powders of a ceramic material such as Al 2 O 3 , mullite, or B 4 C. The mixtures which contain a homogeneous disperson of the SiC whiskers are hot pressed at pressures in a range of about 28 to 70 MPa and temperatures in the range of about 1600° to 1950° C. with pressing times varying from about 0.75 to 2.5 hours. The resulting ceramic composites show an increase in fracture toughness of up to about 9 MP.am 1/2 which represents as much as a two-fold increase over that of the matrix material.

I claim: 
     
       1. A ceramic composite characterized by increased toughness and resistance to fracture, comprising a composite defined by a matrix of ceramic material, .Iadd.wherein said matrix is selected from the group consisting of A 2  O 3 , mullite and B 4  C .Iaddend.having homogeneously dispersed therein about 5 to 60 vol. % of silicon carbide whiskers, with said silicon carbide whiskers having a microcrystalline structure and .[.in.]. a size range of about 0.6 μm in diameter and a length of 10 to 80 μm .Iadd.and which are characterized by crack bridging pull-out from said matrix when said matrix is subjected to crack-forming stresses, .Iaddend.said composite being of a density greater than about 99% of the theoretical density. 
     
     
       2. The ceramic composite claimed in claim 1, wherein the silicon carbide whiskers are .[.essentially unidirectionally oriented.]. .Iadd.preferentially aligned and randomly distributed in planes .Iaddend.in the matrix. 
     
     
       3. The ceramic composite claimed in claim 2, wherein the matrix is Al 2  O 3 , the concentration of silicon carbide whiskers in the matrix is about 20 vol. %, wherein fracture toughness of the composite is in the range of about 8 to 9 MPa.m 1/2   at room temperature, and wherein fracture strength of the composite is in the range of about 590 to 899 MPa at room temperature. 
     
     
       4. The ceramic composite claimed in claim 2 wherein the matrix is 3Al 2  O 3 .2SiO 2 , the concentration of silicon carbide whiskers in the composite is about 20 vol. % wherein fracture toughness of the composition is 4.6 MPa.m 1/2   at room temperature, and wherein fracture strength of the composite is about 438 MPa at room temperature. 
     
     
       5. The ceramic composite claimed in claim 1, wherein the silicon carbide whiskers are essentially omnidirectionally oriented in the matrix. .[.6. The ceramic composite claimed in claim 1, wherein the ceramic matrix material is selected from the group consisting of Al 2  O 3 , 
     
     
        3Al 2  O 3 .2SiO 2 , B 4  C..]. 7. A method for preparing a ceramic composite having increased toughness and resistance to fracture, comprising the steps of forming a homogeneous mixture of .[.particulate ceramic material with.]. about 5 to 60 vol. % of silicon carbide whiskers having a monocrystalline structure and .[.in.]. a size range of about 0.6 μm in diameter and a length of 10 to 80 μm .Iadd.with particulate matrix wherein said matrix is selected from the group consisting of Al 2  O 3 , mullite and B 4  C, .Iaddend.and hot pressing the mixture at a pressure in the range of about 28 to 70 MPa and temperature in the range of about 1600° to 1900° C. for a duration of 0.75 to 2.5 hours to provide a composite with a density greater than about 99% of the theoretical density of the ceramic material .Iadd.and in which composite the silicon carbide whiskers are characterized by crack bridging pull-out from the ceramic material when said composite is subjected to 
     
     
        crack-forming stresses..Iaddend. 8. A method for preparing a ceramic composite as claimed in claim 7, wherein the ceramic material is of a particle size less than about 44 m. .[.9. A method for preparing a cermamic composite as claimed in claim 8, wherein the ceramic material is selected from the group consisting of Al 2  O 3 , 3Al 2   
     
     
        O 3 .2SiO 2 , and B 4  C..]. 10. A method for preparing a ceramic composite as claimed in claim .[.9.]. .Iadd.7.Iaddend., wherein the silicon carbide whiskers are preferentially aligned along an axis of the composite by unidirectionally hot pressing the mixture along an axis 
     
     
        peripendicular to the first-mentioned axis. 11. The method for preparing a ceramic composite as claimed in claim .[.9.]. .Iadd.7.Iaddend., wherein the silicon carbide whiskers are omnidirectionally aligned with the composite by isostatically hot pressing the mixture.