Ceramics containing dispersants for improved fracture toughness

Abstract

The invention is a ceramic composition containing a new class of dispersant for hindering crack propagation by means of one or more energy-dissipative mechanisms. The composition is composed of a ceramic matrix with dispersed particles of a transformation-prone rare-earth niobate, tantalate or mixtures of these with each other and/or with a rare-earth vanadate. The dispersants, having a generic composition tRMO 4 , where R is a rare-earth element, B is Nb or Ta and O is oxygen, are mixed in powder form with a powder of the matrix ceramic and sintered to produce a ceramic form or body. The crack-hindering mechanisms operates to provide improved performance over a wide range of temperature and operating conditions.

The embodiments of this invention in which an exclusive property or privilege is claimed are defined as follows: 
     
       1. A ceramic composition consisting essentially of a ceramic matrix of Al 2  O 3 , mullite, ZrO 2 , MgO or mixtures thereof as a continuous phase with a hardness above about 800 and as a second component a tetragonal rare-earth niobate, tantalate or mixtures of these with each other and/or with a rare-earth vanadate, the second embodiment being present in an amount in the range of about 3-25 wt. % of the composition and dispersed in the matrix as particles sized below about 5 microns, the particles of the second component in the matrix having a transition temperature T D  being below the transition temperature for the second component in bulk or T C . 
     
     
       2. The ceramic composition of claim 1 wherein the second component has a depressed transition temperature T D  above about 800° C. and below T C  by at least about 100° C., a melting temperature above about 1600° and a hardness below about 500 (Vickers pyramid). 
     
     
       3. The ceramic composition of claim 1 wherein the second component has the formula tRMO 4  wherein R is a rare earth, M is Nb or Ta or mixtures thereof with or without V, and R has an atomic number in the range of 39 and 57-71 when M is Nb and 39 and 60-71 where M is Ta. 
     
     
       4. The ceramic composition of claim 3 wherein the second component has a depressed transition temperature above about 800° C., a melting temperature above about 1600° C., and a hardness below about 500. 
     
     
       5. The ceramic composition of claim 5 wherein the second component has a stress-free transition temperature between 800° and 1200° C. and is present in the amount of about 8-15 wt. %. 
     
     
       6. The ceramic composition of claim 5 wherein the particles of the second component are sized in the range of about 0.1-2 microns. 
     
     
       7. The ceramic composition of claim 5 wherein R is Ho and M is Ta. 
     
     
       8. The ceramic composition of claim 5 wherein R is Nd and M is Ta. 
     
     
       9. The ceramic composition of claim 5 wherein R is Nd and M is Nb 0 .6 Ta 0 .4. 
     
     
       10. The ceramic composition of claim 5 wherein R is Nd and M is Ta 0 .8 V 0 .2.