A METHOD FOR PREPARING CERAMIC MIXED-OXIDE MATERIALS, PARTICULARLY INTENDED TO BE USED AS MATRIX MATERIAL IN COMPOSITE CERAMIC PRODUCTS

Abstract: The invention refers to a method for preparing ceramic mixed oxide materials, particularly intended to be used as matrix material in composite ceramic products, by mixing and co-milling of a metal and a ceramic oxide material and subsequent reaction-sintering in an oxidizing atmosphere. For obtaining an improved homogeneity between metal and oxide materials, the invention suggests that the metal is used in the form of an alloy with an element which is to be present in the final mixed-oxide material.

Description: A method for preparing ceramic mixed-oxide materials, particularly intended to be used as matrix material in composite ceramic products. The present invention refers to a method for pre- paring ceramic mixed-oxide materials, particularly inten¬ ded to be used as matrix material in composite ceramic products, by mixing and co-milling of a metal and a cera¬ mic oxide material and subsequent reaction-sintering in an oxidizing atmosphere. In the preparation of ceramic oxide materials of the kind referred to by mixing and co-milling of a metal and a ceramic oxide material and subsequent reaction-sin¬ tering it has always been desirable to use the metal in a particulate form with as small particle size as ever pos- sible. Therefore, in many cases it has been attempted to use milled aluminium of flake shape. A disadvantage has been, however, that said flakes smear and adhere to the milling bodies due to the softness of the metal and hence also mostly remain in flake shape. See e.g. Nils Claussen et al. "Low-shrinkage Reaction-Bonded Alumina", J. Europ. Ceram. Soc. , 5, 1989, pages 29-35. It might also be referred to other articles of and with Nils Claussen and his research group such as "Tailoring of Reaction-Bonded A1 2 0 3 Ceramics", Ceram. Eng. Sci. Proc, 11, pages 806- 820, 1990. Another way to achieve a better homogeneity bet¬ ween metal and oxide materials in the present connection has been suggested e.g. by the US firm Lanxide Corp., Newark, Del., USA and implies that an oxidation of the metal is carried out in melted form. See "Formation of Lanxide ceramic composite materials", J. Mater. Res., 1, 1986, pages 81-89. Furthermore it is to be referred to the Swedish patent No. 8103269-0 which refers to a method in preparing shapes of silicon nitride based materials. Since it thus is the fine-division of the metal which in most cases constitutes the step which is diffi- cult to surmount for achieving the goals aimed at, the present invention now suggests a different way of over¬ coming the difficulties and the feature essentially dis¬ tinguishing the present invention is that the metal is used in the form of an alloy with an element which is to be present in the final oxide material. In this connection it is a particular advantage of the invention when the element with which the metal is to be alloyed, is selected such that the alloy obtained becomes brittle so as to facilitate the milling. The subsequent reaction-sintering furthermore can be improved by adding a catalyst metal to the alloy. Thus the basic idea of the present invention is to mill, together with a suitable oxide compound, a brittle metal alloy into fine particle sizes. In the subsequent reaction-sintering the metal then is oxidized to its cor¬ responding ceramic composition. As examples thereof it can be referred to a test with an Al:Si-alloy (88:12), which together with Al 2 0 3 was successfully milled to fine-size particles. Dry-pressed bodies of this composition were oxidized and it turned out that all of the Si together with a portion of Al formed mullite. Furthermore, this occurs already at a temperature below 1400°C in air atmosphere. Other ceramic materials which can be manufactured according to the same principles are Al 2 Ti0 5 , 3Y 2 0 3' 5A1 2 0 3 (YAG) , g 2 Al 4 Si 5 0 18 (coerdirite) , MgAl 2 0 4 (spinel) , LiAlSi 2 Og (/3-spodumene) and CaZr0 3 (T >2300°C) . By the invention it might thus be achieved that the milling of a pulverulent metal alloy with predetermi- ned brittleness provides particles of sub-micron size (< lμm) . This provides for a really homogenous distribu¬ tion of the metals in the alloy and hence reduces the temperature of the transformation to the corresponding ceramic composition. The homogenous distribution further¬ more promotes this transformation. Moreover, when prepa¬ ring an alloy powder having an added catalyst metal the reactivity might be further increased, e.g. by adding Mg to Al:Si. It also becomes possible to select the alloy such that an exactly correct ceramic composition is obtai¬ ned. Owing to said correct ceramic composition the shrinkage during the oxidization also might be controlled and minimized. Should the metal be made still more brittle so as to promote the milling thereof a gas-atomized alloy powder may be produced having a certain quantity of added ceramic powder. Example 1 Preparation of mullite Two powder mixtures were prepared. One of them (reference AlSi) consisted of 30% by weight of Al:Si- alloy, weight ratio 88:12, (Johnson Matthey, <44 μm) , 69% by weight of 1 2 0 3 (Alcoa, A152SG, BET = 3,2 m 2 /g) and 1% by weight of Mg (Merck, <1 mm) , as auxiliary oxidization agent. The other (reference AlSi-Zr0 2 ) consisted of 30% by weight of Al:Si, 49% by weight of A1 2 0 3 , 20% by weight of Zr0 2 and 1% by weight of Mg. Zr0 2 was added as oxygen- diffusion-increasing agent. The two mixtures were milled in a ball mill with Si 3 N 4 ~balls in an organic solvent to which was added dispersing and pressing agents. After milling the specific surface (BET) of AlSi had increased from 5,9 m 2 /g to 10,0 m 2 /g and for AlSi-Zr0 2 from 6,5 m 2 /g to 10,5 m 2/g and studi.es i.n a scanning-electron microscope disclose that the alloys were milled into sub-micron par¬ ticles. The organic solvent was removed in a thin-film evaporator and after that the mixtures were screen-granu¬ lated. The powder mixtures were subjected to cold isosta- tic pressing at 300 MPa to form green bodies with green densities of 71% of the theoretic density. Oxidation tests were carried out in a thermo-gra- vimetric analyzing equipment (TGA) . At 1400°C all Si with a portion of Al had been oxidized to mullite (Al g Si 2 0 13 ), as proved by X-ray analyses and the density of said sam¬ ples amounted to 84% of the theoretic value (valid for both AlSi and AlSi-Zr0 2 ) . In AlSi-Zr0 2 ~samples Zr0 2 reac- ted with Si and was oxidized to ZrSiO. in the temperature range between 1100 and 1300°C, after which ZrSi0 4 reacted with A1 2 0 3 so as to form mullite at 1400°C. The results also showed that in tests with Zr0 2 -addition the oxidation rate of the alloy increased deeper in the inner of the body. Example 2 Y 2 A1 (Alfa Products) was melted and spray-atomi¬ zed. The powder obtained was weighed together with A1 2 0 3 (Alcoa, A165G) in required quantities. The mixture was milled in a mill grinder with Al 2 0 3 -balls in an organic solvent with added dispersing and pressing agent. Studies in scanning-electron microscope showed that the alloy had been milled to sub-micron particles. The organic solvent was removed by means of a thin-film evaporator and after that the mixture was screen-granulated. The powder mixture was subjected to cold isostatic pressing at 300 MPa to form test bodies. Oxidation tests were made in a thermo-gravimetric analyzing equipment (TGA) at 1400-1700°C. The resulting material consisted of yttrium-aluminium garnet (YAG) , which was identified by means of X-ray diffraction. Example 3 Ti 2 Al-powder (Alfa Products) was weighed together with A1 2 0 3 (Alcoa, A165G) in required quantities. The mixture was milled in a ball mill with Al 2 0 3 ~balls in an organic solvent with added dispersing and pressing agents. Studies in scanning-electron microscope disclosed that the alloy had been milled to sub-micron particles. The organic solvent was removed by means of a thin-film evaporator and after that the mixture was screen-granulated. The powder mixture was subjected to cold isostatic pressing at 300 MPa to form green bodies. Oxidation tests were carried out in a thermo- gravi etric analyzing equipment (TGA) at 1400-1500°C. The resulting material consisted of aluminium titanate (Al 2 Ti0 5 ) , which was identified by means of X-ray diff¬ raction.

Claims: C l a i m s 1. A method for preparing ceramic mixed-oxide ate- rials, particularly intended to be used as matrix material in composite ceramic products, by mixing and co-milling of a metal and a ceramic oxide material and subsequent reac¬ tion-sintering in an oxidizing atmosphere, c h a r a c ¬ t e r i z e d i n that the metal is used in the form of an alloy with an element which is to be present in the final mixed-oxide material. 2. A method according to claim 1, c h a r a c t e ¬ r i z e d i n that the element, with which the metal is to be alloyed, furthermore is selected such that the alloy formed is brittle for facilitating the milling. 3. A method according to claim l or 2, c h a r a c ¬ te r i z e d i n that a catalyst metal is added to the alloy.

Inventors: BRANDT JESPER, LUNDBERG ROBERT, PEJRYD LARS, BRANDT, JESPER, LUNDBERG, ROBERT, PEJRYD, LARS
Applicants: BRANDT JESPER, LUNDBERG ROBERT, PEJRYD LARS, VOLVO FLYGMOTOR AB
Application year: 1993
Main classification: C04B35
IPC codes: C04B35