Compositions: ceramic – Ceramic compositions – Refractory
FIELD OF THE INVENTION
This invention relates to sintered alumina-based ceramics and a process for producing same in which alumina grains are rendered to be grown anisotropically and which enables to achieve both high flexural strength and high fracture toughness. More specifically, this invention relates to sintered alumina-based ceramics and a process for producing same which are preferable for use as structural materials, wear resistant materials or cutting tools, each of which requires high flexural strength and fracture toughness, or suitable for use under high-temperature conditions.
The grouping of the elements is based on the international periodic table of the IUPAC version.
BACKGROUND OF THE INVENTION
Alumina is a material which is thermally stable because of its high melting point and excellent in wear resistant properties and chemical stability. However, the fracture toughness of sintered alumina is generally low of around 3 MPa·m
so that the main current of a structural material is non-oxide ceramics having high fracture toughness such as silicon nitride and the like now. However, it is considered that alumina is one of the essentially excellent materials in anti-oxidizing and anti-corrosive properties as compared with non-oxide ceramics since alumina is an oxide. Further, alumina can be sintered in the air, and therefor, it has the advantage of being possible to regulate its production cost low as compared with non-oxide ceramics which require the control of the sintering atmosphere. Accordingly, it is expected that the improvement of sintered alumina to have high fracture toughness will broaden the range of its applications still more.
The following processes have been disclosed for improving the fracture toughness of alumina, the lack of which has been one of the drawbacks of alumina, by increasing the resistance to crack propagation with anisotropic oxide grains introduced into sintered alumina:
(1) dispersing process of platelet-like alumina grains into alumina matrices (Japanese Patent Kokai Publication JP-A-61-256963 (1986));
(2) growing process of platelet-like alumina grains comprising the steps of admixing fluoride such as LiF, NaF, KF etc. with alumina and providing a liquid phase in the course of sintering (Japanese Patent Kokai Publication JP-A-6-87649 (1994));
(3) growing process of platelet-like alumina grains by adding a very small amount, e.g., around several hundred ppm of SiO
to alumina (PROGRESS IN CERAMIC BASIC SCIENCE: CHALLENGE TOWARD THE 21ST CENTURY, 161-169(1996));
(4) growing process of platelet-like alumina grains by heat-treating sintered alumina after sintering (Japanese Patent Kokai Publication JP-A-7-257963 (1995));
(5) depositing process of platelet-like lanthanum &bgr;-aluminate into alumina matrices (Japanese Patent Kokai Publication JP-A-63-134551 (1988));
(6) adding process of SIO
in addition to the process (5) (Japanese Patent Kokai Publication JP-A-7-277814 (1995));
(7) depositing process of needle-like AlNbO
grains by admixing around 40% of niobium pentoxide into alumina and sintering the resultant mixture (J.Am.Ceram.Soc.,79,(9),p. 2266-2270 (1996)); and
(8) mixing process of isotropic alumina crystal grains having a long diameter of not more than 3 &mgr;m and an aspect ratio of not more than 1.5, with anisotropic alumina crystal grains having a long diameter of not less than 10 &mgr;m and an aspect ratio of not less than 3 at a predetermined mixing ratio, respectively, by way of e.g., admixing alumina powder with metal oxide having an eutectic point with alumina of not more than 1600° C. to mold the resultant admixture, and heating the molded product from the room temperature up to its sintering temperature at a rate of not less than 8° C./min. by means of e.g., microwave heating in the course of sintering (Japanese Patent Kokai Publication JP-A-9-87008 (1997)).
DISCUSSIONS ON THE RELATED ART
In the course of eager investigations toward the present invention it has turned out that the aforementioned conventional processes involve the following problems.
In the process (1), platelet-like grains are added in order to improve the flexural strength. However, these platelet-like grains act as an inhibitor of sintering in this process wherein the platelet-like grains are premixed so that a compact sintered product cannot be obtained when large platelet-like grains are added, or the platelet-like grains are added in an increased amount for the purpose of improving the fracture toughness.
In the process (2), when alumina grains are grown in the platelet-like shape only by being effected by fluoride, that is, without admixing any disperse-phase-forming agent such as zirconia, carbides, whisker and the like (samples Nos. 8 to 12 in Example 2), fracture toughness is 4.8 MPa·m
or less. Accordingly, it is difficult to say that these samples have sufficient fracture toughness.
In the process (3), the fracture toughness is 3.5 MPa·m
at most, which can be obtained by the addition of 300 ppm of SiO
, and accordingly, the improving effect of this process in fracture toughness is very little. Further, the addition of only SiO
results in producing a heterogeneous sintered product in which coarse plate like grains and fine platelet-like alumina grains are co-existing. This causes lowering both the flexural strength and fracture toughness.
In the process (4), the size of platelet-like grains is large and flexural strength value remains low to be in the range of from 380 to 530 MPa. Further, this process requires heat-treating after sintering so that production steps correspondingly increase, which are not preferable in the aspect of production cost.
In the process (5), the fracture toughness is 3.4 MPa·m
at most, which indicates that the improving effect of this process in fracture toughness is little.
In the process (6), simultaneous admixing of SiO
additionally forms alumina grains into platelet-like shape and makes possible to attain high fracture toughness. However, lanthanum-&bgr;-aluminate is a material which has a lower Young's modulus than that of alumina. Thus it would be better to grow anisotropically alumina which has high Young's modulus for increasing the resistance to crack propagation (improving fracture toughness).
In the process (7), the flexural strength value is limited as low as 320 MPa due to the large size of needle-like AlNbO
grains in spite that fracture toughness indicates a high value of 5.3Pa·m
. Further, this process is not preferable in the aspect of cost, since the product contains 40% of costly niobium pentoxide.
The process (8) does not always provide products having satisfied properties, besides it is not preferable in the aspect of cost and the like since it requires the use of a special microwave heating apparatus.
SUMMARY OF THE DISCLOSURE
It is a primary object of the present invention to provide sintered alumina-based ceramics having high flexural strength and fracture toughness free from the disadvantages encountered in the prior art. Further objects of the present invention will become apparent in the entire disclosure.
In order to provide sintered alumina-based ceramics having high flexural strength and fracture toughness, the present inventors advanced the steps of their studies as follows.
The present inventors expected improving the fracture toughness of alumina by growing anisotropically alumina grains in their sintered product because the anisotropic grains deflect the propagation path of cracks. In other words, the larger the degree of this deflection of the cracks, the larger the fracture toughness. Accordingly, the sintered product of anisotropically shaped grains exhibits a large extent of deflection as compared with the sintered product of equiaxial grains so that the sintered product having anisotropically shaped grains has high toughness.
Therefore, the present inventors studied assiduously to attain the purpose of growing alumina grains anisotropically in the course of sintering. Consequently, they found that sintering a forme
Morrison & Foerster / LLP
NGK Spark Plug Co., Ltd
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