1. Field of the Invention
The present invention relates to an improved fibrous ceramics fabrication method by a room temperature extrusion shaping process and a fibrous monolithic ceramic fabrication method using the same.
2. Description of the Background Art
A fibrous monolithic ceramics serving as a target ceramic composite in the present invention has a construction in which a high hardness fibrous ceramics is enclosed by a ceramics boundary layer which is vulnerable to a shearing stress, so that a crack path that grows during a rupturing is deflected for thereby preventing a catastrophic fracture mechanism from occurring.
In general, the ceramics shows a good thermal, mechanical, and chemical characteristics, whereas it is sensitive to the existing defects and its fracture toughness is low, thereby exhibiting an catastrophic fracture mechanism and disadvantageously lowering its reliability when applied to a structural material. In order to effectively improve the fracture toughness of ceramics, there has been considered a fiber-reinforced ceramics in which a matrix phase is reinforced as a fiber having an improved mechanical property. The fiber-reinforced ceramics does not incur a stress concentration on a fractured portion thereof although the fracture occurs at a maximum stress thereof so as for the fiber to disperse the stress by supporting the load, whereby the catastrophic fracture mechanism which is witnessed in a general ceramics does not occur. Further, there appears a graceful failure mechanism in a load-displacement curve. Also, the matrix phase is preferentially fractured prior to the fiber fracture so that the fiber can play a bridge, and when the interface bonding of the fiber and the matrix phase becomes disconnected, the crack becomes deflected along the interface or the path becomes changed. Ultimately, there occurs a fracture such as a pull-out wherein it seems as if a fiber is pulled out. While passing through the above steps, the energy consumed for the fracture is increases, whereby the toughness becomes improved. However, the fiber-reinforced ceramics is expensive and requires longer composite fabrication process, thereby exhibiting its practical application.
Therefore, as a method for improving the ceramic toughness by using relatively simple steps, a ceramics composite concept has been proposed in which the ceramics having a high strength is formed into a plate shape or a fiber shape and the other ceramics that is weak to the shearing stress and relatively low in strength is introduced. In the material having such a structure, a fibrous or plate-like high strength polycrystalline ceramics becomes enclosed by the relatively weak ceramics, and when there grows a crack the path is deflected to the boundary layer and the fibrous or plate-like pull-out characteristic is induced to improve the fracture energy while preventing the catastrophic fracture mechanism.
FIG. 1 is a view illustrating a fibrous monolithic ceramics which is composed of an axially arrayed fibrous ceramics and a boundary layer ceramics.
In fabricating the material having such a structure, it is important to follow the steps wherein there is formed a fibrous or plate-like ceramics which is precise and whose thickness is adjusted, wherein the ceramics forming a weak boundary layer is spread, and wherein a stack is carried out to obtain a desirably shaped body.
In particular, in order to fabricate a large length-width ratio such as a fibrous ceramics required in the fibrous monolithic ceramics, a bonding agent is added to ceramic powders to improve the strength for maintaining its form, and a plasticizer is added to improve a flexibility, a forming property and a treatment facility of the shaped body.
As a method for fabricating the ceramics as a fiber formed shape, there have been employed a dry-spin method and a melt-spin method.
In the dry-spin method, the ceramic powders, a dispersant, a bonding agent and a plasticizer are admixed in a solvent, and the admixture is maintained at an appropriate viscosity, thereby being extruded by an extrusion machine and at the same time the solvent becomes rapidly evaporated using a dry zone of 85.about.110.degree. C., thereby fabricating a fibrous ceramics. The above method, however, is not facilitating in modeling the fibrous slurry having an appropriate viscosity for the extrusion. Further, in order to improve the strength by removing the solvent within the extruded fibrous slurry, there is required a sudden dry for thereby resulting in a longer dry zone. Also, there is disadvantageously included a process in which the temperature should be raised using a thermal line. The dry speed of the extruded body formed in a cylindrical fiber shape is uniformed depending upon the location within the dry zone in the dry step, whereby the fibrous ceramics section becomes largely deviated from one fabricated by the dry extrusion.
FIG. 2 is a picture taken along a cross-section of the fibrous ceramics fabricated by a dry-spin method. As shown therein, the fibrous ceramics fabricated by the dry-spin method has a large degree of contraction in the fibrous thereof due to the solvent evaporation according to the sudden dry, and it does not maintain its circular shape resulting from its uneven state according to the position.
Meanwhile, as techniques for spreading the ceramics in which a boundary layer is included in the extruded fiber ceramics, there has been employed a method in which a coating-purposed slurry is released from above in a state where the fibrous ceramics is suspended for the spreading, or another method in which a extruded fibrous ceramics is passed through the coating-purposed slurry. The former has a disadvantage in that the coating layer thickness cannot be evenly maintained.
In the melt-spin method, the ceramic powders and the thermal plastic bonding agent are admixed while being heated and then being extruded, wherein a thermal plastic bonding agent having a plasticity resulting from a thermal effect is employed to thereby fabricate the extrusion-purpose slurry and it is maintained at a room temperature at the same time with the extrusion for thereby obtaining the plasticizing. In the above steps, there is required a temperature ranging from 170.about.180.degree. C. in case of using polypropylene, and a temperature ranging from 180.about.190.degree. C. in case of using ethylene vinyl acetate, for thereby assigning a plasticity by melting the bonding agent. The melt-spin method maintains a circular surface of the extruded fibrous ceramic, and the treatment strength and the plasticity are good and the spreading step can be easily applicable.
However, in the mulling process of the ceramic powders and the bonding agent, the temperature of the mixing machine should be disadvantageously raised so as to improve the plasticity of the bonding agent and the temperature should be disadvantageously precisely controlled so as to adjust the extrusion-purpose slurry viscosity for the product modeling.