In this paper, CVD SiCf/5A02 composites were prepared by different coatings of SiC fiber and manufacture processes, and SiCf/TiAl composite was made by a novel process called elemental coating method under 800℃ in order to improve the interface of the composites.
Anodic oxidation method was used to treat the surface of carbon coated SiC fiber. The results of analysis by scan electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) indicated that hydroxy, carbonyl and carboxy were formed in the carbon coating and the oxygen (O) content on the surface of coating increased after the treatment. The dimension swelling of coating created a press stress on the surface of the fiber and the tensile stress on the surface of the fiber was reduced. Meanwhile，the damage sensitivity of the fiber relaxed since the surface of fiber was smoothed by the etching effect of the electrochemical treatment. As the result, the tensile strength of the fiber was improved in average and the dispersing ratio was reduced.
SiCf/5A02 composite were prepared by the SiC fibers with carbon, O-rich carbon and SiO2 surface coatings respectively. The interface microstructure and tensile strength of the composites are affected greatly by coatings. The tensile strength of composite with carbon coating SiCf is the highest. Hexagonal Al4C3 plates formed by a mild diffusion reaction at the interface of composite，which resulted in a moderate bonding between the fiber and matrix. The fibers were lightly pulled out after composite broke under tensile stress. In the interface of O-rich carbon coating SiCf/5A02 composite, continuous MgO layer was formed, which restrict the diffusion of C and formation of Al4C3 and strengthen the bonding of interface. However, the volume of the coating swelled by the diffusion of Mg which increased a shear stress in coating or between coating and SiC columnar crystal. As the results, crack on the fiber surface can be deflected at weak bonding zone of the interface and the coating can be delaminated and fiber debonded. The tensile strength of the composite is lower ，the fibers were pulled out severely after the failure of the composite. In the interface of SiCf/5A02 composite with the SiO2 coating fiber, MgO and Al4C3 grew simultaneously and strong bonding interface formed. Because there is no buffer layer between SiC columnar crystal and the brittle interface, if a crack formed on the surface of fiber, it can be extend through grain boundary directly. The strength of the composite is the lowest.
A novel processing called semi-solid diffusing (SSD) method was used to manufactured CVD SiCf /5A02 composite in order to improve the bonding between fiber and matrix, in which the diffusion bonding temperature is partly in the semi-solid zone of the Al alloy. SEM and TEM analysis shows after a treatment at solidas curve the size and density of Al4C3 in the interface of the composite is higher than that of prepared by soild diffusing method. The bonding of interface became stronger, so the fracture of the fiber shows multiple step cracks. After treating at semi-solid zone, Mg enriched around the surface of SiC fiber and reacted with Al2O3 film on the surface of Al alloy matrix to form fine MgO grains. The diffusion of C element and nucleation of brittle Al4C3 was restricted. But the size of Al4C3 was promoted by the increase of temperature, and the surface damage sensitivity of fiber was also increased as well. Crack tends to generate on the surface of fiber under the low stress. The tensile strength of composite is reduced. After shortenning the total heating time, the dimension of Al4C3 reduced and the strength of composite improved.
At the last part of the paper, a SiCf/TiAl composite was made by a novel process called elemental coating method. Firstly, Ti and Al layers about 1μm in thickness were deposited on the SiC fiber alternately by magnetron sputtering to form a Ti-Al elemental coating SiC fiber. Then the SiCf/TiAl composite was prepared by diffusing bonding method at lower temperature with excellent plasticity of elemental coating. The preparation of composite has four steps: removing the glue at 390℃/2h＋heat pressing at 640℃/80MPa/2h＋uniform at 800℃/0MPa/2h＋cooling with 1.5℃/min. The diffusion reaction between elemental coating is almost completed and TiAl intermetallic was formed. The experience shows that composite with uniform matrix could be prepared by this method, hot crack in the matrix and reaction in the interface can be controlled.
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