| Zn O and Zn Al2O4 were coated on the surface of aluminum borate whiskers(ABOw) by a hydrothermal synthesis method. The coating could not only improve the wettability between ceramic whisker and metal substrates, but also avoid the interface reaction between ABOw and magnesium element in the aluminum alloy substrate.Therefore it could improve the interface bonding state and then aluminum matrix composites with excellent properties were fabricated squeeze casting method.It has been proved that Zn O could be successfully coated on the surface of ABOw using hydrothermal synthesis method. And studies show that hydrothermal process parameters, including zinc salt types, hydrothermal temperature, hydrothermal time and the p H value of solution as well as solution concentration, and the sintering temperature of coated whiskers have effects on the distribution and morphology of the coating as well as phase composition. As the increase of hydrothermal temperature, hydrothermal time, p H value or solution concentration, the size and the amount of the coating particle increase gradually. When the mass ratio of ABOw and Zn NO3 is 10/1, and the solution p H value is 8, and the hydrothermal temperature is maintained at 200℃ for 6 hours, the Zn O coating on the surface of whiskers would distribute uniformly. In addition, it is found that the Zn O coating presents a sphere-like when the solution is Zn SO4, and that the morphology of the coating turns to be block-shaped when the solution is Zn NO3.The result of differential scanning calorimetry and thermogravimetry(DSC-TG)shows that ABOw can react with the Zn O coating, whose reaction product is Zn Al2O4,when the sintering temperature reaches 870℃. In order to explore the effects of the structure of the coating on the whisker’s surface on the mechanical properties of the composite, the sintering temperature is set as 700 ℃ and 1000 ℃. The infiltration pressure test results indicate that the infiltration pressure of the preform with coated whiskers decreases apparently, and that the infiltration pressure in the preform with Zn O coating is lower than that of in the preform with Zn Al2O4 coating.It has been proved by the outcome of transmission electron microscope(TEM) that the Zn O coating would have replacement reaction with molten aluminum to produceγ-Al2O3 while the Zn Al2O4 coating has no change during the squeeze casting process.Moreover, it is found that there is no definite orientation relationship between the Zn O coating and the metal matrix while there exists an obvious orientation relationship between Al and Zn Al2O4, <111>Zn Al2O4//<111>Al.The result of tensile test shows that the tensile strength of as-cast ABOw/S-Zn O/Al composite is 380 MPa while as-cast ABOw/N-Zn O/Al composite is 385 MPa. It could be explained by the difference of coatings’ morphology. Then it can be concluded that the mechanical properties of the composite contained the Zn Al2O4 coating is higher than that of the composite contained the Zn O coating.Heat treatment could optimize the mechanical property of the composites. After ageing treatment, θ phase, Cu Al2, appears in the matrix. Because of the existence of θ phase, the strength of matrix rises greatly. Compared with the ABOw/N-Zn O/Al, the elongation of ABOw/S-Zn O/Al has less change while its strength has a wider range ability. |
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