Effects of different SiO

2

addition on microstructure

crystal growth and mechanical properties of Al

2

O

3

/33ZrO2（4Y） composites prepared by combustion synthesis under high gravity were investigated by XRD（X-ray diffraction）

SEM （scanning electron microscope） and EDS （energy dispersive scalometry）. The results reveal that phase constituent in the ceramic composites can not be changed with SiO

2

addition

however

with increasing in SiO

2

addition

the structure of ceramic composites is converted from cell eutectic colonies into rod-like ones

in which the length-diameter ratio of rod-like eutectic colonies is gradually raised with the decrease of volumetric fraction. As a result of the dual effects of the SiO

2

particle on viscosity of the ceramic melts

the relative density of the composites reaches the maximum value of 97.3% with 6% SiO

2

addition. Meanwhile

the hardness of ceramic composites is declined

which is attributed to the formation of SiO

2

amorphous phase in ceramic composites. In addition

due to the enhanced toughening effects of crack-bridging and crack deflection contributed by the rod-like eutectic colonies

the fracture toughness of the ceramic composites can reach up to 15.9MPa·m1/2 with 6% SiO

2

addition.