The
Al 2O 3 coating is prepared by the sol gel process on SG4 engineering ceramics. The heat treatment process of Al
2O 3 gel powders is studied by DTA. The
bending strength of the samples after wire EDM cutting, rough grinding, finishing grinding, coating once with Al 2O 3 sol and coating twice with Al 2O 3 sol is tested and compared.The cause of the surface modification is analyzed by observing SEM photographs of surface and section of the samples coated once and twice with Al 2O 3 sol. It is shown that sol coating closing the microcracks of the sample surface is the main reason of increasing the bending strength of substrates. Because the quality of the samples coated twice with Al 2O 3 sol is better than that of the samples coated once with Al 2O 3 sol, the microcracks of the samples coated twice with Al 2O 3 sol can be closed more evidently and make the bending strength higher. In conclusion, sol gel is an effective method for the surface modification of ceramics.