On the Impact Strength Characteristics of Fine-Ceramics involving the Surface Damage by Repeated Impact Load.

Engineering ceramics have two opposite aspects in their mechanical properties；low fracture toughness and high strength at high temperature together with excellent wear properties.In order to characterize the fracture toughness of fine ceramics，a series of experiments was carried out from two different points of view：One is impact3-point bending test at room and high temperatures to up1200°c，and another is face damtage points of view.The results of the former test were evaluated statistically.Brazed joints of ceramics/steel were used for the latter experiment considering the future application of ceramics.In this study were used three sorts of engineering ceramics；sintered silicon nitride，reaction sintered silicon carbide and alumina.Major conclusions of this study are summarized as follows：(1)Results of impact3-point bending test at room thigh temperature：The impact bending strength of silicon nitride is higher than the static bending strength when compared at each temperature level.And further the scatter of strength under impact bending is almost the same as that of static bending.Thus，it is concluded that silicon nitride has excellent impact strength at high temperature.On the other hand，silicon carbide shows low impact strength at high temperature when evaluated at low failure probability.(2)Results of surface damage tests：Observations of surface damage of silicon nitride indicated that macro-cracks propageted radially from center of ceramics disc；the indentation by steel hammer.

Engineering ceramics have two opposite aspects in their mechanical properties; low fracture toughness and high strength at high temperature together with excellent wear properties.In order to characterize the fracture toughness of fine ceramics, a series of experiments was carried out from two different points of view: One is impact 3-point bending test at room and high temperatures upto 1200゜c, and another is surface damage test by repeated impact load. The results of the former test were evaluated statistically. Brazed joints of ceramics/steel were used for the latter experiment considering the future application of ceramics. In this study were used three sorts of engineering ceramics; sintered silicon nitride, reaction sintered silicon carbide and alumina.Major conclusions of this study are summarized as follows: (1) Results of impact 3-point bending test at room thigh temperature: The impact bending strength of silicon nitride is higher than the static bending strength when compared at each temperature level. And further the scatter of strength under impact bending is almost the same as that of static bending. Thus, it is concluded that silicon nitride has excellent impact strength at high temperature. On the other hand, silicon carbide shows low impact strength at high temperature when evaluated at low failure probability. (2) Results of surface damage tests: Observations of surface damage of silicon nitride indicated that macro-cracks propageted radially from center of ceramics disc; the indentation by steel hammer.