Shoji Ito

Dependence of high-temperature tensile strength on displacement rate for hot-pressed silicon nitride

Tensile tests of hot-pressed silicon nitride were conducted in a wide range of displacement rates from 0.0005 to 50.0 mm min−1 at 1260 and 1380 ° C, and the dependence of strength on displacement rate was discussed in conjunction with the change of fracture surface features. As displacement rate was lowered, strength was degraded and stress-displacement diagrams altered from linearity to non-linearity, accompanying appearances of yielding phenomena. Strength was largely dependent on displacement rate in the high-rate region and rather independent in the low-rate region. The fracture surface features were drastically changed from brittle fracture to subcritical crack-growth fracture with decreasing displacement rate. A further increase in stress following the lower yielding point in the low-rate region was attributed to increments of fracture resistance due to the pull-out contribution of elongated grains.

Fatigue Behaviour of Structural Ceramics

4-point bending static and cyclic fatigue tests are carried out on several structural ceramics both at room and elevated temperatures. Measured fatigue behaviours are compared with the lifetime predicted from the dynamic fatigue properties and cyclic loading effects on delayed failure are discussed. Experimental results indicate that a delayed failure of ceramics under a cyclic loading is caused mainly by extension of a pre-existed flaw and the lifetime is substantially predictable from a subcritical crack growth behaviour estimated under a static or quasi-static loading condition. It is also pointed out that at elevated temperatures the crystallization of glassy phase at grain boundaries extends the lifetime and that a crack growth is accelerated by compressive stress if grain (cluster) bridgings and/or crack interlockings are formed on a crack surface. Humidity controlled stress corrosion of a glassy phase at grain boundaries and mechanical fracture through the grain boundaries due to softening of the glassy phase are considered as main crack growth mechanisms at room and elevated temperatures, respectively. From these results it is concluded that the fatigue behaviour of ceramics is strongly affected by the properties of grain boundaries.

Tensile strength and fracture defects expanded by subcritical crack growth of silicon nitride at high temperatures

On a realise une etude fractographique pour etudier les effets des defauts de rupture developpes par la croissance de fissure sub-critique a 1200 o C sur les proprietes mecaniques de ceramiques de nitrure de silicium

Effect of Machining Conditions on the Strength Distributions of Ground Ceramics

The relationship between strength distribution or ground ceramics and machining condition was examined. The strength of ground silicon carbide ceramics at the failure probability of 1 percent, which is termed 1 % strength, was estimated as a function of the maximum grain depth of cut g, which is used as an index of grinding conditions. It is assumed that the fracture is governed by the competing failure mechanism between pre-existing flaws and machining cracks and that the Weibull modulus of machining cracks was independent of grinding condition. The deviation from estimated value is noticeable in small g region although experimental data of the 1% strength of ground specimen approaches that of polished ones with diminishing g. The observed discrepancy is inferred to be attributed to an increase in stress intensity factor by the interaction between a pre-existing flaw and its adjacent machining crack.Copyright

Estimation for strength of ground ceramics at ambient and elevated temperature

An equation for the strength of ground ceramics has been proposed. The equation is the function of the maximum grain depth of cut g and the fracture toughness of the workpiece material, assuming that the nucleation of median crack during grinding is the same way as that in Vickers indentation test. Experimental results of the flexural strength of sintered silicon carbide ceramics ground with different kinds of grit diamond wheels under various setting conditions showed good agreement with the calculated ones. Assuming that temperature increase does not affect the size and shape of the crack, the strength of ground ceramics at elevated temperature is obtained.

Influence of Surface Crack Density on Strength of Ceramics

Bending test specimens made of silicon nitride ceramics were indented with a Knoop pyramid at intervals of 100-5000μm, and four-point bending strength was measured at room temperature. The bending strength increased with decreasing crack interval for the crack interval smaller than 500μm. This was attributed to the interference of stress field around the Knoop cracks. The influence of surface layer removal on the bending strength was studied for the specimens with many surface cracks introduced during the previous machining process. No difference was observed in the bending strength when the surface layer of less than 10μm was removed. This invariability of strength with the surface removal was also attributed to the interference of the stress field.