Some Aspects of Plastic Flow in Silicon Nitride

The different scales of plastic flow in silicon nitride were investigated either by indentation experiments and compression under hydrostatic pressure in the 20--850 C temperature range, and by stress relaxation and creep above 1,350 C. [0001], 1/3{l_angle}11--20{r_angle} and 1/3{l_angle}11--23{r_angle} dislocations were evidenced by Transmission Electron Microscopy (TEM) in the low temperature range. Cross-slip events in {l_brace}10--10{r_brace} prismatic planes were observed at temperatures as low as 20 C by Atomic Force Microscopy (AFM) on micro-hardness indents. By increasing the temperature, the deviation plane becomes {l_brace}--20{r_brace} prismatic planes. The easiest slip system is by far the [0001]{l_brace}--10{r_brace} system. Above 1,350 C, the creep strain could be fitted by the sum of a transient component, {var_epsilon}{sub t}={var_epsilon}{sup {infinity}}[1-exp-(t/{tau}{sub c}){sup b{sub c}}], where {tau}{sub c} reflects the duration of the transient creep stage, and b{sub c} is between 0 and 1, and a stationary component, {var_epsilon}{sub s} = {var_epsilon}{sub s}t = A{sigma}{sup n}t, where {sigma} is the stress and n is the stress exponent. The increase of {var_epsilon}{sup {infinity}} with temperature is interpreted on the basis of the formation of liquid intergranulary phases above 1,400 C by progressive melting of some of the grains. A creep exponent of 1.8 was determined. A single valuemore » could hardly be given to the activation energy since a S-shape curve was observed in the lN {dot {var_epsilon}}{sub s} versus 1/T plot, as for most glasses over large temperature ranges. The stress relaxation kinetics was found to follow the Kohlrausch-Williams-Watt expression: {sigma}/{sigma}{sub 0} = exp [{minus}(t/{tau}{sub r}){sup b{sub r}}], where b{sub r} ranges between 0 (solid state) and 1 (liquid state) and {tau}{sub r} is a characteristic relaxation time constant. As in the case of glasses, {tau}{sub r} decreases rapidly whereas b{sub r} increases from about 0.2 to 0.7 as the temperature increases from 1,400 to 1,650 C. But again, it is very difficult to get a single value for the activation energy from the ln {tau}{sub r} versus 1/T plot.« less