The choice of technologies and parameters for manufacturing materials for improving the properties of ceramics with a limited plasticity in a broad temperature range should be performed taking its operation parameters into account. For materials of

the first sections of the HRA operating in the brittle-damage temperature range, it is necessary to increase, along with the strength, the fracture toughness, which can be done first of all by decreasing the defectiveness of the materials and producing a structural state that increases the amount of energy required to produce damage. The strength in this temperature region is increased by the following methods: elimination of structural defects by optimizing the condensation and sintering regimes [15, 16], healing of defects and thermomechanical programmed control [21], modification of a stressed surface state [39], or carbon doping [17, 18, 27], or metallic band [14]. Essential increase of strength can be reached at use of nanocrystalline technologies [40, 41].

The elimination of surface defects by healing at temperatures T/Tm > 0.5 can increase the strength appreciably. The healing of radial cracks to the depth up to half a radius of the ZrC sample begins when the crack edges contact by surface diffusion. The healing is intensified with increasing the number of contacts on one length unit and is decreased with crack edge opening 8. At T/Tm= 0.6 the healing of the cracks with 8 = 1-2 ^ is completed within several hours and strength returns to the initial level (Fig.4.32). The cracks with 8 > 3 ^ and very small number of edges contacts are not healed within the same length of time. Under thermal treatment conditions the healing process has two stages.

The kinetics of the first stage (the increased intensity of the strength reduction process) is mainly defined by surface self-diffusion of Zr atoms in ZrC. After the first stage of healing, the crack is a system of isolated cavities. Its healing is performed at a lower rate (the second stage) owing to a viscous flow of the material as the surface diffusion cannot provide their volume decrease.

Another way of ceramic materials strengthening is the formation (in surface lay­ers) of compressive residual stresses preventing the appearance and spreading of cracks. The formation of residual stresses on the surface of a sample or an article is based on relaxation of thermo-elastic stresses nonuniformly distributed along the section.

Fig. 4.32 The strength reduc­tion of ZrC0.95 samples after healing of the surface cracks with the width of crack edge opening (1—I Ц, 2—1.5 Ц, 5-3-4 /Ї) at T = 2,800 K

Fig. 4.33 The bending strength change of ZrC0.95 (1, 2) and SC2O3 (5) in relation to quenching temperature (Tq) and cooling methods. 1 radia­tion cooling; 2 gaseous helium flow; 5 cooling in silicon oil

As a result of poor ductility of ceramic materials, in contrast to metals, their strengthening is performed at rather small values of criteria Bio~10-1 e. g. by blowing off a cold gas stream over the heated sample or by radiation cooling [6]. At higher Bio values the rate of thermal-elastic stress relaxation turns to be less than the rate of their increase, this leads to cracking. The temperature range of strengthening is limited in the lower domain by the temperature of the brittle — ductile transition and in the upper domain by causing the strength decrease. The increase of temperature in the range Tb-d < T < Ts rises the strengthening effectiveness. The gain in strength of strengthened ceramics is, as a rule, 20-40 % (Fig. 4.31).

A decrease of the critical volume defects, as stress concentrators, is possible by using thermal-mechanical treatment (TMT) at T/Tm > 0.6, based on stress relaxation near the concentrators [21].

The preliminary small deformation (є < 0.15%) at the low deformation rate є’ < 10-1 s-1 (Fig.4.34a) or static loading at stresses up to 0.6-0.8 a/amax (Fig. 4.34b) can increase the strength about two times. As said above there is a wide

spectrum of volume defects in materials prepared by powder metallurgy methods. It is possible to evaluate the integral structural defects by measuring the value of temporary microstresses appearing under loading in the elastic region by broadening of X-ray lines and rejecting the defective samples. The more intensive is the line broadening with growth of load, the more defective is the material. In the temper­ature region where the macro-plastic deformation becomes possible, the short — and long-term mechanical thermal-strength parameters can be optimized by almost all methods used for metals. In the temperature region where the deformation process is controlled by the motion of dislocations, the methods are used that reduce the dislocation mobility by substructural strengthening, doping of solid solutions result­ing in the formation of stronger chemical bonds in compounds, and doping with the formation of second phases. Under loading conditions, when deformation is mainly caused by grain-boundary sliding in the case of high-temperature creep, the strength can be efficiently increased by recrystallization, providing a consid­erable decrease in the length of boundaries due to the increase in the grain size. In using strengthening methods, it is necessary to take into account that struc­tural changes intended to improve mechanical characteristics at high temperatures should not impair mechanical properties at temperatures T < Tb-d. The optimal choice of the parameters of traditional isothermal sintering of ZrC samples in dif­ferent media (hydrogen. argon. and vacuum) allows achieving a high density (no less than 95%) and the bending strength about 550MPa at sintering temperatures 2,500-2,700 K [16].