Furnaces developed and constructed at the institute for sintering and consolidating powder samples and deposition of coatings on products at high temperatures (up to 2,500-2,700K), which were unique for that time, were used to prepare samples required for studies and, later, components of the NRE core.

The general ceramics processing technology involves the following steps [14]:

1. Synthesis of powders with a given chemical composition, phase composition and grain size distribution;

2. Preparation of powders for forming;

3. Green part forming;

4. High-temperature consolidation;

5. Process quality.

Sintering is a crucial stage for structure evolution. Sintering is a thermally activated process (spontaneous or involving the application of external force) which features transformation of a system of contacting solids or porous media into thermodynam­ically more stable state through minimization of the free surface area. The driving

Composition	Diffusion element	Temperature range (K)	Do (sm2/s)	Q (kcal/gform)	D (sm2/s) at T, K 2,500 2,800	3,100
ZrC0.97	C14	2,500-3,100	14.1	1089.9 ± 6.1	4.26-10-9	4.35-10-8	2.878-10-7
	Zr95	2,500-3,100	1,030	172 ± 10.7	9.93-10-13	3.83-10-11	7.62-10-10
ZrC0.70	C14	2,500-3,100	2.8	164 ± 5.1	1.40-10-10	5.09-10-9	8.095-10-8
	Zr95	2,500-3,100	1,030	172 ± 10.7	9.93-10-13	3.83-10-11	7.62-10-10
NbC0.97	C14	2,500-3,100	0.11	94 ± 1.8	6.62-10-10	5.25-10-9	3.24-10-8
	Nb95	2,500-3,100	1.47	127 ± 7	8.3-10-13	1.32-10-11	1.23-10-10
NbC0.79	C14	2,500-3,100	0.11	100 ± 1.7	2.61-10-9	2.27-10-8	1.31-10-7
	Nb95	2,500-3,100	0.11	127 ± 7	8.73-10-13	1.32-10-11	1.23-10-10
(Zr0.48Nb0.52)C0.9	C14	2,500-3,100	2.28	96.5 ± 2.5	8.26-10-9	6.72-10-8	3.56-10-7
	Nb95	2,500-3,100	51.0	153.0 ± 9.8	2.12-10-12	5.71 • 10-11	8.26-10-10
(Zr0.48Nb0.52)C0.8	C14	2,500-3,100	0.84	100.5 ± 3.4	1.36-10-9	1.20-10-8	6.91-10-8
	Nb95	2,500-3,100	51.0	153.0 ± 9.8	2.12-10-12	5.71 • 10-11	8.26-10-10
TaC0.98	C14	2,650-3,200	3.9	118.7	1.66-10-10	2.58-10-9	1.66-10-8

Table 4.3 Diffusion parameters of the carbide compounds

4.2 Processing Technology of the Structural Ceramic Materials 35

Fig. 4.4 Installation for hot pressing (IHP-4) with pressing force up to 200tons and working space 900 x 1800 mm

force for this transformation is the excess free energy. Sintering is evident both as a dimensional change (mostly as shrinkage), and as significant change in structure and properties, both approaching these of compact materials. At the RIHRE “Luch”, there were developed special electrothermal vacuum systems for synthesis, sinter­ing, and hot pressing of refractory carbides, and systems for synthesizing fine carbide powders (Figs.4.3 and 4.4).

Details of kinetics of density, structure, and properties evolution during zirconium carbide sintering are described [15, 16]. ZrC powders containing 87.8 % Zr, 11.4 % of Ctotai, 0.15 % of Cfree, 0.018 % N, 0.62 % O were synthesized by carbon-thermal reduction at T = 2,100 °C. The synthesized powders were ball-milled in a vessel lined with refractory ZrC plates. Average particle diameter of powders was 4.5­5 ^, with BET surface area of 4.35m2/g. Green parts were formed by thermoplastic extrusion, dewaxed in a vacuum furnace at final temperature of 350 °C with heating rate of 10K/h, and dwell time of 2h. Final sintering was carried out in the graphite

Fig. 4.5 Kinetics of a relative density change (Y), electrical resistance (p), bending strength (aj), and magnitudes of grain size (L): at sintering of zirconium carbide in an argon, hydrogen and vacuum of 10-3 mm Hg (temperature in °C)

heater furnace at 1,400-2,800°C with dwell time of 0, 15, 30, 60, and 120min; heating and cooling rates were 800-900K/h (Fig.4.5).

High values of Y, a and electrical conductivity were obtained at 2,200-2,400 °C. Sintering in hydrogen and argon atmospheres yielded similar results, while vacuum sintering yielded slightly poorer properties, which is attributable to greater gas evo­lution during sintering possibly leading to more porous compact structure. Similar results were observed during sintering of ZrC+NbC and ZrC+NbC+UC systems [11]. The further increase of sintering temperature to 2,600-2,800 °C did not lead to better density or strength of the sintered samples. Poorer sample consolidation could be due to the effect of gases in the closed pores. At the overall porosity of ~6-7 %, the majority of pores are closed which greatly hinders further consolidation.

Higher porosity of samples sintered at 2,800 °C may be attributed both to gas pressure and to coalescence of gas-filled pores. Lesser strength of samples sintered at 2,600-2,800 °C is due not only to poorer density and significant grain coars­ening, but also to pore segregation at grain boundaries leading to formation of the

Fig. 4.6 Features of micro­scopic ZrC structure sintered at different temperatures and various medium [16]. a Vacuum, Ts = 2,000 °C,

T = 15 min x1000. b Vacuum, Ts = 2,200 °C,

T = 15min x1000. c Vacuum, Ts = 2,400 °C,

T = 15 min x1000. d Vacuum, Ts = 2,600 °C,

T = 15 min x1000. e Hydro­gen, Ts = 2,700 ° C, t = 5min x 1000. f Argon, Ts = 2,700 °C, t = 5min x 1000

highly specific crack patterns (Fig. 4.6). Samples sintered at T > 2,600 °C show pre­dominantly intergranular fracture, while those sintered at lower temperatures show mostly transgranular fracture. The relative density and strength a of fuel rods based on ZrC+UC and sintered at 2,200 °C in hydrogen reach 99 % and 650 MPa, respec­tively, while vacuum sintering at the same temperature yields values of 88% and 360 MPa, respectively [11].

When analyzing the sintering process one should consider porosity features. At relative density nearing 90 % of TD, the residual porosity is nearly always an open one, i. e., pores are connected to a surface. At overall porosity of 7-8 %, all pores became closed. As temperature rises or as sintering time increases, the powder compact becomes denser and the shrinkage increases, possibly reaching 20-25%. Simultaneously, there is a change in properties: density change could be traced by monitoring the decrease of the electrical resistance.

During sintering in gas atmosphere at 2,000-2,400 °C, and in vacuum at 2,300­2,500 °C densification mainly proceeds by decreasing the closed porosity; further temperature rise to 2,600 °C yields virtually no change in closed porosity. At this stage, pores coalesce and become spherical (see Fig. 4.6c, d). The number of pores decreases, while their size increases. At sintering temperatures above 2,600 °C, the closed porosity and overalls porosity values grow. Sometimes surface cracks appear on samples, leading to decrease of the closed porosity. This particular phenomenon was most frequently observed in samples sintered in argon at temperatures above 2,700 °C.

Bubble-shaped pores at this stage of sintering are pinned by boundaries at which these pores tend to segregate and coalesce (see Fig.4.6e, f). The shape of the pores changes, vast cavities appear, and part of bubble pores migrates to the sample surface as a result of coalescence of two gas filled pores of equal radius; the volume of a newly created pore may increase (provided that the volume if gas before and after the coalescence remains constant), which leads to the swelling of samples. In ZrC samples sintered at temperatures above 2,600 °C, the volume increase and density drop were observed. In some samples, this phenomenon was observed already at sintering temperature of 2,550-2600 °C, especially in argon atmosphere. The results of analysis of the furnace chamber atmosphere show that during sintering in Ar at pore closing stage (T at 2,000-2,200 °C) the major impurity is CO which may become trapped in the closed pores of the sample. Moreover, CO also forms during sintering through the reaction of free carbon with oxygen, and during rapid heating at 2,000-2,200 °C this CO may also become trapped in pores.

Samples with the green density p of 52-57 % were sintered in vacuum at tem­peratures of 1,000-2,250 °C, and subsequently in Ar atmosphere at Tmax= 2,500­2,700 °C. The shrinkage kinetics during sintering in a furnace with the special fixture for controlling the linear dimensions of the sample was constantly measured with an accuracy of ±10 ^.

The sintering behavior of the system “ZrC+ 2^10 % C (diamond) up to the tem­perature of 1600-1800 ° C was similar to that of the pure ZrC [17]. In the temperature range of 1,650-1750 °C corresponding to the maximum volume change of the dia­mond (due to its polymorphic transformation), the shrinkage rate dropped nearly to zero. Further temperature increase for this composition leads to significantly accel­erated densification. At this stage, the sample shrinkage curve of the ZrC-2.5 % C system is similar to the one of pure ZrC. Changing the diamond particle size from 50 to 1 ^ leads to a change in shrinkage curves. The absolute value of the density loss (caused by polymorphic transformation of diamond) decreases. The sintered compositions being studied show predominantly aggregated structure with carbon phase particles located primarily on the grain boundaries of the carbide matrix.

The general regularity of structure formation of ZrC, ZrC-C, NbC-C is the high speed recrystallization at sintering temperature above 2,250 °C: The grain sizes of compositions increase to 10-20 ^ with growth of carbon phase after sintering at 2,500 °C. Below this sintering temperature, the grain size of sintered composition are comparable with initial sizes of carbon corpuscles (5 and 2.5 ^).

The pores placed on boundary lines of grains, suppress grain growth at sinter­ing. An average grain size of carbides increases slightly (from 20 to 25 ^) at the sintering temperature interval 2,500-2,700 °C. As a whole the structures of sintered compositions ZrC, NbC with carbon additives (diamond, graphite) is characterized by the uniform distribution of pores in a matrix [18]. Estimations of the sintering energy activations Q of compositions ZrC-C, NbC-C show that Q in the investi­gated interval of temperatures is changeable [17]. At temperatures 1,300-1,400 °C Q = 65-80 kkal/g-form for compositions ZrC-C and NbC-C Q = 70-75 kkal/g — forms. At higher sintering temperature Q increases to 115-125kkal/g-forms for (ZrC-C) and to 120-135 kkal/g-forms for (NbC-C). The received values Q at

sintering of the compositions ZrC-C, NbC-C proceeding from the parameters of a self-diffusion component in carbides ZrC, NbC [2], testify to predominance of the boundary diffusion contribution in mass transfer at sintering to 1,300-1,400 °C. Role of the volume diffusion controlling the shrinkage at sintering becomes appreciable at temperature growth.

It is known that the temperature pattern formed in a powder pressing at heating­cooling can essentially influence on the character and intensity of sintering. However, the data on an agency of heating and cooling speeds on a process of carbide materials sintering in the literature are restricted by consideration of small speeds of heating­cooling near 500-1,000K/h. In this connection, an undertaken attempt [19] to use high-speed heating-cooling when temperature-kinetic parameters of heat treatment can become crucial. High temperature heating up to 2800 K is used for the carbide pressings ZrC0.95 in diameter of 2.5 mm with a speed 600K/s allows for increment sharply to 103 times speed of a shrinkage and to raise an initial density from 71 to 98% (Fig. 4.7).

The influence of sintering parameters on the sintered ZrC structure and properties are showed on Figs.4.8 and 4.9 accordingly. Sintering of ZrC samples by rapid nonisothermal heating takes place in a complicated manner. In the first stage of the sample heating to 1,300 K, for the first 2 s thermal elastic stresses of the order 30- 40MPa (Fig. 4.7) are developed and are capable of creating local plastic deformation of the material on rough surfaces of the particles. The presence of this phenomenon is supported by an experimental study of the dislocation start from the indentor print on the single crystal ZrC0.9 performed at a temperature of 290 K and subsequent heating to temperatures 600 and 850 K. The initial stresses as for dislocation movement at these temperatures are at the limit of 5-8 and 2-3 MPa, respectively [20]. It should be noted that local stresses on roughnesses with the size of hundreds of thousands of angstroms can exceed average-mass thermal elastic stresses by one to two orders. And the latter can relax at the very initial stages of sintering.

In the second stage, which occurred with further temperature increase up to 2,000 K, the shrinkage, perhaps, is defined by mutual slip of particles as a whole

Fig. 4.8 Influence of a maximum heating temperature T at high-speed sintering §T/§t = 600 K/s on the ZrC structure. a T = 2,300 °C; 4 p x300. b T = 2,500 °C; 7 p x300. c T = 2,700°C; 12 p x300. d T = 2,900°C; 30p x300

along the boundaries between them. They move into the voids and to the free surface of the sample (Fig.4.8a-d) according to the mechanism proposed in [19, 21].

After overcoming the resistance of retainers on the definite part of the surface of the particles and their ‘activation’ the boundary diffusion viscosity becomes low and shrinkage sharply increases. Random boundaries that are not connected with initial boundaries between particles are formed. Shrinkage rate decreases with porosity decrease.

At the third stage of heating to temperatures higher than 2,200 K in the central part of the sample, the final stage, coagulation of pores takes place on far from the free surface, isolated pores appear. Further densification takes place due to decrease of the peripheral porous layer with open pores and a reduction in the number and volume nonisothermal sintering of the samples at mainly of the isolated pores (Fig. 4.8d).

An appropriate choice of the nonisothermal heating parameters can provide a fine — grain regular-pore structure (Fig. 4.8b), with high level of strength at appreciable time decrease of sintering.

Fig. 4.9 Influence of a heating temperature at high-speed sintering ST/St = 600 K/s on ZrC properties: grain size d3, electrical resistance p and bending strength o^. Dotted line is the strength of ZrC sample sintered at ordinary speed after an exposure 1.75h at 2,500 °C