Materials Science, Vol. 33, No. 3, 1997

ENHANCEMENT

OF THE C R A C K R E S I S T A N C E

AND ITS STRUCTURAL

AND METALLURGICAL

OF Si3N4-Y203 CERAMICS ASPECTS

V. D. Vasyliv, O. M. Romaniv, I. V. Zalite, and M. I. Yurkevieh

UDC 666.3:539.4.015

We experimentally establish quantitative characteristics of the influence of the technological tJrocedure ofsintering of Si3N4-10wt.% Y203 ceramic compositions on their mechanical properties, crack resistance, and micromechanisms of fracture.

Contemporary technological procedures guarantee high degrees of dispersion of ceramic powders and constant concentrations of their components (nitrogen, free and bound silicon, etc.) [1, 3]. The characteristics of strength of ceramic compositions made of silicon-nitride powders essentially depend on their phase composition and structure, which are controlled by the parameters of technological processes used in manufacturing materials of these sort. We studied the influence of the technology of sintering on the initial structure, mechanical properties, and crack resistance of Si 3 N4-Y 2 03 ceramics.

Experimental Procedure and Materials Compositions were prepared by mechanical mixing of Si3N4 with Y203 (in what follows, they are called mechanical compositions) and by introducing yttrium oxide in plasma jets in the process of synthesis of silicon nitride (plasma compositions). The content of Y203 was as high as 10 wt.%. This content is remarkable by the possibility of preparation of a relatively dense material by simple sintering at 1700~ in an atmosphere of nitrogen. The specific surfaces of the source powders of Si3N 4 and Y2 03 were equal to 40-45 m2/g and 20 m2/g, respectively. The characteristics of the process of sintering (temperatures T, times of exothermal holding x, cooling rates V, and contents of the plasticizer, i.e., of a solution of oleic acid in acetone Cm) as well as some other parameters such as density p, open porosity Ho, and hardness HV20 are presented in Table 1 [4, 5].The values of ultimate strength and fracture toughness were determined by using standard procedures [6-8] for prismatic beams 3.8 • 3.8 • 36 mm in size under the conditions of three-point bending. Straight lateral notches (for the determination of Klc) were made with a 50-I.tm-thick diamond disk to a depth of 1.90 + 0.01 mm. The distance between the supports was equal to 30 ram. The tests were performed in air at room temperature. Under long-term loading, ceramic parts often fail as a result of slow subcritical crack growth. This is why, by analyzing the resistance of a material to subcritical crack growth, one can evaluate the time for which the crack attains its critical size, i.e., the strength of the material. This estimate is used in the stage of manufacturing of materials. The specimens used for long-term static tests had the same geometry as those tested for fracture toughness. Lateral chevron notches with an angle of 110 ~ at the tip were made with a 50-~m-thick diamond disk so that the tip of the chevron was located at a depth of 0.60 + 0.01 mm from the surface. The kinetic fracture diagrams were constructed by using the procedure suggested in [9]. It was shown that, for specimens with straight lateral notches, the range of stress intensity factors in the v - K I diagrams is relatively narrow. Thus, it is very difficult to apply the initial load. On the contrary, for specimens with chevron notches loaded to a level higher than the threshold stress intensity factor, one can decelerate the propagation of a crack initiated from the chevron tip [10-12] and obtain more correct characteristics of its subcritical growth. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv; Institute of Inorganic Chemistry, Latvian Academy of Sciences, Riga, Latvia. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 33, No. 3, pp. 65-71, May-June, 1997. Original article submitted March 14, 1996. 1068-820X/97/3303-0323 $18.00

9 1998

Plenum Publishing Corporation

323

V. D. VASYLIV, O. M~ ROMANIV, I. V. ZALITE, AND ~1. I. YURKEVICH

324

Table 1

Technological procedure C m,

wt.%

p, g/cm 3

I-Io, %

HV20, GPa

Mode

T, ~

"~, h

V, ~

I

1650

2

10

5

2.66 3.13

18.2 2.6

5.7 9 15.7

II

1750

2

10

5

3.29 3.38

0.2 0.2

19.1 18.7

1II

1700

0.5

10

5

2.75 2.88

13. I 9.9

9.4 14.1

IV

1700

5

10

5

3.32 3.37

0.1 0.2

18.4 18.7

V

1700

2

3

5

2.93 3.33

12.3 0.2

7.7 15.6

VI

1700

2

20

5

3.08 3.31

6.6 0.2

12.3 17.4

VII

1700

2

10

2

3.22 3.30

0.1 0.2

17.3 17.8

VHI

1700

2

10

10

3.20 3.31

0. I 0.1

16.0 18.1

Comment: The parameters of mechanical and plasma compositions are presented in the numerator and denominator, respectively. The phase compositions were determined by the methods of microstructural (Neophot-30) and X-ray diffraction (DRON-2.0, Cu-Ka radiation) analyses. Fractographic investigations were carried out with a Jeol T-20 scanning microscope. E x p e r i m e n t a l Results

For compositions with the same contents of constituents, the mode of sintering, which includes the process of mutual ordering of particles, dissolution-diffusion-recrystallization, and the coalescence of particles, is a factor playing the predominant role in the formation of the initial structure. High densities are typical of materials with fine-grained structure formed when the or-13 transition of silicon nitride, which governs the process of growth of grains, is quite rapid. If the process of crystallization of ~ S i 3 N 4 is more intense and the grain-growth rate is higher than the rate of compaction, then we observe the formation of a more porous structure with coarser grains, which deteriorates the mechanical properties of the material [4]. The data presented in Table 1 demonstrate that the densities of these two types of compositions increase and their open porosities decrease as the temperature of sintering increases. Thus, for mechanical compositions, the gradient of open porosity is high. These quantities correlate with the hardness of the material which is three times lower for mode I (T = 1650~ "~ = 2 h) than for plasma compositions. At the same time, the complex comparison of the characteristics of hardness, strength, and fracture toughness of compositions in the process of changes in the temperature and time conditions of sintering estimated by using the Larsen-Miller parameter [ 13] shows that their changes are ambiguous (Fig. I).

ENHANCEMENT OF THE CRACK RESISTANCE OF SI3N4-Y203 CERAMICS, ITs STRUCTURAL AND METALLURGICAL ASPECTS

,_,7:

325

Y /_7"

6 5"

350 3 250 /50 I

3.J

J.4

I

J.ff

P= T("20+log

36

/0

Fig. 1. Dependences of the hardness HVm, bendingstrength crb, and fracture toughness Kic of mechanical (blank symbols) and plasmachemical (dark symbols)compositionsof Si3N-4-10% Y203 ceramics on the temperature T and time x of sintering. The content of plasticizer was equal to 2% (O, 9 5% (I~, 9 and 10% (A, 9 modes of sintering I-VIII are the same as in Table I. In the process of sintering of the material of mechanical compositions at 1650~

we mainly observe the growth

of I]-Si3N 4 crystals, although the a-Si3N 4 phase is also formed. This mode does not lead to a significant compaction of the composition, which is reflected in its mechanical properties and fracture resistance. The analysis of the fracture surfaces of specimens of mechanical compositions tested for fracture toughness (Fig. 2a) demonstrates that cracks grow along the boundaries of isolated agglomerates of sintered particles. On the fracture surfaces, we see numerous pores and coarse grains of ~-Si3N 4. If the same mode of sintering is used for plasma-composition materials, then we observe the formation of c~-Si3 N 4 grains. Amorphous globular particles formed in the course of plasma-chemical synthesis of Si3N 4 and Y203 play the role of centers of crystallization [4]. Further, we observe the a-I] transition of silicon nitride. In the fracture surface of this composition (Fig. 2b), we also detect the results of fracture processes running mainly along the boundaries of some agglomerates, although these aggregates, as welt as isolated ~-grains and pores, are much smaller than those formed in the materials obtained from mechanical compositions (Fig. 2a). The fact that the density of plasma compositions is higher can be explained by a more uniform distribution of oxide impurities between the particles of silicon nitride, unlike the case of mechanical compositions where it is not always possible to destroy agglomerates of highly dispersed particles [4]. However, as the temperature of sintering increases to 1700~ (modes VII and VIII), mechanical compositions undergo more intense compaction. This effect is explained by the fact that yttrium oxide reacts with oxygen-containing compounds on the surface of silicon nitride [14]. As a result, oxygen is removed from the surface and we observe the formation of the liquid phase of the Y203-Si O2-Si 3 N 4 eutectic composition, which promotes the pro-

326

V.D. VASYHV, O. M. ROMANIV, I. V. ZALITE, AND M. I. YURKEVICit

cesses of compactification of the material and crystallization of small globular grains of 13-Si3 N 4 from the amorphous phase and accelerates the o~-13 transition. The material obtained as a result is characterized by high hardness (16-17 GPa) and strength (250-300 MPa) as well as by relatively high crack resistance (K~c = 4.8-5.4 MPa ~ (see Fig. 1).

.

.

.

.

~f~i,

~e?

~

,

,

)

,

a//dmj

Fig. 2.

Microstructure of the fracture surfaces of Si3N4-10%Y203 ceramic specimens: (a) mechanical composition (T = 1650~ x = 2h, and V= 10~ (b) plasma-chemical composition (T= 1650~ x = 2h, and V= 10~ (c) plasma-chemical composition ( T = 1700~

x = 2h, and V= 3~

In plasma-chemical compositions, for these modes of sintering, oxygen is released as a result of the dissociation of Y2 03 and partly penetrates into the structure of nitride suppressing the process of crystallization. In the course of synthesis, we mainly observe the crystallization of c~-Si3 N4. In the presence of the liquid phase, it transforms into the 13-phase according to the mechanism of dissociation, diffusion, and recrystallization. The porosity of the material is somewhat higher than for mechanical compositions because the process of compaction is inhibited in the presence of isolated needle-shaped 13-grains formed if the amount of the liquid phase is insufficient. The hardness of this composition is higher but its strength and crack resistance are lower than those exhibited by mechanical compositions (see Fig. 1). It should be noted that a positive role is played by the plasticizer, which promotes better mixing, compaction and sintering of powders. This enhances the crack resistance of the material and decreases the spread of values of its ultimate strength. These changes were observed for compositions with contents of the plasticizer of 2% (mode VII) and 10% (mode VIII) sintered under the same conditions. The dashed regions in Fig. 1 serve as an indication of an increase in crack resistance accompanied by a decrease in hardness (mechanical compositions) and strength (both mechanical and plasma compositions). The effects of the time of isothermal holding and cooling rate on the strength and crack resistance of ceramics are ambiguous (Fig. 3). Thus, mechanical compositions are very sensitive to the cooling rate. Their maximum hardness (Table 1) and crack resistance (Fig. 3) are attained in modes VII and VIII. This is explained by the incomplete recrystallization of intermediate phases in the process of rapid cooling and by the formation of isolated elongated 13S i 3 N 4 grains in the process of slow cooling. In both cases, mechanical compositions do not attain a sufficient degree of (in mode V, the open porosity can be as high as 12.3%) and elongated 13-grains adjacent to pores turn into nuclei of macrocracks.

ENIlANCEMENT OF THE CRACK RESISTANCE OF SI3N4-Y203 CERAMICS, ITS STRUCTURAL AND ~IETALLURGICAL ASPECTS

280

600 t

fO00

~

I

t

327

Y/V.O0 d

....

1

6

/5 5

,:750 3

0

/

2

I

f

r

3

4,

5

t50

Fig. 3. Dependences of hardness HV20, bending suength o b, and fracture toughness Ktr of Si3N4-10% Y203 ceramics on the time of isothermal holding x (dashed curves) and cooling rate V (solid curves) for specimens of mechanical and plasma compositions (we use the same notation as in Fig. 1).

The materials obtained from plasma compositions are characterized by the maximum value of fracture toughness corresponding to the case of low cooling rates. The compositions treated in mode V are characterized by high fracture resistance (Fig. 3) and hardness and low porosity (Table 1). On the fracture surfaces of specimens tested for fracture toughness, cracks propagate along the boundaries of recrystallized needle-shaped [3-Si3 N 4 grains, although one also encounters small globular zrains and isolated large elongated crystals (Fig. 2c). Sometimes, cracks propagate through elongated ~-Si3N 4 crystals. In this case, pores and agglomerates of Si3N4 and Y203 particles detected in materials synthesized in modes I and III are absent. Irregular surface patterns appear as a result of the action of the intergranular mechanism of fracture caused by the phase composition, morphology of grains, and strength of intergranular boundaries. Note that just these factors are responsible for high crack growth resistance. The effect of the time of sintering on the mechanical behavior of compositions was analyzed under the following conditions: T = 1700~ and V = 10~ (modes Ill, IV, and VII). It was shown that their mechanical properties improve as the time of isothermal holding increases. Moreover, an increase in the time of isothermal holding from 0.5 to 2 h leads to a sharp increase in the hardness (almost by a factor of two), strength, and crack resistance of mechanical compositions. At the same time, for plasma compositions, we observe an insignificant growth of these parameters (Table 1 and Fig. 3). Long-term sintering facilitates the complete recrystallization of ~-Si 3 N 4 in the presence of the liquid phase. In addition, the crack resistance of this material is higher than that exhibited by the materials obtained from plasma compositions by 0.7-1.1 MPa ~m (Fig. 3, z = 2, 5 h).

328

V.D. VASYLIV,O. M. ROMANIV,I. V. ZALITE, AND M. I. YURKEVICH

3

4

y

83

4

Y J

4

Y

Fig. 4. Kinetic diagrams of fracture of mechanical (a) and plasma-chemical (b) compositions of

Si 3N4-10% Y203 ceramics plotted according to the results of long-term tests for static crack resistance and their comparative analysis (c). Applied loads: (a) 46N (O), 51N (1), 64N(~), 68N(fll), 72N(A), 8IN(O), 82N (I--1), 84N(A); (b)48N (O), 53N(11), 61N((}), 63N(DI), 69N (O), 73N(Kl); (c) 61N (&), 64N (A), 67N (11), 71N (El), 73N (O), 81N (O). Here, I-VIII are the modes of sintern

ing, the numbers in brackets denote the values of the exponent n of the function v = A KI for the relevant modes, and the arrows correspond to the exit of a crack from the chevron. As temperature increases further, the structure of materials undergoes qualitative changes and, hence, the mechanisms of fracture also change. Although both materials are characterized by high hardness and low porosity, their strength and crack resistance decrease (Fig. 1, mode II). This is explained by the formation of the Y2 03 * Si3N4 phase in the process of heating performed in the presence of insignificant amounts of oxygen [ 1 4 - 1 6 ] . This phase does not undergo melting up to 1700~

which suppresses the process of crystallization o f silicon nitride. At tem-

peratures above 1700~ this phase decomposes, which leads to the deterioration of mechanical properties of the material (Fig. 1). This effect can be prevented if we use source powders with large specific surface to increase the amount of adsorbed oxygen. In compositions that are rich with oxygen, the amount of the Y2 O 3 - S i O x - Si 3 N 4 eutectic is sufficient for the required compaction of the liquid phase. The kinetic fracture diagrams of compositions plotted according to the results of the investigation of long-term static crack resistance reflect typical features of the processes of initiation and subcritical growth of cracks (Fig. 4). For the same temperature of synthesis (1700~ the kinetic fracture diagrams of materials obtained from plasma compositions by sintering for 0.5 h are located in the region of higher K I (Fig. 4b) as compared to the case of mechanical compositions (Fig. 4a). Qualitative changes occur if the time of isothermal holding is equal to 2 h. In this case, mechanical compositions have better characteristics but after 5 h of holding the diagrams shift to the region of high K[ (Fig. 4c).

ENHANCEMENT OF THE CRACK RESISTANCE OF S13N4-Y203 CERAMICS, ITS STRUCTURAL AND METALLURGICAL ASPECTS

329

According to the method for the approximate evaluation of the durability of ceramic materials with cracklike defects suggested in [17], we have the following formula for the time to fracture:

2 tl = Vo2 2A(,,- 2)

/1

,), -2

x ff

(1)

where Y0 is the minimum value of the function, which takes into account the geometry of a specimen, o is the overall load applied to the body, A and n are the parameters of the relevant v - K ! curve obeying the power law, and Kli and K~ are the current and critical values of the stress intensity factor, respectively. For mechanical compositions, the parameter n varies from 51 to 59. For plasma compositions, it varies from 72 to 81, which means that the former are preferable. The comparison of the u - K ! curves plotted for compositions of both types demonstrates that the best results are attained for the material prepared from the mechanical composition by sintering at 1700~ for 5 h. Since the analysis of some other parameters (such as fracture toughness, strength, hardness, and open porosity) also reveals substantial advantages of the indicated material, we can recommend this mode of sintering as the best one for the preparation of materials of this sort. CONCLUSIONS The application of contemporary technological processes promote an increase in the strength and crack resistance of ceramics produced from highly dispersed silicon nitride and yttrium oxide powders. By using the approaches of fracture mechanics, we determined the optimal modes of sintering of ceramics and established the relationships between the parameters of strength, hardness, and crack resistance of ceramic materials on the one hand and the degree of dispersion of the source components and technology of preparation of compositions on the other hand. Fractographic analysis demonstrates that the crack resistance of S i3N4-Y2 03 ceramics becomes much higher due to the predominant propagation of cracks along the boundaries of needle-shaped and elongated 13-Si3 N 4 grains. These grains play the role of reinforcing fibers. They are responsible for irregular patterns of the fracture surfaces and, hence, for the elevated crack-growth resistance of materials. The parameters A and n of the equation of subcritical crack growth depend on thc structure of ceramics and can be useful for the optimization of the structural state of ceramic materials. REFERENCES 1. K. S, Mazdiyasni and Ch. M. Cooke, "Synthesis,characterizationand consolidation of Si3N4 obtained from ammonolysis of SiCI4 ," J. Am. Ceram- Soc., 56, No. 12, 628-633 (1973). 2. A.E. Comus and D. Cleawer, Production of Nitrogen-Containing Compounds, GB Patent No. 1 199 811, CI C0[B 21/06 (1970). 3. G.M. Heidemane, Ya. P. Grabis, and T. N. Miller, "High-lemperature synthesis of highly dispersed silicon nitride," Izv. Akad. Nauk SSSR, Neorg. Mater., 15, No. 4, 595-598 (1979). 4. !. Zalite, T. Miller, A. Lodzina, and J. Plitmanis, "Werkstoffe auf Basis von feindispersen Si3Na-Pulvern," in: Synthesis and Properties of Refractory Compounds and Materials on Their Basis, Institute of Inorganic Chemistry, Latvian Academy of Sciences, Riga (1989), pp. 27-32. 5. V.N. Simin'kovich, I. V. Zalite, and V. D. Vasyliv, "Deformation and fracture of highly dispersed silicon nitride ceramics." in: Fracture Mechanics: Successes and Problems, Abstracts of the 8th lnternat. Conf. on Fracture (Kiev, June 1993), Vol. 2, L'viv (1993), pp. 401-402. 6. MR 232-87. Strength Analysis and Tests. Methods for Mechanical Testing. Determination of the Characteristics of Crack Resistance (Fracture Toughness) of Superhard Materials, Hard Allows, Tool and Structural Ceramics under Static Loading [in Russian]. VNIINMASh, Moscow (1987). 7. W.F. Brown, Jr., and J. E. Srawley, Plane Strain Crack Toughness Testing of High-Strength Metallic Materials [Russian translation], Mir, Moscow (1972). 8. V.V. Panasyuk (editor), Fracture Mechanics and Strength of Materials. A Handbook, Vol. 4: O. N. Romaniv, S. Ya. Yarema, G. N. Nikiforchin, et al., Fatigue and Cyclic Crack Resistance of Structural Materials [in Russian], Naukova Dumka, Kiev (1990).

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V.D. VASYLIV,O. M. ROMANIV,I. V. ZALITE, AND M. I. YURKEVICH

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I0.

J.A. Salem and J. L. Shannon, Jr., "Fracture toughness of Si3N4 measured with short bar chevron-notched specimens," J. Mater. Sci., 22, No. 1,321-324 (1987). V.N. Simin'kovich, A. N. Tkach, A. S. Klimanov, and A. Yu. Rakovskii, " Crack resistance of cermet cutting picks of drilling tools," Fiz.-Khim. Mekh. Mater.. 25, No. 6, 95-98 (1989). R.T. Bubsey, D. Munz, W. S. Pierce, and J. L. Shannon, Jr., "Compliance calibration of the short rod chevron-notch specimen for fracture toughness testing of brittle materials," Int. J. Fract:, 18, No. 2, 125-133 (1982). O.M. Romaniv, I. V. Zalite, V. M. Simin'kovich, et al., "Effect of the concentration of zirconium dioxide on the fracture resistance of AI203-ZrO2 ceramics," Fi~-Khim_ Mekh. Mater., 32, No. 5, 52-57 (1995). A. Lodzina, J. Grabis, and 1. Zalite, "Oxygen in fine silicon nitride," in: Synthesis and Properties of Refractory Compounds and Materials on Their Basis [in Russian], Institute of Inorganic Chemistry, Latvian Academy of Sciences, Riga (1990), pp. 54-59. 1~. A. Palchevskis, G. M. Heidemane, B. Ya, Bondars, et al., "Investigation of some physicochemical properties of highly dispersed composite powders of silicon and titanium nitrides," in: High-Temperature Nitrides and Materials on Their Basis, Institute for Problems in Materials Science, Ukrainian Academy of Sciences (1985), pp. 115-122. M. Fukuhara, "Effect of nitrogen on the a/b phase conversion in silicon nitride," J. Am_ Ceram_ Soc.. 68, No. 9, C226-C228 (1985). V.A. Lavrenko and Yu. G. Gogotsi, Corrosion of Structural Ceramics [in Russian], Metallurgiya, Moscow (1989).

i 1. 12. 13. 14. 15.

16. 17.