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Complex investigation of hot-pressed boron carbide
Journal
of the Less-Common
Metals,
COMPLEX INVESTIGATION G. A. GOGOTSI, The Institute
YA.
117
(1986)
of Strength,
YU. G. GOGOTSI
and V. A. LAVRENKO
Kiev Polytechnical
Institute,
225
- 230
OF HOT-PRESSED
L. GROUSHEVSKY
for Problems
225
BORON CARBIDE*
and 0. B. DASHEVSKAYA Academy
of Sciences,
Ukrainian
S.S.R.
(U.S.S.R.)
Kiev (U.S.S.R.)
Summary In the present work we investigate the thermomechanical properties and oxidation resistance of hot-pressed boron carbide over a wide temperature range: 20 - 1200 “C. It was demonstrated that oxidation reduced the strength of the material. The possibility of using boron carbide as a structural material for components of various high-temperature devices has been evaluated.
1. Introduction Boron carbide with its high strength, hardness, wear resistance and rather low density is quite promising for structural applications. The fourpoint bending strengths of sir&red and hot-pressed boron carbide are usually 300 - 350 MPa [l] and 400 - 500 MPa [l, 21 respectively. As a rule, these values do not decrease up to 1500 “C in an inert atmosphere [2]. But hitherto B&-based materials have been used only at moderate temperatures [ 31, since at higher temperatures a risk of their active oxidation exists. The literature data on oxidation resistance are mainly related to B& powders [4 - 61. . The oxidation of compacted materials has not been studied adequately.
2. Experimental
details
Test samples (Figs. l(a) and 2) were cut from one bar of the material with a diamond saw and afterwards machined with a 100 pm diamond wheel. The elementary composition (wt.%) was determined by chemical analysis (B, 74.00; C, 22.18; Fe, 0.90; Al, 0.37; 0, 0.21; Ca, Mg, Si, Ti and others, 2.34). The high content of impurities is due to the use of technical B& powders for the production of the material investigated. *Paper presented at the 8th Nitrides and Related Compounds,
International Symposium on Boron, Tbilisi, October 8 - 12,1984. Elsevier
Sequoia/Printed
Borides,
Carbides,
in The Netherlands
226
0
T,V
-AQ
Fig. l(a) Strength and critical stress intensity factor of boron carbide as a function of temperature (in air); (b) the DTA curve for boron carbide oxidation.
Fig. 2. Kinetic curves for boron carbide oxidation.
Mechanical tests were performed according to the procedures given in ref. 7. The strength and critical stress intensity factors over the temperature range of 20 - 1200 “C were measured in air by three-point bending in a MIP100 machine. This machine is fitted with a specially designed fixture enabling ten samples to be heated and tested simult~eously. The ultrasonic wave propagation velocity in the material was determined on a UK-lop unit at a frequency of 150 kHz. These data were used to calculate the dynamic modulus of elasticity. The static modulus of elasticity and ultimate strain were determined from the stress-strain curves obtained by four-point bending in a RM-1OlM machine [ 71. The oxidation process was investigated under isothermal conditions on a quartz spring thermobalance and under programmed heating conditions at a rate of 15 “C mm-’ using an OD-103 thermal analyser. The initial material and oxidation products were studied by X-ray diffraction analysis using a
227
DRON-2.0 diffractometer (Cu KCY_ radiation), by secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS) using an LHS10 system as well as by energy dispersive X-ray microanalysis on a Link Systems-860 AnaIyser. The morphology of the surface layer was investigated using an SEM-505 scanning electron microscope and using an MeF-3 metallographic microscope.
3. Results and discussion 3.1. Mechanical properties The results of mechanical tests at room temperature are given in Table 1 and Fig. l(a). Lower bending strength values as compared with those cited in refs. 1 and 2 are apparently explained by the high content of impurities. At the same time the Km values obtained are similar to those given in ref. 1. As can be seen from Table 1, the strength of the material investigated is somewhat lower than that of hot-pressed silicon nitride [8] or silicon carbide [9]. But its strength is higher than that of reaction-sir&red silicon nitride materials [7] of similar density. However considerable strength degradation with temperature is observed for this material in oxidizing atmospheres (Fig. l(a)). The Km value is not greatly changed under such conditions.
TABLE
1
Physicomechanical
characteristics
of boron
carbide
Characteristic Density, g cm-3 Ultrasonic wave propagation velocity, km s-l Strength, MPa three-point bending four-point bending Modulus of elasticity, GPa static dynamic Critical stress intensity factor, MPa m1’2 Ultimate strain, % Microhardness, GPa
at room
temperature
Value 2.51 13.3 310 230 450 460 3.21 0.05 36.6 + 6
3.2. Oxidation It is known, that B& oxidation starts near 600 “C [lo]. From a thermodynamic point of view the following course of the reaction is the most probable one: B& + 30, = 2B,03 + CO2
228
As a result of the reaction, liquid boron oxide is formed (melting point of B203, 577 “C). From X-ray diffraction analysis of samples oxidized at various temperatures only BzOs was revealed. It is seen in Fig. 2 that up to 1100 “C!the oxidation results in a weight gain in the samples. Starting from 1200 “C a linear weight loss is observed. This confirms that all boron oxide formed vaporizes from the surface of the material and the oxidation process at temperatures above 1200 “C is limited by the reaction rate. In the first stage of oxidation (up to 1200 “C) the kinetic curves are described by a logarithmic law. It should be noted that such behaviour is the result of two simultaneous processes, one leading to the weight gain of the samples (B& oxidation), the other resulting in its loss (B203 vaporization). The results obtained under isothermal conditions are confirmed by differential thermal analysis (DTA) (Fig. l(b)). A rather weak exothermal peak on the DTA curve is observed in the temperature range of 650 - 1000 “C. At these temperatures a protective B203 film is formed on the sample surface. At 1000 “C the process becomes more rapid, and above 1200 “C catastrophic B& oxidation starts accompanied by a considerable thermal effect. An investigation of initial and oxidized samples by different spectral methods demonstrated that under the conditions of intensive vaporization of the oxidation products no diffusion of impurities with high affinity for oxygen (calcium, magnesium, aluminium, etc.) was present, as was revealed during the oxidation of Si3N4 [8] and Sic [ 111. The oxidation rate up to 1200 “C is limited by the diffusion of the oxygen and carbide components through the oxide film. The comparison of SIMS and XPS spectra also demonstrated that at the surface of the initial samples an oxide film about 100 A thick existed [12]. Its presence apparently leads to the charging of the carbide surface during the SEM investigations (bright zones in Fig. 3(a)). The smooth glassy BzOs layer formed on the sample surface during oxidation is cracked as a result of cooling (Fig. 3(b)). The B203 hydration leads to the formation of a thin white film on the surface of the sample exposed to air for some period of time (Fig. 3(c)). After boiling in distilled water the B203 film is dissolved, thus the changes in the surface structure of the material can be revealed. The microstructure observations demonstrate (Fig. 3(d)) that, with the liquid oxide layer present at the sample surface, a chemical etching of grain boundaries takes place. Grooves formed along them act as stress concentrators, moreover these grooves can initiate cracks when load is applied. Such a stress corrosion process seems to be the main reason for strength degradation at temperatures up to 1200 “C. Catastrophic oxidation above 1200 “C!results in stronger etching of the surface layer and, therefore, leads to a further strength degradation (Fig. l(a)). However, in fracture toughness tests, where a crack propagates from a previously made notch, a larger number of defects on the sample surface obviously has no influence on the K,, value (Fig. l(a)).
(b)
(a)
(d) Fig. 3. Micrographs of boron carbide sample surfaces: (a), initial sample; (b), after oxidation at 800 “C for 3 h; (c), after oxidation at 1000 “C for 3 h and after two weeks at room temperature in air; (d), after oxidation at 1000 “C for 3 h and oxide layer dissolution.
4. Conclusions Hot-pressed boron carbide has good mechanical properties and can be used as a structural material in oxidizing atmospheres up to about 600 “C. In the temperature range of 600 - 1200 “C this material exhibits tolerable oxidation resistance and preserves satisfactory strength. At higher temperatures catastrophic B& oxidation starts which results in a complete degradation of strength. References K. A. Schwetz and W. Grellner, J. Less-Common Met., 82 (1981) 37. J. De With, J. Mater. Sci., 19 (1984) 457. F. V. Samsonov and I. M. Vinitsky, Refractory Compounds, Metallurgiya, Moscow, 1976, p. 560. R. Ridgway, Trans. Electrochem. Sot., 65 (1934) 117. Yu. L. Krutsky, G. V. Galevsky and A. A. Kornilov, Poroshh. Metall., (2) (1983) 47. V. A. Lavrenko, A. P. Pomytkin, P. S. Kisly and B. L. Grabchuk, Oxid. Met., 10 (1976) 85.
230 7 G. A. Gogotsi, Strength of Machine-Building Nitride Ceramics, The Institute for Problems of Strength, Kiev, 1982, p. 59. 8 F. F. Lange and B. T. Davis, J. Mater. Sci., 18 (1983) 1497. 9 F. F. Lange, J. Am. Cer. Sot., 53 (1976) 290. 10 R. F. Vojtovich, Oxidation of Carbides and Nitrides, Naukova Dumka, Kiev, 1981, p. 192. 11 V. A. Lavrenko, E. A. Pugach, S. I. Filipchenko and Yu. G. Gogotsi, Sverkhtverdye Materialy, (1) (1984) 21. 12 V. A. Lavrenko, Vu. G. Gogotsi and I. N. Frantsevich, Dokl. Akad. Nauk. USSR, 275 (1984) 114.
of the Less-Common
Metals,
COMPLEX INVESTIGATION G. A. GOGOTSI, The Institute
YA.
117
(1986)
of Strength,
YU. G. GOGOTSI
and V. A. LAVRENKO
Kiev Polytechnical
Institute,
225
- 230
OF HOT-PRESSED
L. GROUSHEVSKY
for Problems
225
BORON CARBIDE*
and 0. B. DASHEVSKAYA Academy
of Sciences,
Ukrainian
S.S.R.
(U.S.S.R.)
Kiev (U.S.S.R.)
Summary In the present work we investigate the thermomechanical properties and oxidation resistance of hot-pressed boron carbide over a wide temperature range: 20 - 1200 “C. It was demonstrated that oxidation reduced the strength of the material. The possibility of using boron carbide as a structural material for components of various high-temperature devices has been evaluated.
1. Introduction Boron carbide with its high strength, hardness, wear resistance and rather low density is quite promising for structural applications. The fourpoint bending strengths of sir&red and hot-pressed boron carbide are usually 300 - 350 MPa [l] and 400 - 500 MPa [l, 21 respectively. As a rule, these values do not decrease up to 1500 “C in an inert atmosphere [2]. But hitherto B&-based materials have been used only at moderate temperatures [ 31, since at higher temperatures a risk of their active oxidation exists. The literature data on oxidation resistance are mainly related to B& powders [4 - 61. . The oxidation of compacted materials has not been studied adequately.
2. Experimental
details
Test samples (Figs. l(a) and 2) were cut from one bar of the material with a diamond saw and afterwards machined with a 100 pm diamond wheel. The elementary composition (wt.%) was determined by chemical analysis (B, 74.00; C, 22.18; Fe, 0.90; Al, 0.37; 0, 0.21; Ca, Mg, Si, Ti and others, 2.34). The high content of impurities is due to the use of technical B& powders for the production of the material investigated. *Paper presented at the 8th Nitrides and Related Compounds,
International Symposium on Boron, Tbilisi, October 8 - 12,1984. Elsevier
Sequoia/Printed
Borides,
Carbides,
in The Netherlands
226
0
T,V
-AQ
Fig. l(a) Strength and critical stress intensity factor of boron carbide as a function of temperature (in air); (b) the DTA curve for boron carbide oxidation.
Fig. 2. Kinetic curves for boron carbide oxidation.
Mechanical tests were performed according to the procedures given in ref. 7. The strength and critical stress intensity factors over the temperature range of 20 - 1200 “C were measured in air by three-point bending in a MIP100 machine. This machine is fitted with a specially designed fixture enabling ten samples to be heated and tested simult~eously. The ultrasonic wave propagation velocity in the material was determined on a UK-lop unit at a frequency of 150 kHz. These data were used to calculate the dynamic modulus of elasticity. The static modulus of elasticity and ultimate strain were determined from the stress-strain curves obtained by four-point bending in a RM-1OlM machine [ 71. The oxidation process was investigated under isothermal conditions on a quartz spring thermobalance and under programmed heating conditions at a rate of 15 “C mm-’ using an OD-103 thermal analyser. The initial material and oxidation products were studied by X-ray diffraction analysis using a
227
DRON-2.0 diffractometer (Cu KCY_ radiation), by secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS) using an LHS10 system as well as by energy dispersive X-ray microanalysis on a Link Systems-860 AnaIyser. The morphology of the surface layer was investigated using an SEM-505 scanning electron microscope and using an MeF-3 metallographic microscope.
3. Results and discussion 3.1. Mechanical properties The results of mechanical tests at room temperature are given in Table 1 and Fig. l(a). Lower bending strength values as compared with those cited in refs. 1 and 2 are apparently explained by the high content of impurities. At the same time the Km values obtained are similar to those given in ref. 1. As can be seen from Table 1, the strength of the material investigated is somewhat lower than that of hot-pressed silicon nitride [8] or silicon carbide [9]. But its strength is higher than that of reaction-sir&red silicon nitride materials [7] of similar density. However considerable strength degradation with temperature is observed for this material in oxidizing atmospheres (Fig. l(a)). The Km value is not greatly changed under such conditions.
TABLE
1
Physicomechanical
characteristics
of boron
carbide
Characteristic Density, g cm-3 Ultrasonic wave propagation velocity, km s-l Strength, MPa three-point bending four-point bending Modulus of elasticity, GPa static dynamic Critical stress intensity factor, MPa m1’2 Ultimate strain, % Microhardness, GPa
at room
temperature
Value 2.51 13.3 310 230 450 460 3.21 0.05 36.6 + 6
3.2. Oxidation It is known, that B& oxidation starts near 600 “C [lo]. From a thermodynamic point of view the following course of the reaction is the most probable one: B& + 30, = 2B,03 + CO2
228
As a result of the reaction, liquid boron oxide is formed (melting point of B203, 577 “C). From X-ray diffraction analysis of samples oxidized at various temperatures only BzOs was revealed. It is seen in Fig. 2 that up to 1100 “C!the oxidation results in a weight gain in the samples. Starting from 1200 “C a linear weight loss is observed. This confirms that all boron oxide formed vaporizes from the surface of the material and the oxidation process at temperatures above 1200 “C is limited by the reaction rate. In the first stage of oxidation (up to 1200 “C) the kinetic curves are described by a logarithmic law. It should be noted that such behaviour is the result of two simultaneous processes, one leading to the weight gain of the samples (B& oxidation), the other resulting in its loss (B203 vaporization). The results obtained under isothermal conditions are confirmed by differential thermal analysis (DTA) (Fig. l(b)). A rather weak exothermal peak on the DTA curve is observed in the temperature range of 650 - 1000 “C. At these temperatures a protective B203 film is formed on the sample surface. At 1000 “C the process becomes more rapid, and above 1200 “C catastrophic B& oxidation starts accompanied by a considerable thermal effect. An investigation of initial and oxidized samples by different spectral methods demonstrated that under the conditions of intensive vaporization of the oxidation products no diffusion of impurities with high affinity for oxygen (calcium, magnesium, aluminium, etc.) was present, as was revealed during the oxidation of Si3N4 [8] and Sic [ 111. The oxidation rate up to 1200 “C is limited by the diffusion of the oxygen and carbide components through the oxide film. The comparison of SIMS and XPS spectra also demonstrated that at the surface of the initial samples an oxide film about 100 A thick existed [12]. Its presence apparently leads to the charging of the carbide surface during the SEM investigations (bright zones in Fig. 3(a)). The smooth glassy BzOs layer formed on the sample surface during oxidation is cracked as a result of cooling (Fig. 3(b)). The B203 hydration leads to the formation of a thin white film on the surface of the sample exposed to air for some period of time (Fig. 3(c)). After boiling in distilled water the B203 film is dissolved, thus the changes in the surface structure of the material can be revealed. The microstructure observations demonstrate (Fig. 3(d)) that, with the liquid oxide layer present at the sample surface, a chemical etching of grain boundaries takes place. Grooves formed along them act as stress concentrators, moreover these grooves can initiate cracks when load is applied. Such a stress corrosion process seems to be the main reason for strength degradation at temperatures up to 1200 “C. Catastrophic oxidation above 1200 “C!results in stronger etching of the surface layer and, therefore, leads to a further strength degradation (Fig. l(a)). However, in fracture toughness tests, where a crack propagates from a previously made notch, a larger number of defects on the sample surface obviously has no influence on the K,, value (Fig. l(a)).
(b)
(a)
(d) Fig. 3. Micrographs of boron carbide sample surfaces: (a), initial sample; (b), after oxidation at 800 “C for 3 h; (c), after oxidation at 1000 “C for 3 h and after two weeks at room temperature in air; (d), after oxidation at 1000 “C for 3 h and oxide layer dissolution.
4. Conclusions Hot-pressed boron carbide has good mechanical properties and can be used as a structural material in oxidizing atmospheres up to about 600 “C. In the temperature range of 600 - 1200 “C this material exhibits tolerable oxidation resistance and preserves satisfactory strength. At higher temperatures catastrophic B& oxidation starts which results in a complete degradation of strength. References K. A. Schwetz and W. Grellner, J. Less-Common Met., 82 (1981) 37. J. De With, J. Mater. Sci., 19 (1984) 457. F. V. Samsonov and I. M. Vinitsky, Refractory Compounds, Metallurgiya, Moscow, 1976, p. 560. R. Ridgway, Trans. Electrochem. Sot., 65 (1934) 117. Yu. L. Krutsky, G. V. Galevsky and A. A. Kornilov, Poroshh. Metall., (2) (1983) 47. V. A. Lavrenko, A. P. Pomytkin, P. S. Kisly and B. L. Grabchuk, Oxid. Met., 10 (1976) 85.
230 7 G. A. Gogotsi, Strength of Machine-Building Nitride Ceramics, The Institute for Problems of Strength, Kiev, 1982, p. 59. 8 F. F. Lange and B. T. Davis, J. Mater. Sci., 18 (1983) 1497. 9 F. F. Lange, J. Am. Cer. Sot., 53 (1976) 290. 10 R. F. Vojtovich, Oxidation of Carbides and Nitrides, Naukova Dumka, Kiev, 1981, p. 192. 11 V. A. Lavrenko, E. A. Pugach, S. I. Filipchenko and Yu. G. Gogotsi, Sverkhtverdye Materialy, (1) (1984) 21. 12 V. A. Lavrenko, Vu. G. Gogotsi and I. N. Frantsevich, Dokl. Akad. Nauk. USSR, 275 (1984) 114.