Pressureless-Sintered Boron Nitride with Limited Content of Boric Oxide

Materials Science Research International, Vol.1, No.4, pp. 209-212 (1995)

General paper

PRESSURELESS-SINTERED BORON NITRIDE WITH LIMITED CONTENT OF BORIC OXIDE

Milan HUBACEK and Masanori UEKI AdvancedTechnology Research Labs., Nippon Steel Corporation, 1618Ida, Nakahara-ku,Kawasaki 211, Japan

Abstract:Elemental boron and boron nitride powders were mixed and pressureless-sintered innitrogen. The structure andmechanical parameters ofthe ceramics were investigated and compared with those obtained from boron nitride powder freeof the elemental boron. A positiveeffect of theaddition of boron was found in increasing density, mechani- calstrength and refractoriness, and in suppressing growth of boronnitride grains. The reason of these changes has been explainedby the formation ofa refractorysuboxide with a formulaB6O as a consequenceofa boron-controlledextrac- tionof boricoxide (B2O3). The extraction of thesesquioxide being originally adhered to thesurface of boronnitride grainsallowing a more effective formation of boronnitride ceramic skeleton. From the morphology of the ceramics, it hasbeen suggested that the formation of volatileboron suboxide (B2O2) precedes to localizationof oxygenin B6O grains. Keywords: Hexagonal boron nitride, Boric oxide, Boron, Pressureless sintering, Crystallization, Grain growth

1 INTRODUCTION 2 EXPERIMENTS

Hot pressing has been preferably used to obtain poly- Two grades of hexagonal boron nitride (hBN), a microcrystalline boron nitride bodies with outstanding mechani- crystalline grade powder (average grain size of 0.64ƒÊm, cal properties originating from the density of typically and oxygen content of 1.70% in weight) and a crystalline above 95% [1]. To obtain such a high density, a pressure grade powder (average grain size of 3.32ƒÊm, and oxygen must be applied to boron nitride, since liquid-phase sinter- content of 0.61% in weight), were mixed for 48 h with ing is beyond expectation due to a high melting point of the amorphous boron powder (particle size less than 0.05ƒÊm) material. The sintering itself involves three fundamental under argon. The mixture was compacted in a steel die at a processes - the crystallization and growth of grains [2, 3] pressure of 30MPa and then isostatically pressed in a rub- and releasing of a certain amount of boric oxide by decom- ber sheet under 700MPa to get square-shaped plates with position of boron oxynitride, which is contained in approximate dimensions of 50•~50•~10mm. The green bod- sinterable powders [4]. Consequently, microcrystalline ies were sintered in nitrogen atmosphere at 1950•Ž (heat- powders with a certain amount of oxygen have been pref- ing rate of 5K/min) for 2h. erably used. From the weight and dimensions, density and weight It is impossible to obtain dense boron nitride ceramic changes were estimated. Flexural strength was obtained bodies by pressureless sintering. However, for some appli- under three-point bending. The Vickers hardness was mea- cations, lower density and weaker mechanical perfor- sured at a load of 9.8N (1kgf). The crystal structure and

mance is acceptable. In order to reduce costs of commer- grain size were characterized from XRD patterns and SEM cial boron nitride ceramics, several attempts were made to micrographs taken from both fractured and polished sur- generate a liquid phase during sintereng of boron nitride, faces of specimens. adding various oxides such silica and calcia. Nevertheless, no significant improvement in either of relative density or 3 RESULTS AND DISCUSSION mechanical parameters has been found. The properties of such composites were governed by those of the added sub- Products of pressureless sintering of boron nitride with stances rather than the intrinsic ones of boron nitride. Seri- elemental boron had a color varying from white, which is ous worsening was found especially in the refractoriness typical for pure boron nitride, to dark gray depending on and thermal conductivity [5, 6]. the fraction of metallic boron in starting powder mixture. In the present experiments, we have chosen completely In Fig. 1, the weight change of sintered body relative to reverse approach to eliminate an undesirable effect of bo- the starting weight of green one is plotted. The weight loss ric oxide as much as possible. For that purpose, elemental observed in sintered boron nitride prepared from microc- boron was added to a mixture with boron nitride powder, rystalline powders was shifted towards negative values and its effect on the sintering process and properties of fi- since this grade of boron nitride contained higher fractions nal ceramics has been studied. of boric oxide which easily evaporates at high temperature.

Received September 7, 1995

209 Milan HUBACEK and Masanori UEKI

The weight gain, however, did not correspond to an esti- It can be seen in Fig. 3 that the hardness of sintered bo- mated amount of secondary boron nitride. This means that ron nitride is linearly dependent on the boron fraction in another chemical reaction in addition to the nitriding of el- green body for both grades of the starting boron nitride emental boron nitride should be considered. It was also powder and the boron fraction dependence seem to be found that, although boron fraction dependence of weight roughly linear. In total, higher hardness was found in sin- change is not linear, it tends to descend with the fraction of tered boron nitride prepared from microcrystalline grade metallic boron in the starting powder mixture. powder. Due to excessive expansion and abundant crack formation in the ceramics obtained by sintering of microcrystalline powder, flexural strength could be measured only for the specimens based on the crystalline grade powder

(Fig. 4). The flexural strength measured at room temperature and 1050•Ž exhibits similar tendency, showing significant maximum around 5% of boron fraction. It should be noted that the specimens obtained from the sintering of boron-free powder exhibited lower strength at 1050•Ž than that at room temperature, although the difference was neg- ligible when compared with hot-pressed ceramics [7]. In all specimens containing boron, the strength measured at 1050•Ž was found to be higher than that at room temperature, and exhibited a maximum at 5% of boron fraction.

Fig. 1. Weight changes in sintered boron nitride vs. boron fraction in starting green body.

Relative density of the boron nitride ceramics is plotted as a function of boron fraction in green bodies in Fig. 2. Boron nitride prepared from the crystalline grade powder showed higher density in comparison with that prepared from microcrystalline grade powder. This difference con- firms a negative role of boric oxide in densification due to its evaporation which induces the expansion of boron nitride body in sintering. With increasing boron fraction, both ceramics tended to become more dense. However, in boron nitride ceramic prepared from crystalline powder, a maximum density can be seen at around 10% of boron Fig. 3. The hardness of sintered boron nitride vs. boron fr action. fraction in green body.

Fig. 2. The density of sintered hBN vs. boron fraction Fig. 4. Flexural strength measured at room temperature in green body. and 1050•Ž as a function of boron ratio.

210 PRESSURELESS-SINTERED BORON NITRIDE

In SEM micrographs taken from polished surfaces, another phase in addition to layered boron nitride was observed in the material obtained by sintering of powder mixture of boron nitride and metallic boron. This phase has the size ranging from 3 to 10ƒÊm. Additionally, in the presence of boron, fine-grained boron nitride was obseveed especially, when microcrystalline powder was used (Figs. 5 and 6). This is a contrast to the case of the same grade of boron nitride powder treated in the absence of metallic boron, where the grain sizes after sintering were more than ten times larger than those in the starting powder. On the other hand, we found that heat treatment of crystalline grade, in- (a) fractured surface cluding the sintering process, does not affect on the grain growth significantly [8].

(b) polished surface

Fig. 6. Micrographs of ceramics sintered from BN- B(5%) powder mixture. (a) fractured surface

(b) polished surface

Fig. 5. Micrographs of ceramics sintered from pure boron nitride powder.

From the XRD patterns, the phase accompanying sintered boron nitride with metallic boron in the initial powder mixture was identified as boron suboxide with a for- mula B6O, as shown in Fig. 7. Although just two peaks at- tributed to the suboxide are marked in the lower pattern, a complete set of diffraction peaks enabling such an identifi- cation could be observed by increasing the sensitivity. The appearance of just the two peaks is due to low content of the suboxide in the ceramic, and to extremely high portion Fig. 7. XRD patterns of ceramics sintered from of X-ray radiation diffracted by basal plane (002) of lay- microcrystalline boron nitride powder ered boron nitride. (A - without boron, B - with 5% of boron).

211 Milan HUBACEK and Masanori UEKI

4 SUMMARY the suboxide revealed that the reduction of boric oxide by boron involves at least two stages; a volatile suboxide The effect of metallic boron on the sintering process of B2O2 is formed and later condenses in a form of a refrac- boron nitride powder and properties of the sintered body tory suboxide B6O. were investigated. Two chemical processes promoting the sintering were recognized in the BN-B system; conversion of boric oxide to boron suboxide (B6O), and formation of REFERENCES fine-grained secondary boron nitride. It was shown that, by 1. K.M. Taylor, Ind. and Eng. Chemistry 47, (1955) 2506. the conversion of boric oxide to the suboxide, an undesir- 2. T.A. Ingles and P. Popper, in Special Ceramics, Edited by P. able effect of the oxide on excessive growth of boron ni- Popper. Heywood & Co. Ltd., London (1960) p. 144. tride grains and reducing the refractoriness was prevented. 3. T.E. O'Connor, J. Amer. Chem. Soc. 84 (1962) 1753 It was proved that disproportionation of microcrystal- . line boron oxynitride to crystalline boron nitride and amor- 4. M. Hubacek and T. Sato, J. Solid State Chem. 109 (1994) phous secondary boric oxide is the controlling process for 384. crystallization of BN and subsequent formation of ceramic 5. •gDenka Boron Nitride•h, a catalogue of Denki Kagaku Kogyo skeleton in processed BN powder. By adding elemental Co. Ltd., 1988. boron, the boric oxide was extracted from BN surface, lo- 6. •gShinEtsu Chikka Hoso•h, a catalogue of ShinEtsu Kagaku calized in intergranular sites and finally converted to the refractory boron suboxide. This process was competitive Kogyo Co. Ltd., 1992. to the catalysis of BN grain growth, and moreover helped 7. M. Hubacek and M. Ueki, unpublished results. boron nitride to reveal its intristic mechanical properties at 8. M. Hubacek, M. Ueki and T. Sato, to be published in J. Amer . elevated temperatures. The distribution and grain size of Ceram Soc. 78 (1995).

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