TEM investigation of high-pressure reaction-sintered cBN-Al composites

Abstract

Cubic boron nitride (cBN) has high hardness and excellent thermal conductivity [1]. Cubic BN composites are generally used as cutting tools for many hardened steels. However, fabrication of cBN composites is very difficult, as a high-pressure and a high-temperature are required. Wentorf and Rocco [2] reported that aluminum alloy was added to assist sintering of cBN. Since then, applications of cBN composites have become more and more important. Hibbs and Wentorf [3] developed high cBN content cutting tools, which were infiltrated by molten Al-Co from WC-Co substrate into the cBN layer during high-pressure and high-temperature sintering. Recently, the present authors [4] also reported that cobalt infiltration into a mixed cBN and WC layer played an important role in the densification of cBN. To improve thermal stability of cBN cutting tools, an addition of TiNx or TiN was found to be useful [5, 6]. We synthesized cBN-Al composites sintered at high-pressure and high-temperature, and measured the effect of the amount of Al and sintering temperature on the hardness of the composites, and it was found that reactions occurred between cBN and molten aluminum [7]. In this work, the microstructure of the cBN-Al composites sintered under high pressure was studied by means of transmission electron microscopy and energy dispersive spectrometry, and the reaction mechanism was investigated. Average particle sizes of starting powders, cubic BN (SBN-F, Showa Denko, Japan), and aluminum (Rare Metallic, Japan) were about 2.0 and 3.0 μm, respectively. Mixing composition of the starting materials ranged between 50 and 90 mol% cBN. The rest was aluminum. The mixed powders with different compositions were sintered under 5.8 GPa between 900 and 1400 ◦C for 30 min using a Link-type cubic anvil apparatus [8, 9] with a 10 mm anvil edge length. Experimental details have been described elsewhere [7]. Samples for TEM were ground on polishing cloths with 3.0 and 0.5μm diamond pastes to a thickness of approximately 100 μm using a polishing machine (Model ML-150S, Maruto, Japan). Then the thin sample (4 mm in diameter) was cut into small pieces available for a TEM specimen holder. A hollow was made at the center of the plate by using a dimple grinder (Model 656N, GATAN, USA) with diamond paste. Thickness of the samples as measured at the center of the dimple was about 20 μm. The dimpled sample was then mounted on a molybdenum grid, 3 mm in diameter, and further thinned until perforation by argon-ion thinning, with a beam current of 0.2 mA per gun (ION TECH, England). The foils were examined using an H-9000 electron microscope (TEM, Hitachi, Japan), operating at 300 kV and equipped with an energy dispersive X-ray detector (EDX, Model Delta IV, Kevex, USA). The phase of each grain was determined by the analysis of diffraction patterns and EDX chemical analysis. The phases in the sintered specimens detected by Xray diffractometry (XRD) were as follows: From the 50 to 60 mol% cBN mixing composition specimens (hereafter “composition” means mixing composition), reaction compounds were AlN, AlB2, and α-AlB12; from the 65 to 75 mol% cBN composition specimens, reaction compounds were AlN, AlB2; and from the 80 to 90 mol% cBN composition specimens, reaction compounds were AlN and α-AlB12, after sintering for both 1200 and 1400 ◦C. However, as well as AlN and AlB2, metallic Al phase was also detected for the 90 mol% cBN composition specimen sintered at 900 ◦C. The result indicated that similar reactions occurred during sintering at 1200–1400 ◦C independent of composition. The local composition was not homogeneous, because AlB2 and α-AlB12 did not appear systematically. Furthermore, the result indicated that α-AlB12 was at a high temperature stable phase even under pressure [10, 11]. We reported earlier that cBN reacted with aluminum [7]. Chemical reactions could not go to completion at low temperatures such as 900 ◦C, whereas reactions finished at higher temperatures such as 1200 and 1400 ◦C. Fig. 1A is a dark-field transmission electron micrograph of the 90 mol% cBN specimen sintered under 5.8 GPa at 900 ◦C for 30 min. Fig. 1B and C show selected area diffraction (SAD) patterns of the areas marked by the arrows. The SAD patterns contained rings which corresponded to the diffraction spacings of aluminum. Size measurement of bright particles in Fig. 1A indicated that the grains of aluminum were less than 100 nm in diameter. These areas are believed to be a liquid phase at the sintering temperature of 900 ◦C, because grain size of the raw Al powder is much greater i.e., between 1.0 and 4.0 μm [7], therefore liquid phase sintering at 900 ◦C in the cBN-Al system could be assumed. Presence of small aluminum grains indicated that the cooling rate was relatively high. In the area shown in Fig. 1A, a reaction between cBN and aluminum cannot be seen.