Search

The mixtures of TiN-coated diamond, TiN and alumina were tried to be sintered by an ultrahigh pressure hot isostatic pressing (HIPing) to examine the fabrication of their composites. Though the HIPing condition (800 MPa at 1373 K for 21.6 ks) was thermodynamically unstable for the diamond, it was not transformed into graphite in the sintered composites. The composite containing 40 vol% diamond could be sintered into a sufficient density as well as the specimen with no diamond. The hardness of the composite consisting of 40 vol% diamond, 22 vol% TiN and 38 vol% alumina was about twice that without diamond.

Yttria is an important additive for sintering Si3N4, A1N and other nitride ceramics. Silicon nitride with yttria additive has been widely investigated [1-3]. The addition of yttria influences the density and elastic moduli of products [2]. Also, the sinterability of Si3 N4 is increased by the smaller particle size of yttria powders [3-6]. Yttria addition helps a higher density [7] and higher thermal conductivity [8] of A1N to be obtained. Oxygen solute atoms in A1N are phonon-scattering sites, which reduce the thermal diffusivity [9, 10]. Yttria consists of a crystalline YAG phase with grain boundary oxygen to prevent the formation of the oxygen-aluminium nitride solid solution [11]. Generally, the grainsize of commercial metallic oxide additives is of the order of 100nm. However, commercial yttria powders are likely to agglomerate [6]. Their effective particle size is normally greater than that of sintering raw materials, i.e. Si3N 4 and A1N. Therefore, ultrafine metallic oxide additives are favourable if they do not agglomerate. Another interesting account of ultrafine additives is the liquefying temperature, which can be reduced by using a smaller particle size of metallic oxides [6]. There have been many reports on production processes for ultrafine powders by plasma methods [12-15]. In the present study, ultrafine yttria powder (UFP) was produced by a radio-frequency (r.f.) plasma method from metallic yttrium (with impuritied (in p.p.m.):gadolinium 50, terbium 50, dysprosium 50, erbium 50, ion 440 and magnesium 30) and used as additive for normal sintering. Fig. 1 is a schematic diagram of the r.f. plasma reactor for UFP production. The gas flow used was argon 0.18 m 3 h-J with oxygen 1.2 m 3 h -l , which was introduced through the cooling gas inlet shown in the figure. The pressure in the chamber was about 1 Pa, and the feeding rate of raw materials was 0.15 kg h l The ultrafine and commercial powders used were evaluated by analysing specific surface area (BET method), particle size (microtrac-SPA, Nikkisou), X-ray diffraction pattern and transmission electron micrographs (TEM). Powders Y1 and Y2 denote the commercial yttria powders supplied by Shin-Etsu Chemical and Asahi Chemical respectively. Some of the properties of the ultrafine and commercial powders are listed in Table I. The Y2 powder was likely to form large agglomerates, and consequently had a greater specific surface area with a larger particle size than the other powders.

Ultrafine Y2O3 powder was produced from metallic yttrium powder by a radio frequency (RF) plasma method, and used as an additive to aluminium nitride in normal sintering. The average particle size (Microtrack method), shape (TEM), specific surface area (BET method) and crystalline structure (X-ray diffraction) of the ultrafine Y2O3 powder were evaluated. Density and thermal conductivity were measured for the sintered aluminium nitride containing the ultrafine or commercial Y2O3 powders as additive. Grain boundaries of the sintered aluminium nitride were also examined by X-ray diffraction and SEM. Yttrium distribution in the sintered aluminium nitride was analyzed by EPMA. The results are summarized as follows:(1) The ultrafine Y2O3 powder contained two allotropic forms. The average particle size and specific surface area of the ultrafine Y2O3 powder were 1.63μm and 14.2m2/g respectively. Particles were spherical. One of the commercial powders tended to form agglomerates, and had a higher specific surface area with bigger effective particle size.(2) The aluminium nitride powder was densified with the ultrafine Y2O3 powder more extensively than the one of the commercial Y2O3 powders at 1700°C.(3) Thermal conductivity of the sintered aluminium nitride with the ultrafine Y2O3 powder was higher (120W/(K·m)) than that with the commercial Y2O3 powders (about 50W/(K·m)). Both of them were sintered at 1700°C.(4) Yttrium ions of the sintered aluminium nitride with the ultrafine Y2O3 powder were distributed throughout the sintered bodies compared with the one with the commercial Y2O3 powders.(5) The sintered aluminium nitride with the ultrafine Y2O3 powder shows a lower oxygen content than the one with the commercial ones.