The densification behavior of pressure sintered cubic silicon carbide containing 1 wt% boron carbide was studied as a function of temperature (1750" to 1950°C). Specimens of theoretical density were obtained at 1950" with a pressure of 3000 psi. Experimental results showed that densification proceeded by a plastic flow mechanism. Interpretation of the data in terms of Murray's equation yields an activation energy of (115+ 18) kcal/mol. At 1850°C and above, tabular grains of 6H and 2H SIC were observed in a matrix of fine grains of 3C Sic.HE good mechanical properties and corrosion resistance of T S i c make it a promising candidate for many high temperature applications. Unfortunately, it is very difficult to obtain a fully dense, fine-grained self-bonded material. It was reported that pure Sic could be densified to near theoretical density at pressures >20 kbars above 2000°C.' Densification behaviors at lower temperatures and pressures were investigated by others, who conducted an extensive study on metal additions and found that the most efficient aids were A1 (or A1203) and Fe, which gave relative densities >98% at <2oOO0C and pressures as low as 6000 psi. Densities >99% of theoretical were obtained with additions of A1203 at temperatures above 1950°C under a pressure of 4000 psi3 Effects of boron additions were investigated in Russia4 and more recently dense samples of 0.4% boron-doped S i c were obtained at 1950°C.5 In this paper, the sintering behavior of Sic doped with 1 wt% B4C is reported along with microstructural features observed on specimens hot-pressed at different temperatures. I. Experimental Procedure (1) Powder PreparationA blend of submicron S i c powder+ prepared by high temperature gas phase reaction under controlled atmosphere and 1 wt% submicron B4Ci was used in this study. Thep-Sic phase was present in this mixture; impurities were (in w t 8 ) B 1, Fe 0.03, Mg 0.03, Cr 0.01,Ni0.01,Ti0.02,Al0.05,Cu0.001,Ca0.001,and0,0.16. An Auger characterization of the powders revealed 0, and Si on the surface of the grains. The total O2 content being very low (0.16 wt%) and the surface area large (6.1 m*/g), the oxide layer (not necessarily SiO,) must be extremely thin.Disks 2 in. in diameter and 1/4 in. thick were sintered in a hot pressrj at from 1750" to 1950°C under a constant pressure of 3000 psi uniaxially applied to the powder by both top and bottom rams. The die walls and spacers were coated with a boron nitride film to prevent adherence and diffusion between the specimen and the graphite. All the runs were carried out under a vacuum of < torr. The heating rate selected was 150°C/min and the pressure (3000 psi) was applied at room temperature and removed at the end of the soaking time. The pressure system had previously been carefully calibrated with a load cell. Two W5Re/W26Re thermocouples were used to control and stabilize the temperature to & 2°C and were regularly checked against an optical pyrometer. Densification was monitored by two linear displacement transducers attached to rams and the...
The effect of phonongrain boundary scattering on the lattice thermal conductivity and thermoelectric conversion efficiency of heavily doped finegrained, hotpressed silicon germanium alloy Silicon carbide is difficult to sinter without the aid of additives and/or pressure. Additions of boron carbide, boron or alumina were made to obtain dense materials. This is explained in terms of surface free energy modifications taking place in the grain boundaries. An expression of the chemical potential over the grain boundaries is developed to show that there is no theoretical limitation for the sintering of covalent ceramics if the grain size of the starting powder is small enough.
The unique sensitivity of Auger electron spectrometry (AES) to combined carbon has been exploited in measuring the surface compositions of hot-pressed, conventionally sintered and mixed powders of WC–Co composite materials. AES sensitivity factors for tungsten and carbon (in WC) relative to cobalt were determined. The concentrations of the major elements in hot-pressed samples measured with AES using the relative sensitivity method were compared to those obtained independently by electron microprobe (EMP) and x-ray fluorescence (XRF) techniques. Corollary studies using ion scattering spectrometry (ISS) showed the absence of (1) matrix effects in the AES measurements, (2) preferential sputtering during ion bombardment, and (3) deposition of the easier-to-sputter component (cobalt) onto WC.
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