Hot‐Pressing of Silicon Carbide with 1% Boron Carbide Addition

Abstract

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...

Role of boron on the Spark Plasma Sintering of an α-SiC powder

“…Indeed, to reach fully dense SiC-based materials (without sintering additives) by using conventional pressurized techniques such as hot-pressing or hot isostatic pressing, it is well known that severe experimental conditions are required: 8,24 high sintering temperature (T ≥ 2100 • C) and high soaking time (t ≥ 30 min).…”

“…1,2 To achieve high densities through solid-state sintering and to avoid the appearance of low corrosion resistance intergranular phases, consolidation of silicon carbide with boron and carbon as sintering additives seems to be the most efficient way. [6][7][8][9] Therefore, boron carbide was found to enhance the hotpressing behaviour of silicon carbide that could be so obtained in a fully dense state. [10][11][12] These additives permit to reach high density slightly over 2000 • C by means of the reduction of the surface energy of the grains (boron effect) 10 and reaction with residual silica (carbon effect).…”

Effect of Additives on the Activation Energy for Sintering of Silicon Carbide

“…A wide range of activation energies are reported in the literature for the densification of SiC. Bind and Biggers 25 hot pressed SiC with 1 wt% B 4 C and reported an activation energy of 482±75 kJ/mol, Yang et al 26 reported values of 543–669 kJ/mol for pressureless sintering with B and C, while Hase et al 27 measured values of 837±121 kJ/mol. The 643±37 kJ/mol measured in the present study for the SiC–B–C material is within the range of values reported in the literature and is consistent with high activation energies reported for C diffusion in SiC 28 and creep 29 of this material.…”

Hot pressed SiC ceramics employing polymer-derived SiBCN as sintering aid

“…The assumed densities are 3.21 g/cm 3 for SiC and 2.2 g/cm 3 for amorphous SiBCN. The theoretical density of the samples in this experiment is 3.07 g/cm 3 .…”

“…To obtain high-density SiC ceramics, special sintering additives, such as B þC [2], B4C [3], and Al2O3 þY2O3 [4], are required. Several reports were recently published on the application of polymer-derived ceramics (PDCs) as a sintering aid to prepare non-oxide ceramics.…”