Efficacy of HfN as sintering aid in the manufacture of ultrahigh-temperature metal diborides-matrix ceramics
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B2 and (ZrB2 + HfB2)-based ceramics containing 19.5 vol% SiC particulate were developed from commercially available powders by hot-pressing. With the assistance of 3 vol% HfN as sintering aid, after hot-pressing at 1900 °C and 50 MPa of applied pressure, full density in both the composites was successfully achieved. The materials revealed a homogeneous microstructure, characterized by faceted diboride grains (2 m average size) and SiC particles regularly dispersed. Limited levels of secondary phases were found. The thermomechanical properties of the composites were promising: about 22 GPa microhardness and 500 GPa Young’s modulus for both. The HfB2–SiC composite showed values of strength of 650 ± 50 and 465 ± 40 MPa at 25 and 1500 °C, respectively. Likewise, the (ZrB2–HfB2)–SiC composite exhibited values of strength of 765 ± 20 and 250 ± 45 MPa at 25 and 1500 °C, respectively. The excellent response at high temperature in air was attributed to the refractoriness of the phases constituting the composites and to the resistance to oxidation enhanced by the presence of the SiC particulate.
I. INTRODUCTION
Early transition metal diborides are refractory compounds of particular interest because of their excellent and unique combination of properties such as very high melting point, high electrical and thermal conductivity, chemical attack inertness, and good thermal shock resistance.1 Mainstream applications of this family of diborides in modern industries include high-temperature electrodes and crucibles for molten metal contact uses. In recent years, there has been renewed interest in ZrB2 and HfB2. The ability to withstand temperatures in excess of 1800 °C makes them viable candidates for ultrahigh-temperature aerospace applications,2–7 for instance thermal protection systems for sharp-bodied reentry vehicles. Highly dense ZrB2 ceramics were obtained with the intentional incorporation of metals8,9 or ceramic additives10 at a temperature lower than that required for pure ZrB2.11 However, thermally unstable secondary phases, which derive from the sintering aids, adversely affected the mechanical properties at temperatures above 1200 °C, making this type of sintered material unacceptable for uses at very high temperatures.
One advantageous approach to saving or to better enhancing the inherent thermomechanical stability of HfB2 and ZrB2 has involved the introduction of a refractory carbide like SiC. The adjustment in SiC of the starting powder composition had succeeded in tightening the diboride matrix, in limiting excessive grain growth1,3,6,7,12,13 and in enhancing the overall resistance to oxidation and ablation resistance.1,4,14–17 The fabrication requirement of temperature above 2000 °C for fully densifying massive HfB2–SiC systems was eliminated by using a suitable amount of Si3N4 as sintering aid.18 The purpose of the present article is to report on the fabrication of HfB2–SiC and (HfB2 + ZrB2)–SiC composites. Densification behavior during hot-pressing, microstructural development, and thermomechanical propertie
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