Oxidation Behavior of Zirconium Diboride Silicon Carbide Produced by the Spark Plasma Sintering Method Carmen M. Carney,* ,w,z,y Pavel Mogilvesky,* ,z,y and Triplicane A. Parthasarathy* ,z,y z Air Force Research Laboratory, Materials and Manufacturing Directorate, AFRL/RX y UES Inc., Dayton, Ohio 45432 Dense samples of ZrB 2 –20 vol% SiC were successfully fabri- cated by spark plasma sintering without the use of sintering aids. Oxidation behavior of these samples was characterized by ex- posing them to 14001, 15001, and 16001C in an ambient atmo- sphere for 150 min, and by measuring the weight gains of the sample and crucible, as well as the thickness of the oxide scale and the glassy outer layer. The effects of gravity on the viscous outer layer are shown to result in significant heterogeneity within a sample. The oxidation scales were characterized by scanning electron microscopy and transmission electron micros- copy with energy dispersive spectroscopy analysis. The oxide scale was found to be composed of three layers: (1) a SiO 2 -rich glassy outer layer, (2) an intermediate layer of a ZrO 2 matrix with interpenetrating SiO 2 , and (3) a layer containing a ZrO 2 matrix enclosing partially oxidized ZrB 2 with Si–C–B–O glass inclusions. I. Introduction B ECAUSE of their high melting temperatures and excellent re- sistance to oxidation and evaporative erosion, transition metal diborides (MB 2 ) such as ZrB 2 and HfB 2 , commonly re- ferred to as ultra-high-temperature ceramics (UHTCs), are be- ing considered as principal candidates for the leading edges of sharp-bodied reentry vehicles. 1–6 ZrB 2 -based UHTCs have re- ceived a majority of the attention due to their lower density (6.09 vs 10.5 g/cm 3 for HfB 2 ) and lower cost. Several studies have shown that the addition of SiC improves the oxidation resis- tance of the diborides at temperatures 412001C. 7–12 In terms of mechanisms, it has been shown that ZrB 2 oxidizes to ZrO 2 and liquid B 2 O 3 , which evaporates at higher temperatures (412001C) as B 2 O 3 (g). 13–15 The addition of SiC allows the for- mation of SiO 2 (melting temperature 17101C), which is more resistant to evaporation and has a higher viscosity at elevated temperatures than B 2 O 3 . However, B 2 O 3 continues to flux the silica scale, lowering its viscosity. Processing of diboride UHTCs typically involves the use of elevated temperatures (above 19001C) for extended periods of time (30 min or longer) accompanied by applied pressures. Some success has been met with pressureless sintering of these UHTCs, but this process normally requires a sintering aid such as MoSi 2 or B 4 C. 16,17 Recently, the spark plasma sintering (SPS) method has provided a processing technique to rapidly (o 5 min at the sintering temperature) densify ZrB 2 -based UHTCs. However, many of these studies also use a sintering aid in ad- dition to SiC to ensure full density, 18–24 unless reactive synthesis is used. 19 These sintering aids may affect the oxidation proper- ties of these materials. In this paper we seek to use SPS to pre- pare dense, ZrB 2 -based UHTCs with SiC and without sintering aids. These samples will be tested for their oxidation resistance, with particular attention paid to unanswered questions that re- main in the literature, regarding the oxide scale morphology and chemistry. These questions include the possible effect of exper- imental conditions (geometry of the sample and crucible) on the fluid flow of the viscous glassy outer layer. Although glass flow has been suggested to be important to boria flow in monolithic diborides, 15 it is not fully explained in the two phase materials that contain silica forming compounds. Additionally, there are inconsistencies in the reported work on the phase distribution of the oxidized scale. Some reports suggest a region of the oxide scale that is depleted of SiC due to active oxidation, while others show no such region among samples tested at similar tempera- tures. 7–13 The objectives of this study are to (1) fabricate dense ZrB 2 – SiC without sintering aids using SPS, (2) study the effect of ex- ternal scale viscosity on oxidation mechanisms, and (3) deter- mine the change in oxide scale morphology and composition as a function of temperature. We show that SPS can be used to produce dense ZrB 2 –SiC without the need for sintering aids. Significant sources of error in measuring weight change are shown to arise from the viscosity of the scale at temperatures below 16001C. An analysis of the oxide scale morphology with emphasis on the glassy scale will highlight the importance of the viscosity of this layer. Finally, a compositional analysis of the oxide scale is presented. II. Experimental Procedure ZrB 2 (Millmaster Chemical, New York, NY) and SiC (Reade Advanced Materials, East Providence, RI) were used to mix 80 vol% ZrB 2 and 20 vol% SiC (ZrB 2 –SiC). The b-SiC was a 45– 55 nm (reported) powder with 97.5% purity (o0.15% free Si, o0.15% Cl, o0.75% free C, and o1.25% O). All impurities (C, Fe, Hf, Ti, Al, and Be) in the ZrB 2 powder were o0.02 wt% except for hafnium, which was 1.3 wt%. The ZrB 2 powder had a measured mean starting size of 9.4 mm. Powders were ball-milled using Si 3 N 4 grinding media in isopropanol for 18 h. The powders were then dried while stirring followed by 18 h of dry milling with the same Si 3 N 4 . The dried powders were sieved using an 80-mesh screen. The weight loss of the Si 3 N 4 grinding media (0.03% lost weight) was 0.14 wt%, based on the total powder weight. Nine grams of the dried powder was loaded into a 20-mm- graphite die coated with BN and lined with graphite foil. The sample was sintered using SPS (FCT Systeme GmbH Model HP D 25-1, Rauenstein, Germany) with a heating and cooling rate of 901C/min and a maximum temperature of 20001C (achieved using 5.5 V and 1 kA). The hold time was 5 min. The temper- ature was measured by an optical pyrometer focused on the bottom of a borehole in the punch B5 mm from the powder. A D. Butt—contributing editor This work was supported in part by USAF Contract # FA8650-04-D-5233. *Member, The American Ceramic Society. w Author to whom correspondence should be addressed. e-mail: ccarney@ues.com Manuscript No. 25384. Received October 22, 2008; approved April 3, 2009. J ournal J. Am. Ceram. Soc., 92 [9] 2046–2052 (2009) DOI: 10.1111/j.1551-2916.2009.03134.x r 2009 The American Ceramic Society 2046