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Thermal Analysis of Tantalum Carbide-Hafnium Carbide Solid Solutions from Room Temperature to 1400 ◦C

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Thermal Analysis of Tantalum Carbide-Hafnium Carbide Solid Solutions from Room Temperature to 1400 ◦C coatings Article Thermal Analysis of Tantalum Carbide-Hafnium Carbide Solid Solutions from Room Temperature to 1400 ◦C Cheng Zhang, Archana Loganathan, Benjamin Boesl and Arvind Agarwal * Plasma Forming Laboratory, Department of Mechanical and Materials Engineering, Florida International University, Miami, 33139 FL, USA; czhan009@fiu.edu (C.Z.); aloga006@fiu.edu (A.L.); bboesl@fiu.edu (B.B.) * Correspondence: agarwala@fiu.edu; Tel.: +1-305-348-1701 Received: 5 June 2017; Accepted: 25 July 2017; Published: 28 July 2017 Abstract: The thermogravimetric analysis on TaC, HfC, and their solid solutions has been carried out in air up to 1400 ◦C. Three solid solution compositions have been chosen: 80TaC-20 vol % HfC (T80H20), 50TaC-50 vol % HfC (T50H50), and 20TaC-80 vol % HfC (T20H80), in addition to pure TaC and HfC. Solid solutions exhibit better oxidation resistance than the pure carbides. The onset of oxidation is delayed in solid solutions from 750 ◦C for pure TaC, to 940 ◦C for the T50H50 sample. Moreover, T50H50 samples display the highest resistance to oxidation with the retention of the initial carbides. The oxide scale formed on the T50H50 sample displays mechanical integrity to prevent the oxidation of the underlying carbide solid solution. The improved oxidation resistance of the solid solution is attributed to the reaction between Ta2O5 and HfC, which stabilizes the volume changes induced by the formation of Ta2O5 and diminishes the generation of gaseous products. Also, the formation of solid solutions disturbs the atomic arrangement inside the lattice, which delays the reaction between Ta and O. Both of these mechanisms lead to the improved oxidation resistances of TaC-HfC solid solutions. Keywords: tantalum carbide; hafnium carbide; solid solutions; oxidation; thermogravimetric analysis 1. Introduction The interest in tantalum carbide (TaC) and hafnium carbide (HfC) has been growing in recent years due to their extremely high melting points, high hardness, and elastic moduli, and more importantly, their ability to form solid solutions [1–4]. The major applications of these two carbides are leading edges of reentry vehicles and lining materials for rocket thrusters. In both cases, excellent oxidation resistance is required. However, gaseous products like CO and CO2 are inevitably formed during oxidation, which leads to porous oxide scales that delaminate and spall. The major oxide of TaC is ◦ Ta2O5, which has a melting point of ~1900 C, lower than the desired application temperature of 2000 ◦C or more [5–7]. As a result, the resultant oxide would melt and lose its structural integrity, and fail catastrophically. To reduce the gaseous products as well as retain the integrity of oxide scales under extremely high temperatures, Hafnium diboride (HfB2) and its composites have been investigated as promising candidate materials for use on next-generation hypersonic vehicles [8,9]. During oxidation, HfB2 forms a solid scaffold-like structure that mainly consists of HfO2 and molten B2O3 infiltrated between the HfO2. The resultant oxide scale is dense and crack-free, which provides exceptional ◦ oxidation resistance. Unfortunately, the B2O3 starts to evaporate around 700 C and therefore its protection of the underlying materials is lost. The SiC addition was used to stabilize the B2O3 by forming a borosilicate glass phase, which increases the onset evaporation temperature to 1400 ◦C. However, 1400 ◦C is still not high enough to withstand higher application temperatures of 2000 ◦C or more. Coatings 2017, 7, 111; doi:10.3390/coatings7080111 www.mdpi.com/journal/coatings Coatings 2017, 7, 111 2 of 9 Coatings 2017, 7, 111 2 of 9 TheThe studies studies on on the the solid solid solutions solutions of of TaC-HfCTaC‐HfC beganbegan with the discovery discovery of of a a TaC TaC0.80.8HfCHfC0.2 0.2phasephase ◦ thatthat possesses possesses the the highest highest melting melting point point (~4000(~4000 °C)C) of of known known substances substances [10]. [10 ].Preliminary Preliminary oxidation oxidation studiesstudies have have been been carried carried out out on on TaC TaC0.80.8HfCHfC0.2 andand HfC HfC-rich‐rich compositions, compositions, but but no no improvement improvement in the in the oxidationoxidation behavior behavior was was observed observed compared compared to to pure pure carbides carbides [11 [11–14].–14]. Additionally, Additionally, sintering sintering aids aids were inevitablewere inevitable in those in studies,those studies, which which introduced introduced secondary secondary phases phases that that clouded clouded the the understanding understanding of oxidationof oxidation behaviors. behaviors. Although Although TaC and TaC HfC and can HfC form can solid form solutions solid solutions above 887 above◦C in all887 compositions, °C in all ascompositions, shown in the phaseas shown diagram in the in phase Figure diagram1, oxidation in Figure studies 1, on oxidation TaC-HfC studies solid solutions on TaC‐HfC have solid barely beensolutions investigated. have barely been investigated. FigureFigure 1. 1.Phase Phase diagramdiagram of TaC and HfC HfC [15]. [15]. Copyright Copyright 2013 2013 Elsevier. Elsevier. Recently, Cedillos‐Barraza et al. as well as our research group sintered TaC‐HfC solid solutions Recently, Cedillos-Barraza et al. as well as our research group sintered TaC-HfC solid solutions without sintering additions by spark plasma sintering (SPS) [16,17]. The compositions cover the full without sintering additions by spark plasma sintering (SPS) [16,17]. The compositions cover the spectrum of TaC‐HfC solid solutions, and both studies noticed that TaC0.5HfC0.5 has the highest full spectrum of TaC-HfC solid solutions, and both studies noticed that TaC HfC has the highest hardness and elastic modulus among the solid solutions. Our group conducted0.5 oxidation0.5 testing hardnessusing a andplasma elastic jet by modulus exposing among these the solid solid solutions solutions. and Ourpure group carbides conducted to a temperature oxidation of testing ~3000 using°C ◦ a plasmaat a gas flow jet by rate exposing of sonic thesespeed solid[18]. In solutions general, the and solid pure solutions carbides showed to a temperature better oxidation of ~3000 resistanceC at a gasthan flow the rate pure of soniccarbides. speed The [ 18best]. In oxidation general, resistance the solid solutionswas found showed in the TaC better0.5HfC oxidation0.5 composition. resistance thanAfter the 300 pure s of carbides. exposure The to such best extreme oxidation oxidation resistance conditions, was found the inthickness the TaC of0.5 HfCthe oxide0.5 composition. scale in AfterTaC 3000.5HfC s0.5 of was exposure only 28 to μm, such which extreme is 1/6 and oxidation 1/10 of conditions,the oxide scale the thickness thickness in ofpure the HfC oxide and scale TaC, in TaCrespectively0.5HfC0.5 was [18]. only The 28 improvedµm, which oxidation is 1/6 andmechanism 1/10 of thewas oxide explained scale by thickness a newly in formed pure HfC Hf6Ta and2O TaC,17 respectivelyphase. More [18 importantly,]. The improved we found oxidation a similar mechanism dense solid was explainedscaffold and by liquid a newly phase formed structure Hf6Ta as2O 17 phase.reported More in importantly, HfB2‐SiC/HfB we2‐ foundB4C systems a similar that dense protect solid the scaffold underlying and liquid materials phase [18]. structure In the as case reported of inTaC HfB‐2HfC-SiC/HfB solid 2solutions,-B4C systems the solid that protectscaffold the consists underlying of HfO materials2 and Hf6 [Ta182].O17 In, and the casethe liquid of TaC-HfC phase solidis solutions,made of themolten solid Ta scaffold2O5. Compared consists to of the HfO B2O2 and3 and Hf borosilicate6Ta2O17, and phase the in liquid the diboride phase is system, made ofmolten molten Ta2O5 is a much more stable phase with a higher melting point of 1900 °C. Hence, the carbide solid Ta2O5. Compared to the B2O3 and borosilicate phase in the diboride system, molten Ta2O5 is a much moresolutions stable exhibit phase exceptional with a higher oxidation melting resistance. point of 1900 ◦C. Hence, the carbide solid solutions exhibit exceptionalOne question oxidation arises resistance. after the investigation on the plasma jet oxidation behavior of the carbide solidOne solutions: question How arises would after the the carbide investigation solid solutions on the behave plasma below jet oxidation 1800 °C, where behavior the temperature of the carbide is not high enough to melt the resultant Ta2O5? To address this question, we sought to understand solid solutions: How would the carbide solid solutions behave below 1800 ◦C, where the temperature the oxidation behavior of the carbide solid solutions from room temperature to 1400 °C using is not high enough to melt the resultant Ta O ? To address this question, we sought to understand thermogravimetric analysis (TGA). Five samples,2 5 namely pure TaC, 80TaC‐20 vol % HfC (T80H20), the oxidation behavior of the carbide solid solutions from room temperature to 1400 ◦C using 50TaC‐50 vol % HfC (T50H50), 20TaC‐80 vol % HfC (T20H80), and pure HfC, were chosen. A detailed thermogravimetric analysis (TGA). Five samples, namely pure TaC, 80TaC-20 vol % HfC (T80H20), 50TaC-50 vol % HfC (T50H50), 20TaC-80 vol % HfC (T20H80), and pure HfC, were chosen. A detailed Coatings 2017, 7, 111 3 of 9 analysis of the oxidation behavior is carried out in the present study using TGA followed by scanning electron microscopy (SEM). 2. Experimental Details 2.1. Materials Commercial TaC powder (Inframat Advanced Materials LLC, Manchester, CT, USA) and hafnium carbide powder (Materion LLC, Cleveland, OH, USA) were used as starting powders. Powders for solid solutions were mixed by a high-energy vibratory ball milling machine (Across International LLC, Livingston, NJ, USA) according to their stoichiometric ratio. Pure powders were milled for one hour separately in a tungsten carbide (WC) jar to breakdown the agglomeration. Subsequently, TaC and HfC powders were mixed for another hour. The ball to powder ratio was 1:3, using a 6-mm diameter WC ball.
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