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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. The mixed TaC-HfC powders were consolidated by a spark plasma sintering (SPS) machine (Model 10-4, Thermal Technologies, Santa Rose, CA, USA). The powders were sintered at 1850 ◦C with a heating rate of 100 ◦C/min and a maximum uniaxial pressure of 60 MPa. The holding time was 10 min to ensure the densification. The environment in the vacuum was set at a pressure of 4 Pa. The details of the processing can be found in our previous study [17].

2.2. TGA Testing and Post-Oxidation Characterization The TGA oxidation testing was conducted on a small portion (~30 mg) of the sintered pellets. A thermogravimetric analysis (TGA) analyzer (SDT-Q600, TA Instruments, New Castle, DE, USA) was used to evaluate the oxidation performance of TaC, HfC, and TaC-HfC solid solution samples. Samples were tested in the air at a heating rate of 5 ◦C/min. The maximum temperature was 1400 ◦C for all samples. The morphologies of the post-oxidation samples were examined by a field emission SEM (JSM-6330F, JEOL Ltd., Tokyo, Japan).

3. Results and Discussions

3.1. Microstructure and Phases in Sintered TaC, HfC, and TaC-HfC Solid Solutions The detailed characterization results of the microstructures and phases formed in spark plasma-sintered TaC-HfC solid solutions were published in our previous paper [17]. For the reader’s convenience and the sake of completeness, a summary of the key results is listed in Table1. All five samples had high densities varying between 97% and 99%. With the addition of HfC, the samples’ densification increases and the highest densification is found in T20H80 sample. The average grain size also decreased with the HfC additions. The lattice parameters of the formed solid solutions matched the theoretical values calculated according to Vegard’s Law. [17]

Table 1. Basic characterizations of the TaC, HfC, and TaC-HfC solid solutions.

Name Pellet Density (×103 kg/m3) Densification (%) Average Grain Size (µm) Pure TaC 14.14 96.7 6.8 ± 1.4 T80H20 13.85 97.8 6.2 ± 2.1 T50H50 13.26 98.2 3.8 ± 1.2 T20H80 12.68 98.8 3.1 ± 1.1 Pure HfC 12.21 98.5 2.3 ± 0.7

3.2. Macro State Morphology of Post-Oxidation TaC-HfC Solid Solutions The overall qualitative oxidation resistance of TaC, HfC, and their solid solutions can be inferred by the morphology of the post-oxidation samples. Appearances of the post-oxidation samples from the TGA testing are shown in Figure2. The pure TaC sample showed the worst oxidation resistance, as it turned into a powdery form with no mechanical integrity (Figure2a). The oxidized pure HfC, Coatings 2017, 7, 111 4 of 9 onCoatings the other 2017 hand,, 7, 111 displayed a much better oxidation resistance. Structural integrity can still be4 of seen 9 even though the oxidized sample broke into several pieces (Figure2b). The post-oxidation samples’ even though the oxidized sample broke into several pieces (Figure 2b). The post‐oxidation samples’ appearancesappearances of of T80H20 T80H20 and and T20H80 T20H80 is is the the combination combination of oxidation morphology morphology exhibited exhibited by by pure pure TaCTaC and and HfC. HfC. In In the the oxidized oxidized T80H20 T80H20 sample sample (Figure(Figure2 2c),c), aa largelarge amount of of powder powder is is noticed noticed with with a fewa few solid solid broken broken pieces. pieces. The The appearance appearance ofof thethe post-oxidationpost‐oxidation T20H80 T20H80 (Figure (Figure 2d)2d) is is analogous analogous to to purepure HfC HfC with with a smalla small amount amount of of powder. powder. T50H50 T50H50 is the only sample sample which which does does not not show show significant significant spallationspallation and and delamination. delamination. This This suggests suggests that that outerouter layer oxide oxide scale scale has has good good mechanical mechanical integrity integrity andand can can protect protect the the underlying underlying carbide carbide solid solid solution. solution.

FigureFigure 2. 2.Post-oxidation Post‐oxidation samples: samples: ( a(a)) Pure Pure TaC; TaC; ((bb)) Pure HfC; ( (cc)) T80H20; T80H20; (d (d) )T20H80; T20H80; and and (e ()e T50H50.) T50H50.

3.3. Mass Change during Thermogravimetric Analysis of Carbide Solid Solutions 3.3. Mass Change during Thermogravimetric Analysis of Carbide Solid Solutions The weight change curves of TaC‐HfC solid solutions are presented in Figure 3 as the degree of oxidationThe weight (α) with change the onset curves oxidation of TaC-HfC temperatures solid solutions for five aresamples. presented The degree in Figure of oxidation3 as the degree(α) is of oxidationdefined as ( αthe) with ratio the of onsetthe measured oxidation weight temperatures change over for five the samples.theoretical The weight degree change of oxidation at 100% ( α) is definedconversion. as the The ratio degree of of the oxidation measured (α) weightis calculated change by the over following the theoretical equation: weight change at 100% conversion. The degree of oxidation (α) is calculated by the following equation: ∆ ∝ ∆∆m m − m ∝= = ins i where ∆ is the measured weight change,∆m ∞∆ mis t the− m theoreticali weight change, is the real‐time measured weight, is the initial weight, and is the theoretical weight. The theoretical where ∆m is the measured weight change, ∆m is the theoretical weight change, m is the real-time weight change is based on the below reactions:∞ ins measured weight, mi is the initial weight, and mt is the theoretical weight. The theoretical weight 9 change is based on the below reactions:2TaC O2 →TaO 2CO (1)

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HfC O →HfO CO (2) Coatings 2017, 7, 111 5 of 9 The onset oxidation temperature is defined in Figure 3 when the degree of oxidation increases sharply. Pure TaC starts to oxidize around 750 °C, followed by pure HfC which starts around 800 °C. The solid solution samples showed improved9 oxidation resistance, as the oxidation processes have 2TaC + O2 → Ta2O5 + 2CO2 (1) been delayed compared to the pure carbides,2 as shown in Figure 2. T80H20 began its oxidation process at 850 °C, and T20H80 started around 900 °C. Further delay was observed in T50H50, where HfC + O2 → HfO2 + CO2 (2) the onset oxidation temperature was around 940 °C.

FigureFigure 3. TGTG curve curve of of TaC, TaC, HfC, and TaC TaC-HfC‐HfC solid solutions.

The oxidation process of pure TaC matches the literature description [19,20]. The degree of The onset oxidation temperature is defined in Figure3 when the degree of oxidation increases oxidation increases sharply around 750 °C and is completed near 950 °C. The transformation from sharply. Pure TaC starts to oxidize around 750 ◦C, followed by pure HfC which starts around 800 ◦C. TaC to Ta2O5 involves tremendous volume changes. According to the Pilling‐Bedworth (PB) ratio The solid solution samples showed improved oxidation resistance, as the oxidation processes have theory, the PB ratio for Ta to Ta2O5 is 2.5 [21]. Such large volume mismatch leads to spallation and delamination.been delayed compared Without any to the anchoring pure carbides, structure, as shown the resultant in Figure oxide2. T80H20 will lose began its mechanical its oxidation integrity, process at 850 ◦C, and T20H80 started around 900 ◦C. Further delay was observed in T50H50, where the onset and thus it cannot protect the underlying◦ materials. Consequently, the final product mainly consists ofoxidation fine powder, temperature as shown was in around Figure 9402a. OneC. spike at 780 °C is observed in the TaC oxidation curve in The oxidation process of pure TaC matches the literature description [19,20]. The degree of Figure 3. The measured temperature dropped◦ during the oxidation process.◦ The morphology of the oxidation increasesproduct suggests sharply aroundthat the 750 oxideC andwould is completed delaminate near and 950 exposeC. The unreacted transformation TaC during from TaCoxidation. to Ta2 OThe5 involves unreacted tremendous TaC has a lower volume temperature changes. According than the surface to the of Pilling-Bedworth the delaminated (PB) oxide, ratio so thetheory, measured the PB temperature ratio for Ta todropped. Ta2O5 is 2.5 [21]. Such large volume mismatch leads to spallation and delamination.Figure 3 shows Without that any oxidation anchoring process structure, of HfC the starts resultant at 800 oxide °C. will Comparing lose its mechanical to the oxidation integrity, of and thus it cannot protect the underlying materials. Consequently, the final product mainly consists pure TaC, the weight increases gradually instead of exhibiting◦ a sharp increase, as evident from the slope.of fine Such powder, behavior as shown is in in accordance Figure2a. Onewith spikethe literature at 780 Cdescription is observed of in the the oxidation TaC oxidation behavior curve of HfCin Figure [12,22].3. The HfC measured has the ability temperature to absorb dropped oxygen during without the transforming oxidation process. into oxide. The morphology The dissolved of the oxidation product suggests that the oxide would delaminate and expose unreacted TaC during oxygen sits on the carbon vacancies and forms an oxy‐carbide layer. The formed HfO2, unlike Ta2O5, doesoxidation. not experience The unreacted much volume TaC has change a lower from temperature the HfC. The than PB the ratio surface is only of 1.7 the [21], delaminated which explains oxide, so the measured temperature dropped. 2 the mechanically stable morphology of HfO in Figure 2b. ◦ TheFigure morphologies3 shows that of oxidation the oxidized process TaC of and HfC HfC starts are at studied 800 C. by Comparing SEM and toshown the oxidation in Figure of 4. pure The TaC,grain the size weight of oxidized increases TaC gradually is larger than instead oxidized of exhibiting HfC. Distinct a sharp grains increase, can be as spotted evident in from the theoxidized slope. HfCSuch sample, behavior but is in the accordance surface of with oxidized the literature TaC is description relatively ofsmooth the oxidation with coalesced behavior grains of HfC due [12,22 to]. HfC has the ability to absorb oxygen without transforming into oxide. The dissolved oxygen sits on the localized sintering. Conventionally, sintering occurs at 0.6 Tm (where Tm is the melting point). The carbon vacancies and forms an oxy-carbide layer. The formed HfO , unlike Ta O , does not experience melting point (Tm) of Ta2O5 is 1872 °C; hence, the sintering of2 Ta2O5 will2 start5 around 1250 °C. much volume change from the HfC. The PB ratio is only 1.7 [21], which explains the mechanically No sintering is expected in HfO2, as its melting point is around 2800 °C [5]. The pores in the oxidized HfCstable are morphology from the formation of HfO2 inof Figurea gaseous2b. product during oxidation. The morphologies of the oxidized TaC and HfC are studied by SEM and shown in Figure4. The grain size of oxidized TaC is larger than oxidized HfC. Distinct grains can be spotted in the oxidized HfC sample, but the surface of oxidized TaC is relatively smooth with coalesced grains due Coatings 2017, 7, 111 6 of 9

to localized sintering. Conventionally, sintering occurs at 0.6 Tm (where Tm is the melting point). ◦ ◦ The melting point (Tm) of Ta2O5 is 1872 C; hence, the sintering of Ta2O5 will start around 1250 C. ◦ No sintering is expected in HfO2, as its melting point is around 2800 C[5]. The pores in the oxidized CoatingsHfC are 2017 from, 7, 111 the formation of a gaseous product during oxidation. 6 of 9

Figure 4. PostPost-oxidation‐oxidation SEM images of ( a)) pure TaC and (b) pure HfC.

Although the post‐oxidation samples of T80H20 and T20H80 do not have sufficient mechanical Although the post-oxidation samples of T80H20 and T20H80 do not have sufficient mechanical integrity to protect the underlying materials (Figure 2c,d), these two solid solutions showed integrity to protect the underlying materials (Figure2c,d), these two solid solutions showed improved oxidation resistance by delaying the onset of oxidation temperature, as shown in Figure 3. improved oxidation resistance by delaying the onset of oxidation temperature, as shown in Figure3. The oxidation onset temperatures of T80H20 and T20H80 are 850 and 900 °C, respectively, which is The oxidation onset temperatures of T80H20 and T20H80 are 850 and 900 ◦C, respectively, which is significantly delayed as compared to pure TaC and HfC. However, both T80H20 and T20H80 still significantly delayed as compared to pure TaC and HfC. However, both T80H20 and T20H80 still reached near 100% oxidation. The T50H50 solid solution displayed the best oxidation resistance. Not reached near 100% oxidation. The T50H50 solid solution displayed the best oxidation resistance. only did the onset temperature for oxidation increase to near 940 °C for the T50H50 samples, only Not only did the onset temperature for oxidation increase to near 940 ◦C for the T50H50 samples, only 60% of the oxidation was completed when the temperature reached 1400 °C. 60% of the oxidation was completed when the temperature reached 1400 ◦C. To further understand the mechanism of improved oxidation resistance in the solid solution To further understand the mechanism of improved oxidation resistance in the solid solution samples, especially in the T50H50 sample, the morphologies of the post‐oxidation samples of the samples, especially in the T50H50 sample, the morphologies of the post-oxidation samples of the solid solutions are investigated by SEM and presented in Figure 5. As described earlier, the oxide solid solutions are investigated by SEM and presented in Figure5. As described earlier, the oxide morphologies of pure TaC and HfC (Figure 4) are highly porous due to gaseous products resulting morphologies of pure TaC and HfC (Figure4) are highly porous due to gaseous products resulting in prominent volume change. The key factor in improving the oxidation resistance of the TaC‐HfC in prominent volume change. The key factor in improving the oxidation resistance of the TaC-HfC solid solutions is to suppress the formation of the Ta2O5 phase. In Figure 5a, the main post‐oxidation solid solutions is to suppress the formation of the Ta O phase. In Figure5a, the main post-oxidation product has large elongated grains as compared to the2 5grains in Figure 5b,c. The inset shows the top product has large elongated grains as compared to the grains in Figure5b,c. The inset shows the top view of the elongated grains where grains look more equiaxial. The grain enlargement is caused by view of the elongated grains where grains look more equiaxial. The grain enlargement is caused by the the dramatic volume change associated with the formation of Ta2O5 and the very high Pilling‐ dramatic volume change associated with the formation of Ta2O5 and the very high Pilling-Bedworth Bedworth ratio of 2.5. Additionally, TaC has a cubic crystal structure, whereas Ta2O5 has an orthorhombic ratio of 2.5. Additionally, TaC has a cubic crystal structure, whereas Ta O has an orthorhombic crystal crystal structure. The transformation from cubic to orthorhombic leads2 to5 elongation in one direction, structure. The transformation from cubic to orthorhombic leads to elongation in one direction, which which is also the reason why Ta2O5 has large grain size. Larger grain size is also possible due to grain is also the reason why Ta O has large grain size. Larger grain size is also possible due to grain growth growth because of the earlier2 5 onset of oxidation in the T80H20 sample as compared to the other two because of the earlier onset of oxidation in the T80H20 sample as compared to the other two solid solid solutions. In the oxidation of the TaC‐HfC solid solutions, especially in the T80H20 sample, the solutions. In the oxidation of the TaC-HfC solid solutions, especially in the T80H20 sample, the formed formed Ta2O5 can react with the unreacted HfC. The reaction (Reaction (3)) replaces the Ta2O5 with Ta2O5 can react with the unreacted HfC. The reaction (Reaction (3)) replaces the Ta2O5 with HfO2, HfO2, so the volume change during the oxidation process is reduced. so the volume change during the oxidation process is reduced. The consumption of Ta2O5 reduced the volume change and increased the mechanical integrity of the post-oxidation samples in solid solutions.

 ◦  3Ta2O5 + 7HfC → 7HfO2 + 6TaC + CO ∆G (kJ) = −0.14T − 759.76 T : 500–2000 C [17] (3)

The oxidation behavior of T20H80 is similar to that of pure HfC, as shown in Figure5b. Another beneficial effect of reaction 3 is the delay of the formation of gaseous products in the early oxidation

Figure 5. Cont.

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Figure 4. Post‐oxidation SEM images of (a) pure TaC and (b) pure HfC.

Although the post‐oxidation samples of T80H20 and T20H80 do not have sufficient mechanical integrity to protect the underlying materials (Figure 2c,d), these two solid solutions showed improved oxidation resistance by delaying the onset of oxidation temperature, as shown in Figure 3. The oxidation onset temperatures of T80H20 and T20H80 are 850 and 900 °C, respectively, which is significantly delayed as compared to pure TaC and HfC. However, both T80H20 and T20H80 still reached near 100% oxidation. The T50H50 solid solution displayed the best oxidation resistance. Not only did the onset temperature for oxidation increase to near 940 °C for the T50H50 samples, only 60% of the oxidation was completed when the temperature reached 1400 °C. To further understand the mechanism of improved oxidation resistance in the solid solution samples, especially in the T50H50 sample, the morphologies of the post‐oxidation samples of the solid solutions are investigated by SEM and presented in Figure 5. As described earlier, the oxide Coatings 2017, 7, 111 7 of 9 morphologies of pure TaC and HfC (Figure 4) are highly porous due to gaseous products resulting in prominent volume change. The key factor in improving the oxidation resistance of the TaC‐HfC stage.solid solutions Reaction is 2 showsto suppress that one the moleformation of HfC of would the Ta2 reactO5 phase. with oneIn Figure mole of5a, oxygen the main to formpost‐oxidation one mole ofproduct gaseous has products large elongated (CO or CO grains2). However, as compared in reaction to the grains 3, Ta2O in5 wouldFigure consume5b,c. The seveninset shows moles the of HfC top toview generate of the oneelongated mole ofgrains gaseous where product, grains solook the more gases equiaxial. produced The by grain HfC canenlargement be reduced is orcaused at least by delayed.the dramatic The Gibbsvolume free change energy forassociated reaction with 3 is computed the formation using Factsageof Ta2O5 [ 17and]. The the delay very ofhigh the gaseousPilling‐ productBedworth diminishes ratio of 2.5. the Additionally, cracking within TaC has the a oxides cubic crystal and enhances structure, the whereas mechanical Ta2O5 integrity has an orthorhombic of the oxide scalescrystal in structure. solid solutions. The transformation The well-adhered from cubic oxide to orthorhombic scale can serve leads as a to protection elongation layer in one against direction, the furtherwhich is oxidation also the reason of solid why solutions. Ta2O5 Thehas large morphology grain size. of the Larger post-oxidation grain size is sample also possible of T50H50 due is to a grain vivid proofgrowth of because this concept, of the as earlier shown onset in Figure of oxidation5c. The oxidein the scaleT80H20 of the sample oxidized as compared T50H50 solidto the solution other two is muchsolid solutions. denser as In compared the oxidation to the of other the TaC samples.‐HfC solid The solutions, oxide grains especially are mostly in the equiaxed, T80H20 sample, suggesting the theformed moderate Ta2O5 volumecan react increase. with the No unreacted large cracks HfC. are The noticeable reaction in(Reaction the oxidized (3)) replaces T50H50 the sample. Ta2O5 Thus, with theHfO T50H502, so the solid volume solution change shows during the the best oxidation oxidation process resistance is reduced. among all the solid solutions.

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Figure 5. Cont.

Figure 5.5. PostPost-oxidation‐oxidation SEM images of ( a) T80H20 (inset showing the top view of thethe elongatedelongated grains); (b)) T20H80; and ((cc)) T50H50. (Insets are high magnificationmagnification images).images).

The consumption of Ta2O5 reduced the volume change and increased the mechanical integrity It must be noted that oxidation onset temperature for T50H50 is 940 ◦C. The onset temperature of the post‐oxidation samples in solid solutions. reveals the interaction between the carbides and oxygen. The study of the absorption of oxygen on TaC and HfC (001) planes suggests3Ta thatO the7HfC→7H oxygen tendsfO 6TaCCO to sit on the Hf–C bridge [23]. In the case of TaC, the preferential oxygen site was on the Ta–Ta bridge. After forming a solid solution, the(3) Ta (ΔG (kJ) = −0.14T − 759.76 T: 500–2000 °C) [17] atoms are partially replaced by Hf atoms, so the availability of Ta–Ta is disturbed, and the oxidation of theThe Hf oxidation element remains behavior unaffected. of T20H80 Thus, is similar the formation to that of pure of Ta 2HfC,O5 is as retarded. shown in As Figure discussed 5b. Another earlier, thebeneficial formation effect of of Ta reaction2O5 is detrimental 3 is the delay to oxidation of the formation performance, of gaseous so the products solid solutions in the early are expected oxidation to havestage. superior Reaction oxidation 2 shows resistances.that one mole Among of HfC the would five samples, react with abrupt one mole weight of oxygen increase to is form observed one mole only inof thegaseous pure products TaC sample. (CO The or CO weight2). However, change in reaction the HfC-contained 3, Ta2O5 would samples consume increases seven steadily, moles of which HfC correspondsto generate one to the mole adsorption of gaseous of product, oxygen, aso unique the gases feature produced of HfC. by The HfC solid can solutionsbe reduced should or at haveleast andelayed. abrupt The weight Gibbs increase free energy at a lower for temperaturereaction 3 is similarcomputed to pure using TaC Factsage if the oxidation [17]. The of delay TaC has of notthe beengaseous retarded. product The diminishes maximum the delay cracking in the within onset of the oxidation oxides and is found enhances in the the T50H50 mechanical sample, integrity which of is expectedthe oxide asscales the Ta–Tain solid bridges solutions. have The been well disturbed‐adhered the oxide most scale by can forming serve solid as a protection solutions. layer against the further oxidation of solid solutions. The morphology of the post‐oxidation sample of T50H50 is a vivid proof of this concept, as shown in Figure 5c. The oxide scale of the oxidized T50H50 solid solution is much denser as compared to the other samples. The oxide grains are mostly equiaxed, suggesting the moderate volume increase. No large cracks are noticeable in the oxidized T50H50 sample. Thus, the T50H50 solid solution shows the best oxidation resistance among all the solid solutions. It must be noted that oxidation onset temperature for T50H50 is 940 °C. The onset temperature reveals the interaction between the carbides and oxygen. The study of the absorption of oxygen on TaC and HfC (001) planes suggests that the oxygen tends to sit on the Hf–C bridge [23]. In the case of TaC, the preferential oxygen site was on the Ta–Ta bridge. After forming a solid solution, the Ta atoms are partially replaced by Hf atoms, so the availability of Ta–Ta is disturbed, and the oxidation of the Hf element remains unaffected. Thus, the formation of Ta2O5 is retarded. As discussed earlier, the formation of Ta2O5 is detrimental to oxidation performance, so the solid solutions are expected to have superior oxidation resistances. Among the five samples, abrupt weight increase is observed only in the pure TaC sample. The weight change in the HfC‐contained samples increases steadily, which corresponds to the adsorption of oxygen, a unique feature of HfC. The solid solutions should have an abrupt weight increase at a lower temperature similar to pure TaC if the oxidation of TaC has not been retarded. The maximum delay in the onset of oxidation is found in the T50H50 sample, which is expected as the Ta–Ta bridges have been disturbed the most by forming solid solutions.

Coatings 2017, 7, 111 8 of 9

4. Conclusions Through TGA analyses, we investigated the oxidation behavior of pure TaC and HfC as well as their solid solutions. The solid solutions exhibit improved oxidation resistance compared to the pure carbides. T50H50 is found to have the best oxidation resistance, followed by T20H80 and T80H20. The onset of oxidation in T50H50 increases by 170 and 120 ◦C as compared to pure TaC and pure HfC, respectively. The improved oxidation resistance can be attributed to the formation of the solid solutions that disturbs the atomic arrangement. Such disturbance delays the formation of Ta2O5 and does not affect the formation of HfO2. The reaction between Ta2O5 and HfC is also responsible for the superior oxidation performances in the solid solution samples. It diminishes the generation of gaseous products during oxidation, which reduces the porosity of the oxide scales and leads to the better protection of the underlying materials. The present study showcases SPS-sintered solid solutions as a new class of oxidation-resistant materials within ultrahigh temperature ceramics (UHTCs).

Acknowledgments: Cheng Zhang thanks the Florida International University Graduate School for the Dissertation Year Fellowship (DYF) award. Advanced Materials Engineering Research Institute (AMERI), FIU is acknowledged for the research facilities used and the support from its staff in this study. Author Contributions: Benjamin Boesl and Arvind Agarwal conceived and designed experiments; Archana Loganathan performed the experiments; Cheng Zhang, Archana Loganathan, Benjamin Boesl and Arvind Agarwal analyzed data; Cheng Zhang wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

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