Fabrication and Properties of Reactively Hot Pressed HfB 2. -HfC Ultra-High Temperature Ceramics

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1 Jurnal f the Krean Ceramic Sciety Vl. 7, N. 6, pp. 5~59, 010. DI:10.191/KCERS Review Fabricatin and Prperties f Reactively Ht Pressed -HfC Ultra-High Temperature Ceramics Seung Jun Lee, Yung-Hn Seng, SeungSu Baek*, Eul Sn Kang*, and D Kyung Kim Department f Materials Science and Engineering, Krea Advanced Institute f Science and Technlgy (KAIST), Daejen , Krea *Agency fr Defence Develpment (ADD), Daejen , Krea (Received ctber 9, 010; Revised Nvember 5, 010; Accepted Nvember 8, 010) ABSTRACT -HfC cmpsites were prepared by reactive ht pressing using Hf and B C at temperatures f 1800 and 1900 C fr 60 min under MPa in an Ar atmsphere. The reactin sequences f the -HfC cmpsite were studied thrugh series f pressureless heat treatments ranging frm 800 t 1600 C. The effect f size reductin f the starting pwders n densificatin was investigated by vibratin milling. Fully dense -HfC cmpsites were btained by size reductin f the starting pwders via vibratin milling. The xidatin behaviur f the -HfC cmpsites at 1500 C in air shwed frmatin f a nn-prtective Hf scale with linear mass gain. Examinatin f the mechanical prperties shwed that particle size reductin via vibratin milling als led t imprved flexural strength, hardness and fracture tughness. Key wrds : Reactive ht pressing, Hafnium bride, Hafnium carbide, Ultra high temperature ceramics, xidatin H 1. Intrductin afnium dibride ( ) is an ultra-high temperature ceramic (UHTC) that has a number f unique prperties, including extremely high melting temperature and hardness. 1) The thermdynamic prperties f at high temperature are superir t thse f ther transitin metal brides. In additin, recent furnace xidatin studies have shwn t be mre xidatin resistant than ther dibride materials. Based n this imprved xidatin resistance at high temperature, transitin metal brides have been extensively investigated fr high temperature structural applicatins that include thermal prtectin materials fr atmspheric re-entry, hypersnic flight and rcket prpulsin. 1-) Densificatin f UHTCs typically requires high temperature (>1900 C) and applied pressure f 0 MPa r mre t btain dense samples because f these materials strng cvalent bnding and lw diffusin cefficient. ) Therefre, metallic additives have been used t prmte liquid-phase frmatin, which can reduce densificatin temperature, but this prcess can deterirate the high temperature mechanical prperties. 5) Recently, pressureless sintering f transitin metal brides has been widely investigated by using WC, B C, C and MSi as a sintering additives. 6-8) Anther advantageus alternative methd is reactive ht pressing Crrespnding authr :D Kyung Kim Tel : Fax : (RHP), which cmbines bth the synthesis and the densificatin int a single-step prcess leading t high density ceramics at reduced temperatures and lwer impurity cntents when cmpared with the cnventinal prcess. 9,10) ZrB -SiC and -SiC cmpsites have been successfully sintered using Zr, Hf, Si, and B C as starting pwders. 9-11P Many attempts have been made t enhance the xidatin resistance f UHTCs thrugh apprpriate additives. It is imprtant t cnsider that the mst cmmn additive is SiC, which enhances the xidatin resistance by frmatin f Si. 1,,1) Bth ZrB -SiC and -SiC ceramics exhibit passive xidatin behaviur with parablic mass gain up t the temperature f 1700 C. At higher temperatures (> 00 C) r at lw xygen partial pressure, hwever, the xidatin resistance f SiC is pr due t active xidatin, which results in the frmatin f high vapur pressure xide prducts like Si and C with almst n silica at the surface. 1,1) Therefre, the additin f SiC as a secnd phase is beneficial fr xidatin resistance f UHTCs at high xygen partial pressure, whereas it is detrimental at higher temperatures (>00 C) and at lw xygen partial pressure. The present wrk investigates the effect f HfC as a secnd phase in the matrix n xidatin behaviur. The cmpsite fabricatin temperature range was investigated by reactin f the precursr at different temperatures frm 900 t 1600 C. Als, the effects f precursr pwder n densificatin and n mechanical prperties were investigated. 5

2 55 Jurnal f the Krean Ceramic Sciety - Seung Jun Lee et al. Vl. 7, N. 6 Table 1. Phase Cmpsitins f As-received Hf/B C Mixed Pwders Heat-treated Under Different Temperatures (Heating Rate 10 C/min) Heating Temperature Crystalline End Prducts 800 C Hf, B C 1000 C Hf, HfC 100 C Hf, HfC, Hf 100 C HfC,, Hf 1600 C HfC,. Experiment.1. Pwder Prcessing The precursr pwders were Hf (purity 99.5%, particle size - 5 mesh, Sigma Aldrich, USA) and B C (Grade HS, particle size ~ 0.8 µm, H.C. Stack, Germany). The chemical reactin can be expressed by the fllwing equatin, used t prepare the -HfC cmpsite: Hf + B C + HfC (1) The vlume cntents f and HfC were 70. vl% and 9.7 vl%, respectively. The theretical density f the cmpsite with respect t the rule f mixture is 11.6 g/cm. Tw milling methds were adpted, ne f which was used t mix precursr pwders, and the ther t reduce the particle size. In the first methd, the precursr pwders were ball milled in ethanl fr h using Zr as a milling media; this methd was designated HC. In the secnd methd, the precursr pwders were vibratin milled using Zr as a milling media in a Tefln jar fr h t reduce the particle size f the pwders; this methd was designated MHC. In rder t investigate the reactin mechanisms and sintering cnditins f the -HfC, presusreless heat treatment was cnducted. The pwder mixtures were pressed int disc shaped pellets and heat treated in the temperature range f 800 t 1600 C fr 1 h under an Ar atmsphere with a heating rate f 10 C/min. The heating temperatures and resultant prducts are listed in Table 1. Under the abve basic experimental cnditins the cmpsites were sintered using reactive ht pressing at different temperatures f 1800 and 1900 C fr 1 h under a pressure f MPa in an Ar atmsphere. The relative density f the sintered samples was measured using the Archimedes technique with deinized water as the immersing medium. Phase cmpsitin was determined by X-ray diffractmetry (XRD, Rigaku D/MAM-IIIC) using Cu Kα radiatin. The micrstructures f each specimen were bserved using a scanning electrn micrscpe (SEM Philips XL 0). The plished samples were subjected t an xidatin test under an air atmsphere. Each specimen was heat treated at 1500 C fr 0 min in a tube furnace with a heating rate f 5 C/min. The xidatin behaviur was als studied using thermal gravimetric analysis (TGA, SETA- RAM). The weight change was measured under isthermal hld at 1500 C fr 180 min. Elastic mdulus (E) was measured by the resnance frequency methd (ASTM E1876-1, 001) using an scillscpe (Tektrnix TDS10 w/fft mdule) and nn-cntact transducer (Cirrus ZE:901 CRL LM Preamplifier). Hardness and fracture tughness f the samples were determined using Vickers indentatin at a lad f kg and a dwell time f 15 s. The fracture tughness was estimated thrugh the fllwing equatin 15) : K IC = P al 1 Where P is the applied lad, a is the mean indentatin half-diagnal length, and l is the crack length. The flexural strength was measured by three pint bending methd.. Results and Discussin.1. The pwder synthesis via slid-state precursrs Fig. 1 shws the XRD results f the samples heat-treated at different temperatures. The crystalline end prducts f each temperature are listed Table 1. The patterns revealed that there was n apparent change in the reactin pwder mixture even after samples were heat treated t 800 C. The samples heat treated at 1000 C revealed the frmatin f a cubic phase f HfC, in additin t Hf peaks, which result indicates that the reactin between Hf and B C ccurs at 1000 C. This phenmenn can be explained by the cnventinal reactive ht press mdel. 9,10) At a suitable reactin temperature, it is assumed that B and C atms frm B C diffuse faster than thse f Hf. Amng these reactins, the diffusin rate f C is higher than that f B, and C easily reacts with Hf. As a result, it is evident that the frmatin f HfC ccurs at a temperature lwer than 1000 C, and ccurs faster than the frmatin f. The frmatin f ccurs at a relatively higher tempera- Fig. 1. XRD patterns f the as-received Hf/B C mixed pwders, which are heat-treated at different temperatures fr 60 min in Ar.

3 Nvember 010 Fabricatin and Prperties f Reactively Ht Pressed -HfC Ultra-High Temperature Ceramics 56 Fig.. XRD patterns f the vibratin milled Hf/B C mixed pwders, which are heat-treated at different temperatures fr 60 min in Ar. ture, abve 100 C, thrugh a reactin between Hf and B r residual B C, due t the increment f the diffusin rate f B. By analyzing the reactin sequence, it can be evidently cncluded that reactins () and () may ccur in tw steps, as fllws: Hf+B C HfC+B () Hf+B () The abve reactins () and () pssess negative Gibbs free energies f G 1000 = 16 and 60 kj/ml, respectively, indicating that the reactins are bth thermdynamically favurable. In the XRD patterns, it can be bserved that the sample heat treated at 100 C revealed the frmatin f an Hf phase. The presence f Hf might be attributed t xide impurities in the precursr pwder and/r xygen uptake during the milling prcess. Further increase in the reactin temperature results in the frmatin f as the main phase, based n the peak intensity, as can be seen in Fig. 1. The XRD pattern f the sample reacted at 1600 C shwed the frmatin f the final cmpsites and HfC withut any secndary phase. As a result, it can be cncluded that the verall pwder reactin takes place in tw steps, in which the initial reactin initiated the frmatin f HfC and, at a higher temperature, simultaneus reactins tk place by reactin (), which frmed. Similar steps were fllwed fr the vibratin milled pwder (MHC); stacked XRD patterns after heattreatment are presented in Fig.. During the milling prcess, 1.5 wt% f Zr impurity was intrduced because f wear f the Zr balls. Zr impurity can be remved by B C thrugh the fllwing reactin (). 5B C + 7Zr 7ZrB + 5C(g)+B (l) () Reactin () is well defined in the previus reprtu 6P The Fig.. XRD pattern f MHC after reactive ht-pressing at 1900 C fr h. reactin between B C and Zr was initiated arund 100 C; the cmplete reactin ccurred at 150 C, t frm ZrB. In the literature, there have been reprts that a mutual cntinuus slid slutin frms amng Grup IV and V f dibiride; 16) i.e, ZrB culd frm a cntinuus slid slutin with. Thus, Zr cntaminatin frm Zr balls was accmmdated in the lattice by the frmatin f slid slutin with the additin f extra B C accrding t the stichimetry f reactin (). As shwn in Fig., the reactin sequences f MHC were similar t that f HC except fr the temperature f reactin between Hf and B C(which decreased t 800 C in MHC) and the frmatin f (which decreased t 100 C in MHC). In additin, the reactins between Hf and B C were cmpleted after hlding at 100 C, which is lwer than the hlding temperature f the HC. Particle size reductin via vibratin milling prvides a shrter diffusin path and this might decrease the reactin temperature. The XRD pattern f the reactive ht-pressed MHC sample fabricated at 1900 C fr h is shwn in Fig.. The XRD pattern f MHC shws the presence f and HfC; n secnd phases were detected. In additin, the majr peaks f and HfC are crrespnding t the JCPDS card. The negligible peak shift in XRD patterns indicated that the slid slutin f ZrB frm reactin () did nt have much influence n the XRD peak angle due t the small amunt f Zr cntaminatin during the vibratin milling prcess... Micrstructure SEM analysis was cnducted t investigate the particle size and mrphlgy f HC and MHC after the cmpletin f the reactin. The relative densities f HC and MHC were als measured, densified at 1800 and 1900 C (Fig. ). Fig. 5 (a) shws the fracture surface f HC after the reactin at 1600 C; Fig. 6 (a) shws the micrstructure f HC densified at 1900 C fr 60 min. Due t the large particle size f the Hf precursr pwder, sme f the resultant prducts f and HfC als frmed with similar particle size with Hf pre-

4 57 Jurnal f the Krean Ceramic Sciety - Seung Jun Lee et al. Vl. 7, N. 6 residual pres. It was difficult t distinguish and HfC in the SEM images due t the very slight cntrast between them. EDS analysis cnfirmed that the dark areas represented the and the bright area represented HfC. Frm this analysis, it can be cncluded that the particle size reductin via vibratin milling allwed us t btain small size and HfC cmpared with the as-received pwder. The increased driving frce f densificatin due t the frmatin f small size and HfC, and the applicatin f external pressure during the RHP prcess led t nearly full densificatin withut residual pres. In additin t the frmatin f small particles, the increase in defect cncentratin, wing t the high milling, culd als pssibly be a reasn fr the increased driving frce f densificatin, similar t that fund in the study f particle size effects n the sintering f bride cmpunds. 17) Fig.. Fig. 5. Plt f relative density (%) f -HfC materials densified at different temperatures... xidatin behaviur Fig. 7 (a) shws the results f isthermal TGA analysis f SEM images f the fracture after heat-treatment at 1600 C fr 60 min: (a) HC and (b) MHC. Inset in (a) shws high-magnificatin f HC. Fig. 6. SEM micrgraphs f plished surface f sample (a) HC and (b) MHC. cursr pwder, as shwn in Fig. 5 (a). Althugh, as shwn in the inset f Fig. 5 (a), the primary grains f the frmed and HfC are f sub-micrn meter size, sme f the larger prducts f and HfC (<0 µm) might have led t the frmatin f residual prsity and, therefre, have limited the final densities (Fig. ). The size and mrphlgy f MHC after the reactin at 1600 C fr 60 min is shwn in Fig. 5 (b). It can be bserved in Fig. 5 (b) that the particle size f the frmed and HfC are abut 5 µm, with platelet shaped small particles that cnsist f sub-micrn size primary grains. The plished surface f RHPed MHC is shwn in Fig. 6 (b). The MHC sample sintered via RHP at 1900 C revealed cmplete densificatin (99.% R.D.) with rare Fig. 7. (a) Isthermal TGA analysis f -SiC and HfC at 1500 C in air and (b) XRD patterns f the xidized surface f -HfC cmpsite after xidatin test at 1500 C fr 0 min in air.

5 Nvember 010 Fabricatin and Prperties f Reactively Ht Pressed -HfC Ultra-High Temperature Ceramics 58 Fig. 8. SEM images f MHC shwing a prus Hf layer (I) and un-reacted -HfC layer (II) after expsure f samples at 1500 C fr 0 min in air. Inset shws highmagnificatin f layer (I). MHC at 1500 C in air. Fig. 7 (b) represents the XRD results fr the xidized surface. Fr cmparisn, the TGA results fr the -SiC fabricated by RHP are included in Fig. 7 (a). As shwn in Fig. 7 (a), the -SiC cmpsite exhibited passive xidatin prtectin with parablic mass gain kinetics during the isthermal analysis at 1500 C. At high temperatures (abve 100 C), the SiC phase begins xidizing, resulting in the frmatin f a silica-rich glassy layer n the surface by reactin (5). SiC (s)+/ (g) Si (l)+c (g) (5) The surface silica rich layer prhibits the transprt f xygen thrugh the inner xide scales and makes it pssible fr an -SiC cmpsite t shw parablic mass gain kineticsu 1,11,18P The mass gain f the -HfC samples, hwever, appeared t increase linearly with time. The linear trend suggests a reactin rate-cntrlled kinetics. Expsure f and HfC t air results in the stichimetric xidatin t, B and C by reactin (6). (s) + HfC (s) + Hf (s) + B (l) + C (g) (6) As shwn in Fig. 8, Hf appears t frm a prus skeletn that des nt enhance the xidatin prtectin. In the literature, there are many reprts that describe the high vapur pressure f B at 1500 C 1) ; at this temperature, B will evaprate rapidly, leaving a prus Hf. At 1500 C, the rate f evapratin f B (l) is greater than its rate f prductin, leaving a nn-prtective prus Hf scale. Despite the evapratin f B, the xidatin f and HfC results in mass gain as the mass f Hf frmed is greater than the mass f and HfC. Thus, a linear mass gain f -HfC was bserved during the isthermal TGA analysis. As expected, examinatin f X-ray diffractin results fr the -HfC xidized at 1500 C revealed that the xide layer (Hf JCPDS # 7-506) was made up almst exclusively f prus Hf, which des nt prvide passive xidatin prtectin. Fig. 8 gives a crss-sectinal analysis f the xidized MHC samples (1500 C fr 0 min, in air) shwing a layered structure: xidized layer (I) and un-reacted -HfC (II) layer. The xidized layer shwed a prus structure cnsisting f spherical particles. Recalling the XRD patterns (Fig. 7 (b)), the phases f the spherical particles are Hf frmed by xidatin f and HfC frmed by Reactin (6). At 1500 C, it is nt pssible fr the Hf xide grains t sinter, and as a result they spntaneusly spall ff the parent matrix (see the interface f regin I and II, in Fig. 8), a prcess knwn as pesting. The Hf frms a prus scale (due t the evlutin f C and B during the xidatin); n sign f a cntinuus prtective layer, which culd impart diffusin cntrlled xidatin prtectin t the underlying materials, was bserved. Thus, xidatin f -HfC at 1500 C shwed linear mass gain as shwn Fig. 7 (a)... Mechanical prperties The relatively density and mechanical prperties f the cmpsites are listed in Table. The variatin in flexural strength was well reflected in the change f relative density and in the micrstructure features. The increase f flexural strength in MHC cmpared with HC might be due t the finer precursrs frmed via vibratin and, thus, the higher density and lwer pre distributin. The fracture tughness f MHC is slightly high cmpared with that f HC. And, the fracture tughness f MHC is higher than that f the ther -SiC cmpsites densified by ht pressing. 1) The Vickers hardness f HC was 15. GPa. Cmpared with HC, the hardness f MHC, was increased t 17.5 GPa due t a situatin f full density with n prsity. Hardness has been shwn t decrease expnentially as the prsity increases fr ceramic materials. 19). Cnclusins This study reprts n reactive pwder synthesis via slidstate precursrs; micrstructure, xidatin behaviur and mechanical prperties were investigated in a mixture f Hf/ B C pwders. The reactins f the pwder mixture cmmenced at 1000 C and finished at 1600 C. The relative density, densified with a vibratin milled pwder mixture, Table. Relative Density and Mechanical Prperties f the Samples Material Relative density (% TD) Elastic mdulus (GPa) Flexural strength (MPa) Vickers hardness (GPa) Fracture tughness (MPam 1/ ) HC ± 15.±0..1±0.8 MHC ±6 17.5±0..5±0.7

6 59 Jurnal f the Krean Ceramic Sciety - Seung Jun Lee et al. Vl. 7, N. 6 was 99.% at 1900 C. The enhanced densificatin was attributed t the frmatin f fine and HfC phases. The reductin f the starting pwder als led t higher mechanical prperties. The mechanical prperties fr the fully dense RHP materials were cmparable t the reprted value. T ur knwledge, this is the first time t investigate reactin mechanisms and xidatin behaviur f -HfC fabricated by reactive ht pressing in rder t imprve the xidatin resistance t a temperature higher than 000 C. The xidatin behaviur f the -HfC cmpsite at 1500 C in air, hwever, was linear xidatin behaviur and the mass gain was higher than that f the -SiC cmpsite. Future wrk will fcus n evaluating the xidatin behaviur f -HfC in extreme cnditins (> 000 C) by using an xyacetylene trch. We expect that at the higher temperature, the xidatin kinetics f -HfC will be parablic due t the densificatin f the xidized Hf and the slwer diffusin f xygen thrugh the dense Hf scale. Acknwledgments This wrk was supprted financially by the Pririty Research Centers Prgram thrugh the Natinal Research Fundatin f Krea (NRF) (N ) and by the Brain Krea 1 (BK1) prgram funded by the Krean Gvernment (MEST). REFERENCES 1. F. Mnteverde and A. Bellsi, The Resistance t xidatin f an -SiC Cmpsite, J. Eur. Ceram. Sc., (005).. S-. K. W, I-. S. Han, H-. S. Kim, E- S. Kang, J-. H. Yang, and C-. H. Kim, Sintering f Zircnium Dibride thrugh Fe-based Liquid Phase, J. Kr. Ceram. Sc., (1996).. A. Rezaie, W. G. Fahrenhltz, and G. E. Hillmas, Evlutin f Structure During the xidatin f Zircnium Dibride-Silicn Carbide in Air up t 1500 C, J. Eur. Ceram. Sc., (007).. S. J. Lee, E. S. Kang, S. S. Baek, and D. K. Kim, Reactive Ht Pressing and xidatin Behavir f Hf-based Ultra- High-Temperature Ceramics, Surf. Rev. Lett., (010). 5. D. M. Van Wie, D. G. Drewry Jr., D. E. King, and C. M. Hudsn, The Hypersnic Envirnment: Required Cnditins and Design Challenges, J. Mater. Sci., (00). 6. S. C. Zhang, G. E. Hilmas, and W. G. Fahrenhltz, Pressureless Densificatin f Zircnium Dibride with Brn Carbide Additins, J. Am. Ceram. Sc., (006). 7. S. C. Zhang, G. E. Hilmas, and W. G. Fahrenhltz, Pressureless Sintering f ZrB -SiC Ceramics, J. Am. Ceram. Sc., (008). 8. D. Sciti, S. Guicciardi, and A. Bellsi, Prperties f a Pressureless-Sintered ZrB -MSi Ceramic Cmpsite, J. Am. Ceram. Sc., (006). 9. W-. W Wu, G-. J. Zhang, Y-. M. Kan, and P-. L. Wang, Reactive Ht Pressing f ZrB -SiC-ZrC Ultra High Temperature Ceramics at 1500 C, J. Am. Ceram. Sc., (006). 10. G-. J. Zhang, Z-. Y. Deng, N. Knd, J-. F. Yang, and T. hji, Reactive Ht Pressing f ZrB -SiC Cmpsites, J. Am. Ceram. Sc., 8 0- (000). 11. F. Mnteverde, Prgress in the Fabricatin f Ultra-High- Temperature Ceramics: in situ Synthesis, Micrstructure and Prperties f a Reactive Ht-Pressed -SiC Cmpsite, Cmps. Sci. Technl., (005). 1. S. J. Lee and D. K. Kim, Effect f TaB Additin n the xidatin Behavirs f ZrB -SiC based Ultra-High Temperature Ceramics, Kr. J. Mater. Res., (010). 1. J. Han, P. Hu, X. Zhang, S. Meng, and W. Han, xidatin- Resistant ZrB -SiC Cmpsites at 00 C, Cmps. Sci. Technl., (008). 1. A. Rezaie, W. G. Fahrenhltz, and G. E. Hillmas, xidatin f Zircnium Dibride-Silicn Carbide at 1500 C at a Lw Partial Pressure f xygen, J. Am. Ceram. Sc., (006). 15. D. K. Shetty, I. G. Wright, P. N. Mincer, and A. H. Clauer, Indentatin Fracture f WC-C Cermets, J. Mater. Sci., (1985). 16. M. J. Gasch, D. T. Ellerby, and S. M. Jhnsn, Handbk f Ceramics Cmpsites, Springer, Chap (005). 17. B. Cech, P. liverus, and J. Sejbal, Sintering f Zircnium Bride with Activating Additins, Pwder Metall., (1965). 18. P. Lespade, N. Richet, and P. Gursat, xidatin Resistance f -SiC Cmpsites fr Prtectin f Carbnbased Materials, Acta Astrnaut., (007). 19. Y. V. Milman, S.I. Chugnva, I.V. Gncharva, T. Chudba, W. Ljkwski, and W. Gch, Temperature Dependence f Hardness in Silicn-Carbide Ceramics with Different Prsity, Int. J. Refract. Met. H., (1999).