ZrB2–SiC based composites for thermal protection by reaction sintering of ZrO2+B4C+Si

Journal of Advanced Ceramics 2017, 6(4): 320–329 https://doi.org/10.1007/s40145-017-0244-2 ISSN 2226-4108 CN 10-1154/TQ Research Article ZrB2–SiC based composites for thermal protection by reaction sintering of ZrO2+B4C+Si R. V. KRISHNARAO*, V. V. BHANUPRASAD, G. MADHUSUDHAN REDDY Defence Metallurgical Research Laboratory, Kanchanbagh, Hyderabad-500058, India Received: June 20, 2017; Revised: August 23, 2017; Accepted: August 30, 2017 © The Author(s) 2017. This article is published with open access at Springerlink.com Abstract: Synthesis and sintering of ZrB2–SiC based composites have been carried out in a single-step pressureless reaction sintering (PLRS) of ZrO2, B4C, and Si. Y2O3 and Al2O3 were used as sintering additives. The effect of ratios of ZrO2/B4C, ZrO2/Si, and sintering additives (Y2O3 and Al2O3), was studied by sintering at different temperatures between 1500 and 1680 ℃ in argon atmosphere. ZrB2, SiC, and YAG phases were identified in the sintered compacts. Density as high as 4.2 g/cm3, micro hardness of 12.7 GPa, and flexural strength of 117.6 MPa were obtained for PLRS composites. Filler material was also prepared by PLRS for tungsten inert gas (TIG) welding of the ZrB2–SiC based composites. The shear strength of the weld was 63.5 MPa. The PLRS ZrB2–SiC composites exhibited: (i) resistance to oxidation and thermal shock upon exposure to plasma flame at 2700 ℃ for 600 s, (ii) thermal protection for Cf–SiC composites upon exposure to oxy-propane flame at 2300 ℃ for 600 s. Keywords: ZrB2; SiC; reactive sintering; synthesis; composites 1 Introduction Zirconium diboride (ZrB2) is well known for its unique combination and high values of properties: melting point, chemical stability, hardness, strength, thermal conductivity, and electrical conductivity. It is useful for extreme thermal and chemical environments existed in hypersonic flight, rocket propulsion, and atmospheric re-entry [1–3]. For the last decade, the research on synthesis and sintering of ZrB2 based composites have been accelerated because ZrB2 is being considered for high speed aircraft leading edges, and for structural parts in high temperature environments. The effect of different additives and open porosity on fracture toughness and  *Corresponding author. E-mail: thermal shock resistance of ZrB2–SiC based composites prepared by spark plasma sintering (SPS) was reported [4,5]. Addition of carbon short fibers is shown to affect the densification and grain growth of ZrB2–SiC based composites prepared by hot pressing (HP) [6,7]. Similarly, addition of AlN and nano-sized carbon black effects the densification and mechanical properties of HP ZrB2–SiC based composites [8,9]. However, the high cost of ZrB2 powders and difficulty in shaping large size components by SPS, HP, and fabrication by joining limit the usage of ZrB2–SiC based composites. Variety of synthesis routes which include: (i) reduction processes [10–12], (ii) chemical routes [13], and (iii) reactive processes [14] can be resorted to prepare ZrB2 powders using ZrO2 as a source of zirconium. The reduction route is relatively much cheaper than other routes for ZrB2 synthesis. ZrO2 can be reduced with B2O3+C, B4C+C, or elemental boron. ZrC, C, and B are the typical impurities. ZrB2 obtained www.springer.com/journal/40145 J Adv Ceram 2017, 6(4): 320–329 321 is agglomerated and requires extensive milling/pulverization to decrease the particle size to improve its sinter ability. But impurities from materials used for milling and oxygen from surface oxidation of particles introduced during pulverization deteriorate the densification behavior and properties of ceramics. The reduction of ZrO2 with B4C was studied extensively [15]. Source of carbon and reaction atmosphere affect the synthesis temperature and morphology of ZrB2 [16]. Yuan et al. [17] prepared porous ceramics of ZrB2 by two‐step sintering method, using spark plasma sintering–reactive synthesis. ZrB2 porous ceramics were first synthesized using ZrO2 and B4C as precursors, and then sintered to ZrB2 porous ceramics [18]. In our previous work, B4C reduction of ZrO2 to form impurity (ZrC, C)-free ZrB2 was reported [19]. Further, composite powders of ZrB2–SiC with particle sizes ranging from sub-micron to nanometer have been produced by rapid heating a mixture of ZrO2+B4C+Si, in an air furnace [19] and in air without using any furnace [20]. As mentioned above, ZrB2 is being considered for high speed aircraft leading edges, and for structural parts in high temperature environments. The peak thermal stress of ultra high temperature ceramic (UHTC) wing leading edge (WLE) under re-entry heating conditions is predicted to be 80 MPa. It is well below the strength of pressureless sintered (PLS) UHTCs [21]. Heat resistant ceramic parts like ceramic aero-shell that protects spacecraft or hypersonic aircraft from heat, pressure, and debris are now 3D printable [22]. Ceramic foams are attractive for this application, but their poor mechanical properties make them unsuitable. 3D printed leading edge ceramic lattice structures are 10 times stronger than commercially available foams [23]. For thermal protection system (TPS) application, high mechanical performance is not required while oxidation resistance is the main material requirement. ZrB2–SiC based multilayer materials are produced by tap casting and sintering without pressure assistance for aerospace applications. A three-level multifunctional TPS was developed with external part constituted by ceramic multilayer based on ZrB2–SiC which in turn brazed to Cf–SiC composites and Si–SiC foams [24]. In our previous work, pressureless sintering (PLS) of ZrB2–SiC–B4C composites with Y2O3+Al2O3 addition has been reported [25]. The composites exhibited good dimensional stability and thermal shock resistance at 2200 ℃ in oxy-acetylene flame and at 2700 ℃ in plasma flame. In the present study, an attempt is made to synthesize and sinter ZrB2–SiC based composites in a single-step PLRS using ZrO2, B4C, and Si for synthesis and Y2O3 and Al2O3 for sintering. Similarly, filler rods/wires were made for TIG welding of ZrB2–SiC based composites. The resulted ZrB2–SiC based composite is exposed to plasma flame and oxy-propane flame to study its oxidation and thermal protection of carbon fibre reinforced silicon carbide (Cf–SiC). 2 Experimental PLRS of ZrB2–SiC composites has been carried out using ZrO2 and B4C with two different ratios of 1.6 and 2.0, Si, and sintering additives (Y2O3 and Al2O3). ZrO2 powders of size 325# (97.1%) were supplied by Nuclear Fuel Complex, Hyderabad, India. B4C powders of sinterable grade 1–2 µm size were supplied by China Abrasives, Zing Zhou, China. The details of purity of ZrO2 and B4C were reported elsewhere [19,25]. Elemental Si of 325# was supplied by the Metal Powder Company Ltd., Thirumangalam, India. Al2O3 of super fine size (d50  0.7 μm) obtained from Alcan and submicron-sized Y2O3 were used. Af (...truncated)