Influence of sol–gel derived ZrO2 and ZrC additions on microstructure and properties of ZrB2 composites

https://doi.org/10.1016/j.jeurceramsoc.2014.04.025Get rights and content

Abstract

ZrB2 powder was coated with 5% Zrsingle bondOsingle bondC sol–gel precursor and sintered by SPS. Relative densities >98% were achieved at 1800 °C with minimal grain growth and an intergranular phase of ZrC. Carbon content in the precursor determined the type of reinforcing phase and porosity of the sintered composites. XRD, SEM and EDS studies indicated that carbon deficiency resulted in ZrO2 retention, improving ZrB2 densification with oxide particle reinforcement. Excess carbon resulted in ZrC formation as the reinforcing phase, but could yield porosity and residual carbon at grain boundaries. These two types of ZrB2 composites displayed different densification and microstructural evolution that explain their contrasting properties. In the extreme oxidative environment of oxyacetylene ablation, the composites with ZrC-C maintained superior leading edge geometry; whereas for mechanical strength, a bias towards the residual ZrO2 content was beneficial. This highlighted the sensitivity of processing carbon-precursors in the initial sol–gel process and the carbon content in ZrB2-based composite systems.

Introduction

Ultra High Temperature Ceramics (UHTCs) are materials with melting points exceeding 3000 °C, due to strong covalent bonds that yield excellent hardness and thermo-mechanical strength. ZrB2 materials are candidates for extreme environments such as solar receivers, nuclear energy, Sharp Hot Structures (SHS) or Wing Leading Edges (WLEs) for hypersonic flight (>2000 °C).1, 2, 3 A major processing challenge is attaining high-density without incorporating low melting point sintering additives, or metals that depreciate thermal performance.4, 5 A common example of a highly refractory composition ZrB2–SiC. Addition of SiC improves oxidation resistance up to 1800 °C, but composites may still have limited service temperatures due to a eutectic at 2270 °C.6, 7 These composites are also viable because SiC-precursor technology is mature in fibre, whisker, solution/polymer form, whereas UHTC carbides – and particularly diborides – are still emerging.8, 9 There have been several powder UHTC compositions with very high thermal stability such as NbB2–NbC, ZrB2–TiC and ZrB2–ZrC, which have been densified by B and C diffusion without the addition of metals or silicides.10, 11, 12 In developing high temperature oxidation resistance, there is potential for ZrC oxidation to provide protection at higher temperatures not achievable by SiC.13, 14, 15 However, the composition and microstructure must be carefully controlled; the synergy is focused on improving the intrinsic oxidation resistance of diboride materials. A small vol% UHTC phase dispersed in a powder could provide both improvement in densification and sacrificial oxidation. Previous work on ZrB2 composites reported successful sintering of a ZrB2–ZrC composite, without the milling process that introduces contaminants that can adversely affect other properties. Instead, the powder had a 5 mol% (4.6 wt%) nano-ZrC coating onto the surface of ZrB2 derived from a sol–gel precursor.16 Until pre-polymer UHTC precursors become commercially available, sol–gel is a useful alternative. Control of the decomposition of the carbon precursor – polyfurfuryl alcohol – in sol–gel was not only crucial in the formation of ZrC, but was also used to remove the surface oxide films on ZrB2.16 The differences in properties caused by changing the carbon content were not explored in detail.

Non-oxide ceramics can use a small amount of carbon as a prudent sintering additive if refractory performance is important. Carbon can also improve toughness in boride ceramics through microcracking.17 It can be introduced through other polymer precursors, such as phenolic resin, sucrose and chitosan, which can homogenously coat ceramic powders.18, 19, 20 The decomposition (also called pyrolysis or carbonisation) to carbon must be controlled to ensure subsequent densification is not adversely affected. Control at the precursor stage requires understanding of chemical reactions, polymer pyrolysis and empirical knowledge of carbon yield. After carbon is formed, the interaction of carbon with particles, phase boundaries, and the graphitization process must be considered.21, 22 The processing of carbon and nano-oxides on the surface of ZrB2 powders is the major focus of this paper. The variable carbon yield in this system produces two contrasting phases and morphologies: ZrB2 with 5 mol% ZrO2 or ZrB2 with 5 mol% ZrC + C. This work examines in detail the microstructure of both compositions and briefly explores their properties.

Section snippets

Powder preparation

The powder-gel process is shown schematically in Fig. 1. A block co-polymer surfactant P123 (Sigma–Aldrich) was dissolved in 99.7% ethanol. Zirconium n-propoxide (ZNP) (Sigma–Aldrich) 70 wt% in 1-propanol was chelated with acetylacetone (AcAc). The composition ratio of P123:EtOH:AcAc:ZNP in the sol was 0.05:40:1:1, yielding a slightly yellow sol. Furfuryl alcohol (C5H6O2) (FA) (Sigma–Aldrich) was added at an FA:Zr molar ratio of at least 4:1 turning the sol an intense yellow. This theoretically

Sol–gel coated powders

We previously reported a sol–gel process to produce nano-ZrC particles using ZNP and FA as the respective oxide and carbon precursors.15 Using a surfactant, the gel precursor was fired at 550 °C. This decomposed PFA to amorphous carbon and crystallized 4 nm ZrO2. Heat treatment at 1450 °C in Ar yielded ∼50 nm ZrC nanoparticles, which typically formed micron size agglomerates, particularly if residual carbon was present.34, 35 These needed to be dispersed in a composite, so the sol–gel process was

Conclusions

A sol–gel derived nanoparticle coating can be applied on ZrB2 powders to form <200 nm ZrC on the surface of ZrB2. These can be formed into dense composites by SPS. Carbon is crucial in the evolution of the reinforcing phase. Excess carbon guarantees formation of very fine ZrC nanoparticles due to carbon preventing their coalescence. If carbon is insufficient the carbothermal reduction cannot complete and ZrB2 is coated with ∼300 nm nano-ZrO2 particles. Densification is still assisted by

Acknowledgements

The authors would like to thank Sonya Slater (DSTO), Daniel Curtis (Monash) and Don Rodrigo (Monash) for technical assistance and discussion. The authors acknowledge the use of facilities within the Monash Centre for Electron Microscopy (MCEM), DSTO Aerospace Division and CSIRO Process Science and Engineering. This research used equipment funded by the following Australian Research Council LIEF grants LE0882821 and RIEFP99, and the CoE for Design in Light Metals.

References (45)

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