ZrB2 Powders Research Articles

Oxidation resistance of ZrB2‐based monoliths using polymer‐derived Si(Zr,B)CN as sintering aid

AbstractThe focus of the present work is the investigation of the influence of polymer‐derived ceramics, used as sintering aids for preparing ZrB2‐based monoliths, on their high‐temperature oxidation behavior. For the preparation of the monoliths, ZrB2 powder was coated with polymer‐derived SiCN, SiZrCN, or SiZrBCN and subsequently densified via hot‐pressing at temperatures as low as 1800°C. To investigate the oxidation kinetics, thermogravimetric analysis (TGA) was performed at 1300°C in synthetic air with exposure times of 50 and 100 h. A detailed study of the materials oxide scale and subsurface microstructure was conducted using optical microscopy, electron probe microanalysis, scanning electron microscopy, and X‐ray diffraction. The experimental findings were compared to thermodynamic equilibrium calculations using the CALPHAD method, which led to a better understanding of the oxidation mechanism. In comparison to the literature data of ZrB2–SiC, the results show improved oxidation resistance for all three investigated materials. The formation of gaseous species during oxidation, in particular CO, CO2, B2O3, and SiO, within the oxide scale of the monoliths was rationalized via CALPHAD calculations and used to explain the oxidation behavior and kinetics and also the formation of bubbles in the subsurface region of the oxidized specimens.

Densification mechanism during hot-pressing of single-phase zirconium boride powders with different dislocation density

ZrB2 powders with different dislocation density were synthesised by the self-propagating high-temperature synthesis (SHS) and carbothermal reduction (CTR) method. The defect structure was analysed by transmission electron microscopy (TEM), and the dislocation density was determined though the convolutional multiple whole profile fitting (CMWP-fit). The SHS process exhibited a very high rate of heating and cooling, which introduces a higher dislocation density (8.68 × 1015 m-2) due to thermal stresses caused by the volume change of crystal, and the crystal grew spirally. Comparably, the CTR process depicted a low rate of heating and cooling. The time was sufficient for crystal nucleation and growth, thus the dislocation density of crystal was lower (6.49 × 1011 m-2), and the crystal grew layer-by-layer. By mixing with the SHS and CTR powders, 5 kinds of powders with different dislocation density were prepared, followed by hot-pressing densification. The dislocation density of powder was calculated and result shows that with the increasing of the content of SHS powder from 0 to 1 the dislocation density increases from 6.49 × 1011 m-2 to 8.68 × 1015 m-2, meanwhile the relative density of ZrB2 ceramics increased from 82.1% to 98.1%. The hot-pressing densification can be divided into two stages. In the pre-sintering stage, with increasing of dislocation density of the powders, the densification mechanism changes from bulk diffusion to grain boundary diffusion, and all are of the dislocation-climb mechanism in the post-sintering stage.

Microstructure evolution, enhanced hardness and toughness in the solid-solution ceramic composite by reaction pressureless sintering of ZrB2 and TiC powders

Fully dense ZrB2–TiC composites were produced by reactive pressureless sintering using ZrB2 and TiC as the starting reactants. The effect of TiC content on microstructure and mechanical properties of ZrB2–TiC composites was investigated. The densification and grain refinement were achieved due to the reaction and solid solution coupling effect. Besides, elongated (Ti, Zr)B2 plate-like grains with a high aspect ratio were obtained for the first time by the reaction of ZrB2 and TiC. The ZrB2-70 mol%TiC composite exhibited good comprehensive mechanical properties of high hardness of 29.0 GPa, flexural strength of 555 MPa, and fracture toughness of 5.4 MPa m1/2 due to grain-refinement and solid solution strengthening, and plate-like grains toughening effects.

Comparative study of B4C, Mg2B2O5, and ZrB2 powder additions on the mechanical properties, oxidation, and slag corrosion resistance of MgO–C refractories

Boron-containing additives are used to improve the oxidation resistance of carbon-containing refractories; however, their effects on the mechanical properties and slag corrosion resistance of the refractories have rarely been studied. In this work, B4C, Mg2B2O5, and ZrB2 powders were incorporated into low-carbon MgO–C refractories to study their effects on the mechanical properties, oxidation resistance, and slag corrosion resistance of the refractories. The relationships between these properties and the microstructure and phase evolution were also studied. The results show that the flexural strengths of the MgO–C refractories at high temperatures are closely related to the apparent porosity and formation of an Mg3B2O6 phase. The oxidation resistances are greatly improved after the introduction of boron-containing additives into the MgO–C refractories in terms of both thermodynamical aspects and the filling of voids and pores. The most effective antioxidant is B4C, followed by the ZrB2 and Mg2B2O5 powders. The mechanisms through which the vanadium-containing slag attacks the MgO–C refractories mainly include the dissolution of magnesia to form melting phases, penetration through pores, and redox reaction with carbon.

Effect of silicon/graphite ratio and temperature on oxidation protective properties of SiC/ZrB2–SiC coatings prepared by pack cementation

To investigate the silicon/graphite ratio and temperature on preparation and properties of ZrB2–SiC coatings, ZrB2, silicon, and graphite powders were used as pack powders to prepare ZrB2–SiC coatings on SiC coated graphite samples at different temperatures by pack cementation method. The composition, microstructure, thermal shock, and oxidation resistance of these coatings were characterized and assessed. High silicon/graphite ratio (in this case, 2) did not guarantee higher coating density, instead could be harmful to coating formation and led to the lump of pack powders, especially at temperatures of 2100 and 2200 °C. But residual silicon in the coating is beneficial for high density and oxidation protection ability. The SiC/ZrB2–SiC (ZS50-2) coating prepared at 2000 °C showed excellent oxidation protective ability, owing to the residual silicon in the coating and dense coating structure. The weight loss of ZS50-2 after 15 thermal shocks between 1500 °C and room temperature, and oxidation for 19 h at 1500 °C are 6.5% and 2.9%, respectively.

Pressureless sintering of zirconium diboride with carbon and boron carbide nanopowder

Zirconium diboride ceramics with and without carbon and boron carbide nanopowder additives were prepared by ball milling with ZrB2 grinding media and pressureless sintering. Additions of up to 1 wt% nano-B4C and 0.5 wt% C were made to the ZrB2 powder. The materials were then sintered between 1800 and 2300 °C for between 90 and 360 min in an Ar/10H2 atmosphere. After sintering at 2200 °C for 90 min, densities ranged from 88.3 to 90.7% for the ZrB2 with 0–1.0% nano-B4C addition. Carbon additions of 0.5 wt% and nano-B4C additions from 0 to 1.0 wt% resulted in densities ranging from 90.9 to 91.9% after sintering at 2100 °C for 90 min. Grain size ranged from 16.6 to 21.7 μm for ZrB2 with nano-B4C content increasing from 0 to 1.0 wt%, sintered at 2200 °C. For the ZrB2 with 0.5 wt% C, increasing the nano-B4C content from 0 to 1.0 wt% resulted in a decrease in grain size from 25.4 to 18.5 μm. The densities achieved in this study were lower than previous pressureless sintering studies of ZrB2 that used WC-6Co grinding media, presumably due to the absence of WC and Co that can also act as sintering aids.

Fully dense ZrB2 ceramics by borothermal reduction with ultra‐fine ZrO2 and solid solution

AbstractThe effects of ZrO2 particle size (55 nm and 113 nm) and borothermal reduction routes (borothermal reduction with water‐washing (BRW) and in situ 5 mol% TaB2 solid solution, BRS) on synthesis and densification of ZrB2 were investigated. Irrespective of reduction routes, the use of finer ZrO2 powders as raw materials resulted in finer ZrB2 powders. Compared to the powders derived from BRS, the powders derived from BRW had smaller particle size with higher oxygen content, especially the powders synthesized with finer ZrO2. Irrespective of ZrO2 particle size, the oxygen contents of ZrB2 powders prepared by the BRS route were similar. Because of the high oxygen content, the ZrB2 ceramics synthesized by BRW with finer ZrO2 demonstrated the lowest relative density (90.5%), which resulted in the lowest Vickers’ hardness (14.2 ± 0.9 GPa). Due to the low oxygen content and small particle size of ZrB2 powders, fully dense ZrB2 ceramics (relative density: 99.6%) with highest Vickers’ hardness (16.0 ± 0.2 GPa) were achieved by BRS with finer ZrO2 powders.

Progress in Preparation of ZrB2 Nanopowders Based on Traditional Solid-State Synthesis.

ZrB2 is of particular interest among ultra-high temperature ceramics because it exhibits excellent thermal resistance at high temperature, as well as chemical stability, high hardness, low cost, and good electrical and thermal conductivity, which meet the requirements of high-temperature components of hyper-sonic aircraft in extreme environments. As raw materials and basic units of ultra-high temperature ceramics and their composites, ZrB2 powders provide an important way for researchers to improve material properties and explore new properties by way of synthesis design and innovation. In recent years, the development of ZrB2 powders’ synthesis method has broken through the classification of traditional solid-phase method, liquid-phase method, and gas-phase method, and there is a trend of integration of them. The present review covers the most important methods used in ZrB2 nanopowder synthesis, focusing on the solid-phase synthesis and its improved process, including modified self-propagating high-temperature synthesis, solution-derived precursor method, and plasma-enhanced exothermic reaction. Specific examples and strategies in synthesis of ZrB2 nano powders are introduced, followed by challenges and the perspectives on future directions. The integration of various synthesis methods, the combination of different material components, and the connection between synthesis and its subsequent application process is the trend of development in the future.

Monolithic ZrB2‐based UHTCs using polymer‐derived Si(Zr,B)CN as sintering aid

AbstractIn the present work, dense ZrB2‐based monoliths were produced via hot‐pressing of ZrB2 powders coated with amorphous polymer‐derived SiCN, SiZrCN, or SiZrBCN at relatively moderate temperatures (i.e., 1800℃). Thus, ZrB2 particles were embedded in polymer‐derived ceramic shell were realized by coating their surface with the corresponding polymeric precursors (polysilazane, Zr‐modified polysilazane, Zr‐ and B‐modified polysilazane, as for SiCN, SiZrCN, and SiZrBCN; respectively), followed by cross‐linking at 200℃ and pyrolysis at 1100℃ in Ar atmosphere. The obtained monolithic samples were carefully characterized concerning their phase composition and microstructure. Typically, the densified samples exhibit a homogeneous microstructure consisting of ZrB2 grains and a multi‐phasic polymer‐derived ceramic grain boundary phase (β‐SiC, t‐BCN, ZrO2, and ZrC). The prepared monoliths exhibited fair hardness values, that is, 12.5, 14.2, and 13.3 GPa for ZrB2/SiCN, ZrB2/SiZrCN, and ZrB2/SiZrBCN.

Preparation of high-temperature active zirconium boride powders via precursor route and microwave sintering

ABSTRACT ZrB2 powders were synthesised by microwave heating using the sol–gel method for precursor with eight C6H14O6 (ZrOCl2·8H2O), boric acid (H3BO3), citric acid (I), and mannitol (as raw materials). The results show that the temperature at which the ZrB2 powder is completely synthesised by microwave heating is 1400°C, which is 300°C lower than the traditional way. XRD and SEM were used to characterise the phase composition and morphology of the powders. In addition, the effects of raw material ratios and synthesis temperature on the synthesis of powder were investigated. The effect of temperature on the purity of the synthesised zirconium boride was considerable. The nearly spherical ZrB2 ultra-fine powders of high purity were synthesised successfully at 1400°C by microwave, with the ratios of C:Zr is 7.44:1, and B:Zr is 2.77:1.

Evaluation of the Temperature Range Suitable for the Synthesis of B4C–TiB2 and B4C–ZrB2 Powder Composite Materials

Evaluation of the Temperature Range Suitable for the Synthesis of B4C–TiB2 and B4C–ZrB2 Powder Composite Materials

Effect of dispersants on the physicochemical properties of ultra-fine ZrB2 powder in Sol-gel synthesis

Boride-based ultra-high temperature ceramics have been extensively utilized as high temperature thermal protection materials due to its excellent physical and mechanical properties. Yet, the synthesis of boride-based ceramic powder with high purity, ultra-fine structure and excellent dispersity remains a great challenge to reach. In this work, zirconium diboride (ZrB2) ultra-high temperature ceramic powder with high purity and ultra-fine structure (size of hundreds of nanometers) was prepared by carbothermal reaction combined with sol-gel method. Various dispersants including oleic acid (OA), polyethylene glycol (PEG) and polyvinyl pyrrolidone (PVP) were employed during the preparation procedure to tune the physicochemical properties of the resultant ZrB2. According to the X-ray diffraction and microscopy characterizations, it was revealed that the OA turned up as the most effective dispersant to obtain the ZrB2 powder with hundreds-nanometer-scale and high purity. Besides, the thermogravimetry-differential thermal analysis and Fourier transformed infrared spectroscopy analysis were performed, unveiling the function of OA in enhancing the dispersity of the ZrB2. Such a work on the effect of dispersant on the preparation of ultra-high temperature ceramic powder could shed some important light for the development of high temperature resistant materials.

Investigation of the parameters of carbon nanotube growth on zirconium diboride supported Ni catalyst via CVD

Carbon nanotubes (CNTs) were synthesized on zirconium diboride (ZrB2) through CVD using a Ni catalyst. The influences of the synthesis temperature, time and catalyst amount on the quality and quantity of grown CNTs were studied. The catalyst content determined the temperature at which the CNTs began to grow. When the Ni catalyst content was low, the temperature at which the CNTs began to grow should be higher, and was 1050 °C for 5 wt% Ni content, but was 950 °C for 15 wt% Ni content. Meanwhile, agglomeration of metal atoms was observed if the synthesis temperature was increased until reaching the upper limit temperature for CNT synthesis, which was 1100 °C for 5 wt% and 10 wt% Ni content, but was 1000 °C for 15 wt% Ni content. Increased Ni catalyst content also led to higher catalytic activity as long as the Ni catalyst content did not exceed 15 wt%. Meanwhile, prolonged synthesis time extended the reaction time for the decomposition of C2H2, but resulted in the agglomeration of carbon atoms when the synthesis time was 150 min. The results showed that the uniform distribution of CNTs can be obtained in ZrB2 powder and there was no reaction between ZrB2 and Ni catalyst. The suitable synthesis conditions for obtained uniform size of CNTs grown with a higher degree of graphitization on ZrB2 that identified in this work were: Ni catalyst content of 10 wt%, synthesis temperature of 1050 °C, and synthesis time of 120 min.

Effect of ZrB2 powders on densification, microstructure, mechanical properties and thermal conductivity of ZrB2-SiC ceramics

With the view to improve the densification behaviour and mechanical properties of ZrB2-SiC ceramics, three synthesis routes were investigated for the production of ZrB2, prior to the fabrication of ZrB2-20 vol. % SiC via spark plasma sintering (SPS). Two borothermal reduction routes, modified with a water-washing stage (BRW) and partial solid solution of Ti (BRS), were utilised, alongside a boro/carbothermal mechanism (BRCR) were utilised to synthesise ZrB2, as a precursor material for the production of ZrB2-SiC. It was determined that reduction in the primary ZrB2 particle size, alongside a diminished oxygen content, was capable of improving densification. ZrB2-SiC ceramics, with ZrB2 derived from BRW synthesis, exhibited a favorable combination of high relative density (98.6%), promoting a marked increase in Vickers hardness (21.4 ± 1.7 GPa) and improved thermal conductivity (68.7 W·m-1K-1).

A universal method for the synthesis of refractory metal diborides

Refractory metal diboride is a kind of excellent ultra-high temperature ceramics and could be applied as thermal shielding material for supersonic aircraft. In this paper, a two-step reaction process for the synthesis of refractory metal diboride powders was proposed using refractory metal oxide, carbon black (C), B4C, and calcium Ca as raw materials. Firstly, the carbothermal reduction reaction between oxides and C was taken place at 1773 K; and then, the reduction product was reacted with B4C and Ca for the purposes of boronization and decarburization. After acidic leaching, pure diboride was prepared. The experimental results indicated that diborides of Ti, Zr, Hf, V, Nb and Ta with low residual carbon content can be prepared by this method. In addition, the inheritance relationship of particle size between raw oxide and finally prepared diboride was revealed. By this method, ultra-fine TiB2 (123 nm) and ZrB2 (76 nm) powders were also successfully prepared, with the raw materials of nano TiO2 and nano ZrO2, respectively.

Microstructure and thermal stability of ZrB2 powder infiltrated by molten Cu and CuCr1Zr alloy

Microstructure and thermal stability of ZrB2 powder infiltrated by molten Cu and CuCr1Zr alloy

Tribo-oxidation and tribological behaviour of ZrB2-20%volSiC composites coupled with WC and Al2O3 at high temperatures

ZrB2-20%volSiC composites were hot pressed with ZrB2 and SiC powders and the effect of temperature and friction couples (WC or Al2O3) on the tribological properties of these composites was investigated. The results show that the coefficient of friction (CoF) of ZrB2–SiC composites coupled with WC or Al2O3 was almost constant with respect to temperature (0.5–0.6). Meanwhile, their wear rates (WRs) were of the order of 10−4 mm3 N−1 m−1. Either discontinuous (with WC) or continuous (with Al2O3) tribo-layers were found to have been formed on the worn surfaces of the composites and these tribo-layers did not exhibit any lubricious or anti-wear effects. At temperatures of 25–400 °C, the wear mechanism of the composites coupled with WC and Al2O3 included abrasion, tribo-oxidation, and adhesion. At higher temperatures (600–800 °C), the wear mechanism of both tribo-pairs included tribo-oxidation and adhesion.

Nanostructural and nanoindentation characterization of ZrB2 ceramics toughened with in-situ synthesized ZrC

A near fully-dense ZrB2–ZrC composite was produced by SPS of ZrB2, Zr, and graphite powders. Advanced electron microscopy characterization was used to investigate the densification and sintering mechanisms, as well as the interfacial phenomena. Mechanical properties were studied through nano-indentation. The synthesis of ZrC is mainly controlled by a high fluidity liquid phase, which plays a crucial role in transferring the Zr atoms on the graphite flakes. Although such a liquid may be solidified as an amorphous interfacial phase during the cooling step, it effectively promotes composite toughening by a homogenous distribution of the in-situ synthesized phases. While a relatively weak hardness result (~11.3 GPa) was measured, a remarkably improved fracture toughness (~5.4 MPa.m1/2) was achieved. The outcomes of nano-indentation confirmed the paradoxical influence of amorphous interfacial phase on the mechanical behavior. Related discussions about the toughening mechanisms, interfacial phenomena, and nano-indentation behavior were also included and graphically presented.

Oxidation kinetics of ZrB2-and HfB2-powders and their SiC reinforced composites

Non-isothermal simultaneous thermal analysis technique elicited the peak oxidation and mass gain shifts to higher temperatures with SiC addition prompting enhanced activation energy (323 kJ/mol to 593 kJ/mol in ZrB2 and 199 kJ/mol to 207 kJ/mol in HfB2). On one hand where the peak oxidation temperature in ZrB2-SiC pellet could not be observed affirming its high oxidation resistance with negligible mass gain, whereas on the other hand the activation energy of HfB2-SiC pellet increased to 266 kJ/mol. HfB2-SiC powder is observed to follow 3D diffusion dominated (Jander equation) mechanism in which mass gain increased linearly from peak oxidation temperature ∼710−730 °C at 10–20 °C/min heating rates till the maximum programmed temperature of 1200 °C. Though mass gain starts and almost ends within a range of 5 °C at any heating rate in the case of monolithic ZrB2 and HfB2 powders and in ZrB2-SiC composite powder at peak oxidation temperature, it (mass gain) remains almost negligible in spark plasma sintered ZrB2-SiC and HfB2-SiC composites.

In situ reaction and solid solution induced hardening in (Ti,Zr)B2‐(Zr,Ti)C composites

Abstract(Ti,Zr)B2‐(Zr,Ti)C ceramics were synthesized by reactive hot‐pressing and solid solution coupling effect using ZrB2 and TiC powders as starting materials. Effects of sintering temperature on phase relations, microstructure and mechanical properties were reported. The equimolar ZrB2 and TiC reactants ensured a complete in situ reaction to form (Ti,Zr)B2 and (Zr,Ti)C solid solutions. The (Ti,Zr)B2‐(Zr,Ti)C composite sintered at 1750°C was fully densified, and exhibited a high hardness of 29.1 GPa due to fine‐grain hardening and solid solution hardening. The optimized comprehensive mechanical properties such as a hardness of 27.9 GPa, a strength of 705 MPa and an indentation fracture toughness of 5.3 MPa m1/2 were achieved in (Ti,Zr)B2‐(Zr,Ti)C composites sintered at 1800°C for 1 hour.