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SCIENCE CHINA Materials, Volume 62 , Issue 12 : 1898-1909(2019) https://doi.org/10.1007/s40843-019-9469-4

Dense and pure high-entropy metal diboride ceramics sintered from self-synthesized powders via boro/carbothermal reduction approach

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  • ReceivedApr 9, 2019
  • AcceptedJul 1, 2019
  • PublishedJul 31, 2019

Abstract

Equimolar quinary diboride powders, with nominal composition of (Ti0.2Hf0.2Zr0.2Nb0.2Ta0.2)B2, were synthesized by boro/carbothermal reduction (BCTR) of oxide mixtures (MOx, M = Ti, Hf, Zr, Nb and Ta) using B4C as source of B and C in vacuum. By adjusting the B4C/MOx ratios, diboride mixtures without detectable MOx were obtained at 1600°C, while high-entropy diboride (HEB) powders with particle size of < 1 μm was obtained at 1800°C. The phase, morphology and solid solution evolution process of the HEB powders during the BCTR process were comprehensively investigated. Although X-ray diffraction pattern indicated the powders synthesized at 1800°C were in a single-phase AlB2 structure, elemental mappings showed that (Ta, Ti)-rich and (Zr, Nb)-rich solid solution coexisted in the HEB powders. The distribution of niobium and zirconium atoms in HEB was unable to reach uniform until the HEB powders were spark plasma sintered at 2000°C. (Ti0.2Hf0.2Zr0.2Nb0.2Ta0.2)B2 ceramics with a relative density of 97.9% were obtained after spark plasma sintering the HEB powders at 2050°C under 50 MPa. Rapid grain growth was found in this composition when the sintering temperature was increased from 2000 to 2050°C, and the averaged grain size increased from 6.67 to 41.2 μm. HEB ceramics sintered at 2000°C had a Vickers hardness of 22.44 ± 0.56 GPa (under a load of 1 kg), a Young’s modulus of ~500 GPa and a fracture toughness of 2.83 ± 0.15 MPa m1/2. This is the first report for obtaining high density HEB ceramics without residual oxide phase, benefiting from the high quality HEB powders obtained.


Funded by

the National Natural Science Foundation of China(51521001,51832003)

and the Fundamental Research Funds for the Central Universities.


Acknowledgment

This work was financially supported by the National Natural Science Foundation of China (51521001 and 51832003), and the Fundamental Research Funds for the Central Universities.


Interest statement

These authors declare no conflict of interest.


Contributions statement

Zou J and Fu ZY designed the research. Gu J and Zou J performed the experiments with the help of Wang H, Wang W, Zhang J and Fu ZY. Sun SK performed the XRD refinement. Yu SY and Zou J performed the hardness test. Fu Z supervised and acquired funding for the research. Gu J and Zou J wrote the paper. All authors discussed the results and commented on the manuscript.


Author information

Junfeng Gu is currently a PhD student at the State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology. His research focuses on the sintering mechanisms and properties of ultra-high temperature ceramics.


Ji Zou is a Research Fellow at the University of Birmingham, UK and an adjunct professor at Wuhan Institute of Technology, China. He received his PhD degree from Shanghai Institute of Ceramics, Chinese Academy of Sciences. He has been active in the processing-structure-property correlation of ceramics, especially for boride ceramics.


Zhengyi Fu is a chief professor at the State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology. He received his PhD degree from Wuhan University of Technology in 1994. His current research interests include advanced sintering and bioprocess-inspired fabrication.


Supplement

Supplementary information

Supporting data are available in the online version of this paper.


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  • Figure 1

    (a) Molar content of the products in equilibrium as a function of the temperature from room temperature (RT) to 2000°C at a partial pressure of 10 Pa. The input amounts of the reactants are based on Reaction (1). (b) The effects of B4C amount on the progress of reactions in MOx-B4C system at 1600°C.

  • Figure 2

    XRD patterns of (a, b) HEB powders synthesized at 1800°C and (c, d) HEB3 powders synthesized at 1300–1800°C.

  • Figure 3

    Back scattered electron (BSE) image of the (a) HEB3-13 and (b) HEB3-16 powders, showing the presence of unreacted oxides, unreacted B4C and newly formed ultra-fine diboride particles as-arrowed. (c, d) Higher magnification images of the ultra-fine diboride particles in (a) and (b). (e) SEM image of the HEB3-18 powders.

  • Figure 4

    HRTEM images of HEB3 powders synthesized at (a) 1300 and (c) 1800°C. The fast Fourier transform images of (a) and (c) were inserted as (b) and (d).

  • Figure 5

    (a) TEM image of the HEB3-13 powders shows an agglomeration of newly formed ultra-fine diborides grains. STEM-mappings of one of the agglomerate regions indicate the quinary solid solution reaction had occurred at 1300°C. (b) TEM image and STEM-mappings of the HEB3-18 powders reveal the unevenly atomic distribution within grains. Note that some circled regions have different concentrations of Nb.

  • Figure 6

    (a) BSE image of the HEB3-13 powders and corresponding EDS-mappings, (b–d) point EDS results showing the presence of (Ti, Ta)-rich regions, (Zr,Hf)-rich regions and unreacted (Zr,Hf)O2. The circled regions in the Nb, Zr and Hf mappings revealing the ZrNb-rich smaller particles are superimposed on the ZrHfNb-rich area.

  • Figure 7

    (a) BSE image and corresponding EDS-mappings of HEB3-16, and (b) Rietveld refinement of the XRD patterns of HEB3-16 using MAUD software (Rwp = 9.29%).

  • Figure 8

    Polished surfaces of HEB3 ceramics sintered at (a) 1900°C, (b) 2000°C, and (c) 2050°C.

  • Figure 9

    XRD patterns of the three ceramics.

  • Figure 10

    WDS-mappings of the specimens sintered at (a) 1900°C, (b) 2000°C, and (c) 2050°C, indicating the uneven elemental distribution at 1900°C while the elemental distributions at 2000 and 2050°C are homogenous.

  • Table 1   Sample designations

    Composition

    TiO2/ZrO2/HfO2/Nb2O5/Ta2O5/B4C

    Excess B4C

    1300°C

    1400°C

    1500°C

    1600°C

    1700°C

    1800°C

    HEB1a

    2/2/2/1/1/7.43

    0 wt%

    HEB1-13

    HEB1-14

    HEB1-15

    HEB1-16

    HEB1-17

    HEB1-18

    HEB2

    2/2/2/1/1/7.8

    5 wt%

    HEB2-13

    HEB2-14

    HEB2-15

    HEB2-16

    HEB2-17

    HEB2-18

    HEB3

    2/2/2/1/1/8.17

    10 wt%

    HEB3-13

    HEB3-14

    HEB3-15

    HEB3-16

    HEB3-17

    HEB3-18

    HEB4b

    2/2/2/1/1/8.4

    13 wt%

    HEB4-13

    HEB4-14

    HEB4-15

    HEB4-16

    HEB4-17

    HEB4-18

    According to Reaction (1); b) according to Reaction (2).

  • Table 2   Oxygen and carbon contents in the selected HEB powders

    Composition

    Oxygen content (wt%)

    Carbon content (wt%)

    HEB3-16

    7.52

    0.18

    HEB3-17

    0.64

    0.041

    HEB3-18

    0.52

    0.032

  • Table 3   Properties of the high-entropy diboride ceramics

    Composition

    Sinteringtemperature (°C)

    Relativedensity (%)

    Average grain size (µm)

    E (GPa)

    KIC (MPa m1/2)

    Hardness (GPa)

    Ref.

    (Ti0.2Zr0.2Nb0.2Hf0.2Ta0.2)B2

    2000

    94.4

    6.67 ± 1.20

    500

    2.83 ± 0.15

    25.61 ± 0.83 (HV0.2)

    22.44 ± 0.56 (HV1)

    This work

    (Ti0.2Zr0.2Nb0.2Hf0.2Ta0.2)B2

    2050

    97.9

    41.2 ± 8.1

    527

    /

    26.82 ± 1.77 (HV0.2)

    This work

    (Ti0.2Zr0.2Nb0.2Hf0.2Ta0.2)B2

    2000

    92.4

    /

    /

    /

    17.5 ± 1.2 (HV~1)

    [22]

    (Ti0.2Zr0.2Nb0.2Hf0.2Ta0.2)B2

    2000

    96.3

    1.59

    /

    4.06 ± 0.35

    21.7 ± 1.1 (HV0.2)

    [49]

    TiB2

    1800

    96.1

    /

    497 ± 15

    4.64 ± 0.45

    28.6-32.4 (HV1)

    [50]

    ZrB2

    2000

    99.8

    ~6

    489

    3.5 ± 0.3

    23 ± 0.9 (HV1)

    [51]

    NbB2

    1900

    97.7

    /

    539

    4.0 ± 0.6

    20.3 ± 0.6 (HV0.5)a

    [52]

    HfB2

    1850

    98.8

    8

    /

    3.5 ± 0.5

    18.1 ± 0.9 (HV1)

    [53]

    TaB2

    1850

    96

    5.3

    /

    5.61 ± 0.17

    25.1 ± 0.5 (HV1)

    [54]

    HV1 data is not available.

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