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Development of High Temperature TiB2-Based Ceramics

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Abstract:

In view of potentiality of titanium diboride (TiB2) materials for high temperature applications, this paper presents the state of art on the processing, microstructure and properties of TiB2 ceramics. TiB2 ceramics are of interest for applications such as cutting tools, wear resistant parts, armor material and electrode materials in metal smelting because of their excellent combination of properties including high hardness, elastic modulus, better strength to weight ratio, wear resistance, good thermal and electrical conductivity. However, such broader applications of the monolithic TiB2 are inhibited by factors like high sintering temperature and poor toughness. Hence, research efforts are directed towards processing TiB2 at lower sintering temperature as well as to enhance the properties with the use of various metallic and non-metallic sinter-additives. In the above perspective, we review the existing literature in this article. In addition, our recent research results obtained with TiB2-TiSi2 materials are also presented. A review of research results revealed that a combination of room temperature properties, i.e. maximum Vickers hardness of 31 GPa and indentation toughness of 11 MPa m1/2 and flexural strength of 810 MPa is obtainable with optimally sintered TiB2. More importantly, a maximum hardness of 9 GPa (at 900oC) and flexural strength of 471 MPa can be retained upto 1200oC. From the perspective of oxidation resistance, TiB2 samples exhibit parabolic oxidation kinetics below 1000oC as result of the formation of TiO2 (s), and B2O3 (l) and linear oxidation kinetics above 1000oC in the presence of crystalline TiO2 and volatile B2O3.

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Key Engineering Materials (Volume 395)

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89-124

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https://doi.org/10.4028/www.scientific.net/KEM.395.89

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October 2008

Authors:

G.B. Raju, B. Basu

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Microstructure, Property Analysis, Sintering, TiB2

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[98] 6 31. 3 5. 9 262 (4-P) 33 TiB2-6. 0%Cu SPS, 1500oC, 15 minutes.

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[98] 6 16. 7 10. 9 … 40 TiB25. 0%AlN HP, 1800oC, 1 h, 30 MPa, Ar.

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[98] 0 22. 0 6. 8 650 (4- P) 48 TiB2-2. 5%SiC PS, 1700oC, Vacuum + HIP at 1600o C.

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[99] 0 … 4. 3 660 ( 3- P) 49 TiB25. 0%TaC HP, 2000o C, 60 min.

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[99] 0 27. 0 5. 1 810 (4-P) 46 TiB2-5%-ZrO2 (2Y)-5%SiC HIP, 1600oC 98. 0 23. 1 5. 2 1280 (3- P) 54 TiB2-3 %CrB2 MW, 2100oC, 30 min, Ar.

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[16] 1 11. 3 - 7. 3 4. 8 - 85 4 TiB2-7wt%Ti- 5vol% (Fe+Cr+Ni).

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[20] 4 15. 6 - 10. 3 6. 6 - Bend Strength Material Composition (in wt%) Relative density (% ρρρρth) Bend test conditions RT 500oC 1000oC 1200oC Ref TiB2 >95 3-point, Argon 310 - 370 405 30 TiB2 99. 3 " 290 305 390 400 " TiB2 99. 5 3-point, Air 400 429 459 471 11 TiB2 94. 4 4-point, Air 365 287 268 - 70 TiB2 96. 4 4-point, Air 375 422 546 - " TiB2-2. 5TiSi2 98. 8 4-point, Air 381 - 433 - " TiB2-5. 0TiSi2 99. 6 4-point, Air 426 479 314 - , Table 5. Comparison of the weight gain of TiB2 based materials at various oxidizing conditions. Fig. 1. Ti-B binary equilibrium phase diagram, indicating the formation of TiB, Ti3B4 and TiB2 intermetallic compounds [13]. Material (wt. %) Oxidation conditions Weight gain mg/cm 2 Reference TiB2 1000o C, 30h 9. 3 89 TiB2 1100o C, 20h 9. 6 89 TiB2-10. 54Fe-1. 6Fe, 50Al5. 92Fe, 50Ti-10. 34Ni, 41Al 1000o C, 70h 71. 1 93 TiB2-2. 5Si3N4 1200o C, 10h 11. 0 91 TiB2-20 Vol%B4C-1vol%Ni 1300 o C, 30h 34. 0 92 Fig. 2. The hexagonal unitcell of single crystal TiB2, (AlB2-type, p.6.

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[33] 100 nm 100 nm TiSi2 (b) (a) Fig. 4. (a) Bright field (BF) TEM image of TiB2-20 wt% MoSi2 (hot pressed at 1700 o C) revealing the presence of TiSi2 liquid phase at grain-boundary triple pocket (indicated by an arrow) [42] and (b) BF TEM Image of TiB2-2. 5 wt% Si3N4 exhibiting the presence of crystalline BN phase and amorphous SiO2 [45]. 20 µµµµm Fig. 5. (a) The hardness of TiB2 samples as a function of temperature and (b) the linear fit of hardness data of TiB2-2. 5 wt% TiSi2 shows the exponential dependency of hardness on temperature [70]. 0 300 600 900 1200.

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[3] 8 (b) lnHv (GPa) Temperature (K) TiB2-2. 5TiSi2 ττττ=843K 0 200 400 600 800 1000 5 10 15 20 25 30 (a) Temperature ( o C) Hardness (GPa) TiB2 [11] TiB2-2. 5 TiSi2 [70] TiB2 Cermet [85] TiB2 Cermet [85] Fig. 6. Effect of temperature on the flexural strength of TiB2 samples. 0 200 400 600 800 1000 1200 250 300 350 400 450 500 550 Temperature ( o C) Bend strength (MPa) TiB2 [30] TiB2 [11] TiB2 [70] TiB2 [70] TiB2-2. 5TiSi2 [70] TiB2-5. 0TiSi2 [70] Fig. 7. Fracture surfaces of hot pressed monolithic TiB2 (a) at RT, (b) at 1000oC, and TiB2-5 wt% TiSi2 (c) at RT and (d) at 1000oC [70]. The corresponding EDS of the samples bend tested at 1000oC shows the presence of Ti, O and B elements irrespective of the sample composition. 5 µm (c) 5 µm (d) 5 µm (a) 5 µm (b) Ti Ti O B Ti Ti O B Fig. 8. (a) X-ray mapping of monolithic TiB2 (oxidized at 850 oC for 64 h) cross section image, (b) X- ray map of boron, (c) X- ray map of oxygen and (d) X- ray map of titanium [43]. (a) (b) Fig. 9. (a) SEM images revealing the nature of oxide scale of TiB2 cermet showing large grains of Fe3Ti3O10 and spherical grains of TiO2 after oxidation at 850 o C for 70 h [93] and (b) TiB2 - 2. 5 wt% Si3N4 composite revealing large spheroidal and highly textured crystalline TiO2 phase after oxidation at 1200 o C for 10 h [91].

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