Abstract
For this study, (Ti,Nb)B2 solid solutions were consolidated by spark plasma sintering. In addition, (Ti,Nb)B2 with graphene nanoplatelets (GNPs) and hexagonal boron nitride (h-BN) were produced to evaluate the potential of the new structural materials. The phase formation, microstructure, mechanical properties, oxidation resistance and room temperature reflectance, and absorbance features of (Ti,Nb)B2 were investigated. X-ray diffraction and Transmission electron microscopy observations showed that a complete solid solution phase was formed when the samples were sintered at 1850 °C for 5 min under 50 MPa. Ti0.75Nb0.25B2 exhibited a relative density of ∼98.6%, a hardness of ∼20.5 GPa, and an indentation fracture toughness of ∼3.4 MPa·m1/2. It was found that the presence of 1 vol% h-BN as an additive enhanced the hardness (∼10%) and fracture toughness (∼30%) of Ti0.75Nb0.25B2 by activating toughening mechanisms. The GNP added Ti0.75Nb0.25B2 proved to have better oxidation resistance and optical absorbance than the other materials used in the study.
| Original language | English |
|---|---|
| Pages (from-to) | 5582-5594 |
| Number of pages | 13 |
| Journal | Ceramics International |
| Volume | 49 |
| Issue number | 4 |
| DOIs | |
| Publication status | Published - 15 Feb 2023 |
Bibliographical note
Publisher Copyright:© 2022 Elsevier Ltd and Techna Group S.r.l.
Funding
This work was supported by the Istanbul Technical University [grant number BAP-42163 ]. The authors thank Prof. Dr. G. Goller for production and characterization facilities, Prof. Dr. M. Urgen for reflectance measurements and Raman analysis, Asst. Prof. Dr. Nuri Solak and Doga Bilican, PhD for EDS analysis, and H. Sezer for microstructural investigations. Also, the authors thank Selcuk University, Advanced Technology Research and Application Center for TEM investigations. Moreover, Melis Kaplan Akarsu is thankful to the Scientific and Technological Research Council of Turkey (TUBITAK) and Council of High Education (YOK) for 2211/C Domestic Priority Areas Doctoral Scholarship and 100/2000 PhD Scholarship, respectively. Fig. 3 shows the fracture surfaces of the 25Ti–75Nb, 50Ti–50Nb, and 75Ti–25Nb solid solutions. The backscattered electron (BSE) images of the fracture surfaces have no compositional contrast, and this can also be considered as evidence for the formation of a complete solid solution. The enhanced densification that occurred from ∼95.4% to ∼98.6% was supported by the SEM images of the solid solutions. The pores are located at the grain boundaries of the less densified Ti0.25Nb0.75B2 (Fig. 3a). The fracture mode of the (Ti,Nb)B2 solid solutions was a mixture of intergranular and transgranular fractures. The high magnification image of the Ti0.75Nb0.25B2 shows the characteristic cleavage planes of the transgranular fracture mode (Fig. 3d).The compositional homogeneity and the reinforcement-matrix interphase of 75Ti–25Nb-G (Fig. 6a–e) were investigated by TEM analysis. In addition, a TEM image of the GNPs is given in Fig. 6f. The elemental distribution of the sample is given in Fig. 6b, c, and d. A composition of 53.9% Ti, 17.8% Nb, 19.1% B, and 9.2% C (in at%) were determined by EDS analysis (Fig. 6). A homogenous distribution of elements was observed in the composition of Ti0.75Nb0.25B2. As shown in Fig. 6a and e, the GNPs were located at the grain boundary of the (Ti,Nb)B2 matrix, which is also supported by the EDS mapping of C (Fig. 6b, the white dotted line indicates the sample boundary. Due to the carbon-based structure of the holder, the mapping revealed carbon on the surface of the holder).The EDS analysis of 75Ti–25Nb–B is given in Fig. 6g–i. A uniform distribution of the Ti and Nb elements was observed in 75Ti–25Nb–B (Fig. 6h and i). TEM images showed that h-BN is located at the grain boundaries of the matrix grains (Fig. 6j and k). The multilayer stacked structure of h-BN was observed in the TEM image (indicated by white arrows in Fig. 6j), which also supported the overlapping of h-BN in the SEM image (indicated by the dotted yellow line in Fig. 5d).The oxidation mechanism is affected by chemical composition, testing conditions, processing, and morphological properties (i.e., density and grain size). Oxidation is a diffusion-controlled process and occurs more quickly in higher defect regions such as grain boundaries. When considering that the relative densities are similar for 75Ti–25Nb, 75Ti–25Nb-G and 75Ti–25Nb–B, grain size and composition become effective parameters for the oxidation of a (Ti,Nb)B2 system as samples with a larger average grain size have less oxygen diffusion [62]. As expected, 75Ti–25Nb had the best oxidation resistance due to it having the highest average grain size, but 75Ti–25Nb-G produced the best performance with a ∼30 μm oxide layer thickness (Fig. 10a). Furthermore, a ∼40 μm oxide layer was produced by 75Ti–25Nb (Fig. 9a). Fig. 9b shows the transition metal oxides morphology in 75Ti–25Nb. The point EDS analysis also supported the formation of metal oxides and B2O3 (Fig. 9b). As can be seen in the elemental maps of 75Ti–25Nb, Ti-depleted and Nb-depleted regions were formed (Fig. 9, Ti and Nb mapping). Desmaison et al. [63] reported the formation of a Ti- depleted layer for an equimolar TiB2–AlN composite. The diffusion of the Ti into the sample surface during oxidation could be the cause of a depleted layer [24]. The continuous and dense passivation layer of TiO2 prevents oxygen diffusion into the bulk of the material below 600 °C [64]. The passivation layer deteriorates with continued, and uncontrolled reaction or absorption with both parabolic and linear oxidation above 850 °C. At this stage, ions are transported faster than the chemical reaction rate in a cracked or porous oxide layer [65]. Therefore, the oxide thickness of 75Ti–25Nb is higher than that of 75Ti–25Nb-G.This work was supported by the Istanbul Technical University [grant number BAP-42163]. The authors thank Prof. Dr. G. Goller for production and characterization facilities, Prof. Dr. M. Urgen for reflectance measurements and Raman analysis, Asst. Prof. Dr. Nuri Solak and Doga Bilican, PhD for EDS analysis, and H. Sezer for microstructural investigations. Also, the authors thank Selcuk University, Advanced Technology Research and Application Center for TEM investigations. Moreover, Melis Kaplan Akarsu is thankful to the Scientific and Technological Research Council of Turkey (TUBITAK) and Council of High Education (YOK) for 2211/C Domestic Priority Areas Doctoral Scholarship and 100/2000 PhD Scholarship, respectively.
| Funders | Funder number |
|---|---|
| Council of High Education | |
| Selcuk University | |
| YOK | |
| The Ministry of Economic Affairs and Employment | 75Ti–25Nb-G |
| Türkiye Bilimsel ve Teknolojik Araştırma Kurumu | |
| Istanbul Teknik Üniversitesi | BAP-42163 |
Keywords
- Borides
- GNP
- Solid solution
- h-BN