Effects of refractory metal additives on diboride‐based ultra‐high temperature ceramics: A review

Abstract

Diboride-based ultra-high temperature ceramics (UHTCs) are a special class of ceramics with excellent comprehensive properties, which have extensive potential applications in extreme environments. However, their practical applications are limited, mainly due to the poor fracture toughness and thermal shock resistance. Refractory metals have high melting points, good ductility, and high toughness, which have huge potential to improve the properties of diboridebased ceramics. As a special class of additives, they have been adopted to promote densification, improve microstructure, and properties. However, diboride-based ceramics containing refractory metals have not received adequate attention due to relatively weak practical effects on property improvement. The present review highlights the progress and existing problems of transition metal diborides with refractory metal additives, including W, Ta, Mo, Nb, Hf, V, Cr, and Zr, focusing mainly on the microstructure change and property improvements, followed by challenges and possible future development strategies.

Toughened (Ti 0.2Zr 0.2Hf 0.2Nb 0.2Ta 0.2)B 2–SiC composites fabricated by one-step reactive sintering with a unique SiB 6 additive

“…Borides were oxidized to form liquid B 2 O 3 , which promoted the generation of a liquid bridge between particles known as necking [33]. However, the formation of necking and solid solution has also been reported to facilitate the mass transport [10,25,43], resulting in the particle sintering and agglomeration to form nanoparticle clusters, as observed in STEM images (Figs. reactions during the BCTR described as Eqs.…”

“…The Young's modulus (E) and Vickers hardness (H) of the ceramic sample were estimated from the load-displacement curves. A total of 20 indentations with 60 μm intervals were made using a load of 1 N with dwell time of 10 s. For fracture toughness measurements, indentation tests were also performed at different loads (5,10,15,20, and 25 N) on the well-polished surface of the bulk ceramic samples to induce radial cracking from the corners of the indentation. The length of the radial cracks were then measured from the center of the indention using SEM.…”

“…To overcome this problem, the transitional metals (Hf, Zr, and Cr) were incorporated into HEB to toughen HEB-Hf, HEB-Zr, and HEB-Cr composites and improve K IC up to ~9.2, ~7.1, and ~7.3 MPa•m 1/2 , respectively [10]. Although the addition of Hf, Zr, and Cr contributed to a large toughening effect for HEB ceramics, the phase transformation of Hf metal additive easily occurred from hexagonal close-packed structure to body-centered cubic structure at ~1757 ℃ [11], and the Cr addition can react with diborides and SiC to produce liquid phases at high temperature to form new solid phases [12].…”

Enhancing thermometric efficiency: a wavelength excitation analysis in LiSrGdW3O12:Tb3+ for superior single band ratiometric (SBR) thermometry

“…It uses a single optically active center that is excited in two different ways, resulting in emission signals with opposite temperature dependences. [4][5][6][7] One of the major benets of SBR luminescence thermometry is that the emission is only gathered in the selected spectral region. This allows us to prevent the negative impacts of selective absorption of a tested medium, which can change the shape of the emission spectrum and hence the reliability of the temperature readout.…”

“…Results that were recently published demonstrated how the kind of doping ion, the nature of the host material, and the interionic energy transfer affect the thermally induced ESA process. 4,5 Nonetheless, up to now, the focus has been on process improvement that results in the thermal enhancement of the ESA process.…”