The development of a new class of multicomponent ultra-high temperature ceramics (MC-UHTCs), often referred to as high-entropy UHTCs, has gained increased interest due to the possibility of improved thermomechanical and oxidation properties. In this study, a systematic approach by gradual addition in the UHTC components ranging from a binary to a dense quaternary (Ta,Nb,Hf,Ti)C is synthesized using spark plasma sintering (SPS). The solid solutioning was the critical factor in homogenizing the composition in the multicomponent system. The segregation of NbC and HfC was seen in binary and ternary UHTC systems, while a single-phase homogeneity was observed in the quaternary UHTC improving its hardness up to 34.8 GPa. The presence of closely spaced slip lines in the MC-UHTCs enhances resistance to indentation damage up to 72% at an applied load of 200 N. The formation of complex mixed oxide phase of Hf 6 Ta 2 O 17 ensued in the lower to negligible oxidation even up to 3 min of plasma exposure with temperature exceeding 2800 • C. In sum, though the entropy remains medium (0.96R) for the selected system, the quaternary UHTC system undoubtedly has significantly better thermomechanical performance when compared to established baseline UHTCs. This raises the debate on the justification for calling a multicomponent system a "high entropy" to be seen in a new light. The developed MC-UHTCs elicits the paradigm of this new class of UHTCs expanding their potential in thermal protection systems for hypersonic applications.

Nanometer- and submicrometer-sized fiber have been used as scaffolds for tissue engineering, because of their fundamental load-bearing properties in synergy with mechano-transduction. This study investigates a single biodegradable poly­(lactic-co-glycolic acid) (PLGA) fiber’s load–displacement behavior utilizing the nanoindentation technique coupled with a high-resolution in situ imaging system. It is demonstrated that a maximum force of ∼3 μN in the radial direction and displacement of at least 150% of fiber diameter should be applied to acquire the fiber’s macroscopic mechanical properties for tissue engineering. The adhesion behavior of a single fiber is captured using a high-resolution camera. The digital image correlation (DIC) technique is adopted to quantify the adhesion force (∼25 μN) between the fiber and the tip. Adhesion force has also been quantified for the fiber after immersing in phosphate-buffered saline (PBS) to mimic the bioenvironment. A 4-fold increase in adhesion force after PBS treatment was observed due to water penetration and hydrolysis on the fiber’s surface. A high similarity between mechanical properties of a single fiber and native tissues (elastic modulus of 10–25 kPa) and superior adhesion force (25–107.25 μN) was observed, which is excellent for promoting cell-matrix communication. Overall, this study examines the mechanics of a single fiber using innovative indentation and imaging processing techniques, disclosing its profound and striking roles in tissue engineering.

Boron Nitride Nanotube (BNNT) is integrated in AZ31 magnesium alloy by field-assisted powder metallurgy route. A mat of BNNT is sputter-coated with pure Mg and then sandwiched between AZ31 alloy powders. This layered composite is consolidated by spark plasma sintering. A high processing pressure of 400 MPa aids in intimate alloy-nanotube adhesion due to localized deformation. Thermal diffusion between AZ31 and BNNT (at 400°C sintering temperature) results in the formation of Mg 3 N 2 and AlN nano-phases at the interface. Due to ultrafast Joule heating during SPS, the reactions are kinetically controlled and only trace amounts of products are formed without compromising the characteristics of the nanotubes. The interphases aid in reactive-bonding between AZ31 and BNNT, which is essential for load-bearing applications. In-situ double cantilever loading of the composite inside the scanning electron microscope shows that the nanotubes bridge, resist, and delay crack propagation. These findings demonstrate the promise of thermally-stable BNNTs as reinforcement for engineering lightweight Mg-based structural composites.

The application of ultra‐high‐temperature ceramics (UHTCs) demands effective ways of joining in overcoming the problems associated with the fabrication of complex‐shaped components. In this study, we choose to investigate a new method of rapidly joining pre‐sintered TaC and HfC ceramics without any filler material using the spark plasma sintering (SPS) technique. A well‐bonded TaC–HfC interface was observed with no apparent cracking and porosity at the joint. The joining mechanisms were predominantly driven by solid‐state diffusion and localized plastic deformation. The nanomechanical properties of the TaC‐HfC joint are better than the HfC while comparable to that of the TaC. High‐load indentation (up to 200 N) results suggest that the TaC–HfC interface is stronger than the parent UHTCs with no crack propagating at the interface. Upon comparison with the parent UHTCs, the damaged area and the average crack length at the interface, reduced up to ~94% and ~56%, respectively. This study shows that the SPS technique can also apply to joining other UHTCs without any filler, resulting in the new field of developing complex components for the thermal protection system (TPS).

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