A dream for
many scientists,
engineers and sci-fi
enthusiasts is of an aerospace
vehicle that can
take off from an airport,
fly through the atmosphere
and travel to the
other side of the earth
at hypersonic speeds,
and then return through
the atmosphere to the
same or another airport.
Thanks to programs
like DARPA’s Falcon
Hypersonic Technology
Vehicle 2 program
(Figure 1), the dream is
taking form.

/pdf/uhtc-composites-for-hypersonic-applications-59v44wakt9.pdf

UHTC composites for hypersonic applications

Some aspects of heat transfer upon the interaction between components with a sharp leading edge and high-enthalpy high-speed flow of dissociated air have been considered; some material characteristics, which should be primarily taken into account when prognosticating the behavior of materials that are promising for using as components of hypersonic flight vehicles, have been substantiated; specific features of the oxidation of materials based on zirconium and hafnium diborides have been touched briefly; the methods of increasing oxidation resistance of these materials that have been developed by various groups of researchers have been demonstrate; some works concerning the behavior of samples under the effect of high-enthalpy flows of dissociated air have been described, including those that simulate sharp leading domes and edges of wings of hypersonic flight vehicles.

Promising ultra-high-temperature ceramic materials for aerospace applications

Microstructure evolutions of ZrB 2 –30 vol.% SiC composites, prepared by hot pressing at different processing temperatures (1700, 1850 and 2000 °C) for 30 min under 10 MPa, were investigated by optical microscopy, scanning electron microscopy and transmission electron microscopy (TEM). The microstructures of the fabricated composites were compared with and the effects of the processing temperature on the sintering process and densification behavior during the hot pressing were found. The amount and the orientation of dislocations which were indicated by TEM analysis in the sample hot pressed at 1700 °C showed that no plastic deformation and atomic diffusion occurred. But the presence of amorphous phases and rearrangement of particles are signs of the fact that liquid phase sintering and particle fragmentation/rearrangement is the main densification mechanism. On the other hand, in the sample hot pressed at 1850 °C, aggregation of dislocations behind the grain boundaries and the presence of twinnings addressed wide plastic deformations which were introduced as the main densification mechanism at 1850 °C. Finally in the sample hot pressed at 2000 °C, lower amounts of un-oriented dislocations and also some annealing twinnings were observed in TEM micrographs together with fractographical SEM analysis and showed that the atomic diffusion is the dominant densification mechanism of hot pressed ZrB 2 –30 vol.% SiC composite.

Temperature dependence of microstructure evolution during hot pressing of ZrB2–30 vol.% SiC composites

Reinforcing effects of SiC whiskers and carbon nanoparticles in spark plasma sintered ZrB2 matrix composites

Spark plasma sintering (SPS) process was used to preparation of ZrB2-based composites co-reinforced with SiC whiskers as well as various amounts of pulverized carbon fibers. The effects of CF content on microstructure and mechanical characteristics of ZrB2–SiCw–CF composites were scrutinized. Although all composites approached high densification, a fully-dense sample was fabricated by the addition of 2.5 wt% CF. The growth of ZrB2 grains was remarkably inhibited in CF-reinforced composites. No in-situ formed phase was detected by XRD; however, trace of nano-ZrC clusters was observed in SEM fractographs of ZrB2–SiCw–CF samples. The formation of such nano-sized ZrC refractory phase was also proved by thermodynamics. The hardness of composites slightly decreased from 21.9 to 19 GPa with increasing the CF content. Reversely, the fracture toughness values enhanced from 4.7 to 6 MPa.m½ with increasing the amount of pulverized carbon fibers.

Spark plasma sintering of ZrB2-based composites co-reinforced with SiC whiskers and pulverized carbon fibers

A novel ZrB 2 based material named “hybrid composite” was prepared with the addition of SiC whiskers plus SiC particles by hot-pressing. Microstructure and mechanical properties of the composites were investigated. The introduction of fine SiC particles can increase the relative density and mechanical properties of ZrB 2 . Mechanical properties especially fracture toughness were better than that of ZrB 2 composites reinforced with SiC particles or SiC whiskers alone. Based on the results presented in this work, it can be stated that the combination of SiC whiskers and SiC particles can be used as reinforcing materials for diborides based ultra-high temperature ceramics, due to their cooperative effect in densification and reinforcement.

Preparation of ZrB2 based hybrid composites reinforced with SiC whiskers and SiC particles by hot-pressing

A ZrC-20 vol% SiC based ceramic matrix composite containing 10 vol% graphite flake was fabricated by hot press sintering. The thermal shock resistance of the materials was investigated through the water-quench method and subsequent three-point bend testing of flexural strength diminution. The ZrC-20 vol% SiC composite containing 10 vol% graphite flake exhibited higher critical temperature difference and higher residual ratio of strength compared with that of ZrC-20 vol% SiC composite. It is the main reason that addition of the graphite flake provides a weak interface into the material which acts to deflect propagating cracks, and the bridging of the cracks occurred. Meanwhile, a laminar structure of the graphite flake would relax the interface stress by sliding of interlayers and provide favorable sites for the dissipation of energy associated with crack growth during fracture.

Thermal shock resistance of ZrC matrix ceramics

In this work, ZrC-based composites reinforced with SiC particles were processed using the hot-pressing route. The influence of SiC content on the microstructure and mechanical properties of the hot-pressed composites was investigated. In all composites with different SiC contents, the highest relative density was obtained for ZrC-based composite containing 20 vol% SiC, having a value of 97 per cent. The flexural strength of ZrC-based composites increased modestly from 390 to 452 MPa as the SiC content increased from 10 to 20 vol%. The increase in strength was attributed to a decrease in grain size and improvement of the relative density. But the strength did not show great variation when the content of the SiC particles increased to 30 vol%, which may be because of the inhomogeneous microstructure and the low relative density. In contrast, toughness and hardness did not vary significantly with SiC content.

Fabrication of hot-pressed ZrC-based composites

Strength and plasticity are essential properties in Al-Cu-Mg alloy for the aerospace, automotive, and weapon industries. Both cold rolling and pre-deformation are critical practices to obtain high strength as well as high plasticity. In this study, the microstructure and properties of the Al-Cu-Mg alloy with various deformation treatments were studied by using optical microscope, scanning electron microscope, electron back scatter diffraction technique, transmission electron microscope, Vickers hardness tester, and tensile tester. The results show that the brittle T phases can cause stress concentration which is not conducive to the plasticity of the alloy. The tiny GPB zones distributed homogeneously can increase the ultimate tensile strength. There are fiber {001} < 110 > , Copper {112} < 111 > , Brass {011} < 211 > , S {123} < 634 > , Goss {011} < 100 > , Cube {001} < 100 > , and R {124} < 211 > texture components in the cold-rolled deformed 2324 aluminum alloy. Compared to cold-rolled deformed alloy, the fiber texture component disappears in the pre-stretched deformed alloy. The intensity of Brass and S texture components are helpful to obtaining higher strength of the alloy. The maximum ultimate tensile strength and yield strength are achieved in the alloy with the pre-stretching deformation of 1.4%, enhancing 42.7 MPa and 147.8 MPa compared to the cold-rolled deformed alloy, respectively. However, the elongation is decreased to 13.3% in the alloy with the pre-stretching deformation of 1.4%. 

Effect of Pre-stretching Deformation Treatment Process on Microstructure and Mechanical Properties of Al-Cu-Mg Alloy

In situ synthesis feasibility of ZrB2-SiC-ZrC composite coatings on ZrC ceramics by reactive plasma spraying (RPS) was investigated. To help to understand the phase evolution during plasma spraying process, reaction behavior in the ZrH2-Si-B4C system was explored carefully by differential scanning calorimetry. The results indicated that the phase transformation sequence in the ZrH2-Si-B4C system could be described as ZrH1.66, Zr3O, ZrC, ZrB2, Zr2Si, ZrSi, and SiC. The prior formation of ZrC was due to high diffusion rate of C atoms from B4C. ZrB2 was produced above 1100 °C. As the temperature increased, SiC were finally formed by the reaction of ZrC with ZrSi and B4C. The RPS composite coatings mainly consisted of ZrB2, SiC, and ZrC phases, except for a small fraction of ZrO2 phase. The microstructural characterization exhibited more dense melted splats, which appears to increase gradually with the increase in spraying currents and distances. The coatings had typical lamellar structure and adhered to the substrate well. The microhardness values were higher than 1000 HV1, but there were few variations with varying spraying currents and distances.

https://www.mdpi.com/1996-1944/15/6/2217/pdf?version=1647514135

Baoxia Ma

Papers

Preparation of ZrB2 based hybrid composites reinforced with SiC whiskers and SiC particles by hot-pressing

Thermal shock resistance of ZrC matrix ceramics

Fabrication of hot-pressed ZrC-based composites

Effect of Pre-stretching Deformation Treatment Process on Microstructure and Mechanical Properties of Al-Cu-Mg Alloy

Synthesis of In Situ ZrB2-SiC-ZrC Coating on ZrC-SiC Substrate by Reactive Plasma Spraying