ZrC Grains Research Articles

Structural damage and bubble evolution in SiC-ZrC composite irradiated with 500 keV He-ions at various temperatures

Ceramic composites with high temperature strengths and low neutron cross-sections are promising candidates for core materials in advanced nuclear systems. In present work, SiC-20 vol% ZrC composites were irradiated with 500 keV He-ions at 25, 500 and 800 °C to evaluate the effect of irradiation temperature on the structural damage and bubble evolution in ceramic composites. XRD and Raman spectra analysis give that the irradiation resulted in structural damages of both SiC and ZrC. TEM observations reveal the formation of helium bubbles and defect clusters after irradiation. Moreover, the occurrence of micro-cracks in ZrC grains and amorphization of SiC are observed for the samples irradiated at room temperature. Nanoindentation test showed that there is irradiation induced hardening or softening of the composites which depends on the irradiation temperature or fluence. The correlation between microstructural evolution and mechanical properties response is discussed.

Synthesis of nanoscale zirconium carbide powders via a two-step process

Herein, ZrC powders with the average particle sizes of 100–300 nm were synthesized via a method composed of vacuum carbothermal reduction followed by calcium treatment. The effects of carbon and calcium addition amounts as well as temperature on the micromorphology of obtained products had been studied, and the involved mechanisms were described. Due to the dissolution-transportation-precipitation mechanism, ZrC grains would grow during the calcium treatment process. The finest ZrC with the average particle size of 126.2 nm could be obtained by the combination of vacuum carbothermal reduction at 1773 K for 4 h and calcium treatment at 1173 K for 4 h, with the carbon addition amount of 1.2 times the theoretical value. Furthermore, carbon defect was found in the final products and the lattice spacing was in good agreement with those calculated by proposed formula.

Radio frequency‐assisted zirconium carbide matrix deposition for continuous fiber‐reinforced ultra high temperature ceramic matrix composites

AbstractZirconium carbide (ZrC) is considered to be a potential candidate for ultra high temperature applications due to its high melting point, good chemical inertness, and ablation resistance, but the monolithic form suffers from low fracture toughness and hence poor thermal shock resistance. Reinforcing it using continuous carbon fibers (Cf) to create an ultra high temperature ceramic matrix composite is an obvious solution, however densifying ZrC requires the use of very high temperatures combined with significant pressure, such as obtained by using hot pressing or spark plasma sintering, which risks damaging fibers. In the present work, radio frequency‐assisted chemical vapor infiltration (RF‐CVI) has been investigated with a view to forming Cf/ZrC composites. These initial experiments revealed the ability to deposit pure, nano‐grained, and near stoichiometric ZrC with deposition occurring preferentially from the center of the sample due to the nature of the inverse temperature profile developed. The deposited ZrC grains were in the range of 4–9 nm in size and had a lattice parameter of 0.4750 nm. The work also showed that the use of RF‐CVI enabled the minimization of early pore sealing, a common problem for conventional CVI.

Ar-ion- and electron-irradiated ZrC layers in ZrC-SiC-coated surrogate TRISO fuel particles

Surrogate tristructural isotropic (TRISO) particles, including ZrC and SiC layers, were manufactured through fluidized-bed chemical vapor deposition and irradiated. ZrC layers exhibited C/Zr atomic ratio of 0.95 without distinct crevices at interfaces in microscopy, but underwent significant microstructural changes post-irradiation. Defects included black dots and Frank loops, with increasing irradiation doses augmenting their area fractions. Frank loop density reduced at 9.4dpa due to the formation of fewer, fully circular loops. The ZrC hardness and modulus rose by 25% and 26% following Ar-ion irradiation at 9.4dpa, linked to the formed defects and implanted Ar ions. Electron irradiation expanded the defect-prone zone up to 100nm from the ZrC grain boundary and led to ZrO2 formation. ZrC grains recrystallized post electron irradiation at 700 °C, causing initial grain decomposition, amorphization, and nanocrystal formation. These nanocrystals, formed at 3.3dpa, closely aligned with ZrC diffraction patterns with no preferred orientations.

Structural and mechanical responses of (ZrTiNbTa)C4 and ZrC ceramics under energetic He-ions irradiation

High entropy ceramics are potential structural materials for advanced reactor concepts where numerous helium atoms would be produced due to the (n, α) nuclear transmutation reaction. However, the irradiation behaviors of high entropy ceramics are not well understood until now. In present work, the responses of high entropy (ZrTiNbTa)C4 and its binary constituent ZrC ceramics to energetic He-ions irradiation have been studied. X-ray diffraction analysis showed that He-ions irradiation resulted in peak shift and broadening of diffraction peaks in both (ZrTiNbTa)C4 and ZrC, which denotes lattice expansion and structural damages induced by the irradiation. Transmission electron microscope observation revealed that spherical helium bubbles distribute uniformly in the grain interiors of (ZrTiNbTa)C4 while “string-like” bubbles with a preferred orientation are observed in ZrC grains. Nanoindentation characterization gives that the irradiation induced hardening of (ZrTiNbTa)C4 is less definite than that of ZrC. The irradiation mechanisms of high entropy ceramics have been discussed.

Evolution of structures and internal stress of ZrC-SiC composite under He ion irradiation and post-annealing

Composite ceramics show great potential for utilization in nuclear systems. However, the component phases therein may exhibit large variations in structural response to the ion irradiation including degree of lattice expansion, ability to maintain the crystallinity, types and density of as-formed defects, etc. Such discrepancy may result in the evolution of internal stress and consequently more complicated interactions with respect to the irradiation behavior, which remains less understood. In this study, a ZrC–30 vol% SiC composite is chosen to study the irradiation and helium retention behavior under 540 keV He ion irradiation with a fluence of 1 × 1017/cm2 and subsequent annealing at 1000 °C and 1500 °C. Irradiation-induced volume dilation of ZrC lattice and especially from an amorphization process of SiC leads to a maximum compressive stress of ∼66 GPa at a certain irradiation depth, at which horizontal cracks are generated but only in ZrC grains. During high-temperature annealing, migration of defects, diffusion and coalesce of helium species, growth of He bubbles, recovery of crystal lattices and formation of dislocations are found to be closely correlated to the irradiation-induced gradient of helium retention concentration, lattice damage and internal stress along the irradiation depth along with the recrystallization process of amorphous SiC. The as-observed irradiation behavior and structural evolution during annealing are discussed by theoretical consideration of internal stress in the composite using the COMSOL Multiphysics package.

The microstructural and mechanical behavior of in-situ synthesized ZrB2–ZrC and ZrB2–SiC–ZrC composites: A comparative study

In the present investigation, the ZrB2–ZrC and ZrB2–SiC–ZrC based composites were fabricated via an in-situ reduction mechanism using ZrO2, B4C, Si, and graphite fine powder at 1900 °C in an argon atmosphere. The boro/carbothermal reduction produced nano-sized ZrC grains with ZrB2 particles, whereas the incorporation of Si content produced nano-sized homogeneous distributed SiC grains with ZrB2–ZrC particles. In the ZrB2-based composite densification, microstructure, mechanical, and chemical state identification of elements were investigated. The densification of the samples was enhanced with the formation of ZrC and SiC content, which was correlated with the micrographs and mechanical properties of the samples. The chemical state identification confirms the trace amount of intermediate zirconium oxycarbide in the ZrB2-based composite. Compared to binary ZrB2–ZrC composite, ZrB2–SiC–ZrC composite has finer microstructure and higher densification. The strength of the ZrB2–ZrC and ZrB2–SiC–ZrC composite was decreased by 458 MPa and 406 MPa, respectively. Similarly, the indentation hardness of the samples was increased by 14.2 GPa–15.3 GPa, and fracture toughness was 5.5 ± 0.5 MPa m1/2 to 6.15 ± 0.4 MPa m1/2.

Thermal conductivity of hot-pressed ZrB2-ZrC ceramics

The thermal conductivity of ZrB2 with 0–10 vol.% ZrC was studied from 25 to 2000 °C. The ZrB2-ZrC ceramics were prepared by attrition milling the starting powders and adding 0.5 wt.% C as a sintering aid. The ceramics were hot-pressed to full density at 1900 °C for 30 min in a flowing Ar atmosphere. The ceramics contained graphitic carbon as a secondary phase, with ZrB2 and ZrC grain sizes of approx. 3.5 and 1.5 μm, respectively. At room temperature, thermal conductivity was 75 W/m·K for monolithic ZrB2, increasing to 79 W/m·K for 2.5 vol.% ZrC, and decreasing to 72 W/m·K for 10 vol.% ZrC. At 2000 °C, thermal conductivity was 72 W/m·K for ZrB2, decreasing to 69 W/m·K for 10 vol.% ZrC. Electrical resistivity behavior was similar to thermal conductivity, decreasing with the addition of ZrC up to 2.5 vol.% (9.9 µΩ·cm at 25 °C), then increasing with further ZrC additions. The increase in thermal conductivity with ZrC addition up to 2.5 vol.% was the result of preferential segregation of W from the ZrB2 phase to the ZrC phase. The reduction in thermal conductivity with greater ZrC additions was due to the lower thermal conductivity of ZrC.

TEM exploration of in-situ nanostructured composite coating fabricated by plasma spraying 8YSZ-Al-SiC composite powder

A novel in-situ synthesis method, which introduced 8YSZ-Al-SiC reaction system to atmospheric plasma spraying process, was put forward to prepare ZrC composite coating. Phase transformation between 8YSZ and 15YSZ caused by in-situ reactions was observed. Efforts were focused on TEM investigation in order to explore in-situ generated microstructure containing nanosized ZrC, ZrSi 2 and 15YSZ grains in the composite coating. Moreover, the relationship between the mechanical properties and microstructure of the ZrC composite coating was preliminarily established based on the micron-indentation test. Results confirmed the feasibility of preparing ZrC composite coating by plasma spraying 8YSZ-Al-SiC composite powder and further proved the positive effect of in-situ generated microstructure of the ZrC composite coating on its mechanical properties.

Interfacial reactions between carbon fiber and ceramic matrix in the short carbon fiber reinforced SiC-BN-10%X (X=ZrC, ZrO2 or ZrB2) composites

The short carbon fiber (Csf) reinforced SiC-BN-10%X (X = ZrC, ZrO2 or ZrB2) composites were prepared by hot pressing at 1900 °C and 40 MPa for 1 h. The phase composition and fracture morphology of the prepared composites were investigated, and the interfacial reactions between carbon fiber and ceramic matrix were discussed. The results show that ZrC, ZrB2 and ZrO2 all react with the Csf/SiC-BN system to form a lot of ZrB2 and ZrC, and the carbon fibers in all composites are corroded and damaged. The carbon atoms from ZrC and carbon fiber react with oxide film (ZrO2, B2O3, SiO2) or transition products (ZrSiO4, B4C) to form ZrC, ZrB2, SiC and CO. The interplanar spacing of the ZrC grains in the prepared composites decreases due to the consumption and shortage of carbon atoms. In the Csf/SiC-BN-10%ZrO2 composite, ZrO2 is completely reacted and almost all carbon fibers are exhausted.

Excellent high-temperature strength and ductility of the ZrC nanoparticles dispersed molybdenum

The interface control is critical in the development of high-performance molybdenum (Mo) alloys for high-temperature applications like space reactors. In this work, nanostructured Mo-ZrC alloy with excellent mechanical properties at both room temperature and high temperatures was fabricated by nanoscale ZrC dispersion and interface control. The average grain size of Mo-ZrC alloy is only 0.67 μm owing to the homogeneous dispersion of nanoscale ZrC particles. At room temperature, the ultimate tensile strength (UTS) and total elongation (TE) of the Mo-ZrC alloy are 928 MPa and 34.4%, respectively. At 1000 °C, the UTS is still as high as 562 MPa, which is significantly higher than those reported in oxide dispersion-strengthened Mo alloys, while the TE remains a high value of 23.5%. Additionally, the recrystallization start temperature of Mo-ZrC alloy is about 1400 °C, indicating remarkable thermal stability. First-principles calculations revealed that the interstitial oxygen reduces grain boundary cohesion, while C and ZrC could increase the fracture strength of the boundaries owing to the strong Mo-C bonds. The excellent mechanical properties and thermal stability of the Mo-ZrC alloy can be attributed to a synergistic effect of nanoscale ZrC particles dispersion strengthening, fine-grain strengthening, and grain boundary purification. The strategy could be applied to design other refractory alloys with both high strength and ductility for high-temperature applications.

Segregation of solute atoms in ZrC grain boundaries and their effects on grain boundary strengths

ZrC is a promising candidate for the application in ultra-high temperature regime due to its unique combination of excellent properties, such as high melting point, good chemical inertness and high temperature stability. The rapid decrease of strength at high temperatures, however, is one of the obstacles that impedes its practical services. Strengthening of grain boundaries by solute segregation is believed to be an effective way to improve its high temperature performance. Therefore, the segregation tendency of ten solid solute atoms, including Sc, Ti, V, Cr, Y, Nb, Mo, Hf, Ta, W, in ZrC grain boundaries, and the strengthening/weakening effects on grain boundaries due to segregation are investigated by first-principles calculations. The segregation tendency is found dominated by the size effect, which is confirmed by both a qualitative analysis and a quantitative approach based on support vector regression. It means that big atoms tend to segregate to grain boundary sites with local expansions, while small atoms tend to segregate to grain boundary sites with local compressions. Simulations on stress-strain responses indicate that segregation of small atoms (Ti, V, Cr, Nb, Ta, Mo, W) can usually improve grain boundary strengths by inducing compression strains to grain boundaries, even though there is also an exception. In contrast, segregation of Sc and Y will soften grain boundaries. The results reveal that strengthening of grain boundaries by solute segregation is a valuable avenue to enhance high temperature mechanical properties of ZrC, providing guidelines for further design of ZrC based materials.

Sliding-wear performance of ZrC–Co: A comparative assessment under dry and neutral/non-neutral wet media

The sliding-wear behaviour of a newly-developed fine-grained ZrC–16.693 vol%Co cemented carbide was investigated, and critically compared under frictional contact in four media (i.e., dry wear, and wet wear in neutral water (pH∼7.1) and in highly acidic (pH∼1.1) and alkaline (pH∼13.7) aqueous solutions). It is shown that, under the testing conditions used (i.e., 20 N for 1000 m at 5 cm/s), because this ZrC–Co cemented carbide is superhard (∼18 GPa), it only undergoes mild wear in the four media, with specific wear rates of the order of 10−6 mm3/(N·m) or less. It was also identified that wear occurs by abrasion (wear mode) controlled by plastic deformation plus some localized fracture (wear mechanisms), with varying severities depending on the wear medium. Importantly, wear in water was comparatively the least, mainly attributable to water imposing moderate lubrication without any sign of chemical attack. Wear in alkaline medium was only slightly greater than in water, mostly imputable to the alkaline aqueous solution imposing moderate lubrication without attacking the ZrC grains and only slightly the Co binder. Dry wear was greater than wear in both water and alkaline medium, imputable to the greater severity of the frictional contact. And lastly, wear in acidic medium was comparatively the greatest, principally imputable to the acidic aqueous solution imposing moderate lubrication without attacking the ZrC grains but dominantly leaching the Co binder. In sum, ZrC–Co was found to be at least little susceptible to the non-neutral wet wear, and therefore in principle could form the basis of a subfamily of cemented carbides highly recommendable for use in tribological applications involving harsh environments.

Microstructure, formation mechanism and mechanical properties of ZrC–Fe coating on cast iron via in situ solid-phase diffusion

Abstract To improve the mechanical properties of the surface of iron-based alloys, a ZrC–Fe coating is produced on cast ion via in situ solid-phase diffusion (ISSD) at 1130 °C. The coating is completely dense, reaching a thickness of approximately 18 μm after 10 h of ISSD. The volume fraction of the ZrC phase in the coating reaches approximately 92.3%. The size of the long axis (short axis) of columnar ZrC particles gradually decreases from approximately 4 μm (1.5 μm) to 1 μm (0.3 μm) as the thickness of the coating increases, displaying a gradient microstructure. The formation mechanism of the coating is mainly attributed to the carbon atoms diffuse into the octahedral vacancies of the zirconium lattice to undergo a diffusion-type solid state phase transformation to form ZrC, and α-Fe phase are distributed among the ZrC particles through the Fe atoms diffusion. Due to the carbon concentration gradient, the nucleation rate is reduced, thereby forming a gradient microstructure on the grain scale. The growth of the coating obeys the growth kinetic model d 2 = 30.5 t . In addition, the nanohardness and elastic modulus of the surface of the ZrC–Fe coating reach 29.7 GPa and 425.1 GPa, respectively, showing remarkable improvement compared to those of the cast iron substrate (4.5 GPa and 229.5 GPa, respectively). The fracture toughness gradually increases from 2.9 ± 0.1 MPa m1/2 to 3.6 ± 0.1 MPa m1/2 as the thickness of the coating increases. Moreover, the coating exhibits the superior coating/substrate adhesion. The excellent comprehensive mechanical properties are tightly related to a small amount of high toughness α-Fe phase, high volume fraction of in situ ZrC particles and the gradient change of the ZrC grain scale.

Processing and room temperature mechanical properties of a zirconium carbide ceramic

AbstractZirconium carbide (ZrC) powder, batched to a ratio of 0.98 C/Zr, was prepared by carbothermal reduction of ZrO2 with carbon black. Nominally phase‐pure ZrC powder had a mean particle size of 2.4 μm. The synthesized powder was hot‐pressed at 2150°C to a relative density of > 95%. The mean grain size was 2.7 ± 1.4 μm with a maximum observed grain size of 17.5 μm. The final hot‐pressed billets had a C/Zr ratio of 0.92, and oxygen content of 0.5 wt%, as determined by gas fusion analysis. The mechanical properties of ZrC0.92O0.03 were measured at room temperature. Vickers’ hardness decreased from 19.5 GPa at a load of 0.5 kgf to 17.0 GPa at a load of 1 kgf. Flexural strength was 362.3 ± 46 MPa, Young's modulus was 397 ± 13 MPa, and fracture toughness was 2.9 ± 0.1 MPa•m1/2. Analysis of mechanical behavior revealed that the largest ZrC grains were the strength‐limiting flaw in these ceramics.

A novel ZrB2-based composite manufactured with Ti3AlC2 additive

A novel ZrB2–Ti3AlC2 composite was densified using spark plasma sintering at 1900 °C under pressure of 30 MPa for 7 min. The effect of Ti3AlC2 MAX phase on the densification behavior, microstructural evolutions, phase arrangement, and mechanical properties of the composite were investigated. The phase analysis and microstructural studies revealed the decomposition of the MAX phase at the initial steps of the SPS process. The structural characteristics and surface morphology of the in-situ synthesized reinforcements were verified using X-ray diffraction and scanning electron microscopy, respectively. The formation mechanism of each reinforcement phase was also investigated using thermodynamical assessments. The prepared ZrB2–Ti3AlC2 composite not only possessed a near fully-dense characteristic having an excellent hardness of 31 GPa, but also unexpectedly presented high fracture toughness. The indentation fracture toughness of the composite was calculated as 7.8 MPa m1/2, which is unprecedented compared with the same class of hard ZrB2-based composites. Indeed, the superior mechanical properties of the composite achieved in this study was obtained by the homogenous distribution of Al-based reinforcements, formation of hard interfacial ZrC grains, and solid solutions provided by Ti-based phases. The correlations between the phase arrangement, microstructure, and the attained mechanical properties of the composite were comprehensively discussed.

Precipitations of W/Cu metallic phases in ZrC in the reactive melt infiltrated ZrC/W composite

Abstract W and Cu rich precipitates have been found in ZrC ceramic grains in the reactive melt infiltrated ZrC/W composite prepared at 1300 °C. The microstructures of the precipitates have been identified using transmission electron microscopy and energy dispersive X-ray spectroscopy. The substantial mechanism of the precipitation was studied by calculating and analyzing the lattice parameters of ZrC (aZrC). First-principles calculations were also used to evaluate the formation energies of carbon sub-stoichiometric ZrC (ZrCx) with substitutional W or Cu atoms. The precipitates in ZrC are identified as W, Cu, CuW3 and Cu5Zr phases. The increase in C/Zr ratio in reaction formed ZrC grains causes the loss of W and Cu solutes in ZrCx, which gives rise to an increment in aZrC. The presence of C vacancies is energetically favorable to the solid dissolutions of W and Cu atoms in ZrCx lattice. Thus, the initial formed ZrCx with a large number of C vacancies has an ability to dissolve W and Cu solutes. And the metallic precipitates in ZrC grains are attributed to the reducing solubility limits of W and Cu with the increasing C content in ZrCx.

Microstructure and mechanical properties of ZrC coating on zirconium fabricated by interstitial carburization

Abstract Herein, ZrC coating was fabricated on the surface of zirconium via interstitial carburization. High-carbon steel was selected as the carbon source to supply the interstitial carbon atoms and was hot-pressed with zirconium substrate at 1150 °C and uniaxial pressure of 2 MPa. The interstitial carbon atoms diffused into the surface of Zr forming a ZrC coating. The microstructure of the coating was characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron backscattered diffraction (EBSD). The obtained coating was completely dense and pure ceramic, consisting of a single ZrC phase with a volume fraction of 100%. The diameter of ZrC grains exhibited a gradient distribution with a range of 180 nm to 5.9 μm. The thickness of the coating was proportional to the square root of the carburizing time, following the classical parabolic law, reaching a coating thickness of 7.2 μm after carburizing for 10 h. The microhardness tested on the coating surface reached 1500 HV and nanohardness measured on the cross-section of the ZrC coating reached 27 GPa, displaying a remarkable improvement compared to the zirconium substrate (192 HV and 3.6 GPa). The average fracture toughness of ZrC coating measured via Vickers indentation was 1.9 MPa·m1⁄2. In response to a scratch where the scratch load increased linearly from 0 to 100 N, the coating demonstrated excellent adhesion with the substrate. As per the obtained results, we expect that the interstitial carburization method can also be applied to various Zr-based alloys.

Synthesis, characterization, and ceramization of a carbon-rich SiCw-ZrC-ZrB2 preceramic polymer precursor

Abstract A precursor (PBSZ) for SiCw-ZrC-ZrB2 hybrid powder was synthesized by chemical reaction of phenol, paraformaldehyde, zirconium oxychloride, boric acid and tetraethylorthosilicate. Results show that zirconium, silicon and boron atoms have been successfully introduced into the branched structure. Decomposition of PBSZ is completed at 800 °C, and it gives amorphous carbon, SiO2, B2O3 and ZrO2 with a yield of 38% at 1200 °C. During the pyrolysis process, ZrB2 and SiC form at about 1500 °C, followed by the appearance of ZrC when the amount of B2O3 is limited. Highly crystallized ZrB2–ZrC–C powder with ZrB2 and ZrC grains evenly distributed in the carbon matrix together with randomly distributed SiC whiskers are obtained after heat-treated at 1800 °C. Further heated at 1900 °C, ZrB2 and ZrC grains grow from 200 to 500 nm, while SiC whiskers show a much smaller diameter size and tend to grow on the ZrB2–ZrC–C block surface. The morphology difference is caused by the larger gas supersaturation and accommodation coefficient of the pore channels on the block surface. In addition, defects of the carbon matrix are cumulated to the highest at 1500 °C and the structure-ordered carbon is obtained after heat treated at 1900 °C.

Formation and growth kinetics of ZrC layer in graphite-zirconium system

The reaction behavior of carbon and zirconium in reactive melt infiltration (RMI) process is significantly meaningful for improvement of preparation parameters. Thus, graphite and zirconium were utilized to analyzed the microstructure, formation mechanism and growth kinetics of ZrC layer. During RMI process, continuous ZrC layer formed immediately by heterogeneous nucleation and ZrC grains grew up by solid-liquid reaction. The growth of continuous ZrC layer was governed by diffusion-reaction and dissolution-precipitation mechanism, and the former was the major factor. ZrC layer can grew to 42 μm in 5 min at 2000 °C, indicating that the porous C/C article would be filled up for a short time. The growth kinetics of ZrC layer followed a parabolic law, and the diffusion activation energy ED and effective diffusivity constant were calculated to be ∼251 kJ mol−1 and ∼0.45 × 10−6 cm2 s−1, respectively.