Preparation and properties of dense ZrB2 composite reinforced by elongated SiC and Al3BC3 grains

Dense ZrB₂-SiC-Al₃BC₃ ultra-high temperature ceramic composite was fabricated by hot pressing sintering at 1900°C for 1 hour under a pressure of 20 MPa using Zirconium diboride (ZrB₂) as the raw material and a powder mixture of SiC, B₄C, Al, and carbon as the sintering additive. Al and B₄C underwent in situ reaction with carbon powder to produce Al₃BC₃, which promoted the densification of ZrB₂ ceramic. SiC grains were found to be elongated during sintering. The ZrB₂-SiC-Al₃BC₃ composite exhibited excellent mechanical properties, such as high flexural strength of 589 ± 147 MPa and fracture toughness of 7.81 ± 1.09 MPa m^1/2. Oxidation behavior of the ZrB₂-SiC-Al₃BC₃ composite was studied in air at 1500°C for 1 hour. A continuous layer of oxides consisting of a mixture of SiO₂, Al₂SiO₅, and Al₂O₃ was formed on the surface of the ZrB₂-SiC-Al₃BC₃ composite. This layer of oxides efficiently prevented oxygen from diffusing into the specimens during oxidation, which improved the oxidation resistance of the ZrB₂ ceramics.     

Solidification of welded SiC–ZrB2–ZrC ceramics

A silicon carbide‐based ceramic, containing 50 vol% SiC, 35 vol% ZrB₂, and 15 vol% ZrC was plasma arc welded to produce continuous fusion joints with varying penetration depth. The parent material was preheated to 1450°C and arc welding was successfully implemented for joining of the parent material. A current of 138 A, plasma flow rate of ~1 L/min or ~0.5 L/min, and welding speed of ~8 cm/min were utilized for repeated joining, with full penetration fusion zones along the entire length of the joints. Solidification was determined to occur through the crystallization of β‐SiC (3C), then the simultaneous solidification of SiC and ZrB₂, and lastly through the simultaneous solidification of SiC, ZrB₂, and ZrC through a ternary eutectic reaction. The ternary eutectic composition was determined to be 35.3 ± 2.2 vol% SiC, 39.3 ± 3.8 vol% ZrB₂, and 25.4 ± 3.0 vol% ZrC. A dual fusion zone microstructure was always observed due to convective melt pool mixing. The SiC content at the edge of the fusion zone was 57 vol%, while SiC content at the center of the fusion zone was 42 vol% although the overall SiC content was still nominally 50 vol% throughout the entire fusion zone.     

ZrB2–SiC–G Composite Prepared by Spark Plasma Sintering of In‐Situ Synthesized ZrB2–SiC–C Composite Powders

To avoid introduction of milling media during ball‐milling process and ensure uniform distribution of SiC and graphite in ZrB₂ matrix, ultrafine ZrB₂–SiC–C composite powders were in‐situ synthesized using inorganic–organic hybrid precursors of Zr(OPr)₄, Si(OC₂H₅)₄, H₃BO₃, and excessive C₆H₁₄O₆ as source of zirconium, silicon, boron, and carbon, respectively. To inhabit grain growth, the ZrB₂–SiC–C composite powders were densified by spark plasma sintering (SPS) at 1950°C for 10 min with the heating rate of 100°C/min. The precursor powders were investigated by thermogravimetric analysis–differential scanning calorimetry and Fourier transform infrared spectroscopy. The ceramic powders were analyzed by X‐ray diffraction, X‐ray photoelectron spectroscopy, and scanning electron microscopy. The lamellar substance was found and determined as graphite nanosheet by scanning electron microscopy, Raman spectrum, and X‐ray diffraction. The SiC grains and graphite nanosheets distributed in ZrB₂ matrix uniformly and the grain sizes of ZrB₂ and SiC were about 5 μm and 2 μm, respectively. The carbon converted into graphite nanosheets under high temperature during the process of SPS. The presence of graphite nanosheets alters the load‐displacement curves in the fracture process of ZrB₂–SiC–G composite. A novel way was explored to prepare ZrB₂–SiC–G composite by SPS of in‐situ synthesized ZrB₂–SiC–C composite powders.     

SiC Depletion in ZrB2–30 vol% SiC at Ultrahigh Temperatures

The formation of a porous SiC‐depleted region in ZrB₂–SiC due to active oxidation at ultrahigh temperatures was characterized. The presence/absence of SiC depletion was determined at a series of temperatures (1300°C–1800°C) and times (5 min–100 h). At T < 1627°C, SiC depletion was not observed. Instead, the formation of a ZrO₂ + C/borosilicate oxidation product layer sequence was observed above the ZrB₂–SiC base material. At T ≥ 1627°C, SiC was depleted in the ZrB₂ matrix below the ZrO₂ and borosilicate oxidation products. The SiC depletion was attributed to active oxidation of SiC to form SiO(g). The transition between C formation in ZrO₂ (T < 1627°C) and SiC depletion in ZrB₂ (T ≥ 1627°C) is attributed to variation in the temperature dependence of thermodynamically favored product assemblage influenced by the local microstructural phase distribution. The growth kinetics of the SiC depletion region is consistent with a gas‐phase diffusion‐controlled process.     

Improved Green Strength and Green Machinability of ZrB2–SiC Through Gelcasting Based on a Double Gel Network

ZrB₂–SiC green ceramics were fabricated by aqueous gelcasting based on single AM‐MBAM, single Na‐alginate, and double gel network system. ZrB₂–SiC ceramics obtained by aqueous gelcasting based on AM‐MBAM and Na‐alginate double gel network had a dramatically highest green strength of 98.6 ± 5.1 MPa, which was 103% and 61% higher than that of ZrB₂–SiC ceramics based on single AM‐MBAM system and Na‐alginate system, respectively. A “scratch test” was conducted to evaluate the green machinability of as‐prepared ZrB₂–SiC ceramics. The ZrB₂–SiC ceramics based on this double gel network was found to have the best green machinability.     

Effect of Si3N4 Addition on Compressive Creep Behavior of Hot‐Pressed ZrB2–SiC Composites

Compressive creep studies have been carried out on hot‐pressed ZrB₂–SiC (ZS) and ZrB₂–SiC–Si₃N₄ (ZSS) composites in air under stress and temperature ranges of 93–140 MPa and 1300°C–1425°C, respectively for time durations of ≈20–40 h. The results of these studies have shown the creep resistance of ZS composite to be greater than that of ZSS. As the temperature is increased from 1300°C to 1425°C, the stress exponent of ZS decreases from 1.7 to 1.1, whereas that of ZSS drops from 1.6 to 0.6. The activation energies for these composites have been found as ≈95 ± 32 kJ/mol at temperatures ≤1350°C, and as ≈470 ± 20 kJ/mol in the range of 1350°C–1425°C. Studies of the postcreep microstructures using scanning and transmission electron microscopy have shown the presence of glassy film with cracks at both ZrB₂ grain boundaries and ZrB₂–SiC interfaces. These results along with calculated values of activation volumes suggest grain‐boundary sliding as the major damage mechanism, which is controlled by O^2? diffusion through SiO₂ at ≤1350°C, and by viscoplastic flow of the glassy interfacial film at temperatures ≥1350°C. Studies by transmission electron microscopy have shown formation of crystalline precipitates of Si₂N₂O near ZrB₂–SiC interfaces in ZSS tested at ≥1400°C, which along with stress exponent values <1 suggests that grain‐boundary sliding involving solution‐precipitation‐type mechanism is operative at these temperatures.     

The effect of the SiC content on the high duration erosion behavior of SiC/ZrB2– SiC/ZrB2 functionally gradient coating produced by shielding shrouded plasma spray method

In this research, the effect of the ZrB₂ middle layer and SiC Weight percentage on the erosion behavior of SiC/ZrB₂– SiC/ZrB₂ functionally gradient coating were investigated. For this purpose, SiC gradient coating was prepared via the reactive melt infiltration method (RMI). Afterwards ZrB₂–SiC layers containing 10, 20 and 30 wt% SiC and, ZrB₂ as the outer layer were applied on SiC coated graphite via solid shielding shrouded plasma spraying (SSPS). To investigate the erosion resistance of the coating, the specimens were subjected to oxy-acetylene and propane flame. The results showed that by applying the ZrB₂–SiC layer between SiC and ZrB₂ coating, due to the gradual change of the coefficient of thermal expansion mismatch and reduction of thermal stresses, erosion resistance improves, so that the coating with 20 wt% SiC with mass and linear erosion rate, ?0.072 × 10^?4 g.cm^?2.s^?1, 0.0166 μm s^?1 respectively had the best erosion resistance under oxy propane flame.In the oxyacetylene flame test, a similar result was obtained to the oxy propane test and the SiC/ZrB₂-20% wt. SiC/ZrB₂ coating had the lowest erosion rate.     

ZrB2–SiC laminated ceramic composites

The oxidation behavior for ZrB₂–20 vol% SiC (ZS20) and ZrB₂–30 vol% SiC (ZS30) ceramics at 1500 °C was evaluated by weight gain measurements and cross-sectional microstructure analysis. Based on the oxidation results, laminated ZrB₂–30 vol% SiC (ZS30)/ZrB₂–25 vol% SiC (ZS25)/ZrB₂–30 vol% SiC (ZS30) symmetric structure with ZS30 as the outer layer were prepared. The influence of thermal residual stress and the layer thickness ratio of outer and inner layer on the mechanical properties of ZS30/ZS25/ZS30 composites were studied. It was found that higher surface compressive stress resulted in higher flexural strength. The fracture toughness of ZS30/ZS25/ZS30 laminates was found to reach to 10.73 MPa m^1/2 at the layer thickness ratio of 0.5, which was almost 2 times that of ZS30 monolithic ceramics.     

Fabrication and properties of reactively hot pressed ZrB2–SiC ceramics

ZrB₂–SiC ceramics with relative densities >99% were fabricated by ‘in situ’ reactive hot pressing from ZrH₂, B₄C and Si. The reaction was studied using two processes, (1) powder reactions at temperatures from 1150 to 1400 °C and (2) reactive hot pressing between 1600 and 1900 °C. The products from the reaction of a 2ZrH₂:1B₄C:1Si molar mixture were ZrB₂, SiC, ZrO₂ and ZrC. Modification of the composition to 2ZrH₂:1.07B₄C:1.16Si resulted in the elimination of the undesired ZrO₂ and ZrC phases. The final composition was approximately ZrB₂–27 vol% SiC with no undesired phases detected by X-ray diffraction, and only low concentrations of B₄C detected by scanning electron microscopy. Elimination of the undesired phases was accomplished by removing surface oxides through chemical reactions at elevated temperatures. Reactively hot pressed samples consisting of ZrB₂ with 27 vol% SiC had a Young's modulus of 508 GPa, a flexure strength of 720 MPa, a fracture toughness of 3.5 MPa m^1/2 and a Vickers’ hardness of 22.8 GPa.     

In Situ Synthesis of a ZrB2‐Based Composite Powder Using a Mechanochemical Reaction for the Zircon/Magnesium/Boron Oxide/Graphite System

A ZrSiO₄/B₂O₃/Mg/C system was used to synthesize a ZrB₂‐based composite through a high‐energy ball milling process. As a result of the milling process, a mechanically induced self‐sustaining reaction (MSR) was achieved in this system. A composite powder of ZrB₂–SiC–ZrC was prepared in situ by a magnesiothermic reduction with an ignition time of approximately 6 min. The mechanism for the formation of the product was investigated by studying the relevant subreactions, the stoichiometric amount of B₂O₃, and thermal analysis.     

High‐temperature mechanical behavior of ZrB2‐based composites with micrometer‐ and nano‐sized SiC particles

Using micrometer‐ and nano‐sized SiC particles as reinforcement phase, two ZrB₂‐SiC composites with high strength up to 1600°C were prepared using high‐energy ball milling, followed by hot pressing. The composite microstructure comprised finer equiaxed ZrB₂ and SiC grains and intergranular amorphous phase. The temperature dependency of flexure strength related to the initial particle size of SiC. In the case of micrometer‐sized SiC, the high‐temperature strength was improved up to 1500°C compared to room‐temperature strength, but the strength degraded at 1600°C, with strength values of 600‐770 MPa. In the case of nano‐sized SiC, the enhanced high‐temperature strength was observed up to 1600°C, with strength values of 680‐840 MPa.     

Reactive Hot Pressing and Properties of Zr1−xTixB2–ZrC Composites

Reactive hot pressing was used to prepare Zr_1?xTi_xB₂–ZrC composites with advantageous microstructure and mechanical properties from ZrB₂–TiC powders. The reaction mechanisms and the effects of different levels of TiC on the physical and mechanical properties of the resulting composite were explored in detail and compared to conventionally hot‐pressed ZrB₂ and ZrB₂–ZrC. Incorporation of 10 to 30 vol% TiC enabled full densification and restrained grain growth, reducing the final average grain size from 5.6 μm in pure ZrB₂ to a minimum of 1.4 μm in samples with 30 vol% TiC. The flexural strengths and hardnesses of the composites sintered with TiC were consequently greater than the conventionally processed ZrB₂–ZrC materials, increasing from 440 MPa and 17.4 GPa to a maximum of 670 MPa and 24.2 GPa at 10 vol% TiC. However, despite a decrease in the total average grain size, the flexural strength at higher TiC levels was limited by an increase in ZrC grain growth, which was observed to determine the flexural strength of the reaction sintered composites similar to the case of ZrB₂–SiC.     

Processing of ZrB2 tribo-ceramics by reactive spark plasma sintering of ZrH2+2B subjected to high-energy pre-ball-milling

The reactive spark plasma sintering (RSPS) of monolithic ZrB₂ ceramics from ZrH₂+2B powder mixtures subjected to shaker pre-milling was investigated, and compared with other three sintering approaches. It was found that RSPS is optimal at 1850 °C, which results in fully-dense ZrB₂ ceramics with ∼20 GPa hardness. Comparatively, at 1850 °C RSPS from the simply-mixed ZrH₂+2B powder mixture, SPS from the commercial ZrB₂ powder, and SPS from the shaker-milled ZrB₂ powder result in non-dense (76.7–86.7%) and softer (6.0–11.8 GPa) ZrB₂ ceramics. Furthermore, the optimally RSPS-ed ZrB₂ ceramic was subjected to unlubricated sliding-wear tests against diamond under 40 N load for 1000 m of sliding, demonstrating that it is a promising tribo-ceramic that only undergoes mild tribo-oxidative wear at 10^–8 mm³/(N·m) in the form of a slight plasticity-dominated two-body abrasion with eventual formation and partial loss of a self-lubricating and protective oxide tribolayer.     

Oxidation behavior of ZrB2-MoSi2-SiC composites in air at 1500 °C

We investigated the oxidation behavior and the effect of the amount of SiC added on oxidation resistance in both hot-pressed ZrB₂-MoSi₂-SiC composites, 55ZrB₂-40MoSi₂-5SiC and 40ZrB₂-40MoSi₂-20SiC (vol.%), exposed to dry air at 1500 °C for up to 10 h. Quantitative electron microprobe analysis characterizations of the chemical compounds of post-oxidized composites were carried out. Parabolic oxidation behavior was observed for both composites. The addition of SiC improved the oxidation resistance of ZrB₂-MoSi₂-SiC composites, and the improvement enhanced with amount of SiC added. The microstructure of the post-oxidized composites consisted of two characteristic regions: oxidized reactive region and unreactive bulk material region. The oxidized reactive region divided into an outermost dense silica-rich scale layer and oxidized reactive mixture layer. The improvement of oxidation resistance with SiC addition is associated with the presence of a thicker dense outermost scale layer which inhibited inward diffusion of oxygen through it.     

Stress measurements in ZrB2–SiC composites using Raman spectroscopy and neutron diffraction

Raman spectroscopy and neutron diffraction were used to study the stresses generated in zirconium diboride–silicon carbide (ZrB₂–SiC) ceramics. Dense, hot pressed samples were prepared from ZrB₂ containing 30 vol% α-SiC particles. Raman patterns were acquired from the dispersed SiC particulate phase within the composite and stress values were calculated to be 810 MPa. Neutron diffraction patterns were acquired for the ZrB₂–SiC composite, as well as pure ZrB₂ and SiC powders during cooling from ～1800 °C to room temperature. A residual stress of 775 MPa was calculated as a function of temperature by comparing the lattice parameter values for ZrB₂ and SiC within the composite to those of the individual powders. The temperature at which stresses began to accumulate on cooling was found to be ～1400 °C based on observing the deviation in lattice parameters between pure powder samples and those of the composite.     

Thermal Shock Resistance of Laminated ZrB2–SiC Ceramic Evaluated by Indentation Technique

In this work, the thermal shock behavior of laminated ZrB₂–SiC ceramic has been evaluated using indentation‐quench method based on propagation of Vickers cracks and compared with the monolithic ZrB₂–SiC ceramic. The results showed that the laminated ZrB₂–SiC ceramic exhibited better resistance to crack propagation and thermal shock under water quenching condition, and the critical temperature difference (ΔT_c) of laminated ZrB₂–SiC ceramic (ΔT_c ≈ 590°C) was much higher than that of monolithic ceramic (ΔT_c ≈ 290°C). The significant improvement in thermal shock resistance was attributed to residual stresses enhancing the resistance to crack growth during thermal shock loading.     

Effect of SiC addition on the formation of ZrB2 through reaction of ZrO2, boric acid (H3BO3) and carbon black

ZrB₂-SiC powders with different amounts of SiC (10–30 wt%) were in-situ synthesized at 1600 °C for 90 min in Ar atmosphere. Effects of SiC addition on the formation of ZrB₂ via carbothermal reduction of ZrO₂, H₃BO₃ and carbon black were investigated. The samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and transmission electron microscope (TEM). The grain size of ZrB₂ in final powders decreased with adding SiC. Columnar ZrB₂ and granular SiC were combined interactively when the SiC content was 25 wt%. Layer-like hexagonal SiC was obtained in the product containing 30 wt% SiC, whereas the ZrB₂ grain growth was strongly inhibited. Furthermore, the growth mechanisms of ZrB₂ and SiC were studied.     

On the crystallite size refinement of ZrB2 by high-energy ball-milling in the presence of SiC

The effect of SiC addition (5, 17.5, and 30 vol.%) on the high-energy ball-milling (HEBM) behaviour of ZrB₂ is investigated. It was found that the presence of SiC during HEBM did not alter ZrB₂ refinement mechanism of repeated brittle fracture followed by cold-welding, thereby leading to the formation of agglomerates consisting of primary nano-particles. SiC did, however, slow down the kinetics of crystallite size refinement and promoted the formation of finer agglomerates. Both of these phenomena became more pronounced with increasing SiC content in the ZrB₂ + SiC powder mixtures, and they were attributed to the energy dissipation effect of the nanocrystalline SiC particles during HEBM of the ZrB₂ + SiC powder mixture. This study offers the first evidence that the addition of harder materials to softer materials can slow down the refinement of crystallite sizes, and thus provides a new mechanism to control crystallite sizes during HEBM. The simultaneous attainment of nano-particles of ZrB₂ and SiC, reduced agglomerate sizes, and homogeneous SiC dispersion at the nanometre scale may have important implications for the ultra-high-temperature ceramic community, as it simplifies the processing route and is likely to facilitate the sintering of ZrB₂-SiC composites.     

Reactive hot-pressing of ZrB2-ZrC-SiC ceramics via direct addition of SiC

A series of ZrB₂-ZrC-SiC composites with various SiC content from 0 to 20 vol% were prepared by reactive hot-pressing using Zr, B₄C and SiC as raw materials. Self-propagating high-temperature synthesis (SHS) occurred, and ZrC grains connected each other to form a layered structure when the SiC content is below 20 vol%. The evolution of microstructure has been discussed via reaction processes. The composite with 10 vol% SiC presents the most excellent mechanical properties (four-point bending strength: 828.6±49.9 MPa, Vickers hardness: 19.9±0.2 GPa) and finest grain size (ZrB₂: 1.52 µm, ZrC: 1.07 µm, SiC: 0.79 µm) among ZrB₂-ZrC-SiC composites with various SiC content from 0 to 20 vol%.     

Fabrication and properties of ZrB2–SiC–BN machinable ceramics

ZrB₂–SiC–BN ceramics were fabricated by hot-pressing under argon at 1800 °C and 23 MPa pressure. The microstructure, mechanical and oxidation resistance properties of the composite were investigated. The flexural strength and fracture toughness of ZrB₂–SiC–BN (40 vol%ZrB₂–25 vol%SiC–35 vol%BN) composite were 378 MPa and 4.1 MPa m^1/2, respectively. The former increased by 34% and the latter decreased by 15% compared to those of the conventional ZrB₂–SiC (80 vol%ZrB₂–20 vol%SiC). Noticeably, the hardness decreased tremendously by about 67% and the machinability improved noticeably compared to the relative property of the ZrB₂–SiC ceramic. The anisothermal and isothermal oxidation behaviors of ZrB₂–SiC–BN composites from 1100 to 1500 °C in air atmosphere showed that the weight gain of the 80 vol%ZrB₂–20 vol%SiC and 43.1 vol%ZrB₂–26.9 vol%SiC–30 vol%BN composites after oxidation at 1500 °C for 5 h were 0.0714 and 0.0268 g/cm², respectively, which indicates that the addition of the BN enhances oxidation resistance of ZrB₂–SiC composite. The improved oxidation resistance is attributed to the formation of ample liquid borosilicate film below 1300 °C and a compact film of zirconium silicate above 1300 °C. The formed borosilicate and zirconium silicate on the surface of ZrB₂–SiC–BN ceramics act as an effective barriers for further diffusion of oxygen into the fresh interface of ZrB₂–SiC–BN.