Monolithic ZrB2 Research Articles

Microstructure and shear strength of self-joined ZrB 2 and ZrB 2–SiC with pure Ni

Monolithic ZrB2 ceramic and ZrB2–20 vol.% SiC composite were joined to themselves using pure nickel powder. The interfacial phenomena were observed and analyzed. The joining effects were evaluated by shear strength. Resulting from the special interfacial microstructure, the shear strength of ZrB2/Ni/ZrB2 system was as high as 59.7 ± 5.3 MPa. In the ZrB2–SiC/Ni/ZrB2–SiC system, the reaction between Ni and SiC resulted in a homogeneous joint composition, which was beneficial to the joining effects.

The effects of modulation period, modulation ratio, and deposition temperature on microstructure and mechanical properties of ZrB2/W multilayers

Monolithic ZrB2, W coatings and ZrB2/W multilayers with different modulation periods and modulation ratios were synthesized by ion beam assisted deposition at room temperature and 400°C. X-ray diffraction (XRD), scanning electron microscopy (SEM), surface profiler, and nanoindention were employed to investigate the influences of the deposition temperature and the modulation period on the growth, textures, interface structure, and mechanical properties of the multilayers. The results indicated that the multilayer with modulation period of 13 nm synthesized at room temperature possessed a higher hardness of 23.8 GPa. Deposition temperature gave a significant contribution to mechanical property enhancement. The 400°C-deposition temperature led to a maximum hardness and elastic modulus value of 32.1 and 399.1 GPa for ZrB2/W multilayer with a modulation period of 6.7 nm. Its critical load increased to 42.8 mN and residual stress decreased to −0.7 GPa. A higher deposition temperature can cause an increase in interfacial atomic mixture and mobility of surface species, which induceds an increase in areal atomic density and dislocation pinning. These results as well as small nanoscale grain sizes should be related to hardness increase.

Investigations on synthesis of ZrB 2 and development of new composites with HfB 2 and TiSi 2

This paper presents the results of experimental investigations carried out on the synthesis of pure ZrB 2 by boron carbide reduction of ZrO 2 and densification with the addition of HfB 2 and TiSi 2. Process parameters and charge composition were optimized to obtain pure ZrB 2 powder. Monolithic ZrB 2 was hot pressed to full density and characterized. Effects of HfB 2 and TiSi 2 addition on densification and properties of ZrB 2 composites were studied. Four compositions namely monolithic ZrB 2, ZrB 2 + 10% TiSi 2, ZrB 2 + 10% TiSi 2 + 10% HfB 2 and ZrB 2 + 10% TiSi 2 + 20% HfB 2 were prepared by hot pressing. Near theoretical density (99.8%) was obtained in the case of monolithic ZrB 2 by hot pressing at 1850 °C and 35 MPa. Addition of 10 wt.% TiSi 2 resulted in an equally high density of 98.9% at a lower temperature (1650 °C) and pressure (20 MPa). Similar densities were obtained for ZrB 2 + HfB 2 mixtures also with TiSi 2 under similar conditions. The hardness of monolithic ZrB 2 was measured as 23.95 GPa which decreased to 19.45 GPa on addition of 10% TiSi 2. With the addition of 10% HfB 2 to this composition, the hardness increased to 23.08 GPa, close to that of monolithic ZrB 2. Increase of HfB 2 content to 20% did not change the hardness value. Fracture toughness of monolithic sample was measured as 3.31 MPa m 1/2, which increased to 6.36 MPa m 1/2 on addition of 10% TiSi 2. With 10% HfB 2 addition the value of K IC was measured as 6.44 MPa m 1/2, which further improved to 6.59 MPa m 1/2 with higher addition of HfB 2 (20%). Fracture surface of the dense bodies was examined by scanning electron microscope. Intergranular fracture was found to be a predominant mode in all the samples. Crack propagation in composites has shown considerable deflection indicating high fracture toughness. An oxidation study of ZrB 2 composites was carried out at 900 °C in air for 64 h. Specific weight gain vs time plot was obtained and the oxidized surface was examined by XRD and SEM. ZrB 2 composites have shown a much better resistance to oxidation as compared to monolithic ZrB 2. A protective glassy layer was seen on the oxidized surfaces of the composites.

Effect of BN grain size on microstructure and mechanical properties of the ZrB 2–SiC–BN composites

ZrB 2–20 vol.%SiC composites containing 10 vol.% h-BN particles (ZSB) with average grain sizes ranging from 1 μm to 10 μm were hot-pressed. The fracture toughness of the ZSB composites was higher than reported results of monolithic ZrB 2 (2.3–3.5 MPa m 1/2) and SiC particle reinforced ZrB 2 composites (4.0–4.5 MPa m 1/2). The improvement in the fracture toughness of the ZSB composites was due to the high aspect ratio of h-BN and weaker interface bonding, which could enhance crack deflection and stress relaxation near the crack-tip. Compared with the flexural strength of the ZrB 2–SiC composites, the reduction in the flexural strength of the ZSB composites was attributed to the weaker interface bonding and the lower relative density. Furthermore, improvement in toughness and the reduction in the strength were valuable to improve the thermal shock resistance of the ZSB composites. The Δ T c of ZSB5 material is 400 °C which is higher than ZrB 2–20%SiC and ZrB 2–15%SiC–5%AlN.

ZrB 2-ceramic toughened by refractory metal Nb prepared by hot-pressing

ZrB 2–Nb (ZN) composites were prepared through hot-pressing at a temperature of 1800 °C. A contribution of Nb was believed a significant influence on the sinterability, microstructure and mechanical properties of ZN composites. The values of flexural strength of ZN composites rang from 395 to 773 MPa, who are dependent on Nb contents. The highest strength obtained for the ZN composite containing 25 vol.% Nb (773 MPa). A fracture toughness of 7.1 MPa m 1/2 of ZN was revealed, which was much higher than that of monolithic ZrB 2. The improvement in fracture toughness strongly depended on an introduction of Nb–ZrB 2 matrix. Crack deflection and branching were believed to be the toughening mechanism of ZN.

Fabrication of SiC w reinforced ZrB 2-based ceramics

ZrB 2-based ceramics with SiC w were produced by hot pressing at 1750 °C for 1 h from mixed powders after adding liquid polycarbosilane. The obtained ZrB 2-SiC w composites had toughness up to 7.57 MPa m 1/2, which was much higher than those for monolithic ZrB 2, SiC particles reinforced ZrB 2 composites, and other ZrB 2–SiC w composites directly sintered at high temperatures. The added liquid polycarbosilane could reduce the sintering temperatures and restrict the reaction of matrix with whisker, which led to fewer damages to the whisker and high fracture toughness.

Microstructure, mechanical properties and thermal shock resistance of zirconium diboride containing silicon carbide ceramic toughened by carbon black

ZrB 2–20 vol.%SiC ultrahigh temperature ceramic containing 5 vol.% carbon black of nanometer size (ZSC) was prepared by hot-pressing at 1900 °C. The effect of carbon black addition on the microstructure and mechanical properties was investigated. Compared to the ZrB 2–SiC ceramics, the fracture toughness was significantly improved whereas flexural strength and hardness of the ZSC ceramic were slightly degraded. The fracture toughness as high as 6.6 ± 0.5 MPa m 1/2 was much higher than that of monolithic ZrB 2 (2.3–3.5 MPa m 1/2) and ZrB 2–SiC ceramics (4.0–4.5 MPa m 1/2). The toughening mechanisms were the crack deflection, bridging and branching. Furthermore, the effect of carbon black addition on the thermal shock resistance was systematically investigated and discussed.

The influence of N + beam bombardment and deposition temperature on the growth of ZrB 2/WN x nanomultilayers

ZrB 2, WN x and ZrB 2/WN x multilayered coatings were synthesized by ion beam assisted deposition at room temperature and 430 °C. Extensive measurements were taken to investigate the influences of N + bombarding energy and deposition temperature on the microstructure and mechanical properties of the multilayers with different modulation periods. A well-defined composition modulation and layer structure with a mixed polycrystalline was found in the multilayers by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Nano indenter analysis indicated that almost all the multilayers synthesized under N + bombardment at different energies possessed higher hardness and elastic modulus than the rule-of-mixtures value of monolithic ZrB 2 and WN x coatings. The highest hardness and critical fracture load were over 30 GPa and 75.7 mN at 200 eV. Deposition temperature also contributed to mechanical properties. At a constant N + bombarding energy of 200 eV, a deposition temperature of 430 °C produced a maximum hardness value of 36.6 GPa for the ZrB 2/WN x multilayer with an estimated modulation period of 5.2 nm. This hardest multilayer also showed the improved elastic modulus and residual stress.

STRUCTURES AND MECHANICAL PROPERTIES OF MODULATED ZrB2/W AND ZrB2/WNx NANOMULTILAYERS

ZrB 2, W , WN x coatings and ZrB 2/ W , ZrB 2/ WN x multilayered coatings have been synthesized by ion beam assisted deposition at room temperature. X-ray diffraction (XRD), XP-2 surface profiler, scanning electron microscopy (SEM) and nano indenter were employed to investigate the influence of modulation periods and N + beam bombardment on microstructure and mechanical properties of the coatings. The low-angle XRD patterns and cross-sectional SEM indicate a well-defined composition modulation and layer structure of the multilayers. The multilayers with modulation periods ranging from 9 to 16 nm without N + bombardment possessed higher hardness and elastic modulus than the rule-of-mixtures value of monolithic ZrB 2 and W coatings. The highest hardness was 24 GPa. N + bombardment to growing multilayers gave a significant contribution to mechanical property enhancement. When modulation period is 9.6 nm, ZrB 2/ WN x multilayer with 200 eV N + bombardment reveals the highest hardness (30.2 GPa) and elastic modulus. This hardest multilayer also showed the improved residual stress and fracture resistance.

Microstructure and mechanical properties of ZrB2–Nb composite

The high relative density of the ZrB2-based composite toughened by 25 vol.%Nb (ZN) was hot-pressed at reduced temperatures with low pressure of 30 MPa. Compared with the toughness of 2.3–3.5 MPa m1/2 and strength of 350 MPa of the monolithic ZrB2, the toughness and strength of the ZN composite were improved to 6.7 MPa m1/2 and 773 MPa, respectively, due to the addition of ductile Nb. The toughening mechanisms are crack deflection and branching as well as stress relaxation near the crack tip. Furthermore, the densification mechanism was analyzed and discussed. The results here pointed to a potential method for improving fracture toughness and strength of ZrB2-based ceramics.

Modulation periods and ratios induced changes of microstructure and mechanical properties of AlN/ZrB 2 multilayered coatings

AlN/ZrB 2 multilayered coatings were synthesized in a magnetron sputtering system. The extensive measurements were employed to investigate the influence of different nanoscale modulation periods and modulation ratios on microstructure and mechanical properties of the coatings. Analysis of X-ray diffraction, profiler and nanoindention indicated that multilayered coatings possessed much higher hardness and elastic modulus than monolithic AlN and ZrB 2 coatings. At the substrate negative bias of −80 V, maximum hardness (34.1 GPa) and elastic modulus (469.8 GPa) were obtained in the multilayer with Λ = 30 nm and t AlN: t ZrB2 = 1:3. This hardest multilayer showed a marked polycrystalline structure with the strong mixture of ZrB 2 (001), ZrB 2 (100), ZrB 2 (101), AlN (100) textures.

Damage tolerance and R-curve behavior of ZrB 2–ZrO 2 composites

Four ZrB 2 composites were prepared by hot-pressing with different content of ZrO 2, i.e., 0, 10, 20 and 30 vol%. By means of the indentation-strength method, damage tolerance and resistance-curve ( R-curve) behavior were investigated for ZrB 2–ZrO 2 composites. Under the indentation load of 196 N, retained strength of monolithic ZrB 2 was only ∼89 MPa, however, that value could reach ∼281 MPa for ZrB 2–30 vol%ZrO 2 composite. Obvious R-curve behavior was further found in ZrB 2–ZrO 2 composites compared with monolithic ZrB 2. Regarding ZrB 2–30 vol%ZrO 2, the steady-state toughness was ∼6.5 MPa m 1/2. Such improved toughening effect was attributed to the crack bridging and transformation toughening of ZrO 2.

Microstructure and mechanical properties of ZrB 2-SiC composites

A ZrB 2-based composite containing 20 vol.% nanosized SiC particles (ZSN) was fabricated at 1900 °C for 30 min under a uniaxed load of 30 MPa by hot-pressing. The microstructure and mechanical properties of the composite were investigated. It was shown that the grain growth of ZrB 2 matrix was effectively suppressed by submicrosized SiC particles located along the grain boundaries. In addition, the mechanical properties of ZSN composite were strongly improved by incorporating the nanosized SiC particles into a ZrB 2 matrix, especially for flexural strength (925 ± 28 MPa) and fracture toughness (6.4 ± 0.3 MPa•m 1/2), which was much higher than that of monolithic ZrB 2 and ZrB 2-based composite with microsized SiC particles, respectively. The formation of intragranular nanostructures plays an important role in the strengthening and toughening of ZrB 2 ceramic.

Microstructure and thermal shock behavior of ZrB2–SiC–graphite composite

A water-quenching technique was adopted to evaluate the thermal shock behavior of hot-pressed ZrB2–SiC and ZrB2–SiC–graphite composites in an air atmosphere. The strength retention of the two kinds of composites was measured after varying temperature (ΔTc) up to 1000°C. Monolithic ZrB2–SiC showed rapidly drop in flexural strength above ΔT of 400°C, however the critical thermal shock temperature difference (ΔTc) is enhanced by adding graphite, and the ZrB2–SiC–graphite composite showed a higher retained strengths up to 1000°C. The main reasons for a great improvement of thermal shock resistance (TSR) were also discussed and analyzed.

Effect of graphite flake on the mechanical properties of hot pressed ZrB 2–SiC ceramics

A ZrB 2 ceramic containing 20 vol.% SiC and 10 vol.% graphite flake (ZrB 2–SiC–G) was fabricated by hot pressing. It was shown that the fracture toughness was improved due to the introduction of graphite flake, whereas the flexure strength and hardness decreased slightly. The fracture toughness of ZrB 2–SiC–G composite was 6.1 ± 0.3 MPa·m 1/2, which was much higher than that of monolithic ZrB 2, ZrB 2–SiC composite and similar ZrB 2–SiC–C composite. The toughening mechanisms are crack deflection and branching as well as stress relaxation near the crack tip. The results here pointed to a potential method for improving fracture toughness of ZrB 2-based ceramics.

Microstructure and properties of silicon carbide whisker reinforced zirconium diboride ultra-high temperature ceramics

Hot-pressed zirconium diboride (ZrB 2) matrix composites containing 0–30 vol% silicon carbide (SiC) whiskers have been investigated to determine the effect of composition (i.e. amount of SiC whiskers) on the microstructure, mechanical properties and thermal properties. With increasing SiC whisker volume contents, the flexural strength and fracture toughness of the composites were improved compared to those of monolithic ZrB 2. Flexural strength increased from 629 MPa for pure ZrB 2 to 767 MPa for ZrB 2–30 vol%SiCw. Likewise, fracture toughness ranged from 5.4 to 7.1 MPa m 1/2 over the same composition range. Specific heat capacity increased with SiC whisker addition, while thermal diffusivity and thermal conductivity decreased slightly with the increase of SiC whisker content.

Simultaneous Synthesis and Consolidation of W-Added ZrB<sub>2</sub> by Pulsed Electric Current Pressure Sintering and their Mechanical Properties

Dense zirconium boride (ZrB2)-based materials with and without tungsten (W) have been fabricated directly from mixtures of constituent elemental powders by pulsed electric current pressure sintering (PECPS) at 1800°C for 10 min under 30 MPa in a vacuum. Formation processes of monolithic, W-doped ZrB2 solid solutions (Zr1-xWx)B2 (0<x≤0.12), and composites consisting of ZrB2(ss) and WB2 were investigated. Their mechanical properties of Vickers hardness (Hv), fracture toughness (KIC), and bending strength (σb) at room temperature were evaluated. Solid solution and composite materials gave higher Hv (~20.7 GPa), KIC (∼4.4 MPam1/2), and σb (~600 and 690 MPa for the former and the latter, respectively) than those (14.1 GPa, 3.21 MPam1/2, and ~500 MPa) of the monolithic ZrB2 fabricated under the same conditions. Furthermore, the latter two materials exhibited excellent high-temperature σb values (~550-600 MPa) up to 1600°C in N2, in comparison with that (~320 MPa) of monolithic ZrB2 materials.

Mechanical Properties of ZrB<sub>2</sub>-Based Ceramics Reinforced by Nano-SiC Whiskers

In this paper, ZrB2-based ceramics containing up to 15 vol% nano-SiC whiskers were prepared by hot pressing at 1950°C under 20MPa pressure in flow argon. SEM and XRD techniques were used to characterize the sintered compacts. A fine and homogeneous microstructure was observed. The relative density of ZrB2-based ceramic containing 10vol% SiC whiskers reached to 97.7%. The bending strength and fracture toughness of the composite were 550 MPa and 8.08 MPa·m1/2 respectively, while those of the monolithic ZrB2 ceramic (0 vol% SiC whiskers added) were 424 MPa and 4.52 MPa·m1/2 respectively. The grain size of the ZrB2-based ceramics was reduced greatly by the addition of nano-SiC whiskers during the sintering process.

Fabrication and mechanical properties of ZrB2–SiCw ceramic matrix composite

A ZrB2 based ceramic matrix composite containing 20 vol.% SiC whisker was fabricated at 1800 °C for 1 h under a uniaxial load of 40 MPa by hot-pressing. The microstructure and mechanical properties of the composite were investigated. It was shown that mechanical properties were improved by introduction of SiC whisker, especially for the fracture toughness. The fracture toughness was ∼5.97±0.3 MPa m0.5, which was much higher than that of monolithic ZrB2 and SiC particle reinforced ZrB2 composite. The toughening mechanisms were whisker bridging, crack deflection around whiskers as well as whisker pull-out and fracture. The results presented here point to a potential method for improving fracture toughness of ultra-high temperature ceramic.

Processing and Mechanical Properties of Zirconium Diboride‐Based Ceramics Prepared by Spark Plasma Sintering

Zirconium diboride (ZrB2) reinforced by nano‐SiC whiskers has been prepared by spark plasma sintering (SPS). Of most interest is the densification of ZrB2–SiCw composites accomplished by SPS at a temperature as low as 1550°C. The relative density of ZrB2–SiCw composites could reach to 97% with an average grain size of 2–3 μm. Both flexural strength and fracture toughness of the composites were improved with increasing amount of SiCw. Flexural strengths ranged from 416 MPa for monolithic ZrB2to over 545 MPa for ZrB2–15 vol% SiCw composites. Similarly, fracture toughness also increased from 5.46 MPa·m1/2to more than 6.81 MPa·m1/2in the same composition range. The relative density of ZrB2–SiCw composites could be further improved to near 100% by adding some sintering aids such as AlN and Si3N4; however, the effects of different sintering additives on the mechanical properties of the composites were different.