Mechanical properties of ZrB2–SiC ceramics prepared by polymeric precursor route

ZrB₂–SiC composite ceramics with varying compositions (6.4, 22.3, and 61.5 vol% ZrB₂–SiC) were synthesized and spark plasma sintered (SPS) for 30 min under argon atmosphere. Ceramics showed relatively uniformly distributed phases with small spherical crystallized grains. Vickers hardness and fracture toughness of ceramics were measured, and scratch and tribological behaviors of sintered ceramic specimens were also investigated. According to experimental results, materials having different inter- and trans-granular fractures showed different wear loss, friction efficient, and tribofilm morphology. Ceramics chemically reacted with moisture while being tribotested, leading to the formation of a tribofilm on the bottom of wear track. Characteristics of silica/hydride silica revealed the formation of tribofilms with different morphologies, thereby implying that several key factors are involved in determining the efficiency of this process.     

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.     

High-temperature bending strength,internal friction and stiffness of ZrB2–20 vol% SiC ceramics

Dense ZrB₂–20 vol% SiC ceramics (ZS) were fabricated by hot pressing using self-synthesized high purity ZrB₂ and commercial SiC powders as raw materials. The high temperature flexural strength of ZS and its degradation mechanisms up to 1600 °C in high purity argon were investigated. According to the fracture mode, crack origin and internal friction curve of ZS ceramics, its strength degradation above 1000 °C is considered to result from a combination of phenomena such as grain boundary softening, grain sliding and the formation of cavitations and cracks around the SiC grains on the tensile side of the specimens. The ZS material at 1600 °C remains 84% of its strength at room temperature, which is obviously higher than the values reported in literature. The benefit is mainly derived from the high purity of the ZrB₂ powders.     

Tribological performance of SiC coating for carbon/carbon composites at elevated temperatures

SiC coating was deposited on carbon/carbon (C/C) composites by chemical vapor deposition (CVD). The effects of elevated temperatures on tribological performance of SiC coating were investigated. The related microstructure and wear mechanism were analyzed. The results show that the as-deposited SiC coating consists of uniformity of β-SiC phase. The mild abrasive and slight adhesive wear were the main wear mechanisms at room temperature, and the SiC coating presented the maximum friction coefficient and the minimum wear rate. Slight oxidation of debris was occurred when the temperature rose to 300?°C. As the temperature was above 600?°C, dense oxide film formed on the worn surface. The silica tribo-film replaced the mechanical fracture and dominated the frication process. However, the aggravation of oxidation at elevated temperatures was responsible for the decrease of friction coefficient and the deterioration of wear rate. The SiC coating presented the minimum friction coefficient and the maximum wear rate when the temperature was 800?°C.     

Aqueous Gelcasting and Pressureless Sintering of Zirconium Diboride Ceramics

Dense (97.3%) zirconium diboride (ZrB₂) ceramics were obtained via gelcasting and pressureless sintering. Four wt% B₄C was used as sintering aid. ZrB₂, SiC, and B₄C can codisperse well in the alkaline region, using a polyacrylate dispersant. Compared with monolithic ZrB₂ (Z), the mechanical properties of ZrB₂‐SiC (ZS) were enhanced. The Vickers hardness and fracture toughness of ZS were (13.1 ± 0.6) GPa and (2.5 ± 0.4) MPa m^1/2, respectively.     

Processing and properties of ZrB2-SiC composites obtained by aqueous tape casting and hot pressing

ZrB₂-SiC composites with different SiC content were prepared through aqueous tape casting and hot pressing. The influences of dispersant, SiC content and binder content on the rheological properties of slurries were investigated and the conditions for preparing stable ZrB₂-SiC suspensions were optimized. After tape casting and drying, the green ZrB₂-SiC tapes showed good flexibility, lubricious surface and homogeneous microstructure. The ZrB₂ ceramics could be densified to 97.2% after hot-pressing, while the ZrB₂ containing 20 and 30 vol% SiC ceramics were nearly fully densified (>99%). The sintered ZrB₂-20 vol% SiC ceramic had improved mechanical properties compared with ZrB₂ ceramic. Further increase in SiC content resulted in lower flexural strength and fracture toughness. SEM and TEM showed a fine microstructure with a clear grain boundary. The fracture mode changed from intragranular type for ZrB₂ to both intragranular and intergranular type for ZrB₂-SiC composites.     

Tribological behavior of carbon fiber reinforced ZrB2 based ultra high temperature ceramics

The tribological behavior of ultra-high temperature ceramic matrix composites (UHTCMCs) was investigated to understand these materials in friction applications. Samples consisting of pitch-based randomly orientated chopped carbon fiber (CF) reinforced ZrB₂-10 vol% SiC were prepared (ZS). The tribological behavior was tested on a self-designed dynamometer, coupling the UHTCMC pads with either carbon fiber reinforced carbon−silicon carbide (C/C-SiC) or steel disks, with two applied contact pressures (1 and 3 MPa) and the surface microstructures were analyzed to unravel the wear mechanisms. Even at high mechanical stresses, tests against the C/C-SiC disk showed stable braking performance and wear. The abraded material from a steel disk formed a stable friction film by fusing together harder pad particles with abraded steel, which reduced wear and stabilized the braking performance. The high values of coefficient of friction obtained (0.5–0.7), their stability during the braking and the acceptable wear rate make these materials appealing for automotive brake applications.     

Mechanical properties of ZrB2–SiC(ZrSi2) ceramics

Mechanical properties of ZrB₂–SiC and ZrB₂–ZrSi₂–SiC ceramics in the temperature range from 20 to 1400 °C were studied. It was found that the introduction of zirconium silicide resulted in pore-free ceramics having bending strengths of 400–500 MPa over a wide range of boride–carbide compositions. Zirconium silicide additive did not lead to significant strength and hardness changes at low temperature, but essentially increased Weibull modulus, and, therefore, the reliability of the ceramics. However, zirconium silicide additions resulted in noticeably reduced bending strength in ZrB₂–SiC based composites at 1400 °C.     

Mechanical,tribological and thermal properties of hot pressed ZrB2–SiC composite with SiC of different morphology

ZrB₂–SiC composites were prepared by hot pressing with different sources of SiC to study the effect of SiC with different morphology on densification, microstructure, phase composition and mechanical properties like hardness, fracture toughness and tribological properties (namely, scratch resistance, wear parameters) and thermal behaviour of the composites. Three different ZrB₂–SiC composites, i.e. ZrB₂–SiC_P (polycarbosilane derived SiC), ZrB₂–SiC_C (SiC from CUMI, India) and ZrB₂–SiC_H (SiC from H. C. Starck, Germany), were studied. It is found that ZrB₂–SiC_C composite shows highest hardness (19·13 GPa) and fracture toughness (5·30 MPa m^1/2 at 1 kgf load) in comparison with other composites. Interconnected network, better contiguity between grains of ZrB₂–SiC composites and impurity content in starting powders can play significant roles for achieving high mechanical, tribological and thermal properties of the composites. Coefficient of friction and wear parameters of all ZrB₂–SiC composites are very low, and thermal conductivity of ZrB₂–SiC composites varied from 52·71 to 65·53 W (m K)^?1 (ZrB₂–SiC_P), 54·30 to 71·55 W (m K)^?1 (ZrB₂–SiC_C) and 64·25 to 88·02 W (m K)^?1 (ZrB₂–SiC_H), respectively and also calculate the interfacial resistance of all the composites.     

Microstructure and densification of ZrB2–SiC composites prepared by spark plasma sintering

ZrB₂–SiC composites were prepared by spark plasma sintering (SPS) at temperatures of 1800–2100 °C for 180–300 s under a pressure of 20 MPa and at higher temperatures of above 2100 °C without a holding time under 10 MPa. Densification, microstructure and mechanical properties of ZrB₂–SiC composites were investigated. Fully dense ZrB₂–SiC composites containing 20–60 mass% SiC with a relative density of more than 99% were obtained at 2000 and 2100 °C for 180 s. Below 2120 °C, microstructures consisted of equiaxed ZrB₂ grains with a size of 2–5 μm and α-SiC grains with a size of 2–4 μm. Morphological change from equiaxed to elongated α-SiC grains was observed at higher temperatures. Vickers hardness of ZrB₂–SiC composites increased with increasing sintering temperature and SiC content up to 60 mass%, and ZrB₂–SiC composite containing 60 mass% SiC sintered at 2100 °C for 180 s had the highest value of 26.8 GPa. The highest fracture toughness was observed for ZrB₂–SiC composites containing 50 mass% SiC independent of sintering temperatures.     

Mechanical properties of sintered ZrB2-SiC ceramics

ZrB₂ ceramics containing 10-30 vol% SiC were pressurelessly sintered to near full density (relative density >97%). The effects of carbon content, SiC volume fraction and SiC starting particle size on the mechanical properties were evaluated. Microstructure analysis indicated that higher levels of carbon additions (10 wt% based on SiC content) resulted in excess carbon at the grain boundaries, which decreased flexure strength. Elastic modulus, hardness, flexure strength and fracture toughness values all increased with increasing SiC content for compositions with 5 wt% carbon. Reducing the size of the starting SiC particles decreased the ZrB₂ grain size and changed the morphology of the final SiC grains from equiaxed to whisker-like, also affecting the flexure strength. The ceramics prepared from middle starting powder with an equiaxed SiC grain morphology had the highest flexure strength (600 MPa) compared with ceramics prepared from finer or coarser SiC powders.     

Hot-pressed ZrB2–SiC composite ceramics: Effect of various Ta-containing additives on the microstructure and mechanical properties

The ZrB₂–SiC composites have been commercially used at ultrahigh temperatures, but it often failed due to their poor toughness. In order to solve this problem, four types of Ta-containing additives (Ta, TaC, TaB₂ and TaSi₂) were used as the “third phase” to regulate the microstructure, so as to enhance the mechanical properties of hot-pressed ZrB₂–20SiC-based ceramics (in vol. %). The incorporation of the additives generated a core–shell structure, which comprised of a ZrB₂ core and a (Zr, Ta)B₂ solid solution shell. The additives helped refine the ZrB₂ grains in addition to the metallic Ta and release the internal stress field generated by the thermal misfit. The interfacial structure was modified by the formation of the coherent core/shell interface and the semi-coherent interface of adjacent ZrB₂ grains and the semi-coherent ZrB₂/(Zr, Ta)C interface in the TaC-additive composite. The addition of TaB₂ or TaC hardened the ZrB₂–20SiC ceramics, whereas the addition of Ta or TaSi₂ reduced the hardness. The fracture toughness was enhanced by the formation of the Ta-containing phases. These phases reduced the stress intensity factor of the crack tip, which was proportional to the intrinsic residual stress. However, the crack-propagation mechanism would be changed by the incorporation of various Ta-containing additives. The decrease in the crack deflection, which was induced by the stronger interfacial bonding force and the significant consumption of SiC, resulted in relatively low toughness in the Ta- and TaC-included samples. The weaker interfacial bonding force in the TaB₂- and TaSi₂-included samples caused an increase in deflection and generated branching, which enhanced the toughness of the TaSi₂-included composites to ～4.72 MPa?m^1/2.     

Dry sliding wear behaviour of ZrB2-based ceramics: Self-mated and cross coupling with alumina

Systematic dry sliding wear tests with monolithic ZrB₂ and Al₂O₃ pins coupled to ZrB₂, ZrB₂-20 vol% SiC and Al₂O₃ discs were carried out in a disc-on-pin configuration. The steady state friction of ZrB₂ self-mated or cross coupled with Al₂O₃ was about 1.1. Self-mated monolithic ZrB₂ discs worn about three orders of magnitude more than self-mated Al₂O₃ discs. ZrB₂ pin wear rate was almost double when coupled to ZrB₂ or ZrB₂-20 vol% SiC discs than when coupled to Al₂O₃ discs. The wear track of ZrB₂-based materials showed an oxygen increment due to humidity-driven tribo-reaction. In all the systems, the main wear mechanisms observed were microfracture and abrasion. Numerical calculations and fracture models were employed to describe the wear mechanisms. By nanoindentation tests on worn and unworn areas, a significant lower hardness of the debris layer formed when ZrB₂ materials were involved.     

Tribological properties of SiC-B4C ceramics under dry sliding condition

SiC-B₄C ceramic composites with different ratios of SiC to B₄C were produced. The relative density, mechanical properties, initial surface characteristics, dry sliding tribological properties against SiC ball and worn surface characteristics of the SiC-B₄C ceramics were studied. Results of dry sliding tribological tests showed that, 40 wt. % SiC-60 wt. % B₄C ceramic composite had the best tribological properties in SiC-B₄C ceramic composites. A relief structure with height difference of 10−30 nm between B₄C grains and SiC grains is formed after dry sliding. This relief structure, on the one hand, can reduce real contact area on interface, decreasing adhesion effect, and on the other hand, can fix or trap the wear pieces formed on sliding interface during the dry sliding process, reducing the abrasive wear. However, there is a limit to the beneficial influence of decreased adhesion effect and reduced abrasive wear, and an optimum proportion of relief structure. Pores can also fix or trap some wear pieces, reducing the abrasive wear. Under the condition of strong bonding between SiC grains and B₄C grains, the SiC-B₄C ceramic composites with higher porosity can obtain better tribological properties. In addition, it is observed by AFM that the depth of scratch on B₄C grains is shallower than that on SiC grains. Hence, it is demonstrated by micro scale measurement that the wear rate of B₄C is lower than that of SiC in this study.     

Effect of ZrB2 on the microstructure,mechanical and tribological properties of Mo–Si–B matrix composites

Composites with good mechanical and tribological properties are in high demand for engineering applications. Toward this aim, the Mo–12Si–8.5B alloy with 2.5–10 wt% ZrB₂ ceramic was prepared. The effects of the ZrB₂ content on the microstructure, mechanical properties, and tribological behavior were thoroughly investigated. The composites exhibited reduced density and enhanced hardness and strength owing to the dispersion strengthening of ZrB₂ particles, thus resulting in improved wear resistance. The frictional properties are highly dependent on the ZrB₂ content and counterpart materials. When coupled with GCr15 steel, it shows much slighter abrasive and adhesive wear; therefore, it presents a more preferable anti-wear performance. The wear rate of the composite with 7.5 wt% ZrB₂ showed a minimum value of 2.71 × 10⁻⁷ mm³N⁻¹m⁻¹.     

Effects of TaSi2 addition on room temperature mechanical properties of ZrB2-20SiC composites

The effects of TaSi₂ addition on the room temperature mechanical properties of ZrB₂–20SiC (volume fraction, %) composite were investigated. Dense ZrB₂–20SiC-xTaSi₂ (x?=?3, 6, 10) composites were prepared by hot pressing the mixture of ZrB₂, SiC and TaSi₂ powders at 1850–1900?°C under 30?MPa in flowing Ar. In the as-prepared composites, apart from ZrB₂, SiC and TaSi₂, (Zr,Ta)B₂ solid solution also existed around ZrB₂ grains. It became a continuous layer and the thickness increased gradually with the increase of TaSi₂ content. When the addition of TaSi₂ was within the range of 3–10%, Vickers hardness of ZrB₂–20SiC reduced slightly by about 1–2?GPa, but the flexural strength and fracture toughness increased simultaneously by more than 35%. These phenomena could be attributed to the relatively low hardness of TaSi₂, and the decrease of ZrB₂ grain size as well as the presence of a continuous (Zr,Ta)B₂ solid solution layer around ZrB₂ grains.     

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.     

Fabrication and characterisation of high-performance joints made of ZrB2-SiC ultra-high temperature ceramics

Joining is crucial for ultra-high temperature ceramics (UHTCs) to be used in demanding environments due to the difficulty in manufacturing large and complex ceramic components. In this study, ZrB₂-SiC composite UHTCs parts were joined via Ni foil as filler, and the mechanical properties and oxidation behaviour of the fabricated ZrB₂-SiC/Ni/ZrB₂-SiC (ZS/Ni/ZS) joint were investigated. Firstly, dense ZrB₂-SiC composites were prepared from nano-sized powders by spark plasma sintering (SPS). The ZrB₂-SiC parts were then joined using SPS. Furthermore, the elastic modulus, hardness, shear strength and high temperature oxidation behaviour of the ZS/Ni/ZS joint were examined to evaluate its properties and performance. The experimental results showed that the ZrB₂-SiC parts were effectively joined via Ni foil using SPS and the resultant microstructures were free from any marked defects or residual metallic layers in the joint. Although the elastic modulus and hardness in the joining zone were lower than those in the base ZrB₂-SiC ceramics, the shear strength of the joint reached ∼161 MPa, demonstrating satisfactory mechanical properties. Oxidation tests revealed that the ZS/Ni/ZS joint possesses good oxidation resistance for a wide range of elevated temperatures (800–1600 ^oC), paving the way for its employment in extreme environments.     

Highly-efficient preparation of anisotropic ZrB2–SiC powders and dense ceramics with outstanding mechanical properties

Highly-dense ZrB₂–SiC ceramics with excellent mechanical properties including Vickers hardness of 24.5 GPa and fracture toughness of 4.8 MPa/m^1/2 were successfully prepared, by spark plasma sintering of the raw powders synthesized by a novel molten-salt and microwave co-assisted boro/carbothermal reduction (MSM-BCTR) method. Compared with the processing conditions required for synthesizing ZrB₂–SiC by conventional reduction method, the present MSM-BCTR method possessed a variety of significant merits including the smaller material cost, lower processing temperature (1200°C), and remarkably higher efficiency (soaking time as short as 20 minutes). More importantly, the ZrB₂–SiC powders, resultant from MSM-BCTR treatment, were verified to have single-crystalline nature and uniform well-grown anisotropic morphologies (rod-like ZrB₂ and sheet-like SiC) as well as great potential in promoting the mechanical properties of their bulk counterparts. This great achievement was mainly ascribed to the specific MSM-BCTR conditions characterized by microwave heating and molten-salt medium.     

Microstructure and mechanical properties of (Zr,Ti)B2–SiC composites obtained by pressureless sintering of ZrB2–SiC–TiO2 powder mixtures

Multiphase ceramics have been highlighted due to the combination of different properties. This work proposes to obtain the multiphase composite of (Zr,Ti)B₂–SiC based on the mixture of ZrB₂, SiC, and TiO₂ sintered without pressure. The effect of TiO₂ addition on solid solution formation with ZrB₂, densification, microstructure, and mechanical properties was investigated. For this, 2.0 wt% TiO₂ was added to ZrB₂–SiC composites with 10–30 vol% SiC and processed by reactive pressureless sintering at 2050 °C with a 2 h holding time. Sinterability, crystalline phases, microstructure, Vickers hardness, and indentation fracture toughness of these composites were analyzed and compared to the non-doped ZrB₂–SiC samples. The XRD analysis and EDS elemental map images indicated the incorporation of Ti atoms into the ZrB₂ crystalline structure with solid solution generation of (Zr,Ti)B₂. The addition of TiO₂ resulted in matrix grain size refinement and a predominant intergranular fracture mode. The relative densities were not significantly modified with the TiO₂ addition, though a higher weight loss was detected after the sample sintering process. The composites doped with TiO₂ showed an increase in fracture toughness but exhibited a slightly lower Vickers hardness compared to composites without TiO₂ addition.