Pressureless sintered in situ toughened ZrB2-SiC platelets ceramics

ZrB₂-SiC composite ceramics were densified by pressureless sintering with addition of Si₃N₄ or MoSi₂ at temperatures that induced SiC anisotropic growth from particles to platelets, within a ZrB₂ matrix with rounded grains. Si₃N₄ addition resulted in the formation of large amounts of liquid phase which enhanced mass transfer mechanisms in terms of matrix grain growth and homogeneous distribution of SiC platelets having an aspect ratio of 3. On the contrary, MoSi₂ helped the densification with local formation of liquid phases leading to a finer matrix with finer SiC platelets, though more agglomerated and with a lower aspect ratio (about 2). These different microstructures had very different fracture properties values, namely a toughness of 3.8 MPa m^1/2 and a strength of 300 MPa for the Si₃N₄-doped composite; toughness of 5 MPa m^1/2 and strength of 410 MPa for the MoSi₂-doped one.     

Ablation behavior of laminated Graphite/ZrB2-SiC ceramics in two different directions

Laminated Graphite/ZrB₂-SiC ceramics were fabricated by tape casting and hot pressing. The ablation properties of the ceramics in the parallel and the perpendicular directions were studied using an oxyacetylene torch. The mass ablation rates were 8.1?±?0.4?mg/s in the parallel direction and 0.2?±?0.1?mg/s in the perpendicular direction. The linear ablation rates were 3.1?±?0.2?µm/s in the parallel direction and 1.2?±?0.1?µm/s in the perpendicular direction. Thus, the ablation resistance of the laminated Graphite/ZrB₂-SiC ceramics in the perpendicular direction was higher than that in the parallel direction. This anisotropy was mainly attributed to the lower surface temperature in the perpendicular direction resulted from higher thermal conductivity, as well as the orientation of the weak graphite interface layer perpendicular to the ablation surface.     

Preparation,microstructure, and mechanical properties of ZrB2-based ceramics with layered Zr2Al4C5 inclusions

ZrB₂/Zr₂Al₄C₅ composite ceramics with different volume contents of Zr₂Al₄C₅ formed in situ were fabricated by the spark plasma sintering technique at 1800 ^°C. The content of Zr₂Al₄C₅ was found to have an evident effect on the preparation, phase constitution, microstructure as well as the mechanical properties of ZrB₂/Zr₂Al₄C₅ ceramics. The results indicated that sinterability of the composites was remarkably improved by the addition of Zr₂Al₄C₅ compared to the single-phase ZrB₂ ceramic. The microstructure of the resulting composites was fine and homogeneous, the average grain size of the ZrB₂ decreased, and the average aspect ratio of the Zr₂Al₄C₅ increased with the increase in the amount of Zr₂Al₄C₅. As the content of Zr₂Al₄C₅ increased, both the Vickers hardness and Young's modulus of the composites first increased and then decreased. The fracture toughness of the ZrB₂–40 vol% Zr₂Al₄C₅ composite was 4.25 MPa m^1/2, which increased by approximately 70% compared to the monolithic ZrB₂ ceramic. The improvement was mainly attributed to the toughening mechanisms such as the layered structure toughening, crack deflection and crack bridging, caused by the in situ formed layered Zr₂Al₄C₅ inclusions.     

Measurement of thermal residual stresses in ZrB2-SiC composites

Neutron diffraction, Raman spectroscopy, and x-ray diffraction were employed to measure the stresses generated in the ZrB₂ matrix and SiC dispersed particulate phase in ZrB₂-30 vol% SiC composites produced by hot pressing at 1900 °C. Neutron diffraction measurements indicated that stresses begin to accumulate at ∼1400 °C during cooling from the processing temperature and increased to 880 MPa compressive in the SiC phase and 450 MPa tensile in the ZrB₂ phase at room temperature. Stresses measured via Raman spectroscopy revealed the stress in SiC particles on the surface of the composite was ∼390 MPa compressive, which is ∼40% of that measured in the bulk by neutron diffraction. Grazing incidence x-ray diffraction was performed to further characterize the stress state in SiC particles near the surface. Using this technique, an average compressive stress of 350 MPa was measured in the SiC phase, which is in good agreement with that measured by Raman spectroscopy.     

Scratch behavior of boron nitride nanotube/boron nitride nanoplatelet hybrid reinforced ZrB2-SiC composites

Spherical instrumented scratch behavior of ZrB₂-SiC composites with and without hybrid boron nitride nanotubes (BNNTs) and boron nitride nanoplatelets (BNNPs) was investigated in this research. Typical brittle fracture such as microcracks both in and beyond the residual groove and grain dislodgement was observed in ZrB₂-SiC composite, while hybrid BN nanofiller reinforced ZrB₂-SiC composite exhibited predominantly ductile deformation. The peculiar three-dimensional hybrid structure in which BNNPs retain their high specific surface area and de-bundled BNNTs extend as tentacles contributes to the improved tolerance to brittle damage. Additionally, easier grain sliding due to BN hybrid nanofillers located at grain boundaries and these BN hybrid nanofillers attached on the scratch surface would provide significant self-lubricating effect to reduce lateral force during scratch and to alleviate contact damage.     

Effect of chromium additive on sintering and oxidation behavior of HfB2-SiC ceramics

The effect of chromium admixture on the processes in the HfB₂-SiC ceramic powder system during its pressureless sintering at 1600?°C was studied. It was shown that an increase in chromium content from 0% to 15.5% in the HfB₂-SiC ceramic powder mixture leads to a continuous increase in its relative density up to 90%. A transient liquid phase Cr-Si-C-B is formed at 1600?°C, and it promotes intense sintering of HfB₂ and SiC powders. The oxidation resistance of HfB₂-SiC-Cr ceramics was studied in static air at 1000–1500?°C. It was shown that the oxidation resistance is greatly improved due to a decrease in the porosity of the sintered ceramic system because of chromium additive. The presence of chromium oxide in the formed surface glassy layer can also lead to the increase in the oxidation resistance. These results suggest that chromium can be considered as a promising sintering additive for HfB₂-SiC and similar systems.     

Microstructures and mechanical properties of 8Y-FSZ ceramics with BaTiO3 additive

The microstructure and mechanical properties of 8 mol% Y₂O₃ fully stabilized zirconia (8Y-FSZ) with BaTiO₃ additive were investigated. The introduction of BaTiO₃ additive would significantly increase the density and the grain size of 8Y-FSZ ceramics. XRD, Raman spectroscopy, and dielectric measurement were performed. A rhombohedral Ba(Ti_1−xZr_x)O₃ ferroelectric phase resulted in the composite with 5 mol% additive, while for those with higher additive content, the secondary phase changes to cubic Ba(Ti_1−xZr_x)O₃. The fracture toughness of the xBaTiO₃/(1−x)8Y-FSZ composites reached a maximum and then decreased with increasing the amount of additive. The highest value reached 6.1 MPa m^1/2 for 0.05BaTiO₃/0.95(8Y-FSZ) sintered at 1475 °C for 3 h, where the piezoelectric/ferroelectric secondary phase toughening played an important role. Moreover, the fracture toughness of the composites increased firstly and then decreased with increasing sintering temperature.     

Temperature-dependent fracture strength and the effect of oxidation for ZrB2-SiC ceramics

Using the stress distribution of the body containing a spherical inclusion, the stress intensity factor at the tip of the annular flaw emanating from the inclusion is formulized. Since the thermal expansion coefficient of matrix and inclusion is not matched, the residual stress is also taken into account. Introducing into the proposed temperature-dependent fracture surface energy or fracture toughness, the temperature-dependent fracture strength for ZrB₂-SiC is obtained. The influence of oxidation on the fracture strength is also discussed and the analysis reveals that the oxidation has significant effect on the fracture strength under some circumstances. The calculated results are compared with the experimental data and they have very good consistency.     

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.     

Improved fracture toughness of laminated ZrB2-SiC-MoSi2 ceramics using SiC whisker

In this study, SiC whisker (SiC_w) was introduced to ZrB₂ matrix layer of laminated ZrB₂/BN ceramics to improve fracture toughness. Laminated ZrB₂-SiC_w/BN ceramics were prepared by tape casting and spark plasma sintering. For comparison, monolithic ZrB₂-SiC_w and laminated ZrB₂-SiC_p/BN ceramics were also prepared using the same method. The introduction of SiC whiskers increased fracture toughness of laminated ZrB₂-SiC_w/BN ceramics to 13.31?±?0.33?MPa?m^1/2 for all samples. This was related to the multi-scale toughening mechanism, including delamination and crack deflection issued from the laminate structure at the macroscopic level, as well as whiskers bridging and pullout at the microscopic view. The R-curve behaviors of all samples revealed improved resistance to crack propagation of laminated ZrB₂-SiC_w/BN when compared to ZrB₂-SiC_p/BN and ZrB₂-SiC_w issued from multi-scale toughening design.     

Study of tribological properties of polymer derived ZrB2-SiC ceramics

ZrB₂–SiC composite ceramics with different compositions (20 and 60?vol% ZrB₂–SiC, 20ZS and 60 ZS, respectively) were prepared. Wear tests were conducted on the obtained ceramics in multiple distances using ball-on-flat tribotester. Volume loss and cross-sectional profiles of samples were measured by three–dimensional (3D) profilometer to study the onset of track wear damages. Pressure–depth curves and hardness were measured by indentation to investigate defects produced in the tribo-film by the debris. The debris of 20ZS was found to be joined to the tribo-film and accumulated with distance, shifting from microcrack (＜10,000 cycles) to abrasive wear (50,000 cycles). Compared to 20ZS, lower debris accumulation of 60ZS resulted in better wear resistance, leading to thinner and more stable non-substrate regions for this sample. These differences between both samples basically resulted from different particle sizes. Fine grains were easily pulled out in the experiment, resulting in abrasive wear of the specimen. While transgranular fracture of grains and the pinning led to larger grains with less debris, the damage mode remained transgranular fracture.     

Comparative evaluation of two different methods for thermal shock resistance of laminated ZrB2-SiCw/BN ceramics

The thermal shock resistance (TSR) of laminated ZrB₂–SiC_w/BN ceramic was evaluated through indentation-quench and quenching-strengthening methods. It was correspondingly compared to monolithic ZrB₂–SiC_w ceramic. In the indentation-quench method with consideration to crack propagation on the surface layer, the critical thermal shock temperature of laminated ZrB₂–SiC_w/BN ceramic with surface residual tensile stress was 550?°C, which was lower than monolithic ZrB₂–SiC_w ceramic (600?°C). Unlike the microscopic method of crack growth measurement through indentation-quench testing, the quenching-strengthening method, which was based on the macroscopic properties of the material, mainly characterizing the residual strength subsequently to thermal shock, the critical thermal shock temperatures of the laminates and monolithic were 609?°C and 452?°C, respectively. Compared to the brittle fracture of ZrB₂–SiC_w ceramics, the deflection, bifurcation and delamination of the cracks as the main TSR mechanisms of the laminated ceramics, were revealed through quenching-strengthening method, which was more suitable for the TSR characterization of laminated ceramics.     

ZrB2-SiC复相陶瓷的制备及其性能的研究

以ZrB2和SiC粉为原料,采用Si3N4球为球磨介质,通过等静压成型及无压烧结制备了ZrB2-SiC复相陶瓷。实验确定了ZrB2-SiC复相陶瓷的烧结制度,并研究了SiC含量与球磨时间对ZrB2-SiC复相陶瓷体积密度的影响。结果表明:随着ZrB2球磨时间的增加,ZrB2颗粒粒径逐渐减小,复相陶瓷的体积密度逐渐增加;随着SiC含量的增加,复相陶瓷体积密度降低。ZrB2最佳球磨时间为6h,SiC最佳含量为20%,ZrB2-SiC20%(体积分数)复相陶瓷体积密度达到4.98g/cm3。     

High temperature oxidation behavior of ZrB2-SiC added MoSi2 ceramics

In this paper, MoSi₂, MoSi₂-20?vol% (ZrB₂-20?vol% SiC) and MoSi₂-40?vol% (ZrB₂-20?vol% SiC) ceramics were prepared using pressureless sintering. The oxidation behaviors of these MoSi₂-(ZrB₂-SiC) ceramics were investigated at 1600?°C for different soaking time of 60, 180 and 300?min, respectively. The oxidation behaviors of the MoSi₂-(ZrB₂-SiC) ceramics were studied through weight change test, oxide layer thickness measurement, and microstructure analysis. Further investigation of the oxidation behaviors of the MoSi₂-(ZrB₂-SiC) ceramics was conducted at a higher temperature of 1800?°C for 10?min. The microstructure evolution of the ceramics was also analyzed. It was finally found that the oxidation resistance of MoSi₂ was improved by adding ZrB₂-SiC additives, and the MoSi₂-20?vol% (ZrB₂-20?vol% SiC) ceramic exhibited the optimal oxidation resistance behavior at elevated temperatures. From this study, it is believe that it can give some fundamental understanding and promote the engineering application of MoSi₂-based ceramics at high temperatures.     

Toughening and strengthening zirconia through the addition of a transient solid solution additive

A very small amount of nickel oxide (NiO), 0.18 mol%, could dissolve into yttria-stabilized zirconia (YSZ) during sintering in air at elevated temperature. The presence of Ni solutes enhances both the densification and grain growth of YSZ specimens. By heat-treating the NiO-doped YSZ specimen in a reducing atmosphere, nano-sized Ni particles are produced at the grain boundaries. The NiO thus acts as a transient solid solution additive for the YSZ-Ni nanocomposite. The formation of Ni nano-particles introduces an extra ferromagnetic performance into the YSZ specimen. Furthermore, the toughness and strength of YSZ are enhanced respectively by 120% and 40%. The toughness enhancement shows strong dependence on the size of ZrO₂ grains. Nevertheless, the strengthening is contributed by many factors.     

Development of Al2O3–SiC composite tool for machining application

Al₂O₃–SiC composites containing up to 30 wt.% of dispersed SiC particles (280 nm) were fabricated via hot-pressing and machined as cutting tools. The Al₂O₃–SiC particulate composites exhibit higher hardness than their unreinforced matrix because of the inhibited grain growth by adding SiC and the presence of hard secondary phase (SiC). The fracture toughness of the composites remains constant up to 10 wt.% loading of SiC. For machining heat-treated AISI 4144140 steel, the Al₂O₃–10 wt.% SiC composite tool showed the longest tool life, seven times longer than a commercial tool made of Al₂O₃–TiC composite, while the composite tool with 5 wt.% SiC showed the longest tool life for machining gray cast iron. The improved performance of the Al₂O₃–SiC composite tools attributes to the transformation of fracture mode from intergranular fracture for Al₂O₃ to intragranular fracture for Al₂O₃–SiC composites.     

Fracture strength and toughness of chemical-vapor-deposited polycrystalline diamond films

The strength of diamond films prepared by chemical vapor deposition (CVD) is usually much lower than that of natural-type IIa single-crystal diamonds. In this work, the fracture strength of free-standing diamond films deposited by direct current arc plasma jet CVD has been examined by conducting three-point bending experiments. The results obtained for both the polished and as-grown samples were in good agreement with a well-known Hall–Petch equation describing the relationship between the fracture strength and grain size, indicating that grain refinement represented an effective way of improving the mechanical properties of CVD diamond films. Furthermore, the diversification of the crystalline texture of the films achieved by polishing apparently increased their fracture strength, which was inversely proportional to the film thickness. A theoretical method for estimating the fracture strength of free-standing CVD diamond films by approximating their intrinsic strength was proposed, whereas their fracture toughness was determined by conducting simplified four-point bending tests at room temperature, 25?°C, using a single-edge pre-cracked beam method.     

Damage resistance and R-curve behavior of multilayer Al2O3/SiC ceramics

Damage resistance and R-curve behavior of multilayer Al₂O₃/SiC ceramics were evaluated in bending by the indentation-strength and the single-edge-notched-beam methods. Due to the crack deflection at the Al₂O₃/SiC interfaces, a plateau indentation strength response was achieved, suggesting an exceptional resistance to contact-induced damage. Moreover, fracture toughness was observed to increase from 8.0 to 15.5 MPa m^1/2 with increasing notch depth from 0.5 to 2.0 mm, indicative of a strong R-curve behavior.     

Stress induced environmental damage in precursor derived SiBCN ceramics

Boron modified polyvinylsilazane was thermolyzed at 1300 °C under argon atmosphere and subsequently spark plasma sintered at 1600 °C in vacuum. The material was subjected to highly saline and constant vapor pressure environments individually to understand the influence of chemistry in propagating the subcritical cracks. The failure behavior in chloride environment was compared against humid environment using the data extracted from Raman spectroscopy, the thermodynamic characteristics of the environment and the micro-sized pore concentrations. The implications of the subcritical crack growth were derived by plotting ν–K_I graphs for each of the samples to clearly distinguish between the effects of the surrounding mediums. The impulse to crack propagation was found to be resistive with variations in degree of crack growth alternating directly with the changes in porosity, surrounding atmosphere and defect density. The rate of energy dissipation and its hindrance at the crack tip were found to be a major player, among others, in reducing the crack propagation rate.     

Fracture behaviour of Mg-PSZ ceramics: Comparative estimates

The mechanical behaviour of different Mg-PSZ ceramics is studied. Results of their edge fracture (EF) and single edge V-notch beam (SEVNB) tests are discussed. These inelastic ceramics exhibit nonlinear relations between the fracture load and the distance from the extreme point on the chip scar to the specimen edge. They also possess nonlinearly rising R-lines. It is established that the data points plotted in the EF base diagram fall below the baseline (lower barrier to the onset of fracture). By projecting these data points onto the baseline, fracture toughness values close to those of the matrix are determined. The limitations of conventional procedures for evaluating the mechanical behaviour of these ceramics were found out. It has been demonstrated that the EF test method can be quite adequate for enhancing the reliability of comparative fracture resistance estimates.