Thermo-mechanical simulation of ultrahigh temperature ceramic composites as alternative materials for gas turbine stator blades

The efficiency of the Bryton cycle strongly depends on the maximum temperature of the cycle. However, restrictions on metallurgical problems deprive engineers from the benefit of high temperatures. Ultrahigh temperature ceramics can be considered in such cases, instead of traditional materials like M152 superalloy. In this study, SiC reinforced HfB₂ and ZrB₂ ultrahigh temperature ceramics were proposed as gas turbine stator blades. The heat transfer and stress-strain equations were solved numerically by the finite element method to obtain temperature and stress distributions. The results showed that the maximum thermal stress occurs in vicinity of the cooling ducts where the temperature gradient is maximum. The maximum displacements of 1.2 mm (for HfB₂–SiC) and 1.14 mm (for ZrB₂–SiC) occur in the upper wall. It can be noticed that the ZrB₂–SiC made blade showed lower maximum stress and displacement than those for the HfB₂–SiC made one, as a result of lower expansion coefficient of ZrB₂–SiC system. The addition of SiC to monolithic HfB₂ and ZrB₂ ceramics decreases their thermal conductivity and following that, the temperature uniformity in blades reduces. Although the thermal stresses and the probability of failure in these stator blades enhance, the ZrB₂–SiC material presented the best performance among the other investigated samples. Both Coulomb-Mohr and Von Mises failure analyses were employed. It was understood that both blades made of HfB₂–SiC and ZrB₂–SiC composites simply withstand the applied stresses with the safety factors of about 1.5.     

Microstructural characterization of ZrB2–SiC based UHTC tested in the MESOX plasma facility

Microstructures were investigated for ZrB₂–SiC and ZrB₂–HfB₂–SiC ultra high temperature ceramics that were subjected to a high temperature plasma environment. Both materials were tested in the MESOX facility to determine the recombination coefficient for atomic oxygen up to 1750 °C in subsonic air plasma flow. Surfaces were analyzed before and after testing to gain a deeper insight of the surface catalytic properties of these materials. Microstructural analyses highlighted oxidation induced surface modification. Oxide layers were composed of silica with trace amounts of boron oxide and zirconia if the maximum temperature was lower than about 1550 °C and zirconia for higher temperatures. The differences in the oxide layer composition may account for the different catalytic behavior. In particular, the presence of a borosilicate glass layer on the surface of ZrB₂–SiC materials guarantees atomic oxygen recombination coefficients that are relatively lower than the coefficients measured when only zirconia is present. The oxidation processes of ZrB₂–HfB₂–SiC materials, associated with catalytic tests carried out up to 1550 °C, lead to the formation of hafnia as well as silica, and zirconia. The higher recombination coefficients measured in the case of ZrB₂–HfB₂–SiC materials can be correlated with the presence of hafnia which is probably characterized by higher catalytic activity compared to zirconia. In any case, the investigated materials demonstrate a low catalytic activity over the inspected temperature range with maximum values of recombination coefficients close to 0.1.     

Mechanical,Thermal, and Oxidation Properties of Refractory Hafnium and zirconium Compounds

The thermal conductivity, thermal expansion, Youngs Modulus, flexural strength, and brittle–plastic deformation transition temperature were determined for HfB₂, HfC_0·98, HfC_0·67, and HfN_0·92 ceramics. The oxidation resistance of ceramics in the ZrB₂–ZrC–SiC system was characterized as a function of composition and processing technique. The thermal conductivity of HfB₂ exceeded that of the other materials by a factor of 5 at room temperature and by a factor of 2·5 at 820°C. The transition temperature of HfC exhibited a strong stoichiometry dependence, decreasing from 2200°C for HfC_0·98 to 1100°C for HfC_0·67 ceramics. The transition temperature of HfB₂ was 1100°C. The ZrB₂/ZrC/SiC ceramics were prepared from mixtures of Zr (or ZrC), SiB₄, and C using displacement reactions. The ceramics with ZrB₂ as a predominant phase had high oxidation resistance up to 1500°C compared to pure ZrB₂ and ZrC ceramics. The ceramics with ZrB₂/SiC molar ratio of 2 (25 vol% SiC), containing little or no ZrC, were the most oxidation resistant.     

Influence of silicon carbide addition on the microstructural development of hot pressed zirconium and titanium diborides

The influences of adding SiC on the microstructure and densification behavior of ZrB₂ and TiB₂ ceramics, hot pressed at 1850 °C for 60 min under 20 MPa, were investigated. The sintered samples were characterized by SEM, EDS and XRD methods. A fully dense TiB₂-based ceramic was obtained by adding 30 vol% SiC. The grain size of ZrB₂ or TiB₂ matrices in the final microstructures decreased with increasing SiC content. The XRD analyses, microstructural characterization as well as thermodynamical calculations proved the in-situ formation of TiC in the SiC reinforced TiB₂-based composites. The interfaces between ZrB₂ and SiC grains in the SiC reinforced ZrB₂-based composites were free of any impurities or tertiary interfacial phases such as ZrC. This result was consistent with the X-ray diffraction pattern and thermodynamics.     

Cutting performance improvement of Si3N4 ceramic cutting tools by adding boride

Cutting performances of silicon nitride (Si₃N₄) ceramic cutting tools with and without boride additive (2.5 vol% ZrB₂ or TiB₂) prepared by hot-pressing at 1500°C were investigated. Due to the α- to β-Si₃N₄ phase transformation and low densification temperature, boride-containing Si₃N₄ ceramics with high hardness and high toughness were obtained. The turning tests showed that the effective cutting lengths of the Si₃N₄–2.5 vol% TiB₂ ceramic (∼2480 m) and Si₃N₄–2.5 vol% ZrB₂ ceramic (∼2200 m) were higher than the monolithic Si₃N₄ ceramic (∼1780 m). As the toughness was improved while maintaining relative high hardness, the cutting performances of the boride-containing Si₃N₄-based inserts were improved by adding 2.5 vol% ZrB₂ or TiB₂. The improved cutting performance indicated that the boride-containing Si₃N₄ ceramics are expected to be used in the field of ceramic cutting tools.     

Thermal properties of La2O3-doped ZrB2- and HfB2-based ultra-high temperature ceramics

Thermal properties of La₂O₃-doped ZrB₂- and HfB₂-based ultra high temperature ceramics (UHTCs) have been measured at temperatures from room temperature to 2000 °C and compared with SiC-doped ZrB₂- and HfB₂-based UHTCs and monolithic ZrB₂ and HfB₂. Thermal conductivities of La₂O₃-doped UHTCs remain constant around 55–60 W/mK from 1500 °C to 1900 °C while SiC-doped UHTCs showed a trend to decreasing values over this range.     

Heat conduction mechanisms in hot pressed ZrB2 and ZrB2–SiC composites

Thermal diffusivity and conductivity of hot pressed ZrB₂ with different amounts of B₄C (0–5 wt%) and ZrB₂–SiC composites (10–30 vol% SiC) were investigated experimentally over a wide range of temperature (25–1500 °C). Both thermal diffusivity and thermal conductivity were found to decrease with increase in temperature for all the hot pressed ZrB₂ and ZrB₂–SiC composites. At around 200 °C, thermal conductivity of ZrB₂–SiC composites was found to be composition independent. Thermal conductivity of ZrB₂–SiC composites was also correlated with theoretical predictions of the Maxwell–Eucken relation. The dominated mechanisms of heat transport for all hot pressed ZrB₂ and ZrB₂–SiC composites at room temperature were confirmed by Wiedemann–Franz analysis by using measured electrical conductivity of these materials at room temperature. It was found that electronic thermal conductivity dominated for all monolithic ZrB₂ whereas the phonon contribution to thermal conductivity increased with SiC contents for ZrB₂–SiC composites.     

Combined role of SiC particles and SiC whiskers on the characteristics of spark plasma sintered ZrB2 ceramics

In this research work, the effects of silicon carbide (SiC) as the most important reinforcement phase on the densification percentage and mechanical characteristics of zirconium diboride (ZrB₂)-matrix composites were studied. In this way, a monolithic ZrB₂ ceramic (as the baseline) and three ZrB₂ matrix specimens each of which contains 25 vol% SiC as reinforcement in various morphologies (SiC particulates, SiC whiskers, and a mixture of SiC particulates/SiC whiskers), have been processed through spark plasma sintering (SPS) technology. The sintering parameters were 1900 °C as sintering temperature, 7 min as the dwell time, and 40 MPa as external pressure in vacuum conditions. After spark plasma sintering, a relative density of ~96% was obtained (using the Archimedes principles and mixture rule for evaluation of relative density) for the unreinforced ZrB₂ specimen, but the porosity of composites containing SiC approached zero. Also, the assessment of sintered materials mechanical properties has shown that the existence of silicon carbide in ZrB₂ matrix ceramics results in fracture toughness and microhardness improvement, compared to those measured for the monolithic one. The simultaneous addition of silicon carbide particulates (SiC_p) and whiskers (SiC_w) showed a synergistic effect on the enhancement of mechanical performance of ZrB₂-based composites.     

Transition metal diboride-silicon carbide-boron carbide ceramics with super-high hardness and strength

Three phase boride and carbide ceramics were found to have remarkably high hardness values. Six different compositions were produced by hot pressing ternary mixtures of Group IVB transition metal diborides, SiC, and B₄C. Vickers’ hardness at 9.8 N was ~31 GPa for a ceramic containing 70 vol% TiB₂, 15 vol% SiC, and 15 vol% B₄C, increasing to ~33 GPa for a ceramic containing equal volume fractions of the three constituents. Hardness values for the ceramics containing ZrB₂ and HfB₂ were ~30% and 20% lower than the corresponding TiB₂ containing ceramics, respectively. Hardness values also increased as indentation load decreased due to the indentation size effect. At an indentation load of 0.49 N, the hardness of the previously reported ceramic containing equal volume fractions of TiB₂, SiC and B₄C was ~54 GPa, the highest of the ceramics in the present study and higher than the hardness values reported for so-called “superhard” ceramics at comparable indentation loads. The previously reported ceramic containing 70 vol% TiB₂, 15 vol% SiC, and 15 vol% B₄C also displayed the highest flexural strength of ~1.3 GPa and fracture toughness of 5.7 MPa·m^1/2, decreasing to ~0.9 GPa and 4.5 MPa·m^1/2 for a ceramic containing equal volume fractions of the constituents.     

Electrical and thermophysical properties of ZrB2 and HfB2 based composites

Electrical resistivities, thermal conductivities and thermal expansion coefficients of hot-pressed ZrB₂–SiC, ZrB₂–SiC–Si₃N₄, ZrB₂–ZrC–SiC–Si₃N₄ and HfB₂–SiC composites have been evaluated. Effects of Si₃N₄ and ZrC additions on electrical and thermophysical properties of ZrB₂–SiC composite have been investigated. Further, properties of ZrB₂–SiC and HfB₂–SiC composites have been compared. Electrical resistivities (at 25 °C), thermal conductivities (between 25 and 1300 °C) and thermal expansion coefficients (over 25–1000 °C) have been determined by four-probe method, laser flash method and thermo-mechanical analyzer, respectively. Experimental results have shown reasonable agreement with theoretical predictions. Electrical resistivities of ZrB₂-based composites are lower than that of HfB₂–SiC composite. Thermal conductivity of ZrB₂ increases with addition of SiC, while it decreases on ZrC addition, which is explained considering relative contributions of electrons and phonons to thermal transport. As expected, thermal expansion coefficient of each composite is reduced by SiC additions in 25–200 °C range, while it exceeds theoretical values at higher temperatures.     

Shock wave-material interaction in ZrB2–SiC based ultra high temperature ceramics for hypersonic applications

We investigate the thermochemical stability of ZrB₂–SiC based multiphase ceramics to hypersonic aerothermodynamic conditions in free piston shock tube with an objective to understand quantitatively the role of thermal shock and pressure. The developed ceramics sustained impulsive thermomechanical shock, under reflected shock pressure of 6.5 MPa and reflected shock temperature of 4160 K in dissociated oxygen, without structural failure. The conjugate heat transfer analysis predicts the surface temperature of ZrB₂–SiC to reach a maximum of 693 and 865 K, for ZrB₂–SiC–Ti. The transient shock-material response is characterized by surface oxidation of the investigated ceramics, when exposed to high enthalpy gaseous environment, as a consequence of the interaction with ultrafast-heated (10⁶ K/s) gas for ~5 ms. Spectroscopic and structural characterization reveals that addition of Ti improves thermomechanical shock resistance, which is attributed to the assemblage of refractory phases. Taken together, ZrB₂–SiC–Ti based multiphase ceramics exhibit favorable shock-material response under impulse loading.     

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.     

Fabrication of SiCw reinforced ZrB2-based ceramics

ZrB₂-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 ZrB₂-SiC_w composites had toughness up to 7.57 MPa m^1/2, which was much higher than those for monolithic ZrB₂, SiC particles reinforced ZrB₂ composites, and other ZrB₂–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.     

Toughening of laminated ZrB2-SiC ceramics with residual surface compression

Laminated ZrB₂-SiC ceramics with residual surface compression were prepared by stacking layers with different SiC contents. The maximum apparent fracture toughness of these laminated ZrB₂-SiC ceramics was 10.4 MPam^1/2, which was much higher than that of monolithic ZrB₂-SiC ceramics. The theoretical predictions showed that the apparent fracture toughness was strongly dependent on the position of the notch tip, which was confirmed by the SENB tests. Moreover, laminated ceramics showed a higher fracture load when the notch tip located in the compressive layer, whereas showed a lower fracture load as the notch tip within the tensile layer. The toughening effect of residual compressive stresses was verified by the appearance of crack deflection and pop-in event. The influence of geometrical parameters on the apparent fracture toughness and residual stresses was analyzed. The results of theoretical calculation indicated that the highest residual compressive stress did not correspond to the highest apparent fracture toughness.     

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.     

Microstructure,mechanical behavior and oxidation resistance of disorderly assembled ZrB2-based short fibrous monolithic ceramics

Axially aligned fibrous monolithic ceramics present non-catastrophic fracture behavior via crack deflection and delamination along cell boundaries. However, severe in-plane anisotropy and time-consuming preparation procedures prevent their extensive promotion. The introduction of high content of weak phase components with poor oxidation resistance in weak interface destroys the excellent oxidation resistance of ceramic matrix. In this work, ZrB₂-based short fibrous monolithic (SFM) ceramics with in-plane isotropic mechanical properties and excellent oxidation resistance were easily prepared by hot pressing randomly assembled short ceramic fibers. The microstructure and mechanical behavior of ZrB₂-based SFM ceramics densified at various temperatures were systematically investigated. The mechanical properties of ZrB₂-based SFM ceramics slightly improved with the increase of sintering temperature. ZrB₂-based SFM ceramics exhibited excellent oxidation resistance and remained intact without macroscopic cracks after ablation for 615 s in oxyacetylene flame with maximum temperatures exceeding 2150 °C. The oxidation behavior was analyzed in detail.     

Effect of sintering temperature on electrical properties of SiC/ZrB2 ceramics

SiC/20?wt% ZrB₂ composite ceramics were fabricated via pressureless solid phase sintering in argon atmosphere at different temperature. The effect of sintering temperature on microstructure, electrical properties and mechanical properties of SiC/ZrB₂ ceramics was investigated. Electrical resistivity exhibits twice significant decreases with increasing sintering temperature. The first decrease from 1900?°C to 2000?°C is attributed to the obvious decrease of continuous pore channels in as-sintered materials. The second decrease from 2100?°C to 2200?°C results from the improvement of carbon crystallization and the disappearance of amorphous layers enveloping ZrB₂ grains. Additionally, the increase of sintered density with increasing temperature caused greatly advance of flexural strength, elastic modulus and Vickers hardness. But excessive temperature is detrimental to flexural strength because of SiC grain growth.     

Thermal properties of ZrB2-TiB2 solid solutions

Zirconium diboride ceramics were prepared with additions of up to 50 vol.% TiB₂. The resulting (Zr,Ti)B₂ ceramics formed complete solid solutions based on x-ray diffraction. The addition of TiB₂ resulted in grain size decreasing from 22 μm for nominally pure ZrB₂ to 7 μm for ZrB₂–50 vol.% TiB₂. The thermal conductivity at 25°C ranged from 93 W/m⋅K for nominally pure ZrB₂ to 58 W/m⋅K for ZrB₂–50 vol.% TiB₂. Thermal conductivity was as high as 67 W/m⋅K for nominally pure ZrB₂ at 2000°C, but dropped to 59 W/m K with the addition of 50 vol.% TiB₂. Electrical resistivity measurements were used to calculate the electron contribution to thermal conductivity, which was 76 W/m⋅K for nominally pure ZrB₂ decreasing to 57 W/m⋅K when 50 vol.% TiB₂ was added. The phonon contribution to thermal conductivity did not change significantly for ≤10 vol.% TiB₂. Additions of ≥25 vol.% TiB₂ reduced the phonon contribution to nearly zero for all temperatures.     

Densification improvement of spark plasma sintered TiB2-based composites with micron-, submicron- and nano-sized SiC particulates

Taguchi design of experiments methodology was used to determine the most influential spark plasma sintering (SPS) parameters on densification of TiB₂–SiC ceramic composites. In this case, four processing factors (SPS temperature, soaking time, applied external pressure and SiC particle size) at three levels were examined in order to acquire the optimum conditions. The statistical analysis identified the sintering temperature as the most effective factor influencing the relative density of TiB₂–SiC ceramics. A relative density of 99.5% was achieved at the optimal SPS conditions; i.e. temperature of 1800?°C, soaking time of 15?min and pressure of 30?MPa by adding 200-nm SiC particulates to the TiB₂ matrix. The experimental measurements and predicted values for the relative density of composite fabricated at the optimum SPS conditions and reinforced with the proper SiC particle size were almost similar. The mechanisms of sintering and densification of spark plasma sintered TiB₂–SiC composites were discussed in details.     

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.