Preparation of high solid loading,low viscosity ZrB2–SiC aqueous suspensions using PEI as dispersant

PEI was used as dispersant for ZrB₂ and SiC powders in water. The dispersion behavior of ZrB₂ and SiC in water was studied by zeta potential measurements, particle size distribution measurements and interparticle interaction calculations. Well-dispersed ZrB₂ and SiC aqueous suspensions were obtained using 0.6 wt% PEI at pH 6. The rheological behavior of ZrB₂–SiC aqueous suspensions was also investigated. Finally, a high solid loading (52 vol%), low viscosity (980 mPa s at 100 s⁻¹) ZrB₂–SiC aqueous suspension was successfully prepared.     

Thermal conductivity in mullite/ZrO2 composite coatings

Mullite-based multilayered structures have been suggested as promising environmental barrier coatings for Si₃N₄ and SiC ceramics. Mullite has been used as bottom layer because its thermal expansion coefficient closely matches those of the Si-based substrates, whereas Y–ZrO₂ has been tried as top layer due to its stability in combustion environments. In addition, mullite/ZrO₂ compositions may work as middle layers to reduce the thermal expansion coefficient mismatch between the ZrO₂ and mullite layers. Present work studies the thermal behaviour of a flame sprayed mullite/ZrO₂ (75/25, v/v) composite coating. The changes in crystallinity, microstructure and thermal conductivity of free-standing coatings heat treated at two different temperatures (1000 and 1300 °C) are comparatively discussed. The as-sprayed and 1000 °C treated coatings showed an almost constant thermal conductivity (K) of 1.5 W m⁻¹ K⁻¹. The K of the 1300 °C treated specimen increased up to twice due to the extensive mullite crystallization without any cracking.     

ZrB2–SiC gradient oxidation protective coating for carbon/carbon composites

To improve the oxidation protective ability of carbon/carbon composites, ZrB₂–SiC gradient coating was prepared on the surface of C/C composites by an in-situ reaction method. The ZrB₂–SiC gradient coating consisted of an inner ZrB₂–SiC layer and an outer ZrB₂–SiC–Si coating. The phase composition and microstructures of the multiphase coating were characterized by XRD, EDS and SEM. Results showed that the inner coating is mainly composed of ZrB₂ and SiC, while the outer multiphase coating is composed of ZrB₂, SiC and Si. The multilayer coating is about 200 μm in thickness, which has no penetration crack or big hole. The oxidation behavior of the coated C/C composites at 1773 K in air was investigated. Results show that the gradient ZrB₂–SiC oxidation protective coating could protect C/C from oxidation for 207 h with only (4.56±1.2)×10⁻³ g/cm² weight loss, owing to the compound silicate glass layer with the existence of thermally stable phase ZrSiO₄.     

Ablation behavior of C/SiC with YSZ and ZrB2 coatings under high-energy laser irradiation

Carbon fiber-reinforced silicon carbide (C/SiC) composites are important candidates for laser protection materials. In this study, ablation mechanism of C/SiC coated with ZrO₂/Mo and ZrB₂–SiC/ZrO₂/Mo under laser irradiation was studied. ZrB₂–SiC multiphase ceramic and ZrO₂ ceramic were successfully coated on C/SiC composite by atmospheric plasma spraying technology with Mo as transition layer. Phase evolution and morphology of composite were investigated by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Moreover, ablation behavior of the composite was investigated by laser confocal microscopy. Results showed that ablation mechanism of C/SiC composite was controlled by phase transformation, thermal reaction, and thermal diffusion, with solid–liquid transition of ZrB₂ and ZrO₂ being dominant factor. Endothermic reaction and good thermal diffusivity of coatings were also important factors affecting ablation performance. Reflectivity effect of ZrO₂ coating was limited under high-energy laser irradiation. Compared with ZrO₂/Mo single-phase-monolayer coating, designed ZrB₂–SiC/ZrO₂/Mo coating showed better ablation performance, and breakdown time of C/SiC increased from 10 to 40 s. The depletion of liquid phase in molten pool was identified as an important factor responsible for rapid failure of C/SiC. The coating failed when the entire liquid phase was consumed within molten pool, followed by rapid damage of C/SiC substrate. Results of this study can provide theoretical guidance and research ideas for design and application of laser protective materials.     

Effect of SiC content,additives and process parameters on densification and structure–property relations of pressureless sintered ZrB2–SiC composites

The effect of SiC content, additives, and process parameters on densification and structure–property relations of pressureless sintered ZrB₂–(10–40 vol%) SiC particulate composites have been studied. The ZrB₂–SiC composite powders mixed by ball-milling with 1.2 wt% C (added as phenolic resin) and 3 wt% B₄C have been uniaxially cold-compacted and sintered in argon environment at 1950–2050 °C for 2 h, or at 2000 °C for durations between 1/2 and 3 h. The amount of densification is found to increase with sintering duration, and by prior holding at 1250 and 1600 °C for reduction of oxide impurities (ZrO₂, B₂O₃ and SiO₂) on powder particle surfaces by the aforementioned additives. Presence of SiC with average size smaller than that of ZrB₂ appears to aid in densification by enhancing green density, increasing WC content by erosion of milling media, and inhibiting matrix grain growth. Both SiC and WC appear to aid in reduction of oxide impurities. Furthermore, the impurities enriched in W, Fe and Co obtained from milling media are found to be segregated at ZrB₂ grain boundaries, and appear to assist in densification by forming liquid phase, which completely wets the ZrB₂ grains. Hardness increases with SiC content or with sintering duration till 1 h, but decreases for periods ≥2 h due to grain growth. The experimentally measured elastic moduli approaches corresponding theoretically predicted values with increasing SiC content due to reduction in porosity.     

TEM Study of the High‐Temperature Oxidation Behavior of Hot‐Pressed ZrB2–SiC Composites

The oxidation behaviors of ZrB₂‐ 30 vol% SiC composites were investigated at 1500°C in air and under reducing conditions with oxygen partial pressures of 10⁴ and 10 ^{? 8} Pa, respectively. The oxidation of ZrB₂ and SiC were analyzed using transmission electron microscopy (TEM). Due to kinetic difference of oxidation behavior, the three layers (surface silica‐rich layer, oxide layer, and unreacted layer) were observed over a wide area of specimen in air, while the two layers (oxide layer, and unreacted layer) were observed over a narrow area in specimen under reducing condition. In oxide layer, the ZrB₂ was oxidized to ZrO₂ accompanied by division into small grains and the shape was also changed from faceted to round. This layer also consisted of amorphous SiO₂ with residual SiC and found dispersed in TEM. Based on TEM analysis of ZrB₂ – SiC composites tested under air and low oxygen partial pressure, the ZrB₂ begins to oxidize preferentially and the SiC remained without any changes at the interface between oxidized layer and unreacted layer.     

Reactive infiltration processing (RIP) of ultra high temperature ceramics (UHTC) into porous C/C composite tubes

A novel reactive infiltration processing (RIP) technique was employed to infiltrate porous carbon fibre reinforced carbon (C/C) composite hollow tubes with ultra high temperature ceramic (UHTC) particles such as ZrB₂. The C/C composite tubes had initial porosity of ∼60% with a bimodal (10 μm and 100 μm) pore size distribution. A slurry with 40-50% ZrB₂ solid loading particles was used to infiltrate the C/C tubes. Our approach combines in situ ZrB₂ formation with coating of fine ZrB₂ particles on carbon fibre surfaces by a reactive processing method. A Zr and B containing diphasic gel was first prepared using inorganic-organic hybrid precursors of zirconium oxychloride (ZrOCl₂·8H₂O), boric acid, and phenolic resin as sources of zirconia, boron oxide, and carbon, respectively. Then commercially available ZrB₂ powder was added to this diphasic gel and milled for 6 h. The resultant hybrid slurry was vacuum infiltrated into the porous hollow C/C tubes. The infiltrated tubes were dried and fired for 3 h at 1400 °C in flowing Ar atmosphere to form and coat ZrB₂ on the carbon fibres in situ by carbothermal reaction. Microstructural observation of infiltrated porous C/C composites revealed carbon fibres coating with fine nanosized (∼100 nm) ZrB₂ particles infiltrated to a depth exceeding 2 mm. Ultra high temperature ablation testing for 60 s at 2190 °C suggested formation of ZrO₂ around the inner bore of the downstream surface.     

Supersonic atmospheric plasma sprayed ZrO2 and ZrB2-SiC/ZrO2 coatings on Cf/Mg composites for anticorrosion

To improve the corrosion resistance of the carbon fiber reinforced magnesium matrix composites (C_f/Mg composites), ZrO₂ and ZrB₂-SiC/ZrO₂ composite coatings were prepared by supersonic atmospheric plasma spraying (SAPS) on C_f/Mg composites. The microstructure and phase composition of the coatings before and after the corrosion test were investigated. Open circuit potential and potentiodynamic polarization tests were measured at room temperature. Results revealed that the corrosion current density ($i_{\mathit{corr}}$) of the ZrO₂ coated C_f/Mg composites decreased by one order while the ZrB_2-SiC/ZrO₂ coated C_f/Mg composites reduced by two orders. Compared with C_f/Mg composites, the corrosion potential ($E_{\mathit{corr}}$) of the ZrO₂ and ZrB₂-SiC/ZrO₂ coated C_f/Mg composites increased by 220.5?mV and 1021.8?mV respectively, indicating that the ZrB₂-SiC/ZrO₂ composite coatings greatly improve the corrosion resistance of C_f/Mg composites. The uniform distribution of the SiC particles with small grain size in ZrB₂ is responsible for the densification of the coating. The ZrB₂-SiC/ZrO₂ composite coatings provide a barrier for the substrate to impede the entry of Cl^- in the corrosion solution, thus exhibiting a better corrosion resistance than the ZrO₂ coating.     

Compressive strength degradation in ZrB2-based ultra-high temperature ceramic composites

The high temperature compressive strength behavior of zirconium diboride (ZrB₂)-silicon carbide (SiC) particulate composites containing either carbon powder or SCS-9a silicon carbide fibers was evaluated in air. Constant strain rate compression tests have been performed on these materials at room temperature, 1400, and 1550 °C. The degradation of the mechanical properties as a result of atmospheric air exposure at high temperatures has also been studied as a function of exposure time. The ZrB₂-SiC material shows excellent strength of 3.1 ± 0.2 GPa at room temperature and 0.9 ± 0.1 GPa at 1400 °C when external defects are eliminated by surface finishing. The presence of C is detrimental to the compressive strength of the ZrB₂-SiC-C material, as carbon burns out at high temperatures in air. As-fabricated SCS-9a SiC fiber reinforced ZrB₂-SiC composites contain significant matrix microcracking due to residual thermal stresses, and show poor mechanical properties and oxidation resistance. After exposure to air at high temperatures an external SiO₂ layer is formed, beneath which ZrB₂ oxidizes to ZrO₂. A significant reduction in room temperature strength occurs after 16-24 h of exposure to air at 1400 °C for the ZrB₂-SiC material, while for the ZrB₂-SiC-C composition this reduction is observed after less than 16 h. The thickness of the oxide layer was measured as a function of exposure time and temperatures and the details of oxidation process has been discussed.     

Evaluation of Rare‐Earth Modified ZrB2–SiC Ablation Resistance Using an Oxyacetylene Torch

Rare‐earth modified ZrB₂–SiC coatings were prepared via mechanical mixing Sm₂O₃ or Tm₂O₃ powders with spray‐dried ZrB₂, or by chemically doping samarium ions into spray‐dried ZrB₂. In either approach, SiC powders were also added and coatings were fabricated via shrouded air plasma spray. An oxyacetylene torch was utilized to evaluate the coatings under high heat flux conditions for hold times of 30 and 60 s. The resulting phases and microstructures were evaluated as a function of rare‐earth type, modification approach, and ablation time. A brittle m‐ZrO₂ scale was observed in the ZrB₂/SiC‐only coating after ablative tests; during cooling this scale detached from the unreacted coating. In contrast, rare‐earth modified coatings formed a protective oxide scale consisting primarily of either Sm_0.2Zr_0.8O_1.9 or Tm_0.2Zr_0.8O_1.9, along with small amount of m‐ZrO₂. These rare‐earth oxide scales displayed high thermal stability and remained adhered to the unreacted coating during heating and cooling, offering additional oxidation protection.     

Pressureless sintering of silicon carbide ceramics containing zirconium diboride

SiC-5 wt.% ZrB₂ composite ceramics with 10 wt.% Al₂O₃ and Y₂O₃ as sintering aids were prepared by presureless liquid-phase sintering at temperature ranging from 1850 to 1950 °C. The effect of sintering temperature on phase composition, sintering behavior, microstructure and mechanical properties of SiC/ZrB₂ ceramic was investigated. Main phases of SiC/ZrB₂ composite ceramics are all 6H-SiC, 4H-SiC, ZrB₂ and YAG. The grain size, densification and mechanical properties of the composite ceramic all increase with the increase of sintering temperatures. The values of flexural strength, hardness and fracture toughness were 565.70 MPa, 19.94 GPa and 6.68 MPa m^1/2 at 1950 °C, respectively. The addition of ZrB₂ proves to enhance the properties of SiC ceramic by crack deflection and bridging.     

Ablation of C/SiC,C/SiC–ZrO2 and C/SiC–ZrB2 composites in dry air and air mixed with water vapor

C/SiC composites with different additives (ZrO₂ and ZrB₂) were fabricated by CVI and CVD and their oxidation and ablation properties at 1700–1800 °C were investigated. Two different ablation test conditions, dry air and air mixed with water vapor, are compared. The ablation test results are reviewed, the weight loss rates are presented and the corresponding micro-structures are investigated in detail. The results show that in dry air, the weight loss rate of C/SiC composites is greater than those with ZrO₂ and ZrB₂ additives. However, in air mixed with water vapor (5 wt%) to simulate the hygrothermal condition, the weight loss rates of these three composites all become relatively smaller. A model is proposed to predict the weight loss of C/SiC composites and it agrees well with the experimental data.     

Evaluation of oxy-acetylene flame angle effect on the erosion resistance of SiC/ZrB2–SiC/ZrB2 multilayer coatings fabricated by the shielding shrouded plasma spray technique

In the present study, the effect of oxy-acetylene flame angle on the erosion resistance of SiC/ZrB₂–SiC/ZrB₂ multilayer coatings with the gradient structure was investigated. To this aim, first, the SiC inner layer was applied by the reactive melt infiltration (RMI) technique; then ZrB₂ and ZrB₂–SiC layers with 10, 20 and 30%wt. SiC were applied on graphite by the plasma spraying technique. To prevent the oxidation of ZrB₂ and SiC particles, the plasma spraying process was performed by a solid protective shield. To evaluate the performance of the coatings in erosive environments, the samples were exposed to oxy-acetylene flame at the angles of 30°, 60° and 90° for 360 s; the destruction mechanism of SiC/ZrB₂–SiC/ZrB₂ multilayer coatings appeared to be controlled mechanically and chemically. The results of the erosion test showed that at the low flame angles of about 30°, due to the shear forces of oxy-acetylene flame, mechanical erosion overcame the chemical one. With increasing the flame angle, due to raising the surface temperature, chemical erosion overcame the mechanical one; so, most chemical destruction occurred at the flame angle of 90°. Also, the results of the erosion test showed that the total chemical and mechanical destruction at the angle of 60° was greater than that in other angles. Also, among the coatings tested, SiC/ZrB₂- 20% wt. SiC/ZrB₂ coatings had the best erosion resistance; so, the weight changes under the oxy-acetylene flame at the angles of 30° and 60°, respectively, were about ?0.038%. and ?0.355%; meanwhile, at the angle of 90°, it was about +4.3%.     

Lanthanum-titanium-aluminum oxide: A novel thermal barrier coating material for applications at 1300 °C

Considerable efforts are being invested to explore new thermal barrier coating (TBC) materials with higher temperature capability to meet the demand of advanced turbine engines. In this work, LaTi₂Al₉O₁₉ (LTA) is proposed and investigated as a novel TBC material for application at 1300 °C. LTA showed excellent phase stability up to 1600 °C. The thermal conductivities for LTA coating are in a range of 1.0-1.3 W m⁻¹ K⁻¹ (300-1500 °C) and the values of thermal expansion coefficients increase from 8.0 to 11.2 × 10⁻⁶ K⁻¹ (200-1400 °C), which are comparable to those of yttria stabilized zirconia (YSZ). The microhardness of LTA and YSZ coatings were in the similar level of ∼7 GPa, however, the fracture toughness value was relatively lower than that of YSZ. The lower fracture toughness was compensated by the double-ceramic LTA/YSZ layer design, and the LTA/YSZ TBC exhibited desirable thermal cycling life of nearly 700 h at 1300 °C.     

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.     

Preliminary study on suspension plasma sprayed ZrO2 + 8 wt.% Y2O3 coatings

ZrO₂ + 8 wt.% Y₂O₃ powder of a mean diameter d_VS = 38 μm was milled to obtain fine particles having mean size of d_VS = 1 μm. The fine powder was used to formulate a suspension with water, ethanol and their mixtures. The zeta potential of obtained suspensions was measured and found out to be in the range from −22 to −2 mV depending on suspension formulation. The suspension was injected through a nozzle into plasma jet and sprayed onto stainless steel substrates. The plasma spray experimental parameters included two variables: (i) spray distance varying from 40 to 60 mm and (ii) torch linear speed varying from 300 to 500 mm/s. The microstructure of obtained coatings was characterized with scanning electron microscope (SEM) and X-ray diffraction (XRD). The coatings had porosity in the range from 10% to 17% and the main crystal phase was tetragonal zirconium oxide. The scratch test enabled to find the critical load in the range of 9-11 N. Finally, thermal diffusivity of the samples at room temperature, determined by thermographic method, was in the range from 2.95 × 10⁻⁷ to 3.79 × 10⁻⁷ m²/s what corresponds to thermal conductivities of 0.69 W/(mK) and 0.97 W/(mK) respectively.     

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.     

Electrical resistivity of silicon nitride-silicon carbide based ternary composites

New electrically conductive ternary composites were developed by adding 8 vol.% of ZrN or ZrB₂ to a Si₃N₄-SiC matrix. During hot pressing, ZrB₂ reacted with Si₃N₄ to form ZrSi₂, ZrN, Si and BN whereas added ZrN did not undergo any reactions in the Si₃N₄-SiC-ZrN composite. The composites modified by ZrN or ZrB₂ addition showed a lower resistivity (7 × 10³ Ω cm and 3 × 10⁻¹ Ω cm) compared to the matrix (3 × 10⁴ Ω cm). Further studies on the grain size distribution and the volume ratio of conducting and non-conducting phases excluded a percolation network of ZrN and ZrSi₂ grains. In fact, doping of SiC grains and modified grain boundaries as a consequence of the formation of liquid phases during sintering are suggested to be the reason for the significantly lower resistivity of materials containing ZrSi₂.A decrease in the composite resistivity due to a subsequent heat treatment was obtained for all hot-pressed composites.     

Electrochemical formation of nanometric layers of tin selenides on Ti surface

Photoelectrically active tin selenide coatings of nanometric thickness were manufactured by electrodeposition from separate solutions of Sn and Se precursors. Sn was deposited from acidic SnSO₄ electrolytes and Se was deposited from H₂SeO₃ solutions. Fine-grained Sn coatings were deposited at potential φ = −0.3 V with 100% current efficiency. Se coatings were formed at two potentials: φ = −0.5 V, forming Se⁰, and φ = −0.85 V, forming Se²⁻ ions. After the Sn coating was immersed into H₂SeO₃ solution, small quantities (∼2 at.%) of SnSe were formed and SeO₃²⁻ was adsorbed on the surface. A short-time deposition of Se at φ = −0.5 V passivated the surface, so no Sn dissolution is observed upon anodic polarization. XPS and Auger data indicated that under those conditions 20 at.% of Se⁰ and only 2 at.% of SnSe were formed. Thickening of Sn and Se layers led to formation of larger quantities of Se⁰ (75 at.%) and SnSe (4-5 at.%) on the surface, whereas deeper layers contained up to 10 times more of SnSe phase. Upon deposition of Se at φ = −0.85 V, new SnSe₂ phase was formed and the quantity of SnSe phase is increased and that of Se⁰ was reduced. All coatings formed exhibited photoelectric properties.     

Fabrication and characterization of ZrB2–SiC ceramic electrodes coated with a proton conducting, SiO2-rich glass layer

3.5 μm thin layers of dense, crack-free, proton conducting, SiO₂-rich glass have been developed on ZrB₂–SiC ceramic composites, by thermal oxidation at 1400 °C for 30 min in air. A conductivity of 2 mS cm⁻¹ at 25 °C was found, as measured by AC impedance and steady-state voltammetry, and was estimated at ca. 2 × 10⁻² S cm⁻¹ at 80 °C. A striking behaviour of the oxidized ZrB₂–SiC composites is also pointed out: underneath the glass layer, there is a porous layer rich in electronic conductive ZrB₂, without well-defined interface between them, i.e., exhibiting a composition gradient in oxygen. In other words, protonic half-fuel cells could be fabricated under such conditions, for future use in hydrogen or direct alcohol fuel cells.