Thermal cycling and hot corrosion behavior of a novel LaPO4/YSZ double-ceramic-layer thermal barrier coating

LaPO₄ powders were produced by a chemical co-precipitation and calcination method. The ceramic exhibited a monazite structure, kept phase stability at 1400?°C for 100?h, and had low thermal conductivity (~ 1.41?W/m?K, 1000?°C). LaPO₄/Y₂O₃ partially stabilized ZrO₂ (LaPO₄/YSZ) double-ceramic-layer (DCL) thermal barrier coatings (TBCs) were fabricated by air plasma spray. The LaPO₄ coating contained many nanozones. Thermal cycling tests indicated that the spallation of LaPO₄/YSZ DCL TBCs initially occurred in the LaPO₄ coating. The failure mode was similar to those of many newly developed TBCs, probably due to the low toughness of the ceramics. LaPO₄/YSZ DCL TBCs were highly resistant to V₂O₅ corrosion. Exposed to V₂O₅ at 700–900?°C for 4?h, La(P,V)O₄ formed as the corrosion product, which had little detrimental effect on the coating microstructure. At 1000?°C for 4?h, a minor amount of LaVO₄ was generated.     

Thermal cycling performance of La2Ce2O7/50 vol.% YSZ composite thermal barrier coating with CMAS corrosion

La₂Ce₂O₇ (LC) is receiving increasing attention due to its lower thermal conductivity, better phase stability and higher sintering resistance than yttria partially stabilized zirconia (YSZ). However, the low fracture toughness and the sudden drop of CTE at approximately 350?°C greatly limit its application. In this study, the LC/50?vol.% YSZ composite TBC was deposited by supersonic atmospheric plasma spraying (SAPS). Compared to YSZ or double layered LC/YSZ coating, the thermal cycling life of LC/50?vol.% YSZ coating with CMAS attack increased by 93% or 91%. The latter possessed higher fracture toughness (1.48?±?0.26?MPa?m^1/2) than LC (0.72?±?0.15?MPa?m^1/2) and better CMAS corrosion resistance than YSZ owing to the formation of Ca₂(La_xCe_1-x)₈(SiO₄)₆O_6–4x with <001> orientation perpendicular to the coating surface. The sudden CTE decrease of LC was fully suppressed in LC/50?vol.% YSZ coating due to the change of temperature dependent residual stresses induced by YSZ.     

Enhancement of mechanical properties of hydroxyapatite coating prepared by electrophoretic deposition method

The addition of bio-inert ceramics such as alumina and zirconia can significantly improve the mechanical properties of hydroxyapatite bioactive coatings and increase their biocompatibility. In the present study, the surface of a titanium substrate was coated by the electrophoretic deposition method (EPD). Moreover, the reaction bonding process has been used to precipitate the nanocomposite containing the hydroxyapatite (HA), alumina, yitteria-stabilized zirconia (YSZ). The coating process was performed by an electrical power supply and a suspension of hydroxyapatite, aluminum, and YSZ nanopowders. For preparing a suspension consisting of 50% isopropanol and 50% acetone, 0.6 g/L of iodine was used as a stabilizer. Green and sintered coatings were analyzed by FE-SEM and XRD. In addition, the mechanical properties such as bonding strength, hardness, and toughness were measured. The hardness, bonding strength, and toughness of the HA coating were 107 ± 10.3 HV, 10.8 ± 3.2MPa, and 0.72MPa√m, respectively, while those of the HA-Al₂O₃-YSZ nanocomposite coating were 213 ± 1.8 HV, 35 ± 1.6MPa, and 1.6MPa√m, respectively.     

Porosity analysis and oxidation behavior of plasma sprayed YSZ and YSZ/LaPO4 abradable thermal barrier coatings

In this research, the high temperature oxidation behavior, porosity, and microstructure of four abradable thermal barrier coatings (ATBCs) consisting of micro- and nanostructured YSZ, YSZ-10%LaPO₄, and YSZ-20%LaPO₄ coatings produced by atmospheric (APS) method were evaluated. Results show that the volume percentage of porosity in the coatings containing LaPO₄ was higher than the monolithic YSZ sample. It was probably due to less thermal conductivity of LaPO₄ phases. Furthermore, the results showed that the amount of the remaining porosity in the composite coatings was higher than the monolithic YSZ at 1000 °C for 120 h. After 120 h isothermal oxidation, the thickness of thermally growth oxide (TGO) layer in composite coatings was higher than that of YSZ coating due to higher porosity and sintering resistance of composite coatings. Finally, the isothermal oxidation resistance of conventional YSZ and nanostructured YSZ coating was investigated.     

Microstructure and mechanical properties improvement of the Nextel™ 610 fiber reinforced alumina composite

The alumina slurry with high solid content was prepared, and a rapid lamination route for fabricate the Nextel? 610 fiber reinforced alumina composite was proposed in this work. The microstructure and mechanical properties of the as-received all-oxide composite were investigated by a series of techniques. The shrinkage cracks in matrix were reduced, while porous structure in composite was maintained. The N610/alumina composite has weak matrix and weak interface, as the Young’s modulus of the alumina matrix and the interfacial shear strength of the composite are 140.8±2.5GPa and 129.1±14.6MPa. The mechanical properties of the composite are much higher than lots of oxide/oxide composites, given its flexural strength, interlaminar shear strength and the fracture toughness are 398.4±5.7MPa、27.0±0.5MPa and 14.1±0.9MPa·m^1/2, respectively. The flexural strength of the virgin composite keep stable at 25–1050 °C, while dramatically decrease at 1100–1200 °C.     

Assessment of the performance of Y2SiO5-YSZ/YSZ double-layered thermal barrier coatings

Y₂SiO₅ is a promising material for the thermal barrier coatings due to its low thermal conductivity, high temperature stability and exceptional resistance for molten silicate attack. However, it suffers low fracture toughness and low coefficient of thermal expansion compared with yttria-stabilized zirconia (YSZ). In this study, a composite coating approach, i.e., incorporating YSZ into Y₂SiO₅ coating, was employed to overcome those limitations. The double-layered Y₂SiO₅-YSZ/YSZ coatings were fabricated using atomospheric plasma spraying and tested under thermal cycling at 1150 °C. The phase compositions, microstructure, mechanical properties and the failure behavior were evaluated. It was found that the amorphous phase during spraying would crystallize at high temperature accompanied by volume shrinkage, leading to cracks and spallation in the coating. With YSZ addition, the composite coatings exhibited a much longer lifetime than the single phase Y₂SiO₅ coating due to a lower volume shrinkage and enhanced toughness.     

Architectural engineering inspired method of preparing Cf/ZrC–SiC with graceful mechanical responses

Inspired by grouting technique in architectural engineering, an innovative method of slurry injection and vacuum impregnation was put forward to introduce nanosized ZrC–SiC ceramics into PyC modified 3-D needle-punched carbon fiber preform homogeneously and continuously, followed by spark plasma sintering to prepare C_f/ZrC–SiC with graceful mechanical responses. The composite possessed improved fracture toughness and work of fracture at 5.37 ± 0.25 MPa∙m^1/2 and 951 ± 12 J/m², 50% and nearly one order of magnitude higher than those of ZrC–SiC composite, respectively, with rivaling flexural strength at 177 ± 8 MPa synchronously. A graceful fracture mode was embodied in an obviously extended yield plateau with increased failure displacement by 300%. This enhancement was attributed to the uniform and continuous combination of ZrC–SiC with carbon fiber preform as well as protection and interface tailoring from PyC coating. The study offered an easy and effective method of preparing 3-D fiber reinforced ceramic matrix composites.     

Characterization and mechanical properties of Cf/ZrB2-SiC composites fabricated by a hybrid technique based on slurry impregnation,polymer infiltration and pyrolysis and low-temperature hot pressing

The continuous carbon fiber reinforced ZrB₂-SiC composite was fabricated successfully via a hybrid technique based on nano ceramic slurry impregnation, polymer infiltration and pyrolysis and low-temperature hot pressing. The C_f/ZrB₂-SiC composites exhibited non-brittle fracture modes and the chemical interaction at the fiber/matrix interfaces was effectively inhibited owing to the low sintering temperature. The S2-C_f/ZrB₂-SiC composite presented the highest mechanical properties with fracture toughness of 4.47?±?0.15?MPa?m^1/2 and the work of fracture of 877?J/m², which was attributed to the multiple length-scale toughening mechanisms including the macroscopic toughening mechanisms of crack deflection and crack branching, the micro toughening mechanisms of fiber bridging and fiber pull-out. This work presented a novel and effective method to fabricate high-performance continuous carbon fiber reinforced ceramic matrix composites.     

Enhanced mechanical properties and thermal shock resistance of Cf/ZrB2-SiC composite via an efficient slurry injection combined with vibration-assisted vacuum infiltration

An efficient slurry injection combined with vibration-assisted vacuum infiltration process has been developed to fabricate 3D continuous carbon fiber reinforced ZrB₂-SiC ceramics. Homogenous distribution between carbon fiber and ceramic was achieved successfully, leading to an enhancement in mechanical properties. The C_f-PyC/ZrB₂-SiC composite exhibited a typical non-brittle fracture mode with a superior fracture toughness of 6.72 ± 0.21 MPa·m^1/2 and an extraordinary work of fracture of 2270 J/m², respectively, increasing by nearly 14.8 % and 36 % as compared with those of a parent composite fabricated by only slurry injection and slurry infiltration. The enhancement in fracture toughness and work of fracture were attributed to multiple toughening mechanism including crack deflection, PyC coated fiber bundles pull-out and fiber bridging. Moreover, a critical thermal shock temperature difference of 814 °C was achieved, higher than that of traditional ZrB₂-based ceramics. This work presents an efficient approach to fabricate high-performance C_f/UHTCs with uniform architecture.     

Microstructure stabilization of a novel glass/YSZ composite coating material by adding alumina particles

Glass-based composite coating materials incorporating particles of alumina or YSZ were prepared by reaction sintering. It was revealed that phase evolution played a key role on thermal expansion behavior of the composite coating materials. Both precipitating of t-ZrO₂ crystals and adding YSZ inclusions could raise CTEs of the glass-based matrix, while the formation of zircon produced the reverse effect. Especially, alumina additives retarded the crystallization of the base glass and reduced reaction rates between YSZ and the glass matrix remarkably. Thus, the Al₂O₃/YSZ/glass tri-composites could serve as an environmental barrier coating for intermetallics and superalloys because of the stabilized microstructure.     

Effect of high-temperature heat treatment on the microstructure and mechanical behavior of PIP-based C/C-SiC composites with SiC filler

C/C-SiC composites were fabricated by a combined process of chemical vapor deposition (CVD), slurry infiltration(SI), and precursor infiltration and pyrolysis (PIP). The microstructure and mechanical behavior were investigated for the dense C/C-SiC composites before and after high-temperature heat treatment. The results indicated that the sintering of the SiC matrix and the migration of the SiC matrix/fiber bundles weak interface occurred after high-temperature heat treatment at 1900 ℃. The SiC sintering resulted in an increase in the flexural strength of the C/C-SiC composites from 298.9 ± 35.0 MPa to 411.1 ± 57.3 MPa. The migration of the weak interface changed the direction of crack propagation, making the fracture toughness of the C/C-SiC composites decrease from 13.3 ± 1.7 MPa⋅m ^1/2 to 9.02 ± 1.5 MPa⋅m ^1/2.     

Effect of pyrolytic carbon coating on the microstructure and fracture behavior of the Cf/ZrB2-SiC composite

The high sintering temperature and interface interaction seriously degraded the toughening effects of continuous carbon fiber in ZrB₂-SiC ceramic. The pyrolytic carbon coated carbon fiber reinforced ZrB₂-SiC composite (C_f-PyC/ZrB₂-SiC) with desirable properties was successfully achieved via brushing nano ZrB₂-SiC slurry followed by spark plasma sintering at relatively low sintering temperature. The fabricated C_f-PyC/ZrB₂-SiC composite presented a non-brittle fracture feature and a remarkable enhancement in comparison with the ZrB₂-SiC composite reinforced by the as-received carbon fiber (C_f-AS/ZrB₂-SiC). The fracture toughness and critical crack size were increased from 5.97?±?0.18–7.66?±?0.24?MPa?m^1/2 and from 91.6 to 164.5?µm, respectively. A high work of fracture of 1915?J/m² for C_f-PyC/ZrB₂-SiC composite was achieved, almost four times higher than that of the C_f-AS/ZrB₂-SiC composite (463?J/m²). Multiple toughening mechanisms contributed to such enhancement, such as crack deflection, fiber bridging, fiber pull-out and crack branching. This work provides a feasible approach to fabricate high-performance fiber reinforced ceramic composites having a high work of fracture.     

Additive manufacturing of continuous carbon fiber reinforced high entropy ceramic matrix composites via paper laminating,direct slurry writing,and precursor infiltration and pyrolysis

In this study, continuous carbon reinforced C_f/(Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C–SiC high entropy ceramic matrix composites were additively manufactured through paper laminating (PL), direct slurry writing (DSW), and precursor infiltration and pyrolysis (PIP). (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C high entropy ceramic (HEC) powders were synthesized by pressureless sintering and ball milling. A certain proportion of HEC powder, SiC powder, water, binder, and dispersant were mixed to prepare the HEC-SiC slurry. Meanwhile, BN coating was prepared on the 2D fiber cloth surface by the boric acid-urea method and then the cloth was cut into required shape. Additive manufacturing were conducted subsequently. Firstly, one piece of the as-treated carbon fiber cloth was auto-placed on the workbench by paper laminating (PL). Then, the HEC-SiC slurry was extruded onto the surface of the cloth by direct slurry writing (DSW). PL and DSW process were repeated, and a C_f/HEC-SiC preform was obtained after 3 cycles. At last, the preform was densified by precursor infiltration and pyrolysis (PIP) and the final C_f/HEC-SiC composite was prepared. The open porosity of the C_f/HEC-SiC composites, with the HEC volume fractions of 15, 30 and 45%, were 7.7, 10.6, and 11.3%, respectively. And the density of the C_f/HEC-SiC composites, with the HEC volume fractions of 15, 30 and 45%, were 2.9, 2.7 and 2.3 g/cm³, respectively. The mechanical properties of the C_f/HEC-SiC composites increased firstly and then decreased with the HEC content increase, reaching the maximum value when the HEC volume fraction was 30%. The mechanical properties of the C_f/HEC-SiC composites containing 45, 30 and 15% HEC were as follows: flexural strength (180.4 ± 14 MPa, 183.7 ± 4 MPa, and 173.9 ± 4 MPa), fracture toughness (11.9 ± 0.17 MPa m^1/2, 14.6 ± 2.89 MPa m^1/2, and 11.3 ± 1.88 MPa m^1/2), and tensile strength (71.5 ± 4.9 MPa, 98.4 ± 12.2 MPa, and 73.4 ± 8.5 MPa). From this study, the additive manufacturing of continuous carbon fiber reinforced high entropy ceramic matrix composites was achieved, opening a new insight into the manufacturing of ceramic matrix composites.     

Improvement in heat resistivity of alkaline earth silicate fiber boards by Al4SiC4 coating

Alkaline earth silicate (AES) fiber ceramic board was immersed in a novel coating slurry consisting of Al₄SiC₄ particles and silica-sol, forming a 500-μm coating layer on the AES fiber board. Linear shrinkage of the uncoated AES fiber board was over 3.0% after heating at 1100°C or higher for 8 hours, whereas the linear shrinkage of the AES fiber board with an Al₄SiC₄ coating was below 2.0%. The coating layer of Al₄SiC₄ changed to a hard shell structure consisting of cristobalite, alumina, and mullite after heating. When AES fiber board with an Al₄SiC₄ coating layer was heated at 1200°C for 8 hours, the compressive strength of the board reached 0.45 MPa, 2.5 times greater than that of the original uncoated board.     

Microstructure and properties of a graphene platelets toughened boron carbide composite ceramic by spark plasma sintering

A kind of B₄C/SiC composite ceramic toughened by graphene platelets and Al was fabricated by spark plasma sintering. The effects of graphene platelets and Al on densification, microstructure and mechanical properties were studied. The sintering temperature was decreased about 125–300?°C with the addition of 3–10?wt% Al. Al can also improve fracture toughness but decrease hardness. The B₄C/SiC composite ceramic with 3?wt%Al and 1.5?wt% graphene platelets sintered at 1825?°C for 5?min had the optimal performances. It was fully densified, and the Vickers hardness and fracture toughness were 30.09?±?0.39?GPa and 5.88?±?0.49?MPa?m^1/2, respectively. The fracture toughness was 25.6% higher than that of the composite without graphene platelets. The toughening mechanism of graphene platelets was also studied. Pulling-out of graphene platelets, crack deflection, bridging and branching contributed to the toughness enhancement of the B₄C-based ceramic.     

Structural characterization of YSZ/Al2O3 nanostructured composite coating fabricated by electrophoretic deposition and reaction bonding

Suspension of YSZ and Al particles in acetone in presence of 1.2 g/l iodine as dispersant was used for electrophoretic deposition of green form YSZ/Al coating. Results revealed that applied voltage of 6 V and deposition time of 3 min were appropriate for deposition of green composite form coating. After deposition, a nanostructured dense YSZ/Al₂O₃ composite coating was fabricated by oxidation of Al particles at 600 °C for 2 h and subsequently sintering heat treatment at 1000 °C for 2 h. Melting and oxidation of Al particles in the green form composite coating not only caused reaction bonding between the particles but also lowered the sintering temperature of the ceramic coating about 200 °C. The EDS maps confirmed that the composition of fabricated coating was uniform and Al₂O₃ particles were dispersed homogenously in YSZ matrix.     

Processing and mechanical properties of Al2O3–5 vol.% Cr nanocomposites

The use of chromium (III) acetylacetonate as a source of nanometre sized chromium particles for the production of Al₂O₃–5 vol.% Cr nanocomposites has been investigated. The details of the processing procedure are crucial in determining the mechanical properties of the composite. The highest strength and fracture toughness, 736±29 MPa and 4.0±0.2 MPa m^1/2, respectively, were obtained for the nanocomposite hot pressed at 1450 °C. It is shown that the strengthening in Al₂O₃–5% Cr nanocomposites mainly results from microstructure refinement in that the mean alumina matrix grain size in the optimum composite was 0.68 μm compared with a grain size of 3.6 μm in the monolithic alumina hot pressed under identical conditions. Crack bridging and crack deflection by the nano-sized Cr particles did not occur to any significant extent. The slight improvement in fracture toughness may result from the observed change in fracture mode from intergranular fracture for monolithic alumina to transgranular failure for the nanocomposites.     

LaPO4 coating on alumina-based fiber: Strength retention of fiber and improvement of interfacial performances

The characteristics of the interface are the key factors that determine the mechanical properties and fracture behavior of fiber-reinforced ceramic matrix composites. Design and preparation of coatings which can preserve fiber strength and maintain appropriate interfacial bonding strength are of great challenges. LaPO₄ coating is a promising weak interface coating for oxide fiber reinforced oxide ceramic matrix composites. Through this coating, the toughening mechanism of the composite such as fiber pulling out and fiber debonding is triggered. The LaPO₄ coating was deposited on the surface of alumina-based fibers by a solution precursor heterogeneous precipitation method. The effects of different precursors and different deposition temperatures on fiber strength were studied, and the mechanism of the strength degradation of the coated fiber was analyzed. It was found that the fibers coated with phytic acid precursor and deposited at 90 °C had the highest tensile strength compared to other coated fibers. The retention of strength is attributed to its loosely stacked coating. Besides, a single fiber pullout test was carried out to evaluate the effect of the coating on the interface of the composites. The results show that the composites coated by depositing citric acid precursor and phytic acid precursor at 90 °C can reduce the interfacial bonding strength by 32.5% and 46.7%, respectively compared to uncoated composites. This study has potential application value in the preparation of ceramic matrix composites used in oxidation and high temperature environments.     

Fabrication of Porous Ceramics with Unidirectionally Aligned Continuous Pores

Porous alumina ceramics with unidirectionally aligned continuous pores were fabricated via the slurry coating of fugitive fiber. Cotton thread was coated with ceramic slurry by pulling it through the slurry, and specimens were produced by spooling the coated thread. The obtained porous alumina ceramics had an average pore diameter of 165 μm, 35% open porosity, and a bending strength of 160 MPa. It was suggested that the pore size and the porosity could be adjusted using the diameter of the cotton thread and the solids concentration of the slurry, respectively.     

Preparation of mullite-TiB2-CNTs hybrid composite through spark plasma sintering

A near fully dense mullite-TiB₂-CNTs hybrid composite was prepared successfully trough spark plasma sintering. 1 wt%CNT and 10 wt%TiB₂ were mixed with nano-sized mullite powders using a high energy mixer mill. Spark plasma sintering was carried out at 1350 °C under the primary and final pressure of 10 MPa and 30 MPa, respectively. XRD results showed mullite and TiB₂ as dominant crystalline phases accompanied by tiny peaks of alumina. The microstructure of prepared composites demonstrated uniform distribution of TiB₂ reinforcements in mullite matrix without any pores and porosities as a result of near fully densified spark plasma sintered composite. The fracture surface of composite revealed a proper bonding of TiB₂ with mullite matrix and also areas with CNTs tunneling and superficies as a result of pulling-out phenomenon. The flexural strength of 531 ± 28 MPa, Vickers harness of 18.31 ± 0.3 GPa, and fracture toughness of 5.46 ± 0.12 MPa m^−1/2 were achieved for prepared composites as the measured mechanical properties.