Fabrication and mechanical behavior of 2D-C_(f)/Ta_(x)Hf_(1−x)C–SiC composites by a low-temperature and highly-efficient route

C_(f)/Ta_(x)Hf_(1−x)C–SiC composites are ideal thermal structural materials for service under extreme conditions of hypersonic vehicles.However,how to synthesize TaxHf1-xC powders and efficiently fabricate C_(f)/Ta_(x)Hf_(1−x)C–SiC composites still faces some challenges.Furthermore,mechanical properties and thermophysical properties of Ta_(x)Hf_(1−x)C vary with the composition,but not monotonically.In-depth analysis of mechanical behaviors of the C_(f)/Ta_(x)Hf_(1−x)C–SiC composites is extremely important for their development and applications.In this study,the Ta_(x)Hf_(1−x)C powders(x=0.2,0.5,0.8)were successfully synthesized via solid solution of TaC and HfC at a relatively low temperature of 1800℃,with a small amount of Si as an additive.Subsequently,the efficient fabrication of 2D-C_(f)/Ta_(x)Hf_(1−x)C–SiC composites was achieved by slurry impregnation and lamination(SIL)combined with precursor infiltration and pyrolysis(PIP).In addition,the mechanical behavior of the composites was investigated systematically.It is demonstrated that the composites present remarkable non-brittle fractures,including a large number of fiber pull out and interphase debonding.Also,the fracture failure involves a complex process of microcrack generation and propagation,matrix cracking,and layer fracture.Moreover,the interfacial bonding between the fibers and the matrix is enhanced as the Ta∶Hf ratio decreases from 4∶1 to 1∶4.As a result,C_(f)/Ta_(0.2)Hf_(0.8)C–SiC composites exhibit exceptional flexural strength of 437±19 MPa,improved by 46%compared with C_(f)/Ta_(0.8)Hf_(0.2)C–SiC(299±19 MPa).This study provides a new perception of design and fabrication of ultra-high-temperature ceramic(UHTC)matrix composites with high performance.

Reaction mechanism and microstructure development of ZrSi_(2) melt-infiltrated C_(f)/SiC-ZrC-ZrB_(2) composites:The influence of preform pore structures

Reactive melt infiltration(RMI)is an effective method for fabrication of highly dense carbon fiber reinforced ultra-high temperature ceramic matrix composites(Cf/UHTCs).In this work,C_(f)/SiC-ZrC-ZrB_(2)composites were fabricated by infiltrating ZrSi_(2)melt into porous C_(f)/B_(4)C-C preforms,where the physical and chemical reactions involved during the RMI process were identified and analyzed.Inhomogeneous infiltration between the inter-and intra-bundle pores was revealed,and was found to be strongly related to the pore structures of the C_(f)/B_(4)C-C preform.It is indicated that the inhomogeneous infiltration can be mitigated remarkably with increasing porosity and pore size of the preform.The effect of pore size on the RMI process was also investigated by a quantitative model,which agrees very well with the experiment results.It further indicates that the inhomogeneous infiltration can also be relieved at elevated RMI temperature.However,excessive infiltration at elevated temperature or more porous preform may cause serious erosion on interphase and fibers,leading to mechanical properties deterioration of the final composites.

Microstructure and mechanical properties of 3D Cf/SiBCN composites fabricated by polymer infiltration and pyrolysis

In this work,three-dimensional(3D)Cf/SiBCN composites were fabricated by polymer infiltration and pyrolysis(PIP)with poly(methylvinyl)borosilazane as SiBCN precursor.The 3D microstructure evolution process of the composites was investigated by an advanced X-ray computed tomography(XCT).The effect of dicumyl peroxide(DCP)initiator addition on the crosslinking process,microstructure evolution,and mechanical properties of the composites were uncovered.With the addition of a DCP initiator,the liquid precursor can cross-linking to solid-state at 120℃.Moreover,DCP addition decreases the release of small molecule gas during pyrolysis,leading to an improved ceramic yield 4.67 times higher than that without DCP addition.After 7 PIP cycles,density and open porosity of the final Cf/SiBCN composite with DCP addition are 1.73 g.cm^(-3)and〜10%,respectively,which are 143.0%higher and 30.3%lower compared with the composites without DCP addition.As a result,the flexural strength and elastic modulus of Cf/SiBCN composites with DCP addition(371 MPa and 31 GPa)are 1.74 and 1.60 times higher than that without DCP addition(213 MPa and 19.4 GPa),respectively.

Advances in ultra-high temperature ceramics,composites,and coatings

Ultra-high temperature ceramics(UHTCs)are generally referred to the carbides,nitrides,and borides of the transition metals,with the Group IVB compounds(Zr&Hf)and TaC as the main focus.The UHTCs are endowed with ultra-high melting points,excellent mechanical properties,and ablation resistance at elevated temperatures.These unique combinations of properties make them promising materials for extremely environmental structural applications in rocket and hypersonic vehicles,particularly nozzles,leading edges,and engine components,etc.In addition to bulk UHTCs,UHTC coatings and fiber reinforced UHTC composites are extensively developed and applied to avoid the intrinsic brittleness and poor thermal shock resistance of bulk ceramics.Recently,high-entropy UHTCs are developed rapidly and attract a lot of attention as an emerging direction for ultra-high temperature materials.This review presents the state of the art of processing approaches,microstructure design and properties of UHTCs from bulk materials to composites and coatings,as well as the future directions.

3D C_f/SiBCN composites prepared by an improved polymer infiltration and pyrolysis

Using liquid poly(methylvinyl)borosilazanes(PMVBSZ)as precursor,carbon fiber reinforced SiBCN matrix composites(C_f/SiBCN)were fabricated by a modified polymer infiltration and pyrolysis(PIP)process.With dicumyl peroxide added as cross-linking agent,the PMVBSZ could be solidified at a low temperature of 120℃,leading to a high ceramic yield of～70%.The cross-linking mechanism and ceramization processes of the precursor were investigated in detail.Moreover,a modified infiltration technology was developed,which improved the efficiency and protected the precursor against moist air during PIP.Consequently,the obtained C_f/SiBCN composites had an oxygen content of around 1.22 wt%.Benefiting from the high ceramic yield and high efficiency of the modified PIP,C_f/SiBCN composites with an open porosity of～10%and uniform microstructure were obtained after only 7 cycles of PIP.The flexural strength and fracture toughness of the derived C_f/SiBCN composites were 371 MPa and 12.9 MPa·m^(1/2),respectively.This work provides a potential route for the fabrication of high performance C_f/SiBCN composites.

Fabrication and microstructure evolution of C_(sf)/ZrB_(2)-SiC composites via direct ink writing and reactive melt infiltration

Fiber damage and uniform interphase preparation are the main challenges in conventional short fiber reinforced ceramic matrix composites.In this work,we develop a novel processing route in fabrication of short carbon fiber reinforced ZrB_(2)-SiC composites(C_(sf)/ZrB_(2)-SiC)overcoming the above two issues.At first,C_(sf) preforms with oriented designation and uniform PyC/SiC interphase are fabricated via direct ink writing(DIW)of short carbon fiber paste followed by chemical vapor infiltration.After that,ZrB_(2) and SiC are introduced into the preforms by slurry impregnation and reactive melt infiltration,respectively.Microstructure evolution and optimization of the composites during fabrication are investigated in detail.The as-fabricated C_(sf)/ZrB_(2)-SiC composites have a bulk density of 2.47 g/cm^(3),with uniform weak interphase and without serious fiber damage.Consequently,non-brittle fracture occurs in the C_(sf)/ZrB_(2)-SiC composites with widespread toughening mechanisms such as crack deflection and bridging,interphase debonding,and fiber pull-out.This work provides a new opportunity to the material design and selection of short fiber reinforced composites.