Preparation And Properties Of SiBCN(Zr) Gradient Porous Ceramics Prepared By Ice Molding And Their Bicontinuous Composites

In this paper,the process parameters of preparing SiBCN?Zr?ceramic slurry with excellent stability and fluidity were firstly explored.Then the porous Si BCN?Zr?with different freezing temperatures and different initial solid contents were prepared by freeze-casting and pressureless-sintering.Then the effects of freezing temperature on the lamellar structure,porosity and mechanical properties of SiBCN?Zr?porous ceramics were studied.With optimized freezing and sintering parameters,porous SiBCN?Zr?ceramics with gradient lamellar structure and good mechanical properties were prepared.SiBCN porous ceramics were used as prefabricated bodies and Cu/SiBCN bicontinuous composites were prepared by pressureless-infiltration.The effects of solid content on the microstructure and mechanical properties of the composites were investigated,and the interface between metal and ceramics was also analyzed.The Cu/SiBCN with 35 vol.%initial solid content of SiBCN was used to study its ablation resistance under oxyacetylene for 10 s.And the ablation behavior and mechanism of the composites were analyzed.The SiBCN and SiBCNZr powder prepared by the mechanical alloying are sufficiently refined and amorphized.By measuring the settling rate,viscosity and Zeta potential of SiBCN and SiBCNZr slurries,the process parameters that ensure perfect stability and fluidity of the two ceramic slurries were determined:the dispersant PAA contents of the of SiBCN and SiBCNZr slurries were 0.3 wt.%and 1.5 wt.%,the pH of the two slurries were around 7 and 10 respectively.The main phases of porous SiBCN ceramic at different freezing temperatures prepared by the freeze-casting and pressureless sintering are?-SiC,BNC phase and BN.The two structural undulation parameters?lamellar spacing?and lamellar pore size??of the sample with a freezing temperature of-60°C have distinct gradient distribution at different heights in the bulk.The relationship between the parameters and the sample height h satisfies:??h^0.88?lg?=0.88lgh+1.61?,??h^0.37?lg?=0.37lgh+1.81?.With the decrease of the freezing temperature,the size of the porous ceramics becomes thinner and the gradient distribution are not obvious;the densities of the SiBCN porous ceramics are close and the open porosities are all close to about 40%,the compressive strength decreases with the decrease of the freezing temperature?lamellar size decreases?.The porous SiBCN?Zr?porous ceramics with different solid contents were prepared using freezing temperature:-60°C and improved sintering process.All the three kinds of solid-content porous ceramics have gradient lamellar structure and high open porosity.The mechanical properties of porous ceramics increase with the increase of solid content.The open porosity and compressive strength of Si BCN porous ceramics with initial solid content of 40 vol.%can reach 48.70±0.37%and 23.07±1.79 MPa,respectively.After the introduction of ZrB₂,the mechanical properties of the porous ceramics increased.The compressive strength of the three solid-content SiBCNZr porous ceramics was increased by about 37%,12%,and 23%,respectively,compared with SiBCN porous ceramics.By studying the wetting and infiltration behaviour of Cu-Ti mixtures and porous SiBCN ceramics,it was determined that the composition of the metal mixture was Cu₈₀Ti₂₀.The phase composition of bicontinuous composites with initial SiBCN solid content of 30,35,40 vol.%prepared by pressureless-infiltration is mainly?-SiC,BNC,Cu,Ti and interfacial reaction product TiC?N?,Cu₅Si,TiB₂.The metal and ceramic layers in the bicontinuous composites are alternately arranged and the density of the material increases as the metal content increases.The density of the composite material with a ceramic solid content of 35 vol.%?5.320±0.027 g/cm³?is closest to the theoretical value.In the composites,an interfacial reaction layer with a width of 1-14?m is formed.The reaction of Ti and SiBCN matrix promotes the contact and counter-diffusion between Cu and Si.The reaction product TiC_xN_y and TiB_xN_y intersperse in the interfacial layer.The trend of the mechanical properties obtained by the two loading modes:indenter displacement parallel/perpendicular to the freezing direction is consistent with the change of the solid content.The flexural strength of the Cu/SiBCN bicontinuous composites decreases with the increase of the solid content of the ceramics,and the maximum flexural strength can reach 252.36±3.32 MPa.With the increase of the initial solid content of the composite ceramics,the elastic modulus slightly increases,the fracture toughness first increases and then decreases,and the fracture toughness reaches 11.24±0.31 MPa·m^1/2.The Cu/SiBCN composite with 35 vol.%ceramics contents have a good overall performance thus was selected for the ablation test.After ablation for 10 s,the mass ablation rate and line ablation rate were 0.06 g/s and 0.25 mm/s,respectively.The ablation mechanism of the oxygenacetylene flame on the sample surface is mainly:mechanical erosion,airflow erosion and high temperature oxidation.The ablated surface of the composite was covered with a layer of TiO₂ oxide film after ablation,and the ablation center A region released a large amount of Cu liquid and formed an ablation pit.A part of the Cu liquid leaved the sample under TiO₂ package.Cu melted and flew out to consume a lot of heat;then Cu accumulated on the B region around the ablation pit,protecting the matrix from the scouring.The C region at the periphery of the sample was barely influenced by the ablation flow at macro level whereas the surface forms a porous TiO₂oxide layer.