Performance Studies Of Laminated Graphite/（ZrB₂-SiC） Composites

Zirconium diboride（Zr B₂） based ceramics, as a kind of ultra-high temperature ceramics, has become the focus of research of ultra-high temperature ceramics due to its advantages of low density, high melting point and extremely thermal shock resistance. But the intrinsic brittleness of ceramics has been restricting its wide application. In order to improve its toughness, laminated Graphite/（Zr B₂-Si C） composites which used the powder of Zr B₂, Si C and graphite as raw materials were successfully fabricated by tape casting,stacking and hot pressing in this paper. Final sintering performed at 1950℃ for 1 h under an applied pressure of 30 MPa. The composites microstructure, mechanical properties,thermal shock resistance, oxidation resistance and high temperature bending properties of the materials were studied.In this paper, three kinds of laminated Graphite/（Zr B₂-Si C） composites with different interface layers were fabricated by tape casting, stacking and hot-pressing sintering. The Zr B₂-Si C matrix layers and the graphite interface layers of the laminated composites were alternately stacked with straight boundary and the thicknesses were about 330 μm and30μm, respectively. With the increase of Zr B₂ content in the interface layers, the boundaries between the matrix layers and the interface layers became from clear to blurred while the elastic modulus, flexural strength and fracture toughness increased and the fracture work decreased. The flexural strength of laminated Graphite/（Zr B₂-Si C）composites decreased,but the fracture toughness significantly increased, reaching to10.3MPa·m1/2,compared with monolithic Zr B₂-Si C ceramics. Toughening mechanism was mainly that the weak interface layers made the crack tip produce stress relaxation which weaken the driving force of the crack propagation. At the same time, crack frequent bifurcation and deflection increased the crack propagating path and absorbed doubled fracture energy. The fracture work decreased with the increase of Zr B₂ content in the interface layers which could be attributed to that bonding strength increased between layers and the deflection largely decreased.Thermal shock resistance test showed that the retained strength of laminated Graphite/（Zr B₂-Si C） composites decreased which could be attributed to the mismatch between thermodynamic parameters and the defect of crack. ΔTC for LSZ-1,LSZ-2 and LSZ-3 was calculated as 433℃,463℃ and 343℃, respectively. Moderate bonding strength and interface strength made LSZ-2 have better thermal shock resistance.The oxidation weight gain of the laminated Graphite/（Zr B₂-Si C） composites increasedwith the increase of time at 1300℃.When the oxidation time increased to 10 h,the weight was 2.04mg/cm2. The speed of oxidize increasing weight gradually slowed down with the increase of time. The main reason was that the materials were oxidized to form a dense layer of glass which inhibited the diffusion of oxygen into the materials on the surface of the samples. The flexural strength of the laminated Graphite/（Zr B₂-Si C） composites decreased with the increase of oxidation time. When the oxidation time reached to 10 h,the flexural strength was reduced to 240 MPa.The decrease of the flexural strength was mainly due to the loose structure of oxide layer which had lots of defects and cracks.The flexural strength of the laminated Graphite/（Zr B₂-Si C） composites decreased firstly and then increased at high temperature. The minimum value of the flexural strength was 195 MPa in air atmosphere at 1300℃.The reason that the flexural strength decreased was that the thermal stress in the materials relaxed before fracture and produced plastic deformation. The repair of the dense glass phase on the surface to the cracks led to the increase of the flexural strength.