Research On Design Fabrication And Properties Of Gr/Al-Si Dissipative Thermal Protective Materials

In this thesis, the different fabrication processes were used to prepare the Gr/Al25 Si and Gr/Al40 Si dissipation thermal protective composite s by means of pressure impregnation. Optical microscopy(OM), scanning electron microscopy(SEM) and transmission electron microscopy(TEM) were employed to analyse the material microstructure and the interface of the composites. The characterization of matrix graphite was analyzed qualitatively and quantitatively. Thermal physical properties at room temperature and high temperatures of graphite and two kinds of dissipation thermal protective composites were tested, Mechanical properties of materials were also tested at room temperature and high temperatures. With the help of thermal shock test, the influence of different consumption powders against heat shock has been analyzed. The ablative properties of the composites were tested by means of oxyacetylene ablation test. The surface ablation behavior and volumetric ablation behavior were analysed combined with the microstructure.Compared with the ordinary electrode graphites, the structure of high purity flake graphite which adopted in this paper presented for 3D mesh, and the graphite was more integrated and its graphitization degree(nearly 92.15%) was also higher. Graphite consists of fibrous tissue and roses tissue. The microstructure of two dissipation thermal protective composites was homogeneous and its density was high. The interface between consumption powder and matrix graphite was clear without interfacial reaction product. C elements in graphite will diffuse to the Al and Si without any chemical reaction.At room temperature, the bending strength of Gr/Al25 Si and Gr/Al40 Si materials were 122.32 MPa and 119.46 MPa respectively. Compared with the matrix graphite, the strength increased by 98.56% and 98.56%. With the increase of temperature, their bending strength decreased. Through the analysis of bending fracture, the fracture mechanism of materials was mixture of cavitation and cleavage fracture. The compression strength of Gr/Al25 Si and Gr/Al40 Si dissipation composite materials were 240.89 MPa and 201 MPa. No matter bending strength or compression strength, the strength of these two kinds of dissipation materials were both higher than those of matrix graphite within 1000℃.With the increase of temprature, the thermal expansion coefficient of graphite increased. But the variation was between 4.8×10-6K-1 to 6×10-6K-1, the thermal expansion coefficients of these two dissipation thermal protective composites were higher than matrix graphite. The thermal expansion coefficient of Gr/Al40 Si was slightly smaller than that of Gr/Al25 Si material and they were both kept between7.5×10-6K-1 to 8×10-6K-1 under 300 ℃. Above 300 ℃, the thermal expansion coefficient of Gr/Al25 Si was on the decline, the thermal expansion coefficient of Gr/Al40 Si waved obviously and reached maximum of 10.8×10-6K-1 at 400 ℃. As the temperature increased, the thermal conductivity of three materials decreased linearly. The thermal conductivity of the composites were higher than those of matrix graphite, and the thermal conductivity of Gr/Al25 Si was 121.66 W/m K at room temperature.The thermal shock test of materials showed that the critical thermal shock temperature of Gr/Al25 Si material was about 820 ℃, and Gr/Al40 Si about 980 ℃. It means that thermal shock resistance of Gr/Al40 Si material was superior to the Gr/Al25 Si material.The mass ablation rate of the Gr/Al25 Si composite was 10-3g/s orders of magnitude at different ablation time. The mass ablation rate of Gr/Al40 Si was higher than that of Gr/Al25 Si material at 30 s, but the mass ablation rate of Gr/Al25 Si was higher than that of Gr/Al40 Si and the mass ablation rate of the matrix graphite was higher than the two kinds of composites after 30 s. The mass ablation rate of two composites were one or two orders of magnitude less than graphite. The surface ablation mechanism of the dissipation thermal protective composites was that Al oxidation reaction occured preferentially and the oxide was spherical; Si oxidation reaction occurs secondly and has well wettability with the matrix; Dense oxide film prevented oxygen from entering and withstanded intense scouring action on the surface of the material stream. The ablation mechanism of the dissipation thermal protective composite was that interface reaction occurred under the surface without oxidation reaction. The products of interface reaction were Al4C3 and Si C, and they reacted and formed Al4 Si C4 which was a refractory material at 1600℃.