Abstract: As an emerging brand new type of engineering material for a variety of important high technology applications, the deep understanding of the mechanical behavior of freestanding diamond films has become an emergent task of vital importance. Unfortunately the mechanical behavior of this brand new material is not fully understood. Effects that affect the fracture strength are still not very clear except that the fracture strength of freestanding diamond films is only depended on the grain size (film thickness), and is insensitive to the microscopic defects, and the strength is considerably higher when the nucleation side is in tension than that when the growth side is in tension. However, this is not the full story. Based on the experimental date accumulated in USTB (University of Science and Technology Beijing) for high power dc arcjet diamond films, other factors that may affect the fracture strength of freestanding diamond films are discussed in detail. Effects of the quality of the diamond films, impurities (nitrogen, hydrogen, non-diamond carbon etc.), film morphology and texture, on fracture strength are discussed. Effects of the deposition parameters are explained. Advantages for small amount of nitrogen addition, and for the use of higher substrate temperatures in the increase in fracture strength are demonstrated, which have already been applied in the mass production of tool grade freestanding diamond films. It is hoped that the present paper will be helpful for those who wish to understand and use this brand new type of engineering material
Abstract: A dense ceramic oxide coating approximately 30µm thick was prepared on a Ly12 Al alloy by microarc oxidation in an alkali-silicate electrolytic solution. The morphology and microstructure were analyzed by scanning electron microscope (SEM) and X-ray diffraction (XRD). Coating thickness and surface roughness (Ra) were measured after the coating had been synthesized. The tribological performance of the coatings was evaluated using a dry sand abrasion test and a solid particle erosion test. The results show that microarc oxidation coatings consist of the loose superficial layer and the inner dense layer. Both inner layer and out layer are composed of α-Al2O3 and γ-Al2O3, While the Al6Si2O3 phase is observed only in out loose layer. The average of the microhardness of the coating is 2096Hv.
Abstract: Under different growth conditions, silicon nitride (SiNx) thin films were deposited successfully on Si(100) substrates and glass substrates by plasma enhanced chemical vapor deposition (PECVD). The thickness, refractive index and growth rate of the thin films were tested by ellipsometer. The surface morphologies of the thin films were investigated using atomic force microscope (AFM). The average transmittance in the visible region was over 90%.
Abstract: The surface of porous nitride radome became very compact by AlPO4-Si3N4-SiO2(n) coating treatment. Coating could immerse into the matrix about 20μm deep. This could also make material be moisture resistance for long time placement. Coating effectively healed the micro cracks on the matrix and combined with it by crossed occlusion, so the strength and the fracture toughness of material increased.
Abstract: Hole-sealing coating was fabricated on the porous silicon nitridesubstrate by sol-gel method, with Li2O-Al2O3-SiO2 as the basic materials and other additives were added. The morphology of the coating was tested by scanning electron microscope(SEM). The density,water absorption and porosity of porous silicon nitride before and after coating were tested by Archimedes method.The bending strength was tested by universal testing machine．The results indicated that the water absorption of porous silicon nitride was decreaced by 93.73% to 96.74%, the bending strength of porous silicon nitride was increaced by 8.21％ to 15.56％.
Abstract: The SiC coating on C/C composites was prepared by CVD and pack-cementation method in this paper, then SiC coated C/C composites was jointed with LAS glass ceramic at 1423K for 30min and the pressure of 15 MPa. The structure and morphology of the SiC coating and as-received joints were analyzed by SEM and XRD; shear strength of joint was examined through the Instron universal testing tensile machine. The results show that the thickness of SiC coating prepared by CVD is about 30μm, while the thickness of SiC layer by pack-cementation is nearly 100μm, and the layer is easier to connect with LAS glass ceramic owing to the rough surface and wedge-shaped structure on the SiC coating; the shear strength of joint between SiC coating coated C/C and LAS by pack-cementation is greater than the joint of C/C- LAS with SiC coating prepared by CVD.
Abstract: In order to further study the cracks initiation and distribution mechanism of SiC coating, then optimize the coating structure and composition on surface of C/C composites, CVD method is used to deposit SiC coating on C/C composites and pyrolytic carbon. Through the analysis on the reasons of cracks initiation via SEM, the influence of different kinds of substrates and the toughening of SiC whisker on the cracks initiation and distribution are researched. The results show that the cracks easily form in the area near the defects and sample edge, and the cracks also mainly distribute in the direction which are perpendicular to the first fiber layer. Compared with pyrolytic carbon matrix, cracks on C/C composites matrix are width and numerous, and SiC whisker toughening coating can significantly reduce the number of cracks.
Abstract: In order to improve the anti-oxidation of C/C composites, a C/SiC functionally graded coating for C/C composites was prepared by low pressure chemical vapor deposition (LPCVD) using methyltrichlorosilane (MTS, CH3SiCl3) and H2 as precursors. The relative amount of C and SiC in coatings was varied by controlling the input ratio of H2 to MTS. The phase composition and morphology were examined by scanning electron microscope (SEM) and X-ray diffraction (XRD), and the content distribution of C and SiC phases were investigated by energy dispersive spectroscopy (EDS). The isothermal oxidation test was evaluated at 900 °C and 1500 °C respectively. The results showed that the as-obtained coatings were possessed of a dense and uniform structure, and the SiC content in coatings increased with an increment of the molar ratio of H2 to MTS. The C/SiC coating had a good oxidation resistance.
Abstract: C⁄SiC composites were plasma sprayed with Yb2SiO5⁄LaMgAl11O19 (LMA) coatings with varying Yb2SiO5 layer thickness. The effect of Yb2SiO5 layer thickness on the thermal cycling life of the Yb2SiO5LMA coatings was investigated. The results showed that the thermal cycling life is significantly dependent on the Yb2SiO5 layer thickness. It decreased from 130 cycles to 35 cycles as Yb2SiO5 layer thickness increased from 50 µm to 100 µm. Further increasing Yb2SiO5 layer thickness to 200 µm made it decrease to 2 cycles. The influencing mechanism of Yb2SiO5 layer thickness for the thermal cycling life was clarified based on the thermal expansion behavior, the chemical stability at high temperature and the microstructure analysis.