Shoko Suyama

Development of high-strength reaction-sintered silicon carbide

Reaction sintering is one of the most attractive manufacturing processes of silicon carbide (SiC), because of dense structure, low processing temperature, good shape capability, low cost and high purity. However, mechanical properties of reaction-sintered SiC (RS-SiC) were typically much lower than normal sintered one. Particularly, the bending strength was approximately 300 MPa. In this study, in order to develop the high-strength RS-SiC, effect of the microstructure on the bending strength was examined. The bending strength of RS-SiC was recognized to be increased with decreasing the residual silicon (Si) size, and high-strength RS-SiC has been newly developed. The strength over 1000 MPa was obtained to control the residual Si size under 100 nm. Some other properties of developed high-strength RS-SiC were also evaluated.

DEVELOPMENT OF A REACTION-SINTERED SILICON CARBIDE MATRIX COMPOSITE

Abstract SiC matrix composites reinforced with continuous SiC-based fibres using reaction sintering (RS) for matrix processing were produced and their mechanical and physical properties were studied. Mechanical behaviour of SiCf/SiC (RS) composites in tension and in flexure exhibits improved toughness and a non-catastrophic failure due to fibre crack bridging and pullout from the matrix, and the composites exhibit high thermal conductivity, high Youngs modulus and reduced porosity. Moreover, SiCf/SiC (RS) composites showed improved thermal shock resistance in comparison to monolithic RS-SiC. SiC matrix processing by RS leads to reduced production times and lower costs when compared with other methods such as polymer impregnation and pyrolysis (PIP) or chemical vapour infiltration (CVI). Composite prototypes were also produced for feasibility demonstration, and it was verified that the method could be applied to produce large parts and complex shapes.

Microstructural and mechanical characteristics of SiC/SiC composites with modified-RS process

Abstract This paper deals with the efficiency of modified-reaction sintering techniques including a impregnation pressure and a cold pressure as well as the applicability of new high performance fiber, in order to develop high performance SiC/SiC composites. SiC/BN coated Hi-Nicalon fiber, SiC coated Tyranno ZMI fiber and Tyranno SA fiber were used in this composite system. Microstructural characteristics of reaction sintered SiC/SiC composites were investigated by means of SEM, EDS and TEM. Mechanical properties and fracture mechanisms of SiC/SiC composites were also evaluated through the three-point bending test. Improved SiC/SiC composites with high density and high strength were successfully fabricated by the sequential application of low pressure impregnation and cold pressing processes. An excellent chemical stability of Tyranno SA fiber during the reaction sintering process was also revealed. Tyranno SA fiber reinforced SiC/SiC composites with no interfacial coating exhibited good flexural strength and elastic modulus, compared to those reinforced by BN/SiC coated Hi-Nicalon and SiC coated Tyranno ZMI fibers. However, this composite system needed an appropriate interfacial coating for proper CMC composite behavior.

Effect of structure of interfacial coating layer on mechanical properties of continuous fiber reinforced reaction sintered silicon carbide matrix composite

A dense silicon carbide matrix composite reinforced by Hi-Nicalon fibers CVD coated with boron nitride and silicon carbide was fabricated by slurry impregnation and subsequent reaction sintering with molten silicon. The effect of the structure and the thickness of the silicon carbide layer of the fiber coating on the mechanical properties of the composite were investigated. That is, three types of silicon carbide layers, namely a dense structure with a thickness of 0.15 μm and two porous structures with a thickness of 0.15 μm and 0.48 μm, respectively, were investigated. As a result, excellent strength property of ceramic matrix composite (CMC) was obtained in the case of the dense silicon carbide (SiC) layer. The thickness effect of the SiC layer on the strength was smaller than that of the structure.

NTSIC (New-Technology Silicon Carbide): Evaluation of microstructure of high-strength reaction-sintered silicon carbide for optical mirror

Silicon carbide (SiC) is the most advantageous as the material of various telescope mirrors, because of high stiffness, low thermal expansion, high thermal conductivity, low density and excellent environmental stability. Newly developed high-strength reaction-sintered SiC, which has two to three times higher strength than a conventional sintered SiC, is one of the most promising candidates in applications such as lightweight substrates of optical mirrors, due to being fully dense and having small sintering shrinkage (±1 %), and low sintering temperature. In this study, in order to improve nano-scale homogeneity of the high-strength reaction-sintered SiC, the microstructure of high-strength reaction-sintered SiC was investigated using scanning electron microscopy (SEM) and microscope type interferometer in comparison with the conventional sintered SiC. And also, the microstructure was investigated by focusing on the crystal structures and the interface of each crystal through transmission electron microscopy and X-ray diffraction analysis. As a result, it was the confirmed that the high-strength reaction-sintered SiC was fully dense in comparison with the conventional sintered SiC, and the finer-scale microstructure consisted of large particles (~1 μm in diameter) of α-SiC starting powder and small particles (<1 μm in diameter) of β-SiC synthesized during the reaction-sintering (Si+C→SiC) with residual silicon (Si) filling the remaining pores. In addition, the β-SiC synthesized during the reaction-sintering was identified as the cubic type (3C), and the α-SiC of the starting powder was identified as the hexagonal type (6H).

NT-SiC (new-technology silicon carbide) : Φ 650mm optical space mirror substrate of high-strength reaction-sintered silicon carbide

Silicon carbide (SiC) is the most advantageous as the material of various telescope mirrors, because of high stiffness, low density, low coefficient of thermal expansion, high thermal conductivity and thermal stability. Newly developed high-strength reaction-sintered silicon carbide (NTSIC), which has two times higher strength than sintered SiC, is one of the most promising candidates for lightweight optical mirror substrate, because of fully dense, lightweight, small sintering shrinkage (±1 %), good shape capability and low processing temperature. In this study, 650mm in diameter mirror substrate of NTSIC was developed for space telescope applications. Three developed points describe below. The first point was to realize the lightweight to thin the thickness of green bodies. Ribs down to 3mm thickness can be obtained by strengthen the green body. The second point was to enlarge the mirror size. 650mm in diameter of mirror substrate can be fabricated with enlarging the diameter in order. The final point was to realize the homogeneity of mirror substrate. Some properties, such as density, bending strength, coefficient of thermal expansion, Youngs modulus, Poissons ratio, fracture toughness, were measured by the test pieces cutting from the fabricated mirror substrates.

New-technology silicon carbide (NT-SiC): demonstration of new material for large lightweight optical mirror

Newly developed high-strength reaction-sintered silicon carbide, called New-Technology Silicon Carbide (NT-SiC) is an attractive material for lightweight optical mirror with two times higher bending strength than other SiC materials. The material has advantages in its fabrication process. The sintering temperature is significantly lower than that of pure silicon carbide ceramics and its sintering shrinkage is smaller than one percent. These advantages will provide rapid progress to fabricate large structures. The characteristics of the material are also investigated. The polish of the test piece demonstrated that the polished surface has no pore and is suited to visible region as well as infrared without CVD SiC coating. It is concluded that NT-SiC has potential to provide large lightweight optical mirror.

Reaction-sintered silicon carbide: newly developed material for lightweight mirrors

Newly developed high-strength reaction-sintered silicon carbide is an attractive material for lightweight optical mirror with two times higher bending strength than other SiC materials. The polished surface has no pore and is suited to visible region as well as infrared without CVD SiC coating. The fabrication process, with low temperature and small shrinkage, is also suited to develop large scale objects.

Strengthening Mechanism of High-Strength Reaction-Sintered Silicon Carbide

A newly developed high-strength reaction-sintered silicon carbide (SiC), which has two or three times higher strength than conventional sintered SiC, is one of the most promising candidates for lightweight substrates of optical mirrors, because of its fully dense structure, small sintering shrinkage ( < 0.5 %), good shape capability, and low sintering temperature. In this paper, in order to improve the performance of the newly developed reaction-sintered SiC, the effect of the microstructure on the bending strength was investigated by focusing on a physical fracture model using observations from transmission electron microscopy and X-ray stress measurement. As a result, it was confirmed that the bending strength of the newly developed reaction-sintered SiC could be improved by reducing the size of residual silicon. The strengthening mechanism of the newly developed reaction-sintered SiC was assumed to be due to piled-up dislocations at the grain-boundary of residual silicon sites, based on Stroh’s fracture model of polycrystalline solids.

NTSIC(New Technology SiC): the progress of recent two years

New Technology Silicon Carbide (NTSIC®) is a reaction sintered silicon carbide with very high bending strength. Two times higher bending strength than other SiC materials is important characteristics in an optical mirror for space application. The space optics is to endure the launch environment such as mechanical vibration and shock as well as lightweight and good thermal stability of their figure. NTSIC has no open pore. It provides good surface roughness for infrared and visible application, when its surface is polished without additional coatings. Additional advantages are in the fabrication process. The sintering temperature is significantly lower than that of a sintered silicon carbide ceramics and its sintering shrinkage is less than one percent. These advantages will provide rapid progress to fabricate large structures and will enable that one meter mirror will put practical use. The fabrication capability has developed from 250mm to about one meter in these two years, after previous report of NTSIC. It is concluded that NTSIC has potential to provide large lightweight optical mirror.