Shuqi Guo

Microstructure and role of outermost coating for tensile strength of SiC fiber

Abstract The detailed microstructure of the SiC fiber surface and the outermost coating of SiC(SCS-6) fiber are observed using transmission electron microscopy (TEM) and high resolution electron microscopy (HREM). The tensile strengths of the SiC fibers: uncoated fiber (SCS-0), coated fiber (SCS-6) and extracted fiber from fatigue-loaded SiC(SCS-6) fiber-reinforced Ti-15-3 composite are determined. Fractographic analysis is done on these fibers and the mirror radius is compared with the tensile strength. Thickness of the outermost coating is ≈3.6xa0 μ m and it consists of three different layers (i.e. sublayers I, II and III). Basically, these sublayers consist of a carbon matrix in which β -SiC crystallites are dispersed. The fracture toughness of the SiC fiber is ≈3.3xa0MPaxa0m 1/2 . The outermost coating increases the fiber strength twofold because it reduces stress concentration at the surface of the SiC fiber. The tensile strength of the extracted fiber (SCS-6) from fatigue-loaded specimens shows a reduced strength which is attributed to the debonding of the outermost coating while the composite is loaded.

Microstructural characterization of interface in SiC fiber-reinforced Ti-15V-3Cr-3Al-3Sn matrix composite

Abstract The detailed microstructure of the interfacial reaction layer in SiC (SCS-6) fiber-reinforced Ti-15-3 (Ti–15V–3Cr–3Al–3Sn) alloy matrix composite has been studied using transmission electron microscopy. The interfacial reaction occurs between the Ti matrix and the outermost coating layer of the SiC fiber. The reaction layer grows at the expense of both the Ti-15-3 matrix and the outermost coating layer. The reaction front on the coating layer side is relatively smooth while that adjacent to the matrix is irregular due to a difference in reaction rate between the coating layer and α, β phases of the matrix. The mean thickness of the reaction layer follows a parabolic growth law. The results of transmission electron microscopy with energy dispersive spectroscopy observations indicate that the reaction layer consists of a mixture of single crystal stoichiometric Ti5Si3, nonstoichiometric TiC1−x, and the size of these reaction products varies with the location of the layer. The product size adjacent to the matrix is larger than that adjacent to the unreacted outermost coating layer. In addition, vacancy ordering in the TiC1−x phase is identified and microvoids, which result from the volume contraction during the reaction, are also observed in the interfacial reaction layer.

Tensile fracture behavior of continuous SiC fiber-reinforced SiC matrix composites at elevated temperatures and correlation to in situ constituent properties

Abstract The tensile fracture behavior and tensile mechanical properties of polymer infiltration pyrolysis (PIP)-processed two-dimensional plain-woven fabric carbon-coated Nicalon™ SiC fiber and BN-coated Hi-Nicalon™ SiC fiber-reinforced SiC matrix composites have been investigated. Tensile testing of the composites was carried out in air between 298 and 1400 K. In situ fiber strength and interface shear stress were determined by fracture mirror size and pulled-out fiber length measurements. For the Nicalon/C/SiC, tensile strength remained nearly constant up to 800 K, and while the strength dropped from 140 MPa at 800 K to 41 MPa at 1200 K, with weakest link failure mode. For the Hi-Nicalon/BN/SiC, the tensile strength increased slightly with increase in test temperature up to 1200 K; however, a large decrease in the strength was observed at 1400 K. In the case of the Hi-Nicalon/BN/SiC, the fracture was governed by fiber bundle strength. The temperature dependence of tensile strength and fracture behavior of both composites was attributed to change of the in situ constituent properties with temperature.

Fatigue damage evolution in SiC fiber-reinforced Ti-15-3 alloy matrix composite

Abstract Tension-tension fatigue damage behavior of an unnotched SiC (SCS-6) fiber-reinforced Ti-15-3 alloy matrix composite at room temperature was examined, applying maximum stresses of 450, 670 and 880 MPa with R = 0.1. The change in stress-strain hysteresis curves was measured. Fiber fracture behavior and matrix cracking behavior were observed in situ and the results were compared with the change of unloading modulus obtained from the hysteresis curves. The fiber fracture behavior inside the specimen was also determined by dissolving the Ti alloy matrix. The results showed abrupt reductions in the unloading modulus of the composite at stresses of 450, 670 and 880 MPa; the normalized unloading modulus decreased by 8%, 12% and 17%, respectively, in the initial stage ( N ⩽ 10 cycles). This reduction was caused by the multiple fiber fragmentation. Thereafter, the unloading modulus maintained a nearly constant value; and non-propagating matrix cracks were initiated adjacent to the end of fractured fiber. The propagation of the matrix crack again led to a rapid reduction of the unloading modulus, and the composite then failed. With higher applied stress, the fatigue life was reduced. The fracture behavior of the composite was discussed with special attention to the fiber fracture behavior and its effect on the modulus of the composite.

Effect of loading rate and holding time on hardness and Young's modulus of EB-PVD thermal barrier coating

Abstract The effect of the loading rates and duration of holding periods at the maximum load on hardness and Youngs modulus in thermal barrier coatings fabricated by the electron beam physical vapor deposition process has been studied using an ultra-micro-indentation technique. The indentation tests were carried out with three different loading rates and durations of holding time at a maximum load. The average hardness of the coating significantly increased with increasing loading rate, in contrast, it slightly decreased with holding time. However, a substantial decrease in Youngs modulus was observed with the increase in loading rate. A slight increased Youngs modulus was found with an increase in holding time. The different dependences of both the hardness and Youngs modulus on loading rate and holding time were associated with creep deformation occurring during indentation testing.

Observation of short fatigue crack-growth process in SiC-fiber-reinforced Ti-15-3 alloy composite

The microscopic fatigue damage characteristics and short fatigue crack growth of an unnotched SiC(SCS-6) fiber-reinforced Ti-15-3 alloy composite were investigated in tension-tension fatigue tests (R = 0.1) carried out at room temperature for applied maximum stress of 450, 670, and 880 MPa.In situ observation of the damage-evolution process was done using optical and scanning laser microscopies, which were attached in the fatigue machine. The first damage for the composite started from a cracking of the reaction layer followed by fiber fracture. The matrix cracking initiated near the broken fiber when the microhardness of the matrix just to the side of the fracture fiber reached ≈6 GPa, and the number of cycles for the initiation of this cracking decreased with the increase of applied stress. The slope of the relation of surface crack growth lengthvs number of cycles fell into two characteristic stages; in the first stage, the rate was lower than the second stage and accelerated. The surface crack growth rate,d(2c)/dN,vs surface crack length relation also fell into two stages (stages I and II). With the increase in surface crack length, the crack-growth rate,d(2c)/dN, decreased in stage I and increased in stage II. The transition from stage I to stage II occurred due to the fracture of fibers located around the first fractured fiber. It was concluded that the fatigue crack growth resistance of the composite in the short-crack region was controlled by the fiber fracture and matrix work hardening near the fractured fiber. When the fiber fracture occurred, the surface crack growth rate was accelerated and became faster than that of the monolithic matrix.

In situ observation of cyclic fatigue crack propagation of SiC-fiber/SiC composite at room temperature

Abstract In situ observation of cyclic fatigue crack propagation of SiC-fiber reinforced SiC composite at room temperature has been carried out by laser microscopy. Both smooth (unnotched) and notched specimens are used for tension-tension cyclic fatigue tests. Cracks initiate at the comers of large pores during loading in smooth specimens. In notched specimens cracks are formed at the interfaces between fibers and matrix that are connected to the notch. The balance between the fiber bridging in the wake of propagating crack tip and the breakage of bridged fibers by the degradation of interfaces maintains a steady cyclic crack propagation. Crack propagation rate gradually decreases with time after the maximum load being applied.

High-temperature thermal stability of Hi-Nicalon™ SiC fiber/SiC matrix composites under long term cyclic heating

Abstract The high-temperature thermal stability of C-coated standard grade Nicalon™ SiC fiber and of C or BN-coated Hi-Nicalon™ SiC fiber-reinforced SiC matrix composites under long-term cyclic heat exposure in air has been studied. Woven fabric composites fabricated by the PIP process were cyclic heat-exposed at 1273 and 1673 K for up to 1000 h with one heat exposure cycle of 200 h. The degradation of the composites was evaluated by a three-point flexure test at room temperature. The oxidation of the composite occurred from the matrix phase due to the existence of macro- and micro-pores that exist in the matrix. Degradation of the strength was observed for the C-coated SiC fiber/SiC composites after heat exposure for 200 h at 1273 K. On the contrary, the BN-coated Hi-Nicalon™ SiC fiber/SiC composite showed only a slight degradation even after a 1000-h exposure at 1273 K. However, at 1673 K, this composites strength degraded after an 800-h exposure.

Interfacial compatibility of C/Au-coated SiTiCo fiber-reinforced Ti matrix composite

Abstract The feasibility of incorporating C/Au-coated SiTiCO fiber into a Ti matrix has been investigated. The integrity and effectiveness of the coating were examined. The tensile strengths of the as-coated fiber and extracted fiber from the Ti matrix were measured. The critical coating thickness needed for preventing the chemical reaction between the fiber and the matrix and for preserving the strength of the SiTiCO fiber has been determined.

Effect of Cu/Ta duplex metal coating on interface characterization in SiC fibre-reinforced Ti-15 wt% V-3 wt% Cr-3 wt% AI-3 wt% Sn matrix composite

Abstract The interfacial reaction behaviour, alloying element diffusion behaviour and interface shear mechanical properties in Cu/Ta-duplex-metal-coated SiC(SCS-6)-fibre-reinforced Ti—15-3 matrix composite (SiC/Cu/Ta/(Ti—15-3)) have been studied. The results are compared with the original SiC(SCS-6)-fibre-reinforced Ti—15-3 matrix composite (SiC/(Ti—15-3)), and the effect of the Cu/Ta duplex metal coating on the interface characterizations has been discussed. The interface reaction occurs between the coating layer and the matrix, and the thickness of the reaction layer in the SiC/Cu/Ta/(Ti—15-3) increases slightly more than that of the SiC/(Ti—15-3). Electron probe microanalysis shows that the Cu and Ta diffuse into both the SCS coating and the matrix and that a large amount of Cu and Ta exists in the interface reaction region. The Cu/Ta coating acts as a diffusion barrier layer on Ti; however, the coating has no significant effect on the diffusion behaviours of C and Si. Fibre push-out test results indicate that the Cu Ta coating layer reduced significantly the interfacial shear debond stress and frictional stress. The interfacial shear mechanical properties of the composite have significantly degenerated after thermal exposure. This degeneration is attributed to a microstructure change of the matrix, which is associated with thermal exposure and the diffusion of alloying elements during fabrication and/or following heat treatment and/or thermal exposure processes.