Thermal stability of low-oxygen silicon carbide fibers (Hi-Nicalon) in carbon monoxide

The effect of CO treatments on thermal stability of low-oxygen SiC fibers (Hi-Nicalon) was examined at 1273–1773 K using mass change measurements, X-ray diffraction (XRD) analysis, Auger electron spectroscopy (AES) analysis, resistivity measurements, scanning electron microscopy (SEM) observation and tensile tests. The fiber properties remained unchanged by heating below 1573 K. In addition to the grain growth of SiC, reduction of resistivity and degradation of strength above 1573 K, mass loss was observed above 1673 K. AES analysis showed carbon film formation on fiber surfaces at high temperature. The carbon film was formed by the following reaction:CO(g) + SiC(s) = SiO(g) + 2C(s)A Tensile strength of 1.83 GPa was retained even after exposure at 1773 K for 10 h, owing to the suppressing effect of the carbon film on the thermal decomposition of SiC _X O _Y phase.     

High-temperature stability of Nicalon under Ar or O2 atmosphere

The thermal stability of Nicalon NL400 at 1573, 1673 and 1773 K has been studied by TGA and XRD. Under an Ar atmosphere, Nicalon pyrolysed severely to crystallize into -SiC involving the generation of both SiO and CO. Under an O₂ atmosphere, the oxide film which formed around Nicalon retarded the pyrolytic reaction of the core. Nicalon coated with a silica film by the previous oxidation treatment pyrolysed hardly at all under an Ar atmosphere, because the film restricted the escape of SiO and CO. Nicalon which was oxidation-treated at 1773 K retained 63% of its original strength after heating at 1773 K under an Ar atmosphere. The amorphous silica film was found to resist rapid thermal cycling between room temperature and elevated temperatures.     

Effect of thermal treatment on the reactivity of SiC-based fibres

The evolution of the chemical composition of the outer layer of SiC-based fibres was studied under different conditions of temperature and gas environment. After heating the fibres under vacuum or in argon at 1270, 1470, and 1570 K, their reactivity was determined with molecular oxygen at 1170 K and their surface composition was analysed by X-ray photoelectron spectroscopy. Treatment under a vacuum led to severe degradation of the fibres due to chemical reactions between SiC, SiO₂ and carbon. As a result, the silica layer initially present at the surface of the fibre disappeared and an oxygen-deficient silica compound was formed. On the contrary, the formation of silica at the fibre surface was observed during the treatment in argon and the chemical changes were more limited.     

Active-to-passive oxidation transition for polycarbosilane-derived silicon carbide fibers heated in Ar-O2 gas mixtures

The active-to-passive oxidation transition for three types of polycarbosilane-derived SiC fibers (Nicalon, Hi-Nicalon and Hi-Nicalon S) was examined at 1773 K through TG, XRD analysis, SEM observation and tensile tests. The oxygen partial pressure for the active-to-passive oxidation transition decreased in the following order: p _{O 2} = 100–250 Pa for Nicalon, p _{O 2} = 10–25 Pa for Hi-Nicalon and p _{O 2} = 1–2.5 Pa for Hi-Nicalon S. Considerable strength was retained in the passive-oxidation region. The active-oxidation produced a marked decrease in strength of Nicalon and Hi-Nicalon ( 0 GPa). On the other hand, the strength of Hi-Nicalon S after active-oxidation was nearly identical to that after passive-oxidation ( 1 GPa).     

A new type of fiber-bonded-ceramic material synthesized from pre-oxidized Si-Ti-C-O fiber

Two types of fiber-bonded-ceramic material (FBC₂₁₂₃ or FBC₁₈₇₃) were synthesized from preoxidized Si-Ti-C-O fibers with oxide layers of 150 to 500 nm in thickness at 2123 K or 1873 K under 50 to 70 MPa. The interstices in both types of the materials were packed by an oxide material, which had existed on the surface of the pre-oxidized Si-Ti-C-O fiber. So, the dense, fiber-bonded-ceramic materials with small amount of the oxide matrix were obtained. During hot-pressing, carbon in excess of the non-stoichiometric ratio was released from the fiber and formed an interfacial layer on the surface of the fiber, beneath the pre-existing oxide material. Both FBC₂₁₂₃ and FBC₁₈₇₃ showed fibrous fracture patterns with high fracture energies at temperatures up to 1573 K and 1773 K, respectively. FBC₂₁₂₃ exhibited some plasticity in air at a temperature of 1673 K or over, due to the existence of amorphous silica in the matrix, and then a reduction in bending strength was observed at 1773 K in air. On the other hand, FBC₁₈₇₃ maintained its initial bending strength up to 1773 K in air, which is attributed to reduced crystallization of Si-Ti-C-O fiber and to the formation of cristobalite in the matrix.     

Oxidation behavior and mechanical properties of low-oxygen SiC fibers prepared by vacuum heat-treatment of electron-beam-cured poly(carbosilane) precursor

EB-cured PCS fibers were heat-treated at 1273–1673 K under a reduced pressure of 1.33 Pa, and subsequently they were exposed to 1773 K in air. The thermal stability and oxidation resistance of the fibers were investigated through TG, XRD analysis, specific resistivity measurement, SEM observation and tensile tests. The oxidation rates at initial stage are thought to be strongly dependent upon the microstructure of the fibers in the as-heat treated state and the presence of water vapor. Incomplete ceramization and the occurrence of active-oxidation during heat-treatment yielded poor oxidation resistance to the fibers. The oxidation caused the grain growth of SiC, drop of resistivity and degradation of strength. The oxidation of the fibers was retarded at later stage. The fibers heat-treated at 1573 K had high strength and high oxidation resistance.     

Effect of CO2 partial pressure on oxidation of low-oxygen SiC fibers (Hi-Nicalon) in Ar-CO2 gas mixtures

The oxidation behavior and thermal stability of Si—C fibers (Hi-Nicalon) in Ar-CO₂ gas mixtures were investigated at 1773 K, through mass change determination, XRD analysis, resistivity measurement, SEM observation and tensile tests. Mass gain and cristobalite formation were observed at p _{CO 2} 10³ Pa, showing the occurrence of passive-oxidation of the fibers. On the other hand, the active-oxidation was characterized by the mass loss, no formation of SiO₂ film and a marked increase in resistivity at p _{CO 2} 5 × 10² Pa. The oxygen potential for the active-to-passive oxidation transition in Ar-CO₂ gas mixtures was nearly identical to that in Ar-O₂ gas mixtures. About 50% of the strength in the as-received state was retained after the active-oxidation in Ar-CO₂ gas mixtures.     

Characterization and properties of TiN-containing amorphous Ti-Si-N nanocomposite coatings prepared by arc assisted middle frequency magnetron sputtering

TiN-containing amorphous Ti-Si-N (nc-TiN/a-Si₃N₄) nanocomposite coatings were deposited by using a modified closed field unbalanced middle frequency magnetron sputtering system which is arc assisted and consists of two circles of targets, at a substrate temperature of 400 °C. The coatings exhibit good mechanical properties that are greatly influenced by the total gas pressure and N₂/Ar ratios. For coatings prepared at a N₂/Ar ratio of 3:1, the hardness increases from 24 GPa at a total gas pressure of 0.2 Pa-58 GPa at 0.4 Pa, and then, the hardness decreases gradually when the total gas pressure was further increased. On the other hand, the friction coefficient decreases monotonously with increasing total gas pressure. XRD, XPS and high resolution TEM experiments showed that the coatings contain TiN nanocrystals embedded in the amorphous Si₃N₄ matrix. The coating deposited under optimum conditions exhibits excellent tribological performance with a low friction coefficient of 0.42 and a high hardness of 58 GPa. These properties make it possible for industrial applications.     

Effect of fibre surface morphology on stability of C/TiBx coatings in Ti–6Al–4V/σ (SM1240) fibre metal matrix composite

Abstract

Sigmafibres (SM1240) produced by a chemical vapour deposition process using a 15 μm tungsten wire corefor SiC deposition have a duplex coating of graphitic carbon and TiBx. Nodules present on the fibre surface are attributed to the deposition of the carbon coating over soot particles present on the substrate. Both the carbon and TiB_x coatings were stable in vacuum or air at temperatures up to 973 K. The nodules werefound to be sites of preferential attack by the titanium alloy matrix. The average number of nodules per fibre decreased more rapidly when the specimens were heated in air than in vacuum. It is suggested that the nodules may reduce the stability temperature of the coatings.

MST/2028     

Dense outer layers formed by plasma treatments of silica coatings produced by the sol–gel method

Silica gel coatings prepared by the sol-gel method were subjected to low-temperature plasma treatments in air and argon. This was found to give rise to the formation of a dense outer layer, whose thickness increased with the duration of the treatment and decreased when the pressure in the plasma chamber was reduced. The formation of a dense layer in the coatings was confirmed from measurements of the overall thickness and the refractive index by ellipsometry, and also from TEM examination and light transmission experiments. The coatings were found to contain an uppermost layer of alumina, which was formed through secondary sputtering of aluminium from the electrodes in the plasma chamber. The concentration of alumina in the mixture with silica decreased through the thickness of the coating and became zero at a distance slightly smaller than the overall thickness of the dense layer. The thickness of the dense layer was found to be slightly higher with argon than with air plasma treatments. Whereas the aluminium concentration through the thickness of the coating was the same for the two types of treatment, a carbon residue was found only in the case of argon plasma treatments. The composition of the underlying silica layers was found to correspond to SiO_1.6. This revised version was published online in November 2006 with corrections to the Cover Date.     

Characterisation of thin silica films deposited on carbon fibre by an atmospheric pressure non-equilibrium plasma (APNEP)

Low modulus carbon fibres were used to investigate the potential of a low cost novel atmospheric pressure microwave plasma technique for continuously depositing silica coatings onto carbon fibres, in a tow form. The objective was to improve the interfacial properties of aluminium/carbon fibre composites produced by liquid metal infiltration techniques. Amorphous silica coatings, approximately 50–400 nm thick, were successfully produced in a continuous process. The nature and morphology of the coatings were determined using transmission electron microscopy. Squeeze cast and gas pressure infiltrated samples were manufactured to investigate the fibre/matrix interface using tensile tests and a short beam interlaminar shear test. The tensile samples displayed brittle fracture with ultimate tensile stresses of 223 and 251 MPa for the uncoated and coated fibres, respectively. The shear samples did not show any interlaminar shear, but failed in tension. It is concluded that the silica coating did not have a significant effect on improving the carbon fibre/aluminium matrix interface. However, the microwave plasma technique was very successful and can be modified to produce low cost alternative coatings for future development.     

Application of nitriding to electroless nickel–boron coatings: Chemical and structural effects; mechanical characterization; corrosion resistance

Electroless nickel–boron coatings, synthesized on mild steel, were submitted to nitridation treatments in varying conditions of pressure, temperature and atmosphere composition. One treatment was carried out under a reduced pressure of a nitrogen-based gas, the other under ambient pressure in a ammonia-based atmosphere.The modifications of the samples’ chemistry after those treatments were investigated by ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectroscopy), GD-OES (Glow Discharge-Optical Emission Spectroscopy) and ToF-SIMS (Time of flight-Secondary Ions Mass Spectroscopy) analysis. Their structures and morphology were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and optical microscopy. The mechanical properties of the samples were investigated by micro- and nanohardness measurements successively on the free surface of the sample and on polished cross-sections; their roughness and resistance to scratch test were measured. Electrochemical corrosion tests were also carried out.The effects of both treatments were then compared: after the treatment carried out under lowered pressure, the coatings are dense, present signs of solution hardening and are characterized by a high hardness (close to 1600 hv₁₀₀). A combination layer is observable on the samples treated under an ammonia-based atmosphere. This outer layer possesses poorer mechanical properties but the inner layer of the coating presents properties similar to those of vacuum nitrided coatings. The corrosion resistance of the coatings is as good as that of heat treated coatings.     

SiC-Si-ZrSiO_4 Multiphase Oxidation Protective Coating for Carbon/Carbon Composites

In order to improve the anti-oxidation property of carbon/carbon （C/C） composites, a novel SiC-Si-ZrSiO4 multiphase oxidation protective coating was produced on the surface of C/SiC coated carbon/carbon compo ites by a pack cementation technique. The phase composition and microstructure of the as-prepared coatings were characterized by XRD （X-ray diffraction）, SEM （scanning electron microscopy） and EDS （energy dispersive spectroscopy）. Oxidation behavior of the multiphase coated C/C composites was also investigated. It showed that the as-prepared coating characterized by excellent oxidation resistance and thermal shock re- sistance could effectively protect C/C composites from oxidation at 1773 K for 57 h in air and endure the thermal cycle between 1773 K and room temperature for 12 times, whereas the corresponding weight loss is only 1.47%. The excellent oxidation protective ability of the SiC-Si-ZrSiO4 coating could be attributed to the C/SiC gradient inner layer and the multiphase microstructure of the coating.     

Effect of reduced pressure on oxidation and thermal stability of polycarbosilane-derived SiC fibers

To clarify the effects of reduced pressure on the thermal stability of polycarbosilane-derived SiC fibers (Nicalon, Hi-Nicalon and Hi-Nicalon S), the heat-treatments were conducted at 1623 and 1723 K under total pressures (p _T) of 1–10⁵ Pa. The oxidation behavior and thermal stability of the fibers were investigated through examinations of mass change, grain growth, specific resistivity, fiber morphology and tensile strength. All the fibers were definitely oxidized in the active-oxidation regime at p _T 10² Pa and T = 1723 K. Nicalon and Hi-Nicalon S were subjected to serious degradation of fiber strength. Hi-Nicalon had a strength of 1.2 GPa even after heat-treatment at p _T = 1 Pa.     

Characterization of TiCr(C,N)/amorphous carbon coatings synthesized by a cathodic arc deposition process

Ternary TiCrN and nanocomposite TiCr(C,N)/amorphous carbon (a-C) coatings with different carbon contents (0-26.6 at.%) were synthesized by cathodic arc evaporation with plasma enhanced duct equipment. The structural, chemical, and mechanical properties of the deposited films were studied by X-ray diffraction, X-ray photoelectron spectroscopy (XPS), and nanoindentation measurement. The atomic content ratios of carbon/(Ti + Cr) and carbon/nitrogen increased with increasing C₂H₂ flow rate. A nanocomposite structure of coexisting metastable hard TiCr(C,N) crystallites and amorphous carbon phases was found in the TiCr(C,N)/a-C coatings, those possessed smaller crystallite sizes than the ternary TiCrN film. XPS analyses revealed the concentration of a-C increased with increasing carbon content from 8.9 at.% to 26.6 at.%. Exceeding the metastable solubility range of carbon within the TiCrN lattice, the carbon formed a-C phase in the deposited coatings. The nanocomposite TiCr(C,N)/a-C coatings exhibited higher hardness value of 29-31 GPa than the deposited TiCrN coating (26 ± 1 GPa). It has been found that the structural and mechanical properties of the films were correlated with the carbon content in the TiCr(C,N)/a-C coatings.     

Thermal properties of chemical vapour-deposition SiC-C nanocomposites

The relationship between the thermal properties and the microstructure of chemical vapour-deposition (CVD) SiC-C nanocomposites, covering the entire composition range from SiC to C, was investigated after measuring thermal conductivity and thermal expansion. The samples were prepared under deposition temperatures (T _dep) of 1673 and 1773 K and total gas pressure (P_tot) of 40 kPa. The thermal conductivity of CVD SiC-C nanocomposites decreased as C content increased. For the deposits containing 24.3 to 71 mol % C prepared atT _dep = 1773 K, some parts of the C phase formed a layered structure having its plane parallel to the deposition surface. This arrangement reduced the thermal conductivity in the direction perpendicular to the deposition surface to a much lower value. The CVD C and CVD C-SiC containing < 1.5 mol % SiC showed strong anisotropic thermal expansion. However, the thermal expansion of CVD SiC-C nanocomposites having a C content up to about 70 mol % was isotropic and nearly equal to that of CVD SiC. The low preferred orientation and the low modulus of elasticity of the C phase may be reasons for these results.     

Slip‐rolling resistance of ta‐C and a‐C coatings up to 3,000 MPa of maximum Hertzian contact pressure

The slip‐rolling resistances of hard and stiff thin films under high Hertzian contact pressures can be improved by optimizing the “coating/substrate systems”. It is known from former investigations that the so‐called “egg‐shell” effect is no general hindrance for high slip‐rolling resistance of thin hard coatings. The coating stability depends more on specific deposition process and coating/substrate interface design. In this article it is experimentally shown, that pure amorphous carbon thin films with hardness between 15 and 63 GPa can be slip‐rolling resistant several million load cycles under a maximum Hertzian contact pressures of up to 3.0 GPa. Whereas all coatings were stable up to 10 million load cycles in paraffin oil at room temperature, reduced coating lifetime was found in SAE 0W‐30 engine oil at 120°C. It was shown how the coating hardness and the initial coating surface roughness influence the running‐in process and coating lifetime. No clear correlation between coating hardness and coating lifetime could be observed, but friction coefficients seem to be reduced with higher coating hardness. Very low friction down to ?0.03 in unmodified engine oils was found for the hardest ta‐C film.     

Inter-diffusion of carbon into niobium coatings deposited on graphite

The inter-diffusion of carbon (originating from a graphite substrate) into a niobium coating and the fabrication of its carbides by heat treatment in the temperature range of 1073-1773 K was studied. The thickness of the Nb₂C and NbC phases formed after heat treatment as well as the inter-diffusion coefficients for the formation of the carbide layers were also studied. It was found that the carbide layer growth displayed parabolic behavior patterns inherent in the growth rate constants (K) of Nb₂C and NbC layers.By assuming that the inter-diffusion coefficients are independent of concentration, it was possible to determine the inter-diffusion coefficients of carbon D^c into Nb₂C and NbC layers as a function of temperature.     

Thermal stability of SiO2-coated SiC fiber (Hi-Nicalon) under argon atmosphere

Thermal stability of low-oxygen SiC fiber (Hi-Nicalon) coated with SiO₂ film was investigated. The SiO₂ film of same thickness but different crystal structure was formed by heating low-oxygen SiC fiber (Hi-Nicalon) under different oxidation conditions. The oxidation treatment and the subsequent exposure at 1773 K in argon caused very little loss of strength for unoxidized core. For as-oxidized fiber with SiO₂ film which containedimperfections, further loss of strength was caused after exposure in argon. There was little degradation of core strength on being exposed repeatedly at rapid heating and cooling rate in argon. The fiber oxidized at 1773 K kept high level of strength even after exposure at 1823 K. This is because the change in crystal structure of SiO₂ film before and after exposure in argon, which was the controlling factor in the degradation of strength, was diminished with increasing oxidation temperature.     

Microstructure and oxidation-resistant of ZrO2/Ni coatings applied by high-speed jet electroplating

ZrO₂/Ni composite coatings with different contents of ZrO₂ particles were deposited on superalloy K17 substrate using high-speed jet electroplating process. The microhardness and microstructure of composite coatings were studied. The oxidation kinetic curves of uncoated and coated K17 alloys were obtained. The results indicated that ZrO₂/Ni composite coatings exhibit higher microhardness than that of pure nickel coatings under the same high-speed jet electrodeposition conditions. ZrO₂/Ni composite coatings exposure to air at 1000 °C for 5 h formed scale containing NiO and Cr₂O₃; after exposure to air at 1000 °C for 100 h the scale was comprised NiO, NiCr₂O₄, and Cr₂O₃. The formation of Cr₂O₃ scales on the ZrO₂/Ni composite coating directly improved the oxidation resistance of superalloy K17.