Enhancing the strength-ductility synergy of Fe-based composites by changing interface bonding between matrix and TiCx with various stoichiometric ratios

The increase of strength in composites usually comes at the cost of ductility. This work proposed a strategy of design and synthesis of TiC_x nanoparticles with controlled Ti/C stoichiometric ratios to adjust the microstructure and thus enhance the mechanical properties of the TiC_x nanoparticles reinforced iron matrix composites. The changes of size and morphology of TiC_x nanoparticles were investigated by self-propagating combustion synthesis (SHS) in the Fe–Ti–C reaction system. The influences of Ti/C stoichiometric ratios on the interface bonding and mechanical properties of Fe/TiC_x were analyzed by lattice mismatch calculation. The results indicate that the morphology of TiC_x nanoparticles changes from polyhedral to near-spherical sometimes accompanied by a few cubes with an increase in Ti/C stoichiometric ratios. The maximum yield strength (1525 MPa) compressive strength (2224 MPa) and microhardness (691.5 HV) of the TiC_x/Fe composites are obtained when the C/Ti stoichiometric ratio reaches 1.0. The TiC_0.8/Fe composites present a remarkable ductility up to 8.34%. Based on the lattice mismatch calculation, the good mechanical properties are mainly because the low lattice mismatch reduces the interfacial energy and thus leads to the higher mechanical properties. Effective design and control of the morphology of TiC particles and the interface bonding in Fe melt can maximize comprehensive performances and applications.     

Investigation of Ti3AlC2 formation mechanism through diffusional reaction between carbon and Mo-modified Ti6Al4V

Carbon and Mo-coated Ti6Al4V alloy diffusion couple was used to investigate Mo-modified diffusion reaction between carbon and Ti6Al4V. Randomly dispersed carbide particles were observed in Ti6Al4V alloy after 900℃ exposure. Carbide particles were found evolving from defective TiC_x (x<1) to mixture of TiC_x and defective Ti₃AlC₂. Although Mo atoms were hardly detected in particles, their dilution effect along Ti alloy grain boundary (GB) is beneficial to carbon diffusion and carbide formation along GB. Based on high-resolution TEM (HRTEM) imaging, high density stacking faults (SFs) and nanotwins were observed in TiC_x particles which explain Raman activation of defects in TiC. Formation of SFs and nanotwins in TiC_x is attributed to carbon vacancies, which inversely promotes incoherent twin boundary (ITB) formation. Transformation from TiC_x to Ti₃AlC₂ is considered driven by Al indiffusion along ITBs. Intergrowth of TiC_x in defective Ti₃AlC₂ is the compromise to low Al concentration in Ti6Al4V.     

Nanostructure,mechanical and tribological properties of reactive magnetron sputtered TiCx coatings

This study describes the correlation between microstructure, mechanical and tribological properties of TiC_x coatings (with x being in the range of 0–1.4), deposited by reactive magnetron sputtering from a Ti target in Ar/C₂H₂ mixtures at ~ 200 °C. The mechanical and tribological properties were found to strongly depend on the chemical composition and the microstructure present. Very dense structures and high hardness, combined with low wear rates and friction coefficients, were observed for coatings with chemical composition close to TiC. X-ray diffraction and X-ray photoelectron spectroscopy analysis, used to evaluate coating microstructure, composition and relative phase fraction, showed that low carbon contents in the coatings lead to sub-stoichiometric nanocrystalline TiC_x coatings being deposited, whilst higher carbon contents gave rise to dual phase nanocomposite coatings consisting of stoichiometric TiC nanocrystallites and free amorphous carbon. Optimum performance was observed for nanocomposite TiC_1.1 coatings, comprised of nanocrystalline nc-TiC (with an average grain size of ~ 15 nm) separated by 2–3 monolayers of an amorphous a-DLC matrix phase.     

Joining of SiCf/SiC using a layered Ti3SiC2-SiCw and TiC gradient filler

A layered filler consisting of Ti₃SiC₂-SiC whiskers and TiC transition layer was used to join SiC_f/SiC. The effects of SiC_w reinforcement in Ti₃SiC₂ filler were examined after joining at 1400 or 1500 °C in terms of the microstructural evolution, joining strength, and oxidation/chemical resistances. The TiC transition layer formed by an in-situ reaction of Ti coating resulted in a decrease in thermal expansion mismatch between SiC_f/SiC and Ti₃SiC₂, revealing a sound joint without cracks formation. However, SiC_f/SiC joint without TiC layer showed formation of cracks and low joining strength. The incorporation of SiC_w in Ti₃SiC₂ filler showed an increase in joining strength, oxidation, and chemical etching resistance due to the strengthening effect. The Ti₃SiC₂ filler containing 10 wt.% SiC_w along with the formation of TiC was the optimal condition for joining of SiC_f/SiC at 1400 °C, showing the highest joining strength of 198 MPa as well as improved oxidation and chemical resistance.     

Raman Investigation of Interfacial Reaction Product of SiCf/Ti43Al9V Composite

Interficial reaction products of 800°C heat‐treated SiC_f/Ti43Al9V are studied. EDS mapping indicates two layers existence in the interface with different element enrichment. Raman line scanning along interface also shows two layers with TiC_x next to SiC fiber and Ti₂AlC next to matrix. Excess carbon throughout the interface and accumulation in TiC_x layer is confirmed by Raman. FWHH of both D and G peaks indicate carbon in Ti2AlC layer is more disordered than that in TiC_x layer. According to G peak and I(D)/I(G), carbon in interface can be assigned to disordered carbon with perfect graphene units size (L_a) stays around 0.7–0.9 nm. Raman peaks of Ti₂AlC vary with same trend throughout Ti₂AlC layer is detected, and Raman spectra of Ti₂AlC near TiC_x layer is different from that near TiAl matrix, which results from residual stress and TiC intergrowth with Ti₂AlC due to lack of Al.     

Mechanical properties of SiCf/SiC composites with alternating PyC/BN multilayer interfaces

Alternating pyrolytic carbon/boron nitride (PyC/BN)_n multilayer coatings were applied to the KD–II silicon carbide (SiC) fibres by chemical vapour deposition technique to fabricate continuous SiC fibre-reinforced SiC matrix (SiC_f/SiC) composites with improved flexural strength and fracture toughness. Three-dimensional SiC_f/SiC composites with different interfaces were fabricated by polymer infiltration and pyrolysis process. The microstructure of the coating was characterised by scanning electron microscopy, X–photoelectron spectroscopy and transmission electron microscopy. The interfacial shear strength was determined by the single-fibre push-out test. Single-edge notched beam (SENB) test and three-point bending test were used to evaluate the influence of multilayer interfaces on the mechanical properties of SiC_f/SiC composites. The results indicated that the (PyC/BN)_n multilayer interface led to optimum flexural strength and fracture toughness of 566.0?MPa and 21.5?MPa?m^1/2, respectively, thus the fracture toughness of the composites was significantly improved.     

Microstructure and microindentation of Ti3SiC2 – Titanium filler brazed joints by tungsten inert gas (TIG) process

Herein we study the joining of Ti₃SiC₂ - a MAX phase - with a Ti filler (Ti₃SiC₂/Ti-filler) using a TIG-brazing process. The microstructures of the interfaces were investigated by scanning electron microscopy and energy dispersive spectrometry. When Ti₃SiC₂ comes into contact with the molten Ti - filler during the TIG-brazing operation, it starts decomposing into TiC_x and a Si-rich liquid. Simultaneously, the molten Ti infiltrates into the Ti₃SiC₂ resulting in a 200 µm thick duplex region, comprised of TiC_x and a Ti-rich phase with some dissolved Si. Both Si and C are found in the solidified Ti; the Si source is from the Si-rich liquid, while the presence of C indicates that some of the C diffused into the Ti. Upon cooling, C- containing Ti- rich lamellae form the solidified Ti. Microindentation results of the decomposed Ti₃SiC₂ layer show an increase in hardness and a decrease in elastic modulus relative to T₃SiC₂. Notably, no cracks were observed.     

Effect of C/Ti ratio on densification,microstructure and mechanical properties of TiCx prepared by reactive spark plasma sintering

Titanium carbide (TiC) has been widely used as reinforcement in metal matrix composites and is known to exist over a wide range of stoichiometry. In this study, the effect of C/Ti ratio on the densification kinetics, grain size, lattice parameter, hardness and elastic modulus of TiC_x prepared by reactive spark plasma sintering (RSPS) is presented. Commercial purity titanium was ball milled with 5, 7.5, 10, 12.5, 15 and 17.5 wt% carbon black powder for 5 h and subjected to RSPS to prepare TiC_x samples with different C/Ti ratio. Dense TiC_x samples with ‘x′ ranging from 0.34 to 0.78 could be prepared by RSPS at 1400 °C. Increasing C/Ti ratio was found to increase the activation energy thereby reducing the rate of sintering and also resulted in finer grain size. The lattice parameter and the ratio of intensities of (200) to (111) peaks were correlated with the C/Ti ratio. The hardness and elastic modulus were shown to increase significantly with increase in C/Ti ratio.     

Synthesis of coatings on SiC fibers and their effects on microstructure and mechanical properties of SiCf/SiBCN composites

In this study, the amorphous C, ZrB₂, and BN single-layer coatings as well as C/BN, C/ZrB₂, ZrB₂/BN, and C/ZrB₂/BN composite coatings were prepared on SiC fibers (SiC_f) by an in situ synthesis and solution impregnation–pyrolysis method. Subsequently, SiC_f/SiBCN composites were fabricated by hot-pressing sintering at 1900℃/60 MPa/30 min to explore the influence of different coatings on the microstructure and mechanical performance of resulting composites. After the preparation of single-layer-coated SiC_f, the SiC_f(BN) or SiC_f(ZrB₂) tended to be overlapped with each other, whereas the dispersion of amorphous C–coated SiC_f was satisfying. Besides, some uneven areas and attached particles have appeared on fiber surfaces of the SiC_f(BN) or SiC_f(ZrB₂), whereas smooth and dense surfaces of amorphous C–coated SiC_f were observed. Because the uniformity of ZrB₂ coatings can be partially damaged by the subsequent coating process of BN, the composite coatings of ZrB₂/BN and C/ZrB₂/BN were thereby not suitable for strengthening SiBCN matrix. The SiC_f/SiBCN composites with C/ZrB₂ coatings have desirable comprehensive mechanical properties. Nevertheless, the conventional toughening mechanisms such as fiber pull-out and bridging, and crack deflection are not available for these composites because the serious crystallization of SiC_f leading to great strength loss, resulting in catastrophic brittle fracture.     

Effect of intermediate heat treatment on mechanical properties of SiCf/SiC composites with BN interphase prepared by ICVI

SiC_f/SiC composites with BN interface were prepared through isothermal-isobaric chemical vapour infiltration process. Room temperature mechanical properties such as tensile, flexural, inter-laminar shear strength and fracture toughness (K_IC) were studied for the composites. The tensile strength of the SiC_f/SiC composites with stabilised BN interface was almost 3.5 times higher than that of SiC_f/SiC composites with un-stabilised BN interphase. The fracture toughness is similarly enhanced to 23 MPa m^1/2 by stabilisation treatment. Fibre push-through test results showed that the interfacial bond strength between fibre and matrix for the composite with un-stabilised BN interface was too strong (>48 MPa) and it has been modified to a weaker bond (10 MPa) due to intermediate heat treatment. In the case of composite in which BN interface was subjected to thermal treatment soon after the interface coating, the interfacial bond strength between fibre and matrix was relatively stronger (29 MPa) and facilitated limited fibre pull-out.     

Oxidation behavior and kinetics of in situ (TiB2 + TiC)/Ti3SiC2 composites in air

The isothermal oxidation behavior of in situ (TiB₂ + TiC)/Ti₃SiC₂ composite ceramics with different TiB₂ content has been investigated at 900-1200 °C in air for exposure times up to 20 h by means of X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, and energy dispersive spectroscopy. The oxidation of (TiB₂ + TiC)/Ti₃SiC₂ composites follows a parabolic rate law. With the increase in TiB₂ content, the oxidation weight gain, thickness of the oxidation scale, and parabolic rate constant decrease dramatically, which suggests that the incorporation of TiB₂ greatly improves the oxidation resistance of the composites. With the increase in oxidation temperature, the enhancement effect becomes more pronounced. Due to the incorporation of TiB₂, the oxidation scale of (TiB₂ + TiC)/Ti₃SiC₂ composites is generally composed of an outer layer of coarse-grained TiO₂ and an inner layer of amorphous boron silicate and fine-grained TiO₂. Only the dense inner layer formed on the surface acts as a diffusion barrier, retarding the inward diffusion of O, and consequently contributing to the improved oxidation resistance of the (TiB₂ + TiC)/Ti₃SiC₂ composites.     

Effects of additives on the synthesis of TiCxN1−x by a solid-gas mechanically induced self-sustaining reaction

The synthesis of TiC_xN_1?x from Ti/C mixtures in a N₂ atmosphere performed in a high-energy planetary mill was used as example to study the influence of the use of additives in mechanically induced self-sustaining reaction (MSR) processes. In particular, the effect of the addition of TiN, TiC, Si₃N₄ and SiC was analyzed. The self-sustaining reaction was extinguished when additive contents of 50, 40, 40 and 30?wt% for TiN, TiC, Si₃N₄ and SiC, respectively, were employed. These additives cannot be regarded as real inert since they served as an extra solid source for nitrogen and carbon, modifying the final stoichiometry of the TiC_xN_1?x phase. The adiabatic temperature (T_ad) determined for the mixtures with no MSR effect was well above the empirical limit value of 1800?K adopted as criterion for the occurrence of the self-propagating high-temperature synthesis (SHS) process. The ignition time (t_ig) of the MSR process was practically invariant for low additive contents (approximately 50?min) and tended to increase up to maximum values of 85–95?min for the larger additive contents.     

SiCn/SiC oxidation protective coating for carbon/carbon composites

Nano-SiC (SiC_n) coating was deposited on SiC pre-coated C/C composites by a hydrothermal electrophoretic deposition. The phase compositions, surface and cross-section microstructures, and anti-oxidation properties of the multilayer coatings were investigated. Results show that hydrothermal electrophoretic deposition is an effective route to prepare smooth and homogeneous SiC_n coating on SiC-C/C composites. The as-prepared SiC_n/SiC multilayer coatings can effectively protect C/C composites from oxidation in air at 1773 K for 202 h with a weight loss of 0.79% and at 1873 K for 64 h with a weight loss of 1.3%.     

Phase analysis and wear behavior of in-situ spark plasma sintered Ti3SiC2

In-situ synthesis of dense near-single phase Ti₃SiC₂ ceramics from 3Ti/SiC/C/0.15Al starting powder using spark plasma sintering (SPS) at 1250 °C is reported. Systematic analysis of the phase development over a range of sintering temperatures (1050–1450 °C) suggested that solid state reactions between intermediate TiC and Ti₅Si₃ phases lead to the formations of Ti₃SiC₂. The effect of starting powder composition on phase development after SPS at 1150 °C was also investigated using three distinct compositions (3Ti/SiC/C, 2Ti/SiC/TiC, and Ti/Si/2TiC). The results indicate that the starting powder compositions, with higher amounts of intermediate phase such as TiC, favor the formation of Ti₃SiC₂ at relatively lower sintering temperature. Detailed analysis of wear behavior indicated that samples with higher percentage of TiC, present either as an intermediate phase or a product of Ti₃SiC₂ decomposition, exhibited higher microhardness and better wear resistance compared to near single phase Ti₃SiC₂.     

Temperature-driven deintercalation and structure evolution of Ag/Ti3AlC2 composites

Ag/Ti₃AlC₂ composites are promising sliding contact material. Here, Ag/Ti₃AlC₂ composites were obtained via the hot pressing technique and their structure evolutions upon sintering temperature were investigated. Sintering temperature controlled the deintercalation of Al layers from Ti₃AlC₂, thus controlling the interfacial structure of Ag/Ti₃AlC₂ composites. Amorphous interface was found after sintering at 750?°C. TiC_x particles with a size of around 10?nm were found in the interfacial region after sintering at 800?°C. Increasing sintering temperature to 850?°C, stripy structures composed of alternately arranged silver-rich phase and TiC_x phase appeared around the edges of Ti₃AlC₂ particles. The over-saturated Al precipitates found in 850?°C sintered composites are cubic μ-Ag₃Al inter-metallic compounds, which have the coherent relationship with Ag matrix.     

Oxidation resistance of multi-walled carbon nanotubes coated with polycarbosilane-derived SiCxOy ceramic

Multi-walled carbon nanotubes (MWCNTs) have been successfully coated with a thin SiC_xO_y coating when polycarbosilane (PCS) was used as precursor and pyrolyzed in a coke bed. Meanwhile, effect of PCS concentration on oxidation resistance of the coated MWCNTs is studied. The results showed that the pyrolysis products of PCS were composed of amorphous SiC_xO_y as the main phase, together with β-SiC and SiO₂ as the minor phases whose amount increased a little with the increase of temperature from 1000 °C to 1500 °C. The thickness of SiC_xO_y coating on the surface of MWCNTs increased a little from 1 wt.% to 5 wt.%, but decreased dramatically with PCS concentration in the range of 10-30 wt.%. The oxidation resistance of the coated MWCNTs was greatly improved in comparison with as-received ones. The oxidation peak temperature of the coated MWCNTs reached 783.7 °C, much higher than 652.2 °C for as-received ones.     

Effects of Mo addition on tribological performance of plasma-sprayed Ti–Si–C coatings

Ti–Si–C–Mo composite coatings were fabricated by plasma spraying using Ti, Si, graphite and Mo powders. The effect of Mo on microstructure and tribological performance of the Ti–Si–C coatings were investigated. The results showed that the Ti–Si–C coating consisted of TiC, Ti₃SiC₂, Ti₅Si₃, and residual graphite. The Ti–Si–C–Mo coatings consisted of TiC, Ti₃SiC₂, Ti₅Si₃, residual graphite, Mo and Mo₅Si₃ phases. With increasing Mo contents, the fractions of Mo and Mo₅Si₃ phases increased, and the fractions of Ti₃SiC₂ and Ti₅Si₃ phases decreased. All the coatings existed a typical lamellar structure. The addition of Mo enhanced the hardness and fracture toughness of Ti–Si–C coating by 16% and 52%, respectively. The coating porosity decreased by 57.6%. The wear resistance of the Ti–Si–C coating was also improved and the mass loss decreased by 83%. The wear mechanism of the Ti–Si–C–Mo coatings was the combination of abrasive wear, adhesive wear, and tribo-oxidation wear.     

Tribocorrosion behaviour of TiCxOy thin films in bio-fluids

Decorative coatings require not only an attractive appearance for market applications, but also an ability to protect the surface underneath. Because of this, corrosion, wear and their combined effects (termed tribocorrosion) are particularly important for lifetime prediction. In this paper, the tribocorrosion behaviour of a range of single layered titanium oxycarbide, TiC_xO_y, coatings, produced by DC reactive magnetron sputtering, has been studied and reported as a function of electrode potential and applied load. The study was conducted in a reciprocating sliding tribo-system (Plint TE 67/E) in a bio-fluid (an artificial perspiration solution) at room temperature. During the wear tests, both the open-circuit potential and the corrosion current were monitored. The results showed that electrode potential and load have a significant influence on the total material loss. The variations in R_p (polarization resistance) and C_f (capacitance) before and after sliding, obtained by Electrochemical Impedance Spectroscopy (EIS) were evaluated in order to provide an understanding of the resistance of the film in such conditions. Tribocorrosion maps were generated, based on the results, indicating the change in mechanisms of the tribological and corrosion parameters for such coatings, as a function of load and applied potential.     

Molten salt synthesis of in-situ TiC coating on graphite flakes

TiC coating was synthesized on graphite flakes (G_f) by molten salt synthesis (MSS) using metal Ti powder and alkali salts. Three different alkali chloride salts of KCl, NaCl, and NH₄Cl were selected as the molten salt media substrate. Two mass ratios of 1:3 and 1:5 were chosen for Ti: G_f ratio, and the mass ratio of the powder (Ti + G_f) to the salt was 1:1. The synthesis was carried out at a temperature of 1100°? for 4 h. XRD was used to study the effect of alkali chloride salts and the Ti: G_f mass ratio on the synthesized coating. FE-SEM and AFM were accomplished to investigate the carbide formation and microstructure of the samples. Results showed that TiC coating was formed at 1100 °C for 4 h with both mass ratios in all three alkali chloride salts, but KCl was found to be the optimum alkali chloride salt or reaction medium. FE-SEM results displayed the formation of uniform coatings, and results from AFM indicated that the surface roughness increased from 0.72 for G_f to 4.94 nm for TiC coated G_f.     

Spark plasma sintering of TiC–SiCw ceramics

Silicon carbide whiskers (SiC_w) in TiC had impressive impacts on the properties and made it possible for special applications which generally would not be conceivable with TiC alone. In the present work, SiC_w reinforced TiC based composites were prepared by spark plasma sintering (SPS) technique, at the temperature of 1900 °C under the pressure of 40 MPa for sintering time of 7 min. To test out the effects of different amount of SiC whisker (0, 10, 20 and 30 vol%) on the characteristics of TiC, the sintered samples were investigated about sinterability and physical-mechanical properties. Microstructure observations and density measurements confirmed that the composites were dense with uniformly distributed reinforcement, and the specimen doped with higher than 10 vol% SiC_w could attain higher relative density (>100%) than pure TiC and TiC–10 vol% SiC_w. Also, the highest values for hardness (29.04 GPa) and thermal conductivity (39.2 W/mK) were achieved in specimen containing 30 vol% SiC_w, whereas the optimum bending strength (644 MPa) was recorded in material containing 20 vol% SiC_w. It seems that one of the reasons which contributes to this trend of properties variation is the generation of near-stoichiometric TiC_x phase and new Ti₃SiC₂ compound.