Interfacial reactions in a Ti-6Al-4V based composite: role of the TiB2 coating

The characterization of TiB₂/C-coated SiC fibres and their interface region in a Ti-6Al-4V based composite has been performed by using scanning electron microscopy (SEM), energy-dispersion X-rays (EDX) and Auger electron spectroscopy (AES). The features of the as-received fibre and the reactivity between fibre and matrix occurring during preparation of the composite have been studied in this paper. The interaction of the TiB₂ external coating of the fibre with both the adjacent carbon layer and the titanium-based matrix is already appreciable in the as-received composite: TiB needles grow from TiB₂ towards the matrix and a new layer containing C, Ti and B appears between TiB₂ and C. The thicknesses of the original carbon and TiB₂ fibre coatings decrease in the composite from 1000 nm to 400 and 800 nm, respectively. The TiB₂ inhibits the reaction between SiC and Ti: there is no evidence of Si _x Ti _y brittle phases.     

TEM investigations of interfacial processes in SCS-6 SiC/TiB2/Super α2 composites

Interfacial processes in SCS-6 SiC/TiB₂/Super α₂ composites were investigated by means of analytical transmission electron microscopy (TEM). No reaction between the TiB₂ coating and the carbon coating of the fibres can be found in the as-processed specimen. At the TiB₂/matrix interface the TiB₂ transforms into both a homogeneous layer and needles of TiB which are distributed in the matrix. This process takes place according to the reaction TiB₂+Ti=2TiB. The TiB₂ coating is consumed completely in the specimen heat treated at 900°C for 64 h. At this time TiC and Ti₃AlC form at the TiB/C-coating and the TiB/matrix interface, respectively. Prolonging the holding time at 900°C, reaction layers thicken continuously following parabolic reaction kinetics. The paper additionally discusses the formation mechanisms of the reaction products.     

Chemical vapour deposition of tungsten and TiN on SiC fibres

Handling of uncoated SiC/W (Sigma) fibres resulted in a drop in their strengths. Coating with TiB₂ or boron at about 1000 °C by chemical vapour deposition resulted in a further reduction in strength. Coating with tungsten or TiN by lower temperature chemical vapour deposition processes did not degrade the fibre and the coated fibres also retained their strengths after heating in hydrogen at 1040 °C.     

Evaluation of the efficiency of TiB2 and TiC as protective coatings for SiC monofilament in titanium-based composites

The vigorous interfacial reactions in SiC/Ti-6Al-4V composites at elevated temperatures lead to the deterioration of the mechanical properties of the composites. TiB₂ and TiC were selected as potential protective coatings for SiC fibres in titanium-based composites. These coatings were deposited on to fibres by the chemical vapour deposition technique. Comparisons and evaluations have been made of the effectiveness of these ceramics as protective coatings for SiC fibres by incorporating the coated fibres into a Ti-6Al-4V matrix using the diffusion bonding method. Emphasis has been placed on the chemical compatibility of the candidate coating with SiC and Ti-6Al-4V by examining the interfaces of the fibre/coating/matrix using microscopic methods and chemical analysis. A stoichiometric TiB₂ coating was found to be stable with SiC and has proved an effective barrier to prevent the SiC fibre from reacting with the Ti-6Al-4V. The TiC coating showed no apparent reaction with a titanium-alloy matrix under the conditions studied, but was found to react with the SiC fibre substrate.     

Micro-structural evolution of phenol-formaldehyde resin modified by boron carbide at elevated temperatures

The phenol-formaldehyde (PF) resin was modified by boron carbide (B₄C). In order to investigate the modification effect of B₄C, the residue values of pure PF resin and B₄C modified PF resin were measured using thermal gravity. It was shown that the residue values of B₄C modified PF resin are 71.9% and 68.4% after being pyrolyzed at 700 and 1000 °C, respectively, which are obviously higher than those of the pure PF resin (62.9% and 60.5% at 700 and 1000 °C, respectively). The microstructure evolution of the modified resin at high temperatures was also investigated by scanning electron microscopy and energy dispersive analysis of X-rays. By means of the microstructure characterization, the modification reactions between the B₄C additives and the oxygen-containing volatiles, such as CO and H₂O, are demonstrated. The carbon and oxygen elements remained in the resin matrix in the forms of amorphous carbon and B₂O₃, respectively, resulting in the improvement of residue values and stability of the PF resin at high temperatures. The distribution of modification particles became well-proportioned gradually at the elevated temperatures, and the shape of ceramic additives changed into white spherules due to the surface tension.     

Isothermal oxidation behavior of reactive hot-pressed TiN–TiB2 ceramics at elevated temperatures

Isothermal oxidation behavior of reactive hot-pressed TiN–TiB₂ ceramics with various TiN/TiB₂ molar ratios of 2/1, 1/1 and 1/2 was evaluated in the temperature range of 500–800 °C in air. TiN–TiB₂ ceramics have a relative density of 97–98.6%. The oxidation weight gains of TiN–TiB₂ ceramics depend upon the composition, oxidation temperature and exposure time. The structure and morphology of oxidized layers of TiN–TiB₂ ceramics were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). During isothermal oxidation of TiN–TiB₂ ceramics, anatase and rutile-TiO₂ form as the oxidized products at 500 °C. However, phase transformation from anatase to rutile occurs at temperatures between 500 and 600 °C, and therefore rutile-TiO₂ becomes the only crystalline phase after oxidation at temperatures of 600–800 °C for 10 h. The oxidation mechanism was proposed with reference to thermodynamically feasible oxidation reactions. The influence of composition on oxidation behavior of TiN–TiB₂ ceramics varies with temperature.     

Reaction control of TiB2 formation from titanium metal and amorphous boron

TiB₂ powder was synthesized by a controlled formation reaction from titanium metal and amorphous boron. Precursory TiB₂ formed by the pretreatment of the mixed powder (mole ratio: B/Ti=2.0) at 600° C for 60 min in an argon stream. Hollow TiB₂ powder with an average grain size of 15m was obtained by subsequent heat treatment above 900° C for more than 60 min in an argon stream. The formation reaction of TiB₂ powder was further controlled by pretreatment of the mixed powder at 600° C for 60 min in a hydrogen and argon stream and subsequent heat treatment at 1000° C for 360 min in an argon stream, when hollow-free TiB₂ powder was formed by a milder formation reaction between amorphous boron and the reformed titanium metal with hydrogen diffused lattice.     

Structure-property relations for silicon nitride matrix composites reinforced with pyrolytic carbon pre-coated Hi-Nicalon fibers

Si₃N₄ matrix composites reinforced with pyrolytic carbon pre-coated Hi-Nicalon (SiC) fibers, were studied using tensile testing and transmission electron microscopy. Three types of samples were evaluated all with a nominal coating thickness of 200 nm. The composites were densified by hot pressing at 1550 °C (type I and II) and at 1600 °C (type III). The fibers were coated with pyrolytic carbon via CVD with identical (sample I) and opposite (samples II and III) directions of the gas flow and of the fiber movement through the reactor. Tensile testing indicated for the three sample types respectively: brittle behaviour with huge pull out of the fibers, pseudo-plastic behaviour and brittle behaviour with little pull out. TEM indicated for the three sample types debonding typically at the fiber/coating interface, at the coating/matrix interface and in the coating, respectively. The relation between processing, structure, particularly of the coating and its interfaces with the matrix and the fibers and mechanical properties is addressed.     

Oxidation of monolithic TiB2 and of Al2O3-TiB2 composite

In view of the susceptibility of TiB₂ to oxidation, the thermal stability of monolithic TiB₂ and Al₂O₃-TiB₂ composite was investigated. The temperature at which TiB₂ ceramic starts to oxidize is about 400°C, oxidation kinetic being controlled by diffusion up toT900°C and in the first stage of the oxidation at 1000 and 1100°C (up to 800 and 500 min, respectively), and by a linear law at higher temperatures and longer periods. Weight gains of Al₂O₃-TiB₂ composite can be detected only at temperatures above 700°C and the rate-governing step of the oxidation reaction is characterized by a one-dimensional diffusion mechanism atT=700 and 800°C and by two-dimensional diffusion at higher temperatures. The composition and morphology of the oxidized surfaces were analysed.     

TiO2 coatings on silicon carbide and carbon fibre substrates by electrophoretic deposition

Electrophoretic deposition (EPD) has been used to obtain TiO₂ coatings on three dimensional (3-D) SiC fibre (Nicalon ®-type) and carbon fibre substrates. Colloidal suspensions of commercially available TiO₂ nanoparticles in acetylaceton with addition of iodine were used. The EPD parameters, i.e., deposition time and voltage, were optimised for each fibre type. Strongly adhered TiO₂ deposits with high particle packing density were obtained. Scanning electron microscopy observations revealed high penetration of the titania nanoparticles into the fibre preforms. The TiO₂ deposits were sintered at 800°C for 1 h in order to produce relatively dense and uniform TiO₂ coatings covering completely the SiC or carbon fibres. For the carbon fibre/TiO₂ system, an effort was made to produce a 3-D titania matrix composite by further infiltration of the porous fibrous preform with TiO₂ by slurry dipping and subsequent pressureless sintering. The 3-D carbon fibre reinforced TiO₂ matrix composites fabricated contained residual porosity, indicating further infiltration and densification steps are required to produce dense composites of adequate structural integrity. For SiC fibre fabrics, oxidation tests in air established the effectiveness of the TiO₂ coating as oxidation protective barrier at 1000°C. After 120 h the increase of weight due to oxidation of coated fibres was more than twice lower than that of the uncoated fibres. TiO₂ coated SiC fibre preforms are attractive materials for manufacturing hot gas filters and as reinforcing elements for ceramic matrix composites.     

High temperature behaviour of a Ti-6Al-4V/TiCp composite processed by BE-CIP-HIP method

The high temperature behaviour of a Ti-6Al-4V/TiC_p composite (10% Vol. of TiC) was investigated. A composite produced by Dynamet Technology according to the blended-elemental-cold-hot isostatic pressing (BE-CHIP) method was used. The stress-strain properties of the material were tested at 25, 200, 400, 500, 600 and 800°C. Composite specimens were aged in air at 500 and 700°C or under vacuum at 500, 700 and 1050°C, for periods ranging between 100 and 500 hours. The thermal stability of the matrix/ceramic interfaces was studied by using scanning electron microscope, electron probe microanalysis and x-ray diffraction. Carbon diffusion from the ceramic particles towards the composite matrix occurred (very likely already during the composite fabrication) because the metal matrix of all the composite samples (either in the as received or thermally treated conditions) showed a high content of carbon (more than 1% at.). However, the thermal treatments carried out at both 500 and 700°C under vacuum did not result in a ceramic-metal reaction. In spite of this, the formation of an ordered phase of formula Ti₂C can be inferred. Long period aging under vacuum at 700°C (500 h) did not lower the composite tensile strength. On the other hand, above 500°C in air the titanium matrix rapidly underwent oxidation, which gave rise to the formation of a thick surface reaction layer; this confirms that the composite material cannot be used above this temperature. Furthermore, the thermal treatment performed at 1050°C (under vacuum) resulted in a strong composite microstructure modification: the formation of new mixed carbides of Al and Ti was observed.     

As received Ti–6AI–4V/σ-SiC fibre composite

Abstract

A Ti–6Al–4V/σ (SM 1240) composite prepared by diffusion bonding has been studied in the as received condition, using Auger electron spectroscopy, transmission electron energy loss spectroscopy, and scanning electron microscopy. The SiC based σ fibre has a tungsten core, and a duplex coating of carbon (adjacent to the SiC deposit) and TiB_x. It is shown that boron from the TiB_x layer diffused into the matrix and formed TiB needles. Carbon was detected in the TiB_x layer and was present in elemental free form. A continuous SiO₂+ carbon layer was detected at the SiC/carbon layer interface. Analysis of in situ fracture composite surfaces in an Auger spectrometer has shown that the tensile failure was initiated within the carbon layer or at the TiB_x/matrix interface. An oxide layer detected at the TiB_x/matrix interface influenced the fracture behaviour of the composite.

MST/2027     

Optimization of the interfaces in Nicalon-fibre-reinforced-AIPO-matrix composite materials

The optimization of fibre/matrix interfaces in Nicalon-fibre-reinforced aluminium-phosphatematrix composite materials is addressed. First, the structure and chemical composition of the fibre/matrix interfaces were characterized for the as-fabricated composite materials and for the same materials after high-temperature exposures simulating the conditions of their intended application. Transmission electron microscopy (TEM) showed considerable Si diffusion from the fibres into the matrix accompanied by the formation of an interfacial diffusion/reaction zone in the process of heat treatment at 816 °C and higher temperatures. A BN-fibre coating did not prevent Si diffusion. Next, the interfacial bond strength was measured for the uncoated and some of the coated interfaces. The measurements showed a much lower bond strength in the carbon and carbon/BN coated interfaces than in the uncoated and BN-coated interfaces. Finite-element modelling was used to evaluate the interfacial bond strength which would result in the highest strength of the composite material. This optimal bond strength was found to be characterized by a critical energy-release rate close to 50 J m^–2. Further increase in the interfacial bond strength above 50 Jm^–2 resulted in brittle failure of the composite materials. Finally, the potential fibre coatings which were stable and not reactive with the fibres and the matrix at elevated temperatures were identified for the projected service temperatures of 816 and 1093 °C.     

Improvements in refractoriness and properties of Nicalon fibres by high-temperature heat-treatment

Nicalon SiC fibres were heat-treated in various atmospheres and at various pressures. Initially CO, nitrogen and air were used as the heat-treatment environment at one atmosphere pressure. Microstructural changes and any related strength degradation or improvement were measured for the heat-treated fibres. After heat-treatment in the temperature range 1000°C–1600°C, each sample showed different weight changes. Thus, in air, a weight gain was observed with increasing temperature, whereas in CO and N₂, weight losses were observed but with a smaller weight loss observed for CO. Moreover, carbon monoxide had a significant effect on the strength retention of the fibres. Since the lowest weight loss was observed after heat-treatment in CO at one atmosphere, high pressure CO gas was used to heat-treat Nicalon fibres between 1000°C and 1700°C and the resulting fibres were analysed by single-filament strength testing, scanning electron microscopy, and X-ray diffraction. The results were completely different compared with those in one atmosphere of CO. As the temperature increased, weight and strength increased whereas at one atmosphere pressure, both weight and strength had decreased. The weight increase was because of surface reaction between the CO atmosphere and the SiC fibre and/or because of deposition of carbon from the pressurised CO gas, giving the fibre a surface carbon coating. Carbon coating of a fibre is a beneficial property for CMCs since it provides a weak interface which facilitates pull-out during fracture.     

In situ reacted TiB2-reinforced mullite

In situ formation of TiB₂ in mullite matrix through the reaction of TiO₂, boron and carbon has been studied. In hot-pressed and pressureless-sintered samples, in addition to TiB₂, TiC was also found to be dispersed phases in mullite matrix. However, in the case of pressurelesssintered samples, mullite/TiB₂ composite with 98% relative density can be obtained through a preheating step held at 1300 °C for longer than 3 h and then sintering at a temperature above 1600 °C. Hot-pressed composite containing 30 vol% TiB₂ gives a flexural strength of 427 MPa and a fracture toughness of 4.3 MPam^1/2. Pressureless-sintered composite containing 20 vol% TiB₂ gives a flexural strength of 384 MPa and a fracture toughness of 3.87 MPam^1/2.     

The stability of hydroxyapatite in an optimized bioactive glass matrix at sintering temperatures

The stability of sintered hydroxylapatite particles was studied in glass matrices of the system SiO₂-CaO-P₂O₅-Na₂O-Al₂O₃-B₂O₃ at sintering temperatures between 700 and 1000 °C. The results from X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectroscopy (EDX) analysis, transmission electron microscopy (TEM) and electron diffraction showed that above 700 °C the Na⁺ ions diffuse into the hydroxylapatite particles which then transform into rhenanite. The glass matrix undergoes crystallization yielding wollastonite crystals and a silica rich matrix.     

Microstructure–mechanical properties relations in SiC–TiB2 composite

Densification and mechanical properties (fracture toughness, flexural strength and hardness) of SiC–TiB₂ composite were studied. Pressureless sintering experiments were carried out on samples containing 0–50 vol% of TiB₂ created by in situ reaction between TiO₂, B₄C and carbon. Al₂O₃ and Y₂O₃ were used as sintering additives to create liquid phase and promote densification at sintering temperature of 1940 °C. The sintered samples were subsequently heat treated at 1970 °C. It was found that the presence of TiB₂ serves as an effective obstacle to SiC grain growth as well as crack propagation thus increasing both strength and fracture toughness of sintered SiC–TiB₂ composite. The subsequent heat treatment of sintered samples promoted the elongation of SiC matrix and further improved mechanical properties of the composite. The best mechanical properties were measured in heat-treated samples containing 12–24 vol% TiB₂. The maximum flexural strength of ∼600 MPa was obtained in samples with 12 vol% TiB₂ whereas the maximum fracture toughness of 6.6 MPa m^1/2 was obtained in samples with 24 vol% TiB₂. Typical microstructures of samples with the mentioned volume fractions of TiB₂ consist of TiB₂ particles (<5 μm) uniformly dispersed in a matrix of elongated SiC plates.     

Interfacial reactions for the non-stoichiometric TiB x /(100)Si system

In order to evaluate the interfacial reactions in the TiB _x /(100)Si system and the thermal stability of non-stoichiometric TiB _x films (0 B/Ti 2.5), TiB _x /Si samples prepared by a co-evaporation process were annealed in vacuum at temperatures between 300 and 1000°C. The solid phase reactions were investigated by means of sheet resistance, X-ray diffraction, transmission electron microscopy, X-ray photo-electron spectroscopy, and stress measurement. For TiB _x samples with a ratio of B/Ti 2.0, an apparent structural change is not observed even after annealing at 1000°C for 1 h. For samples with a ratio of B/Ti < 2.0, however, there are two competitive solid phase reactions: the formation of a titanium silicide layer at the interface and the formation of a stoichiometric TiB₂ layer at the surface, indicating the salicide process. The sheet resistance and the film stress in the Ti/Si and TiB _x /Si systems are well explained by the solid phase reactions.     

Assessment of reactions occurring during the solid state processing of iron-based titanium diboride composites

The reactions occurring during the solid-state processing of Fe-C/TiB₂ composite materials have been assessed. Optical microscopy and X-ray diffraction have been used to identify the products of reaction after sintering and hot isostatic pressing of such materials in the temperature range 1000–1200 °C. TiC has been seen to form readily at the TiB₂/Fe interface: an apparently continuous layer of TiC forms on the surface of the TiB₂, hindering further reaction. Solid state processing appears to be a potentially viable route for the production of iron-based TiB₂ composite materials.     

Interface analysis in Al and Al alloys/Ni/carbon composites

Nature of fibre/matrix interfaces existing in Al/C composites were investigated depending on the presence of a nickel interlayer deposited on carbon fibres and on the composition of the aluminium matrix. Auger and electron microprobe analyses were used. The role of the nickel layer on the chemical evolution of the system after a 96 h heat treatment at 600°C is discussed. The presence of this nickel layer limits the diffusion of carbon into aluminium, and thereby, eliminates the formation of a carbide interphase, Al₃C₄, which is known to lower the mechanical properties of Al/C composites. The mechanisms differ according to the composition of the matrix. In the case of pure aluminium, an Al-Ni intermetallic is formed after thermal annealing. It does not react with the carbon fibre and so inhibits the growth of Al₃C₄. In the case of the alloyed matrix (AS7G0.6), the dissolution of the Ni sacrificial layer, after annealing, does not lead to the same Al-Ni intermetallic but a thin nickel layer remain in contact with the carbon fibre avoiding formation and growth of Al₃C₄ carbide. This difference of behaviour is tentatively ascribed to the presence of silicon that segregates at the fibre/matrix interface.