Synthesis of TiB2 powder from a mixture of TiN and amorphous boron

TiB₂ powder was synthesized by solid state reaction using amorphous boron and TiN as a source of titanium. The TiB₂ formation did not occur at all in a nitrogen atmosphere even at 1400° C. TiB₂ formed above 1100° C in argon and hydrogen atmospheres. The only crystalline phase of TiB₂ powder was favourably synthesized at 1400° C for 360 min in an argon atmosphere from a starting powder with a composition containing excess boron (B/Ti = 2.2). The synthesized powder was well dispersed and had a particle size of 0.5 to 2 µm. The powder activity was evaluated by sintering at 4 G Pa and 1300 to 1600° C for 15 min.     

Preparation of TiB2 sintered compacts by hot pressing

A sintered compact of titanium diboride (TiB₂) was prepared by hot pressing of the synthesized TiB₂ powder, which was obtained by a solid-state reaction between TiN and amorphous boron. Densification of the sintered compact occurred at 20 MPa and 1800° C for 5 to 60 min with the aid of a reaction sintering, including the TiB₂ formation reaction between excess 20 at % amorphous boron in the as-synthesized powder (TiB₂ + 0.2B) and intentionally added 10 at % titanium metal. A homogeneous sintered compact of a single phase of TiB₂, which was prepared by hot pressing for 30 min from the starting powder composition [(TiB₂ + 0.2B) + 0.1 Ti], had a fine-grained microstructure composed of TiB₂ grains with diameters of 2 to 3 m. The bulk density was 4.47 g cm^–3, i.e. 98% of the theoretical density. The microhardness, transverse rupture strength and fracture toughness of the TiB₂ sintered compact were 2850 kg mm^–2, 48 kg mm^–2 and 2.4 MN m^–3/2, respectively. The thermal expansion coefficient increased with increasing temperature up to 400° C and had a constant value of 8.8 x 10^–6 deg^–1 above 500° C.     

SiC matrix composites reinforced with internally synthesized TiB2

A new process of preparing particulate-reinforced ceramic composites by internal synthesis has been developed. SiC powder mixed with TiN and amorphous boron was hot-pressed above 2000° C in an argon atmosphere. The boron molar content in the mixture was designed to be more than twice that of TiN. In the process of hot-pressing, the following reaction took place between 1100 and 1700° C TiN+2B TiB₂+1/2N₂ The synthesis of TiB₂ was followed by the densification of SiC matrix with the aid of the excess boron. The new process provides SiC matrix composites in which fine TiB₂ particulates are dispersed. Compared with hot-pressed monolithic SiC, the composite containing 20 vol % TiB₂ exhibits a 80% increase in fracture toughness and about the same flexural strength of 490 MPa at 20° C in air and 750 MPa at 1400° C in a vacuum.     

Preparation of TiB2 and ZrB2. influence of a mechano-chemical treatment on the borothermic reduction of titania and zirconia

TiB₂ and ZrB₂ have been synthesized by a mechano-chemical treatment of a mixture of titania or zirconia powder and amorphous boron followed by a relatively low temperature annealing (1100°C). Both the temperature and the kinetics of the borothermic transformations are affected by the mechano-chemical treatment when the size of the particles obtained after thermal annealing of the sample milled for a short time remains in the sub-micron range. The reaction paths are different for TiB₂ and ZrB₂ with the formation of TiBO₃ and Ti₂O₃ as intermediate compounds in the case of the borothermic reduction of titania, while in the zirconia/boron system a direct borothermic reduction of zirconium oxide is observed.     

Formation process of tungsten borides by solid state reaction between tungsten and amorphous boron

Various kinds of tungsten borides were synthesized by solid state reaction between tungsten and amorphous boron powders. The mixed powders with various compositions (B/W = 0.4 to 13.0) were treated at 800 to 1550° C for 0 to 120 min in a stream of argon. Four kinds of boride phases such as W₂B, WB, W₂B₅ and WB₄ were formed, although the boride phase having the composition of the highest boride, WB₁₂, did not appear. The formation of W₂B was initiated approximately at 1000° C in excess of tungsten content. On the other hand, in the excess boron content, the formation of WB, W₂B₅ and WB₄ was initiated approximately at 800, 950 and 1200° C, respectively. The maximum formation amount and crystallinity of WB and W₂B₅ was found in nearly 10 at % excess boron content in their own stoichiometric compositions. The only crystalline phase of WB₄ was prepared with a large excess boron content. However, the formation behaviour of WB₄ showed that WB₄ is metastable above 1400° C. The stability of WB₄ phase could be increased by the presence of excess boron.     

Production of titanium particulate metal matrix composite by mechanical milling

Abstract

Mechanical milling is an established production method for aluminium particulate metal matrix composites (MMCs). There are examples of its use for high performance automotive applications and within the aerospace industry. The production of a titanium particulate MMC is still in the developmental stage. However, compared to conventional titanium alloys such materials offer improvements in stiffness, strength, fatigue and creep properties, high temperature capability, and wear resistance. This paper describes the use of mechanical milling for the production of titanium particulate MMCs with the addition of 10 vol.-%TiB. Gas atomised titanium powders with additions of either boron or TiB₂ were milled in a high purity argon atmosphere to avoid contamination of the powders by oxygen or nitrogen. The distribution of the boron or TiB₂ with increasing milling time is discussed along with the effect of the alloy composition. Gas atomised, hydride dehydride, and sponge fine powder blends are also compared. The powders were subsequently hot isostatically pressed at 500°C for 2 h at 150 MPa followed by 900°C for 2 h at 150 MPa. During this consolidation process TiB was formed by an in situ reaction between either the TiB₂ or boron and the titanium matrix.     

Thermal stability of interfaces in Ti-6Al-4V reinforced by SiC Sigma fibres

Interfacial reactions in Ti-6Al-4V/SiC Sigma fibres (coated with carbon and TiB₂) were studied at different temperatures (600, 700 and 1000 °C). Interface microstructure was investigated by scanning electron microscopy and Auger electron spectroscopy. A simulation of the chemical phenomena occurring at the interfaces was carried out using powders of pure titanium, carbon and TiB₂; the reaction products were identified by X-ray diffraction. The double coating of Sigma fibres is effective in delaying detrimental reactions with the matrix. At the interfaces matrix/TiB₂ and TiB₂/C, the TiB and TiC_x phases form, respectively. The protective coating of fibres shows a lifetime greater than 1000 and 750 h at 600 and 700 °C, respectively.     

Synthesis of boron nitride from triammoniadecaborane and hydrazine under pressure

Boron nitride (BN) of low crystallinity was synthesized from triammoniadecaborane (TAD) and hydrazine at 125 MPa below 650° C. TAD itself was pyrolysed at 600° C and 125 MPa to form a mixture of amorphous boron and boron nitride containing BH and NH bonds. The infrared spectrum of the pyrolysed product of TAD itself at 600° C and 125 MPa showed the BNB absorption at 800cm^–1 due to the formation of B₃N₃ structures. The X-ray diffraction (XRD) of the reaction product from TAD and hydrazine at 600° C had broad diffractions centred at 2=25.5° and 43.0° (CuK). The BH absorption at 2500cm^–1 decreased in intensity on increasing the N/B ratio from 0.3 to 0.85, and disappeared finally at a ratio of N/B=1.3. The reaction product at 125 MPa had a porous structure. The electron diffraction of the specimen changed from faint rings to spots on circular rings after heat treatment at 800° C for 10 h. The heat-treated specimen, however, did not give sharp reflections corresponding to hexagonal BN in the XRD profile. BN of low crystallinity was transferred to cubic BN at 1200° C and 6.5 GPa in a 90% yield, which was higher than that of well-crystallized BN in the presence of AIN.     

Preparation of titanium diboride nanopowders of different particle sizes

Reactions between titanium and microcrystalline boron powders in a Na₂B₄O₇ ionic melt at temperatures from 700 to 850°C and those between TiCl₄ and NaBH₄ at temperatures from 300 to 750°C and hydrogen pressures of up to 10 MPa, with no solvent, have been studied by X-ray diffraction, scanning electron microscopy, thermogravimetry, and elemental analysis. The results demonstrate that TiB₂ formation occurs at t 〉 730°C and 550°C, respectively. According to scanning electron microscopy data, the TiB₂ powder consists of particles 70–75 and 35–50 nm size, and the crystallite size evaluated from X-ray diffraction data is 55 and 30 nm, respectively, in agreement with the equivalent particle diameters obtained from the specific surface area of the TiB₂ powders: 60 and 45 nm, respectively.     

Preparation of titanium nitride coated powders by rotary powder bed chemical vapour deposition

Titanium nitride coated powders were prepared by rotary powder bed chemical vapour deposition (CVD) in which a powder in a rotary specimen cell was heated by infrared radiation in a reactant gas stream. Titanium powder covered with TiN or Ti₂N thin film was obtained by diffusion coating treatment of titanium particles (grain size 10 to 50 µm) at 900 to 1000°C and 0.5 to 1.0 atm for 60 min in a nitrogen stream. TiN was coated on to the surface of scaly graphite particles (grain size 30 to 100 µm or 100 to 1000 µm) as well as titanium particles by CVD in the reactant system TiCl₄-N₂-H₂ at 900° C and 1 atm for 40 min. The uniformity of the coating (composition and film thickness) and the dispersability of the coated particles were considerably promoted by rotating the powder bed at about 90 r.p.m. compared with nonrotary powder bed CVD.     

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.     

Microstructure and mechanical properties of pulsed electric current sintered B4C-TiB2 composites

Monolithic B₄C, TiB₂ and B₄C-TiB₂ particulate composites were consolidated without sintering additives by means of pulsed electric current sintering in vacuum. Sintering studies on B₄C-TiB₂ composites were carried out to reveal the influence of the pressure loading cycle during pulsed electrical current sintering (PECS) on the removal of oxide impurities, i.e. boron oxide and titanium oxide, hereby influencing the densification behavior as well as microstructure evolvement. The critical temperature to evaporate the boron oxide impurities was determined to be 2000 °C. Fully dense B₄C-TiB₂ composites were achieved by PECS for 4 min at 2000 °C when applying the maximum external pressure of 60 MPa after volatilization of the oxide impurities, whereas a relative density of 95-97% was obtained when applying the external pressure below 2000 °C. Microstructural analysis showed that B₄C and TiB₂ grain growth was substantially suppressed due to the pinning effect of the secondary phase and the rapid sintering cycle, resulting in micrometer sized and homogeneous microstructures. Excellent properties were obtained for the 60 vol% TiB₂ composite, combining a Vickers hardness of 29 GPa, a fracture toughness of 4.5 MPa m^1/2 and a flexural strength of 867 MPa, as well as electrical conductivity of 3.39E+6 S/m.     

Solid-state interfacial reactions and compound morphology of cBN grain and surface Ti coating

In order to research thoroughly the mechanism of the solid-state interfacial reactions and the induced compound morphology of cubic boron nitride (cBN) abrasive grains and the surface Ti-coating layer, annealing experiments of Ti-coated cBN grains were conducted at different temperatures of 550-950 °C for dwell times of 60 min under high-vacuum conditions. The corresponding interfacial characteristics were investigated by differential thermal analysis, X-ray diffraction and scanning electron microscopy. The results show that the relevant interfacial reactions and compound morphology between cBN and Ti are significantly influenced by the heat-treatment temperature. No reaction occurs below 550 °C, and TiN is the sole reaction product during heating from 650-750 °C. Three kinds of compounds, TiN, TiB₂ and TiB, can exist in the interfacial region at 950 °C. Here, the favorable interfacial structures, cBN/TiB₂/TiB/(TiB+TiN)/TiN/Ti, are developed for the excellent mechanical and chemical transition effects between cBN grains and Ti coating. The thermodynamic analysis finally predicts that there is a reasonable probability for the formation of such a special interfacial transition layer.     

In situ reacted TiB2-reinforced alumina

In situ formation of TiB₂ in Al₂O₃ matrix through the reaction of TiO₂, boron and carbon has been studied. In hot-pressed samples, in addition to TiB₂, TiC and Al₂TiO₅ were also found to be dispersed phases in Al₂O₃ matrix. However, in the case of pressureless-sintered samples, pure Al₂O₃/TiB₂ composite with > 99% relative density can be obtained through a preheating step held at 1300°C for longer than 30 min and then sintering at a temperature above 1500°C. Pressureless-sintered composite containing 20vol% TiB₂ gives a flexural strength of 580 MPa and a fracture toughness of 7.2 MPa m^1/2.     

Characteristics of TiSi2 contact to BF2-doped single-silicon

The reaction mechanism of titanium silicide was investigated for varying amounts, of BF₂ dopant on a Si-substrate. Titanium thin films were prepared by direct current sputtering on non-doped and BF₂-implanted silicon wafers. The heat treatment temperatures, by rapid thermal annealing (RTA), were varied in the range 600–800 °C for 20 s. C49 TiSi₂ forms at 700 °C and almost all of its phase is transformed into C54 TiSi₂ with a very low resistivity value (16 cm) at 800 °C. When the amount of impurities is increased, the sheet resistance of Ti-silicides also increased while its thickness decreased. The main cause of the thickness reduction of Ti-silicide is the growth of enhanced native oxide. Dopants are chiefly redistributed in the interface between the Ti-silicide and the Si-substrate. It is believed that the formation of titanium boride increases the contact resistance during the Ti-silicide formation for samples annealed at 750 °C and 800 °C.     

Crystallization and structure of high boron content iron-boron ultrafine amorphous alloy particles

Amorphous to crystalline transformation of chemically prepared Fe₆₄B₃₆ ultrafine amorphous alloy particles has been investigated by Mössbauer spectroscopy, Brunauer-Emmett-Teller surface area measurements and transmission electron microscopy. Structural relaxation was observed below 350°C, which resulted in narrowing the full width at half maximum for the hyperfine field distribution from 13.0 to 10.6 T, while the average hyperfine field kept unchanged, to be about 20.3 T. Crystallization started on the surface at about 300°C and proceeded into the bulk at about 400°C. Partial crystallization between 400 and 450°C resulted in increasing the average hyperfine field for the remaining Fe-B amorphous matrix to 21.6 T. -Fe and Fe₂B were the only iron containing phases related to bulk crystallization, with the latter as a predominant component, accompanied by the segregation of about 19% boron atoms. Above 500°C, sintering of the particles became very remarkable and a solid state reaction between diffusing iron and boron atoms to form Fe₂B took place making the spectral area ratio for Fe₂B to -Fe components increase accordingly. A locally distorted non-stoichiometric Fe₂B quausicrystalline structure for the high boron content sample was proposed.     

Plasma spray synthesis of TiB2-Fe coatings

A limiting feature of the plasma spray process is the need for the powder to melt during its passage through the plasma flame. It is quite impossible to obtain coatings with materials that are difficult to melt. However, metal borides, particularly titanium boride, are attractive. Because of their high melting point, satisfactory coatings based on these materials have not been achieved.To overcome this problem, a process for making TiB₂-Fe coatings was studied. The TiB₂-based coatings were produced by the reaction of ferrotitanium with boron. The TiB₂ formation was first studied by thermal differential analysis and X-ray diffraction. It was observed that TiB₂ is formed at low temperature by an exothermic reaction. The characteristics of the reaction products obtained at different reaction temperatures are described.Agglomeration techniques were used to prepare the reagents: fine powders of ferrotitanium alloy and boron. TiB₂-Fe coatings were produced by plasma spraying the agglomerated powders. The influence of the plasma spray process parameters and the powder preparation techniques on the coating microstructure is discussed.Thick hard coatings comprising compounds of the reagent materials are produced during spraying by this synergetic process. Such coatings may be suitable for wear resistance applications.     

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.     

A facile one-step route to nanocrystalline TiB2 powders

Nanocrystalline titanium diboride (TiB₂) has been prepared by the reaction of TiCl₄ with NaBH₄ in the temperature range of 500–700 °C in an autoclave. X-ray powder diffraction (XRD) patterns can be indexed as hexagonal TiB₂ with the lattice constants of a=3.032 and c=3.229 Å. Transmission electron microscopy (TEM) image shows particle morphology, with average size of 15 nm for the powder obtained at 600 °C. Selected area electron diffraction (SAED) pattern confirms the prepared hexagonal TiB₂. The oxidation behavior of TiB₂ is studied by thermogravimetric analysis (TGA) and differential thermal analysis (DTA).     

Preparation of titanium diboride powders from titanium alkoxide and boron carbide powder

Titanium diboride powders were prepared through a sol-gel and boron carbide reduction route by using TTIP and B₄C as titanium and boron sources. The influence of TTIP concentration, reaction temperature and molar ratio of precursors on the synthesis of titanium diboride was investigated. Three different concentrations of TTIP solution, 0.033/0.05/0.1, were prepared and the molar ratio of B₄C to TTIP varied from 1.3 to 2.5. The results indicated that as the TTIP concentration had an important role in gel formation, the reaction temperature and B₄C to TTIP molar ratio showed obvious effects on the formation of TiB₂. Pure TiB₂ was prepared using molar composition of Ti: B₄C = 1: 2.3 and the optimum synthesis temperature was 1200°C.