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A superior anti Si-poisoning Al-Ti-Nb-B refiner with Ti:Nb=1:4 and M:B = 8:1 (M=Ti, Nb) was developed by a comprehensive data screening on the Al-10Si/Al-Ti-Nb-B system with thermodynamic calculations, by which the refinement level for Al-10Si alloys was upgraded to 109~125 μm, significantly extending the grain size tailoring scope for the aluminum casting industry. The synergistic effect of Ti/Nb not only bestowed an appreciable amount of high-potency aluminides during solidification but also formed NbB2-TiB2 core-shell structure which created an effective diboride substrate for nucleating α-Al. These results shed new lights on designing innovative anti Si-poisoning refiners by composition engineering.

High-entropy boride ceramics of (Zr0.2Ta0.2Ti0.2Nb0.2Hf0.2)B2 and (Zr0.2W0.2Ti0.2Mo0.2Hf0.2)B2 were prepared by combining powder synthesis via borothermal reduction process at 1600 °C and subsequent ceramic densification by spark plasma sintering at 2000 °C in argon. High-entropy diboride powdered products exhibited ultra-fine particle size (

The production of composite electrolytic coatings on a nickel substrate using binary titanium–chromium diboride obtained by mechanical synthesis was studied. The influence of heat treatment parameters on the phase composition and structure of the coatings was examined. It is shown that these coatings substantially increase the wear resistance of structural medium-carbon steel.

The composite coatings consisting of immiscible components of TiB 2 -Cu and Mo-Cu systems are formed by method of electro-explosion spraying. The structure of coatings is studied by method of transmission electron microscopy. It is established that the coatings are the particles of titanium, tungsten or molybdenum diboride in copper matrix. The coatings range in thickness from 90 to 100 µm.

Friction and wear behaviour of the hot pressed monolithic chromium diboride (CrB 2 ) of 94% theoretical density were studied at different loads (3, 5 and 10 N) and frequencies (10 and 15 Hz) under dry reciprocative sliding condition against cemented tungsten carbide (WC–Co) ball. Coefficient of friction and wear rate were calculated. While the value of coefficient of friction (COF) is decreased from 0.50 to 0.36 with increasing load from 3 N to 10 N; it is observed to be nearly independent with respect to sliding frequency. The measured wear volume is in the range of 0.67 × 10 −3 –2.24 × 10 −3 mm 3 for the investigated regime. The lowest specific wear rate of 4.63 × 10 −7 mm 3 /N m was observed at 10 N load and 15 Hz frequency. Abrasive and tribo-oxidative wear with delamination were the active mechanisms manifested in both flat and counter body materials.

Sinterability of monolithic CrB 2 was investigated under pressureless sintering condition using spark plasma sintering facility (SPS). Monolithic chromium diboride (CrB 2 ) was sintered in a modified die setup instead of traditional/conventional plunger and die assembly. This kind of assembly creates pressureless sintering conditions in spark plasma sintering unit. The main objective of this modified setup is to couple the combined aspect of conventional pressureless sintering with fast heating. Densification experiments using CrB 2 were conducted at temperatures in the range of 1600 °C to 2000 °C with different dwelling period (1–30 min) under no load condition. The effect of multi-step sintering treatment on the densification behaviour of CrB 2 was also investigated. The density and the resulted microstructures of the sintered samples are presented and discussed in this paper.

The development of composite materials (CM) in the systems “metal-refractory compound” is one of the up-to-date trends in design of novel materials aimed at operating under the conditions of significant loads at high temperature. To design such material, NiAl, which is widely used for deposition of protective coatings on parts of gas-turbine engines, was selected for a matrix. To strengthen a NiAl under the conditions of intense wear and a broad temperature range (up to 1000 °C), it is reasonable to add refractory inclusions. Introduction of refractory borides into matrix leads to a marked increase in metal wear resistance. In order to research the behavior of the designed composites at high temperatures and to study the influence of oxides on the friction processes, the authors carried out high temperature oxidation of CM of the above systems at 1000 °С for 90 min. It was determined that all of the composites were oxidized selectively and that the thickness of oxide layers formed on the boride inclusions is 3 – 7 times that on the oxides formed on the NiAl matrix. The mechanism of wear of gas-thermal coatings of the NiAl – МеB2 systems was studied for conditions of high temperature tribotests using the «pin-on-disc» technique. The obtained results indicate that introduction of TiB2, CrB2 and ZrB2 leads to their more intense oxidation during high temperature tribotests as compared to the matrix. The oxides formed on refractory borides act as solid lubricants, which promote a decrease in wear of the contact friction pairs. For more detailed investigation of the effect of tribo-oxidation products on the friction processes, tribotests were conducted for prior oxidized (at 900 °С) coatings NiAl – 15 wt.% CrB2 (TiB2, ZrB2).DOI: http://dx.doi.org/10.5755/j01.ms.22.1.8093

Solid solutions of tungsten diboride (WB2) with increasing substitution of tungsten (W) by tantalum (Ta) and niobium (Nb)—ranging from 0 to 50 at. % on a metals basis—were synthesized through resis...

Spark plasma sintering (SPS) experiments were conducted to investigate the effect of the processing parameters such as temperature, mechanical pressure and dwell time on densification behavior of monolithic chromium diboride. The sintering experiments were performed at different temperatures ranging from 1100 °C to 1900 °C under the mechanical pressure of 30 MPa–70 MPa for 1 min–15 min duration. The onset temperature for the densification of CrB2 is observed to be 1300 °C at 50 MPa. High dense chromium diboride (98.4%ρth) compact was obtained when processed at 1900 °C under a mechanical pressure of 70 MPa for 15 min duration. Hardness and fracture toughness of high density monolithic CrB2 (98.4%ρth) sample were measured to be 15.89 ± 1.3 GPa and 1.8 ± 0.14 MPa·m1/2 respectively.

The influence of chromium diboride additions on the tribotechnical characteristics of NiAl–CrB2 composite coatings in friction against a counterface made of NiAl plasma-sprayed coating is studied. The friction surfaces of the plasma-sprayed composite NiAl coatings with different chromium diboride contents (5, 15, and 30 wt.%) are examined. It is established that wear substantially depends on the amount of refractory component.

The relationships of diboride phases in Al–Ti(Zr)–B alloys with a variable Ti/B ratio close to the stoichiometry of TiB2 were studied. The formation of diboride solid solutions was confirmed. A grain refinement mechanism is proposed as that diboride particles in the Al–Ti–B master alloys reacting with aluminium upon adding into an aluminium melt and release titanium into the melt through forming a (Ti,Al)B2 solid solution and maintain a thin dynamic Ti rich layer on the surfaces of the (Ti,Al)B2 particles, which nucleates α-Al grains in solidification. The poisoning effect of zirconium on grain refinement of aluminium by Al–Ti–B master alloys is also discussed.

Cermet coatings based on Ni–Al alloys and titanium–chromium diboride are electrospark-deposited onto 40Kh steel. It is analyzed how the metal-to-refractory phase ratio in the electrode material and the deposition conditions influence the mechanical and tribotechnical characteristics of the coatings. The wear resistance and friction coefficients of the electrospark-deposited coatings are shown to depend on the friction speed and loading. The cermet coatings show high tribotechnical characteristics (I = 1–5 μm/km, f = 0.2–0.25) at speeds to 12 m/sec and loads to 4 MPa in dry friction conditions. These characteristics can be controlled by varying electrode chemical composition and deposition conditions.

Niobium diboride (NbB 2 ) is synthesized and consolidated by the spark plasma sintering technique. Elemental reactants such as niobium (Nb) and boron (B) were subjected to two stage heat treatment, initially at 1200 °C for synthesis and followed by densification at the temperatures in the range of 1700 °C to 1900 °C. High dense NbB 2 (~ 97.7%ρ th ) is obtained at 1900 °C after 15 min holding period. Load application during heat treatment stage is found to improve the sinterability of the niobium diboride compacts. Hardness, elastic modulus and indentation fracture toughness of the high dense NbB 2 are measured as 20.25 GPa, 539 GPa and 4 MPa m 1/2 respectively.

Single phase erbium borides ErB2, ErB4, and ErB12 show Seebeck coefficients and power factors with absolute values that are significantly lower than those of a stable Er-B multi phase composite obtained through high temperature solid-solid reaction from the elements (molar ratio Er:B = 1:6). According to quantitative Rietveld analysis the composite consists of erbium diboride (1 %), tetraboride (83 %), and dodecaboride (16 %), and the measurement of the electrical conductivities, Seebeck coefficients, and thermal conductivities leads to ZT values as high as 0.53 at 830 K. Such refractory materials can be used for energy conversion in a range of high temperatures that are otherwise difficult to address.

The paper examines the wear mechanisms of the VT-3 titanium alloy and TBKhN composite in fretting corrosion conditions. It is shown that the oxide film formed on the titanium alloy during friction fails and further wear occurs on the juvenile surface. This leads to seizure and removal of the material from the surface, indicating that adhesive wear prevails in friction of the titanium alloy. The TBKhN40 composite is characterized by oxidative wear. The oxide film with grain-oriented fine structure forms in the friction process through selective oxidation of refractory and metal components.

In this study, AlB 2 powders were synthesized by using a combined method of mechanical alloying (MA) and annealing of elemental aluminum (Al) and boron (B) powders. Milling was performed in a planetary ball-mill (Fritsch™ Pulverisette 7 Premium Line) up to 15 h under argon (Ar) atmosphere. Annealing process was carried out in a tube furnace at 650 °C for 6 h under Ar atmosphere. The effects of MA durations on the annealing process and AlB 2 formation were investigated. The conversion of Al and B powders to AlB 2 starts after only MA for 3 h or after MA for 1 h and subsequent annealing. A slight formation of AlB 12 occurs at 242 °C for as-blended powders and it shifts to about 272 °C for MA’d powders. Al–B powder blends MA’d for 9 h and annealed have AlB 2 particles in size between 35 and 75 nm in the presence of Al 13 Fe 4 , Fe 3 B and Fe 2 B contaminations.

AbstractZirconium diboride is a strategic material, finding applications as thermal protection system in reentry vehicles, high temperature electrode, thermowell tubes, etc. Solid state reactions require high temperature and thus result in coarse powders which are difficult to sinter. Densification of ZrB2 is extremely difficult due to its refractory nature. Sinter additives help in densification but deteriorate the high temperature properties. In the recent past, various researchers have made attempts to improve the existing methods and also invented new processes for synthesis of pure and fine powders as well as for densification of ZrB2. All the techniques on synthesis and consolidation of ZrB2 are discussed in detail and critically reviewed in this article.

The resistance to electrochemical corrosion and high-temperature oxidation of titanium-chromium diboride and materials with a metal binder based upon it is studied. The mechanism of processes that occur in compacted specimens and powders is studied, and the composition of reaction products in relation to specimen heating temperature is determined. It is established that the oxide films that form at a specimen surface during oxidation are protective and prevent further material oxidation.

Experimental investigations were carried out on synthesis of chromium diboride through boron carbide reduction of Cr2O3. The products obtained were characterized by X-ray diffraction and the process optimized to prepare single-phase CrB2 powder. Densification of CrB2 was investigated by pressureless sintering and hot pressing. A maximum of 93% ρth was obtained by pressureless sintering at 1850 °C after a prolonged duration of 360 min. However, near theoretical density was achieved by hot pressing at 1600 °C and 35 MPa pressure for 2 h. The hardness and fracture toughness of fully dense CrB2 was measured as 22 GPa and 3.5 MPa m1/2, respectively. The mode of fracture in pressureless sintered samples is intergranular whereas that in hot pressed is transgranular.

The paper addresses the effect of pressure and a chromium diboride additive on shrinkage and shrinkage rate in hot pressing and on mechanical properties of the resulting diamond-containing composites based on Fe, Cu, Ni, and Sn powders. The shrinkage rate curves are found to have a stepwise trend. The shrinkage rate variations are shown to be associated with the phase and structural changes that occur in this system. The authors have determined the initial composition and p,T-conditions that ensure process activation and simultaneous improvement in mechanical properties of the composite.

Electrode composite materials of the TiCrB2-(Fe-Cr) and TiCrC-(Fe-Cr) systems are discussed. The composition, structure, mechanical and tribotechnical properties of the materials have been studied. Coatings from the materials developed have been applied to the 30KHGSA steel and VT3-1 titanium alloy. The effect of the length of electric-spark alloying and electrode and substrate materials on the mass-transfer kinetics has been analyzed. It has been shown that the materials developed can be successfully deposited onto steels and titanium alloys by the electric-spark method.

Over the years, the self-propagating high-temperature synthesis (SHS) has become an interesting research field to prepare a large numbers of advanced materials. Recently, the demands for high temperature advanced ceramics have further intensified the research on SHS for efficient material preparation. Several reviews, large numbers of papers and patents on various aspects of material production by SHS are available in literature. These are scattered and it is desirable to have a comprehensive review of the literatures that not only helps the researchers but also guide the beginners in this area. In this paper, we have emphasized our contributions on synthesis of various advanced high temperature ceramics, the borides, carbides, oxides and their composites by SHS processes. Several advantages and disadvantages of the SHS technique for advanced high temperature (HT) materials are highlighted. The preparation of nano-sized powders and finegrained in-situ high temperature ceramic composites through SHS is specially mentioned.

The addition of TiC (5–10 wt.%) and C (5 wt.%) was found to enhance the sinterability of SHS produced ZrB2 powder. The densification increased to 94% of the theoretical density with the addition of carbon and TiC. The oxygen impurities normally present on the surface of this diboride powder leads to exaggerated grain-growth during sintering and hinders densification. The addition of C and TiC reduced the surface oxide layer and resulted in improved densification. Less grain-growth was also observed during sintering.

The paper studies properties of an ultrafine aluminum powder produced by electric explosions of conductors, whose particles are stabilized by coating with aluminum diboride immediately during the synthesis of the powder. The ultrafine aluminum powder stabilized in such a manner has special properties: narrow particle size distribution, increased dispersity, and higher resistance to oxidation upon heating.

Results are provided for a study of the reaction of MoSi2 with niobium and tantalum diborides. It is established that sections of MoSi2–Nb(Ta)B2 within the quaternary system Mo–Si–B–Nb(Ta) are described by eutectic composition diagrams with a eutectic temperature equal to 1870 and (1890 ± 30)°C. The diboride phase content is 20 and 14 mol.% respectively in systems with NbB2 and TaB2.

The synthesis and consolidation in one processing step of single phase tantalum diboride by Spark Plasma Sintering (SPS) is proposed. TaB 2 formation from Ta and B elemental powders takes place at about 800 °C through a solid-state combustion reaction while product densification requires higher temperatures. Consolidation is significantly improved (~ 96% density) when increasing the applied pressure from 20 to 60 MPa immediately after the synthesis reaction occurs. Simplicity, short duration, milder temperatures and pressure conditions, in comparison with other routes utilized so far for obtaining bulk TaB 2 are, along with the good mechanical properties of the obtained product, the main benefits of the adopted approach.

Tantalum diboride (TaB 2 ) was synthesized by reducing Ta 2 O 5 using B 4 C and graphite at 1600 °C under flowing Ar. The powder had an average particle size of 0.4 μm with both needle-like and rounded particles. The TaB 2 powder was hot pressed to relative densities of 97% at 2000 °C (3.6 μm grain size) and 98% at 2100 °C (5.3 μm grain size). Mechanical properties were measured for TaB 2 hot pressed at 2100 °C and were comparable to those of the commonly studied diborides, ZrB 2 and HfB 2 . The Young's modulus was 551 GPa, Vickers' hardness was 25.6 GPa, flexure strength was 555 MPa, and fracture toughness was 4.5 MPa-m 1/2 .

Diborides present in Al-Ti-B alloys with a weight ratio TilB < 2.2 are investigated in the arc melted and annealed conditions as well as in the controlled heated and cooled specimens by means of light and scanning electron microscopy, energy dispersive spectrometry and X-ray diffraction. In Al- Ti-B alloys with composition close to the AlB2-TiB2 tieline; apparently pure spheroidal and platelike TiB2 particles are present. In the as cast condition of all other investigated alloys the formation of AlB2 on primary TiB2 particles is observed regardless of the applied cooling rate. In the investigated alloys apparently pure TiB2 and AlB2 coexist even after 1000 h exposure at 800° C, and the formation of mixed diboride (Al, Ti) B2 was never observed. The results of this work are discussed together with the results of previous work in this area and it seems very likely that the mixed diboride (Al, Ti)B2 is not a thermodynamically stable phase in the aluminium rich corner of the Al-Ti-B system, but onlyap...

It has been shown that by directional crystallization of pseudobinary compositions of molybdenum disilicide (MoSi 2 ) with some other refractory compounds, particularly with diborides of transition metals (composite materials having a disilicide matrix and extended in crystallization direction diboride inclusions) can be obtained that may improve the mechanical properties of the matrix material.

The distribution and redistribution of impurities, alloying elements and components of binders in the rutile (TiO 2-x ) scales during high-temperature oxidation of titanium, titanium alloys, refractory compounds of titanium (carbide and diboride) and materials based on them have been studied by SEM, EPMA and XRD methods. The correlation between defect and crystallographic structures of rutile with diffusion processes in the scales and redistribution of elements at the matrix/scale, scale/scale and scale/gas interfaces is ascertained. Mechanisms of diffusion processes in scales during high-temperature oxidation are discussed.

The ultra high temperature performance of ceramic-based complex structures may require the development of liquid-assisted joining techniques; this in turn requires the definition of the wettability of these materials by various metals over a wide range of compositions and temperatures. After a short description of the relevant experimental aspects of wettability studies at high temperatures, a discussion is presented on how these results can be used to derive chemical and structural information on the solid-liquid interactions. Reference is made mainly to metal-ceramic systems; a summary of the results of sessile drop tests under carefully controlled conditions is given in relation to the wettability and the interfacial characteristics of systems based on transition metals (Zr, Hf) diboride ceramics in contact with liquid Ag, Cu, Au and Ni and of some of their alloys with Ti, Zr, Hf and B to promote/control wettability. In particular, the utilization of phase diagrams is discussed, as one of the most powerful tool to design the filler alloy compositions for the optimization of joining (brazing) processes.

Sliding friction experiments were conducted on SiC, Si3N4 B4C-TiB2 and Al2O3 ceramics under the condition of sliding contact with themselves in air. The test conditions included temperature up to 800°C., maximum surface velocity of 1.14m/min, and normal forces ranging from 4.31N(0.44kgf) to 104.57N(10.67kgf). Friction coefficient as low as 0.09 to 0.1 was obtained at 800°C for boron carbide-titanium diboride. The wear rates of Si3N4, SiC and Al2O3 ceramics increased with increasing temperature. However, the wear rate of B, C-TiB, decreased with increasing temperature. For most of the sliding pairs, the wear rate was in the range of 10-5mm3/Nm to 10-4mm3/N-m, the single exception being a value of 10-8mm3/N-m recorded for B4C-TiB2 at 800°C. The implications of wear mechanism for these results were shown.

§** A joint research work financed by CIRA and performed with the support of the CNR-ISTEC have been performed with the aim to evaluate and to assess the use of non-conventional metal diboride compounds in the manufacturing of slender-shaped hot structures for re-entry vehicles. The results of the metal diborides trade-off and the characterisation of the selected material are reported in this paper. A description of the material process employed is also provided. Different diboride compositions, both hafnium- and zirconium- based, have been considered and characterized in their mechanical, thermal and oxidation resistance behaviour: as a result of this trade-off, the most promising composition was chosen and further characterised. The obtained results highlight an excellent potential for the selected material. A scaled hot structure demonstrator, to be on-ground tested at CIRA Scirocco Plasma Wind Tunnel, have also been manufactured.

In conventional Al-electrolysis the electrodes suffer from degradation when used in the harsh environment of an aluminium electrolysis cell. The carbon cathode, which is placed at the cell bottom, is not well wetted by aluminium, and must be covered by a pad of liquid Al in order to establish a good electrical contact. To avoid short circuiting of the cell the anode-to-cathode distance (ACD) must be kept above 4-6 centimetres. Replacing the carbon cathode with a new, inert cathode, well wetted by liquid aluminium, would allow new cell designs where only a thin film of aluminium covers the cathode during cell operation, thereby allowing the ACD to be lowered. This in turn opens for substantial reductions in the energy consumption of an electrolysis cell, and combined with an inert anode the outlet of CO2 and fluorides harmful to the environment could be reduced.The most promising candidate for use as an inert cathode is titaniumdiboride (TiB2), a ceramic material which exhibits promising properties such as a high electrical conductivity, sufficient mechanical strength, good resistance to chemical attack and low solubility in liquid aluminium, and which is also well wetted by liquid aluminium.

Niobium diboride was prepared by chemical vapour deposition from a gas mixture of NbCl 5 , BCl 3 , H 2 and Ar on a quartz substrate heated to a temperature of 950–1200°C. Homogeneous films were formed from 950 to 1050°C, and pillar crystal from 1050 to 1200°C. Niobium diboride was found to react with the quartz substrate above 1200°C to form a silicide, Nb 5 Si 3 . The optimum molar compositions of the gas mixture used to grow crystals of NbB 2 were 2.5–5.0 vol% of BCl 3 , and 2.5–5.5 vol% of NbCl 5 . In the presence of impurities of nickel regular hexagonal shaped pillar crystals of maximum 10 μm in a edge and 50 μm in length were obtained.

The chemical bonding in AlB2-type metal diboride phases is considered from a crystal chemical viewpoint. The ability of metals to deform from a spherical shape appears to be very important in explaining why the diboride is formed by such a wide variety of metals. Crystal chemical parameters are used to compare relative bond strengths.

A study was made of the zones of diffusional reaction of chromium diboride with carbon steels, cast iron, and Armco iron, forming under glow-discharge conditions. An attempt was made to discover the laws governing the formation of diffusion layers in these systems. It was established that with rise in carbon concentration in the steel the width of the diffusional reaction zone grows, attaining a maximum for a steel with a carbon content of 0.8%. At a carbon concentration in the steel of 1.6% or more, the width of the diffusion zone sharply decreases. In a CrB2-steel system a strong inhibiting influence on diffusion is exerted by the higher chromium carbide Cr3C2.