Boron-Containing Nanocrystalline Ceramic and Metal–Ceramic Materials

Chapter 2

Boron-Containing Nanocrystalline Ceramic and Metal
Ceramic Materials Levan Chkhartishvili, Archil Mikeladze, Otar Tsagareishvili, Archil Gachechiladze, Anatoly Oakley and Boris Margiev Laboratory for Boron-Containing & Composite Materials, Ferdinand Tavadze Institute of Metallurgy & Materials Science, Tbilisi, Georgia

2.1 BORON CARBIDE-BASED MATERIALS 2.1.1 On Assembling of Boron Carbide Based Nanocomposite Materials Boron carbides, with approximated chemical formula B4C, are one of the most useful nonoxide ceramics used in modern engineering because of the unique combination of their physicochemical and mechanical properties, such as high elasticity modulus, high hardness, low density, heightened wear-resistance, high melting point, low thermal expansion coefficient (TEC), etc. Among the known materials, it is characterized by the highest value of hardness-to-density ratio [1]. All of these properties make boron carbide attractive for numerous important industrial applications including abrasives, wear-resistant coatings, lightweight ballistic body-armors, nuclear industry materials, neutron detectors, etc. However, further technological applications of boron carbide are limited by other properties characteristic of this material, such as brittleness, low impact strength, low thermal conductivity, and instability against thermal stresses. For this reason, today more attention is paid to the creation of so-called heteromodulus composite metal
ceramics based on boron carbide, in which hardness and wear-resistivity of the ceramic matrix are combined with the impact toughness and ductility of the metal binder. Based on the theory of continuous media and micromechanical theory, in the seminal work [2] it was shown how it is possible to significantly improve the properties of high-temperature structures with low resistivity against thermal stresses by introducing into them a dispersive phase with low modulus of elasticity. Creation of a boron carbide-based heteromodulus material is possible when (1) initial boron carbide is highly dispersive and (2) metallic binder possesses high

adhesion ability and low chemical reactivity toward boron carbide. For the creation of metal
ceramic composite systems based on boron carbide, it is essential to know how to choose the metallic binder. One should take into account wetting of the ceramic surfaces of particles by the melted binder, its adhesion ability, the possibility to control processes at phase interfaces, and the difference between TECs of ceramic and metal, etc. In addition, the metal should not degrade properties of the ceramic matrix. In general, the manipulation of mechanical and thermal interfaces is certainly essential for the design of effective boron carbide-based nanocomposites. If the assembling of the ceramic component—boron carbide—is conducted in the nanocrystalline state, it should lead to a qualitative improvement in physical
mechanical properties of the material, since the contribution from the surface layers will be decisive in the energy balance of the system. In such conditions, it should drastically change the spectrum of atomic vibrations affecting the diffusion and all the transport processes. Accordingly, the adhesion ability of the relatively passive metal components should be substantially improved. One more obstacle to a wider use of the attractive complex of properties of boron carbide is related to the difficulty of pressing its powders. Currently, the main way to obtain the sufficiently consolidated samples is high-temperature (B2000 C) pressing. By this method, it is possible to form only samples of small sizes and simple geometries. Moreover, since under such temperatures the agglomeration of boron carbide crystallites is intensified, the retaining of the nanostructure in consolidated samples is too complicated and thus the quality of the material degrades. Accordingly, it is advisable to search for assembling processes of boron carbide based nanocomposites that will be conducted at not too elevated temperatures.

Handbook of Nanomaterials for Industrial Applications. DOI: https://doi.org/10.1016/B978-0-12-813351-4.00002-X © 2018 Elsevier Inc. All rights reserved.

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PART | I Different Kinds Of Engineered Nanomaterial For Industrial Use

In this Section, a brief review of the available data on already obtained boron carbide based metal
ceramic nanocomposites is given. There are only listed metal components (not additions of boron and/or carbon themselves) and then characterized powdered boron carbides. There is also described an original production technology developed for hard nanocrystalline alloys, which includes sputtering of mixtures of soluble compounds of corresponding elements and high-molecular liquid hydrocarbons inside a reactor with reduction environment and subsequent combining of reduction and selective carbonization processes within a certain temperature interval. It allows obtaining of the products with components maintained in highly dispersive—nanocrystalline state.

2.1.2 Available Data on Boron CarbideBased Metal
Ceramic Nanocomposites 2.1.2.1 Al—Aluminum One of the basic approaches to forming metal
ceramic composites means the infiltration of molten metal or metal alloy into a preform with a controlled porosity. Because of its light weight, particular attention is paid to the boron carbide composites with aluminum—infiltration of aluminum with some, e.g., silicon, additives—see [3]. It is known that the use of boron carbide for hardening of aluminum leads to excellent physical
mechanical properties. Although, there are problems related to wetting by liquid aluminum the boron carbide and formation of undesirable phases (ternary compounds of the Al/B/C-system: B13C2Al0.5 with boron carbide-, B48Al3C2 with close to so-called I-tetragonal boron-, and also Al3BC3 and Al3BC with Al4C3- or Al4BxC32x-like structures) at the interface. It was found [4] that, high isostatic pressure treatment at 1300
1400 C leads to the improvement of B4C
Al ceramics structure and, correspondingly, their mechanical properties. The effect of adding aluminum without and with titanium on the compacting kinetics, structure, and properties of materials in the systems B4C
Al and B4C
(Ti
Al) was studied in [5]. Chemical oven method was shown [6] to be viable for formation of B4C powder-based materials. It can readily heat reactants used in combustion synthesis as well as forming itself in a sintered TiC
Al2O3 system as a precursor for a composite material. The Al2O3
B4C-whisker composites were successfully compacted by the pressureless sintering [7]. Maximum density and increased fracture toughness were achieved at 1800 C and 10
20 vol.% B4C, respectively. Morphology and phase composition of friction surfaces and the tribotechnical properties of the (B4C
Al2O3)
steel system were studied under dry

friction conditions [8]. It was found that fine-grained secondary structures are formed on the friction surfaces. Their structure and morphology determine the tribotechnical properties of ceramic
steel couples. The maximum wear resistance of B4C ceramics containing 5
20 wt.% Al2O3 was determined by the formation of dense secondary-phase thin films on the friction surface. A simple casting-technique proposed in [9] uses B4Cpowder for the aluminum-hardening. Lets emphasize that these are composites with metal- and not ceramic-matrix. The work [10] examined groups of samples based on B4C and B13C2 powders with additions of aluminum and Al2O3 in the amounts of 2 and 5 wt.%, respectively. The increase in strength of B4C-based samples is revealed over the range from 1200 to 1600 C, mainly for highporous materials (10%
12%). Presumably, this is due to the higher relaxation properties of the porous material microstructure. The mechanical properties of boron carbide particulatereinforced metal
matrix composites based on aluminum alloys at high temperatures were studied in [11]. Influence of changes in pressure and other sintering conditions on the microstructure and mechanical properties of the aluminum matrix composites (AMCs) reinforced with boron carbide (10 wt.%) have been specifically studied [12]. Porous samples cold-pressed in vacuum from powdered B4C were being filled with liquid aluminum at 1200
1400 C. Saturation was achieved at B1350 C. The subsequent heat treatment causes a chemical interaction to form aluminum carboborides resulting in the increased material strength. The weakness of such a material is related to the production of the aluminum carbide Al4C3, which in the atmosphere leads to hydrolysis of aluminum and its conversion into a powder causing degradation of the sample. The systematic investigation of the microstructural origins and the strengthening mechanisms as appropriate to each phase constituent in aluminum-based metal matrix nanocomposite with boron carbide reinforcement was reported [13]. Phase composition, structure, and erosion properties of B4C 2 Al composite materials produced by hot pulse pressing were also examined [14]. It was shown that powder components actively interact to form new phases during pressing, with AlB12 being the main phase. The erosion properties are greatly dependent of the powder mixtures composition and compaction temperature. The nanosized secondary phases at B4C grain boundaries intensify the anode mass transfer to the substrate. B4C
TiB2
Al2O3 powdered ceramics with phosphate binder obtained using self-propagating high-temperature synthesis (SHS) was reported [15]. The unique properties which are typical for the composites fabricated in Ti
Al
B
C systems makes them

Boron-Containing Nanocrystalline Ceramic and Metal
Ceramic Materials Chapter | 2

attractive for aerospace, power engineering, machine and chemical and other industrial applications. Besides, AMCs have great potential as structural materials again due to their excellent physical, mechanical, and tribological properties. Because of the good combination of thermal conductivity and dimensional stability AMCs are found to be potential materials for electronic packaging. The methodology and technology for the fabrication of bulk materials from ultrafine grained powders of Ti
Al
B
C system are described in [16]. It includes results of theoretical and experimental investigation for selection of powder compositions and determination of thermodynamic conditions for blend preparation, as well as optimal technological parameters for mechanical alloying and adiabatic compaction. Due to the unique combination of their properties, MAX compounds Mn11AXn, where M is a transition metal, A is a group IIIA or IVA element, and X 5 C or N, are thought to be very promising for applications. It was reported [17] on the SHS of a “MAX” phase with X 5 B from 3Ti 1 2Al 1 2((1 2 x)C 1 xB) green powder compacts (x 5 0, 0.15, 0.25, 0.50, and 0.75). Combustion reaction in such Ti
Al
C
B system was monitored by timeresolved X-ray diffraction (XRD). The influence of Al-impurities on properties of a boron carbide based composite was studied in [18]. Alumina Al2O3 granules served for an Al-source. They had no direct contact with boron carbide preform, but were placed within the heating zone. Aluminum was found to be able to intensify the transformation of initial boron carbide particles into a new phase-state and, correspondingly, growing-kinetics of shell-layers. The dependence of the phase constitution and strength of B4C
AlN composites on the charge formulation and hot pressing temperature was studied in [19]. The structure and phase formation during hot pressing of boron carbide powder mixtures containing 20
50 vol. % Al were examined and the microhardness and fracture toughness of hot-pressed samples analyzed [20]. It was established that the particle size of the starting powder has an essential effect on the structure and mechanical properties of composites. A matrix structure consisting of aluminum carboboride and borates as well as inclusions of boron carbide was formed from the mixtures containing coarse B4C powder, while homogeneous fine-grained structure mainly consisting of aluminum nitride and borates mixtures containing fine boron carbide. A search technique to optimize the fabrication process conditions of the high wear resistance Al alloy matrix composites reinforced with different volume percentages of boron carbide particles was described in [21].

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2.1.2.2 Ca—Calcium The occurrence in stages and the regularities of the interaction of boron carbide with calcium oxide CaO in the production of the material B4C
CaB6 was studied [22]. Conditions under which the hot-pressed composite materials B4C
(5
10 wt.%)Ca
Si can be fabricated, their structure, nature of failure, and mechanical properties were investigated [23]. Maximal mechanical characteristics are attained at hot-pressing temperatures in the range 2000
2100 C.

2.1.2.3 Co—Cobalt The synthesis of cobalt borides by boron carbide reduction of oxides was studied [24] and it was shown that the reaction occurs through a stages involving formation of cobalt, its lower boride phases, carbides, and borates. A characteristic feature of such a reaction is the higher reactivity of boron carbide compared with carbon in the initial stages, leading to the appearance of B2O3 and C which react at higher temperatures to form boron carbide.

2.1.2.4 Cr—Chromium Composites B4C
CrB were suitably prepared [25] by carbide reduction of oxide Cr2O3 when B4C serves as a reducing agent and remains one of the components of the composite. The composite material with the composition 95% B4C
5% CrB2 was obtained by boron carbide reduction of chromium oxide Cr2O3 with an addition of carbon [26]. The thermogravimetric method was used to study the effect of B4C, (Ti, Cr)C, and phosphorus additives on the kinetics of oxidation of chromium carbide alloys of the type 85% Cr3C2
15% Ni and 85% Cr3C2
15% (Ni
P) in air under isothermal heating conditions at 1000 C [27].

2.1.2.5 Cu—Copper One should expect obtaining denser material if boron carbide is unfiltered with copper instead of aluminum. The problem is that molten Cu does not wet B4C. It can be resolved by applying pressure-stimulated infiltration [28] or utilizing a powder cladded with copper. It was also used [29] as an essentially different approach based on microstructural investigation of the B4C
Cu interface. When instead of pristine Cu, its alloy, e.g., with Si-additive, is used as copper, it does not form compounds with boron or with carbon, thus it can be considered as a silicon-dissolving agent.

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PART | I Different Kinds Of Engineered Nanomaterial For Industrial Use

2.1.2.6 Fe—Iron The mechanism of phase and structure formation in sintering a Fe
C
B composite, i.e., in Fe
B4C interaction, with the liquid phase taking part was examined [30]. Boriding in a mixture of B4C with additions of oxygen-containing activators leads to formation of a coating in which FeB phase propagates as a layer to some distance from the surface [31]. The synthesis of iron borides by boron carbide reduction of oxideswas studied [24] and shown that the reaction occurs through a stages involving formation of iron, its lower boride phases, carbides, and borates. Fully dense boron carbide can be processed from an initial mixture of 5.5 vol.% Fe and low-cost B4C powder by SPS at 2000 C [32]. At this temperature, Fe-free boron carbide can be consolidated only to 96% of the theoretical density. The effect of the Fe addition on the densities is even more pronounced at lower processing temperatures and related to the presence of a liquid phase in the Fecontaining material. The paper [33] examined the thermal synthesis of master alloys from a mixture of iron and boron carbide powders. It was shown that the content of boron decreases and the boron/carbon ratio in the master alloy powder changes with increasing synthesis temperature.

2.1.2.7 Hf—Hafnium By thermodynamic modeling [34] of phase-equilibrium conditions in the ternary system Hf
B
C, the set of liquid-phase reactions leading to the HfB2
“B4C” ceramic material and its isoplet Hf
“B4C” were investigated.

2.1.2.8 Mo—Molybdenum The effects of B4C and also SiC additives in the MoSi2 matrix on the microstructures and mechanical properties were investigated [35] and it was found that the Mo2B5 reinforced phase was formed. By the method of directed crystallization the eutectic composite B4C
MoB2 was obtained [36]. In the Mo
B
C system, at sufficiently high temperature the Mo2BC phase can appear, mostly with C-defects, i.e., Mo2BC12x phase, which then converts into the parasite phase Mo2C (frequently these are intra-granular sediments).

2.1.2.9 Nb—Niobium By the method of directed crystallization, the eutectic composite B4C
NbB2 was obtained [36]. No new compound is formed in the Nb
B
C system. However, it contains precrystallization fields of 10 types of solid solutions and intermediate phases, in which reactions of eight

types take place and the eutectic, consisting of metallic NbB2-columns and semiconducting B4C-matrix, is formed. The microstructures and mechanical properties of multiphase Nb-based composites from Nb
Ti
C
B system were investigated [37]. The composites contain Nb solid solution, (Nb, Ti)B boride, and (Nb, Ti)C carbide phases. The (Nb, Ti)B and (Nb, Ti)C hybrid together with Nb solid solution form the network microstructure. Boride and carbide phases effectively improve the strength and hardness of the composites, while Nb solid solution toughens the composites. Higher boron contents increase the hardness and strength, but decrease the toughness.

2.1.2.10 Ni—Nickel The thermogravimetric method was used to study the effect of B4C, (Ti, Cr)C, and phosphorus additives on the kinetics of oxidation of nickel alloys of the type 85% Cr3C2
15% Ni and 85% Cr3C2
15% (Ni
P), in air under isothermal heating conditions at 1000 C [27]. Thermodynamic methods were applied to the chemical interactions in mixtures 0.9625B4C 1 0.0500NiO 1 0.0375C, B4C 1 NiO, and B4C 1 4NiO 1 3C [38]. It was found that NiB always contains small amounts (,2 wt.%) of lower borides. The synthesis of nickel borides by boron carbide reduction of oxides was studied [24] and it was shown that the reaction occurs through a stage involving formation of nickel, its lower boride phases, carbides, and borates. The B4C-microparticles were successfully coated [39] with Ni
B using an electrolysis method of obtaining the metallic nickel.

2.1.2.11 Sc—Scandium The goal of the investigation [40] was to study the stages in the interaction of B4C with scandium oxide Sc2O3 in the production process of composite B4C
ScB2. Composites B4C
ScB were suitably prepared [25] by carbide reduction of a Sc oxide when B4C serves as a reducing agent and also remains one of the components of the composite.

2.1.2.12 Si—Silicon An addition of SiO2 to the charge helps to obtain by the hot-pressing the highly dense heterophase materials based on boron carbide [41]. This additive activates sintering and makes it possible to obtain ceramic materials of the system B4C
SiC with stable physical
mechanical properties. It was found [4] that high isostatic pressure treatment at 1300
1400 C leads to the improvement of B4C
SiC

Boron-Containing Nanocrystalline Ceramic and Metal
Ceramic Materials Chapter | 2

ceramics structure and, correspondingly, their mechanical properties. The purpose of the investigation [42] was the production of a semiconducting B4C 2 SiC composite phase. The effect of impurities and additives of titanium and zirconium borides on the structure and mechanical properties of SiC
B4C ceramics over a broad temperature range was investigated in [43]. A series of test materials were produced from boron carbide B4C powders with additions of either boron, silicon, or silicon and silicon carbide in amounts up to 60, 4, or 4 and 30 wt.%, respectively [44]. The erosion resistance was significantly improved by additions of silicon and silicon carbide. If boron carbide B4C powder is added in initial mixture for obtaining the silicon carbide SiC composites, it serves as an alternative carbon-source as well [45]. It is clear that the ratio of intensities of these two carbonsources significantly affects morphology of B4C-particles grown in process of composite-formation and also its mechanical properties. The a-SiC
B4C composite was formed by a pressureless sintering [46]. The B4C micrograins were found welldispersed in SiC-matrix. The effects of SiC and also B4C additives in the MoSi2 matrix on the microstructures and mechanical properties were investigated [35]. How the structure of electrically conducting ceramic composites in the SiC
B4C system is formed has been studied in [47]. Preparation, microstructure, and mechanical properties of SiC 2 SiC/B4C 2 B4C laminates were studied in [48]. A self-healing multilayered matrix was designed, deposited by chemical vapor infiltration, and used [49] in a model composite, which combines B-doped pyrocarbon mechanical fuse layers and B- and Si-bearing compound, namely B4C and SiC, layers forming B2O3-based fluid healing phases when exposed to an oxidizing atmosphere. Conditions under which the hot-pressed composite materials B4C
(5
10 wt.%)Ca
Si can be fabricated, their structure, nature of failure, and mechanical properties were investigated [23]. When instead of pristine Cu, its alloy, e.g., with Siadditive, is used for infiltration there is formed an interface promoting the wetting of the boron carbide particles [29]. However, according to the thermodynamic analysis at .15 at.% Si in the B
C
Cu-system, there is presented the SiC-phase creating a diffusion-barrier for the reaction. It was investigated how the composition, grain morphology, and method of preparing of the starting mixture affect the processes that form the structure and phase composition of B4C
SiC composites during hot pressing [50] and it was found that, when B4C
SiC composites with a low SiC content are made from initial

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B4C
B4Si
B
C powder mixtures those composites have a high cracking resistance because of their fine grain structure. In [51], two types of samples were compared. They were obtained from partially sintered boron carbide preforms with porosity of 20%
40%, and preforms saturated with carbon. When free-carbon source is absent boron carbide reveals core
shell structures. Particles of initial boron carbide powder serve for the core, while the shell has increased Si-content. The static and dynamic mechanical properties at strain of ceramic composites based on porous B4C infiltrated with molten Si were studied in [52]. The region of growth of these compositions was analyzed in [53]. The influence of Al-impurities on properties of a boron carbide-based composite containing silicon was studied [18]. The carbide binder-phase Bx(C, Si, Al)y is obtained instead of the ternary phase BxSiyC. The dynamic high-strain-rate behavior of boron carbide-based composites with similar phase composition was investigated [54] as a function of the planar impact strength as well. The dynamic response of the composites depends strongly on the amount of residual silicon, on the average size of the boron carbide grains, and on the morphology of the SiC particles. A composition B4C 2 TiB2 2 SiC was also proposed [55]. Polarization curves and Auger spectroscopy were used to study the kinetics, formation mechanism, and phase composition of oxide films resulting from the anodic oxidation of SiC 2 TiB2 2 B4C ceramics [56]. In [57], the kinetics and mechanism of combustion of Zr
Si
B
(C) mixtures and the staging of chemical reactions in the combustion wave during synthesis of ceramics based on ZrB2 with silicon containing dopants were studied.

2.1.2.13 Ti—Titanium Boron carbide composites with changing vol.% of Ti and monolithic boron carbide were fabricated by powder metallurgy technique and SPS method consequently [58]. The density results show that the densification is improved by the metallic additions and when the Ti addition increases to 20 vol.%, the sintering temperature is required to be decreased in order to avoid failure occurring due to melting. Within a certain volume in the composites, the amount of boron carbide is replaced by secondary phases occurring due to Ti metal existence. As a result, the metallic titanium addition in boron carbide seems to be favorable on sinterability and density. In [59], a study was made of B4C 2 TiB2 2 TiO2 ceramics produced by hot-pressing from B4C 2 TiO2 2 C charges (C—carbon black).

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PART | I Different Kinds Of Engineered Nanomaterial For Industrial Use

An addition of TiO2 to the charge helps to obtain by hot-pressing the highly dense heterophase materials based on boron carbide [41]. This additive activates sintering and makes it possible to obtain ceramic materials of the system B4C
TiB2 with stable physical
mechanical properties. The thermogravimetric method was used to study the effect of B4C, (Ti, Cr)C, and phosphorus additives on the kinetics of oxidation of chromium carbide alloys in air under isothermal heating conditions at 1000 C [27]. The effect of adding aluminum with titanium on the compacting kinetics, structure, and properties of materials in the system B4C
(Ti
Al) was studied in [5]. The methodology and technology for the fabrication of bulk materials from ultrafine-grained powders of Ti
Al
B
C system are described in [16]. It includes results on determination of thermodynamic conditions for blend preparation, as well as optimal technological parameters for mechanical alloying and adiabatic compaction. Due to the unique combination of their properties, MAX compounds Mn11AXn, where M is a transition metal, A is a group IIIA or IVA element, and X 5 C or N, are thought to be very promising for applications. In was reported [17] on the SHS of a “MAX” phase with X 5 B from 3Ti 1 2Al 1 2((1
x)C 1 xB) green powder compacts (x 5 0, 0.15, 0.25, 0.50, and 0.75). Combustion reaction in such Ti
Al
C
B system was monitored by time-resolved XRD. The reaction of boron carbide with titanium carbide TiC was examined [60] under various conditions. Reactive sintering of the initial mixtures produces a heterophase material based on B4C
TiB2 via a stage of lower boride formation. Adiabatic temperatures, thermal characteristics, and the concentration of reaction products in the system Ti
B4C containing 1:99 wt.% Ti:B4C were determined by thermodynamic analysis [61]. In order to prepare alloys of titanium with titanium boride and titanium carbide, a SHS regime mixture was recommended. Pressureless sintering of boron carbide ceramics containing 0
25 vol.% TiB2 phase, produced via an in situ chemical reaction between B4C, TiO2, and elemental carbon, was studied in the isothermal and constant heatingrate regimes [62]. The presence of TiB2 results in a decrease in activation energy for sintering. The fracture response of these ceramics was studied in [63]. Both strength and fracture toughness depend on TiB2 volume fraction. Additions of TiO2 affect significantly the sintering behavior of B4C [64]. The two powders react at approximately 1500 C according to the reaction: B4 C 1 TiO2 -B4 C1
x 1 TiB2 1 COðor CO2 Þm

the resulting two-phase Above 2000 C, B4C12x 1 TiB2 mixture sinters at a higher rate yielding a dense fine-grained composite material consisting of substoichiometric B4C12x and TiB2. The reaction hot-pressing of B4C ceramics with the addition of TiO2 and C enables the production of high strength high toughness B4C ceramics at temperatures significantly lower than the traditional sintering temperature involving C as an additive [65]. The carbon-coated TiO2 precursors containing B4C were capable of producing high quality TiB2 powders suitable for making ceramics and composites of highly pure powder from the submicrometer particles [66]. Chemical oven method is viable for formation of B4C powder-based materials [6]. It can readily heat reactants used in combustion synthesis as well as forming itself into a sintered TiC
Al2O3 system as a precursor. The effect of impurities and additive of titanium boride on the structure and mechanical properties of SiC 2 B4C ceramics over a broad temperature range was investigated in [43]. The W2B 2 TiB2 2 B4C ceramics studied were hotpressed from composite powders synthesized by thermal reduction of boron carbide [67]. The B4C/(W, Ti)C ceramic composites with different content of solid-solution (W, Ti)C were produced by hot pressing [68]. A chemical reaction took place for this system during hot pressing, and resulted in a B4C/TiB2/ W2B5 composite with high density and improved mechanical properties. The B4C-based composites with 20 mol.% TiB2 were fabricated by reaction hot-pressing of different submicron-size B4C powders with the addition of nanometer-size TiO2 and C powders at 2000 C [69]. It seems that their extremely high strength is attributed to the fine-grained B4C microstructure and uniform dispersion of TiB2 particles. Measurements were conducted on the erosion resistance of B4C
15% TiB2 materials made by reactive sintering on hot pressing of B4C 2 TiO2 2 C powder mixtures [70]. Boron carbide
titanium diboride composites were synthesized and consolidated by SPS of mechanically milled elemental Ti
B
C powder mixtures [71]. B4C
TiB2
Al2O3 powdered ceramics with phosphate binder obtained using SPS was reported in [15]. Directionally crystallized eutectic B4C
TiB2 composite was prepared by a floating zone method based on crucible-less zone melting of compacted powders [72]. In the final product, the fibers of titanium diboride were uniformly distributed in the B4C matrix thereby reducing its brittleness. Phase composition, structure, and erosion properties of B4C 2 Al composite materials produced by hot pulse pressing were examined [14]. It was shown that powder

Boron-Containing Nanocrystalline Ceramic and Metal
Ceramic Materials Chapter | 2

components actively interact to form new phases during pressing including TiB2 because of titanium substrate. A composition B4C 2 TiB2 2 SiC was also proposed [55]. Polarization curves and Auger spectroscopy were used to study the kinetics, formation mechanism, and phase composition of oxide films resulting from the anodic oxidation of SiC 2 TiB2 2 B4C ceramics [56]. The microstructures and mechanical properties of multiphase composites from Nb
Ti
C
B system were investigated [37]. The composites contain (Nb, Ti)B boride and (Nb, Ti)C carbide phases. The hybrid of (Nb, Ti)B and (Nb, Ti)C forms the network microstructure. Boride and carbide phases effectively improve the strength and hardness of the composites. As is known, the SHS is a combustion synthesis process which presents some advantages—high quality of production, low cost, low processing temperatures, low energy requirement, very short processing time, and simple operation—against other production methods for the synthesis of advanced composite ceramics (such as carbothermic reduction, synthesis from elements, gas phase reactions, etc.). This method allows synthesizing advanced boron-containing materials such as ceramics, e.g., ZrB2, TiB2, and B4C. However, the disadvantages of the process, such as unreacted products due to undesirable reaction rates, need to be overcome by changing some parameters such as ignition temperature, particle size, additive, atmosphere, etc. On the one hand, titanium diboride TiB2 is an important transition metal boride with its unique properties such as high strength, hardness, durability, melting point, wear resistance, thermal conductivity, and low electric resistivity. On the second hand, boron carbide B4C is one of the hardest materials known, ranking third behind diamond and cubic boron nitride. It is the hardest material produced in tonnage quantities. TiB2 and B4C are being used in various industrial areas from space technology to nuclear industry owing to the combination of their unique properties. The study [73] was conducted in two main stages: SHS reactions and leaching. TiO2, carbon black C, B2O3 were used as starting materials to produce TiB2
B4C
TiC powders by SHS. Also magnesium powder was used as the reductant. Eleven mixtures were prepared at different rates (from 100% TiB2
0% B4C to 0% TiB2
100% B4C). Powder mixtures were charged into a copper crucible and compacted. Tungsten wire was placed at the top of the copper crucible and the reaction was realized by passing current through the wire. Leaching of the obtained SHS products was performed at 80 C for 1 hour.

2.1.2.14 V—Vanadium Composites B4C
CrB were suitably prepared [25] by carbide reduction of a V oxide when B4C served as a

19

reducing agent and remained one of the components of the composite. The nature of the interaction of boron carbide with vanadium oxide in obtaining the composite material B4C
VB2 was studied so as to establish the stages of the process, the composition of the intermediate products, and to determine the limits of solubility of the reacting components [74]. The properties of composites in the B4C
VB2
C system obtained by reaction synthesis with hot pressing were studied [75] and it was established that the presence of free carbon and vanadium boride in the ceramic makes it possible to activate the sintering process and to obtain a dense highly dispersed ceramic with good structural homogeneity parameters for lower isothermal holding temperatures.

2.1.2.15 W—Tungsten In situ formed W2B5 and graphite containing B4C composites were produced [76] by SPS technique. 5 vol.% W contained B4C starting powders were shaped into square cross-sectioned bulk composites. The sintering process was carried out at temperatures of 1500, 1550, and 1600 C under vacuum. The effects of tungsten addition and different sintering temperatures on densification, hardness, fracture toughness, and microstructural properties were examined. Composites with high hardness and improved fracture toughness were obtained. The W2B
TiB2
B4C ceramics studied were hotpressed from composite powders synthesized by thermal reduction of boron carbide [67]. The B4C/(W, Ti)C ceramic composites with different content of solid-solution (W, Ti)C were produced by hot pressing [68]. A chemical reaction took place for this system during hot pressing, and resulted in a B4C/TiB2/ W2B5 composite with high density and improved mechanical properties.

2.1.2.16 Zr—Zirconium An addition of ZrO2 to the charge helps to obtain by the hot-pressing the highly dense heterophase materials based on boron carbide [41]. This additive activates sintering and makes it possible to obtain ceramic materials of the system B4C
ZrB2 with stable physical
mechanical properties. The reaction of boron carbide with zirconium carbide ZrC was examined [59] under various conditions. Reactive sintering of the initial mixtures produces a heterophase material based on B4C
ZrB2 via a stage of lower boride formation. The Zr-doped boron carbide (of composition B4.3C) semiconductor was prepared by hot pressing of mixture of boron carbide powder and Zr nanocrystals (0.5 at.%) [77].

20

PART | I Different Kinds Of Engineered Nanomaterial For Industrial Use

An investigation was made of reactions in the Zr
B
C system [78]. The existence of a eutectic in the quasibinary section B4.3C
ZrB2 was confirmed. A directionally solidified eutectic alloy was obtained by zonemelting. Zirconium diboride ZrB2 possesses a complex of unique physical and mechanical properties: high melting point, thermal conductivity, hardness, abrasion resistance, and resistance to aggressive environments. Constructional high-temperature ceramics based on ZrB2, which is capable of long-term operation in an oxidizing environment at temperatures above 1500  C, has low density and high strength characteristics at elevated temperatures. The use of such materials is promising in engineering industry, as well as for protective coatings deposition. The main disadvantage of ceramics based on ZrB2 is low crack resistance. To improve the oxidation resistance and strength of ceramics based on ZrB2 various alloying additives are used. Prospective methods for obtaining of ZrB2-based ceramics are hot pressing, SPS, and SHS. In [57], the kinetics and mechanism of combustion of Zr
Si
B
(C) mixtures and the staging of chemical reactions in the combustion wave during synthesis of ceramics based on ZrB2 with silicon containing dopants were studied. The effect of additive of zirconium boride on the structure and mechanical properties of SiC
B4C ceramics over a broad temperature range was investigated in [43]. Zirconium diboride
zirconium carbide based composites were fabricated by reactive melt infiltration with boron carbide and Zr2Cu at 1200 C [79].

2.1.3 Powdered Boron Carbide The critical stage of formation of such composites is obtaining of powdered boron carbide. Lets list some recent reports in this direction. The processes of accumulative recrystallization of grains of hot-pressed boron carbide (during subsequent annealing in vacuum) were investigated [80]. The morphology of a number of ceramic, graphite, and diamond powders including B4C have been studied by scanning electron microscopy and the specific area was determined [81]. Heat-treatment of the powdered hybrids at 1500 C in Ar led to the formation of B4C powders [82]. The free C content in the powders was demonstrated to be variable with the C/B ratio. The morphology and structure of B4C nanocrystals in a thin layer produced on Si substrate by plasma-enhanced vapor deposition from trimethylamineborane were studied [83] before and after synchrotron radiation. Lattice parameters of nanocrystals were found to be different from those of bulk material.

The B4C ultrafine powders were successfully synthesized at 450 C through the so-called coreduction route [84]. The synthesis was carried out in an autoclave by using BBr3 and CCl4 as the reactants and metallic Na as the coreductant. Typical B4C crystallites were composed of uniform ultrafine spherical and rod-like particles. The formation of whiskers and platelets of B4C was studied through carbothermal reduction of B2O3 [85]. In the absence of any additive, neither whiskers nor platelets were formed from B2O3 and carbon black. NiCl2 and K2CO3 were found to accelerate their growth. Boron carbide nanoparticles were made via a reaction of boron, obtained from thermal decomposition of magnesium diboride with multiwall carbon nanotubes at 1150 C in vacuum [86]. The size of the nanoparticles was smaller than 100 nm. The single-crystal nature of each nanoparticle was evidenced. Commonly, the obtaining of boron carbide needs high temperature, good vacuum, and utilization of the toxic substances. These problems were partially resolved in the work [87]. Boron carbide was obtained in form of spherical particles in ambient conditions using laser-irradiation of boron particles dissolved in organic solvent (ethyl acetate), which serves as a carbon-source. Generally, boron carbide-based ceramics can be compacted using a two-stage process: heating the powder up to 2000 C and then sintering at 2190 C in argon atmosphere [88]. If the initial powder is too fine-grained it can be achieved at 95% of the theoretical density. Frequently, before these stages boron carbide powder is treated in nitrogen at 1900 C, where it forms boron nitride BN and residual carbon in the form of graphite-sediments. At first, thermal treatment in vacuum leads to the decomposition of BN, but then free boron interacts with graphite and again forms boron carbide particles, but of much lesser sizes. The work of [89] aimed to develop a cost-effective and low-temperature manufacturing process to synthesize boron carbide from polymer precursors. The polymeric precursor was synthesized by the reaction of boric acid and polyvinyl alcohol that after pyrolysis at 400 and 800 C gives boron carbide. The influence of deformation pretreatment—milling in a planetary mill and shock-wave treatment—of B4C powders of different size-composition on their structure as well as the structure and mechanical properties of samples that have been hot-pressed from these powders was investigated [90]. The production of boron carbide nanoparticles was investigated in a conventional high temperature furnace reactor [91]. The reaction was carried out by heating a mixture of amorphous carbon and amorphous boron at 1550 C to efficiently obtain a quantity of B4C.

Boron-Containing Nanocrystalline Ceramic and Metal
Ceramic Materials Chapter | 2

Separately was studied the effect of the liquid media on the process of obtaining of boron carbide: a number of organic compounds were tested [92]. It was found out that, the average size of particles is inversely proportional to the dielectric constants of the solvent. The relatively high cost of raw materials, as well as the complexity of obtaining densely-sintered products from boron carbide, greatly limits its wide application. In [93], the object of the study was: boron carbide powder obtained by mechanosynthesis of mixtures of soot and amorphous boron and B4C powder obtained by SHS method. By standard technology the parts made from B4C powder were pressed and then sintered. The process of hot pressing powders was studied. The process of hot pressing was carried out at temperature 2000 6 50 C. The main phases of the materials obtained were B4C and carbon. The most uniform and fine-grained structure was observed in the nanomodified powder B4C obtained by mechanochemical synthesis of mixtures of soot and amorphous boron. Due to the low density and extremely high hardness, boron carbide ceramics are strong candidates for personal body armors. However achieving full density in final compacts of strong covalently bonded boron carbide is difficult. In order to overcome this problem in boron carbide, not just some additives were included to the structure but also SPS which proved its success on sintering final products with very high relative densities was used in the study [94]. The powders were prepared at corresponding rates with the ball milling technique, then dried and granulated. After powder preparation, without any preshaping application or any binder addition, the powders were directly SPS at the temperatures between 1400 and 1550 C for 4 minute under an applying pressure of 40 MPa in vacuum atmosphere. The density and ballistic performances of the samples with different additions were compared with monolithic boron carbide ceramics. SHS is an energy saving and attractive method for production of a variety of advanced micron- and nanoscale materials with properties that are superior to those manufactured by conventional methods. In turn, SPS, also known as the field-assisted sintering technique, is a relatively novel sintering technique. Combination of SHS and SPS in one-step method to produce pore-free ceramics is a promising technique for fabrication of advanced materials. In the work by [95], bulk boron carbide was fabricated from a mixture of elements (boron and carbon) by using this one-step, so-called, reactive SPS approach. It was demonstrated that preliminary high-energy ball milling of the B 1 C powder mixture leads to the formation of composite particles with enhanced reactivity. Using these reactive composites in reactive SPS permits tuning of the microstructure for the synthesized ceramic and thus produces materials with desired properties. Optimization

21

of conditions allowed rapid fabrication of a B4C ceramic with porosity less than 1%, high hardness (B35 GPa), and good fracture toughness.

2.1.4 Case Study: Composite B4C
TiB2 The goal is that this case study can be formulated as the development of a method of assembling a boron carbidebased nanocomposite material, which optimally combines the high elasticity modulus, high hardness, and abrasive ability of boron carbide with the ductility and impact toughness of the metal binder. Achieving this goal means both experimental and theoretical—via modeling—fundamental studying of the physical
chemical processes that take place at interfaces between boron carbide and metallic binders. Also, it is necessary to predict and measure the mechanical properties of samples with high hardness and satisfactory impact toughness and resistance to cracking obtained by consolidation of nanopowdered products. There should be investigations into the nanopowders of boron carbide, its additives, e.g., TiB2 in the case under the consideration, and metallic binders, as well as physical
chemical processes that take place when nanocomposite materials with advanced mechanical properties are assembled from these components. This means a preliminary choice of composition for boron carbide based nanocomposite materials with advanced mechanical properties taking into account physical
chemical properties of likely components, subsequent adjustment of composition by measuring the parameters of components assembling processes, and their modeling. To obtain boron carbide, one can use amorphous boron, which forms B4C above 1200 C. According to preliminary data, nanograde (B70 nm) boron carbide powder can be obtained by the interaction of amorphous boron with carbonizing reagents such as organic compounds (polyvinyl alcohol, hydrocarbons, etc.). Usually, the reaction is conducted in an argon atmosphere. At the initial stage, pyrolysis of organic reagents forms the activated carbon: Cx Hy Oz -C 1 n H2 O 1 m CO2 : From 800 C argon has to be replaced by hydrogen and temperature can increase up to 1400 C. Formation of boron carbide starts at 1100 C. The B4C
TiB2 composites can be obtained by several methods. Among them is the high energy milling of the mixture of boron, titanium, and carbon, pressing and then sintering at .1200 C. Formation of the B4C
TiB2 composite powder by the hydrolysis of mixture of titanium alkoxides and boron carbide takes place at relatively lower temperature: obtained gel is annealed at 850 C. The reactions are:

22

PART | I Different Kinds Of Engineered Nanomaterial For Industrial Use

TiðORÞ4 1 B4 C 1 H2 O-TiðOHÞ4 =B4 C 1 4 ROH; TiðOHÞ4 =B4 C-TiO2 =B4 C; TiO2 =B4 C-TiB2 =B4 C 1 ðCÞ: In this case, boron carbide serves as a boriding agent and, correspondingly, in the system equivalent amount of carbon is released. Superdispersive boron carbide powders intended for producing ceramic nanomaterials with and without inclusions of free carbon can be obtained in the technological process previously proposed by us in [96]. Boron anhydride and glucose serve for boron- and carbon-sources, respectively. An analogous method is described in other our work [97]. This method enables the obtaining of boron carbide nanopowder at relatively lower temperature. The technology that we have developed for producing boron carbide based metal
ceramic materials can also be used. It provides pyrolysis (900
1100 C) of the precursors comprised of amorphous boron, carbon-chain polymers, and appropriate metal salts [98]. The structure and mechanical properties of obtained through this way hard samples with good impact toughness can be studied by a complex of standard methods as we have done previously [99] for compacted superdispersive boron carbide specimens. As for the theoretical modeling of the process of assembling boron carbide-based nanocomposite materials, it can be carried out on the basis of quasi-classical method of calculation of substance atomic and electronic structures, which was developed [100] (see also [101]) and successfully tested for interactions of boron both in ceramics, in particular, boron nitrides [102
111], and with metals [112
116]. Values of static parameters found in this way then should be used in current kinetic models. Below we list some of them. The work [117] focuses on the understanding gained from the investigation of the physical properties of boron carbides with theoretical methods based on density functional theory. It has been emphasized that simple quasibinary eutectics of B4C
TiB2-type should be considered as examples of systems with phase diagrams essentially different at nanoscale [118]. The models suggested in the paper [119] can also be used, which summarizes the utilization of analytical and numerical computations of ceramic/metal and ceramic/ composite add-on armor failure process as valuable tools for armor design optimization. The thermodynamic potentials of binary TiB2
SiC, B4C
SiC, and B4C
TiB2 systems were constructed with the pseudopotential method in [120]. The minimum thermodynamic potential can be used to determine the eutectic temperatures and concentrations of the components.

The method of granular dynamics, also known as the method of discrete elements, simulates the processes of cold compacting nanopowders in 2D geometry [121]. Model systems corresponding to nanopowders exhibiting both weak and strong tendency to agglomeration were investigated. The interaction of particles, in addition to the widely used laws of contact interaction, includes the dispersion forces of attraction and the possibility of the formation of strong interparticle bonds of chemical nature. The latter arise from the strong pressing of particles to each other, which is initiated either by the action of high dispersion interactions or by the process of compaction. In computer experiments, the effect of the particle size-distribution on the processes of cold compaction of a nanopowder was analyzed. To this end, a uniaxial and comprehensive compaction of various model systems has been simulated: mono-size systems with a particle diameter of 10, 20, and 30 nm, bidisperse systems with different contents of large (30 nm) and fine (10 nm) particles, and polydisperse systems described by the lognormal distribution function with different widths. In such polydisperse systems, a small deviation of the density of compacts from the density of the corresponding monodisperse system was found to be not more than 1%. The latter result, in particular, justifies the use of monodisperse systems in the framework of the granular dynamics method for modeling the properties of polydisperse nanopowders. The study [122] is a continuation of the modification of the granular dynamics method, intended to describe the mechanical properties of nanoscale powder compacts and their cold pressing processes. Unlike traditional approaches used to describe micron-sized powders, in the description of nanopowders, the interaction of particles, in addition to the widely used contact laws, should include dispersion forces of attraction and the possibility of formation of strong interparticle bonds of a chemical nature, the possibility of completely relaxing, and achieving a new equilibrium state. However, in connection with the development of effective methods of pulse pressing, when the rate of deformation is too high, the modeling of dynamic processes becomes highly relevant. The final speed of the process requires strict consideration of the viscous stresses arising in the contact region of the interacting particles. To describe viscous stresses, viscous forces are obtained that arise in the contact area of two spherical elastic granules at their normal motion relative to the plane of contact, tangential shifts, rotation along the contact axis and in the perpendicular direction (rolling). The shear viscosity of the particle material is estimated from the ultrasound attenuation. The decay time of collective oscillations in the powder system can become comparable with the characteristic times of the impulse processes. For the purpose of qualitative analysis and to

Boron-Containing Nanocrystalline Ceramic and Metal
Ceramic Materials Chapter | 2

facilitate calculations, the first test numerical experiments were conducted in a two-dimensional setting. Based on existing models for particles rearrangement and sliding, the contributions of different processes in SPS in conjunction with nanoparticle properties and process parameters were highlighted in [123]. So-called special composites as objects for modeling were described in [124] and multicriteria synthesis of composite materials was presented as a task of the management theory. Most part of technologies developed for nanostructured materials use thermal treatment. This process has been considered in [125]. The procedure has to be conducted at lower temperatures, taking into account features of structures and phase-transitions. In a single technological process the two-phase B4C
TiB2 nanoscale powdered ceramic compositions were obtained [126] by a chemical method—dehydration in special mixers of a suspension solution of amorphous boron, titanium(IV) oxide, glycerol as dispersible medium, and distilled water as a media supporting homogeneity in the mixture, and further stepwise heat treatment at temperature of 1250 C. The liquid charge was processed into several stages at different temperatures from the range 120
1300 C and in different exposure conditions. In order to achieve the most uniform distribution of amorphous boron and titanium oxide in the mixture, they were long mixed in the attritors, and then the resulting powdered mixture again was long stirred with a solution of glycerol and water at room temperature in the mixers.

23

For the formation of the desired product, in addition, it is important to fit the optimal parameters of further processes of drying and heat-treatment of the liquid mixture. The change of amounts of precursors in the charge makes it possible to control the components ratio in the final product. X-ray phase analysis of the final product shows that the two-phase system comprising boron carbide and titanium diboride was obtained (see Fig. 2.1). On the XRD image the presence of other phases is not observed. The electron-microscopic studies found that grain size in the powdered mass is less than 100 nm (see Fig. 2.2). Increased synthesis temperature to 1350 C significantly increases (above 200 nm) crystallites size (see Fig. 2.3). Obtained B4C
TiB2 composite powders were compacted by using the SPS method. For the basic composition is chosen the ceramic nanostructured composite B4C 1 30% TiB2 obtained by the technology considered above. The dispersity of a binary composite powder chemically synthesized from a liquid charge is ,100 nm. The technological parameters of compacting process are pressing temperature 1750 C, pressure 18 MPa, duration 3 min, sizes of pressed specimens: diameter 30 mm and height 6
10 mm. It is established that by changing the ratios of initial components, it is possible to adjust the amounts of the constituent phases (up to the ratio B4C:TiB2  2:1). The ceramic composition B4C 1 30 wt.% TiB2 has a high strength of 95 HRA (the maximum microhardness of the composite is 4650 kg/mm2) and a flexural strength of 85 kg/mm2.

FIGURE 2.1 XRD-image of nanostructured powdered ceramic material produced on basis of boron carbide and titanium diboride. XRD, X-ray diffraction.

24

PART | I Different Kinds Of Engineered Nanomaterial For Industrial Use

FIGURE 2.2 Electron microscopic image of nanostructured powdered ceramic material produced on basis of boron carbide and titanium diboride.

FIGURE 2.3 Electron microscopic image of B4C
TiB2 nanostructured powdered ceramic material synthesized at temperature 1350 C.

2.2 BORON NITRIDE-CONTAINING MATERIALS 2.2.1 Mini Review on Metal
Boron Nitride Tribological Composites As is known, wear is a common problem for different types of surfaces of an engine component subjected to dynamic loadings. In the industry, a number of lubricants are used in order to reduce friction and wear in machines with moving parts. Typically, lubricants utilized are in the form of liquids or greases. But, materials of such consistency do not meet all the requirements for heavy-loaded friction pairs operating under extreme conditions— extremely high or extremely low temperatures, in

ultra-high vacuum, under extreme contact pressures, at high or low sliding speeds, etc. Usually, this problem is solved by the preparation of liquid lubricants containing solid additives of layered crystalline materials. However, widely used additives of such kind, e.g., molybdenum disulfide (MoS2), graphite, and the like, contain heavy metals, sulfur, carbon (in form of graphite), etc. and then are environmental pollutants. In many cases, they can be successfully replaced by promising boronbased “green” lubricants [127] or boriding the rubbing surfaces [128]. In particular, the addition of about 1 wt.% hexagonal boron nitride (h-BN) powder in oil, grease, or fuel is sufficient to provide quite good tribological properties. For the first time, Kimura et al. conducted [129] a series of detailed sliding experiments, which reveal the behavior of h-BN when it is added to the lubricating oil. In the case of sliding of bearing steel vs steel, it slightly increases the coefficient of friction, but dramatically decreases wear; and boron is found to remain on rubbing surfaces in the form of nonstoichiometric oxide. But, when bearing steel is sliding against cast iron, the powdered h-BN decreases the coefficient of friction and the remnant is mostly BN. There are suggestions from a number of more recent results confirming that h-BN is effective in reducing wear if used as a liquid lubricant additive—see the references in [130]. The increase in dispersion of h-BN suppresses the sedimentation processes in a liquid lubricant that improves its performance. This is particularly important in the metalwork, when the cutting or grinding processes are accompanied by the significant heat-releasing. A graphite-like lamellar structure of h-BN predetermines its tribological properties: intralayer bonds are strong covalent bonds with a deal of ionicity, whereas weak van der Waals polarization-forces are responsible for the interlayer interaction. Therefore, layers of h-BN are easily moving relative to each other. In the process of rubbing, the single-crystalline h-BN particles are spontaneously aligned so that their basal planes are parallel to the rubbing metal surfaces. Thus, the friction between metal surfaces is replaced by the internal friction between BN-hexagonal layers. Sometimes in nanoscale junctions containing h-BN, even the effect of superlubricity occurs, which means the almost frictionless tribological state. From the friction experiments conducted with h-BN in sliding contacts with various metals, this material is known as a good solid lubricant as well [131]. An alternative way to reduce the wear and friction is the production of self-lubricating materials by introducing h-BN particulates onto the surface layer or into the bulk of rubbing metallic materials. Such an approach has been a major topic of interest in the last decade. It has been successfully examined in different Fe-based alloys—low-carbon [132],

Boron-Containing Nanocrystalline Ceramic and Metal
Ceramic Materials Chapter | 2

austenitic [133], stainless [134], high-Cr [135], CrMo[136], and TiC-steels [137], and distaloy [138]—as well as in some Cu-based pseudoalloys [139]. This issue is closely related to the processes of growth of h-BN layers on Fe- [140
144] or Cu-surfaces [145
152] and vice versa—formation of Fe- [153
155] and Cu-coatings [156,157] onto the h-BN basal plane.

2.2.2 Case Study: Self-Lubricating Brass and Iron Modified by Boron Nitride Here we propose a novel approach to fabrication of antifrictional, i.e., self-lubricating, metallic materials containing brass or iron matrices and modifier particulates of h-BN. In particular, we study tribological properties of composite materials based on brass (Cu—68.5 wt.% and Zn—31.3 wt.%) and iron (Fe). Nanosized h-BN obtained by a specially developed technology was as an antifriction component. Among the numbers of methods useful for production of boron nitride, increasing attention is being paid to the method of chemical synthesis using organic and inorganic precursors. Above all, it is interesting because its easiness to perform, the availability of components used, and the ability to reduce energy consumption during the process implementation. Aiming to increase the dispersity of products and decrease the processing temperature, for chemical synthesis of the nanostructured boron nitride we used charges with different contents of components. Their utilization has showed the following results. G

G

G

If polyvinyl alcohol and gaseous nitrogen N2 are used in amorphous boron charges, a catalyst is necessary to initiate process. For this reason, a certain amount of BN nanopowder, up to 5 wt.%, should be introduced in the charge as a catalyst. Such a synthesis process is carried out in three stages in the temperature range 300
1250 C. If a mixture of boric acid H3BO3 and urea NH2CONH2 are used to synthesize nanocrystalline BN, the process should be carried out in N2 atmosphere in the temperature range 1000
1300 C. Changes in temperature parameters of the process make it possible to obtain both amorphous and nanocrystalline boron nitride. If a charge consisting of a mixture of borax Na2B4O7  nH2O with ammonium chloride NH4Cl is prepared by mixing of components mechanically (dry method) or their aqueous solutions, the process of boron nitride formation begins at 700 C and ends at 900 C. Increasing the synthesis temperature up to 1300 C does not change significantly the phase composition and particle sizes of obtained crystals.

G

G

25

If charge is prepared in the form of mixture consisting of borax and urea, it allows obtaining of the fully formed h-BN in ammonia NH4 flow at 1000 C. If one uses a charge consisting of a mixture of borax and sodium nitrite, the formation of BN begins at a temperature of 700 C. However, the liquid phase formed in the process of synthesis blocks the diffusion of nitrogen into the reaction region and then prevents further intensification of the synthesizing process.

Investigation of products synthesized from the above listed different sets of reagents shows that the use of the gaseous ammonia is more effective than that of nitrogen because ammonia reduces formation temperature of the nanocrystalline BN. It should be also noted that the dispersity of powders obtained from aqueous solutions is higher than that of products obtained by mechanical mixing of components. Based on geometric characteristics and morphology of the synthesized BN powders, a typical particle size is found to be 100
150 nm, while that for their agglomerates is 2
3 μm. Maximum surface area of the products of synthesis estimated theoretically corresponds to around 9 m2/g. So, the obtained BN-product can be used as a solid lubricant or an additive to liquid lubricant materials. Here we use h-BN nanopowders synthesized from a mixture of boric acid and urea or sodium tetraborate. Processes were conducted in a temperature range of 850
1100 C in atmosphere of ammonia or ammonium chloride, respectively. The resulting product is a textured nanocrystalline boron nitride with an average density of B2.1 g/cm3. Fig. 2.4 presents the XRD pattern (Θ is the Bragg angle) of the boron nitride powder synthesized at temperature of 870 C in ammonia atmosphere using as precursors sodium tetraborate and ammonium chloride, and washed in distilled water. This diffraction pattern shows that, obtained boron nitride has a hexagonal structure. The presence of diffraction peaks broadened to varying degrees indicates the nonspherical form of crystals. As is known from the literature (see, e.g., [158,159] and references therein), they should be disk-shaped. Sizes of disk-shaped particles were evaluated by the Selyakov
Scherer method [160]. It means that, on the basis of measurements of the peak broadenings and the formula Δð2ΘÞ 5

Kλ ; Dhkl cos Θ

there is estimated the size of a substance crystallite. Here Δ(2Θ) is the half-width of the interference peak (in radians), K is the factor of shape of the crystal (in general, its value can vary in the range from 0.98 to 1.39), which for crystals of h-BN can be taken as 1.20, λ is the

26

PART | I Different Kinds Of Engineered Nanomaterial For Industrial Use

FIGURE 2.4 XRD pattern of boron nitride powder synthesized at 870 C in ammonia atmosphere using as precursors sodium tetraborate and ammonium chloride and washed in distilled water. XRD, X-ray diffraction.

FIGURE 2.5 XRD pattern of copper-plated h-BN. XRD, X-ray diffraction; h-BN, hexagonal boron nitride.

wavelength (in our case, i.e., for Cu Kα radiation, it equals to 0.1539 nm), Dhkl is the grain size along the normal to the (hkl)-plane (in nm). The peak broadening can include both broadenings due to small grain sizes and socalled apparatus broadening, and the broadening caused by the deformations and defects of different types. The sizes of the grains were estimated from the interference maxima (0 0 4) and (1 0 0), for which the apparatus broadenings were  0.5 and  0.3 , respectively. If we neglect the broadening caused by strains and defects, we find that the average thickness of the disk-shaped grains of boron nitride  25 nm and their average diameter  240 nm.

Investigations of the products synthesized from various reagents showed that, among the nitrogen compounds in the process of synthesis the most effective is ammonia because if compared with the nitrogen gas its use reduces the starting temperature of the synthesis. In order to improve adhesion to the matrix alloy, brass or iron, boron nitride was chemically plated by copper or iron, respectively. XRD image of the copper-plated h-BN is shown in Fig. 2.5. Plated nanocrystalline boron nitride introduced into molten matrix metal (brass or low-carbon iron). The microstructures of the obtained composite material based on brass are shown in Fig. 2.6.

Boron-Containing Nanocrystalline Ceramic and Metal
Ceramic Materials Chapter | 2

(A)

27

(B)

FIGURE 2.6 Microstructures of (A) mechanically and (B) chemically polished samples of composite brass 1 1 wt.% h-BN ( 3 400). h-BN, hexagonal boron nitride.

The mentioned antifrictional additive has been distributed in the matrix without any preferential accumulation at the grain boundaries or other structural defects. Determination of the tribological properties of metallic materials modified with boron nitride was conducted using an installation SMC-2 specially designed for the study of friction and wear processes in metals and alloys. Its principle of operation is based on the wear of pairs of samples pressed together by a force of a prespecified value. During the tests, the friction torquewas measured using a remote inductive coupler. The value of the electromotive force, which was assumed to be proportional to the friction torque, was fixed by the multimeter X-18 connected to a computer for a software-elaboration of the obtained data. The frequency of measurements was 1 Hz. The method used was “disk drive.” The test sample served for the first disk, and the second, so-called pinbody, was selected respectively to a task. The geometrical dimensions of disks, diameter and thickness, were D 5 45
50 and h 5 10
12 mm, respectively. Slip ratio was B10%. Normal load on the drive was controlled by means of a bagged spring and its value was within the range 200
1000 N in accordance with the characteristics of the material. As is known, due to wavy surfaces of machine parts actual contact area (ACA)—area at which the microirregularities touch each other—is less than that of nominal contact surface: usually ACA is small and amounts to 0.01%
0.10% of the nominal contact area. Accordingly, for the evaluation of actual pressure on parts in contact, it is very important to know their ACA. From the modern methods of determining the ACA, we have chosen the method of thin plates, which is widely used in industrial practice. The method consists of the following. Between the contacting surfaces, the dye-containing plate is placed and after unloading, the ACA is estimated by area and geometry of the left imprint. These estimates were conducted using the special software. Photos of typical imprints are shown in Fig. 2.7. ACA values estimated before and after grinding of contacting parts at different loads. Duration of grinding was determined based on the dynamics of changing in the friction

FIGURE 2.7 The imprint in conditions of actual contact load of 400 N.

torque. Stable (constant over time) value of friction torque of rubbing parts is reached after about 30 min. After a stable value of the friction torque is reached, the ACA is estimated and based on it the value of the actual load is selected. Estimates of ACA obtained at normal load of 200 N lead to the actual pressure value of 3660 N/mm2. As is known, the stabilization of geometric sizes is considered as a very important factor in the operation of friction pairs working in extreme conditions. In order to determine the extent to which it is succeeded in achieving such kind of stabilization, the TEC of the obtained composite was measured. Fig. 2.8 shows (T is the temperature) that the introduction of a friction modifier has an insignificant effect on the TEC. This fact reveals the stabilization of the geometric sizes of friction pair made of the composite brass 1 1 wt.% h-BN. Introducing of 5 wt.% h-BN in the matrix alloy increases the room temperature thermal conductivity of the matrix from 25 to 50 W/m K. The increase in the thermal conductivity indicates a possible intensification of heat transfer processes in heavily loaded friction pairs made of the composite material brass 1 5 wt.% h-BN. As it has been noted, the study of the tribological properties of the materials was carried out on a “disk drive.” The sliding speed was 0.9 m/second. Figs. 2.9 and 2.10 show the dependence of the degree of wear Δm on the number of revolutions N and friction modifier content, respectively. The last of them shows that the introduction of the friction modifier, h-BN, is essential to reduce the mass losses in the composite brass 1 1 wt.% h-BN due to changing the mechanism of wear if compared to the initial sample. Fig. 2.11 shows the optical microscopic image of wear for brass L67 and composite L67 1 1 wt.% h-BN for extreme load conditions. Testing under extreme load conditions is carried out at a “catastrophic” wear as well. Sizes and morphology of wear products indicate that the

PART | I Different Kinds Of Engineered Nanomaterial For Industrial Use

20

0.03

15

0.025

10

0.02 brass

brass + 1 wt.% h-BN

5

0

ΔM (g)

TEC (106 K−1)

28

brass + h-BN Fe + h-BN

0.015

0.01 0

100

200

300

400

T (°C)

0.005

FIGURE 2.8 Temperature dependence of thermal expansion coefficient of brass and composite material brass 1 1 wt.% h-BN. h-BN, hexagonal boron nitride.

0

0

2

4

6

h-BN (wt.%) 1.6 FIGURE 2.10 Dependence of degree of wear of composite materials from content of boron nitride (340 rpm).

1.4 1.2

(A)

(B)

ΔM (g)

1 0.8 0.6 0.4

brass brass + 1 wt.% h-BN

0.2

10 mm

brass + 5 wt.% h-BN 0

0

5000

10,000

15,000

20,000

2 mm

25,000

N

FIGURE 2.11 Wear products of (A) brass and (B) brass 1 1 wt.% h-BN at almost identical loads. h-BN, hexagonal boron nitride.

FIGURE 2.9 Dependence of degree of wear on friction modifier content and number of revolutions.

1 brass, 51 N

0.8

brass + 1 wt.% h-BN, 51 N ΔM (g)

friction modifier changes the mechanism of wear and greatly reduces the intensity of the wear-process. From Fig. 2.11, one can see that differences in the morphology and the linear sizes of wear particles at almost identical loads are too radical. Further increase in the content of boron nitride up to 5 wt.% increases the mass losses in the process of friction. Probably, it is due to the decrease in the composite shear modulus and the structure loosening. Fig. 2.11 shows the wear products, while Fig. 2.12 shows the dependence of the degree of wear of the investigated samples on the speed at different normal loads. In Table 2.1, there are presented the values of the linear wear rate k 5 Δd/s for the investigated samples at 12,500 rpm and normal load of 117 N. D0 and D are the

0.6

brass, 20 N brass + 1 wt.% h-BN, 20 N

0.4 0.2 0

0

10,000 20,000 N (rpm)

30,000

FIGURE 2.12 Dependence of degree of wear of studied samples on speed at different normal loads.

Boron-Containing Nanocrystalline Ceramic and Metal
Ceramic Materials Chapter | 2

29

TABLE 2.1 Linear Wear of Brass and Iron, and Their Composites Sample

D0 (mm)

D (mm)

Δd (mm)

k

Brass

49.76

49.63

0.065

0.21  1027

Brass 1 1 wt.% h-BN

50.33

50.24

0.045

0.14  1027

Fe

50.12

49.76

0.180

0.92  1027

Fe 1 1 wt.% h-BN

50.28

50.02

0.130

0.66  1027

h-BN, hexagonal boron nitride.

TABLE 2.2 Friction Coefficient and Efficiency Resource Third Body

Friction Coefficient Without “Third Body”

Friction Coefficient With “Third Body”

Resource (km)

Gear oil

0.039

0.006

3.0

PEG

0.039

0.007

14.7

PEG 1 5 wt.% h-BN

0.039

0.006

17.7

PEG, polyethylene glycol; h-BN, hexagonal boron nitride.

initial and final diameters of a test disk-shaped sample, respectively, Δd 5 (D0 2 D)/2 is the thickness of the removed layer, and s is the distance traveled in mm. From the above results obtained, it implies that the introduction of the friction modifier, the h-BN, in a metal alloy significantly reduces wear of the matrix
metal due to changing the mechanism of wear and the optimal amount of modifier should be around 1 wt.%. For a “third body,” the environmentally friendly polymer—PEG C2nH4n12On11 modified with boron nitride was selected. We used PEG with density of 1.1
1.2 g/ cm3, which is a gelled solid phase substance. Thickness of the coating of the “third body” was B200 μm. For a “third body,” standard gear oil was also tested. BN layers were obtained by the aerosol spraying. From the results obtained for the friction coefficient, one can see that gear oil and PEG with and without a modifier do not affect significantly the value of the friction coefficient, but PEG significantly (by 5
6 times) increases the efficiency resource of the “third body”—the distance passed to restore the original moment of friction—see Table 2.2. Thus, a relatively cheap chemical technology of producing nanograde h-BN directly usable for solid lubrication has been developed. For so-called third bodies with nanopowdered h-BN friction-modifier, the gelled PEG (with the density of 1.1
1.2 g/cm3) based eco-friendly compositions are selected. Tests show that their working resource significantly exceeds that of other materials utilized in practice with the same purpose. Some new brass-

and iron-based composites with h-BN nanoinclusions have been obtained. At optimal contents (from 1 up to 4 wt.%) of this friction modifier, wear resistance of a composite compared to its metallic matrix is enhanced by several times.

2.3 BORON-CONTAINING NANOCRYSTALLINE MASTER-ALLOYS Prospects for the development of ferrous metallurgy are determined not only by the improvement of metallurgical aggregates and processes, but also by the possibility of satisfying the ever-growing requirements for the quality of metal with minimal expenditure of alloying components. The use of boron opens up new possibilities for obtaining economically alloyed steels whose performance characteristics in many cases not only are not inferior, but also exceed the level of properties of steels obtained using the traditional alloying system. Boron, as a microalloying element, has long been known and widely used in steelmaking plants in the form of ferroboron and boron-containing ligatures. However, with the addition of these alloys, significant differences in the phase density and a significant overheating of the metal (300
500 C) above the liquidus temperature before it leaves the furnace lead to a segregation of practically all the controlled elements along the height and crosssection of the steel ingot. This causes a decrease in the

30

PART | I Different Kinds Of Engineered Nanomaterial For Industrial Use

efficiency of the “action” of boron and the stability of the results obtained. It is clear that the new approaches and the search for new technological solutions for obtaining and using boron-containing microalloys are needed. Boron significantly improves the quality of the metal when added to cast iron and steel in an amount of 1024%
1023%, which is 2
4 orders of magnitude lower than the consumption of other alloying elements (e.g., chromium, manganese, molybdenum, or nickel), the use of which makes it possible to obtain corresponding results on increasing hardenability and strength of low- and medium-alloyed steels [161]. The examination [35] of the thermal synthesis of master alloys from a mixture of iron and boron carbide powders has shown that the content of boron decreases and the boron/carbon ratio in the master alloy powder changes with increasing synthesis temperature. We are conducting research on the production of nanocrystalline alloys for the steel modification [162]. Below the results [163] of research on obtaining alloys Fe
B and Fe
B
Al2O3 are given. The charge materials used were: G

G

for obtaining Fe
B alloy: ferric chloride FeCl3  6H2O, boric anhydride B2O3, and sucrose C12H22O11; for obtaining Fe
B
Al2O3 alloy: ferric chloride FeCl3  6H2O, aluminum chloride Al2O3  6H2O, boric anhydride B2O3, and sucrose C12H22O11.

The three-stage technological process of obtaining Fe
B and Fe
B
Al2O3 alloys, in general terms, is as follows.

At the first stage, the initial charge mixture of the given stoichiometric composition is sprayed onto a corundum substrate heated to 250
300 C. The resulting powder is placed in a retort where hydrogen reduction takes place at a temperature of 800
870 C—the second stage. In the third stage, the reconstituted powder is placed in another retort, where it is maintained at 1100
1200 C in argon atmosphere. The FeB phase begins to form at 1000 C. As the temperature rises, the Fe2B phase begins to form and at 1200 C a two-phase FeB
Fe2B system is formed (Figs. 2.13 and 2.14). An estimate of the geometric dimensions of nanoparticles by the expansion of XRD maxima (Scherer’s method) showed that they are in the range of 40
80 nm. This result is confirmed by electron microscopic studies as well. The compounds of iron and boron formed in the test master alloy are characterized by high dispersibility and, probably, will play a role of dispersed inclusions in the metal matrix, and consequently are effective barriers for dislocations moving during plastic deformation. In this place, they do not block the movement of dislocations, which determines an increase in the strength of the material with preservation of the initial plastic characteristics of the matrix [164]. The establishment of a mechanism of disperse hardening that is realizable in the studied alloys is the subject of subsequent research. The use of nanocrystalline ligatures containing highly dispersed iron and boron compounds will increase the ultimate strength of alloying steels while maintaining

FIGURE 2.13 XRD pattern of ferroboron obtained at 1100 C (*—FeB and  —Fe2B). XRD, X-ray diffraction.

Boron-Containing Nanocrystalline Ceramic and Metal
Ceramic Materials Chapter | 2

31

FIGURE 2.14 XRD pattern of ferroboron obtained at 1200 C (*—FeB and  —Fe2B). XRD, X-ray diffraction.

their plastic characteristics. The finely dispersed structure of steels alloyed with the developed nanocrystalline ligature will undoubtedly increase the mechanical properties of steels along with the effect of dispersed hardening. The obtaining ferroboron ligatures also is possible by the SHS-metallurgy, which is characterized by simple and small-sized equipment, high productivity and purity of the products, and environmental safety of the process. The centrifugal machine was used [165] as processing equipment for carrying out works under the influence of centrifugal force at overloads. There were used a boroncontaining material in the form of B2O3, iron oxides and as restoring metals powders of aluminum, magnesium and magnesium
aluminum alloy. On the basis of the received results it was established, that extraction of boron is more than 95 wt.%. The analysis of the samples showed the presence of a high content of carbon in the casted ligatures. The problem of the high content of carbon in the melt can be minimized by creating a protective barrier between the reaction mixture and used graphite cup. It is important, because the content of carbon in boron steels should not exceed 1.0 wt.%. Analysis of the structure and content of the alloys showed that content of boron in the ferroboron alloys does not exceed 22 wt.%. There was no boron in the slag practically. Minimization of boron loss is an important condition. Thus ferroboron ligature enriched with boron has been obtained in the quantities sufficient for carrying out melting and receiving of boron steel.

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