formation and properties of tib2–ni composite ceramics · electron microscope and leo 1450 vp...

Science of Sintering, 48 (2016) 137-146 ________________________________________________________________________

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*) Corresponding author: [email protected]

doi: 10.2298/SOS1602137V UDK 692.533.1; 622.785 Formation and Properties of TiB2Ni Composite Ceramics Marina Vlasova1*), Alexander Bykov2, Mykola Kakazey1, Pedro Antonio Marquez Aguilar1, Igor Melnikov3, Isai Rosales1, Rene Guardian Tapia1 1Center of Investigation in Engineering and Applied Sciences of the Autonomous University of the State of Morelos (CIICApUAEMor), Av. Universidad, 1001, Cuernavaca, Mexico. 2Institute for Problems of Materials Science, National Academy of Sciences of Ukraine, 3, Krzhyzhanovsky St., Kiev, 252680, Ukraine. 3Department of Electronic Materials, National Research University of Electronic Technology, Zelenograd, Moscow 124498, Russian Federation. Abstract:

An analysis of physical and chemical processes occurring during hot pressing of the 95 wt. % TiB25 wt. % NiCl2By powder mixture in the temperature range 18002000 C has been performed by Xray diffraction, scanning electron microscopy, an electron-probe microanalysis. It has been established that, in the process of heat treatment, sintering, TiB2 grain growth, diffusion of boron and titanium into nickel layers, and the formation of NixByTiz layers between TiB2 grains occur. These layers act as a grains binder TiB2. It is shown that the drilling of the obtained high-strength ceramics can be performed by laser machining. Keywords: TiB B2Ni, Sintering, TiB2NixByTiz ceramics 1. Introduction

Titanium diboride (TiB2) exhibits a combination of an extremely high melting point, high hardness, and low density (~2730 C, 35 GPa, and 4.52 g/cm3, respectively). Moreover, TiB2 has the highest lattice rigidity, as evidenced by its small CTE, poor compressibility, high Young's modulus, and the phonon component of the thermal conductivity [14]. These unique properties of TiB2 determined its wide range of applications: as armor material, cathodes in HallHeroult cells for primary aluminum smelting, and electrode materials in metal melting. It is also used as a constituent in materials for cutting tools and coatings for protection against hightemperature corrosion, in seals, wear parts, and parts operating under high-temperature conditions. Titanium diboride is a particularly useful constituent of composite materials, the addition of which increases the strength and fracture toughness of the matrix [1, 48].

In view of the high melting point of TiBB2, it is usually sintered by hot pressing [911]. At present, the following modern methods of high-temperature sintering are used: plasma and microwave sintering, hot isostatic pressing, high-pressure high-temperature sintering, spark plasma sintering, and combustion synthesis [1217]. To reduce its sintering temperature (Tsint ~ 2/3Tmelt), additives of different types are introduced into TiB2 powders [1824]. Using additives, one can obtain ceramic specimens even at Tsint ~ 1600 C. A large number of previous investigations were devoted to NiTiB2-based ceramics [18, 2530]. In the present

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paper, the formation of a TiB2-based composite with an addition of Ni in the form of an aqueous NiCl2 solution was investigated.

Taking into account the high hardness and the difficulty of mechanical machining of titanium diboride, in this investigation, we used laser drilling. Moreover, an investigation of the main physical and chemical processes occurring in the laser irradiation zone was carried out.

2. Experimental Technique

Titanium diboride powders were prepared by the reaction of titanium oxide, boron carbide, and an addition of carbon black in. As a result of this synthesis, aggregates of TiB2 grains of different sizes (~ 150 m) formed. The resultant powder was added into an aqueous NiCl solution 2 to obtain a 95 wt. % TiB25 wt. % NiCl2 mixture. Ceramic specimens were prepared from the dried mixtures by hot pressing in vacuum (104105 mm Hg) at 1800 C, 1900 C, and 2000 C for 45 min. The obtained cylindrical specimens had a diameter d = 5 mm and a length l = 10 mm. Note that the decomposition of NiCl2 takes place at T > 700 C.

Drilling of the specimens was performed by laser machining in a pulse irradiation regime (l = 1064 nm) in air on an YLPN501204005 installation. The pulse energy was 31 J, and the pulse duration was 15 ms. The diameter of the laser spot was 0.3 mm. Ablation products were deposited on quartz collective plates. Such plates were located in parallel with the surface of a target at a distance of 20 mm from it.

An Xray diffraction (XRD) examination of the obtained specimens was carried out in Cu K radiation with a Bruker D8 Advance diffractometer. An electron microscopy study and an electronprobe microanalysis (EPMA) were performed on an HU200F type scanning electron microscope and LEO 1450 VP unit. The mechanical properties (microhardness) were determined by using a LECO LM-300AT microhardness tester and a Vickers indenter under a load of 10 N with a holding time of 15 s. An Auger electron spectroscopy study was carried out on a PHI 670xi Scanning Auger Nanoprobe (Physical Electronics Inc.) at an accelerating voltage of the primary electron beam of 5 kV and a primary current I = 18 mA. The ion etching by Ar ions was performed at an accelerating voltage of 2 kV and a current of 0.5 mA.

p+

3. Results and discussion 3.1. Characterization of ceramics

According to the XRD analysis data, the main phase of the ceramics under

investigation is TiB2. However, with increase in the sintering temperature up to 1900 C, the Ni3B phase appears (Figs. 1, 2 a). This corresponds to the beginning of the interaction between titanium diboride and nickel. The interaction is further enhanced at Tsint. > 1900 C. At a sintering temperature Tsint.= 2000 C, the appearance of weak diffraction lines of the nickel-containing compounds Ni3B and NiB, and titanium boride is observed. In the XRD pattern of the specimen obtained at Tsint.= 2000 C, lines of TiB2 shifted to higher angles, which indicates a decrease in the lattice parameters of titanium diboride due to the loss of boron and titanium, which are consumed on the formation of a new compound (Fig. 2 b). An abrupt decrease in the intensity of the (101) peak of TiB2, which coincides with the (111) peak of Ni, confirms the formation of TiB and a new Ni-containing phase at 2000 C (Fig. 2 a). The formation of a metal and a boron vacancy in the crystal lattice of TiB2 during the interaction with nickel is confirmed by the change in the ratio of the reflection intensities I(110)/I(100) of TiB2 [31]. This intensity ratio is assured by the metal and boron ions scattering [31]:

M. Vlasova et al./Science of Sintering, 48 (2016) 137-146 ___________________________________________________________________________

139

+

=

+

+

CuKB

CuKTi

CuKB

CuKTi

ff

ffC

II

100100

110110

100

110

sinsin

sin2sin

2

2

(1),

where: I is the intensity of the diffraction peak; C is a coefficient; fTi2+ and fB are the scattering factors for Ti and B ions.

Fig. 1. Xray diffraction patterns of specimens sintered at 1800 C (a), 1900 C (b),

and 2000 C (c).

Fig. 2. Change in the intensity of the diffraction peaks (a) and d(102) for TiB2 (b) depending on

temperature sintering. On a: (1) for TiB2 (d = 0.20278 nm); (2) for Ni3B (0.19689 nm).

The increase in the intensity ratio (1) may be attributed to the predominant formation of titanium vacancies. For the ceramics sintered at temperatures of 1800, 1900, and 2000 C, the ratio I(110)/I(100) for TiB2 is equal to 0.45, 0.6, and 0.8, respectively. Obviously, the increase in the sintering temperature from 1800 to 1900 C causes an increase in the aforementioned ratio, which is about 30%. As noted above, two additional NiB phases, namely, Ni3B and Ni4BB3, form simultaneously. At a temperature of 2000 C, the interaction becomes even more intensive. The results of the analysis of phase formation in the sintered material also indicate


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the appearance of the following phases of the TiB and the NiB system: TiB, Ti3B4B (traces), NiB, and Ni3B.

Fig. 3. SEM micrographs of a TiB2 surface. (a, a). Specimen sintered at 1800 C, (b, b) specimen sintered at 2000 C. sint = 45 min. (a, b) secondary-electron image.

Fig. 4. SEM micrographs of a surface area of a TiB2 specimen sintered at 1900 C (a) and

local microanalysis (b) at the points marked in Fig. 4 a.


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From the micrographs of specimens (Fig. 3) it is clear that an increase in the sintering temperature is accompanied by the associationconsolidation of TiB2 grains (dark gray), a decrease in the pore (black) size, the disappearance of large regions of nickel (white), and the formation of thin layers of a new intermediate compound (gray) between TiB2 grains. A thorough microanalysis showed that in moving from the TiB grain to the zone of Ni 2localization, the contents of boron, titanium, and nickel change gradually (Figs. 4, 5). In the vicinity of TiB2 grains, the boron content (CB) is higher than inside grains (TiBB 2 contains ~68.8 wt. % Ti and ~34 wt. % B) . Along with B and Ni, in the interlayers, Ti is registered(see Figs. 4 a, b) It can be concluded that the diffusion of boron into the region of nickel .localization takes place. Along with NixByB [2], NixBByTiz ternary compounds [3234] can form (see Figs. 5 a, b). As can be seen from Fig. 6, with the increase in Tsint., the distribution of titanium and boron atoms becomes more uniform in the whole volume of the material. The distribution of nickel atoms is somewhat different: at 1800 C, Ni atoms are distributed along the grain boundaries of TiB2B ; at 1900 C, the initial stage of formation of clusters of Ni atoms is noticed; at Tsint. ~ 2000 C, denser clusters form. These transformations agree with the XRD data, namely, with the gradual formation of Ni3B.

Fig. 5. Distribution of elements in a thin section of a TiB2 specimen sintered at 1900 C.

This means that borides of different composition, namely, TiB, Ti3BB4, and others, can

form [2]. Thus, with increase in the sintering temperature, layers consisting of different nickel

and titanium borides, and a TiB2Ni composite form along grain boundaries of TiB2. Since with increase in the sintering temperature, the titanium atoms are registered in

the whole volume of the specimen, it can be concluded that, after the diffusion of boron into the nickel melt, the diffusion of titanium into the nickel melt enriched in boron should occur.

The results of testing the mechanical properties of the specimens are presented in Tab. I. It should be noted that the hardness (HV) increases proportionally with the sintering temperature (on the average, by nearly 25 percent). These results correlate with the obtained data, which indicates that, in sintering, the TiB2 grain growth and the formation of intergranular NixBByTiz interlayers occur as Tsint increases. The increase in the microhardness was also caused by the formation of microstresses in the TiB2 crystalline structure at a sintering temperature 2000 C. This is reflected on diffraction spreading of titanium diboride peaks. As it is seen from Tab. I, in specimens obtained at Tsint. = 1800 C, HV is close to the hardness of pure (unalloyed) TiB2, whereas at Tsint. > 1800 C, HV increases with increasing content of Ni3B (or, more precisely, NixByB Tiz). The appearance of NixBByTiz interlayers can be considered as the formation of self-bonded titanium borides, by analogy with the self-bonded silicon carbide [35-37].


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Fig. 6. Distribution of elements in analyzed regions of ceramics.

Tab. I Hardness behavior of sintered specimens. tsint. = 45 min.

Tsint., oC Vickers Hardness, GPa 1800 25.76 1900 31.06 2000 34.54

Note: for pure TiB2 25 GPa [3]; for diamond (60 -120) GPa [38]

3.2. Laser machining of TiB2 ceramics

Since TiB2 is a brittle material, cracks are initiated in it under loading, (Fig. 7), which complicates its drilling by traditional methods. For this reason, it was of interest to carry out laser machining of such ceramics. In laser drilling in a selected pulse mode for 10 h, a crater with a depth of 500 m and a diameter in the upper part of the hole of ~800 m was formed (Fig. 8). Figure 8 a shows that from the zone of laser irradiation the "eruption" of products in


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the liquid and the vapor state occurs. The first of them form a rampart around the hole, and the second of them are deposited on the surface of the specimen and glass substrate.

Fig. 7. View of a TiB2 grain after loading by a diamond pyramid. Tsint. = 2000 C.

Fig. 8. View of a laser drilling zone of a TiB2 specimen. (a, b) at different magnifications.

Fig. 9. Surface of TiB2 specimens (a, c) and Auger analysis (b, d) for the places marked in (a)

and (c). (a) Before ion etching; (b) after ion etching for 1 h.


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The Auger spectroscopy data and ion etching (Figs. 9, 10), showed that part of precipitated products of ablation are firmly bonded to the surface of the specimens. Since drilling is carried out in air, during passage of ablation products (such as Ti and B), the absorptionadsorption process of gases present in the atmosphere (O, N, and CO2) occurs. As a result of their deposition on the hot surface, not only titanium and boron oxides, but also more complex compounds such as titanium and boron oxycarbides and oxynitrides can form. This is why they are difficult to remove during ion etching from the surface (see (Figs. 8, b, c). Ablation products that deposit on the surface of the specimen later from "cold zones of flight" [38] can be easily removed. Directly in the region of drilling (in the crater) (Fig. 8 b), the appearance of pores indicates the melting and boiling of the ceramic material. This means that the heating temperature of TiB2 exceeds 3000 C. However, the small depth of the hole obtained in this mode suggests that it is necessary to use more powerful sources of laser drilling.

Fig. 10. View of ablation products on a substrate (a) and Auger analysis (b, d) of ablation

products. (b) Before ion etching; (c) after ion etching for 1 h. 4. Conclusions

The performed investigation has shown that, during hot pressing of TiB2 powder with a NiCl2 additive in the temperature range 18002000 C, not only sintering and TiB2 grain growth, but also the diffusion of boron atoms and subsequent diffusion of titanium ions into the intergranular space between TiB2 grains, where the nickel melt is localized, occur. This treatment has enabled us to form a heterophase structure on the basis of the main phases such as TiB2, TiB, NiB, and Ni3B. This leads to the formation NixBByTiz layers, the mechanical and


145

temperature properties of which are similar to those of TiB2. The drilling of such high-strength ceramics can be performed by laser machining.

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: 95 wt. % TiB25 wt. % NiCl2By 18002000 C , . : , TiB2, , NixByTiz TiB2. TiB2. . : TiB2Ni, , TiB2NixByTiz

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formation and properties of tib2–ni composite ceramics · electron microscope and leo 1450 vp...

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