Formation and Properties of TiB 2 – Ni Composite Ceramics

An analysis of physical and chemical processes occurring during hot pressing of the 95 wt. % TiB2–5 wt. % NiCl2By powder mixture in the temperature range 1800–2000 °C has been performed by X–ray 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.


Introduction
Titanium diboride (TiB 2 ) exhibits a combination of an extremely high melting point, high hardness, and low density (~2730 °C, 35 GPa, and 4.52 g/cm 3 , respectively).Moreover, TiB 2 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 [1][2][3][4].These unique properties of TiB 2 determined its wide range of applications: as armor material, cathodes in Hall-Heroult 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 high-temperature 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,[4][5][6][7][8].
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.

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 TiB 2 grains of different sizes (~ 1-50 μm) formed.The resultant powder was added into an aqueous NiCl solution 2 to obtain a 95 wt.% TiB 2 -5 wt.% NiCl 2 mixture.Ceramic specimens were prepared from the dried mixtures by hot pressing in vacuum (10 -4 -10 -5 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 NiCl 2 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 YLPN-50-120-400-5 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 X-ray diffraction (XRD) examination of the obtained specimens was carried out in Cu K α radiation with a Bruker D8 Advance diffractometer.An electron microscopy study and an electron-probe microanalysis (EPMA) were performed on an HU-200F 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 +

Characterization of ceramics
According to the XRD analysis data, the main phase of the ceramics under investigation is TiB 2 .However, with increase in the sintering temperature up to 1900 °C, the Ni 3 B 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 T sint.> 1900 °C.At a sintering temperature T sint.= 2000 °C, the appearance of weak diffraction lines of the nickel-containing compounds Ni 3 B and NiB, and titanium boride is observed.In the XRD pattern of the specimen obtained at T sint.= 2000 °C, lines of TiB 2 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 TiB 2 , 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 TiB 2 during the interaction with nickel is confirmed by the change in the ratio of the reflection intensities I (110) /I (100) of TiB 2 [31].This intensity ratio is assured by the metal and boron ions scattering [31]:   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 TiB 2 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 Ni-B phases, namely, Ni   From the micrographs of specimens (Fig. 3) it is clear that an increase in the sintering temperature is accompanied by the association-consolidation of TiB 2 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 TiB 2 grains.A thorough microanalysis showed that in moving from the TiB grain to the zone of Ni 2 localization, the contents of boron, titanium, and nickel change gradually (Figs. 4, 5).In the vicinity of TiB 2 grains, the boron content (C B ) is higher than inside grains (TiB  This means that borides of different composition, namely, TiB, Ti 3 BB 4 , and others, can form [2].
Thus, with increase in the sintering temperature, layers consisting of different nickel and titanium borides, and a TiB 2 -Ni composite form along grain boundaries of TiB 2 .
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 TiB 2 grain growth and the formation of intergranular Ni x BB y Ti z interlayers occur as T sint increases.The increase in the microhardness was also caused by the formation of microstresses in the TiB 2 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 T sint.= 1800 °C, HV is close to the hardness of pure (unalloyed) TiB 2 , whereas at T sint.> 1800 °C, HV increases with increasing content of Ni 3 B (or, more precisely, Ni x B y B Ti z ).The appearance of Ni x BB y Ti z interlayers can be considered as the formation of self-bonded titanium borides, by analogy with the self-bonded silicon carbide [35][36][37].

Laser machining of TiB 2 ceramics
Since TiB 2 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 μm and a diameter in the upper part of the hole of ~800 μm was formed (Fig. 8). Figure 8 a shows that from the zone of laser irradiation the "eruption" of products in 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.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 absorption-adsorption process of gases present in the atmosphere (O, N, and CO 2 ) 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 TiB 2 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.

Conclusions
The performed investigation has shown that, during hot pressing of TiB 2 powder with a NiCl 2 additive in the temperature range 1800-2000 °C, not only sintering and TiB 2 grain growth, but also the diffusion of boron atoms and subsequent diffusion of titanium ions into the intergranular space between TiB 2 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 TiB 2 , TiB, NiB, and Ni 3 B.This leads to the formation Ni x BB y Ti z layers, the mechanical and temperature properties of which are similar to those of TiB 2 .The drilling of such highstrength ceramics can be performed by laser machining.
is the intensity of the diffraction peak; C is a coefficient; f Ti 2+ and f B -are the scattering factors for Ti and B ions.
3 B and Ni 4 BB 3 , 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 the appearance of the following phases of the Ti-B and the Ni-B system: TiB, Ti 3 B 4 B (traces), NiB, and Ni 3 B.

Fig. 4 .
Fig. 4. SEM micrographs of a surface area of a TiB 2 specimen sintered at 1900 °C (a) and local microanalysis (b) at the points marked in Fig. 4 a.
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 Ni x B y B [2], Ni x BB y Ti z ternary compounds [32-34] can form (see Figs. 5 a, b).As can be seen from Fig. 6, with the increase in T sint., 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 TiB 2 B ; at 1900 °C, the initial stage of formation of clusters of Ni atoms is noticed; at T sint.~ 2000 °C, denser clusters form.These transformations agree with the XRD data, namely, with the gradual formation of Ni 3 B.

Fig. 5 .
Fig. 5. Distribution of elements in a thin section of a TiB 2 specimen sintered at 1900 °C.

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

Fig. 8 .
Fig. 8. View of a laser drilling zone of a TiB 2 specimen.(a, b) at different magnifications.

Fig. 9 .
Fig. 9. Surface of TiB 2 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.

Fig. 10 .
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.