Improvement of Boron Carbide Mechanical Properties in B 4 C-TiB 2 and B 4 C-ZrB 2 Systems

Experimental works have been conducted the objective of which was to improve mechanical properties of boron carbide by introduction of doping elements into the system. Titanium and Zirconium were selected as doping elements, which were introduced into the system in the form of TiB2 and ZrB2. Four types of boron carbide-titanium and zirconium mixture with various titanium and zirconium diboride content were used in experiments. Optimal process parameters, as well as doping elements concentration, necessary to provide required high mechanical parameters in the composite were defined.


Introduction
Due to outstanding features of its basic componentoron, boron carbide (B 4 C), finds wide application in various fields, and in particular, thanks to its unique nuclear features it is widely used in control rods of nuclear power plants, as neutron absorbing material, as hard material (9.35 by Moos scale) it finds application in abrasive and finishing materials [1][2][3], in nozzles of sand-blasting and gas-jet facilities.Boron carbide preserves its hardness at high temperatures, which enables to use it at temperatures up to 2000˚C [4,5].
One of the ways to increase impact elasticity is to dope boron carbide with metal elements, which provides the growth of free electron fraction in boron carbide.In this work impact elasticity growth was achieved by introduc-tion of titanium and zirconium diborides additives into B-C system.

Experimental
Samples of pure boron carbide as well as samples of containing titanium and zirconium diborides were prepared.Boron carbide powder produced by German company H.C.Stark was used in experiment.Boron carbide powder specifications are presented in Table 1.
Quantities of powders were selected in such way that doping metal weight portion in the composite made 1%, 3% and 5% which is equivalent to 1.45, 4.35 and 7.25 wt% of titanium diboride and 2.35, 7.05 and 11.75 wt% of zirconium diboride.Fine-grained graphite, type APB, with inner surface covered with graphite foil "Sigraflex" was used for press moulds.The powders were pressed by hot pressing method in vacuum at 2150˚C -2200˚C temperature and 20 -25 MPa pressure, pressing duration at final temperature was 5 -8 min.
In order to determine each property, the separate composition cylinders with dimensions Ø70 × 5.7 mm were pressed and various size samples were cut out to measure water absorption, open porosity, density, impact elasticity, thermal expansion coefficient, compressive and bending resistance.Samples surfaces were processed with fine grain diamond grinding wheel.
Density of the samples made about 96% of theoretical value.X-ray-structural analysis was done using DRON-3 diffractometer.X-ray pattern clearly shows boron carbide and titanium diboride sharply drawn peaks.Figure 2 presents diffraction patterns of boron carbide and titanium diboride, while Figures 3 and 4 present X-ray patterns of boron carbide doped with titanium and zirconnium, respectively.
Diffraction patters prove that chemical interaction of boron carbide and doping compound does not give some new phase.The material is two phased which is also proved with electron microscope research.
Figures 5 and 6 present the results of electron microscope research.Test was performed on scanning microscope SEM Cam Scan.The surfaces are not but the figures clearly show that material consists of two phases-   Besides, the samples were tested for bending strength and compressive resistance, their thermal expansion co-efficients, hardness, microhardness and impact elasticity were measured.The obtained results are presented in Table 2.
It is seen from Table 2 that when doped with titanium and zirconium diborides, mechanical properties of boron carbide increased.Especially remarkable is the increase of mechanical compressive strength and impact elasticity.The increase of impact elasticity is the main purpose of Table 2.The results of samples testing.

Specification B 4 C B-C-T, 1% Ti B-C-T, 3% Ti B-C-T, 5% Ti B-C-Zr, 1% Zr B-C-Zr, 3% Zr B-C-Zr, 5% Zr
Water   ), cracks are not observed, accordingly, there happens no energy dissipation [16,17].Here is noticed elastic property of material (Table 2) to endure loading so that not generate cracks and not turn loading energy into dissipation phenomena.The unalloyed boron carbide pressed with the same technological mode at 2150˚C is characterized with more porosity than doped one (Figure 8(a)).The imprint shows lateral cracks in right upper and lower corners.In spite of equal loading (0.5 N) the imprint form is not clear compared to the sample doped with zirconium diboride.Energy dissipation is explicit which decreased to some extent the sharpness of imprint contours.The figures show that matrix as well as second phase grain dimension is of micron size which is one of the preconditions of high mechanical strength.Zirconium diboride (white grains) is equally apportioned in matrix which creates dispersive strengthened composite material.This enables significant improvement of ceramic composite decomposition elasticity (Kic), also of strength (Table 2) and wear resistance.
In large grains (35 -40 mcm, Figure 9(b)) on the whole diameter of the imprint the cracks appear.In small  g shows that the second phase is apportioned on matrix grain boundary, as well as, in grain (in large grain).At the same time around zirconium diboride inclusions cracks are not observed or crack avoids the inclusion which indicates that at doping the grain brittleness also decreases.

Conclusions
Composites B C-TiB rameters are obtained.The above composites can be used when measuring refractory material hardness at temperatures up to 2000˚C.Besides, possibility of its application in nuclear engineering, in abrasive materials, etc. also extends.Simultaneo ss parameters provides important possibility to use this material as construction ceramics.
work.The mention phen n ably conditioned with the increase of free electrons' share in boron carbide which is caused with metal solubility increase.The latter process provides the increase of elasticity preserving covalent type bonds.Only in such a case may high hardness be preserved which is clearly seen in the Table.

Figure 7
presents in diagram form the conditions and results of hardness test by Vikers method of boron carbide doped with zirconium diboride (3% Zn), while Figure 8 presents optical microscope patterns of the same sample (b) and boron carbide (a) surfaces after hardness testing.As shown in Figure 7 under 0.5 N loading indenter is implanted in material for 1.35 mcm.Test continued for 27 sec.In this case imprint form is clear, with well-defined edges (Figure 8(b)

Figure 9
Zr) and the same sample after testing for microhardness (b).The sample was contaminated with 0% water solution of NaNO 3 for 15 seconds.