Mechanical Properties of Cr3B4 Cermets Cemented by Different Metallic Binders

Cr3B4 cermets cemented by cobalt and iron as a metallic b inder up to 20 wt% were prepared by hot pressing at 1500 and 1800°C under a pressure of 14 and 41 MPa for 1hr. The binder addition enhanced the densification of Cr3B4 cermets due to the formation of liquid metal and its flow into the pores. It is also found that the temperature and pressure have a pronounced effect on densificat ion. The densification of Cr3B4 is enhanced by the addition of cobalt and iron due to the liquid phase format ion, therefore, the hardness and fracture toughness were improved, to a certain extent. A composite with a both high Vickers hardness of 35.98 GPa and a modest fracture toughness of 2.5 MPa.m could be obtained by the addition of 8 wt% cobalt at 1500C and 14 MPa. There is an overgrowth of the boride grains which surly reduces the hardness but has less impact on the fracture toughness.


Introduction
Based on their unique physical and mechanical p roperties, particularly at elevated temperatures, binary borides have a great interest for practical and industrial applications such as thermocouple jackets, crucib les, cutting tools, and jets of airplanes and rockets (1,2). Ch ro me Boride is a pro mising can d id ate fo r stru ctu ral ap p licatio ns esp ecially th ose requ iring h igh -temperatu re st rength and stab ility under severe erosion conditions. Because the chrome boride is a b ritt le material, one o f t he met hods o f increas ing t he toughness is to alloy it with elements which form limited or unlimited substitutional solid solution with chromiu m (3). By add ing a s mall amo u nt o f metal b in der, the fractu re toughness of the boride will increase and hence improve the mechanical strength of the compact. However, the presence of the metal binder will decrease its hardness and toughness; th erefo re the amo un t o f th e metal b ind er sh ou ld be compro mised with the hardness and toughness (4)(5)(6)(7)(8). On the other hand, the presence of the metal binder can accelerates the compact densificat ion during the hot pressing process through the format ion of a liqu id phase that flo ws under pressure and fills the pores. Cobalt, Nickel, and Iron are good cand idates as b ind in g materials that can imp ro ve the mech an ical p rop erties and th e thermal res istan ce o f transition metal borides (9,10). The extent of application of the chrome borides is still quite limited because of the lack of reliable data on its behavior at high temperatures.
The aim of this work is main ly to enhance the densificat ion by adding different amounts of metallic binder to Cr 3 B 4 cermets and, thereby, study the effect of these different process variables on the mechanical properties of it.

Experimental
The Cr 3 B 4 powder imported fro m Riedal de Haen having 91 wt% Cr 3 B 4 and 9 wt% Al 2 O 3 . The theoretical density of this powder as measured by the null pycnometer was 5.059 g/cm 3 and the median particle size was 7 µm as measured by the 3D image analy zer and a surface area of 3.08 m 2 /g. The iron powder used in this work was imported fro m Merk with med ian particle size 0.5 µm, specific surface area 1.2218 m 2 /g and, a theoretical density 7.73 g/cm 3 and, purity 97.745 %. The cobalt powder density 8.22 g/cm 3 , median particle size 8.5 µm and specific surface area 1.2 m 2 /g. The Cr 3 B 4 powder was mixed with the required proportion of Fe or Co by tumb ling for 6 hrs and mounted equally to a poco graphite die having six cy lindrical holes 1 cm diameter each. It was hot pressed for 1 hr at different temperatures and pressures and stagnant Argon atmosphere. The compacts are characterized after that and the micro hardness was measured using a Shiwadzu type M No. 80413 using 50g load. The hardness values represent the mediu m of 10 measured points. The fracture toughness was evaluated by the Anstis crack length method which is dependent on the elastic modulus of the material, the micro hardness, the crack length and the applied load. Anstis et. al. (11,12) used a simplified t wo dimensional fracture mechanics analysis and obtained 2 3 where F is the load in Newtons, c is the crack length from the center of the indent to the crack tip in meters, E is the Young's modulus in GPa estimated by a nondestructive testing, and H is the Vickers hardness in GPa measured experimentally.

Hot Pressing
The relat ive densities of Cr 3 B 4 cermets cemented by cobalt and iron respectively hot pressed, at 1500 and 1800 ℃ at 14 MPa and 41 M Pa for 1hr., as a function of the binder content are shown at figures.1, 2, 3, 4. Fo r these compacts hot pressed at 1500 ℃ and 14 MPa , it is clear that the relative density increases as the binder content and or the time increases. The increase in the relative density by the addition of b inder cobalt and or iron is more pronounced than the increase of time as it may be clear fro m these figures. With the addition of more than 8 wt% both cobalt and or iron, high relative densities, over 95% were obtained. In particular, the highest value of 98% could be reached for the specimens with 12 wt% of both cobalt and or iron after one hour hot pressing at 1500℃. For the samples containing 20 wt% binder, a relative density more than 0.96 is reached at the beginning of the pressing time.
Fro m Figs. 3 and 4 it is clear that the theoretical density is obtained after one hour using 41 MPa at 1800℃ whatever the amount of binder up to 10 wt % b inder which indicates that the pressure has a pronounced effect on the densificat ion.

Microstructure
The microstructure of Cr 3 B 4 cermets polished sections, hot pressed at 1500℃, 14 MPa and cemented by 1 , 2 , 12 wt% Co and or 1 , 2 , 12 wt% Fe, is shown in fig. 5 and 6 respectively. The Cr 3 B 4 phase appears as areas of high contrast in the darker cobalt phase. It can be seen that the size and the number of pores decreased with increasing the binder content. The morphology and size of Cr 3 B 4 specimens were quite different fro m those of Cr 3 B 4 starting powder and seem that the binder in the liquid state penetrates among the Cr 3 B 4 grains. Also from the micrograph, it is clear that with the increasing of metallic binder the co mposite became denser as it clearly observed from the densificat ion curves Fig. 1 and the same trend is observed also for the Cr 3 B 4 iron cermets but the porosity is a little bit h igher.   The cracks that appeared at the corners of the indentation are used to calculate the fracture toughness of the material.
Increasing of the amount of cobalt added as a cementing material to the Cr 3 B 4 cermets, the hardness increased for both the two pressing temperatures, 1500 and 1800℃ using a pressure of 14 and 41 MPa respectively, up to 8 wt% Co.
The Cr 3 B 4 cermets exhib it the highest hardness across cobalt content taking the maximu m at 8wt%, 35.98 GPa, at 1500℃ with a pressure of 14 MPa. While the hardness showed a decreasing across the iron content added to the Cr 3 B 4 cermets taking the lowest at 12 wt% of iron at 1500 ℃ with a pressure of 14 MPa. At 1800℃ with a pressure of 41 MPa, there was no a significant change on the decreasing of the hardness values with the iron content, i.e, the hardness was decreased and reached the lowest value at 12 wt% of iron, nearly the same value for 1500℃ with a pressure of 14 MPa. This can be explained by the insufficient densificat ion which occurred during the hot pressing operation at the two different temperatures or due to the solubility of iron in the boride phase.  The fracture toughness values as calculated from equation (1) are shown in figures. 10, and 11, as a function of the binder content, cobalt and or iron, for these specimens that hot pressed at 1500℃ and 1800℃ respectively.
The fracture toughness of the specimens that has a cobalt content less than 2 wt% is very low due to insufficient densificat ion. The fracture toughness measurements for the specimens that pressed at 1500℃ with a pressure of 14 M Pa increased to 3 MPa.m 1/2 with the increasing of cobalt content to 12wt%. On the other hand, the fracture toughness increased to 2.53 MPa.m 1/2 with increasing the iron content up to 8 wt% and there after it decreased to 1.51 MPa.m 1/2 .
At the meantime, for the specimens which pressed at 1800℃ with a pressure of 41 MPa, the Cr 3 B 4 cermets exhibit an increase in the fracture toughness measurements taking the highest, 1.92 MPa.m 1/2 , at 12wt% of cobalt and 1.56 MPa.m 1/2 at 8wt% of iron.
There was an overgrowth of the boride grains occurred with increasing the cementing content and the pressing temperature and pressure surely reduces the Vickers hardness but in the meantime has less impact on the fracture toughness.

Conclusions
(1) Cr 3 B 4 cermets with different amounts of cobalt and iron as a metallic binder were fabricated by hot pressing of a powder mixture at 1500 ℃ and 1800 ℃ with an applied pressure of 14 and 41 MPa respectively.
(2) The densification of Cr 3 B 4 was enhanced by the addition of Cobalt and or iron due to the liquid phase formation, and therefore, the Vickers hardness and fracture toughness were imp roved, a change that was ascribed to the densificat ion and residual stress caused by the thermal expansion mismatch of Cr 3 B 4 and the metallic binder.
(3) A co mposite with both high Vickers hardness of 35.98 GPa and a modest fracture toughness of 2.53 MPa.m 1/2 could be obtained by the addition of 8wt% cobalt.