Gd₂O₃ (99.9% purity, supplied by M/s Advanced Technology Materials, Mumbai), B₄C (78.5% B, 19.5% C, < 1% O, 0.02% Fe, 0.02% Si; 5.34 μm particle median diameter; supplied by M/s Boron Carbide India) and petroleum coke (99.4% purity; 13.9 μm median particle diameter, supplied by M/s Assam Carbon, India) were used as starting materials. The starting materials were characterized by X-ray diffraction (XRD) analysis using Cu Kα (λ = 1.5404 Å) radiation; this method has a detection limit of 4 vol%. Fig. 1 presents the XRD patterns of the starting powders, and confirms that there is no impurity phase in the starting powders, within the detection limit of XRD.

For the synthesis of GdB₄, weighed quantities of gadolinium oxide, boron carbide and carbon in stoichiometric ratios were mixed thoroughly in a motorized mortar and pestle. The powder mixture was then pelletized under a pressure of about 200 MPa to obtain green pellets of 12 mm diameter. The pellets were then placed in a graphite crucible and heated in an induction furnace at a fixed temperature between 1200 to 1500°C for dwelling period of 2 h under a dynamic vacuum of 2 × 10⁻⁵ mbar. The temperature of the charge was measured using a two-colour pyrometer with an accuracy of ± 20°C. After the dwelling period, the furnace was allowed to cool to room temperature under vacuum; then, the partially sintered pellets were taken out, crushed and ground to a fine size using a high energy tungsten carbide lined cup grinding mill.

Major phases of powders were identified by XRD using Cu Kα (λ = 1.5404 Å) radiation. Carbon and oxygen were measured by combustion method and by inert gas fusion (IGF) method, respectively. Particle morphology was examined by scanning electron microscope (SEM).

In the pressureless sintering (PS) experiments, GdB₄ powder was cold compacted under a pressure of about 280 MPa to form pellets of 12 mm diameter and 5 mm height with a green density of ~ 62% of the theoretical density (ρ_th). These compacts were sintered at a temperature between 1200 to 1800°C under a dynamic vacuum of 2 × 10⁻⁵ mbar in an induction furnace for holding periods of 2 h. In the hot pressing experiments, fine powders were hot pressed under a vacuum of 2 × 10⁻⁵ mbar in a high density graphite die (17 mm hole) by applying 35 MPa pressure at 1700°C for 2 h. The densities of the pellets were measured by the Archimedes principle. The fracture surfaces of the dense pellets was examined by SEM. The samples were broken by hammering to obtain a fracture surface.

Densified samples were polished to a mirror finish using diamond powder of various grades from 15 to 0.25 μm in an auto polisher (laboforce-3, Struers). Microhardness was measured at a load of 100 g and dwell time of 10 sec. Indentation fracture toughness (K_Ic) data were evaluated by crack length measurement of the crack pattern formed around the Vickers indents (using 10 kg load), adopting the model formulation proposed by Anstis et al.¹⁶⁾ K_IC = 0.016(E/H)^1/2P/c^3/2, where E is the Young’s modulus, H the Vickers’s hardness, P the applied indentation load, and c the half crack length.

Isothermal oxidation study of the dense GdB₄ was carried out at a temperature of 900°C for different time intervals. A hot pressed pellet of diameter 17 mm was cut into thin slices of 3 mm thickness. All the surfaces of the cut sample were ground with emery papers (1/0, 2/0, 3/0, 4/0) and finally polished with diamond paste to achieve a 1 μm finish. Oxidation tests were conducted in a resistance heated furnace. In order to avoid oxidation during heating, the sample was directly inserted into the hot zone after the furnace had reached the set temperature. Sample was placed in an alumina crucible loaded inside the furnace. The sample was oxidized in air for different time intervals (4, 8, 16, 32) at the set temperature of 900°C. The sample was carefully weighed to an accuracy of 0.01 mg before and after exposure, to determine the weight change during the oxidation process. The morphology and nature of the oxide layer were established by observing the surface in a field emission scanning electron microscope (FE-SEM) with energy dispersive spectroscopy (EDS). The phases present on the oxidized surface were analysed by XRD.

Table 2 presents the processing parameters and results of synthesis experiments carried out in a vacuum induction furnace. At 1200 and 1300°C, weight loss noticed is about 5.7% and 16.8%, respectively, which values are lower than the theoretical weigh loss (17.3%), indicating that the reaction had started but did not complete. The product was analysed and found to contain Gd₂O_3, GdB₄ and GdB₆ phases. The presence of Gd₂O₃ confirms the incomplete reduction. Sample processed at 1400°C was observed to contain GdB₄ and GdB₆ phases. Only at a temperature of 1500°C was single phase GdB₄ obtained. The carbon and oxygen content of GdB₄ were measured and found to be 0.5 wt.% each.

Figure 2 presents the XRD patterns of the samples processed at different temperature. The XRD patterns reveal that the product obtained at 1500°C is pure GdB₄, whereas products obtained at lower temperatures also contain the Gd₂O₃ and GdB₆ phase. The GdB₆ phase could have formed via the following side reaction (reaction (2)).

At higher temperatures, the reaction kinetics for reaction (1) was fast and all the Gd₂O₃ was converted to GdB₄. In the Gd-B system, GdB₄ is the most stable phase. Free energy calculations for reactions (1) and (2) could not be done as thermodynamic data for GdB₄ and GdB₆ are not available.

Though the reaction starts at a lower temperature (< 1200°C), pure GdB₄ is obtained only at 1500°C. At low temperature, the reaction kinetics is very slow and hence a high temperature is required for faster diffusion of the solid reactant atoms through the product that formed at the interface.

The scientific literature on the processing of GdB₄ is scant, which was the motivation for this work. A summary of past studies is provided in Table 3.^5,11,12) The borothermic reduction process involves the use of elemental boron, which is expensive and pyrophoric. Preparation of GdB₄ by boron cage compounds and metal hydrides requires one additional step, as these materials are not readily available. Handling of metal hydride is also difficult due to its pyrophoric nature. Preparation by reaction between Gd₂O₃, B₄C and carbon seems to be the best process as it involves readily available charge material and does not have any handling issues. The process is also suitable for large scale production of boride powder. In this study, single phase GdB₄ was obtained.

Synthesis of other rare earth borides by this route has been reported in literature. Zaykoski et al. have prepared aYB₄ ceramic by this route at 1800°C in helium atmosphere.¹⁷⁾ Liu et al.¹⁸⁾ have reported that NdB₆ powder can be prepared by boron carbide reduction of Nd₂O₃ at 1500°C in vacuum. Xu et al.¹⁹⁾ have prepared LaB₆ by this route at 1650°C at atmospheric pressure. Sonber et al. have prepared LaB₆, EuB₆ and CeB₆ by this route.^13,20,21)

The particle size distribution of synthesized GdB₄ powder that was ground using a high energy vibratory cup mill is presented in Fig. 3, which shows a bimodal distribution of particles with peaks at 2.5 mm and 10 mm. Mean particle diameter was found to be 5.6 mm. An SEM image (Fig. 4) of the powder shows particle agglomerates of 4 - 7 mm size.

Results of pressureless sintering and hot pressing experiments are summarized in Table 4. Results of pressure less sintering at temperatures upto 1600°C have shown that the obtained density is close to the green density of 63%, and that no densification took place during sintering. At 1800°C, densification started and a density of 77.1% of the theoretical value was obtained. The presence of high porosity (~ 23%) after sintering at 1800°C indicated that the densification of GdB₄ by pressureless sintering is extremely difficult. Hot pressing of GdB₄ at 1700°C under 35 MPa pressure resulted in a density that was 95.5% of the theoretical density.

The poor sinterability observed in monolithic GdB₄ is due to this material’s high refractoriness and low intrinsic self diffusivity, which are also observed in other borides.^13,14) Arabei et al.¹⁵⁾ have reported that the densification processes of hexaborides becomes activated at temperatures in the range of 1800 - 2000°C. Zaykoski et al.¹⁷⁾ have prepared fully dense YB₄ by hot pressing at 1800°C.

Figure 5 provides an SEM image of the fracture surfaces of the pressureless sintered samples. Porosity is present in all the samples. The image also shows that there is no grain growth in the sample processed at 1200 and 1400°C. Grain growth was observed in the sample processed at 1800°C. Fig. 6 shows the fractured surface of the hot pressed GdB₄; this surface shows a dense microstructure.