Processing and Characterization of CrB₂-Based Novel Composites

Starting material

In-house synthesized CrB₂ (D₅₀: 6.7 µm, “C”: 0.9 wt.%, “O”: 0.6 wt.%) and commercial Nb (99.7% pure) powder were used as starting materials. CrB₂ powder was prepared by boron carbide reduction of Cr₂O₃ in presence of carbon. Preparation details of CrB₂ powder are presented elsewhere [15]. Figure 1 presents the XRD pattern of the starting powders.

Densification and characterization

For densification, weighed quantities of fine chromium diboride and Nb powder were mixed thoroughly using a motorized mortar and pestle in dry condition for 1 h to obtain samples of different compositions. Figure 2(a) and (b) presents the particle size distribution of CrB₂ and Nb powders respectively. CrB₂ powder has tri-modal distribution and particle size in the range of 0.1 μm to 20 μm. Median particle diameter was measured as 6.79 μm. Distribution of Niobium powder was monomodal and the particle size ranges from 10 μm to 70 μm. Median particle diameter was measured as 28.16 μm.

Figure 1

XRD pattern of starting CrB₂ and Nb powder

Figure 2

Particle size distribution of (a) CrB₂ and (b) Nb powder

The powder mixtures were then loaded in a high density graphite die (12 mm dia) and hot pressed at a temperature of 1,600°C under a pressure of 35 MPa for 2 h in a high vacuum (1×10⁻⁵ mbar) chamber. The pellets were ejected from the die after cooling and the density measured by liquid displacement method. Densified samples were polished to mirror finish using diamond powder of various grades from 15 to 0.25 μm in an auto polisher (Laboforce-3, Struers). Microhardness was measured on the polished surface at a load of 100 g and dwell time of 10 s. The indentation fracture toughness (K_IC) data were evaluated by crack length measurement of the crack pattern formed around Vickers indents (using 10 Kg load), adopting the model formulation proposed by Anstis et al. [19],

where E=Young’s modulus, H=Vickers’s hardness, P=Applied indentation load, c=Half crack length.

Elastic modulus of the samples was measured by ultrasonic method. The reported value of hardness and fracture toughness are the average of five measured values. Polished and fractured surfaces of dense pellets were analyzed by scanning electron microscope and energy dispersive spectroscopy (EDS). X-ray diffraction was also used for phase analysis.

Densification

Table 1 summarizes the results of densification experiments carried out by hot pressing. CrB₂ powder containing 0, 2.5, 10 and 20 wt.% Nb was hot pressed. A density of higher than 95% ρ_th was achieved in all the composites consolidated by hot pressing at 1,600°C and 34 MPa pressure. For monolithic CrB₂, only 89.42% density was obtained under similar hot pressing condition. The enhanced densification by addition of Nb could be due to reaction sintering caused by reaction between CrB₂ and Nb which resulted in formation of NbB₂. More details of the reaction are discussed in Section “Phase Analysis and Microstructure”. Iizumi et al. [16] have reported that chromium borides (Cr₂B, CrB and CrB₂) obtained by solid state reaction of Cr and B in the temperature range of 1,400 to 1,500°C. These powders [16] could not be consolidated by pressureless sintering process and were densified by hot pressing at about 1,700°C.

Phase analysis and microstructure

XRD patterns of sintered CrB₂ pellet processed with different content are shown in Figure 3. It indicates the presence of crystalline CrB₂ and the reaction products such as Cr₂B₃, Cr₃B₄ and NbB₂. The presence of NbB₂ in XRD pattern indicates that niobium is converted into niobium boride. Niobium boride formation takes place due to reaction of niobium with chromium boride. Required boron to form NbB₂ will be obtained from the matrix CrB₂. As a result, boron-deficient chromium boride such as Cr₂B₃ and Cr₃B₄ is formed. The formation of NbB₂, Cr₂B₃ and Cr₃B₄ can be explained by following possible reactions:

Reaction (1) is thermodynamically feasible at hot pressing temperature of 1,400°C in vacuum (1×10⁻⁵ mbar). Gibbs free energy change obtained using FactSage database for reaction 1 is presented in Figure 4. It shows that free energy change is negative for all the temperature. Elemental chromium, which formed as per reaction (1), reacts with CrB₂ and forms Cr₃B₄ (reaction 2) and Cr₂B₃ (reaction 3). Simultaneously, CrB₂ and Nb can react and result in formation of NbB₂, Cr₂B₃ and Cr₃B₄ as per reaction (4). Thermodynamic calculations for reactions (2 to 4) could not be computed as data for Cr₃B₄ and Cr₂B₃ are not available.

Figure 3

XRD pattern of hot pressed CrB₂ composites prepared by addition of Nb

Figure 4

Free energy change for reaction between CrB₂ and Niobium (calculated using FactSage software)

From XRD pattern, it is observed that as the percentage of niobium increases in chromium boride, the intensity of niobium boride is higher in composite material.

Figure 5 presents the BSE image of CrB₂-based composite prepared by addition of 20% Nb, which shows the presence of gray matrix and in which, second phase (lighter shade) is dispersed. In EDS spectra, the gray matrix was analyzed to contain only Cr and B indicating that, it is CrB₂. The lighter shade was analyzed to contain mainly Nb in which very little amount of Cr (~4 atom %) is present. Chromium, which formed as per the reaction (1), could be diffused into Nb. Other phases such as Cr₃B₄ and Cr₂B₃ could not be seen in microstructure, due to difference in atomic number is not sufficient to distinguish the contrast of different chromium boride phases.

Figure 5

SEM image and elemental analysis of different phases in CrB₂ composite prepared by addition of 20% Nb

Mechanical properties and fractography

Measured Vickers hardness, fracture toughness and elastic modulus of CrB₂ composites are presented in Table 1. Hardness of CrB₂ prepared with 2.5% Nb was measured as 18.46 GPa which increases to 21.89 GPa with increasing Niobium content to 10 mass%.This could be due to the higher hardness (25.5 GPa) of NbB₂ phase as compared to monolithic CrB₂(22 GPa) [2]. Further increase of Nb content decreased the hardness to 17.34 GPa. This could be attributed to the formation of Cr₂B₃ and Cr₃B₄ phases in significant amounts. Hardness of monolithic CrB₂ was measured as 11.45 GPa. The low hardness obtained is due to the lower density (89.42%) of the sample. Microhardness of single crystal CrB₂ has been reported as 22.6 GPa (1N load) by Okada et al. [20]. Sonber et al. [15] have reported hardness of hot pressed CrB₂ as 22 GPa.

Fracture toughness of monolithic CrB₂ was measured as 3.10 MPa.m^1/2. Fracture toughness of composites prepared by addition of 2.5 and 10 wt.% Nb was measured as 3.11 and 3.38 MPa.m^1/2 respectively. Addition of 20% Nb resulted in increased fracture toughness of 4.32 MPa.m^1/2. The increase in the fracture toughness is due to presence of second phase. Figure 6 presents the fracture surfaces of CrB₂ composites. Dense surfaces were observed in all the composites. The mode of fracture is observed to be transgranular.

Figure 6

Fracture surface of CrB₂ composites prepared by addition of (a) 2.5% Nb (b) 10% Nb (c) 20% Nb

Reinforcement of metallic particles in ceramic material enhances fracture toughness of ceramics. Addition of Nb and Mo into ZrB₂ has been reported to result in increased fracture toughness [18, 21]. This enhancement is due to plastic deformation of metals which consumes available energy. In the present study, Niobium metal was added to CrB₂ matrix to form new composite. During hot pressing, Nb has reacted with CrB₂ and formed NbB₂ and thus in composite, ductile Nb metal is not present. However, presence of NbB₂ resulted in enhancement of hardness and fracture toughness but effect is not as high as achieved by Nb addition in ZrB₂ by Sun et al. [18].