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Consolidation Behaviors of FeB–25Ni Powders in Spark Sintering and Mechanical Properties of Their Compacts
Shaoming KangZhefeng XuYong Bum ChoiKazuhiro MatsugiHideaki KuramotoJinku Yu
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2016 Volume 57 Issue 12 Pages 2139-2145

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Abstract

25 vol% Ni electroless-plated FeB powders were consolidated by spark sintering for the development of hard materials. The sintering curves of the FeB–25Ni powders were between those of as-received FeB without Ni addition and pure Ni powders. The maximum densification rate in the FeB–25Ni was achieved at an apparent relative density of 0.79, which was higher than that of the as-received FeB (0.6) and close to that of pure Ni (0.74). The densification of FeB–25Ni was predominantly a result of plastic deformation and power-law creep deformation of the Ni binder. The change in densification mechanism occurred roughly at the maximum densification rate. The sintering curve and densification rate of the FeB–25Ni powders could be explained by the combination of sintering curve and densification rate obtained from the as-received FeB and pure Ni powders. The increase in maximum holding temperature led to the improvement in hardness and compressive and fracture toughness properties, which resulted from the increase in apparent relative density due to the activation of diffusion at the interfaces between particles.

1. Introduction

WC–Co hard materials are widely used for the cutting tools, wear resistant, high-temperature and corrosion-resistant components13). A broad range of WC grades with different particle sizes and the addition of various types of carbides to WC have been investigated to improve the mechanical properties of WC–Co hard materials4). However, the application of such hard materials has been limited due to the low reserves and poor refining efficiency of W. Simultaneously, Co is low reserves and expensive5). Furthermore, the binder phase has issues related to its environmental toxicity6). Therefore, the development of environmentally friendly hard materials consisting of ubiquitous elements to substitute for WC–Co is necessary.

Metal carbides, nitrides and borides are commonly used as hard materials7). Matsugi et al. reported that inexpensive iron borides (FeB and Fe2B) could act as substitutes for WC810). However, the sintering of iron borides to full density is very difficult without a binder phase. The achievement of full density requires the application of high temperature and long holding times, which leads to high energy consumption and low productivity. However, sintering can be enhanced by introducing binder phases such as Fe or Ni11,12). Actually, the use of FeB with Fe hard materials consolidated by spark sintering was investigated9), where the FeB–Fe powders were mixed by an elemental powder blending method. Although a high apparent relative density was obtained, a large number of pores were present between the FeB hard phases. It was considered that the heterogeneous distribution of the Fe binder in the FeB–Fe mixed powders resulted in the residual irregular-shaped pores.

Electroless Ni plating solves the abovementioned drawbacks. Electroless Ni plating is a well-established method to obtain homogeneous distribution of Ni over the entire substrate surface, regardless of the shape and size of the substrate13). Furthermore, the coating of Ni on the surfaces of FeB particles prevents direct contact between the hard phases, making it possible to obtain full density. In addition, spark sintering, which has the advantage of rapid densification and energy saving8,1416), is introduced to consolidate the FeB–25 vol% Ni (hereafter called FeB–25Ni) powders.

In this study, FeB–25Ni powders were prepared by electroless plating. The powders were then consolidated by spark sintering for the development of hard materials consisting of ubiquitous elements. The relation between the process parameters and characteristics of the powders were also investigated to elucidate the consolidation behaviors of FeB–25Ni powders.

2. Experimental Procedure

2.1 Powder preparation

FeB powders with a composition of 78.4% Fe, 19.1% B, 0.04% C, 0.48% Si, 0.02% P, 0.003% S, and 1.9% Al (mass%) were received from Fukuda Metal Foil and Powder Co. Ltd. The constructed phases of the as-received FeB powders were mainly FeB and Fe2B. The powders showed a mean particle size of 45 μm. The powders were cleaned by acetone and immersed in hydrochloric acid (50 mL/L) for 60 s for activation. Then, the activated FeB powders were put into a beaker placed in a water bath for electroless plating. The plating solution, which contained less than 1% B, was purchased from Okuno Chemical Industries Co. Ltd. During the electroless plating, the plating solution was continuously stirred to ensure the uniform dispersion of powders. The pH and temperature of the electroless plating bath were 6.5 and 333 K, respectively17). After the electroless plating, the powders were cleaned with de-ionized water and ethanol and dried in a vacuum oven at 323 K for 7.2 ks.

2.2 Spark-sintering process

The FeB–25Ni powders were consolidated by spark sintering. The powders were placed in a graphite die (outside diameter, 40 mm; inside diameter, 10 mm; height, 60 mm) with two graphite punches (diameter, 10 mm; height, 30 mm) and then introduced into the spark-sintering system. The spark-sintering process had two modes, a pulse discharge sintering mode and a continuous discharge sintering mode. The two modes were applied to all sintered compacts. In other words, all compacts were consolidated by the continuous discharge mode after the pulse discharge mode. In the pulse discharge sintering mode, the powders were spark sintered for 0.9 ks with a pulse current of 100 A, pulse-discharge and pulse-cut times of 0.1 s, and a punch pressure of 15 MPa. In the continuous discharge sintering mode, the following steps were used. First, a heating rate of 0.83 K/s was used to achieve temperatures of 1183, 1233, and 1283 K, which were 90 K lower than the maximum temperatures of 1273, 1323, and 1373 K. Then, a heating rate of 0.083 K/s was used to achieve the maximum temperatures without overshooting. The compacts were kept at the maximum temperature for 0.6 ks. Finally, the compacts were cooled to 293 K in the chamber. A uniaxial pressure of 50 MPa was applied for this mode. The spark-sintering process was carried out under vacuum (<10−2 Pa). The sintering temperature was measured using an R-type thermocouple inserted into the die cylinder. The thermocouple tip was approximately 2 mm from the compact. The temperature of a compact was estimated from that of the die in the spark-sintering process using a method that had been reported earlier18).

2.3 Characterization

The densities of the compacts were measured by Archimedes' method. The apparent relative density was calculated by dividing the height of the compacts by the ideal height of 10 mm. The thermal analysis was carried out by differential scanning calorimetry (DSC, NETZSCH STA 449 C, Germany) at a constant heating and cooling rate of 0.083 K/s at 298–1000 K in an atmosphere of pure argon with a flow rate of 0.83 mL/s. The microstructures of the compacts were observed by scanning electron microscopy (SEM, TOPCON SM-520; Japan). The quantitative analysis was conducted on selected compacts using an electron probe micro analyzer (EPMA, JXA-8900; Japan). The porosity of compacts was measured by the image analysis method19). Phases in the compacts were characterized by the X-ray powder diffraction method (XRD, D/max-2500/PC, Japan) using Cu Kα radiation (λ = 15.406 nm) at 40 kV and 0.1 A. The hardness of the compacts was measured by a Rockwell hardness tester (A scale, HR-40752, Japan). The fracture toughness, KIC, of the FeB–25Ni compacts was determined by measuring the crack length near the indent made by a Vickers indentation (MHT-1, Japan) load of 9.8 N and calculated by using the following eq. (1)20):

 $K_{\rm IC} = 0.064\sqrt{\frac{E}{H}} P/c^{3/2}$ (1)

The Young's modulus, E, was 307 GPa according to the rule of mixtures calculation between the Young's modulus of FeB and that of Ni. The Young's moduli of FeB and Ni were approximately 343 GPa and 200 GPa, respectively21). H is the Vickers hardness, P is the indentation load, and c is the half of the indentation crack length near the indent. The compressive strength of the compacts was measured at room temperature by using a mechanical testing machine (Autograph DCS-R-5000, Shimadzu Corporation, Japan) with a constant crosshead speed of 0.83 μm/s.

3. Results and Discussion

3.1 Characteristics of the electroless-plated Ni layer

Figure 1(a) shows a SEM image of the as-received FeB powder of Fe–19 mass% B, which consisted of both FeB and Fe2B phases, as shown in Fig. 2(a). The SEM image of the as-received FeB powder shows irregular shapes and sharp surface. Figure 1(b) shows the morphology of the FeB particles after electroless Ni plating, which consisted of both FeB, Fe2B and Ni phases, as shown in Fig. 2(b). Furthermore, the Ni peak is broad, which indicates the presence of an amorphous Ni layer. Figure 1(c) and (d) show SEM image and the corresponding Ni concentration profile of the cross section of the FeB–Ni powder, which means the homogeneous Ni plating on FeB particles. The Ni content of the electroless-plated FeB powder was controlled to be 25 vol% by adjustment of the plating conditions17). Figure 3 shows part of the DSC heating curve obtained from the FeB–25Ni powders. There is an exothermic peak between 700 K and 873 K, which is considered to correspond to the crystallization of the Ni layer22).

Fig. 1

SEM images of (a) as-received FeB and (b) FeB–25Ni powders. (c) SEM image and (d) Ni concentration profile of the cross section of the FeB–25Ni powder.

Fig. 2

X-ray patterns of (a) as-received FeB and (b) FeB–25Ni powders.

Fig. 3

DSC curve obtained from FeB–25Ni powders.

3.2 Spark-sintering behaviors

Figure 4 shows the relation between the apparent relative density and sintering temperature of FeB–25Ni, with those of as-received FeB (hereafter called FeB–0Ni), FeB–10Fe/Ni, and pure Ni powders as references. The sintering curves of FeB–25Ni are located between those of FeB–0Ni and pure Ni. The sintering behavior of FeB–25Ni compacts in the temperature range below 1273 K are the same, showing good reproducibility among the three curves with different maximum temperatures of 1273, 1323, and 1373 K. The apparent relative density of the FeB–25Ni compacts increased monotonically in the temperature range below 700 K and increased rapidly in the temperature range of 700–873 K. It then increased again to 1200 K and remained constant above 1200 K. The slope of the sintering curves below 1200 K for the FeB–25Ni compacts is located between that of the FeB–0Ni and pure Ni compacts.

Fig. 4

Relation between apparent relative density and sintering temperature of FeB–25Ni, FeB–0Ni, pure Ni and FeB–10Fe/Ni.

The deformation of the FeB hard phases was not expected at these sintering temperatures because of the slight increase of the apparent relative density according to the sintering curve of FeB–0Ni. It was estimated that the increase of the apparent relative density of FeB–25Ni compacts was mainly determined by the plastic deformation of the Ni layers surrounding the FeB particles. In other words, the plastic deformation of the Ni binder phases caused by Joule heating played a dominant role in increasing the apparent relative density of the FeB–25Ni compacts, which resulted in the similar sintering curves of the pure Ni and FeB–25Ni compacts. In addition, it was considered that the rapid increase of the apparent relative density observed in the temperature range of 700–873 K was due to the volume change of the FeB–25Ni compacts caused by the crystallization reaction of the Ni layers, as seen in Fig. 3.

Figure 5 shows the relation between the densification rate and the apparent relative density of FeB–25Ni, FeB–0Ni, and pure Ni compacts. The experimental densification rate, , was obtained by dividing the increase in the apparent relative density, dD, by the time increase, dt, as represented in eq. (2).

 $\dot{D} = dD/dt$ (2)

Some studies2325) have explained the densification process of pure Cu and a mixed powder of Cu and Al2O3 using experimental and theoretical values of . The theoretical analysis of was performed using eq. (3) for plastic deformation and eq. (4) for power-law creep deformation.

 $\dot{D} = \left[ \left(\frac{d\sigma_{\rm yield}}{dT}\right)/ \left(\frac{d\kappa ({\rm D})}{dD}\right) \right] \left(\frac{\kappa({\rm D})}{\sigma_{\rm yield}}\right) \dot{T}$ (3)

 $\dot{D} = AV_{ex}^{m} D\kappa (D)^{n + 1} {\rm e}^{\left(-\frac{Q}{RT}\right)} P^{n}$ (4)
where T, , P, σyield, A, Q, n, m and R are the temperature, heating rate of the compact, applied pressure, yield stress of the powder material, creep constant, activation energy of power law creep, stress exponent, coefficient representing the contribution of macroscopic stress and gas constant, respectively. κ(D) is a function of the apparent relative density and Vex is the extended volume fraction for densification26). For Cu and Cu–Al2O3, the experimental results of were consistent with the results of theoretical calculation by eqs. (3) and (4). It was found that the plastic deformation of the pure Cu powders occurred before reaching the maximum point of , and then the power-law creep deformation of just the Cu powders occurred, even in the Cu–Al2O3 mixture.
Fig. 5

Relation between densification rate and apparent relative density of FeB–25Ni sintered at 1373 K, FeB–0Ni sintered at 1418 K, and pure Ni sintered at 1418 K.

In this study, it was also hypothesized that the consolidation and sintering of the FeB–25Ni compacts proceeded by the deformation of just the Ni binder, as seen with the Cu–Al2O3 compacts. The experimental results of of the pure Ni and FeB–25Ni compacts were consistent with the theoretical results using eqs. (3) and (4). It was considered that the deformation of FeB–0Ni contributed little to the densification of the FeB–25Ni compacts. Therefore, it was considered that the increase of of the FeB-25Ni compacts was caused by the plastic and power-law creep deformation of the Ni binder phases before and after reaching the maximum point of at the D of 0.79, respectively. The value of D at the maximum in the FeB–25Ni compacts was 0.79, which was higher than that for the FeB–0Ni compact (0.6) and close to that for pure Ni (0.74). The densification mechanisms mentioned above were also applied to the Cu–Al2O3 composites because the content of the deformable binder Cu was more than 70 vol%23). It was found that the proposed mechanisms could also be applied to FeB–Ni composites having 25 vol% Ni as a deformable binder.

3.3 Microstructures

Figure 6 shows compositional images of the FeB–25Ni compacts sintered at different temperatures. The porosity decreased when the sintering temperature increased, and the highest temperature, 1373 K, led to almost full density, as seen in Fig. 6(a)–(c). The magnified microstructure of the sintered compact at 1323 K is shown in Fig. 6(d) as a typical example. Four different phases are observed and their compositions as measured by EPMA are also denoted in Fig. 6(d). The four phases are identified to be FeB, Fe2B, Ni, and (Ni,Fe)2B, which agrees with the XRD results shown in Fig. 7. The volume fractions of the phases are almost the same among the three compacts, as listed in Table 1. The tetragonal Ni2B phase in Fig. 7 corresponds to the (Ni,Fe)2B phase, with 36 mol% Fe, 32 mol% Ni and 32 mol% B. The Ni binder content containing small amounts of pure Ni and the (Ni,Fe)2B phase shown in Fig. 6(d) are also listed in Table 1. It was reported that the formation of a tetragonal M2B phase was predicted in the compositions between Ni2B and Fe2B27). The Ni binders were distributed uniformly in the compacts, which corresponded to the appearance of FeB powders surrounded by Ni phases, as seen in Fig. 1.

Fig. 6

Compositional images of FeB–25Ni compacts sintered at (a) 1273, (b) 1323, and (c) 1373 K and (d) high-magnification image of (b) showing the amounts of the constructed elements in four phases.

Fig. 7

XRD patterns of (a) FeB–0Ni sintered at 1418 K and FeB–25Ni compacts sintered at (b) 1273, (c) 1323, and (d) 1373 K.

Table 1 Volume fractions of the constructed phases in three FeB–25Ni compacts sintered at 1273, 1323, and 1373 K.
Compacts Constructed phases, vol%
FeB Fe2B Ni binder
(pure Ni and
(Ni,Fe)2B)
FeB–25Ni@1273 K 67.8 5.4 26.8
FeB–25Ni@1323 K 66.4 6.9 26.7
FeB–25Ni@1373 K 67.1 6.6 26.3

3.4 Mechanical properties

Figure 8 shows the HRA hardness values for the FeB–0Ni and FeB–25Ni compacts. The HRA value of WC–7.8Co sintered at 1573 K was also used as a reference. The HRA values increased with increasing sintering temperature for FeB–25Ni. The porosity is also shown in this figure. Furthermore, the mean density values were 6.6, 6.8, and 6.9 × 103 kg/m3 for the three Ni-added compacts sintered at 1273, 1323, and 1373 K, respectively. The hardness of WC–7.8Co with 9% porosity is comparable to that of the FeB–25Ni compacts with similar porosity levels. The hardness values increased as the porosity decreased in the FeB–25Ni compacts, which resulted in a good correlation between hardness and density under almost the same constructed phases, as listed in Table 1. The HRA value of both FeB–25Ni and WC–7.8Co sintered at 1323 and 1573 K, respectively, was 82, although the porosity of WC–7.8Co was two times higher than that of FeB–25Ni. In contrast, the FeB–25Ni compact sintered at 1373 K showed 1.1 and 1.15 times higher HRA values as compared to the WC–7.8Co and FeB–0Ni compacts, respectively. Therefore, it was found on the basis of comparison among FeB–0Ni, FeB–25Ni, and WC–7.8Co compacts that sufficient sintering was carried out on the 25% Ni-containing compacts as a result of the homogeneous Ni plating and usage of the FeB hard phase. Thus, FeB–25Ni compacts are effective substitutes for WC–Co hard materials.

Fig. 8

Rockwell hardness values on A scale and porosity of FeB–25Ni compacts sintered at 1273, 1323, and 1373 K and FeB–0Ni and WC–7.8Co compacts.

Figure 9 shows the KIC values for the FeB–25Ni, FeB–0Ni, and WC–7.8Co compact. The values increased with increasing sintering temperature, which agreed with the hardness properties. The increase of both the hardness and KIC resulted from the decrease of porosity and the improvement of interfacial strength among the particles by diffusion. Furthermore, the FeB–25Ni compact sintered at 1373 K showed 1.3 and 2.9 times higher KIC as compared to FeB–0Ni and WC–7.8Co, respectively. The effects of the usage of the FeB hard phase and Ni plating on KIC were larger than those on the hardness properties, as seen in Figs. 8 and 9.

Fig. 9

Fracture toughness of FeB–25Ni, FeB–0Ni and WC–7.8Co compacts.

Figure 10 shows the compressive stress–strain curves of the FeB–25Ni compacts. As shown in this figure, the maximum compressive stress and strain of the compacts increased with increasing sintering temperature. The maximum compressive stress of the FeB–25Ni compact sintered at 1373 K with the highest density was 1.6 and 2.1 times larger than that of the compacts sintered at 1323 and 1273 K, respectively. In contrast, the FeB–25Ni compact sintered at 1373 K showed a slight decrease in Young's modulus as compared to the compacts sintered at 1273 and 1323 K.

Fig. 10

Compressive stress–strain curves of FeB–25Ni compacts sintered at 1273, 1323, and 1373 K.

Figure 11 shows SEM images of fracture surfaces obtained from compressive tests using FeB–25Ni compacts sintered at 1273, 1323, and 1373 K. Many pores are clearly observed between particles in the compacts sintered at 1273 K, indicating insufficient sintering. Crack propagation was encouraged due to the existence of pores. In contrast, no obvious pores between particles are observed in the FeB–25Ni compacts sintered at 1373 K, which indicates sufficient sintering or a higher interfacial strength. Plastic deformation of the Ni phases was observed on the fracture surface of the compacts sintered more than 1323 K, which led to the higher compressive strain.

Fig. 11

SEM images of fracture surfaces obtained from compressive tests using FeB–25Ni compacts sintered at (a) 1273, (b) 1323, and (c) 1373 K.

The compressive stress, strain, and KIC values increased by improvement of the apparent relative density and interfacial strength between particles. In contrast, the hardness value increased with increasing apparent relative density28). It was considered on the basis of these results that the higher sintering temperatures combined with both homogeneous Ni plating and usage of the FeB hard phase resulted in the improvement of the compressive and fracture toughness properties.

3.5 Comparison of the effects of Ni and Fe binders on consolidation and properties of respective compacts

Sintering and mechanical characteristics are compared in Table 2 between FeB–25Ni and FeB–10Fe29) sintered at 1373 and 1505 K, respectively, in which the solid-state processing parameters were optimized. The sintering curve of FeB–10Fe, a mixture of both of FeB and pure Fe powders, was also shown in Fig. 4. FeB–10Fe showed improved sintering behavior as compared to FeB–0Ni, which corresponded to the promotion of densification by the deformation of Fe binders29) as well as Ni binders. The FeB–10Ni30) compact showed an apparent relative density of 0.8 at 1190 K, in the region of high temperatures above 1000 K. In contrast, the FeB–10Fe compact showed an apparent relative density of 0.8 at 1260 K. The addition of Ni therefore resulted in the improvement of sinter-ability achieving the same density level at a temperature 70 K lower. The results also show the homogeneous distribution of Ni by electroless plating. The maximum value at the apparent relative density of 0.79 for FeB–25Ni, 7.3 × 10−4 s−1, was 2.1 times higher than that at 0.75 for FeB–10Fe, which indicated to the usability of spark sintering in order to control the process parameters and the usage of particles with a homogeneous Ni coating.

Table 2 Comparison of some characteristics of FeB–25Ni and FeB–10Fe compacts sintered at 1373 and 1505 K, respectively.
Compacts Grain size
(μm)
Relative
density
Max. Ḋ in
Fig. 5
(10−4s−1)
RD at Max.
Ḋ in Fig. 5
HRA Max.
compressive
stress (MPa)
Max.
compressive
strain (%)
FeB–25Ni
@1373K
46.1 0.99 7.3 0.79 85 1216 58.6
FeB–10Fe
@1505K
13.8 0.75 3.4 0.75 73 451 18.1

The grain size in the FeB–25Ni compacts of 46 μm was the same as that of the as-received particle size, 45 μm. In contrast, the grain size of the FeB–10Fe compacts, 14 μm, was similar to that of a finer starting powder with a size of 10 μm obtained by ball milling. It has been found31) that high-density compacts with minimal grain growth could be obtained in a short time at low temperatures under applied pressure by spark sintering. The FeB–25Ni compact sintered at 1373 K showed such a high apparent relative density and minimal grain growth with spark sintering. Also, this compact showed the excellent hardness and compressive values.

4. Conclusions

The spark sintering behaviors and mechanical properties of FeB–25Ni were investigated in this study, and the following results were found:

• (1)   The sinter-ability of FeB was improved by the addition of 25 vol% Ni, and the apparent relative density of the FeB–25Ni compacts increased with increasing sintering temperature from 1273 K to 1373 K. The maximum densification rates of FeB–25Ni and pure Ni were obtained experimentally, and the compacts exhibited apparent relative densities of 0.79 and 0.74, respectively. The plastic deformation and power-law creep deformation of the Ni binder phase in the FeB–25Ni compacts occurred before and after reaching the maximum densification rate, respectively.
• (2)   The Rockwell hardness values of the FeB–25Ni compacts increased with increasing sintering temperature because of the decrease in porosity. The KIC values increased with increasing sintering temperature, which agreed with the hardness properties. The effects of the usage of FeB hard phase and Ni plating on the KIC values were larger than those on the hardness properties.
• (3)   The maximum compressive stress and strain of the FeB–25Ni compacts increased with increasing sintering temperature due to the promotion of the sintering at the interfaces between particles.
• (4)   The FeB–25Ni compact sintered at 1373 K showed a high apparent relative density and minimal grain growth as a result of spark sintering and good mechanical properties due to the optimization of both the process parameters and the powders used, which had a homogeneous Ni coating, as compared to the FeB–10Fe compact.

Acknowledgement

This work was supported in part by JSPS KAKENHI Grant Number JP26340101.

© 2016 The Japan Institute of Metals and Materials
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