Production and wear property of electroless Ni-plated B₄C-AstaloyCr-M composites

1 Introduction

Electroless plating produces a high-performance product with high hardness, elevated wear, and corrosion resistance for critical applications. Several advantages, such as easy formation of a continuous and uniform coating on the surface of a substrate even with a complex shape and the ability of depositing on both conductive and nonconductive parts, have attracted a lot of interests from the industry [1]. Electroless nickel (Ni) coatings are widely used to protect surfaces of engineering metal components against degradation by corrosion and wear or to repair worn areas [2]. The hardness of electroless Ni composite coatings increases with the incorporation of ceramic (hard) particles, whereas, with soft particles, the hardness tends to decrease [3–7]. The carbides of W, B, Si, Ti, and Ta have been used for a variety of applications over the years, since they are extremely hard and possess excellent wear and oxidation resistance [1, 8]. Metallic materials with small grains exhibiting high strength are interesting from both theoretical and experimental points of view. Further enhancement of their mechanical properties is possible because of reinforcement by ceramic particles [9–13]. Such composites are widely used as carbide ceramics and wear resistance cutting tools [14–17].

In this study, a ceramic plus metal composite [i.e., AstaloyCr-M+boron carbide (B₄C)+Ni] was obtained by using electroless Ni plating of B₄C) and AstaloyCr-M powders. The aim of this study is to characterize and investigate the wear property of the AstaloyCr-M+B₄C+Ni composite.

2 Materials and methods

In this study, AstaloyCr-M and B₄C were used as metallic and ceramic starting powders. NiCl₂ was used as Ni source in Ni plating of ceramic and metallic powders. Furthermore, 99.5% purity powders of AstaloyCr-M with 50 μm particle size and B₄C powders with 20 μm particle size were both provided from Johnson Matthey Materials Technology Co. (London, UK). AstaloyCr-M powder composition used in this study is given in Table 1.

In the experimental study, the samples were prepared in different ways. B₄C and AstaloyCr-M powders were electroless Ni plated and then shaped in a hydraulic press under 400 bar pressure. The preformed samples were sintered for 2 h at temperatures of 1000°C, 1050°C, 1100°C, 1150°C, and 1200°C under argon atmosphere in a conventional furnace. Sintered samples were prepared for mechanical and metallographic analysis. The contents of the plating bath and conditions are given in Table 2.

Scanning electron microscopy (SEM) photographs were taken using LEO 1430VP SEM attached with Röntec EDS analyzer. After securing the samples within resin and completing their surface polishing processes, a metallographic examination was made using SEM at different magnifications. Shimadzu-AG/IS 100 kN universal tensile testing machine and Shimadzu HMV 2 L microhardness tester were used to measure the compression strength and the hardness of the samples, respectively. The hardness measures were obtained by taking the mean of hardness values taken from 10 different areas for each sample. Pin-on-disk method was used for wear test investigation in accordance to ASTM G99. The wear test was applied to composites with parameters of 0.1 m/s travelling speed for 200 m wear distance with a load of 10 N. In this work, cast steel was used as a reference sample. Volume changes were measured using a pycnometer before and after wear tests.

3 Results and discussion

3.1 Metallographic and X-ray diffraction (XRD) analysis

SEM and XRD analyses were carried out on the specimens to reveal the effect of Ni plating and to characterize the phases present within the specimen. It was observed that Ni-coated particles were strongly bonded to each other due probably to the well wetting of Ni matrix and this caused an excessive growth of some particles, causing a decrease in total volume. Such a contraction within the matrix can lead to the formation of a network of pores around islands of strongly bonded particles as seen in Figure 1. Some of the pores may be originating from increasing curvature of powder islands; however, some of the pores may be a result of dropping out of B₄C particles during metallographic specimen preparation or examination. There were also pores exhibiting homogeneous dispersion among the particles.

Figure 1

SEM image of AstaloyCr-M-B₄C-Ni composite at 1150°C.

In Figure 2, among many peaks, B₄C and FeNi/FeCr peaks can be seen in the XRD spectrogram of AstaloyCr-M+B₄C+Ni composite sintered in furnace at 1150°C. However, peaks for pure elements do not exist, suggesting that the formation of FeNi compound is a dominant phase together with Ni₂B peaks. The formation of Ni₂B and FeC also profoundly suggests that some B₄C may have been dissolved at high temperatures and caused complex nonequilibrium diffusion reactions. The reactivity between B₄C and AstaloyCr-M powders and Ni coating to form compounds is controlled through Ni layer existing on the starting powders; hence, compounds should be mainly formed on the powders as a thin reaction layer. The reactions appear to be complicated. Although there are a few unidentified peaks in the spectrogram, it is possible that some amount of unidentified compounds within the reaction zone between Fe, Ni, B, C, and Cr may be the source for those peaks.

Figure 2

XRD spectrogram AstaloyCr-M-B₄C-Ni composite sintered in furnace at 1150°C.

3.2 Physical, wear, and mechanical properties

In the study, the samples prepared and shaped were sintered at temperatures ranging from 1000°C to 1200°C in a conventional furnace and prepared for physical, mechanical, and metallographic analyses. Density-temperature change curve is linearly dependent on the sintering temperature as shown in Figure 3. The highest sintered density was achieved at 1150°C as 5.12 g/cm³, suggesting that binding-phase Ni plays an important role in increasing the density due to spreading of the phase between the powders.

Figure 3

Density changes with respect to sintering temperature.

Following the wear tests, volume changes were measured in both the reference material (cast iron) and the AstaloyCr-M+B₄C+Ni composites. The cast iron was eroded as much as 9.31×10^-3 mm³. The volume change in composite (AstaloyCr-M+B₄C+Ni; 1150°C) was 8.22×10^-3 mm³. This result shows that AstaloyCr-M+B₄C+Ni composites possess better wear resistance than cast iron. The volume loss with respect to sintering temperature is given in Figure 4. Increasing the sintering temperature leads to lower wear volume loss. This may suggest that the bonding between B₄C and pure elements becomes stronger and the hardness of the matrix may be higher with increasing sintering temperature, since the wear volume loss is inversely proportional to hardness [18]. Indeed, Figures 5 and 6 indicate that the matrix strength of AstaloyCr-M+B₄C+Ni composite is elevated with sintering temperature, causing the hardness and compressive strength increase with higher diffusion temperatures. The highest and lowest hardness values in the composite AstaloyCr-M+B₄C+Ni samples were observed to be 135.18 HB at 1150°C and approximately 85 HB at 1000°C, corresponding to 90 and 128 MPa of compressive strength, respectively. The rise in the hardness and strength values is encouraging for a low wear volume with relatively high hardness composite materials having a limited ductile property. Since the amount of B₄C is not modified, it is possible that the diffusion reaction thickness between the powders and Ni coating is enlarged with the increase of sintering temperature, which accelerates the process of diffusion.

Figure 4

Wear volume loss change in the sintered samples depending on the temperature.

Figure 5

Hardness measurements results with respect to sintering temperatures.

Figure 6

Compression strength results from samples sintered at different temperatures.

4 Conclusion

AstaloyCr-M powders and B₄C powders were electroless Ni plated and then sintered in a conventional furnace. After sintering, a gradual increase in the mechanical properties of specimens sintered at 1000°C and 1200°C was observed. The wear volume loss results show that composite has good wear resistance compared to the reference material (cast iron). From XRD results of specimens sintered at 1150°C, FeNi intermetallic phase and B₄C appear to be coexisting as two separate constituents. Hardness results and compressive strength of AstaloyCr-M+B₄C+Ni composite appear to be higher with increasing sintering temperature. It is also observed that various reactions occurred at the interface of contacting surfaces (i.e., between Ni plating and the other constituents – AstaloyCr-M and B₄C powders), easing the formation of composite structure.