Synthesis and Characterization of Electroless NiP/Nano h-BN Composite Coatings

Electroless nickel coating is a technique in which nickel atoms are obtained from an aqueous solution containing a reducing agent through an autocatalytic chemical reduction method and applied to the material surface [1]. Electroless NiP coatings have found numerous applications in many fields due to their excellent properties such as high corrosion resistance, high wear resistance, good lubrication and high hardness [2]. The addition of micro and nano-sized particles to the matrix structure alongside the coating reaction is an effective method to enhance the mechanical and physical properties of electroless nickel coatings. Literature studies demonstrate that researchers have been focusing intensively on the addition of hard particles (Al2O3, B4C, WC, SiC, Si3N4 and c-BN) to the matrix structure in order to increase the hardness and wear properties of coatings [2–4]. However, composite coatings in which lubricating soft particles (graphite, PTFE, MoS2 and hexagonal boron nitride (h-BN)) are added to the coating matrix structure to improve the friction properties of coatings have been studied in a more limited manner. In this study, work was carried out on the synthesis and characterization of NiP/h-BN composite coatings by ensuring that nano-sized h-BN particles added to the coating bath accumulate on the substrate material surface along with the coating reaction, in order to improve the friction properties of electroless NiP alloy coatings with high hardness, wear and corrosion resistance.

In the experimental study, AISI 1040 steel with dimensions of 5x10x20 mm was used as the substrate material. To obtain NiP/h-BN composite coatings, hexagonal BN (hBN) particles below 100 nm (<100 nm), whose SEM image is shown in Figure 1, were used.

AISI 1040 substrate materials were subjected to a coating process for 1 hour in an electroless NiP coating bath with the composition and working parameters given in Table 1. To obtain NiP/hBN composite coatings, particles were added to the coating baths at different rates of 0.25, 0.5 and 1 g/l. The structure, morphology and chemical composition of the coatings were analyzed using a scanning electron microscope (SEM/EDX), (Carl Zeiss ULTRA FESEM). The hardness of the coatings was measured using a Qness Q10 Micro Vickers Tester microhardness device by applying a load of 50 gf to the coating surface for 10 seconds.

3. Experimental Results and Discussion

The surface morphologies obtained by SEM analysis of NiP alloy and NiP/h-BN particle-reinforced composite coatings are shown in Figure 2. While NiP coating exhibits a well-known typical surface morphology, h-BN particle-reinforced composite coatings have been found to have a rougher and more porous surface morphology. Upon closer examination, the pores on the surface are more clearly evident (Figure 3). EDX analyses conducted reveal the presence of nano h-BN particles in the porous regions (Figure 4). In Figure 4, the first three of four point chemical analyses conducted on the NiP/h-BN (1 g/l) coating surface were taken from porous regions, while the fourth analysis was taken from a smoother region. Boron nitride was detected in the first three analyses from the porous regions, while no boron nitride was found in the fourth analysis. The nickel-phosphorus content of the NiP/hBN (1 g/l) coating was determined to be approximately 9% P and 91% Ni by weight. In the NiP alloy coating, the coating composition was determined to be 12.7% P and 87.3% Ni by weight. The results obtained show that an increase in the amount of particles added to the coating bath causes a decrease in the phosphorus content of the coating. Chih et al. [2] demonstrated in their work that an increase in BN content in composite coatings causes a decrease in the phosphorus content of the coating, revealing that the coating compositions have changed. Upon examination of the coating cross-sections, it was found that NiP alloy coating has an average thickness of approximately 13.5-15 µm, while NiP/h-BN coatings have an average coating thickness of 11.8-12.5 µm (Figure 5). It is considered that h-BN particles with passive surface properties added to the coating solution may affect the reduction of the coating metal negatively as a result of their binding to the surface, creating passive regions that may contribute to the coating thicknesses remaining at low levels. In the cross-section image of the NiP/h-BN (1 g/l) composite coating, h-BN particles in the matrix structure appear as black regions. Upon examination of the cross-section images, it is observed that nano-sized h-BN particles are distributed homogeneously within the coating matrix structure. In microhardness measurements performed by applying a 50 gf load to the uncoated AISI 1040 steel sample and coating surfaces, it was found that the coatings caused a significant increase in the hardness of AISI 1040 steel (Figure 6). The highest hardness was obtained in NiP alloy coatings. In composite coatings, hardness values decreased depending on the increase in h-BN particle content of the coating, and the microhardness value in NiP/h-BN (1 g/l) composite coating decreased to levels of 485 HV0.05. The lower hardness of composite coatings compared to NiP coating can be explained by the soft structure possessed by h-BN in the coating composition. Furthermore, the poor bonding structure caused by agglomeration of reinforcing particles in the coating film may be another reason for the decrease in hardness values. However, one of the important factors determining the hardness of NiP alloy coatings is the composition of the coating. As mentioned earlier, an increase in h-BN particle content in the matrix structure in composite coatings leads to a decrease in the phosphorus content of the coating. For this reason, when explaining the hardness value changes in composite coatings, changes in the coating composition must also be evaluated. Microhardness measurements showed that AISI 1040 steel has approximately 300 HV0.05 and NiP coating has hardness values of 530 HV0.05. In composite coatings, a decrease in microhardness values occurred depending on the increase in nano h-BN particle addition to the coating bath.

The results obtained as a result of characterization tests applied to NiP alloy and NiP/h-BN composite coatings with different compositions are given below as bullet points:

1. NiP alloy and NiP/h-BN composite coatings can be successfully applied over AISI 1040 steel. 2. Although the coating time is the same, the coating thickness in composite coatings is 10-15% lower than in NiP coatings. 3. As the amount of nano h-BN particles added to the NiP matrix structure increases, the coating roughness increases. 4. Depending on the amount of particles added to the coating bath, the secondary phase particle content of the composite coating also increases. 5. An increase in nano h-BN particle content negatively affects the coating hardness. 6. Electroless NiP alloy and NiP/h-BN composite coatings have caused a significant increase in the surface hardness of AISI 1040 steel. Acknowledgement This work was supported within the scope of Karabük University Scientific Research Projects. (Project No: KBÜ-BAP16/1-KP-159)

References [1] A. Kılıçarslan, F. Toptan, I. Kerti, "Electroless nickel coating method and its application to ceramic particles," Metallurgy Journal, vol. 154, pp. 33-37, 2010. [2] C.I. Hsu, K.H. Hou, M. Der Ger and G.L. Wang, "The effect of incorporated self-lubricated BN(h) particles on the tribological properties of Ni-P/BN(h) composite coatings," Applied Surface Science. vol. 357, pp. 1727–1735, 2015. [3] U. Matik, "Structural and wear properties of heat-treated electroless Ni-P alloy and Ni-P-Si3N4 composite coatings on iron based PM compacts," Surface and Coatings Technology, vol. 302, pp. 528–534, 2016. [4] O.A. León, M.H. Staia, and H.E. Hintermann, "High temperature wear of an electroless Ni-P-BN (h) composite coating," Surface and Coatings Technology, vol. 163–164, pp. 578–584, 2003.

    Dr. Ulaş Matik, Assistant Professor Karabük University Department of Machinery and Metal Technologies Metallurgy Programme