1. Introduction
Electroless nickel plating is a coating technique that relies on the reduction of nickel ions with an autocatalytic chemical reduction process, in an aqueous solution containing a reducing agent [1].
Electroless nickel coatings find extensive applications in aircraft, automotive, chemical and electrical industries owing to their excellent unique properties such as decent solderability, high hardness, good corrosion and wear resistance [2,3].
In addition to pure metal coatings with electroless coating methods, it can be obtained in coatings with binary and multiple alloy structures. Depending on the properties expected from the coating, coatings with the same or different alloy structures can be applied in a single layer, duplex or multilayer.
The electroless coating method can be applied to materials of different shapes and sizes, such as glass, ceramic and plastic, as well as metal surfaces [4]. Due to their superior properties, especially electroless NiP and NiB coatings are widely applied.
Electroless NiP alloy coatings are applied in many areas due to their excellent properties such as high corrosion and wear resistance, good lubrication and high hardness [5]. NiB alloy coatings are preferred due to their high hardness and superior wear resistance [6,7].
However, electroless coatings are sensitive to thermal effects, and heat treatments have a strong effect on the hardness and corrosion resistance, especially the structural properties of the coatings.
Studies show that the heat treatment applied at 400 °C causes a significant change in the structure of the electroless coatings and thus their properties.
2. Experimental
In the experimental study, iron-based powder metal (PM) compacts were used as substrate. TM parts composition and operation parameters were coated in electroless NiP and NiB coating baths given in Table 1 and Table 2.
Considering the service conditions of iron-based powder metal compact, hardness and wear resistance gain importance. For this reason, the NiB coating film, which stands out with its hardness and wear resistance, is designed as a thick top layer, while the NiP coating film, which stands out with its corrosion resistance, is designed to form a thin and bottom layer.
The structure, morphology and chemical composition of the coatings were analyzed by scanning electron microscopy (SEM / EDX). Phase structures of the coatings were determined by applying X-ray diffraction analysis (XRD). Hardness values were determined by microhardness tests applied to the coating section. Corrosion resistance was determined by using Parstat 4000 potentiostat/galvanostat system in 3.5% NaCl solution.
3. Results and Discussion
3.1. Morphology and Structure of the Coating
The cross-sectional views of the electroless NiP/NiB duplex coating PM compact are shown in Figure 1. NiP thin film (<500 nm) formed a compatible interface between TM compact and NiB coating. The average composition of the NiB coating is 5.7 wt.% B, 0.5 wt.% Pb and 94.3 wt.% Ni and that of NiP is 4.8 wt.% P and 95.2 wt.% Ni. NiB coating, which showed a columnar growth, was found to have a coating thickness of 20 μm. Figure 2 shows the SEM image of the cross-section and surface morphology of the NiP/NiB coating film. Electroless NiB coating has a well-known and cauliflower- like surface morphology. In friction conditions, the cauliflower-like structure of NiB coatings provides a natural property of lubricity increasing the wear strength due to the reduction of the contact area.
Fig. 1. The cross-section morphologies of NiP/NiB coating.
Fig. 2. Surface and cross-section morphologies of NiP/NiB film.
XRD patterns of the as-deposited and annealed at 400ºC samples are shown in Fig. 3. The XRD pattern of the as-deposited sample obtained a broad hump characteristic of an amorphous phase. Upon heat-treatment at 400ºC for 1 h, NiB deposit crystallized and produced Ni, Ni2B and Ni3B phases in NiB deposit (Fig. 3). Ni3B and Ni2B phases in the Ni matrix cause a significant increase in the hardness of the coating. In XRD analysis, it could not be detected Ni and P phases in NiP bottom layer.
Fig. 3. XRD patterns of the NiP/NiB duplex coatings.
3.2. Microhardness of the Coating
The hardness of heat-treated NiB coating increased from 668 to 1143 HV100 due to precipitation hardening by the formation of intermetallic Ni3B and Ni2B phases (Fig. 4). This increase in the surface hardness of the TM compact will positively affect its wear resistance.
Fig. 4. Microhardness of PM compact and the coatings.
Fig. 5. Polarization curves of PM compact and
coatings
3.3. Corrosion resistance of NiP/Ni-B coatings ,
Polarization curves of PM compact and coatings are given in Figure 5. As can be seen from the curves, the Tafel line of the as-plated NiP/NiB coating was obtained at a lower polarization current density value compared to the PM compact and heat treated NiP/NiB coating. This result shows that the corrosion resistance of the NiP/ iB coating is higher than that of the TM compact and heat treated NiP/NiB coating. The crystallization of the amorphous structure as a result of heat treatment caused to decrease in the corrosion resistance of the coating.
Conclusions
In this work, electroless NiP/NiB duplex coatings were examined by microstructure analysis, microhardness, SEM, XRD, and corrosion test techniques. NiP/NiB duplex coating process has been successfully applied to the substrate with the electroless coating method. Heat treatment caused a significant increase in the microhardness of coatings due to precipitation hardening by the formation of Ni3B and Ni2B phases in the nickel matrix. Structural changes caused by heat treatment reduce the corrosion resistance of the coating.
Asst. Prof. Dr. Ulaş Matik
Karabük University
Machine and Metal Technologies Department
Metalurji Programı
References
[1] Kılıçarslan A, Toptan F and Kerti I 2010 Akımsız nikel kaplama yöntemi ve seramik partiküllerine uygulanması 33–7.
[2] Matik U 2016 Structural and wear properties of heat-treated electroless Ni-P alloy and Ni-P-Si3N4 composite coatings on iron
based PM compacts Surf. Coatings Technol. 302 528–34.
[3] Abdel-Gawad S A, Sadik M A and Shoeib M A 2019 Preparation and properties of a novel nano Ni-B-Sn by electroless deposition
on 7075-T6 aluminum alloy for aerospace application J. Alloys Compd. 785 1284–92.
[4] Delaunois F, Petitjean J P, Lienard P and Jacob-Duliere M 2000 Autocatalytic electroless nickel-boron plating on light alloys Surf.
Coatings Technol. 124 201–9.
[5] Hsu C I, Hou K H, Ger M Der and Wang G L 2015 The effect of incorporated self-lubricated BN(h) particles on the tribological properties
of Ni-P/BN(h) composite coatings Appl. Surf. Sci. 357 1727–35.
[6] Bekish Y N, Poznyak S K, Tsybulskaya L S and Gaevskaya T V. 2010 Electrodeposited Ni-B alloy coatings: Structure, corrosion resistance
and mechanical properties Electrochim. Acta 55 2223–31.
[7] Matik U 2018 Akımsız Ni-B kaplama ile demir esaslı toz metal kompaktların yüzey özelliklerinin geliştirilmesi Gazi Üniversitesi
Mühendislik-Mimarlık Fakültesi Derg. 2018 1603–10.
