Interaction Mechanism and Wear Resistance of Ni-encapsulated Al

Abstract

Ni coating was prepared on the surface of Al₂O₃ by chemical deposition method. Ni coated Al₂O₃ particles (Al₂O₃p@Ni) was used as particle-reinforcement for iron matrix. The Al₂O₃p@Ni/Fe composites were prepared by SPS. Results show that by optimizing electroless plating process, the surface of Al₂O₃ is uniformly covered by Ni. Ni coating presents a typical cauliflower structure with the size of 1~4 μm, which is deposited in pits and holes on the surface of Al₂O₃ and then gradually extends outwardly. The thickness of Ni layer is up to 100.55 μm, and Ni coating is closely bounded to Al₂O₃. In the process of sintering, Ni coatings not only improve the wettability between Al₂O₃ and iron matrix, but also promote the diffusion and reaction of Al₂O₃ and iron matrix at the interface. Finally, Al₂O₃/NiAl₂O₄/(Al_0.8Cr_0.2)₂O₃/NiFe₂O₄/Ni/iron matrix interface layer is formed by mechanical bonding, interdiffusion and chemical reactions, which can improve interface bonding strength greatly. The wear tests of Al₂O₃p@Ni/Fe composites and Al₂O₃p/Fe composites were carried out. Compared with Al₂O₃p/Fe composites, the wear mass loss of Al₂O₃p@Ni/Fe composites is decreased by 50%, and the friction coefficient is decreased by 12.5%. The wear resistance of Al₂O₃p@Ni/Fe composites is greatly improved.

Keywords

electroless plating; Al₂O₃@Ni/Fe composites; SPS; wear resistance

Science Press

Ceramic particles reinforced metal matrix composites (MMCs) have become a research hotspot due to their high hardness, high temperature strength and good wear resistance. And ceramic particles reinforced iron matrix composites have become a new trend to fabricate wear resistant materials instead of iron and steel.

Different from traditional smelting method, which may have segregation and porosity defects, powder metallurgy method is the main method for the fabrication of high-content ceramic-reinforced metal matrix composites. Fan et al^[1] fabricated alumina particles reinforced iron matrix composites by powder metallurgy. And they found out that by adding element C, the microstructure of matrix changes from ferrite to ferrite and pearlite, the hardness increases significantly which can reach about 9000 MPa, and the wear resistance improves significantly. Dang et al^[2] fabricated Al₂O₃/Cu composites by near melting point casting method, and the wear resistance is increased by 36.6% with addition of 0.6wt% La₂O₃. NASA has fabricated B/Al composite materials by powder metallurgy method, which have been applied to aircraft cargo compartment truss. Toyota has fabricated SiC-Al₂O₃ reinforced aluminum matrix composites by powder metallurgy, which have been applied to wear-resistant piston, and the wear resistance is improved greatly^[3].

However, due to the poor wettability of ceramics and metals, for example, the wetting angle of Al₂O₃ to iron solution is 140°, so it is difficult for ceramics to connect ceramics with metals^[4]. Unlike metals, which are composed of metallic bonds, ceramics are composed of covalent bonds. The different bond types also make it hard to react. Therefore, the bonding strength of the ceramic-metal interface is poor, so the ceramic particles are easy to fall off in service. At present, numerous surface modification technologies are used to improve the wettability of ceramic and metal, such as ball-milling, sol and electroless platings. Hong et al^[5] successfully fabricated a homogeneous nickel layer on the surface of zirconia toughened alumina ceramics (ZTA) by electroless plating. Ru et al^[6,7] fabricated uniform and continuous nickel-coated ZTA powder by ionic liquid assisted deposition method. The thickness of Ni coating reaches 7~10 μm. They measured the wettability of molten iron on the surface of nickel-plated zirconia toughened alumina ceramic (ZTA@Ni). The results show that, compared with ZTA without surface treatment, the wetting angle of 65Mn liquid on ZTA@Ni plate decreases from 104.1° to 83.6°, and that of the high chromium cast iron liquid on ZTA@Ni plate decreases from 102.3° to 88.2°. At the same time, they also analyzed the interface behavior of ZTA@Ni reinforced iron matrix composites. At a high casting temperature, Ni diffuses into the iron solution and reacts with Al₂O₃ to generate Al₂NiO₄. The mutual diffusion of Ni and other elements at the interface between ZTA and iron can enhance the interfacial bonding strength. Olgun et al^[8] successfully coated a ZrB₂ layer with a thickness of 15~20 μm on the surface of copper by ball milling. Guo et al^[9] fabricated ultrafine WC/Co composite powders by electroless plating. The Co layer with an average thickness of 50~100 nm was fabricated on the surface of WC particles with a diameter of 0.3~0.5 μm. Results show that the rate of electroless plating has an exponential relation with temperature, and the coating thickness has a certain relation with pH value. Wang et al^[10] successfully fabricated TiO₂ film on stainless steel surface by plasma Ti-thermal oxidation method.

At present, Al₂O₃ is widely used as reinforcement particles to fabricate ceramic reinforced iron-based wear-resistant materials because of its low price and the possibility to manufacture complex geometry parts. Considering the coating quality, pricing factor and industrializable preparation, electroless plating is an ideal surface treatment method, which has been successfully carried out on the surface of Al₂O₃ ^[11]. Ni is good as an interlayer considering the phenomenon of infinite solution of Ni layer and molten iron, which can greatly improve the wettability between ceramic and metal, and promote the interface connection greatly^[12,13].

In this study, copper ion auxiliary solution was used for pretreatment firstly, Ni coating was fabricated on the surface of Al₂O₃ by chemical deposition method, and sodium hypophosphite was used to reduce nickel ions. The Ni coated Al₂O₃ particle (Al₂O₃@Ni) was used as the precursor for reinforcing iron matrix composite. The morphology of the Ni coating on the Al₂O₃ surface was observed. Then Al₂O₃@Ni reinforced iron matrix composites (Al₂O₃@Ni/Fe) was prepared by powder sintering method, and the morphology, phase composition and element distribution of Al₂O₃@Ni/Fe composites were investigated. The interaction mechanism between Al₂O₃@Ni and iron matrix was described in combination with the sintering process. In addition, the wear resistance of the composites was also measured.

1 Experiment

1.1 Chemicals

In this study, 95wt% Al₂O₃ with a diameter of 3 mm was used as the reinforcing particle, purchased from Aladdin, and high-chromium steel was used as the matrix. The composition of high-chromium steel is shown in Table 1.

Table 1 Chemical composition of steel matrix (wt%)

C	Cr	Si	B	Ni	Fe
0.4~1.2	14~20	2.5~3.5	1.2~2.0	10~15	Bal.

1.2 Fabrication of Ni-encapsulated Al₂O₃ particles

The preparation of Al₂O₃@Ni particles includes pretreatment and electroless plating. Firstly, the Al₂O₃ particles were cleaned, coarsened, sensitized and activated, then soaked in deionized water and cleaned by ultrasonic vibration, and finally the surface with catalytic activity was obtained. The optimal pretreatment process is shown in Table 2.

Table 2 Composition and content of the chemicals used in pretreatment process

Component	Content	Stage
Acetone	-	Washing
NaOH	20 g/L	Washing
HCl	20 mL/L	Coarsening
SnCl2	20 g/L	Sensitizer
PdCl2	0.5 g/L	Activation

The pretreated Al₂O₃ particles were pre-plated and placed into the copper ion auxiliary solution for 3 min, and then put into the nickel plating solution for 1 h. The optimal plating solution process is shown in Table 3. The pH of the solution was 5.7, the temperature was 60 °C, and the stirring rate of agitation was 200 r/min. After the electroless plating, the samples were cleaned with deionized water and dried in the drying oven.

Table 3 Chemical composition of electroless plating

Component	Content/ g·L-1	Role in bath or operating parameters
Ion auxiliary solution (CuSO4)	10	Main salt
NiCl2	60	Main salt
NaH2PO2	80	Reducing agent
C6H5Na3O7	48	Complexing agent
NaKC4H4O6	144	Buffering agent

1.3 Preparation of Al₂O₃@Ni/Fe composites

Al₂O₃@Ni particles and iron matrix were uniformly mixed in a volume ratio of 1:5, and then the mixture was sintered by SPS, and the sintering temperature of 960 °C, the holding time of 10 min and the pressure of 30 MPa were used to fabricate Al₂O₃@Ni/Fe composites.

1.4 Characterization

The distribution of Al₂O₃ particles and the interfacial connection between Al₂O₃ and iron matrix were observed by metallographic microscope (DMC2900 OM). Y-2000 X-ray diffractometer (XRD) was used to analyze the phase at the interface between Al₂O₃ and iron matrix. The microstructure and element distribution at the interface were characterized by scanning electron microscopic (SEM TESCAN MIRA3 LMH). To evaluate the wear resistance of Al₂O₃@Ni/Fe composite, reciprocating circular motion tests for Al₂O₃@Ni/Fe composites were carried out using a pin-disk friction and wear tester. The friction pair adopted Si₃N₄ with a diameter of 5 mm, and the friction rate was 200 r/min; the load was 20 N, the wear mark diameter was 8 mm, and the time was 3600 s.

2 Results and Discussion

2.1 Characteristics of Ni-encapsulated Al₂O₃ particles

A tightly bonded nickel coating on Al₂O₃ is the premise for preparing Al₂O₃@Ni/Fe composites. So it is necessary to study the quality of Ni on the surface of Al₂O₃ for improving the interfacial bonding strength of composites. Because it is difficult to accurately characterize due to the limitation in shape and size of Al₂O₃ particles, we used Al₂O₃ plate to do the same experiment to complete the characterization analysis instead of Al₂O₃ particle, and the size of Al₂O₃ plate was 12 mm×18 mm×2 mm. After electroless plating was conducted on Al₂O₃ using the parameters in Table 2 and Table 3, Al₂O₃@Ni plate was obtained. The plate was hung with a string and suspended in solution to make operation easy, so the area around the hole was not observed.

The specimen of Al₂O₃@Ni prepared by ionic liquid assisted electroless plating and the OM micrograph are presented in Fig.1. From Fig.1a, Ni coatings covers the surface of Al₂O₃ uniformly and smoothly with no burr and crack. It is speculated that the deposition of Ni coatings begins in pits and holes on the surface of Al₂O₃ and then gradually extends outward, The thickness of Ni coatings is 100.55 μm which is closely bonded to the Al₂O₃ plate (Fig.1b), which meets the realization of promoting interface reaction and improving interface wettability during next sintering. The morphology of Ni coatings was observed by SEM, as shown in Fig.2 and Fig.3. It can be seen that the spore of Ni coatings presents a typical cauliflower structure^[14], which means that the Ni coating is amorphous, and the size of Ni spores is 1~4 μm (Fig.2), indicating that the Ni coating has stable structure, and does not fall off. This compact structure completely covers the Al₂O₃ matrix. Fig.3 shows a cauliflower Ni, and the mapping result demonstrates that the Ni is distributed uniformly without aggregation. Finally the dense sedimentary Ni layers are obtained.

Fig.1 Macrograph (a) and OM image (b) of Al₂O₃@Ni plate prepared by electroless plating

Fig.2 SEM morphologies of Al₂O₃@Ni plate prepared by electroless plating

Fig.3 SEM images of Al₂O₃@Ni plate prepared by electroless plaing (a, d) and corresponding EDS mappings of Ni (b), P (c), Al (e), and O (f)

The coating obtained is Ni-P coating, and the particular reason for this circumstance is the reducing agent (NaH₂PO₂). Therefore, the element P appears in the coating. The element P increases the hardness of coating, but decreases the bonding with substrate. The bonding strength between the coating and substrate is sufficient while the content of P in the coating is 9%~14%^[15]. Therefore, we selected five regions on the coating to calculate the content of P. As shown in Table 4, the content of P in the coating is about 10.5%, which indicates that the element P has no adverse effects on the interface.

Table 4 Chemical composition of coating fabricated by electroless plating (wt%)

Element	1	2	3	4	5	Average
Ni	87.9	90.28	90.8	89.8	88.6	89.5
P	12.1	9.72	9.2	10.2	11.4	10.5

2.2 Interaction mechanism of Al₂O₃p@Ni/Fe composites

The same electroless plating process was applied to Al₂O₃ particles, and Ni coated Al₂O₃ particles (Al₂O₃p@Ni) was obtained. The surface morphology of Al₂O₃p@Ni was observed by SEM, as shown in Fig.4. There are continuous and compact Ni coatings on the surface of Al₂O₃ particles, and the Ni coatings have cauliflower structure, which is similar to Ni coatings on Al₂O₃ plate (Fig.2). The EDS mappings of Ni coatings on Al₂O₃ particles are shown in Fig.5, which demonstrates that the Ni coatings are distributed uniformly and densely.

Fig.4 SEM micrographs of Al₂O₃@Ni particles prepared by electroless plating

Fig.5 SEM images of Al₂O₃@Ni particles prepared by electroless plating (a, d) and corresponding EDS mappings of Ni (b), P (c), Al (e), and O (f)

We mixed Al₂O₃p@Ni with iron powders at a volume ratio of 1:5, and the mixture was sintered by SPS. Al₂O₃p@Ni/Fe composites are obtained (Fig.6), and Al₂O₃p@Ni is distributed uniformly in the iron matrix.

Fig.6 Macrograph of Al₂O₃p@Ni/Fe composites fabricated by SPS

The bonding quantity of Al₂O₃ particles and iron matrix will affect the wear resistance of the composites directly, so it is significant to observe the microstructure of interface between Al₂O₃ and iron matrix. The OM micrograph of interface between Al₂O₃ and iron matrix is presented in Fig.7, and about 60 μm of diffusion layer is observed at the interface. Take a further research by SEM in Fig.8, it can be seen that at the interface, Ni and Al spread to the side of iron matrix, while Cr diffuses to the side of Al₂O₃.

Fig.7 OM micrograph of Al₂O₃p@Ni/Fe composites fabricated by SPS

Fig.8 SEM images of Al₂O₃p@Ni/Fe composites fabricated by SPS (a, e) and corresponding EDS mappings of Fe (b), Al (c), Cr (d), C (f), Ni (g) and O (h)

Fig.9 shows the XRD patterns of interface products of Al₂O₃p@Ni/Fe composite and Al₂O₃p/Fe composite, which demonstrates that there are not only (Al_0.8Cr_0.2)₂O₃ but also FeNi₃ and NiAl₂O₄ at the interface.

Fig.9 XRD pattern of Al₂O₃p@Ni/Fe composites and Al₂O₃p/Fe composites fabricated by SPS

Based on SEM-EDS mapping (Fig.8) and XRD results (Fig.9), the combined mechanism of interface of Al₂O₃p@Ni/Fe composites is analyzed as follows. During sintering, Ni coatings diffuse into iron marix and form infinitude solid solution because of the same fcc crystal structure, which facilitates the diffusion of some elements in Al₂O₃ and iron matrix, and there are (Al_0.8Cr_0.2)₂O₃, FeNi₃ and NiAl₂O₄ in the interface, which can be wetted easily with Fe to achieve good wettability at the Al₂O₃/(Al_0.8Cr_0.2)₂O₃/NiAl₂O₄/FeNi₃/Fe interface.

The schematic illustration of combined mechanism of interface of Al₂O₃p@Ni/Fe composites is demonstrated in Fig.10. At the initial stage, the iron is in contact with Al₂O₃ melts, and when Ni is coated on Al₂O₃ contacts with molten iron, they will become soft and migrate into iron matrix (Fig.10a) and the empty surface of Al₂O₃ is occupied by iron under high pressure of SPS. Meanwhile, metallic Ni has the same crystal structure with austenitic Fe, so it is easy to form infinitude solid solution with Fe. This is in favor of the interdiffusion of Ni and alloying elements between Al₂O₃ surface and iron matrix during the sintering process, so the wettability of ceramic and matrix can be improved (Fig.10b). In addition, the element Cr diffuses into Al₂O₃ and forms (Al_0.8Cr_0.2)₂O₃, and a small amount of NiAl₂O₄ and NiFe₃ may form at the interface of Al₂O₃ and iron matrix, which forms a Al₂O₃/(Al_0.8Cr_0.2)₂O₃/NiAl₂O₄/FeNi₃/Fe interface (Fig.10c). The reinforced interface of Al₂O₃p@Ni/Fe is constructed through mechanical bonding, interdiffusion of elements and chemical reactions.

Fig.10 Schematic diagrams of interface connection mechanism of Al₂O₃p@Ni/Fe composites: (a) initial stage,

(b) reaction stage, and (c) end stage

2.3 Friction mechanism of Al₂O₃p@Ni/Fe composites

The effects of Ni coating on wear properties of Al₂O₃p@Ni/Fe composites were investigated by abrasion test under dry friction conditions. The friction mechanism of Al₂O₃p@Ni/Fe composites is further studied. The modified formula of friction coefficient is as follows^[16]:

$$f=\frac{{\tau }_{\mathrm{f}}}{\sigma }$$

(1)

where f is friction coefficient, τ_f is ultimate shear strength,σ is normal compressive stress.

As shown in Fig.11, the curve shows the fluctuation of friction coefficient over time for the two samples. In the initial stage, the friction coefficient increases, which is due to the bare Al₂O₃ particles on the surface of composites. The high hardness of Al₂O₃ particles makes ultimate shear strength τ_f higher to overcome during the friction, which results in an increase in the friction coefficient, and then the curves tend to be stable. In the stable stage, the friction coefficient of Al₂O₃p@Ni/Fe composites is lower than that of Al₂O₃p/Fe composites. Ni coating diffuses into iron matrix in sintering, and the matrix metal near the interface is alloyed to form ferronickel^[17]. The ductility at the interface is improved. Under the action of cyclic loading, the abrasive particles fill into the groove and void to form a solid lubrication layer, thus reducing the friction coefficient. At the same time, Ni coating improves the interface bonding strength between Al₂O₃ and iron matrix, making Al₂O₃ particles difficult to flake off. Therefore, Ni plating treatment on the surface of Al₂O₃ improves the wear resistance.

Fig.11 Friction coefficient of Al₂O₃p@Ni/Fe composites and Al₂O₃p/Fe composites

The stable friction coefficient and mass loss of Al₂O₃p@Ni/Fe and Al₂O₃p/Fe composites are shown in Fig.12. The results demonstrate that Al₂O₃p@Ni/Fe composites have less abrasion mass loss than Al₂O₃p/Fe composites, and Al₂O₃p@Ni/Fe composites have lower friction coefficient, indicating that Al₂O₃p@Ni/Fe composites have smoother friction surface and smaller roughness, which is conducive to resistance to abrasion, so the wear resistance of Al₂O₃p@Ni/Fe composites is relatively better, which means that the layer of Ni has a more positive effect on wear resistance.

Fig.12 Friction coefficient and mass loss of Al₂O₃p@Ni/Fe and Al₂O₃p/Fe composites

3 Conclusions

1) Al₂O₃@Ni particles are prepared by electroless plating. The Ni coatings present a typical cauliflower structure, and the deposition of Ni coatings begins from the pits and holes on the surface of Al₂O₃ and then gradually extends outwards. Finally, the compact structure completely covers the Al₂O₃ matrix.

2) Al₂O₃p@Ni/Fe composites are prepared by SPS. The Ni coating improves the wettability of Al₂O₃ particles and Fe. The interface layer of Al₂O₃/(Al_0.8Cr_0.2)₂O₃/NiAl₂O₄/NiFe₂O₄/Ni/Fe is formed by mechanical bonding, interdiffusion and chemical reactions, and the interface bonding strength is improved greatly.

3) In the Al₂O₃p@Ni/Fe composites, the diffusion of Ni increases the ductility of interface, and a solid lubrication layer forms, thus reducing the attrition. At the same time, Ni coating improves the interface bonding strength between Al₂O₃ and iron matrix, making Al₂O₃ particles difficult to flake off. Therefore, Ni plating treatment on the surface of Al₂O₃ can improve the wear resistance of Al₂O₃p@Ni/Fe composites.