Fabrication and High Temperature Friction Behavior and Oxidation Resistance of Ni-Co-ZrO₂ Composite Coating

Abstract

Ni-Co alloy and ZrO₂ micron particles were codeposited on 45 carbon steel by electrodeposition. The composition and microstructure of the composite coating were characterized. The high temperature tribological properties were carried out by a pin-on-disk tribo-tester. Additionally, the oxidation resistance was evaluated via high temperature circulating oxidation test. The results indicated that the deposited composite coating showed dispersed ZrO₂ particles and continuous Ni-Co matrix, and there were no obvious pores, cracks and other defects at the interface between the composite coating and the substrate. The embedded ZrO₂ particles changed the friction mechanism from adhesive wear to abrasive wear, the wear loss rate and friction coefficient of Ni-Co-ZrO₂ composite coating were lower in comparison with that of Ni-Co alloy coating and carbon steel substrate. In addition, the embedded ZrO₂ particles exerted a reactive-phase effect on the growth of nickel oxide and cobalt oxide, and effectively reduced the oxidation rate of the substrate at high temperature. Therefore, the Ni-Co-ZrO₂ composite coating presents better oxidation resistance, when compared with Ni-Co coating.

Keywords:
Electrodeposition; Ni-Co-ZrO₂ composite coatings; High temperature tribological behavior; Oxidation resistance

1 Introduction

In modern industries, carbon steels are widely used as a structural material for a variety of engineering applications. However, carbon steels are easy to suffer an attack in aggressive solutions and atmospheres because of its high corrosion and oxidation rate ¹. Moreover, many carbon steel mechanical products and parts should be able to work steadily for long term, especially under high temperature, high-pressure and high-speed conditions. For example, in hot forging processing, works failed due to high temperature wear is a serious problem, and 70-80% of dies in hot environments are damaged by wear. The prolonged life and reduced cost of die is achieved by the use of the anti-wear materials in high temperature environments ². Therefore, in order to improve the wear, corrosion and high temperature oxidation resistance of carbon steels, surface strengthening techniques are still needed to increase the service life and reliability and to improve the performance and quality of mechanical equipment. Fine particles reinforced metal matrix composite coatings are made up of matrix metal and evenly dispersed second-phase particles such as ZrO₂³, B₄C ⁴, Si₃N₄⁵, WC ⁶, Al₂O₃^3,5,7, CeO₂⁸, SiC ^5,9,10, possessing excellent comprehensive performance. ZrO₂ is commonly used as the second ceramic phase due to its high hardness, low thermal conductivity and high temperature oxidation resistance. At the same time, Ni-Co alloys are important as they possess high temperature wear and corrosion resistance because it is hardened by the addition of cobalt (Co) into nickel (Ni) in a form of solid solution which does not embrittle during the heat treatment ^11-12. For Ni-Co alloys possess excellent mechanical and chemical properties, incorporating ZrO₂particles into Ni-Co matrix achieved by electrodeposition presents particular chemical and physical properties, leading to a new class of composite coating ^7-9. Currently, most research works focus on improving the hardness, corrosion and wear resistance, and electrocatalytic activity of Ni-Co alloy matrix composite coatings ^7-9,13. However, the researches on high temperature tribological behavior and the high temperature oxidation resistance of Ni-Co-ZrO₂ composite coating have been less reported yet. In this work, submicron ZrO₂ particles reinforced Ni-Co alloy composite coating was prepared by electrodeposition and its high temperature tribological behavior and its high temperature oxidation resistance were investigated. In the meantime, the phase structure and surface morphologies were also analyzed. For comparison, the Ni-Co alloy coating was also prepared.

2 Experimental details

The electrodeposition was carried out using a ZD-A direct current power supply. Two nickel samples with the size of 70×20×5 mm³ were used as the anode, and 45 steel sample with the size of 12×12×5 mm³ was used as the cathode. The average particle size was about 0.58 µm. ZrO₂ particles were subjected to hydrochloric acid with a concentration of 37.5% for 6 h to remove metal impurities that may exist. The particles were flushed with distilled water till neutral and dried for later use. Prior to the co-deposition, the agglomerated particles were dispersed by suspending the ZrO₂ particles in the electrolyte and subjecting to ultrasonic in the bath for 4 h. The bath was stirred by a magnetic stirrer and maintained at the required temperature. The duration time for plating was maintained so as to obtain a thickness of 65 µm. Ni-Co alloy coating and Ni-Co-ZrO₂ composite coating were prepared under the same conditions of deposition (seen from Table 1).

Table 1
Composition of plating solution and experiment conditions

NiSO4·6H2O	210 g/L
CoSO4·7H2O	95 g/L
H3BO4	20 g/L
Sulfourea	10 mg/L
ZrO2 (0.58 μm)	60 g/L
Parameters for equipment setup
Current density	8 A/dm2
Magnetic stirring rate	250 rpm
Temperature	328 K
pH	2.5~3.5
Plating duration	80 min

High temperature friction and wear test was performed under dry sliding condition at 873 K for 15 min by a rotational pin-on-disk tribo-tester (HT-1000, Lanzhou Zhongkai LTD. China). Si₃N₄ ball (hardness > HV1300) was used as the counter-body. The tests were conducted on a track radius of 5 mm under a load of 10 N and a sliding speed of 0.293 m/s for 15 min. The friction coefficients and sliding time were recorded automatically during the test. Each of friction pairs was cleaned by ultrasonic washing in acetone before and after each test. An electrical balance was used to weigh the samples before and after each wear test, so as to calculate the weight loss of the coatings.

High temperature oxidation tests were carried out in a muffle furnace in static air. The temperature of the furnace was maintained at a set value by an automatic controller with precision of ±2 K. The oxidation experiments were carried out at 573 K, 873 K and 1173 K for 2 h, respectively, and at 873 K for different exposure time^14-16. After oxidation, the samples were withdrew from the furnace and cooled in air without air flow, and then weighed the samples by using an electronic balance with an accuracy of 0.01 mg.

The surface and cross-section micrographs of Ni-Co coatings and Ni-Co-ZrO₂composite coatings before and after high temperature oxidation test, as well as the surface morphologies of the coatings after wear test were observed by Scanning Electron Microscopy (SEM, JSM-6390A, Japan)) and the chemical composition of the Ni-Co and Ni-Co-ZrO₂ composite coatings before and after high temperature oxidation test were probed by Energy Dispersive Spectroscopy (EDS) attached to the above-mentioned SEM. Phase analysis of the Ni-Co-ZrO₂ composite coating after high temperature friction test was identified by X-ray diffraction (XRD) (XRD-6000, Japan) with a CuKα radiation.

3 Results and discussion

3.1 Morphology observation and EDS analysis

Fig. 1(a) shows the surface SEM micrographs of the Ni-Co coating. It is clearly that Ni-Co coating presents typical crystal morphology with average uniform particle size of about 4 µm. The larger crystal particles size makes the coating surface coarse relatively. Fig. 1(b) shows the surface SEM micrographs of the Ni-Co-ZrO₂ composite coating prepared on 45 steel substrate. It can be seen that the obtained composite coating possesses a mat-gray smooth metallic surface with fine and compact white spots which are visible to naked eye. And in Fig. 1(d), we can see that each white ZrO₂ particle size is less than 1 um, it means the distribution of ZrO₂ particles is favorable. Fig. 2(a) presents the cross-section micrograph of Ni-Co composite coating, showing continuous and good combinations between the coating and the substrate, and there are no obvious defects between the composite coating and the substrate. And Fig. 2(b)shows the cross-section micrograph of Ni-Co-ZrO₂ composite coating, which presents dispersed ZrO₂ particles and continuous Ni-Co matrix, and there are no pores and cracks and other defects at the interface.

Fig. 1
SEM micrographs of surface: (a) Ni-Co coating (b) Ni-Co-ZrO₂ composite coating.

Fig. 2
SEM micrographs of the cross-section: (a) Ni-Co coating (b) Ni-Co-ZrO₂ composite coating.

Fig. 3 (a) shows the EDS analysis which shows that the composition of coating is Ni-Co alloy and there are no impurities in the coating. Fig. 3 (b) illustrates the EDS analysis data, which strongly suggest that the composition of coating are nickel cobalt (Ni-Co) alloy and ZrO₂ particles, and it also reveals much higher content of cobalt (67.69wt%) than that of nickel (29.68wt%) in the composite coating. Co-rich Ni-Co alloy possesses better thermostability compared with Ni-rich Ni-Co alloy ¹⁷, so the coating was designed with higher Co content than Ni in content. Fig. 3(b) also shows that the embedded ZrO₂ particles with irregular shape homogeneously disperse within the Ni-Co matrix.

Fig. 3
EDS analysis of (a) Ni-Co coating (b) Ni-Co-ZrO₂ composite coating.

3.2 High temperature friction behavior of Ni-Co-ZrO2 composite coating

Fig. 4 illustrates the variations of wear weight loss for 45 steel substrates, Ni-Co alloy coating and Ni-Co-ZrO₂ composite coating. The wear rate decreases from 25.1×10^-3 mg/m for 45 steel down to 22.3×10^-3 mg/m for Ni-Co coating and further down to 14.8×10^-3 mg/m for Ni-Co-ZrO₂ composite coating. Fig. 4 demonstrates that Ni-Co-ZrO₂ composite coating possesses the best wear resistance. The decrease of the wear rate of Ni-Co-ZrO₂composite coating, as compared with Ni-Co coating, is rationally understood because of the plastic deformation of the matrix material under the load by way of the ZrO₂ particles dispersion strengthening.

Fig. 4
Wear weight loss of 45 steel, Ni-Co alloy coating and Ni-Co-ZrO₂ composite coating at 873 K for 15 min.

Fig. 5 shows the relationships between friction coefficient and the sliding time for 45 steel, Ni-Co alloy coating and Ni-Co-ZrO₂ composite coating at 873 K. It shows that the friction coefficient of Ni-Co-ZrO₂ composite coatings is lower than that of Ni-Co coating and much lower than that of 45 steel, which agrees well with the results shown in Fig. 4.

Fig. 5
Friction coefficient of 45 steel substrate, Ni-Co alloy coating and Ni-Co-ZrO₂ composite coatings at 873 K.

Fig. 6 presents the XRD pattern of Ni-Co-ZrO₂ composite coatings after high temperature friction at 873 K for 15 min. The peaks of NiO and CoO phases which could further prevent inward oxygen diffusion, appear on the XRD pattern, and there are no ferric (Fe) oxides, confirmed by Fig. 3, where no Fe was probed, It shows that the carbon steel substrate is well protected by the composite coating.

Fig. 6
XRD pattern of Ni-Co-ZrO₂ composite coatings after high temperature friction for 15 min at 873 K.

Fig. 7 shows the morphologies of the wear traces surface of 45 steel with and without coatings. Fig. 7(a-b) presents photographs of a typical worn surface of 45 carbon steel at 873 K, Fig. 7(a)shows that there is a wide and dark furrow along the sliding direction. FromFig. 7(b), it can be seen that the entire surface of 45 steel after oxidation is dark, the oxidation is heavy, and it further identifies that the darker areas in Fig. 7(a) are indeed a deep groove. The groove means that the wear loss will be high. The result as just mentioned could be attributed to the brittle oxide layer, which cannot effectively prevent the substrate from sliding cut from the micro-contact surface of counter-part ¹⁶.

Fig. 7
SEM morphologies of the surface of 45 carbon steel (a-b), Ni-Co alloy coating (c-d) and Ni-Co-ZrO₂ composite coating (e-f) after friction with the load of 10 N for 264 m at 873 K.

Fig. 7(c-d) presents the wear trace of Ni-Co coating. Arrows in Fig. 7(d) shows that there are many cracks vertical with the sliding tracks, and the integrity of coating was poor. The darker areas directed by the arrows were oxides, which were formed during the high temperature tribo-test. The indents generated in the alloy coating surface along the sliding direction were formed by the welding phenomenon, which was caused by imprisoning the fragments between the specimen surface and the pin. Fig. 7(e-f) presents the worn morphologies of Ni-Co-ZrO₂ composite coatings. During the wear test, the ZrO₂ particles carry the load and prevent Ni-Co matrix from wearing by way of dispersive strengthening as discussed in paragraph 1 of section 3.2, and when a large number of ultra-fine ZrO₂ particles were dug out by counter-body from the coating surface. The friction mechanism was transformed from pure sliding to partly rolling, which reducing the friction force and smoothing the counter surface in a similar way by polishing it with ultra-fine particles. Compared with Ni-Co coating, Ni-Co-ZrO₂composite coatings had the small and slight cracks, it is because ZrO₂ particles could reinforce composite coating and hinder the growth of the cracks, and hence the friction coefficient became lower, it was also confirmed by Fig. 5. Ni-Co-ZrO₂ composite coating possesses better oxidation resistance than that of Ni-Co coating and 45 steel as discussed earlier, so Ni-Co-ZrO₂ composite coating was not oxidized so heavily as Ni-Co coating and 45 steel. Compared with Fig. 7(a-b) and (c-d), it can be seen from all SEM morphologies that the surface of Ni-Co-ZrO₂ composite coating is the smoothest, the integrity is the best and the oxidation is the lightest. So the Ni-Co-ZrO₂ composite coating possesses higher wear resistance and lower wear rate than that of Ni-Co alloy coating and 45 steel.

3.3 Oxidation resistance

The oxidation experiments were carried out at 573 K, 873 K and 1173 K for 2 h, respectively. Fig. 8 shows the weight gain of 45 carbon steel, Ni-Co alloy coating and Ni-Co-ZrO₂ composite coating at different temperatures. From Fig. 8, it can be seen that with the increase of the temperature, the weight gain of 45 steel, Ni-Co alloy coating and Ni-Co-ZrO₂ composite coating are all increased. When the oxidation temperature is 873 K, the weight gain of 45 steel, Ni-Co alloy coating and Ni-Co-ZrO₂ composite coating is lower. While it is up to 1173 K, the weight gain of 45 steel rapidly increased by 13.4 times, higher than that of Ni-Co coating (9.7 times), and much higher than that of Ni-Co-ZrO₂ composite coating (7.8 times). It indicates that Ni-Co-ZrO₂ composite coating possesses the best high temperature oxidation resistance.

Fig. 8
Weight gain of 45 steel, Ni-Co alloy coating and Ni-Co-ZrO₂ composite coating after oxidation test for 2 h at different temperatures.

Fig. 9 gives the oxidation weight gain curves for 45 steel, Ni-Co coating and Ni-Co-ZrO₂ composite coating samples at 873 K. From Fig. 9, it can be seen that the weight gain of 45 steel, Ni-Co alloy coating and Ni-Co-ZrO₂ composite coating increase with the increase of oxidation time. Compared with 45 steel, the oxidation rate of Ni-Co coating and Ni-Co-ZrO₂ composite coating reduce significantly after 2 h. Meanwhile, the weight gain and the oxidation rate of Ni-Co-ZrO₂composite coating are lower than that of Ni-Co alloy coating, and it means that Ni-Co-ZrO₂ composite coating possesses better oxidation resistance than that of Ni-Co coating.

Fig. 9
Weight gain of 45 steel, Ni-Co alloy coating and Ni-Co-ZrO₂ composite coating after high temperature oxidation test at 873 K.

Fig. 10 shows the cross-section micrographs of the Ni-Co coating and Ni-Co-ZrO₂ composite coating after high temperature oxidation test at 873 K for 6 h. It was found that the surface of Ni-Co coating appears black oxide, accompanied by a little crack. In contrast, Ni-Co-ZrO₂ composite coating is still intact, and there are no crack and fall off. And in Fig. 10(b), we can see that the oxide layers obviously. It can be confirmed that the oxide layer of Ni-Co-ZrO₂ composite coating is smooth and uniform in comparison with that of Ni-Co coating after high temperature oxidation test.

Fig. 10
Cross-section micrographs after high temperature oxidation test for 6 h at 873 K: (a) Ni-Co coating (b) Ni-Co-ZrO₂ composite coating.

Fig. 11 gives EDS analysis of the cross-section micrographs of Ni-Co coating and Ni-Co-ZrO₂ composite coating after high temperature oxidation test at 873 K for 6 h. It shows that the oxide layers contain the elements of Ni, Co and O. Fig. 12 shows the line scanning analysis diagram, suggesting that the oxygen content in the coating increases and the content of Co is still high. It can be demonstrated that the oxide layer has formed on the surface of Ni-Co coating and Ni-Co-ZrO₂ composite coating. However, the content of embedded ZrO₂ particles in the Ni-Co-ZrO₂composite coating decreases after high temperature oxidation test, the reason why the ZrO₂ particle decreases needs to be further explored.

Fig. 11
EDS analysis of the cross-section: (a) Ni-Co coating (b) Ni-Co-ZrO₂ composite coating after high temperature oxidation test for 6 h at 873 K.

Fig. 12
Line scanning analysis diagram after high temperature oxidation test for 6 h at 873 K (a) Ni-Co coating (b) Ni-Co-ZrO₂ composite coating.

Fig. 13 shows the SEM micrographs of surface morphology of Ni-Co coating and Ni-Co-ZrO₂ composite coating after high temperature oxidation test. The aggregation particles of Ni-Co composite coating on the surface are uneven, where appears some irregular lumps; however, the particles of Ni-Co-ZrO₂ composite coating on the surface are uniform and tiny, which confirms the analysis of the oxidation weight gain curves of two coatings.

Fig. 13
SEM micrographs of the surface after high temperature oxidation test for 6 h at 873 K (a) Ni-Co coating (b) Ni-Co-ZrO₂ composite coating.

Fig. 14 presents the X-ray diffraction patterns of Ni-Co coating and Ni-Co-ZrO₂ composite coating. Fig. 14(a),(c) show XRD patterns of Ni-Co coating and Ni-Co-ZrO₂ composite coating before high temperature oxidation test, and Fig. 14(b),(d) show XRD patterns of Ni-Co coating and Ni-Co-ZrO₂ composite coating after high temperature oxidation test for 6 h at 873 K. It can be seen that NiO and CoO formed after high temperature oxidation test.

Fig. 14
XRD pattern before and after high temperature oxidation test for 6 h at 873 K (a) (b)of Ni-Co coating (c) (d) Ni-Co-ZrO₂ composite coating.

In summary, it is well known that the oxygen diffusion through the coating is the dominant diffusion mechanism during the oxidation process ³. The reason why Ni-Co-ZrO₂composite coatings can improve the high temperature oxidation resistance is that the dispersive ZrO₂ particles in the Ni-Co matrix reduce the effective area of Ni-Co alloy contacting with ambient oxygen. Furthermore, the ZrO₂ particles exert a reactive-phase effect on the growth of NiO and CoO in the composite coating and effectively reduce the oxidation rate of the composite coating at high temperature. In addition, new phases like NiO and CoO further prevent inward oxygen diffusion. It is also one of the important factors that enable the composite coating to achieve high temperature oxidation resistance.

4 Conclusions

• 1)

  Ni-Co alloy and ZrO₂ micron particles were codeposited on 45 steel by electrodeposition and the deposited composite coating shows dispersed ZrO₂ particles and continuous Ni-Co matrix, and there are no pores and cracks and other defects at the interface between the composite coating and the substrate.

• 2)

  The embedded ZrO₂ particles change the friction mechanism from adhesive wear to abrasive wear. The wear loss rate and coefficient of Ni-Co-ZrO₂ composite coating are lower than that of Ni-Co alloy and much lower than that of 45 steel, and the Ni-Co-ZrO₂composite coating possesses the best high temperature wear resistance.

• 3)

  The embedded ZrO₂ particles exert a reactive-phase effect on the growth of nickel oxide (NiO) and cobalt oxide (CoO) and effectively reduce the oxidation rate of the substrate at high temperature. The oxidation weight gain of Ni-Co-ZrO₂ composite coating is lower than that of Ni-Co coating, demonstrating that the Ni-Co-ZrO₂ composite coating presents better high temperature oxidation resistance.

Acknowledgments

This work is supported by Shaanxi Industrial Science and Technology Research (2014K08-09) and National Nature Science Foundation of China (50172023) and the Scientific Research Program Funded by Yulin city (2012) and the College Students' Innovation Training Program of Xi'an University of Science and Technology (201510704070).

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Publication Dates

• Publication in this collection
  May-Jun 2016

History

• Received
  06 Feb 2015
• Reviewed
  08 Jan 2016
• Accepted
  07 Mar 2016