Electroless nickel plating on AZ91 Mg alloy substrate

Surface & Coatings Technology 200 (2006) 5087 – 5093 www.elsevier.com/locate/surfcoat

Electroless nickel plating on AZ91 Mg alloy substrate Zhenmin LiuT, Wei Gao Department of Chemical and Materials Engineering, The University of Auckland, Auckland 1001, New Zealand Received 9 January 2005; accepted 21 May 2005 Available online 23 June 2005

Abstract Electroless Ni-plating on Mg alloy AZ91 has been studied to understand the effect of the substrate microstructure and roughness on the deposition rate, nucleation, coating microstructure, and mechanical property of the coatings. Experimental results indicate that the growth of Ni deposit in the early stage was influenced by the substrate microstructure and roughness. The electroless Ni-plating on the abrasive blasted AZ91 (rough) substrate showed a higher deposition rate than that on the finely polished one, indicating that the mechanical roughening enhances the nucleation and coalescence of Ni crystallites. Scratching tests showed that higher coating adhesion is achieved on the roughened AZ91 substrate. Wear tests, however, showed that the Ni plating on the rough substrate has a higher friction coefficient than that on the polished surface. The hardness and adhesion property of Ni coatings before and after heat treatment were also characterised. D 2005 Elsevier B.V. All rights reserved. Keywords: Magnesium alloy; Electroless Ni plating; Substrate microstructure; Substrate roughness; Scratch adhesion test

1. Introduction Mg alloy is presently used in a wide range of structural applications such as aerospace, automotive, electronics and other industries. It is expected that applications, in particular involving motion or portability of the component, will increase in the future because of the material’s high strength-to-weight ratio and a relatively high stiffness [1,2]. One of the challenges in using Mg-based materials is their poor corrosion and wear resistance. Corrosion and wear resistance of Mg parts is often enhanced by means of surface coatings or treatment. Among the various surface techniques, electroless Ni (EN) coating is of special interest due to its good properties and convenience. However, being a highly chemically active alloy, plating on Mg alloys needs special bath formulation and pretreatments. Hence, direct plating of Mg is still a challenge for producers and researchers. The process becomes more complicated on AZ91 alloy due to the microstructure * Corresponding author. Tel.: +64 9 3737599x88667; fax: +64 9 3737463. 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.05.023

heterogeneity owing to the uneven distribution of Al in the three constituent phases (primary a, eutectic a and h phases) [3], which often leads to the non-uniform coating growth. In order for a coating to provide adequate protection, the coating must be uniform, well adhered and pore-free [1]. So far there are a few patents on EN plating processing and bath formula [4 – 6]. Several papers are on the research of corrosion and wear proTable 1 Chemical composition of AZ91D Mg alloy (wt.%) Al

Zn

Mn

Ni

Cu

Fe

Si

Ca

K

Mg

9.0

0.64

0.17

0.001

0.001

<0.001

<0.01

<0.01

<0.01

Bal

Table 2 Surface roughness value Ra (Am) and coating wear properties Sample

Before etching

After etching

After plating (110 min)

As-plated Friction coating coefficient hardness

Polished AZ91 0.05 T 0.01 1.2 T 0.2 0.32 T 0.05 580 – 610 0.18 Blasted AZ91 4.5 T 0.5 5.5 T 0.5 1.2 T 0.2 0.34

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Z. Liu, W. Gao / Surface & Coatings Technology 200 (2006) 5087 – 5093

Fig. 1. Cross-section of EN coatings on (a) polished smooth AZ91and (b) blasted rough AZ91R substrates (plated for 110 min).

perties of the deposits on Mg alloys [2,3,7 – 9]. One of them reports that the EN deposits become more compact and defect-free as the substrate surface roughness increases [9]. Although EN coatings were studied since 1970s on steels, Cu, Al and later on Mg alloys [2,3,7 –15], most of these studies were focused on the relationships among the plating bath compositions, temperature, additives, pH value and the deposition mechanisms. Very few of them paid attention to the substrate roughness and its effects on the deposition mechanisms and coating properties. This paper reports a study on EN coatings on AZ91D Mg alloy, specifically, the effects of substrate roughness on coating nucleation and some mechanical properties.

half of the samples were mechanically polished with sand paper to a roughness value (Ra) of ¨0.05 Am. Another half was blasted with a pressurized-air sand blaster, using ceramic beads sized from 50 to 300 Am. The blaster air pressure was ¨1.6 bar. The roughness value (Ra) of the matte-finished substrate is about 4.5 Am. Electroless coating on Mg and Mg alloys requires controlled surface pre-treatment to insure integrity and adhesion of the coating layers. These steps include cleaning, etching and fluoridization before EN plating. The detailed operation processes can be found elsewhere [16]. The deposition rate on the substrates was measured as the mass gains at the time of 4, 10, 20, 30, 45, 60, 90, 110 and 150 min immersion. Triplicate experiments were conducted under the same plating processing and the coating rate reported is the average of three experiments. It is found that the reproducibility of the deposition is reasonably good. The differences of deposition mass gains measured from the three experiments are within 5% of the total mass gains. The microstructure of the deposits and substrates were examined by optical microscopy and high-resolution scanning electron microscopy (HRSEM). The phase structure of the deposits was analysed using X-ray diffraction technique (XRD). The surface roughness and mechanical properties of the substrates were measured before and after etching and after EN coating (Table 2). Wear property of the Ni coatings was tested on a fully computerized micro-tribometer (Model UMT-2), with a friction counterpart of a cemented carbide ball of U = 5 mm. A load of 5 N and a sliding speed of 2 mm/s were used without lubrication and the total elapsed time was 15 min. Scratch adhesion tests was performed using a CSR-01 scratch tester fitted with a Rockwell ‘‘C’’ diamond (120- cone with a 200 Am radius hemispherical tip). The applied load was from 0 to 5 kg in a uniform accelerating motion with a sliding speed of 10 mm/min. The scratch length was 10 mm.

3. Results and discussions 3.1. Surface roughness and wear properties

2. Experimental procedure The substrate material used in the present investigation is AZ91 sand-cast alloy. The chemical composition of the alloy is given in Table 1. Rectangular coupons sized 10  15  3 mm3 were used as the substrates. Before plating,

The surface roughness values, coating hardness and friction coefficient are given in Table 2. Etching makes the surface more chemically active and rougher on both types of substrate, but the degree of chemical roughening is far below that of sand blasting roughening (1.2 versus 4.5 Am). In other words, sand blasting produced a much

Table 3 Hardness and adhesion strength of EN coatings Substrate sample

As-plated hardness (HV50g)

Adhesion strength (L C, N)

523 K annealed coating hardness (HV50g)

Adhesion strength (L C, N)

673 K annealed coating hardness (HV50g)

Adhesion strength (L C, N)

Polished (fine) AZ91 Blasted (rough) AZ91R

580 – 610

10.2 14.0

870 – 930

10.6 16.5

980 – 1040

9.7 14.8

Z. Liu, W. Gao / Surface & Coatings Technology 200 (2006) 5087 – 5093

rougher surface than chemical treatment (see Table 2 and Fig. 1). This can improve the adhesion of coatings further (see Table 3). It can also be seen from Fig. 1a that the microstructure heterogeneity contributes to the increase of the polished substrate surface roughness but does not have significant effect on the surface roughness of the blasted substrates (Fig. 1b). Moreover, it can be found from Fig. 1 that the EN coating on blasted substrate is thicker and more uniform than that on the polished one. It should be expected that the EN plating on a rougher substrate is more compact and defect-free [9]. Therefore it might be concluded that the sand blasting reduces the negative effect of microstructural heterogeneity of AZ91 alloy on EN plating, even though it increases the EN coating surface roughness. Nevertheless, the coating surface roughness on both polished and blasted samples decreased significantly compared with the substrates, indicating the leveling ability of EN plating. The micro-hardness of EN coatings was measured using a diamond indenter. Vickers hardness values were obtained with a load of 50 g on the Ni plating by averaging five measurements. The hardness of the coatings is listed in Table 2, indicating that the EN coatings have much higher hardness than the substrates, which was in the range of 85– 95 HV for AZ91 alloy. Moreover, the micro-hardness of the coatings increased to ¨900 HV after 1 h heat treatment at 523 K and 1040 HV after 1 h at 673 K. This hardness is similar to the EN coatings on steels [17,18]. It is believed that the hardness increase during annealing is due to the phase changes, i.e. formation of coherent Ni3P. Friction coefficient curves of the EN coatings on AZ91 substrates with different roughness are shown in Fig. 2 and the wear tracks are shown in Fig. 3. The friction coefficients of EN coatings on the polished and rough AZ91 substrates are in the range of 0.15– 0.2 and 0.3 – 0.4, respectively. The rough coating showed a higher friction coefficient and large variation at the beginning of

5089

Fig. 3. Wear tracks observed with SEM on EN coatings: (a) on polished and (b) rough AZ91 substrate.

wear and took a few seconds to reach a stable condition. The micrographs in Fig. 3 also show that the rougher EN plating surface had narrower wear tracks (¨100 Am) than those on the polished AZ91 substrate (¨200 Am). This is probably due to the EN coating surface on the blasted AZ91 substrate being rougher than that on the polished AZ91 (Table 2). It can be concluded from Fig. 3 that a load of 5 N does not damage the coating severely,

Fig. 2. Friction coefficient of Ni plating on polished and rough AZ91 substrates.

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implying that the EN coating on AZ91 may resist a load larger than 5 N. More detailed investigations on wear and friction property are in progress. 3.2. Scratch adhesion test and the effect of heat treatment The American Society for Testing and Materials defines adhesion as ‘‘two surfaces are held together by interfacial forces’’ (ASTM D 907-70). These forces can be FVan der Waals_ forces, electrostatic forces or chemical bonding forces that are effective across the interface. A method to assess the quantitative value of adhesion of coatings on substrate is the scratch test, which is measured by the critical load L C. L C and the effect of heat treatment on L C were summarized in Table 3.

Fig. 4 shows the scratch testing result including the scratch track images and friction plots which can be used to assess the adhesion strength between the coatings and substrates. In terms of the scratch track morphology there is a similar trend to that of wear tracks, i.e. the scratch track width on the rough surface is narrower than that on polished surface (Fig. 4a and b). Furthermore, in both cases the track width becomes slightly narrower after 1 h annealing at 523 K. This is probably due to the increase of coating hardness from ¨600 to ¨900 HV. It can be seen from Fig. 4 that the wear, throughthickness cracking and spallation failures appear at different specimens and scratch testing stages. It is accepted that small oscillations often appear with the wear failure and the oscillations grow with cracking and spallation. In our

Fig. 4. Friction plot for the scratch tests of EN. The upper-left SEM micrograph is the track at the beginning, the lower-right one is the end track image: (a) on the polished AZ91 substrate, (b) on rough substrate, (c) on polished substrate after 1 h annealing at 523 K and (d) on rough substrate after 1 h annealing at 523 K.

Z. Liu, W. Gao / Surface & Coatings Technology 200 (2006) 5087 – 5093

5091

Fig. 4 (continued).

experiments polynomial fit of the primary data and tangential approximation were used to estimate the critical load value. It can be seen that the critical load L C of the coating on rough AZ91 substrate is ¨1400 g, higher than L C for the polished ones (¨1000 g), indicating that the coatings on the rough substrate has a better adhesion strength. This can be explained by the effective mechanical interlocking between the EN and rough substrate. The friction plots and scratch SEM images also demonstrate that severe wedge spallation took place on the coatings of the polished substrate (Fig. 4a). It is interesting to see that the adhesion strength (L C) of the coating on rough substrate was increased to 1650 g after 1 h annealing at 523 K (Fig. 4d). It is believed that this increase was caused by the combination of the increase of coating hardness and the effect of the rough

interface. The scratch failure on the rough substrates shows brittle cracking of the coating while the wedge spallation of the coating was found on the polished substrates (Fig. 4c and d). This further demonstrates that the adhesion of coatings on the rough substrate is stronger than that on the polished one. It can be seen from Table 3 that the annealing at 523 K produced the maximum adhesion strength. Adhesion property usually depends on factors including the coating hardness, ductility and substrate surface conditions [19,20]. From the SEM micrographs of the scratch tracks, the EN coatings become more brittle as the annealing temperature increases up to 673 K [17]. Therefore the adhesion strength did not increase after an annealing at 673 K although the coating hardness has increased a little.

Mass gain (mg/cm2)

5092

Z. Liu, W. Gao / Surface & Coatings Technology 200 (2006) 5087 – 5093 AZ91 AZ91R

28 26 24 22 20 18 16 14 12 10 8 6 4 2 0 0

20

40

60

80

100 120

140

160

Time (minutes) Fig. 5. The kinetics of EN deposition on AZ91 substrates with different roughness.

3.3. Deposition rate and deposit morphology Fig. 5 shows the deposition mass gain as a function of time on AZ91 alloy with different surface roughness. The deposit mass gain curves did not follow a linear relation as reported in some literatures [7]. The reason of this has been discussed in detail in a previous paper [16]. It can also be seen that the deposition curves show an incubation period of time (¨10 min). This can be

explained as the chemical replacement reactions take place before the nickel autocatalytic reaction and the rate of the first stage replacement reaction is usually low [16,21]. It has also shown that the deposit mass gain on the rough AZ91 alloy substrate surface (AZ91R) is higher than that on the polished substrate (AZ91), specifically during the early deposition stage (20 – 30 min). Microstructure was studied to understand the deposition mechanisms and kinetics on the two types of substrates. It is generally accepted that the first stage of EN deposition consists of three simultaneous crystalbuilding processes: nucleation, growth, and coalescence of three-dimensional crystallites (3DCs), which results in the rapid deposit mass gain on substrates [18]. Fig. 6 shows the effects of substrate roughness on the nucleation, growth and coalescence processes. It can be found that the EN nucleus initially formed around the a and h phase boundaries from Fig. 6a and c. There are more and larger Ni crystals formed on the rough surface than on the polished surface, resulting in the higher deposition rate on the rough surface. This is probably due to the fact that the specific surface area of the roughened substrate is larger than that of the polished substrate, which consequently provides more nucleus sites for the nucleation of 3D crystals. This result is also consistent with the results observed in vapour deposition that low substrate roughness led to less nucleation sites [22]. At the later stage (after 30 min in Fig. 5), however, the

Fig. 6. SEM image of Ni – P coatings on AZ91 substrate: (a) polished substrate after 4 min deposition, (b) rough substrate after 4 min deposition, (c) polished substrate 10 min deposition, and (d) rough substrate 10 min deposition.

Z. Liu, W. Gao / Surface & Coatings Technology 200 (2006) 5087 – 5093 1 2 3 4

a

500

EN on rough AZ91 EN on polished AZ91 EN at 523 K annealing EN at 673 K annealing

CPS

(3) Ni plating on the sand-blasted substrate has a higher friction coefficient than that on the polished surface. The coating surface on the polished substrate appeared flatter than that on the rough substrate, resulting in the different wear behaviors.

a: Ni b: Ni3P

400

5093

300

Acknowledgments 200 b

100

b a b

b

b

4 3 2

1

0 30

40

50

60

70

80

90

2 Theta Fig. 7. X-ray diffraction spectra of Ni coatings with and without annealing.

deposition rates were becoming closer between the rough and fine polished surfaces, probably due to the leveling effect of EN deposition which reduces the surface roughness of the coating, making the difference of deposition rates on both substrates smaller after 30 min plating. 3.4. Crystal structure of EN coatings Fig. 7 showed the X-ray diffraction spectra of EN coatings on AZ91 substrate. The as-deposited coatings formed on both polished and rough substrates showed a broad peak in the spectra, indicating an amorphous structure possibly with small microcrystalline areas. The substrate roughness does not have significant effect on the crystal structure of the coatings (see curves 1 and 2). Crystallisation took place after ¨1 h annealing at 523 K and a higher annealing temperature produced sharper Ni peak and more Ni3P precipitate (curves 3 and 4), which contributed to the increase of micro-hardness discussed before [17,18,23,24].

4. Conclusions (1) Electroless nickel coatings on AZ91 Mg alloy possess an amorphous structure. Crystallisation takes place after 1 h annealing at 523 K. An annealing at 673 K results in better crystallinity of Ni coating and more Ni3P precipitates, which contribute to the increase of hardness up to 1040 HV. However, the maximum adhesion strength was obtained after 1 h annealing at 523 K. (2) A strong influence of substrate surface roughness was observed. The deposition rate on the sand-blasted AZ91 substrate is higher than that on the polished substrate. Consequently, a thicker coating was obtained on the rough substrate, with improved adhesion due to the enhanced interlocking force.

The authors gratefully acknowledge the support of the Foundation for Science, Research and Technology of New Zealand on the RFI Programme entitled Transforming Light Metals. We also appreciate the technical supports from the Department of Chemical and Materials Engineering, the Research Centre for Surface and Materials Science, and Dalian University of Technology China, especially from Dr. X. Mei, C. Hobbis, S. Strover and A. Asadov.

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