Microstructure and erosion characteristics of Ni-AlN thin films prepared by electrodeposition

1 Introduction

Aluminum nitride is a ceramic material used in many applications, such as in optoelectronics, ceramic filters, insulating films, and dielectric materials. AlN nanoparticles are usually introduced to Ni-based thin films to enhance their properties, including microhardness, corrosion resistance, and wear resistance [1–5]. Electrodeposition is relatively easy to control and is widely used to incorporate a variety of ceramic particles into different metals [6–12]. In recent years, metallic thin films have been produced successfully. Chen et al. [13] electrodeposited Ni-Al₂O₃ thin films via a Watts-type bath. The addition of a cationic surfactant hexadecylpyridinium bromide reduces the agglomeration of particles, resulting in the uniform distribution of Al₂O₃ particles in the Ni matrix. Niu et al. [14] fabricated Ni-SiC composite through direct-current (DC) electroplating technique. The effects of the principal technological parameters, including solution agitation speed, concentration of SiC nanoparticles, and current density, on the microstructure of the Ni-SiC composite film were investigated and optimized. Ma et al. [15] prepared Ni-SiC composite thin films via ultrasonic-assisted pulse-current (UAPC) deposition. They found that the average grain diameters of Ni and SiC in the UAPC-deposited thin film were 63.6 and 38.5 nm, respectively. Amadeh et al. [16] prepared Ni-SiC nanocomposite coatings with various SiC contents by pulse electrodeposition from a modified Watts bath containing SiC nanoparticles. The properties of thin films have been shown to mainly depend on both the matrix phases and the amount and distribution of co-deposited ceramic particles. This distribution is related to many process parameters, such as ceramic particle concentration in solution, electrolyte composition, and applied current (DC, PC, and current density) [17–20]. Although many investigations on DC or PC deposition of metals have been reported [21–23], few reports are available concerning the application of UAPC deposition in preparing Ni-AlN thin films. Thus, exploring the preparation method and characterization of Ni-AlN coatings fabricated by the UAPC deposition technique is necessary. Our current research indicates that Ni-AlN thin films with superior properties can be prepared by UAPC deposition. In this study, three types of Ni-AlN thin films were produced via different techniques, namely, DC, PC, and UAPC deposition. The morphology and mechanical properties of the fabricated Ni-AlN thin films were discussed. The erosion characteristics of the films were also investigated.

2 Materials and methods

Figure 1 shows the electrodeposition equipment used. The ultrasonic generator (XL-500, Beijing Xieli Science and Technology Development Co., Ltd, Beijing, PR China) could generate high-frequency (20 kHz) axial vibration signals, which were converted to mechanical vibrations with the use of a transducer. The maximum power of the ultrasonic generator was 500 W. A metal frame (150×150×120 mm) was used to support the plating bath. A plastic plating bath (90×90×100 mm) was utilized to fill the electrolyte. A pulse plating power source (SMD-30, Handan Dashun Electroplating Equipment Co., Ltd, Handan Hebei, PR China) was operated in DC, PC, and UAPC electrodeposition.

Figure 1

Schematic of the experimental setup.

Ni-AlN thin films (∼50 μm thick) were prepared on steel substrates (20×20×5 mm) by DC, PC, and UAPC deposition. The thickness of the films was surveyed by an ultrasonic thickness detector (TT100, Beijing Time High Technology Co., Ltd, Beijing, PR China). The steel substrates were used as cathodes. Prior to deposition, the substrates were polished, and a roughness tester (SV-RT110, Guangzhou Huazhi Instruments and Apparatuses Co., Ltd, Guangzhou, Guangdong, PR China) was employed to measure the surface roughness (Ra) of the substrates. After polishing, the substrates exhibited an Ra value of 0.15 μm. Two Ni plates (40×40×5 mm) were used as anodes. The following electrolytes were utilized to obtain electrodeposited Ni-AlN thin films: 35 g/l nickel chloride, 200 g/l nickel sulfate, 28 g/l boric acid, and 8 g/l AlN particles. The temperature was maintained at 50°C and pH 4.5, which was adjusted using ammonium hydroxide or sulfuric acid dilutions. Table 1 provides the plating parameters for the electrodeposition of Ni-AlN thin films. During electrodeposition, the AlN particles (∼30 nm) were introduced into the electrolyte. AlN particles were observed using transmission electronic microscopy (TEM, Tecnai-G2-20-S-Twin, FEI Co., Ltd, Columbia, MD, USA). Figure 2 shows a representative TEM image of the AlN particles. This image confirms the nanometer size, shape regularity, and agglomeration of the AlN particles.

Figure 2

TEM image of AlN nanoparticles.

The surface morphology of the Ni-AlN thin films was determined by scanning probe microscopy (SPM, Nanoscope IIIa, Veeco Co., Ltd, Plainview, NY, USA) and scanning electronic microscopy (SEM, JSM-5610LV, JEOL Co., Ltd, Mitaka, Tokyo, Japan) with energy-dispersive X-ray analysis (EDS, IE-300X, Oxford Semiconductor Co., Ltd, Oxford, UK). The operating voltage for the SEM was 20 kV. To determine the phase structure of the Ni-AlN thin films, X-ray diffraction analysis was performed with the use of a D/Max-2400 instrument (Rigaku Co., Ltd, Akishima-shi, Tokyo, Japan) with Cu Kα radiation (λ=0.15418 nm). Vickers hardness was measured with the use of a 401 MVT microhardness tester (Shanghai Precision Instruments Co., Ltd, Shanghai, PR China) at loads of 100 gf for 15 s. The Ni-AlN composite thin films were subjected to corrosion tests in an aerated 3.0 wt.% NaCl solution for 30 h at ambient temperature by using a CHI 650B electrochemical station (Shanghai Huachen Instruments Co., Ltd, Shanghai, PR China) at 0.06 mV/s scan rate. A saturated calomel electrode (SCE) ending in a lugging capillary was used to measure the electrode potential without IR drops. Weight loss was measured with an electronic analytical balance [BS210S, ±0.01 mg, Sartorius Scientific Instruments (Beijing) Co., Ltd, Beijing, PR China].

3 Results and discussion

3.1 Microstructures of Ni-AlN thin films

Figure 3 presents the SPM images of the three types of Ni-AlN thin films. The Ni-AlN film fabricated by PC deposition exhibits a relatively uniform and compact morphology, whereas the film produced by DC deposition displays relatively coarse and non-uniform structure. PC can increase the nuclei number for nucleation of Ni grains and inhibit grain growth. Moreover, the Ni-AlN film prepared by UAPC deposition exhibits a compact and exiguous morphology. The grain size in this film is smaller than that in the other films because of ultrasonication. PC also breaks the normal growth of Ni crystals and disrupts larger crystals from producing smaller nuclei. The average grain diameters of Ni and AlN in the UAPC-deposited film are approximately 97.7 and 40.2 nm, respectively. These results establish that the film synthesized by UAPC deposition is the most excellent and that by DC deposition is the worst in terms of morphology.

Figure 3

SPM images of Ni-AlN thin films deposited by (A) DC, (B) PC, and (C) UAPC methods.

The XRD patterns of the Ni-AlN films were obtained to confirm the existence of AlN particles. The patterns were recorded from 2θ=20° to 80° with a 0.02° scan step. Figure 4 shows the XRD patterns of the three types of Ni-AlN thin films. The thin films consist of Ni and AlN phases. For the Ni grain, the diffraction peaks at 44.8°, 52.2°, and 76.8° correspond to the (1 1 1), (2 0 0), and (2 2 0) planes, respectively. For the AlN particles, the diffraction peaks at 36.7°, 42.6°, and 61.8° correspond to the (1 1 1), (2 0 0), and (2 2 0) planes, respectively.

Figure 4

XRD patterns of Ni-AlN thin films deposited by (A) DC, (B) PC, and (D) UAPC methods.

3.2 Microhardness of Ni-AlN thin films

Figure 5 indicates the microhardness of the three types of Ni-AlN thin films as functions of the applied current density. The microhardness of the films increased remarkably when the applied current density increased from 3 to 5 A/dm². The Ni-AlN thin films prepared by DC, PC, and UAPC deposition at 4.5 A/dm² current density showed an optimum microhardness value of 904, 943, and 987 HV, respectively. The improvement in the microhardness of the thin films is related to the dispersion-hardening effect caused by the AlN particles. These particles have high microhardness, which enhances the properties of the Ni-AlN thin films.

Figure 5

Plots of microhardness versus applied current density used for Ni-AlN thin film electrodeposition: DC, PC, and UAPC methods.

3.3 Erosion dynamics of Ni-AlN thin films

Figure 6 displays the weight loss of the three Ni-AlN thin films with respect to time. Weight loss was determined every 3 h for 30 h under erosion condition. The corrosion curves for Ni-AlN thin films produced by DC and PC deposition are basically the same: the curves increased rapidly at the beginning and then changed slowly. After erosion testing for 21 h, the mass losses of the films synthesized by DC, PC, and UAPC deposition were 2.14, 1.95, and 0.60 mg, respectively. Thus, the Ni-AlN thin film prepared by UAPC deposition has better corrosion resistance than those prepared by DC or PC deposition because of the moderate ultrasonication, which is conducive to homogeneous dispersion of AlN particles in the Ni-AlN films. In addition, the AlN particles embedded in the films improve the compactness of the films and enhance their corrosion resistance.

Figure 6

Weight loss curves of Ni-AlN thin films after corrosion: (A) DC, (B) PC, and (C) UAPC methods.

3.4 Corrosion morphologies of Ni-AlN thin films

Figure 7 shows the SEM images of the samples after corrosion tests for 30 h. Some large pores are visible in the Ni-AlN films deposited by DC and PC methods, whereas only a few small pits are observed on the surface of the Ni-AlN film fabricated by UAPC deposition. Thus, ultrasonication facilitates efficient dispersion of tiny particles in the films [24]. The AlN nanoparticles embedded in the films improve the film structure and produce smooth and compact surfaces, which can impede solution contact with the films. Consequently, the corrosion resistance of the Ni-AlN thin films is enhanced.

Figure 7

SEM images of samples after corrosion test: (A) DC, (B) PC, and (C) UAPC.

3.5 Polarization curves of Ni-AlN thin films

Figure 8 exhibits the representative anodic polarization curves of the Ni-AlN films obtained through potentiodynamic polarization method for various deposits. Table 2 lists the electrochemical erosion parameters obtained from the polarization curves. The corrosion potentials of the DC-, PC-, and UAPC-deposited films were -0.552, -0.473, and -0.446 V vs. SCE, respectively. Moreover, the corrosion current density of the UAPC-prepared film was 3.52×10^-5 A/cm², which was the lowest among the samples.

Figure 8

Polarization curves of Ni-AlN thin films deposited by (A) DC, (B) PC, and (C) UAPC methods.

Table 2

Erosion characteristics of Ni-AlN thin films in 3.0 wt.% NaCl solution.

Types of films	(A) DC	(B) PC	(C) UPC
βa (V/dec)	0.03	0.01	0.01
βc (V/dec)	0.02	0.02	0.02
R (Ω/cm2)	2536	3941	6862
E (V) vs. SCE	-0.552	-0.473	-0.446
I (A/cm2)	5.48×10-5	4.67×10-5	3.52×10-5

4 Conclusions

Ni-AlN thin films were produced via DC, PC, and UAPC deposition. SPM results revealed that the Ni-AlN film prepared by UAPC deposition have a compact and fine morphology; the average grain diameters of the Ni and AlN particles in this film are approximately 97.7 and 40.2 nm, respectively. XRD results showed that the Ni-AlN thin films consist of Ni and AlN phases. For the AlN particles, the diffraction peaks at 36.7°, 42.6°, and 61.8° correspond to the (1 1 1), (2 0 0), and (2 2 0) planes, respectively. The Ni-AlN thin films prepared by DC, PC, and UAPC deposition at 4.5 A/dm² current density exhibited an optimum microhardness value of 904, 943, and 987 HV, respectively. Erosion test results revealed that the film prepared by UAPC deposition possesses the highest corrosion resistance among the three types of thin films. The corrosion potentials of the DC-, PC-, and UAPC-deposited films were -0.552, -0.473, and -0.446 V vs. SCE, respectively.