Sol–gel derived nickel-doped TiO₂ films as wear protection coatings

Abstract

Thin films of pure and Ni-doped TiO₂ were prepared on a glass substrate by sol–gel and spin-coating process from specially formulated ethanol sols. The morphologies of the films surface were observed with atomic force microscope (AFM). The tribological properties of the obtained thin films sliding against steel ball were evaluated on a one-way reciprocating friction tester. AFM results show that by the addition of the Ni in TiO₂, smooth surfaces were obtained. As a result, the Ni-doped TiO₂ films exhibit better wear protection properties than pure TiO₂. The best protection was observed for 5% Ni-doped TiO₂ films in this study.

Key Words:

• thin film
• sol–gel
• Ni-doped TiO₂
• friction

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Erratum

1. Introduction

Titania (TiO₂) has received a lot of attention in the past two decades, due to many advanced applications of the material in photocatalysis, solar energy cells, gas sensors 1–3, etc. Research study has been carried out on the mechanical properties of the TiO₂ thin film. Jämting et al. 4,5 reported that TiO₂ thin films have higher mechanical strength. Zhang et al. 6,7 found that they have excellent wear resistance capability under lower load.

Several methods have been employed to fabricate TiO₂ thin films, including e-beam evaporation 8, sputtering 9, chemical vapour deposition 10 and sol–gel process 11. Due to several advantages such as low processing temperature, low equipment cost and good homogeneity, the sol–gel process is one of the most appropriate technologies to prepare thin films.

In addition to pure titania, the interest has been steadily grown to study metal ion-doped titania. The doping modifies the electronic structure of the material, which can cause more effective electron–hole pair generation that is especially important for applications that are based on light-induced phenomena. Sol–gel method is also used for the preparation of doped TiO₂ films 12,13.

Nickel-doped TiO₂ has shown several interesting properties like photo-catalytic activity and UV light-induced hydrophilicity 14–18. However, information regarding the mechanical and tribological performance of Ni-doped TiO₂ is not available, though this is a crucial problem faced during the engineering application of films. In this article, we present the results of the systematic study of the tribological properties of nickel-doped TiO₂ films prepared by sol–gel method.

2. Experimental section

2.1. Preparation of TiO₂ thin film

Nickel-doped titania thin films on glass substrate were prepared by sol–gel method using commercially available tetrabutyl titanate and nickel nitrate as the precursor materials and triethanolamine as the stabilising agent in anhydrous ethanol. The chemical reagents are all AR grade and used without further purification. First, tetrabutyl titanate was added to an anhydrous ethanol solution. The mixture was stirred at room temperature for 15 min and then triethanolamine dissolved in the mixture. After stirring for 5 min, distilled water was slowly dripped into the solution under continuous vigorous stirring. The volume ratio of the composition in mixed solution is Ti(OBu)₄:EtOH:H₂O:N(C₂H₅OH)₃ = 3:12:1:1. Then, nickel nitrate was dissolved into some distilled water. The solution was incorporated into the TiO₂ sol. The mixed solution was stirred for 1 h and then aged overnight at room temperature. A clear, transparent and light yellow solution was ready for film preparation. The precursor sol will not precipitate even after several months.

Glass plates with size of 25 × 76 × 1 mm³ were pretreated in piranha solution (in which the volume ratio of 98% H₂SO₄ and 30% H₂O₂ was 7:3) at 90°C for 30 min. After being rinsed with distilled water, these plates were blow dried by nitrogen. TiO₂ thin films were prepared on these plates by spin-coating method with speed 2000 cycle/s and time 30 s. The films were dried at 80°C for 15 min, and then sintered at 480°C for 60 min in an atmospheric oven. These films were cooled to room temperature in the oven. Finally, uniform and transparent TiO₂ thin films were obtained. The TiO₂ films containing Ni molar concentrations 2%, 5% and 10% were denoted as 2%, 5% and 10% Ni–TiO₂, respectively.

2.2. Characterisation of films

Because the thickness of films is too thin, the crystal structures of pure and doped TiO₂ powders, prepared after employing the same annealing process used for thin films, were analysed using a SHIMADZU Rigaku D/max-rB X-ray diffractometer. Cu-Kα radiation was produced using the tube's 40 kV and 20 mA regime.

The morphological properties of the samples’ surfaces were performed with a Shimadzu SPM-9500 Atomic Force Microscope (AFM). Typically, a tapping mode was utilised, which is known to provide optimal performance in such cases.

Friction and wear tests were carried out on a UMT-3 tester (CETR Corporation Ltd, USA) under reciprocating distance of 5 mm per pass. The counterpart was GCr15 steel ball with a diameter of 4 mm. All the tests were conducted under the following conditions: room temperature, relative humidity about 40%, applied load 0.5 N with sliding speed 10 mm/s. The friction coefficient was recorded automatically by a computer. During the whole friction process, friction coefficient remained stable for a particular period and then abruptly rose. This period was regarded as the wear life of the films. Three replicated tests were carried out for each specimen and the average friction coefficient and wear life are used in this article.

3. Results and discussion

3.1. Characterisation of the films using AFM and XRD

AFM was used to characterise the uniformity and particle size of pure and doped films. The results are shown in Figure 1. Compared with the AFM image of pure TiO₂ film that we reported before 19, Ni-doped TiO₂ films exhibit similar surface morphology with no crack, however with a smoother surface. Roughness average (Ra), values are 4.24, 2.74, 2.34 and 2.71 nm for pure and 2%, 5% and 10% Ni-doped TiO₂ films, respectively. It indicates the reduction in particle size in the films because of Ni doping. The smoothest surface is observed for 5% Ni-doped TiO₂ films. Further increase in Ni concentration to 10% will damage the surface and increase Ra value. The similar results were observed for Cu-doped TiO₂ films prepared by sol–gel technology 19,20.

Figure 1. (Colour online) AFM images of: (a) 0%, (b) 2%, (c) 5% and (d) 10% Ni-doped TiO₂ films.

Figure 2 shows the XRD patterns of pure and Ni-doped TiO₂ powders annealed at 480°C. For TiO₂ powder, typical peaks in XRD pattern are observed at 2θ values 25.3°, 38.1°, 48.0° and 55.0°, assigned to (1 0 1), (1 1 2), (2 0 0) and (2 1 1) planes, respectively. Calculations show that the observed crystalline peaks correspond to anatase TiO₂ phase. There is no indication of rutile phase formation up to a sintering temperature of 480°C.

Figure 2. (Colour online) XRD spectra of pure and Ni-doped TiO₂ powder.

In the case of Ni-doped TiO₂ films, similar XRD patterns were observed. Crystalline phases of Ni ion on these films have not been detected in the XRD pattern of doped film up to 10 mol% concentration. However, significant change in the XRD peak broadening has been observed. The peak broadening of XRD spectra show the decrease in the average crystal size of TiO₂ particles. According to Scherrer's formula 21

where L is the crystallite size, λ the wavelength of the X-ray radiation, K a constant taken as the default value of 0.89, β the line width at half maximum height and θ the Bragg angle; estimated crystallite sizes were 11.8, 10.5 and 11.6 nm for pure, 5% and 10% Ni-doped TiO₂ powders, respectively. This finding generally agrees with the AFM observations.

3.2. Tribological properties of films

Tribological properties of the TiO₂ films were tested on a UMT-3 friction and wear tester. Figure 3 and Table 1 present the tribological performance of the TiO₂ and Ni-doped TiO₂ films at a normal load of 0.5 N.

Figure 3. (Colour online) Friction coefficients of pure and doped TiO₂ as a function of sliding time.

Table 1. Tribological properties of the films.

Samples	Friction coefficient
	Initial value	Plateau value	Wear life (s)
TiO2	0.24	0.81	120
2% Ni–TiO2	0.16	0.76	650
5% Ni–TiO2	0.15	0.74	1050
10% Ni–TiO2	0.16	0.74	430

In this experiment, it was found that the glass substrate is easily worn off when sliding against the GCr15 steel ball. The initial friction coefficient is 0.24 and this remains for only few seconds and then sharply increases to 0.81. Pure TiO₂ thin films prepared in the experiment can protect the glass substrate from severe wear. Before the film is worn out, the friction coefficient of the films sliding against GCr15 steel ball remains stable for a longer period of time (∼150 s). All three Ni-doped TiO₂ films show better wear protection performance than pure TiO₂ film. However, the improvement in wear protection for Ni-doped TiO₂ films depends on the concentration of Ni in TiO₂ films. The performance of Ni-doped TiO₂ films improved with increase in the Ni concentration until 5%. Afterwards, the further increase in of Ni concentration damage the performance.

The tribological performance of pure and Ni-doped TiO₂ films can be explained by AFM observation for films. The improvement in tribological performance for Ni-doped TiO₂ films can be explained by the change in film surface when Ni is doped into TiO₂ films. It has been observed in AFM and XRD experiments that Ni doping results in the smaller average size of TiO₂ particles and smoother films surface. It is well known that the roughness plays a great role in determining the tribological performance of surface. The Ni-doped TiO₂ films are more smooth and therefore have better friction-reducing and wear protection performance than pure TiO₂ films. However, the performances depend on the concentration of Ni dopant. When Ni is at 5% concentration in TiO₂ films, the smallest Ra value is obtained, whereas best friction-reducing and wear protection behaviour is observed. Further increase in Ni concentration to 10% greatly damages the uniform surface. It inevitably affects the tribological performance of Ni-doped TiO₂ films.

4. Conclusions

Pure and Ni-doped TiO₂ films have been successfully obtained by sol–gel and spin-coating process. Results show that the addition of inorganic Ni salts to the starting sol of TiO₂ can improve the tribological properties of the corresponding films. However, the wear life of Ni-doped films does not increase monotonically with increasing Ni concentration in TiO₂ matrix. The 5% Ni-doped TiO₂ shows the best wear protection in this study.

Acknowledgements

This study was supported by the Natural Science Foundation of Shandong Province (Y2008F05) and Applied and Basic Research Foundation of Qingdao City (09-1-3-35-jch).

This article was originally published with errors. This version has been corrected. P lease see Erratum (http://dx.doi.org/10.1080/17458080.2013.829704).