Article pubs.acs.org/IECR
Preparation, Friction, and Wear Behaviors of Cerium-Doped Anatase Nanophases in Rapeseed Oil Kecheng Gu,† Boshui Chen,*,† Xuemei Wang,‡ Jiu Wang,† Jianhua Fang,† Jiang Wu,† and Xin Yang† †
Department of Oil Application and Management Engineering and ‡Department of Chemistry and Materials Engineering, Logistical Engineering University, Chongqing 401311, China ABSTRACT: Composite cerium-doped anatase nanophases with an average diameter of 20 nm were prepared via sol−gel method followed by surface modification with stearic acid, abbreviated SA/Ce-TiO2. The microstructure of materials was characterized by XRD, FTIR, and SEM. Furthermore, the antiwear and friction-reducing capacities of SA/Ce-TiO2 as lubrication additives in rapeseed oil were evaluated on a four-ball tribotester. The chemical characteristics, elemental composition, and morphology of worn surfaces of steel balls were investigated by XPS and SEM. The results show that SA/Ce-TiO2 could obviously improve the antiwear and friction-reducing capacities of rapeseed oil, especially for the nanophases doped with 2.0 and 3.0% cerium in molar ratio, respectively. The excellent tribological performance of SA/Ce-TiO2 in rapeseed oil can be attributed to the formation of a complex boundary lubrication film mainly consisting of Fe2O3, TiO2, and CeO2 as well as the formation of an adsorption film of SA and base oil on the friction surfaces.
1. INTRODUCTION
In this study, we prepared cerium-doped anatase nanophases modified by stearic acid (SA) and investigated their tribological behaviors in rapeseed oil.
Over the past decades, nanostructure materials have attracted much interest due to the special chemical and physical properties.1−4 In the field of tribology, much effort has been paid to the development of inorganic nanoparticles as lubricant additives, such as sulfide,5,6 fluoride,7−9 oxide,10−12 metal,13 and borate nanoparticles.14,15 However, the poor dispersibility of inorganic nanoparticles in base oil has restricted their applications as lubricant additives. The surface modification technology has also been widely adopted to improve the dispersibility of lubricant additives in base oil.16,17 Among various inorganic nanoparticles as lubricant additives, TiO2 nanoparticles have obtained great attention owing to excellent tribological properties, nontoxicity, low cost, and high chemical stability.11,12 Previous investigations about the tribological performance of TiO2 mainly focused on that of rutile.18−25 Relatively few reports are currently available about the tribological behaviors of anatase nanophases as lubricant additives. Therefore, further research on the surface modification and tribological behaviors of anatase nanophases is still needed. Furthermore, rare earth elements and compounds have also been widely used as mineral lubricant additives because of their special physical and chemical properties.26,27 However, the tribological property of rare earth element−anatase composites as lubricant additives is rarely reported. During the past decades, the investigation of environmentally acceptable and compatible lubricant oils has become a topic of research interest due to the increasing public attention to and awareness of environment protection.28−31 The key issue in formulating environmentally compliant lubricant oils is the choice of suitable capacity additives and reliable base stocks. Currently, many base oils such as vegetable oils have found practical applications in the formulation of biodegradable lubricant oils because of their nontoxicity and excellent biodegradability.32−34 © 2014 American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Materials. All reagents used in our experiment were of analytical grade and employed without further purification. Nanocerium was obtained from ref 35 (30 nm). Distilled water was used in all experiments. 2.2. Preparation and Surface Modification of CeriumDoped Anatase Nanophases. Preparation.36 Thirty milliliters of Ti(OC4H9)4 and 48 mL of anhydrous ethanol were set in an erlenmeyer flask, followed by the addition of 5 mL of acetic acid under vigorous agitation at room temperature. The resulting mixture was denoted A1. A certain amount of cerium nitrate was dissolved into the mixed solution of 4.8 mL of distilled water and 14 mL of anhydrous ethanol, and the resulting mixture was marked A2. Thereafter, A2 was added dropwise into A1 under vigorous stirring at room temperature, and then a yellow sol was obtained, which could be transformed into a gelatin after 3 days of aging. The as-prepared gelatin was dried at 373 K for 4 h in a vacuum desiccator and then calcined at 673 K for 2 h in air to obtain cerium-doped anatase nanophases (denoted Ce-TiO2), which were ground with a mortar and pestle before use. Anatase nanophases without cerium doping (denoted TiO2) were prepared without the addition of cerium nitrate while keeping all other processes the same. Anatase nanophases doped with 0.5, 1, 2, 3, and 4% cerium in molar ratio were prepared and abbreviated 0.5%CeTiO2, 1%Ce-TiO2, 2%Ce-TiO2, 3%Ce-TiO2, and 4%Ce-TiO2, respectively. Received: Revised: Accepted: Published: 6249
October 27, 2013 March 3, 2014 March 17, 2014 March 17, 2014 dx.doi.org/10.1021/ie403621k | Ind. Eng. Chem. Res. 2014, 53, 6249−6254
Industrial & Engineering Chemistry Research
Article
Surface Modification. As-prepared products were separately set into 100 mL of anhydrous ethanol solution containing 1 wt % of SA. Resultant dispersions were heated at 348 K for 2 h in an ultrasonic bath. At the end of reactions, resultant products were separated by centrifugation, rinsed with methanol, and dried at 348 K for 2 h in a vacuum desiccator to yield SA surface-capped products (SA/TiO2, SA/0.5%Ce-TiO2, SA/1% Ce-TiO2, SA/2%Ce-TiO2, SA/3%Ce-TiO2, SA/4%Ce-TiO2). 2.3. Friction and Wear Tests. As-prepared products were separately dispersed ultrasonically into 100 mL of rapeseed oil (RP). The friction coefficients and wear scar diameters (WSD) were measured with an MMW-1P universal four-ball tribotester under 1200 rpm and 392 N for 30 min. The frictional pair for the present test comprised GCr15 bearing steel balls (composition: C, 0.95−1.05%; Si, 0.15−0.35%; Mn, 0.25− 0.45%; P,
