Report of Self-lubricating Al2O3-based nanocomposites

Student: Shubing LU

Module: Nanomaterials (CME8034)School: Mechanical and Systems Engineering

Major: Biomedical Engineering

Student No: 14049776

Technical report (Self-lubricating Al2O3-based nanocomposites)1. Introduction:

Self-lubricating Al2O3-based nanocomposites are being viewed as a new generation of materials widely used in engineering. According to serial search done by experts, Al2O3-based nanocomposites in essence consist of at least two different components in which one ingredient is commonly nano-sized Al2O3 particles and other elements may contain metal matrix such as Ni, Ti, Al, Cr or Cu[1]., ceramics like SiC or polymer. Thanks to the material combination, the Self-lubricating Al2O3-based nanocomposites present the properties of high weight bearing ratio, high stiffness high elastic modulus, and wear resistance (Tribological behavior and property) mainly offered by Al2O3 nano-sized particles to overcome the shortcomings of metal matrix [2]. Additionally, it is sparking that Al2O3-based nanocomposites showed off the property of toxicity absorbability in the field of environmental friendly [3]. Currently, this type of nanocomposites is being frequently used in engineering field like automotive industry. Especially, taking advantages of wear resistance property, the Al2O3 nano-sized particles usually form a layer of coating on the surface of metal matrix to avoid the mechanical wear in production, whose ultimate products usually are Ni-Al2O3 or Al- Al2O3 composite. In this report, there are four parts of Preparation, Microstructure, Mechanical and tribological properties introduced.2. Preparation:

Depending on different components in Al2O3-based composites, the fabrications were variable.

2.1 Electroless plating method (Stirring method)One of presentitive nano-materials was 5vol.% Al2O3/A206 composite [4], and its fabrication was through semi-solid and liquid stirring process under the protection of pure argon atmosphere.Except to 5vol.% Al2O3/A206 composite, another material of NiPAl2O3 nanocomposite coatings [5] was manufactured in another way for the ground that another sorts of metallic components including Ni and P were added into the precursor of Al2O3 nanocomposite. In detail, since the disc had standard shape, the substrate samples can be told out and selected from mild steel, and then surface preparation of the sample was through a series of method containing the use of 600-grade SiC paper, degreasing in an alkaline bath and acetone with intermediate water rinses, respectively. The commercial Ni-P eletroless solution used in fabrication contained nickel, NaH2PO2 and additive and stabilizer. The plating bath was stirred using a magnetic stirrer. The plating process was achieved in a double wall beaker surrounded by a thermostated circulating water bath. Finally, Al2O3 particles previously dispersed in some electroless solution under the effect of 1h ultrasound was entered into the plating bath. The codeposition of Al2O3 nanoparticles lasted for 2h in plating bath, and the modified nanocomposite was ultimately obtained. 2.2 Electrode position method (Composite electroplating) The method of electrode position is a way to integrate the Al2O3 with extra metallic matrix, like mainly-used Ni, Cu, Cr, physically and chemically changing the properties of Al2O3 nanocomposites. According to many experts[6-8], the plating electrolyte for electrodeposition was a Watts-type bath, and Al2O3 was applied as reinforcing phase. In detail, the polished substrate of Ni matrix and copper foil respectively as anode and cathode (Fig.1)were placed in electrolytic solution uniformly containing Al2O3 nano-particles, and after the treatments of long-time magnetic sirring and ultrasonic dispersing, the Ni matrix with Al2O3 began to be obtained under the effect of 2h or electro co-deposition. Every condition such as temperature, pH value or ion concentration of bath was optimized in experiment. Based on the research by H. Gl et al. [6], the electrode position methods were assorted into two methods, namely conventional direct current (DC) electroplating and pulse current (PC) plating, resulting from the different application of current. 2.3 Hot pressing methodIn a piece of research by Parchoviansk, M. et al. [9], the Al2O3/SiC nanocomposite was prepared via mixing a sub-micron Al2O3 powder with nano-sized silicon carbide powders, the mixture of which was hot pressed, and all the reaction processed in the atmosphere of Ar with suitable conditions that the temperature was 1740 and pressure was 30MPa.

2.4 Vacuum coating methodIn accordance with Wang, Zhijiang et al. [10], the formation of Al2O3/PTFE (polytetrafluoroethylene)composite coating came true via vacuum coating technology where coating samples immersed into water-based PTFE suspension were placed into a vacuum oven, deposited and heated for a long period. 3. Microstructure: Base on the extra ingredients such as added into nanosized Al2O3, the internal structure of Al2O3-based nanocomposites are variable.

For instance, after the evaluation of the effect of duty cycle on morphological properties of Ni-Al2O3 composite coating, in which the ingredient of Ni was electrodeposited to the surface of Al2O3 substrate. As the Fig. 2 indicated, as a result of ultrasonic agitation the Al2O3 particles were distributed uniformly in nickel matrix in all composite coating, and a truncated pyramidal structure with big-sized grains, which is a typical morphology for Ni deposited coatings, were also presented in Fig.3. Internally (Fig.4), Al2O3 particles could forme five layers of cross section, in which the volume percentage of Al2O3 decreased from innermost layer to topmost layer . (Fig.5)

Additionally, the microstructure of Al2O3/SiC nanocomposites by Parchoviansk, M. et al.[9] differed from the Ni-Al2O3 nanocomposites. In term of its structure, it was illustrated that SiC content increased with the mean size of Al2O3 decreasing. Among different outcomes, the composite AS 15c contained SiC particles situating both along the grain boundaries and inside the Al2O3 matrix grains (Fig. 6), and AS 15f had the SiC particles dispersing mainly within the Al2O3 matrix grains (Fig.7 ).

4. Mechanical and tribological properties:

Al2O3-based nanocomposites in general performs a series of properties, namely improved hardness and tensile strength, and excellent wear resistance ability, all of which are mainly focused in this part. 4.1 HardnessSpeaking of hardness of Al2O3-based nanocomposites, Al2O3 is a contributory factor to the increasing of microhardness. In a research by Alirezaei, S. et al. [5], the reinforcement content showed a direct relationship with hardness of Ni-P-Al2O3 coating with the property of lubricity, the microhardness value dropped due to the rising content of alumina micronsized particles, on the contrary, when the content of Al2O3 nanosized particles in coating increased, total hardness of coating, to some extent, indicated a higher value than the one without Al2O3 nanosized particles. As the Fig.8 shown, under both as-plated and heat-treated condition the microhardness value had an obvious decrease when Al2O3 content varied from 150 to 500 mg/L, however, the total hardness property of modified coating with any content of nanosized Al2O3 was comparatively better than the one having a lack of Al2O3.

4.2 Tensile strengthThere was a distinctive improvement on tensile strength, when the particle size of Al2O3 alters into nanometer, S. Tahamtan et al. [4] showed when measured by application of load, more damages appeared in the particle-rich regions in the Al/A206Al2O3 nanocomposite fabricated by researchers rather than in the particle lacking regions that were full of Al2O3 nanocomposites. The scientific explanation for this phenomenon was that the strength of particle-rich regions dramatically dropped because of high amount of Al2O3 microsized particles, further performing a low bearing ability to resist load when experiencing high load of tension.4.3 Tribological behavior and propertyAl2O3-based nanocomposites occupy a well-performing wear resistance property compared with other one-ingredient matrix like Ni, for example, in a piece of research by Chen, Li et al. [11], Ni-Al2O3 composite obtained from electrodeposition was observed to have lower wear rates than pure Ni coating.

It was interesting that the property is affected by the conditions in experiments, the electrodeposition in particular. Current density is related to wear and friction properties of Al2O3-based nanocomposites. Drawing NiAl2O3 nanocomposite coatings as an example, in the research by Gl, H. et al. [6], the nanocomposite coatings were put into two groups in which one group of coating was PC (pulse current) -deposited with another being synthesized by DC (direct current) -deposited method, after analysis, the PC -deposited group contained more Al2O3 nanosized particles than another group. Consequently, the PC-deposited NiAl2O3 composite coatings withstood wear better than DC-deposited coatings via the testing of a reciprocating ball-ondisk CSM tribometer (Fig. 9 and 10), and it was because that the both increase to Al2O3 nanosized particle content and homogenous particle distribution had already influenced the tribological behavior. Moreover, another influent factor attributed to growing Ni matrix grain size that increased wear resistance in the PC-deposited coating owing to higher Ni matrix grain size improving plastic deformation energy absorbability and preventing microcrack formation.

5. Conclusion: Al2O3-based nanocomposites belong to novel engineering materials. Different Al2O3-based nanocomposites were manufactured via different physical and chemical method, and present a series of mechanical and tribological properties. howcver, at present, a large amount of scientists and experts are making effect to largely improve the properties of Al2O3-based nanocomposites and widen their application rangements. Reference

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Fig. 8: In this picture, owing to the increase to Al2O3 content, the hardeness value of nanocomposite rose obviously.

Fig. 9 and 10: The first picture represented the wear rate of DC with second one representing the wear rate of PC.

Fig. 2 and 3: Both of the two pictures showed the different layers of surface morphology of composite coatings, and first picture represented the top layer with the second one representing the second layer.

Fig. 1: Both of two electrodes were placed in plating bath.

Fig. 4: This picture represented the five different layers of nano-sized coating with different amount of Al2O3 particles from the angle of cross section, and the leftmost substrate was Ni matrix.

Fig. 5: In this picture, the hardness of nanocomposite was positively related to the amount of Al2O3 particles in layers.

Fig. 6 and 7: Both of the two pictures represented microstructure of Al2O3/SiC nanocomposites.