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Understanding lubrication mechanisms in Engineered Al2O3-B2O3 Composites
AuthorKasar, Ashish Kumar
AdvisorMenezes, Pradeep L.
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Technology advancements in aerospace, nuclear, marine, and other endeavors heighten the need for more capable self-lubricating and wear-resistant materials. Ceramics have the potential to satisfy such extreme environment application challenges but require improvements in friction and wear properties. Alumina (Al2O3) is a widely used ceramic material that is available in abundance and demonstrates a combination of properties, such as high melting point, hardness, and corrosion resistance. However, the friction and wear properties of alumina limit its tribological applications. A common practice to improve the tribological properties of alumina is by reinforcing materials to form composites. Based on thermodynamic considerations, in the present work, boron oxide (B2O3) was added to Al2O3 matrix to form a double oxide system, i.e., aluminum borate (9Al2O3.2B2O3). Four methods for enhancing the lubrication mechanisms have been developed and investigated for the Al2O3-B2O3 composite. First, the lubrication behavior of Al2O3-B2O3 composite was evaluated in terms of friction and wear. The formation of double oxide (aluminum borate) led to improved wear resistance with a deteriorated coefficient of friction compared to porous alumina. Also, the formation of aluminum borate during sintering generated porosity in the Al2O3-B2O3 composite. One of the methods to reduce the porosity in the Al2O3-B2O3 composite is the incorporation of sintering additives. In this work, CuO and CaO were selected as sintering additive due to their preferential reactivity of Al2O3 and B2O3, respectively in the matrix. Although adding CuO/CaO improved the density and further improved wear resistance of the composites, the manufactured composite adversely affected the COF again, particularly with CaO addition. The observed friction and wear behavior of CuO/CaO incorporated Al2O3-B2O3 composites are discussed by combining the crystal-chemical approach and polarization theory for the multi-oxide systems. Due to the adverse effect of CaO addition on COF, a new method has been adapted by embedding the solid lubricants in the Al2O3-B2O3-CaO matrix to reduce friction. This work chose hBN as secondary reinforcement additive to the composite due to its chemical suitability. The dispersion of hBN in the Al2O3-B2O3-CaO matrix led to less diffused microstructure, resulting in smaller sintered particles. During sliding, smaller sintered particles were smeared with embedded hBN solid lubricant that enhanced the lubricating properties by reducing COF with an increase in wear rate. The amount hBN in the composite can be engineered to achieve optimum friction and wear. To further enhance the lubricating properties of the Al2O3-B2O3 composite, B2O3 coating on the Al2O3-B2O3 composites were made through laser processing to avoid the reaction between B2O3 coating with Al2O3-B2O3 substrate. The B2O3 was chosen as a coating because it reacts with moisture to form boric acid phase, which is a solid lubricant. The rapid heating and cooling during laser processing led to the formation of amorphous B2O3 and boric acid phases (H3BO3/HBO2). The amount of B2O3 phases was controlled by using different laser powers. The amount of amorphous B2O3 was quantified using the crystallinity index. The formation of amorphous B2O3 coating was advantageous to reduce COF while retaining a lower wear rate because it can instantaneously react with moisture to form a boric acid phase.The last method to enhance the lubrication performance involves the utilization of in-situ formed pores in the Al2O3-B2O3 composite that formed during sintering to capture liquid lubricants. A new class of liquid lubricants, ionic liquids (IL), were selected due to their superior tribological performance. The lubrication performance of the ILs impregnated Al2O3-B2O3 composites were tested at four different temperatures and sliding velocities. The lubrication mechanism for this study was observed to be a function of thermal expansion of stored liquid lubricants and their viscosity. The results revealed that the thermal expansion of liquid lubricant, either by frictional heat or external heat, controls the flow of liquid from pores to the sliding interface. Whereas viscosity determines the lubrication performance at the sliding interface. Different sliding velocities were used to evaluate the effect of frictional heat that causes temperature generation at the sliding interface to cause overflow of liquid lubricants. The outcome from this work suggests that the phosphonium saccharinate ILs are suitable for high-temperature applications due to their higher viscosity, which helps achieve lower friction and wear. Whereas phosphonium salicylate and phosphonium benzoate ILs are provided lower friction and wear at low temperatures. The studied approaches of multi-oxide system theory, the addition of solid lubricant, achieving an amorphous phase by laser processing of coating, and effective utilization of in-situ generated pores provide a roadmap for the development of lubrication mechanisms in the oxide ceramic materials.
|Wang, Yan; Zhang, Jun; Ramachandran, Rahul; Mushongera, Leslie T.; Chandra, Dhanesh
|Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 United States