Adsorption and Tribochemical Reactions
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ADSORPTION AND TRIBOCHEMICAL REACTIONS W. M. MULLINSt AND T. E. FISCHERt
f Purdue University, West Lafayette, IN 47907 t Stevens Institute of Technology, Hoboken, NJ 07030 ABSTRACT A model for chemisorption based on the Lewis theory is proposed and developed. The model reproduces the observed relationship between aqueous solubility and band gap for covalent oxides. A relationship is shown between the calculated free-energy of the surface reactions and the observed effect of water on the wear of these materials. This relationship is explained in terms of surface reaction rate and equilibrium. Similar calculations for the adsorption of -CH 3 functional groups are presented. The results are compared to water adsorption calculations and related to experimental wear test results for model fluids on covalent oxides. The effect of water on lubricant adsorption is discussed. 1. Introduction Ceramic materials are attracting much interest in friction and wear applications because of their high hardness and apparent chemical inertness. Significant chemical effects on wear, tribochemistry, have been demonstrated in the literature. Relative humidity has been demonstrated to dramatically effect the wear rate of Si3 NN 4 [1] and in certain conditions produce surface lubricity [2]. Water has also been shown to produce chemisorption embrittlement (CE) in silica and glasses [3], A120 3 [4] and partially stabilized zirconia (psz) [5]. In addition, highly polar organics such as stearic acid, traditional boundary layer lubricants for metals, have been shown to increase wear in A12 0 3 and psz. Non-polar organics such as paraffin that are inert to metals have been shown to be effective boundary layer lubricants for many ceramics [5,6]. In general, surface reactions during wear are very important to wear behavior of ceramics. The surface chemistries of these materials are very different from metals so that little of the traditional lubrication technology is applicable. In addition, surface chemistries vary significantly in these materials so that an empirical understanding of one or two is not sufficient to design effective lubricants for general application. A systematic approach to lubricant molecule design must start with the fundamentals of the surface adsorption reaction. Surface chemistry is determined by the electronic structure of the constituents through the Lewis acid-base theory. The adsorption reaction is modelled as a simple chemical bonding problem between a single molecule and a surface, approximated by the linear combination of orbitals (LCO) method [7]. Thermodynamic equilibrium behavior for a large ensemble are determined by using Fermi statistics in the single molecule model to determine relative probabilities of orbital occupations. The model was used to calculate the extent of the surface adsorption reaction as a function of band gap for H 20 and -CH 3 on a series of covalent oxides. Regions for lubricity, CE and boundary layer lubrication are suggested for the results. Criteria are suggested for synergistic effects
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