Chemical analyses of mechanochemical reaction products of α-Si3N4 in ethanol and other lower alcohols

Abstract

Recently, ceramic engines made of silicon nitride have attracted widespread interest because the exhaust does not pollute the environment [1]. To develop the ceramic engine, it is necessary to understand the tribological characteristics of silicon nitride in an alcohol fuel such as ethanol. In our previous report [2], silicon nitride showed lower friction in lower alcohols such as methanol and ethanol, even at low sliding speed. To explain the results, we considered the formation of a lubricating film due to the tribochemical reaction of the silicon nitride with alcohols, as shown in Fig. 1. That is, the surface of the silicon nitride is oxidized and esterified to form soluble silicon alkoxides which are effective in reducing the friction. We need to clarify the nature of the tribochemical films. Generally, the yields of tribochemical reaction products are too small to allow their nature and properties to be characterized. To overcome this difficulty, Gates and Hsu [3] used micro-sample wear tests, in which silicon nitride was slid in trace amounts of higher alcohols, octanol and decanol, and they detected secondary ion mass spectroscopy (SIMS) peaks of Si(OR)4 (R: C8H17 and C10H21) and gel permeation chromatography/graphite furnace atomic absorption (GPC/GFAA) peaks of silicon containing polymers. It is quite difficult, however, to detect trace amounts of the reaction product of Si3N4 in lower alcohols because lower alcohols are volatile. Systematic studies of the tribochemical products of silicon nitride in lower alcohols have not yet been made. Milling tests were therefore conducted to accelerate the mechanochemical reaction of silicon nitride with ethanol and other lower alcohols. Milling tests were performed using a planetarytype micro-pulverizer (Fritsch Ltd, p-7). AE-Silicon nitride powder (10 mmol, 1.4 g, diameter 1.5 im, purity 99.9‡%) was milled for 60 h with 25 ml ethanol (H2O , 0.005%) in a pot made of silicon nitride. After the milling test, the mechanochemical reaction mixture was filtered under reduced pressure through a PTFE filter with pore size of 0.2 im to remove any unreacted silicon nitride powder. The filtrate was concentrated, then chloroform was added to the concentrate. A precipitate of a pale yellow solid separated out. The pale yellow solid was filtered off, washed with chloroform, and dried. After the evaporation of solvent from the filtrate, a yellow residue remained. These separated substances were characterized by Fourier transform infrared (FTIR) absorption spectroscopy, proton nuclear magnetic resonance spectroscopy (1H-NMR), gel permeation chromatography (GPC) and mass spectroscopy (MS). Pale yellow solids precipitated from the reaction mixture (milling time: 60 h) were analysed by means of FTIR, of which the spectrum is shown in Fig. 2. The peaks of the pale yellow solid are generally identical with those of an authentic sample of silica gel [4, 5] except for the absorption bands of 3000– 2800 cmy1, which are attributed to methyl (CH3) or methylene (CH2) stretching vibration. These absorption bands are observed when silica gel is esterified by alcohol [6, 7]. Precipitation of silica gel from the liquid phase suggested that a silicon component dissolved in ethanol during milling. Two possible mechanisms of silica gel formation were proposed, as shown in Fig. 3. In route 1, silicon nitride undergoes oxidation by oxygen to form silicon oxide. The silicon oxide reacts with ethanol and the adsorbed hydroxyl group to produce silicon alkoxide (Si(OR)n(OH)4yn), which is soluble in ethanol. Silicon alkoxide is hydrolysed and condensed to form long-chain polyoxysilane and twoor three-dimensional cross-linked polyoxysilane. Further condensation of polyoxysilane resulted in the precipitation of silica gel. In route 2, the