Abstract
In view of their possible application as high temperature solid lubricants, the tribological and thermochemical properties of several organosilica networks were investigated over a range of temperatures between 25 and 580 °C. Organosilica networks, obtained from monomers with terminal and bridging organic groups, were synthesized by a sol-gel process. The influence of carbon content, crosslink density, rotational freedom of incorporated hydrocarbon groups, and network connectivity on the high temperature friction properties of the polymer was studied for condensed materials from silicon alkoxide precursors with terminating organic groups, i.e., methyltrimethoxysilane, propyltrimethoxysilane, diisopropyldimethoxysilane, cyclohexyltrimethoxysilane, phenyltrimethoxysilane and 4-biphenylyltriethoxysilane networks, as well as precursors with organic bridging groups between Si centers, i.e., 1,4-bis(triethoxysilyl)benzene and 4,4′-bis(triethoxysilyl)-1,1′-biphenyl. Pin-on-disc measurements were performed using all selected solid lubricants. It was found that materials obtained from phenyltrimethoxysilane and cyclohexyltrimethoxysilane precursors showed softening above 120 °C and performed best in terms of friction reduction, reaching friction coefficients as low as 0.01. This value is lower than that of graphite films (0.050 ± 0.005), a common bench mark for solid lubricants.
Highlights
Organosilica materials are silicon oxide networks that contain organic groups distributed at the molecular level, i.e., a fraction of the Si–O bonds is replaced by Si–R bonds (R is alkyl, alkylene, phenyl, phenylene, etc.)
The lubricative of different organosilica with terminal bridging organic agroups looselywere bound powder obtained from pin-on-disc evaporation(PoD)
The introduction of organic groups to the silicon oxide network affects the tribological properties of the lubricants
Summary
Organosilica materials are silicon oxide networks that contain organic groups distributed at the molecular level, i.e., a fraction of the Si–O bonds is replaced by Si–R bonds (R is alkyl, alkylene, phenyl, phenylene, etc.). The mixed organic-inorganic nature of these materials provides an interesting combination of material properties, such as the high thermal and chemical stability of silica and the hydrothermal stability and flexibility of the organic groups [1,2]. The position of the organic groups can be terminal (Si–R) or bridging (Si–R’–Si), which affects their exposure at the (internal) surface and influences the crosslink density and microporosity of the network [3]. The rigidity and strength of silica networks composed of connected silicate SiO4 units is supplemented by the flexibility and elasticity provided by organic segments.
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