Wear behaviour of a REFEL SiC containing fluorides up to 900°C

In certain applications involving high temperatures or vacuum environments, liquid lubricants are not suitable and structural ceramics containing solid lubricants may provide an alternate lubrication method. The aim of this work was to develop a self-lubricating silicon carbide for use up to 900°C. The main sequential steps in the experimental study included: (i) preparation of several grades of reaction bonded silicon carbide (RB-SiC) with different free silicon content (10–22 wt.%), (ii) removal of free silicon to make porous RB-SiC and (iii) infiltration of the porous RB-SiC bulk with CaF2, to act as a lubricant. Reaction bonded silicon carbide containing 10 wt.% of free silicon (REFEL), RB-SiC/CaF2 composites with 12–20 wt.% CaF2 were selected for tribological studies. Friction and wear characteristics of self-mated couples of these compositions were studied under dry sliding conditions in a pin-on-disk tribometer at 25, 500, 700 and 900°C. The morphology of the worn surfaces was studied by means of optical and scanning electron microscopy. The friction and wear results show that the wear coefficient of REFEL increases more than two order of magnitude with increasing temperature (from 25 to 700°C) and then decreases slightly with further increase in temperature (at 900°C), the friction coefficient lies between 0.5 and 1.0 under steady state conditions. Scanning electron microscopy revealed that (i) at room temperature, a tribochemical reaction between silicon carbide and the humid environment led to the formation of shallow grooves, (ii) at 500–700°C, inter-granular fracture and delamination predominate, (iii) at 900°C, a tribo-oxidation mechanism occurs. In contrast, the RB-SiC/CaF2 composites showed only mild wear to be operative at all temperatures. The reduction in friction and wear coefficient of these composites (i) at 25, 500 and 700°C, is shown to be related to the smearing of calcium fluoride on the contacting surfaces, (ii) at 900°C, the chemical reaction of calcium fluoride with silica (under the normal and tangential stresses) and a possible change in the wear characteristics of silica is a dominating factor in the wear behaviour of these composites.