Tribological behavior of silicon nitride that is compositionally modified by high-temperature electrolysis

This paper describes the effect of grain boundary chemistry, which can be easily altered by the amount of introduced sintering additives or impurities, on the high-temperature wear behavior of silicon nitride. Two different kinds of silicon nitride were made to investigate the effect of minor additives on the high-temperature wear behavior. A small amount of either CaO or Fe₂O₃ was added to the silicon nitride. Specimens were cold pressed and then sintered by gas pressure sintering. An electric field was applied to the silicon nitride in a furnace at 1300°C. Additive ions were expected to be drawn out of the intergranular glassy phase during the electrolysis treatment. The wear tests were conducted at temperatures up to 600°C with specimens before and after ion release. The wear tests were carried out on a high temperature tribometer with ball on plate geometry. Sliding velocity and applied load were 5 × 10^-3 ms^-1 and 20 N. Examination by electron probe micro-analysis revealed that the concentration of the cations was influenced by the electrical treatment. A detrimental effect on the high temperature wear resistance due to addition of the minor additives was observed. The wear test results showed that the change in composition of the grain boundary phase with the electric treatment at 1300°C resulted in a 50% decrease in wear rate at 600°C compared with otherwise identical material. This study showed that the composition of the glass phase present greatly influences the high-temperature wear behavior of silicon nitride.