The wear properties of two types of silicon nitride($Si_3N_4$) ceramics exposed to high(above RH 90%) and low humidity(below RH 10%) were examined experimentally at various sliding speeds(0.25 to 4.0 m/s). AISI 52100 Bearing steel was used as the disk material under pin-on-disk type sliding conditions. The wear rate of the pressureless sintered plus hot isostatic pressed(HIP+PLS) $Si_3N_4$ was slightly lower than that of the pressureless sintered(PLS) $Si_3N_4$. It was observed that humidity and sliding speed markedly affect the wear properties of $Si_3N_4$. The wear rate was higher at a high humidity than a low humidity. The highest wear rate was obtained with a high humidity and low sliding speed. As the sliding speed was increased, the wear rate was decreased and the effect of humidity on the wear rate of the $Si_3N_4$ was reduced. Surface of the pin became smoother at low sliding speed. As the sliding speed was increased, deep and uniform grooves were found on the surface. The smooth surface is considered to be the result of micro-plowing and polishing process by the mechanical action of the water and small hard particles. Protection film was not formed on the surface of the pin.
The result that the $Si_3N_4$ pin showed a higher wear rate under high humidity was explained as follows. First, chemically adsorbed moisture on the sliding surface of the $Si_3N_4$ pin has decreased the microhardness of $Si_3N_4$ pin surface and thus the wear rate of the $Si_3N_4$ pin by the plowing action of the bearing steel disk has been increased. Another effect of high humidity is the increased plowing action of $Fe_2O_3$ particles formed as a result of the oxidation of bearing steel, which can be accelerated by the presence of moisture.
An increase in the sliding speed is supposed to have reduced the humidity effect on the wear rate of $Si_3N_4$ pin by raising the bulk temperature of the bearing steel disk and the temperature at the pin-disk contact point. Also, with the rise in sliding speed, hardness decrease due to the temperature rise at the contact point should have been much larger for the bearing steel disk than for the $Si_3N_4$ pin, leading to a decrease in the wear rate of $Si_3N_4$.
