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Effect of the humidity on the friction and wear characteristics of Si3N4–hBN composite ceramics

The tribological behaviors of Si3N4–hBN composites sliding against austenitic stainless steel at different relative humidities (RH25∼35%, RH55∼65%, and RH90% or higher) were investigated by using an MMW-1 type vertical universal tribometer. The morphological and chemical characterizations of the worn surfaces of the specimens were conducted by using scanning electron microscopy, energy-dispersive X-ray spectroscopy, laser scanning microscope, and X-ray photoelectron spectroscopy. The results showed that the friction coefficients of Si3N4–10%hBN/ASS sliding pairs were lowered to about 0.03 under the relative humidity of RH55∼65%. This was mainly attributed to the formation of a surface film (consisting of B2O3, SiO2, and Fe2O3) on the wear surface of the SN10/ASS pairs. The surface film lubricated and protected the wear surfaces of pin and disc. Under the relative humidity of RH25∼35%, the formation of the surface film was limited because of less water molecules. Under the relative humidity of RH90% or higher, a part of the water molecules in the air can easily deposit on the wear surfaces, so that the microcracks on the wear surfaces would be promoted to expand, resulting in the inability to form a continuous and stable surface film. Besides, for Si3N4/austenitic stainless steel pair, no tribofilm was formed on the surfaces, and higher friction coefficient and wear rate were obtained. When hBN content increased to 20% or higher, rough wear surface was covered by the wear debris layer mainly due to their poor physical and mechanical properties.