Fig. 2

a Schematic representation of the humidity control system. The relative humidity was controlled by the temperature at the sample holder and steam generator. b Schematic representation of the sample setting in the frictional testing machine. The sample holder was heated with an oil and rubber heater, and the temperature of the sample holder was held constant. The horizontal ram applied a constant normal force and the vertical ram applied a shear force at a constant loading velocity. The thermocouple was placed central to the frictional surfaces