Superlubricity phenomena, which were previously achieved merely under point contact conditions, have been observed under the linear contact condition between the silicon nitride (Si3N4) cylinder and the silica (SiO2) ring in phosphoric acid solutions. After a total 800 s running-in period and low friction stage, the friction coefficient finally declined to about 0.003. The ultralow friction coefficient could last for more than 5 h. In these experiments, a torque sensor of high accuracy was used to make sure that the measuring result was credible. The differences between superlubricity experiments in our group and others were discussed. Unlike the situations in other superlubricity experiments about silicon nitride, where the specimens were immersed in water totally, the phosphoric acid solution with volume of only 150 mu L was used in these tests. It was proved that free water in the solution had a great relationship to the necessary time of acquiring superlubricity and phosphoric acid played a unique role in the superlubricity, which could not be explained properly by the hydrodynamic lubrication model proposed by others. Finally, based on the results of the friction tests and the analysis with scanning electron microscope (SEM), a modified hydrogen-bonded network and hydrated shearing layer model was proposed to explain the mechanism of superlubricity.