Abstract: With the increasing enhancement of people's awareness of environmental protection, new and high requirements have been posed to the upgrading and updating of production activities and equipment that may lead to environment pollution or destruction and resources waste. In the medium lubrication field, especially for water lubrication, it is used more widely, energy saving and environmental protection comparing with the traditional oil lubrication. While the most remarkable characteristic is green, eco-friendly and low cost for water lubrication, in addition to its high safety and well cooling effect. However, pure water has also some shortcomings, such as low viscosity, poor extreme pressure resistance, poor bearing capacity and poor lubrication performance, etc. This leads to pure water not suitable for direct use as a lubricant without modification, which to a large extent restricts the development of industries related to water lubrication. Based on this, we synthesized the penniform carbon nitride (PFCN) material by a one-step hydrothermal method adopting citric acid and urea as the raw materials. The characteristic of hydrothermal method is that carbon nitride can be dispersed in water in situ, gaining a water-based lubricant. Under this premise, it not only can realize the uniform dispersion of solute and effectively inhibit the agglomeration, but also can obtain the micron distribution of water-soluble carbon nitride materials. The tribological properties of PFCN with different mass fractions on epoxy resin (EP)-stainless steel tribopair under harsh boundary water lubrication conditions were investigated by using a block-on-ring friction and wear testing machine. The micro-structure characterization results of the PFCN material show that the bulk carbon nitride was directly exfoliated between layers under hydrothermal conditions and formed the materials with fluffy interlayer structures. Due to the high temperature and high pressure under hydrothermal conditions, the interlayer bonding strength decreased significantly. At the same time, the fluffy interlaminar structure is conducive to the transfer of the carbon nitride material onto the interface during the friction process, forming a thin and continuous transfer film on the steel counterface. And more importantly, Raman results of the transferred materials showed that ordering degrees of the carbon based materials formed on the counterface significantly increased when lubricated with PFCN as the water lubricant, comparing with deionized water, and furthermore the ordering degrees gradually increased with the content of carbon nitride in the water lubricant, indicating indirectly that the orderly transfer film was mainly composed of carbon nitride materials. In addition, the carbon nitride based transfer film possessed excellent bearing capacity and lubrication performance, which can effectively protect the EP-stainless steel tribopair and avoid the severe wear of EP specimens, comparing with the poor bearing capacity and lubrication performance of deionized water. When only deionized water was employed as the lubricant, friction coefficient of the tribopair and wear rate of the EP specimen were 0.56 and 2.92×10⁻⁴ mm³/(Nm), respectively. By contrast, the friction coefficient and wear rate were remarkably reduced by 71.4% and 78.1% respectively, with dropwise adding 20% PFCN as the water lubricant into the contact interface. As a new type of green water lubricant, in-situ water-based carbon nitride has certain research value and application potential in the lubrication design and service-life extension of polymer-metal pairs.