Preparation Of Sulfate Solid Lubricant Materials And The Lubricating Mechanisms At High Temperatures

Sulfate has good high temperature stability and good lubricating property, and it is a kind of high performance and environmentally-friendly high temperature solid lubricant, which has bro ad prospect of application（such as missiles, space vehicles, gas turbine engines, turbo engine s, nuclear industry, etc.）. The tribological performance of the sulfate solid lubricant was investigated and its self-lubricating mechanism was clarified. This r esearch can provide theoretical support for the practical application of the sulfate solid lubricant, and it also has important significance for enriching solid lubricant system.SrSO₄ block material which has high relative density was fabricated by hot-pressing sintering method. SrSO₄ ceramic has poor tribological properties at low-medium temperature, and brittle fracture and delamination wear are found to be the dominating wear mechanisms. The friction coefficient and wear rate of SrSO₄ ceramic are greatly reduced at high temperature.The average friction coefficients of the material decrease from 0.65 to 0.37, and the wear rates decrease from the order of magnitude of 10-3 mm3/（Nm） to 10-4 mm3/（Nm） over a temperature range from room temperature to 800 ℃. Combining with the observation and analysis of TEM, the high temperature self-lubricating mechanism of sulfate solid lubricating material is obtained. The slip systems of SrSO₄ material become active at high temperature, and the plastic deformation ability is enhanced. A layer of lubricating film with low shear strength forms on the worn surface of the material, which is mainly consisted of nanoscale crystalline. A layer of continuous and smooth transfer film is also formed on the surface of the grinding ball. Therefore, it can effectively alleviate the friction between the friction pairs, thus improve high-temperature tribological properties of material.SrSO₄ solid lubricant has excellent self-lubrication at elevated temperatures. In this paper, SrSO₄ self-lubricating coatings with different granularity were prepared by spark plasma sintering method. This kind of SrSO₄ coatings can be used in high-temperature environment. The friction coefficients and wear rates of the coatings with different granularity at 600℃ are lower than those at room temperature. The tribological properties of nanoscale SrSO₄ self-lubricating coating at different temperatures are better than another two different coatings. In order to improve the sintering properties of Sr SO 4 solid lubricating materials, the nanoscale SrSO₄-Ag composite coatings with different Ag content were prepared by spark plasma sintering method. The average friction coefficients of nanoscale SrSO₄-Ag composite coating decrease with the increase of Ag content at different temperature, and the wear rates increase first and then decrease with the increase of Ag content. The nanoscale SrSO₄-Ag composite coating with 20 wt.% Ag has the best tribological properties. At room temperature, the average friction coefficient of th e composite coating is 0.57, and the wear rate is in the order of magnitude of 10-3 mm3/（Nm）. The friction coefficient then reduces to 0.22, and the wear rate decreases by an order of magnitude. Synergistic effect of SrSO₄ and Ag lubricant can improve the tribological properties of SrSO₄-Ag composite coating over a broad temperature range.