Research progress on external friction coefficient test of powders

The external friction behavior between the interface and powders is affected by the properties of the powders, the surface roughness of dies, the relative motion speed, temperature, and pressure. Unreasonable external friction behavior may cause the wear of powder processing equipment and the uneven density distribution of products. To explore the external friction behavior of powders, the research progress of powder external friction coefficient was thoroughly investigated, and the testing methods of powder external friction coefficient were summarized. According to the loads, the testing methods of powder external friction coefficient are composed of small load testing methods and heavy load testing methods. The small load testing methods include slope method and plate method, and the heavy load testing methods include rotation method, shear method, and closed mold method. The principle, testing equipment, and important conclusions of those various testing methods were briefly described in this paper. The results show that, the test methods under the small load are only suitable for the external friction coefficient of powders with low relative density, and the pressing force in the test is generally less than 100 times of powder weight. The test methods under the heavy load are more commonly used for measuring the external friction coefficient of powders with high relative density; the pressing force of polymer materials is usually 0.5×10³~1.0×10⁵ times of powder weight, while that of metal materials is 10⁵~10⁷ times of powder weight.