DLC for tools protection in warm massive forming

Abstract

In order to find an alternative to solid lubricants in bulk metal forming with contact temperatures between work piece and tool up to 600 °C, low wear friction coatings like hydrogenated amorphous diamond-like carbon (DLC), silicon doped diamond-like carbon (Si-DLC), sputtered molybdenum disulfide and a multilayer system of TiC and TiN (TiCN) were tested in a modified forced-in test at contact temperatures between room temperature and 500 °C. Caused by the high normal pressure between work piece and tool, this test models heavy sliding friction which can lead to adhesive wear. The tested solid lubricants showed, for all tested temperatures, lower coefficients of friction than the four coatings, but their values were very low too. Only the bolt coated with Si-DLC failed the test at room temperature and the TiCN coating did not work at all temperatures. The lowest press force of the coatings at 400 °C was needed for the Si-DLC, while for the other temperatures MoS 2 was best. The surfaces of the sleeves showed no visible failures. In endurance tests with the same bolt in different sleeves, surfaces formed with a Si-DLC coated bolt had no mistakes after 16 repetitions at 500 °C, while tests with the MoS 2 coating failed after nine times and with DLC after two times. At 400 °C the Si-DLC coating showed the best results too, while MoS 2 was best at room temperature.