Silicon doped diamond like carbon as substitute for lubrication in spin-extrusion

To protect tools and to fulfil the request of good surface quality of the workpiece, it is normally necessary to use solid lubricants like molybdenum disulfide (MoS2) or graphite in warm bulk metal forming. Caused by economical and ecological aspects, it is desirable to avoid lubrication and to substitute it by systems like coatings without the necessity to renew the protecting and lubricating film after every forming process.So called low wear friction coatings like Diamond like Carbon (DLC) are candidates for substituting lubricants or combinations of lubricants and hard coatings like TiN. The advantages of DLC are its high hardness, high Young's modulus and low coefficient of friction. Another important aspect is the property of DLC to graphitise the surface at high pressures, so the coating becomes a kind of self-lubricating. In massive forming at low temperatures like cold extrusion, DLC is applied today.The aim of the work is to show the suitability of DLC as a protective low wear friction coating for warm massive forming, especially for a technique called spin-extrusion. The greatest tribological problems of this forming process are a low sliding velocity and high temperatures in the working zone between tool and workpiece, which can reach a maximum temperature of about 800 °C. Normally, the maximum working temperatures of DLC should be lower than 400 °C when working in air atmosphere. So, the DLC was doped with silicon to raise the maximum working temperature of the coating. Also, the time a-C:H:Si is subjected to this temperature is short, and the air content in the working zone is reduced during the process. The behaviour of a-C:H:Si coated tools during spin-extrusion without additional lubrication of the tool is compared with the forming behaviour of uncoated, but lubricated tools.