For the purpose of reliably testing materials for sliding in paper machine conditions, a special high-velocity sliding test rig was developed by Valmet Technologies in close collaboration with The Department of Materials Science at Tampere University of Technology. The key features of the equipment are the presence of process fluids and the capability to reach sliding velocities of 2500 m/min. The device is one of a kind prototype in the world.
Testing was carried out for ceramics and hard coatings in conditions closely resembling those in the paper machine. It was established that in the sliding velocities above 1000 m/min, a material must have high fracture toughness as well as high hardness. The balance between hardness and toughness as a dominant material property leans heavily towards favouring the fracture toughness, when sliding velocity is increased further. In velocities above 1500 m/min a very hard material without sufficient fracture toughness would show signs of severe wear.
Studies conducted in TUT show, that the required material properties can be achieved by a nanocomposite structure. Introducing nano-sized nickel particles into alumina ceramic improved the fracture toughness by 16% without any decrease in hardness.
Wear-related stoppages on a paper machine scale are very expensive. Wear also makes sliding surfaces less optimal for sliding, which translates into increased friction; i.e. energy going to waste. Better understanding of wear mechanisms guides material development to more wear resistant and low friction solutions in the application.
The research results are directly applicable to material screening and product development by Valmet Technologies to produce wear resistant low friction surfaces with long life time all over paper machinery. Potential applications include water removal elements, hard tipped doctor/application blades and roll coatings. Overall, the results show the importance of testing new materials and surfaces in conditions mimicking the actual application as closely as possible.
Erkki Levänen, Tampere University of Technology
Valmet Technologies, Tampere University of Technology