Advanced and forward-looking large bore gas engine concepts have to fulfil challenging tasks which arise from higher efficiency accompanied by tightened emission legislations. In this context improved combustion characteristics, higher temperatures, increasing peak cylinder pressures and lower combustion residues lead to a new load collective, which exhibits previously unknown excessive valve wear. In particular, hybrid power generation plants using large bore gas engines in combination with solar or wind energy are characterised by several thousands of combustion engine shut downs per year which change the operating conditions of the wear pair valve spindle/seat ring significantly in comparison to conventional power plant applications.
Hardfacings of gas exchange valves on sealing interfaces are tribologically highly loaded and mostly wearing areas. For this reason, two different well-established hardfacing materials were investigated: Cobalt base alloys Stellite®12 and Tribaloy®T400. The worn surfaces of the sealing interfaces were analysed in reference to their wear behaviour, oxygen penetration depth in the wear scars and formation of a protective tribofilm after operation in the same large bore gas engine type. It is shown that the wear rate is significantly influenced by the different microstructures of the cobalt-based alloys. The identified wear mechanisms are a combination of flow wear by accumulated plastic shear flow and surface fatigue by crack initiation and propagation. The formation of various metal oxides on the seating faces was proven as well. Localised traces of transferred material indicate adhesion but are not necessarily part of the dominant wear mechanism. Samples which show low wear feature protective tribofilms of thickness varying from 100 nm to 1 µm consisting of Co, Cr, Mo, Si, Ca, Zn, P and oxygen-containing hydrocarbons. It may be assumed that the layers preserve the surface from wear and the sub-surface microstructure from fatigue.