ELECTRON BEAM LIQUID PHASE SURFACE TREATMENTS – FACILITATING THE USE OF PVD HARD COATINGS AS SURFACE PROTECTION FOR CAST IRONS

摘要

Hard coating is a well-known and industrially established technology for generating tribological protection layers on hardened steels. For cast irons, the typically soft graphite in the microstructure gives rise to limitations with respect to the deposition and the load-bearing capacity of hard coatings. As a result, surface defects and breakthrough of coatings occur. An electron beam remelting (EBR) or alloying (EBA) process with Ni-based additives for cast iron results in surface layers with higher hardness levels over a wide range of from 450 to 800 HV0.30. Due to the liquid phase surface treatments and the high process-specific cooling rates, the graphite of the base material is eliminated and hard ledeburitic carbides are formed. Furthermore, the rising Ni content leads to increased austenite contents in the alloyed layer. The EB treated layers serve as a supporting layer for the subsequently deposited thin (2 - 4μm), hard PVD coatings (TiN, TiAlN). Comparative investigations of the wear behaviour with pin-on-disc testing and the corrosion behaviour in a chloridic solution (5 wt.-% NaCl) under these conditions showed an overall improvement in the case of the combined treatment. This resulted from the deposition of defect-free PVD hard coatings on the graphite-free EB treated layer. The scope of this paper covers the influence of different hardness levels as well as the microstructural composition (cementite, austenite) of the EB treated layers on wear and corrosion behaviour after single and combined treatments of cast iron with spheroidal graphite (EN-GJS-400). Therefore, the influence of the toughness and/or brittleness of the alloyed layer on supporting behaviour and adhesive strength are discussed.