Wear Phenomena in Tunnel Boring Machine (TBM) Hard Rock Drilling—Reasons and Consequences

In hard rock tunnel boring machine (TBM) drilling, wear issues play a crucial role in the success of a tunneling project. Thus, excavation tools must be selected carefully and adapted to ground conditions. The shape and the material-quality of the cutters have to be adjusted and possible alterations of the tool, caused by the excavation process, must also be taken into account. This is because of the stress applied on the steel during the drilling process. Due to the knowledge of the wear processes by the interaction of the steel with the rock surface and the resulting material changes, it is possible to draw conclusions about the choice of the ideal excavation tool. In our paper, five basic macroscopically wear types will be distinguished: (1) abrasive/normal wear, (2) tapering, (3) mushrooming as (4) brittle failure of the cutter ring and (5) blockade of the roller bearing. These basic wear types lead to distinct cutter ring shapes which allow deriving some fundamental characteristics of the interacting rock types. In addition to an assessment on this macroscopic level, a microscopic analysis is necessary too. For this purpose, the surfaces and associated metallographic sections of worn disc cutters are analyzed. As a result, the tribological processes under the disc cutter, leading to tool wear, can be better understood. In summary, the investigations may contribute to a better understanding to make it easier choosing the proper TBM disc cutter for a distinct rock type in order to minimize the risk of tool related downtimes.