Wear damage of cemented carbides with different combinations of WC mean grain size and Co content. Part II: Laboratory performance tests on rock cutting and drilling

In the present work we carried out laboratory performance tests of cemented carbides with very different WC grain sizes varying from nearly 0.2 mu m to 5 mu m and Co contents varying from 6 to 33 wt.% in rock cutting and drilling. These tests are determined to well simulate the real operation of cemented carbides in mining and construction applications, particularly in road-planing and rock-cutting as well as in percussive drilling and rotary drilling. The major objective of the present work was to examine the wear damage, wear behavior and wear mechanisms of cemented carbides having a similar hardness but greatly varying with respect to their WC mean grain size and Co content in rock evacuation. Phenomena of micro-fatigue, micro-fracturing and micro-chipping of cemented carbides are found to play the decisive role in rock-cutting. As a result the wear-resistance of the near-nano carbides having a reduced fracture resistance is significantly lower than that of the medium and coarse grades having similar hardness values. The major wear mechanism of the near-nano grade in rock-cutting is related to micro-cracking and microchipping leading to the detachment of huge WC-Co agglomerates. This is presumably a result of reduced fracture toughness of the near-nano cemented carbides containing much grain growth inhibitors, which is caused by the segregation of grain growth inhibitors at the WC-Co and WC-WC interfaces. In contrast to the near-nano grades, the major wear mechanism of the coarse grade in rock-cutting is related to wearing off the binder interlayers among the large WC grains and their microchipping and fragmentation. Phenomena of micro-fatigue, micro-fracturing and micro-chipping of cemented carbides also play a very important role in rock-drilling. The wear-resistance of the near-nano grade is noticeably lower than that of the corresponding straight coarse-grain grade having nearly the same hardness but significantly higher fracture resistance. As in the case of rock-cutting, the major wear mechanism of the near-nano grade in rock-drilling is related to micro-cracking and micro-chipping caused by micro-fatigue phenomena, which leads to the detachment of huge WC-Co agglomerates. In contrast to that, the following wear mechanisms take place on the surface of the medium and medium-coarse grades during percussive drilling: (1) wearing the binder among WC grains, which leaves them unsupported, (2) abrasion of the surface of the WC grains, (3) micro-chipping the WC grains, (4) detachment of separate WC grains, and (5) detachment of relatively small WC-Co fragments. There are some regions on the worn surface of the medium and medium-coarse grades, where the WC grains were presumably in direct contact with the rock during drilling (first regions), and there are some regions lying slightly below the first regions (second regions). The worn surfaces in the first regions are characterized by a significantly lower Co content (less than 1 wt.%) than on average. The Co contents on the surface of the second regions, which were not presumably in direct contact with the rock when drilling, are noticeably greater and even slightly higher than the average Co content in the bulk of the medium and medium-coarse grades, therefore, the binder was not predominately worn out from the carbide surface in these regions. The second regions presumably comprise Co interlayers on the surface on underlying WC grains after the detachment of WC-Co fragments due to micro-fatigue, as there are no traces of abrasive wear on their surface. Therefore, the presence of the first and the second region on the worn surface with various Co contents is a clear indication of the completely different wear mechanisms in these two regions. (C) 2014 Elsevier Ltd. All rights reserved.