Cemented carbide blades Wear Characteristics in Drilling limestoneⅥ

3.3. Wear mechanism of cemented carbide blade drill rock with water jet

The failure type of the cemented carbide blades in water jet drilling is not the same as that in dry drilling. Fig. 6 shows the SEM micrographs (500×) of damaged surfaces of blades in the experiments. Fig. 6 (a) is the typical brittle fracture morphology in dry drilling. No serious fractures are detected in the drilling experiments with water jet under the same zoom scope and the surfaces mainly show wear morphology.

There are mainly three reasons to explain the different results. Firstly, water jet can effectively decrease the surface temperature and thermal stress. Secondly, the water jet provides impact force to crack the limestone, and it helps to decrease the mechanical force on the blade. Thus, the sum of thermal stress and mechanical stress which can induce serious brittle fractures could be lower than the material strength of the blade in drilling with water. In the third place, the water jet with higher pressure could form a comparatively cooler water layer to lubricate the blade, and could rush away the hard abrasive particles in the rock like a polisher. Therefore, the surface of the blade in water jet drilling is much smoother than that in dry drilling, and the wear rate will decrease while the water jet pressure increases.

Although wide range of brittle fractures is avoided, but there will still be surface damages on the blades in rock drilling with water jet. Fig. 7 shows micrographs of typical wear surfaces of cemented carbide blades in limestone drilling with water jet. It is obvious that point 1, on the edge of the blade, is rougher than point 2. In fact, the two points could reflect the wearing process of cemented carbide blades in limestone drilling with water jet.

The wearing process of cemented carbide blades in limestone drilling with water jet could be divided into two stages. Initially, under water jet-assisted condition, micro cracks appear on the edge of the blade, probably caused by local mechanical abrasion and thermal stress which is induced by the fl ash temperature. The grain size and its distribution on the surface can be seen in Fig. 7 (d). The Co phase is much softer than the WC phase and it is easy to be worn. So when the blade mills the rock, Co phase is worn firstly and with particles washed away by the water jet, the porosities between grains are larger and the surface of the blade becomes more uneven.

Then, this kind of micro surface damage expands from the edge to the center of the blade surface. While the drill bit is rotating in the rock, the edges and corners of grains will be polished as mentioned before, shown in Fig. 7(e). And this polishing process continues from the edge to the center of the blade surface. When the drill bit drills into the rock continuously, the polished surface on the edges will form new micro cracks which then extend to the center of the blade surface because of mechanical abrasion and thermal stress caused by flash temperature.

Therefore, this roughing-polishing process is repeated from the edge to the center of the blade surface constantly, and the blade will became thinner and thinner until it cannot work.

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