Thermally Stable Polycrystalline Diamond Bit Cutter

Thermally stable polycrystalline diamond bit cutters were introduced when it was found that PDC bit cutters were sometimes chipped during drilling. This failure was due to internal stresses caused by the differential expansion of the diamond and binder material.

Cobalt is the most widely used binder in sintered PCD products. This material has a thermal coefficient of expansion of 1.2 x 10 ^-5 deg. C compared to 2.7 x 10 ^-6 for diamond. Therefore cobalt expands faster than diamond. As the bulk temperature of the cutter rises above 730 deg C internal stresses caused by the different rates of expansion leads to severe intergranular cracking, macro chipping, and rapid failure of the cutter. 

These temperatures are much higher than the temperatures to be found at the bottom of the borehole (typically 100 deg C at 8000 ft). They arise from the friction generated by the shearing action by which these bits cut the rock. 

This temperature barrier of 730 deg C presented serious barriers to improved performance of PCD cutter bits. 

Manufacturers experimented with improving the thermal stability of the cutters and thermally stable polycrystalline diamond bits cutters were developed. 

These bits cutters are more stable at higher temperatures because the cobalt binder has been removed and this eliminates internal stresses caused by differential expansion. Since most of the binder is interconnected, extended treatment with acids can leach most of it out. The bonds between adjacent diamond particles are unaffected, retaining 50-80% of the compacts’ strength. Leached PCD is thermally stable in an inert or reducing atmospheres to 1200 deg C but will degrade at 875 deg C in the presence of oxygen. 

It was proved that if the cobalt material can be removed from the grain gap, the thermal stability of the PDC teeth will be significantly improved so that the bit can drill better in harder and more abrasive formations. This cobalt removal technology enhances the wear resistance of PDC teeth in highly abrasive hard rock formations and further broadens the application range of PDC bits.

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