A further development of the PDC bit concept was the introduction in the later 1980’s of Thermally Stable Polycrystalline (TSP) diamond bits. These bits are manufactured in a similar fashion to PDC bits but are tolerant of much higher temperatures than PDC bits.
Thermally Stable Polycrystralline – TSP – Diamond bits were introduced when it was found, soon after their introduction, that PDC bit cutters were sometimes chipped during drilling. It was found that 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 7300 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 1000 C at 8000 ft). They, in fact, arise from the friction generated by the shearing action by which these bits cut the rock.
This temperature barrier of 7300 C presented serious barriers to improved performance of PCD cutter bits. Manufacturers experimented with improving the thermal stability of the cutters and Thermally Stable Polycrystralline Diamond Bits were developed. These bits 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 inert or reducing atmospheres to 12000 C but will degrade at 8750 C in the presence of oxygen. Due to the nature of the manufacturing process the thermally stable polycrystalline (TSP) diamond cannot be integrally bonded to a WC substrate. Therefore, not only is the PCD itself weaker, but the excellent strength of an integrally bonded Tungsten Carbide (WC) substrate is sacrificed.Without the WC substrate, the TSP diamond is restricted to small sizes (Figure 20) and must be set into a matrix similar to natural diamonds.