Thermal wear and cobalt removal of PDC

I. Thermal wear and cobalt removal of PDC

In the high pressure sintering process of PDC, cobalt acts as a catalyst to promote the direct combination of diamond and diamond, and make the diamond layer and tungsten carbide matrix become a whole, resulting in PDC cutting teeth suitable for oilfield geological drilling with high toughness and excellent wear resistance,

Diamonds heat resistance is quite limited. Under atmospheric pressure, the surface of diamond can transform at temperatures around 900℃ or higher. During use, traditional PDCs tend to degrade at about 750℃. When drilling through hard and abrasive rock layers, PDCs can easily reach this temperature due to frictional heat, and the instantaneous temperature (i.e., localized temperature at the microscopic level) can be even higher, far exceeding the melting point of cobalt (1495°C).

Compared to pure diamond, due to the presence of cobalt, diamond converts to graphite at lower temperatures. As a result, wear on diamond is caused by the graphitization resulting from localized frictional heat. Additionally, the thermal expansion coefficient of cobalt is much higher than that of diamond, so during heating, the bonding between diamond grains can be disrupted by the expansion of cobalt.

In 1983, two researchers performed diamond removal treatment on the surface of standard PDC diamond layers, significantly enhancing the performance of PDC teeth. However, this invention did not receive the attention it deserved. It wasnt until after 2000 that, with a deeper understanding of PDC diamond layers, drill suppliers began applying this technology to PDC teeth used in rock drilling. Teeth treated with this method are suitable for highly abrasive formations with significant thermal mechanical wear and are commonly referred to as “de-cobalted” teeth.

The so-called “de-cobalt” is made in the traditional way to make PDC, and then the surface of its diamond layer is immersed in strong acid to remove the cobalt phase through the acid etching process. The depth of cobalt removal can reach about 200 microns.

A heavy-duty wear test was conducted on two identical PDC teeth (one of which had undergone cobalt removal treatment on the diamond layer surface). After cutting 5000m of granite, it was found that the wear rate of the non-cobalt-removed PDC began to increase sharply. In contrast, the cobalt-removed PDC maintained a relatively stable cutting speed while cutting approximately 15000m of rock.

2. Detection method of PDC

There are two kinds of methods to detect PDC teeth, namely destructive testing and non-destructive testing.

1. Destructive testing

These tests are intended to simulate downhole conditions as realistically as possible to evaluate the performance of cutting teeth under such conditions. The two main forms of destructive testing are wear resistance tests and impact resistance tests.

(1) Wear resistance test

Three types of equipment are used to perform PDC wear resistance tests:

A. Vertical lathe (VTL)

During the test, first fix the PDC bit to the VTL lathe and place a rock sample (usually granite) next to the PDC bit. Then rotate the rock sample around the lathe axis at a certain speed. The PDC bit cuts into the rock sample with a specific depth. When using granite for testing, this cutting depth is generally less than 1 mm. This test can be either dry or wet. In “dry VTL testing,” when the PDC bit cuts through the rock, no cooling is applied; all the frictional heat generated enters the PDC, accelerating the graphitization process of the diamond. This testing method yields excellent results when evaluating PDC bits under conditions requiring high drilling pressure or high rotational speed.

The “wet VTL test” detects the life of PDC under moderate heating conditions by cooling the PDC teeth with water or air during testing. Therefore, the main wear source of this test is the grinding of the rock sample rather than the heating factor.

B, horizontal lathe

This test is also carried out with granite, and the principle of the test is basically the same as VTL. The test time is only a few minutes, and the thermal shock between granite and PDC teeth is very limited.

The granite test parameters used by PDC gear suppliers will vary. For example, the test parameters used by Synthetic Corporation and DI Company in the United States are not exactly the same, but they use the same granite material for their tests, a coarse to medium grade polycrystalline igneous rock with very little porosity and a compressive strength of 190MPa.

C. Abrasion ratio measuring instrument

Under the specified conditions, the diamond layer of PDC is used to trim silicon carbide grinding wheel, and the ratio of the wear rate of grinding wheel and the wear rate of PDC is taken as the wear index of PDC, which is called wear ratio.

(2) Impact resistance test

The method for impact testing involves installing PDC teeth at an angle of 15-25 degrees and then dropping an object from a certain height to strike the diamond layer on the PDC teeth vertically. The weight and height of the falling object indicate the impact energy level experienced by the test tooth, which can gradually increase up to 100 joules. Each tooth can be impacted 3-7 times until it cannot be tested further. Generally, at least 10 samples of each type of tooth are tested at each energy level. Since there is a range in the resistance of teeth to impact, the test results at each energy level are the average area of diamond spalling after impact for each tooth.

2. Non-destructive testing

The most widely used non-destructive testing technique (other than visual and microscopic inspection) is ultrasonic scanning (Cscan).

C scanning technology can detect small defects and determine the location and size of defects. When doing this test, first place the PDC tooth in a water tank, and then scan with an ultrasonic probe;

This article is reprinted from “International Metalworking Network“