How Are PDC Cutters Made?

Understanding how PDC cutters are made is a great way to increase your drill bit knowledge. 

For some of us in the oil industry the term PDC may be a daily acronym used in the office or in the field but what does PDC really stand for?

It stands for Polycrystalline Diamond Compact.

A PDC cutter is the heart and sole of a PDC drill bit. It makes up more than 90% of all drill bits used today for mining, geothermal, oil and gas drilling.

PDC Cutters, Trendon Bit Service, Drilling Corner, Oil and Gas Drilling, Geothermal Drilling

What are PDC cutters made from?

PDC cutters come in many sizes, shapes and forms.

The most common are the cylindrical planar type in the image above.

A PDC cutter consists of two parts, the diamond table and the substrate. 
The thin disc at the top is the  diamond responsible for removing rock.

Diamond is one of the hardest materials on earth.
The larger portion below is the metal tungsten carbide alloy that is responsible for securing the cutter to the drill bit.

Natural diamonds can be found in the earth’s crust and require high pressure, heat and time to form.

For the last 125 years methods have been discovered to create synthetic diamond.
The first commercially successful synthetic  diamonds were created in 1954 by Tracy Hall of General Electric.
Since then, the manufacture of synthetic diamond has been vastly improved for uses in many different industries including mining, automotive machining, jewelry, and lasers for medical procedures.  

In order to make a PDC cutter, diamond grit is placed in a small cannister placed in a press.

The most common press machine is called a cubic press standing 9 feet tall and weighing 40 tons.
The cubic press uses 6 anvils which converge to create pressures higher than 1,000,000 PSI, but this pressure alone is not enough to create diamond.
As the diamond is pressed, it is also heated to 2300 degrees Celsius. A catalyst is added before pressing to help speed up the process.

The catalyst is usually cobalt.

The diamond will sit in this heated and pressed state for several minutes,  whilst diamond to diamond bonds are created.

How Are PDC Cutters Made

Diamond is known as a non-wettable material. This means that you cannot weld or braze to it.

The diamond is secured to the metal substrate during the pressing process. Before the grit is pressed it is put into a refractory metal can along with the tungsten carbide and cobalt.

What’s inside a PDC Cutter?

How are PDC Cutters Made?

Cobalt, which helps to catalyze the diamond sintering process, also helps to “glue” the diamond to the substate through molecular joining of the diamond and the substate.

Below are images showing loose diamond particles and joined diamond grains after the pressing process.
Cobalt can be seen as white specks between the black visible diamond grains.

This method of creating diamond gives the PDC cutter greater toughness (impact resistance) than a single diamond crystal.

The grains and boundary lines of each individual crystal is randomly oriented.
We see more uniform wear than a single diamond crystal but thermal conductivity and hardness is near that of single crystal values.

This multigranular diamond structure is extremely wear resistant but also gives it a significant boost in crack resistance.

After the cutter is pressed it is removed from the cube assembly.

A raw PDC cutter, which is very crude, is then sent for machining. Since diamond is the hardest stable material on earth, diamond tools are required to grind and machine the PDC cutter into tolerance. It’s like cutting a piece of lumber with a wood saw.

It’s a long arduous process.

Raw Unfinished PDC Cutter

Machined and Final PDC Cutter

There are various steps and changes that can be made to alter the properties of the PDC cutter in order to increase or decrease the cutters ability to withstand impact, wear, heat and corrosion.

Some of these include altering the diamond grain size, diamond table thickness, metallurgy of the tungsten carbide substrate, press pressure, press time, press temperature as well as many others.

The most important step that can vastly affect the performance of the cutter is the amount of cobalt left in between the diamond grains.
Cobalt when heated expands at a rate 13 times greater than diamond.

As a cutter begins to heat up while drilling the cobalt will expand, pushing and pressing between each diamond grain.

If the pressure becomes too great the diamond-to-diamond bonds will be severed leading to rapid wear and failure of the cutter.

One technique to increase cutter life and durability is to remove the cobalt from the diamond itself. Since the cobalt successfully completed its job during the manufacturing process its no longer needed when drilling takes place.

The cobalt is removed through a leaching process where it is placed in an acid bath. The vulnerable substrate is carefully protected with an acid resistant paint to stop rapid corrosion of the metal beneath the diamond.

The cutter can sit in an acid bath up to 4 weeks before being removed. The amount of time the cutter sits in the acid is dependent on how much or how deep into the diamond table the cobalt is to be removed.

In most cases the cobalt is removed 600 to 900 microns down. You can think of leaching as aging a fine wine. All good things take time.

Leached PDC Cutter Cross Section

After leaching, the PDC cutter is inspected for defects. Some PDC cutters are cut in half and viewed under scanning electron microscopes in order to ensure that the leach depths are correct.

Once the cutters pass inspection they are shipped to the drill bit supplier and are brazed into a PDC bit.

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