Ordinary Carbide

Smaller Grains

Tight Packed Grains

Locked Grains (note lines)

Ordinary tungsten carbide -  individual  grains glued together with cobalt

Comet grades – A solid substance with reinforcing like rebars (dark lines) in concrete

2. & 3.  Adhesion and Diffusion

The materials used in tungsten carbide have an affinity to the materials being cut.  This functions two ways.  One way is adhesion where the material being cut actually sticks to the tungsten carbide in a sort of welding process.   The picture to the right we call “surfing dinosaur.”   It is a 60 x picture of an aluminum chip welded to piece of tungsten carbide.   We found this in an aerospace machine shop milling aluminum. 

The second way is where the material being cut dissolves one or more of the materials in the tungsten carbide.  Usually it is the cobalt binder, in the tungsten carbide.  This is very readily seem cutting high acid woods.  It is also important cutting metals.  The solid solubility of nickel in aluminum does not exceed 0.04% while cobalt can have a factor several times that.  In addition a nickel / chromium binder chemically locks the nickel to the chrome which makes it much less reactive than elemental cobalt.    Right is representation of the electron configuration  of cobalt.  ((Cobalt 27 (2:8:15:2))  With only two electrons in the outer shell it is highly reactive.   

4.  Fatigue

This is standard metal fatigue. On a large scale you see it by bending piece of metal repeatedly until it snaps or tears.   The binder in tungsten carbide work hardens and fails much like any other metal.  Its susceptibility to metal fatigue can be changed by minor changes in chemistry and processing.  

5.  Tribology  

Tribology is a term covering the results of a combination of the above causes of wear.  When you take a saw tip and get it hot by using it to cut it becomes  much more sensitive to chemical attack.  High heat also contributes towards the tendency for adhesion or welding.  There is an additional effect that is sort of like an electroetching to destroy the material by takign advantage of the differeng electrical potentials in the materials. 

Additional wear factors

Tungsten carbide is actually tungsten carbide grains cemented with a metal, usually cobalt. What follows are failure mechanisms we have seen in industry.

The following list is open for discussion but we have found it to be a useful tool for developing new grades.

1. Wear – the grains and the binder just plain wear down

2. Macrofracture – big chunks break off or the whole part breaks

3. Microfracture – edge chipping

4. Crack Initiation – How hard it is to start a crack

5. Crack propagation - how fast and how far the crack runs once started

6. Individual grains breaking

7. Individual grains pulling out

8. Chemical leaching that will dissolve the binder and let the grains fall out

9. Rubbing can also generate an electrical potential that will accelerate grain loss 10. Part deformation - If there is too much binder the part can deform

11. Friction Welding between the carbide and the material being cut

12. Physical Adhesion – the grains get physically pulled out. Think of sharp edges of the grains getting pulled by wood fibers.

13. Chemical adhesion – think of the grains as getting glued to the material being cut such as MDF, fibreboard, etc

14. Metal fatigue – The metal binder gets bent and fatigues like bending a piece of steel or other metal

15. Heat – adds to the whole thing especially as a saw goes in and out of a cut. The outside gets hotter faster than the inside. As the outside grows rapidly with the heat the inside doesn’t grow as fast and this creates stress that tends to cause flaking (spalling) on the outside.

16. Compression / Tension Cycling - in interrupted cuts the carbide rapidly goes though this cycle. There is good evidence that most damage is done as the carbide tip leaves the cut and pressure is released.

17. Tribology – as the tip moves though the material it is an acid environment and the heat and friction of the cutting create a combination of forces.

Notes:

As with any chemical reaction of this sort the acids create a salt that protects underlying binder until the salt is abraded away so grain size and binder chemistry are also important.

Electrochemical effect – erosion compounded by the differences in electrical resistivity between carbide and cobalt

Heat from rubbing can affect carbide so a slicker grade can increase life.