Mechanisms for the high-temperature strength degradation of diamond are:
Occurs around metallic inclusions comprising solvent/catalyst metal. It can begin to occur at around 700°C where diamond starts to be re-catalysed back to graphite. Graphite is more stable at these temperatures and pressures and has a greater volume per unit mass, therefore, the diamond starts to expand internally → stress.
Heat energises the breaking of carbon–carbon bonds on the diamond surface and oxygen reacts with the carbon → carbon oxides. This creates pitting on the diamond surface; this reaction occurs more readily at defects (e.g. dislocations).
Is caused by catalyst metals (Co, Fe) in the segment matrix. Diamond is converted back to graphite which darkens the diamond surface. Chemical removal of this darkened surface reveals surface etching. Reactions occur more readily at defects (e.g. dislocations).
Effect of heating in flowing argon @ 1,100°C
Effect of heating in static air @ 1,100°C
Effect of sintering in Fe bond @ 900°C
Reduced graphitsation due to coating
Height of diamond protrusion
The theoretical height of diamond protrusion, as a function of cutting rate is shown in the diagram.
Initially, protrusion height increases rapidly, due to bond erosion, and then flattens off.
Diamond size determines an upper limit to the maximum protrusion height (if the adhesion between the diamond and the bond is sufficient). The diamond's structure and properties limit the protrusion height if wear occurs by microfracture.
How coatings work
Retention - the weak link theory:
"A chain is only as strong as its weakest link".
The role of coatings is to protect diamond during tool manufacture and enhance diamond retention in abrasive applications.
Successful implementation of coatings depends on a balanced coating-bond interaction, tool design which considers the effect of coatings on performance (higher power) and that the toolmaker employs good tool making practices.