Toughness decides whether a grit survives the first minute in a metal bond; crystal shape decides how it cuts while it survives. Both matter more than nominal size for saw blades, core drills and CMP pad conditioners, and both can be measured before you place an order.
What Toughness Actually Measures
Toughness index, usually written TI, is the share of grit that survives a standard impact test without breaking, expressed as a percentage. A higher number means the particle resists fracture. It is not a hardness measurement: all synthetic diamond is hard, but not all of it survives being clamped in a metal matrix and slammed into stone.
Measured TI on high-strength grit separates the grades clearly. In our own test data, MBD4 sits in the 43% to 69% range depending on size, MBD6 around 65% to 68%, MBD8 between 69% and 80%, MBD10 at 82% to 84%, and MBD12 up to 86%. SMD25 measures 88.2% to 88.4% and SMD30 reaches 90.2% at the fine end.
Crystal Shape And What It Does
Shape is usually described on a scale from blocky to sharp, and it is read from a microscope image rather than from a certificate number. The two extremes behave differently:
- Blocky, well-formed crystals resist fracture and hold position in the bond. They are the right starting point for sawing and drilling, where the grit must stay put.
- Irregular, sharp crystals cut freely and self-sharpen quickly. They suit free-abrasive lapping and electroplated tools, where fresh edges matter more than retention.
- Spherical material is a separate category. MBDS is shape-corrected to coarser than 30 micrometres and is used where uniform protrusion matters.
Shape also explains a common field complaint. If a saw segment glazes instead of cutting, the grit may be too blocky for the stone and not sharpening; if segments wear out in a day, the grit may be sharp and simply fracturing away. The fix is a different crystal habit, not a different mesh.
Ash Content: The Quiet Variable
Ash is what is left after the diamond burns away, and it represents non-diamond residue from the synthesis and processing route. In our measured data it ranges from 0.0002% to 0.0093% across MBD sizes.
For a saw blade, a few thousandths of a percent of ash is irrelevant. For an electroplated dental burr, a CMP pad conditioner or a thermal interface material, it is not: metallic residue changes plating adhesion, introduces contamination into a clean process, and in thermal work it sits exactly at the interface you are trying to control. Buyers in those applications should ask for the ash figure by lot, not by catalogue.
Matching Grade To Tool
| Application | What the tool needs | Grade direction | Typical size |
|---|---|---|---|
| Saw blade segments | High TI, blocky, survives impact | MBD, SMD class | 30/35 – 50/60 |
| Core drills | High TI, good retention in matrix | MBD, SMD class | 30/35 – 60/70 |
| Electroplated tools | Sharp edges, clean surface, low ash | HDY crushed, RVD class | 80 – 400 mesh |
| CMP pad conditioners | Regular shape, sharp tips, tight size | MBD-6 to MBD-12 | 80 – 270 mesh |
| Free abrasive lapping | Sharp, fast cutting, low cost | HDY, HDG micron | W7 – W40 |
Common Mistakes
- Buying the highest TI available. A grit that is too tough for a soft stone will glaze and stop cutting. Toughness has to match the work, not exceed it.
- Judging shape from a catalogue photo. Catalogue images are usually taken at one size. Ask for an image at the size you are buying.
- Ignoring coating interaction. A titanium coating at 0.1% to 0.3% by weight can improve retention in a metal bond, but it does not fix a grit that is fracturing because the TI is too low.
How To Verify
Three documents settle most arguments: a TI value at the exact size, a morphology image at the same size, and an ash figure for the lot. If the supplier measures TI, ask which standard and which size, because a number without those two details cannot be compared with anyone.
Next Step
High-strength grit: MBD 18/20 – 500/600. Crushed sharp grit for plating: HDHR and HDYR. Then match the tool: saw blades and core drills, electroplated tools, CMP pad conditioners.