Coating exists to solve a specific failure. In resin and electroplated tools it solves grit pull-out, and nickel is the answer. In sintered metal bonds it solves oxidation and heat damage, and titanium is the answer. Using the wrong one adds cost without changing the failure.
What Bare Diamond Cannot Do
Diamond is chemically inert and does not wet metals or resins. In a bond it is held mechanically, by whatever shrinkage and friction the matrix provides. That works until the load rises, at which point the particle either pulls out whole or fractures at the interface.
A metallic coating changes the interface. The coating bonds to the diamond surface on one side and to the matrix on the other, so the load transfers through a continuous layer instead of through a mechanical grip.
Nickel In Resin And Electroplated Tools
Nickel is applied by chemical or electrolytic plating and forms a thick, continuous shell. Two weight levels are standard: about 30% for general retention work, and about 56% for wire sawing, where one carat of diamond becomes roughly 1.56 carats after coating.
Where nickel earns its place:
- Resin bond wheels. The coating gives the resin a mechanical key, reducing pull-out before the grit has done work. This is the standard fix for wheels that shed grit early.
- Electroplated tools. Nickel plates onto nickel, so a pre-coated particle and the plating layer become one matrix. Adhesion improves and the grit sits higher and sharper.
- Wire sawing. At 6 to 12 micrometre core size, only the heavy coating holds the particle on a wire moving at speed.
- Graphite and carbon cutting. Retention again, in a bond that otherwise releases grit prematurely.
Nickel grades here run from W7 and coarser, with a 2000 carat minimum for coated product.
Titanium In Metal Bonds
Titanium is a thin chemical plating, typically 0.1% to 0.3% by weight. It is not a structural shell, so it does not change particle size meaningfully.
Its job is thermal and chemical. During sintering and during dry cutting, diamond oxidises and graphitises at the surface, and the altered surface no longer bonds to the matrix. The titanium layer limits oxygen access, improves heat transfer away from the edge, and reduces that thermal degradation. In saw segments and drills this shows up as longer life rather than higher initial cut rate.
This is the coating to specify when the tool is sintered or brazed and the bond is metallic.
Choosing By Failure Mode
| What you see | Likely cause | Coating to try |
|---|---|---|
| Grit pulled out with resin still attached | Poor retention in resin | Nickel, 30% |
| Bare grit sitting in the swarf | Plating adhesion failure | Nickel pre-coat |
| Segment glazes, grit looks burnt | Oxidation during sintering or dry cut | Titanium |
| Grit fractures at low load | Toughness too low | No coating fixes this |
| Wire loses grit along its length | Retention on wire | Nickel, 56% |
How To Tell A Good Coating From A Patchy One
The average weight gain can look correct while a share of particles is barely covered. Patchy coating is the most common hidden defect and it fails in service, not in incoming inspection.
Ask for a cross-section or SEM image at the size you are buying. A continuous shell looks continuous around the particle perimeter. If the image shows bare diamond facets, the coating is incomplete regardless of what the weight gain says.
Common Mistakes
- Using coated grit in a slurry. Coating adds metal and cost. Free-abrasive processes rarely need it.
- Assuming titanium adds strength. It protects the surface; it does not raise toughness. If grit is fracturing, change the grade.
- Forgetting the size shift at 56%. The coated particle is measurably larger than the core grit. Confirm both numbers.
How To Verify
Request weight gain percentage, coating thickness where available, a morphology image of the coated particle, and a note on which plating process was used. Then run the trial in the actual bond, because retention cannot be evaluated in a free-abrasive test.
Next Step
Products: N nickel-coated, T titanium-coated. Base grit: HDY, HDYR.
Related reading: nickel versus titanium, coating selection and resin versus metal bond.