Diamond is the hardest material on Earth, but it is not eternal. At high temperature, diamond begins to oxidize and graphitize, losing its unique properties. For PCD sintering at 1400–1600°C, the thermal stability of the source powder is one of the key parameters determining the quality of finished PCD.
What Happens to Diamond When Heated
When heated in air, diamond is stable up to approximately 600–700°C. Above this temperature, oxidation begins: carbon reacts with oxygen in air forming CO and CO2, powder mass decreases. At 850°C, oxidation rate becomes noticeable: 1–3% mass loss in 1 hour in air. At 1000°C and above, diamond actively oxidizes, and in inert atmosphere (without oxygen), direct graphitization begins — transformation of diamond lattice into graphite. For PCD sintering, which takes place in inert atmosphere at 1400–1600°C, resistance to graphitization is critical.
What Thermal Stability Depends On
- Grain surface purity: residual metal catalyst (Fe/Ni/Co) on the surface accelerates graphitization, acting as a catalyst. The cleaner the surface, the higher the thermal stability.
- Crystal lattice defects: cracks, inclusions and dislocations are graphitization centers. Defective grains begin graphitizing at lower temperature.
- Grain size: fine grains have larger specific surface area, so they oxidize faster. For fine W5 fractions, thermal stability is lower than for W20.
Why Catalyst Metals Are the Worst Offenders
Diamond itself behaves predictably at high temperature, but contact with iron, nickel or cobalt changes the picture. These metals dissolve carbon and act as a "transport" by which carbon atoms leave the diamond lattice into graphite. The reaction starts not across the whole grain at once but at contact points, so even trace surface metal (tens of ppm) locally destroys the structure during sintering — locally, not uniformly. Hence thermal stability and cleaning depth are two sides of one coin. A powder with only visual cleaning may show an acceptable TGA number on a single heating run, but will drift under the sustained load of PCD sintering. That is why we measure both Fe/Ni/Co content and thermal-impact mass loss, rather than relying on either parameter alone.
Indicative Temperature Limits by Grade
| Grade | Onset of oxidation in air | Typical application |
|---|---|---|
| SCMD-AA | 850°C and above | PCD, metal bonds |
| SCMD-A | 750–850°C | diamond wheels, pastes |
| regular industrial | 650–750°C | rough machining |
Figures are indicative and depend on fraction: the finer the grain, the larger the specific surface and the lower the absolute oxidation threshold, so for thin W1–W5 limits should be confirmed against the actual report rather than a generic table.
How Thermal Stability Is Measured
The standard method is thermogravimetric analysis (TGA). A powder sample (usually 10–50 mg) is placed in a crucible, heated at a constant rate (5–20°C/min) to 1000–1200°C in air or inert atmosphere. The entire process is continuously weighed, plotting mass loss vs temperature. From the graph, the oxidation onset temperature (usually at 1% mass loss) and oxidation rate at a given temperature are determined. PCD-grade powder should retain ≥99% mass at 850°C in air for 1 hour.
How Manufacturers Improve Thermal Stability
The main approach is deep cleaning of grain surfaces from metallic inclusions. As described in our article on Fe/Ni/Co impurities, repeated acid treatment + high-gradient magnetic separation remove catalyst from surfaces and microcracks. Additionally, some manufacturers apply heat treatment of powder in vacuum or inert atmosphere at 800–1000°C to burn out residual impurities and anneal stresses in the crystal lattice.
How to Read a Thermal-Stability Report
What matters in a report is not a single "850°C" figure but the conditions behind it. Check three things: test atmosphere (air or inert gas), hold time (one hour or more) and acceptance criterion (1% or 5% mass loss). A figure quoted in inert gas with no hold does not equal powder behavior under real PCD sintering with dwell. If the report gives both mass loss and retained strength after thermal impact, that is more reliable than an oxidation-onset temperature alone. When buying, ask the COA to state the atmosphere and hold time explicitly; a "high thermal stability" claim without these details gives the buyer no comparable data.
What to Ask Your Supplier
- A TGA report stating atmosphere, heating rate and hold, not just the final temperature.
- ICP Fe/Ni/Co data — an indirect but fast indicator of future thermal stability.
- Thermal-impact (TI/TTI) results for critical PCD batches.
- Confirmation that the figure is for the same fraction you buy: the threshold depends on grain size.
850°C thermal stability in air is the entry ticket to PCD-grade. If powder begins actively oxidizing at 700°C, it won't work for PCD sintering, no matter how good other parameters are.
Frequently Asked Questions
How is thermal stability measured?
Diamond micron powder thermal stability is measured by thermogravimetric analysis (TGA): the sample is heated in oxidizing environment to 1000–1200 °C, and the temperature of onset of intensive mass loss (oxidation/graphitization) is recorded. Standard PCD-grade micron powder has thermal stability of 850 °C.
Does thermal stability depend on grain size?
Yes, fine grains have larger specific surface area and usually lower thermal stability compared to coarse grains. This is important for PCD sintering: fine fractions (W1–W5) require more careful thermal stability control. We specify thermal stability for each fraction in reports.
Can thermal stability be improved?
Thermal stability is a characteristic of the diamond grain itself, determined by synthesis conditions (temperature, pressure, hold time, catalyst). It cannot be improved in finished micron powder, but fractions with higher thermal stability can be selected through per-batch control. For PCD applications, we recommend specially selected PCD-grade micron powder.
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Practical tip
For a first purchase we recommend a trial batch from 1 kg: test the fraction on your equipment before a bulk order. Every trial batch comes with a report (particle size, Fe+Ni+Co). Standard W5/W10/W20 fractions are always in stock — ships within 48 hours.
How Thermal Stability is Measured: The TI/TTI Method
Thermal stability of diamond micron powder is measured using the thermal impact test method (TI — Thermal Impact, TTI — Thermal Toughness Index):
- A diamond powder sample is heated to 1100°C in an inert atmosphere
- Held at this temperature for 2–3 hours
- Mass loss (ash content) and impact strength change are measured
- Strength before and after heat treatment is compared
High-quality PCD micron powder retains over 85% of its strength after 1100°C exposure. Mass loss (ash content) indicates non-metallic inclusions (graphite, amorphous carbon).
Application Temperature Limits
When selecting micron powder for high-temperature applications (PCD, metal bonds), always request TI/TTI data, not just purity percentage.
Bottom line: thermal stability is not a separate "option" but a property built in by cleaning and per-batch control. Roughing can use a generic industrial grade, but PCD and high-temperature metal bonds need powder with a verified TGA and TI/TTI figure. Start with a trial batch from 1 kg and request a trial COA with thermal stability for your fraction — faster than arguing numbers verbally.
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