in evaluating the quality of synthetic diamonds, thermal stability is the most important and practical property. It directly dictates the actual performance and service life of diamond tools during high-temperature sintering and cutting operations.
Scientific Definition and Measurement: Thermal stability is defined as the ratio of thermal impact strength (TI1) to the original impact strength (TI).
Original Impact Strength (TI): Measured by checking the impact resistance of diamonds using a steel ball inside a hardened steel container, which is vibrated at a set frequency by a precision-controlled system, followed by sieving to test the breakdown rate.
Thermal Impact Strength (TI1): Measured by first heat-treating the diamonds at 1100°C under the protection of 99.99% pure argon gas for 15 minutes, cooling them to room temperature, and then testing their impact strength using the same method.A ratio closer to 1 indicates that the diamond loses less strength after exposure to high temperatures, representing better thermal stability and superior quality.
Positive Correlation with Quality Grade: Under the same grain size conditions, the thermal stability of diamond grains increases gradually as their grade rises. For example, within GE's diamond series, thermal stability shows a clear upward trend from the lower-grade MBS913 to the high-grade MBS970. Thus, the thermal stability value serves as a critical benchmark to evaluate and grade diamond quality.
Indirect Reflection of Crystal Purity: The primary driver for the degradation of thermal impact strength (i.e., poor thermal stability) is the presence of metallic inclusions (magnetic impurities) inside the crystal. Because metal expands much more than diamond at high temperatures, it induces severe internal stress and graphitization. Therefore, higher thermal stability typically indicates lower magnetic impurity content and a more integrated internal crystal structure.
An Evaluation Dimension Independent of Grit Size: Research demonstrates that for the same grade (such as MBS950), there is no obvious linear relationship between thermal stability and grain size. This means that thermal stability is an independent quality indicator that purely assesses the crystal's intrinsic material quality and internal defect states, separate from the physical grit size.