With the rapid development of synthetic diamond powder products and materials across various countries, the quality and performance testing technology of synthetic diamond powders have also correspondingly advanced and increasingly attracted significant attention from relevant professionals. The primary testing items for synthetic diamond powder single crystals include: impact toughness (impact strength), thermal stability, magnetic susceptibility, etc.
The definition of the impact toughness of diamond abrasives is the ability of diamond abrasives to resist fracture under specified impact conditions. Specifically, after sieving, basic grains of the sample are taken and placed into a steel sample tube along with a specified number of steel balls. This assembly is then subjected to a certain number of impacts at a specific frequency on an impact device, followed by sieving again.
The impact toughness value is then expressed as the percentage of unbroken grains (the ratio of the amount of material remaining on the sieve after impact to the original amount). A second method was later proposed, which represents the impact toughness of the sample as the number of impacts (also known as the half-fracture number) required for the unbroken rate to reach 50% after further calculations and adjustments based on the first method's results.
Determining the impact toughness (impact strength) of synthetic diamond powder single crystals provides a more objective reflection of their actual performance and quality compared to measuring their single-particle compressive strength. Additionally, measuring impact toughness offers advantages such as good sample representation and minimal measurement errors.
Diamonds are composed of carbon atoms, and pure diamonds should be non-magnetic. However, synthetic diamond powders contain varying amounts of metallic impurities due to the use of metals as catalysts during synthesis, especially inclusions within the crystals, resulting in magnetic properties. The level of magnetic susceptibility in synthetic diamond powders is directly related to the content and composition of internal impurities and inclusions, and it is closely related to other physical and chemical properties and quality indicators (such as toughness, strength, specific gravity, thermal stability, color, transparency, etc.) as well as their applications and classifications.
There are many methods and instruments for measuring magnetic susceptibility, such as the electromagnetic force balance method, permanent magnetic force balance method, isomagnetic force balance method, electromagnetic induction method, impact method, and bridge method, etc.
The objects processed by various synthetic diamond powder products typically have very high hardness and generate a large amount of heat during the processing, with the working environment sometimes exceeding 1000°C. The thermal stability temperature of abrasive-grade diamonds (the temperature of graphitization and oxidation) generally ranges between 750°C and 1000°C, while the thermal stability temperature of micro-powder grade diamonds is even lower.
If the operating environment temperature of the diamond product exceeds its thermal stability temperature, the graphitization and oxidation of the diamond will cause a rapid decline in strength, significantly reducing processing efficiency and effectiveness.
Additionally, most diamond products require pressing and sintering at high temperatures during manufacturing, and the pressing and sintering temperatures must be controlled below the thermal stability temperature (critical strength change temperature) of the diamond. Therefore, determining the thermal stability of diamonds is crucial.
Large synthetic diamond powder production enterprises and companies consider the thermal stability of synthetic diamond powders as one of the key indicators for evaluating their quality and performance and an essential item for testing.
Both differential thermal analysis (DTA) and thermogravimetric analysis (TGA) can be used to determine the thermal stability of diamonds. They can also be used simultaneously for mutual verification.
The thermogravimetric analysis method provides reliable and highly accurate results with minimal sample usage. It is quick, convenient, and can directly calculate the weight loss (in milligrams) and weight loss rate (percentage) of diamond samples at each temperature point throughout the heating process. Therefore, it is an effective method that should be promoted.
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