Synthetic diamond powder is an important modern engineering and functional material with unparalleled excellent properties. It is primarily used to manufacture sawing tools, abrasives, cutting tools, and geological drills. The usage scenarios demand high strength, hardness, thermal resistance, chemical inertness, and wear resistance of the diamond. Most synthetic diamond powder products undergo high-temperature sintering and are threatened by frictional heat during use.
Therefore, studying the chemical stability, strength, thermal resistance, and other changes of synthetic diamond powder in air at high temperatures has significant theoretical and practical implications. This article will explore the factors affecting the thermal stability of diamond.
Under working conditions above 750℃, the performance of synthetic diamond powders will be affected to varying degrees, manifested as a decrease in single-particle compressive strength, surface corrosion or graphitization, cracks or fragmentation within the particles, leading to reduced cutting and grinding performance.
In fact, the degree of thermal damage to diamond depends on the quality of the diamond itself, but more importantly, on the environment the diamond is in, that is, the medium in contact with the diamond.
In different atmospheric media, the temperature at which pure diamond undergoes this transformation also varies. For example, in air, the conversion starts at 800℃; in nitrogen-hydrogen mixed atmosphere, it starts at 1200℃; in vacuum and inert gas, it starts at 1500℃. Obviously, the presence of oxygen is detrimental to the conversion of diamond to graphite.
Solution
Applying an appropriate coating to the surface of synthetic diamond powder to isolate the contact between oxygen and diamond will eliminate the harm of oxygen to diamond during the tool sintering process. The coating used as a protective layer has two requirements:
Firstly, the coating must not contain any of the elements iron, cobalt, or nickel, because at temperatures above 800℃, these elements will corrode the diamond and convert it into graphite or amorphous carbon, causing greater harm to the diamond.
Secondly, the coating must have sufficient oxidation resistance. Studies have shown that the oxidation resistance temperature of Ti or Ti-Cr alloy coatings are 1024℃ and 1041℃ respectively, meeting the above requirements.
Synthetic diamond powders with a more complete crystal form and fewer impurities have higher compressive strength, and thus the best thermal resistance and thermal stability. Diamonds with incomplete crystal forms and surface defects have poor thermal stability. The thermal stability of synthetic diamond powder micro-powder is the worst, beginning to oxidize exothermically around 600℃ and undergoing intense oxidation at 780℃.
This is due to its fine particle size and large specific surface area, making it easy to oxidize to form graphite and generate CO2 and CO gases. Additionally, micro-powder is formed by crushing diamond single crystals, and there are micro-cracks in the crystal structure, making it prone to thermal decomposition reactions and also reducing thermal resistance.
Various synthetic diamond powder abrasive and tool products typically process objects with high hardness, generating a large amount of grinding heat during processing. The working environment temperature can instantly reach above 1000℃, but the thermal stability temperature of diamond abrasives is below 1000℃, and the thermal stability temperature of diamond micro-powder is even lower.
If the use temperature of diamond abrasives and other products exceeds their thermal stability temperature, the diamond will undergo graphitization and oxidation, causing strength to decrease and grinding efficiency to decline.
Solution
Cool the diamond tools using coolant during use. In addition, the sintering temperature of diamond products should be controlled as much as possible within the thermal stability temperature range of the diamond; otherwise, it will also reduce the strength of synthetic diamond powder, affecting the usage effect.
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