Effect of heat treated tungsten interlayer on microcrystalline diamond coatings

Objectives: Microcrystalline diamond coatings have extremely high hardness and excellent wear resistance, but its application field is limited by poor tribological and binding properties. To improve the tribological properties of microcrystalline diamond coatings on cemented carbide substrates, we have chosen tungsten metal as the interlayer material and constructing microscopic texture by heat treatment on the surface of tungsten interlayer.


Methods: We used evaporation method to deposit a tungsten interlayer on the surface of cemented carbide, and heat treated the tungsten interlayer in a reducing atmosphere mixed with argon and hydrogen gas. After heat treatment for 30 minutes at various temperatures (700 / 800 / 900 / 1000 °C), the effects of different heat treatment temperatures on the composition, morphology, and microstructure of the tungsten transition layer were studied using Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD). Diamond coatings were deposited using hot filament chemical vapor deposition (HFCVD) method on substrates without any treatment and tungsten interlayers which were teated by different temperature. The substrate temperature was controlled at 800 ± 50 ℃, and the growth time was 6 hours. Analyze the morphology and quality of diamond coatings using SEM, X-ray diffraction, and laser Raman spectroscopy. Using the reciprocating friction and wear tester, Si₃N₄ ceramic balls were subjected to reciprocating friction with diamond coatings for 120 minutes to evaluate the friction performance of each coating sample.


Results: The results show that the tungsten interlayer deposited by vapor deposition exhibits an amorphous structure, and the crystallinity of the tungsten interlayer significantly increases after heat treatment. Cracks are generated on the surface of the interlayer, and different sizes of "island - gully" structures are formed. The crystallinity of the tungsten interlayer after heat treatment at 700~800 ℃ is not good, and the surface has larger "islands" and narrower "gully". The structure size of the tungsten interlayer after heat treatment at 900 ℃ is more moderate. The tungsten interlayer after heat treatment at 1000 ℃ has the best crystallinity and the smallest size of "islands". The SEM surface morphology, X-ray diffraction patterns, and Raman spectroscopy show that the diamond grown on the substrate surface without tungsten transition layer has the largest average grain size and uneven grain size distribution. The crystallinity and content of diamond coatings grown on the tungsten interlayer during heat treatment are better, which was reflected in the higher diffraction intensity of diamond peak in the X-ray diffraction spectrum and the narrower full width at half maximum (FWHM) of diamond peak in the Raman spectrum. The grain size of diamond shows a trend of first decreasing and then increasing with the increase of heat treatment temperature, but all are smaller than the sample without tungsten interlayer. The friction and wear results indicate that the diamond coating grown on the tungsten interlayer after 700 ℃ heat treatment has large drop. The diamond coating on the tungsten interlayer after heat treatment at 800~1000 ℃ ensures good bonding performance while improving friction performance in varying degrees. Among them, the diamond coating grown on the tungsten interlayer after 900 ℃ heat treatment has the smoothest wear mark, with an average friction coefficient as low as 0.062, and the corresponding Si₃N₄ friction pair has the smallest wear mark diameter and wear rate.


Conclusions: The tungsten interlayer and its "island - gully" structure after heat treatment can significantly improve the growth and crystal state of diamond, resulting in grain refinement and improved friction properties. The surface of the tungsten interayer treated at 900 ℃ for 30 minutes has a moderately sized "island-gully" structure and the best uniformity. The average grain size of the diamond coating grown on it is about 1.97 μm. The maximum and average friction coefficients are as low as 0.217 and 0.062, and the wear of friction pair is only 19.2% of the sample without tungsten interlayer.