Preparation of diamond-gradient film and analysis of its mechanical properties

In order to prepare the diamond gradient film with different structure to reduce the internal stress of the diamond coating surface, the diamond coating was prepared on the cemented-carbide substrate by hot-filament chemical vapor deposition to form three types of diamond gradients: 2 layers, 3 layers and 5 layers. And they have the same total thickness (3.2 mu m). Then, the films' bond strength and friction and wear properties were investigated by using an optical microscope, x-ray diffractometer (Cu K alpha), laser Raman spectrometer, LCR500 Rockwell hardness tester, and RTEC-MFT5000 friction and wear tester. Laser Raman spectrometer display that the two-, three-, and five-layer diamond-gradient film coatings showed internal stress values of -1.644, -0.737, and -0.567 GPa, respectively, with the five-layer coating showing the minimum value. Indentation tests revealed that the two- and three-layer coatings were less detached whereas the five-layer one peeled off easily. Friction and wear tests indicated that the friction coefficient of the two-layer diamond-gradient film was the lowest, that of the three-layer film was the highest, and that of the five-layer film had an intermediate value. The conclusion is that under the condition of gradually changing the concentration of methane and ensuring the thickness of micron diamond, appropriately increasing the gradient layer can reduce the internal stress effectively. Adding nanodiamond in the gradient layer can reduce the friction coefficient.