Substrate selection and pretreatment
- Selection of substrate: Usually, a cemented carbide die is chosen as the substrate, such as YG6 cemented carbide die, because it has good strength and wear resistance, and can provide stable support for the subsequent growth of diamond film1.
- Surface Cobalt Removal: Metal cobalt has the effect of catalyzing graphitization in the process of CVD diamond deposition, which will make the coating structure loose and decrease the adhesion. Generally, acid corrosion method is used to remove cobalt from the substrate surface, such as placing the substrate in dilute hydrochloric acid solution and adding a small amount of hydrogen peroxide as a corrosion promoter, in order to promote and regulate the process of corrosion de-cobaltization, to effectively remove cobalt from the substrate surface layer, and to prevent excessive corrosion 1.
- Grinding: Grind the cobalt removal processed substrate surface with diamond powder to remove the poorly bonded tungsten carbide particles on the surface and to increase the nucleation density of the CVD deposited diamond1.
- Cleaning and drying: The ground substrate is ultrasonically washed, rinsed and dried to ensure that the substrate surface is clean and free of impurities, providing good surface conditions for the subsequent deposition process.
Hot Filament CVD Deposition of Diamond Films
- Installation of hot wire: A single perforated tantalum or tungsten wire is used as the hot wire, which is straightened by a high-temperature resistant spring after passing through the hole of the wire drawing die and placed exactly at the axial center of the die, so that the temperature of the inner hole surface is basically maintained uniformly during the whole coating process.
- Vacuuming and gassing: The pre-treated substrate is placed in a CVD reaction chamber, which is vacuumed and then gassed with reaction gases, usually hydrogen, acetone and a small amount of carbon tetrachloride. Hydrogen and acetone are excited by a hot filament to produce sufficient concentrations of reactive hydrogen atoms and carbon-containing reactive groups, while carbon tetrachloride is used to increase the nucleation density of the film in order to improve the surface finish of the coating.
- Adjustment of process parameters: Adjust the reaction chamber pressure to 2 - 6 Kpa, control the total flow rate of reaction gas at 200 - 400 ml/min, the volume ratio of acetone to hydrogen is 1 - 3%, the ratio of carbon tetrachloride to acetone is about 2%, the filament temperature is maintained at about 2200°C, and a DC bias voltage of 50 - 150 volts is applied to the surface of the hot filament and the inner holes of the die to create a 0.6 - 2 Ampere DC bias voltage. A DC discharge current of 0.6 - 2 amperes is applied to the surface of the hot wire and the hole in the wire drawing die to accelerate the growth of the diamond film.
- Deposition of Diamond Film: Under the above process conditions, a uniform diamond coating can be obtained on the surface of the die bore after a certain period of deposition. For example, after 4 hours of deposition, a diamond coating with a thickness of about 10 microns can be grown.
Post-processing
- Cooling and Removal: After the deposition is completed, the heating and gassing are stopped, the reaction chamber is allowed to cool down to room temperature naturally, and then the wire drawing die core is removed.
- Inspection and Evaluation: The prepared CVD polycrystalline diamond wire drawing die cores are inspected, including measuring the thickness, hardness, adhesion and other performance indexes of the diamond coating, as well as checking the dimensional accuracy and surface quality of the cores, to make sure that they meet the requirements for use as wire drawing dies.
- Packaging and storage: Pack the tested wire drawing die cores and store them in a dry and clean environment to prevent them from being damaged or corroded
