A method called negative additive manufacturing is used to produce near fully dense complex shaped boron carbide parts of various length scales. This technique is possible via the formulation of a novel suspension involving resorcinol-formaldehyde as a unique gelling agent that leaves behind a homogenous carbon sintering aid after pyrolysis.
Negative additive manufacturing, or AM, is a cost-effective method to produce ultra hard, dense ceramic parts into complex shapes, that would not have been possible by traditional casting methods. The use of an aerogel in this technique is unique. It serves both as a gelling agent, with little to no shrinkage during casting, and also is a well-dispersed carbon source for sintering after pyrolysis.
Gel casting into 3D printed molds is a versatile technique that can be extended to other materials. The resorcinol formaldehyde gelling agent is best suited for carbon bearing systems. To begin, prepare a two part suspension to help prolong the shelf life of the suspensions before casting.
To do so, create the R mix by dissolving 0.88 grams of polyethyleneimine in 25.00 grams of water, using a planetary mixer. To create a separate F mix, dissolve 0.88 grams of polyethyleneimine in 16.83 grams of water, using a planetary mixer. Now, dissolve 12.60 grams of resorcinol powder into the R mix.
The solution should turn from a cloudy white to a clear transparent solution after complete dissolution of the powder from mixing. Also, add 17.03 grams of formaldehyde solution to the f-mix, and ensure complete mixing. At this point, incrementally add 5.25 grams of boron carbide powder into both the R mix and F mix separately.
Then, add 6.50 grams of acetic acid to the R mix and F mix, and ensure complete mixing in each. To prepare molds for casting, print the molds using a fused deposition modeling 3D printer with acrylonitrile butadiene styrene, or ABS, filaments. After thoroughly agitating the suspensions, combine the R mix and F mix to obtain the final suspension.
Before casting, mix and apply vacuum to the final suspension mixture for about 10 minutes, to remove air bubbles without boiling the water. This can be a accomplished by using a stirring plate at 200 to 300 RPM, with a vacuum jar. This following step is the most critical, because the gelling agent will start to react, and cause the suspension's viscosity to rapidly increase.
Being aware of this time-sensitive step is crucial for best results. Immediately pour the de-aired suspension into the 3D printed mold, and seal the glass container. Place the mold inside the sealable glass container to prevent moisture loss during the curing process.
Place the sealed container with the mold into a 60 to 80 degree Celsius oven to initiate the curing process. Allow the cast to cure for at least eight hours for parts that are several centimeters in length scale. After eight hours, remove the sealed container with the mold from the oven, and allow it to cool to room temperature.
Transfer to a new container, and then add enough acetone until the mold is fully submersed. This process might take up to two to four days, depending on the amount of volume that needs to be dissolved away. Minimal agitation of the acetone bath, or heating it slightly to 40 degrees C, may help speed up the process.
Extract the free green body from the acetone bath, after the ABS plastic is dissolved away. Place the green body in an oven at 80 degrees Celsius to ensure complete drying and removal of all moisture. After drying, place each green body in a two-inch quartz tube lined with graphite foil.
Place the quartz tubes into a furnace with flowing gas at 250 SCCM, consisting of four weight percent hydrogen gas, and 96 weight percent argon gas, to create a reducing atmosphere during the pyrolysis treatment. Heat the green body inside the furnace at five degrees Celsius per minute, until 1, 050 degrees Celsius, and hold for three hours. Ensure that the green body comes out uniformly darker in color, indicating the presence of carbon from the pyrolysis treatment.
Now, place the carbonized parts in a graphite furnace with vacuum backfilled flowing helium gas for sintering. Heat up the furnace 2, 290 degrees Celsius, and hold for one hour to achieve optimal densification of the parts. Remove after one hour.
Viscosity of the suspension is optimized for gelcasting at pH 2.8, to provide the longest working time before significant gelation occurs. There is approximately 20 minutes until a viscosity of 1.0 pascal seconds. Scanning electron microscope images show evidence of the spatially uniform carbon network that forms on the boron carbide particles after the pyrolysis of resorcinol formaldehyde.
X-ray diffraction of the samples, at various processing stages, also confirms the growth of a graphite peak after pyrolysis. Additional microscopy of a cross-sectioned sample reveals the low porosity of the final product, after being sintered at 2, 280 degrees Celsius. Don't forget that formaldehyde is carcinogenic, and working with it can be extremely hazardous.
Therefore, wearing proper personal protective equipment, and working in a well-ventilated area, is essential when handling the suspensions. After watching this video, hopefully you are now familiar with the negative additive manufacturing process, and can successfully gelcast boron carbide suspensions into complex shaped green bodies, and later sinter them to their full densities.
