The materials used to build aircrafts need to be very strong, able to withstand high temperatures, and lightweight. Carbon fiber composites have attracted a lot of interest because they are durable at high temperature and their low density keeps the materials lightweight. However, oxygen molecules in the air can react with and weaken the surface of carbon fiber composites. This is called oxidation damage.
Some studies have attempted to add an anti-oxidation layer to the material, but this leads to a weakened, embrittled material that would not hold up well in aerospace applications. One way to strengthen a material is to reinforce it with the addition of small pieces of another material. Nanowires have attracted a lot of attention as an additive material because their small size keeps them lightweight, yet still very strong.
The scientists in this study explored a unique method of including nanowires made of silicon carbide, a material similar to a diamond in hardness, to a carbon fiber composite. Rather than simply adding the nanowires into the composite, they grew the nanowires directly on the carbon fibers in the composite. They used a process where the composite material was first exposed to certain chemicals, called precursors, and then placed in a very hot furnace. This process is called precursor impregnation and pyrolysis, or PiP for short.
The scientists referred to their starting material as a carbon fiber “felt,” which consisted of fibers randomly distributed in a layer, kind of like felt fabric. These fibers were approximately 10 microns in diameter, or 1/10 the width of a human hair. Before beginning the PiP process, the felts were placed in a furnace at 600°C (1112°F) and then washed with nitric acid. This would remove any contaminants on the felts. Next they placed the felts in a solution containing iron for 24 hours. Iron is added to speed up the chemical reaction, it is called the catalyst. They then moved each carbon fiber felt sample to a solution containing silicon, oxygen, and carbon. This solution was the precursor.
After all the residual solvent evaporated over two days, the scientists placed the samples in an extremely hot furnace (1500 °C or 2732°F) containing argon gas for 2 hours. This was the pyrolysis part of the PiP process, which means “fire- or heat-separating.” In this furnace, a chemical reaction occurred between the catalyst and the precursor that caused small silicon carbide nanowires to grow all over the surfaces of the carbon fibers in the felts.
After the nanowires were grown, the scientists added an anti-oxidation layer to the felts through another PiP process. The felts, now with silicon carbide nanowires embedded, were again placed in the precursor solution containing silicon, oxygen, and carbon. They then moved the samples to a furnace at 1200°C (2192°F) containing argon gas for 1 hour. This PiP process was carried out for two cycles to grow a sufficient anti-oxidation layer. The scientists also prepared a felt sample with no nanowires as a control. This sample was subjected to the above PiP process to create an anti-oxidation layer as well.
The scientists used a high powered microscope called a transmission electron microscope to examine the nanowires. They found that the nanowires were thicker on the top end (500-600 nm), followed by a longer thinner segment (80-90 nm). The thin ends of the nanowires were attached to the carbon fibers throughout the felt sample. These nanowires were much smaller, about 1/100 the size of the carbon fibers in the felt samples.
In order to measure if the nanowires made the felts stronger, the scientists compressed the samples in a mechanical testing machine. This machine measured the force per area (unit MPa) for the samples to break under compression. They found that the felts with nanowires broke at 6.89 MPa compared to 1.47 MPa for the felts with no nanowires when compressed in one orientation. When the materials were tested in a different orientation, the felts with nanowires broke at 10.36 MPa compared to 3.79 MPa in the sample with no nanowires. This means that the material reinforced with nanowires can withstand three to four times the amount of force before breaking.
The materials used to build aircrafts need to be able to withstand harsh environments while remaining light weight. This study demonstrated that it is possible to use a chemical process to directly grow silicon carbide nanowires on a carbon fiber composite. Compression experiments revealed that the nanowires increased the strength of the materials, making them attractive candidates for building lightweight, durable aircraft.
