Effect of Si Content on Deposition and High-Temperature Oxidation of Al-Si Coatings Obtained by Magnetron Sputtering PVD Method

Intermetallic Al-Si-based coatings can greatly increase the oxidation resistance of gamma-TiAl alloys. However, the effects of the Si addition are not fully understood. Therefore, it is difficult to determine the Si content that is optimal for oxidation resistance. Therefore, pure Al and several Al-Si coatings with varying Si contents between 1 and 81 at.% were studied. The coatings were produced using a combinatorial magnetron sputtering process. Scanning electron microscopy and energy dispersive X-ray spectroscopy were used for structure and chemical analysis. The phases were identified by X-ray diffraction. Cyclic oxidation tests at 900 degrees C were conducted up to 5000 cycles of 1 h each and subsequently evaluated by thermogravimetric analysis. Si addition in the range of 1 to 12 at.% did not deteriorate the oxidation resistance compared to a pure Al coating up for 1000 cycles (1 h) of oxidation at 900 degrees C, while higher Si contents led to a high mass gain. For oxidation times up to 5000 cycles (1 h), a sufficient thickness of the coatings is crucial for good oxidation resistance. The main effect of Si addition is to enhance the transformation speed of the deposited Al and Si to the high temperature stable Ti(Al,Si)(3) phase during the heat treatment. Si additions of up to 12 at.% led to increased initial mass gains and a decrease in the oxidation rates during subsequent exposures compared to pure Al coatings.

Automated summary

Intermetallic Al-Si-based coatings can improve the oxidation resistance of gamma-TiAl alloys. The optimal Si content for oxidation resistance is still uncertain. Si additions in the range of 1 to 12 at.% do not deteriorate the oxidation resistance, while higher Si contents lead to greater mass gain. The thickness of the coatings is important for good oxidation resistance.