Development of Improved Fatigue Resistant Nanocomposite Coatings for Rotary Machinery Components, 18-R9622

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Principal Investigators
Ronghua Wei
Sastry Cheruvu

Inclusive Dates:  04/01/06 – 06/30/07

Background - Key components of modern machinery in various fields including turbine and compressor blades, vanes of aero and land-based engines, and impellers of fluid pumps are often plagued by soft particle erosion. Hard coatings are increasingly being considered for minimizing the damage to these components. In this way conventional materials can still be used while a hard ceramic coating can withstand the harsh environment.

Under an internal research program completed in 2005, SwRI developed a plasma-enhanced magnetron sputtering (PEMS) technology, by which the thickest nanocomposite coatings ever reported were produced. Evaluations conducted at SwRI and a number of outside institutions showed that the coatings exhibited superior performance against solid particle erosion and heavy load wear over "the-state-of-the-art" coatings (approximately an order of magnitude). However, the high cycle fatigue (HCF) life of TiSiCN coated samples was found to be 100 times lower than the uncoated Ti-6Al-4V in Boeing's tests. Maintaining fatigue life is essential for rotating components. The specific goal of this IR project was to bring the coating fatigue life close to that of the uncoated material (Ti-6Al-4V) without significantly compromising wear/erosion resistance.

Approach - Under this internal research project, the deposition process of a Ti-Si-C-N nanocomposite coating system was studied using a design of experiment method. Using this method, a systematic study was conducted to correlate the influence of the processing parameters to the coating quality including the coating micro-nanostructure, morphology, composition, adhesion, erosion resistance and hardness. Seven critical parameters and three levels of each were considered to influence coating quality. The design of experiment method reduced the total experiments down to 18. Considering the overall microstructure, erosion resistance, and adhesion of the coatings, the most optimum set of deposition parameters was used to apply coatings on samples to investigate the fatigue strength of the coated samples. Fatigue tests were conducted on the coated and un-coated samples. In addition, SwRI also deposited coatings on the Custom 450 and Inconel 718 samples for commercial companies to evaluate the fatigue strength of the coating specifically for aero and land-based engine applications.

Accomplishments - The TiSiCN coating deposited using optimum parameters showed both excellent erosion resistance and a significant increase in fatigue life of coated Ti-6Al-4V samples. The coated fatigue samples were tested by Boeing again. Now the HCF life of the coated specimens has been improved by nearly 100 times over the previous coated specimens. Though the HCF life of the coated samples is still about 20 to 30 percent lower than that of the uncoated Ti-6Al-4V, it is better than any PVD coatings Boeing has studied. This is a significant improvement because these coatings can be accepted for use when both erosion resistance and fatigue resistance are required. Boeing showed an interest in SwRI's TiSiCN coating for their engine applications.

In parallel to the internal research work, fatigue tests were conducted on the TiSiCN coated samples of Custom 450 and Inconel 718 prepared for two commercial companies. The fatigue life of the TiSiCN coatings on Custom 450 and Inconel 718 is identical to that of base metal. Considering the modulus of elasticity of these three materials, Ti-6Al-4V, Custom 450 and Inconel 718, it is believed that the high coating strain induced due to low modulus of elasticity of Ti-6Al-4V is presumably responsible for premature crack initiation in the superhard nanocomposite coating. The premature crack initiation in the superhard nanocomposite coating is responsible for the 20 to 30 percent reduction in HCF life from the uncoated Ti-6Al-4V. The results from both Custom 450 and Inconel 718 lead us to believe that further tailoring of the deposition process for Ti-6Al-4V, such as using a gradient coating structure starting with a more ductile coating composition near the substrate and ending with the superhard coating at the top, even if compromising the erosion resistance slightly, may bring up the fatigue resistance of the coated specimens.

Based on both the erosion resistance and the fatigue resistance of these nanocomposite coatings, EPRI has already signed a licensing agreement with SwRI, while the aero engine company is conducting the last test an engine test before the technology is transferred to its designated commercial coating company. SwRI has also demonstrated that these coatings are excellent for the protection of helicopter rotor blades.

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