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URN etd-0729104-112641 Statistics This thesis had been viewed 3610 times. Download 3190 times. Author Sheng-Chien Chiu Author's Email Address firstname.lastname@example.org Department Materials Engineering Year 2003 Semester 2 Degree Ph.D. Type of Document Doctoral Dissertation Language English Page Count 138 Title Surface Modification Processes and Fracture Behaviors of 7075-T6 Aerospace Al-Alloy Keyword Physical vapor disposition Hard Anodizing Erosion Electroless nickel phosphorous Ductile fracture mechanism Diamond like carbon 7075-T6 Al-alloy 7075-T6 Al-alloy Diamond like carbon Ductile fracture mechanism Electroless nickel phosphorous Erosion Hard Anodizing Physical vapor disposition Abstract In this investigation the adhesion strength, corrosion resistance, fatigue strength, and the erosion resistance are studied for various surface treatment applied on the high strength aerospace 7075-T6 Al-alloy. The morphology of the fracture surfaces, were also studied afterwards.
There are three objectives in this investigation: the first objective is to identify the optimum manufacturing processes of the three coating systems for possibly raising the fatigue strength of the coated system. The selected coatings for the experiments are the Hard Anodizing (HA) coating, commonly used in the aerospace industry for its corrosion resistance, the electroless nickel phosphorous (ENi) coating for its wear and corrosion resistance and the duplex coating system with the ENi as the under coat and the diamond like carbon top coating through physical vapor disposition (PVD) method. The second objective is to study the impact of the salt spray corrosion to the fatigue strength of the ENi coating in three thicknesses. The third objective is to investigate the fracture mechanism and wear loss of the 7075-T6 Al-alloy coated with three different thicknesses of ENi coating and sandblast them with solid particles.
The test results indicate that ENi/PVD-DLC duplex coating (7μm/3μm) has high micro-hardness (Hv), low surface roughness and high adhesion strength that improve the fatigue strength of the 7075-T6 Al-alloy by about 22.2%. The ENi coating, on the other hand, lowers the fatigue strength of the 7075-T6 Al-alloy by about 3 to 5.5%. The worst fatigue performance among the three coating systems is the HA coating. These test results help to conclude the importance of high hardness, good adhesion strength and low surface roughness of a coating system in reducing the chance of crack initiation at the treated surface thus improves the fatigue strength of the treated material.
The surface appearances and the adhesion strength of the most cost-effective and high deposition efficiency ENi coating are influenced by the amount of phosphor in the coating. The high phosphorous (Hi-P) ENi coating has lower surface roughness and higher adhesion strength when compared to the low phosphorous ENi coating. It is found that ENi coated to a 25μm thick is sufficient to prevent the corrosion from happening when exposing the coated 7075-T6 Al-alloy to a long term salt spray corrosion test. Also found in this investigation is that the ENi coating film when built up to 25μm thick can efficiently reduce the corrosion induced fatigue strength loss but doubling the coating thickness to 50μm thick it is surprising to find out that the fatigue strength is lower than the 25μm after exposing them to salt spray corrosion.
A wide and deep cutting with plowing like fracture mechanism was found on the 7075-T6 Al-alloy test specimen when exposed at lower angle Al2O3 solid particle erosion test. Crater and plowing like fracture mechanism was identified for the mid angle erosion test that has the highest erosion force for producing the Gaussian distribution profile. At the high angle erosion test, the fracture mechanism revealed an indented crater with plastic deformation fracture mechanism. Also found was the subsurface work hardened zone increases with the erosion amount of the solid particles in sandblasting test. The resistance to the wear is lower for ENi coating under low angle erosion than the high angle erosion. In addition, the coating film is thinner than others and the uniqueness of the substrates mechanical properties, overall, result in a ductile fracture mechanism.
Advisor Committee Cheng-Hsun Hsu - advisor
Fan-Shiong Chen - co-chair
Hong-Ming Lin - co-chair
Hong-Shin Chiou - co-chair
Jin-Kao Wu - co-chair
Shen-Ji Lee - co-chair
Files Date of Defense 2004-07-26 Date of Submission 2004-07-29