الفهرس 
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Abstract
Plasma spraying due to the high temperature, enables to deposite all different kinds of materials. However, this method is suited for high melting materials. The development of new coating system and the optimization of the mechanical and tribological properties of the coatings is still mainly carried out through time intensive and trial and error approaches. The use of multilayer coatings has been cited as the way forward to improve the mechanical and tribological properties of the coatings. For some applications, the bulk wear resistant materials may not suitable, perhaps for reason of cost, overall weight, difficulty of fabrication or its mechanical properties. Plasma spraying method can, then be used to apply a coating of wear resistant material onto a cheap substrate with lower wear resistance with the bulk properties that satisfy the system demand as well as high economy.
For more development of plasma sprayed coatings, this investigations is a further study in the field of multilayer plasma coating for wear resistance, dealing with the most famous oxide and carbide powder types used for this purpose. The experiments of this investigation have been conducted to investigate the abrasive wear behavior of aluminum oxide (A12O3), and tungsten carbide with cobalt (WC-CO) plasma coating. The effect of coating layer characteristics, substrate material, and operating conditions of wear test, on the wear resistance of the coatings have been investigated. Coating characteristics including coating material, coating thickness, powder particle size before spraying and, spraying direction of the coating layers .Operation conditions of wear test including sliding speed, and applied normal load. The coating thickness was in range of 100-300µ. Multilayer coating was sprayed in two different procedures depending on the spraying direction of the top-layer with respect to the inner-layer. The wear tests were achieved under load range of 5 to 20 N, and speed range of 0.5 to 2.25ms-1. The tests were carried out using specially designed block-on-ring abrasive wear tester in the dry conditions.
The present work introduces a new mathematical model for abrasive wear of plasma coating depending on the physical properties of coated powder. The present mathematical model allows to quantify wear loss using easy measurable factors such as powder grain size before spraying, applied normal load, and hardness of the coatings.
The results of this study showed that, plasma spraying of multilayer improves the wear resistant for both the two materials used, especially when the coarse particles. The results also, showed that the wear resistance of plasma coating is affected by sliding speed and the applied normal load. The applied normal had the most significant effect on wear resistance of the aluminum oxide coating than the sliding speed under the test conditions in this investigation. While for the tungsten carbide coatings the wear resistance affected by sliding speed more than the normal load. The coating thickness variation in this investigation had no effect on the wear resistance in the investigated range. Finally, the results displayed that, the aluminum oxide plasma coated is more wear resistant than tungsten crbide in all conditions applied in this study.