Improve Magnesium Alloy AZ91 Technical Characteristics By Using Magnetron Sputtering Coating Process

Magnesium and its alloys received increasing attention over the past years for they possess remarkable technical characteristics. With a density of 1.74 g/cm3 magnesium is one of the lightest construction metal, it is also non-magnetic and has a high thermal and electrical conductivity. Besides, regarding the materials shaping, magnesium presents an excellent castability and workability, a low price and it is easily recycled. However, if magnesium has indeed numerous advantages, it also possesses important drawbacks which prevent its wider use. Indeed, its high susceptibility to corrosion and wear prevent its use in many fields. Thus, in order to overcome magnesium drawbacks scientists are currently investigating coating techniques. One of the main problem that they have to overcome is the adhesion between the substrate and the coated layers. Recently, PVD techniques received increasing attention. Indeed, this technique is environmental friendly and does not generate wastes. That is why this research will focus on these techniques and more specifically in magnetron sputtering deposition process. Based on previous researches the performances of the coating system Al/Al N/Ti N will be investigated. In order to do so this research was organized in 3 steps:1. Study of a single Al coating layer: the influence of the bias voltage was studied. 6 groups were investigated: 20 V, 40 V, 60 V, 90 V, 120 V and 150 V.2. Study of Al/Al N coating system: the influence of the N2 flow rate was studied. 4 groups were investigated: 10 sccm, 15 sccm, 35 sccm, 40 sccm.3. Study of Al/Al N/Ti N coating system: the influence of the transition time was studied. 3 groups were investigated: 0s, 900s(15 minutes) and 1800s(30 minutes).In order to analyze the coating layers SEM and EDS observations along with adhesion tests were conducted. The following conclusions were drawn: the higher thickness and best deposition are obtained for a N2 flow rate of 35 sccm and a transition time of 1800 s, while the best adhesion between the coating film and the substrate is obtained for a bias voltage ranged from 60 V to 90 V, a N2 flow rate ranged from 35 sccm to 40 sccm and no transition time. Furthermore, in order to appreciate the technical performances of the coated sample wear resistance, nano-hardness and corrosion resistance tests were conducted. The conclusions were drawn as follows: the best wear resistance is obtained for a N2 flow rate of 40 sccm and a transition time of 1800 s, the higher hardness is obtained for a N2 flow rate of 35 sccm, and finally the best corrosion resistance is obtained for a bias voltage of 60 V, a N2 flow rate of 35 sccm and no transition time.It seems that the best parameters configuration is bias voltage=60V, N2 flow rate=35sccm and no transition time. As the adhesion was the main problem to solve a transition time of 0s was chosen in order to obtain the best adhesion possible. Thus, when coating the layers Al/Al N the bias voltage was set to 60 V and when coating the layers Al/Al N/Ti N the bias voltage was set to 60 V and the N2 flow rate to 35 sccm.