Design of wear and corrosion resistant PVD‐coatings for magnesium alloys

Abstract

Although magnesium alloys became popular in the first half of the 20 th century, the bad corrosion properties prevented their breakthrough in industrial mass production. Since the technology for the production of high purity alloys was introduced in the 1970s, magnesium alloys became more and more in the focus of industrial attention. Today magnesium alloys are state of the art in structural parts in automotive industry. Despite its outstanding properties like good castability, low density and nearly unlimited availability the negative aspects like weak corrosion and wear behaviour still limit the application of magnesium in industry [1].So, the only economic solution is the deposition of a coating or a suitable surface treatment which provides both, wear and corrosion resistance. Today, plasma electrolytic anodisations are state of the art [2 -5]. They provide acceptable corrosion resistance and protect the magnesium from mechanical damage due to their high hardness. On the other hand, their high porosity limits their use in combination with electrochemically noble materials, leading to galvanic corrosion [6]. In addition, the high surface roughness of the plasma electrolytic anodisations restricts their use in tribological applications, particularly under dry sliding conditions [7].On the other hand, due to the high life time recommendations the application of magnesium in the automotive industries motion component field is a long term process. Nevertheless, there is a quite high industrial interest to apply magnesium in the motion component field in consumer applications like do-it-yourself or gardenig. Some examples are motor components of lawnmovers, motor saws or drills. Especially for these fields of application there are quite high demands on the corrosion properties due to undefined storage and the conditions during usage.In order to achieve smooth surfaces with high quality, the PVD technology moves into the centre of interest. Since the 1980s PVD coatings are well established and widely used for different industrial applications, mainly for steel and tool coatings. The authors were the first who carried out serious studies on the development of PVD coatings for magnesium alloys since 1999 [6,7].The extensive research activities lead to the recent development of a coating system, which provides both, good wear properties as well as good corrosion behaviour.

PVD Coating of Mg–AZ31 by Thin Layer of Al and Al–Si

“…As a result, the vapor stream condenses on the substrate surface and loses most of the energy as the heat of condensation, thus providing a thermal load on the substrate. [17][18][19] Figure 2 indicates the temperature rise measured for several specimens coated with different substrate speeds and under different conditions. Such conditions include cases of Al or Al-12.6% Si, either with or without plasma activation.…”

“…18,20 The bombardment of argon atoms on the substrate resulted in the removal of nanoparticles or fine oxides and was responsible for activating the surface and enhancing adhesion between the substrate and coating. 18,20 In a group of experiments for optimizing the ionetching procedure, the temperature of the substrate was monitored. It was found that during ion-etching, the substrate temperature increased, as shown in Fig.…”

“…Plasma activation is a source of extra energy provided to the deposited atoms that heats the vapor stream more. 17,18 It was used in a previous work 17 on steel substrates to provide sufficient energy for structure rearrangement, thus obtaining a dense coat layer instead of a columnar porous structure.…”

“…Ultrasonic cleaning was then conducted in a solution of sodium carbonate (Na 2 CO 3 ) 25 g/L and sodium hydroxide (NaOH) 15 g/L at 65°C for 3 min, followed by water rinsing. 18 Before ion etching and coating, a thermocouple was welded to the substrate sample to record its thermal history during coating. The sample was then put inside the lock vacuum etching chamber.…”

Calcium phosphate coating on magnesium alloy by biomimetic method: Investigation of morphology, composition and formation process

“…However, magnesium is highly susceptible to corrosion in the electrolytic physiological environment and could loose mechanical integrity before new tissue sufficiently heals which limits its orthopedic application [11,14,18,19]. Many surface coating techniques were developed to mitigate corrosion by separating the metal surface from its surroundings or incorporation of corrosion inhibiting chemicals [20][21][22][23].…”