Tribological coatings containing nanomaterials as solid lubricants produced by means of electrochemical methods

The significant use of aluminum alloys as engineering materials is supported by the fact that they can be coated to form specific surface layers. Metallic and oxide coatings contribute to new qualitative applications of the aluminum alloys as elements of sliding contacts. This is possible mainly due to the properties of the composite coatings containing nanomaterials as solid lubricants. In hard anodic treatment the following properties are important: high hardness, good adherence to base material, good abrasion resistance, and high lubricant absorption ability. Due to these properties, oxide coatings are used in contacts in machines working under friction-in- air conditions and limited lubrication conditions (poor lubrication). When such layers slide against cast iron, the latter undergoes an extensive wear at the initial stage. The oxide cracks due to its high brittleness and the lack of shock-resistance. Reduction of the above mentioned limitations is possible due to nanotechnology using two methods for improving the elasticity of hard, but brittle anodic oxide layers, it is: 1) nickelizing using metal nanowires by two stages process, 2) modifying using co-deposition one stage process. The first method, called coating modification, consists of two steps, it is oxidizing of aluminum alloy (first step) and nickelzing of the oxide (second step). It is based on electrolytic reinforcement of hard but brittle AHC by a ductile, elastic metal nanowires for example nickel, silver or copper. As the result of this method, the composite coating with ceramic matrix containing metal nanowires in its pores is obtained. These nanowires act as solid lubricants. The second method, called co-deposition, consists of one step only. It is based on oxidizing with simultaneous introduction of solid lubricant substances. As the result of this method the composite coating with ceramic matrix containing nanoparticles of solid lubricants distributed in their whole volume, is obtained. In this chapter, both methods as well as the selected properties of coatings will be described.