Metal Matrix Composites (MMC) are very interesting materials for applications in the automotive and aerospace fields, since they combine the lightness of Aluminium with the strength of the ceramic reinforcement. These materials are very difficult to join and conventional welding techniques are not applicable, whereas solid-state welding techniques, like Friction Stir Welding (FSW), could be a solution. However very hard tool materials will need to be chosen in order to overcome the problem of heavy abrasive tool wear. In this work the wear behaviour of coated and uncoated steel tools has been investigated in the Friction Stir Processing of extruded bars in A359 + 20%SiC. AISI 1040 steel was used as the tool material in the hardened uncoated and annealed and coated versions. In the latter case the steel was coated with multilayer PVD AlSiCrN or AlSiTiN. Coatings were fully characterised, in terms of thickness, hardness, adhesion and tribological behaviour. Further, tool wear analysis has been carried out using optical and electronic microscopy, with EDS analysis. The results gave some interesting information on the choice of tool in the FSW of MMC materials.