Production of steel matrix composites by mechanical milling and spark plasma sintering

Hot work tool steels (HWTSs) are ferrous alloys for tooling application, particularly developed to meet high toughness and good hot hardness. Increasing hardness generally leads to a decrease in toughness, therefore metal matrix composite (MMC) coatings and functionally graded materials have been proposed as a good solution for improving wear resistance. In this PhD thesis powder metallurgy has been applied for the production of particle reinforced HWTSs. Mechanical milling (MM) and mechanical alloying (MA) have been considered as suited techniques for the production of powders showing higher sinterability and finer microstructure. Spark plasma sintering (SPS) has been used for the consolidation. As reinforcement a harder high speed steel (HSS) and different ceramic powders (TiB2, TiC and TiN) have been selected. The production of HWTS/HSS blends has highlighted the negative interaction on densification of the two components due to their different sintering kinetics. This interference can be minimised by selecting powders with smaller particles size. With this respect MM was proved to be a very useful method, which enhances sintering. Fully dense blends with good dispersion of the reinforcing particles can be sintered using small sized powders and setting the particle size ratio (PSR) smaller than 1. For the production of MMCs the formation of aggregates has been overcome by MA which promotes a uniform dispersion of hard particles into the parent steel. Among the reinforcement considered in this work, TiB2 is not suitable because it reacts with steel depleting carbon and producing TiC and brittle Fe2B. HWTS composites with 20%vol of TiC can be fully densified by SPS at 1100 °C for 30 minutes and 60 MPa uniaxial pressure. On the other hand TiN-reinforced MMC shows high resistance to densification and fully dense materials could not be produced.