Fracture in ceramic-reinforced metal matrix composites based on high-speed steel

A series of metal matrix composites based upon M3/2 high-speed steel was produced by a powder metal sintering route. Hard ceramic titanium carbide or niobium carbide additions and a self-lubricant in the form of manganese sulfide, were added as a basis for achieving improved wear resistance and reduced friction. After sintering, the composites were given a full standard high-speed steel heat treatment and subjected to mechanical tests. All three particulate additions had a deleterious effect on three-point bend strength, particularly in the case of MnS addition, mainly due to the ease of initiating cracks at or near to the particulate additions. Bend strengths were further reduced by the simultaneous addition of both MnS and either TiC or NbC especially when a high volume fraction of approximately 25 vol % MnS was added. Single, low volume fraction (less than or equal to 8%) additions, of TiC, NbC, or MnS, had little effect on fracture toughness and K-IC values were comparable to those found in the baseline M3/2 steel. Slight improvements in fracture toughness shown to occur in the 7.74% NbC composites were attributed to energy dissipation caused by the effects of crack branching during crack propagation. Composites with the higher volume fraction additions of MnS and ceramic carbide gave poor fracture toughness by forming MnS/carbide clusters which provided an easy path for crack propagation. (C) 1998 Chapman & Hall.