A new ceramic composite cutting tool material (YST)+, based on a YAG (Y3Al5O12) matrix, was developed for the improved machining of titanium alloys. Reaction couple studies of the cutting tool material, as well as YAG single crystals, with commercial grade titanium showed the formation of a thin passivating layer of Y2O3 next to the composite which is exceptionally resistant to attack by titanium. The YST cutting tools developed in this study are electrically conductive and were easily machined using electro-discharge machining. Microstructural analysis of fully dense YST, using optical, scanning, and transmission electron microscopies, revealed the presence of three distinct constituent phases (YAG, SiCw, and TiC) with no discernible reaction amongst the phases. The average four-point flexural strength of the YST was approximately 700 MPa and the fracture origins were identified as large TiC particles or TiC particle-agglomerates. Analysis of fracture toughness measurements, based on the indentation induced controlled surface flaw/strength technique, showed a pronounced rising Rcurve behavior of YST. The SEM analyses showed clear evidence of SiC whisker pullouts and bridging, which support the rising R-curve behavior of YST. Preliminary machining experiments using the YST cutting tool (not yet optimized) on Ti-6Al- 4V alloy showed similar machining characteristics as conventional cutting tool materials (e.g., cemented carbides). It is suggested that process optimization will yield significantly improved cutting tools for titanium machining.

Keywords: YAG based new ceramic composite, cutting tool, titanium alloy