New process for improving silicon nitride cutting tools: Coating by pulsed high energy density plasma

Recently, we proposed a new technique, pulsed high energy density plasma (PHEDP), to modify the surfaces of industrial cutting tools. Using this technique, hard and wear-resistant TiN and TiCN coatings were tentatively deposited onto silicon nitride ceramic cutting tools at ambient temperature. The mechanical properties of the coatings were determined by nanoindentation and nanoscratch tests. Under optimum conditions, the maximum nanohardnesses of the TiN- and TiCN-coated tools were more than 28 GPa and 50 GPa, respectively; the Young's moduli were about 350 GPa and 300-350 GPa, respectively; the maximum critical loads measured by nanoscratch tests were more than 80 mN and 100 mN, respectively. The tribological properties were evaluated by the cutting performances of the coated tools applied in turning of hardened CrWMn steel and cast iron under industrial conditions. Compared with the uncoated Si3N4 ceramic cutting tools, the flank wear was reduced by a factor of 2-3 for TiN-coated tools and 6-8 for TiCN-coated tools when applied in turning hardened CrWMn steel, and by a factor of 2-5 for TiN-coated tools and more than 8 for TiCN-coated tools when used in turning cast iron. The excellent mechanical properties of the as-coated tools were attributed to the significantly improved microstructures. There were wide (240-330 nm) and continuously graded interfaces between the coatings and silicon nitride ceramic tool substrates, and the grain size of the coatings fell in nanometer scale (60-80 nm) with dense structures.