On the effects of B4C addition on the microstructure, mechanical properties, wear and thermal stability of TiCN- Co -Cr3C2-Si3N4 cermets developed by vacuum hot pressing

Abstract

This paper reports the development of TiCN - Co - Cr3C2 - Si3N4 cermets modified with the addition of B4C. Cermets of three different compositions with varying B4C in TiCN - Co - Cr3C2 - Si3N4 were fabricated. The TiCN, Co, Cr3C2, Si3N4 and B4C powders in the combinations 80%TiCN-5%Co-5%Cr3C2–5%Si3N4–5%B4C; 75%TiCN-5%Co-5%Cr3C2–5%Si3N4–10%B4C; 70%TiCN-5%Co-5%Cr3C2–5%Si3N4–15%B4C were ground in a ball milling unit for six hours at a speed of 300 rpm at a ball-to-powder ratio of 10:1. The powders were then sintered in a vacuum hot press at a temperature of 1600 °C and a pressure of 50 MPa with soaking time of 90 min. For comparison purpose, 90%TiCN-5%Co-5%Cr3C2 cermet was prepared. The microstructure of the sintered cermets revealed the core-rim structure. The Vickers hardness test was done with a load of 10 Kg and for duration of 10 s. Fracture toughness was measured using the crack length data obtained from the hardness test using scanning electron microscopy. The cermet with a composition 75% TiCN-5%Co-5% Cr3C2–5% Si3N4–10% B4C produced higher hardness of 26.53 ± 0.56 GPa and fracture toughness of 10.29 ± 0.18 MPam and 70% TiCN-5%Co-5% Cr3C2–5% Si3N4–15% B4C showed high fracture toughness value of 11.23 ± 0.09 MPam with hardness value of 21.05 ± 0.10 GPa. The cermet composition 75% TiCN-5%Co-5% Cr3C2–5% Si3N4–10% B4C retained the hardness of 16.85 GPa even after annealing at 600 °C. The sintered cermets tend to form TiO2 which determines the hardness of the annealed cermets at a range of 600 °C – 1000 °C. The presence of these oxides tends to decrease the hardness. The wear test was performed taking the cermet as the pin material. The abrasive and adhesive wear mechanisms were dominant in the cermets. The harder precipitates formed tend to increase wear resistance by reducing the wear rate. The harder precipitates (TiN and TiB2) formed mainly due to the addition of Si3N4 and B4C, this addition promotes adhesive wear by a tribolayer formation.