Fracture toughness of hard ceramic phases produced on Nimonic 80A-alloy by gas boriding

A hard boride layer was produced on Nimonic 80A-alloy using gas boriding in N2–H2–BCl3 atmosphere. This process was carried out at 920 °C (1193 K) for 2 h. Proposed gas boriding accelerated the diffusion of boron into the surface of workpiece in comparison with other acceptable methods of diffusion boriding. The comparable thickness of boride layer was obtained after considerably shorter duration and at lower temperature. The microstructure, microhardness and fracture toughness were in detail studied. The boride layer consisted of a mixture of hard ceramic phases, i.e. nickel and chromium borides. The fracture toughness of borided layer was measured by microindentation technique using a Vickers diamond indenture under loading 100 gf (about 0.981 N). The measurements were carried out through a cross-section of the specimen in order to determine the differences in brittleness distribution. The changeable surface percentage of hard ceramic phases (chromium and nickel borides׳ mixtures) strongly influenced the fracture toughness which obtained the values from 0.609 to 4.436 MPa m1/2.