Effect of the ZrO2 Content on the Strength Characteristics of the Matrix Material of Cdiamond–(WC–Co) Composites Synthesized by Spark Plasma Sintering

Abstract—For structurally homotypical specimens different in the ZrO2 content from the (94WC–6Co) + ZrO2 matrix material used in diamond-containing Сdiamond–(WC–Co) composites formed by spark plasma sintering, the dependences of the relative density ρrel, the ultimate strength under compression Rcm and bending Rbm, the microhardness НV, and the fracture toughness KIс on the zirconia content have been established. The addition of 6 wt % zirconia to the WC–6Co composite leads to an increase in the relative density from 0.948 to 0.990, the ultimate compression strength Rcm from 4950 ± 110 to 5600 ± 120 MPa, the ultimate bending strength Rbm from 1935 ± 80 to 2660 ± 115 MPa, and the fracture toughness KIс from 13.8 ± 0.71 to 16.9 ± 0.76 MPa m0.5 at a slight decrease in hardness (from 15.9 ± 0.72 to 15.1 ± 0.33 GPa). Such values are caused by the decrease of main WC phase grains in size with tetragonal t-ZrO2 phase transformation and, correspondingly, by the growing role of transformation strengthening mechanism and the active action of inner mechanical compressive microstresses. When the ZrO2 additive to the WC–6Co composite is increased to 10%, the parameters ρrel, Rcm, Rbm, and KIс are gradually decreased. At the same time, the material at the indentor imprint edge begins to destruct, and crack propagate in a chaotic way. It has been revealed that the properties ρrel, Rcm, Rbm, and KIc are worsened at a zirconia nanopowder content above 6 wt % in the WC–Co composite due to the formation of agglomerates during the mixing of components, their separation under sintering, and the formation of micropores and microcracks.