In spite of chemically vapor deposited (CVD) TiC-coated cemented tungsten carbide cutting tools being widely used, the influence of intermediate zones of eta phase (e.g. M12C) and interface porosity on their toughness behavior is still controversial. The effect of eta phase on both transverse rupture strength (TRS) and toughness (as measured by test turning) was investigated. Results showed that although small zones of eta phase decreased the TRS, only relatively thick zones (over 5 influenced the toughness behavior in turning. Carburization with mixtures of CH4 + H2 before TiC coating was found to be effective in reducing or completely eliminating the formation of eta phase zones. By varying the degree of carburization, inserts with various thicknesses of eta phase were obtained which were then tested for toughness in test turning. It was found that complete elimination of eta phase was unnecessary in order to obtain good toughness behavior in turning. The extra amount of carbon taken up by the TiC layers agrees with what can be expected using Fick's second law and the assumption that carbon is dissolved in the cemented carbide binder as an intermediate stage between the precarburization and TiC coating. © 1979, The Electrochemical Society, Inc. All rights reserved.
