Effect Of TiCN/(Ti,W)C Ratio On Microstructure,Properties And Corrosion Behavior Of Cermets

Based on the low formation energy and stable structure of(Ti_0.88W_0.12)C solid solution,it can be inferred that(Ti_0.88W_0.12)C solid solution can be used as an additive for TiCN-based cermets to regulate the microstructure and properties of the hard phase.In this study,two kinds of cermets 28TiC_0.7N_0.3–35(Ti_0.88W_0.12)C–4.5TaC–4.5NbC–8Mo₂C–10Ni–10Coand 48TiC_0.7N_0.3–15(Ti_0.88W_0.12)C–4.5TaC–4.5NbC–8Mo₂C–10Ni–10Co were prepared by pressure sintering（named A and B respectively）.The effects of TiC_0.7N_0.3/(Ti_0.88W_0.12)C ratio on the microstructure,mechanical properties,electrochemical corrosion behavior and high temperature oxidation behavior of TiCN-based cermets were studied.The electrochemical corrosion mechanism and high temperature oxidation mechanism are analyzed and discussed.The main conclusions are as follows:（1）(Ti_0.88W_0.12)C solid-solution can strongly drive atomic recombination of hard phase and promote the formation of（Ti,W,Mo,Ta,Nb）（C,N）,so as to inhibit the formation of black core/rim structure.Low TiC_0.7N_0.3/(Ti_0.88W_0.12)C ratio can improve the hardness and Palmqvist toughness of cermets,while high TiC_0.7N_0.3/(Ti_0.88W_0.12)C ratio can inhibit the grain growth of hard phase and greatly improve the transverse fracture strength of cermets through fine grain strengthening;hardness,Palmqvist toughness and transverse rupture strength of cermet B with higher TiC_0.7N_0.3/(Ti_0.88W_0.12)C ratio are 90.5 HRA,12.1 MPa·m^1/2and 2616 MPa,respectively.（2）The corrosion resistance of cermet B with high TiC_0.7N_0.3/(Ti_0.88W_0.12)C ratio is better than that of cermet A with low TiC_0.7N_0.3/(Ti_0.88W_0.12)C ratio in three different pH corrosive medium.The corrosion resistance of the two cermets is the strongest in Na OH solution（pH=13）,followed by Na₂SO₄ solution（pH=7）,while the corrosion resistance of the two cermets is the worst in H₂SO₄ solution（pH=1）.（3）In H₂SO₄ solution（pH=1）,due to the active dissolution of Co and Ni,the binder phase is obviously corroded.In Na OH solution（pH=13）and Na₂SO₄ solution（pH=7）,Co（OH）₂ and Ni（OH）₂ produced by the corrosion reaction of Co and Ni are insoluble,and adhere to the surface of hard phase and binder phase together with other oxidation products,which can slow down the corrosion process of cermet surface.The corrosion of the binder phase decreases with the increase of pH value.（4）After oxidation at 1100°C,dense oxide films are formed on the surfaces of the two cermets,and there are no obvious defects such as oxidation cracks.The structure of oxide layer can be divided into porous external oxide layer,intermediate oxide layer and internal oxide layer.Compared with the cermet A with low proportion of TiC_0.7N_0.3/(Ti_0.88W_0.12)C,cermet B with high TiC_0.7N_0.3/(Ti_0.88W_0.12)C ratio has fine and uniform microstructure,which promotes the formation of denser oxide film and its oxidation resistance is significantly increased.（5）The alloy elements are preferentially oxidized to form binary oxides,and then the binary oxides undergo composite reaction to form more complex multicomponent oxides.Co and Ni in the binder phase preferentially migrate outward and combine with O,preferentially forming CoO and NiO.The Tielement in（Ti,M）（C,N）multicomponent solid-solution combines with O to form TiO₂,and then TiO₂ reacts with CoO and NiO to form CoTiO₃,Co₂TiO₄ and Ni TiO₃.W and Mo elements migrate outward and combine with O to form WO₃ and MoO₃.Then,WO₃ and MoO₃ react with CoO and NiO to form CoWO₄,NiWO₄,CoMoO₄ and NiMoO₄.