Microstructure And Properties Of WC-Al₂O₃-ZrO₂ Cemented Carbides Synergistic Toughened By In-situ Elongated Grains

Traditional Co bonded WC-based cemented carbide has excellent properties such as high strength and high wear resistance.As a tool material,it is widely used in metal processing,mining,food industry,construction engineering,and other applications.With the continuous improvement of service requirements,the performance degradation of WC-Co cemented carbide caused by the metal bonding phase under high temperature,corrosion,and other environments have gradually become prominent.The high cost and the carcinogenicity of WC-Co alloys have made the research of new Co-free cemented carbides a hot spot in the field of cemented carbides.In recent years,metal-free cemented carbide has abandoned the metal phase that brings performance degradation and carcinogenicity and has gradually become a potential substitute for traditional WC-Co alloys.Based on the research group’s previous research on WC–Al₂O₃–ZrO₂,WC-Si₃N₄ cemented carbide and using ceramics toughening method,this paper uses in-situβ-Sialon whiskers and in-situ aluminum borate whiskers as toughening phases.It is proposed to use spark plasma sintering（SPS）technology to prepare multiphase synergistically toughened metal-free binder phase cemented carbide.In this paper,the interaction between the in-situ self-generated whiskers and the cemented carbide matrix and the original toughening phase and its influence on the sintering characteristics,microstructure,and mechanical properties of the cemented carbide are studied.Chemical corrosion,high temperature oxidation properties,etc.,systematically explored the feasibility of metal-free binder phase cemented carbide to replace WC-Co cemented carbide.In this paper,a two-step spark plasma sintering technique was used to prepare in-situβ-Sialon whiskers with fine WC grains to toughen the non-metallic bonding phase WC-Al₂O₃-ZrO₂ cemented carbide.Compared with the initial WC–10 wt.%Si₃N₄ cemented carbide,the addition of Al₂O₃-ZrO₂ can significantly reduce the initial densification temperature and sintering temperature,accelerating the densification process.The designed two-step sintering process（heating to 1650°C and immediately cooling to 1500°C for 5 minutes）combined with Al₂O₃-ZrO₂ makes most of theα-Si₃N₄ conversion,and in-situ 10 wt.%β-Si₅AlON₇ toughened WC material is obtained.The toughening mechanisms of the in-situβ-Sialon phase are whisker pull-out,crack deflection and crack bridging.When the Al₂O₃-ZrO₂ content is 4.5 wt.%,the obtained composite material has an optimal hardness of～18.93 GPa and a fracture toughness of～10.43 MPa·m^1/2.To further reduce the sintering temperature,on the basis of WC-Al₂O₃-ZrO₂ cemented carbide,the in-situ aluminum borate whisker toughened WC-Al₂O₃-ZrO₂ cemented carbide（referred to as WEB cemented carbide）has been developed.With the help of B₂O₃ melting and in-situ aluminum borate whisker reaction,a completely dense block was prepared at a relatively low temperature of 1350°C.The formation process of Al₄B₂O₉ and Al₁₈B₄O₃₃ aluminum borate whiskers and the spiral growth process of whiskers in the composite by the liquid-solid reaction was revealed by experiments.The coherent interface of{10-10}_WC//{104}_Al2O3 was found by high-resolution transmission electron microscopy.The in-situ aluminum borate whisker and good interface bonding increase the optimal fracture toughness of the material to 11.57MPa·m^1/2,and the Vickers hardness reaches 20.20±0.42 GPa.The main toughening mechanism of the material is that the aluminum borate whisker breaks and deflects the crack to absorb the crack energy.To further compare the difference between the microstructure of the metal-free cemented carbide and the traditional WC-Co cemented carbide,clarify whether the high-strength orientation texture of the special 90°WC/WC grain boundary angle is essentially related to Co,statistics of the strength and distribution of a variety of special grain boundaries of cemented carbide without metal binders are gathered through EBSD technology.It is found that the distribution of special WC/WC grain boundaries in metal-free cemented carbide is similar to the distribution of WC/WC grain boundaries in WC-Co materials.There are also two preferential orientation angles of the WC/WC grain boundary of 30°and 90°.In the metal-free binder phase WC,∑=2 grain boundary distribution is still a strong preferred orientation.The high-strength orientation texture at 90°grain boundary angle has no essential relationship with cobalt but is closely related to the average grain size of WC.The chemical and electrochemical corrosion behaviors of the prepared WEB cemented carbide in several different corrosive media were further studied.The corrosion mechanisms of WEB cemented carbide in nitric acid solution,hydrochloric acid solution,sodium hydroxide solution,and sodium chloride solution are revealed.It is found that the corrosion resistance performance of WEB cemented carbide in these four corrosive media is better than that of Co-containing YT15 cemented carbide with the same WC volume fraction.The corrosion process of WEB cemented carbide is mainly that WC is corroded to form WO₃,WO₄^2-phase,which in turn causes part of Al₂O₃ and ZrO₂ to fall off and peel off the surface of WEB cemented carbide material.Finally,the oxidation behavior of WEB cemented carbide and YT15 cemented carbide was compared.The research shows that the oxidation activation energy of WEB cemented carbide is 125.10 k J/mol in the temperature range of 600℃～900℃,which is higher than the oxidation activation energy of YT15 cemented carbide 103.91 k J/mol,indicating that in this temperature range,WEB is more difficult to oxidize than YT15.The study also found that regardless of the oxidation temperature,the oxidation process of WEB cemented carbide is mainly the oxidation of the WC phase to form WO₃.The residual Al₂O₃ or ZrO₂ phase of the matrix will be retained in the oxide layer,and the residual Al₂O₃ at high temperature will form Al₂（WO₄）₃ with WO₃.When the oxidation temperature is higher than 800℃,the oxidation process of WEB cemented carbide will be inhibited by the generated Al₂（WO₄）₃ phase.