The Effect Of Al₂O₃ With Different Morphology On The Synthesis And Properties Of WC-Al₂O₃ Composites

Cemented carbide with superior hardness and strength, high elastic modulus and low thermal expansion coefficient, has been playing an important role in the areas of tool material, Wear resistant material, high temperature resistant and corrosion resistant materials and wildly used in cutting tools, wear parts, high pressure diamond tools, mining and road construction machinery and other fields.WC-Co is one of the important research topics of the field of materials, Co is the guarantee of its fracture toughness. However, Co is rare, expensive and non-renewable strategic resource, a large amount of cobalt is imported from foreign countries every year. Meanwhile, due to its excellent physical, chemical and mechanical properties, the demand of Co in catalyst, batteries, electronic components and other aspects is increasing rapidly.Moreover, because of the low melting point and high chemical activity of Co, it will be soften and react with other elements easily, deteriorating the hardness and corrosion resistance of WC-Co composite. Therefore, it is of great significance and urgency to consolidate new WC matrix composite with cheap raw materials but high hardness and strength to instead of the WC-Co composite.In this paper, Al₂O₃ was chose as the substitute of Co, WC powders with nano structure were prepared by high energy planetary ball milling method, Al₂O₃ powderswere prepared by molten salt method and hydrothermal synthesis method, then WC-Al₂O₃ composite had been consolidated through hot pressing sintering.The main aims of this study are: Preparation of Al₂O₃ powder and Al₂O₃ precursor powder prepared by molten salt method and hydrothermal method, Al₂O₃, effect of various processing conditions on the morphology and size of Al₂O₃ in the process of melting salt was observed; Effect of grain inhibitor VC and Cr₃C₂ on the microstructure and mechanical properties of WC-Al₂O₃ composites were investigated and the mechanism of grain growth of WC in composites with grain inhibitor was studied; The process of WC-Al₂O₃ composites prepared by Al₂O₃ platelet and Al₂O₃ whisker was optimized,and the content was optimized and the mechanism of Al₂O₃ platelet and Al₂O₃ whisker reinforced and strengthen WC-Al₂O₃ composite was discussed. The main results are as follows:1. Preparation of Al₂O₃ platelet were prepared by molten salt method,The effects of type of molten salt, amount of salt, temperature of molten salt, holding time, and the effect of adding seed on the preparation of Al₂O₃ platelet were studied. Al₂O₃ platelet and Al₂O₃ whisker precursor were prepared by hydrothermal method,the influence of water temperature, p H value, holding time on the morphology of alumina was studied, and the mechanism of formation of Al₂O₃ with different morphologies was explored.The main conclusions are as follows: The particle size range of WC powder prepared by high energy ball milling was 20nm-968 nm, the average particle size was 325 nm, which reached the sub micron level; Na Cl, KCl, Na2SO4 and K2SO4 were used as the molten salt, and the Al₂O₃ powder prepared by mixed salt? Na₂SO₄-K₂SO₄ and Na Cl-KCl? is better than that by a single molten salt and the molar ratio of the molten salt to powder was 4; With the addition of ?-A₁₂O₃ seed, the nucleation density was improved and the energy barrier of nucleation was reduced,the phase transition temperature decreased significantly, when the temperature of the molten salt to 1100? and holding time to 4h, Al₂O₃ crystal growth will more complete; In the process of synthesis of Al₂O₃ precursor by hydrothermal method,with the increase of PH value of the reaction solution, the one-dimensionalnanostructures are gradually reduced, the two-dimensional nanostructures is gradually increased. When the PH value is 5, it is a one-dimensional nanostructures; when the PH value is 9, it is the two-dimensional nanostructures; when the PH value is 7, it is a mixture of one-dimensional nanostructures and two-dimensional nanostructures;When the p H value is 5, with the increase of water temperature and heating time, the amount of granular product is gradually reduced, and the morphology of one-dimensional nanostructures is gradually increased, and the surface is more smooth and morphology becomes more regular, with the increase of water temperature and water heating time, when the water temperature is 200 and the holding time is 24 h, the morphology of the alumina whisker is the best.2. VC and Cr₃C₂ were selected as grain growth inhibitors, WC-Al₂O₃ composite powder fabricated by high energy ball milling, the effects of grain growth inhibitors on densification,decarburization were studied, WC grain size during sintering process were measured, the activation energy of WC grain growth in the sintering process was calculated by Arrenheinus equation and the least square method, and the effect of VC and Cr₃C₂ on the microstructure and mechanical properties of WC-Al₂O₃ composites were observed, the main conclusions are as follows: VC 0.5wt% and Cr₃C₂ were added in the composites, the density of sintered samples with different holding time was studied and compared, the addition of VC and Cr₃C₂ is beneficial to the densification of WC-Al₂O₃ composites; The diffraction peaks of W2 C in the composites with VC and Cr₃C₂ were significantly weaken, the addition of VC and Cr₃C₂ can effectively inhibit the oxidation and decarburization during sintering,mainly because VC and Cr₃C₂ more easily oxidized than WC in high temperature; The activation energy of WC grain growth in the composites with VC and Cr₃C₂ can be increased, the growth of WC grains was effectively inhibited, when 0.5wt% VC and1wt% Cr₃C₂ were added to the composite, the activation energy required for WC grain growth was increased from 380.75 k J/mol 446.68 k J/mol and 423.47 k J/mol respectively; The hardness, fracture toughness and flexural strength of compacts0.5wt% VC were improved to 20.61 GPa,11.54 MPa m1/2 and 1238±72 MPa,respectively, while with 1wt% Cr₃C₂ addition, a hardness of 20.23 GPa, a fracture toughness of 11.02 MPa m1/2 and a flexural strength of 1180±67 MPa were achieved.3. Nano Al₂O₃ platelet prepared by hydrothermal method and WC powder prepared by high energy ball milling were mixed by mechanical milling, ultrasonic dispersion and magnetic stirring, the influence of different dispersing methods on the microstructure of the composites was studied; Because of the special structure of the Al₂O₃, the cross section and the longitudinal section microstructure of composites were observed, and the dispersion state of Al₂O₃ in the WC matrix is studied;the optimum amount of Al₂O₃ platelet was calculated by theory, and the optimum content of Al₂O₃ was confirmed by experiments, and the optimum sintering process of WC composites reinforced by Al₂O₃ and VC was studied. The main results were as follows:Through the observation of the microstructure of the composite, it can be seen that the mechanical milling method is better, the cross section and longitudinal section of the sintered specimen were observed, it is found that there is no difference in the distribution of the Al₂O₃ platelet during hot pressing sintering; The optimum content of Al₂O₃ was calculated by theory, and the optimum addition amount of Al₂O₃ was10wt%; The sintering temperature and holding time were optimized. When the sintering temperature was 1650?, holding time was 90 min, the relative density were up to 98.2%, the hardness reached 19.3GPa, the fracture toughness reached11.8MPam1/2, the flexural strength reached1289Mpa; The addition of grain inhibitor VC increased the density and mechanical properties of composite. Mainly because of refined the microstructure of composite, increased number of grain boundaries, and the exist of?V, W? C solid solution, which makes the combination of the WC and the Al₂O₃ interface and improve The density, hardness, fracture toughness and flexural strength of the composite.4. Nano Al₂O₃ whisker prepared by hydrothermal method and WC powder prepared by high energy ball milling were mixed by mechanical milling, the morphology of Al₂O₃ whisker after sintering was studied by the fracture of the composite; The density, hardness, fracture toughness and flexural strength of thesintered bulk with different content of whisker were tested, the optimum amount of Al₂O₃ whisker was explored; The mechanism of Al₂O₃ whisker toughening and reinforcing was explored; The sintering process of WC-Al₂O₃ w composites was optimized,the main conclusions are as follows: When the sintering temperature is1600?, the holding time is 90 min, the composite was composed of Al₂O₃ and WC phases, the Al₂O₃ whisker precursor??-Al OOH? was transformed into Al₂O₃ after sintering; When the content of the whisker was less than 10 wt.%, the composite can be fully density;The hardness of WC-Al₂O₃ w composites decreased with the increase of Al₂O₃ whisker, the fracture toughness and flexural strength increased first and then decreased with the increase of the content of whiskers. When the whisker content is10 wt.%, the maximum value of the fracture toughness and flexural strength are13.8MPa.m1/2 and 1335±42 MParespectively; The toughening mechanism of whisker toughing composites is the pullout of the whisker, the deflection of the crack and bridge; The main mechanism of whisker reinforced composite materials is related to the content of the whisker, When the content of whisker is 0-10 wt.%, the increase of flexural strength is mainly due to the increase of whisker, When the whisker content is over 10 wt.%, the decrease of flexural strength mainly due to the aggregation of large amount of the whisker which induced the decrease of relative density?In this paper, micro-nano WC powder and Al₂O₃ powder prepared with different morphology were used to fabricated WC-Al₂O₃ composites by hot pressing sintering,The microstructure and properties of the composites were improved by the grain inhibitor VC and Cr₃C₂. The morphology and particle size of the powders were studied by means of modern test technology, the microstructure and mechanical properties of the composites were studied, the effect of different conditions of molten salts method and hydrothermal method on the morphology of Al₂O₃ and Al₂O₃ precursor powders, the effect of grain growth inhibitors on the microstructure and mechanical properties of composites and the effect of Al₂O₃ with different morphology on the sintering process and mechanical properties of WC-Al₂O₃ Composites were discussed.It has laid a solid foundation for the further research andextension of WC-Al₂O₃ composites.