Study On Dispersion Mechanisms And Forming Process Of ZrO₂-Based Ceramic Composite Powder

In order to enhance the properties of ZrO₂ and Fe-Al/ZrO₂ composite further, improve the dispersing homogeneity of powders, and lay the foundation for preparing complicated shape parts, studies were carried out in the following respects on the dispersing mechanisms, gelcasting process and sintering process of nano ZrO₂ powder and Fe-Al/ZrO₂ composite powders.Nano sized Fe-Al powders were at first prepared by hydrogen arc-plasma and mechanical alloying. XRD results revealed that the Fe-Al powders prepared by hydrogen arc-plasma were Fe₃Al with B2 structure and the Fe-Al powders prepared by mechanical alloying were a kind of amorphous α-Fe(Al) solid solution. SEM indicated that increasing the milling time led to the occurrence of cold-welding, the increase of which decreased the powder size with the increase of milling time.After being annealed at low temperature, the rules of phase transition of Fe-Al powders were investigated using DSC: at 390℃, Fe-Al powders prepared by mechanical alloying exhibited conversion of disordered structure to DO₃ structure, which occurred in the Fe-Al powders prepared by hydrogen arc-plasma at 483 ℃. Therefore, the Fe-Al powders prepared by mechanical alloying were converted to DO₃ structure by calcinations at 750 ℃. TEM observations revealed that the particle size was less than 200 nm.This paper investigated the dispersion characteristics of nano ZrO₂ powder and Fe-Al/ZrO₂ composite powder in water or organic liquid medium.When ZrO₂ powders were dispersed into water, both the amount of the dispersant NH₄-PMAA and, the pH value of the suspension could affect greatly the dispersion characteristic of nano ZrO₂. The results showed that the optimum content of NH₄-PMAA was 0.6% at pH=10, at which the dispersion effect is optimum and the highest solid content suspension of 54.33 wt% ZrO₂ obtained. TEM observation revealed that the ZrO₂ particle size is 20-50 nm in suspension well-dispersed.However, adding the Fe-Al powders into the well-dispersed ZrC>2 suspension promoted the reaction between the nano-sized Fe-Al intermetallics, which originally possessed high corrosion resistance, and water, with the product of brown Fe2O3-like substance, preventing the dispersion of Fe-Al/ZrC>2 composite powders.Above results indicated that Fe-Al/ZrC^ composite powders couldn't well-dispersed in water and nano ZrC>2 powder was able to well-dispersed in water by adjusting both the pH value and the amount of dispersant.When ZrC>2 powders or Fe-Al/ZrO2 composite powders were dispersed into the organic liquid using dibutyl phthalate (DBP), n-octanol or n-butanol as dispersion medium and acrylic acid (AA), methacrylic acid (MAA), oleic acid (OA), stearic acid (SA) or Y3000 being the dispersant, the results of viscosity measurements indicated that: (1) MAA or SA can fairly disperse the nano ZrC>2 powder and Fe-Al/ZrO2 composite powders and obtain 57.28-66.67 wt% suspension in n-butanol or n-octanol. (2) Y3000 has the best dispersion effect for dispersing the nano ZrO2 powder and Fe-Al/ZrC>2 composite powders in n-butanol or n-octanol and 61.9-70.22 wt% suspension obtained.FTIR results indicated that RCOO-Zr or RCOOH HO-Zr coordination bond formed on the surface of ZrC>2 particles or Fe-Al/ZrO2 particles after absorbing dispersant NH^-PMAA, MAA, SA or Y3000, which constituted the chemical absorption. Dispersion mechanisms revealed that (1) adjusting pH value could achieve electrostatic stabilization suspension when dispersing ZrC>2 powders in water and electrosteric stabilization suspension was obtained by both adding polyelectrolyte dispersant NH4-PMAA and adjusting pH value; (2) adding superdispersant Y3000 could attain steric stabilization suspension when dispersing ZrC>2 powders or Fe-Al/ZrCh composite powders in the organic liquid, while the dispersion stabilization should be attributed to the principle that substance is apt to be wetted in similar polarity liquid when incorporating the small molecular weight organic compound MAA or SA; (3) for the high solid content Fe-Al/ZrCh composite powder suspension prepared in this work, the dispersion stabilization was due to rheological stabilization mechanism.The results of dispersion tests showed that the well-dispersed ZrC>2 suspension in water could be prepared by using polyelectrolyte dispersant NH4-PMAA and the superdispersant Y3000 possess the best dispersing effect for Zr(>2 powders or Fe-Al/ZrCh composite powders dispersed in n-butanol or n-octanol.This paper studied the gelcasting process of ZrC>2 or Fe-Al/ZrC^ in aqueous or nonaqueous media on the basis of dispersion tests results.For gelcasting ZrC>2 in water, the influences of the corresponding factors on the strength of ZrC>2 green body were investigated through adjusting the content of main monomer AM, cross-linker monomer MBAM, initiator APS, catalyst TEMED in the acrylamide aqueous gelcasting, the ratio of main monomer to cross-linker monomer and deairing slurry in vacuum. The dry green body prepared on the basis of these parameters exhibited the bending strength of above 20 MPa, which were able to satisfy the requirements of machining on strength fully. The optimal parameters were found to be: the total amount of AM and MBAM in the premix solution is 20 wt%; the ratio of AM to MBAM is 24. In the slurry (50 wt%) containing 50 g ZrC>2 powders, the content of dispersant MN is 6.0 ml, initiator ASP solution is 0.4-0.5 ml and catalyst TEMED is 0.4-1.2 ml.When fabricating ZrC>2 or Fe-Al/ZrO2 by non-aqueous gelcasting, the total amount of monomer in the premix solution is 20 vol%, where the ratio of TMPTA and HDODA is 1:4 and the initiator BPO solution is 1.0 vol%. Green bodies with homogeneous microstructure were prepared by introducing adequate dispersant. EPMA results indicated that the green body obtained by the gelcasting presented more uniform component distribution than that achieved by press molding. SEM analysis showed that gelcasting process enabled the solid particle in the suspension in-situ and consequently reserved the favorable dispersion effects in the green body just as in the suspension.This paper also studied the microstructures and mechanical properties of Fe-Al/ZrC>2 composites.The microstructures of composites were investigated by SEM, TEM, EDS andHREM. The SEM showed that the Fe-Al/ZrO2 composite possessed more uniform microstructure prepared by using dispersed powders than powders milled without liquid. The average grain size of the ZrC>2 and the Fe-Al were 350 nm and 1 urn, respectively. Most of the ZrO2 and Fe-Al grains had an equiaxed morphology and that part of the Fe-Al was clubbed. The The HREM image of Fe-Al/ZrO2 interface along the [001]Fe-Ai/[0H]zrO2 projection shows that the interfaces are very clean, and no reaction phase existed at the interface. The atom arrangement beside interface between Fe-Al and ZrO2 is partly corresponding, existing at semicoherent state. The mismatch is observed at interface.The measurement of mechanical properties indicated that the fracture toughness and bending strength of Fe-Al/ZrO2 composites increase with the increasing of Fe-Al content. The oj, Kic and hardness are 1265MPa, 30.8 MPa ? m1/2 and 87.3HRA respectively when Fe-Al content is 30 vol%. The K/c value of composite is 2.2 times as that of ZrO2, and aj increase 23% compared with ZrO2. the hardness of composites decline slightly with the increasing of Fe-Al content.According to the systematically study, the well-dispersed suspension of Fe-Al/ZrO2 composite powders with high solid content were successfully prepared, and the green body with uniform microstructure had been shaped by gelcasting, and the Fe-Al/ZrO2 composites with uniform microstructure, high fracture toughness, high bending strength and high hardness were sintered. For the excellent properties, the Fe-Al/ZrO2 composites can be applied in some occasions such as high performance ceramic cutting tools and hot-press mould with its high fracture toughness, bending strength and hardness. Now this material has been developing for application by the support of the National Natural Science Fund (No.50242008) and the "863" Program (No.2002AA332100).