Preparation And Performance Of Nanocrystalline Corundum Abrasive

Ceramic corundum abrasive via sol-gel route is a new kind of alumina-based abrasive. It is a promising abrasive of high performance due to excellent self-sharpening, high precision grinding with lower grinding cost and long service life used for superior machining field, as compared to the conventional single crystalline fused-corundum abrasive. In this article, the synthesis processing of nanocrystalline corundum abrasive precursor was studied. Influence of MgO-CaO-SiO₂ ternary additive and synergistic effect of co-doped seed on the properties of nanocrystalline corundum abrasive were investigated. A two-step sintering technique as an efficient route to fabricate high performance corundum abrasive was first introduced and two-step sintering mechanism was discussed. And the effect of heat-treatment process on the microstructure and mechanical properties of ceramic corundum abrasive were also investigated.The results of synthetic experiments revealed that the synthesis method had obvious effect on the properties of precursor and corundum abrasive. The sample prepared with modified sol-gel processing had uniform microstructure and narrow grain size distribution, its average grain size was less than 100nm. The mol ratio and addition amount of MgO-CaO-SiO₂ ternary additive had significant effect on the evolution of microstructure and mechanical properties. As amount of sintering additive components was adjusted to be equivalent to 2.5 wt.% with MgO ?CaO ?SiO₂ molar ratio 5 ?1 ?5, corundum abrasive with fine grain microstructure and high mechanical properties was developed. Liquid-sintering activation energy calculated of sample with MgO-CaO-SiO₂ ternary additive was 195.6kJ/mol. It indicated that diffusion mechanism dominated the densification kinetics of corundum abrasive. Co-doped seed had potential synergistic effects for reducing the temperature of phase transformation fromθ-Al2O3→α-Al2O3, increasing density of samples and improving the mechanical properties of alumina abrasive particle. With addition of 1.5wt.% fine crystalα-Al2O3+1.5wt.% (NH4)3AlF6 co-doped seed, the temperature ofα-Al2O3 phase transformation could be decreased by 270°C in contrast to the unseeded sample. With high single particle compressive strength and 99.5% relative density of sample were obtained when sample was sintered at 1300°C. Grinding wheel sample exhibited superior wear-resisting property.Optimum two-step sintering technique: At first step, samples were heated up to the higher temperature (named hereafter as T1) (1250¹300°C) without holding time. And sample need to reach a sufficient starting density (>75%) at T1 in order to obtain enough thermodynamics driving force of grain-boundary diffusion. They were then cooled down to the lower temperature (named hereafter as T2) and held for 5h at T2 in order to attain grain-boundary diffusion mechanism as the densification kinetics. Two-step sintering significantly prohibited the grain growth in the final stage of sintering (at lower temperature) without deteriorating the densification process due to promoting grain-boundary diffusion and suppress grain-boundary migration. The two-step sintering technique yielded the nanocrystalline corundum abrasive with smaller grain size value (less than 100 nm) and higher performance, compared with ceramic corundum abrasive via traditional sintering processing and white corundum abrasive via fused processing.Thermal treatment temperature had obvious influence on crystal size and mechanical strength of corundum abrasive. And a critical temperature (950°C) of abnormal crystal growth of heated treatment corundum abrasive was found. Below 950°C, thermal treatment processing had not obvious influence on mechanical properties and microstructure of corundum abrasive. However, beyond the critical temperature, abnormal crystal growth of heated treatment corundum abrasives was occurred, single particle compressive strength of abrasive decreased and the microstructure and mechanical properties of abrasive degraded. During the heat-treatment process, the growth of a crystal was not only related to the thermal treatment temperatures, but also very strongly connected with the surface state of the abrasive particle (i.e., abrasive particles coated by vitrified bond).