Research On Surface Coating Of Ultrafine Diamond And Preparation Of Vitrified Bond Composite Powders For Ultra-precision Gringding Tools

With the rapid development of modern science and technology,the precision of ultra-precision machining was developing gradually to nano level.Thus,more and more ultrafine abrasive were required in ultra-precision grinding wheel.However,due to the poor thermal stability and dispersion of ultrafine diamond,there were still some difficulties in the production of UFD-vitrified bond wheels.In this paper,surface coated UFD were employed to improve its thermal stability and dispersion in organic salts solution,and then UFD-vitrified bond compsite powders were prepared via polymer network gel mothod,which laid a good foundation for the preparation of vitrified bond grinding wheel with a uniform distribution of UFD.The effects of ammonia solution,TEOS and reaction time on the thickness and morphology of SiO₂ shell were studied.UFD@SiO₂ with shell thickness of 13 nm,24 nm and 41 nm were prepared successfully via the optimized preparation process.The results of DSC-TG,particle-size analysis and sedimentation test suggests that the thermal stability and dispersion of UFD were improved significantly after coated by SiO₂.The initial oxidation temperature?in air?of UFD@SiO₂ was increased about 150?than that of UFD.The dispersion stability of UFD@SiO₂ decreased slightly with the increase of shell thickness,however,UFD@SiO₂ which with a shell thickness of 15 nm could keep stable dispersion above 48 h in the organic salts solution.The effect of pH on preparation of UFD@TiO₂ coating was discussed.It was found that when the pH of reaction solution was 5.0,a uniform shell could be coated on the UFD surface by the hydrolysis polymerization of TEOS with the assistance of PVP.The results of XRD and TEM showed that the TiO₂ shell was amorphous,with a thickness of about 5nm and transformed to anatase after calcination at 450.The results of DSC-TG and sedimentation tests show that the initial oxidation temperature of the UFD@TiO₂ was 157?higher than that of original UFD,and could be kept stable dispersion in the organic salts solution for more than 12 h.In the process of preparation of multi component vitrified bond by P-G method,the relationship among the polymerization temperature,the proportion of monomer,crosslinker,initiator and gelation time was explored.A good performance gel could be prepared in a relatively short time,when the polymerization temperature was 80?,the proportion of monomer,crosslinker,initiator was 6:1:1.The thermal decomposition process of the gel,and the influence of calcination temperature on the morphology,phase composition of the vitrified bond powders and the bending strength and apparent porosity of sintered vitrified bond were investigated.The results indicate that a purity vitrified bond could be prepared at the calcination temperature of 520?,the main components of vitrified bond was amorphous phase with a small amount of LiNa?SO₄?crystal phase,the bending strength of vitrified bond?sintered at 630??was 66.1 MPa and the apparent porosity was 21.1%.UFD@SiO₂-vitrified bond and UFD@TiO₂-vitrified bond composite powders were prepared by introducing coated UFD in the process of preparing multi component vitrified bond powders via P-G method.The effects of sintering temperature and coated UFD concentration on the bending strength,apparent porosity,micro-morphology and phase of the sintered composite powders were investigated.The results show that when the concentration of coated UFD was 25-100%,UFD@SiO₂ particles could be distributed uniformly in the gel.However,the dispersibility of UFD@TiO₂ particles is slightly lower than that of UFD@SiO₂,and the agglomerate size is about 0.5-1?m.The sintering temperature range of UFD@SiO₂-vitrified bond composite powder is 670-720?,and the maximum bending strength range is 66.4-87.6MPa.For UFD@TiO₂-vitrified bond composite powders,these two values range were 660-700?and 78.1-96.9MPa respectively.