Preparation And Properties Of Graphite Surface Oxidation Resistant Coatings

Graphite material has high temperature strength,good electrical conductivity,thermal shock resistance,corrosion resistance and selflubricating properties,so it is an excellent aluminum liquid purification rotor material.Due to the high temperature of the aluminum liquid,the melt purification temperature in the aluminum and aluminum alloy melt casting production is from 640? to 760?,and the critical temperature of carbon oxidation in air is 400?,which means that carbon is easily oxidized at the temperature range from 640? to 760?.This demands the oxidation resistance for the graphite rotor.In this thesis,the immersion methods in the solutions of phosphate and the phosphate/borate composite,respectively,were used to treat the graphite rotor material in order to form anti-oxidation substances on their surface,thereby improving the using time of the graphite rotor in the aluminum liquid.The protective mechanism is briefly analyzed to provide a reference for further application of the graphite rotor immersion treatment process.The research results are as follows:(1)For the phosphate immersion solution,the optimum compositions of the solution contained 20 ml of water,20 ml of phosphoric acid and 15 g of disodium hydrogen phosphate and 6ml of glycerol.The optimal treatment conditions included immersion time of 45 min and heat treatment:room temperature?200 ?(10 min)?300 0C(30 min)?400 ?(10min).(2)For phosphate and borate immersion solution,The best compositions contained 20 ml of water,20 ml of phosphoric acid,15 g of disodium hydrogen phosphate,and 12 g of sodium tetraborate and 8 ml of glycerol.The optimum treatment process included immersion time of 30 min and heat treatment:room temperature?200 ?(10 min)?300 ?(20 min)?400 ?(30 min).(3)The results of oxidation at different temperatures(600 ?,700?,800?)for 3 h showed that the weight loss rates of the untreated graphite samples were 3.57%,10.7%,26.1%,respectively.The weight loss rates of graphite samples treated with phosphate were 1.18%,3.53%,and 5.56%,respectively.The weight loss rates of graphite samples treated with phosphate and borate were 0.05%,1.37%,and 2.86%,respectively.Both of treated methods effectively reduced the oxidation weight loss rate of the sample at high temperature,and the graphite sample treated with phosphate and borate immersion solution has better oxidation resistance and lower oxidation weight loss rate at high temperature.(4)The results of oxidation experiments at 800? for different times showed that the normal use time of the graphite sample treated with phosphate immersion solution was 12 h,and the time for the graphite sample treated with phosphate and borate was normal 18 h,the graphite using time was extended by 6 h.