Preparation And Properties Of High Temperature Microwave Absorbing Coatings By Plasma Spraying Technology

With the radar tec hnology i mpr oving dramat ically,the function of radar absorbing materials in a stealth defense system for all military pl atforms has become more important.Radar absorbing coatings have attracted much attention all over the world due to their lots of advantages suc h as the relative simplici ty of fabrication,the capa cit y to form on wide-ranging work piece,and have been widely empl oyed in most stealth weapon sys tems.Plasma spraying i s an attracti ve technique t o prepare coatings for wear-resistant,corrosion-re sistant and t hermal barrie r coatings due to the simplicity of application,the re levant number of sprayable materials,high achievable deposition efficiency,and cost effectiveness.Therefore,it has great pot enti al in preparation of high tempera ture microwave absorbing materials.The aim of this paper i s preparation of high te mperature microwave absor bing coatings by plasma spray system with an internally fed powder torch after the agglomer ate with carbon bl ack（CB）,multi ple wall carbon nanot ubes（M WCNTs）,and Ti₃SiC₂ as absorbers filled with cor dierite（2MgO·2Al₂O₃·5SiO₂,MAS）as matrix,respectively.The microstructure,phase composition,density and porosity,microhardness of the microwave absorbing c oati ngs were observed and characterized.The dielectric and microwave absor ption properties at room t emperature and high temperature of the coatings were investigated.Microwave absorbing coatings with requirements were prepared via adjusting the content of the abs orbers and coating thicknes s,according t o electromagnetic theory.Meanwhile,in order to improve the hi gh te mperature ant i-oxidation property of the coatings,the single-layer and double-layer gl ass coat ings were prepared on the surface of the CB/MAS mi crowave absorbing coating.The a nti-oxidation mechanism and the failure reason of the glass coatings wer e analyzed and di scussed.M oreover,the effect of glass coatings on the dielect ric and microwave absor ption property of the CB/MAS coatings was a lso investi gated.The main researches and conclusions ar e as follows :（1）CB/MAS and MWCNTs/MAS mic rowave absorbing coatings we r e fabricated via spray-drying technique and pl asma spraying technology using CB and MWCNTs as absorbers and MAS as matrix,respectively.Many pores and a few cracks exi st in t he coatings.The microstructure of coatings shows a uniform dispers ion of ca rbon agglomerat ed particles.According to the XRD analysis,the coatings are composed of C and M AS wit h no new phase forming during the spraying process.Because of the carbon absorbers oxidation during the spraying process,t he actual ca r bon absorbers content in the sprayed c oatings is less the addedcarbon absorbers content.The density,porosity and mic r o-hardness of the coatings are related t o the a ctual carbon absorbers content.In CB/MAS coatings,with the actural CB content i ncreasing from 2.11% t o 9.98%,the density of the as-sprayed coatings decreases from 2.073g·cm-3to 1.893 g·cm-3,the porosity increases from 13.4% t o 20.1%,whi le the micro-hardness decreases from 296 kgf · mm-2 to 210 kgf · mm-2,respectively.In MWCNTs/MAS coatings,wit h the actural MWCNTs content increasing from 0.81% to 5.01%,the density of the as-sprayed coatings decreases fr om 2.082 g·cm-3to 1.984 g·c m-3,the porosity increases from12.9% to 15.8%,while the micro-hardnes s decreases fr om 281 kgf · mm-2 to 256 kgf · mm-2,respectively.The real and imaginary part of the complex permittivit y for the coatings i ncreases with the increas e of the actual c arbon absorbers conte nt,which can be asc ribed to t he pola rization effect and formation of conductive networ k.It can be found that MWCNTs absorber is more easily agglomera gated and oxidized.Therefore,the CB/MAS coatings show a better microwave absorpti on propert y,compared with the MWCN Ts/MAS coatings.By calculating the mi crowave absorption of the coatings with 6% CB c ontent,the refl ection loss values exceeding-5dB and-10 dB are achieved in the frequency r ange of 8.2-12.4GHz and8.39-12.4GHz,respectively,when the coating thickness is 3.0mm,and the mini mum value i s-23.9dB at 10.13 GHz.（2）Single-layer borosilicate glass coatings applied at 400°C were pre pared on t he surface of CB/MAS microwave absorbing coating via slurry br ushi ng method.After oxida tion at 400°C for 210 min in air,the weight loss of the c oating s ampl es is 0.019%.The real and i maginary of the comple x permitti vity for the coating samples are 8.33-7.04 a nd 10.24-7.64,reducing 0.17 and 0.01 compared with the original samples,respectively.（3）Double-layer glass coatings applied at 700°C were prepared on the sur face of CB/MAS microwave a bsorbi ng coat ing vi a slurry brushing method.The inner layer is phosphate glass coat ing and the outer layer is boros ilicat e glass coating.After oxidation at700°C for 210 min in air,the weight loss of the coating sample s is 0.115%.The real and i maginary of the complex permittivity for the coating sampl es are 8.38-7.09 and 9.80-7.35,reducing 0.12 and 0.38 compared with the original samples,respectively.（4）Double-layer glass coatings were prepared on the surface of CB/ MAS microwave absorbing coating via slurry br ushing method.The inner layer is phosphate glass coating and t he outer layer is borosilicate glass c oating.It concluded t hat coatings brushing 3 times can be applied at 900°C.After oxidation at 900°C for 210 min in a ir,the weight loss of the coating samples is 0.143%.The real and i maginary of the complex per mittivity for the coating samples are 8.1-6.9 and 10.1-7.4,reducing 0.35 and 0.2 compa red with the original samples,respectively.（5）Ti₃SiC₂/MAS micr owave absorbing coatings were fabricated via spray-drying t echnique and plasma spraying te chnology using Ti3 SiC2 as absorbers and MAS as matrix,respectively.The melton degree of spraying particles which attributed to the s praying par ameters depend on the microstructure,phase constitution,de nsity,por osity,mic r o-hardness and c omple x permitti vity of the coat ings.The micros tructure of coatings shows a uniform di spersion of Ti₃SiC₂ in MAS matrix wi thout agglomerated.It can be found a few pores and cr acks exi st in the coatings.Because of chemical reaction at high temperature during spraying process,new phase of TiC,SiO₂ and TiO₂ appeared in the as-sprayed coatings.The density,porosit y and mi cro-hardness of the coatings are rel ated to the Ti₃SiC₂ content.The density and porosit y of the coatings increases and the micro-har dness of the coatings decreases with the i ncreasing Ti₃SiC₂ content.The real and imaginary part of the compl ex permittivity for the coatings increases with t he i ncreasing Ti₃SiC₂ cont ent,which can be ascribed to t he space charge polarization and conduc tive loss.When the Ti₃SiC₂ content is higher than 40%,the complex permit tivity of coatings wi ll increase dramatically,which c an be ascribed to the per colation effect.The real and imaginary part of the complex permittivity for the coatings i ncreases with the increasing te mperature because of relaxation polarization.Considering the i nvestigated temper ature and frequency,the Ti₃SiC₂/MAS mi crowave absorbing coatings with a 1.5mm thickness will show a proper mi crowave absorption property wit h a relatively low reflectivity.