Preparation And Characterization Of Waterproof Coating On The Surface Of Fused Silica Ceramics

The fused silica ceramics have small coefficient of thermal expansion (CTE), lowdielectric constant and loss, low thermal conductivity and good thermal shock resistance.They can be used as electromagnetic windows for missiles and rockets, thermal-shieldingmaterials for aerocrafts. However, fused silica ceramics generally have high porosities and thesurfaces of the pores are hydrophilic. As a result, water and vapor can be easily adsorbed andsucked in by the fused silica ceramics. This could do harm to the functions of the fused silicaceramics in real applications. Preparation of waterproof coating on the surfaces of the fusedsilica ceramic substrates can effectively solve above-mentioned problems.Since the fused silica ceramic substrate has a low CTE, the coating material on it shouldalso have a lower CTE. Lithium aluminum silicate (LAS) system glass-ceramic possesses lowor even zero and negative CTE, and its CTE can be adjusted over a wide range. Based onprevious research results, LAS glasses with four different composition were prepared. Andtheir crystallization and sintering behavior has been studied. Then, three of them wereselected for coating preparation. Using slurry painting method, LAS glass-ceramic coatingswere sintered on a fused silica ceramic substrates with a porosity of87.28%. The main crystalphase of the three LAS coatings are β-quartz solid solution and β-spodumene, β-spodumeneand β-spodumene solid solution, respectively. The LAS coatings effectively solve thewaterproof problems of the fused silica ceramics.Experiments and analysis show that the crystallization temperatures of LAS glasspowders are low, their densification is difficult between the crystallization temperatures.Sintering at a temperature above the crystallization peak temperature so that a relativecompact coating could be obtained and the crystallization phases of the resultantglass-ceramics are not changed. The painting slurry was prepared using a solvent to the glasspowder with a mass ratio of1:2. The solvent consists of turpentine and terpineol with a massratio of1:4. The optimum heat-treatment temperatures are1160oC,1100oC and1150oC,respectively. And the soak time is2h. SEM measurement results for the three selected glasspoweders show that the surface of LAS glass-ceramic coating is very dense and no pores wasformed. The glass-ceramic coatings show excellent waterproofing performance. In addition, isolated and closed pores can be found in the internal part of the coating. The surface coatinglayer partially penetrates into the open pores of the substrate, forming a well interconnectedinterface. The CTE of fused silica ceramics is1.05×10-6/oC, higher than the preparedglass-ceramic coatings. The CTE of the three glass-ceramic coatings are0.73×10-6/oC,1.00×10-6/oC and0.53×10-6/oC, respectively. Therefore, the coatings have compressive stress,making them difficult to crack or peel off. Since the porosity of the coated substrate wasreduced, the dielectric constant is slightly increased. The fused silica ceramics substrate withLAS glass-ceramic coatings were subject to ablation at1700oC for7s. It’s enidenced that theLAS glass-ceramic coatings protected the substrate surface from collapse and reduced the lossof weight of the substrate. When the ablation time increased to the3min, the protective effectof the coating decreased. Thermal shock resistance measurements proved that the coatedsamples were sustainable subjected to15cycles of sudden heating-cooling between roomtemperature and1100oC. The coating and the substrate still combined very well, and thesurface of the coating became more dense.On the surface of the fused silica ceramics with a porosity of14.06%, LAS glass-ceramiccoating could not be formed. In this case, sol-gel method was used to prepare SiO2gel coatingon the fused silica ceramic substrates. The sol-gel reaction was performed under acidicconditions. The optimal molar retion of the reactants are at room temperature with the molarratio of TEOS: EtOH: H2O=1:7:4. The results show that after5times repetition ofsol-dipping, drying and sintering at550oC, the waterproof properties of the coating aregreatly improved.