A parametric study on processing of scratch resistant hybrid sol-gel silica coatings on polycarbonate

Scratch resistant silica-based hybrid coatings on polycarbonate substrates were formed by dip-coating of acid-catalyzed tetramethyl ortohosilicate (TMOS):diethylenetriamine (DETA):H2O:2-propanol sols. The sol formulation and dip-coating process parameters on microstructural and performance properties-here optical transmittance and scratch resistance-of the coatings were evaluated. The effect of water quantity, total aqueous component (H2O + 2-propanol) amount and relative proportion of TMOS:DETA on film formation behavior and on performance properties have been investigated in a systematic way. It was found that an effectively polymerized hybrid coating rich in silica content, as realized for high TMOS or abundant water containing sols, resulted in defective films with microcracking and adhesion problems. High 2-propanol content on the other hand led to incomplete film coverage. It was shown that 5 ± 1 μm-thick, scratch resistant and pristine coatings exhibiting a visible transmittance of 86-88% can be formed with a single deposition process using an optimized sol formulation of TMOS:DETA:H2O:2-propanol of 30:30:20:20 in wt.%. Meanwhile, the hardness of the PC has increased from an initial value of 13.9 ± 2 to 70 ± 25 (Vickers hardness, HV1) upon coating. A surface hardness approaching to 250 HV1 can be attained by for the thicker coatings (8 ± 1 μm) deposited at higher withdrawal speeds. However, such films suffered from non-uniform coverage and poor surface/optical quality. The transmittance values reduced by a factor of 20-30% for thicker coatings.