Performance of products using thin films greatly depends on their durability. Wear resistance of thin films is a critical factor when it comes to products exposed to abrasive wear and other severe conditions. In fact, the application of thin films to provide an extra function to a selected substrate or protect it from abrasive forces, is on the rise.
Titanium oxide is preferable for the industry owing to an array of applications in self-cleaning surfaces, optical filters, sensors, and antibacterial coatings. Titanium oxide films exhibit good adhesion to glass, resist abrasion, and reduce friction. However, titanium oxide has been replaced by SiO2-TiO2 blends in a bid to enhance its performance for the various applications. It has been found that silicon oxide addition to titanium oxide films enhances its wear resistance, improves adhesion to glass and reduces friction.
However, adhesion concerns between the titanium oxide and silicon oxide layers and a sharp interface between them have been identified. These might result in detrimental effects on wear resistance of the top active film and subsequently injure its lifetime performance. Researchers led by professor Alfonso Perez-Garcıa from Center for Materials Research, Mexico, determined the optimum titanium oxide film thickness that if effective for improved wear resistance and adhesion. They as well investigated the influence of silicon oxide addition onto the titanium oxide top film composition. Their work is now published in Applied ceramic technology.
Diluted titanium oxide was prepared from titanium butoxide hydrolysis in hydrochloric acid and anhydrous isopropanol. The resulting sol was applied to deposit thinner titanium oxide films. Another sol was prepared with the amount of titanium butoxide doubled. The resulting sol was then used to deposit thick titanium oxide films.
The authors prepared silicon oxide sol from a mixture of hydrochloric acid, tetraethyl orthosilicate and anhydrous ethanol. Diluted SiO2-TiO2 sols were synthesized for thinner depositions. Another SiO2-TiO2 was synthesized for thicker depositions.
The authors deposited the sol-gel bilayers on SLS glass. The bilayers were composed of a silica film and a top hydrophilic surface which was composed of either titanium oxide or SiO2-TiO2. The films were deposited implementing the dip coating approach with varying pull extraction velocities to obtain the film thickness required. The authors deposited 30-40nm thick films for the case of silicon dioxide. For the case of titanium oxide and SiO2-TiO2, they deposited 19-123 nm thick films.
The team observed that titanium oxide bilayer did not meet the stipulated abrasion criterion. Therefore, the bilayer wasn’t a good candidate for automotive applications despite having better abrasion and adhesive performances with reduced titanium oxide film thickness. Modifying the top layer composition by adding silica raised its abrasion and adhesion resistance. However, active top films based in the SiO2-TiO2 with molar ratios 2:8 and 1:9 met the abrasion resistance stipulations by the American automotive norm. Micro-hardness and friction coefficient measurements supported the abrasion results.
Chemical composition, adherence, thin film thickness, and interface microstructure were found to be critical for abrasion resistance as well as durability of titanium oxide and SiO2-TiO2 thin films. The outcomes of the study will be helpful when selecting film thickness in silica glass applications including interference filters, optical waveguides, and antireflective films.
Rosa Elena, Ramırez-Garcıa, Sergio Alfonso, Alfonso Perez-Garcıa, Jesus Alberto, Gonz alez-Rodrıguez, Miguel Arroyo-Ortega, Liliana Licea-Jimenez. Engineered TiO2 and SiO2-TiO2 films on silica-coated glass for increased thin film durability under abrasive conditions. Applied Ceramic Technology 2017; 14: 39–49.
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