Transparent Hard Coatings Research Articles

Transparent Hard Coatings with SiON-Encapsulated N-Doped Carbon Dots for Complete UV Blocking and White Light Emission

The solid-state hard encapsulation of carbon dots (CDs) can introduce an additional dimension for the tuning of their optical properties and effective promotion of multifunctional applications as well. In this study, simple hydrothermal and soft-sintering processes have been demonstrated to obtain silicon oxynitride-encapsulated N-doped carbon dots (SiON@NCDs) as well as organosilane-functionalized NCDs (OSi-NCDs). The surface-decorating long alkyl chains on NCDs help to prevent luminescence quenching in solid state and emit blue to orange color while forming a smooth film coating on the substrate. Interestingly, these OSi-NCDs can be readily converted into SiON-encapsulated NCDs (SiON@NCDs) and form a smooth, hard, and visible light transparent coating on a glass surface after soft sintering. This hard coating has the ability to emit white light and 100% block all types of toxic UV light (UV-A, B, and C). The process reported here provides a unique strategy to modulate the NCD luminance properties from blue to white light emission (WLE) by transforming the organosilane matrix together with the NCDs themselves, possibly modified at their surface defects or functional groups. The transformation process involves the conversion of organosilane into a SiON hard-coat matrix via the reaction with the amine surface groups on NCDs. Subsequently, the hard-coat-embedded NCD films emit WLE under UV excitation with good color stability and adjustable correlated color temperature (CCT) from warm white to cool white (3900–7300 K) depending upon their thicknesses. The process proposed here can be adopted for glass hard coating for white light emission (WLE) and UV-light blocking applications. Also, with our proposed hard encapsulation scheme, NCDs are endowed with long-term stability in harsh environmental conditions, which enables these materials for practical applications in many fields, including automotive, illumination industrial sectors, and luminescent solar cells (solar cell windows).

Structural, optical and mechanical properties of thin diamond and silicon carbide layers grown by low pressure microwave linear antenna plasma enhanced chemical vapour deposition

In this work, we detail the properties of thin silicon carbide and polycrystalline diamond layers grown by microwave plasma enhanced chemical vapour deposition with linear antenna delivery. Structural, mechanical and optical properties are compared for their potential use as transparent hard coatings. Silicon carbide layers exhibit mechanical properties comparable to thin diamond layers but with a significantly higher adhesion and lower optical absorption coefficient over a wide spectral range.

Preparation of hydrophobic nanosilica-filled polyacrylate hard coatings on plastic substrates

Colloidal silica nanoparticles synthesized from tetraethoxysilane via a sol–gel process were surface-modified first by 3-(trimethoxysilyl)propyl methacrylate, and then by trimethylethoxysilane (TMES). The former agent acts both as a coupling agent and as a C=C provider, whereas the latter agent is used to prevent particle aggregation and to increase hydrophobicity of the coating. The modified silica particles were UV-cured together with the monomer, 2-hydroxyethyl methacrylate (2-HEMA), and the crosslinking agent, dipentaerythritol hexaacrylate (DPHA), to form highly transparent hard coatings on plastic (PMMA and PET) substrates. Both differential scanning calorimetric and thermal gravimetric analyses of the hybrid materials indicated enhanced thermal stability with respect to the neat HEMA–DPHA copolymer. Furthermore, due to the incorporation of TMES, hydrophobicity of the hybrid coating increased considerably with increasing modified silica content. In the extreme case, an antiabrasive hard coating (7H on PMMA) with a water contact angle of 99° was obtained at the silica content of 15 wt%.

Microstructure and mechanical properties of Al–Si–N transparent hard coatings deposited by magnetron sputtering

Motivated by the success of transition metal nanocomposite hard coatings, a new optically transparent coating based on the Al–Si–N ternary system was developed. Al–Si–N thin films were deposited by reactive DC magnetron co-sputtering of Al and Si targets in an Ar/N 2 atmosphere at 200 °C and 500 °C sample temperature. The structure and mechanical properties of the coatings were investigated by XPS, XRD, TEM, nanoindentation and mechanical profilometry. The chemical composition was varied from pure AlN to Al–Si–N with 23 at.% of Si. The films are crystalline with the hexagonal AlN structure up to 12–16 at.% of Si as found from XRD and TEM analyses. A shift of X-ray diffraction peaks indicates a substitutional incorporation of silicon in the h-AlN lattice up to a solubility limit identified at 6 at.% of Si. By further increasing the silicon content, a nanocomposite nc-Al 0.44Si 0.06N 0.5/ a-SiN x is formed. From TEM analysis it follows that the crystalline material consists of columnar grains composed of crystallites with a (002) texture. The mean crystallite size decreases from 60 nm to about 5 nm upon addition of silicon, as revealed by XRD. Hardness measurements show a diffuse hardness maximum exceeding 30 GPa around 10 at.% of Si. At this value a negligible residual stress in the coating is measured.

Hard coatings by plasma CVD on polycarbonate for automotive and optical applications

In many applications, plastic surfaces need coatings as a protection against abrasion or weathering. Leybold Optics is developing Plasma CVD processes and machinery for transparent hard coatings (THC) for polycarbonate parts. In this paper we present the current features and remaining challenges of this technique. The coatings generally show excellent adhesion. Abrasion resistance is superior to commonly used lacquers. Climate durability of the coating has been improved to pass the tests demanded by automotive specifications. Current activities are focused on improving the durability under exposure to UV radiation. Estimations show that our high-rate plasma CVD hard coating process is also economically competitive to lacquering.

Silicone/Amine Resin Hybrid Materials as Abrasion Resistant Coatings

Hydroxyl organofunctional silicone solutions were prepared by acid-catalyzed hydrolysis and condensation of tetraalkoxysilanes, alkoxysilane modified organic compounds, and hydroxyl functional trialkoxysilanes or epoxy functional silanes. These silicone solutions were mixed with methoxylated urea formaldehyde and methoxylated melamine formaldehyde to form silicone/amine resin hybrids and applied on bisphenol A polycarbonate substrates as coatings. After thermal curing at 100−125 °C for 2−3 h, several optically transparent hard coatings with excellent abrasion resistance were obtained.

Deposition of dense hydrocarbon films from a nonbiased microwave plasma

An electron cyclotron resonance plasma was used to prepare C:H layers from methane. The temperature dependence of the deposition rate was investigated at substrate temperatures ranging from room temperature to 700 K, at a gas pressure of 1.6 Pa. Despite low ion energies corresponding to the plasma potential, transparent hard coatings were obtained at elevated temperature with a density up to 2 g cm−3. A deposition model is proposed which describes the growth from an adsorbed layer, including surface reactions with radicals and atomic hydrogen as well as the direct incorporation of ions. Two different deposition processes can be identified, yielding polymerlike films in the temperature range up to 450 K and dense hydrocarbons above this temperature. The observed temperature dependence of the film properties such as H/C ratio, index of refraction, and density is consistent with the predictions of the model.