Hafnia Coatings Research Articles

Microstructural evolution and fracture toughness of plasma sprayed CNT reinforced Yttria‐stabilized Hafnia coating

AbstractYttria (8 wt%)‐stabilized hafnia (YSH) and carbon nanotubes (CNTs) (1 wt%) reinforced yttria‐stabilized hafnia (YSHC) coatings were fabricated on alumina substrate using atmospheric plasma spray technique. Raman spectra confirmed the survival of CNTs in plasma sprayed YSHC coating and indicated about graphitization of CNTs. Whereas, the FE‐SEM micrograph infers the presence of few 2‐D graphene platelet‐like structure in plasma sprayed YSHC coating. Addition of 1 wt% CNTs has significantly increased the densification of YSH coating from 86% to 92%, whereas average hardness and elastic modulus increased by ~57% and ~16%, respectively. A phenomenal increase of ~125% in relative fracture toughness was observed in YSHC coating, which is attributed to three major factors viz. (a) Enhanced densification (b) High fraction of fully melted regions and (c) Various toughening mechanisms, like CNTs pull out, CNTs braiding, graphene splat wrapping, CNTs anchoring.

Formation and investigation of plasma nanostructured hafnia coatings

Nanostructured coatings HfO2-9wt%Y2O3 were obtained by plasma spraying in vacuum using supersonic nozzle creating Prandtl-Mayer expansion fan. The coatings were investigated by scanning electron microscopy, X-ray diffraction analysis, and synchronous thermal analysis (differential scanning calorimetry, thermogravimetric analysis, and differential thermogravimetric analysis) in the temperature range of 40÷1600°C. The coatings are predominantly characterized by the nanostructured globular structure. Thermal analysis showed a peak corresponding to the exothermic reaction in the temperature range of 1400÷1600 °C, accompanied by a mass loss. c-HfO2 phase remained in the coating after thermal analysis with a decrease in the lattice parameter a from 5.18 Å to 5.13 Å.

HfO2 Nanorod Array as High‐Performance and High‐Temperature Antireflective Coating

Antireflective (AR) coating with both high antireflectance and omnidirectional characteristic is desired for many advanced optical device applications and photovoltaic thin‐film solar cells. The application of double‐layer antireflective (DLAR) coating to reduce reflectance is largely constrained at wide wavelength range due to refractive index variation within individual layer and between the air–substrate interfaces. In this article, high performance gradient refractive index double‐layer AR coatings with periodically patterned hierarchical porous hafnia nanorods that mimic moth compound eyes are fabricated on a dense hafnia matrix layer using glancing angle deposition technique. The refractive indices for the top and bottom layers of the DLAR hafnia coatings are 1.36 and 1.87. The fabricated films possess high transparency showing zero reflectance at calculated wavelength of 550 nm but also give <1% reflectance over broad range of visible spectrum at different angles of light incidence displaying its omnidirectional characteristic. The shifting and tuning of reflectance to nearly <1% in visible, infrared, and ultraviolet regions was achieved for wide range of substrate materials by tailoring refractive index through porosity and film thickness of the two layers. Moreover, the DLAR hafnia thin films show high thermal stability at high temperatures up to 300 °C.

Laser-induced damage of hafnia coatings as a function of pulse duration in the femtosecond to nanosecond range

Laser-damage thresholds and morphologies of hafnia single layers exposed under femtosecond, picosecond, and nanosecond single pulses (1030/1064 nm) are reported. The samples were made with different deposition parameters in order to study how the damage behavior of the samples evolves with the pulse duration and how it is linked to the deposition process. In the femtosecond to picosecond regime, the scaling law of the laser-induced damage threshold as a function of pulse duration is in good agreement with the models of photo and avalanche ionization based on the rate equation for free electron generation. However, differences in the damage morphologies between samples are shown. No correlation between the nanosecond and femtosecond/picosecond laser-damage resistance of hafnia coatings could be established. We also report evidence of the transition in damage mechanisms for hafnia, from an ablation process linked to intrinsic properties of the material to a defect-induced process, that exists between a few picoseconds and a few tens of picoseconds.

Reactive evaporation of low-defect density hafnia

Motivation for this work includes observations at Lawrence Livermore National Laboratory of a correlation between laser damage thresholds and both the absorption and the nodular-defect density of coatings. Activated oxygen is used to increase the metal-oxidation kinetics at the coated surface during electron-beam deposition. A series of hafnia layers are made with various conditions: two µ-wave configuations, two sources (hafnium and hafnia), and two reactive oxygen pressures. Laser damage thresholds (1064-nm, 10-ns pulses), absorption (at 511 nm), and nodular-defect densities from these coatings are reported. The damage thresholds are observed to increase as the absorption of the coatings decreases. However, no significant increase in damage thresholds are observed with the coatings made from a low nodular-defect density source material (hafnium). Hafnia coatings can be made from hafnium sources that have lower nodular-defect densities, lower absorption, and damage thresholds thatare comparable with coatings made from a conventional hafnia source.