Reactive Magnetron Sputtering of ZrO2/Al2O3 Coatings: Alumina Content and Structure Stability

Ternary zirconia-alumina coatings with different compositional ratios, ranging from pure zirconia to 50% alumina content, were deposited by reactive sputtering from two targets, Zr and Al, in argon-oxygen mixtures. The coating composition was controlled by the Zr/Al target power ratio provided by two pulsed-DC power supplies. The coatings were ~1 µm thick and they were deposited on floating potential substrates at a temperature of 650±3K. XRD indicated that the pure zirconia coatings possessed a monoclinic structure with a grain size of 35-40 nm. Adding alumina to the zirconia coating stabilized the cubic zirconia phase and decreased the grain size to 10-15 nm. The alumina phase in the coatings remained amorphous. The hardness of the nanocomposite structure increased from 11.6±0.5 GPa to 16.1±0.5 GPa for an alumina content of 17%. At higher alumina concentrations, the zirconia phase became amorphous and the hardness decreased to 10-11 GPa. Structure stability of the zirconia-alumina coatings was studied by measuring the coating structure and hardness after annealing at temperatures up to 1173 K. Pure zirconia (m-ZrO2) coatings had low structure stability; the hardness reached a maximum value of 18±1 GPa after annealing at a temperature of 773-873K; however, at higher annealing temperatures the hardness decreased, reaching a minimum value of 12.3±0.6 GPa after annealing at 1173K. The hardness of the nanocomposite ZrO2/Al2O3 coating with various compositions increased with annealing temperature. The hardness of a coating with an alumina content of 17% reached a high value of 19.2±0.5 GPa after annealing at 1073-1173 K. Measurements of post annealing XRD analyses indicated that the stabilization of the coating structure with c-ZrO2/a-Al2O3 phases is the reason for the higher structure stability. From the analyses of phase stability and hardness before and after annealing, we conclude that adding alumina to the zirconia phase promotes the formation of nanocomposite c-ZrO2/a-Al2O3 coatings with a markedly higher stability than single-phase m-ZrO2. Highlights: 1. ZrO2/Al2O3 nanocomposite coatings were deposited by co-sputtering from Zr and Al targets. 2. Adding alumina to the zirconia coating stabilized the cubic zirconia phase. 3. ZrO2-17% Al2O3 coatings had a grain size of 10-15 nm and a hardness of 16.1±0.5 GPa. 4. ZrO2/Al2O3 coatings maintained a high hardness after annealing at 1173K with a high value of 19 GPa for alumina content of 17%. 5. The ZrO2/Al2O3 nanocomposite coatings were crack-free after annealing at 1173K

Thermal Stability of Filtered Vacuum Arc Deposited Er2O3 Coatings

Erbium oxide (Er2O3) coatings were deposited using filtered vacuum arc deposition (FVAD) and their structure and thermal stability were studied as a function of fabrication parameters. The coatings were deposited on silicon wafer and tantalum substrates with an arc current of 50 A and a deposition rate of 1.6 ± 0.4 nm/s. The arc was sustained on truncated cone Er cathodes. The influence of oxygen pressure (P= 0.40-0.93 Pa), bias voltage (Vb= -20, -40 or grounded) and substrate temperature (room temperature (RT) or 673K) on film properties was studied before and after post deposition annealing (1273K for 1 hour, at P~ 1.33 Pa). The coatings were characterized using X-ray diffraction (XRD), optical microscopy, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and Knoop Hardness. Optical microscope images indicated that the coatings had very low macroparticle concentration on their surface. The macroparticle diameters were less than 2.5 μm. The coatings were composed of only Er2O3 without any metallic phase under all deposition parameters tested. The coatings deposited on RT substrates were XRD amorphous and had a featureless cross-section microstructure. However, the coatings deposited on 673K heated substrates had a C-Er2O3 structure with (222) preferred orientation and weak columnar microstructure. The coating hardness varied with deposition pressure and substrate bias, and reached a maximum value of 10 GPa at P = 0.4 Pa and Vb = -40 V. The post-deposition annealing caused crystallization, and the coatings hardness dropped to 4 GPa with thermal treatment. However, after post-deposition annealing, no peeling or cracking appeared at the coating surface or the interface with the substrate

Optical properties of transparent ZnO-SnO2 thin films deposited by filtered vacuum arc

ZnO-SnO2 films were deposited onto glass substrates by filtered vacuum arc deposition. The source was equipped with a 70 at% Zn and 30 at% Sn cathode that was the source of Zn and Sn ion beam. Arc current was 200-300 A. The oxygen background pressure was 4-8mTorr. The deposition time was 60 or 120s, resulting in film thickness in the range 100-900 nm. The maximum deposition rate was 7.6 nm s-1. All films were found to be amorphous. The transmission of the film in the VIS was 80%-90%, affected by interference. The refractive index and the extinction coefficient were determined from the measured optical transmission in the range 300-1100 nm by fitting a theoretically calculated film transmission to the measured one, using a single oscillator model. The values of n and k were determined from spectroscopic ellipsometry data and were in the ranges 2.38-1.97 and 0.24-0.013, respectively, depending on wavelengths and deposition parameters. The optical band gap (Eg) was determined by the dependence of the absorption coefficient on the photon energy at short wavelengths. Its values were in the range 3.5-3.62 eV, depending on the deposition conditions. © 2006 IOP Publishing Ltd

The effect of substrate temperature on filtered vacuum arc deposited zinc oxide and tin oxide thin films

Zinc oxide (ZnO) and tin oxide (SnO2) thin films were deposited on commercial microscope glass and UV-fused silica (UVFS) substrates using a filtered vacuum arc deposition (FVAD) system. During the deposition, the substrates temperature was kept at room temperature (RT) or at 400 °C. The film structure, surface morphology, and composition were determined using X-ray diffraction (XRD), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS), respectively. The XRD patterns of the ZnO films deposited on RT substrates contained lines of strong c-axis orientation, whereas the intensity of the XRD lines of ZnO films deposited on 400 °C substrates was significantly stronger. The XRD patterns of SnO2 films deposited on RT substrates did not contain any diffraction lines, indicating an amorphous film structure, whereas the XRD patterns of SnO2 films deposited on hot substrates contained diffraction lines indicating that the films are polycrystalline. The ZnO and SnO2 film thickness was in the range 100-363 nm. The surface roughness (RMS) of ZnO film was 1.3 nm at RT and increased to 5 nm at 400 °C, whereas that of SnO2 films was 1.5 nm and decreased to 0.5 nm at RT and at 400 °C, respectively. The films' optical constants in the 250-1100 nm wavelength range were determined by variable angle spectroscopic ellipsometry and by transmission measurements. The peak transmission of the ZnO and SnO2 films in the VIS was 80-90%. The refractive index n of the films deposited on RT and hot substrates were in the range 1.72-2.23 for ZnO samples, and in the range 1.87-2.20 for SnO2 samples, as function of wavelength. The extinction coefficient of all films in the VIS was in the range 0.001 to 0.05, depending on wavelengths and deposition parameters. The optical band gap (Eg) was determined from the dependence of the absorption coefficient on the photon energy at short wavelengths. Depending on the deposition conditions, the values of Eg of ZnO and SnO2 were in the range 3.25-3.30 and 3.60-3.98 eV, respectively. © 2007 Elsevier B.V. All rights reserved