Nanostructured Zirconia Coatings Research Articles

Thermal shock resistances of nanostructured and conventional zirconia coatings deposited by atmospheric plasma spraying

Nanostructured and conventional zirconia coatings were deposited by atmospheric plasma spraying and the thermal shock resistances of as-sprayed coatings were investigated by the water quenching method. The results showed that the nanostructured as-sprayed coating possessed better thermal shock resistance than the conventional coating. This phenomenon is explained in terms of the difference in microstructure and microstructural changes occurring during thermal shock cycling.

Phase composition of nanostructured zirconia coatings deposited by air plasma spraying

Plasma sprayed nanostructured yttria stabilized zirconia coatings were deposited using nanostructured powder. The phase composition and microstructure characterization of the nanostructured as-sprayed zirconia coatings were studied by TEM, FESEM and XRD. The results showed the as-sprayed nanostructured zirconia coatings exhibited the typical lamellar structure, which was mainly composed of column grains with a diameter around 100 nm. The primary phase of as-sprayed nanostructured coatings was non-transformable tetragonal zirconia (T′ phase) having near the overall yttria content of nanostructured as-sprayed coatings. In addition, the typical superlattice-type reflections of the tetragonal phase were observed in the as-sprayed nanostructured coatings. No monoclinic phase was observed in the nanostructured as-sprayed coatings. There existed small segmentation cracks with lengths in the size range around 10 μm in the coatings.

Transmission Electron Microscopy Observation of Microstructure of Plasma-Sprayed Nanostructured Zirconia Coating

Nanostructured zirconia coatings deposited by plasma spraying technique were observed using transmission electron microscopy (TEM). It was found that the as-sprayed nanostructured zirconia coating had bimodal microstructures in terms of grain size distribution in the direction parallel to substrate surface. One was in the range 30–120 nm, which was the dominative structure of the coating, and the other was about 150–400 nm. The cross-section micrograph of the plasma sprayed nanostructured zirconia coating revealed that the coating still exhibited lamellar structure with columnar grains extending through its thickness. In conjunction with partially molten zirconia grains, amorphous regions were found. Domain structure and superlattice structure were observed in the plasma-sprayed nanostructured zirconia coating. The formations of the domain and superlattice structures are discussed in this paper.

Influence of feedstock and spraying parameters on the depositing efficiency and microhardness of plasma-sprayed zirconia coatings

In this work, nanostructured and conventional zirconia coatings were deposited by atmospheric air plasma spraying (APS) under different spraying conditions using spray-dried nanostructured powders and conventional powders as feedstocks, respectively. The microstructures of the feedstocks were characterized by TEM and SEM. The depositing efficiencies for both nanostructured and conventional zirconia coatings deposited under different spraying parameters were comparatively measured. The obtained results revealed that the spray-dried nanostructured powders possessed higher deposition efficiency than that of conventional powders. It is significant from economic view of reducing cost. In addition, the influence of spraying parameters on microstructure and microhardness of zirconia coatings were discussed also. It was found that the microhardness of nanostructured zirconia coatings was larger than that of its counterparts.

Wear of plasma-sprayed nanostructured zirconia coatings against stainless steel under distilled-water conditions

The friction and wear properties of plasma-sprayed nanostructured and traditional zirconia coatings against stainless steel were investigated with a sliding, reciprocating and vibrating test machine under water-lubricated conditions. The counterface was a 10-mm diameter AISI316 stainless steel ball. It was found that the plasma-sprayed nanostructured zirconia coating possessed better wear resistance than traditional zirconia coating. The wear rates of the nanostructured zirconia coatings are in the range from one-fourth to four-fifths of the traditional zirconia coating under loads ranging from 20 to 50 N. The plasma-sprayed nanostructured zirconia coating also reduces wear rate of the friction pair materials. The higher wear resistance of the plasma-sprayed nanostructured zirconia coating is attributed to its enhanced cohesion, improved microhardness and homogenous microstructure. The wear mechanisms of nanostructured and traditional zirconia coatings under water-lubricated conditions are discussed.

Investigation of the thermomechanical properties of a plasma-sprayed nanostructured zirconia coating

Abstract Yttria partially stabilized nanostructured zirconia coatings were deposited by atmospherical plasma spraying (APS). The microstructure of the as-sprayed nanostructured coating was characterized with Scanning electronic microscopy (SEM), Transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectrum (RS). The laser-flash diffusivity method and push-rod method were used to examine the thermomechanical properties of the nanostructured zirconia coatings. The results obtained indicated that the plasma-sprayed zirconia coating possessed nano-structure and its average grain size was about 73 nm. The average thermal expansion coefficients of the nanostructured coating at the first thermal cycle and second thermal cycle from room temperature to 1200 °C are 11.0 and 11.6×10−6 °C−1, respectively. The thermal diffusivity of the nanostructured zirconia coating was 1.80–2.54×10−3 cm2/s between 200 and 1200 °C. The microhardness of the nanostructured zirconia coating was 8.6 GPa, which was 1.6 times as large as that of traditional zirconia coating.

Tribological properties of nanostructured zirconia coatings deposited by plasma spraying

The nanostructured and traditional zirconia coatings were deposited by atmospheric plasma spray. The friction and wear properties of these two zirconia coatings against stainless steel were investigated with a block-on-ring tester under dry friction conditions. It was found that the plasma-sprayed nanostructured zirconia coating possessed better wear resistance than that of traditional zirconia coating. The wear rates of the nanostructured zirconia coatings are about two-fifths of those of traditional zirconia coatings under loads from 20 to 80 N. The improved wear resistance of the nanostructured zirconia coatings is attributed to the optimization of microstructure and the enhancement of mechanical properties, which improved the ability of plastic deformation of materials. The wear for both of two zirconia coatings is abrasive wear resulting from brittle fracture under low load (20 N). But under high load (80 N), the wear of nanostructured zirconia coating is explained in terms of plastic deformation and microcracking, while the wear of traditional zirconia coating is interpreted in terms of plastic deformation and brittle fracture.

Microstructural characterization of plasma-sprayed nanostructured zirconia powders and coatings

Abstract The objective of this paper was to study the melting state of nanoscale zirconia powders in plasma jet, examine the crystalline phase and microstructural characterization of the plasma sprayed nanostructured zirconia powders and coatings. The cross-section morphology of water-quenched powders revealed that most of the starting powders were molten in the plasma jet. Phase analysis using XRD spectra proved that the monoclinic phase zirconia of the starting powders transferred into tetragonal phase zirconia after the plasma spraying. SEM analysis indicated that the as-sprayed coatings exhibited lamellar structure, about 0.5–5 μm in thickness of a single lamella, which was found to be a characteristic feature of the plasma-sprayed nanostructured zirconia coating. Inside of the lamellae, columnar grains were observed. The mean grain size of the as-sprayed zirconia coating is about 80 nm.

Atmospheric plasma sprayed coatings of nanostructured zirconia

Abstract Nanostructured zirconia coating has been fabricated by atmospheric plasma spraying. The microstructure of the coating has been characterized with SEM and XRD, and the microhardness has been measured. The results show ZrO 2 coating possessed two kinds of structure. One was the poorly consolidatedly structure, which was composed of nanosized particles. The other was an overlapping structure, which consisted of micrometer size particles. The former was the main structure in the coating. The zirconia coating had the same phase composition as the starting powder and the thickness of coating was quite heterogeneous. The as-sprayed nano zirconia coating had a similar microhardness to sintered ZrO 2 , which was much higher than the conventional couterpart.

Evaluation of microhardness and elastic modulus of thermally sprayed nanostructured zirconia coatings

Results concerning microhardness and roughness ( R a ) of plasma sprayed coatings fabricated from nanostructured partially stabilized zirconia (PSZ) feedstock are presented. Nanostructured zirconia particles were plasma sprayed (Ar/H 2) at three power levels, with two argon flow rates at two spray distances. The results indicate that the microhardness, elastic modulus and roughness of the nanostructured zirconia coatings exhibit the following trends: the smoother the roughness, the higher the microhardness and elastic modulus. It was found that roughness is an indicator of the coating state that reflects the intrinsic microstructure of the coatings. It was ascertained that a surface profilometer could be used to determine the level of microhardness and elastic modulus as a non-destructive and in situ test by simple comparison with standard samples.