Ahmet Kucuk

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

Abstract 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.

Bimodal distribution of mechanical properties on plasma sprayed nanostructured partially stabilized zirconia

The mechanical behavior of nanostructured partially stabilized zirconia (PSZ) coatings was evaluated via Knoop microhardness. The distribution of the microhardness values of the feedstock particles and coatings under a 10 g load were analyzed via Weibull statistics. The percentage of non-molten material was determined using scanning electron microcopy and image analysis. It was observed that the nanostructured coatings present a bimodal distribution in their Weibull plots, indicating the presence of two phases which are described as molten and non-molten. The presence of the bimodal distribution in the mechanical properties allows the prediction of microhardness values of these nanostructured coatings.

Integrity of nanostructured partially stabilized zirconia after plasma spray processing

Nanostructured partially stabilized zirconia powders with different particle size distributions were plasma sprayed under a range of thermal spray parameters. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were applied to analyze the nanostructured powder feedstocks and coatings. These results demonstrated that a feedstock with a broad particle size distribution maintained some of its nanostructure during spraying; whereas a feedstock with a narrow particle size distribution lost a major proportion of its nanostructure character. The apparent success in spraying nanostructured particles can be explained by employing the SEM and XRD analyses.

Influence of plasma spray parameters on mechanical properties of yttria stabilized zirconia coatings I: four point bend test

Abstract Yttria (8 wt.%) stabilized zirconia (YSZ) with a NiCrAlY bond coat was atmosphere plasma sprayed on mild steel substrates. The bond coat thickness (100–250 μm), YSZ coating thickness (300–500 μm), stand off distance (80–100 mm), and substrate temperature (273–393 K) were changed in a four by 17 experimental design matrix to investigate the influence of each spray parameter on the mechanical properties of coatings. Coatings were tested using a four point bend test arrangement. Coatings sprayed with thinner bond coat on a cold substrate exhibited higher yield strength and stiffness under bending. Change in the stand off distance and the top coat thickness did not statistically influence either yield strength or stiffness of the coatings.

Deposition efficiency, mechanical properties and coating roughness in cold-sprayed titanium

In the cold-spray process, metal powder particles develop into a coating as a result of ballistic impingment on a substrate. In cold-spray, compressed gas (air, nitrogen or helium), at pressures ranging between 1.4– 3.4 MPa (200–500 psi), but typically around 1.7 MPa (250 psi), flows through a manifold system containing a gas heater and a powder feeder. The pressurized gas is heated electrically to around 100–600 ◦C then passed through a Laval-type converging/diverging nozzle until the gas velocities reach supersonic speeds. The powder particles are introduced into the gas stream just in front of the converging section of the nozzle and are accelerated by the expanding gas. The powder feedstock is delivered on the high-pressure side of the nozzle by the metering device, which is heated and maintained at the elevated pressure of the manifold. During the supersonic expansion through the Laval nozzle, there is a temperature reduction. Thus, the temperature of the gas stream is always below the melting point of the particulate material, providing coatings developed primarily from particles in the solid state with very little oxidation [1–5]. As cold-spray is a 100% solid-state process, the deposition “in air” of titanium coatings without significant oxidation represent an important technical achievement. Titanium and its alloys are employed in corrosive environments, aerospace and bio-implants [6]. Beyond the solid-state characteristic, a fundamental feature of the cold-spray method is the concept of critical velocity (V ∗). For each coating and substrate combination there is a V ∗. Above the V ∗ the particles will have enough kinetic energy to be incorporated into a coating. Below the V ∗, the particles will be either reflected from the surface (bounced-off) or cause erosion of the substrate and any coating buildup which had begun. For particle velocities V > V ∗, the coating process occurs and the deposition efficiency is seen to increase with increasing V [1, 4, 5]. The actual mechanisms by which the solid-state particles deform and bond has not been well characterized. It seems plausible, though it has not yet been demonstrated, that plastic deformation may disrupt thin surface films, such as oxides, and provide intimate conformal contact under high local pressure, thus per-

Porosity determinations in thermally sprayed hydroxyapatite coatings

The porosity of various thermally sprayed coatings was determined using an Archimedian Method. The method relies on density determinations and buoyancy effects. First, measurements were carried out on standard samples in order to verify the accuracy and reliability of the Archimedian method. Then, porosity measurements were taken for hydroxyapatite samples and correlated to its physical properties. The results from the experiments show that as the crystallinity of the HAp increases, the porosity increases. Also, as the amorphous content increases, porosity decreases. Other relationships indicated that the porosity decreased as the stand off distance increased and it decreased with a power increase.

Influence of plasma spray parameters on mechanical properties of yttria stabilized zirconia coatings. II: Acoustic emission response

Abstract Yttria (8 wt.%) stabilized zirconia (YSZ) with a NiCrAlY bond coat was atmospherically plasma sprayed on mild steel substrates using various processing parameters including YSZ coating thickness, bond coat thickness, stand off distance, and substrate temperature. The cracking behavior of these coatings under four point bending load was examined using an acoustic emission (AE) recorder. The numbers of AE events exhibited during the elastic and plastic deformation of coatings were analyzed. Using multi-linear regression analysis, the number of AE events was correlated to the spray parameters. This analysis revealed that coatings with thicker YSZ top coat and NiCrYAl bond coat sprayed on a heated substrate at shorter stand off distance exhibited more AE activity and released higher AE energy under the bending. The greater emission activity and higher AE energy were evidence of severe cracking.

Composite coatings of Si3N4-Soda lime silica produced by the thermal spray process

Silicate glass and glass-silicon nitride composite coatings were atmospheric plasma sprayed onto mild carbon steel substrates under varying process parameters such as torch power and H2/Ar ratios. The x-ray analysis revealed that Si3N4 in the composite coatings could be preserved under the harsh environmental conditions of the plasma spray process. The presence of Si3N4 as the reinforcement phase to the glass matrix conferred higher hardness properties to the coatings.