Rogerio S. Lima

Microstructural characteristics of cold-sprayed nanostructured WC-Co coatings

Abstract The cold-spray process was used to prepare nanostructured WC–Co coatings. The coating microstructural characteristics and phase composition were analyzed via optical microscopy, scanning electron microscopy (SEM) and X-ray diffraction (XRD). The morphology and microstructure of the nanostructured WC–Co powder were also analyzed by SEM and XRD. A 10 μm thick coating was achieved. The powder particles and coating microhardness were also evaluated and compared. The results show that there is no degradation of the WC–Co powder during the cold-spray process and well bonded and phase pure WC coating can be produced by the cold-spray process.

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.

Elastic modulus measurements via laser-ultrasonic and knoop indentation techniques in thermally sprayed coatings

Nondestructive techniques for evaluating and characterizing coatings were extensively demanded by the thermal spray community; nonetheless, few results have been produced in practice due to difficulties in analyzing the complex structure of thermal spray coatings. Of particular interest is knowledge of the elastic modulus values and Poisson’s ratios, which are very important when seeking to understand and/or model the mechanical behavior or to develop life prediction models of thermal spray coatings used in various applications (e.g., wear, fatigue, and high temperatures). In the current study, two techniques, laser-ultrasonics and Knoop indentation, were used to determine the elastic modulus of thermal spray coatings. Laser-ultrasonics is a noncontact and nondestructive evaluation method that uses lasers to generate and detect ultrasound. Ultrasonic velocities in a material are directly related to its elastic modulus value. The Knoop indentation technique, which has been widely used as a method for determining elastic modulus values, was used to compare and validate the measurements of the laser-ultrasonic technique. The determination of elastic modulus values via the Knoop indentation technique is based on the measurement of elastic recovery of the dimensions of the Knoop indentation impression. The approach used in the current study was to focus on evaluating the elastic modulus of very uniform, dense, and near-isotropic titania and WC-Co thermal spray coatings using these two techniques. Four different coatings were evaluated: two titania coatings produced by air plasma spray (APS) and high-velocity oxyfuel (HVOF) and two types of WC-Co coatings, conventional and multimodal (nanostructured and microsized particles), deposited by HVOF.

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.

Process temperature/velocity-hardness-wear relationships for high-velocity oxyfuel sprayed nanostructured and conventional cermet coatings

High-velocity oxyfuel (HVOF) spraying of WC-12Co was performed using a feedstock in which the WC phase was either principally in the micron size range (conventional) or was engineered to contain a significant fraction of nanosized grains (multimodal). Three different HVOF systems and a wide range of spray parameter settings were used to study the effect of in-flight particle characteristics on coating properties. A process window with respect to particle temperature was identified for producing coatings with the highest resistance to dry abrasion. Although the use of a feedstock containing a nanosized WC phase produced harder coatings, there was little difference in the abrasion resistance of the best-performing conventional and multimodal coatings. However, there is a potential benefit in using the multimodal feedstock due to higher deposition efficiencies and a larger processing window.

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-

Diagnostics for advanced materials processing by plasma spraying

Advanced coatings deposited by plasma spraying are used in a large variety of industrial applications. The sprayed coatings are employed typically in industry to protect parts from severe operating conditions or to produce surfaces with specific functions. Applications are found in many industrial sectors such as aerospace, automobile, energy generation, and biomedical implants. Coatings are built by the successive deposition of molten or partially molten particles that flatten and solidify upon contact on the substrate, forming lamellae. The coating properties are intimately linked to the properties of these lamellae, which in turn depend on in-flight particle properties as well as substrate temperature during spraying. Consequently, the development of diagnostic tools for monitoring and controlling these spray parameters will help provide the necessary information to study the coating formation process, optimize the coating properties, and, eventually, control the spray process in production. In this paper, a review of some recent developments of optical diagnostic techniques applied to monitor plasma-sprayed particles is presented. In the first part of the paper, two different sensing techniques for in-flight particle measurement are described. First, time-resolved diagnostics on individual particles is described. This technique is used to study the instabilities of the particle characteristics associated with the plasma fluctuations. Secondly, a technique adapted for use in an industrial production environment for measuring the particle jet characteristics as an ensemble is presented. In the second part of the paper, the use of an optical system to study the influence of the substrate temperature on the flattening and solidification of sprayed particles impacting on a flat substrate is described. The last part of this paper describes the optimization of nanostructured coatings based on a tight control of the temperature and velocity of the plasma-sprayed particles.

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.