This research demonstrates the use of cold spray (CS) as an additive manufacturing process to manufacture reflective aluminium coatings. Nitrogen was used as a carrier gas at various gas heating temperatures. Following deposition, the coatings were finished using a number of machining and/or polishing processes to surface roughness values of 20-150 nm. The samples were characterised with respect to total reflectivity within the wavelength range of 400-1800 nm, porosity, surface roughness, and density. The reflectivity of the coatings approached that of bulk material, and 99% dense coatings were obtained. Increasing the gas heating temperature did not decrease the porosity with the lowest gas heating temperature found to deliver the best reflectivity. This work demonstrates that CS can be used to coat thin layers of aluminium onto various materials, which can be subsequently polished to create composite reflectors. This provides a novel reflector with the reflectivity of aluminium, and the structural and thermal properties of the substrate material, allowing for greater flexibility in the manufacture of reflectors.

This study assesses the effect of acid corrosion on the luminescence of YAG:Ce coatings. The feedstock powder is prepared by high-temperature solid phase synthesis and the coatings are deposited by air plasma spraying. Microstructure and phase composition are characterized and the effect of acid immersion duration on luminescent intensity is measured. It is found that the luminescent properties of YAG:Ce 3+ coatings have a tendency to fluctuate with immersion time, which appears to be related to phase composition.

This study investigates the feasibility of using solution precursor plasma sprayed (SPPS) zinc oxide to fabricate NO 2 gas sensors. In the experiments, thin ZnO layers were deposited on Al 2 O 3 substrates that had been printed with interdigitated gold electrodes. FE-SEM images show that the as-sprayed films are highly porous and nanostructured as desired. Diffuse reflectance measurements reveal that significant absorption occurs in the visible light range. In gas sensing tests, the SPPS ZnO films were responsive to concentrations of NO 2 gas down to 0.4 ppm. The performance is attributed to the porous nanostructure and the presence of oxygen vacancies, or mid-bandgap defects, as confirmed by XPS analysis.

One approach for controlling the twin wire arc spray (TWAS) process is to use optical properties of the particle beam like length or brightness of the beam as input parameters for a process control. The idea is that changes in the process like eroded contact nozzles or variations of current, voltage and/or atomizing gas pressure can be detected through observation of optical properties of the particle beam. It can be assumed that if these properties deviate significantly from those obtained from a beam recorded for an optimal coating process the spray particle and so the coating properties change significantly. Thus, the goal is to detect these optical deviations and compensate occurring errors by adjusting appropriate process parameters for the wire arc spray unit. One cost effective method for monitoring optical properties of the particle beam is to apply the process diagnostic system PFI (Particle Flux Imaging): PFI fits an ellipse to an image of a particle beam thereby defining easy to analyze characteristic parameters by relating optical beam properties to ellipse parameters. Using artificial neural networks (ANN) mathematical relations between ellipse and process parameters can be defined. Thus in the case of a process disturbance through the use of an ANN-based control new process parameters can be computed to compensate particle beam deviations. In this paper, it will be shown that different process parameters can lead to particle beams with the same PFI parameters.

In this study, gas- and water-stabilized plasma torches were used to spray cesium-doped yttrium aluminum garnet (Ce:YAG) on different substrate materials and in large-area free-standing layers. The coatings were evaluated based on microstructure, crystallinity, and thermal stability, and tests were performed to measure porosity, hardness, phase composition, band-gap energy, and the presence of defects. Some coatings were also heat treated to determine how it changes their spectral response. The results of the examinations and tests are presented in the paper.

Plastic dye-sensitized solar cells (DSCs) are a promising photochemical solar technology owing to their light weight, flexibility, and suitability for low-cost roll-to-roll production. In order to improve cell efficiency, a light scattering layer was prepared on the surface of nanocrystalline (NC) TiO 2 by vacuum cold spraying (VCS) at room temperature using submicron-sized TiO 2 particles. The influence of the scattering layer and its connection with the underlying TiO 2 was investigated to understand the improvement in cell efficiency. For comparison, two scattering layers were prepared by spray deposition using ethanol-based suspensions with either submicron Al 2 O 3 or TiO 2 particles. All three layers exhibited a higher light-reflecting capacity than the TiO 2 nanocrystalline layer, although dark current test results showed that the biggest efficiency improvement was achieved in the NC-VCS-TiO 2 cells.

Submicro-sized anatase TiO 2 powder was used to prepare light scattering layers by vacuum cold spraying at room temperature. The microstructure of the TiO 2 coatings was characterized to be correlated with optical property and adhesion/cohesion of the coatings. Results show that the light-reflecting ability of the coatings decreased with the increase of accelerating gas flow rate from 3.5 to 7.5 L/min and increased with the increase of coating thickness from 2 to 10 µm. Both the cohesion and adhesion of the coatings increased with the increase of the accelerating gas flow rate, due to the improvement of the particle-particle contact and the particle-substrate contact by the high velocity impact resulting from the increased powder particle velocity.

The aim of this work is to measure the optical reflective properties of plasma sprayed ceramic coatings on glass substrates. The coatings are made with Al 2 O 3 -TiO 2 powders and are assessed by scanning them with an optical computer mouse. It is shown that mouse movements are smoother on ceramic surfaces than on a standard test pad and tracking performance is more stable, indicating that the reflective properties of ceramic-coated glass are better than those of a typical mouse pad. The ceramic coatings are also shown to have the potential to improve resolution in image recording devices.

In this study, Ni-Al powder is plasma sprayed on various substrates prepared by in-situ laser ablation and preheating. Surface topography and splat shape are examined in order to assess how laser induced heat flux affects surface modifications and variations in splat shape. The causes of laser induced surface modifications are identified for Al, Ti, and stainless steel substrates and it is shown that laser preheating improves surface absorptivity, which effectively subjects more defects to the effects of the laser.

This study analyzes the effects of laser remelting on the morphological structure and adherence of flame-sprayed PEEK coatings on stainless steel. Different types of lasers were used with wavelengths of 1.064 mm (Nd:YAG), 10.6 mm (CO 2 ), and 0.88 mm (diode). It was found that the longer the wavelength, the more compact the coating, but the less well-adhered. By making adjustments to compensate for the wavelength-dependent absorption coefficient of PEEK, both coating properties were improved.

Normally the conventional thermal spray processes as the atmospheric plasma spraying (APS) have to use easily flowable powders in a size range between 10 and 100 µm. In contrast the suspension plasma spraying (SPS) makes it possible to process nano sized particles directly. Due to the use of nano materials new microstructures and properties could be generated. One point is the possibility to influence the porosity level, its size range and micro crack densities in a wide band. Microstructure features like the porosity and cracks serve as scattering centres and lead to changes of optical properties. Furthermore the thermal conductivity is affected by the porosity level. In this work yttria partially stabilized zirconia coatings were generated by the SPS and APS process. The influence of the different microstructures on the thermal conductivity, the hemispherical reflectance and transmittance for wavelengths between 0.3 to 2.5 µm has been investigated. Due to the higher porosity and crack level of the SPS coatings the thermal conductivity and hemispherical transmittance was significant reduced.

The objective of this study is to establish relationships between the optical properties of yttria-stabilized zirconia (YSZ) plasma-sprayed coatings and their microstructure to improve YSZ thermal barriers. So, coatings with significant microstructural differences were manufactured and their hemispherical reflectance and transmittance were investigated over the 0.25-20µm wavelength range. However as plasma-sprayed coatings are heterogeneous materials, they exhibit optical properties different from those of homogeneous ones. Indeed, the pores and grain boundaries act as scattering centers and cause modifications in optical properties that are closely linked both to intrinsic properties of materials and microstructural characteristics. Therefore, the extinction coefficient which describes the decrease in light intensity due to the light absorption and diffusion processes in the matter, was determined by using the Kubelka-Munk two-flux model. The absorption and diffusion coefficients were infered from the comparison between the experimental data and model predictions of reflectance and transmittance.

Optical selective surfaces, i.e., surfaces with optical properties varying according to the frequency of the impinging radiation, have been exploited in several technical fields. These surfaces consist generally of doped semiconducting films, such as mixed oxides of Indium and Tin as well as Aluminium and Zinc. Thay are currently obtained by physical vapour deposition or sol-gel techniques. The present work aimed at demonstrating that coatings retaining optical selectivity can be obtained also by plasma spraying. Powders of Indium Tin Oxide (ITO) were prepared by an agglomeration technique and sprayed with a plasma torch under air and inert gas atmospheres. Both powders and coatings were characterised by X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. Optical reflection coefficients of the coatings were determined in the wavelength range 0.3-20 µm, i.e., in the visible and in the infrared regions of the spectrum. The experimental results indicated that it was possible to deposit, by plasma spraying, coatings possessing optically selective properties.

Several compositions of agglomerated ZrO 2 (Y 2 O 3 ), Al 2 O 3 and MgAl 2 O 4 powders were plasma sprayed through an axial injection torch into a water quenching chamber in order to obtain a fully melted and homogenized, metastable particle nanostructure. This metastable extended solid solution morphology allows the powder to be subsequently heat treated and superplastically formed into ceramic parts with potentially excellent optical and mechanical properties. Light microscopy, SEM and XRD analyses were used to evaluate the melt-quenched powder properties, and some comparison was made to previous work performed with traditional radial injection plasma torches. Thick build-ups were also sprayed from the melt-quenched powders to test their ability to superplastically flow. Results showed that during a single processing through the torch the axial injection equipment produced powders with a significantly higher fraction of fully homogenized powders than previously attainable. Homogenization of powders comprised of a larger particle size range was also achieved.

Spraying distance (SD) is one of the main parameters that can affect the spraying process - its efficiency as well as deposit's character. The reason lies in a different thermal history of particles corresponding to different spraying distances. Variation in the structure, preferred orientation, variation of the phase contents and/or in the chemical composition of deposits can be then found for different SDs. Consequently, properties of coatings can greatly vary, not only in the resistance state but also on deposit's annealing. Some materials are, however, more and some less sensitive to that effect. The goal of this work is to compare CaTiO 3 samples produced by plasma spraying with WSP at SD = 350 and 450mm with sintered samples. The following properties were compared: microhardness, thermal expansion coefficient, permittivity and reflectivity. Porosity and the differential thermal analysis of resistance and annealed deposits were also compared. Perovskite CaTiO 3 belongs among materials very stable during spraying - neither chemical nor phase differences were found between the feedstock powder, free-flight particles, as-sprayed deposit and annealed deposit. Despite that there are significant differences in behavior of deposits and freestanding parts sprayed from different SDs. All the recorded differences for CaTiO 3 plasma sprayed deposits with varying SDs must be therefore accounted to the deposit's structural differences, such as pore and splat sizes and shapes and preferred orientation. Additional differences on annealing can be probably attributed to the different amount of "stored" thermal energy in deposits due to the varying SD. However, there is a lower limit for SD assuring a quality deposit on spraying.