Andreas Killinger

The 2016 Thermal Spray Roadmap

Considerable progress has been made over the last decades in thermal spray technologies, practices and applications. However, like other technologies, they have to continuously evolve to meet new problems and market requirements. This article aims to identify the current challenges limiting the evolution of these technologies and to propose research directions and priorities to meet these challenges. It was prepared on the basis of a collection of short articles written by experts in thermal spray who were asked to present a snapshot of the current state of their specific field, give their views on current challenges faced by the field and provide some guidance as to the R&D required to meet these challenges. The article is divided in three sections that deal with the emerging thermal spray processes, coating properties and function, and biomedical, electronic, aerospace and energy generation applications.

Suspension thermal spraying of hydroxyapatite: microstructure and in vitro behaviour.

In cementless fixation of metallic prostheses, bony ingrowth onto the implant surface is often promoted by osteoconductive plasma-sprayed hydroxyapatite coatings. The present work explores the use of the innovative High Velocity Suspension Flame Spraying (HVSFS) process to coat Ti substrates with thin homogeneous hydroxyapatite coatings. The HVSFS hydroxyapatite coatings studied were dense, 27-37μm thick, with some transverse microcracks. Lamellae were sintered together and nearly unidentifiable, unlike conventional plasma-sprayed hydroxyapatite. Crystallinities of 10%-70% were obtained, depending on the deposition parameters and the use of a TiO2 bond coat. The average hardness of layers with low (<24%) and high (70%) crystallinity was ≈3.5GPa and ≈4.5GPa respectively. The distributions of hardness values, all characterised by Weibull modulus in the 5-7 range, were narrower than that of conventional plasma-sprayed hydroxyapatite, with a Weibull modulus of ≈3.3. During soaking in simulated body fluid, glassy coatings were progressively resorbed and replaced by a new, precipitated hydroxyapatite layer, whereas coatings with 70% crystallinity were stable up to 14days of immersion. The interpretation of the precipitation behaviour was also assisted by surface charge assessments, performed through Z-potential measurements. During in vitro tests, HA coatings showed no cytotoxicity towards the SAOS-2 osteoblast cell line, and surface cell proliferation was comparable with proliferation on reference polystyrene culture plates.

What Do We Know, What are the Current Limitations of Suspension HVOF Spraying?

Suspension spraying has evolved during the past decades and now is at the threshold of a commercial utilization. Compared to standard powder spray methods, mainly DC plasma spraying and (high velocity) flame spraying, it is quite clear that suspension spraying will not replace these well-established technologies but can extend them by adding new coating properties. Still there remain many issues to be solved. Suspension interaction with the hot gas stream is much more complex than in ordinary powder spray processes. In case of HVOF when axial injection into the combustion chamber is used, a direct observation of the liquid flame interaction is not possible. This paper discusses the present status of suspension HVOF-spraying (high velocity suspension flame spraying) including torch concepts, torch configuration in case of a TopGun system as well as different injector concepts and their influence on suspension atomization. The role of suspensions is discussed regarding their rheological and thermodynamical properties, mainly given by the solvent type and the solid content. An overview of different available diagnostic methods and systems and the respective applicability is given. Coating properties are shown and discussed for several oxide ceramics in respect to their possible applications.

UV-A and UV-C light induced hydrophilization of dental implants.

OBJECTIVES Wettability is increasingly considered to be an important factor determining biological responses to implant materials. In this context, the purpose of this study was to compare the dynamic wettability of dental implants made from different bulk materials and modified by different surface modifications, and to analyze the respective changes of wettability upon irradiating these implants by UV-A or UV-C light. METHODS Four original screw-type implants were investigated: One grit-blasted/acid-etched and one anodically oxidized titanium, one zirconia and one polyetheretherketone implant. Additionally, experimental, screwless, machined titanium cylinders were included in the study. Part of that cylinders and of blasted/etched implants were further modified by a magnetron-sputtered photocatalytic anatase thin film. Scanning electron microscopy was used to investigate the surface micro- and nanostructures. Samples were treated by UV-A (382nm, 25mWcm(-2)) and UV-C (260nm, 15mWcm(-2)) for entire 40min, respectively, and their wettability was quantified by dynamic contact angle (CA) analysis from multi-loop Wilhelmy experiments. RESULTS All implants are characterized by submicron- and nanosized surface features. Unexposed implants were hydrophobic (CA>90°). Upon UV-A, solely the implants with anatase coating became superhydrophilic (CA<5°). Upon UV-C, the blasted/etched implants turned superhydrophilic, the anodized titanium and the zirconia implants were considerably (CA=34° and 27°, respectively) and the PEEK implants slightly (CA=79°) hydrophilized. SIGNIFICANCE The wettability of implant surfaces can be improved by UV irradiation. The efficiency of UV-A and UV-C irradiation to lower the CA by photocatalysis or photolysis, however, is strongly dependent on the specific material and surface. Thus, attempts to photofunctionalize these surfaces by irradiation is expected to result in a different pattern of bioresponses.

Comparison Between High-Velocity Suspension Flame Spraying and Suspension Plasma Spraying of Alumina

Two different spray processes—suspension plasma spraying (SPS) and high-velocity suspension flame spraying (HVSFS)—are under focus in the field of suspension spraying. Both techniques are suitable for manufacturing finely structured coatings. The differences in the particle velocity and temperature of these two processes cause varying coating characteristics. The high particle velocity of the HVSFS process leads to more dense coatings with low porosity values. Coatings with a higher and also homogeneous porosity, which can be generated by SPS, have also high potential, for example, for thermal barrier coatings. In this study, both the processes—SPS and HVSFS—were compared using alumina as feedstock material mixed with different solvents. Besides the characterization of the microstructure and phase composition of the applied coatings, the focus of this study was the investigation of the melting behavior of the particles in-flight and of single splat characteristics.

Advanced Ceramic Tribological Layers by Thermal Spray Routes

Protective and functional coatings featuring outstanding tribological performance are of general interest for all kinds of industrial applications i.e for high performance automotive and mechanical applications. Thermal spray coating technologies play a key role in fabricating hard layers based on ceramic, metal - ceramic and further multiphase materials. Additional functionality can be achieved by combining these coatings with polymer based top coats with low friction coefficient or anti adhesive behaviour. Combined coatings feature also designed thermophysical and electrophysical properties. Several case studies will be discussed, ranging from automotive applications to paper and printing industry. Thermally sprayed coatings were applied using APS, HVOF and the newly developed HVSFS processes (High Velocity Suspension Spraying) with a special focus on nanoceramic feedstocks. In some applications polymer top coats with dispersed solid phases are applied to enhance functional properties. Special aspects in manufacturing engineering are addressed with particular importance not only of the influence of spray process parameters on coating properties but also of spray torch kinematic and robot trajectories on hardness, residual stress distributions, dimensional tolerances and porosity distributions will be discussed.

New Approach to Ceramic/Metal-Polymer Multilayered Coatings for High Performance Dry Sliding Applications

The combination of thermally sprayed hard coatings with a polymer based top coat leads to multilayered coating systems with tailored functionalities concerning wear resistance, friction, adhesion, wettability or specific electrical properties. The basic concept is to combine the mechanical properties of the hard base coating with the tribological or chemical abilities of the polymer top coat suitable for the respective application. This paper gives an overview of different types of recently developed multilayer coatings and their application in power transmission under dry sliding conditions. State of the art coatings for dry sliding applications in power transmission are mostly based on thin film coatings like diamond-like carbon or solid lubricants, e.g. MoS2. A new approach is the combination of thin film coatings with combined multilayer coatings. To evaluate the capability of these tribological systems, a multi-stage investigation has been carried out. In the first stage the performance of the sliding lacquers and surface topography of the steel substrate has been evaluated. In the following stage thermally sprayed hard coatings were tested in combination with different sliding lacquers. Wear resistance and friction coefficients of combined coatings were determined using a twin disc test-bed.

Application of Supersonic Flame Spraying for Next Generation Cylinder Liner Coatings

Rising demands for ecologically friendly automotive engines require a significant decrease in fuel consumption and emissions. Also the recent trend of downsizing engines demands for high performance materials for internal combustion engine applications. Tribologically functional coatings applied by supersonic flame spraying help in boosting the engine efficiency by reducing the internal friction and improving the durability and wear resistance of the cylinder running surface much-needed for engine downsizing tasks together with a high corrosion resistance enabling the use of bio fuels. In addition, the tailored surface topography of the thermal spray coatings help in supporting advantageous friction states and thereby show the benefit of reducing the oil consumption resulting in reduced emissions. The thermally sprayed coatings were applied using HVOF and HVSFS processes together with a specially designed spray gun trajectory in order to achieve a fast and cost efficient coating procedure. Several different coating materials, including novel nanostructured powders, have been investigated and compared to state-of-the-art cylinder liners. The performance of the coated cylinder liners regarding wear and corrosion resistance, friction coefficient and effects of the surface topography have been investigated in various test setups including engine tests.

High Velocity Thermal Spraying of Powders and Suspensions Containing Micron, Submicron and Nanoparticles for Functional Coatings

Thermal Spraying of submicron and nanoscaled powder materials require the application of agglomerated powders that can be processed with standard HVOLF (High Velocity Oxy Liquid Fuel Flame Spraying) equipment, or the conversion of these powders into a finely dispersed suspension together with an organic solvent, appropriate for suspension spraying (HVSFS: High Velocity Suspension Flame Spraying). Both methods are suitable for the manufacturing of finely structured and dense coatings and offer new possibilities in functional coating development for new application fields. Due to their refined microstructure, mechanical and physical coating properties can differ significantly from their conventionally sprayed counterparts. The paper gives an overview of HVOLF sprayed WC-Co cermets containing submicron and nanocarbides and HVSF sprayed nanooxide ceramics used in numerous technical applications.

Thermally Sprayed Coating Composites for Film Heating Devices

Direct heated systems in white goods for medium temperature applications (approx. 250° -350° C) can be applied onto glass, glass ceramic and metal substrates by means of thermal spray techniques. Essentially, such a system requires at least two functional layers to work properly: (1) The metal coating that works as the heating element and (2) an insulator coating that electrically separates the substrate from the film heater. Therefore, the heating device combines three materials (substrate, insulator and film heater) with rather different thermophysical properties. An optimized spray deposition process with proper guidance of the heat flow is required to produce coatings that can operate under cyclic thermal load conditions. The paper discusses the influence of thermal spray process parameters as well as the applied spray powders on the electrical properties of the achieved layer structures of the respective insulator and film heater coatings.