Tomi Suhonen

Dielectric properties of HVOF sprayed ceramic coatings

Thermally sprayed ceramic coatings can be used as electrical insulators for example in high temperature applications (e.g. fuel cells) or in other demanding conditions. In electrical insulation applications the mostly used coating materials are aluminum oxide, magnesium oxide and magnesium aluminate. In general, only few reports of dielectric properties of thermally sprayed ceramic coatings can be found in literature and further analysis is thus needed. In addition, the measurement methods and conditions in previous research are often not fully documented, complicating the evaluation and comparison of the properties of different coatings. The aim of this paper was to characterize dielectric properties of thermally sprayed ceramic spinel coating sprayed with high-velocity oxygen fuel (HVOF) technique. The studied dielectric properties are DC resistivity, DC dielectric breakdown strength, as well as permittivity and dielectric losses at different frequencies. All measurements were made at temperature of 20 °C and at relative humidity of 20 %. Dielectric properties and the composition of coating material are presented and analyzed.

Pulsed Electric Current Sintering of the Al2O3 - 15wt% ZrO2 Nanocomposites with 3wt% of Different Solid Lubricants

High temperature low friction materials are sought for use in engines in order to reduce energy consumption of the machines. Due to the high service temperatures solid lubricating materials are necessary. This study is designed to find the optimal processing conditions for preparing these materials by pulsed electric current sintering. In this study, the Al2O3 - 15wt% ZrO2 (AZ) nanocomposite was modified with 3 wt% of self-lubricating component (CaF2, BaF2, MoS2, WS2, h-BN, or graphite). After the preparation of the alumina-zirconia powder mixture solid lubricant powder was added. Powders were then mixed in ethanol for 24 h, dried in a rotary evaporator, and in oven at 80°C for 24 h. The particle size distribution of the powders was established with the laser method. Powders were compacted by using pulsed electric current sintering technique at 1300 °C with 50 MPa for 5 min in vacuum. The structure of the materials was studied with XRD and SEM. Density of the compacts was measured with the Archimedes method and their hardness was evaluated by applying HV1 hardness with the instrumented indentation techniques. Their mechanical behavior was further studied with the instrumented scratch testing.

MICROSTRUCTURES, MICROHARDNESS, AND CRYSTALLIZATION BEHAVIORS OF AMORPHOUS Al2O3-YSZ COATINGS PREPARED BY AIR PLASMA SPRAYING

Three types of amorphous Al2O3-yttria-stabilized zirconia (YSZ) coatings were prepared using air plasma spraying (APS). The effects of the APS parameters on the microstructure, porosity, amorphous-phase content, and microhardness of the coatings were investigated using scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and transmission electron microscopy (TEM). The results indicated that the microstructure of the coatings was dependent on the APS parameters used. The porosity of the coatings and the number of unmelted particles deposited increased with the flow rate of the primary plasma gas increasing. The amorphous-phase contents and microhardness values of the coatings exhibited a trend opposite to that of their porosity and the number of unmelted particles present in them. The crystallization behaviors and microhardness values of Al2O3-YSZ coatings annealed at different temperatures were also investigated. The results indicated that the coatings with higher amorphous-phase contents possessed homogeneous nanocrystalline structure and exhibited higher microhardness values after the heat treatments.

Electric field dependency of dielectric behavior of thermally sprayed ceramic coatings

High temperature applications e.g. fuel cells require ceramic based insulation solutions instead of polymers. The aim of this paper was to characterize the dielectric properties of thermally sprayed ceramic coatings; especially the electric field dependency of AC and DC behavior of thermally sprayed ceramic coatings. One of the spinel samples and one of the alumina samples have quite similar lamellar microstructure which may partly explain their similar type of behavior in DC resistivity as well as in AC loss indexes at low frequencies. These two samples had smaller lamellar size than the other alumina and spinel samples which also had quite similar behavior of AC losses at low frequencies, respectively. In addition to the lamellar size and structure, also micro cracks in the coating microstructure are proposed to have an effect on the dielectric behavior and its electric field dependency.

Influence of humidity and temperature on the dielectric properties of thermally sprayed ceramic MgAl 2 O 4 coatings

Thermally sprayed ceramic coatings can be used as electrically insulating materials for example in high temperature applications (e.g. fuel cells) or in other demanding conditions where ceramic-based solutions are needed instead of e.g. polymers. The dielectric properties of thermally sprayed ceramic coatings are strongly affected by external conditions. The aim of this paper is to characterize the dielectric properties of thermally sprayed ceramic MgAl2O4 coatings; especially the effects of ambient conditions on certain dielectric properties of thermally sprayed coatings are studied. DC resistivity at various electric field strengths as well as permittivity and losses at different frequencies is reported in the paper for MgAl2O4 samples made by three different thermal spray techniques. These measurements were performed at three temperatures as well as at two different relative humidities. The DC breakdown strength was studied at one condition. Due to the slightly open porous microstructure of the studied coatings, increasing humidity particularly increases the dc conductivity and relative permittivity.

Role of microstructure in dielectric properties of thermally sprayed ceramic coatings

Thermally sprayed insulating ceramic coatings can be utilized in conditions where polymers are inapplicable. The coatings exhibit a special lamellar microstructure with interfaces and some defects (e.g. voids, cracks) in between. The aim of this study was to analyze the relationship between the microstructural features and the dielectric properties of various thermally sprayed ceramics. The structural characterization of the ceramic coatings was made based on following properties: porosity, volumetric gas permeability and the characteristic size of crystalline lamella. High gas permeability of the coatings decreased the breakdown strength but similar effect cannot be seen in the DC resistivity and the permittivity results. Decrease of DC resistivity at high humidity did not correlate to microstructural properties; rather it is speculated to indicate the hydrophilic nature of the coatings. The characteristic crystalline domain sizes showed no clear correlation with the dielectric properties.

Material Challenges in the Manufacturing of Tailored Structures with Direct Write Technologies

Two different direct write technologies, Direct Write Paste (DWP) and Direct Write Thermal Spray (DWTS), and the general material challenges related to these technologies are described in this paper. These new technologies open new routes to produce highly functional products that can be tailored and customised according to the demands of the applications. The DWP process is based on dispensing paste-like materials through a tip onto a substrate. Depending on the paste material, either 2 or 3 dimensional structures can be printed. The DWTS technique is based on thermal spraying, which is a spray process where the powder material is accelerated in molten form and impinged onto a surface. Products made using DWTS are especially targeted at harsh environment solutions. Results related to development of printable materials and printed products are presented.

DC conduction and breakdown behavior of thermally sprayed ceramic coatings

In this study, the DC conductivity from low electric fields up to breakdown fields is studied for several different thermally sprayed ceramic coatings. Although the DC conductivity of bulk alumina ceramic has been observed to follow the space charge limited current conduction mechanism, the studied ceramic coatings do not follow or follow only partly this mechanism. Possible reason for this is their different microstructure since bulk alumina exhibits fully crystalline microstructure while the ceramic coating consists of crystalline and amorphous regions as well as voids, defects and numerous interfaces. A possible conduction mechanism of the ceramic coatings based on the different conductivities of the amorphous and crystalline regions of the coatings is proposed. The microstructural features (e.g. volumetric porosity) are found to affect the breakdown strength for some of the studied coatings. The step-test breakdown strengths of the coatings were lower than the ramp-test ones due to the longer stress durations in step tests giving an indication of effects of electrical stress duration and possible short-term degradation of the coatings.