Ulla Kanerva

Development of thermal spray powders for improved tribological and corrosive applications and cost-effective solutions

HVOF sprayed carbide/metal coatings are used widely to prevent wear and corrosion of engineering components. However, in some specific applications additional properties, such as low friction surfaces with good sliding properties and/or more improved corrosion resistances with simultaneous high wear resistance are required. Often such properties are not obtained by using conventional commercial spray powder compositions. The cost of the spray powders and sprayed coating plays very often also an important role. Therefore, cost-effective solutions are also needed. The aim of the present work is to describe the development of special hard metal powders with tailored powder compositions and structures. Experimental Cr3C2-25Ni + 10vol-% solid lubricant powders, WC-15% corrosion resistant metal binder powders, and WC-10Co-4Cr powder manufactured from recycled hard metal scrap were prepared by using the spray drying and sintering process. The powders were sprayed by High-Velocity Oxy-Fuel (HVOF) spray process. The microstructures of the developed powders and coatings are reported.

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.

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.

Thermal stability of PECS-compacted Cu-composites

In this paper pulsed electric current sintering (PECS) is applied for submicron-sized copper (sm-Cu) based composite-powders aiming to produce MMC’s with higher strength and better temperature stability than reference sm-Cu. Incorporation of cuprite (Cu2O), alumina (Al2O3), titaniumdiboride (TiB2) and nano- and submicronsized diamonds (ND’s and SMD’s) improved noticeably the room temperature mechanical properties and the high-temperature stability of copper the effects becoming more noticeable with smaller dispersion size and higher amount of reinforcement. The hardness increment was at highest, when using ND’s or Al2O3. E.g., the microhardness for the reference sm-Cu sample and Cu with 3 vol.% ND’s, 6 vol.% ND’s and 2.5 vol.% Al2O3 were 1.02, 1.43, 1.77 and 1.58 GPa, respectively. Similar trend was noted also in the case of thermal stability and CTE. The study shows that Cu-ND, Cu-SMD and Cu-Cu2O are suitable for use at moderate temperatures around 623 - 673 K, whereas Cu-Al2O3 and Cu-TiB2 are suitable above 1023 K. In conclusion, PECS is suitable method to produce high quality Cu-composites having superior thermal and mechanical properties compared to those of sm-Cu.

Materials for Electronics by Thermal Spraying

In this paper, dielectric and conductive properties of thermally sprayed Al2O3- and Cu-based coatings on steel and alumina substrates were studied. Alumina powders with nanoand micro-sized additions of Ni, NiO, TiO2, silica, and commercial glass were used in High Velocity Oxygen Fuel (HVOF) deposition. The conventional commercial copper powder and three Ag, WC and H2 -modified powders were used in Direct Write Thermal Spray (DWTS) deposition. Mixed phases of α-Al2O3 and γ-Al2O3 were found to be present in the as-deposited coatings. Sprayed alumina-based composites exhibited dielectric permittivity of 5.3-13.9 and losses of 0.002-0.178 at 1 MHz and 1 GHz while the additions tend to increase the values. Sprayed compositions with glass-type additions were found to retain α-Al2O3 crystalline phase after the deposition. Cu depositions, especially modified ones, realised by Direct Write Thermal Spray (DWTS) showed conductivity values as high as 4256 % of IACS values. The results demonstrate that ceramic and conductive coatings fabricated by thermal spray techniques show feasible properties for electrical applications, such as low-frequency components and insulation layers to be utilised in embedded 3D circuitry, in a way that is not possible through traditional manufacturing methods.