Papers by Author: Tommi Varis

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Authors: Simo Pekka Hannula, Erja Turunen, Jari Keskinen, Tommi Varis, Teppo Fält, Tom E. Gustafsson, Roman Nowak
Abstract: HVOF thermal spraying has been developed to deposit dense Al2O3-coatings for improved protective properties. As compared to generally used plasma sprayed coatings HVOF coatings can be prepared much denser and thus are better suited for applications where protective properties of the coating are needed. In this paper we describe the development of HVOF spraying technologies for nanocrystalline Al2O3- and Al2O3-Ni-coatings. The microstructure and the mechanical properties of these novel coatings are reported and compared to a conventionally processed Al2O3-coating.
Authors: Erja Turunen, Tommi Varis, Jari Keskinen, Teppo Fält, Simo Pekka Hannula
Abstract: Special nanoreinforced ceramic composite coatings were produced. Dense nanostructured alumina and chromia coatings alloyed with various nanofraction elements such as Ni, NiO, ZrO2 and SiC were manufactured by HV2000 HVOF spraying. As a result coatings with nearly 100% improvements relative fracture toughness, were produced.
Authors: Helena Ronkainen, Ulla Kanerva, Tommi Varis, Kimmo Ruusuvuori, Erja Turunen, Jani Peräntie, Jussi Putaala, Jari Juuti, Heli Jantunen
Abstract: 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.
Authors: Erja Turunen, Tommi Varis, Tom E. Gustafsson, Jari Keskinen, Pertti Lintunen, Teppo Fält, Roman Nowak, Simo Pekka Hannula
Abstract: Special mechanical properties have widely been demonstrated with bulk nanocrystalline materials. An increasing effort has been made to transfer such improvements also into thermal sprayed ceramic coatings. This paper focuses on such efforts in alumina-based ceramic coatings. The optimization of process conditions and effect of different process parameters on the mechanical performance of high velocity oxy-fuel (HVOF) sprayed ceramic coatings is discussed.
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