Optimization of wear protective coatings for heat transferring components
The laser-flash method is used to determine the thermal diffusivity of HVOF sprayed WC-Co(Cr) and Cr3C2-Ni20Cr as well as APS sprayed Cr2O3 and electroplated hard chromium coatings in the temperature range between RT and 600°C. Additionally bond and/or corrosion protective coatings like Ni5Al, Ni20Cr and 316L are characterized taking into account the different manufacturing methods twin wire arc spraying, HVCW and HVOF. With respect to the application example of drying rollers in paper industries the Taber-Abraser wear test is applied to evaluate the wear resistance. Finally the coatings are characterized concerning their corrosion resistance by salt fog test and by exposure to humid SO2 environment. For WC-CoCr feedstock the effect of carbide size and micro hardness on thermal, wear and corrosion properties are studied. WC-CoCr coatings with maximum micro hardness and fine carbides show the best thermal conductivity. The use of coarse carbide feedstock permits manufacturing of coatings with the highest resistance against dry abrasive wear, but the protective function depends severely on the processing conditions.
B. Wielage, G. Reisel, A. Wank, Chemnitz / D, U. Gross, G. Barth, Freiberg / D
Local wear protection for demountable automotive aluminum draw bars by thermal spray coatings
There are constantly increasing requirements concerning light-weight concepts in automotive design due to energysaving demands. Additionally reduction of component weight is desired for demountable parts to permit easier handling. One innovative development example in this field is a draw bar consisting of precipitation hardened aluminum featuring significant weight reduction compared to presently applied steel draw bars. However, low wear resistance of aluminum alloys makes sophisticated addition of wear resistant parts in the area of the positive fit necessary. This leads to increased machining time, further costs for purchasing and machining of adapted additional components and further assembling steps. A study on the capability of thermal spray coating deposition in the positive fit area for substitution of the sophisticated wear protection system is carried out. Different HVOF coating systems with variable thicknesses are tested concerning their capability to withstand bending stress. The optimum combination of Ni20Cr bond coats and Cr3C2 - 25 Ni20Cr top coats is applied for prototype production. The prototypes are tested concerning their performance under dynamical load and in corrosion tests. All produced prototypes pass both mechanical and corrosion tests. Prototypes, on which arc-sprayed Ni20Cr coatings are deposited as a low-cost solution, are still in tests.
A. Wank, B. Wielage, T. Grund, C. Rupprecht, Chemnitz / D, K. Angermann, Hartha / D, T. Schnick, Kriftel / D
Performance of thermal spray coatings under dry abrasive wear conditions
The performance under dry abrasive wear conditions is studied for HVOF sprayed cermet coatings in comparison to APS Cr2O3 and cermet as well as electroplated hard chromium coatings in Taber Abraser tests. Investigations on the wear behavior depending on the use of different abrasive wheels as well as on the influence of hard phase size, microhardness and chemical composition of the metallic matrix alloy in HVOF sprayed cermet coatings are carried out. In general cermet coatings show superior wear resistance compared to APS Cr2O3 and electroplated hard chromium coatings in Taber Abraser tests with H-22 abrasive wheels. The chemical composition of the metallic matrix of HVOF cermet coatings proves to be decisive concerning optimized wear resistance. The size of hard phases shows only minor influence. Coatings produced with coarse carbides make the highest wear resistance accessible, but the wear behavior depends much more severely on an optimized processing compared to coatings with fine carbides.
A. Wank, B. Wielage, G. Reisel, T. Grund, E. Friesen
Institute of Composite Materials, Chemnitz University of Technology, GERMANY
Influence of thermal spraying method on the propersties of tungsten carbide coating
The main tendencies in the development of tungsten carbide coating applications are presented. General properties of WC based coatings and their dependence on different factors are discussed. For comparative examination of the abrasive wear resistance of thermal spray coatings detailed Taber Abraser wear tests are carried out. Besides HVOF sprayed cermet and APS Cr2O3 also electroplated hard chromium coatings are studied. The best results have been obtained for HVOF sprayed cermet coatings. For this kind of wear conditions the difference in wear behaviour depending on carbide size in WC-CoCr coatings is insignificant compared to the difference depending on the matrix composition.
B. WIELAGE, H. POKHMURSKA, A. WANK, G. REISEL, S. STEINHAEUSER, M. WOEZEL
Institute of Composite Materials, Chemnitz University of Technology, Chemnitz, Germany