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
High energy ball milling - a promising route for production of tailored thermal spray consumables
The high energy ball milling technique permits production of composite powders suitable for application in thermal spray processes. Different milling systems are compared concerning their potential for production of composite powder feedstock for spraying processes. Hard phase materials like carbides or oxides are incorporated into various metallic matrices including light weight alloys based on titanium and aluminum. For optimized choice of matrix alloy and processing parameters a homogeneous distribution of hard phases with a size mostly smaller than 100 nm and perfect embedding in the metal matrix are achieved. Examples of oxide reinforced aluminum, carbide and boride reinforced self fluxing nickel based alloys and boride reinforced titanium alloy powders are presented. For optimized milling process roughly 90 wt.-% of the applied powder constituents form composite powder particles with suitable size for application in thermal spray processes. The use of HVOF spraying permits to maintain the extremely fine structure of the composite powders in the produced coatings. Characterization of the composite powders and the sprayed coatings is carried out by optical microscopy, SEM and XRD. Only negligible increase of the hard phase crystallite size but significant reduction of lattice distortion is observed after HVOF spraying. The composite coatings are characterized concerning their performance under severe wear stress. In addition to the Taber Abraser wear test an oscillating and a dry erosion wear test at elevated temperature are applied. The high potential of HVOF coatings produced from high energy ball milled composite powders is proved although
A. WANK, B. WIELAGE
Institute of Composite Materials, Chemnitz University of Technology, Chemnitz, Germany
Characterization of the particle jet of different HVOF guns by means of LDA
For constant conditions concerning substrate state, feedstock and environment properties of thermal spray coatings depend only on temperature and velocity of particles at impact on the substrate. Two different HVOF spraying guns, the kerosene fuel system Tafa JP5000 with radial powder injection and the ethylene fuel system Sulzer Metco Diamond Jet Hybrid 2700 with axial powder feeding, are characterized concerning the evolution of space resolved velocity and density of particles by LDA. Also influence of process parameter variations is examined. The region of shock diamonds is studied specifically. The influence of different characteristics concerning impact velocity and trajectories on the coatings microstructure is determined by means of optical microscopy and microhardness testing.
A. Wank, B. Wielage, T. Grund, C. Rupprecht, Chemnitz / D, S. Zimmermann, K. Landes, München / D
Corrrosion properties of X - HVOF corrosion protective coatings
Titanium grade 2 as well as steel 446 and 316L powders are applied for production of X-HVOF coatings on mild steel substrates. Deposition efficiency is determined by process parameters. Microstructural investigations are carried out by means of optical microscopy, SEM and XRD. In addition comparisons of oxygen and nitrogen content in titanium coatings and powder feedstock are drawn. Corrosion protection capability of produced coatings is studied by current density-potential measurements and by salt fog tests. Depending on the process parameters increase of oxygen and nitrogen content can be restricted to factor of 2 compared to the powder feedstock. Coatings showing nearly theoretical density in metallographical inspections are possible. In direct comparison to wrought titanium grade 2 material the corrosion behavior of the titanium coatings is very promising. Keeping in mind that coatings have been produced under atmospheric conditions the observed increase of the corrosion current density by factor four is regarded an excellent result. During the corrosion tests no damage, neither to the surface nor the substrate - indicated by rust precipitates on the specimen surface, is observed. So penetration of corrosive medium to the substrate is securely avoided.
A. Wank, B. Wielage, Chemnitz / D, F. Jansen, D. Wieczorek, Salzgitter / D, H.-M. Höhle, Hattersheim / D