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
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
High resolution microstructural investigations of interfaces between light weight alloy substrates and cold gas sprayed coatings.
Investigations on interfaces between light weight alloys (aluminum AA7022 and magnesium AZ91) and optimized cold gas sprayed (CGS) zinc-based coatings are carried out. The analyses include standard optical and scanning electron microscopy (SEM) as well as transmission electron microscopy (TEM). By using TEM resolution within nanometric dimensions is realized.
Investigations by SEM show areas of an intensive mixing between substrate and coating material with a number of different grey values due to element contrast. By EDXS analyses in a 1 µm broad transition zone increased zinc concentration compared to the substrate material is detected. TEM investigations prove that in this transition zone no homogeneous solid solution exists, but additionally submicron and nanosized intermetallic phases with different concentration of aluminum, zinc and magnesium are found. Due to these results an intensive mechanical interaction must take place at the substrate-coating interface during CGS. Thus bond strength does not only depend on physical and chemical adhesion or thin metallurgically affected zones, but also highly on effects of mechanical alloying.
Wielage, B., H. Podlesak, T. Grund, A. Wank
Proceedings of the International Thermal Spray Conference, Basel, Switzerland, May 2nd - 4th 2005, pp. 154-157, ISBN 3-87155-793-5
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