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Exploitable results
The main general industrial objective of the project was to develop the practical recommendations enabling formation of the reliable and practice oriented quality control system (QCS) giving high quality coatings by HVOF spraying of the WC-Co-Cr and WC-Ni-Cr powders due to selection of the optimum parameters of the powders and the spraying process.
The Quality Control System (QCS) was developed based upon the physical interpretation of the results obtained. It allows the sprayer to achieve the highest quality of the coating by controlling the whole spray process including selection of the best powders which correspond to each specific gun, checking the powder characteristics and adjusting the spray parameters. The spray guns used in the project were classified into three groups according to the heat and momentum that they transfer to the powder particles. There were introduced six groups of the powders related to their characteristics which affect the heat and momentum transfer from the HVOF jet. The parameters responsible for the final properties of the coatings (wear and corrosion resistance) were identified and the influence of the spray parameters on the coating quality was assessed. The detailed description of the QCS is given in the final technical report. The general recommendations of the QCS are as follows. The gun-substrate relative velocity must be set in the range of 500-1000 mm/s. The transverse velocity, where applicable, should be calculated to have a partial overlapping of the gun passes which depends on the spot size of the jet. Typical values are in the range of 4-6 mm per pass. Both velocities and the powder feed rate must be adjusted to obtain the coating thickness per pass not greater than 20-25 ?m. Smaller thickness per pass (10-15 ?m) is preferred because the risk of the thermal cracks is reduced. Cracks and porosity are the critical parameters when the corrosion protection is required.
The main general industrial objective of the project was to develop the practical recommendations enabling formation of the reliable and practice oriented quality control system (QCS) giving high quality coatings by HVOF spraying of the WC-Co-Cr and WC-Ni-Cr powders due to selection of the optimum parameters of the powders and the spraying process.
The powders were characterized in relation to their chemical composition, density, X-ray phase distribution, flowability, specific surface area, size distribution, magnetic saturation, SEM and EDS. The coatings obtained with these powders were characterized with respect to their microstructure and properties (basically microhardness, wear and corrosion resistance). The coatings developed with the powders of the group of WC-Co-Cr had different contents of carbon depending on the type of the gun. The coatings sprayed with the powders of the group of WC-Ni-Cr have an increased corrosion resistance and slightly reduced abrasion resistance compared with the case of the WC-Co-Cr powders.
Three new powders were manufactures for spraying by the CDS and DJH guns which were selected to be the most representative. Concerning the powers of WC-Co-Cr it was found that the best coatings were sprayed with the powders having an increased size distribution (45-63?m). The corrosion resistance increases when Cr is introduced in the form of Cr3C2, although it is smaller than in the case of the powders of WC-Ni-Cr. In relation to the coatings sprayed with the powders of WC-Ni-Cr it was established that the results on the weight loss are better than those corresponding to the samples with WC-Co-Cr. The tests on abrasion resistance with the sintered samples show the best results for the following compositions: 3.47Cr3C2-0.72VC-12Ni, 20Cr3C2-12Ni and 8Cr-12Ni.
The main general industrial objective of the project was to develop the practical recommendations enabling formation of the reliable and practice oriented quality control system (QCS) giving high quality coatings by HVOF spraying of the WC-Co-Cr and WC-Ni-Cr powders due to selection of the optimum parameters of the powders and the spraying process.
The CDS gun together with the DJH gun were selected as the most representative in the actual state-of-the art of the HVOF spraying. Optimization of the coatings sprayed by the DJH gun was provided by variation of the gas flow rates, spray distance and transverse gun velocity.
The coatings developed by the DGS gun don't exhibit great variations in the properties with changes in the powder characteristics The geometry of the spray barrel used in this system makes it possible to obtain good results with the powders having finer and greater size distributions. In the case of the coatings sprayed by the Top Gun the best general properties correspond to the coatings with the metallic matrix of Co-Cr obtained with the agglomerated and sintered and sintered and crushed powders having the high carbon content.
For the coatings sprayed by the DJG gun there exists a general tendency in the development of the coatings with the best properties using the powders with a fine size distribution corresponding to characteristics of the spray system used.
The results obtained with JP5000 gun are similar to those corresponding to the DJH system. Result 9.
For the coatings sprayed by the OSU modified system the coating roughness is much higher than in the case of the coatings sprayed by the DHJ gun. The weight losses are significantly greater than for the coatings developed by the DJH gun or JP-5000 system. The coatings have the structure which is more homogeneous than in the case of the DJH gun.
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