Determining the Influence of Chromium Concentration on the Erosive Wear of Ni-Cr-B-Si Coatings Applied by Supersonic Flame Jet (HVOF)

The object of the present research work is to study the influence of chromium concentration in powder composites on the erosion wear of Ni-Cr-B-Si coatings, deposited by supersonic flame jet upon the substrate without thermal treatment and on substrate with thermal treatment. Without any prior heat treatment of the sub-strate and with some preliminary thermal treatment of the substrate up to 650 C, coatings were produced from each powder composite. The coatings were tested under identical circumstances of abrasive particle erosion in an air stream. The work also represents comparative results for erosion wear and erosion wear resistance of coatings, deposited by supersonic flame jet (HVOF), which contain composites Ni-Cr-B-Si having various chromium concentrations 9.9%;13.2%;14%;16% and 20%, at identical size of the particles 45 m and the same content of the remaining elements boron and silicon. The coatings have been tested in identical regime of erosion using air stream, carrying abrasive particles under the same conditions - nature, sizes and velocity of the particles, duration of erosion, air pressure, distance between the nozzle and the surface, angle of the jet axis with respect to the surface. It has been shown that the angle of the jet stream influences the wear resistance of all the coatings: at jet angle 600 the wear resistance grows up within the in-terval from 1.5 up to 2.6 times for the coatings without thermal treatment of the substrate and within the interval from 1.6 up to 3.5 times for the coatings under-going thermal treatment of the substrate in comparison with the wear resistance at angle 900. This effect is strongest -3.5 times for the coating No 8 of 16%Cr. At jet angle 900 the wear resistance increases up to 10% and this increase is one and the same for all the coatings. The found relationship between erosive wear resistance and Cr concentration exhibits a considerable non-linear character. Wear resistance increases as the Cr concentration increases, reaching a maximum for 16 percent Cr coatings with thermal treatment of the substrate at jet angle of 60°. It has been found out that the wear resistance of the coatings, having the highest Cr content -20%, but not containing the elements boron and silicon, is lower or almost equal to the wear resistance of the coatings having the lowest Cr concentration -9.9%.