プラズマジェットによる溶射の研究(第1報)

予熱シールド溶射法とNi-Cr-B-Si合金の溶射皮膜の性質について

抄録

Recently, plasma spraying has been applied to various purposes in the industrial field. Plasma process is applicable to various materials which have been considered impossible to spray by oxy-acetylene flame spraying. However, properties of coating made by ordinary plasma process are not always so good because of porosity or weak bonding between the substrate and the coating.
The authors have developed a plasma spraying method which is characterized by the following:
1. Application of shield nozzle to insure sufficient protection of substrate and spraying material from the chemical attack of atmospheric gases.
2. Effective and local preheating of the substrate to promote the metallurgical bonding between the coating and the substrate.
3. Selection of proper size of particle to secure their perfect melting.
4. Determination of the stand-off distance of torch from the substrate to assure the proper flying velocity of particle.
This new method was applied to the coating of mild steel with the Ni-Cr-B-Si wear resistant alloy using the plasma spraying torch and shield nozzle as shown in Fig. 1.
Various examinations of coating were carried out with the following successfull results;
(1) Sprayed layer has a dense and fine grained structure, and a defect is hardly seen. Grain growth and carburizing of substrate, which are observed in sprayed and remelted one, are not observed.
(2) Hardness of coating is in range of VHN 850-1100, and is much higher than that of remelted one.
(3) Density of coating is as high as 99.2% of cast one, and the porosity is less than 1%.
(4) X-ray diffraction analysis of coating has shown that detectable change of alloy did not occured during the spraying.
(5) Adhesive strength between the coating and the substrate was more than 40 kg/mm² as a result of metallurgical bonding between them.
(6) Sand wear tests of shield sprayed specimen show that the average weight loss was about 1/2-2/3 of remelted one.