Shogo Tobe

Peening action and residual stresses in high-velocity oxygen fuel thermal spraying of 316L stainless steel

Abstract316L stainless steel powder was sprayed by a high-pressure high-velocity oxygen fuel (HVOF) process. Effects of powder size and the pressure in the combustion chamber on the velocity and temperature of sprayed particles were studied by using an optical instrument, first, at the substrate position. A strong negative correlation between the particle temperature and the diameter was found, whereas the correlation between the velocity and the diameter was not significant. The pressure in the combustion chamber affected the velocity of sprayed particles significantly, whereas the particle temperature remained largely unchanged. In-situ curvature measurement was employed in order to study the process of stress generation during HVOF spraying. From the measured curvature changes, the intensity of peening action and the resultant compressive stress by HVOF sprayed particles were found to increase with the kinetic energy of the sprayed particles. The results were further used to estimate the stress distribution within the coatings. X-ray stress measurement revealed that the residual stress on the surface of the HVOF coatings is low and often in tension, but the stress inside the coatings is in a high level of compression.

The effect of spraying distance on wear resistance of molybdenum coatings

Abstract Thermally sprayed molybdenum coating is one of the most wear resistance coatings and, therefore, widely used in many practical applications. The paper reports a significant dependency of wear resistance of molybdenum coating on the spraying distance. Also, the influence of test configuration and conditions on the wear performance ranking of molybdenum coatings is outlined. In this study a plasma spraying process was used to deposit molybdenum coating from four different distances. Wear resistance of the coatings was assessed under a number of test condditions and with the help of three different test configurations.

Surface fatigue of molybdenum and Al-bronze coatings in unlubricated rolling/sliding contact

Surface fatigue phenomenon of Mo and Al-bronze coatings was investigated by using two-disc machine under various loadings. Life to flaking and life to complete surface failure of two different types of coatings on low carbon steel substrates were determined in rolling line contact with hardened steel disc as the counter surface. The experimental findings revealed that Mo coatings had superior life to surface failure when compared to Al-bronze coatings under pure rolling and rolling/sliding contacts. The life to flaking decreased for Al-bronze particularly with an increase in the load. An increase in the slip ratio had a negative effect on the Al-bronze coatings giving rise to a very severe wear and sudden surface failures. Optical microscopy and scanning electron microscopy studies also indicated that Mo coatings were subjected to relatively mild wear as compared to those of Al-bronze coatings.

Subsurface crack investigation on delaminated ceramic elements

Abstract This paper presents an experimental investigation of subsurface cracks on delamination fatigue failure of ceramic rolling elements. Three failure cases are considered on hot isostatically pressed silicon nitride surfaces. These cases consist of two failures from ceramic/ceramic contact and one from pre-cracked ceramic/bearing steel contact. The extent of subsurface crack length and depth is investigated using a dye penetration method and acoustic microscopy. Quantitative measurements of subsurface crack shapes, depths and lenghts are presented. The information from this study relates to the fatigue failure modes.

Corrosion resistance of thermal sprayed titanium coatings in chloride solution

The corrosion behavior of a resin-sealed flame-sprayed titanium coating in 3.5% NaCl solution was investigated by electrochemical polarization measurements. The composition and structure of the sprayed film was also analyzed by scanning electron microscopy (SEM) and electron probe x-ray microanalysis (EPMA). Although an as-sprayed titanium coating exhibited no resistance to corrosion because of its porosity, the sprayed titanium sealed with epoxy or silicon resin showed an excellent resistivity with respect to chloride corrosion. Although almost half of the titanium changed to oxides, nitrides, and carbides through the wire flame spraying, the conversion of the metal to those compounds had little effect on decreasing the corrosion resistivity. The sprayed and sealed titanium coating obtained by conventional on-site thermal spraying is expected to be an economical material for chloride containing environments.

Effect of Pressure on Surface Roughness Treated by Cathode Spots of Low Pressure Arc

Cathode spots of a low-pressure arc can remove oxide layers and evaporate impurities on metal surfaces. Removal of the oxide layer using cathode spots is expected to solve recent obstacles to chemical and mechanical cleaning methods. Various phenomena of cathode spots have been investigated for pre-treatment of atmospheric pressure plasma spray (APPS). This study addressed the relation of pressure and initial surface roughness treated by cathode spots of low pressure arc. The test pieces (SS 400) are compared with the ones treated by cathode spots and mechanically blasted surfaces. Results show that the roughness on the sample surfaces is most suitable at 1000 Pa of atmospheric pressure. This result can be applied for the pre-treatment of plasma spray

Surface fatigue of polymers in rolling contact

Results of the study into the forms of surface failure of cross-linked polystyrene, polycarbonate and polymethylmethacrylate resulting from rolling contact are presented. Also the influence of two different lubricating liquids on the time to failure is quantitatively assessed and discussed. Under the contact stress ranged from 1.5 to 2.6 GPa it was found that in case of certain polymers the surface failure is of similar type and morphology to that observed for some ferrous materials. The type and chemical nature of a lubricating liquid both have a significant influence on the number of load cycles to failure.

Residual stress field of HIPed silicon nitride rolling elements

Abstract The residual stress field of HIPed Si 3 N 4 rolling elements were studied. Two kinds of HIPed Si 3 N 4 ball blanks self-finished at different nominal lapping loads ranging from 1.3 to 10.87 kgf/ball and four kinds of commercially finished 1/2 in (12.7 mm) HIPed Si 3 N 4 balls before, during and after RCF tests were investigated. The experimental results showed that in the finishing process of HIPed Si 3 N 4 rolling elements, the surface and subsurface compressive residual stress induced is proportional to the lapping load applied. There was initially a high compressive residual stress layer on the HIPed Si 3 N 4 ball blanks and this layer is mostly removed during the finishing process. During the rolling contact fatigue process of HIPed Si 3 N 4 rolling elements, the residual stresses on the rolling track will change dramatically as RCF proceeds.

Resistance of thermal-sprayed duplex coating composed of aluminum and 80Ni-20Cr alloy against aqueous corrosion

The development of corrosion-resistant sprayed coatings without sealing is required to increase the reliability of the thermal spray coating method and to expand the field of application for wet corrosion environments. The conventional wire flame-sprayed aluminum coating on steel without sealing has poor resistance against aqueous corrosion and has restricted practical use. A duplex coating composed of sprayed aluminum on an 80Ni-20Cr alloy undercoat exhibited sufficient resistance in a hot, near-neutral aqueous environment through a trial use in a vegetable oil process. In this paper, the mechanism of corrosion resistance of the duplex coating is investigated by electrochemical polarization measurements and electron probe microchemical analysis (EPMA) to examine the individual role of each layer and the change of the microstructure with time.

Friction and wear of polytetrafluoroethylene reservoirs embedded into metallurgical coatings

Abstract Assessment of tribological performance of polytetrafluoroethylene (PTFE) reservoirs embedded into two metallurgical coatings [copper-aluminium (Cu-Al) alloy and molybdenum] and arranged in three different patterns was the main aim of studies reported in this paper. PTFE reservoirs were arranged in such a way that the load on the contact, at any given moment, was supported by both PTFE and hard coating. Two different contact configurations were used. Of the two coating materials used, the Cu-Al performed better in terms of friction force magnitude. Performance of the three different shapes of PTFE reservoirs depends on both the coating material and the contact configuration.