Tapio Mäntylä

Friction and adhesion of stainless steel strip against tool steels in unlubricated sliding with high contact load

The strong tendency of stainless steel to adhere to other metals and to work harden makes it particularly difficult to form. A new slider-on-strip tribometer was used to study the tribological behaviour of traditional and new tool materials. The tribometer allows friction, workpiece adhesion on the tool surface and wear to be studied under conditions with low sliding speeds and high contact pressures. Several tool steels were tested using cold rolled stainless steel strip as a workpiece material. The results showed that the composition of the tool steel does not have a marked effect on the friction between the tool and the workpiece. However, the surface roughness and topography of the tool have a marked effect. Polishing of the tool surface to reduce the surface roughness reduces the friction between the tool and the workpiece.

Rolling contact fatigue failure mechanisms in plasma and HVOF sprayed WC-Co coatings

Abstract The rolling contact fatigue (RCF) behaviour of thermally sprayed WC-Co coatings with nominal compositions of WC-12%Co, WC-10%Co-4%Cr and WC-17%Co was studied with a two-roll configuration roll-against-roll testing apparatus under 420–600 MPa Hertzian contact stresses in unlubricated pure rolling conditions. The coatings were prepared by atmospheric plasma spray (APS) and two high-velocity oxyfuel (HVOF) spray processes. In the APS sprayed WC-12%Co coating the RCF damage was dominated by an increased surface roughness due to spallation of flakes and a formation of a network of cracks within the coating layer. HVOF sprayed WC-12%Co and WC-10%Co-4%Cr coatings were damaged either by the formation of vertical, linear cracks or pitting of the contact surface. The formation of pits in the HVOF sprayed coatings was significantly less than that found in the APS sprayed coatings. The HVOF sprayed WC-17%Co coating showed the best RCF behaviour among the studied coatings with unchanged surface roughness, no formation of cracks and only a few pits were found on the contact surface. The good resistance of this coating against formation of failure in the RCF testing is caused by its higher ductility and fracture toughness due to a higher metallic binder content in comparison with the other coatings. Other characteristics such as a low amount of brittle Co-W-C carbides and a dense microstructure are also believed to be beneficial for a RCF resistant coating.

Comparison of the Semiconductive Properties of Sputter‐Deposited Iron Oxides with the Passive Film on Iron

The semiconducting properties of sputtered magnetite (Fe{sub 3}O{sub 4}) and oxidized magnetite layers with different Fe(II) contents were compared with the passive film on iron. Electrochemical impedance spectroscopy and photoelectrochemical experiments were carried out in borate buffer, pH 8.4. An evaluation of the impedance data according to the Mott-Schottky concept showed that the capacitance of all films is linked with their doping concentrations rather than their thicknesses and that the doping species of the passive film on iron is Fe(II). For the passive film a potential-dependent doping concentration was found. Photoelectrochemical investigation of the passive film and the sputtered oxide layers showed that for low doping concentrations, the photocurrent increases with doping, whereas for high doping concentrations, an increase in doping leads to a decrease of the photocurrent. Possible causes for this effect are discussed. Further, the combination of a light reflectance technique with photocurrent measurements allowed consideration of light absorption effects in the data treatment and separation of photocurrent contributions from the space-charge layer from film-thickness effects.

Environmental effects on microstructure and strength of SiC-based hot gas filters

The aggressive process environment in advanced coal-fired power generation systems causes microstructural changes in the ceramic hot gas filters used to clean the fuel gas. Changes in microstructure and their effect on strength were studied for commercial SiC-based clay bonded hot gas filters exposed to high temperature, water vapour and gaseous sodium compound. Exposures caused significant crystallization of the binder phase and oxidation of SiC. Loss in strength also occurred. The microstructural changes and their effect on strength are discussed.

Improved sealing treatments for thick thermal barrier coatings

Abstract Zirconia-based 8Y 2 O 3 –ZrO 2 , 22MgO–ZrO 2 and 25CeO 2 –2.5Y 2 O 3 –ZrO 2 thick thermal barrier coatings (1000 μm) were studied with different sealing methods for diesel engine and gas turbine applications. The aim of the sealing procedure was to improve the hot corrosion-resistance and mechanical properties of porous, thick thermal barrier coatings (TTBC). The surface of the coatings was sealed with three different methods: (1) laser glazing; (2) an aluminium phosphate sealing treatment; and (3) detonation gun spraying of a dense top coating on the TTBC. Sealant penetration into the coating and the coating microstructure were determined by scanning electron microscopy/energy-dispersive spectrometry (SEM/EDS) and optical microscopy. Coatings were characterised by X-ray diffraction (XRD), microhardness and porosity measurements. The thickness of the densified top layers in all cases was 50–400 μm. XRD analysis showed some minor phase changes and reaction products caused by the phosphate-based sealing treatment and some crystal orientation and phase changes in laser-glazed coatings. The porosity of the outer layer of the sealed coating decreased in all cases, which led to increased microhardness values.

High temperature corrosion of coatings and boiler steels in reducing chlorine-containing atmosphere

Abstract Unacceptably high corrosion rates are often experienced, when chlorine-containing fuels are combusted. Reducing conditions that may occur in various boilers accelerate corrosion even further. Protective oxide scales are not formed on low-alloy steels if partial pressure of oxygen is too low. Materials rich in oxide formers, such as chromium and aluminum, are needed to resist corrosion in reducing combustion atmospheres, but processibility of such bulk alloys is very limited. Various coating technologies are considered as potential solution for corrosion problems in high temperature combustion environments with low partial pressure of oxygen. High temperature corrosion tests were performed on one ferritic boiler steel, one austenitic boiler steel, five high velocity oxy-fuel (HVOF) coatings, one laser-melted HVOF coating, and one diffusion chromized steel. Synthetic atmosphere simulating reducing conditions in combustion of chlorine-containing fuels was created for the tests. The test atmosphere contained 500 ppm HCl, 600 ppm H 2 S, 20% H 2 O, 5% CO, and Ar as a balance. The test temperature was 550 °C and the test duration was 1000 h. Corrosion resistance of steels and homogeneous coatings was mainly determined by chromium content. Homogeneous and dense coatings with high chromium content performed well and were able to protect the substrate. Some of the HVOF coatings were attacked by corrosive species through interconnected network of voids and oxides at splat boundaries.

Electrostatic and electrosteric stabilization of aqueous slips of 3Y-ZrO2 powder

Abstract Yttria-doped zirconia powder (3Y-ZrO2) was characterized and dispersed in distilled water. The state of dispersion was evaluated in terms of zeta potential, apparent viscosity and the mean particle size of solid phase in the slip. Zeta potential, apparent viscosity and the mean particle size as a function of pH indicated the pH range of electrostatic stabilization. These results showed that electrostatic stabilization of the slip can be accomplished in low acidic and high basic range of pH. Dissolution of yttria from the powder surface in acidic pH was found to be high and fast, risky to the stability of the slip as well as the microstructure of the sintered body. Electrosteric stabilisation by addition of an anionic polyelectrolyte (PMAA-NH3) shifted the isoelectric point (IEP) to lower pH. The state of dispersion was further investigated by particle size measurements of the solid phase in the slip. The optimum amount of dispersant is discussed in terms of zeta potential and viscosity. The adsorption of polyelectrolyte is considered at pH 4, native pH of suspension and at pH 10, below and above the IEP of powder and correlated with the dissociation rate of polymer and the net surface charge of particles at given pH. Higher adsorption of dissociated polymer to the positively charged surfaces justifies the higher optimum amount of polyelectrolyte at pH 4.

Modified thick thermal barrier coatings: Microstructural characterization

Thick thermal barrier coatings were modified with laser glazing and phosphate based sealing treatments. Surface porosityof the sealed coatings decreased significantly in all cases. Structural analysis showed a strong preferred crystal orientation of the t 0 ZrO2 phase in direction [002] in laser-glazed 25CeO2–2.5Y2O3–ZrO2 coating. In laser-glazed 22MgO–ZrO2 coating the major phase was rhombohedral Mg2Zr5O12. In phosphate sealed 8Y2O3–ZrO2 coating the strengthening mechanism was identified as adhesive binding without chemical bonding. Coating microstructures were determined byscanning electron microscopy , energydispersive spectroscopy, transmission electron microscopy and optical microscopy. Coatings were also characterized by X-ray diffraction, microhardness and porosity. # 2003 Elsevier Ltd. All rights reserved.

Aluminum phosphate sealed alumina coating: characterization of microstructure

Abstract The microstructure of aluminum phosphate sealed plasma-sprayed alumina coating was characterized by X-ray diffractometry, scanning electron microscopy, and analytical transmission electron microscopy. Microstructural characterization was carried out to identify the phases of the coating and to understand better the strengthening effect of aluminum phosphate sealant in the coating. The main phases in the coating are metastable γ-Al 2 O 3 and stable α-Al 2 O 3 . The overall structure of the coating is lamellar with columnar γ-Al 2 O 3 grains. The aluminum phosphate sealant shows good penetration into the coating to the depth of about 300 μm filling the structural defects such as pores, cracks and gaps between the lamellae. The sealant in the coating has the relative composition of 26 at.% aluminum and 74 at.% phosphorus giving the molar ratio P:Al of 3, which refers to the metaphosphates Al(PO 3 ) 3 . There is also some crystalline aluminum phosphate in the coating, in the form of berlinite-type orthophosphate AlPO 4 , owing to the reaction between the sealant and the alumina coating. Thus, the phosphate bonding in the alumina coating is based both on chemical bonding resulting from the chemical reaction with the alumina coating and on adhesive binding resulting from the formation of the condensed phosphates in the structural defects of the coating.

Elevated temperature erosion-corrosion of thermal sprayed coatings in chlorine containing environments

Abstract A series of hot erosion and erosion–corrosion (E–C) tests was carried out on thermal sprayed coatings, diffusion coatings and boiler steels using a burner-rig type elevated temperature E–C tester in order to evaluate the possibility to utilise thermal sprayed coatings in shielding of boiler components. Test conditions simulated the E–C conditions in the superheater section of a circulating fluidised bed combustor (CFBC). Carbide containing HVOF coatings performed well in erosion tests, as expected. Also diffusion coatings and nickel-based, high-chromium HVOF coatings performed well. In E–C tests in presence of chlorine, nickel-based HVOF coatings performed the best, whereas carbide containing HVOF coatings and diffusion coatings wore away.