Petri Vuoristo

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.

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.

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.

The 2016 Thermal Spray Roadmap

Considerable progress has been made over the last decades in thermal spray technologies, practices and applications. However, like other technologies, they have to continuously evolve to meet new problems and market requirements. This article aims to identify the current challenges limiting the evolution of these technologies and to propose research directions and priorities to meet these challenges. It was prepared on the basis of a collection of short articles written by experts in thermal spray who were asked to present a snapshot of the current state of their specific field, give their views on current challenges faced by the field and provide some guidance as to the R&D required to meet these challenges. The article is divided in three sections that deal with the emerging thermal spray processes, coating properties and function, and biomedical, electronic, aerospace and energy generation applications.

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.

Artificial Cr- and Fe-oxide passive layers prepared by sputter deposition

The electrochemical behavior of synthetic Cr- and Fe-oxide films with well-known chemical composition and structure, prepared by sputter deposition, has been studied. Photoelectrochemistry and Mott-Schottky analyses show that the parameters during the sputter deposition process influence the stoichiometry of Cr- and Fe-oxides. The bandgap energy of Cr-oxides is independent of the solution pH, thus the values of the bandgap energy lower than those found for ideal Cr{sub 2}O{sub 3} are due mainly to nonstoichiometry. The defect structure influences the electrochemical reactivity of the Cr-oxide. The photocurrent behavior of Fe{sup 3+}-oxides is strongly determined by the presence of Fe{sup 2+} in the oxide, which can be introduced by a partial electrochemical reduction of the Fe-oxides. Chloride ions did not show an effect on the electrochemical reactivity of stoichiometric Fe{sub 2}O{sub 3}. Mott-Schottky analysis shows changes in the capacitance behavior if chloride is added to the solution; this suggests that a specific adsorption of chloride takes place on the surface. A comparison of the stability of the synthetic oxides and natural passive films may lead to new insights into the role of stoichiometry and defects on the breakdown of passive films.

Novel method for in-flight particle temperature and velocity measurements in plasma spraying using a single CCD camera

A novel, technically simple imaging system for individual, in-flight particle temperature and velocity measurements for plasma and other thermal spray processes is described. A custom double dichroic mirror is used to add spectral resolving capability to a single, black-and-white, fast-shutter digital charge coupled device (CCD) camera. The spectral double images produced by the individual in-flight particles are processed using specialized image processing algorithms. Particle temperature determination is based on two-color pyrometry, and particle velocities are measured from the length of the particle traces during known exposure times. In this paper, experimental results using the first prototype system are presented. Laboratory tests were performed using rotating pinholes to simulate in-flight particles, and plasma spraying experiments were performed with commercial, standard spraying equipment operated with Al2O3 and NiCrAlY powders. The prototype instrument can be readily used to determine velocity and temperature distributions of individual in-flight particles from the imaged region of interest of the plume. Dividing the imaged area into smaller sections, spatial distributions of particle temperature, velocity, and number of detected particles can be studied. The study aims to develop a technically simple, single imaging instrument, which can provide a visual overview of the spray plume in combination with quantitative evaluation of the most important spray particle parameters.