Effect of surface conditions on internal oxidation and nitridation of HVOF MCrAlY coatings

This study deals with the isothermal oxidation behaviour of high velocity oxy-fuel sprayed MCrAlY coatings. Both free-standing coatings and coatings attached to IN738 and CMSX4 substrates undergo isothermal oxidation at 1100°C for up to 100h. The effect of surface conditions (as-sprayed and polished) has been investigated. Scanning electron microscopy and energy dispersive X-ray analysis were used to characterise the details of oxidation behaviour. The results have shown that both surface conditions and substrate type have a pronounced effect on oxidation behaviour. Extensive internal oxidation and nitridation is observed for polished coatings on Inconel 738 and is attributed to the combined effects of titanium diffusion from the substrate and enhanced surface diffusion due to polishing

A study of wire breakup and in-flight particle behaviour during wire flame spraying of aluminium

Although wire flame spraying has been used for many years, there has been relatively little attention given to understanding the process dynamics. In this work, imaging of the molten wire tip, particle imaging using the Oseir Spray Watch system and particle capture (wipe tests) have all been employed to quantify plume behaviour. Aluminium wire feedstock is melted and then breaks-up close to the exit of the spray nozzle in a non-axisymmetric manor. The mean velocity and diameter of the particles detected by the Spray Watch system change little with stand-off distance with values of approximately 280 m/s and 70 μm respectively for the spray parameters employed. The particle diagnostic system could not detect particles ⪅ 45 μm in diameter and it is estimated that these account for no more than 53 % of the sprayed material. Overall, wire flame spraying generates a surprisingly stable particle stream

influence of microstructure on the ductile to brittle transition and fracture behaviour of HVOF NiCoCrAlY coatings determined via small punch tensile testing

The development of new thermal barrier coatings (TBC’s) capable of increasing the efficiency of gas-powered turbines requires an understanding of how the tensile behavior and ductile to brittle transition temperature (DBTT) of MCrAlY bond coats are influenced by the coating microstructure. High velocity oxy-fuel (HVOF) thermal spraying was used to manufacture free-standing coatings from two NiCoCrAlY alloys that have potential as new bond coat alloys, referred to as coatings BC1 and BC2. The coatings were vacuum heat treated at 1100 °C for 2 hours and characterised using X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron back-scatter diffraction (EBSD). Both coatings comprised a BCC β-NiAl matrix with FCC γ-Ni and TCP σ-Cr2Co as secondary phases. Coating BC2 also contained FCC γ’-Ni3Al. Small punch tensile (SPT) tests were conducted on the coatings between RT and 750 °C. The DBTT’s of coatings BC1 and BC2 were found to be 600-700 °C and 650-750 °C respectively. Lower phase fractions of γ-Ni were shown to increase the DBTT. The main mode of crack propagation in both coatings was intergranular fracture along the grain boundaries of differing phases. The influence of coating microstructure on the fracture behavior of both coatings was discussed using schematic models

X-ray photoelectron spectroscopy study of the passive films formed on thermally sprayed and wrought Inconel 625

There is a well known performance gap in corrosion resistance between thermally sprayed corrosion resistant coatings and the equivalent bulk materials. Interconnected porosity has an important and well known effect, however there are additional relevant microstructural effects. Previous work has shown that a compositional difference exists between the regions of resolidified and non-melted material that exist in the as-sprayed coatings. The resolidified regions are depleted in oxide forming elements due to formation of oxides during coating deposition. Formation of galvanic cells between these different regions is believed to decrease the corrosion resistance of the coating. In order to increase understanding of the details of this effect, this work uses X-ray photoelectron spectroscopy (XPS) to study the passive films formed on thermally sprayed coatings (HVOF) and bulk Inconel 625, a commercially available corrosion resistant Ni-Cr-Mo-Nb alloy. Passive films produced by potentiodynamic scanning to 400mV in 0.5M sulphuric acid were compared with air formed films. The poorer corrosion performance of the thermally sprayed coatings was attributed to Ni(OH)2, which forms a loose, non-adherent and therefore non-protective film. The good corrosion resistance of wrought Inconel 625 is due to formation of Cr, Mo and Nb oxides. Keywords

Corrosion behaviour of crystalline and amorphous forms of the glass forming alloy Fe43Cr16Mo16C15B10

The corrosion behaviour of both crystalline and largely amorphous forms of the Fe-based glass forming alloy, Fe43Cr16Mo16C15B10 alloy was investigated. Two different methods were used to induce transformation to the amorphous form of the alloy: laser melting and HVOF spraying. Both methods produced largely amorphous material, however the high brittleness of the alloy makes it susceptible to cracking during laser treatment, hence this technique is not suitable for largescale application. Potentiodynamic scanning showed that in 0.5M H2SO4 and 3.5% NaCl electrolytes both amorphous forms of the alloy had better corrosion resistance (lower current densities for -200 to +1000mV SCE) compared to the crystalline material. The laser treated material and HVOF coating performed similarly in 3.5% NaCl. In 0.5M H2SO4 the HVOF coating had a lower current density than the laser melted material for almost all of the potential range -300 to +1000mV SCE. The improved corrosion behaviour of the largely amorphous material is attributed to its homogeneity, and particularly to the elimination of the Mo-rich phase that underwent preferential corrosion in the crystalline form of the material

The application of the small punch tensile test to evaluate the ductile to brittle transition of a thermally sprayed CoNiCrAlY coating

Thermally sprayed MCrAlY bond coats are important elements of thermal barrier coating (TBC) systems which are applied to the surface of gas turbine components to protect them in high temperature environments. Knowledge of their mechanical properties is essential in preventing TBC failure which can have catastrophic consequences. However, limited data on modulus, strength and ductility are available for such coatings. In this work, the ductile to brittle transition behaviour of a CoNiCrAlY coating has been investigated via the small punch tensile test (SPTT). Displacement controlled tests were carried out on free standing coatings at room temperature (RT) and between 400-750 °C at a rate of 1 μms-1. At low temperatures there was evidence of elastic-brittle behaviour and at high temperatures there was clear evidence of yielding and plastic deformation. The ductile to brittle transition temperature was found to be between 500-750 °C. The yield stress ranged from 1000-1500 MPa below 600 °C to less than 500 MPa above 650 °C. The elastic modulus was found to be approximately 200-230 GPa at 500 °C and 55 GPa above 700 °C. At room temperature the fracture surface showed flat, smooth features indicating brittle failure whereas at 700 °C there was evidence of ductile tearing

Sliding wear of a self-mated thermally sprayed chromium oxide coating in a simulated PWR water environment

Bearing surfaces in the primary circuit of pressurized water reactors (PWR) are prone to damage due to aggressive chemical and tribological conditions under which they operate, and a wide range of materials have been examined in this regard. One of the most promising candidates is chromium oxide in the form of a thermally spayed coating, and in this work, the behaviour of a commercially available Cr2O3 coating in self-mated sliding was considered. Tests consisted of a number of start-stop cycles of sliding between a crowned pin and a rotating disc in a water environment in an autoclave in an attempt to simulate the most aggressive phase of bearing run-up and run-down. Wear and damage mechanisms were examined at temperatures from ambient up to 250 C (a representative PWR environment). Samples were characterized before and after wear testing using mass measurements, profilometry, X-ray diffraction, scanning and transmission electron microscopy (SEM and TEM) and X-ray photoelectron spectroscopy (XPS). Across the temperature range, wear was mild, with no evidence of coating delamination. A five-fold increase in wear was observed between 80 C and 250 C (with wear depths of generally less than 8 µm being observed on the disc samples even at the higher temperature), despite there being only very small changes in hardness of the coating over the same temperature range. Debris was observed on the wear tracks following testing, with the evidence together suggesting that this debris was a very fine-grained mixture of Cr2O3 and amorphous -CrOOH, a corrosion product of Cr2O3

The high temperature creep properties of a thermally sprayed CoNiCrAlY coating via small punch creep testing

Thermal barrier coatings (TBC’s) protect superalloy components from excessively high temperatures in gas turbines. TBC’s comprise of a ceramic top coat, a metallic bond coat and a thermally grown oxide (TGO). The creep behaviour of the MCrAlY bond coat, which is sensitive to the composition and the method of deposition, has a significant effect on the life of the TBC. High velocity oxy-fuel (HVOF) thermal spraying is a popular deposition method for MCrAlY bond coats however the creep properties of HVOF MCrAlY coatings are not well documented. The creep behaviour of a HVOF thermally sprayed CoNiCrAlY coating has been determined by small punch creep (SPC) testing. Tests were conducted between an equivalent uniaxial stress range of 37-80 MPa at 750 °C on two different SPC rigs and between 30-49 MPa at 850 °C on a single SPC rig. The measured steady-state creep deformation rates at 750 °C were consistent across the two rigs. A comparison with previous work demonstrated that the creep behaviour of HVOF CoNiCrAlY coatings is not sensitive to the manufacturing variability associated with HVOF thermal spraying. The CoNiCrAlY coating exhibited typical SPC deformation at 750 °C. At 850 °C the CoNiCrAlY coating showed significantly different creep behaviour which could be attributed to the onset of superplasticity

An evaluation of the capability of data conversion of impression creep test

High temperature power plant components are now working far beyond their operative designed life. Establishing their in-service material properties has become a matter of significant concern for power generation companies. Advantages for the assessment of creep material properties may come from miniature specimen creep testing techniques, like impression creep testing method, which can be treated as a quasistatic non-destructive technique and requires a small volume of material that can be scooped from in-service critical components, and can produce reliable secondary creep data. This paper presents an overview of impression creep testing method to highlight the capability in determining the minimum creep strain rate data by use of conversion relationships that relates uniaxial creep test data and impression creep test data. Stepped-load and stepped-temperature impression creep tests are also briefly described. Furthermore, the paper presents some new impression creep test data and their correlation with uniaxial data, obtained from P91, P92 and ½CrMoV steels at different stresses and temperatures. The presented data, in terms of creep strain rate against the reference uniaxial stress, are useful for calibration of impression creep testing technique and provide further comparative results for the evaluation of the reliability of the method in determining secondary creep properties

Investigation into the effect of beam shape on melt pool characteristics using analytical modelling

An established analytical model is used to simulate an extended laser beam. Multiple Gaussian sources are superimposed to form a rectangular beam and results are compared with a single circular Gaussian source model as well as experimental results from a high power diode laser with a rectangular beam. Melt depth, and melt pool profile and progression have been predicted by modelling which are compared with experimental results from melting of Inconel 625. The model produced is shown to give a reasonable prediction of melt pool shape and can be usefully employed to help optimise overlap required for laser surface processing applications. The value of absorptivity used in the model can be used as a fitting parameter to optimise the match between experimental and predicted results