This paper examines and compares the microstructure and oxidation behaviour of CoNiCrAlY coatings manufactured by the APS, HVOF and CGDS deposition techniques. The coatings microstructural features were characterized by means of SEM and XRD analyses. Coating samples were then subjected to isothermal heat treatments at 1000 °C. Oxide growth rates were obtained from a series of mass gain measurements while oxide scale compositions were determined from SEM, XRD and EDS analyses. Results obtained in this study show that the as-sprayed CGDS and HVOF coatings exhibit similar microstructures, whereas the APS coating features high levels of visible defects and oxide content. Oxidation experiments revealed low oxide growth rates for both the CGDS and HVOF coatings as a result of low porosity and oxide content. The oxide scale on the CGDS and HVOF coatings after 100 h of oxidation were composed mainly of alumina without the presence of detrimental fast-growing mixed oxides. The presence of Cr 2 O 3 and dispersed NiO was however also observed for the HVOF coating. As expected, the APS coating featured the onset of mixed oxides in the early stages of oxidation. From these results, it appears that potential improvements to the bond coat oxidation behaviour can be achieved using low-temperature processing methods such as CGDS.

Oxidation behaviour of CoNiCrAlY bond coats produced by plasma, HVOF and cold gas dynamic spraying

The growth of thermally grown oxide (TGO) layers and their influence on crack formation were studied for two thermal barrier coating (TBC) systems with CoNiCrAlY bond coats produced by (i) air plasma spray (APS) and (ii) high-velocity oxy-fuel (HVOF) techniques. All samples received a vacuum heat treatment and were subsequently subjected to thermal cycling in air. The TGOs were predominantly comprised of layered alumina, along with some oxide clusters of chromia, spinel and nickel oxide. However, after extended oxidation, the alumina layer formed in the APS-CoNiCrAlY bond coat transformed to chromia/spinel, while that formed in the HVOF-CoNiCrAlY bond coat remained stable. TGO thickening in the APS-CoNiCrAlY bond coat generally exhibited a three-stage growth behavior, which resembles a high temperature creep curve, whereas growth of the alumina layer in the HVOF-CoNiCrAlY bond coat showed an extended steady-state stage. Crack propagation in these two TBCs was found to be related to the growth and coalescence of oxide-induced cracking, connecting with pre-existing discontinuities in the topcoat. Hence, crack propagation during thermal cycling appeared to be controlled by TGO growth.

TGO growth behaviour in TBCs with APS and HVOF bond coats

Detailed damage analyses of a plasma sprayed ZrO 2 /8 wt–% Y 2 O 3 –MCrAlY–CMSX-4 TBC system during isothermal and cyclic oxidation tests with different dwell times at high temperature have been performed The resulting failure mode, ie the particular delamination crack path, is strongly dependent on the temperature cycle applied Isothermal exposure promotes crack propagation within the TGO, whereas thermal cycling shifts the crack path towards the TBC Thermal cycling with dwell time at high temperature leads to a mixed delamination crack path (partly within TBC and TGO) The respective correlation between TBC lifetimes and duration of high temperature dwell time per cycle (cycle frequency) is shown and discussed

Damage mechanisms and lifetime behavior of plasma-sprayed thermal barrier coating systems for gas turbines — Part II: Modeling

https://www.research.manchester.ac.uk/portal/files/76397348/A_18_1526R1.pdf

Effect of microstructure on early oxidation of MCrAlY coatings

In this work High Velocity Oxy-fuel (HVOF) thermal spray techniques, spraying process optimization, and characterization of coatings are reviewed. Different variants of the technology are described and the main differences in spray conditions in terms of particle kinetics and thermal energy are rationalized. Methods and tools for controlling the spray process are presented as well as their use in optimizing the coating process. It will be shown how the differences from the starting powder to the final coating formation affect the coating microstructure and performance. Typical properties of HVOF sprayed coatings and coating performance is described. Also development of testing methods used for the evaluation of coating properties and current status of standardization is presented. Short discussion of typical applications is done.

https://www.mdpi.com/2079-6412/1/1/17/pdf

Optimization and Characterization of High Velocity Oxy-fuel Sprayed Coatings: Techniques, Materials, and Applications

Turbine blades are protected against high temperature oxidation by thermal barrier coating (TBC) systems, which consist of a ceramic top coating (ZrO 2 /Y 2 O 3 ) and a metal bond coating (MCrAlY, M  Ni, Co). At high temperatures and under oxidative conditions, between the MCrAlY and the ceramic top coating an oxide scale is formed, which protects the metal against further oxidation. The oxidation behaviour of the thermally sprayed MCrAlY is influenced by the coating process and the composition of the metal alloys. This work is concerned with the isothermal oxidation behaviour of vacuum plasma sprayed (VPS) MCrAlY coatings. The MCrAlY powders used have different aluminium contents: 8 and 12 wt.%. The MCrAlY specimens are oxidized at 1050 °C in air as well as in helium with 1% O 2 and the oxidation kinetics are determined thermogravimetrically. The microstructure, morphology and thickness of the oxide scales formed are characterized by metallography, SEM, TEM and XRD. After short time oxidation (6 h) θ-Al 2 O 3 is the main constituent of the oxide scale. Exposure times of 500 h and more lead to oxide scales consisting of α-Al 2 O 3 . Moreover, after a long time oxidation, Cr 2 O 3 and CoO (CoO on the coatings with 8 wt.% Al) are formed. The oxidation rates of both MCrAlY coatings are the same. Beneath the oxide scale an Al-depleted zone is formed and this zone is considerably thicker with the coating with 8 wt.% Al, because the amount of β-NiAl phase in this coating is lower than that in the coating with 12 wt.% Al. The oxide scale formed in He—1% O 2 consists of α-Al 2 O 3 and Cr 2 O 3 on both MCrAlY coatings.

The oxidation behaviour of sprayed MCrAlY coatings

A new high velocity oxygen fuel (HVOF) spraying process to produce MCrAlY coatings was developed and optimized. The HVOF sprayed MCrAlY coatings were isothermally oxidized at 950 °C and 1050 °C in air as well as in different oxidizing atmospheres (synthetic air, He-10% O2 and He-10% synthetic air). The oxidation behaviour of HVOF-sprayed coatings is compared with that of VPS coatings. Under the chosen oxidation conditions, which assure a high oxygen partial pressure, the oxidation kinetics of the two coatings are very different, i.e. the oxidation rate of the HVOF sprayed coating is considerably lower than that of the VPS coating. In the present paper, this observation is explained by the presence of finely divided α-Al2O3 particles in the HVOF-sprayed coating, which are formed during spraying. The Al2O3 probably hinders the grain boundary diffusion of the elements. As a consequence, the oxide scale growth is very low. This effect is more evident at high temperature. The as-sprayed coating as well as the oxidized coatings were examined by optical microscopy, X-ray diffraction, scanning electron microscopy and transmission electron microscopy to study the appearance and the composition of the oxide scales and the phase transformations in the MCrAlY coating.

The oxidation behaviour of HVOF thermal-sprayed MCrAlY coatings

Turbine blades are protected against high-temperature oxidation by thermal barrier coating (TBC) systems, which consist of a ceramic top coating (ZrO 2 /Y 2 O 3 ) and a metal bond coating (MCrAlY, M=Ni, Co). At high temperatures and under oxidative conditions between the bond coating and the ceramic top coating an oxide scale is formed, which protects the metal against further oxidation. The oxidation behavior of thermally sprayed MCrAlY coatings is influenced by the coating process and the composition of the metal alloy. This work is concerned with the isothermal oxidation behavior of high-velocity oxygen fuel (HVOF) MCrAlY (M=Ni, Co) coatings. During thermal spraying two elements, yttrium and aluminum, oxidize. As a consequence the HVOF MCrAlY coatings exhibit a microstructure with fine dispersed Al 2 O 3 and aluminum yttrium oxides. Free-standing bodies of HVOF coatings were isothermally oxidized in synthetic air between 850 and 1050°C for different periods. Oxidation experiments show that the oxidation rate of HVOF coatings is two times slower than the oxidation rate of corresponding VPS MCrAlY coatings. The oxidation mechanism is mainly changed in the transient stage (no metastable modification of Al 2 O 3 was formed). It is supposed that the oxide dispersion favors the formation of α-Al 2 O 3 scales. The presence of the fine oxide dispersion also influences the adherence of the oxide scale. The microstructure of the transient oxide scales were examined by X-ray diffraction, scanning electron microscopy and transmission electron microscopy.

The Characteristics of Alumina Scales Formed on HVOF-Sprayed MCrAlY Coatings

In view of the frequent occurrences of mycotoxins in cereals, this study assessed the presence of trichothecenes in 121 samples from Romanian markets. These samples were divided into five groups based on product type: (1) bread and bakery products containing white flour, (2) half-brown bread with whole wheat flour, (3) brown bread containing rye flour, (4) pasta, and (5) raw wheat. Gas Chromatography-Mass Spectrometry was used to detect 13 different mycotoxins, which included the Type A compounds HT-2 toxin and T-2 toxin, as well as the Type B compounds deoxynivalenol and nivalenol. Results indicated trichothecene contamination in 90.08% of our samples, with deoxynivalenol predominating by at least 78% in each examined group. Co-occurrence of three or four trichothecenes were found in 23.85% of our samples. Our study underscores the necessity of consistent monitoring of staple foods to prevent the intake of harmful trichothecenes by consumers.

D. Toma

Papers

The oxidation behaviour of sprayed MCrAlY coatings

The oxidation behaviour of HVOF thermal-sprayed MCrAlY coatings

The Characteristics of Alumina Scales Formed on HVOF-Sprayed MCrAlY Coatings

Occurrence of Types A and B Trichothecenes in Cereal Products Sold in Romanian Markets