Overlay Coatings Research Articles

Microstructural characterization of metallic overlay coatings by high resolution analytical electron microscopy

The microstructures of MCrAlY coatings have been investigated using high resolution analytical electron microscopy. Phase identification was accomplished by X-ray diffraction, energy-dispersive X-ray spectroscopy, selected area diffraction and convergent beam diffraction of extractions and foils. The techniques for sample preparation and analysis will be emphasized. The phase distribution in the NiCrAlY coatings was observed to be a region of β-NiAl with fine spherical α-Cr along with a region of γ phase (nickel solid solution) and γ′-Ni 3Al. Fissure-like growth defects were associated with the formation of coarse α-Cr. In addition, Y 2O 3 formation shows a strong affinity for Al 2O 3.

The effects of ion implantation on the initial stages of hot corrosion attack of cast Co22Cr11Al at 700 °C

The initiation and early propagation stages of low temperature hot corrosion (LTHC) attack of CoCrAlY-type overlay coatings is under study so that improved coatings can be developed through an understanding of the deterioration mechanisms. In an attempt to resolve the role played by the active element addition ( i.e. yttrium, hafnium etc.), the LTHC of both a cast Co22Cr11Al alloy and ion-implanted Co22Cr11Al alloys was studied. In this paper, results for yttrium-implanted and cobalt-implanted Co22Cr11Al are presented. Implanted and unimplanted alloy surfaces were pre-oxidized for 1 h at 700 °C before exposure to conditions that simulate the LTHC attack that is typically observed on hot-end components of gas turbines operated in a marine environment. Scanning electron microscopy and energy-dispersive X-ray analysis were used to characterize the oxide scales both prior to and after LTHC exposures. The LTHC attack was markedly more severe in the yttrium-implanted surfaces than in the unimplanted or the cobalt-implanted surfaces. Moreover, little difference was observed in the LTHC response between the cobalt-implanted and the unimplanted alloy. Both for yttrium-implanted and cobalt-implanted alloys and for the unimplanted alloy, the β-CoAl matrix phase was preferentially attacked whereas the α-Co solid solution phase showed little attack; the relative degree of LTHC attack on the alloy surfaces increased with exposure time. Mechanisms are proposed to explain the enhanced LTHC attack observed in the yttrium-implanted CoCrAl alloy.

Microstructures and compositions of oxide films formed on CoCrAlY and Y-ion-implanted CoCrAl alloys

Hot stage components in many gas turbine engines require surface coatings to protect them from aggressive atmospheres at elevated temperatures. The most commonly employed protection scheme is to modify the surface composition of the components so that high temperature exposure in an oxidizing atmosphere will produce a dense, slow-growing, and adherent aluminum and/or chromium oxide film. Alloys of the MCrAlX-type, where M = Co, Ni, Fe, or combinations thereof, and X = a highly oxygen-active element such as Y, Hf, or Ce, are widely employed as overlay coatings in these applications. In the present investigation, several microanalytical techniques were applied to examine the mechanisms by which the microstructure and microchemistry of a CoCrAlY coating alloy affects the growth and adherence of protective oxide scales.Two cast alloy compositions were examined: Co-22Cr-llAl, and Co-22Cr-l1A1- 0.5Y (nominal compositions, wt. %). Selected specimens of the CoCrAl alloy were implanted with 2 × 1016 Y+/cm2 or 5 × 1016 Co+/cm2 at 150 keV.

Sputter-ion plating of coatings for protection of gas-turbine blades against high-temperature oxidation and corrosion

Considerable effort is being devoted to the development of overlay coatings for protecting critical components such as turbine blades against high-temperature oxidation, corrosion, and erosion damage in service. The most commercially advanced methods for depositing coatings are electron-beam evaporation and plasma spraying. Sputter-ion plating (SIP) offers a potentially cheaper and simpler alternative method for depositing overlays. Experimental work on SIP of Co-Cr-Al-Y and Ni-Cr-Al-Ti alloy coatings is described. Results are presented of metal/ographic assessment of these coatings, and of the results obtained from high-velocity testing using a gas-turbine simulator rig.

New results on the oxidation and hot corrosion of silicide overlay coatings on nickel-based alloys

Silicon-containing coatings with a matrix consisting of the saturated γ solid solution Ni 6Cr 2Si 2 and which also contain intermetallics of complex tantalum silicides as precipitates show great promise as protective layers for nickel-based alloys with improved oxidation and hot corrosion resistance. We present results of oxidation tests performed at 1000°C for up to 4000 h in air and of hot corrosion tests in burner gas with intermediate dipping in Na 2SO 4 and V 2O 5 melts. From silicon diffusion data, lifetimes greater than 10 000 h at 1000°C can be expected.

Coatings for the prevention of localized corrosion of M-50 bearing steel

In this study we deal with the improvements in localized corrosion resistance and wear reduction of type M-50 steel used for engineering components such as bearings by the use of overlay coatings. The overlay coating materials selected to resist localized pitting corrosion were titanium, chromium, Nichrome (Ni20Cr) and molybdenum deposited by physical vapor deposition techniques of direct evaporation, activated evaporation ( i.e. evaporation in the presence of a plasma) and ion plating, and TiC, TiN and HfN deposited by the activated reactive evaporation process. Coated and control (uncoated) samples of M-50 steel were corrosion tested, the test having been designed to stimulate the bearing environment inside an engine with a specified roller-bearing-on-race geometry. The results showed that the control (uncoated) samples were severley pitted along the contact line and that general corrosion occurred outside this region, whereas overlay coatings of titanium, chromium, Nichrome, TiC and TiN completely eliminated the localized pitting corrosion. Coatings of these materials deposited by different techniques showed equally good results. Wear test samples with TiC and other hard coatings were prepared and submitted to the Naval Engineering Support Office, San Diego, for rolling contact fatigue wear tests. The results showed that a TiC coating produces no decrease in the fatigue life of the M-50 alloy. It appears that overlay coatings are a viable way to prevent localized pitting corrosion whichh not only is a cheaper alternative to ion implantation but also has a significant advantage in that hard coatings such as TiN and TiC can provide corrosion and wear resistance simultaneously.

The influence of overlay coatings on high cycle fatigue of an advanced tantalum carbide strengthened eutectic composite

High cycle fatigue of coated and uncoated NiTaC 3-116A2, a directionally solidified tantalum carbide strengthened eutectic, has been examined over a range of temperatures. In the uncoated condition, the fatigue strength of the alloy increased with temperature from room temperature to 600 °C, but was slightly lower at 900 °C than at 600 °C, which is consistent with the variation in 0.2 pct yield strength with temperature. At room temperature and 600 °C, the influence of several plasma sprayed coatings in reducing the fatigue life was found to be sensitive to the coating composition and alternating stress level. A Ni-20Cr coating resisted fatigue damage on the basis of its ductility and therefore performed better at high cyclic stress levels than at low cyclic stress levels. By contrast, Ni-20Cr-10Al-2Hf-0.1C. and Ni-20Co-20Cr-10Al-2Hf-0.1C coatings were stronger and so exhibited high fatigue strengths at long lives but, because of their limited ductility, cracked readily when tested at high stress levels. At 900 °C the coatings had little effect on fatigue life. On the basis of the present results and the metallographic observations of early crack growth through the coatings, it is concluded that the selection of a mechanically compatible coating requires a knowledge of the fatigue cycle. To ensure a minimum effect on fatigue lives for the anticipated stress cycle, it is necessary to match the coating and substrate mechanical properties as closely as possible.

Methods for characterization of coating microstructures

The use of deposition methods to produce both overlay coatings and free-standing sheets or pieces of metals offers the possibility of producing materials with unique properties not previously obtained by conventional materials processing. These properties are closely related to the internal microstructural features of the deposited film and can be varied over wide ranges by the selection of appropriate deposition parameters and film composition. Coatings with estremely fine grain size or amorphous structures can thus be produced as well as coatings with unusual distributions of voids and cavities or with dispersions of particles of non-equilibrium phase compositions. It is thus important in the evaluation of the coating properties to take these structural features into consideration in an effort to optimize the result of the deposition process. A variety of analysing tools exist for these structural investigations, each exhibiting advantages and limitations. The purpose of the present paper is to discuss some of the methods that have proved to be useful in the course of the development of coatings.

Friction and adhesive wear of titanium carbide and titanium nitride overlay coatings

The coefficient of friction and self-adhesive wear were studied for hard overlay coatings (4–8 microm thick) of TiC and TiN deposited on 304 stainless steel, titanium and aluminum discs and 440 C stainless steel riders by activated reactive evaporation process. The hard overlay coatings were characterized by X-ray diffraction and indentation hardness techniques. The frictional and adhesive wear behavior were evaluated using a standard pin-on-disc tribotester in dry and lubricated conditions. The coefficient of friction and the wear were very much lower when the test couple consisted of a hard coating rubbing against a hard coating than when one or both of the components were uncoated metals, even in dry (unlubricated) conditions. In some instances of the hard-hard couple, very low coefficients of friction (0.05–0.1) and very low wear, i.e. wear groove depths less than 1 × 10 −5 in, were observed for test conditions of a run distance of 500 m under a load of 0.4 kgf in dry conditions. From examination of the wear scar the failure mode of the coating was observed to be microfragmentation resulting from the initiation and propagation of microcracks at the rider-disc interface.

Advanced coating development for industrial/utility gas turbine engines

Laboratory hot corrosion rig tests were used to identify new superalloy overlay coatings which are more resistant to hot corrosion than current CoCrAlY (Co 20Cr12Al0.5Y) or NiCoCrAlY (Ni20Co20Cr12Al0.5Y) coatings. These resistant coatings consist of two layers, an inner layer of CoCrAlY or NiCo CrAlY and an outer layer of MCrAlY containing 20–35 wt.% Si. Procedures are described in which the outer layer is developed on the surface of conventional CoCrAlY or NiCoCrAlY coatings using electron beam physical vapor deposition techniques. The combustor rig tests used to evaluate these coatings are discussed in terms of their relevance to industrial or utility turbine hot corrosion.

Thermal expansion study of plasma-sprayed oxide coatings

The thermal expansion characteristics of overlay coatings is of considerable importance when the coated parts are exposed to thermal cycling. In this study we investigated the thermal cycling behavior of oxide coatings plasma sprayed onto mild steel. Thermal expansion measurements were carried out on Al 2O 3TiO 2 and zirconia coatings under different conditions of coating-substrate adherence and on coatings that were substrate free. A study was initiated of the effects on the expansion coefficient of variations in composition, porosity, heating rate and thermal cycling. In addition, metallographic and acoustic emission methods were employed to examine the compatibility of these oxide coatings with steel substrates under varying conditions.

Gas Turbine Bucket Corrosion Protection Developments

This paper describes a joint General Electric/ARAMCO program for improving rotating gas turbine parts lives in desert environments with high concentrations of alkali salts and sulfur-containing fuels. A description of the environment is given along with a definition of the hot corrosion problem. A series of buckets coated with experimental or developmental coatings produced by a variety of processes such as pack cementation, physical vapor deposition, plasma spray and sheet claddings are described and results from actual field trials are discussed. Pt-Cr-Al pack cementation coatings were observed to perform well. Next generation overlay coatings of the MCr-AlY-type applied by plasma spraying electron beam and sheet cladding processes also performed well.

Influence of ion implantation and overlay coatings on various physico-mechanical and wear properties of stainless steel, titanium and aluminium

Abstract This work represents the data on the wear behaviour of 304 stainless steel, commercial grade titanium and commercial grade aluminium without and with different surface treatments, namely ion implantation of boron and nitrogen and overlay coatings of titanium carbide and nitride. The surface treatments were characterized by phase identification, hardness, bend strength, as well as adhesion of overlay coatings. Wear properties were evaluated in adhesive, erosive and abrasive modes of wear. The experimental results showed that surface treatments produced measurable changes in hardness and strength. The results of adhesive wear tests indicated that the wear resistance of all the substrate materials can be considerably improved by overlay coating with superhard materials in dry as well as lubricated test conditions. Ion implantation resulted in improvement of wear properties for only a limited regime of adhesive wear under lubricated conditions and for the abrasive mode of wear. Overlay coatings produced a marked improvement in abrasive wear tests under lubricated conditions on all substrate materials.

Aluminium and silicon base coatings for high temperature alloys—Process development and comparison of properties

Abstract The paper is concerned with systematic studies on the formation of overlay coatings on nickel and iron base superalloys to improve their resistance against high temperature oxidation and hot corrosion. In contrast to the simpler case of aluminizing nickel base alloys, the problems arising in aluminizing iron base superalloys and in siliconizing nickel base alloys have not yet been solved. A new and economical coating procedure is presented, which involves a reaction sintering process of unalloyed powder mixtures to obtain overlay coatings. The influence of the compositions of the layer and the substrate on the chemical compatibility of the whole layer composite is described in detail. It is shown that overlay coatings containing high concentrations of silicon can only be applied on nickel base superalloys if elements that are able spontaneously to form reaction barriers are present within the substrate alloy ( e.g. aluminium) or in the as-preformed interlayer. Refractory metals have proved to be the most advantageous. To obtain a ductile coating, silicon-rich donor phases in the form of isolated precipitates were incorporated into a matrix containing low concentrations of silicon. These coating systems can also be applied to improve the aluminization of iron base alloys. The high temperature oxidation and corrosion behaviours of the coated samples were tested in burner gas or air at 1000°C and in molten salts at 900°C.

Electron-beam-deposited overlay coatings

Electron-beam-deposited overlay coatings

Hot corrosion evaluation of high temperature metal protective coatings for combustion turbines

An experimental program was carried out to determine the degree that variations of critical elements and microstructural characteristics of MCrAlY overlay coatings affect hot corrosion resistance in land-based combustion turbines. The primary gas path surface coatings were exposed to a simulated sea-salt- contaminated combustion environment. It was found that high chromium (24 wt.%) and aluminum (13 wt.%) concentrations are essential for corrosion-resistant CoCrAlY. Oxide-filled leaders could be tolerated in thick coatings (above 6 × 10 -3 in, 150 μm) and peening was not essential.

Hot Corrosion of Gas Turbine Components

Hot corrosion is the accelerated rate of oxidation of coatings and substrates, which occurs when a condensed phase is present on the surface. The major constituent of the corrosive deposit is sodium sulfate, although when fuels other than light distillates are employed, the deposit can contain various amounts of vanadium pentaoxide, an impurity present in many heavy distillates and most residual oils. Other corrodents that can be present in varying concentrations are lead, carbon and alkali halides. The coatings employed to extend the life of gas turbine alloys, and the alloys themselves exhibit varying degrees of resistance to hot corrosion. In our study of the “Parameter Monitoring for Corrosion Control of Utility Gas Turbines” we are determining the relative effect of each of the aforementioned corrodents with respect to simple aluminides, overlay coatings and precious metal coatings on three nickel base superalloys and one cobalt base superalloy. Although these studies are being conducted over the temperature range from 1500 to 1800°F, this presentation primarily emphasizes the completed high temperature results with some examples of the ongoing lower temperature studies.

Vapour deposition of wear-resistant surfaces

Many hard and wear-resistant coatings can deposited from the vapour phase. With the exception of gaseous nitriding and carburising, these are overlay coatings and generally consist of refractory carbides and nitrides, although bearing metals can also be deposited in this way. Some of the wear-resistant metals and compounds deposited from the gas phase are listed in Table 1. In general these materials are extremely hard ie > 2000 VPN, ~md are used when this p}operty is of utmost importance. They are also chemically inert and therefore combine some corrosion resistance with their hardness. It is probably this combination which results in their interesting tribological properties. For example a titanium carbide coating protects a cutting tool from wear because it reduces the tendency for cold welding between tool and work piece.

Composite coatings of CoCrAlY plus platinum

The addition of noble metals has been reported to improve the corrosion resistance of aluminide and overlay coatings for gas turbine airfoils. The production and structure of a CoCrAlY + Pt coating are described. This coating is superior in oxidation and hot corrosion resistance to a coating made of the same CoCrAlY material without platinum. The 0.6% ductility temperature T d for these two coating is similar. However, mechanical damage greatly raises the T d value of CoCrAlY + Pt. This is attributed to a favorable residual stress system locally destroyed by the damage.

Some effects of structure and composition on the properties of electron beam vapor-deposited coatings for gas turbine superalloys II. The origin of defects

The advent of the gas turbine engine has imposed serious materials problems with respect to oxidation and hot corrosion. One solution has been the application of multi-element electron beam overlay coatings of the MCrAlY type. However, state-of-the-art overlay coatings inherently exhibit such structural imperfections as spits, flakes and linear defects (leaders). This paper presents the results of the continued investigation to determine the origin of these imperfections. Microprobe and laboratory data are presented which characterize these defects. Spits were found to consist of ejected pool material, exhibiting enrichment in impurity elements of low vapor pressure. Investigation indicates spitting may be the result of non-metallic (carbide or oxide) particles within the melt, which upon superheating initiate ejection of pool material. Flakes, generally cone shaped, were found to originate at non-metallic particles loosely attached to the surface. Leader formation was found to be weakly dependent upon surface finish (to 100 AA) and strongly dependent upon the angle of incidence of the arriving vapor flux. Both flakes and leaders seem to be enhanced by preferential growth and shadowing phenomena.