METAL 2002 14. – 16.5.2002, Hradec nad Moravicí - 1 - MECHANICAL PROPERTIES OF THERMALLY SPRAYED COATINGS Šárka Houdková a Radek Enžl b Olga Bláhová c Petra Pechmanová a Jitka Hlinková c a Institute of Interdisciplinary Studies, University of West Bohemia, Husova 11, Plzeň,31600, E-mail: [email protected]b Škoda Research Ltd., Tylova 57,Plzeň,31600 c Department of Material Science and Metallurgy, University of West Bohemia, Univerzitní 22, Plzeň Abstract: The present work is concerning with the mechanical properties determination of cermets thermally sprayed coatings, namely WC-Co and Cr 3 C 2 -NiCr coatings. The basic mechanical properties (e.g. superficial hardness, microhardness, elastic-plastic properties and indentation fracture toughness) were evaluated in dependence on process parameters, particularly on in-flight particle velocity and temperature. Particular attention was paid to the changes of coatings microstructure and mechanical properties, caused by heat influence. High temperature behavior of cermets coatings were evaluated in terms of hardness and microhardness. 1.1. Introduction Thermal spray coatings are one of many methods for modification of part’s surface properties, taking advantage of possible combination of wide range of substrate materials and coatings. The technology is based on principle of melting and accelerating of fine particles (pure metals, alloys, cermets or ceramics) and their rapid solidification after impacting the substrate. Coatings with properties varying from very hard wear-resistant coatings to coatings with special physical properties on most of commonly used substrate materials can be created. The main purpose of application of cermet coatings, such as WC-Co and Cr 3 C 2 -NiCr, is the increase of coated parts wear and oxidation resistance. The HP/HVOF sprayed cermet coatings are characterized by almost ideal combination of properties utilizing high hardness of carbides embedded in ductile metal matrix, which makes them the best choice in the case of abrasive and erosive wear. The superior quality of cermets, in comparison with other types of coatings, is reached thanks to the technology of their deposition, High Pressure High Velocity Oxygen Fuel (HP/HVOF). HP/HVOF process represented by the systém TAFA JP-5000 has been proven in its ability to produce the best quality cermet coatings. The main advantage of this process is high powder kinetic energy of particle upon impact on a substrate and the suppression of powder particle overheating in the flame jet. It results in elimination of detrimental phase-chemical changes of sprayed material during particle flight dwell time and dense well-bonded, hard deposits [1,3,5,6].
a Institute of Interdisciplinary Studies, University of West Bohemia, Husova 11, Plzeň,31600,E-mail: [email protected]
b Škoda Research Ltd., Tylova 57,Plzeň,31600c Department of Material Science and Metallurgy, University of West Bohemia, Univerzitní 22,
Plzeň
Abstract:
The present work is concerning with the mechanical properties determination of cermetsthermally sprayed coatings, namely WC-Co and Cr3C2-NiCr coatings. The basic mechanicalproperties (e.g. superficial hardness, microhardness, elastic-plastic properties and indentationfracture toughness) were evaluated in dependence on process parameters, particularly on in-flightparticle velocity and temperature. Particular attention was paid to the changes of coatingsmicrostructure and mechanical properties, caused by heat influence. High temperature behaviorof cermets coatings were evaluated in terms of hardness and microhardness.
1.1. Introduction
Thermal spray coatings are one of many methods for modification of part’s surface properties,taking advantage of possible combination of wide range of substrate materials and coatings. Thetechnology is based on principle of melting and accelerating of fine particles (pure metals, alloys,cermets or ceramics) and their rapid solidification after impacting the substrate. Coatings withproperties varying from very hard wear-resistant coatings to coatings with special physicalproperties on most of commonly used substrate materials can be created.
The main purpose of application of cermet coatings, such as WC-Co and Cr3C2-NiCr, is theincrease of coated parts wear and oxidation resistance. The HP/HVOF sprayed cermet coatingsare characterized by almost ideal combination of properties utilizing high hardness of carbidesembedded in ductile metal matrix, which makes them the best choice in the case of abrasive anderosive wear.
The superior quality of cermets, in comparison with other types of coatings, is reached thanksto the technology of their deposition, High Pressure High Velocity Oxygen Fuel (HP/HVOF).HP/HVOF process represented by the systém TAFA JP-5000 has been proven in its ability toproduce the best quality cermet coatings. The main advantage of this process is high powderkinetic energy of particle upon impact on a substrate and the suppression of powder particleoverheating in the flame jet. It results in elimination of detrimental phase-chemical changes ofsprayed material during particle flight dwell time and dense well-bonded, hard deposits [1,3,5,6].
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The most important parameters affecting the quality of thermal sprayed coatings are particletemperature and velocity. These parameters can be altered over a significant range of conditionsas a result of various chamber pressures and fuel/oxygen ratios at JP-5000.
In many applications (e.g. in power industry), the high temperature behavior of coatings is themain point of interest. Coatings behavior at high temperatures is usually evaluated according toamount of coatings surface oxidation [2,4]. Coatings oxidation resistance is determined by theability of oxidation products to protect the surface against further oxidation. For example Cr3C2-NiCr creates on its surface a protective film of Cr2O3, that enables to use the coating up to hightemperatures (cca 800°C [2]). On the contrary the oxidation products of WC-Co does not haveany protective properties, so WC-Co starts to oxidize rapidly at about 400°C [1]. Thetemperature, at which coatings start to oxidize, is usually taken as a limit temperature for theirapplications. Exposition of coatings, which contain unstable amorphous and nanocrystallinephases thanks to rapid solidification, to the influence of high temperature caused beside oxidationalso the change in coatings microstructure and related changes of coatings properties.Crystallization of amorphous phases, precipitation of carbides from their supersaturated solidsolution in metal matrix and similar phenomenon can occur with increasing temperature [8,9].The changes of coatings wear resistance with connections to microhardness changes can be alsopresumed [5,7].
2. Experimental
2.1. Coating Preparation
Coatings were sprayed onto grit blasted stainless steel substrates.The deposition parameters are summarized in table 1. The kerosene and oxygen flow rates werevaried by reason of studying the influence of chambre pressure and equivalence ratio(representing the flame stechiometry) on coatings properties.
Coating WC-17%Co Cr3C2-25%NiCrPowder -type -grain sizeSpraying system
The denotation of coatings according the combustion chamber pressure (p) and equivalence ratio(Φ) for WC-Co and Cr3C2-NiCr is summarized in table 2a and table 2b, respectively.
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p ΦΦΦΦ 0,74 1 1,274 1 2 3103 4 5 6116 7 8 9
p ΦΦΦΦ 0,7 0,8 0,976 1 4 786 2 5 896 3 6 9
Tab.2. a) denotation of WC-Co coatings and b) denotation of Cr3C2-NiCr coatings with respect to combustion chamber pressure p [psi] and equivalence ratio Φ
The deposition parameters, showing best results (marked in table 2 and 3), were used to createcoatings that were further examinated in terms of high temperature behavior.Selected coatings were annealed for 1 hour at 400, 500, 550, 600, 650, 700, and 800°C to studychanges of microstructure, superficial hardness and microhardness of the coatings.
2.2. Testing Procedure
Superficial hardness HR15N and HR30N was measured according to ČSN ISO 1024003 usinghardness tester Amsler - Wolpert Testor HT 2. At least five measurements were performed foreach sample.Microhardness HV0,3 was measured on the cross section of the coatings, using LECO DM 400Aequipment. At least ten measurements were performed to for each sample.Microhardness HV0,1 were determined on the cross sections using an ultramicrohardness testerSHIMADZU DUH 202. For better understanding of elastic-plastic properties the load-depthdependence was recorded during indentation. A minimum of ten readings was taken for eachcoating.The indentation fracture toughness of WC-Co and Cr3C2-NiCr was measured using Vickersindentor at a load of 120N and 75N, respectively. At least seven indents were made for eachcoating. Lawn equation [12] was used to calculate the value of coatings indentation fracturetoughness.
3. Results and discussion
The results of superficial hardness HR15N and Vickers microhardness HV0,3 for WC-Co andCr3C2-NiCr coatings in dependence on combustion chamber pressure and equivalence ratio canbe seen from the following figures.
WC-17%Co Cr3C2-25%NiCr
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82
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Figure1: Superficial hardness and microhardness for WC-Co and Cr3C2-NiCr coatings independence on combustion chambre pressure and equivalence ratio
It is significant from the figures, that superficial hardness and microhardness increase with theflame temperature (equivalence ratio) and with the in-flight particle velocity (combustionchamber pressure) for both cermet systems [10,11].
The indentation fracture toughness results (fig.2) do not show such a significant dependence onthe deposition parameters as in the case of microhardness.
Figure2: Indentation fracture toughness of WC-Co and Cr3C2-NiCr coatings in dependence on thecombustion chambre pressure and equivalence ratio
With respect to the results of overall analyzes, including the microstructure evaluation,deposition efficiency evaluation etc., the optimal spraying parameters were chosen for furtherexamination. Arrows in figure 1 and 2 mark the optimized coatings.
High temperature behavior of the optimized coatings was studied in terms of microstructureand mechanical properties.
In the case of WC-Co coatingsthe superficial hardness decreasedrapidly above 650°C. Thisphenomenon is caused by oxidationof WC-Co coatings, starting above670°C. The oxidation products ofWC-Co do not have any protectiveproperties against further oxidationand make the superficial hardnesseven immeasurable. Theseobservations are in a goodcorrelation with previous thermal-thermophysical measurements [12].
Fig.3. Microhardness and superficial hardness of heat treated WC-Co coating
The WC-Co microhardness measurement are indicative of microstructure changes above600°C, which can be explained as the amorphous phase crystallization and WC and Wprecipitation from the supersaturated solid solution in the matrix. The changes of microstructureare further confirmed by the changes of elastic-plastic properties of WC-Co coatings, that showsa distinct increase of indentation stiffness above 550°C (see fig.4). The shape of indentationcurves represents the amount of elastic and plastic energy absorbed during indentation and torefers to H/E ratio of the measured coatings. The material stiffness can be also expressed by aratio of an elastic energy (We) to the total amount of energy stored during indentation (Wg)(fig.5).
Fig. 4.: Load-depth curve of heat treated Fig.5: Elastic recovery of heat treated WC-Co WC-Co coating coating
In the case of Cr3C2-NiCr the increase of the microhardness above 600°C is caused by thecreation of Cr3C2 from the matrix. A decrease of microhardness at lower temperatures is probablycaused by relieving plastic deformation of the matrix introduced during impacts of particles(fig.6).
The microhardness decrease ofCr3C2-NiCr coatings in the rangeof 550 – 600°C is, as in the caseof WC-Co, followed by thechange of the unloadingindentation curve slope. Thesuperficial hardness of Cr3C2-NiCr coating follows the sametendency as microhardnessbecause the existence of a thinprotective Cr2O3 layer at highertemperatures does not affect thenumber of superficial hardness.
Fig.6. Microhardness and superficial hardness of heat treated Cr3C2-NiCr coating
The change of unloading indentation slope is followed by an increase of coating stiffness. Thisphenomenon can be also connected with the relieving plastic deformation of the matrix (fig 7.and 8.).
Fig.7.: Load-depth curve of heat treated Fig.8: Elastic recovery of heat treatedCr3C2-NiCr coating Cr3C2- NiCr coating
The microstructure changes can be seen from following micrographs. The as sprayed WC-Cocoating shows denser microstructure compared to Cr3C2-NiCr. At higher temperatures, theporosity does not change significantly, but in the case of WC-Co a decrease of thickness wasobserved at 700°C and 800°C due to oxidation of coating surface.
0
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WC-Co as sprayed Cr3C2-NiCr as sprayed
WC-Co at 600°C Cr3C2-NiCr at 600°C
WC-Co at 800°C Cr3C2-NiCr at 800°CFig.9.: Microstructure of WC-Co and Cr3C2-NiCr as sprayed coatings and coatings at elevatedtemperatures
4.Conclusions
The dependence of WC-Co and Cr3C2-NiCr coatings mechanical properties on depositionparameters, namely combustion chambre pressure and equivalence ratio was evaluated. Based onthe results, the optimal deposition parameters were chosen for further examination.
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The high temperature behavior of selected coatings were examined in terms of indentation tests.The changes of superficial hardness, microhardness and elastic-plastic behavior with temperaturewere determined. The changes of coatings microstructure were observed by means of lightmicroscopy. It was found that at temperature above 600°C some changes of microstructure occurin the case of both selected coatings, which can cause the changes mechanical properties. Thesechanges can be explained by a creation of carbides from supersaturated matrix and by relieving ofmatrix plastic deformation.
AcknowledgementThis paper was prepared thanks to financial support of project no.: MSM 230000009
References
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