112 1068-1302/12/0102-0112 2012 Springer Science+Business Media, Inc. Powder Metallurgy and Metal Ceramics, Vol. 51, Nos. 1-2, May, 2012 (Russian Original Vol. 51, Jan.-Feb., 2012) STRUCTURAL MATERIALS RESEARCH STRUCTURAL AND PHASE TRANSFORMATIONS ON SPARK-LASER COATINGS UNDER FRETTING CORROSION IN AIR V. M. Panashenko, 1,2 I. A. Podchernyaeva, 1 A. I. Dukhota, 1 and A. D. Panasyuk 1 UDC [620.18+544.015]:669.295.69:620.178.16 The structure and phase transformations on the surface of a ZrB 2 -based combined laser–spark coating on titanium alloy are studied. The transformations are induced by triboprocesses under fretting corrosion in air. It is shown that the surface transformations result from a combination of three mechanisms: plastic deformation (surface texturing, fatigue, crack formation, brittle fracture); oxygen diffusion from the environment, chemical reactions of tribooxidation, diffusion of substrate elements (Al) toward the wear surface; and removal of particles from the surface. Two-phase nanostructured eutectic is found on the wear surface of the combined coating. Keywords: laser–spark coating, zirconium diboride, fretting, tribofilm, structure and phase transformations. INTRODUCTION Tribological processes cause virtually any surface to change its constitution, structure, hardness, and other properties. The modified surface as a rule fundamentally differs from the original one by tribological parameters [1]. Four types of surface transformations are proposed in [2], reflecting the formation of secondary structures in the tribocontact area: mechanisms without material transfer (plastic deformation, fatigue, crack formation, etc.); mechanisms with material transfer (diffusion and chemical reactions between surface atoms and counterface/ environment); formation of a tribofilm or coating on the starting surface (scale formation, material transfer from contacting surfaces, compaction of free particles, adsorption of lubricant admixtures); and wear (removal of particles or atoms from the surface). In reality, these types of surface modifications are combined, resulting in a polyoxide secondary structure that forms in the friction process in air and serves as solid lubrication. In view of a combined action of the above surface transformations in the tribocontact area, two approaches may be applied to deal with the wear resistance of * This category should necessarily account for tribooxidation of components. 1 Frantsevich Institute for Problems of Materials Science, National Academy of Sciences of Ukraine, Kiev, Ukraine. 2 To whom correspondence should be addressed; e-mail: [email protected]. Translated from Poroshkovaya Metallurgiya, Vol. 51, No. 1–2 (483), pp. 142–152, 2012. Original article submitted April 14, 2011.
Transcript
112 1068-1302/12/0102-0112 2012 Springer Science+Business Media, Inc.
Powder Metallurgy and Metal Ceramics, Vol. 51, Nos. 1-2, May, 2012 (Russian Original Vol. 51, Jan.-Feb., 2012)
STRUCTURAL MATERIALS RESEARCH
STRUCTURAL AND PHASE TRANSFORMATIONS ON SPARK-LASER COATINGS UNDER FRETTING CORROSION IN AIR
V. M. Panashenko,1,2 I. A. Podchernyaeva,1 A. I. Dukhota,1 and A. D. Panasyuk1
UDC [620.18+544.015]:669.295.69:620.178.16
The structure and phase transformations on the surface of a ZrB2-based combined laser–spark
coating on titanium alloy are studied. The transformations are induced by triboprocesses under fretting corrosion in air. It is shown that the surface transformations result from a combination of three mechanisms: plastic deformation (surface texturing, fatigue, crack formation, brittle fracture); oxygen diffusion from the environment, chemical reactions of tribooxidation, diffusion of substrate elements (Al) toward the wear surface; and removal of particles from the surface. Two-phase nanostructured eutectic is found on the wear surface of the combined coating.
Tribological processes cause virtually any surface to change its constitution, structure, hardness, and other properties. The modified surface as a rule fundamentally differs from the original one by tribological parameters [1]. Four types of surface transformations are proposed in [2], reflecting the formation of secondary structures in the tribocontact area: mechanisms without material transfer (plastic deformation, fatigue, crack formation, etc.); mechanisms with material transfer (diffusion and chemical reactions between surface atoms and counterface/ environment); formation of a tribofilm or coating on the starting surface (scale formation, material transfer from contacting surfaces, compaction of free particles, adsorption of lubricant admixtures); and wear (removal of particles or atoms from the surface).
In reality, these types of surface modifications are combined, resulting in a polyoxide secondary structure that forms in the friction process in air and serves as solid lubrication. In view of a combined action of the above surface transformations in the tribocontact area, two approaches may be applied to deal with the wear resistance of
* This category should necessarily account for tribooxidation of components.
1Frantsevich Institute for Problems of Materials Science, National Academy of Sciences of Ukraine, Kiev, Ukraine.
2To whom correspondence should be addressed; e-mail: [email protected].
Translated from Poroshkovaya Metallurgiya, Vol. 51, No. 1–2 (483), pp. 142–152, 2012. Original article submitted April 14, 2011.
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coatings intended to perform in extreme conditions: materials science and mechanical–mathematical. The former approach establishes the relation of the tribotechnical characteristics to the properties of the secondary structure in the tribocontact area and the latter approach to the stress-stress state of the wearing surface. The ideal case should combine both approaches.
The development of chameleon coatings, whose surface improves its tribological properties in harsh operating conditions under loading in air, is a promising area of tribology [3]. These coatings/materials include composites based on diborides of transition metals in group IV (TiB2, ZrB2, HfB2), forming in the
oxidation/tribooxidation process a scale consisting of boron oxide that is precipitation-hardened with a respective metal dioxide [4, 5]. Higher hardness of zirconium dioxide (16 GPa) compared with titanium oxide phases (8–10 GPa) suggests that the precipitation hardening effect produced by boron oxide would be greater for zirconium diboride coatings. There are very scarce data on ZrB coatings produced by electrospark deposition (ESD).
It is believed [6–8] that laser melting (LM) of a spark-deposited coating increases its wear resistance through composition homogenization and grain refinement in the surface layer. For composite ceramic spark-deposited coatings, one should also consider that their surface phases transform during LM from the ceramic layer to the metal matrix through mixing of the components.
The objective of this paper is to apply the materials science approach to examine the structural and phase transformations on the surface of a ZrB2 spark coating on titanium alloy that occur after laser melting and fretting
wear.
EXPERIMENTAL PROCEDURE
The electrode materials for ESD were new high-temperature composite ceramics, TsLAB-2, based on the ZrB2–ZrSi2–LaB6 (59.05 vol.% ZrB2) system developed at the Frantsevich Institute for Problems of Materials
Science. The electrodes produced by hot pressing had 2–3% porosity and grains 2–3 µm in size. The coatings were spark-deposited using an Élitron-21 high-frequency laboratory unit with a manual
oscillator. The substrates were samples from heat-resistant titanium alloy VT3-1 of composition (wt.%): 5.5–7.0 Al, 2.0–3.0 Mo, 0.8–2.3 Cr, 0.15–0.4 Si, and 0.2–0.7 Fe. Laser melting (LMpulse) proceeded in air in pulsed mode using
a KVANT-15 laser unit (wavelength 1.06 µm, pulse duration = 5 10–3 sec, pulse frequency f = 10 Hz, spot diameter 1 mm, power density 6.5 104 W/cm2).
The combined (ESD + LMpulse) coatings on standard VT3-1 samples were subjected to a fretting-corrosion
(FC) ring-on-disk test (ring being the counterface) in dry friction in air, using an MFK-1 setup and the procedure [9], in compliance with GOST 23.211–80. Fretting corrosion of the “spark-deposited + laser molten coating–spark-deposited coating (counterface)” system proceeded at oscillation amplitude A = 87 µm, frequency f = 25 Hz, pressure P = 19.8 MPa, and number of cycles N = 5 105.
The surface microstructure was examined with scanning electron microscopy (SEM) using a JAMP-9500-S Auger microprobe in SEM mode. The surface composition was determined with energy-dispersive X-ray analysis (EDX) and quantitative Auger electron spectroscopy (AES) with a LAS-2000 spectrometer (Riber, France).
EXPERIMENTAL RESULTS
The surface microstructure and composition of the starting (ESD) and combined (ESD + LMpulse)
TsLAB-2/VT3-1 coatings fundamentally differ [10]. Laser melting causes the rough surface of the spark-deposited ZrB2 ceramic coating (Fig. 1a, b) to transform into a smooth surface on titanium alloy (Fig. 1c, d) with prevailing
dendrite structure and thermal cracks. Fretting corrosion of the combined coating results in structurally inhomogeneous surface (Fig. 2a)
consisting of dark smoothed areas caused by sliding friction of the (ESD + LMpulse) coating (spectra 1–4) and light
areas (spectra 5 and 6) containing oxidized wear products (Fig. 2b). The wear products are located along the edges of sliding areas and in hollows or cracks. The dark sliding areas contain a nanostructured surface layer of sintered wear products forming a tribofilm (Fig. 3). Figure 3a shows delamination of this tribofilm (with a short arrow). The thickness of tribofilm may vary from several tens to several hundreds of nanometers [11].
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a b
c d
Fig. 1. Surface microstructure of the starting spark-deposited coating (a, b) and combined spark-deposited + laser-molten TsLAB-2/VT3-1 coating (c, d)
a b
Fig. 2. Surface microstructure of the combined TsLAB-2/VT3-1 coating after fretting corrosion: a) general view (spectra 1–4—sliding areas; 5, 6—wear products); b) area of wear products
Table 1 compares EDX results averaged over several spectra for the starting surface, sliding areas, and wear products. A large amount of oxygen on the starting surface results from laser melting in air. Titanium and oxygen are the main components of the surface.
The fretting corrosion products for the friction system in question represent partially nanostructured disperse particles from several micrometers to tens of nanometers in size, with an oval shape resulting from friction (Fig. 2b). The extent of elongation increases with refinement of particles. The oval shape of wear particles greatly decreases their abrasive action on the surface.
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TABLE 1. Chemical Composition, at.%, of the Combined TsLAB-2/VT3-1 Coating before and after Fretting Corrosion (According to EDX)
The removal of material during fretting corrosion increases the amount of Al on worn areas by one order of magnitude, which becomes close to that in the VT3-1 alloy, increases the amount of Zr by two to three times, and leads to admixture elements (Si, Fe, Mo) contained in the titanium alloy.
The wear products are significantly enriched by B and Zr, which may originate from both the counterface and combined coating. The Al content of the wear products is higher than that of the tribofilm. Aluminum may originate only from the combined coating, being evidence of friction-induced diffusion of Al atoms to the surface. This is indicative of active physicochemical interaction between components in the tribocontact area, which is accompanied by mass transfer of Al, B, and Si from the substrate due to their high diffusion mobility and by structural and phase transformations: oxidation, dispersion, and elongation of wear particles.
Table 2 shows amounts of phases on the surface of the combined TsLAB-2/VT3-1 coating before and after fretting corrosion, resulting from analysis of chemical composition (Table 1). The main phase components of the surfaces and wear products are represented by Ti–O oxides. These data allow us to represent the formation of phase composition of fretting wear products in the “combined coating–spark-deposited coating (counterface)” system
a b
Fig. 3. Microstructure of the sliding area with a surface layer of sintered wear products: a) general view (fragment of Fig. 2a), a long arrow indicates the direction of reverse sliding; b) local area
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Fig. 4. Formation of phase composition of fretting wear products in the “combined coating—spark-deposited coating” system
(Fig. 4). The main sources of B and Zr oxides are combined and spark-deposited coatings, while Ti and Al result from wear of the combined coating.
The results show that a polyoxide secondary structure (tribofilm) forms on the worn combined coating. This film consists of titanium and aluminum oxides and admixtures of zirconium oxides and impurity elements of the titanium alloy. Titanium and zirconium oxides seem to form complex Ti–Zr–O oxides.
Figure 5 shows the distribution of the main elements (Ti, Zr, Al, O) at a depth of 600 nm of the friction track. Increased oxygen content at the edges of the friction track (Fig. 5g) testifies that oxide phases mainly accumulate at the periphery.
The microstructure of different areas of the friction track (Fig. 6) indicates that the friction surface is structurally inhomogeneous. The friction surface is mainly represented by a fine structure, consisting of dark grains 2–4 µm in size and a light phase bordering the dark grains with a layer 1–1.5 µm thick (Fig. 6a). This structure results from surface crystallization in laser melting. The VT3-1 alloy is known [12] to consist of two phases: low-temperature -titanium and high-temperature -titanium. The -phase differs from the -phase in higher oxygen solubility. Keeping this in mind and considering that the dark phase contains twice as much oxygen as the light phase (Table 3), we assume that the dark phase corresponds to -titanium and the light phase (bordering) to -titanium.
The surface shows (Fig. 6b) light spherical inclusions 4 µm in size. Analysis of EDX spectra shows that these inclusions (41.28 O, 30.4 La, 16.78 Ti, 7.71 Zr, 2.47 Al, 1.36 at.% B) represent a polyoxide phase based on lanthanum oxide and including complex Ti–Zr oxides and aluminum and boron oxides. It is noteworthy that these inclusions form exceptionally in the area of the high-temperature light phase bordering the dark grains of -titanium.
The oval shape of inclusions testifies that they emerge in a liquid phase during laser melting of the spark-deposited coating in air.
TABLE 3. Chemical Composition, at.%, of Grain Structure on Worn Surface of the Combined TsLAB-2/VT3-1 Coating after Fretting Corrosion (Fig. 5a)
Fig. 5. Microstructure of the fretting wear track on the (ESD + LM) coating in secondary electrons (a) and characteristic radiation of elements (b–g) after etching with Ar+ ions for 30 min at a rate of
20 nm/min
High stresses induced by sliding friction during fretting corrosion lead to plastic deformation of outer
coating layers. This results in surface texturing and formation of grooves in the sliding friction direction (Fig. 6c, d). There are also local nanostructured areas of two-phase eutectic on the surface (Fig. 6e, f), probably resulting from
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a b
c d
e f
Fig. 6. Microstructure of sliding areas: a) surface after recrystallization, b) elongated inclusions, c) fatigue cracks, d) plastic deformation along the sliding friction direction, e, f) eutectic
low-temperature eutectic precipitations from Ti, Al, and Si oxides. This eutectic forms on the starting surface of the combined coating during laser melting.
Local deformation causes local fatigue cracks (Fig. 6c) leading to wear particles, increasing the surface roughness, and resulting in subsequent removal of the surface layer.
Surface transformations of four types occur on the laser–spark coating resulting from triboprocesses in the “spark-deposited + laser-molten coating–spark-deposited coating (counterface)” system. These transformations are schematized in Fig. 7: mechanical damage induced by plastic deformation and fatigue changes without material transfer; material (oxygen) transfer from the environment through tribooxidation and diffusion of elements (aluminum) from the substrate; formation of the secondary structure (tribofilm) based on titanium–zirconium and aluminum oxides; removal of particles/atoms from the surface (wear).
Analysis of the results has led to a conclusion that is important for predicting the tribological behavior of contacting surfaces in the “spark-deposited + laser-molten coating–spark-deposited coating (counterface)” system. Fretting corrosion of the combined coating in air results in a tribofilm based on titanium oxides and admixture of Zr
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a b
c d
Fig. 7. Surface transformations under triboprocesses during fretting corrosion in air: a) mechanical processes (plastic deformation, fatigue changes, crack formation), b) tribooxidation, diffusion mass
transfer, c) formation of tribofilm, d) removal of particles
and Al oxides with typical structural changes caused by plastic deformation of the surface layer (texturing, fatigue cracks). Fretting corrosion initiates diffusion of substrate elements (Al) into the surface layer. Oxidized and partially elongated wear products are concentrated at the periphery of the friction track.
CONCLUSIONS
Fretting corrosion of the combined coating leads to structurally inhomogeneous surface consisting of smoothed sliding areas and wear products located at their periphery.
The wear products as elongated disperse particles contain boron and zirconium oxides, originating from both surfaces (counterface and combined coating), as well as oxides of titanium and elements present in the titanium alloy generated by the combined coating.
Fretting corrosion results in a polyoxide film based on titanium–zirconium and aluminum oxides on sliding areas.
The structural and phase transformations are a combination of three mechanisms: (i) plastic deformation (leads to surface texturing, fatigue, crack formation, brittle fracture); (ii) diffusion of oxygen from the environment and chemical reactions of tribooxidation, diffusion of substrate elements (Al) to wear surface; and (iii) removal of particles from the surface. Two-phase nanostructured eutectic is found on the worn surface of the combined coating.
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