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Wear resistance of Ti–6Al–4V alloy treated by means of glow-discharge and furnace treatments
Wear 240 Ž2000. 199–206 www.elsevier.comrlocaterwear
Wear resistance of Ti–6Al–4V alloy treated by means of glow-discharge and furnace treatments T. Bacci a,) , F. Borgioli a , E. Galvanetto a , F. Galliano a , B. Tesi b a
Dip. di Meccanica e Tecnologie Industriali, UniÕersita` di Firenze, Õia S. Marta 3, 50139, Florence, Italy b Dip. di Ingegneria dei Materiali, UniÕersita` di Trento, Õia di Mesiano 77, 38050, Trent, Italy Received 25 November 1999; received in revised form 1 March 2000; accepted 1 March 2000
Abstract The contemporary glow-discharge oxidising and nitriding treatment has shown to produce modified surface layers with enhanced hardness properties, in comparison with the conventional ion-nitriding process. The aim of the present study was to evaluate the tribological properties of ion-oxinitrided samples and to compare their wear behaviour with the one of furnace oxinitrided and ion-nitrided samples. At low coupling loads Ž50 N. the wear volumes of the treated samples result small and comparable for all the tested velocities. On the other hand, when high coupling loads are used Ž100 N., the wear of the ion-nitrided samples is higher than that of the oxinitrided ones, this effect becoming more remarkable as sliding velocity increases; moreover, the ion-oxinitriding treatment allows to achieve lower wear volumes than the ones obtained by means of the furnace oxinitriding process. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Ti–6Al–4V alloy; Surface treatments; Oxinitriding; Wear behaviour
1. Introduction Titanium and its alloys are widely employed in many industrial fields, due to their high specific strength and good corrosion resistance. On the other hand, these materials show relatively low shear strength and high coefficient of friction: owing to the low tensile strength, material transfer to counterfaces is easily produced, so that adhesive wear occurs; moreover, their low hardness allows damage by abrasive wear mechanisms w1x. As a matter of fact, the poor wear resistance of these materials is still a limit to a wider use. In order to improve the poor tribological properties of titanium and its alloys, many thermochemical processes have been studied in the last decades w2x. The glow-discharge ion-nitriding process allows to obtain modified surface layers, consisting of an outer compound layer and an inner diffusion layer w3–6x. In the compound layer,
) Corresponding author. Tel.: q39-55-479-6503; fax: q39-55-4796400. E-mail address: [email protected] ŽT. Bacci..
titanium nitrides, TiN Žf.c.c.. and Ti 2 N Žtetragonal. are present and very high hardness values Žup to ; 2000 HK. are obtained. The diffusion layer consists of nitrogen-rich a-Ti Žh.c.p.. crystals, embedded in the substrate; the decreasing nitrogen concentration in the solid solution produces gradually decreasing hardness values, from ; 1000 HK to the matrix values. Owing to the presence of hard nitrides layers, the wear resistance of ion-nitrided components is remarkably improved in many working conditions, especially when low and medium coupling loads and velocities are used. Titanium nitride TiN shows very high hardness values Ž; 2400 HK., but it is very brittle: when high loads and velocities are employed, a microfragmentation of the nitrided layers may occur, so that the surface is abraded by the nitride fragments w7–9x. As reported in previous papers w10,11x, the contemporary glow-discharge oxidising and nitriding treatment is able to produce, on Ti–6Al–4V samples, modified surface layers with enhanced hardness properties, if compared with a conventional ion-nitriding process. The working conditions Žtreatment atmosphere, temperature and time. have a remarkable influence on the microstructure and on the mechanical properties of the hardened layers. Thicker dif-
0043-1648r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 Ž 0 0 . 0 0 3 6 2 - 8
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fusion layers, characterised by higher hardness values Žup to ; 1300 HK., and smoother gradients can be achieved by increasing not only treatment temperature and time, but also oxygen concentration. As a matter of fact, the oxinitriding treatment allows to produce hardened layers having thickness comparable with the one of the layers obtained by ion-nitriding process, but with shorter treatment time. Preliminary wear tests carried out on Ti–6Al–4V alloy untreated, ion-nitrided and ion-oxinitrided samples show that the oxinitriding process improves the wear resistance of the samples w12x. Owing to the lack of complete and extensive information on wear properties of samples treated with the oxinitriding process, this study evaluates the wear behaviour of Ti–6Al–4V oxinitrided samples, obtained by treatments performed in both glow-discharge equipment and air circulating furnace. Wear tests were carried out in sliding conditions, without lubricants and in air, by means of a tribometer in a block-on-disk configuration, with AISI O2 hardened and stress relieved disks as counterparts. The morphology and constitution of the worn samples and the debris were examined by means of optical and electronic microscopy and X-ray diffraction techniques, in order to establish the wear mechanisms in the different test conditions. Moreover, the wear behaviour of ion- and furnace oxinitrided samples was compared with the one of ion-nitrided samples.
2. Experimental procedure Prismatic samples Ž50 = 12 = 20 mm. of annealed Ti– 6Al–4V alloy ŽTable 1. were prepared by cutting and grinding. Glow-discharge treatments were performed in a plasma equipment, as described in previous papers w6,11x. Each sample was connected to the cathode of a dc power supply; the treatment temperature was controlled by a chromel–alumel thermocouple inserted into the sample. Treatments were carried out at 1173 K, with a pressure of 1 kPa. Ion-nitriding was performed for 8 h, by using a gas composition of 80 vol.% N2 and 20 vol.% H 2 ; ionoxinitriding was carried out for 2 h by introducing air in the chamber Ž21 vol.% O 2 , 79 vol.% N2 .. Furnace oxinitriding treatments were performed in an air circulating furnace at 1173 K for 2 h.
Table 2 Chemical composition of AISI O2 steel counterfaces Material
AISIO 2
Composition Žwt.%. C
Cr
Mn
Si
P
S
1.08
0.47
1.80
0.20
0.04
0.017
After the treatment, the samples were sectioned perpendicularly to the external surface and mounted in thermosetting resin. Polishing was performed by using SiC papers up to 1200 grit and diamond pastes up to 1-mm grain size. The polished sections of the samples were etched by using Kroll’s reagent. The microstructure of the treated samples was examined by optical metallographic techniques and scanning electron microscopy ŽSEM.. X-ray diffraction analysis ŽCu K a radiation. was performed to identify the phases constituting the surface layers, and diffraction spectra were analysed by means of a fitting program using Rietveld method. This analysis was repeated after a progressive removal of very thin surface layers Ž; 5 mm, every time. with abrasive papers. Microhardness measurements ŽKnoop indenter, 25 gf. were carried out on the modified layers and on the matrix. Roughness analysis was performed on the surface of the samples before and after the treatments by using a stylus surface tester. Wear resistance was tested by means of a tribometer in a block-on-disk configuration, in sliding conditions, without lubricants and in air. The blocks for wear tests Ž8 = 12 = 20 mm. were obtained by cutting the treated samples. AISI O2 ŽTable 2. hardened and stress relieved disks Ždiameter: 50 mm; width: 14 mm. Hardness: 62.5 HRC. were used as counterparts; in order to avoid edge effects and to ensure a correct coupling between block and disk, the width of counteracting disk was larger Ž14 mm. than the block one Ž8 mm.. Tests were carried out by using coupling loads of 50 and 100 N, at sliding velocities in the range 0.4–2.0 mrs; the sliding distance was 3000 m. At least three repetitions of each test were carried out. The wear volumes of the tested samples were determined by measuring the width and the depth of the worn regions by optical microscopy techniques. The morphology and constitution of the worn samples and the debris were examined by means of optical and electronic microscopy, microprobe analysis and X-ray diffraction.
3. Results and discussion Table 1 Chemical composition of Ti–6Al–4V alloy samples
3.1. Morphology and microstructure
Material
Composition Žwt.%. Al
Fe
V
N
O
Ti–6Al–4V
5.98
0.09
3.25
0.16
0.03
The modified surface layers of the treated samples consist of an outer compound layer and an inner diffusion layer.
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Fig. 1. X-ray diffraction spectra of samples treated at 1173 K for 2 h by means of ion-oxinitriding Ža. and furnace oxinitriding Žb. processes, after the removal of the scarcely adherent part of the compound layer. Inserts Ž1. and Ž2. of Žb. show the TiN x O y diffraction peaks as obtained by increasing the scan time.
Ion-nitriding process produces a compound layer well adherent to the substrate and consisting of titanium nitrides TiN Žc.f.c.. and Ti 2 N Žtetragonal.. Both furnace and ionoxinitrided samples show a compound layer with a stratified structure; it consists essentially of TiO 2 Žrutile type;
201
tetragonal., TiN xO y Žf.c.c.. and small amounts of Al 2 O 3 Žalumina type; rhombohedral.. The stratified structure of the compound layer can be ascribed to the high content of TiO 2 in the layer, as observed by other authors w13,14x. In the oxinitrided samples, a scarcely adherent part of the compound layer has been easily removed before wear tests by ultrasonic cleaning; X-ray diffraction spectra show that, in the adherent layer, TiO 2 and small amounts of TiN xO y are present ŽFig. 1a,b.. The diffusion layer of the treated samples consists of a-Ti Žh.c.p.. crystals, rich in interstitial atoms, that are embedded in the substrate. In the oxinitrided samples, X-ray diffraction shows the presence of titanium– aluminium intermetallic phase Ti 3 Al Žhexagonal. ŽFig. 1a,b.; in ion-oxinitrided samples, small amounts of TiAl Žtetragonal. are also detected. The repetition of diffraction analysis after the progressive removal of thin layers Ž; 5 mm, every time. has shown that the intermetallic phases are present near the compound layer-diffusion layer interface. The presence of Ti–Al intermetallic phases can be ascribed to a wider stability field of these phases due to the presence of oxygen in the layer, as reported by other authors w15–17x. The lattice parameters, a, c, of a-Ti and Ti 3 Al and the Ti 3 Al relative volume amount in respect of a-Ti were evaluated, for the oxinitrided samples, by means of a fitting program using Rietveld method and they are reported in Table 3; as reference, a, c values of a-Ti for untreated samples are also shown. After the treatments, the a-Ti a, c parameters increase, as a consequence of interstitials solubilization, and result higher in furnace oxinitrided samples than in ion-oxinitrided ones; on the contrary, Ti 3 Al lattice parameters are comparable. Moreover, in the ion-oxinitrided samples, higher volume concentrations of Ti 3 Al are detected. The lower lattice parameters values shown by ion-oxinitrided samples, in comparison with furnace oxinitrided ones, can be related to a lower concentration of interstitial atoms in solid solution. This lower solubilization of interstitials may be due to the presence of intermetallic phases, that are able to solubilize oxygen w18,19x and can act as a barrier to the diffusion of interstitial atoms. The modified layer microstructures of ion-nitrided Ža., ion-oxinitrided Žb. and furnace oxinitrided Žc. samples are shown in Fig. 2. The oxinitrided samples show similar microstructures; moreover, the treatments produce diffusion layers of comparable thickness. It must be pointed out
Table 3 a-Ti and Ti 3 Al lattice parameters, a, c, and Ti 3 Al relative volume amount for ion- and furnace oxinitrided samples Sample type
a-Ti
Ti 3 AlrŽTi 3 Al q a-Ti.
a Žnm. " 0.0004
c Žnm. " 0.0006
a Žnm. " 0.0004
c Žnm. " 0.0006
Untreated Ion-oxinitrided Furnace oxinitrided
0.2925 0.2936 0.2941
0.4677 0.4728 0.4753
– 0.5787 0.5783
– 0.4695 0.4695
Ti 3 Al
– 16 9
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Fig. 2. Micrographs of the modified layer of ion-nitrided Ž t s 8 h. Ža., ion-oxinitrided Ž t s 2 h. Žb. and furnace oxinitrided Ž t s 2 h. Žc. samples treated at 1173 K.
that smaller grain size is observed in the oxinitrided samples, in comparison with the ion-nitrided ones, owing to the shorter treatment times: thus, it is expected that the oxinitrided samples result tougher and show better mechanical properties than ion-nitrided ones. Roughness analysis was performed on the samples before and after the treatments; on oxinitrided samples the analysis was carried out after the removal of the scarcely adherent part of the compound layer. Mean roughness values are reported in Table 4. In spite of shorter treatment time, oxinitrided samples show a rougher surface, in comparison with ion-nitrided ones; moreover, furnace treated samples show the highest roughness values. The rougher surface of oxinitrided samples can be ascribed to the growth mechanism of the oxide layer: in fact, as time increases, this layer becomes porous and develops a stratified structure, increasing the surface roughness w13,14x. 3.2. Microhardness analysis Microhardness measurements were performed on the diffusion layer of the treated samples. In Fig. 3, the microhardness profiles of the samples are shown; depth is measured from the compound layer–diffusion layer interface towards the substratum. The oxinitrided samples show comparable profiles and higher hardness values near the interface than ion-nitrided samples; moreover, all the treated samples show comparable case depth thickness
Ž; 40 mm.. It must be pointed out that the oxinitriding process produces the highest hardness values in the diffusion layer, while, by using the ion-nitriding process, the highest hardness values are measured in the compound layer Ž; 2000 HK., owing to the presence of titanium nitrides. Microhardness measurements were also carried out on the surface of the treated samples by using different loads, in order to qualitatively evaluate the load bearing capacity of the surface layers. The hardness values vs. applied load, in logarithmic scale, are shown in Fig. 4. The hard and brittle compound layer of the ion-nitrided samples is able to bear an increasing load up to a critical value Ž; 100 gf.; beyond this value, the compound layer fails and the load is born by the inner diffusion layer, as it is shown by a steep decrease in the measured hardness values. Thus, when high specific loads are applied, the layer can break down and the hard nitride particles can cause a severe abrasive wear. Both ion- and furnace oxinitrided samples show similar trends of hardness values vs. applied load. The surface hardness values continuously decrease as the applied load increases, due to graded hardness properties of the diffu-
Table 4 Mean roughness values Sample type
R a Žmm.
Untreated Ion-nitrided Ion-oxinitrideda Furnace oxinitrideda
0.1 1.1 1.2 1.4
a
layer.
After the removal of the scarcely adherent part of the compound
Fig. 3. Microhardness profiles of ion-nitrided Ž t s8 h. and ion- and furnace oxinitrided Ž t s 2 h. samples treated at 1173 K.
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Table 5 Phases present in the debris of the treated samples tested with 50 N coupling load Õ Žmrs.
Phases
0.4 0.8 1.6
Ti 2 O 3a , Fe 2 O 3 , Fe 3 O4 Ti 2 O 3a , Fe 2 O 3 , Fe 3 O4 , a-Fe a Ti 2 O 3a , Fe 2 O 3 , Fe 3 O4 , a-Fe a
Fig. 4. Surface microhardness vs. applied load of ion-nitrided Ž t s8 h. and ion- and furnace oxinitrided Ž t s 2 h. samples treated at 1173 K.
sion layer and to the absence of a hard compound layer. At the same critical load of the ion-nitrided samples the fail of the layer is not produced. 3.3. Wear tests Wear tests put in light that wear behaviour of the samples depends on both the test conditions Žcoupling load, sliding velocity. and the treatment conditions. 3.3.1. Tests with 50 N coupling load In Fig. 5, the wear volumes of the treated samples, as a function of sliding speed are shown; the wear volumes of all the tested samples increase as sliding velocity increases. The treated samples exhibit essentially the same wear behaviour: their wear volumes are very small and comparable, and the worn surfaces show only minor broken wear tracks. A plastically deformed subsurface layer is present on the ion-nitrided samples, but not on the oxinitrided ones. The wear of the counteracting disk is low, but it results higher when oxinitrided samples are used as counterfaces. In order to evaluate the tribological behaviour of the tested samples, X-ray diffraction and EDS microprobe
Fig. 5. Wear volumes vs. sliding velocity of ion-nitrided, ion-oxinitrided and furnace oxinitrided samples tested with 50 N coupling load and 3000 m sliding distance.
Traces.
analysis of the wear debris were carried out; the constituent phases of the debris are summarised in Table 5. At low and medium sliding velocities Ž0.4 and 0.8 mrs. wear debris consist of oxides particles: the analyses show that they are mainly iron oxides, Fe 2 O 3 and Fe 3 O4 , but also small amounts of titanium oxide, Ti 2 O 3 , are detected. Moreover, at 0.8 mrs, the diffraction analysis shows that very small amount of iron fragments is also present. At 1.6 mrs, oxides particles and iron fragments are detected in the debris. On all the tested samples the wear mechanism is essentially the same: oxidation prevails at low sliding velocities, while, at higher velocities, a combination of oxidation and delamination occurs; the wear prevails on the counteracting disks. No evident microfragmentation of the nitrided layers and no evident effects of third-body abrasion by nitride fragments have been observed, in accordance with the good wear resistance of plasma treated samples at low coupling loads w4,7–9x. The extended plastic deformation in the subsurface layers, shown by ion-nitrided specimens, can be related to a scarcely protective oxide layer. In fact, with low coupling loads, the surface heating is low and not sufficient to promote the formation of a protective oxide layer; thus, the frictional force causes a deformation of the metal surface, shearing it in the sliding direction w20x. On the contrary, the hard diffusion layer of the oxinitrided samples succeeds in preventing any plastic deformation of the subsurface layers. 3.3.2. Tests with 100 N coupling load For 100 N coupling load, the wear behaviour of the tested samples and of the counteracting disks is dependent on the sliding velocity and on the different counteracting couple, as it is shown in Fig. 6. The constituent phases of the wear debris are reported in Table 6; X-ray diffraction spectra of the debris of ion-oxinitrided samples tested with Ža. 0.4 and Žb. 1.6 mrs are shown in Fig. 7. While at low sliding velocity Ž0.4 mrs. the wear volumes of all the tested samples are comparable, at medium and high velocities the volumes of all the oxinitrided samples result smaller than those of the ion-nitrided ones; moreover, the ion-oxinitrided samples show the lowest values. The wear of oxinitrided samples shows a minimum at 0.8 mrs, while, for the ion-nitrided samples, it increases with the sliding velocity.
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Fig. 6. Wear volumes vs. sliding velocity of ion-nitrided, ion-oxinitrided and furnace oxinitrided samples tested with 100 N coupling load and 3000 m sliding distance.
At 0.4 mrs, all the treated samples have worn surfaces with superficial and broken wear tracks and show a plastically deformed subsurface layer of comparable thickness. The morphology and composition of the debris are essentially the same: they consist of oxides particles, mainly Fe 2 O 3 and Fe 3 O4 with small amounts of Ti 2 O 3 ; very small iron fragments are also detected. At 0.8 mrs, the worn surfaces show only small and broken tracks; an extended plastically deformed subsurface layer is present on the ion-nitrided samples, but not on the oxinitrided ones ŽFig. 8a,b.. The debris consist of iron and titanium oxides, Fe 2 O 3 , Fe 3 O4 and Ti 2 O 3 ; small iron fragments and compound layer particles ŽTiN, TiN xO y . are also observed. At 1.6 mrs, the ion-nitrided samples show deep wear tracks, along the whole worn surface, and the worn layer results ; 110 mm, about three times deeper than the case depth Ž; 40 mm.; a plastically deformed subsurface layer is observed and its thickness results comparable with the one on the 0.8 mrs tested samples ŽFig. 9a.. The debris consist of iron and titanium oxide particles, Fe 2 O 3 , Fe 3 O4 , Ti 2 O 3 , and TiO, and of small fragments, consisting of titanium nitride, TiN, and metal iron. Both the ion- and furnace oxinitrided samples show small and broken tracks on the worn surfaces; the worn layer thickness is comparable with the case depth Ž; 40 mm. and shows an extended
Fig. 7. X-ray diffraction spectra of wear debris of ion-oxinitrided samples tested with Ža. 0.4 and Žb. 1.6 mrs sliding velocities, 100 N coupling load and 3000 m sliding distance.
plastic shearing in the subsurface layers ŽFig. 9b,c.. The debris consist essentially of iron oxide particles and relatively large iron fragments; also small titanium oxide particles and TiN xO y fragments are detected. At 2.0 mrs, deep wear tracks are present on the worn surface of the oxinitrided samples; the worn layer thickness is comparable with the case depth and an extended plastic shearing in the subsurface layers is observed. The debris consist of iron oxide particles, Fe 2 O 3 , and Fe 3 O4 ,
Table 6 Phases present in the debris of the treated samples tested with 100 N coupling load Sample type
Õ Žmrs.
Phases
Ion-nitrided
0.4 0.8 1.6 0.4 0.8 1.6 2.0
Ti 2 O 3a , Fe 2 O 3 , Fe 3 O4a , a-Fe a Ti 2 O 3a , TiN a , Fe 2 O 3 , Fe 3 O4 , a-Fe a Ti 2 O 3 , TiN a , TiO a , Fe 2 O 3 , Fe 3 O4 , a-Fe Ti 2 O 3a , Fe 2 O 3 , Fe 3 O4a , a-Fe a Ti 2 O 3a , TiN x O ay , Fe 2 O 3 , Fe 3 O4 , a-Fe a Ti 2 O 3a , TiN x O ay , Fe 2 O 3 , Fe 3 O4 , a-Fe Ti 2 O 3a , TiN x Oy , Fe 2 O 3 , Fe 3 O4 , a-Fe
Ion- and furnace oxinitrided
a
Traces.
Fig. 8. Cross-sections of the worn ion-nitrided Ža. and ion-oxinitrided Žb. samples after testing with 100 N coupling load, 0.8 mrs sliding velocity and 3000 m sliding distance.
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Fig. 9. Cross-sections of the worn ion-nitrided Ža., ion-oxinitrided Žb. and furnace oxinitrided Žc. samples after testing with 100 N coupling load, 1.6 mrs sliding velocity and 3000 m sliding distance.
and fragments of TiN xO y and metal iron; moreover, small titanium oxide particles, Ti 2 O 3 , are also detected. With reference to disk wear, it is not negligible and shows the same trend for all the tested couples, with a maximum at 0.8 mrs; moreover, it results higher when oxinitrided specimens are used as counterparts. The observed wear trend can be interpreted on a basis of an oxidation-dominated wear w20x; the effects of adhesive wear become important only at high sliding velocities. The different counteracting couple influences the wear behaviour of the tested samples and oxidative, abrasive and adhesive wear occurs, one mechanism prevailing in different test conditions. For ion-nitrided samples at low and medium velocities, the wear mechanism is essentially oxidative, the oxidation involving mainly the disk; a moderate microfragmentation of the compound layer is observed at 0.8 mrs, but it is not sufficient to cause third-body abrasion. At high velocity, the wear of the specimens and of the disks results comparable. The nitrides layer fails and a microfragmentation of the layer occurs, causing abrasive wear; owing to the high local temperatures, titanium oxidises and titanium oxides, Ti 2 O 3 and TiO, are produced. For both ion- and furnace oxinitrided samples, the wear behaviour is the same and it can be ascribed to oxidative and adhesive wear processes; a microfragmentation of the compound layer occurs at medium and high velocities, but it is not able to promote an abrasive wear. It can be suggested that the wear minimum, as observed at 0.8 mrs sliding velocity, can be due to the transition from oxidative wear to delamination, in accordance also with literature w20,21x; moreover, the absence of a plastically deformed subsurface layer can be ascribed to the formation of a protective oxide layer which inhibits the plastic shearing. At high velocities, the high local temperatures cause a thermal softening of the counteracting material and wear occurs mainly on the disks, by oxidation and delamination processes. On the contrary, an extended plastic deformation of the subsurface layers occurs on the titanium sam-
ples, but the high hardness of the diffusion layer succeeds in preventing a delamination. The lower wear volume observed on the ion-oxinitrided samples, in comparison with furnace oxinitrided ones, may be ascribed to the presence, in the upper parts of the diffusion layer, of a higher content of titanium–aluminium intermetallic phases, that are able to reinforce the metal but they do not cause an abrasive wear. 4. Conclusions The experimental results allow to draw the following main conclusions Žlisted below.. Ž1. The oxinitriding treatment, carried out in both a glow-discharge equipment and an air circulating furnace, allows to obtain modified surface layers with enhanced mechanical properties. By performing oxinitriding treatments, on Ti–6Al–4V samples, for 2 h at 1173 K, hardened layers are produced having ; 40 mm thickness; comparable case depth is obtained with a conventional ion-nitriding treatment for 8 h at the same temperature. Microhardness analysis shows that, on the oxinitrided samples, the highest hardness values are measured on the diffusion layer Ž; 1150 HK.. On the contrary, on the ion-nitrided samples the compound layer has the highest values Ž; 2000 HK., owing to the presence of titanium nitrides, while the maximum values of the diffusion layer are lower Ž; 950 HK.. Ž2. The wear tests, performed in sliding conditions and without lubricants for 3000 m sliding distance, show that, at low coupling loads Ž50 N., the wear of all the treated samples is very small and comparable, and it increases as sliding velocity is higher. The wear mechanism is mainly oxidative at low sliding velocities, while, at higher velocities, it can be ascribed to a combination of oxidation and delamination. No evident effects of third-body abrasion by nitride fragments have been observed. Ž3. At high coupling loads Ž100 N., the wear volumes of the treated samples are comparable when low sliding
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velocity Ž0.4 mrs. is employed. At medium and high velocities, the wear of both ion- and furnace oxinitrided samples results lower than that of the ion-nitrided ones; moreover, the ion-oxinitrided samples show lower values than the ones of the samples obtained by means of the furnace oxinitriding process. Wear behaviour of the ionnitrided samples is mainly oxidative at low sliding velocities Ž0.4 mrs., while, at higher velocities, a microfragmentation of the compound layer occurs and wear is severe due to third-body abrasion. On both the ion- and furnace oxinitrided samples wear can be ascribed to oxidative and adhesive processes. The high hardness values of the diffusion layer succeed in inhibiting the wear of the samples by delamination and adhesive wear occurs mainly on the counteracting disks. Ž4. It is suggested that the lower wear volumes measured on the ion-oxinitrided samples, in comparison with furnace oxinitrided ones, can be ascribed to the presence, in the upper parts of the diffusion layer, of a higher content of titanium–aluminium intermetallic phases, that are able to reinforce the metal without causing an abrasive wear.
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Wear resistance of Ti–6Al–4V alloy treated by means of glow-discharge and furnace treatments T. Bacci a,) , F. Borgioli a , E. Galvanetto a , F. Galliano a , B. Tesi b a
Dip. di Meccanica e Tecnologie Industriali, UniÕersita` di Firenze, Õia S. Marta 3, 50139, Florence, Italy b Dip. di Ingegneria dei Materiali, UniÕersita` di Trento, Õia di Mesiano 77, 38050, Trent, Italy Received 25 November 1999; received in revised form 1 March 2000; accepted 1 March 2000
Abstract The contemporary glow-discharge oxidising and nitriding treatment has shown to produce modified surface layers with enhanced hardness properties, in comparison with the conventional ion-nitriding process. The aim of the present study was to evaluate the tribological properties of ion-oxinitrided samples and to compare their wear behaviour with the one of furnace oxinitrided and ion-nitrided samples. At low coupling loads Ž50 N. the wear volumes of the treated samples result small and comparable for all the tested velocities. On the other hand, when high coupling loads are used Ž100 N., the wear of the ion-nitrided samples is higher than that of the oxinitrided ones, this effect becoming more remarkable as sliding velocity increases; moreover, the ion-oxinitriding treatment allows to achieve lower wear volumes than the ones obtained by means of the furnace oxinitriding process. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Ti–6Al–4V alloy; Surface treatments; Oxinitriding; Wear behaviour
1. Introduction Titanium and its alloys are widely employed in many industrial fields, due to their high specific strength and good corrosion resistance. On the other hand, these materials show relatively low shear strength and high coefficient of friction: owing to the low tensile strength, material transfer to counterfaces is easily produced, so that adhesive wear occurs; moreover, their low hardness allows damage by abrasive wear mechanisms w1x. As a matter of fact, the poor wear resistance of these materials is still a limit to a wider use. In order to improve the poor tribological properties of titanium and its alloys, many thermochemical processes have been studied in the last decades w2x. The glow-discharge ion-nitriding process allows to obtain modified surface layers, consisting of an outer compound layer and an inner diffusion layer w3–6x. In the compound layer,
) Corresponding author. Tel.: q39-55-479-6503; fax: q39-55-4796400. E-mail address: [email protected] ŽT. Bacci..
titanium nitrides, TiN Žf.c.c.. and Ti 2 N Žtetragonal. are present and very high hardness values Žup to ; 2000 HK. are obtained. The diffusion layer consists of nitrogen-rich a-Ti Žh.c.p.. crystals, embedded in the substrate; the decreasing nitrogen concentration in the solid solution produces gradually decreasing hardness values, from ; 1000 HK to the matrix values. Owing to the presence of hard nitrides layers, the wear resistance of ion-nitrided components is remarkably improved in many working conditions, especially when low and medium coupling loads and velocities are used. Titanium nitride TiN shows very high hardness values Ž; 2400 HK., but it is very brittle: when high loads and velocities are employed, a microfragmentation of the nitrided layers may occur, so that the surface is abraded by the nitride fragments w7–9x. As reported in previous papers w10,11x, the contemporary glow-discharge oxidising and nitriding treatment is able to produce, on Ti–6Al–4V samples, modified surface layers with enhanced hardness properties, if compared with a conventional ion-nitriding process. The working conditions Žtreatment atmosphere, temperature and time. have a remarkable influence on the microstructure and on the mechanical properties of the hardened layers. Thicker dif-
0043-1648r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 Ž 0 0 . 0 0 3 6 2 - 8
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fusion layers, characterised by higher hardness values Žup to ; 1300 HK., and smoother gradients can be achieved by increasing not only treatment temperature and time, but also oxygen concentration. As a matter of fact, the oxinitriding treatment allows to produce hardened layers having thickness comparable with the one of the layers obtained by ion-nitriding process, but with shorter treatment time. Preliminary wear tests carried out on Ti–6Al–4V alloy untreated, ion-nitrided and ion-oxinitrided samples show that the oxinitriding process improves the wear resistance of the samples w12x. Owing to the lack of complete and extensive information on wear properties of samples treated with the oxinitriding process, this study evaluates the wear behaviour of Ti–6Al–4V oxinitrided samples, obtained by treatments performed in both glow-discharge equipment and air circulating furnace. Wear tests were carried out in sliding conditions, without lubricants and in air, by means of a tribometer in a block-on-disk configuration, with AISI O2 hardened and stress relieved disks as counterparts. The morphology and constitution of the worn samples and the debris were examined by means of optical and electronic microscopy and X-ray diffraction techniques, in order to establish the wear mechanisms in the different test conditions. Moreover, the wear behaviour of ion- and furnace oxinitrided samples was compared with the one of ion-nitrided samples.
2. Experimental procedure Prismatic samples Ž50 = 12 = 20 mm. of annealed Ti– 6Al–4V alloy ŽTable 1. were prepared by cutting and grinding. Glow-discharge treatments were performed in a plasma equipment, as described in previous papers w6,11x. Each sample was connected to the cathode of a dc power supply; the treatment temperature was controlled by a chromel–alumel thermocouple inserted into the sample. Treatments were carried out at 1173 K, with a pressure of 1 kPa. Ion-nitriding was performed for 8 h, by using a gas composition of 80 vol.% N2 and 20 vol.% H 2 ; ionoxinitriding was carried out for 2 h by introducing air in the chamber Ž21 vol.% O 2 , 79 vol.% N2 .. Furnace oxinitriding treatments were performed in an air circulating furnace at 1173 K for 2 h.
Table 2 Chemical composition of AISI O2 steel counterfaces Material
AISIO 2
Composition Žwt.%. C
Cr
Mn
Si
P
S
1.08
0.47
1.80
0.20
0.04
0.017
After the treatment, the samples were sectioned perpendicularly to the external surface and mounted in thermosetting resin. Polishing was performed by using SiC papers up to 1200 grit and diamond pastes up to 1-mm grain size. The polished sections of the samples were etched by using Kroll’s reagent. The microstructure of the treated samples was examined by optical metallographic techniques and scanning electron microscopy ŽSEM.. X-ray diffraction analysis ŽCu K a radiation. was performed to identify the phases constituting the surface layers, and diffraction spectra were analysed by means of a fitting program using Rietveld method. This analysis was repeated after a progressive removal of very thin surface layers Ž; 5 mm, every time. with abrasive papers. Microhardness measurements ŽKnoop indenter, 25 gf. were carried out on the modified layers and on the matrix. Roughness analysis was performed on the surface of the samples before and after the treatments by using a stylus surface tester. Wear resistance was tested by means of a tribometer in a block-on-disk configuration, in sliding conditions, without lubricants and in air. The blocks for wear tests Ž8 = 12 = 20 mm. were obtained by cutting the treated samples. AISI O2 ŽTable 2. hardened and stress relieved disks Ždiameter: 50 mm; width: 14 mm. Hardness: 62.5 HRC. were used as counterparts; in order to avoid edge effects and to ensure a correct coupling between block and disk, the width of counteracting disk was larger Ž14 mm. than the block one Ž8 mm.. Tests were carried out by using coupling loads of 50 and 100 N, at sliding velocities in the range 0.4–2.0 mrs; the sliding distance was 3000 m. At least three repetitions of each test were carried out. The wear volumes of the tested samples were determined by measuring the width and the depth of the worn regions by optical microscopy techniques. The morphology and constitution of the worn samples and the debris were examined by means of optical and electronic microscopy, microprobe analysis and X-ray diffraction.
3. Results and discussion Table 1 Chemical composition of Ti–6Al–4V alloy samples
3.1. Morphology and microstructure
Material
Composition Žwt.%. Al
Fe
V
N
O
Ti–6Al–4V
5.98
0.09
3.25
0.16
0.03
The modified surface layers of the treated samples consist of an outer compound layer and an inner diffusion layer.
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Fig. 1. X-ray diffraction spectra of samples treated at 1173 K for 2 h by means of ion-oxinitriding Ža. and furnace oxinitriding Žb. processes, after the removal of the scarcely adherent part of the compound layer. Inserts Ž1. and Ž2. of Žb. show the TiN x O y diffraction peaks as obtained by increasing the scan time.
Ion-nitriding process produces a compound layer well adherent to the substrate and consisting of titanium nitrides TiN Žc.f.c.. and Ti 2 N Žtetragonal.. Both furnace and ionoxinitrided samples show a compound layer with a stratified structure; it consists essentially of TiO 2 Žrutile type;
201
tetragonal., TiN xO y Žf.c.c.. and small amounts of Al 2 O 3 Žalumina type; rhombohedral.. The stratified structure of the compound layer can be ascribed to the high content of TiO 2 in the layer, as observed by other authors w13,14x. In the oxinitrided samples, a scarcely adherent part of the compound layer has been easily removed before wear tests by ultrasonic cleaning; X-ray diffraction spectra show that, in the adherent layer, TiO 2 and small amounts of TiN xO y are present ŽFig. 1a,b.. The diffusion layer of the treated samples consists of a-Ti Žh.c.p.. crystals, rich in interstitial atoms, that are embedded in the substrate. In the oxinitrided samples, X-ray diffraction shows the presence of titanium– aluminium intermetallic phase Ti 3 Al Žhexagonal. ŽFig. 1a,b.; in ion-oxinitrided samples, small amounts of TiAl Žtetragonal. are also detected. The repetition of diffraction analysis after the progressive removal of thin layers Ž; 5 mm, every time. has shown that the intermetallic phases are present near the compound layer-diffusion layer interface. The presence of Ti–Al intermetallic phases can be ascribed to a wider stability field of these phases due to the presence of oxygen in the layer, as reported by other authors w15–17x. The lattice parameters, a, c, of a-Ti and Ti 3 Al and the Ti 3 Al relative volume amount in respect of a-Ti were evaluated, for the oxinitrided samples, by means of a fitting program using Rietveld method and they are reported in Table 3; as reference, a, c values of a-Ti for untreated samples are also shown. After the treatments, the a-Ti a, c parameters increase, as a consequence of interstitials solubilization, and result higher in furnace oxinitrided samples than in ion-oxinitrided ones; on the contrary, Ti 3 Al lattice parameters are comparable. Moreover, in the ion-oxinitrided samples, higher volume concentrations of Ti 3 Al are detected. The lower lattice parameters values shown by ion-oxinitrided samples, in comparison with furnace oxinitrided ones, can be related to a lower concentration of interstitial atoms in solid solution. This lower solubilization of interstitials may be due to the presence of intermetallic phases, that are able to solubilize oxygen w18,19x and can act as a barrier to the diffusion of interstitial atoms. The modified layer microstructures of ion-nitrided Ža., ion-oxinitrided Žb. and furnace oxinitrided Žc. samples are shown in Fig. 2. The oxinitrided samples show similar microstructures; moreover, the treatments produce diffusion layers of comparable thickness. It must be pointed out
Table 3 a-Ti and Ti 3 Al lattice parameters, a, c, and Ti 3 Al relative volume amount for ion- and furnace oxinitrided samples Sample type
a-Ti
Ti 3 AlrŽTi 3 Al q a-Ti.
a Žnm. " 0.0004
c Žnm. " 0.0006
a Žnm. " 0.0004
c Žnm. " 0.0006
Untreated Ion-oxinitrided Furnace oxinitrided
0.2925 0.2936 0.2941
0.4677 0.4728 0.4753
– 0.5787 0.5783
– 0.4695 0.4695
Ti 3 Al
– 16 9
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Fig. 2. Micrographs of the modified layer of ion-nitrided Ž t s 8 h. Ža., ion-oxinitrided Ž t s 2 h. Žb. and furnace oxinitrided Ž t s 2 h. Žc. samples treated at 1173 K.
that smaller grain size is observed in the oxinitrided samples, in comparison with the ion-nitrided ones, owing to the shorter treatment times: thus, it is expected that the oxinitrided samples result tougher and show better mechanical properties than ion-nitrided ones. Roughness analysis was performed on the samples before and after the treatments; on oxinitrided samples the analysis was carried out after the removal of the scarcely adherent part of the compound layer. Mean roughness values are reported in Table 4. In spite of shorter treatment time, oxinitrided samples show a rougher surface, in comparison with ion-nitrided ones; moreover, furnace treated samples show the highest roughness values. The rougher surface of oxinitrided samples can be ascribed to the growth mechanism of the oxide layer: in fact, as time increases, this layer becomes porous and develops a stratified structure, increasing the surface roughness w13,14x. 3.2. Microhardness analysis Microhardness measurements were performed on the diffusion layer of the treated samples. In Fig. 3, the microhardness profiles of the samples are shown; depth is measured from the compound layer–diffusion layer interface towards the substratum. The oxinitrided samples show comparable profiles and higher hardness values near the interface than ion-nitrided samples; moreover, all the treated samples show comparable case depth thickness
Ž; 40 mm.. It must be pointed out that the oxinitriding process produces the highest hardness values in the diffusion layer, while, by using the ion-nitriding process, the highest hardness values are measured in the compound layer Ž; 2000 HK., owing to the presence of titanium nitrides. Microhardness measurements were also carried out on the surface of the treated samples by using different loads, in order to qualitatively evaluate the load bearing capacity of the surface layers. The hardness values vs. applied load, in logarithmic scale, are shown in Fig. 4. The hard and brittle compound layer of the ion-nitrided samples is able to bear an increasing load up to a critical value Ž; 100 gf.; beyond this value, the compound layer fails and the load is born by the inner diffusion layer, as it is shown by a steep decrease in the measured hardness values. Thus, when high specific loads are applied, the layer can break down and the hard nitride particles can cause a severe abrasive wear. Both ion- and furnace oxinitrided samples show similar trends of hardness values vs. applied load. The surface hardness values continuously decrease as the applied load increases, due to graded hardness properties of the diffu-
Table 4 Mean roughness values Sample type
R a Žmm.
Untreated Ion-nitrided Ion-oxinitrideda Furnace oxinitrideda
0.1 1.1 1.2 1.4
a
layer.
After the removal of the scarcely adherent part of the compound
Fig. 3. Microhardness profiles of ion-nitrided Ž t s8 h. and ion- and furnace oxinitrided Ž t s 2 h. samples treated at 1173 K.
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Table 5 Phases present in the debris of the treated samples tested with 50 N coupling load Õ Žmrs.
Phases
0.4 0.8 1.6
Ti 2 O 3a , Fe 2 O 3 , Fe 3 O4 Ti 2 O 3a , Fe 2 O 3 , Fe 3 O4 , a-Fe a Ti 2 O 3a , Fe 2 O 3 , Fe 3 O4 , a-Fe a
Fig. 4. Surface microhardness vs. applied load of ion-nitrided Ž t s8 h. and ion- and furnace oxinitrided Ž t s 2 h. samples treated at 1173 K.
sion layer and to the absence of a hard compound layer. At the same critical load of the ion-nitrided samples the fail of the layer is not produced. 3.3. Wear tests Wear tests put in light that wear behaviour of the samples depends on both the test conditions Žcoupling load, sliding velocity. and the treatment conditions. 3.3.1. Tests with 50 N coupling load In Fig. 5, the wear volumes of the treated samples, as a function of sliding speed are shown; the wear volumes of all the tested samples increase as sliding velocity increases. The treated samples exhibit essentially the same wear behaviour: their wear volumes are very small and comparable, and the worn surfaces show only minor broken wear tracks. A plastically deformed subsurface layer is present on the ion-nitrided samples, but not on the oxinitrided ones. The wear of the counteracting disk is low, but it results higher when oxinitrided samples are used as counterfaces. In order to evaluate the tribological behaviour of the tested samples, X-ray diffraction and EDS microprobe
Fig. 5. Wear volumes vs. sliding velocity of ion-nitrided, ion-oxinitrided and furnace oxinitrided samples tested with 50 N coupling load and 3000 m sliding distance.
Traces.
analysis of the wear debris were carried out; the constituent phases of the debris are summarised in Table 5. At low and medium sliding velocities Ž0.4 and 0.8 mrs. wear debris consist of oxides particles: the analyses show that they are mainly iron oxides, Fe 2 O 3 and Fe 3 O4 , but also small amounts of titanium oxide, Ti 2 O 3 , are detected. Moreover, at 0.8 mrs, the diffraction analysis shows that very small amount of iron fragments is also present. At 1.6 mrs, oxides particles and iron fragments are detected in the debris. On all the tested samples the wear mechanism is essentially the same: oxidation prevails at low sliding velocities, while, at higher velocities, a combination of oxidation and delamination occurs; the wear prevails on the counteracting disks. No evident microfragmentation of the nitrided layers and no evident effects of third-body abrasion by nitride fragments have been observed, in accordance with the good wear resistance of plasma treated samples at low coupling loads w4,7–9x. The extended plastic deformation in the subsurface layers, shown by ion-nitrided specimens, can be related to a scarcely protective oxide layer. In fact, with low coupling loads, the surface heating is low and not sufficient to promote the formation of a protective oxide layer; thus, the frictional force causes a deformation of the metal surface, shearing it in the sliding direction w20x. On the contrary, the hard diffusion layer of the oxinitrided samples succeeds in preventing any plastic deformation of the subsurface layers. 3.3.2. Tests with 100 N coupling load For 100 N coupling load, the wear behaviour of the tested samples and of the counteracting disks is dependent on the sliding velocity and on the different counteracting couple, as it is shown in Fig. 6. The constituent phases of the wear debris are reported in Table 6; X-ray diffraction spectra of the debris of ion-oxinitrided samples tested with Ža. 0.4 and Žb. 1.6 mrs are shown in Fig. 7. While at low sliding velocity Ž0.4 mrs. the wear volumes of all the tested samples are comparable, at medium and high velocities the volumes of all the oxinitrided samples result smaller than those of the ion-nitrided ones; moreover, the ion-oxinitrided samples show the lowest values. The wear of oxinitrided samples shows a minimum at 0.8 mrs, while, for the ion-nitrided samples, it increases with the sliding velocity.
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Fig. 6. Wear volumes vs. sliding velocity of ion-nitrided, ion-oxinitrided and furnace oxinitrided samples tested with 100 N coupling load and 3000 m sliding distance.
At 0.4 mrs, all the treated samples have worn surfaces with superficial and broken wear tracks and show a plastically deformed subsurface layer of comparable thickness. The morphology and composition of the debris are essentially the same: they consist of oxides particles, mainly Fe 2 O 3 and Fe 3 O4 with small amounts of Ti 2 O 3 ; very small iron fragments are also detected. At 0.8 mrs, the worn surfaces show only small and broken tracks; an extended plastically deformed subsurface layer is present on the ion-nitrided samples, but not on the oxinitrided ones ŽFig. 8a,b.. The debris consist of iron and titanium oxides, Fe 2 O 3 , Fe 3 O4 and Ti 2 O 3 ; small iron fragments and compound layer particles ŽTiN, TiN xO y . are also observed. At 1.6 mrs, the ion-nitrided samples show deep wear tracks, along the whole worn surface, and the worn layer results ; 110 mm, about three times deeper than the case depth Ž; 40 mm.; a plastically deformed subsurface layer is observed and its thickness results comparable with the one on the 0.8 mrs tested samples ŽFig. 9a.. The debris consist of iron and titanium oxide particles, Fe 2 O 3 , Fe 3 O4 , Ti 2 O 3 , and TiO, and of small fragments, consisting of titanium nitride, TiN, and metal iron. Both the ion- and furnace oxinitrided samples show small and broken tracks on the worn surfaces; the worn layer thickness is comparable with the case depth Ž; 40 mm. and shows an extended
Fig. 7. X-ray diffraction spectra of wear debris of ion-oxinitrided samples tested with Ža. 0.4 and Žb. 1.6 mrs sliding velocities, 100 N coupling load and 3000 m sliding distance.
plastic shearing in the subsurface layers ŽFig. 9b,c.. The debris consist essentially of iron oxide particles and relatively large iron fragments; also small titanium oxide particles and TiN xO y fragments are detected. At 2.0 mrs, deep wear tracks are present on the worn surface of the oxinitrided samples; the worn layer thickness is comparable with the case depth and an extended plastic shearing in the subsurface layers is observed. The debris consist of iron oxide particles, Fe 2 O 3 , and Fe 3 O4 ,
Table 6 Phases present in the debris of the treated samples tested with 100 N coupling load Sample type
Õ Žmrs.
Phases
Ion-nitrided
0.4 0.8 1.6 0.4 0.8 1.6 2.0
Ti 2 O 3a , Fe 2 O 3 , Fe 3 O4a , a-Fe a Ti 2 O 3a , TiN a , Fe 2 O 3 , Fe 3 O4 , a-Fe a Ti 2 O 3 , TiN a , TiO a , Fe 2 O 3 , Fe 3 O4 , a-Fe Ti 2 O 3a , Fe 2 O 3 , Fe 3 O4a , a-Fe a Ti 2 O 3a , TiN x O ay , Fe 2 O 3 , Fe 3 O4 , a-Fe a Ti 2 O 3a , TiN x O ay , Fe 2 O 3 , Fe 3 O4 , a-Fe Ti 2 O 3a , TiN x Oy , Fe 2 O 3 , Fe 3 O4 , a-Fe
Ion- and furnace oxinitrided
a
Traces.
Fig. 8. Cross-sections of the worn ion-nitrided Ža. and ion-oxinitrided Žb. samples after testing with 100 N coupling load, 0.8 mrs sliding velocity and 3000 m sliding distance.
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Fig. 9. Cross-sections of the worn ion-nitrided Ža., ion-oxinitrided Žb. and furnace oxinitrided Žc. samples after testing with 100 N coupling load, 1.6 mrs sliding velocity and 3000 m sliding distance.
and fragments of TiN xO y and metal iron; moreover, small titanium oxide particles, Ti 2 O 3 , are also detected. With reference to disk wear, it is not negligible and shows the same trend for all the tested couples, with a maximum at 0.8 mrs; moreover, it results higher when oxinitrided specimens are used as counterparts. The observed wear trend can be interpreted on a basis of an oxidation-dominated wear w20x; the effects of adhesive wear become important only at high sliding velocities. The different counteracting couple influences the wear behaviour of the tested samples and oxidative, abrasive and adhesive wear occurs, one mechanism prevailing in different test conditions. For ion-nitrided samples at low and medium velocities, the wear mechanism is essentially oxidative, the oxidation involving mainly the disk; a moderate microfragmentation of the compound layer is observed at 0.8 mrs, but it is not sufficient to cause third-body abrasion. At high velocity, the wear of the specimens and of the disks results comparable. The nitrides layer fails and a microfragmentation of the layer occurs, causing abrasive wear; owing to the high local temperatures, titanium oxidises and titanium oxides, Ti 2 O 3 and TiO, are produced. For both ion- and furnace oxinitrided samples, the wear behaviour is the same and it can be ascribed to oxidative and adhesive wear processes; a microfragmentation of the compound layer occurs at medium and high velocities, but it is not able to promote an abrasive wear. It can be suggested that the wear minimum, as observed at 0.8 mrs sliding velocity, can be due to the transition from oxidative wear to delamination, in accordance also with literature w20,21x; moreover, the absence of a plastically deformed subsurface layer can be ascribed to the formation of a protective oxide layer which inhibits the plastic shearing. At high velocities, the high local temperatures cause a thermal softening of the counteracting material and wear occurs mainly on the disks, by oxidation and delamination processes. On the contrary, an extended plastic deformation of the subsurface layers occurs on the titanium sam-
ples, but the high hardness of the diffusion layer succeeds in preventing a delamination. The lower wear volume observed on the ion-oxinitrided samples, in comparison with furnace oxinitrided ones, may be ascribed to the presence, in the upper parts of the diffusion layer, of a higher content of titanium–aluminium intermetallic phases, that are able to reinforce the metal but they do not cause an abrasive wear. 4. Conclusions The experimental results allow to draw the following main conclusions Žlisted below.. Ž1. The oxinitriding treatment, carried out in both a glow-discharge equipment and an air circulating furnace, allows to obtain modified surface layers with enhanced mechanical properties. By performing oxinitriding treatments, on Ti–6Al–4V samples, for 2 h at 1173 K, hardened layers are produced having ; 40 mm thickness; comparable case depth is obtained with a conventional ion-nitriding treatment for 8 h at the same temperature. Microhardness analysis shows that, on the oxinitrided samples, the highest hardness values are measured on the diffusion layer Ž; 1150 HK.. On the contrary, on the ion-nitrided samples the compound layer has the highest values Ž; 2000 HK., owing to the presence of titanium nitrides, while the maximum values of the diffusion layer are lower Ž; 950 HK.. Ž2. The wear tests, performed in sliding conditions and without lubricants for 3000 m sliding distance, show that, at low coupling loads Ž50 N., the wear of all the treated samples is very small and comparable, and it increases as sliding velocity is higher. The wear mechanism is mainly oxidative at low sliding velocities, while, at higher velocities, it can be ascribed to a combination of oxidation and delamination. No evident effects of third-body abrasion by nitride fragments have been observed. Ž3. At high coupling loads Ž100 N., the wear volumes of the treated samples are comparable when low sliding
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velocity Ž0.4 mrs. is employed. At medium and high velocities, the wear of both ion- and furnace oxinitrided samples results lower than that of the ion-nitrided ones; moreover, the ion-oxinitrided samples show lower values than the ones of the samples obtained by means of the furnace oxinitriding process. Wear behaviour of the ionnitrided samples is mainly oxidative at low sliding velocities Ž0.4 mrs., while, at higher velocities, a microfragmentation of the compound layer occurs and wear is severe due to third-body abrasion. On both the ion- and furnace oxinitrided samples wear can be ascribed to oxidative and adhesive processes. The high hardness values of the diffusion layer succeed in inhibiting the wear of the samples by delamination and adhesive wear occurs mainly on the counteracting disks. Ž4. It is suggested that the lower wear volumes measured on the ion-oxinitrided samples, in comparison with furnace oxinitrided ones, can be ascribed to the presence, in the upper parts of the diffusion layer, of a higher content of titanium–aluminium intermetallic phases, that are able to reinforce the metal without causing an abrasive wear.
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