Surface chemical state of Ti powders and its alloys: Effect of storage conditions and alloy composition

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Surface chemical state of Ti powders and its alloys: Effect of storage conditions and alloy composition Eduard Hryha a,∗ , Ruslan Shvab a , Martin Bram b , Martin Bitzer b , Lars Nyborg a a b

Department of Materials and Manufacturing Technology, Chalmers University of Technology, Rännvägen 2A, SE - 412 96 Gothenburg, Sweden Forschungszentrum Jülich GmbH, Institute of Energy and Climate Research, Materials Synthesis and Processing (IEK-1), D-52425 Jülich, Germany

a r t i c l e

i n f o

Article history: Received 14 October 2015 Received in revised form 15 December 2015 Accepted 6 January 2016 Available online xxx Keywords: Titanium powder Titanium alloy powder Surface chemical state Oxide layer thickness Storage conditions

a b s t r a c t High affinity of titanium to oxygen in combination with the high surface area of the powder results in tremendous powder reactivity and almost inevitable presence of passivation oxide film on the powder surface. Oxide film is formed during the short exposure of the powder to the environment at even a trace amount of oxygen. Hence, surface state of the powder determines its usefulness for powder metallurgy processing. Present study is focused on the evaluation of the surface oxide state of the Ti, NiTi and Ti6Al4V powders in as-atomized state and after storage under air or Ar for up to eight years. Powder surface oxide state was studied by X-ray photoelectron spectroscopy (XPS) and high resolution scanning electron microscopy (HR SEM). Results indicate that powder in as-atomized state is covered by homogeneous Ti-oxide layer with the thickness of ∼2.9 nm for Ti, ∼3.2 nm and ∼4.2 nm in case of Ti6Al4V and NiTi powders, respectively. Exposure to the air results in oxide growth of about 30% in case of Ti and only about 10% in case of NiTi and Ti6Al4V. After the storage under the dry air for two years oxide growth of only about 3-4% was detected in case of both, Ti and NiTi powders. NiTi powder, stored under the dry air for eight years, indicates oxide thickness of about 5.3 nm, which is about 30% thicker in comparison with the as-atomized powder. Oxide thickness increase of only ∼15% during the storage for eight years in comparison with the powder, shortly exposed to the air after manufacturing, was detected. Results indicate a high passivation of the Ti, Ti6Al4V and NiTi powder surface by homogeneous layer of Ti-oxide formed even during short exposure of the powder to the air. © 2016 Elsevier B.V. All rights reserved.

1. Introduction High strength and ductility, corrosion resistance and high melting point combined with low density are some of the advantages of the titanium that are responsible for increasing utilization of this metal and its alloys during the last decades [1]. Thanks to the combination of the light-weight and high mechanical performance, titanium became very popular material for usage in various areas of human’s life – from the chemical and energy industry to aerospace and orthopaedics [1–7]. Alloying of titanium with aluminium, vanadium, nickel, chromium, etc., allows to significantly improve properties of the titanium alloys that makes them indispensable materials in the most demanding applications. One of such alloys – nickel titanium or nitinol (NiTi) – is a shape memory alloy that due to its excellent biocompatibility has become very popular material for biocompatible applications

∗ Corresponding author. Tel.: +46317722741; fax: +46317721313. E-mail address: [email protected] (E. Hryha).

as e.g. orthodontics, orthopaedics, urology etc. [8,9]. Ti6Al4V is a workhorse of the titanium industry and accounts for about 60% of the total titanium production [10]. Due to its excellent mechanical properties and biocompatibility, it is used in the variety of structural and aerospace applications as well as for manufacturing of the implants, including additive manufacturing [10–12]. The main disadvantage of the titanium and its alloys is their high cost in combination with manufacturing difficulties connected to the tremendous reactivity of the titanium [6,10]. Powder metallurgy (PM) can reduce production costs by possibility to produce near-net shape components/implants in one step [13]. Moreover, some complex-shape components cannot be produced by other techniques than PM. Particularly additive manufacturing (AM) is becoming more and more used for manufacturing of implants and high-performance components for aerospace applications where Ti6Al4V is the most used material nowadays [14–17]. Taking into account a high specific surface area of the powder in comparison to the bulk material, surface chemical state is the most important aspect determining usability of powder for PM and AM processing [18]. Titanium has very high reactivity meaning that

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even short exposure of the powder to the environment containing even trace amount of active species as e.g. oxygen or nitrogen will result in immediate formation of surface species. Oxidation process is characterized by extremely strong thermodynamic driving force due to a high stability of the titanium oxide. Hence, the creation of the surface species (e.g. oxides) takes place in the system in order to minimize high surface energy of the powder. Formation and stability of the oxide covering powder surface is of special importance in case of powder material [18]. Chemical composition, morphology and distribution of the oxide phases on the powder surface determine the conditions required for oxide removal/redistribution in order to form strong inter-particle necks. Strong and defect-free inter-particle connections are required to reach high mechanical performance of the final component. Oxide layer, inevitably covering powder particles, has good passivation properties but can change its thickness and composition depending on the time and temperature. Hence, handling and storage conditions as well as any pre-consolidation manufacturing steps are of the significant importance for robust manufacturing of titanium alloys by powder metallurgy routes. One more important aspect that can influence the powder and final component quality is extremely high oxygen solubility in titanium [19–22]. Hence, improper handling and storage conditions can result in increased oxide content in the powder that will be dissolved in the Ti matrix during the high-temperature consolidation. From one side this leads to a good sinterability of the powder but at the same time results in inferior mechanical properties. On the other hand, nickel titanium is characterized by the low solubility of oxygen that is assumed to be responsible for the bad sintering activity of such powder [23,24]. Investigation of the powder surface by X-ray photoelectron spectroscopy (XPS) has been proved to be very effective tool for the determination of the chemical state of the elements present on the powder surface [18,25–27]. Combination of the XPS with highresolution scanning electron microscopy (HR SEM) and energy dispersive X-ray analysis (EDX) allows to obtain information on composition of the different phases on the powder surface. These techniques enable evaluation of the distribution of the secondary phases on the nano-level (thickness of the surface oxide films, presence, size and distribution of the secondary phases, etc.) as well as powder surface morphology [18,25–27]. Developed by authors model for determination of the surface oxide thickness in case of the powder particles [25] is currently implemented in the developed software for estimation of the thickness of the homogeneous oxide layer, covering powder particle surface. Model developed is based on experimental measurements of the relative intensity of the metallic peak of the base metal utilizing XPS [26]. Present study is focused on the evaluation of the alloy composition and storage conditions (storage time and environment – dry air or inert argon atmosphere) on the surface oxide state of the Ti, NiTi and Ti6Al4V powders using described above surface analysis methodology.

2. Material and methods The powders of Ti, NiTi and Ti6Al4V, used in this study, were produced by TLS Technik GmbH & Co Spezialpulver KG (Bitterfeld, Germany). As the manufacturing route, electrode induction inert gas atomization (EIGA) of Ti, NiTi or Ti6Al4V rods was used. In the process, atomization of the melt heated by induction is done by highly pressurized argon (purity 99.9999 vol.%, oxygen below 0.5 ppm) without contact to graphite crucible or ceramic nozzle. After gas atomization, the powder batches were fractionized by the manufacturer by a sieving process to small (<25 ␮m), intermediate (25–45 ␮m) and coarse sized (45–100 ␮m) powder fractions. Powders were analyzed in as-received state shortly after manufacturing – two to four weeks after manufacturing – that is the time required for powder logistics between powder manufacturer and research organisation. Hence, storage time for such a powder is marked as “0 years” in the Table 1. Powders of the same alloys, produced using the same method by the same manufacturer, atomized two and eight years prior to analysis and stored in argon and air were also analyzed. This was done in order to estimate the effect of the storage time and conditions on the surface chemistry of the powder. It is important to note that all the powders were shortly exposed to the ambient air atmosphere during sample introduction into the XPS chamber. Chemical composition from the point of view of interstitials content (oxygen, carbon and nitrogen), powder size fraction used for analysis as well as storage conditions and time are summarized in Table 1. Specimens were prepared by mounting of the experimental powder on a carbon tape prior to analysis by X-ray photoelectron spectroscopy (XPS). Prepared samples were directly introduced into XPS analysis chamber in order to minimize sample exposure to the ambient air. For the HR SEM + EDX analysis of the powder, powder samples were prepared by lightly pressing the powder into soft aluminum plate. The surface chemical analysis of the powder was performed by means of X-ray photoelectron spectroscopy (XPS) using a PHI 5500 instrument (Perkin Elmer, Waltham, Massachusetts, USA). The base pressure in the analysis chamber was ∼10−9 mbar. The analyzed area during XPS analysis was about 0.8 mm in diameter. Hence, about 100 powder particles were analyzed at the same time giving statistically reliable average result that represent the general powder surface composition. Photoelectrons were generated by monochromatic Al K␣ source (1486.6 eV). Selected region spectra were recorded covering the Ti2p, Al2p, V2p, Ni2p, O1s, C1s and N1s photoelectron peaks. The acquisition conditions for such highresolution spectra were 23.5 eV pass energy with the step of 0.1 eV and nominal take-off angle of 45◦ . Energy calibration using Au 4f7/2 (84.0 eV), Ag 3d5/2 (368.3 eV) and Cu 2p3/2 (932.7 eV) was carried out on the daily basis under the same conditions as high-resolution spectra. It was estimated that the experimental error of the binding energy was below ±0.1 eV. Therefore, experimental error of the binding energy shift was smaller than this value. The recorded

Table 1 Summary of the studied materials – interstitials content, powder size fraction and storage conditions used prior to powder analysis. Alloy

Storage time (years)

Storage gas

Size fraction, ␮m

O, wt.%

C, wt.%

N, wt.%

Ti

2 0 0 8 2 0 0 0 0

Air Ar Air Air Air Ar Air Ar Air

−45 −45 −45 +45 −25 −25 −25 −45 −45

0,15 0,16 0,17 0,04 0,09 0,07 0,08 0,18 0,18

0,003 0,003 0,002 0,017 0,022 0,029 0,027 0,004 0,004

0,011 0,005 0,001 0,001 0,001 0,001 0,001 0,020 0,021

NiTi

Ti6Al4V

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photoelectron peaks were curve fitted utilizing the PHI Multipak software (Perkin Elmer, Waltham, Massachusetts, USA) with asymmetric curves and assuming Shirley background. Chemical shift of the elements in compounds as well as XPS fitting parameters for the Ti2p3/2 and O1s peaks, required for determination of the chemical composition of the surface oxide layer, were obtained by XPS analysis of the annealed standards for TiO2 (rutile and anatase) taken from the recent research published elsewhere [28]. Determination of the surface oxide layer thickness and compositional profiles was done by altering of ion etching and XPS analysis. The ion etching was performed in argon gas with an accelerating voltage of 4 kV and angle between the ion incidence and the sample surface of ∼50◦ . The Ar+ beam was rastered on the area from 4 × 5 mm to 2 × 3 mm giving an etching rate from 3 to 5 nm min−1 . The etch rate was calibrated on a flat oxidized tantalum foil with the well-known Ta2 O5 thickness (100 nm). Hence, the oxide thickness refers to Ta2 O5 units. The thickness of the homogeneous titanium oxide layer was evaluated utilizing representative narrow scans of titanium using the relation between the normalized intensity of the titanium metal peak and etch depth according to the procedure described elsewhere [18,25,26]. Evaluation of the thickness of the surface oxide layer was performed using theoretical model involving the effect of the setup geometry (etching and measurement angles) with respect to the surface geometry (roughness) on the etching profiles and integrated in the software “Powder XPS calculator v.1.2.4”, described elsewhere [26]. The morphology of the powder surface and composition of the secondary phases were analyzed on the powder samples by means of high resolution scanning electron microscopy (HR SEM) utilizing LEO Gemini 1550 (LEO GmbH, Oberkochen, Germany). 3. Results The spectra of XPS analysis of the powders of Ti, NiTi and Ti6Al4V shortly after manufacturing that were sealed in Ar until XPS analysis indicate presence of carbon (C1s), strong titanium (Ti2p) and oxygen (O1s) peaks, see Fig. 1. Beside these elements, traces of nitrogen (N1s) were registered in the case of Ti and Ti6Al4V powders but not in case of NiTi powder. Surprisingly enough, only traces of nickel were found in case of NiTi powder on the powder surface, see Fig. 1. In case of Ti6Al4V powder, peaks of aluminum (Al2p and Al2s) were clearly observed. However, no traces of vanadium were registered on the powder surface, see Fig. 1. This allows to assume that the powder surface is predominantly covered by titanium oxide layer [18,25–27]. Chemical state of the elements of interest – metallic or oxide state, presence of other species as nitrides, carbides, etc., on the powder surface – can be evaluated from the high-resolution narrow

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scans over the binding energy ranges of the corresponding peaks. Analysis of the shifting in the binding energy gives a possibility to deconvolute the corresponding contributions of the element in every state to the total intensity of measured peak. Concentration of the elements of interest at each etch depth was also calculated by using a high-resolution spectra. Analysis of the peak position of titanium indicates its presence in oxide state that is removed after ion etching up to 5 nm. At the final etch depth only pure metallic peak of titanium is observed, see Fig. 2(a). Oxygen peak O1s with the peak position at 530.75 eV was used as internal reference for titanium oxide peak, as described in [28]. This was done in order to take into account small shift in the energy scale connected to a possibility of the poor conductivity between some powder particles and sample holder as well as instrument instability. It is important to note that carbon peak can’t be used for internal referencing as it is very weak at larger etch depths and also shifts with the etching depth due to formation of TiC. The analysis of the high-resolution titanium peak Ti2p on the as-received powder surface indicates that in case of all the powders titanium is present in oxide state only, see Fig. 2(b). Position of the Ti2p3/2 peak on the as-received powder surface corresponds to the peak position characteristic for rutile [28]. Weak shoulder at around 454 eV in Fig. 2(b) corresponds to the metallic peak of titanium and is increasing with the etching depth. However, as it is shown in [28], titanium oxide is very sensitive to Ar+ ion etching. Hence, due to preferred etching of the oxygen atoms significant widening of the Ti2p peak happens after even slight ion etching. At the final etch depth of 20 nm, only pure metallic peak of titanium is observed in all cases, see Fig. 2(c), assuming the presence of the thin titanium oxide layer on the powder surface according to the model described elsewhere [18,25,26]. Careful analysis of the Ti2p3/2 peak position for different powders at the final etch depth of 20 nm, see Fig. 2(c), clearly indicates a shift in peak position in case of NiTi of up to about 0.7 eV whereas titanium peak position in case of Ti and Ti6Al4V powders is in excellent agreement with the peak position for the Ti standard (plate). All the peak positions for the oxide peak (as-received powder surface) and metallic peaks (after etching to 20 nm etch depth) for studied powders and their comparison with the standards, measured by the same instrument at the same conditions, are summarized in Table 2. Change in the chemical composition of the surface with etching depth for studied powders indicates the enrichment of the powder surface in oxygen in case of all three studied powders, see Fig. 3 and Table 3. Steep decrease in oxygen concentration is observed up to above 5 nm etch depth after which slope of the decrease in oxygen content becomes weaker. Removal of the surface contaminants – etching of 1 nm – results in significant decrease in the carbon peak that results in the increase in intensity of the titanium peak. Further etching leads to the significant decrease in the oxygen peak intensity that also results in increasing of the intensity of the peaks of the metallic elements. Nitrogen has almost constant concentration of around 4 at.% at all etching depths in case of Ti and Ti6Al4V powders. This indicates the presence of particulate nitrides, size of which is larger than the etching depth applied in this study. Only traces of nitrogen that were close to the detection limit of the instrument Table 2 Positions of the Ti2p3/2 peak on the as-received powder surface (oxide) and after etching to 20 nm (metal) together with standard peak positions for TiO2 (rutile) and metallic Ti.

Fig. 1. XPS survey spectra of Ti, NiTi and Ti6Al4V powders shortly after manufacturing and sealed in Ar after removal of the surface contaminants (1 nm Ar+ etching).

Ti-Standard TiO2 -Standard Ti powder NiTi powder Ti6Al4V powder

Ti2p3/2 oxide, eV

Ti2p3/2 metal, eV

O1s, eV

– 459.52 459.26 459.24 459.15

454.11 – 454.11 454.82 454.07

530.75

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Fig. 2. Position of the Ti2p peak in case of pure titanium powder in as-received state and after etching to 20 nm (a); position of the Ti2p peak on the as-received surface of the Ti, NiTi and Ti6Al4V powders shortly after manufacturing together with the peak position for TiO2 standard (rutile) (b); position of the Ti2p peak after 20 nm ion etching of the surface of the Ti, NiTi and Ti6Al4V powders and together with the peak position for Ti standard (c).

Fig. 3. Chemical composition of the surface of the Ti, NiTi and Ti6Al4V powders shortly after manufacturing vs. etch depth.

are observed in case of NiTi powder, see Table 3. It is important to note that only elements, present in the alloy, were observed on the powder surface. This is the indication of the very clean powders – no traces of Ca, Si and Al that can be observed on the surface of industrial Ti-based powders. Hence, it allows to make a clear conclusion on the effect of the alloy composition on the surface state of the powder. Analysis of the concentration profiles of the metallic elements in case of alloyed powders – NiTi and Ti6Al4V – indicates inhomogeneous distribution of the alloying elements on the powder surface as well, see Fig. 3 and Table 3. This is even more pronounced if only metallic concentration of the alloying elements is taken into account, see Fig. 4. Depletion of the surface layer in nickel in case of NiTi powder is clearly evident, see Fig. 4, where compositions are equalized only after above 10 nm ion etching. Concentration profiles also indicate that as titanium is enriched in the top 10 nm of the powder surface, it is further depleted at depths below 10 nm. In case of Ti6Al4V powder, enrichment of the top layer of up to 3 nm in aluminum is clearly evident, see Fig. 3, after which

concentration of aluminum is stabilized with further etch depth. Vanadium is absent on the as-received powder surface. Detectable traces of vanadium appear after about 2 nm of ion etching after which its concentration is close to the alloy composition, see Fig. 4. Apparent relative cation concentration can be obtained from the high-resolution scans over the binding energy of the metallic elements, able to form oxides in the system [18]. In order to get relative cation composition of the oxide phases at different etch depths, oxide peak areas were extracted from the superimposed metal/oxide peaks and normalized to 100%. Hence, this allows to evaluate what is the oxide composition at different etching depth taking into account that the amount of the oxides is significantly decreasing with the etching depth. In case of the pure titanium powder, titanium is the only element forming oxide on the powder surface according to the XPS analysis of the powder at different etch depths, see Figs. 1 and 2. In case of NiTi powder, only traces of Ni were observed on the asreceived powder surface, see Fig. 4. After even slight etching nickel was present only in metallic state, as Ni2p3/2 XPS peak position at 852.3 eV corresponds to metallic nickel. This indicates that oxide in

Table 3 Chemical composition of the surface of the Ti, NiTi and Ti6Al4V powders shortly after manufacturing (at.%). Etch depth

as-rec. 1 2 3 4 5 7.5 10 20

Ti

NiTi

Ti6Al4V

Ti

O

C

N

Ti

O

C

N

Ni

Ti

O

C

N

Al

V

18.8 29.5 35.5 36.7 39.3 38.6 42.7 43.5 55.4

41.2 49.7 46.8 44.5 41.1 39.1 36.8 32.3 22.3

38.7 18.3 14.8 15.3 15.8 19.2 16.3 20.4 18.9

1.3 2.5 2.9 3.5 3.8 3.1 4.2 3.8 3.4

16.6 29.0 31.4 33.3 33.3 31.1 32.5 28.6 30.1

42.9 52.6 51.7 47.8 42.4 38.4 27.6 21.0 14.2

40.1 17.1 14.0 14.0 15.7 17.2 12.6 18.3 12.5

0.1 0.2 0.4 0.5 0.4 0.6 1.2 1.4 1.7

0.3 1.1 2.5 4.4 8.2 12.7 26.1 30.7 41.5

16.8 26.5 29.1 30.9 32.5 32.5 40.1 36.3 44.6

46.6 46.0 43.4 40.3 38.7 38.0 32.8 31.1 25.0

30.1 18.2 19.9 20.2 19.9 21.1 18.0 21.7 15.6

1.8 3.4 3.5 4.0 3.9 4.0 4.5 4.5 7.7

4.4 5.2 3.5 3.8 4.1 3.8 3.4 4.9 5.3

0.0 0.0 0.6 0.8 0.9 0.6 1.2 1.5 1.8

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Fig. 4. Chemical composition (only metallic elements) of the surface of the NiTi and Ti6Al4V powders shortly after manufacturing vs. etch depth.

the case of NiTi powder is also formed only by titanium. The situation is more complicated in case of the Ti6Al4V powder. This alloys contains three alloying elements characterized by strong tendency to the oxide formation [29] and all of them are present in the top surface layer of the powder, see Fig. 4. Evaluation of the oxide state of the Ti, Al and V indicates that the relative concentration of the titanium cation decreases with etching depth with significant drop after above 5 nm, see Fig. 5. This is the indication of the presence of continuous Ti-oxide layer [18,25]. Un-etched (shaded) areas of the powder particles during Ar-etching are assumed to be responsible for traces of residual titanium oxide which are present after final etching to 20 nm. Vanadium is observed only in metallic state and in nitrides at all etch depths. Hence, it does not take part in the formation of oxide phases in the system. Aluminum, on the other hand, shows significant enrichment on the top powder surface with the initial decrease up to above 3 nm. Further increase up to ∼65% at final etch depth of 20 nm was registered, see Fig. 5. In general, relative concentration of the aluminum in the surface oxide phases is significantly higher (5 to 15 times) than its content in the alloy. This can indicate the presence of fine particulate oxides rich in Al-oxide embedded into the homogeneous Ti-oxide layer, covering powder surface [18]. The thickness of the surface titanium oxide layer was evaluated using representative narrow scans of titanium utilizing methodology developed in [25,26] and “Powder XPS calculator v.1.2.4” [26]. This methodology uses the relationship between the normalized intensity of the titanium metal peak and etch depth. Intensity of the metallic peak of titanium at every etch depth was obtained by extraction of the metal intensity peak from the intensity of superimposed Ti peaks from high energy resolution narrow scans. Electron mean free path  = 17 A˚ for titanium oxide was used and exact value of the metal intensity ratio was evaluated by “Powder XPS calculator v.1.2.4”. As an example, Fig. 6 illustrates relative

Fig. 6. The normalized intensity of Ti-metal (Ti2p3/2 -peak) and oxygen (O1s–peak) vs. etch depth, indicating surface oxide layer thickness of ∼2.9 nm in the case of Ti6Al4V powder (as-produced, Ar).

intensities of metallic titanium and oxygen vs. etch depth. Evaluation indicates the thickness of the homogeneous titanium oxide layer to be 2.9 nm for Ti6Al4V powder shortly after manufacturing and sealed in Ar. Thicknesses of the titanium oxide layer for different powders and storage conditions are summarized in Table 4. The analysis of the surface composition of the powder, stored in the air during the time between powder production and surface analysis by XPS, see Fig. 7, does not indicate any significant differences in comparison to the powder stored in Ar, see Fig. 3. Only one difference that can be observed is the slower decrease of the oxygen peak in case of the powder stored in dry air, see Figs. 7 and 3. This is especially evident in case of NiTi powder that indicates thicker oxide layer formed on the powder. This is confirmed by the measurements of the oxide thickness, see Table 4. More careful analysis of the distribution of the metallic elements with etch depth for powders, stored in air, indicates slightly higher enrichment in Ti on surface of the NiTi powder. This is also in agreement with the increasing in thickness of the titanium oxide layer on the powder surface. In case of Ti6Al4V powder, slightly higher enrichment in Al on the surface of the powder, exposed to the air, was also detected.

Table 4 Effect of alloy composition and storage conditions (time and gas) on the thickness of the titanium surface oxide layer in case of the Ti, NiTi and Ti6Al4V powders (in nm). Time (years)

Fig. 5. Relative cation concentration vs. etch depth for Ti6Al4V powder.

0 2 8

Ti

NiTi

Ti6Al4V

Ar

Air

Ar

Air

Ar

Air

2.9 – –

3.8 3.9 –

4.15 – –

4.6 4.8 5.3

2.9 – –

3.2 – –

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Fig. 7. Chemical composition of the surface of the Ti, NiTi and Ti6Al4V powders shortly after manufacturing vs. etch depth, stored in the air.

Fig. 8. Chemical composition (only metallic elements) of the surface of the NiTi powders stored in the air for up to 8 years vs. etch depth.

Storage for longer time has a stronger effect on the chemical composition of the surface layer of the powder but still the effect is not significant. Almost no change in composition was detected for the powder stored in dry air for 2 years in comparison with the fresh powder. The most pronounced differences were observed in case of NiTi powder, see Fig. 8. Presence of higher content of Ni in top surface layer of NiTi powder stored for 8 years in air in comparison with traces of Ni in case of powders, stored for 0 and 2 years, was observed, see Fig. 8. This can be connected to the slight difference in manufacturing conditions eight years ago with probably modernized powder atomization facility these days. Difference in powder size – larger particle size (>45 ␮m) for powder stored for 8 years in comparison with finer powder (<25 ␮m) manufactured two years ago and fresh powder, see Table 1, can have some minor effect as well. Enrichment of surface layer in Ti increases with the storage time in the air, see Fig. 8, that is connected to the increase in the thickness of the titanium oxide as well, see Table 4. Increase in thickness of the titanium oxide

layer, covering powder particles, with the storage time is the most pronounced in case of NiTi powders, see Table 4. SEM observation of the powder surface indicates high purity of the powder particles surface with a clearly visible solidification structure, see Fig. 9. High purity of the powder surface was observed even in case of the powders exposed to the air and even stored in air for couple of years, see Fig. 10. However, observation of the surface of the NiTi powder manufactured 8 years ago and stored in air indicates the presence of some irregularities on the powder surface. Structure of the powder is microcrystalline and this is why solidification structure is not visible. However, powder surface is clean. Microcrystalline structure is characteristic for all studied powders. For Ti and Ti6Al4V powders, see Figs. 9 and 10, solidification structure can be seen for many powder particles indicating that cooling rate during powder manufacturing is just on a border between cellular/dendritic and planar solidification. It is important to note that no any particulate phases were registered on the powder surface.

Fig. 9. SEM images of particles of Ti6Al4V powder analyzed shortly after manufacturing, showing powder morphology and solidification structure visible on the powder surface at high magnification.

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Fig. 10. SEM images of particles of Ti powder, stored in air for 2 years, showing powder morphology and solidification structure visible on the powder surface at high magnification.

The microcrystalline structure of the powders was confirmed by Xray diffraction (XRD) analysis which is in the correspondence with the powder producer specification [30]. 4. Discussion Distribution of the metallic elements with etching depth and their correlation with the distribution of the oxygen, see Figs. 3 and 7, clearly indicates presence of the surface oxide, covering powder particle surface. Presence of only weak shoulder of titanium metallic peak on the as-received powder surface, see Fig. 2, assumes that surface oxide is of continuous nature. Thickness of surface oxide layer is in the range of no more than three electron free path length for TiO2 (otherwise shoulder of Ti-metallic peak will not be visible) according to model presented elsewhere [18,25,26]. Analysis of the distribution of the metallic elements with etching depth, see Fig. 4, clearly indicates that in case of Ti and NiTi powders, surface oxide is formed by titanium oxide (rutile). In case of Ti6Al4V powder aluminum is additionally present in the surface oxide layer. Enrichment of the surface oxide layer in Al is confirmed by evaluation of the relative cation concentration, see Fig. 5, that clearly indicates 5 to 15 times higher content of Al in surface oxide than concentration of Al in the alloy. It is important to note the absence/trace amount of nickel on the powder surface in case of NiTi alloys, see Figs. 3, 4 and 7. Nickel concentration is increasing with etching depth and its peak position corresponds to metallic nickel in the alloy matrix. This means that nickel does not take part in the formation of the surface oxide layer and powder surface is passivated by titanium oxide (rutile). In case of vanadium, a high number of vanadium oxides with different stability [31] can also lead to the presence of some complex oxide phases. However, results of the XPS analysis indicate the absence of vanadium in the surface oxide layer, see Figs. 4 and 5. Increasing intensity of vanadium with increasing etching depth is solely connected to

the V2p3/2 peak position at ∼512.15 eV, corresponding to metallic vanadium [31]. Detected surface oxide composition, observed in case of titanium powders as described above, is in agreement with the thermodynamic stability of oxides, see Fig. 12, where TiO2 has higher stability than oxides of Ni or V. This describes the formation of layer of titanium oxide on the surface of the powders. Only oxide of aluminum has higher stability that is in agreement with observed Al-enrichment on the Ti6Al4V powder surface. Steep increase of the relative intensity of metallic titanium peak with etching depth, see Fig. 6, clearly indicates the presence of the titanium oxide layer, covering powder particles in case of all three studied alloys. Absence of any particulate phases on the surface of the powders, see Figs. 9–11, confirms this conclusion. Presence of the homogeneous titanium oxide layer, composed of rutile, on the surface of the Ti and NiTi powder, is confirmed by the elements distribution, Figs. 3, 4, 7 and 8, and high-resolution SEM analysis, presented in Figs. 10-11. This is in agreement with the thermodynamic stability of oxides, Fig. 12, as discussed above. However, situation is more complicated when it comes to the oxide composition in case of Ti6Al4V powder. Distribution of the alloying elements, see Fig. 4, and especially cation composition of surface oxide with etching depth, see Fig. 5, assumes the presence of fine secondary phases reach in aluminum oxide according to the model, developed in [18,27]. However, observation of the powder surface at high resolution does not reveal the presence of any particulate phases on the powder surface, see Fig. 9. Hence, there are two possibilities of the distribution of the aluminum oxide on the powder surface: i) aluminum oxide can be dissolved in the homogeneous oxide layer on the powder surface formed by titanium oxide (rutile); ii) aluminum oxide particles are present as very fine precipitates (up to 50 nm), located in the inter-cell/interdendritic areas, not possible to reveal by SEM. Critical evaluation of the TiO2 -Al2 O3 phase diagram [32] indicates the presence of

Fig. 11. SEM images of particles of NiTi powder, stored in air for 8 years, showing irregular powder morphology visible on the powder surface at high magnification.

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Fig. 12. Ellingham-Richardson diagram indicating stability of the oxides (HSC Chemistry 8.0).

the aluminum titanate (Al2 TiO5 ) that is formed at high temperatures (above 1200 ◦ C) and is characterized by wide stability range. The same authors also emphasize high thermodynamic stability of aluminum titanate in wide range of oxygen partial pressures – between 0.21 and 2.5 × 10−9 bar. This means that there is a high probability that this compound was formed during initial stages of metal particle solidification even in such low oxygen partial pressures as used during powder manufacturing. However, aluminum titanate undergoes eutectoid-type decomposition at the temperatures below ∼1200 ◦ C into TiO2 (rutile) and ␣-Al2 O3 . Solubility of the aluminum in the rutile is close to zero. Therefore eutectoid-type decomposition of the aluminum titanate results in the presence of the two-phase field (rutile + corundum). Hence, based on the described above thermodynamics of the TiO2 -Al2 O3 system, it can be assumed that aluminum oxide is present as very fine precipitates (< 50 nm) in homogeneous oxide layer, most probably located in the inter-cellular/inter-dendritic regions on the powder surface. Evaluation of the surface oxide thickness, see Table 4, indicates similar thickness of the homogeneous surface oxide layer, formed by TiO2 , for Ti and Ti6Al4V powders, sealed in Ar and analyzed shortly after manufacturing. However, about 40% thicker layer of titanium oxide is observed in case of NiTi powder, indicating a better passivation of the Ti-rich powders in comparison with equiatomic intermetallic powder. Taking into account thickness of the surface oxide, it can be expected that oxygen content in asmanufactured powder has to be higher in case of NiTi powder and rather similar in case of Ti and Ti6Al4V powders. However, this is not the case – chemical analysis of the powder, see Table 1, indicates similar oxygen content for Ti and Ti6Al4V powders (∼0.17 wt.% O) but oxygen content in NiTi powder is approximately the half of this value (∼0.08 wt.%O). In addition, it has to be emphasized that powders of Ti and Ti6Al4V have similar powder size – <45 ␮m, whereas NiTi powder particles are even finer – <25 ␮m. This means that due to finer powder – larger surface area has to result in higher amount of the oxygen, enclosed in the surface oxide layer [18]. This can be explained by careful analysis of the development of the oxygen O1s peak with etch depth during XPS analysis, see Fig. 13. At the as-received surface, chemical shift of titanium indicates its presence in oxide state at approximately the same position for all the powders – ∼459.5 eV, see Fig. 2, corresponding to the rutile according to [25]. Oxygen peak O1s with the peak position at 530.75 eV was used as the internal reference for titanium oxide peak during all etching depths, as described in [28]. However, in case of Ti and Ti6Al4V powders, widening of the oxygen O1s peak was detected with the shift in peak position to higher binding energy, see Fig. 13. Deconvolution of the peak at larger etch depths indicates the presence of two peaks, one at 530.75 eV, corresponding to the oxygen from the residues of the surface oxide

Fig. 13. Deconvoluted peak of oxygen O1s for Ti-powder in as-received state and after etching to 20 nm, indicating presence of two oxygen peaks at larger etch depths.

layer – un-etched areas of the powder surface that contribute to the measured signal [18,25–27]. Second oxygen peak, with about 1 eV higher binding energy, is most probably corresponding to the oxygen, dissolved in the Ti-matrix. Thermodynamic simulation of the oxygen solubility in titanium, see Fig. 14, indicates high solubility of oxygen in titanium matrix – up to 33 at.%. This confirms the indication by XPS analysis concerning the presence of the dissolved oxygen in case of Ti and Ti6Al4V powders, see Fig. 13. Intermetallic phases as NiTi are characterized by low solubility to oxygen that is also confirmed by single oxygen peak during XPS analysis. Hence, it can be concluded that higher oxygen content, registered for the Ti and Ti6Al4V powders in comparison with NiTi powder, is connected to higher amount of the dissolved oxygen and not more extensive oxidation of the powder. Vice versa, powders of Ti and Ti6Al4V are characterized by even thinner layer of titanium oxide due to better passivation of the powder surface. Mechanism of the dissolution of the oxygen in the powder matrix is not clear. It can be connected to the initial

Fig. 14. Phase diagram Ti O, indicating high solubility of oxygen in titanium – up to 33 at.%.

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oxygen content in the rods, used for electrode induction inert gas atomization (EIGA). Another reason could be oxygen dissolution during the powder solidification process by dissolution of the oxygen from the gas in the atomizer. There is a possibility of “knockingin” oxygen from the surface oxide layer during Ar+ -etching. However, in that case similar situation could be expected in case of NiTi. Better passivation of the powder surface in case of titanium-rich powders in comparison with the NiTi powder is confirmed by the characterization of the change in surface oxide thickness with time, see Table 4. From the presented results it is clear that exposure to the air results in surface oxide layer thickness growth of about 10% in case of Ti6Al4V and NiTi powder and about 30% in case of pure Ti-powder. However, pure Ti-powder indicates better stability of the Ti-oxide layer with time – only ∼3–4% increase in the thickness of the Ti-oxide layer after storage for 2 years in case of both, Ti and NiTi. Storage under dry air in case of NiTi powder resulted in surface oxide thickness increase up to 5.3 nm, that is about 30% thicker in comparison with the as-atomized powder. Hence, oxide thickness increase of only ∼4% and ∼15% after storage for two and eight years, respectively, in comparison with the powder, shortly exposed to the air after manufacturing, indicates a significant importance of the initial powder exposure to the oxidizing environment, even in case of trace amount of oxygen. Whenever passivation oxide layer of titanium of about 3 to 4 nm is created on the powder surface, it possesses good passivation properties, able to protect powder from further oxidation during following powder handling. However, exposure of the powder to the higher temperatures during handling happens during some manufacturing processes (e.g. powder preheating/pre-sintering during additive manufacturing, drying of the powder, etc.). This can change passivation properties of the surface oxide layer as well as affect dissolution of the oxygen in the powder matrix and requires more detailed studies. 5. Conclusions Results of the X-ray photoelectron spectroscopy and highresolution electron microscopy analysis of the Ti, NiTi and Ti6Al4V powders in as-manufactured state and stored for different time (0 to 8 years) in dry argon and air atmospheres combined with thermodynamic simulation of the oxide stability in different systems was performed. Obtained results clearly indicate that the amount of oxides, their composition and spatial distribution for studied metal powders are determined by the alloy composition and exposure to the oxygen containing atmospheres as well as time of the exposure. In case of the all three studied Ti-based alloys, due to the high stability of the Ti-oxide, surface of the powder is covered by homogeneous Ti-oxide layer with the thickness of couple nanometers. Oxide thickness is determined by the alloy composition, manufacturing and handling conditions. Results indicate that powder in as-atomized state is covered by homogeneous Ti-oxide layer with the thickness of ∼2.9 nm in case of Ti and Ti6Al4V and ∼4.2 nm in case of NiTi powder. Exposure to the air results in oxide growth of about 30% in case of Ti and about 10% in case of NiTi and Ti6Al4V. Storage under the dry air for two years results in oxide growth of only about 3–5% in case of Ti and NiTi. NiTi powder, stored under the dry air for eight years, indicates oxide thickness that is about 30% thicker in comparison with the as-atomized powder. Oxide thickness increase of only ∼15% during storage for eight years in comparison with the powder, shortly exposed to the air after manufacturing, was detected in case of NiTi powder. Hence, high passivation of the Ti, NiTi and Ti6Al4V powder surface by homogeneous layer of Ti-oxide formed even during the short exposure of the powder to the oxygen containing atmosphere, even at ppm levels of oxygen, can be concluded from the performed studies. Presence of fine precipitates of aluminum oxide (<50 nm) in the inter-cellular/inter-dendritic areas on the powder surface is

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assumed in case of Ti6Al4V powder. Only a trace amount of nickel was detected on the surface of the NiTi powder. Oxygen O1s peak at the binding energy of ∼531.75 eV, that is about 1 eV higher than the binding energy of oxygen peak characteristic for rutile (∼530.75 eV), was detected in the powder at large etch depths, that is assumed to be connected to the oxygen, dissolved in the Ti-matrix of the powder.

Acknowledgements Support from the Chalmers Areas of Advance in Materials Science and Production as well as funding from the strategic innovation program LIGHTer, provided by Vinnova, are gratefully acknowledged.

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