metals Article
Hydrogen Reduction in MEP Niobium Studied by Secondary Ion Mass Spectrometry (SIMS) Tadeusz Hryniewicz 1, *, Piotr Konarski 2 1 2 3
*
ID
and Ryszard Rokicki 3
Division of BioEngineering and Surface Electrochemistry, Department of Engineering and Informatics Systems, Faculty of Mechanical Engineering; Koszalin University of Technology, Koszalin PL 75-620, Poland Vacuum Measurement Laboratory, Institute of Tele- and Radio Technology, Warszawa PL 03-450, Poland;
[email protected] Electrobright, Macungie, PA 18062, USA;
[email protected] Correspondence:
[email protected]; Tel.: +48-94-347-8244
Received: 4 September 2017; Accepted: 10 October 2017; Published: 20 October 2017
Abstract: Niobium, as pure metal and alloying element, is used in a variety of applications, among them in nuclear industries. Niobium is incorporated into nuclear fission reactors due to its enormous strength and low density. Surface finishing of niobium is often performed in electrochemical polishing processes in view of improving its smoothness, corrosion resistance and its surface cleanability. However, the presently used electropolishing process (EP) is intrinsically linked to the subsurface hydrogenation of niobium, which measurably degrades its properties. This is why the annealing operation is used to remove hydrogen from electropolished niobium that is a costly and time-consuming process. The traditional electrolyte consisting of a mixture of 96% H2 SO4 /49% HF acids by volume in a 9:1 ratio has been substituted for the new one, being a mixture of 70% methanesulfonic acid with 49% hydrofluoric acid by volume in a 3:1 ratio. Moreover, the additional imposition of a magnetic field during the electropolishing process (MEP) further increases hydrogen removal, when compared to the hydrogen content achieved by the electropolishing process alone. The aim of the study is to reveal a methodic approach and showing decreasing hydrogenation of niobium samples after consecutive steps of electrochemical polishing. Secondary ion mass spectrometry (SIMS) was used to measure the hydrogen content in the surface layer of as-received AR niobium and in the samples after EP and MEP processes. Keywords: niobium; magnetoelectropolishing (MEP); SIMS; hydrogen content; de-hydrogenation
1. Introduction Niobium, formerly columbium, is a rare, soft, grey and ductile transition metal with the symbol Nb. It is used in a variety of applications, such as superconducting magnets, medical devices, capacitors, optical lenses, barometers, superconducting radio frequency cavities, or electromagnetic radiation detectors, and nuclear industries [1,2]. Moreover, as the alloying element it is used in stainless steels, nickel-, cobalt-, zirconium-, and iron-based super-alloys which are used in aviation industry for jet engines components, rocket sub-assemblies, heat resistant, combustion equipment, etc. [2–4]. Niobium improves the strength of the alloys, especially at low temperatures. It acts as a key element in nuclear fission reactors, due to its enormous strength, low density, high melting point (2477 ◦ C), and low neutron absorption [5]. Niobium is very resistant to corrosion due to a layer of oxide formed on its surface. Although its corrosion resistance is not as outstanding as that of tantalum, the lower price and greater availability make niobium attractive for numerous demanding applications. Niobium forms oxides in the oxidation states +5 (Nb2 O5 ), +4, +3, and the rarer oxidation state +2 (NbO). Most common is the pentoxide, precursor to almost all niobium compounds and alloys used in engineering. Pure niobium is used Metals 2017, 7, 442; doi:10.3390/met7100442
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specifically for superconducting radio frequency (SRF) cavities [6,7], and as niobium-zirconium alloys, for core elements of pressurized water reactors [8,9]. Developed recently, the Zr-Nb alloys provide reliable operation of fuel elements and rod arrays in active reactors and are a basis for new modifications of these alloys created for raising the service properties [8]. Surface finishing of niobium covers both mechanical/abrasive polishing, chemical, and electrochemical polishing processes, with the last also improving surface cleanability. The smoothness and subsurface hydrogen concentration are the most important and often critical factors of the niobium quality. Nowadays, the best method to obtain the smoothest surface of niobium is the electropolishing process. However, the presently used electropolishing process is intrinsically linked to the subsurface hydrogenation of niobium, possibly resulting in the creation of nanohydrides. The annealing operations used to remove hydrogen from electropolished niobium are costly and time-consuming processes. One should mention that the firing of niobium under a vacuum at very high temperature results in a considerable recrystallization [6–9]. Electropolishing processes have been developed for eight decades now to improve surface finishing of metals and alloys [10–24]. Beginning from surface roughness decay and gloss effects appearance, many more interesting features in the metal surface were revealed [25,26], including corrosion resistance and a high improvement of mechanical properties of the treated parts. In recent years, attention was drawn to the problem of hydrogenation of the surface layer after electropolishing. The presently used process for niobium electropolishing is based on the electrolyte consisting of a mixture of 96% H2 SO4 /49% HF acids by volume in a 9:1 ratio. The process gives a satisfactory smooth finish of niobium (average maximum height of the profile Rz in the range of 300 nm), however hydrogenation of subsurface layer [6–8] often appears difficult to avoid. Our earlier studies have shown that by using different electrolytes, which consist of a mixture of 70% methanesulfonic acid with 49% hydrofluoric acid by volume in a 3:1 ratio, the comparable smoothness of niobium surface is obtained [24]. The new electrolyte formula does not introduce hydrogen to the subsurface of an electropolished niobium layer with concentration as the currently used electrolyte does. Additionally, improved surface roughness, by reducing Rz down to below 0.2 µm, was achieved [24] by the imposition of a magnetic field upon the electropolishing process, by the process named magnetoelectropolishing MEP [18,19,24,27–34]. The required removal of metal is sped up around 15 times when compared to metal removal by the presently used process [24]. Concerning hydrogen behavior in metals, Pundt and R. Kirchheim [35] provide an interesting insight into metal-hydrogen (M-H) systems. They show that the hydrogen solubility of M-H systems is strongly affected by the morphology and microstructure and the stress between regions of different hydrogen concentration. These problems have been investigated for years now by many researchers, with the obvious conclusion that, in thin films deposited on stiff substrates, compressive stresses evolve during hydrogen loading because the films are effectively clamped to substrates. However, much less attention was directed toward de-hydrogenation of electropolished surfaces, with no extended studies on the effect of magnetic field on M-H system during and after MEP treatment. Problems associated with hydrogen detection by SIMS are discussed in several papers [36–45], with only two of them [44,45] directed straight at the hydrogenation of metal surfaces after magnetoelectropolishing MEP. The main problems found in the literature include: residual gas contamination, primary ion beam contamination and instrumental background. SIMS analysis of hydrogen plays an important role in corrosion studies [40], transistor structure characterization [41], geology studies [42], and hard coating analysis [43]. Electrochemical treatment processes are often characteristic with hydrogenation of metal surface layer. Apart from the electrolyte used, it appears, the additional imposition of a magnetic field during the electropolishing process (MEP) further increases hydrogen removal, when compared to the hydrogen content achieved by the electropolishing process (EP) alone. The aim of the study is to reveal a methodic approach and show decreasing hydrogenation of niobium samples after consecutive
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after consecutive steps of electrochemical polishing. Secondary ion mass spectrometry (SIMS) was steps to of measure electrochemical polishing. Secondary mass spectrometry (SIMS) wasAR used to measure the used the hydrogen content [44,45] ion in the surface layer of as-received niobium and the hydrogen content [44,45] in the surface layer of as-received AR niobium and the samples after EP and samples after EP and MEP processes. MEP processes. 2. Method 2. Method 2.1. 2.1. Niobium Niobium Samples Samples and and Surface Surface Treatment Treatment Samples dimensions 12 mm × × 13 Samples of of dimensions 12 mm 13 mm mm (Residual (Residual Resistance Resistance Ratio Ratio 200 200 (RRR) (RRR) >> 99% 99% pure pure niobium, WAN CHANG—Albany, OR, USA), cut off from a niobium sheet 3.2 mm thick, were used niobium, WAN CHANG—Albany, OR, USA), cut off from a niobium sheet 3.2 mm thick, were used for prepared forfor the studies, two samples in for the the experiments. experiments.Three Threegroups groupsofofniobium niobiumsamples sampleswere were prepared the studies, two samples each group. The first two samples, without any further treatment were marked AR—as received, in each group. The first two samples, without any further treatment were marked AR—as received, next and two two others others were were magnetoelectropolished—marked magnetoelectropolished—marked next two two were were electropolished—marked electropolished—marked EP, EP, and MEP. The MEP MEP set-up set-up is is presented presented in in Figure Figure 1. 1. Niobium placed inside MEP. The Niobium sample sample SS as as anode anode A A is is placed inside the the electrolytic cell between two aluminum rods R connected as cathode C. The electrolytic electrolytic cell between two aluminum rods R connected as cathode C. The electrolytic cell cell with with electrolyte electrolyte E E is is surrounded surrounded by by aa container container with with cooling cooling water water CW CW to to keep keep aa constant constant temperature. temperature. For magnetoelectropolishing MEP, a stack of magnetic rings MS surrounding For magnetoelectropolishing MEP, a stack of magnetic rings MS surrounding the the system system was was used used (Figure 1). (Figure 1).
electrolytic cell with magnetic magnetic field: A—anode, Figure 1.1.Electropolishing Electropolishing electrolytic cell superimposed with superimposed field: C—cathode, A—anode, MS—stack of MS—stack ring magnets, water, E—electrolyte, R—aluminum rod, S—niobium sample. C—cathode, of CW—cooling ring magnets, CW—cooling water, E—electrolyte, R—aluminum rod,
S—niobium sample.
The electropolishing processes, EP and MEP, were performed for two hours in stagnant electrolyte mixture 70% methanesulfonic acid (CH H) andwere 49% hydrofluoric acid (HF) by volume in 3:1 3 SO3MEP, Theofelectropolishing processes, EP and performed for two hours in stagnant volume ratio. They were carried out in potentiostatic conditions 5 V49% in constant temperature of 25 ◦by C. electrolyte mixture of 70% methanesulfonic acid (CH 3SO3H) of and hydrofluoric acid (HF) The magnetoelectropolishing MEP was performed in the same electrolyte under the same voltage, volume in 3:1 volume ratio. They were carried out in potentiostatic conditions of 5 V in constant temperature, and field ≈100 mT wasMEP superimposed on the in electropolishing process temperature of 25time. °C. The Themagnetic magnetoelectropolishing was performed the same electrolyte by using ring ceramictemperature, (ferrite) magnets together: thefield electrolytic cellwas wassuperimposed positioned inside under thefour same voltage, and stacked time. The magnetic ≈100 mT on ring magnets [27,33]. the electropolishing process by using four ring ceramic (ferrite) magnets stacked together: the
electrolytic cell was positioned inside ring magnets [27,33]. 2.2. Hydrogen Content Measurement 2.2. Hydrogen Content Measurement The niobium samples, both as received AR, electropolished EP, and magnetoelectropolished MEP, wereThe usedniobium to perform measurements of hydrogen in the surface layer before electrochemical samples, both as received AR, content electropolished EP, and magnetoelectropolished surfacewere treatment, andperform after these processes. The hydrogen content secondary depth MEP, used to measurements of hydrogen in ion the currents surface versus layer before profiles were measured on a SAJW-02 analyzer, designed and built in the Institute of Vacuum electrochemical surface treatment, and after these processes. The hydrogen secondary ion currents versus depth profiles were measured on a SAJW-02 analyzer, designed and built in the Institute of
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Metals 2017,Technology 7, 442 Vacuum
4 of 19 (IVT), Warsaw, Poland, equipped with a 06-350E ion gun Physical Electronics (Chanhassen, MN, USA), and 16 mm QMA-410 Balzers quadrupole mass spectrometer (Balzers, Liechtenstein). The ultra-high vacuum system, sample manipulator and load/lock system were done Technology (IVT), Warsaw, Poland, equipped with a 06-350E ion gun Physical Electronics+(Chanhassen, in IVT, Warsaw, Poland. This SIMS system was described elsewhere [36]. 5 keV Ar primary ion MN, USA), and 16 mm QMA-410 Balzers quadrupole mass spectrometer (Balzers, Liechtenstein). beam was digitally scanned over 3.5 mm × 3.5 mm area while the emission of secondary ions was The ultra-high vacuum system, sample manipulator and load/lock system were done in IVT, Warsaw, measured from 1.35 mm × 1.35 mm area. Niobium sputtering rate 0.016 nm·s−1 was calculated basing Poland. This SIMS system was described elsewhere [36]. 5 keV Ar+ primary ion beam was digitally on Tencor α-step 100 stylus profilometry measurements of much smaller craters (1.2 mm × 1.2 mm) scanned over 3.5 mm × 3.5 mm area while the emission of secondary ions was measured from eroded with the same ion beam. 1.35 mm × 1.35 mm area. Niobium sputtering rate 0.016 nm·s−1 was calculated basing on Tencor In the SIMS measurements, spectrograms of both the secondary positive and negative ions were α-step 100 stylus profilometry measurements of much smaller craters (1.2 mm × 1.2 mm) eroded with registered during Nb samples surface bombardment with Ar+ ions. For the depth profile analysis, the same ion beam. the following secondary ions were chosen: 1H+, 16O+, 93Nb+ and 1H−, 16O−, 93Nb−. SIMS studies were In the SIMS measurements, spectrograms of both the secondary positive and negative ions were performed by using parameters presented in Table 1. registered during Nb samples surface bombardment with Ar+ ions. For the depth profile analysis, 1 H+ , 16 O+ , 93 Nb+ and 1 H− , 16 O− , 93 Nb− . SIMS studies the followingTable secondary ions were 1. Parameters usedchosen: in the Secondary Ion Mass Spectrometry (SIMS) studies. were performed by using parameters presented in Table 1.
Series Primary Series
I Primary
II III I
II III
3. Results
Mass Secondary Ions Selected for the Studies Scanning Area Spectrograms Table 1. Parameters used in the Secondary Ion Mass Spectrometry (SIMS) studies. 0–140 D (+) 0–200 MassD (−) Secondary Ions for the Studies Scanning Area 1H+ (1.27), 16O+Selected Spectrograms , 93Nb+ and 1H−(1.38), 16O−, 2 mm × 2 mm 93Nb− 0–140 D (+) 0–200 D 1 mm × 1 mm - (−) H+ (1,41), H2+, O+, 93Nb+ 1 H+ (1.27), 16 O+ , 93+Nb+ and 1 H − (1.38), 16 O − , 93 Nb− + 93 + + -2 mm × mm 3.5 mm ×23.5 mm H (1,41), H2 , O , Nb H+ (1,41), H2 + , O+ , 93 Nb+ H+ (1,41), H2 + , O+ , 93 Nb+
-
1 mm × 1 mm 3.5 mm × 3.5 mm
Ion Beam Parameters 5 keV, Ar+ Ion Beam Parameters
5 keV, Ar+
5 keV, Ar+
5 keV, Ar+
5 keV, Ar+Ar+ 5 keV, 5 keV, Ar+ 5 keV, Ar+
3. Results 3.1. First Series of Experiments
Positive secondary ion current [counts]
3.1. First of Experiments The Series registered mass spectrograms are given in Figure 2. Spectrograms of the secondary positive ion mass are presented Figure 2a, and mass2.inSpectrograms Figure 2b. Results Figure The registered massinspectrograms arenegative given inion Figure of thedisplayed secondaryinpositive 2 show that the AR sample surface is much more contaminated by foreign particles than those two ion mass are presented in Figure 2a, and negative ion mass in Figure 2b. Results displayed in Figure 2 + + + other that Nb samples, after EP and MEP. Higher are currents ofbythe positive ions of Nathose , K and . In show the AR sample surface is much more contaminated foreign particles than two O other casesamples, of the analysis negative ions, one also note higherions areof currents ionsO+of. In C−,case O−, of F− Nb after EPofand MEP. Higher arecan currents of thethat positive Na+ , K+ofand − andanalysis Cl . the of negative ions, one can also note that higher are currents of ions of C− , O− , F− and Cl− . 10000 1000 100 10 1
+
H
+
+
Na
+
C
K
AR
+
Nb
+
NbO +
NbC
0
20
+
10000 1000 100 10 1
H
40
+
Na
+
C
60
80
100
120
+
Nb
+
K
140
EP
+
NbO +
NbC
0
10000 1000 100 10 1
20
+
H
+
C
40
+
Na
60
80
+
K
100
120
MEP
+
Nb
140
+
NbO +
NbC
0
20
40
60
80
m/z [D]
(a) Figure 2. Cont.
100
120
140
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Negative Negativesecondary secondaryion ioncurrent current[counts] [counts]
of 19 19 55 of 5 of 19 1000000 1000000 100000 10000 100000 10000 1000 100 1000 100 10 1 10 1
1000000 100000 1000000 10000 100000 10000 1000 100 1000 100 10 1 10 1
1000000 1000000 100000 10000 100000 10000 1000 100 1000 10 100 1 10 1
-
-
HH
0 0
- O C- O- FF C
-
ClCl
20 20
40 40
AR AR
-
-
-
-
NbONbO
NbNb
60 60
80 80
100 100
NbO2NbO2
120 120
NbO3NbO3 NbO2FNbO2F
140 140
160 160
180 180
-
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HH
0 0
O- FC- O F C
20 20
-
-
HH
- O C- O- FF C
-
-
ClCl
40 40
-
-
NbNb
60 60
80 80
100 100
EP EP
-
NbO3NbO3 NbO2FNbO2F
140 140
160 160
180 180
-
-
40 40
60 60
80 80
-
NbONbO
100 100
m/z [D] m/z [D]
NbO2NbO2
120 120
NbO3NbO3NbO F2 NbO2F
140 140
160 160
200 200
MEP MEP
-
NbNb
20 20
NbO2NbO2
120 120
ClCl
-
0 0
-
NbONbO
200 200
180 180
200 200
(b) (b) Figure 2. Mass spectrograms of positive (a) and negative (b) secondary ion currents. Results taken of Figure 2. spectrograms of (a) and negative Figure 2. Mass Masssamples: spectrograms of positive positive and negative (b) (b) secondary secondary ion ion currents. currents. Results Results taken taken of of three niobium AR, EP, and MEP(a) are presented. three and MEP MEP are are presented. presented. three niobium niobium samples: samples: AR, AR, EP, EP, and
Results presented in Figure 3 confirm that the near-surface layer of the AR sample is more Results Results presented presented in in Figure Figure 33 confirm confirm that that the the near-surface near-surface layer layer of of the the AR AR sample sample is is more more contaminated in comparison with those of the EP and MEP samples. It is also apparent that currents contaminated in comparison with those of the EP and MEP samples. It is also apparent that currents contaminated in comparison with those of the EP and It is also apparent that currents of the secondary ions of hydrogen and oxygen in MEP sample are higher in comparison with of the secondary secondaryions ionsofof hydrogen oxygen in MEP sample are higher in comparison with of the hydrogen andand oxygen in MEP sample are higher in comparison with currents currents measured on EP niobium sample. Higher−level of O−− ions measured on MEP sample, in currents on EP sample. niobiumHigher sample. Higher O ions on measured on MEP sample, in measuredmeasured on EP niobium level of O level ions of measured MEP sample, in comparison comparison with EP sample, is meaningful. comparison withisEP sample, is meaningful. with EP sample, meaningful. The ion etching area in the first series of SIMS measurements was equal 2 mm × 2 mm. One can The measurements was was equal equal 22 mm mm × × 22 mm. The ion ion etching area in the first series of SIMS measurements mm. One One can can easily conclude that in case of ion etching of the area 2 mm × 2 mm with the same ion beam (previous easily conclude that in case of ion etching of the area 2 mm × 2 mm with the same ion beam (previous easily × 2 mm with the same ion beam (previous −1. series of results) the rate of ion etching was 0.05 nm·s−1 series series of of results) results) the rate of ion etching was 0.05 nm·s nm·s−.1 . AR AR
EP EP
MEP MEP
Positive Positivesecondary secondaryion ioncurrent current[counts] [counts]
100000 100000
+
H+ H+ O+ O + Nb+ Nb
10000 10000
1000 1000
100 100
10 10
1 1
0 0
2000 2000
4000 4000
0 0
2000 2000
4000 4000
Time [s] Time [s]
(a) (a) Figure 3. Cont.
0 0
2000 2000
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AR AR
EP EP
MEP MEP -
HHOONb Nb
Negativesecondary secondaryion ioncurrent current[counts] [counts] Negative
1000000 1000000 100000 100000 10000 10000 1000 1000 100 100 10 10 1 1
0 0
2000 2000
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6000 6000
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Time [s] Time [s]
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6000 6000
(b) (b) Figure 3. Rough results of SIMS depth analyses for positive (a), and negative (b) ions studies. The Figure 3. depth analyses forfor positive (a), (a), andand negative (b) ions studies. The Figure 3. Rough Roughresults resultsofofSIMS SIMS depth analyses positive negative (b) ions studies. values of currents of the secondary ions vs. etching time are presented (at the electron beam of the values of currents of theofsecondary ions vs. time are presented (at the(at electron beam of the The values of currents the secondary ionsetching vs. etching time are presented the electron beam energy of 5 keV). The ion etching area: 2 × 2 mm, analysis area: 0.8 × 0.8 mm. Three niobium samples energy of 5 keV). The ion etching area: 2 × 2 mm, analysis area: 0.8 × 0.8 mm. Three niobium samples of the energy of 5 keV). The ion etching area: 2 × 2 mm, analysis area: 0.8 × 0.8 mm. Three niobium were studied: AR, EP, and MEP. were studied: AR, EP, and samples were studied: AR, MEP. EP, and MEP.
To compare emission of hydrogen ions for the studied Nb the of To compare compareemission emission hydrogen for three the three three studied Nb samples, samples, theofratios ratios of these these To of of hydrogen ionsions for the studied Nb samples, the ratios these currents currents and niobium ions currents were calculated. The results are displayed in Figure 4. One may currents and niobium ions currents were calculated. The are results are displayed in Figure Oneeasily may and niobium ions currents were calculated. The results displayed in Figure 4. One 4. may easily notice that the relative emission for hydrogen ions is slightly higher on MEP sample easily that notice the emission relative emission for hydrogen ions is slightly higher on MEP sample in in notice thethat relative for hydrogen ions is slightly higher on MEP sample in comparison comparison with that one measured on EP sample. comparison that one on EP sample. with that onewith measured onmeasured EP sample. 2 2
AR AR EP EP MEP MEP
+ +
+ +
IHIH/I/INbNb
+ +
IHIH/I/INbNb
+ +
1 1
0,1 0,1
0 0
1000 1000
2000 2000
Time [s] Time [s]
3000 3000
(a) (a) Figure 4. Cont.
1 1
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Time [s] Time [s]
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AR EP MEP
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IH /INb
IH /INb
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-
20
10 10
0 0
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0
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Time [s]
Time [s]
(b) Figure 4. 4. Dependence Dependence of of secondary secondary ions current of of hydrogen hydrogen to to secondary secondary ions ions current current of of Figure of ratios ratios of ions current niobium on onthe thetime time ionic etching. Etching of s5500 s corresponds the thickness of niobium of of ionic etching. Etching timetime of 5500 corresponds with thewith thickness of niobium niobium etched layer equaling a few hundreds of nanometers; in (a) there are given ratios of positive etched layer equaling a few hundreds of nanometers; in (a) there are given ratios of positive secondary secondary ionsand currents, and inof(b) ratios of negative ions secondary ions currents. ions currents, in (b) ratios negative secondary currents.
Results obtained for positive secondary ions present the difference in the whole range of ion etching. On On the the other other hand, hand, the the results results obtained obtained for for negative negative secondary secondary ions ions relate to the depth corresponding with half of the ion ion etching etching depth. depth. 3.2. Second Series of Experiments The second second series seriesof ofSIMS SIMSmeasurements measurementswas was carried niobium samples, EP carried outout on on twotwo niobium samples, afterafter EP and + + + + and Positioning H Hand H2 on peaks on the mass spectrogram werevery chosen very carefully. MEP.MEP. Positioning of H of and 2 peaks the mass spectrogram were chosen carefully. Positive Positive secondary with four masses: 2, 16, were chosen/appointed the profile depth secondary ions withions four masses: 1, 2, 16, 1,and 93 and were93chosen/appointed for the for depth + was performed. The area of ion etching was + profile analysis. Ion etching using beam 5 keV Ar analysis. Ion etching using beam 5 keV Ar was performed. The area of ion etching was equal 1 mm × equal mm × mm. Timewas of the etching was 5000 s so thatwas obtained crater was 1 micrometer in 1 mm.1Time of 1the etching 5000 s so that obtained crater 1 micrometer in depth. Therefore, depth. the rate ofunder niobium etching under these conditions of ion etching process the rateTherefore, of niobium etching these CONDITIONS OF ION ETCHING PROCESS WAS EQUAL 0.2was NM·equal S−1. IN 1 . in the first series of sims measurements, under ionic etching of area 2 mm × 2 mm using 0.2 ·s−SERIES THEnm FIRST OF SIMS MEASUREMENTS, UNDER IONIC ETCHING OF AREA 2 MM × 2 MM USING THE SAME the beam, rate of ionic etching was equal 0.05 ION same BEAM,ion THE RATE the OF IONIC ETCHING WAS EQUAL 0.05 NM ·S−1nm . · s−1 . Rough results ofof measurements areare given in results of of SIMS SIMSdepth depthanalysis analysisobtained obtainedininthe thesecond secondseries series measurements given Figure 5. Changes in the values of ion secondary currents vs. vs. time of etching are are apparent, using the in Figure 5. Changes in the values of ion secondary currents time of etching apparent, using etching beam 5 keV. With thethe assumed area of ion etching 1 mm × ×11mm, the etching beam 5 keV. With assumed area of ion etching 1 mm mm,the thearea areaof of analysis analysis was 0.4 mm × 0.4 mm. mm. The The obtained obtained results results show show that that investigated investigated positive positive secondary secondary currents are very × 0.4 similar on both samples, EP and MEP. MEP.
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Positive Positivesecondary secondaryion ioncurrent current[counts] [counts]
EP EP 1000 1000
1000 1000
100 100
100 100
10 10
10 10
1 1 0 0
H H H H22 O O Nb Nb
MEP MEP
2000 2000
1 1 0 0
4000 4000
Time [s] Time [s]
2000 2000
4000 4000
Time [s] Time [s]
Figure 5. 5. Rough results ofofSIMS profile analysis obtained in the 2nd series of measurements. The Figure Roughresults resultsof SIMS profile analysis obtained in the series of measurements. Figure 5. Rough SIMS profile analysis obtained in the 2nd 2nd series of measurements. The secondary ion currents are displayed. Surface area of ion etching: 1 mm × 1 mm, analysis area: 0.4 The secondary ion currents are displayed. Surface of ion etching: 1 mm 1 mm, analysis secondary ion currents are displayed. Surface areaarea of ion etching: 1 mm × 1× mm, analysis area:area: 0.4 mm × 0.4 mm, two Nb samples were used for the study: EP—on the left, and MEP—on the right. 0.4 ×mm, 0.4 mm, Nb samples for the study: EP—on MEP—on right. mmmm × 0.4 two two Nb samples werewere usedused for the study: EP—on the the left,left, andand MEP—on the the right. + +and InFigure Figure6,6,the theratios ratiosof ofsecondary secondaryion ioncurrents currentsfor forpositive positivehydrogen hydrogen(H (H andHH+2+) to secondary secondary In In Figure 6, the ratios of secondary ion currents for positive hydrogen (H+ and H22+) to secondary ion currents currentsof ofniobium niobiumon ontime time(a) (a)and anddepth depth(b) (b)of of the the ion ion etching etching are are given, given, related relatedto toEP EP and and MEP MEP ion ion currents of niobium on time (a) and depth (b) of the ion etching are given, related to EP and MEP niobiumsamples. samples. Time Timeof of ion ion etching etching was was 5000 5000 ss (Figure (Figure 6a), 6a), with with the the depth depth of of etched etched niobium niobium layer layer niobium niobium samples. Time of ion etching was 5000 s (Figure 6a), with the depth of etched niobium layer equalto to11micrometer micrometer(Figure (Figure6b). 6b). equal equal to 1 micrometer (Figure 6b).
EP EP
MEP MEP 10 10
1 1
1 1
0,1 0,1
0,1 0,1
0,01 0,01
0,01 0,01
+
IX/I I Nb /I
X Nb
+
10 10
H H H H22
0 0
2000 2000
4000 4000
0 0
Time [s] Time [s]
(a) (a) Figure 6. Cont.
2000 2000
4000 4000
Time [s] Time [s]
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EP EP MEP MEP
1 1
+
Nb
IH2 I/INb/I H2 Nb
+
0,1 0,1
+
1 1
+
H
+
+
IH /IINb/I
+
+
10 10
0,01 0,01
0,1 0,1 0 0
0 0
400 800 1200 400 800 1200 Depth [nm]
Depth [nm]
(b) (b)
400
800
1200
400 800 Depth [nm] 1200 Depth [nm]
Figure 6. Dependence of ratio of secondary ion currents for positive hydrogen (H+ and H2+) to + ++ to Figure 6. Dependence of positive hydrogen (H+5000 H Figure Dependence of ratio of secondary secondary for positive (H ands H to 22 ))the secondary ion currentsof of ratio niobium on time ofion thecurrents ion etching. Time ofhydrogen ion etching: (a), secondary currents of niobium on time of the ion etching. Time of ion etching: 5000 s (a), the secondary ion currents of niobium on time of the ion etching. Time of ion etching: 5000 s (a), the depth depth of etched niobium layer equals to 1 micrometer (b). depth of etched niobium layer equals to 1 micrometer of etched niobium layer equals to 1 micrometer (b). (b).
In Figure 7, the results of the ratios of secondary ion currents for positive hydrogen (H+ + and H2++) In Figure Figure 7, 7, the the results results of of the the ratios ratios of of secondary secondary ion currents for positive hydrogen (H H2+)) In ionion currents positive (H+toand and to secondary ion currents of niobium on depth of the etchingforare given, hydrogen also related EPH and to secondary ion currents of niobium on depth of the ion etching are given, also related to EP and MEP to secondary currents of niobium on depth of the ion are etching are given,form. also related to EP and MEP samples.ion Here the values of ordinates are presented in a smoothed samples. Here Here the values of ordinates are presented are inare a smoothed form.form. MEP samples. the values of ordinates are presented in a smoothed +
EP, H + + EP, EP,HH2 + EP, H2 + MEP, H + + MEP, MEP,HH2 + MEP, H2
+
IX/IINb/I
X Nb
+
10 10
1 1
+
H + H
0,1 0,1
0,01 0,01 0 0
+
H2 + H2
200 200
400
600
400 [nm] 600 Depth Depth [nm]
800 800
Figure 7. Dependence of ratio of secondary ion currents for positive hydrogen (H+ and H2 + ) to Figure 7. Dependence of ratio of secondary ion currents for positive hydrogen (H+ and H2+) to secondary ion currents of niobium on time of the ion etching. Values of ordinates presented a + and H2+)into Figure 7. Dependence of secondary for positive hydrogen (Hpresented secondary ion currentsofof ratio niobium on time ofion thecurrents ion etching. Values of ordinates in a smoothed form. secondary ion currents of niobium on time of the ion etching. Values of ordinates presented in a smoothed form. smoothed form.
3.3. Third Series of Experiments 3.3. Third Series of Experiments 3.3. Third Seriesseries of Experiments The third of SIMS measurements was carried out on AR, EP, and MEP samples taking into The third series of SIMS measurements was carried out on AR, EP, and MEP samples taking into account the same mass parameters of secondary ions alike those the second The positive The the thirdsame series of SIMS measurements was carried out on AR,of EP, and MEP series. samples taking into account mass parameters of secondary ions alike those of the second series. The positive secondary ions of four masses: 1, 2, 16, and 93 were studied. Ion mass etching with the beam of account the same mass parameters of secondary ions alike those of the second series. The positive secondary+ ions of four masses: 1, 2, 16, and 93 were studied. Ion mass etching with the beam of 5 keV 5 keV Ar ions wasof used. view of reducing thewere edgestudied. effects during ionetching etching, thethe etching area was secondary four In masses: 1, 2, 16, and Ionetching, mass with of 5 keV Ar+ was used. In view of reducing the edge93 effects during ion the etching areabeam was enlarged. enlarged. For the studies, the etching area was 3.5 mm × 3.5 mm. Etching time of the process was + Ar used. Inthe view of reducing the3.5edge during ion etching, the process etching was area3600 was senlarged. Forwas the studies, etching area was mmeffects × 3.5 mm. Etching time of the resulting 3600 s resulting in the crater of depth equaling to about 65 nm. Herewith, the ion-etching rate of For the studies, etching area was mm ×653.5nm. mm.Herewith, Etching time the processrate wasof 3600 s resulting in the crater ofthedepth equaling to3.5 about the of ion-etching niobium was − 1 niobium was 0.016 nm·s . in thenm·s crater −1. of depth equaling to about 65 nm. Herewith, the ion-etching rate of niobium was 0.016 −1 0.016 nm·s .
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SIMS studies with the craters 3.5 mm × 3.5 mm were carried out to register positive secondary ions. In Figure 8, the holder with three niobium samples, AR, EP, and MEP, and slightly visible Metals 2017, 7, 442 10 of 19 craters are shown (see one of them indicated by an arrow). The study results of this series are Metals 2017, 7, x FOR PEER REVIEW 10 of 19 displayed in Figure 9. SIMS studies 3.5 mm were carried out to register positive secondary studies with with the craters craters 3.5 3.5 mm mm × × 3.5 ions. In Figure 8, the holder with three niobium samples, AR, AR, EP, and and slightly visible craters In Figure 8, the holder with three niobium samples, EP,MEP, and MEP, and slightly visible are shown one (see of them by an arrow). Thearrow). study results of this seriesofare displayed in craters are(see shown one indicated of them indicated by an The study results this series are Figure 9. in Figure 9. displayed
Figure 8. Holder with with three Nb Nb samples fixed fixed onto it: it: EP—up on on the left left side, and and MEP—upon on the Figure Figure 8. Holder Holder with three three Nb samples samples fixed onto onto EP—up EP—up on the the left side, side, and MEP—up MEP—up on the the right side, and AR—down below. Well visible crater 3.5 mm × 3.5 mm, as marked by arrow, appears right AR—downbelow. below.Well Wellvisible visiblecrater crater3.53.5 mm 3.5mm, mm,asasmarked marked arrow, appears right side, and AR—down mm × ×3.5 byby arrow, appears on onEP EP niobium sample. on niobium sample. EP niobium sample. AR AR
Positive [counts] current [counts] ion current secondary ion Positive secondary
100000 100000
EP EP
MEP MEP
HH HH2 2
O O Nb Nb
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11
00
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00
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Figure 9. Primary results depth profile analysis. Presented are of ions 9.Primary Primaryresults results of SIMS SIMS depth profile analysis. Presented are values values of secondary secondary ions Figure 9. ofof SIMS depth profile analysis. Presented are values of secondary ions currents currents on etching time using 55 keV. area surface ion 3.5 mm ×× dependent on etching using beam of beam 5 keV.of The areaThe of surface etching: 3.5 mm × 3.5 mm, currents dependent dependent ontime etching time using beam of keV. The area of ofion surface ion etching: etching: 3.5 mm the area the of 1.35 mm1.35 × 1.35 mm. 3.5 area mm ××1.35 3.5mm, mm, theanalysis: area of ofanalysis: analysis: 1.35 mm 1.35mm. mm. + ions emission is the Results obtained in this series (Figure 9) show that the of Results number the H Results obtained obtained in in this this series series (Figure (Figure 9) 9) show show that that the the number number of of the the H H++ ions ions emission emission is is the the lowest are somewhat different from those obtained on the with lowest on MEP sample. These results are somewhat different from those obtained on the lowest on onMEP MEPsample. sample.These Theseresults results are somewhat different from those obtained onsample the sample sample craters 1 mm × 1 mm, andand much different from those with craters 2 mm ×××222mm. reasons lie with 11 mm ×× 11 mm, much different from those with craters 22 mm mm. The with craters craters mm mm, and much different from those with craters mm mm.The The reasons reasons lie lie + ions currents during probably in edge effects and other selection in measurement parameters of the H + + probably probably in in edge edge effects effects and and other other selection selection in in measurement measurement parameters parameters of of the the H H ions ions currents currents 2during mm ×222 mm mm interesting here is a sharp drop content during ×× 22 mm craters’ analysis. Another interesting here is aa oxygen sharp in mm craters’ mmanalysis. craters’Another analysis. Another observation interesting observation observation here isin sharp drop drop in on the MEP sample. Here also thickness of the oxide film is much lesser on MEP sample in comparison oxygen oxygen content content on on the the MEP MEP sample. sample. Here Here also also thickness thickness of of the the oxide oxide film film is is much much lesser lesser on on MEP MEP with that measured on EP sample. sample in comparison with that one sample inone comparison with that one measured measured on on EP EP sample. sample. One ions currents onon niobium samples (Figure 8) are One may may easily easilynotice noticethat thatthe theruns runsofofsecondary secondary ions currents niobium samples (Figure 8) clearly different in three samples (AR, EP, MEP). To check and confirm these results, another additional are clearly different in three samples (AR, EP, MEP). To check and confirm these results, another measurements were carried out carried on two niobium samples, EP samples, and MEP,EP taking time of 600 s. additional measurements were out on two niobium and etching MEP, taking etching Results of these measurements are given in Figures 10 and 11. time of 600 s. Results of these measurements are given in Figures 10 and 11. In Figure 10 the smoothed results (B-Spline) in the range of 900 s time of ion etching are given, and for two of the samples, EP and MEP, results of rough SIMS profile analyses are displayed
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One may easily notice that the runs of secondary ions currents on niobium samples (Figure 8) One may easily in notice the runs of EP, secondary ionscheck currents niobium samples (Figure 8) are clearly different threethat samples (AR, MEP). To and on confirm these results, another are clearly measurements different in three samples MEP). To check and confirm additional were carried(AR, out EP, on two niobium samples, EP andthese MEP,results, taking another etching Metals 2017, 7, measurements 442Results of these 11 of 19 additional were carried out on niobium samples, time of 600 s. measurements aretwo given in Figures 10 andEP 11.and MEP, taking etching time In of Figure 600 s. Results of these measurements are given Figures 10 and 11. of ion etching are given, 10 the smoothed results (B-Spline) in theinrange of 900 s time In Figure 10 thesamples, smoothed results (B-Spline) in the rough range of 900 profile s time of ion etching displayed are given, and In for two of and(B-Spline) MEP, results SIMS analyses Figure 10 the the smoothed EP results in theof range of 900 s time of ion etching are are given, and and for 11). twoThe of the samples, EP and MEP,were results of scattering rough SIMS profile analyses are displayed (Figure same analysis parameters used: area of 3.5 mm × 3.5 mm, and 11). the for two of the samples, EP and MEP, results of rough SIMS profile analyses are displayed (Figure (Figure 11). The same analysis parameters were used: scattering area of 3.5 mm × 3.5 mm, and analysis of 1.35 mm × 1.35were mm.used: scattering area of 3.5 mm × 3.5 mm, and the analysis areathe The samearea analysis parameters of analysis area of 1.35 mm × 1.35 mm. 1.35 mm × 1.35 mm.
Positive secondary current [counts] Positive secondary ion ion current [counts]
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H H H2 O H2 Nb O
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Figure 10. Results of rough SIMS depth analyses in the range of 900 s time of ion etching. Smoothed Figure 10. rough SIMS depth analyses in the range 900 s time of ion etching. Smoothed data: B-Spline. Figure 10. Results Resultsofof rough SIMS depth analyses in theofrange of 900 s time of ion etching. Smoothed data: B-Spline. data: B-Spline. EP 10000
MEP
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11.Results Resultsofof rough SIMS depth analyses the range 600ofsion time of ionTwo etching. Two Figure 11. rough SIMS depth analyses in theinrange of 600 softime etching. additional Figure 11. measurements Results rough SIMS analyses in same the range of parameters 600 s analysis time used: of ion etching.used: Two additional of EP anddepth MEP samples are shown. The same parameters measurements of EPof and MEP samples are shown. The analysis scattering area: additional measurements of EP and MEP samples are shown. The same analysis parameters used: 3.5 mm × 3.5 mm, the analysis area: 1.35 mm × 1.35 mm. scattering area: 3.5 mm × 3.5 mm, the analysis area: 1.35 mm × 1.35 mm. scattering area: 3.5 mm × 3.5 mm, the analysis area: 1.35 mm × 1.35 mm.
Results to those those displayed displayed in in Figures Figures 99 and and 10. 10. Results presented presented in in Figure Figure 11 11 appear appear to to be be very very similar similar to + + currents Ion currents with that EP presented Figure 11 appear tosample be veryinsimilar to those displayed 9 and 10. Ion H HResults drop is in falling faster on MEP comparison with that one oneinof ofFigures EP sample. sample. + currents To study the distribution of hydrogen concentration in the near-surface layers, another approach Ion H drop is falling faster on MEP sample in comparison with that one of EP sample. To study the distribution of hydrogen concentration in the near-surface layers, another + and H + +ions emission + ions currents + ions was done, presenting comparison of H with niobium To study the distribution of hydrogen concentration thethe near-surface layers, another approach was done, presenting comparison of 2H and H2+ ionsin emission with the Nb niobium Nb + + + + + ions emission. In Figure 12,presenting change the ratio positive ions currents approachemission. was done, comparison ofratio H and H2 ionssecondary emissionsecondary with(H the and niobium 2 +) Nb currents In Figure 12,inchange inof thehydrogen of hydrogen positive ionsH(H and H2+) to the niobium secondary ions ionofetching timepositive is given. Here theions etching timeHof currents emission. In Figure 12, currents change invs. thethe ratio hydrogen secondary (H+ and 2+) 3600 s corresponds with depth of the niobium etched layer equaling to about 70 nm.
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currents to the niobium secondary ions currents vs. the ion etching time is given. Here the etching currents to the niobium secondary ions currents vs. the ion etching time is given. Here the etching time of 3600 s corresponds with depth of the niobium etched layer equaling to about 70 nm. Metals 2017, 7, 442 time of 3600 s corresponds with depth of the niobium etched layer equaling to about 70 nm. 12 of 19 AR AR
EP EP
MEP MEP
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+
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+
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+
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Figure 12. Change in the ratio of hydrogen positive secondary ions (H++ and H2++) currents to the Figure 12. currents to the 12. Change in the ratio ratio of of hydrogen hydrogen positive positive secondary secondary ions ions (H (H+ and and H H22+)) currents niobium secondary ions currents vs. the ion etching time. Etching time 3600 s corresponds with niobium secondary ions currents vs. the ion etching time. Etching time 3600 s corresponds with depth secondary ions currents vs. the ion etching time. Etching time 3600 s corresponds with depth of the niobium etched layer equaling to about 70 nm. of the niobium etched layer equaling to about 70 nm. depth of the niobium etched layer equaling to about 70 nm.
In Figure 13, 13, there are are results presenting presenting the positive positive secondary ion ion currents dependent dependent on the the In In Figure Figure 13, there there are results results presenting the the positive secondary secondary ion currents currents dependent on on the depth ofpenetration penetration in the range from top surface down to 17in-depth. nm in-depth. A constant rate of depth range from toptop surface down to 17to nm17 A constant rate of etching depth of of penetrationininthe the range from surface down nm in-depth. A constant rate of etching was assumed. In Figure 13a, there are rough data presented, and in Figure 13b—smoothed was assumed. In Figure there arethere rough data presented, and in Figure plots by etching was assumed. In13a, Figure 13a, are rough data presented, and in13b—smoothed Figure 13b—smoothed plots by B-Spline are given. B-Spline are given. plots by B-Spline are given. AR AR
Positive secondary ionion current [counts] Positive secondary current [counts]
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(a) (a) Figure 13. Cont.
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Depth Depth [nm] [nm]
(b) (b) Figure 13. Results secondary ion currents values dependent on the of ion in the Figure 13. 13.Results Resultsofof ofsecondary secondary currents values dependent on depth the depth depth ion etching etching the Figure ionion currents values dependent on the of ionofetching in the in range range from top surface down to 17 nm in-depth: (a) rough data, and (b) smoothed plots: B-Spline. range from top surface down to 17 nm in-depth: (a) rough data, and (b) smoothed plots: B-Spline. from top surface down to 17 nm in-depth: (a) rough data, and (b) smoothed plots: B-Spline.
One thinnest oxide layer was obtained on MEP, after One can can clearly notice that thethe thinnest oxide layer was was obtained on sample sample MEP, MEP, after One can clearly clearly notice noticethat thatthe thinnest oxide layer obtained on sample magnetoelectropolishing. Also the hydrogen ion current emission decreases sharply during etching magnetoelectropolishing. Also the hydrogen ion current emission decreases sharplysharply during etching after magnetoelectropolishing. Also the hydrogen ion current emission decreases during of the sample MEP. thickness is 1–2 sample this of this this layer layer onlayer the on sample MEP. Its Its thickness is only only about 1–2 nm. nm. On EP niobium sample this etching of thison the sample MEP. Its thickness is about only about 1–2 On nm.EP Onniobium EP niobium sample oxide layer is about 3 nm thick, and on AR sample—about 4 nm. One should notice here that they oxide layer is about 3 nm thick, and on AR sample—about 4 nm. One should notice here that they this oxide layer is about 3 nm thick, and on AR sample—about 4 nm. One should notice here that + are values only, obtained basing on the drop the are estimated estimated values only, obtained basing onon thethe drop ofofH HH+ +secondary secondary ion currents, with the the they are estimated values only, obtained basing dropof secondaryion ioncurrents, currents, with simultaneous assumption that the rate of etching of this oxide layer is the same as that one in case of simultaneous assumption assumption that that the the rate rate of of etching etching of of this this oxide oxide layer layer is is the the same same as as that that one one in in case case of of simultaneous etching pure/metallic niobium. etching pure/metallic niobium. etching pure/metallic niobium. +/Nb+ and H2+/Nb+) currents indicate that the Changes of rates of hydrogen niobium ions + /Nb + +and +/Nb Changes of ofrates ratesof ofhydrogen hydrogentoto toniobium niobium ions (H andHH2 +2+/Nb /Nb++))currents the Changes ions (H(H currents indicate indicate that the highest possible content of hydrogen is in sample AR—after cold-rolling, highest possible possible content of hydrogen is in sample AR—after cold-rolling, some highest some lower lower in in EP EP sample, sample, + and H2+ ions, and the least one—in MEP sample (Figure 14). In Figure 15, separated results for and the the least least one—in one—in MEP MEP sample sample (Figure (Figure14). 14). In In Figure Figure15, 15,separated separatedresults resultsfor forHH H++ and andH H22++ ions, and related to AR, EP, and MEP niobium samples, are presented. related to to AR, AR, EP, EP,and andMEP MEPniobium niobiumsamples, samples,are arepresented. presented. related AR AR
MEP MEP
EP EP
+
+
H +/Nb + H +/Nb + H2 +/Nb + H2 /Nb
IXIX/I/INbNb
+ +
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++ and H ++ ) currents to the Figure 14. Dependence of rates of hydrogen positive secondary ions (H Figure Figure 14. 14. Dependence Dependence of of rates rates of of hydrogen hydrogen positive positive secondary secondary ions ions (H (H+ and and H H222+)) currents currents to to the the secondary niobium ion currents on the depth of ion etching. secondary niobium ion currents on the depth of ion etching. secondary niobium ion currents on the depth of ion etching.
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AR EP AR MEP EP MEP
AR EP AR MEP EP MEP 0,1 +
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Figure 15. Dependence of rates of hydrogen positive secondary ions (H++ and H2++) currents to the Figure Dependence of of hydrogen positive secondary ions (H H22++)) currents to Figure 15. 15.niobium Dependence of rates rateson ofthe hydrogen secondary ionsresults (H+and and currents to the the H2+ ions are secondary ion currents depth ofpositive ion etching. Separated for H H and + and H + ions secondary niobium ion currents on the depth of ion etching. Separated results for H + and H2+ ions 2 are secondary niobium ion currents on the depth of ion etching. Separated results for H displayed. are displayed. displayed.
+
+ secondary current [counts] H H secondary ionion current [counts]
Figure is provided provided to to illustrate, illustrate, how how the the hydrogen hydrogen ion ion emission emission is is changing changing during during SIMS SIMS Figure 16 16 is Figure 16 is provided to illustrate, how the hydrogen ion emission is changing during SIMS etching of near-surface near-surfacelayer layerofofthree threeinvestigated investigatedNb Nbsamples: samples: AR, and MEP. assuming etching of AR, EP,EP, and MEP. By By assuming thatthat the etching of near-surface layer of three investigated Nb samples: AR, EP, and MEP. By assuming that the thickness of oxide may be assessed into consideration half-value of thecurrent, falling thickness of oxide layerlayer may be assessed takingtaking into consideration half-value of the falling the thickness of oxidethicknesses layer mayofbetheassessed taking niobium into consideration ofAR—about the falling current, the estimated layers’ samples arehalf-value as follows: the estimated thicknesses of the surface surface layers’ niobium samples are as follows: AR—about 4 nm, current, the estimated thicknesses of the surface layers’ niobium samples are as follows: AR—about 4EP—3 nm, EP—3 nm, and MEP—about nm, and MEP—about 1.5 nm.1.5 nm. 4 nm, EP—3 nm, and MEP—about 1.5 nm. AR EP AR MEP EP MEP
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Figure 16. Cont.
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(b) Figure 16. Comparison of plots obtained for H+ ion currents against the depth of ion etching: (a) Figure 16. Comparison of plots obtained for H+ ion currents against the depth of ion etching: (a) range range of 0 to 65 nm, and (b) results covering range of etching from 0 down to 15 nm. In Figure of 0 to 65 nm, and (b) results covering range of etching from 0 down to 15 nm. In Figure 16b—smoothed 16b—smoothed data (B-Spline) are presented.
data (B-Spline) are presented. 4. Discussion
4. Discussion
Quantitative SIMS analysis of hydrogen in materials is very difficult, as there are number of factors influencing theanalysis measurement result [46]. main problem contamination of theare sample’s Quantitative SIMS of hydrogen inThe materials is veryis difficult, as there number of surface with hydrogen (mainly water). additional problem is is in contamination situ contamination in sample’s the factors influencing the measurement resultAn [46]. The main problem of the vacuum the masswater). spectrometer. The purpose of this paper not to describeinabsolute surface with chamber hydrogenof(mainly An additional problem is in situ iscontamination the vacuum hydrogen concentration but to compare the hydrogen content within the surface layers of the tested chamber of the mass spectrometer. The purpose of this paper is not to describe absolute hydrogen samples. Thus, all the samples after the electropolishing process were subjected to the same careful concentration but to compare the hydrogen content within the surface layers of the tested samples. storage procedure. In order to limit “in situ” contamination, SIMS measurements were done in 3 × Thus,10all the samples after the electropolishing process were subjected to the same careful storage −8 Pa base vacuum. procedure. order to “in situ” contamination, SIMS measurements weresamples, done inby3 using × 10−8 Pa TheInstudies on limit hydrogenation level of electrochemically polished niobium base secondary vacuum. ion mass spectrometry (SIMS), are presented in the paper. The program of the study The studies hydrogenation level of(1)electrochemically polishedafter niobium samples, by using covered three on kinds of niobium samples: AR—as received, material cold-rolling, without any surface treatment, (2) EP—Nb(SIMS), samples are afterpresented a standard in electropolishing, and program (3) MEP—samples secondary ion mass spectrometry the paper. The of the study afterthree magnetoelectropolishing. Time of this SIMS analysis was 6000 s. covered kinds of niobium samples: (1) AR—as received, material after cold-rolling, without any The first series of our SIMS studies of niobium appeared to be charged with(3) strong edge effects, after surface treatment, (2) EP—Nb samples after a standard electropolishing, and MEP—samples and with these presenting rather unreliable results. One could find very similar level of H+ ion magnetoelectropolishing. Time of this SIMS analysis was 6000 s. emission in both EP and MEP electrochemically treated samples. The assumed depth of penetration The first series of our SIMS studies of niobium appeared to be charged with strong edge effects, in the SIMS studies could suggest there is no essential difference in hydrogen concentration of EP + and and withMEP these presenting rather unreliable results. One could find very similar level of H ion samples. Also the character of courses of H+ and H2+ ion currents is very similar, but the emission in both and MEP is electrochemically samples. The assumed depth of penetration + ion + ion currents. intensity of H2EP currents ten times less thantreated that of H in the SIMS studies couldafter suggest there is no essentialwere difference Niobium samples EP and MEP treatments appliedintohydrogen the secondconcentration series of SIMSof EP + and MEP samples.In Also character of courses H+ and very but the measurements. this the series, a lesser area of ionic of etching wasHapplied, equaling 1ismm × 1similar, mm, with 2 ion currents + ion the area analysis reduced down to than 0.4 mm 0.4Hmm. The time of the SIMS analysis was intensity of Hof2 +SIMS ion currents is ten times less that× of currents. assumed be 5000 s.after The results obtained in this serieswere show applied that investigated positive series secondary Niobiumtosamples EP and MEP treatments to the second of SIMS currents are very similar on both samples, EP and MEP (Figures 5 and 6). However, the dependence measurements. In this series, a lesser area of ionic etching was applied, equaling 1 mm × 1 mm, ions currents for positive hydrogen (H+ and H2+) to the secondary ion currents with of theratios areaofofsecondary SIMS analysis reduced down to 0.4 mm × 0.4 mm. The time of the SIMS analysis was of niobium on time of the ion etching reveals a slight difference between EP and MEP niobium assumed to be 5000 s. The results obtained in this series show that investigated positive secondary samples (Figure 7). Smoothed forms of the ordinates values presented by dots in Figure 7 indicate currents are veryvalues similar on bothMEP samples, EP and MEP 5 and 6). However, the dependence of the reduced regarding samples, which can (Figures be easily noticed. + + ratios of secondary ions for positive hydrogen (Hout and the secondary ion currents of 2 ) to This behavior ofcurrents ion currents prompted us to carry theHthird series of SIMS experiments niobium on atime of the ion etching reveals a slight difference and MEP niobium samples paying special attention to the edge effects. With this seriesbetween of SIMS EP measurements, all niobium (Figure 7). Smoothed forms of the ordinates values presented by dots in Figure 7 indicate the reduced samples—AR, EP, and MEP—were taken into consideration, with the mass parameters of secondary
values regarding MEP samples, which can be easily noticed. This behavior of ion currents prompted us to carry out the third series of SIMS experiments paying a special attention to the edge effects. With this series of SIMS measurements, all niobium samples—AR,
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Metals 2017, 7, 442 16 of 19 EP, and MEP—were taken into consideration, with the mass parameters of secondary ions same as those of the second series. The etching area in this series was 3.5 mm × 3.5 mm, assuming 3600 s as the ions same as those of the second series. The etching area in this series was 3.5 mm × 3.5 mm, etching time of the process. The crater of depth of about 65 nm, and the ion etching rate of niobium assuming 3600 s as the etching time of the process. The crater of depth of about 65 nm, and the ion equaling 0.016 nm·s−1 resulted in getting clarification of the data obtained and much more meaningful etching rate of niobium equaling 0.016 nm·s−1 resulted in getting clarification of the data obtained effects of the studies. and much more meaningful effects of the studies. In Figure 17, an of estimated depth to to which thethe hydrogen In Figure 17,illustration an illustration of estimated depth which hydrogencontent contentisisexpected expected has has been provided, with the highest concentration measured on AR sample. The electrochemical treatments been provided, with the highest concentration measured on AR sample. The electrochemical with the EP andwith MEPthe processes in lowering the hydrogenation and a significant and shiftain the treatments EP andresult MEP both processes result both in lowering the hydrogenation significant shift measurement in the beginninginofall SIMS measurement in all plots presented above. beginning of SIMS plots presented above.
Figure Comparablezone zoneaffected affected by by hydrogen hydrogen in received, EP—after Figure 17. 17. Comparable in Nb Nb sample: sample:AR—as AR—as received, EP—after electropolishing, MEP—after magnetoelectropolishing. electropolishing, MEP—after magnetoelectropolishing.
By limiting edge effectsand andother otherselections selections in parameters of the H+ ions By limiting the the edge effects in measurement measurement parameters of the H+ ions + currents for this analysis, level theHH+ ions ions emission to to be be thethe lowest on MEP sample, currents for this analysis, thethe level ofofthe emissionappeared appeared lowest on MEP sample, as expected from our earlier studies (see refs. of the study on 316L SS [44] and Ti [45]). One more as expected from our earlier studies (see refs. of the study on 316L SS [44] and Ti [45]). One more interesting phenomenon noticed here was a sharp decrease in oxygen content in the surface film of interesting phenomenon noticed here was a sharp decrease in oxygen content in the surface film of the the MEP sample. All herewith presented results show that the thickness of the oxide film is much MEP lower sample. AllMEP herewith presented results showwith that that the one thickness of the oxide film is much lower on the niobium sample in comparison found on EP sample. on the MEP niobium sample in comparison with that one found on EP sample. The next two niobium samples, EP and MEP, were investigated by assuming etching time of The two samples, EP and MEP, byThey assuming etching of 3600 s 3600next s and theniobium SIMS measurements results arewere giveninvestigated in Figure 12. allowed us to time estimate thicknesses of surface oxide film are on the niobium samples to beallowed of the magnitudes as follows: and the SIMS measurements results given in Figure 12. They us to estimate thicknesses AR—about nm, EP—3 and MEP—about 1.5be nm. of surface oxide4 film on thenm, niobium samples to of the magnitudes as follows: AR—about 4 nm, EP—3 nm, and MEP—about 1.5 nm. 5. Conclusions
5. Conclusions The studies carried out on niobium allowed us to formulate the following conclusions: The out on niobium us to formulate themagnetoelectropolished following conclusions: • studies Niobiumcarried (AR—as received) may beallowed electropolished (EP) and/or (MEP)
•
•
•
using compounded electrolyte consisting of mixture of 70% methanesulfonic acid with 49%
Niobium (AR—as received) may electropolished (EP) and/or magnetoelectropolished (MEP) hydrofluoric acid by volume in abe3:1 ratio using compounded electrolyte consisting of mixture of 70% methanesulfonic acid • SIMS measurements have shown that the lowest hydrogenation level was achieved on with MEP 49% hydrofluoric acid by volume in a 3:1 ratio niobium samples in comparison with that one measured before (AR samples) and after other treatments (EP samples) SIMSelectrochemical measurements have shown that the lowest hydrogenation level was achieved on MEP • The estimated of thewith surface niobium samples as samples) follows: AR—about niobium samples thicknesses in comparison thatlayers’ one measured beforeare (AR and after4other nm, EP—3 nm, and MEP—about 1.5 nm electrochemical treatments (EP samples) The estimated thicknesses of the surface layers’ niobium samples are as follows: AR—about 4 nm, EP—3 nm, and MEP—about 1.5 nm
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The MEP process in new electrolyte is advised for surface finishing of niobium with the sped-up metal removal in-depth on request. Author Contributions: Tadeusz Hryniewicz and Ryszard Rokicki conceived and designed the experiments; Piotr Konarski and Ryszard Rokicki performed the experiments; Tadeusz Hryniewicz and Piotr Konarski analyzed the data; Tadeusz Hryniewicz, Piotr Konarski, and Ryszard Rokicki contributed reagents, materials, analysis tools; Tadeusz Hryniewicz and Piotr Konarski wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.
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