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Ni-Cr-Ti protective coating
Surface and Coatings Technology, 60 (1993) 603—608
Ni—Cr—Ti protective coating Boris Navin~ek J. Stefan Institute, Jamova 39, 61000 Ljubljana (Slovenia)
Abstract Ni—Cr--Ti protective films were prepared by a plasma beam sputter deposition process at 250-C. Ni—Cr—Ti thin films (50 nm) have been used to study the microstructure by transmission electron microscopy and the grain growth during heating in the heating stage of an electron microscope and after baking in an oxygen flow in an oven. Energy-dispersive spectroscopy analysis in the transmission electron microscope was used to study compositional variations in the films. Ni—Cr—Ti films 300 nm thick deposited on polished hotworked tool steel (AISI Hil) substrates have been studied by Auger electron spectroscopy and depth profiling before and after heat treatment in oxygen in an oven at temperatures from 600 to 800 ~C. Basic microstructural, morphological, compositional and mechanical properties have been studied on Ni—Cr—Ti protective coatings 3 ~smthick. Oxidation and electrochemical properties have also been studied.
1. Introduction Hard, wear-resistant TiN, TiCN, (Ti, Al)N and CrN PVD (physical vapour deposition) coatings 1 —5 jim thick have been successfully introduced in practice in the last decade. Current development is directed at finding alternative coatings not only with increased wear resistance but also with improved corrosion and oxidation resistance. The use of such coatings at elevated temperatures, in various corrosion media, with liquid metals or under conditions incorporating a combination of these parameters requires optimization of the complete coating procedure. In particular, one must control the appearance of microporosity, which is directly correlated with undesired structural and morphological changes leading to defects in and destruction of coatings. In many tribological systems one also finds materials that should not be heated during deposition to above 250 ~C. At temperatures below 250 °C surface cleaning in situ by plasma etching and an intermediate layer play a decisive role in protective coating production. Alloy PVD coatings are now considered for specific applications in which environmental attack necessitates good resistance to and protection from high temperature oxidation and corrosion [1]. For such applications good adhesion between the substrate and a coating a few microns thick is one of the most important prerequisites. In recent years it has been shown that the adhesive strength of ion-plated TiN coatings to tool and steel substrate materials is substantially improved if one uses a thin layer of titanium at the interface [2]. It was also reported that this titanium interlayer transforms at elevated temneratures into a Ti—C nhase. Such a new.
graded interface avoids sharp interface changes and turn increases the adhesion of the coating via a bet interface contact and stronger chemical bonds [3]. In this research we studied Ni—Cr--Ti coatings dep ited at 250 °C. Our preliminary investigations show that this type of coating could resist various corrosi media and oxidation in pure oxygen up to 800 Structural, compositional, interfacial and morphologii investigations have been performed by transmissi electron microscopy (TEM), scanning electron mici scopy (SEM), Auger electron spectroscopy (AES) a glancing incidence X-ray analysis. Oxidation has be studied in an oxygen flow in an oven at temperatu~ from 600 to 800 °C,while corrosion measurements h~ been made in acetate buffer at pH 5.7 and 0.5 M Na solution.
2. Experimental details 2.1. Preparation of bulk substrates The substrates used for the Ni—Cr—Ti coatings w stainless steel (AISI 304), powder metallurgically pi duced high speed steel (HSS; ASP 30, Speedst~ Sweden), hot-worked tool steel (AISI Hl 1) and sapphi Polished sapphire substrates were used for weight g~ measurements. All the steel substrates were prepared by cutting ir discs of thickness 3 mm. The discs were progressiv~ ground and polished down to a roughness Ra of b than 0.05 jim. After ultrasonic cleaning, the substra were fixed in the planetary substrate holder of a Balzi SDutron tetrode nlasma beam denosition annaratus 1
604
B
Naj,it’/~
Cr Ti
Ni
The substrates were heated with a radiant heater up to 350 C and then sputter cleaned with an argon plasma for tO mm. The Ni--Cr--Ti films were sputtered from a single-strip-type source. Thicknesses of 50. 300 and 3000 nm were deposited at a substrate temperature of 250 C and a constant deposition rate of 12.5 nm mm No heat treatment was given to the Ni- Cr Ti films after deposition. ‘.
2.2. 7ransinission electron microscopy Films 50 nm thick were deposited on a carbon film supported by a platinum mesh. Several specimens were used to determine the microstructure of as-deposited and ex-situ oxidized Ni—Cr-Ti films by TEM and energy-dispersive spectroscopy (EDS) in a Jeol 2000 FX electron microscope and to study the recrystallization process when they were continuously heated in the heating stage of a Philips 301 electron microscope from room temperature to 650 C. The specimen surroundings were cooled with liquid nitrogen. For ex-situ oxidation the thin films were heated in an oxygen flow in an oven at 600, 700 and 800 C. The recrystallization growth was compared for both kinds of heating process. Dark field images revealed the crystallite dimensions in the films. Ni-- Cr—Ti coatings 3 p.m thick were also thinned in a dual-ion-beam milling machine and the microstructure investigated by scanning transmission electron microscopy (STEM). 2.3. Auger electron spectroscopv For chemical analyses Ni—Cr—Ti films 300 nm thick were sputter deposited on polished hot-worked tool steel H II. Auger surface and sputter depth profile analyses were performed on a PHI SAM 545 A analyser at the IEVT Institute in Ljubljana [5]. A static primary electron beam of 3 keV energy, I jiA beam current and 40 lim diameter was used. Samples were ion sputtered by a I keV argon ion beam rastered on the surface over an area larger than 5 mm x 5 mm. Auger peak-to-peak heights of Cr (529 eV), Ni (848 eV), Ti (418 eV), Fe (703 eV). 0 (512 eV) and C (271 eV) were recorded during depth profiling. Auger depth profiling has been performed on as-deposited and oxidized Ni—Cr—Ti samples. For all analyses a sputtering rate of approximately 3.0 nm mm 1 was used. -
2.4. Scanning electron microscopy and roughness measurements Topography of as-deposited and oxidized Ni—Cr--Ti was performed on coatings 3 p.m thick. These analyses were correlated with roughness measurements using a Talysurf apparatus (Taylor—Hobson, UK). Morphological changes and crystallite growth in an oxygen flow as a function of heating temperature and oxidation time were determined,
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ag
2.5. Oxidation of Ni--Cr Ti coatings Ni -Cr Ti coatings 3 jim thick on sapphire substrai were oxidized in an oven with an oxygen flow at consta temperatures of 600. 700 and 800 C for I. 4 and 16 The weight gain due to oxide layer formation w determined. 2.6. Laser Roman spectroscopv and corrosion tests The identification of the thin mixed oxides Ni Cr Ti we alloy 3 p.m thick is complicatr Therefore also coatings used Raman spectroscopy. The spect were correlated with the available spectra of vario oxides which we coatings expect to inhe present oxide lay on Ni—Cr-Ti oxygen inat thetemperatur between 600 and 800 ~C. The corrosion resistance was evaluated using 0.5 I NaCI solution and acetate buffer at pH 5.7 and ron temperature relative to a saturated calomel electro (SCE) and a sweep rate of 5 mV s From potentiod namic curves the critical corrosion potential and corr sion current density and polarization resistance we determined. Data were accumulated continuously usit ~.
a PAR Model 273 galvanostat potentiostat controll by Model 342C corrosion software and Model 2 electrochemical software. Data for Ni --Cr Ti coatings were compared wi those for a PVD TiN coating 3 p.m thick produced in Balzers BAI 730 ion-plating apparatus. which h~ undergone the same electrochemical treatment. -
3. Results and discussion 3.1. Microstructure of Ni--Cr--Ti Ii lins The initial stage of Ni--Cr Ti growth was studied f thin films sputtered at 250 C on a carbon film. All t] analysed films 50 nm thick were amorphous (s Fig. 1(a)). Bright and dark field images indicated a hi1 probability of fractural crystallization. Heating of the Ni-- Cr--Ti films in an oxygen flow an oven at 600-C for I h induced recrystallization at the appearance of various alloy and mixed oxide phas (see Fig. 1(b)). After I h poorly developed grain boun aries were observed, while after 16 h at 600 C ti microstructure had changed to a well-developed gra structure (grain size 40—SO nm). When we increased ti heating temperature to 700 ~Cfor 4 h in an oxygen fib a well-developed grain structure was observed (see Fi1 1(c) and I (d)). The grain size was 50 -70 nm and ti film was still homogeneous. but consisted of a mixtu of the following phases: NiTi, NiTiO5. Cr703, CrC NiCrO4 and Ti02. The intensity of these phases chang from grain to grain. From the intensities in the E spectrum one can conclude that the metal pea
B. Navins’ek
/
Ni—Cr—Ti proleitiis’ coating
Onm (h)
(Ci)
~
lOnm
_
tel
(dl
(c)
(F)
Fig. 1. Microstructure of Ni—Cr—Ti film 50 nm thick sputtered on carbon film (Pt + C mesn) at 250 °C:(a) bright field and electron diffracti (b) dark field; (c), (d) after heating for 1 h at 600°Cin oxygen; (e), (f) after heating for 4 h at 700°Cin oxygen.
decreased while the film thickness increased as a result of the slow oxidation process. Additionally, we studied the recrystallization process in a Philips 301 electron microscope using a heating stage. At room temperature the Ni—Cr—Ti films were amorphous agglomerates of grain size 2—3 nm. The film temperature was then increased to 100°Cand held for 15 mm. The start of recrystallization was expected to be seen in the ED spectrum with the appearance of new lines. This occurred at 550 °C,while at 600 °C a fine grain structure was developed (grain size 5—b nm).
When the sample was cooled to room temperature, ED spectrum showed a continuous film with a sm percentage of oxide phases. The picture was simi to Fig. 1(b), indicating no well-developed gri boundaries. 3.2. Composition and homogeneity of Ni—Cr—Ti coatin, 3 jim thick For this study we used semiquantitative analysis iT Jeol 840 A scanning electron microscope. The followi composition was obtained: Cr, 39.5 wt.%; Ni, 28.0 wt.
606
B. Narinsek
Ni - (~r——Ti pt~i’~ft~’ coaling
3.3. Oxidation of Ni-- Cr Ti coatings at temperatures
-
Ni-Cr-Ti
0
~°°----4 600—800T
Ti. 32.5 wt.%. Only traces of oxygen and carbon contamination were found in these films, AES depth profiling of an as-deposited film has been performed through the whole thickness of an Ni—Cr—Ti coating 300 nm thick deposited on a polished H 11 steel substrate. The depth profile in Fig. 2 shows good homogeneity of the Ni—Cr--Ti coating through the whole thickness, while only a small amount of contamination was present at the interface between the coating and the steel substrate (interface width less than 14 nm). The thickness and inicrostructure of the Ni—Cr—Ti coating 3 jim thick have been determined on a crosssectional specimen (Fig. 3(a)). No columnar or fine equiaxed grains were detected within the coating. After thinning of the edge-on sample, we observed the same
Oxidation of Ni--Cr—Ti coatings 3 jim thick at cc stant temperatures of 600, 700 and 800 °Cfor 1, 4 a 16 h induced a weight gain due to oxide layer formati morphology changes (crystallite growth) an increase the surface roughness (Ra) and changes in the elemen distribution over the whole thickness of the Ni—Cr-coating especially at the interface between the coati and the steel substrate. The %4elght gain induced by oxidation increases w increasing temperature and oxidation time (Fig. 4(~ Data were obtained with sapphire substrates, which thermally stable to at least to 900 C. All the oxidized samples have also been investigat by SEM (topography) and profilometry (roughness). T roughness increased almost linearly after 4 h oxidati in oxygen for all baking temperatures (Fig. 4(b)). general, values of Ra less than 0.12 p.m for 16 h oxidati. at 800°C represent a lower surface roughness than generally required and the majority of polished coi form and die-casting tools operating between 250 a 750 °C.This was also confirmed by optical inspection samples oxidized at 800 °C: they were still shiny reflected light. The roughness increase is closely connected wi crystallite growth during the oxidation ~rocess. VaIL of crystallite size as a function of test nperature a substrate material obtained by SEM analysis are shm in Fig. 5. The diagram shows that the lowest values average crystallite size were obtained with ASP 30 su strates (Vickers hardness 860 HVO1), while the larg
result. The coating was nearly amorphous; microdiffraction showed only a few weak lines. The ED spectrum of the same thinned specimen (Fig. 3(b)) showed a similar elemental line distribution and intensity correlation as was observed for the 50 nm Ni—Cr--Ti film,
crystallites were found on sapphire substrates. Proper insight to the oxidation process can obtained by AES depth profile analysis. For this inves gation an Hl I tool steel substrate has been chosen. Or brief comments will be given on the results shown
-
-
I
-~ —
-
tt~r
j
I -
-
l-i~.2. AES depth profiles of Ni--Cr--Ti coating 300 nm thick as •
-,
-
deposited at 50 C on I .~343hot-worked tool steel substrate. •. ( (271 eV(; Ti (418 cV): ~. 0(510 eVt . Cr (529 eVI: x, Ni (848 eV); I-c 703 eV).
Ni (hi
.
A~L
‘CU
Fig. 3. (a) Scanning electron micrograph of Ni—Cr- Ti coating 3 om thick on polished HSS substrate. (h) Energy-dispersive X-ray spectrum same specimen thinned by an ion-milling technique to transparency.
B. Navins°ek / Ni—Cr—Ti protective coating
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1200 NIO~TI .E~
&
Ni-Cr-Ti
80000
700°C oxygen -
1000
800
I~ — 0
÷_4
ooo0I 400 200 0
(a)
OXIDATION TIME [hi NIOrTI
0.12
~ CI)
70000 16 60000 20
8_________+ 12
0.08 0.06
80000
~1”
700°C
-
=
0.04 0
0.02
eoo°o
00
4
8
12
16
20
(b) OXIDATION TIME [hi Fig. 4. (a) Weight gain of Ni—Cr—Ti coating 3 om thick on sapphire substrate induced by oxidation in oxygen flow. (b) Roughness increase as a function of oxidation time.
1.2 NIC~TlOOATING 800°C/4h
1 0.8
0
20
40
60
80
100
120
140
160
Sputter tIme [mini
180
200
220
2
Fig. 6. AES depth profiles of Ni—Cr—Ti coating300 nm thick deposit at 250°Con 1.2343 tool steel substrate and oxidized for 4 h at 700 1 in oxygen. •, C (271 eV); +, Ti (418 eV); ~,0(510 eV); ~, Cr (529 e
x, A Ni microhardness (848 eV);process ~ Fe (703 eV). mechanical oxidation loads. and of 950—l also120 sustain HV relatively larl 0.25various was measur for as-deposited Ni—Cr—Ti coatings on substra materials. After heating the coatings at 700 °Cin oxyg
for 4 h, the samples retained their smooth surface and microhardness higher than 900 HV025. Raman spectra have also been taken from oxidiz Ni—Cr—Ti coatings deposited on sapphire substratc Figure 7 illustrates an ex-situ Raman spectrum obtaim after oxidation at 700°Cfor 16 h. Raman peaks at 44 616 and 717 cm 1 were observed for all samples oxidiz —
at 600, 700 and 800 °C,their intensities increasing wil increasing oxidation time and temperature of oxidatio Because of the known problems with taking represent. tive Raman spectra from transparent oxides (Cr—( Ti—O and mixed Ni—Cr—O and Cr—Ti—O), we tried 1 identify observed energies with data in the literature [~ The peak at 445 cm could correspond to either Ti( -
~
0.0
7Oo~b/4h -J
0.4
or Cr203, while that at 616 cm_t was found to corr spond to Cr2NiO4 (and also to Cr23 and Ti02_,j. Sin
0.2 C
100
it
it
it
_____________________________________________ ASP 30 88 304 1.2343 SAPPHIRE
445
80
SUBSTRATE MATERIAL
Fig. 5. Crystallite size of Ni—Cr—Ti coating as a function of oxidation test temperature and substrate material.
60 0,
~4O
Fig. 6; a detailed analysis will be published elsewhere. Only Ni—Cr—Ti coating 0.3 jim thick was oxidized at 700°C for 4 h in oxygen. The surface was transformed into a mixture of various Cr and Ti-oxides and Cr—Ti phases, while important changes were observed at the interface. Two interface zones were formed, the most important being the Ti—C, Cr—C or Fe—C phases at the substrate surface. There is a high probability that these carbides will increase the adhesion strength during the
20 0
-20
0
200
400
600 800 1000 1200 MC Energy (cm-i) Fig. 7. Raman spectrum of Ni—Cr—Ti coating heated for 16 hat 700 in oxygen flow (ex situ).
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H
000 —
TN
~
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7
-t
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-
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-
N
----
1CrTi
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0 ~o E vs SCE (VI
Fig S Potentiodynamic polarization curves for Ni Cr Ti and TIN coatings on Ta foil obtained with 0.51 M NaCI solution TABLE I Electrochemical parameters for PVD Ni—Cr—Ti and TiN coatings in 0,51 M NaCI solution (scan rate S mV __________________ ________ _________ 2) R(kt2 cmH Coating E~r,mV(SCE) i. (~tAcm TIN m °4~ 6 I 69 10 Ni—Cr—Ti, .3 jim —4210 076 ~
morphological properties and their influence on oxidation behaviour when they are thermally treated an oxygen flow at temperatures up to 800 C. It ~ found by TEM, SEM. EDS and profilometry measu ments that recrystallization of the Ni Cr Ti coatii which as deposited is practically amorphous (or nar crystalline), starts at approximately In paral with the recrystallization process. the 550 outerC.morpholo -
of a 3 p.m coating changed slowly when oxidation ~ performed in oxygen up to 700 C. while an Ra value 0.06 ltm was observed. AES depth profiling showed if at this oxidation temperature the interface between steel substrate and the Ni--Cr Ti coating also contain Ti and Cr carbides, This would probably increase adhesion strength during the oxidation process in va ous media. At 800 C only small additional changes w observed It should be pointed out that a microhardness as hi as 1000 HV 0 isheating typical inforoxygen Ni Cr-atTi700 coatings on re H substrates after C. This tively high value for a metallic coating has to be coi ,~
_____________________
we have made no measurements in situ here, the influence of exposure of the sample surface to air after oxidation in an oxygen flow is not known. 3.4. Corrosion tests The potentiodynamic polarization curves for two coatings, 3pm Ni—Cr—Ti and 3 p.m TiN, deposited on a tantalum foil substrate are presented in Fig. 8. The main electrochemical parameters are listed in Table I. The preliminary general picture shows that an Ni--Cr Ti coating is more corrosion resistant than a TiN hard coating in NaCI media. This is demonstrated by the higher corrosion potential and lower corrosion current densities. The calculations enabled the evaluation of the polarization resistance R~, which was approximately 440% higher for the Ni—Cr—Ti coating. Further evidence of the superiority of Ni—Cr -Ti over TiN coatings has been observed from the relation between corrosion current and corrosion potential using a slow potential scan rate of 0.1 mV ~ In this curve TiN shows a clear oxidation increase between 0.00 and 0.85 V. while Ni--Cr--Ti shows only a small effect at a corrosion potential higher than 0.90 V. -
bined with a low Ra value and good corrosion resistar (higher than that for PVD TiN coatings) when practi applications are of interest. Preliminary tnvestigatio showed that Ni- Cr- Ti coatings could he used succe: fully with some liquid metals and also with aluminii alloys.
Acknowledgments This work was supported by the Ministry of Scier and Technology, Republic of Slovenia, Ljubljana. T author would like to acknowledge the assistance Mrs. M. Peternel, Dr. V. Kra~evecand Mrs. M. Rem~k for the TEM analyses. Dr. A. Zalar and B. Praëek I the AES depth profiling. Mrs. I. Milo~evfor the con sion tests and Dr. D. Mihajlovië for the Ram spectroscopy. R f e erences M. S.~anRickerhy Roode and and L. Stir!. (~oat. Tcchnol..32 1987) [33I 0. P. J.Hsu. Burnett, Thin Solid F urn., /57 1988) ~i C. C. Cheng. A. Erdemir and Ci. R. Ienske. Surf. (oat. Tic/tn -~ —
.19 4(1 (1989) 365.
4. Summary Ni-Cr-Ti (28.5-39.5-32.5 wt.%) alloy coatings have been used to study microstructural, compositional and
4 B. NavinVek and J. Fine. Vactiwn, ./6 (1986) 71 I. S A. Zalar, P. Panjan and S. Hofmann, Thin Solid Films. 101 (19 R - D. oss. Inorganic Infrared and Ranian .Spectra. McGraw- I— London, [986, Chap 4. p. 95.
Ni—Cr—Ti protective coating Boris Navin~ek J. Stefan Institute, Jamova 39, 61000 Ljubljana (Slovenia)
Abstract Ni—Cr--Ti protective films were prepared by a plasma beam sputter deposition process at 250-C. Ni—Cr—Ti thin films (50 nm) have been used to study the microstructure by transmission electron microscopy and the grain growth during heating in the heating stage of an electron microscope and after baking in an oxygen flow in an oven. Energy-dispersive spectroscopy analysis in the transmission electron microscope was used to study compositional variations in the films. Ni—Cr—Ti films 300 nm thick deposited on polished hotworked tool steel (AISI Hil) substrates have been studied by Auger electron spectroscopy and depth profiling before and after heat treatment in oxygen in an oven at temperatures from 600 to 800 ~C. Basic microstructural, morphological, compositional and mechanical properties have been studied on Ni—Cr—Ti protective coatings 3 ~smthick. Oxidation and electrochemical properties have also been studied.
1. Introduction Hard, wear-resistant TiN, TiCN, (Ti, Al)N and CrN PVD (physical vapour deposition) coatings 1 —5 jim thick have been successfully introduced in practice in the last decade. Current development is directed at finding alternative coatings not only with increased wear resistance but also with improved corrosion and oxidation resistance. The use of such coatings at elevated temperatures, in various corrosion media, with liquid metals or under conditions incorporating a combination of these parameters requires optimization of the complete coating procedure. In particular, one must control the appearance of microporosity, which is directly correlated with undesired structural and morphological changes leading to defects in and destruction of coatings. In many tribological systems one also finds materials that should not be heated during deposition to above 250 ~C. At temperatures below 250 °C surface cleaning in situ by plasma etching and an intermediate layer play a decisive role in protective coating production. Alloy PVD coatings are now considered for specific applications in which environmental attack necessitates good resistance to and protection from high temperature oxidation and corrosion [1]. For such applications good adhesion between the substrate and a coating a few microns thick is one of the most important prerequisites. In recent years it has been shown that the adhesive strength of ion-plated TiN coatings to tool and steel substrate materials is substantially improved if one uses a thin layer of titanium at the interface [2]. It was also reported that this titanium interlayer transforms at elevated temneratures into a Ti—C nhase. Such a new.
graded interface avoids sharp interface changes and turn increases the adhesion of the coating via a bet interface contact and stronger chemical bonds [3]. In this research we studied Ni—Cr--Ti coatings dep ited at 250 °C. Our preliminary investigations show that this type of coating could resist various corrosi media and oxidation in pure oxygen up to 800 Structural, compositional, interfacial and morphologii investigations have been performed by transmissi electron microscopy (TEM), scanning electron mici scopy (SEM), Auger electron spectroscopy (AES) a glancing incidence X-ray analysis. Oxidation has be studied in an oxygen flow in an oven at temperatu~ from 600 to 800 °C,while corrosion measurements h~ been made in acetate buffer at pH 5.7 and 0.5 M Na solution.
2. Experimental details 2.1. Preparation of bulk substrates The substrates used for the Ni—Cr—Ti coatings w stainless steel (AISI 304), powder metallurgically pi duced high speed steel (HSS; ASP 30, Speedst~ Sweden), hot-worked tool steel (AISI Hl 1) and sapphi Polished sapphire substrates were used for weight g~ measurements. All the steel substrates were prepared by cutting ir discs of thickness 3 mm. The discs were progressiv~ ground and polished down to a roughness Ra of b than 0.05 jim. After ultrasonic cleaning, the substra were fixed in the planetary substrate holder of a Balzi SDutron tetrode nlasma beam denosition annaratus 1
604
B
Naj,it’/~
Cr Ti
Ni
The substrates were heated with a radiant heater up to 350 C and then sputter cleaned with an argon plasma for tO mm. The Ni--Cr--Ti films were sputtered from a single-strip-type source. Thicknesses of 50. 300 and 3000 nm were deposited at a substrate temperature of 250 C and a constant deposition rate of 12.5 nm mm No heat treatment was given to the Ni- Cr Ti films after deposition. ‘.
2.2. 7ransinission electron microscopy Films 50 nm thick were deposited on a carbon film supported by a platinum mesh. Several specimens were used to determine the microstructure of as-deposited and ex-situ oxidized Ni—Cr-Ti films by TEM and energy-dispersive spectroscopy (EDS) in a Jeol 2000 FX electron microscope and to study the recrystallization process when they were continuously heated in the heating stage of a Philips 301 electron microscope from room temperature to 650 C. The specimen surroundings were cooled with liquid nitrogen. For ex-situ oxidation the thin films were heated in an oxygen flow in an oven at 600, 700 and 800 C. The recrystallization growth was compared for both kinds of heating process. Dark field images revealed the crystallite dimensions in the films. Ni-- Cr—Ti coatings 3 p.m thick were also thinned in a dual-ion-beam milling machine and the microstructure investigated by scanning transmission electron microscopy (STEM). 2.3. Auger electron spectroscopv For chemical analyses Ni—Cr—Ti films 300 nm thick were sputter deposited on polished hot-worked tool steel H II. Auger surface and sputter depth profile analyses were performed on a PHI SAM 545 A analyser at the IEVT Institute in Ljubljana [5]. A static primary electron beam of 3 keV energy, I jiA beam current and 40 lim diameter was used. Samples were ion sputtered by a I keV argon ion beam rastered on the surface over an area larger than 5 mm x 5 mm. Auger peak-to-peak heights of Cr (529 eV), Ni (848 eV), Ti (418 eV), Fe (703 eV). 0 (512 eV) and C (271 eV) were recorded during depth profiling. Auger depth profiling has been performed on as-deposited and oxidized Ni—Cr—Ti samples. For all analyses a sputtering rate of approximately 3.0 nm mm 1 was used. -
2.4. Scanning electron microscopy and roughness measurements Topography of as-deposited and oxidized Ni—Cr--Ti was performed on coatings 3 p.m thick. These analyses were correlated with roughness measurements using a Talysurf apparatus (Taylor—Hobson, UK). Morphological changes and crystallite growth in an oxygen flow as a function of heating temperature and oxidation time were determined,
()roicctflE’ ci,cii
ag
2.5. Oxidation of Ni--Cr Ti coatings Ni -Cr Ti coatings 3 jim thick on sapphire substrai were oxidized in an oven with an oxygen flow at consta temperatures of 600. 700 and 800 C for I. 4 and 16 The weight gain due to oxide layer formation w determined. 2.6. Laser Roman spectroscopv and corrosion tests The identification of the thin mixed oxides Ni Cr Ti we alloy 3 p.m thick is complicatr Therefore also coatings used Raman spectroscopy. The spect were correlated with the available spectra of vario oxides which we coatings expect to inhe present oxide lay on Ni—Cr-Ti oxygen inat thetemperatur between 600 and 800 ~C. The corrosion resistance was evaluated using 0.5 I NaCI solution and acetate buffer at pH 5.7 and ron temperature relative to a saturated calomel electro (SCE) and a sweep rate of 5 mV s From potentiod namic curves the critical corrosion potential and corr sion current density and polarization resistance we determined. Data were accumulated continuously usit ~.
a PAR Model 273 galvanostat potentiostat controll by Model 342C corrosion software and Model 2 electrochemical software. Data for Ni --Cr Ti coatings were compared wi those for a PVD TiN coating 3 p.m thick produced in Balzers BAI 730 ion-plating apparatus. which h~ undergone the same electrochemical treatment. -
3. Results and discussion 3.1. Microstructure of Ni--Cr--Ti Ii lins The initial stage of Ni--Cr Ti growth was studied f thin films sputtered at 250 C on a carbon film. All t] analysed films 50 nm thick were amorphous (s Fig. 1(a)). Bright and dark field images indicated a hi1 probability of fractural crystallization. Heating of the Ni-- Cr--Ti films in an oxygen flow an oven at 600-C for I h induced recrystallization at the appearance of various alloy and mixed oxide phas (see Fig. 1(b)). After I h poorly developed grain boun aries were observed, while after 16 h at 600 C ti microstructure had changed to a well-developed gra structure (grain size 40—SO nm). When we increased ti heating temperature to 700 ~Cfor 4 h in an oxygen fib a well-developed grain structure was observed (see Fi1 1(c) and I (d)). The grain size was 50 -70 nm and ti film was still homogeneous. but consisted of a mixtu of the following phases: NiTi, NiTiO5. Cr703, CrC NiCrO4 and Ti02. The intensity of these phases chang from grain to grain. From the intensities in the E spectrum one can conclude that the metal pea
B. Navins’ek
/
Ni—Cr—Ti proleitiis’ coating
Onm (h)
(Ci)
~
lOnm
_
tel
(dl
(c)
(F)
Fig. 1. Microstructure of Ni—Cr—Ti film 50 nm thick sputtered on carbon film (Pt + C mesn) at 250 °C:(a) bright field and electron diffracti (b) dark field; (c), (d) after heating for 1 h at 600°Cin oxygen; (e), (f) after heating for 4 h at 700°Cin oxygen.
decreased while the film thickness increased as a result of the slow oxidation process. Additionally, we studied the recrystallization process in a Philips 301 electron microscope using a heating stage. At room temperature the Ni—Cr—Ti films were amorphous agglomerates of grain size 2—3 nm. The film temperature was then increased to 100°Cand held for 15 mm. The start of recrystallization was expected to be seen in the ED spectrum with the appearance of new lines. This occurred at 550 °C,while at 600 °C a fine grain structure was developed (grain size 5—b nm).
When the sample was cooled to room temperature, ED spectrum showed a continuous film with a sm percentage of oxide phases. The picture was simi to Fig. 1(b), indicating no well-developed gri boundaries. 3.2. Composition and homogeneity of Ni—Cr—Ti coatin, 3 jim thick For this study we used semiquantitative analysis iT Jeol 840 A scanning electron microscope. The followi composition was obtained: Cr, 39.5 wt.%; Ni, 28.0 wt.
606
B. Narinsek
Ni - (~r——Ti pt~i’~ft~’ coaling
3.3. Oxidation of Ni-- Cr Ti coatings at temperatures
-
Ni-Cr-Ti
0
~°°----4 600—800T
Ti. 32.5 wt.%. Only traces of oxygen and carbon contamination were found in these films, AES depth profiling of an as-deposited film has been performed through the whole thickness of an Ni—Cr—Ti coating 300 nm thick deposited on a polished H 11 steel substrate. The depth profile in Fig. 2 shows good homogeneity of the Ni—Cr--Ti coating through the whole thickness, while only a small amount of contamination was present at the interface between the coating and the steel substrate (interface width less than 14 nm). The thickness and inicrostructure of the Ni—Cr—Ti coating 3 jim thick have been determined on a crosssectional specimen (Fig. 3(a)). No columnar or fine equiaxed grains were detected within the coating. After thinning of the edge-on sample, we observed the same
Oxidation of Ni--Cr—Ti coatings 3 jim thick at cc stant temperatures of 600, 700 and 800 °Cfor 1, 4 a 16 h induced a weight gain due to oxide layer formati morphology changes (crystallite growth) an increase the surface roughness (Ra) and changes in the elemen distribution over the whole thickness of the Ni—Cr-coating especially at the interface between the coati and the steel substrate. The %4elght gain induced by oxidation increases w increasing temperature and oxidation time (Fig. 4(~ Data were obtained with sapphire substrates, which thermally stable to at least to 900 C. All the oxidized samples have also been investigat by SEM (topography) and profilometry (roughness). T roughness increased almost linearly after 4 h oxidati in oxygen for all baking temperatures (Fig. 4(b)). general, values of Ra less than 0.12 p.m for 16 h oxidati. at 800°C represent a lower surface roughness than generally required and the majority of polished coi form and die-casting tools operating between 250 a 750 °C.This was also confirmed by optical inspection samples oxidized at 800 °C: they were still shiny reflected light. The roughness increase is closely connected wi crystallite growth during the oxidation ~rocess. VaIL of crystallite size as a function of test nperature a substrate material obtained by SEM analysis are shm in Fig. 5. The diagram shows that the lowest values average crystallite size were obtained with ASP 30 su strates (Vickers hardness 860 HVO1), while the larg
result. The coating was nearly amorphous; microdiffraction showed only a few weak lines. The ED spectrum of the same thinned specimen (Fig. 3(b)) showed a similar elemental line distribution and intensity correlation as was observed for the 50 nm Ni—Cr--Ti film,
crystallites were found on sapphire substrates. Proper insight to the oxidation process can obtained by AES depth profile analysis. For this inves gation an Hl I tool steel substrate has been chosen. Or brief comments will be given on the results shown
-
-
I
-~ —
-
tt~r
j
I -
-
l-i~.2. AES depth profiles of Ni--Cr--Ti coating 300 nm thick as •
-,
-
deposited at 50 C on I .~343hot-worked tool steel substrate. •. ( (271 eV(; Ti (418 cV): ~. 0(510 eVt . Cr (529 eVI: x, Ni (848 eV); I-c 703 eV).
Ni (hi
.
A~L
‘CU
Fig. 3. (a) Scanning electron micrograph of Ni—Cr- Ti coating 3 om thick on polished HSS substrate. (h) Energy-dispersive X-ray spectrum same specimen thinned by an ion-milling technique to transparency.
B. Navins°ek / Ni—Cr—Ti protective coating
1CC
1200 NIO~TI .E~
&
Ni-Cr-Ti
80000
700°C oxygen -
1000
800
I~ — 0
÷_4
ooo0I 400 200 0
(a)
OXIDATION TIME [hi NIOrTI
0.12
~ CI)
70000 16 60000 20
8_________+ 12
0.08 0.06
80000
~1”
700°C
-
=
0.04 0
0.02
eoo°o
00
4
8
12
16
20
(b) OXIDATION TIME [hi Fig. 4. (a) Weight gain of Ni—Cr—Ti coating 3 om thick on sapphire substrate induced by oxidation in oxygen flow. (b) Roughness increase as a function of oxidation time.
1.2 NIC~TlOOATING 800°C/4h
1 0.8
0
20
40
60
80
100
120
140
160
Sputter tIme [mini
180
200
220
2
Fig. 6. AES depth profiles of Ni—Cr—Ti coating300 nm thick deposit at 250°Con 1.2343 tool steel substrate and oxidized for 4 h at 700 1 in oxygen. •, C (271 eV); +, Ti (418 eV); ~,0(510 eV); ~, Cr (529 e
x, A Ni microhardness (848 eV);process ~ Fe (703 eV). mechanical oxidation loads. and of 950—l also120 sustain HV relatively larl 0.25various was measur for as-deposited Ni—Cr—Ti coatings on substra materials. After heating the coatings at 700 °Cin oxyg
for 4 h, the samples retained their smooth surface and microhardness higher than 900 HV025. Raman spectra have also been taken from oxidiz Ni—Cr—Ti coatings deposited on sapphire substratc Figure 7 illustrates an ex-situ Raman spectrum obtaim after oxidation at 700°Cfor 16 h. Raman peaks at 44 616 and 717 cm 1 were observed for all samples oxidiz —
at 600, 700 and 800 °C,their intensities increasing wil increasing oxidation time and temperature of oxidatio Because of the known problems with taking represent. tive Raman spectra from transparent oxides (Cr—( Ti—O and mixed Ni—Cr—O and Cr—Ti—O), we tried 1 identify observed energies with data in the literature [~ The peak at 445 cm could correspond to either Ti( -
~
0.0
7Oo~b/4h -J
0.4
or Cr203, while that at 616 cm_t was found to corr spond to Cr2NiO4 (and also to Cr23 and Ti02_,j. Sin
0.2 C
100
it
it
it
_____________________________________________ ASP 30 88 304 1.2343 SAPPHIRE
445
80
SUBSTRATE MATERIAL
Fig. 5. Crystallite size of Ni—Cr—Ti coating as a function of oxidation test temperature and substrate material.
60 0,
~4O
Fig. 6; a detailed analysis will be published elsewhere. Only Ni—Cr—Ti coating 0.3 jim thick was oxidized at 700°C for 4 h in oxygen. The surface was transformed into a mixture of various Cr and Ti-oxides and Cr—Ti phases, while important changes were observed at the interface. Two interface zones were formed, the most important being the Ti—C, Cr—C or Fe—C phases at the substrate surface. There is a high probability that these carbides will increase the adhesion strength during the
20 0
-20
0
200
400
600 800 1000 1200 MC Energy (cm-i) Fig. 7. Raman spectrum of Ni—Cr—Ti coating heated for 16 hat 700 in oxygen flow (ex situ).
608
B. Narin.s°ek -Ni—Cr— Ti prolccti-c coat ing 00
H
000 —
TN
~
H <
7
-t
H[
-
30~ -
-
N
----
1CrTi
‘~
~
-~
0 ~o E vs SCE (VI
Fig S Potentiodynamic polarization curves for Ni Cr Ti and TIN coatings on Ta foil obtained with 0.51 M NaCI solution TABLE I Electrochemical parameters for PVD Ni—Cr—Ti and TiN coatings in 0,51 M NaCI solution (scan rate S mV __________________ ________ _________ 2) R(kt2 cmH Coating E~r,mV(SCE) i. (~tAcm TIN m °4~ 6 I 69 10 Ni—Cr—Ti, .3 jim —4210 076 ~
morphological properties and their influence on oxidation behaviour when they are thermally treated an oxygen flow at temperatures up to 800 C. It ~ found by TEM, SEM. EDS and profilometry measu ments that recrystallization of the Ni Cr Ti coatii which as deposited is practically amorphous (or nar crystalline), starts at approximately In paral with the recrystallization process. the 550 outerC.morpholo -
of a 3 p.m coating changed slowly when oxidation ~ performed in oxygen up to 700 C. while an Ra value 0.06 ltm was observed. AES depth profiling showed if at this oxidation temperature the interface between steel substrate and the Ni--Cr Ti coating also contain Ti and Cr carbides, This would probably increase adhesion strength during the oxidation process in va ous media. At 800 C only small additional changes w observed It should be pointed out that a microhardness as hi as 1000 HV 0 isheating typical inforoxygen Ni Cr-atTi700 coatings on re H substrates after C. This tively high value for a metallic coating has to be coi ,~
_____________________
we have made no measurements in situ here, the influence of exposure of the sample surface to air after oxidation in an oxygen flow is not known. 3.4. Corrosion tests The potentiodynamic polarization curves for two coatings, 3pm Ni—Cr—Ti and 3 p.m TiN, deposited on a tantalum foil substrate are presented in Fig. 8. The main electrochemical parameters are listed in Table I. The preliminary general picture shows that an Ni--Cr Ti coating is more corrosion resistant than a TiN hard coating in NaCI media. This is demonstrated by the higher corrosion potential and lower corrosion current densities. The calculations enabled the evaluation of the polarization resistance R~, which was approximately 440% higher for the Ni—Cr—Ti coating. Further evidence of the superiority of Ni—Cr -Ti over TiN coatings has been observed from the relation between corrosion current and corrosion potential using a slow potential scan rate of 0.1 mV ~ In this curve TiN shows a clear oxidation increase between 0.00 and 0.85 V. while Ni--Cr--Ti shows only a small effect at a corrosion potential higher than 0.90 V. -
bined with a low Ra value and good corrosion resistar (higher than that for PVD TiN coatings) when practi applications are of interest. Preliminary tnvestigatio showed that Ni- Cr- Ti coatings could he used succe: fully with some liquid metals and also with aluminii alloys.
Acknowledgments This work was supported by the Ministry of Scier and Technology, Republic of Slovenia, Ljubljana. T author would like to acknowledge the assistance Mrs. M. Peternel, Dr. V. Kra~evecand Mrs. M. Rem~k for the TEM analyses. Dr. A. Zalar and B. Praëek I the AES depth profiling. Mrs. I. Milo~evfor the con sion tests and Dr. D. Mihajlovië for the Ram spectroscopy. R f e erences M. S.~anRickerhy Roode and and L. Stir!. (~oat. Tcchnol..32 1987) [33I 0. P. J.Hsu. Burnett, Thin Solid F urn., /57 1988) ~i C. C. Cheng. A. Erdemir and Ci. R. Ienske. Surf. (oat. Tic/tn -~ —
.19 4(1 (1989) 365.
4. Summary Ni-Cr-Ti (28.5-39.5-32.5 wt.%) alloy coatings have been used to study microstructural, compositional and
4 B. NavinVek and J. Fine. Vactiwn, ./6 (1986) 71 I. S A. Zalar, P. Panjan and S. Hofmann, Thin Solid Films. 101 (19 R - D. oss. Inorganic Infrared and Ranian .Spectra. McGraw- I— London, [986, Chap 4. p. 95.