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Surface modification of stainless steel in plasma environments
MATERIALS SCIENCE & ENGINEERING ELSEVIER
Materials Science and Engineering A198 (1995) 75 83
A
Surface modification of stainless steel in plasma environments M. H a s h i m o t o a, Y. M i y a m o t & , Y. K u b o ", S. T o k u m a r u a, N. O n o b, T. T a k a h a s h i b, I. Ito c ~Advanced Materials and Technology Research Laboratories, Nippon Steel Corporation, 1618 Ida, Nakahara-ku, Kawasaki 211, Japan bHikari Works, Hikari R&D Laboratory, Nippon Steel Corporation, 3434 Shimata, Hikari City, Yamaguchi Pre. 743, Japan CMetal Foil Products Division, New Materials Divisions Group, Nippon Steel Corporation, 6-30temachi 2-chome, Chiyoda-ku, Tokyo 100, Japan
Abstract
Stainless steels are widely used for architecture, vehicles, etc. because of their good corrosion resistance and surface aesthetics. In order to provide additional surface functions, such as decorative colors, we have developed a continuous dry coating process, which can provide stainless steel strip with a multilayer ceramic or metallic coating. In this paper, the role of the environment will be discussed from two different points of view. The first is the performance of these coated stainless steels in corrosive and mechanically stressed environments. The second is the influence of the plasma environment used to synthesize the coating materials on the quality of thin films.
Keywords: Surface modification; Plasma-assisted CVD; Sputtering; Ion plating; Stainless steel
I. Introduction
Stainless steels are used in architectural, automotive and chemical processing applications because of their good corrosion resistance, clean appearance and workability. Coating these metals with ceramics, other metals or alloys adds innovative surface functions, such as insulation or dielectric properties, aesthetic appeal and optical characteristics. In order to develop these additional surface functions, a combined process utilizing sputtering, ion plating and plasmaassisted chemical vapor deposition (PACVD) has been assessed. Thin film processes that use ion and low-temperature plasmas are commonly applied to modify the surface characteristics of materials. Unfortunately, handling problems associated with many toxic coating materials and the severe process requirements, such as high vacuum or material purity, have limited the large-scale production use of ion and plasma deposition techniques. We have developed a dry coating process that combines ion plating, sputtering and PACVD in a tandem arrangement to coat continuously coils of material on a semi-production scale [1]. 0921-5093/95/$09.50 © 1995 SSD1 0921-5093(94)04510-0
Elsevier Science S.A. All rights reserved
The above three processes have been selected since they generally give dense and adherent films. However, each coating process has its own limitations. For example, stainless steel coated with S i O 2 o r SiaN 4 by PACVD shows higher corrosion resistance than that coated by sputtering; in addition, the applicability of PACVD is limited by the availability of coating materials in the form of gases [2]. Therefore the combination of the above-mentioned three processes is particularly meaningful. By eliminating exposure to external environments, the tandem arrangement allows improvements in conventional coating systems. It also creates a procedure for developing new composite materials by allowing different materials to be sequentially coated. Although our coating process is compatible with many materials, we have used it to coat stainless steel strip. In this paper, the role of the environment will be discussed from two different points of view. The first is the performance of the coated stainless steels in corrosive and mechanically stressed environments. The second is the influence of the plasma environment used to synthesize the coating materials on the quality of thin films.
76
M. Hashimoto et al. / Materials Science and Engineering A198 (1995) 75 83 PAY-OFF ION CHAMBER BOMBARDMENT
2. Experimental details
ION
T~t,£- 0P CI'Uq~Eq SPUTTERING DIFFERENTIAL
PLASMA
2.1. Substrate
The materials used in this study were commercially available cold-rolled type 304 and 430 stainless steels, which are the most versatile stainless steels, and YUS180 ( 1 9 C r - 0 . 3 N i - 0 . 4 C u - 0 . 4 N b , low carbon and nitrogen), which is widely used in outdoor applications. These materials were received as bright annealed (BAi sheet. 2.2. Pretreatment
Before deposition, the specimens were pretreated by one or a combination of the following methods: (1) no treatment; (2) acetone ultrasonic cleaning; (3) ion bombardment; (4) sputter etching; (5) nitric acid pretreatment [3]. Ion b o m b a r d m e n t and sputter etching eliminate the surface contamination and, as a result, improve the adhesion of deposited layers. The nitric acid pretreatment was employed to stabilize the pre-existing passive film on stainless steels [4] and thus to make the interface between the deposited layer and the stainless steel more resistant to corrosion. In this treatment, stainless steel was oxidized using 5% HNO3 at 35 m A c m - 2 for 40 s. 2.3. Coating processes and conditions
Three deposition processes, i.e. ion plating, sputtering and PACVD, were employed in this study. The first set of processes was performed in a batch coating apparatus for small sample fabrication. This was used to investigate the basic properties of metal- or ceramiccoated stainless steels. The details of these batch processes and typical operating conditions are listed in Table 1 [2]. Table 1 Batch coating processes and typical deposition conditions [2] Process
Depositionconditions
Ion plating HCD: p = 1.06 Pa (8 mTorr); maximum power, 16 kW Electron beam: maximum power, 1 kW D.c. bias: maximum, 2 kV Maximum temperature, 300 °C Sputtering R.f. or d.c. magnetron sputtering: p=0.67 Pa (5 mTorr) maximum power, 3 kW; maximum temperature, 700 °C; target diameter, 15.2cm PACVD Capacitively coupled plasma: p = 13.3 Pa (0.1 Torr); maximum power, 500 W; maximum temperature, 500 °C d.c. bias: maximum, 1.5 kV; electrode size, 10cmx 10cm HCD: hollow cathode discharge.
U
............ /_[:k L_Jl
Fig. 1. Schematic diagram of the in-line dry coating process [l].
The second of the processes is a continuous dry coating process installed at Hikari Works on a semiproduction scale. This process is illustrated schematically in Fig. 1. The three Units are made continuous through extremely narrow slits, typically 5 - 1 5 mm. The substrate of stainless steel strip is passed along the line in contact with rolls on its top surface and is coated on its bottom surface. The ion plating and sputtering chambers are located adjacent to each other because they have very similar operating pressures. P A C V D differs so much in operating pressure from the other two processes that a four-compartment differential pressure chamber is provided between the sputtering and P A C V D chamber [5]. The substrate conditions are as follows; material, cold-rolled stainless steel in coil; thickness, 0.1-0.5 mm; width, 370 mm; coil length, 300 m. The substrate transfer conditions are as follows: substrate temperature, maximum of 300 °C for ion plating and sputtering, maximum of 420 °C for PACVD. The coating conditions for five different combinations of steel substrate and coating material are summarized in Table 2. 2.4. Evaluation 2.4. I. Measurement o f coating thickness The thickness of the coatings was measured by either a surface profilometer ( D E K T A K ) or ellipsometry in the case of transparent films. 2.4.2. Analysis of coating composition The compositions of the films were analyzed by X-ray photoelectron spectroscopy (XPS) or glow discharge spectroscopy (GDS). The argon content coming from the processing gas was measured by Rutherford backscattering spectrometry (RBS). 2.4.3. Corrosion resistance Specimens were tested by atmospheric exposure on a rack placed 5 m from the sea and 1 m above the ground for coatings. The modified salt spray test (MST, 5%NaC1 + 0.2%H202, 35 °C, 24 h) was also utilized as an accelerated test to assess the saline environment corrosion resistance. The degree of corrosion degradation in the above methods was measured and rated by
M. Hashimoto et al. / Materials Science and Engineering A 198 (1995) 75-83
77
Table 2 In-line dry coating process and deposition conditions for typical coating materials [1] Coating species
Process
R.f. power of HCD current
General applications A1203 Sputtering Cr Sputtering SiO2 PACVD TiN Ion plating TiC Ion plating
5 kW 5 kW 35 kW 280 A ( x 2) 250 A ( x 2)
Insulation AI203
1.25-5 kW
Sputtering
Pressure (Pa)
Substrate Temperature (°C)
Speed (m rain 1)
10-3 Torr) 10-3 Torr) 10-I Torr) 10-3 Torr) 10-3Torr)
200 200 250 260 260
0.1 0.1 0.08 0.4 0.4
0.133 (1 x 10-3 Torr)
200
0.01
0.133 (1 x 0.133 (1 x 26.6 (2 x 0.133 (1 x 0.133 (1 x
visual inspection of the substrate after the corrosion test as described elsewhere [6]. The anodic polarization of the coating was measured with a potentiometer in 0.1 N NaC1 solution. 2.4.4. Wear resistance The wear resistance of each coating was tested by placing alumina paste between a piece of felt and the coated surface of a specimen and repeatedly rubbing the two together under a load of 200 gf c m - 2 . The change in color before and after the test was measured using differential colorimetry. 2.4.5. Mechanical test Adhesion was studied using a modified scratch test with a steadily increasing load. We have modified a conventional CSEM-Revetest unit to observe how the film is scratched in situ [7]. Using this equipment, we can observe the change in acoustic emission (AE) and the real-time image of the film detachment and detect accurately the critical load (Lc) value of the films. The scratch conditions employed in this study were as follows: radius of the diamond indenter tip, 0.8 mm; rate of increase in load, 100 N min ~; rate of traversal of the table on which the specimens were mounted, 10 mm m i n - 1 ; AE sensitivity, 2.0. The hardness was measured by a Vickers microindenration tester using a low indentation load of 5 gf. The internal stresses were measured by the curvature of the bent substrate (a Pyrex glass disk 0.5 mm thick and 25 mm in diameter) caused by the internal stress of the film on it [8]. The curvature of the substrate was measured by a surface profilometer. The internal stress was calculated using Stoney's equation [9]. 2.4.6. Electric insulation Aluminum electrodes, 5 mm in diameter, were evaporated over the AI20 3 coatings and the electric resistance between the aluminum electrodes and the substrates was measured by an insulation resistance tester [10].
3. Results and discussion 3.1. General properties o f the coatings 3.1.1. Coating composition The XPS results are summarized in Table 3 for five typical coating materials produced by the continuous coating process. The [O]/[A1] ratio of the A1203 coating was around 1.5-1.6, which is roughly equal to the stoichiometric ratio of alumina. The [O]/[Si] ratio of the SiO 2 coating was slightly oxygen deficient. The chromium coating contained oxygen at the surface, although it was vapor deposited as metallic chromium. The composition of TiN was approximately equal to the stoichiometric ratio. However, TiC was intentionally deposited with a high carbon content to develop the decorative black color. 3.1.2. Corrosion resistance Of the coating materials and processes investigated in this study, SiO 2 and Si3N 4 coatings produced by P A C V D were found to be the most suitable for corrosion resistance. Table 4 shows the corrosion resistance of SiO2- and Si3N4-coated type 430 stainless steels evaluated by a modified salt spray test. From the table, it is clear that a thickness of 0.1 ~tm or more is enough to increase the corrosion resistance of these ceramiccoated type 430 stainless steels well above the level of Table 3 Atomic ratios of coatings (XPS) [1] Coating species AI203 Cr SiO2 TiN TiC
Atomic ratio [O]/[AI] [O]/[Cr] [O]/[Si] [N]/[Ti] [C]/[Til
Depth (lam) 0.002
0.006
0.02
1.54 12.76 1.74 1.05 3.82
1.53 6.66 1.39 1.05 3.58
1.55 1.50 1.27 1.07 3.20
M. Hashimoto et al. / Materials Science and Engineering A 198 (1995) 75-83
78 Table 4
C o r r o s i o n resistance o f SiO 2- a n d Si3N4-coated type 430 stainless steel by b a t c h process [2] Material
Process
Pretreatment
Thickness (lam)
Corrosion resistance"
430BA
--
--
--
F
430BA SiO 2 SiO 2
-PACVD PACVD
Nitric acid -Nitric acid
-0.1 0A
D E- F A-B
SiO 2
PACVD
Nitric acid
0,5
A-B
SiN4 Si3N 4
PACVD PACVD
-Nitric acid
0.1 0.1
D- E A B
Si 3 N 4
Sputtering
--
0.1
F
304BA
--
--
--
C-D
Rating of corrosion resistance(modifiedsalt spraytest): A, good(no rust); G, poor (severe red rust). type 304. It should be emphasized here that the nitric acid pretreatment is the key step in making the high corrosion resistance possible. Thus a suitable surface pretreatment is a prerequisite for highly corrosion-resistant stainless steels. The Si3N4 coating is also beneficial for improving the corrosion resistance as shown in Table 4. In this table, a comparison of Si3N4 coatings formed by PACVD and sputtering is presented. The results clearly show that the PACVD process is superior to the sputtering process in producing highly corrosion-resistant stainless steel. Detailed observation of the defect population by transmission electron microscopy indicates that the film deposited by sputtering tends to exhibit microcracks, whereas that deposited by PACVD has essentially no microcracks. In this sense, the PACVD technique is the best process examined in this study for depositing corrosion-resistant coatings, although the material selection in PACVD is limited by the availability of gases for the coatings. As described above, certain ceramics are suitable for corrosion-resistant coatings. However, ceramics are generally brittle. Therefore when the coated stainless steels undergo deformation, the formation of microcracks is inevitable. In order to evaluate quantitatively the effect of deformation on the corrosion behavior, atmospheric corrosion tests were performed on bent specimens produced by the continuous coating process. Fig. 2 shows the results of a 1 year exposure test for five kinds of ceramic- or metal-coated stainless steels. The exposure site was a harsh environment where the specimens were exposed directly to seawater splashes on windy days. The SiO2 coating showed the best corrosion resistance, followed by the A1203, Cr, TiN and TiC coatings. It should be noted in particular that the underside portion, labeled 4 in Fig. 2, of the convex part of the specimen is most severely corroded and the vertical portion 1 is least affected. A similar phenomenon was observed with
uncoated stainless steel specimens. The effect of the coating in improving the corrosion resistance is most clearly demonstrated on the underside of the convex bent part of the specimen. The above experimental observation could be explained by the length of time taken for the underside portion to dry after the deposition and condensation of seawater splashes. In addition to the corrosion resistance, a top coating of S i O 2 is a useful decorative color coating, since SiO2 is essentially transparent in the visible range when its thickness is below a critical value; hence this top layer enables the undercoating color to be retained. An example is a stainless steel coated with SiO2 as a transparent corrosion-resistant top coating and TiN as a decorative undercoating. TiN is a decorative gold color, but has poor corrosion resistance as shown in Fig. 2. Thus the top S i O 2 coating increases the corrosion resistance, and the TiN gold color remains unchanged. The anodic polarization curves in 0.1 N NaC1 solution of the two-layer film (TiN overcoated with SiO2) are compared in Fig. 3 with those of a monolayer TiN coating and the substrate (type 304 stainless steel). The two-layer coating has a higher pitting potential than the other coatings. Thus the film overcoated with SiOz has an improved corrosion resistance without spoiling the decorativeness of the TiN coating. This example shows that the multiple, continuous coating system provides considerable freedom in the design of multilayer, i.e. multifunctional, coatings. 3.1.3. Wear resistance
Stainless steels are widely used for architecture, vehicles, etc. because of their good corrosion resistance and surface aesthetics. Thus a further improvement in corrosion resistance by ceramic coatings is beneficial for the development of excellent corrosion-resistant stainless steels. Ceramics can also add color to stainless steels for decorative applications. However, wear resistance is also required since the removal of surface stains by polishing is necessary to maintain a good surface appearance. It should be noted here that wear resistance in this case has to be evaluated in terms of the decorative appeal. In other words, it is necessary to keep the original decorative color even after polishing the surface. Thus the wear resistance in this case was evaluated from the change in color before and after the rubbing test using alumina paste. Fig. 4 shows the results of the wear resistance test of the various coatings. Each coating exhibited a wear resistance higher than that of the chemical conversion coating in aqueous solution. The Cr coating was found to have the best wear resistance, followed by the TiN, SiO2, TiC and A1203 coatings. The best result for the Cr coating is mainly due to the fact that the metal is essentially colorless from the beginning. TiN keeps its good decorative golden color even after rubbing for 100 times.
M. Hashimoto et al./ Materials Science and Engineering A 198 (1995) 75 83
79
Unchanged A Stained
B
f
(1)(2)(3) (3)
Point rusted C
(4)
i °
(1)(2) 1)(2)(3: (4)
Whole rusted G
TiN AI203 type 304
SUBSTRATE
breeze 9mmR
(1)(2)
F -(3)(4) (3)(4)
COATING
, . q e a
(1)(2) (4)
:1)(2)(3
E -(1)(2) (1)(2)
(4)
(3) (4)
(1)(2) (3)(4)
TiC type 430
(3)(4)
Cr 19Cr
(4)
/ SiO 2
type 430
Fig. 2. Results of the atmospheric exposure test at the seashore after 1 year [1].
3.1.4. Mechanical properties of the coatings TiN has been widely used as a decorative and wearresistant coating because of its golden color and high hardness [11,12]. The experimental results clearly show that TiN has a high wear resistance and exhibits a decorative golden color. However, in general, the interface between the ceramic and metal is not well bonded. Therefore, in some cases, peeling off of the TiN films was observed especially when the coated stainless steels underwent deformation. Thus it is important to increase the adhesion of ceramic coatings on metallic substrates. In this section, the mechanical properties of TiN coatings applied by a batch sputtering process are investigated to reveal the basic relationship between the mechanical properties and the deposition parameters. In this experiment, the substrates were ultrasonically cleaned in acetone and sputter cleaned at 500 W for 30 min. During sputtering, the substrate was ground and a substrate bias was not applied [7]. It was found that the operating pressure during sputtering was the most critical deposition parameter in terms of the mechanical properties of the coatings. The
relationship between the internal stress and the pressure is shown in Fig. 5. It can be seen from the figure that the internal stress of the TiN film deposited at over 2.6 Pa (region 2) was slightly tensile, but compressive at less than 2.3 Pa (region 1). In region 1, the color was gold and the coating was dense. In this region, the compressive stress increased with a decrease in pressure, and at a pressure of 0.6 Pa it increased steeply from 3 to 8 GPa. In region 2, the color was dark brown and the coating had an open columnar structure. The critical load L c of TiN in region 1 is given in Fig. 6 as a function of pressure. The critical load, a measure of the adhesion of the films, increased with an increase in the deposition pressure, especially when the pressure was above 0.6-0.7 Pa. The hardness as a function of pressure is also given in Fig. 6. The hardness decreased with an increase in pressure. Such changes in adhesion and hardness correspond well with the change in the internal stress. As a result, TiN films have a high hardness but poor adhesion at low pressures.
F~ AI=O= slurry
103
I
E o j.
/
TiN/type 304 / 102
v
30 "C, Air 100mV/min
/
c a • ,* c
-~'
Substrate
A 101
I /
10o
Si(~2/I'iNnype 304
'71
k.
o
Coating
40
uJ '~
J
/
r/
t
• •
I0-I
-500
+500
Potential
+1000
(mV vs SCE)
Fig. 3. Anodic polarization curves of type 430 substrate, monolayer TiN coating and TiN/SiO2 two-layer coating [1].
v Q 0
30
.Q
20
L.
10
_o o o
I
*~ ~"
I I
o
lo
AI203
20
30
CHEMICALCONVERSION COATING OXIDE
-
40 so
Number
80
ro
so
9o too
of slides
Fig. 4. Comparison of wear resistance [1].
M. Hashimoto et al. / Materials Science and Engineering A 198 (1995) 75 83
80
substrate
2 Gold •
i
O-
)
Dark brown
C~ °
o o
-4-
I
°::iiI i°" 0
1
2
3
4
1 5
6
Fig. 5. Internal stress of TiN films as a function of the pressure during deposition [7].
The film adhesion to the substrate depends on the bonding energy and the internal stress. If the film stress is high, the film will easily be detached by a low external stress. In contrast, the hardness will increase when the film has a high compressive stress, because the stress resists the deformation by the loading indenter• Thus the pressure dependence of the adhesion and hardness can be explained by the change in internal stress. At a fixed pressure, we can make either good adherent films or hard films, but not both. Although with a constant deposition pressure it is difficult to deposit a good adherent and hard film, it is possible by changing the pressure during deposition from high to low. The film with ~ an underlayer deposited at high pressure had a higher critical load than that with an underlayer deposited at low pressure. This result shows that high-pressure deposition for the underlayer improves the adhesion. Even if the low-pressure overlayer is thick, the adhesion is good as long as the underlayer is deposited at high pressure. This indicates that the stress at the interface of the film is a key factor for obtaining good adhesion, although the total stress is also important. On the other hand, the hardness depends on the thickness of the layer deposited at 4000
3000 '~
20-
= ;5
]a000ff ' °°
=
I--o-0.0
0.2
0.4
0.6
0.8
1.0
I
I position B
Fig. 7. Schematic drawing showing the position of the substrate for r.f. magnetron experiments [10].
7
Pressure (Pa)
40
"1 position A
11000 1.2
Pressure (Pa) Fig. 6. Critical load and microhardness of TiN films (2/~m) on type 430 stainless steel as a function of the pressure during deposition [7].
low pressure. As a result, when the underlayer is deposited at high pressure, we can obtain both good adhesion and hard coatings by appropriate design of the thickness ratio. 3.1.5. Electric insulation
Electrically insulated stainless steels can be produced by depositing dielectric materials such a s SiO 2 or A1203. An advantage of using these insulator-coated metals is that they not only insulate, but also possess good thermal conductivity and mechanical flexibility for microelectronic applications in contrast with conventional bulky ceramic substrates. A1203 films show good dielectric properties and a number of attempts have been made to deposit thin films with low leakage current and a high breakdown strength [13,14]. However, a major problem with the preparation of A1203 films on metals by sputtering is pinhole formation, which causes a short circuit or a deterioration of resistivity. Therefore a thicker deposit of at least a few micrometers is required to bury such pinholes [13]. In this section, we show that high-resistivity A1203 films can be prepared by a novel sputter deposition technique in which the first layer and the second layer are controlled independently, thus differentiating it from the usual one-step sputter deposition. R.f. magnetron sputtering was used in this work [10]. Films of 500 nm total thickness were deposited on type 430BA stainless steel; 25 aluminum-sputtered top electrodes, 100 nm thick and 5 mm in diameter, were formed on insulators using a metal mask. In this experiment, the A1203 films were prepared at two different positions: the normal position (position A) and at an oblique angle off the normal (position B) as shown in Fig. 7. Two-step processes were performed in which the first layer and second layer were controlled independently. The first layer was deposited at position B and the second layer at position A. The film properties were estimated by the insulation probability, which was defined as the percentage of the number of insulated electrodes over 20 M ~ at 1 V d.c. Fig. 8 shows the dependence of the insulation probability on the nominal thickness of the first layer deposited at position B.
M. Hashimoto et al. / Materials Science and Engineering A 198 (1995) 75 83
It should be noted that the total thickness was 500 nm in all cases, and the second layer was deposited at position A. The experimental result clearly indicates that the insulation probability is sensitive to the firstlayer thickness. Although no one-step process was able to give an insulating sample, 100% insulation probability was almost always attained by the two-step process, in which the first layer was deposited to a nominal thickness of approximately 20 nm at position B and the second layer, 480 nm thick, was deposited at position A. We also attempted to form films by various one-step processes and other two-step processes, but the good insulation could not be achieved under any conditions except a position B-position A two-step process. The above result suggests that the density of pinholes in insulating films is strongly affected by the substrate positions during deposition. In order to estimate the density of pinholes per unit area in a film, an aluminum-coated silicon wafer was used as a substrate. The silicon substrate was used to eliminate the effect of surface roughness. The density was estimated by electrocrystallization in an electrolyte solution (CuSO4"5H20) at an electrode potential of approximately - 0.2 V vs. a saturated calomel electrode (SCE) by counting the number of Cu electroplated particles on the insulating film using an optical microscope. The pinhole density of the film deposited at position A was 460 cm -2 and that at position B was 800 cm -2 However, the density of pinholes in the film deposited by a two-step process showed the lowest value, 140 cm-2. Thus the novel two-step process creates fewer pinholes, and hence good insulator films.
3.2. Role of plasma in thin film growth A number of interesting experimental observations have been made so far, in particular in terms of the 100
~
o
o~ 80 ",-,"
)~
" \
nominalthickness (thefirstlayerdeposited
~
~•/~'=(.~OeD .,6040 O . ~
0
=0.5pro
atpositionBbyMS)
i 100
i 200
Nominal
i 300
i 400
500
thickness(nm)
Fig. 8. Dependence of the insulation probability on the nominal thickness of the first layer deposited at position B [10].
81
relationship between the performance of the coatings and the deposition parameters. (1) Corrosion-resistant coatings formed by PACVD are superior to those obtained by sputtering since the film deposited by sputtering tends to bear microcracks. (2) The operating pressure during sputtering is the most critical deposition parameter in terms of the mechanical properties of the coatings. The compressive internal stress increases, and hence the hardness increases and the adhesion decreases, with a decrease in pressure. (3) The density of pinholes in insulating films formed by sputtering is strongly affected by the substrate positions during deposition. High-resistivity A1203 films can be prepared by a novel sputter deposition technique in which the first layer and second layer are controlled independently, thus differentiating it from the usual one-step sputter deposition. It is necessary to reveal the basic characteristics of the plasma itself in order to understand the above experimental observations. For example, in the sputtering process, the plasma is created between the sputtering target, called a cathode, and the substrate. In the plasma region, a number of species, such as neutral gases, ions and electrons, are generated by the externally supplied electricity. These species interact with the substrate and cause a variety of surface reactions, such as deposition, chemical reactions with other species, resputtering of the deposited films, surface mixing, enhancement of adatom diffusion and heating of the substrate. Although we can control the external parameters, such as the operating pressure, gas flow rate, power density, substrate position and so on, it is obvious that the environment near the growing film is the most critical factor determining the characteristics and performance of the film. In this sense, the concept used to describe electrochemical reactions may help in the understanding of the thin film growth mechanism in plasma environments, although there are a number of differences in both cases. In this sense, surface science, in particular the role of the environment in surface reactions, is the key issue in understanding, and hence controlling, the plasmaassisted coating process. A plasma can be roughly defined as a gas consisting of ions and electrons both having approximately the same density over the Debye length. Thus electric neutralization is held over this characteristic length. In most cases, the energy necessary to generate the plasma is supplied by accelerated electrons. The accelerated electrons cause a variety of reactions, such as elastic collision, excitation, dissociation, ionization and so on. Since the excited species can react chemically with other species even at low temperatures, chemical vapor deposition can be assisted by a plasma. An external electric field is used for the ease of generation of accelerated
82
M. Hashimoto et al./ Materials Science and Engineering A 198 (1995) 75-83
ions for the sputtering process. Thus the plasma has unique characteristics in both physical and chemical reactions. In the case of a low-pressure plasma, where the frequency of collisions is low, the temperature of the electrons is much higher than that of the ions since light electrons can be easily accelerated and gain high velocity without losing their energy. The typical temperature of electrons is about 104 K or higher, whereas that of ions stays around several hundred kelvin. Thus a lowpressure plasma is also called a low-temperature plasma. However, the situation becomes quite different when a solid is placed in a low-pressure, low-temperature plasma. Since the velocity of electrons is much higher than that of ions, the flux of negative charge towards ' t h e solid surface is also initially higher than that of ':positive ions. Therefore the surface is negatively charged and develops an excess positive ion region called an ion sheath. When an external bias voltage, for example a negative bias, is applied to one of the two electrodes, an electric field is artificially generated near the cathode, and positive ions are accelerated towards this cathode and hit it. If the energy is high enough to sputter the cathode material, the material is deposited onto the anode. The depositing atoms sputtered from the cathode have an energy of several electronvolts. However, it should be noted that Ar ions, which are commonly used as the processing ions accelerated towards the cathode at several hundred electronvolts, are reflected at the cathode. Thus the neutralized and reflected Ar atoms hit the anode and hence affect the thin film growth at the anode. In other words, the growing film is always exposed to the high- energy processing gas as well as the flux of depositing atoms. This is a rough picture of the sputtering process. Thus the mobility of electrons and ions (the electrode potential), which is either unintentionally or intentionally generated, and the flux of various species having different energies are the key factors describing the sputter deposition. Fig. 9 summarizes the reactions and fundamental steps of radicals and ions in a reactive plasma. The above picture of the sputtering process can be used to explain the dependence of the mechanical properties of TiN films on the operating pressure. In Section 3.1.4, it was found that the operating pressure during sputtering was the most critical deposition parameter in terms of the mechanical properties of the coatings, and the compressive internal stress increased (and hence the hardness increased and the adhesion decreased) with a decrease in the pressure. Scanning electron microscopy (SEM) observations also show that the density of TiN films increases with a decrease in pressure. The above result suggests that a decrease in pressure has the same effect as a bias on the substrate. Kumer et al. [15] reported that a brown columnar film changed to
(1) INITt&L~ACTIOI~
GENERATI OOF NPLASMA Ionlza~,on dissociation electroncollision
SECOND RI."JLCTION
12~
chargeexchange mcom~nation
E~mtr°Naaiiii~i~qo~Wpre~e~lsSizJ~' JL
~
*PLASMAPARAMETERS energy distribution of electrons density of electrons, electric field profile
~' "RADICALANDIONREACTIONS density and space distribution of working gas, ions and radicals
(3)TRANSPOR T spacechargelayer sheath
f *DIFFUSION ANDDRIFT transport of radicals and
~4) SURF&CE REACTIO~I
sputter ion implantation degceition surfacediffusion nucleation& growth
1
~ons
T deposition etching surface modification I
Fig. 9. Atomic and molecular reactions in reactive plasma.
a golden dense film when the substrate was negatively biased. They suggested that densification by a high-energy, argon-ion-peening effect could explain the dense structure. Hoffman and Thornton [16] investigated the stress of sputtered metal films, and explained the cause of the stresses by the argon-ion-peening effect. On decreasing the deposition pressure, the argon-ionpeening effect becomes more evident. The energy delivered to the film surface depends on the discharge pressure. After high-pressure deposition without a substrate bias voltage, the deposited films exhibit a rough morphology and slight tensile stress because the working gas atoms or the sputtered atoms are thermalized by collision with working gas atoms, and the energy transferred to the film surface is small. On the other hand, during deposition at low pressure, energy is delivered to the film surface by the energetic gas atoms backscattered from the sputtering target or by the sputtered atoms, and a film with a smooth morphology and a large compressive stress is obtained. The above picture is supported by the measurement of the concentration of Ar atoms in the films using proton backscattering spectroscopy [17]. It was found that the Ar content in the films is high at low operating pressure, typically 9 at.%, but low at high pressure, about 6 at.%. It is considered that the concentration of Ar atoms in the films depends on the energy of the Ar atoms impinging on the film surface. Thus the operating pressure affects the energy of the processing gas atoms delivered to the growing films, and hence the density, morphology, internal stress and adhesion of the films. The above picture can also be used to explain the experimental observation that the density of pinholes in
M. Hashimoto et al. / Materials Science and Engineering A 198 (1995) 75 83
insulating films formed by sputtering is strongly affected by the substrate position during deposition. We showed that high-resistivity A1203 films could be prepared by a novel sputter deposition technique in which the first layer and second layer were controlled independently, thus differentiating it from the usual one-step sputter deposition. At the initial stage, strong bombardment at position A in the plasma leads to large adatomdepleted islands. These zones will cause pinhole-type defects. A thicker deposit over several micrometers is therefore required to bury such pinholes. In contrast, with weak bombardment, i.e. position B, large adatomdepleted zones are difficult to form. The energetic particle assistance at position A can lead to densification of the near-surface region and the burial of smaller defects. It is reasoned that the dependence of the insulation probability on the thickness of the first layer, typically 20 nm, is due to a necessary minimum thickness for the burial of defects. This thickness depends on how the defects arise in the initial stages, i.e. larger defects arising in a film at position A cannot be buried at position B, while smaller defects arising at position B can be covered with a second layer, 480 nm thick, at position A. We believe that it is necessary to optimize the deposition conditions for the formation of the first layer and the second layer independently in order to achieve high insulation probability. This example clearly indicates that the initial stage of thin film growth should be carefully controlled to obtain the most desirable island structure for the latter stage. However, the optimum conditions for the latter stage are not necessarily the same as those for the initial stage. Thus, in general, the initial stage, in which the nucleation of the island structure is critical, and the latter stage, in which the coalescence of islands and morphology development are critical, should be separately optimized in order to obtain high-quality thin films. The two-step process may also be applicable to the design of wear-resistant coatings. As shown in Section 3.1.4, a good adherent and hard film can be obtained by changing the pressure during deposition from high to low even when the same material is deposited. Thus the design of the plasma coating process should be performed by considering the basic aspects of the thin film growth mechanism in the environment, where the behavior of energetic particles is the crucial factor.
4. Conclusions
In this paper, the role of the environment on coated stainless steels was discussed from two different points of view. The first involved the performance of ceramic-
83
coated stainless steels in corrosive and mechanically stressed environments. It was found that a n S i O 2 coating deposited by PACVD led to the largest increase in the corrosion resistance of type 430 stainless steel to well above the level of type 304 stainless steel. It was also possible to coat stainless steel with TiN and other ceramics for decorative applications. The combination of the color of the TiN undercoating and the SiO2 corrosion-resistant top coating provides the potential to develop multifunctional stainless steels. Other examples are wear-resistant coatings and insulating coatings. The second viewpoint was the influence of the plasma environment used to synthesize the coated materials on the quality of thin films. Since the growing film is always exposed to the plasma during deposition, the flux and energy of various species, such as the processing gas, ions and depositing atoms, are the key factors in describing thin film growth. We believe that it is necessary to optimize the deposition conditions for the initial stage and latter stage independently in order to achieve high-quality thin films.
References [1] T. Takahashi, Y. Oikawa, T. Komori, I. Ito and M. Hashimoto, Surf Coat. Technol., 51 (1992) 522. [2] M. Hashimoto, S. Miyajima, W. Ito, S. lto, T. Murata, T. Komori, I. Ito and M. Onoyama, Surf Coat. Technol., 36 (1988) 837. [3] H. Omata, S. lto and T. Murata, U.S. Patent 4,612,095, 1986. [4] H. Omata, S. lto, M. Yabumoto and T. Murata, Proc. 4th Asian-Pac(fic Corrosion Control Conj'., Tokyo, May 2 ~ 31, 1985, Vol. 1, 1985, p. 475. [5] Y. Hosoi, 1. lto, S. Saida, T. Murata, S. Ito and N. Okubo, Japanese Patent Application Publication, No. Sho 62-205272, September 9, 1987. [6] U. Nakata, I. Ito, M. Onoyama and H. Inagaki, Proc. 28th Joint Symp. on Corrosion Science and Technology, Japan Society of Corrosion Engineering, 1981, p. 18. [7] Y. Kubo and M. Hashimoto, Surf Coat. Technol., 49 (1991) 342. [8] J.D. Finegan and R.W. Hoffman, J. Appl. Phys., 30 (1950) 597. [9] G. Stoney, Proc. R. Soc. London, Set. A, 32(1909) 172. [10] S. Tokumaru and M. Hashimoto, Sur[~ Coat. Technol., 54/55 (1992) 303. [11] K. Nakamura, K. Inagawa, K. Tsuruoka and S. Komiya, Thin Solid Films, 40 (1977) 155. [12] B. Zega, M. Kornmann and J. Amiguet, Thin Solid Films, 45 (1977) 577. [13] M.N. Khan, Thin Solid Films, 124 (1985) 55. [14] J. Saraie, S. Goto, Y. Kitao and Y. Yodogawa, J. Electrochem. Soc., 134 (1987) 2805. [15] N. Kumer, J.T. McGinn, K. Pourrezaei, B. Lee and E.C. Douglas, J. Vae. Sci. Teehnol. A, 6 (3) (1988) 1602. [16] D.W. Hoffman and J.A. Thornton, Thin Solid Films, 40 (1976) 355. [17] K. Tanaka, Y. Kubo, N.B. Chilton and M. Kumagai, Nucl. Instrum. Methods B, 83 (1993) 525.
Materials Science and Engineering A198 (1995) 75 83
A
Surface modification of stainless steel in plasma environments M. H a s h i m o t o a, Y. M i y a m o t & , Y. K u b o ", S. T o k u m a r u a, N. O n o b, T. T a k a h a s h i b, I. Ito c ~Advanced Materials and Technology Research Laboratories, Nippon Steel Corporation, 1618 Ida, Nakahara-ku, Kawasaki 211, Japan bHikari Works, Hikari R&D Laboratory, Nippon Steel Corporation, 3434 Shimata, Hikari City, Yamaguchi Pre. 743, Japan CMetal Foil Products Division, New Materials Divisions Group, Nippon Steel Corporation, 6-30temachi 2-chome, Chiyoda-ku, Tokyo 100, Japan
Abstract
Stainless steels are widely used for architecture, vehicles, etc. because of their good corrosion resistance and surface aesthetics. In order to provide additional surface functions, such as decorative colors, we have developed a continuous dry coating process, which can provide stainless steel strip with a multilayer ceramic or metallic coating. In this paper, the role of the environment will be discussed from two different points of view. The first is the performance of these coated stainless steels in corrosive and mechanically stressed environments. The second is the influence of the plasma environment used to synthesize the coating materials on the quality of thin films.
Keywords: Surface modification; Plasma-assisted CVD; Sputtering; Ion plating; Stainless steel
I. Introduction
Stainless steels are used in architectural, automotive and chemical processing applications because of their good corrosion resistance, clean appearance and workability. Coating these metals with ceramics, other metals or alloys adds innovative surface functions, such as insulation or dielectric properties, aesthetic appeal and optical characteristics. In order to develop these additional surface functions, a combined process utilizing sputtering, ion plating and plasmaassisted chemical vapor deposition (PACVD) has been assessed. Thin film processes that use ion and low-temperature plasmas are commonly applied to modify the surface characteristics of materials. Unfortunately, handling problems associated with many toxic coating materials and the severe process requirements, such as high vacuum or material purity, have limited the large-scale production use of ion and plasma deposition techniques. We have developed a dry coating process that combines ion plating, sputtering and PACVD in a tandem arrangement to coat continuously coils of material on a semi-production scale [1]. 0921-5093/95/$09.50 © 1995 SSD1 0921-5093(94)04510-0
Elsevier Science S.A. All rights reserved
The above three processes have been selected since they generally give dense and adherent films. However, each coating process has its own limitations. For example, stainless steel coated with S i O 2 o r SiaN 4 by PACVD shows higher corrosion resistance than that coated by sputtering; in addition, the applicability of PACVD is limited by the availability of coating materials in the form of gases [2]. Therefore the combination of the above-mentioned three processes is particularly meaningful. By eliminating exposure to external environments, the tandem arrangement allows improvements in conventional coating systems. It also creates a procedure for developing new composite materials by allowing different materials to be sequentially coated. Although our coating process is compatible with many materials, we have used it to coat stainless steel strip. In this paper, the role of the environment will be discussed from two different points of view. The first is the performance of the coated stainless steels in corrosive and mechanically stressed environments. The second is the influence of the plasma environment used to synthesize the coating materials on the quality of thin films.
76
M. Hashimoto et al. / Materials Science and Engineering A198 (1995) 75 83 PAY-OFF ION CHAMBER BOMBARDMENT
2. Experimental details
ION
T~t,£- 0P CI'Uq~Eq SPUTTERING DIFFERENTIAL
PLASMA
2.1. Substrate
The materials used in this study were commercially available cold-rolled type 304 and 430 stainless steels, which are the most versatile stainless steels, and YUS180 ( 1 9 C r - 0 . 3 N i - 0 . 4 C u - 0 . 4 N b , low carbon and nitrogen), which is widely used in outdoor applications. These materials were received as bright annealed (BAi sheet. 2.2. Pretreatment
Before deposition, the specimens were pretreated by one or a combination of the following methods: (1) no treatment; (2) acetone ultrasonic cleaning; (3) ion bombardment; (4) sputter etching; (5) nitric acid pretreatment [3]. Ion b o m b a r d m e n t and sputter etching eliminate the surface contamination and, as a result, improve the adhesion of deposited layers. The nitric acid pretreatment was employed to stabilize the pre-existing passive film on stainless steels [4] and thus to make the interface between the deposited layer and the stainless steel more resistant to corrosion. In this treatment, stainless steel was oxidized using 5% HNO3 at 35 m A c m - 2 for 40 s. 2.3. Coating processes and conditions
Three deposition processes, i.e. ion plating, sputtering and PACVD, were employed in this study. The first set of processes was performed in a batch coating apparatus for small sample fabrication. This was used to investigate the basic properties of metal- or ceramiccoated stainless steels. The details of these batch processes and typical operating conditions are listed in Table 1 [2]. Table 1 Batch coating processes and typical deposition conditions [2] Process
Depositionconditions
Ion plating HCD: p = 1.06 Pa (8 mTorr); maximum power, 16 kW Electron beam: maximum power, 1 kW D.c. bias: maximum, 2 kV Maximum temperature, 300 °C Sputtering R.f. or d.c. magnetron sputtering: p=0.67 Pa (5 mTorr) maximum power, 3 kW; maximum temperature, 700 °C; target diameter, 15.2cm PACVD Capacitively coupled plasma: p = 13.3 Pa (0.1 Torr); maximum power, 500 W; maximum temperature, 500 °C d.c. bias: maximum, 1.5 kV; electrode size, 10cmx 10cm HCD: hollow cathode discharge.
U
............ /_[:k L_Jl
Fig. 1. Schematic diagram of the in-line dry coating process [l].
The second of the processes is a continuous dry coating process installed at Hikari Works on a semiproduction scale. This process is illustrated schematically in Fig. 1. The three Units are made continuous through extremely narrow slits, typically 5 - 1 5 mm. The substrate of stainless steel strip is passed along the line in contact with rolls on its top surface and is coated on its bottom surface. The ion plating and sputtering chambers are located adjacent to each other because they have very similar operating pressures. P A C V D differs so much in operating pressure from the other two processes that a four-compartment differential pressure chamber is provided between the sputtering and P A C V D chamber [5]. The substrate conditions are as follows; material, cold-rolled stainless steel in coil; thickness, 0.1-0.5 mm; width, 370 mm; coil length, 300 m. The substrate transfer conditions are as follows: substrate temperature, maximum of 300 °C for ion plating and sputtering, maximum of 420 °C for PACVD. The coating conditions for five different combinations of steel substrate and coating material are summarized in Table 2. 2.4. Evaluation 2.4. I. Measurement o f coating thickness The thickness of the coatings was measured by either a surface profilometer ( D E K T A K ) or ellipsometry in the case of transparent films. 2.4.2. Analysis of coating composition The compositions of the films were analyzed by X-ray photoelectron spectroscopy (XPS) or glow discharge spectroscopy (GDS). The argon content coming from the processing gas was measured by Rutherford backscattering spectrometry (RBS). 2.4.3. Corrosion resistance Specimens were tested by atmospheric exposure on a rack placed 5 m from the sea and 1 m above the ground for coatings. The modified salt spray test (MST, 5%NaC1 + 0.2%H202, 35 °C, 24 h) was also utilized as an accelerated test to assess the saline environment corrosion resistance. The degree of corrosion degradation in the above methods was measured and rated by
M. Hashimoto et al. / Materials Science and Engineering A 198 (1995) 75-83
77
Table 2 In-line dry coating process and deposition conditions for typical coating materials [1] Coating species
Process
R.f. power of HCD current
General applications A1203 Sputtering Cr Sputtering SiO2 PACVD TiN Ion plating TiC Ion plating
5 kW 5 kW 35 kW 280 A ( x 2) 250 A ( x 2)
Insulation AI203
1.25-5 kW
Sputtering
Pressure (Pa)
Substrate Temperature (°C)
Speed (m rain 1)
10-3 Torr) 10-3 Torr) 10-I Torr) 10-3 Torr) 10-3Torr)
200 200 250 260 260
0.1 0.1 0.08 0.4 0.4
0.133 (1 x 10-3 Torr)
200
0.01
0.133 (1 x 0.133 (1 x 26.6 (2 x 0.133 (1 x 0.133 (1 x
visual inspection of the substrate after the corrosion test as described elsewhere [6]. The anodic polarization of the coating was measured with a potentiometer in 0.1 N NaC1 solution. 2.4.4. Wear resistance The wear resistance of each coating was tested by placing alumina paste between a piece of felt and the coated surface of a specimen and repeatedly rubbing the two together under a load of 200 gf c m - 2 . The change in color before and after the test was measured using differential colorimetry. 2.4.5. Mechanical test Adhesion was studied using a modified scratch test with a steadily increasing load. We have modified a conventional CSEM-Revetest unit to observe how the film is scratched in situ [7]. Using this equipment, we can observe the change in acoustic emission (AE) and the real-time image of the film detachment and detect accurately the critical load (Lc) value of the films. The scratch conditions employed in this study were as follows: radius of the diamond indenter tip, 0.8 mm; rate of increase in load, 100 N min ~; rate of traversal of the table on which the specimens were mounted, 10 mm m i n - 1 ; AE sensitivity, 2.0. The hardness was measured by a Vickers microindenration tester using a low indentation load of 5 gf. The internal stresses were measured by the curvature of the bent substrate (a Pyrex glass disk 0.5 mm thick and 25 mm in diameter) caused by the internal stress of the film on it [8]. The curvature of the substrate was measured by a surface profilometer. The internal stress was calculated using Stoney's equation [9]. 2.4.6. Electric insulation Aluminum electrodes, 5 mm in diameter, were evaporated over the AI20 3 coatings and the electric resistance between the aluminum electrodes and the substrates was measured by an insulation resistance tester [10].
3. Results and discussion 3.1. General properties o f the coatings 3.1.1. Coating composition The XPS results are summarized in Table 3 for five typical coating materials produced by the continuous coating process. The [O]/[A1] ratio of the A1203 coating was around 1.5-1.6, which is roughly equal to the stoichiometric ratio of alumina. The [O]/[Si] ratio of the SiO 2 coating was slightly oxygen deficient. The chromium coating contained oxygen at the surface, although it was vapor deposited as metallic chromium. The composition of TiN was approximately equal to the stoichiometric ratio. However, TiC was intentionally deposited with a high carbon content to develop the decorative black color. 3.1.2. Corrosion resistance Of the coating materials and processes investigated in this study, SiO 2 and Si3N 4 coatings produced by P A C V D were found to be the most suitable for corrosion resistance. Table 4 shows the corrosion resistance of SiO2- and Si3N4-coated type 430 stainless steels evaluated by a modified salt spray test. From the table, it is clear that a thickness of 0.1 ~tm or more is enough to increase the corrosion resistance of these ceramiccoated type 430 stainless steels well above the level of Table 3 Atomic ratios of coatings (XPS) [1] Coating species AI203 Cr SiO2 TiN TiC
Atomic ratio [O]/[AI] [O]/[Cr] [O]/[Si] [N]/[Ti] [C]/[Til
Depth (lam) 0.002
0.006
0.02
1.54 12.76 1.74 1.05 3.82
1.53 6.66 1.39 1.05 3.58
1.55 1.50 1.27 1.07 3.20
M. Hashimoto et al. / Materials Science and Engineering A 198 (1995) 75-83
78 Table 4
C o r r o s i o n resistance o f SiO 2- a n d Si3N4-coated type 430 stainless steel by b a t c h process [2] Material
Process
Pretreatment
Thickness (lam)
Corrosion resistance"
430BA
--
--
--
F
430BA SiO 2 SiO 2
-PACVD PACVD
Nitric acid -Nitric acid
-0.1 0A
D E- F A-B
SiO 2
PACVD
Nitric acid
0,5
A-B
SiN4 Si3N 4
PACVD PACVD
-Nitric acid
0.1 0.1
D- E A B
Si 3 N 4
Sputtering
--
0.1
F
304BA
--
--
--
C-D
Rating of corrosion resistance(modifiedsalt spraytest): A, good(no rust); G, poor (severe red rust). type 304. It should be emphasized here that the nitric acid pretreatment is the key step in making the high corrosion resistance possible. Thus a suitable surface pretreatment is a prerequisite for highly corrosion-resistant stainless steels. The Si3N4 coating is also beneficial for improving the corrosion resistance as shown in Table 4. In this table, a comparison of Si3N4 coatings formed by PACVD and sputtering is presented. The results clearly show that the PACVD process is superior to the sputtering process in producing highly corrosion-resistant stainless steel. Detailed observation of the defect population by transmission electron microscopy indicates that the film deposited by sputtering tends to exhibit microcracks, whereas that deposited by PACVD has essentially no microcracks. In this sense, the PACVD technique is the best process examined in this study for depositing corrosion-resistant coatings, although the material selection in PACVD is limited by the availability of gases for the coatings. As described above, certain ceramics are suitable for corrosion-resistant coatings. However, ceramics are generally brittle. Therefore when the coated stainless steels undergo deformation, the formation of microcracks is inevitable. In order to evaluate quantitatively the effect of deformation on the corrosion behavior, atmospheric corrosion tests were performed on bent specimens produced by the continuous coating process. Fig. 2 shows the results of a 1 year exposure test for five kinds of ceramic- or metal-coated stainless steels. The exposure site was a harsh environment where the specimens were exposed directly to seawater splashes on windy days. The SiO2 coating showed the best corrosion resistance, followed by the A1203, Cr, TiN and TiC coatings. It should be noted in particular that the underside portion, labeled 4 in Fig. 2, of the convex part of the specimen is most severely corroded and the vertical portion 1 is least affected. A similar phenomenon was observed with
uncoated stainless steel specimens. The effect of the coating in improving the corrosion resistance is most clearly demonstrated on the underside of the convex bent part of the specimen. The above experimental observation could be explained by the length of time taken for the underside portion to dry after the deposition and condensation of seawater splashes. In addition to the corrosion resistance, a top coating of S i O 2 is a useful decorative color coating, since SiO2 is essentially transparent in the visible range when its thickness is below a critical value; hence this top layer enables the undercoating color to be retained. An example is a stainless steel coated with SiO2 as a transparent corrosion-resistant top coating and TiN as a decorative undercoating. TiN is a decorative gold color, but has poor corrosion resistance as shown in Fig. 2. Thus the top S i O 2 coating increases the corrosion resistance, and the TiN gold color remains unchanged. The anodic polarization curves in 0.1 N NaC1 solution of the two-layer film (TiN overcoated with SiO2) are compared in Fig. 3 with those of a monolayer TiN coating and the substrate (type 304 stainless steel). The two-layer coating has a higher pitting potential than the other coatings. Thus the film overcoated with SiOz has an improved corrosion resistance without spoiling the decorativeness of the TiN coating. This example shows that the multiple, continuous coating system provides considerable freedom in the design of multilayer, i.e. multifunctional, coatings. 3.1.3. Wear resistance
Stainless steels are widely used for architecture, vehicles, etc. because of their good corrosion resistance and surface aesthetics. Thus a further improvement in corrosion resistance by ceramic coatings is beneficial for the development of excellent corrosion-resistant stainless steels. Ceramics can also add color to stainless steels for decorative applications. However, wear resistance is also required since the removal of surface stains by polishing is necessary to maintain a good surface appearance. It should be noted here that wear resistance in this case has to be evaluated in terms of the decorative appeal. In other words, it is necessary to keep the original decorative color even after polishing the surface. Thus the wear resistance in this case was evaluated from the change in color before and after the rubbing test using alumina paste. Fig. 4 shows the results of the wear resistance test of the various coatings. Each coating exhibited a wear resistance higher than that of the chemical conversion coating in aqueous solution. The Cr coating was found to have the best wear resistance, followed by the TiN, SiO2, TiC and A1203 coatings. The best result for the Cr coating is mainly due to the fact that the metal is essentially colorless from the beginning. TiN keeps its good decorative golden color even after rubbing for 100 times.
M. Hashimoto et al./ Materials Science and Engineering A 198 (1995) 75 83
79
Unchanged A Stained
B
f
(1)(2)(3) (3)
Point rusted C
(4)
i °
(1)(2) 1)(2)(3: (4)
Whole rusted G
TiN AI203 type 304
SUBSTRATE
breeze 9mmR
(1)(2)
F -(3)(4) (3)(4)
COATING
, . q e a
(1)(2) (4)
:1)(2)(3
E -(1)(2) (1)(2)
(4)
(3) (4)
(1)(2) (3)(4)
TiC type 430
(3)(4)
Cr 19Cr
(4)
/ SiO 2
type 430
Fig. 2. Results of the atmospheric exposure test at the seashore after 1 year [1].
3.1.4. Mechanical properties of the coatings TiN has been widely used as a decorative and wearresistant coating because of its golden color and high hardness [11,12]. The experimental results clearly show that TiN has a high wear resistance and exhibits a decorative golden color. However, in general, the interface between the ceramic and metal is not well bonded. Therefore, in some cases, peeling off of the TiN films was observed especially when the coated stainless steels underwent deformation. Thus it is important to increase the adhesion of ceramic coatings on metallic substrates. In this section, the mechanical properties of TiN coatings applied by a batch sputtering process are investigated to reveal the basic relationship between the mechanical properties and the deposition parameters. In this experiment, the substrates were ultrasonically cleaned in acetone and sputter cleaned at 500 W for 30 min. During sputtering, the substrate was ground and a substrate bias was not applied [7]. It was found that the operating pressure during sputtering was the most critical deposition parameter in terms of the mechanical properties of the coatings. The
relationship between the internal stress and the pressure is shown in Fig. 5. It can be seen from the figure that the internal stress of the TiN film deposited at over 2.6 Pa (region 2) was slightly tensile, but compressive at less than 2.3 Pa (region 1). In region 1, the color was gold and the coating was dense. In this region, the compressive stress increased with a decrease in pressure, and at a pressure of 0.6 Pa it increased steeply from 3 to 8 GPa. In region 2, the color was dark brown and the coating had an open columnar structure. The critical load L c of TiN in region 1 is given in Fig. 6 as a function of pressure. The critical load, a measure of the adhesion of the films, increased with an increase in the deposition pressure, especially when the pressure was above 0.6-0.7 Pa. The hardness as a function of pressure is also given in Fig. 6. The hardness decreased with an increase in pressure. Such changes in adhesion and hardness correspond well with the change in the internal stress. As a result, TiN films have a high hardness but poor adhesion at low pressures.
F~ AI=O= slurry
103
I
E o j.
/
TiN/type 304 / 102
v
30 "C, Air 100mV/min
/
c a • ,* c
-~'
Substrate
A 101
I /
10o
Si(~2/I'iNnype 304
'71
k.
o
Coating
40
uJ '~
J
/
r/
t
• •
I0-I
-500
+500
Potential
+1000
(mV vs SCE)
Fig. 3. Anodic polarization curves of type 430 substrate, monolayer TiN coating and TiN/SiO2 two-layer coating [1].
v Q 0
30
.Q
20
L.
10
_o o o
I
*~ ~"
I I
o
lo
AI203
20
30
CHEMICALCONVERSION COATING OXIDE
-
40 so
Number
80
ro
so
9o too
of slides
Fig. 4. Comparison of wear resistance [1].
M. Hashimoto et al. / Materials Science and Engineering A 198 (1995) 75 83
80
substrate
2 Gold •
i
O-
)
Dark brown
C~ °
o o
-4-
I
°::iiI i°" 0
1
2
3
4
1 5
6
Fig. 5. Internal stress of TiN films as a function of the pressure during deposition [7].
The film adhesion to the substrate depends on the bonding energy and the internal stress. If the film stress is high, the film will easily be detached by a low external stress. In contrast, the hardness will increase when the film has a high compressive stress, because the stress resists the deformation by the loading indenter• Thus the pressure dependence of the adhesion and hardness can be explained by the change in internal stress. At a fixed pressure, we can make either good adherent films or hard films, but not both. Although with a constant deposition pressure it is difficult to deposit a good adherent and hard film, it is possible by changing the pressure during deposition from high to low. The film with ~ an underlayer deposited at high pressure had a higher critical load than that with an underlayer deposited at low pressure. This result shows that high-pressure deposition for the underlayer improves the adhesion. Even if the low-pressure overlayer is thick, the adhesion is good as long as the underlayer is deposited at high pressure. This indicates that the stress at the interface of the film is a key factor for obtaining good adhesion, although the total stress is also important. On the other hand, the hardness depends on the thickness of the layer deposited at 4000
3000 '~
20-
= ;5
]a000ff ' °°
=
I--o-0.0
0.2
0.4
0.6
0.8
1.0
I
I position B
Fig. 7. Schematic drawing showing the position of the substrate for r.f. magnetron experiments [10].
7
Pressure (Pa)
40
"1 position A
11000 1.2
Pressure (Pa) Fig. 6. Critical load and microhardness of TiN films (2/~m) on type 430 stainless steel as a function of the pressure during deposition [7].
low pressure. As a result, when the underlayer is deposited at high pressure, we can obtain both good adhesion and hard coatings by appropriate design of the thickness ratio. 3.1.5. Electric insulation
Electrically insulated stainless steels can be produced by depositing dielectric materials such a s SiO 2 or A1203. An advantage of using these insulator-coated metals is that they not only insulate, but also possess good thermal conductivity and mechanical flexibility for microelectronic applications in contrast with conventional bulky ceramic substrates. A1203 films show good dielectric properties and a number of attempts have been made to deposit thin films with low leakage current and a high breakdown strength [13,14]. However, a major problem with the preparation of A1203 films on metals by sputtering is pinhole formation, which causes a short circuit or a deterioration of resistivity. Therefore a thicker deposit of at least a few micrometers is required to bury such pinholes [13]. In this section, we show that high-resistivity A1203 films can be prepared by a novel sputter deposition technique in which the first layer and the second layer are controlled independently, thus differentiating it from the usual one-step sputter deposition. R.f. magnetron sputtering was used in this work [10]. Films of 500 nm total thickness were deposited on type 430BA stainless steel; 25 aluminum-sputtered top electrodes, 100 nm thick and 5 mm in diameter, were formed on insulators using a metal mask. In this experiment, the A1203 films were prepared at two different positions: the normal position (position A) and at an oblique angle off the normal (position B) as shown in Fig. 7. Two-step processes were performed in which the first layer and second layer were controlled independently. The first layer was deposited at position B and the second layer at position A. The film properties were estimated by the insulation probability, which was defined as the percentage of the number of insulated electrodes over 20 M ~ at 1 V d.c. Fig. 8 shows the dependence of the insulation probability on the nominal thickness of the first layer deposited at position B.
M. Hashimoto et al. / Materials Science and Engineering A 198 (1995) 75 83
It should be noted that the total thickness was 500 nm in all cases, and the second layer was deposited at position A. The experimental result clearly indicates that the insulation probability is sensitive to the firstlayer thickness. Although no one-step process was able to give an insulating sample, 100% insulation probability was almost always attained by the two-step process, in which the first layer was deposited to a nominal thickness of approximately 20 nm at position B and the second layer, 480 nm thick, was deposited at position A. We also attempted to form films by various one-step processes and other two-step processes, but the good insulation could not be achieved under any conditions except a position B-position A two-step process. The above result suggests that the density of pinholes in insulating films is strongly affected by the substrate positions during deposition. In order to estimate the density of pinholes per unit area in a film, an aluminum-coated silicon wafer was used as a substrate. The silicon substrate was used to eliminate the effect of surface roughness. The density was estimated by electrocrystallization in an electrolyte solution (CuSO4"5H20) at an electrode potential of approximately - 0.2 V vs. a saturated calomel electrode (SCE) by counting the number of Cu electroplated particles on the insulating film using an optical microscope. The pinhole density of the film deposited at position A was 460 cm -2 and that at position B was 800 cm -2 However, the density of pinholes in the film deposited by a two-step process showed the lowest value, 140 cm-2. Thus the novel two-step process creates fewer pinholes, and hence good insulator films.
3.2. Role of plasma in thin film growth A number of interesting experimental observations have been made so far, in particular in terms of the 100
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81
relationship between the performance of the coatings and the deposition parameters. (1) Corrosion-resistant coatings formed by PACVD are superior to those obtained by sputtering since the film deposited by sputtering tends to bear microcracks. (2) The operating pressure during sputtering is the most critical deposition parameter in terms of the mechanical properties of the coatings. The compressive internal stress increases, and hence the hardness increases and the adhesion decreases, with a decrease in pressure. (3) The density of pinholes in insulating films formed by sputtering is strongly affected by the substrate positions during deposition. High-resistivity A1203 films can be prepared by a novel sputter deposition technique in which the first layer and second layer are controlled independently, thus differentiating it from the usual one-step sputter deposition. It is necessary to reveal the basic characteristics of the plasma itself in order to understand the above experimental observations. For example, in the sputtering process, the plasma is created between the sputtering target, called a cathode, and the substrate. In the plasma region, a number of species, such as neutral gases, ions and electrons, are generated by the externally supplied electricity. These species interact with the substrate and cause a variety of surface reactions, such as deposition, chemical reactions with other species, resputtering of the deposited films, surface mixing, enhancement of adatom diffusion and heating of the substrate. Although we can control the external parameters, such as the operating pressure, gas flow rate, power density, substrate position and so on, it is obvious that the environment near the growing film is the most critical factor determining the characteristics and performance of the film. In this sense, the concept used to describe electrochemical reactions may help in the understanding of the thin film growth mechanism in plasma environments, although there are a number of differences in both cases. In this sense, surface science, in particular the role of the environment in surface reactions, is the key issue in understanding, and hence controlling, the plasmaassisted coating process. A plasma can be roughly defined as a gas consisting of ions and electrons both having approximately the same density over the Debye length. Thus electric neutralization is held over this characteristic length. In most cases, the energy necessary to generate the plasma is supplied by accelerated electrons. The accelerated electrons cause a variety of reactions, such as elastic collision, excitation, dissociation, ionization and so on. Since the excited species can react chemically with other species even at low temperatures, chemical vapor deposition can be assisted by a plasma. An external electric field is used for the ease of generation of accelerated
82
M. Hashimoto et al./ Materials Science and Engineering A 198 (1995) 75-83
ions for the sputtering process. Thus the plasma has unique characteristics in both physical and chemical reactions. In the case of a low-pressure plasma, where the frequency of collisions is low, the temperature of the electrons is much higher than that of the ions since light electrons can be easily accelerated and gain high velocity without losing their energy. The typical temperature of electrons is about 104 K or higher, whereas that of ions stays around several hundred kelvin. Thus a lowpressure plasma is also called a low-temperature plasma. However, the situation becomes quite different when a solid is placed in a low-pressure, low-temperature plasma. Since the velocity of electrons is much higher than that of ions, the flux of negative charge towards ' t h e solid surface is also initially higher than that of ':positive ions. Therefore the surface is negatively charged and develops an excess positive ion region called an ion sheath. When an external bias voltage, for example a negative bias, is applied to one of the two electrodes, an electric field is artificially generated near the cathode, and positive ions are accelerated towards this cathode and hit it. If the energy is high enough to sputter the cathode material, the material is deposited onto the anode. The depositing atoms sputtered from the cathode have an energy of several electronvolts. However, it should be noted that Ar ions, which are commonly used as the processing ions accelerated towards the cathode at several hundred electronvolts, are reflected at the cathode. Thus the neutralized and reflected Ar atoms hit the anode and hence affect the thin film growth at the anode. In other words, the growing film is always exposed to the high- energy processing gas as well as the flux of depositing atoms. This is a rough picture of the sputtering process. Thus the mobility of electrons and ions (the electrode potential), which is either unintentionally or intentionally generated, and the flux of various species having different energies are the key factors describing the sputter deposition. Fig. 9 summarizes the reactions and fundamental steps of radicals and ions in a reactive plasma. The above picture of the sputtering process can be used to explain the dependence of the mechanical properties of TiN films on the operating pressure. In Section 3.1.4, it was found that the operating pressure during sputtering was the most critical deposition parameter in terms of the mechanical properties of the coatings, and the compressive internal stress increased (and hence the hardness increased and the adhesion decreased) with a decrease in the pressure. Scanning electron microscopy (SEM) observations also show that the density of TiN films increases with a decrease in pressure. The above result suggests that a decrease in pressure has the same effect as a bias on the substrate. Kumer et al. [15] reported that a brown columnar film changed to
(1) INITt&L~ACTIOI~
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~
*PLASMAPARAMETERS energy distribution of electrons density of electrons, electric field profile
~' "RADICALANDIONREACTIONS density and space distribution of working gas, ions and radicals
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~4) SURF&CE REACTIO~I
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1
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Fig. 9. Atomic and molecular reactions in reactive plasma.
a golden dense film when the substrate was negatively biased. They suggested that densification by a high-energy, argon-ion-peening effect could explain the dense structure. Hoffman and Thornton [16] investigated the stress of sputtered metal films, and explained the cause of the stresses by the argon-ion-peening effect. On decreasing the deposition pressure, the argon-ionpeening effect becomes more evident. The energy delivered to the film surface depends on the discharge pressure. After high-pressure deposition without a substrate bias voltage, the deposited films exhibit a rough morphology and slight tensile stress because the working gas atoms or the sputtered atoms are thermalized by collision with working gas atoms, and the energy transferred to the film surface is small. On the other hand, during deposition at low pressure, energy is delivered to the film surface by the energetic gas atoms backscattered from the sputtering target or by the sputtered atoms, and a film with a smooth morphology and a large compressive stress is obtained. The above picture is supported by the measurement of the concentration of Ar atoms in the films using proton backscattering spectroscopy [17]. It was found that the Ar content in the films is high at low operating pressure, typically 9 at.%, but low at high pressure, about 6 at.%. It is considered that the concentration of Ar atoms in the films depends on the energy of the Ar atoms impinging on the film surface. Thus the operating pressure affects the energy of the processing gas atoms delivered to the growing films, and hence the density, morphology, internal stress and adhesion of the films. The above picture can also be used to explain the experimental observation that the density of pinholes in
M. Hashimoto et al. / Materials Science and Engineering A 198 (1995) 75 83
insulating films formed by sputtering is strongly affected by the substrate position during deposition. We showed that high-resistivity A1203 films could be prepared by a novel sputter deposition technique in which the first layer and second layer were controlled independently, thus differentiating it from the usual one-step sputter deposition. At the initial stage, strong bombardment at position A in the plasma leads to large adatomdepleted islands. These zones will cause pinhole-type defects. A thicker deposit over several micrometers is therefore required to bury such pinholes. In contrast, with weak bombardment, i.e. position B, large adatomdepleted zones are difficult to form. The energetic particle assistance at position A can lead to densification of the near-surface region and the burial of smaller defects. It is reasoned that the dependence of the insulation probability on the thickness of the first layer, typically 20 nm, is due to a necessary minimum thickness for the burial of defects. This thickness depends on how the defects arise in the initial stages, i.e. larger defects arising in a film at position A cannot be buried at position B, while smaller defects arising at position B can be covered with a second layer, 480 nm thick, at position A. We believe that it is necessary to optimize the deposition conditions for the formation of the first layer and the second layer independently in order to achieve high insulation probability. This example clearly indicates that the initial stage of thin film growth should be carefully controlled to obtain the most desirable island structure for the latter stage. However, the optimum conditions for the latter stage are not necessarily the same as those for the initial stage. Thus, in general, the initial stage, in which the nucleation of the island structure is critical, and the latter stage, in which the coalescence of islands and morphology development are critical, should be separately optimized in order to obtain high-quality thin films. The two-step process may also be applicable to the design of wear-resistant coatings. As shown in Section 3.1.4, a good adherent and hard film can be obtained by changing the pressure during deposition from high to low even when the same material is deposited. Thus the design of the plasma coating process should be performed by considering the basic aspects of the thin film growth mechanism in the environment, where the behavior of energetic particles is the crucial factor.
4. Conclusions
In this paper, the role of the environment on coated stainless steels was discussed from two different points of view. The first involved the performance of ceramic-
83
coated stainless steels in corrosive and mechanically stressed environments. It was found that a n S i O 2 coating deposited by PACVD led to the largest increase in the corrosion resistance of type 430 stainless steel to well above the level of type 304 stainless steel. It was also possible to coat stainless steel with TiN and other ceramics for decorative applications. The combination of the color of the TiN undercoating and the SiO2 corrosion-resistant top coating provides the potential to develop multifunctional stainless steels. Other examples are wear-resistant coatings and insulating coatings. The second viewpoint was the influence of the plasma environment used to synthesize the coated materials on the quality of thin films. Since the growing film is always exposed to the plasma during deposition, the flux and energy of various species, such as the processing gas, ions and depositing atoms, are the key factors in describing thin film growth. We believe that it is necessary to optimize the deposition conditions for the initial stage and latter stage independently in order to achieve high-quality thin films.
References [1] T. Takahashi, Y. Oikawa, T. Komori, I. Ito and M. Hashimoto, Surf Coat. Technol., 51 (1992) 522. [2] M. Hashimoto, S. Miyajima, W. Ito, S. lto, T. Murata, T. Komori, I. Ito and M. Onoyama, Surf Coat. Technol., 36 (1988) 837. [3] H. Omata, S. lto and T. Murata, U.S. Patent 4,612,095, 1986. [4] H. Omata, S. lto, M. Yabumoto and T. Murata, Proc. 4th Asian-Pac(fic Corrosion Control Conj'., Tokyo, May 2 ~ 31, 1985, Vol. 1, 1985, p. 475. [5] Y. Hosoi, 1. lto, S. Saida, T. Murata, S. Ito and N. Okubo, Japanese Patent Application Publication, No. Sho 62-205272, September 9, 1987. [6] U. Nakata, I. Ito, M. Onoyama and H. Inagaki, Proc. 28th Joint Symp. on Corrosion Science and Technology, Japan Society of Corrosion Engineering, 1981, p. 18. [7] Y. Kubo and M. Hashimoto, Surf Coat. Technol., 49 (1991) 342. [8] J.D. Finegan and R.W. Hoffman, J. Appl. Phys., 30 (1950) 597. [9] G. Stoney, Proc. R. Soc. London, Set. A, 32(1909) 172. [10] S. Tokumaru and M. Hashimoto, Sur[~ Coat. Technol., 54/55 (1992) 303. [11] K. Nakamura, K. Inagawa, K. Tsuruoka and S. Komiya, Thin Solid Films, 40 (1977) 155. [12] B. Zega, M. Kornmann and J. Amiguet, Thin Solid Films, 45 (1977) 577. [13] M.N. Khan, Thin Solid Films, 124 (1985) 55. [14] J. Saraie, S. Goto, Y. Kitao and Y. Yodogawa, J. Electrochem. Soc., 134 (1987) 2805. [15] N. Kumer, J.T. McGinn, K. Pourrezaei, B. Lee and E.C. Douglas, J. Vae. Sci. Teehnol. A, 6 (3) (1988) 1602. [16] D.W. Hoffman and J.A. Thornton, Thin Solid Films, 40 (1976) 355. [17] K. Tanaka, Y. Kubo, N.B. Chilton and M. Kumagai, Nucl. Instrum. Methods B, 83 (1993) 525.