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Deposition and characterization of nitrogen containing stainless steel coatings prepared by reactive magnetron sputtering
Vacuum/volume
47/number g/pages 1077 to 1080/1996 Copyright Q 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0042-207X/96 $15.00+.00
Pergamon PII: SOO42-207X(96100143-1
Deposition and characterization of nitrogen containing stainless steel coatings prepared by reactive magnetron sputtering J M Schneider,Kb C Rebholz,” A A Voevodin,a,c A Leyland” and A Matthews,” “RCSE, University of Huii, Hull HU6 7RX,UK; ‘BIRL, Northwestern University, Evanston, IL 60210-3135, USA; “Wright Laboratory, WL/MLBT Wright
Patterson
AFB, Dayton,
received
10 October 7995
Ohio 45433, USA
Nitrogen containing austenitic stainless steel coatings were deposited with nitrogen concentrations up to 14.1 at% using electron enhanced closed field unbalanced DC magnetron sputtering. The reactive gas flow was controlled with an optical spectrometer. The effect of the nitrogen to argon flow ratio on the chemical composition, microstructure and hardness of the coatings was investigated. The nitrogen concentration in the coating was measured by Auger Electron Spectroscopy (AES), and was found to be linearly dependent on the relative optical intensity of the iron 428.2 nm line, used as control signal for the reactive gas concentration at the source. Chemical state information was gathered by X-ray photoelectron spectroscopy (XPS). X-ray diffraction (XRD) was used to evaluate the lattice parameter, and peak shape parameters as a function of the nitrogen content. The influence of the nitrogen concentration on the hardness was investigated by nanoindentation measurements. Copyright 0 1996 Elsevier Science Ltd.
Introduction
Current demands on high performance wear and corrosion protective coatings can best be satisfied by advanced multilayer systems.’ Multilayer designs for tribological applications typically include a thin lubricating top layer and a hard load support layer as shown in Figure 1. Recent work on the tribological properties of metal-containing amorphous carbon top layers are given in Ref. 2. However, in order to produce a successful coating substrate composite, adequate attention has to be paid to the load support layer? A load support layer should be hard to prevent substrate deformation and resist abrasive wear. At the same time this layer needs to be tough to prevent crack initiation and propagation as well as fatigue failure. The load support layer should adhere well to both the lubricating top layer as well as the substrate. Plasma nitriding is one possible way to produce load support layers. This concept was realized with the so called duplex coatings.k6 Since this process is diffusion controlled, high temperatures and comparatively long process times are required. The diffusion layer typically consists of several phases. For example D’Haen et ~1.~list in their low angle X-ray diffraction work on plasma nitrided stainless steel c-Fe,_,N as well as y’-Fe,N as possible phases. In load support applications multiphase materials can exhibit certain disadvantages:
l
l
Considerable differences in elastic moduli can favor crack propagation. The difference in electrochemical potential of the individual phases can cause corrosion development.
Thus a single phase interlayer with high hardness and toughness can be employed beneficially as a load support interlayer for protective coatings. The objective of this research is to deposit and characterize a single phase load support layer for stainless steel substrates. This was realized by sputtering stainless steel in a low pressure argon plus nitrogen gas mixture. The utilization of optical emission spectroscopy for process control will also be discussed. Methods of research
Nitrogen containing stainless steel coatings were deposited in a electron enhanced, closed field unbalanced magnetron system described previously. The system base pressure of 10e3 Pa was achieved using a liquid-nitrogen-trapped diffusion pump. The AI.51 316 steel coupons were ultrasonically cleaned in acetone and etched in a 0.6 Pa argon ambient, at a substrate potential of - 600 V. Plasma bombardment in argon was performed until the substrate temperature reached 400°C. Deposition was carried out in argon/nitrogen discharges at a constant current of 3 A and a voltage of - 480 V for the pure Ar discharge and up to - 600 V 1077
J M Schneider et a/: Nitrogen containing stainless steel coatings
detectable in the XPS experiments, and this was corrected by peak fitting and stripping. The film thicknesses and roughness was measured by a DEKTAK3ST surface profilometer. Nanoindentation studies were undertaken with an Ultra Micro-Indentation System UMIS-2000. The Berkovitch indenter was loaded and unloaded in 50 increments, the maximum load was 10 mN. The hardness values at maximum depth were then calculated. The microstructure was studied with a Scintag XDS 2000 PAD V X-ray diffractometer (XRD), using copper Ka radiation. The raw data was stripped from its Ka2 component by the method of Rachinger.‘” The peak profile was evaluated using the Kew function.” Results and discussion
Fig:ure 1. Multilayer design for tribological applications
for the N2/Ar discharge. The total sputtering pressure was 0.1 Pa and the substrate bias potential was - 70 V. The ion current density was 3 mA/cm*. The closed loop feed back process control based on an optical spectrometer, was discussed in detail previously.” The iron line at 428.2 nm was chosen as a control parameter, since this line exhibits both a high intensity as well as no overlaps with emission lines of other species present. In the first seconds of every run the iron line intensity of a pure argon discharge is measured. Then the reactive gas is introduced via a high speed piezo valve. The reaction of the nitrogen on the target surface reduces both the metal flux coming off the target as well as the iron line intensity. The ratio of the iron line intensity of a argon/nitrogen discharge compared to a pure argon discharge is called the relative intensity (RI). The reactive gas flow is adjusted via a high speed piezo valve to keep a constant intensity ratio (RI) and therefore a constant nitrogen partial pressure in the chamber. The chemical composition was estimated using an Auger Electron Spectrometer (AES) at 3 keV primary energy on a Perkin Elmer Phi 3017 Auger System for computer controlled depth profiling. For the material removal via sputter etching, a 4 kV Argon ion beam was focused on the sample with a 3 mm x 3 mm raster drive. Sputter cleaning was performed until a stable nitrogen signal was achieved. The chemical composition was calculated using the sensitivity factors given in Ref. 9. Surface chemistry of coatings was studied with a Surface Science Instruments M-Probe X-ray photoelectron spectroscopy (XPS) instrument operated at 3 x lo-’ Pa using a 400 x 1000 and 150 pm line spot and a 50 eV pass energy to provide a full width at half maximum (FWHM) of 0.77 eV for Au 4f,!2 peak. Binding energy positions were calibrated using the Cu 3s and Cu 2P,.? peaks at 122.39 eV and 932.47 eV, respectively. An Ar ion gun with 1 keV accelerating energy was used to etch the surface of the coatings for 15 min to remove the oxide layer. Ion etching was stopped as soon minimal peak broadening could be detected. At this optimized condition, surface oxides were still 1078
The nitrogen containing stainless steel coatings exhibit a shiny metal surface, but increasing nitrogen content tends to give them a matte appearance. The surface roughness (R,) was 25 A prior to deposition, and increased over 35-45 8, for a reduction of RI 100% to 80%. The sample with the highest nitrogen content (RI 70%) also exhibits the highest roughness with 65 A. The coating thickness was in the range of 3.2-4.1 pm. The nitrogen concentrations of the stainless steel coatings as analyzed by AES as a function of the relative optical intensity (RI) is shown in Figure 2. A linear response of the nitrogen concentration and the relative optical intensity (RI) at 1 = 428.2 nm can be observed in the analyzed concentration range. This linear relationship was used to control the partial pressure of the reactive gas via a high speed piezoelectric valve. Precise control of the nitrogen composition of the coatings was achieved. The microstructure and therefore the mechanical properties of the load support layer, which are reported below, can be engineered in accordance with the requirements of a particular application. This shows that an optical emission spectrometer employed in a closed loop feedback control circuit is a powerful process control tool. The chemical state of the iron, chromium and nickel was characterized by XPS. Figures 3 and 4 show the Cr 2p,,, peak and the Fe2p3,2 peak, respectively as a function of the nitrogen concentration for the samples containing minimum and maximum amount of nitrogen. If either of the metals would form a chemical bond with nitrogen incorporated in the film, the metal peak would shift towards higher energies. On the basis of peak fitting
0
100
‘Z ;
95
sl .z
90
5 2
85
Z .; 0”
80 75 70
0
5 Nitrogen
Concentration
Figure 2. Nitrogen Concentration troscopy vs the relative intensity
10
15
by AES [at%]
as measured by Auger Electron of the iron line at 428.2 nm.
Spec-
JM Schneider et al: Nitrogen containing stainless steel coatings 25000,
1
8 8
0 Nitrogen 570
580
575
590
585
Energy
595
15
10
5 Concentration
by AES [at%]
Figure 5. Hardness vs nitrogen concentration by Auger Electron spectroscopy. The error bars represent the standard deviation.
[HI]
Figure 3. Cr,p,/, XPS spectrum for the samples with minimum and maximum nitrogen content.
calculations no evidence of the formation of the iron nitride could be observed. A peak shift of 0.2 eV at an instrumental resolution of 0.04 eV could indicate the formation of chromium nitride for the RI 70% sample. Figure 5 shows the O-20 scans as a function of the nitrogen concentration. The coating deposited in a pure argon discharge shows both phases, the austenitic stainless steel phase” and a bee ferrite phaseI (not shown). Nickel is known to stabilize the austenite phase and chromium the ferrite phase. The microstructure changes if nitrogen (stabilzes austenite) is introduced to the discharge. The bee structure disappears and only the expanded austenite can be observed. Evidence for the austenitic structure is given by the (11 l), (200) and (311) reflections. A further increase in nitrogen content up to 14.1 at% results in an increased lattice parameter due to nitrogen incorporation into the austenite lattice (Figure 6), as well as a change in preferred orientation (Figure 7)
r” _
Pw
E-
E
10.1 at% nitrcgen-
(WA
-
(200) 4.6at% nitrogen
1 , 32
I
I
40
I
I/ 48
I
I
I
1,
1
I
I
8,
56 64 72 2 theta [deg.]
I
80
,
-
/
88
Figure 6. X-ray diffraction pattern of the expanded austenite at different nitrogen concentrations. 4
,,,,,,‘,,,,,,,,,,,,,,,,,,,,,,‘ O-
3.9
-
90000 80000
70000
0
'380000 1 50000 Z 00 z 40000
Nitroge:
z -
15
0 Concentratk!
[at%]
Figure 7. Lattice parameter vs nitrogen concentration.
30000
OJ 700
710
730
720 Energy
740
[sV]
Figure 4. Fe,p,,, XPS spectrum for the samples with minimum and maximum nitrogen content.
and an increase in full width at half maximum (not shown). No evidence of chromium nitride, iron nitride or related phases could be observed by XRD, which confirms the XPS results. The microstructural changes were quantified: Figure 6 shows the lattice parameter calculated from the (200) reflection as a function of the nitrogen content. Incorporation of - 14.1 at% nitrogen increases the lattice parameter by 10% compared to the coating produced in a pure argon discharge (RI 100%). 1079
J M Schneider
et a/: Nitrogen containing stainless steel coatings
The influence of the nitrogen on the relative X-ray diffraction intensity (I,, I I,/4,, II + &,,, is shown in Figure 7. The texture coefficient of the sample produced in a pure argon discharge is close to the one given for 304 stainless steel.” A strong (200) preferred orientation can be observed for higher nitrogen incorporations. The hardness values measured were in the range of 3.9 GPa for the uncoated substrate to 9.9 GPa for the coating containing 14.1 at% nitrogen. Thus, hardness was increased by factor 2.5 (see Figure 8). It should be possible to achieve further improvements in stainless steel coating hardness by post-coat diffusion if a low (i.e. <4OO”C) treatment temperature is maintained. The production of single phase treatments on stainless steel by low temperature diffusion has previously been possible,‘4.‘5 however the sharp case to core interface does not impart ideal load support characteristics. This effect has been shown to be avoided by coating and post coat diffusion. It has been shown that single phase nitrogen containing austenitic stainless steel coatings were produced. These coatings exhibit the following potential advantageous over steel diffusion layers produced by conventional plasma nitriding techniques: l
l
Homogeneous strain distribution due to homogeneous elastic moduli, as opposed to a multiphase material with drastically different elastic moduli from grain to grain which gives rise to strain peaks at the boundaries. The probability of initiation and propagation of cracks as well as fatigue failure is smaller than in a multiphase system. For the application in a corrosive media, both the single phase material and the homogenous strain distribution are favorable: The driving force for wet corrosion is the difference in electrochemical potential of the corrosion couple.‘” By introducing new phases with different corrosion properties, new corrosion couples are created, which can lead in turn to more severe corrosion attack. It is well known that corrosion rates are drastically increased under both the presence of stress (stress corrosion cracking) and fatigue.” Thus in a multiphase material with localized strain peaks the
effective corrosion rate can be higher than for a material with a homogeneous strain distribution. Conclusions Single phase nitrogen containing austenitic stainless steel coatings were produced by reactive magnetron sputtering. From this research the following conclusions can be drawn: The nitrogen concentration in the coating is linearly dependent on the nitrogen partial pressure in the explored concentration range. This linear relationship is the basis for reliable control of the chemical composition of the coatings, and therefore their microstructure and physical properties. No evidence of metal nitride phases could be found by XRD. However XPS studies indicate the formation of chromium nitride at concentrations higher than 14 at% nitrogen. On the basis of lattice parameter calculations from the XRD data, is can be concluded that the nitrogen was incorporated in the austenite lattice. An expansion of 10% in lattice parameter was observed. The lattice expansion resulted in a hardness increase from 3.9 GPa to 9.9 GPa. The single phase coatings produced, show potential for use in duplex and other multilayer coating systems for improved wear and corrosion protection. Acknowledgements The authors are grateful to Professor W D Sproul for helpful discussions and the provision of laboratory facilities at BIRL (Northwestern University). We also acknowledge the support of the UK EPSRC for the RCSE. References ‘A A Voevodin, E L Erokhin and V V Lyubimov, Phys Stat Sol(u), 565 (1994).
145,
‘A A Voevodin, C Rebholz, J M Schneider, P Stevenson and A Matthews, Surfand Coat Technol, 73, 185 (1995). ‘J M Schneider, A A Voevodin, C Rebholz and A Matthews, J of Vuc Sciund
Tech A, 13(4), 2189-2193
(1995).
‘M Zlatanovic and W D Muenz, Surf and Coat Trchnol, 41, 17 (1990).
0.6 0.5 0.4 0.3 0.2
0.1
0
5 Nitrogen
10 Concentration
15
[at%]
Figure 8. Normalized X-ray diffraction peak intensities vs nitrogen concentration.
1080
‘B Dorn. MSc, The Universitv of Hull (1995). A Leyland. Be Dorn, d R Stevenson, M Bin-Sudin, C Rebholz. A Voevodin and J Schneider. J of Vnc Sci and Technol A, 1313, 1202 (1995). ‘J D’Haen, C Quaeyhaegens and L Stals, Surf and Coat Technol, 60,468 ( 1993). ‘A A Voevodin, P Stevenson, C Rebholz, J M Schneider and A Matthews, Vrrcuunz. 46, 723 (1995). “L E Davis. N C MacDonald, P W Palmberg, G E Riach and R E Weber, Handbook of Au,ger Electron Spectroscopy, Physical Electronics Division. Perkin Elmer Corp, Minnesota, USA. I”W A Rachinger, Jour Sci Znstr, 25, 254 (I 948). “Siemens, Di[~i~~c/ AT Manual, Profile VI .O, 79 (1993). “JCPDS Powder Diffraction File, International Center for Powder Diffraction Data, Swarthmore, PA (I 993), Card 33-0397. ” JCPDS Powder D@wction File, International Center for Powder Diffraction Data, Swarthmore, PA (1993), Card 33-0396. “A Leyland, K S Fancey and A Matthews. Sury Engng, 7,207 ( 199 1). “P R Stevenson. A Leyland, M A Parkin and A Matthews. Su~fund Coot Technol, 63, 135 (I 994). lhM F Ashby and D R H Jones, Engineering Materials I. Pergamon Press. Oxford (I 99 I ).
“A Matthews,
0.7
47/number g/pages 1077 to 1080/1996 Copyright Q 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0042-207X/96 $15.00+.00
Pergamon PII: SOO42-207X(96100143-1
Deposition and characterization of nitrogen containing stainless steel coatings prepared by reactive magnetron sputtering J M Schneider,Kb C Rebholz,” A A Voevodin,a,c A Leyland” and A Matthews,” “RCSE, University of Huii, Hull HU6 7RX,UK; ‘BIRL, Northwestern University, Evanston, IL 60210-3135, USA; “Wright Laboratory, WL/MLBT Wright
Patterson
AFB, Dayton,
received
10 October 7995
Ohio 45433, USA
Nitrogen containing austenitic stainless steel coatings were deposited with nitrogen concentrations up to 14.1 at% using electron enhanced closed field unbalanced DC magnetron sputtering. The reactive gas flow was controlled with an optical spectrometer. The effect of the nitrogen to argon flow ratio on the chemical composition, microstructure and hardness of the coatings was investigated. The nitrogen concentration in the coating was measured by Auger Electron Spectroscopy (AES), and was found to be linearly dependent on the relative optical intensity of the iron 428.2 nm line, used as control signal for the reactive gas concentration at the source. Chemical state information was gathered by X-ray photoelectron spectroscopy (XPS). X-ray diffraction (XRD) was used to evaluate the lattice parameter, and peak shape parameters as a function of the nitrogen content. The influence of the nitrogen concentration on the hardness was investigated by nanoindentation measurements. Copyright 0 1996 Elsevier Science Ltd.
Introduction
Current demands on high performance wear and corrosion protective coatings can best be satisfied by advanced multilayer systems.’ Multilayer designs for tribological applications typically include a thin lubricating top layer and a hard load support layer as shown in Figure 1. Recent work on the tribological properties of metal-containing amorphous carbon top layers are given in Ref. 2. However, in order to produce a successful coating substrate composite, adequate attention has to be paid to the load support layer? A load support layer should be hard to prevent substrate deformation and resist abrasive wear. At the same time this layer needs to be tough to prevent crack initiation and propagation as well as fatigue failure. The load support layer should adhere well to both the lubricating top layer as well as the substrate. Plasma nitriding is one possible way to produce load support layers. This concept was realized with the so called duplex coatings.k6 Since this process is diffusion controlled, high temperatures and comparatively long process times are required. The diffusion layer typically consists of several phases. For example D’Haen et ~1.~list in their low angle X-ray diffraction work on plasma nitrided stainless steel c-Fe,_,N as well as y’-Fe,N as possible phases. In load support applications multiphase materials can exhibit certain disadvantages:
l
l
Considerable differences in elastic moduli can favor crack propagation. The difference in electrochemical potential of the individual phases can cause corrosion development.
Thus a single phase interlayer with high hardness and toughness can be employed beneficially as a load support interlayer for protective coatings. The objective of this research is to deposit and characterize a single phase load support layer for stainless steel substrates. This was realized by sputtering stainless steel in a low pressure argon plus nitrogen gas mixture. The utilization of optical emission spectroscopy for process control will also be discussed. Methods of research
Nitrogen containing stainless steel coatings were deposited in a electron enhanced, closed field unbalanced magnetron system described previously. The system base pressure of 10e3 Pa was achieved using a liquid-nitrogen-trapped diffusion pump. The AI.51 316 steel coupons were ultrasonically cleaned in acetone and etched in a 0.6 Pa argon ambient, at a substrate potential of - 600 V. Plasma bombardment in argon was performed until the substrate temperature reached 400°C. Deposition was carried out in argon/nitrogen discharges at a constant current of 3 A and a voltage of - 480 V for the pure Ar discharge and up to - 600 V 1077
J M Schneider et a/: Nitrogen containing stainless steel coatings
detectable in the XPS experiments, and this was corrected by peak fitting and stripping. The film thicknesses and roughness was measured by a DEKTAK3ST surface profilometer. Nanoindentation studies were undertaken with an Ultra Micro-Indentation System UMIS-2000. The Berkovitch indenter was loaded and unloaded in 50 increments, the maximum load was 10 mN. The hardness values at maximum depth were then calculated. The microstructure was studied with a Scintag XDS 2000 PAD V X-ray diffractometer (XRD), using copper Ka radiation. The raw data was stripped from its Ka2 component by the method of Rachinger.‘” The peak profile was evaluated using the Kew function.” Results and discussion
Fig:ure 1. Multilayer design for tribological applications
for the N2/Ar discharge. The total sputtering pressure was 0.1 Pa and the substrate bias potential was - 70 V. The ion current density was 3 mA/cm*. The closed loop feed back process control based on an optical spectrometer, was discussed in detail previously.” The iron line at 428.2 nm was chosen as a control parameter, since this line exhibits both a high intensity as well as no overlaps with emission lines of other species present. In the first seconds of every run the iron line intensity of a pure argon discharge is measured. Then the reactive gas is introduced via a high speed piezo valve. The reaction of the nitrogen on the target surface reduces both the metal flux coming off the target as well as the iron line intensity. The ratio of the iron line intensity of a argon/nitrogen discharge compared to a pure argon discharge is called the relative intensity (RI). The reactive gas flow is adjusted via a high speed piezo valve to keep a constant intensity ratio (RI) and therefore a constant nitrogen partial pressure in the chamber. The chemical composition was estimated using an Auger Electron Spectrometer (AES) at 3 keV primary energy on a Perkin Elmer Phi 3017 Auger System for computer controlled depth profiling. For the material removal via sputter etching, a 4 kV Argon ion beam was focused on the sample with a 3 mm x 3 mm raster drive. Sputter cleaning was performed until a stable nitrogen signal was achieved. The chemical composition was calculated using the sensitivity factors given in Ref. 9. Surface chemistry of coatings was studied with a Surface Science Instruments M-Probe X-ray photoelectron spectroscopy (XPS) instrument operated at 3 x lo-’ Pa using a 400 x 1000 and 150 pm line spot and a 50 eV pass energy to provide a full width at half maximum (FWHM) of 0.77 eV for Au 4f,!2 peak. Binding energy positions were calibrated using the Cu 3s and Cu 2P,.? peaks at 122.39 eV and 932.47 eV, respectively. An Ar ion gun with 1 keV accelerating energy was used to etch the surface of the coatings for 15 min to remove the oxide layer. Ion etching was stopped as soon minimal peak broadening could be detected. At this optimized condition, surface oxides were still 1078
The nitrogen containing stainless steel coatings exhibit a shiny metal surface, but increasing nitrogen content tends to give them a matte appearance. The surface roughness (R,) was 25 A prior to deposition, and increased over 35-45 8, for a reduction of RI 100% to 80%. The sample with the highest nitrogen content (RI 70%) also exhibits the highest roughness with 65 A. The coating thickness was in the range of 3.2-4.1 pm. The nitrogen concentrations of the stainless steel coatings as analyzed by AES as a function of the relative optical intensity (RI) is shown in Figure 2. A linear response of the nitrogen concentration and the relative optical intensity (RI) at 1 = 428.2 nm can be observed in the analyzed concentration range. This linear relationship was used to control the partial pressure of the reactive gas via a high speed piezoelectric valve. Precise control of the nitrogen composition of the coatings was achieved. The microstructure and therefore the mechanical properties of the load support layer, which are reported below, can be engineered in accordance with the requirements of a particular application. This shows that an optical emission spectrometer employed in a closed loop feedback control circuit is a powerful process control tool. The chemical state of the iron, chromium and nickel was characterized by XPS. Figures 3 and 4 show the Cr 2p,,, peak and the Fe2p3,2 peak, respectively as a function of the nitrogen concentration for the samples containing minimum and maximum amount of nitrogen. If either of the metals would form a chemical bond with nitrogen incorporated in the film, the metal peak would shift towards higher energies. On the basis of peak fitting
0
100
‘Z ;
95
sl .z
90
5 2
85
Z .; 0”
80 75 70
0
5 Nitrogen
Concentration
Figure 2. Nitrogen Concentration troscopy vs the relative intensity
10
15
by AES [at%]
as measured by Auger Electron of the iron line at 428.2 nm.
Spec-
JM Schneider et al: Nitrogen containing stainless steel coatings 25000,
1
8 8
0 Nitrogen 570
580
575
590
585
Energy
595
15
10
5 Concentration
by AES [at%]
Figure 5. Hardness vs nitrogen concentration by Auger Electron spectroscopy. The error bars represent the standard deviation.
[HI]
Figure 3. Cr,p,/, XPS spectrum for the samples with minimum and maximum nitrogen content.
calculations no evidence of the formation of the iron nitride could be observed. A peak shift of 0.2 eV at an instrumental resolution of 0.04 eV could indicate the formation of chromium nitride for the RI 70% sample. Figure 5 shows the O-20 scans as a function of the nitrogen concentration. The coating deposited in a pure argon discharge shows both phases, the austenitic stainless steel phase” and a bee ferrite phaseI (not shown). Nickel is known to stabilize the austenite phase and chromium the ferrite phase. The microstructure changes if nitrogen (stabilzes austenite) is introduced to the discharge. The bee structure disappears and only the expanded austenite can be observed. Evidence for the austenitic structure is given by the (11 l), (200) and (311) reflections. A further increase in nitrogen content up to 14.1 at% results in an increased lattice parameter due to nitrogen incorporation into the austenite lattice (Figure 6), as well as a change in preferred orientation (Figure 7)
r” _
Pw
E-
E
10.1 at% nitrcgen-
(WA
-
(200) 4.6at% nitrogen
1 , 32
I
I
40
I
I/ 48
I
I
I
1,
1
I
I
8,
56 64 72 2 theta [deg.]
I
80
,
-
/
88
Figure 6. X-ray diffraction pattern of the expanded austenite at different nitrogen concentrations. 4
,,,,,,‘,,,,,,,,,,,,,,,,,,,,,,‘ O-
3.9
-
90000 80000
70000
0
'380000 1 50000 Z 00 z 40000
Nitroge:
z -
15
0 Concentratk!
[at%]
Figure 7. Lattice parameter vs nitrogen concentration.
30000
OJ 700
710
730
720 Energy
740
[sV]
Figure 4. Fe,p,,, XPS spectrum for the samples with minimum and maximum nitrogen content.
and an increase in full width at half maximum (not shown). No evidence of chromium nitride, iron nitride or related phases could be observed by XRD, which confirms the XPS results. The microstructural changes were quantified: Figure 6 shows the lattice parameter calculated from the (200) reflection as a function of the nitrogen content. Incorporation of - 14.1 at% nitrogen increases the lattice parameter by 10% compared to the coating produced in a pure argon discharge (RI 100%). 1079
J M Schneider
et a/: Nitrogen containing stainless steel coatings
The influence of the nitrogen on the relative X-ray diffraction intensity (I,, I I,/4,, II + &,,, is shown in Figure 7. The texture coefficient of the sample produced in a pure argon discharge is close to the one given for 304 stainless steel.” A strong (200) preferred orientation can be observed for higher nitrogen incorporations. The hardness values measured were in the range of 3.9 GPa for the uncoated substrate to 9.9 GPa for the coating containing 14.1 at% nitrogen. Thus, hardness was increased by factor 2.5 (see Figure 8). It should be possible to achieve further improvements in stainless steel coating hardness by post-coat diffusion if a low (i.e. <4OO”C) treatment temperature is maintained. The production of single phase treatments on stainless steel by low temperature diffusion has previously been possible,‘4.‘5 however the sharp case to core interface does not impart ideal load support characteristics. This effect has been shown to be avoided by coating and post coat diffusion. It has been shown that single phase nitrogen containing austenitic stainless steel coatings were produced. These coatings exhibit the following potential advantageous over steel diffusion layers produced by conventional plasma nitriding techniques: l
l
Homogeneous strain distribution due to homogeneous elastic moduli, as opposed to a multiphase material with drastically different elastic moduli from grain to grain which gives rise to strain peaks at the boundaries. The probability of initiation and propagation of cracks as well as fatigue failure is smaller than in a multiphase system. For the application in a corrosive media, both the single phase material and the homogenous strain distribution are favorable: The driving force for wet corrosion is the difference in electrochemical potential of the corrosion couple.‘” By introducing new phases with different corrosion properties, new corrosion couples are created, which can lead in turn to more severe corrosion attack. It is well known that corrosion rates are drastically increased under both the presence of stress (stress corrosion cracking) and fatigue.” Thus in a multiphase material with localized strain peaks the
effective corrosion rate can be higher than for a material with a homogeneous strain distribution. Conclusions Single phase nitrogen containing austenitic stainless steel coatings were produced by reactive magnetron sputtering. From this research the following conclusions can be drawn: The nitrogen concentration in the coating is linearly dependent on the nitrogen partial pressure in the explored concentration range. This linear relationship is the basis for reliable control of the chemical composition of the coatings, and therefore their microstructure and physical properties. No evidence of metal nitride phases could be found by XRD. However XPS studies indicate the formation of chromium nitride at concentrations higher than 14 at% nitrogen. On the basis of lattice parameter calculations from the XRD data, is can be concluded that the nitrogen was incorporated in the austenite lattice. An expansion of 10% in lattice parameter was observed. The lattice expansion resulted in a hardness increase from 3.9 GPa to 9.9 GPa. The single phase coatings produced, show potential for use in duplex and other multilayer coating systems for improved wear and corrosion protection. Acknowledgements The authors are grateful to Professor W D Sproul for helpful discussions and the provision of laboratory facilities at BIRL (Northwestern University). We also acknowledge the support of the UK EPSRC for the RCSE. References ‘A A Voevodin, E L Erokhin and V V Lyubimov, Phys Stat Sol(u), 565 (1994).
145,
‘A A Voevodin, C Rebholz, J M Schneider, P Stevenson and A Matthews, Surfand Coat Technol, 73, 185 (1995). ‘J M Schneider, A A Voevodin, C Rebholz and A Matthews, J of Vuc Sciund
Tech A, 13(4), 2189-2193
(1995).
‘M Zlatanovic and W D Muenz, Surf and Coat Trchnol, 41, 17 (1990).
0.6 0.5 0.4 0.3 0.2
0.1
0
5 Nitrogen
10 Concentration
15
[at%]
Figure 8. Normalized X-ray diffraction peak intensities vs nitrogen concentration.
1080
‘B Dorn. MSc, The Universitv of Hull (1995). A Leyland. Be Dorn, d R Stevenson, M Bin-Sudin, C Rebholz. A Voevodin and J Schneider. J of Vnc Sci and Technol A, 1313, 1202 (1995). ‘J D’Haen, C Quaeyhaegens and L Stals, Surf and Coat Technol, 60,468 ( 1993). ‘A A Voevodin, P Stevenson, C Rebholz, J M Schneider and A Matthews, Vrrcuunz. 46, 723 (1995). “L E Davis. N C MacDonald, P W Palmberg, G E Riach and R E Weber, Handbook of Au,ger Electron Spectroscopy, Physical Electronics Division. Perkin Elmer Corp, Minnesota, USA. I”W A Rachinger, Jour Sci Znstr, 25, 254 (I 948). “Siemens, Di[~i~~c/ AT Manual, Profile VI .O, 79 (1993). “JCPDS Powder Diffraction File, International Center for Powder Diffraction Data, Swarthmore, PA (I 993), Card 33-0397. ” JCPDS Powder D@wction File, International Center for Powder Diffraction Data, Swarthmore, PA (1993), Card 33-0396. “A Leyland, K S Fancey and A Matthews. Sury Engng, 7,207 ( 199 1). “P R Stevenson. A Leyland, M A Parkin and A Matthews. Su~fund Coot Technol, 63, 135 (I 994). lhM F Ashby and D R H Jones, Engineering Materials I. Pergamon Press. Oxford (I 99 I ).
“A Matthews,
0.7