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Structural and chemical surface modifications of a stainless steel with implanted molybdenum and carbon ions
Surfaceand Coatings Technology80 (1996) 49-52
Structural and chemical surface modifications of a stainless steel with implanted molybdenum and carbon ions X. de Buchere*, P. Andreazza, C. Andreazza-Vignolle, C. Clinard, R. Erre Centre de Recheyche SW la Mat&e Divide, Universit6 d’OrEans-CNRS,
45071 OdBans Cedex 2, France
Abstract
The surface transformation of stainless steel was investigated in the case of single- and multi-element ion implantation in order to improve the localized corrosion resistance. Molybdenum and carbon ions (MO alone and MO-C) were implanted into austenitic stainless steels (316L type). Several surface techniques (X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and grazing incidence X-ray diffraction (GIXD)) were used to determine the surface modifications, such as phase
transformation (y-ta), amorphization, precipitation and stable or metastable phase formation, e.g. substitution (for MO) or insertion (for C) solid solution. Keywords: Stainless steel; Molybdenum
implantation; XPS; TEM; GIXD
1. Introduction In the last 15 years, ion implantation [ 11 in metals and alloys has provided a new method for modifying surface properties. The effect of a number of species implanted in stainless steels on the corrosion, friction coefficient and wear rate has been investigated. A better understanding of the mechanisms and processes which dominate structural transformations can lead to better means of obtaining the required properties. In stainless steels, nitrogen has been most extensively studied because it is known to improve the electrochemical and tribological properties of the surface. Moreover, it constitutes a very interesting case because of the physical and chemical phenomena observed [a]. However, a few papers [3] have reported the effect of implanting other species, such as MO, Si, Ti, C, etc., in single- or multiimplantation. In this paper, we present the results of implanting molybdenum and carbon in stainless steel surfaces in order to improve the localized corrosion resistance. To investigate the process-inherent surface modifications [4], such as phase transformation (y-austenite to a’martensite), amorphization, precipitation and stable or metastable phase formation, e.g. substitution or insertion solid solution, we used several surface techniques, i.e. X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and grazing incidence X-ray diffraction (GIXD). Our purpose was to discuss the * Corresponding author.Tel.:38515368; fax: 38633796. 11717~8973/9h/‘X15
Ml 0 1995
Flswier
Srimw
8 A
All riohtq
rrwm=d
chemical composition and microstructural modification differences resulting from single-implantation of molybdenum and multi-implantation of molybdenum and carbon.
2. Experimental
procedures
The stainless steel used in this work was a biomedical type (316L) with the following mass per cent composition: Cr, 18; Ni, 12; MO, 2.5; C
X de Buchese et a//Surface
50
and Coatings
the passivation layer. X-Ray analysis (GIXD) was performed with a Philips parallel beam horizontal diffractometer. The GIXD technique allows the determination of the in-depth distribution of various crystalline phases by varying the penetration depth with the X-ray incidence angle [ 51. In our case, the incidence angle a: between the beam and the sampleOsurface varied from 0.2” to 1.5”, i.e. from 50 to 1000OA penetration depth using Cu Ko: radiation (1,= 1.54 A). TEM was carried out after electrochemical attack of the samples and the preparation of grids by replica methods.
3. Chemical and structural results The distribution of all the elements present in the top 1000 A of the surface was determined by XPS analysis. Profiles obtained after abrasion of the surface layers (Fig. 1) and the binding state of the elements detected [6] give information on the composition and nature of the species created during MO and MO + C implantation. In the two types of sample, the depth of the molybdenum pseudo-gaussian distribution is centred at about 300 A, in good agreement with that predicted by the LSS theoretical simulation (TRIM program). This atomic concentration distribution presents a maximum of 30% and a slightly asymmetrical shape, broader than that predicted by the simulation, an effect which could be correlated with the highly textured crystalline character of the steel. The oxide layer (estimated depth, approximately 20 A) contains a relatively important proportion of molybdenum (approximately 10 at.%) as MOO, and MOO,. The particularly important point is that MO substitution in the steel matrix does not induce preferential segregation of iron, chromium or nickel (Fig. 1(b)), but rather a depletion (15%) of the initially high chromium concentration (30% minimum), which is usually found in the passive surface films. This surface depletion of chromium was not observed after implantation of elements such as
Technology
80 (1996)
49-52
Si, Ti, N, etc. A few oxides, such as Fe304 and Cr203, constitute the major part of this surface layer. We can therefore subdivide it into0 an outer part composed of hydroxides (less than 10 A) and an inner part mostly composed of iron and chromium oxides. In addition, only very small quantities of nickel are detected, accumulated at the oxide-metal interface (Fig. 2). These results have allowed us to propose a qualitatively and quantitatively detailed layer model of the zone modified by implantation. It is presented in Fig. 2, which summarizes all the important information, Supplementary implantation of carbon does not seem to induce other modifications in the oxide layer. However, a difference between the binding state of MO in the MO and MO + C implantations is revealed in all implantation in-depth levels. A comparison of the Mead levels in the MO and MO + C implantations shows changes in the binding energy (E =227.7 eV against E = 227.9 eV) and the full width at half-maximum (FWHM) (FWHM= 1.15 eV against FWHM= 1.29 eV) characterizing metallic and carbide bindings. In terms of structural characterization, GIXD analysis seems to be particularly well adapted for the study of the microstructural modifications expected to affect the first 100 A layer. Transmission electron micrographs and diffraction patterns complete this analysis. A 316L stainless steel sample without any implantation is used as a reference. The corresponding diffractograms for different incidence angles CIshow that only y-austenite is present (main lines at 20rr1=43.58”, 20,,,,= 50.79”, 2&, = 74.70”) with an orientation effect assigned to the previous mechanical Ohistory of the sample. The cell parameter is a = 3.598 A. In the MO implantation diffractograms shown in Fig. 3, the austenitic phase is always detected in very important quantities. However, the presence of a small
hydroxide lnver rich iron oxide lnyer m
70
FeO al
Fe304
@
C&,
metallic lnw- : I-;=I
0
300
depth
bulk
Fe 70 % Cr 17% Ni 10%
molybdenum (binding MO-C for MotC implantation)
600
(in A)
Fig. 1. XPS sputter depth profiles of an Mo-implanted stainless steel: (a) all detected elements; (b) steel matrix elements (Fe, Ni and Cr).
Fig. 2. Layer model of the surface zone modified by MO and MO + C implantations.
X de Buchere et al.lSttrfaee and Coatings Technology 80 (1996) 49-52
51
8, Fig. 3. Grazing incidence X-ray stainless steel at different angles to 50, 250 and 1000 A. “J-SS is diffracted intensity on logarithmic
diffractograms of Mo-implanted (OZ’, 0.5” and 1.5”) corresponding the austenite solid solution (X-ray scale).
peak at 2%=44.30”, more important at a =0.5” than at other incidence angles, agrees with the appearance of a structural change produced by ion implantation. TEM supplies more local information: austenitic and martensitic grains are observed by selected area diffraction, distinguished by their (211),* and (loo),* diffraction patterns (Figs. 4(b) and 4(c)). Moreover, some preferential orientation (110)” is observed as in the reference sample, but with grain sizes varying from 10 to 300 nm (Fig. 4(a)). In addition, Fig. 3 shows an important shoulder at small 26 angles on the (111) and (200) main lines of austenite, corresponding to cell expansion produced by substitution of implanted molybdenum, since the MO atomic radius is slightly larger than that of Fe, Cr or Ni. A hard-sphere model will be developed later to explain the evolution of the reticular distance in austenite as a function of the atomic percentage of MO. A very broad base appears on the (11 l), main line at
Fig. 5. Grazing incidence X- ray difkactograms of MO + C-implanted stainless steel at the same angles and with the same notation as in Fig. 3.
0.5” incidence, coinciding with the molybdenum implantation profile. This structural characteristic is ascribed to an amorphization effect due to implantation and corresponds to the disappearance of austenitic long distance order. Finally, no precipitates were detected in any diffractograms; however, in the diffraction patterns, we observed two diffuse rings characteristic of iron oxide, and the presence of precipitates was observed by TEM, sometimes localized at the grain boundaries. Their size was determined by dark field micrographs and is less than 100 nm. Some were identified as CrZ3C6, but the analysis of the others is not easy because of their lack of stability under the electron beam. The MO + C implantation diffractograms (Fig. 5) show a complex shape around the (111) austenitic main Iine and a partial amorphization similar to that observed with MO-implanted samples, which seems to be independent of carbon implantation. However, the shift of the shoulders previously observed on the austenitic main lines, characteristic of substitution during the formation of the solid solution, is larger (20 = 42.53”); it represents the effect of the supplementary insertion of carbon which increases the austenitic cell parameter. The distinguishing feature between the two types of implantation is the important role of martensite or ferrite formation. The two principal lines of this phase are (110) and (200), appearing respectively at 28=44.00” and 63.70”. The relative intensities indicate that it is at least as abundant as austenite, indicating a very important structural evolution.
4. Discussion Fig. 4. (a) Bright field transmission denum-implanted sample showing Two diffraction patterns: (b) [211] (c) [ lOO] zone axis of a martensitic
electron micrograph of a molyba grain size smaller than 300 nm. zone axis of an austenitic grain; grain.
MO implantation does not induce any particular segregation of constituent elements (Fe, Cr and Ni) in the metallic part. However, a loss of chromium is observed in the surface oxide Iayer. This phenomenon has not
52
X de Buchere
et al./Surface
and Coatings
been observed with any other implanted elements and no satisfactory explanation has been found. It may be directly related to the nature of the implanted element, e.g. chemical effects due to the important presence of MO in the layer or the ballistic effects characteristic of ion implantation creating defects and interactions between these defects and the elements. The position profiles of implanted MO and C overlap completely. The MO-C binding energy has been characterized and is different from the energy of the Mogd core level present after MO implantation. No particular binding state different from that observed in the 316L reference could be observed with the other metallic elements: Fe, Cr and Ni. For MO implantation, the major evolution consists of partial substitution and amorphization of the austenitic matrix. The cell parameter of the substitution solid solution is larger than that of austenite, because of the larger size of the MO atoms. We were able to estimate the proportion of MO necessary to yield this lattice parameter expansion using a simple hard-sphere model, assuming that the lattice distances vary linearly with the amount of implanted MO substitution. This hypothesis has been experimentally verified in most binary alloys in which C.C. or f.c.c. structures are mixed. From the 304L cell parameter (equivalent to 316L stainless steel without MO, JCPDF 34-396, a = 3.5911 A), we can define a mean metallic radius I’~ = 1.27 A, with all atoms touching in the principal cell diagonal of the hard-sphere model. From the (110) plane projection, the reticular distance d,,, is calculated to be 2.073 A, in agreement with the value of 2.075 A observed. If R is the proportion of molybdenum substitution, we have (T) = R*r,,+(l-R)*r, dlll=a
sin #
a=4(r)/2/2 dzzO= a’r2/4 where 4 is the angle between the (111) plane and the c axis, and rMo = 1.40 A for 12 coordination. Hence, with all values in angstroms dill =0.0021R+2.074 d,,,=O.O013R+
Technology
80 (1996)
49-52
tration are sources of error which can justify the difference noted with the XPS results. The observed partial amorphization reveals a strongly stressed state attributed to the ballistic effect of implantation. Compared with the 316L reference, the austenitic grains seem to be smaller. The grain size measured by TEM is smaller than 300 nm, which is about onehundredth of the size in the reference sample. Partial amorphized zones were observed. This is characteristic of long distance order loss. The alloy composition change (because of MO substitution) did not lead to an important y-ta transformation. Consequently, MO addition may have shifted the position of the alloy stability domain on the Shaeffler diagram appreciably. MO t C implantation may be distinguished from MO implantation by a much more pronounced general perturbation of the surface. We have shown the presence of a large amount of martensite or ferrite. The solid solution cell parameter increase is greater in this case. The ~-KY. transformation may be attributed to carbon implantation. Several hypotheses may be considered to explain this. First, molybdenum, an a-genie element, cannot be implicated because ferritic transformation after monoimplantation has never been observed. However, carbon, a y-genie element, is known to stabilize the austenitic phase when inserted and is therefore irrelevant. Secondly, this structural transformation may be attributed to a sufficiently severe stress, linked to carbon implantation. The interstitial position has been demonstrated by expan$on of the austenitic cell (rllll = 2.124 A and dZ2e= 1.293 A) which is smaller in the case of MO implantation. Stress induced by the large amount of carbon inserted in the austenitic matrix may cause efficient martensitic transformation. XPS analysis has shown the presence of an MO-C bond. There may be fine molybdenum carbide particles, not detectable by GIXD, which may induce a high stress level sufficient to permit partial y+cc transformation. Furthermore, the change in alloy composition certainly modifies and increases (taking into account the a-genie part of MO) the temperature M, of martensitic formation by a thermal effect and the temperature Md of martensitic formation by the effect of distortion. Thus the y+cc transition is facilitated as soon as implantation stress occurs.
1.270
The reticular distance value corresponding to the maximum of the shoulder is dill =2.106 A for an incidence angle cl=O.5”. The quantity of molybdenum in the solid solution is 15% at 25 nm depth, which is about as expected, and the percentage is the same as determined by energy dispersive X- ray (EDX) analysis with a 50 nm probe. The width and position of the centre of the shoulder corresponding to the gradient in MO concen-
References Cl1 G. Dearnaley, Nucl. Instrum. Methods B, 7/8 (1985) 158. VI R. Leutenecker, G. Wagner, T. Louis, V. Gonser, L. Guzman
and A. Mohari, Mater. Sci. Eng. A, 115 (1989) 229. 131 I.L. Singer, Appl. Sur& Sci., 18 (1984) 28. c41 D.M. Follstaedt, L.E. Pope, J.A. Knapp, S.T. Picraux and F.G. Yost, Thin Solid Films, 107 (1983) 259. c51 Y. Arnaud and M. Brunel, Appl. SurJ. Sci., 35 (1988) 345. C61 P. Marcus and M.E. BUSSEL, Appl. SurJ Sci., 59 (1992) 7.
Structural and chemical surface modifications of a stainless steel with implanted molybdenum and carbon ions X. de Buchere*, P. Andreazza, C. Andreazza-Vignolle, C. Clinard, R. Erre Centre de Recheyche SW la Mat&e Divide, Universit6 d’OrEans-CNRS,
45071 OdBans Cedex 2, France
Abstract
The surface transformation of stainless steel was investigated in the case of single- and multi-element ion implantation in order to improve the localized corrosion resistance. Molybdenum and carbon ions (MO alone and MO-C) were implanted into austenitic stainless steels (316L type). Several surface techniques (X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and grazing incidence X-ray diffraction (GIXD)) were used to determine the surface modifications, such as phase
transformation (y-ta), amorphization, precipitation and stable or metastable phase formation, e.g. substitution (for MO) or insertion (for C) solid solution. Keywords: Stainless steel; Molybdenum
implantation; XPS; TEM; GIXD
1. Introduction In the last 15 years, ion implantation [ 11 in metals and alloys has provided a new method for modifying surface properties. The effect of a number of species implanted in stainless steels on the corrosion, friction coefficient and wear rate has been investigated. A better understanding of the mechanisms and processes which dominate structural transformations can lead to better means of obtaining the required properties. In stainless steels, nitrogen has been most extensively studied because it is known to improve the electrochemical and tribological properties of the surface. Moreover, it constitutes a very interesting case because of the physical and chemical phenomena observed [a]. However, a few papers [3] have reported the effect of implanting other species, such as MO, Si, Ti, C, etc., in single- or multiimplantation. In this paper, we present the results of implanting molybdenum and carbon in stainless steel surfaces in order to improve the localized corrosion resistance. To investigate the process-inherent surface modifications [4], such as phase transformation (y-austenite to a’martensite), amorphization, precipitation and stable or metastable phase formation, e.g. substitution or insertion solid solution, we used several surface techniques, i.e. X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and grazing incidence X-ray diffraction (GIXD). Our purpose was to discuss the * Corresponding author.Tel.:38515368; fax: 38633796. 11717~8973/9h/‘X15
Ml 0 1995
Flswier
Srimw
8 A
All riohtq
rrwm=d
chemical composition and microstructural modification differences resulting from single-implantation of molybdenum and multi-implantation of molybdenum and carbon.
2. Experimental
procedures
The stainless steel used in this work was a biomedical type (316L) with the following mass per cent composition: Cr, 18; Ni, 12; MO, 2.5; C
X de Buchese et a//Surface
50
and Coatings
the passivation layer. X-Ray analysis (GIXD) was performed with a Philips parallel beam horizontal diffractometer. The GIXD technique allows the determination of the in-depth distribution of various crystalline phases by varying the penetration depth with the X-ray incidence angle [ 51. In our case, the incidence angle a: between the beam and the sampleOsurface varied from 0.2” to 1.5”, i.e. from 50 to 1000OA penetration depth using Cu Ko: radiation (1,= 1.54 A). TEM was carried out after electrochemical attack of the samples and the preparation of grids by replica methods.
3. Chemical and structural results The distribution of all the elements present in the top 1000 A of the surface was determined by XPS analysis. Profiles obtained after abrasion of the surface layers (Fig. 1) and the binding state of the elements detected [6] give information on the composition and nature of the species created during MO and MO + C implantation. In the two types of sample, the depth of the molybdenum pseudo-gaussian distribution is centred at about 300 A, in good agreement with that predicted by the LSS theoretical simulation (TRIM program). This atomic concentration distribution presents a maximum of 30% and a slightly asymmetrical shape, broader than that predicted by the simulation, an effect which could be correlated with the highly textured crystalline character of the steel. The oxide layer (estimated depth, approximately 20 A) contains a relatively important proportion of molybdenum (approximately 10 at.%) as MOO, and MOO,. The particularly important point is that MO substitution in the steel matrix does not induce preferential segregation of iron, chromium or nickel (Fig. 1(b)), but rather a depletion (15%) of the initially high chromium concentration (30% minimum), which is usually found in the passive surface films. This surface depletion of chromium was not observed after implantation of elements such as
Technology
80 (1996)
49-52
Si, Ti, N, etc. A few oxides, such as Fe304 and Cr203, constitute the major part of this surface layer. We can therefore subdivide it into0 an outer part composed of hydroxides (less than 10 A) and an inner part mostly composed of iron and chromium oxides. In addition, only very small quantities of nickel are detected, accumulated at the oxide-metal interface (Fig. 2). These results have allowed us to propose a qualitatively and quantitatively detailed layer model of the zone modified by implantation. It is presented in Fig. 2, which summarizes all the important information, Supplementary implantation of carbon does not seem to induce other modifications in the oxide layer. However, a difference between the binding state of MO in the MO and MO + C implantations is revealed in all implantation in-depth levels. A comparison of the Mead levels in the MO and MO + C implantations shows changes in the binding energy (E =227.7 eV against E = 227.9 eV) and the full width at half-maximum (FWHM) (FWHM= 1.15 eV against FWHM= 1.29 eV) characterizing metallic and carbide bindings. In terms of structural characterization, GIXD analysis seems to be particularly well adapted for the study of the microstructural modifications expected to affect the first 100 A layer. Transmission electron micrographs and diffraction patterns complete this analysis. A 316L stainless steel sample without any implantation is used as a reference. The corresponding diffractograms for different incidence angles CIshow that only y-austenite is present (main lines at 20rr1=43.58”, 20,,,,= 50.79”, 2&, = 74.70”) with an orientation effect assigned to the previous mechanical Ohistory of the sample. The cell parameter is a = 3.598 A. In the MO implantation diffractograms shown in Fig. 3, the austenitic phase is always detected in very important quantities. However, the presence of a small
hydroxide lnver rich iron oxide lnyer m
70
FeO al
Fe304
@
C&,
metallic lnw- : I-;=I
0
300
depth
bulk
Fe 70 % Cr 17% Ni 10%
molybdenum (binding MO-C for MotC implantation)
600
(in A)
Fig. 1. XPS sputter depth profiles of an Mo-implanted stainless steel: (a) all detected elements; (b) steel matrix elements (Fe, Ni and Cr).
Fig. 2. Layer model of the surface zone modified by MO and MO + C implantations.
X de Buchere et al.lSttrfaee and Coatings Technology 80 (1996) 49-52
51
8, Fig. 3. Grazing incidence X-ray stainless steel at different angles to 50, 250 and 1000 A. “J-SS is diffracted intensity on logarithmic
diffractograms of Mo-implanted (OZ’, 0.5” and 1.5”) corresponding the austenite solid solution (X-ray scale).
peak at 2%=44.30”, more important at a =0.5” than at other incidence angles, agrees with the appearance of a structural change produced by ion implantation. TEM supplies more local information: austenitic and martensitic grains are observed by selected area diffraction, distinguished by their (211),* and (loo),* diffraction patterns (Figs. 4(b) and 4(c)). Moreover, some preferential orientation (110)” is observed as in the reference sample, but with grain sizes varying from 10 to 300 nm (Fig. 4(a)). In addition, Fig. 3 shows an important shoulder at small 26 angles on the (111) and (200) main lines of austenite, corresponding to cell expansion produced by substitution of implanted molybdenum, since the MO atomic radius is slightly larger than that of Fe, Cr or Ni. A hard-sphere model will be developed later to explain the evolution of the reticular distance in austenite as a function of the atomic percentage of MO. A very broad base appears on the (11 l), main line at
Fig. 5. Grazing incidence X- ray difkactograms of MO + C-implanted stainless steel at the same angles and with the same notation as in Fig. 3.
0.5” incidence, coinciding with the molybdenum implantation profile. This structural characteristic is ascribed to an amorphization effect due to implantation and corresponds to the disappearance of austenitic long distance order. Finally, no precipitates were detected in any diffractograms; however, in the diffraction patterns, we observed two diffuse rings characteristic of iron oxide, and the presence of precipitates was observed by TEM, sometimes localized at the grain boundaries. Their size was determined by dark field micrographs and is less than 100 nm. Some were identified as CrZ3C6, but the analysis of the others is not easy because of their lack of stability under the electron beam. The MO + C implantation diffractograms (Fig. 5) show a complex shape around the (111) austenitic main Iine and a partial amorphization similar to that observed with MO-implanted samples, which seems to be independent of carbon implantation. However, the shift of the shoulders previously observed on the austenitic main lines, characteristic of substitution during the formation of the solid solution, is larger (20 = 42.53”); it represents the effect of the supplementary insertion of carbon which increases the austenitic cell parameter. The distinguishing feature between the two types of implantation is the important role of martensite or ferrite formation. The two principal lines of this phase are (110) and (200), appearing respectively at 28=44.00” and 63.70”. The relative intensities indicate that it is at least as abundant as austenite, indicating a very important structural evolution.
4. Discussion Fig. 4. (a) Bright field transmission denum-implanted sample showing Two diffraction patterns: (b) [211] (c) [ lOO] zone axis of a martensitic
electron micrograph of a molyba grain size smaller than 300 nm. zone axis of an austenitic grain; grain.
MO implantation does not induce any particular segregation of constituent elements (Fe, Cr and Ni) in the metallic part. However, a loss of chromium is observed in the surface oxide Iayer. This phenomenon has not
52
X de Buchere
et al./Surface
and Coatings
been observed with any other implanted elements and no satisfactory explanation has been found. It may be directly related to the nature of the implanted element, e.g. chemical effects due to the important presence of MO in the layer or the ballistic effects characteristic of ion implantation creating defects and interactions between these defects and the elements. The position profiles of implanted MO and C overlap completely. The MO-C binding energy has been characterized and is different from the energy of the Mogd core level present after MO implantation. No particular binding state different from that observed in the 316L reference could be observed with the other metallic elements: Fe, Cr and Ni. For MO implantation, the major evolution consists of partial substitution and amorphization of the austenitic matrix. The cell parameter of the substitution solid solution is larger than that of austenite, because of the larger size of the MO atoms. We were able to estimate the proportion of MO necessary to yield this lattice parameter expansion using a simple hard-sphere model, assuming that the lattice distances vary linearly with the amount of implanted MO substitution. This hypothesis has been experimentally verified in most binary alloys in which C.C. or f.c.c. structures are mixed. From the 304L cell parameter (equivalent to 316L stainless steel without MO, JCPDF 34-396, a = 3.5911 A), we can define a mean metallic radius I’~ = 1.27 A, with all atoms touching in the principal cell diagonal of the hard-sphere model. From the (110) plane projection, the reticular distance d,,, is calculated to be 2.073 A, in agreement with the value of 2.075 A observed. If R is the proportion of molybdenum substitution, we have (T) = R*r,,+(l-R)*r, dlll=a
sin #
a=4(r)/2/2 dzzO= a’r2/4 where 4 is the angle between the (111) plane and the c axis, and rMo = 1.40 A for 12 coordination. Hence, with all values in angstroms dill =0.0021R+2.074 d,,,=O.O013R+
Technology
80 (1996)
49-52
tration are sources of error which can justify the difference noted with the XPS results. The observed partial amorphization reveals a strongly stressed state attributed to the ballistic effect of implantation. Compared with the 316L reference, the austenitic grains seem to be smaller. The grain size measured by TEM is smaller than 300 nm, which is about onehundredth of the size in the reference sample. Partial amorphized zones were observed. This is characteristic of long distance order loss. The alloy composition change (because of MO substitution) did not lead to an important y-ta transformation. Consequently, MO addition may have shifted the position of the alloy stability domain on the Shaeffler diagram appreciably. MO t C implantation may be distinguished from MO implantation by a much more pronounced general perturbation of the surface. We have shown the presence of a large amount of martensite or ferrite. The solid solution cell parameter increase is greater in this case. The ~-KY. transformation may be attributed to carbon implantation. Several hypotheses may be considered to explain this. First, molybdenum, an a-genie element, cannot be implicated because ferritic transformation after monoimplantation has never been observed. However, carbon, a y-genie element, is known to stabilize the austenitic phase when inserted and is therefore irrelevant. Secondly, this structural transformation may be attributed to a sufficiently severe stress, linked to carbon implantation. The interstitial position has been demonstrated by expan$on of the austenitic cell (rllll = 2.124 A and dZ2e= 1.293 A) which is smaller in the case of MO implantation. Stress induced by the large amount of carbon inserted in the austenitic matrix may cause efficient martensitic transformation. XPS analysis has shown the presence of an MO-C bond. There may be fine molybdenum carbide particles, not detectable by GIXD, which may induce a high stress level sufficient to permit partial y+cc transformation. Furthermore, the change in alloy composition certainly modifies and increases (taking into account the a-genie part of MO) the temperature M, of martensitic formation by a thermal effect and the temperature Md of martensitic formation by the effect of distortion. Thus the y+cc transition is facilitated as soon as implantation stress occurs.
1.270
The reticular distance value corresponding to the maximum of the shoulder is dill =2.106 A for an incidence angle cl=O.5”. The quantity of molybdenum in the solid solution is 15% at 25 nm depth, which is about as expected, and the percentage is the same as determined by energy dispersive X- ray (EDX) analysis with a 50 nm probe. The width and position of the centre of the shoulder corresponding to the gradient in MO concen-
References Cl1 G. Dearnaley, Nucl. Instrum. Methods B, 7/8 (1985) 158. VI R. Leutenecker, G. Wagner, T. Louis, V. Gonser, L. Guzman
and A. Mohari, Mater. Sci. Eng. A, 115 (1989) 229. 131 I.L. Singer, Appl. Sur& Sci., 18 (1984) 28. c41 D.M. Follstaedt, L.E. Pope, J.A. Knapp, S.T. Picraux and F.G. Yost, Thin Solid Films, 107 (1983) 259. c51 Y. Arnaud and M. Brunel, Appl. SurJ. Sci., 35 (1988) 345. C61 P. Marcus and M.E. BUSSEL, Appl. SurJ Sci., 59 (1992) 7.