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Carbon layers formed on steel and Ti alloys after ion implantation of C+ at very high doses
PERGAMON
Vacuum 52 (1999) 141±146
Carbon layers formed on steel and Ti alloys after ion implantation of C + at very high doses J.L. Viviente *, A. Garcia, A. Loinaz, F. Alonso, J.I. OnÄate INASMET; Camino de Portuetxe 12, 20009 San Sebastian, Spain
Abstract Ion implantation is a useful technique to tailor surface properties of steel and Ti alloys. In particular, very high dose C + implantation (in the range of 1018 ions cm ÿ 2) oers the possibility of forming carbon layers without a sharp interface with the substrate material. In this study, ion implantation of carbon doses up to 8 1018 ions cm ÿ 2 has been performed on 440C martensitic stainless steel and Ti6Al4V substrates under similar conditions and tribological and surface analysis results have been compared. Surface hardening occurred for all ion implantation conditions up to doses of 1018 ions cm ÿ 2 [1±3]. Higher doses resulted in a dierent behaviour for both materials. The stainless steel showed a softening while a twofold hardness increase was maintained in the Ti alloy. Nevertheless, at the higher implanted dose a decrease in hardness was also observed in the Ti alloy. Small area XPS analyses were performed to evaluate the chemical states after ion implantation and establish a relationship with the observed surface hardening. Depth pro®le XPS analyses showed that for a dose of 4 1018 ions cm ÿ 2 a carbon layer (with concentration over 85% at. C) was formed in the near surface region for both materials. # 1998 Elsevier Science Ltd. All rights reserved.
1. Introduction Ion implantation is a useful method of modifying and improving the mechanical and chemical surface properties of metals. The implantation of interstitial atoms, such as carbon, nitrogen or boron, can modify the surface properties of selected metals like steels [4], titanium alloys [5], or aluminium alloys [6]. Several authors have reported on the eect of carbon implantation at very high doses into selected steels and titanium alloys: Ti6Al4V [7], AISI D2 [8], AISI 52100 [9], AISI 430 [10] or AISI 440C [3]. Under these implantation conditions, the high amount of carbon in the implanted layers favours the formation of C±C chemical bonds which at certain depths below the surface can represent more than the 80% of the chemical states of carbon [3]. This results in the formation of a carbonaceous layer on the surface of the implanted material, which can in¯uence surface properties, for instance, reducing signi®cantly wear and friction [3, 7± 9]. However, a deleterious increase in surface rough* Correspopnding author.
ness and an impairment of mechanical properties has also been observed at very high doses. In the present work, high dose implantation of C + has been carried out on AISI 440C martensitic stainless steel and Ti6Al4V alloy, with doses ranging from 1018 up to 8 1018 ions cm ÿ 2. Mechanical properties have been evaluated and Small Area X-ray Photoelectron Spectroscopy (SAXPS) combined with an Ar + sputtering has been performed to evaluate chemical states of surface carbonaceous layers formed after ion implantation.
2. Experimental procedure The samples used were disc shaped samples of 15 mm diameter and of 5 mm thickness of AISI 440C steel and Ti6Al4V alloy, that were machined from bar materials. The 440C steel was hardened and tempered to a ®nal hardness of 582 1 HRc. Before ion implantation all samples were surface ground and mirror polished to a ®nal roughness ®nish better than 0.02 mm Ra. The discs were ultrasonically cleaned and subsequently implanted with 12C + ions at 75 KeV in a
0042-207X/98/$ - see front matter # 1998 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 9 8 ) 0 0 2 1 3 - 9
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between 3.6 and 13.3 mA cm ÿ 2, and the temperature was kept below 1708C to avoid undue softening of the substrate material. The Ti6Al4V samples were also implanted at three dierent nominal doses: 1018, 4 1018 and 8 1018 ions cm ÿ 2, with ion current densities ranging between 8 and 90 mA cm ÿ 2, and the temperature was kept below 3138C. Implantations were always carried out at normal incidence with a vacuum in the chamber prior to implantation less than 10 ÿ 5 mbar. Composition and chemical states depth dependence of implanted samples were analysed by SAXPS (Small Area X-ray Photoelectron Spectroscopy) combined with 4 keV Ar + sputtering. The analyses were performed in a Microlab MKII VG spectrometer with Al Ka X-ray radiation at 34 mA and 13 kV under a pressure of less than 2 10 ÿ 9 mbar (without Ar). The analysis condition to acquire survey and detailed spectra (Ti 2p, C 1s, Fe 2p, Cr 2p and O 1s) as well as atomic fraction calculation and ®tting procedure to deconvulate the C 1s spectra have been described elsewhere [3, 7]. The theoretical depth distribution of implanted carbon and the sputtering yield during carbon implantation were obtained using the Pro®le Code2 simulation program [11]. Dynamic microhardness experiments were carried out in a Ficherscope H100 apparatus with a load range of 0.4 to 10 mN. The hardness ratio of implanted vs. unimplanted samples was calculated according to the procedure described formerly [12]. 3. Results and their discussion 3.1. Titanium alloy implanted samples
Fig. 1. SEM micrographs of chemically etched cross-sections of high dose C + implanted Ti6A14 V alloy at (a) 1018 C + cm ÿ 2, (b) 4 1018 C + cm ÿ 2 (c) 8 1018 cm ÿ 2.
Danfysik 1090 High-Current ion implanter system. The AISI 440C samples were implanted at three dierent nominal doses: 1018, 2 1018 and 4 1018 ions cm ÿ 2, with ion current densities ranging
After implantation, the samples treated with the lowest dose showed little visible change when compared with the unimplanted material. When the implantation reached 4 or 8 1018 C + cm ÿ 2, the samples began to reveal a colour change towards a blue dark appearance. SEM analyses showed that the surface of the samples implanted at the lowest doses were smooth and had the same morphology as the unimplanted material. Nevertheless, the samples implanted at 8 1018 ions cm ÿ 2 revealed a completely dierent morphology with a micro-cracked structure covering all the implanted surface. Non-standard surface roughness measurements carried out after implantation with an optical pro®lometer showed a slight increase in surface roughness from 0.01 to 0.05 mm Ra on samples implanted at 4 1018 ions cm ÿ 2. However, surface roughness severely increased up to 0.6 mm for samples implanted at 8 1018 ions cm ÿ 2. A pro®lometry scan done onto a partially masked sample implanted at 4 1018 C + cm ÿ 2 showed that the im-
J.L. Viviente et al. / Vacuum 52 (1999) 141±146
plantation increased 0.16 mm the height from the original surface of the sample, clearly indicating outward growth of a surface layer. Furthermore, SEM micrographs of chemically etched cross-sections showed that well de®ned surface layers are formed in the titanium alloy and a good intermixing is observed between the substrate and these layers [Fig. 1(aÐc)]. Thickness of these layers are 0.17, 0.56 and 0.86 mm (as measured in the micrographs) for the samples implanted at 1018, 4 1018 and 8 1018 C + cm ÿ 2, respectively. A typical XPS depth pro®le of the Ti6Al4V alloy material implanted at the highest dose (8 1018 C + cm ÿ 2) is shown in Fig. 2. The carbon concentration on this sample showed a trapezoidal distribution with an almost pure carbon zone. The analysis showed the presence of titanium at the top surface layer but a lower external oxide layer than in the lowest implanted dose. The carbon concentration at the surface was high (around 58% at.) and increased until an almost pure carbon layer which has a maximum carbon content between 99.7% and 95% (sputtering time: 167 min±563 min). Furthermore, a broadened interface between the carbon layers and the titanium alloy were clearly visible, indicating a mixed titanium alloy-carbon zone. Specimens implanted at lower doses showed gaussian-like carbon distribution with a maximum of around 53% and 89.2% at. for samples implanted at 1018 and 4 1018 C + cm ÿ 2, respectively. Deconvolution of the C 1s spectra acquired through
143
the pro®le, indicated that the carbon layer observed in Fig. 2 is formed by a pure C±C or graphitic bonds. In addition, a carbidic contribution was observed at the surface and at the end of the implanted zone. Samples implanted with the lowest dose (1018 C + cm ÿ 2) showed a high carbidic contribution through all the pro®le with a minor C±C contribution. The carbonaceous contribution to the implanted zone increases as the implanted dose is increased until a pure C±C layer is formed. The carbon atoms combine to form carbidic compounds (TiC and carbon-enriched carbides [13]) until a net carbon concentration of about 32.4% at. is achieved; for higher C concentrations the proportion of C±C bonds increases and results in the outward growth of a pure carbonaceous layer at the highest implanted doses. These results agree with those obtained by other workers on Ti implanted samples [13] or steels [8, 10, 14, 15], where the outward growth of a carbonaceous layer has been observed. The experimental depth pro®le of carbon exhibited an approximate gaussian shape corresponding to the theoretical pro®le for implanted doses up to 1018 C + cm ÿ 2. With increasing implanted dose, an outgrowth of a carbonaceous layer is observed. In this case it is dicult to compare experimental and theoretical results, as computer simulation using a ballistic approach does not allow the simulation of the outgrowth of a pure carbonaceous layer.
Fig. 2. XPS depth pro®le of the TigA14 V alloy implanted with carbon at the highest dose (75 keV, 8 1018 ions cm ÿ 2). Al and V pro®le not included.
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3.2. Steel implanted samples AISI 440C implanted samples showed also a colour change as the implanted dose increased. As in the case of the titanium alloy there was an increase in surface roughness due to sputtering (measured by optical pro®lometry, non-standard) from 0.02 to 0.06, 0.14 and 0.63 mm on samples implanted at 1018, 2 1018 and 4 1018 ions cm ÿ 2, respectively. A chemically etched cross-section of the steel sample implanted at 4 1018 C + cm ÿ 2 revealed also a well de®ned surface layer showing a good intermixing between the substrate and this layer. The thickness of the layer was 0.57 mm; similar to the thickness measured on the titanium alloy sample implanted at the same dose. A typical XPS depth pro®le of the AISI 440C steel sample implanted at the highest dose (4 1018 C + cm ÿ 2) is shown in Fig. 3. This analysis clearly shows that the carbon concentration increases gradually until a maximum value of about 89.9% at., remains constant and decreases again after some sputtering. The distribution shape is similar to a gaussian distribution (partially truncated). There is no drastic change in composition between the carbonaceous layer and the steel substrate, evidencing a gradual interface. The chemical state of the carbon, as shown by the C 1s spectra, is nearly 100% graphitic or C±C at the depth where the maximum concentration is measured, moving to a mixture of carbidic and graphitic bonds away from the maximum concentration (at low carbon concentrations a metallic bond is detected, for
instance, at the end of the pro®le). Samples implanted with the lowest doses showed a gaussian-like carbon distribution with maximums around 45.7% and 63.6% at. for samples implanted at 1018 and 2 1018 C + cm ÿ 2, respectively. Samples implanted at the 1018 C + cm ÿ 2 showed a higher carbidic contribution through all the pro®le with a C±C contribution around the averaged projected range. As in the titanium alloy, the carbonaceous contribution to the implanted zone increases as the implanted dose increased until a pure C±C layer is formed. Nevertheless, the carbidic contribution observed in the steel is lower than the carbidic contribution observed in the titanium alloy. The carbon atoms combine to form carbidic compounds until a net carbon concentration of about 23.1% at. is achieved; for higher C concentrations the proportion of C±C bonds increases and results in the outward growing of a pure carbonaceous layer at the highest implanted doses. The SEM analysis and XPS results agree well with the results published by Mikkelsen et al. [8] for an AISI D2 steel implanted with doses in the range of 0.5± 15 1018 C + cm ÿ 2, who reported the outward growing of pure amorphous carbon layers of about 0.5± 1 mm at the highest implanted doses. Follstaedt et al. [14] investigated also the in¯uence of carbon implantation on the structure of AISI 440C and other high Cr alloys. These workers found that steels with at least 12 at. % Cr or other carbide forming elements were amorphized by carbon implantation for C concentrations greater than approximately 20% and that
Fig. 3. XPS depth pro®le of the AISI 440 C stainless steel implanted with carbon at the highest dose (75 keV, 4 1018 ions cm ÿ 2).
J.L. Viviente et al. / Vacuum 52 (1999) 141±146
at lower C concentration crystalline E-carbides were formed. Similar results have been found by other authors when implanting into 52100 bearing steel [9]. A further investigation by TEM or GAXRD techniques would allow a detailed assignments of phases throughout the pro®le. 3.3. Dynamic microindentation Hardness results for high carbon implanted doses on Ti6Al4V alloy samples and AISI 440C stainless steel samples showed a dierent behaviour. The variation in hardness ratio vs. applied load showed a decrease in hardness in the near surface region of all the implanted AISI 440C samples [3]. This region would correspond to the depth of the layer created by the implantation treatment and XPS results explain this softening eect by the increase of the graphitic (C±C) contribution. However, as it is shown in Fig. 4, the implanted titanium alloy showed a clear increase in hardness for the samples implanted in the range 1018 to being maximum at 4 1018 C + cm ÿ 2, 2.6 1018 C + cm ÿ 2 implanted dose, where a hardness ratio of about 3.2 is achieved in the load range (0.4 to 2 mN). As the implanted dose increases the pro®les show a higher graphitic contribution and the hardness values, still higher than in the unimplanted samples, decrease. The titanium alloy implanted at the highest
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dose showed the lowest hardness increment; being nearly equal to that of the unimplanted samples in the ®rst load range and increasing by a factor of around 1.4 at higher loads. At higher loads the hardness value of the highest implanted dose showed a higher value than in the other implanted titanium alloys. This behaviour could be explained by the high carbidic contribution shown in the interface of the substrate and the carbonaceous layer which is higher than the carbidic contribution measured in the 1018 C + cm ÿ 2 implanted titanium alloy. Furthermore, a high increase in the elastic recovery was also measured and values of 17%, 23%, 42% and 50% were obtained for the unimplanted and implanted samples (1018, 4 1018 and 8 1018 C + cm ÿ 2, respectively). The increase in elastic recovery has to be attributed to the formation of a thicker hard structure as the implanted dose increases.
4. Conclusions The present results show the formation of a carbon layer after implantation of C + at very high doses. The major conclusion from this experimental study can be summarized as follows: 1. Ion implantation of C + ions at very high doses, up to 8 1018 C + cm ÿ 2, produces almost pure carbon
Fig. 4. Dynamic indentation curves for unimplanted and carbon implanted Ti6A14 V alloys.
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2.
3.
4.
5.
J.L. Viviente et al. / Vacuum 52 (1999) 141±146
layers, having thicknesses proportional to the ion dose and internal mixed interfaces in which a combination of carbidic and C±C bonds can be observed. Enriched substrate elements compounds (carbides, oxides) are observed at the surface of these carbon layers. Titanium alloy samples show a higher carbidic contribution than the AISI 440C steel samples implanted at the same C + doses, due to the greater anity of Ti to C. Critical carbon concentrations of about 32.4% at. and 23.1% at. are measured in Ti6Al4V alloy and AISI 440C steel samples, respectively. The carbon atoms combine to form carbidic compounds until these critical concentrations are reached; for higher C concentration the proportion of C±C bonds increases and results in the outward growth of pure carbonaceous layers at the highest implanted dose. Implantation of C + ions at very high doses, as expected, did not produce any hardening eect on AISI 440C steel. The graphitic content of these layers accounts for the loss in hardness at the surface. A signi®cant increase in hardness is observed in the implanted Ti6AlV alloy. The maximum hardness ratio (03.2) is obtained at 2.6 1018 C + cm ÿ 2. The increment in hardness has to be attributed to a hard structure created after ion implantation (e.g. by formation of carbidic phases). A further study with the aid of complementary surface analyses is needed for a detailed assignment of phases throughout the pro®le.
Acknowledgement This work has been carried out with the ®nancial support of the Spanish `ComisioÂn Interministerial de Ciencia y TecnologõÂ a' (CICYT) under Project Number MAT95-0980-C02-02.
References [1] Alonso F, Arizaga A, Quaiton S, Ugarte JJ, Viviente JL, OnÄate JI. Surf Coat Technol 1995;74-75:986. [2] Alonso F, Ugarte JJ, Sansom D, Viviente JL, OnÄate JI. Surf Coat Technol 1996;83:301. [3] Alonso F, Garcia A, Ugarte JJ, Viviente JL, OnÄate JI, Baranda PS, Cooper CV. Surf Coat Technol 1996;83:263. [4] Iwaki M. Mater Sci Eng 1987;90:263. [5] Lempert GD, Tsour A. Surf Coat Technol 1992;52:291. [6] Vardiman RG, Kant RA. J Appl Phys 1987;53(1):690. [7] Garcia A, Viviente JL, Alonso F, Loinaz A, OnÄate JI. Surf Coat Technol, in press. [8] Mikkelsen NJ, Eskildsen SS, Straede CA, Chechenin NG. Surf Coat Technol 1994;65:154. [9] Kobs K, Dimigen H, Denissen CJM, Gerritsen E, Politiek J, Oechsner R, Kluge A, Ryssel H. Nucl Instrum Methods Phys Res 1991;B59/60:746. [10] Fujihana T, Ueda Y, Takahashi K, Iwaki M. Surf Coat Technol 1994;66:441. [11] Pro®le Code2. Implant Sciences Corp., Wake®eld, MA, USA. [12] OnÄate JI, Alonso F, Viviente JL, Arizaga A. Surf Coat Technol 1994;65:165. [13] Fujihana T, Okabe Y, Iwaki M. Surf Coat Technol 1994;66:419. [14] Follstaedt DM, Knapp JA, Pope LE. J Appl Phys 1989;66:2743. [15] Straede CA, Poulsen JR, Lund BM, Sùrensen G. Mater Sci Eng 1991;A139:150.
Vacuum 52 (1999) 141±146
Carbon layers formed on steel and Ti alloys after ion implantation of C + at very high doses J.L. Viviente *, A. Garcia, A. Loinaz, F. Alonso, J.I. OnÄate INASMET; Camino de Portuetxe 12, 20009 San Sebastian, Spain
Abstract Ion implantation is a useful technique to tailor surface properties of steel and Ti alloys. In particular, very high dose C + implantation (in the range of 1018 ions cm ÿ 2) oers the possibility of forming carbon layers without a sharp interface with the substrate material. In this study, ion implantation of carbon doses up to 8 1018 ions cm ÿ 2 has been performed on 440C martensitic stainless steel and Ti6Al4V substrates under similar conditions and tribological and surface analysis results have been compared. Surface hardening occurred for all ion implantation conditions up to doses of 1018 ions cm ÿ 2 [1±3]. Higher doses resulted in a dierent behaviour for both materials. The stainless steel showed a softening while a twofold hardness increase was maintained in the Ti alloy. Nevertheless, at the higher implanted dose a decrease in hardness was also observed in the Ti alloy. Small area XPS analyses were performed to evaluate the chemical states after ion implantation and establish a relationship with the observed surface hardening. Depth pro®le XPS analyses showed that for a dose of 4 1018 ions cm ÿ 2 a carbon layer (with concentration over 85% at. C) was formed in the near surface region for both materials. # 1998 Elsevier Science Ltd. All rights reserved.
1. Introduction Ion implantation is a useful method of modifying and improving the mechanical and chemical surface properties of metals. The implantation of interstitial atoms, such as carbon, nitrogen or boron, can modify the surface properties of selected metals like steels [4], titanium alloys [5], or aluminium alloys [6]. Several authors have reported on the eect of carbon implantation at very high doses into selected steels and titanium alloys: Ti6Al4V [7], AISI D2 [8], AISI 52100 [9], AISI 430 [10] or AISI 440C [3]. Under these implantation conditions, the high amount of carbon in the implanted layers favours the formation of C±C chemical bonds which at certain depths below the surface can represent more than the 80% of the chemical states of carbon [3]. This results in the formation of a carbonaceous layer on the surface of the implanted material, which can in¯uence surface properties, for instance, reducing signi®cantly wear and friction [3, 7± 9]. However, a deleterious increase in surface rough* Correspopnding author.
ness and an impairment of mechanical properties has also been observed at very high doses. In the present work, high dose implantation of C + has been carried out on AISI 440C martensitic stainless steel and Ti6Al4V alloy, with doses ranging from 1018 up to 8 1018 ions cm ÿ 2. Mechanical properties have been evaluated and Small Area X-ray Photoelectron Spectroscopy (SAXPS) combined with an Ar + sputtering has been performed to evaluate chemical states of surface carbonaceous layers formed after ion implantation.
2. Experimental procedure The samples used were disc shaped samples of 15 mm diameter and of 5 mm thickness of AISI 440C steel and Ti6Al4V alloy, that were machined from bar materials. The 440C steel was hardened and tempered to a ®nal hardness of 582 1 HRc. Before ion implantation all samples were surface ground and mirror polished to a ®nal roughness ®nish better than 0.02 mm Ra. The discs were ultrasonically cleaned and subsequently implanted with 12C + ions at 75 KeV in a
0042-207X/98/$ - see front matter # 1998 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 9 8 ) 0 0 2 1 3 - 9
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J.L. Viviente et al. / Vacuum 52 (1999) 141±146
between 3.6 and 13.3 mA cm ÿ 2, and the temperature was kept below 1708C to avoid undue softening of the substrate material. The Ti6Al4V samples were also implanted at three dierent nominal doses: 1018, 4 1018 and 8 1018 ions cm ÿ 2, with ion current densities ranging between 8 and 90 mA cm ÿ 2, and the temperature was kept below 3138C. Implantations were always carried out at normal incidence with a vacuum in the chamber prior to implantation less than 10 ÿ 5 mbar. Composition and chemical states depth dependence of implanted samples were analysed by SAXPS (Small Area X-ray Photoelectron Spectroscopy) combined with 4 keV Ar + sputtering. The analyses were performed in a Microlab MKII VG spectrometer with Al Ka X-ray radiation at 34 mA and 13 kV under a pressure of less than 2 10 ÿ 9 mbar (without Ar). The analysis condition to acquire survey and detailed spectra (Ti 2p, C 1s, Fe 2p, Cr 2p and O 1s) as well as atomic fraction calculation and ®tting procedure to deconvulate the C 1s spectra have been described elsewhere [3, 7]. The theoretical depth distribution of implanted carbon and the sputtering yield during carbon implantation were obtained using the Pro®le Code2 simulation program [11]. Dynamic microhardness experiments were carried out in a Ficherscope H100 apparatus with a load range of 0.4 to 10 mN. The hardness ratio of implanted vs. unimplanted samples was calculated according to the procedure described formerly [12]. 3. Results and their discussion 3.1. Titanium alloy implanted samples
Fig. 1. SEM micrographs of chemically etched cross-sections of high dose C + implanted Ti6A14 V alloy at (a) 1018 C + cm ÿ 2, (b) 4 1018 C + cm ÿ 2 (c) 8 1018 cm ÿ 2.
Danfysik 1090 High-Current ion implanter system. The AISI 440C samples were implanted at three dierent nominal doses: 1018, 2 1018 and 4 1018 ions cm ÿ 2, with ion current densities ranging
After implantation, the samples treated with the lowest dose showed little visible change when compared with the unimplanted material. When the implantation reached 4 or 8 1018 C + cm ÿ 2, the samples began to reveal a colour change towards a blue dark appearance. SEM analyses showed that the surface of the samples implanted at the lowest doses were smooth and had the same morphology as the unimplanted material. Nevertheless, the samples implanted at 8 1018 ions cm ÿ 2 revealed a completely dierent morphology with a micro-cracked structure covering all the implanted surface. Non-standard surface roughness measurements carried out after implantation with an optical pro®lometer showed a slight increase in surface roughness from 0.01 to 0.05 mm Ra on samples implanted at 4 1018 ions cm ÿ 2. However, surface roughness severely increased up to 0.6 mm for samples implanted at 8 1018 ions cm ÿ 2. A pro®lometry scan done onto a partially masked sample implanted at 4 1018 C + cm ÿ 2 showed that the im-
J.L. Viviente et al. / Vacuum 52 (1999) 141±146
plantation increased 0.16 mm the height from the original surface of the sample, clearly indicating outward growth of a surface layer. Furthermore, SEM micrographs of chemically etched cross-sections showed that well de®ned surface layers are formed in the titanium alloy and a good intermixing is observed between the substrate and these layers [Fig. 1(aÐc)]. Thickness of these layers are 0.17, 0.56 and 0.86 mm (as measured in the micrographs) for the samples implanted at 1018, 4 1018 and 8 1018 C + cm ÿ 2, respectively. A typical XPS depth pro®le of the Ti6Al4V alloy material implanted at the highest dose (8 1018 C + cm ÿ 2) is shown in Fig. 2. The carbon concentration on this sample showed a trapezoidal distribution with an almost pure carbon zone. The analysis showed the presence of titanium at the top surface layer but a lower external oxide layer than in the lowest implanted dose. The carbon concentration at the surface was high (around 58% at.) and increased until an almost pure carbon layer which has a maximum carbon content between 99.7% and 95% (sputtering time: 167 min±563 min). Furthermore, a broadened interface between the carbon layers and the titanium alloy were clearly visible, indicating a mixed titanium alloy-carbon zone. Specimens implanted at lower doses showed gaussian-like carbon distribution with a maximum of around 53% and 89.2% at. for samples implanted at 1018 and 4 1018 C + cm ÿ 2, respectively. Deconvolution of the C 1s spectra acquired through
143
the pro®le, indicated that the carbon layer observed in Fig. 2 is formed by a pure C±C or graphitic bonds. In addition, a carbidic contribution was observed at the surface and at the end of the implanted zone. Samples implanted with the lowest dose (1018 C + cm ÿ 2) showed a high carbidic contribution through all the pro®le with a minor C±C contribution. The carbonaceous contribution to the implanted zone increases as the implanted dose is increased until a pure C±C layer is formed. The carbon atoms combine to form carbidic compounds (TiC and carbon-enriched carbides [13]) until a net carbon concentration of about 32.4% at. is achieved; for higher C concentrations the proportion of C±C bonds increases and results in the outward growth of a pure carbonaceous layer at the highest implanted doses. These results agree with those obtained by other workers on Ti implanted samples [13] or steels [8, 10, 14, 15], where the outward growth of a carbonaceous layer has been observed. The experimental depth pro®le of carbon exhibited an approximate gaussian shape corresponding to the theoretical pro®le for implanted doses up to 1018 C + cm ÿ 2. With increasing implanted dose, an outgrowth of a carbonaceous layer is observed. In this case it is dicult to compare experimental and theoretical results, as computer simulation using a ballistic approach does not allow the simulation of the outgrowth of a pure carbonaceous layer.
Fig. 2. XPS depth pro®le of the TigA14 V alloy implanted with carbon at the highest dose (75 keV, 8 1018 ions cm ÿ 2). Al and V pro®le not included.
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3.2. Steel implanted samples AISI 440C implanted samples showed also a colour change as the implanted dose increased. As in the case of the titanium alloy there was an increase in surface roughness due to sputtering (measured by optical pro®lometry, non-standard) from 0.02 to 0.06, 0.14 and 0.63 mm on samples implanted at 1018, 2 1018 and 4 1018 ions cm ÿ 2, respectively. A chemically etched cross-section of the steel sample implanted at 4 1018 C + cm ÿ 2 revealed also a well de®ned surface layer showing a good intermixing between the substrate and this layer. The thickness of the layer was 0.57 mm; similar to the thickness measured on the titanium alloy sample implanted at the same dose. A typical XPS depth pro®le of the AISI 440C steel sample implanted at the highest dose (4 1018 C + cm ÿ 2) is shown in Fig. 3. This analysis clearly shows that the carbon concentration increases gradually until a maximum value of about 89.9% at., remains constant and decreases again after some sputtering. The distribution shape is similar to a gaussian distribution (partially truncated). There is no drastic change in composition between the carbonaceous layer and the steel substrate, evidencing a gradual interface. The chemical state of the carbon, as shown by the C 1s spectra, is nearly 100% graphitic or C±C at the depth where the maximum concentration is measured, moving to a mixture of carbidic and graphitic bonds away from the maximum concentration (at low carbon concentrations a metallic bond is detected, for
instance, at the end of the pro®le). Samples implanted with the lowest doses showed a gaussian-like carbon distribution with maximums around 45.7% and 63.6% at. for samples implanted at 1018 and 2 1018 C + cm ÿ 2, respectively. Samples implanted at the 1018 C + cm ÿ 2 showed a higher carbidic contribution through all the pro®le with a C±C contribution around the averaged projected range. As in the titanium alloy, the carbonaceous contribution to the implanted zone increases as the implanted dose increased until a pure C±C layer is formed. Nevertheless, the carbidic contribution observed in the steel is lower than the carbidic contribution observed in the titanium alloy. The carbon atoms combine to form carbidic compounds until a net carbon concentration of about 23.1% at. is achieved; for higher C concentrations the proportion of C±C bonds increases and results in the outward growing of a pure carbonaceous layer at the highest implanted doses. The SEM analysis and XPS results agree well with the results published by Mikkelsen et al. [8] for an AISI D2 steel implanted with doses in the range of 0.5± 15 1018 C + cm ÿ 2, who reported the outward growing of pure amorphous carbon layers of about 0.5± 1 mm at the highest implanted doses. Follstaedt et al. [14] investigated also the in¯uence of carbon implantation on the structure of AISI 440C and other high Cr alloys. These workers found that steels with at least 12 at. % Cr or other carbide forming elements were amorphized by carbon implantation for C concentrations greater than approximately 20% and that
Fig. 3. XPS depth pro®le of the AISI 440 C stainless steel implanted with carbon at the highest dose (75 keV, 4 1018 ions cm ÿ 2).
J.L. Viviente et al. / Vacuum 52 (1999) 141±146
at lower C concentration crystalline E-carbides were formed. Similar results have been found by other authors when implanting into 52100 bearing steel [9]. A further investigation by TEM or GAXRD techniques would allow a detailed assignments of phases throughout the pro®le. 3.3. Dynamic microindentation Hardness results for high carbon implanted doses on Ti6Al4V alloy samples and AISI 440C stainless steel samples showed a dierent behaviour. The variation in hardness ratio vs. applied load showed a decrease in hardness in the near surface region of all the implanted AISI 440C samples [3]. This region would correspond to the depth of the layer created by the implantation treatment and XPS results explain this softening eect by the increase of the graphitic (C±C) contribution. However, as it is shown in Fig. 4, the implanted titanium alloy showed a clear increase in hardness for the samples implanted in the range 1018 to being maximum at 4 1018 C + cm ÿ 2, 2.6 1018 C + cm ÿ 2 implanted dose, where a hardness ratio of about 3.2 is achieved in the load range (0.4 to 2 mN). As the implanted dose increases the pro®les show a higher graphitic contribution and the hardness values, still higher than in the unimplanted samples, decrease. The titanium alloy implanted at the highest
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dose showed the lowest hardness increment; being nearly equal to that of the unimplanted samples in the ®rst load range and increasing by a factor of around 1.4 at higher loads. At higher loads the hardness value of the highest implanted dose showed a higher value than in the other implanted titanium alloys. This behaviour could be explained by the high carbidic contribution shown in the interface of the substrate and the carbonaceous layer which is higher than the carbidic contribution measured in the 1018 C + cm ÿ 2 implanted titanium alloy. Furthermore, a high increase in the elastic recovery was also measured and values of 17%, 23%, 42% and 50% were obtained for the unimplanted and implanted samples (1018, 4 1018 and 8 1018 C + cm ÿ 2, respectively). The increase in elastic recovery has to be attributed to the formation of a thicker hard structure as the implanted dose increases.
4. Conclusions The present results show the formation of a carbon layer after implantation of C + at very high doses. The major conclusion from this experimental study can be summarized as follows: 1. Ion implantation of C + ions at very high doses, up to 8 1018 C + cm ÿ 2, produces almost pure carbon
Fig. 4. Dynamic indentation curves for unimplanted and carbon implanted Ti6A14 V alloys.
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layers, having thicknesses proportional to the ion dose and internal mixed interfaces in which a combination of carbidic and C±C bonds can be observed. Enriched substrate elements compounds (carbides, oxides) are observed at the surface of these carbon layers. Titanium alloy samples show a higher carbidic contribution than the AISI 440C steel samples implanted at the same C + doses, due to the greater anity of Ti to C. Critical carbon concentrations of about 32.4% at. and 23.1% at. are measured in Ti6Al4V alloy and AISI 440C steel samples, respectively. The carbon atoms combine to form carbidic compounds until these critical concentrations are reached; for higher C concentration the proportion of C±C bonds increases and results in the outward growth of pure carbonaceous layers at the highest implanted dose. Implantation of C + ions at very high doses, as expected, did not produce any hardening eect on AISI 440C steel. The graphitic content of these layers accounts for the loss in hardness at the surface. A signi®cant increase in hardness is observed in the implanted Ti6AlV alloy. The maximum hardness ratio (03.2) is obtained at 2.6 1018 C + cm ÿ 2. The increment in hardness has to be attributed to a hard structure created after ion implantation (e.g. by formation of carbidic phases). A further study with the aid of complementary surface analyses is needed for a detailed assignment of phases throughout the pro®le.
Acknowledgement This work has been carried out with the ®nancial support of the Spanish `ComisioÂn Interministerial de Ciencia y TecnologõÂ a' (CICYT) under Project Number MAT95-0980-C02-02.
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