Surface and Coatings Technology, 66 (1994) 283—287
Tribological properties of ion-implanted high-chromium steel V. V. Uglov, V. V. Khodasevich, N. N. Cherenda, I. V. Kasko* and V. A. Kutsanov Byelorussian State University, Department of Solid State Physics, 220050 Minsk (Belarus)
Abstract Changes in the hardness, friction coefficient, and behavior of the element and phase compositions of ion-implanted high-Cr stamp steel XI2M have been investigated. The optimization of the implanted ion (Ti, Ta, W, C or Nb) and irradiation dose 2) has been conducted. The irradiation by Ti, C and Nb ions gave good results in changing the surface mechanical (1016_1017 ion cm properties during the implantation of Ti, Ta W. C and Nb ions (E=18 keY, D=7 x 1015 ion cm2). The friction coefficient decreased by 1.6,4 and 2 times respectively. The implantation by Nb ions (E= 18 keY, D= 1016_1017 ion cm2) led to an increase in microhardness by 1.6 times and to a decrease in friction coefficient by 5 times at an irradiation dose of 7 x 1016 ion cm2. A change in the Cr 7C3 phase volume fraction was the main cause of the changes in the microhardness and friction coefficient.
1. Introduction At present, ion implantation one of theinprospective methods of modifying material isproperties, particular those of construction materials such as steel [1, 2]. This method avoids the problems of adhesion of the modified layer to the substrate, changes in geometric dimensions during processing, and also other problems connected with the property modification of stamps with irregular shapes. In the present paper, the results of an investigation of the compositional and tribological properties of ionimplanted high-Cr stamp XI2M steel are presented.
2. Experimental details Samples of high-Cr stamp X12M steel (D2, USA; SKD11, Japan) have been investigated. The elemental composition of the steel was 1.5 wt.% C, 0.25 wt.% Si, 11.75 wt.% Cr, 0.5wt.% Mo and 0.25 wt.% V. Cr 7C3, Cr23C6 and Fe3C were the main phases in addition to Fe, owing to the high content of Cr and C. Cr carbides of high hardness contribute to2)the and steelhigher hardness of wear about 60 HRC (744 kg mm resistance. Before irradiation, the samples were subjected to the standard thermomechanical processing used in industry. The surface of these samples was then mirror finished with diamond sheets of grain size up to 1 jim. The irradiation was carried out with Ti, Ta, W, C and Nb ~Present address: Frauhofer Society, Institute of Integrated Circuits, Erlangen, Germany.
0257—8972/94/87.00 SSDI 0257-8972(94)07097-B
ions . The energy of the implanted ions was 18 keY, while the doses2.ofThe the density implantations varied from x iO’~ of the pulsating ion 7current to i0’~~ cm and theion frequency of ion impulses were 4 2 and 1.tA cm 300 Hz respectively. Tribological tests were conducted with a TAY-1 device. The features of the tests were as follows: reciprocating motion, sliding. The type of contact was “sphere— plane”. The indenter was made of alloy BK8 (92% WC, 8% Co). The load on the indenter was 0.2 and 1 N. The radius of the indenter curvature was 1 mm. The motion speed of the sample was 4.0 mm s1. A PMT-3 microhardmeter (Vickers microhardness) was used to measure the near-surface layer hardness. The load on the diamond indenter was 1 N. Investigation of the phase composition was carried out using X-ray diffraction (XRD) analysis with a DRON-2 diffractometer, using monochromatic Co Kct radiation. The analysis of the elemental composition was carried out using X-ray microanalysis (XMA). The concentration profiles of implanted and doped impurities were calculated using the program T-DYN , taking into account a change in the element composition during irradiation. 3. Results and discussion As a result of the steel elemental and phase composition differences, the optimization of the doped impurity effect to achieve a maximum improvement in properties is the main task of the steel modification. This is why ions such as Ti and C, as well as W and Nb, were used as special additions to the steel, because the implantation
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Trihologv of ion—implanted hi.gh—Cr steel
of these ions into the majority of stamp steels gives good results. The optimization of the properties was based on tribological tests. The results in the form of the relative friction coefficient (ratio of the friction coefficient of the implanted sample to the friction coefficient of the non-implanted sample) are given in Table 1. The implantation of Ti, C friction coefficient (pt) in comparision with the implantand Nb ions resulted in a considerable decrease in the ation of W and Ta ions. As C and Ti are well-known modifiers (when implanted), the same result for Nb was unexpected, because we could find no similar examples in the literature. This was the main reason for studying the effect of Nb ion implantation on the resistance and tribological properties of X12M steel. The steel samples were irradiated by Nb ions with an energy of 18 keY, within the dose range 1 0i6 I 0i ~ ion cm 2 The results of the measurements of the relative microhardness change during irradiation are given in Fig. 1. As can be seen, the plot reaches a maximum. The variation of the sample friction coefficients as a function of the indenter sliding distance 1, for different irradiation doses, is given in Fig. 2. As can be seen, the sample irradiated up to a dose of 7 x l0~ion cm2 has the longest break-in track. The corresponding change in the friction coefficient with the irradiation dose at 1= 1 m is given in Fig. 3. The maximum decrease in p value corresponds to a dose of 7 x 1016 ion cm2. In addition, the samples irradiated at doses of 1016 and l0’~ion cm2 exhibited occasional changes in p during the sliding distance intervals 1-2 m
Fig. 2. Friction coefficient p dependence on the sliding distance passed by the indenter under a load of 1.0 N for non-irradiated sample ( ioncrn2 A), 7x 1016 ioncm2(A)and 1017 ioncm 22 (V). 4 x 1016 and samples irradiated with Nb at doses of l0’~ion ciii
TABLE I. Relativefriction coefficient of XI2M steel samples irradiated by different ions with an energy of 18 keY, at a dose of 7 x l0~ions cm2
Fig. 3. Dose dependence of the relative friction coefficient during irradiation with Nb ions.
(lO’6ioncm2) and 4—6m (l017ioncm2) of sliding distance. However, these changes did not exceed the limits of measurement error (these changes will be called ~osci1lations”). This is why the load on the indenter was
Indenter sliding distance. 2 m: load. 1.0 N.
decreased to 0.2 N to diminish local temperatures during the contact of the friction pair, and reduce tension during 2 friction. The p dependence on the sliding distance passed by the indenter at an irradiation dose of 1017 ion cm is given in Fig. 4. Here, the occasional changes in p exceed the limits of measurement error for the given
0 1 .7 1 .5 1 .5
0,1016 ions .cm~
Fig. I. Dose dependence of relative microhardness of XI2M steel samples irradiated with Nb ions.
load, which shows the presence of oscillations for an interval of 1=7—b m. Elemental (XMA) and phase (XRD) analyses of the results obtained and also a calculation of the concentration profiles of doped and implanted impurities under the program T-DYN were performed. As Cr is the main impurity in the steel, the distribution of Cr in the near-
V V Uglov et al. / Tribology of ion-implanted high-Cr steel
800 N, cycloc
Fig. 4. Friction coefficient p dependence of sliding the sample irradiated with 2 on the distance passed by Nb ions at a dose of 1017 ion cm the indenter under a load of 0.2 N.
I U IV .1 I
surface region was of interest. Peaks obtained by the XMA (Fig. 5) are caused by the Cr-containing phases. If the Cr content in the calculated depth of X-ray radiation generation does not change (Fig. 6), then the distribution along the surface is characterized by the appearance of new small peaks at a dose of 7 x 10~~ ion cm2 (Fig. 5(b)), with these peaks not apparent for doses of 4 x 1016 ion cm2 (Fig. 5(a)) and 1017 ion cm2 (Fig. 5(c)). This appearance of additional Cr-containing phases (small peaks) can cause a further decrease inp . The analysis of the Cr distribution inside and outside the wear track of the sample irradiated at a dose of 1017 ion cm2 was carried out to reveal the cause of oscillation. The investigation showed the presence of new small peaks of Cr-containing compounds on the surface of the wear track (Fig. 7(a)) in comparison with the surface of the sample outside the track (Fig. 7(b)). It is possible that this is connected with the wear products in the track being formed during friction. It has been shown that the ~ value, which is proportional to the concentration of the element being analysed and is defined as the ratio of the intensity of the characteristic X-ray radiation of an element in the sample to the intensity of the characteristic X-ray radiation of the standard of this element, is 1.2 times higher inside the wear track than outside it. One can say that the increased content of Cr in the wear track is caused by wear products partially remaining in the track during friction. The wear products increase the contact area between the indenter and sample, resulting in increases in the friction coefficients at doses of 1016 ion cm2 and 10~ion cm2, and to greater sample wear (abrasive wear). Abrasive wear may be one reason for the oscillations of p at these doses, because of the changing contact area between the indenter and sample. The phase analysis of the initial sample showed the
Fig. 5. Distribution of the characteristic X-ray radiation of Cr Ka along the surface of the samples irradiated with Nb ions at doses of (a)4xloi6ioncm_2,(b)7xloi6ioncm2 and(c) 10171oncm2.
a o 20
D,1c/6 ions .cm2
Fig. 6. Dose dependence of Cr concentration ~ (ratio of the intensity of the characteristic X-ray radiation of element i of the sample to the intensity of the characteristic X-ray radiation of the standard of this element).
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Fig. 5. Dose dependence of the relative intensity of the X—ray lines I ratio of the intensity of the X—ray line to the intensity of tile Fe (d = 0.202 68 nm, l00°/o) line): (a) Cr, 5C,,. J=0. 108 nm: I hi Fe,C, il= 0.206 nrn: Ic) 11=0.201 nm: (dl i/=0.210 nm; )e) Cr~C).d=0. 127 nm: If)
___________________________________________ hi 100
Fig. 7. Distribution of the characteristic X-ray radiation of Cr K-i along the surface of the samples irradiated with Nb ions at a dose (If 101 ion cm —. measured (a) in the wear track and (hi outside the wear track
existence of the same phases (Cr7C3, Cr23C6, Fe3C) which are described in the literature. After the irradiation, the phase composition remained unchanged. The dose dependences of the intensities of the separate X-ray lines of the phases Fe3C, Cr7C3, Cr53C6, are shown in Fig. 8. The intensity of the X-ray lines of some phases in the multiphase composition is proportional to the volume fraction given factors phase (i) the mixture, in addition (V1) to allofthethe intensity .inThus, we observe a change in V~during irradiation. Because the dimensions in the area probed on the surface layer by the XRD method in Bragg—Brentano geometry are of the order of micrometers, they essentially exceed R~=7.2nm . Such behavior of the phases perhaps results from the effect of elastic waves [1, 8] on the substructural constituents of X12M (i.e. the effect of the elastic waves, generated by ions, on the defect structure of the polycrystalline materials at a large depth).
8, it is clear that an increase in V~at a dose
10i6 iOfl C~ ~ is characteristic for all the phases in comparison with V~for the initial sample. This would appear to be connected with additional precipitation of Cr7C5, Cr23C6 and Fe3C at the expense of the redistribution of C and Cr (which are a part of the composition of the irradiated steel martensite). However, further behavior of the intensity of these phase lines essentially differs. For the Cr7C3 phase, which is responsible for the wear resistance of X12M steel , the intensity goes
through the maximum at a dose of 7 x lO~~ ion cm similarly to the dose dependence of the relative microhardness (Fig. 1). The intensities of the Fe3C and Cr3C6 lines decrease with increasing dose. This dose dependence of the intensity of Cr7C X-ray .
lines (Fig. 8)is well correlated with the dose dependences of the relative microhardness (Fig. 1) and friction coefficient (Fig. 3). Therefore, the change in the volume fraction content of the Cr7C3 phase is the main reason for the changes in the microhardness and friction coefficient during irradiation. The phase analysis indicates that there is a large quantity of Cr in the carbides, so the results of the Cr distribution along the surface (Fig. 7) can be attributed to the distribution of Cr carbides, i.e. Cr7C3 Cr23C6. 2)and is 1.5 times The hardness of Cr7C3 (Hv= 1600 kg mm higher than that of Cr, 2). This 1100 kg mmof Cr is why we can assume 3C6 that (Hv= the destruction 23C1, rather than Cr7C takes place during friction, and abrasive wear is mainly caused by the particles of this phase during dry friction. Thus, the wear products (small peaks in Fig. 7(a)) belong to the main phase, i.e. Cr2C1,. The processes connected with the formation of Fe—Nb solid solutions, blocking, stopping of dislocations of induced and doped impurities, radiation defects, and Cr7C3, Cr7 3C6 and Fe3C precipitates-—and also with the formation of dispersion-hardened structures, such as
V V Uglov et al. C,
Tribology of ion-implanted high-Cr steel
(1) During the implantation of Ti, Ta, W, C and Nb ions at a dose of 7 x iO’~ion cm2 with an energy of 30~ 20
18 keY X12Mthe stamp steel,coefficient. irradiationThe by respective Ti, C and Nb ionsinto improve friction
decreases 2 times. (2) During in the thefriction implantation coefficient of Nb are by ions1.6,with 4 and an
Fig. 9. Distribution of (a) Fe (scale 0.5), (b) Nb and (c) Cr by depth during the implantation of Nb ions with an energy 18 keY at a dose of i0~ion cm2 (calculation using T-DYN).
C, at.% 12 lo
energy of 18 keY in the dose interval 2, 10t6~~10i7 ion cm a maximum increase in the microhardness by 1.5 times and a maximum decrease in the friction coefficient by 5 times were revealed at an irradiation dose of 7 x 1016 ion cm2. A change in the volume fraction content of Cr 7C3 is the main reason for the change in the microhardness. (3) During the implantation of Nb ions with an 2, energy of 18 keY at doses of 1016 and io’~ ion cm oscillations of the friction coefficient were observed, resulting from abrasive wear.
The authors are grateful to J. P. Biersack for calculaAcknowledgment tions using the T-DYN program.
Fig. 10. Distribution of C by depth during the implantation of Nb ions with an energy of 18 keY at doses of (a) 2 and (b) 10~ion cm2 (calculation using T-DYN), and (c)10i6produced ion cm vacancies (calculation using TRiM-88).
References 1 F. F. Komarov, Ion Implantations of Metals, Metallurgy, Moscow, 1990, p. 216. 2 J. M. Poate, G. Foti and D. C. Jacobson, Surface Modification and
NbC and Fe 2Nb, contribute to the change in the microhardness. The concentration profiles of implanted and doped impurities 2, were calculated doses The of 1016 using the T-DYNforprogram. resultsand of 10’~ ion cm the calculation are given in Figs. 9 (Nb, Fe and Cr) and 10 (C). In the surface layer, the necessary stoichiometry is created for the formation of Fe 2Nb (Fig. 9). According to the XRD data, Nb is probably in a solid solution. The calculations revealed enrichment of the surface layer by C (Fig. 10). The maximum in the distribution of vacancies produced during implantation (the calculation was made under the TRIM-88 program) coincides with the maximum in the C distribution (Fig. 10).
Alloying by Laser, Ion and Electron Beams, Plenum, New York, 1983, p. 424. 3 Y. A. Geller, Instrument Steels, Metallurgy, Moscow, 1983, p. 544.
4 A. M. Mazurkevich, Y. Y. Khodasevich, V. Y. Uglov, Y. A. Kutsanov, A. Serov, Y. Y. Pankratov, Vac.G.Tech. Technol., 1(1988)18.A. G. Kobyak and Y. V. Bobkov, ~ j, P. Biersack, S. Berg and C. Nender, NucI. Instrum. Methods B, 59—60 (1991) 21. 6 Y. S. Umansky, Y. A. Skakov, A. N. Ivanov and L. N. Rastorguev, Crystallography, Roentgenography and Electron microscopy, Metallurgy, Moscow, 1982, p. 632. 7 A. F. Burenkov, F. F. Komarov, M. A. Kumakhov and M. M. Tsikin, Tables of Spatial Distribution of ion-implanted Impurities, Byelorussian State University, Minsk, 1980, p. 352. 8 Y. A. Semin, The amplification of ion bombardment generated elastic waves during spreading in crystal with defect clusters, JTF Lett., 14(3) (1988) 273.