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High current density ion implantation processing of 440C steel Part 1: Implantation parameters and rolling contact fatigue improvement
Surface
and Coatings
Technology,
37 (1989)
1
1
- 13
HIGH CURRENT DENSITY ION IMPLANTATION PROCESSING OF 440C STEEL PART 1: IMPLANTATION PARAMETERS AND ROLLING CONTACT FATIGUE IMPROVEMENT F. M. KUSTAS,
M. S. MISRA
Martin
Astronautics
Marietta
and S. R. SMITH Group,
Denver,
CO (U.S.A.)
P. J. WILBUR Department (U.S.A.)
of Mechanical
Engineering,
Colorado
State
University,
Fort
Collins,
CO
R. L. THOM NASA
Marshall
(Received
Space
August
Flight
Center,
Huntsville,
AL
(U.S.A.)
3, 1987)
High alloy steels, such as 440C steel, are commonly used for high speed, high load turbopump bearing applications. The low fatigue lifetimes of this steel have been attributed to cracking of the large Fe-Cr carbide particles and subsequent spalling. Surface modification by high current density (107 PA cm-‘) ion implantation processing with nitrogen (N2+) was performed to assess the effects of ion implantation on rolling contact fatigue performance. Process parameter studies showed that implantation using a current density of 107 /..LAcme2 prevented excessive thermal exposure and substrate tempering. Rolling contact fatigue tests were performed under lubricated conditions at two different stress levels: 4.04 GPa (586 klbf in2) and 5.42 GPa (786 klbf inM2)hertzian stress. Test results showed improvements in the rolling contact fatigue lifetimes for the N,+-implanted specimens compared with those for unimplanted 440C steel.
1. Introduction Ion implantation is a proven technique to increase the tribological properties of engineering alloys. Significant improvements in wear resistance [l] and fatigue resistance [2, 31 have been reported for ion-implanted low alloy steels. For the severe application of pure rolling, common for high speed, high load turbopump bearings, inconsistent test results have been reported for ion-implanted bearing steels. A review of rolling contact fatigue (RCF) results on implanted bearing steels, compiled by Hubler [4], showed increases in RCF lifetimes for nitrogen-implanted M50 and 52100 steels but 0257-8972/89/$3.50
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Sequoia/Printed
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2
inconsistent results for nitrogen-implanted 440C steel [5, 61. For the latter material, Kustas and coworkers showed increased RCF lifetimes for low dose nitrogen implantation [5] and reduced lifetimes for higher dose specimens
[61.
One of the possible reasons for the non-reproducible RCF results for nitrogen-implanted 440C steel is the use of non-optimal implantation parameters, resulting in degradation of near-surface mechanical properties. For example, improper heat sink fixturing may allow excessive temperature excursions, and abnormally high doses can blister the surface, inducing plastic deformation [ 71. The reduced RCF lifetimes observed for higher dose nitrogen-implanted 440C steel were attributed to a hardness reduction of about 2.5 Rockwell C points, presumably because of excessive tempering during implantation. Another concern with implantation of quenched and tempered martensitic steels is the preferential sputtering of the matrix and exposure of the hard Fe-Cr carbide particles which can raise the shear stresses near the surface during hertzian contact [8]. In this investigation, systematic studies were performed to define better a more optimum range of implantation parameters to prevent degradation of near-surface material properties. Implantation was performed using high ion beam current densities (greater than 100 PA cm-*) to reduce processing time and potentially to increase depth of penetration [9] (Fig. 1) from radiationenhanced diffusion effects [lo]. RCF tests were performed to evaluate the effects of the more optimum implantation conditions. A companion paper [ll] presents failure analyses of RCF specimens to establish the modification in the fatigue failure mode from N,+ implantation processing.
0
t
100
200 DEPTH
300
400
(NM)
Fig. 1. AES concentration-depth profiles high ion beam current density implantation
showing enhanced (from ref. 9).
nitrogen
penetration
from
2. Experimental 2.1.
details
Test specimens
A 6.35 cm (2.5 in) diameter annealed bar of 440C steel (nominal composition in weight per cent: Cr, 17.0; C, 1.08; Si, 1.0 max; Mn, 1.0 max; MO, 1.0; Fe, balance) was rough machined into oversized solid cylindrical rods and heat treated according to the following NASA specification [ 121: (1) austenitize at 1054 “C for 1 h; (2) oil quench (50 “C); (3) temper at 163 “C for 1 h; (4) air cool to ambient temperature, (5) liquid nitrogen soak for 0.5 h; (6) temper at 163 “C for 1 h. Specimen surfaces were ground to final dimensions (0.953 cm diameter by 8.26 cm long) and then buffed to produce a surface finish of 100 nm (4 pin) r.m.s. 2.2. Ion implantation Implantation processing was performed in the Mechanical Engineering Department of Colorado State University, Fort Collins, CO, using a broadbeam system developed from ion thruster technology [13]. A gaseous nitrogen source, emitting primarily molecular N,+ ions, was accurately controlled to maintain uniform dose exposures to within *lo% of the desired levels. High current densities of up to 1500 PA cm-’ can be achieved with this broad-beam system, although current densities of this magnitude were not utilized in this preliminary study. Selection of the most optimum ion current density to prevent excessive tempering is discussed in Section 3. RCF specimens were mounted in a water-cooled heat sink fixture as illustrated in Fig. 2. A graphite mask was used to provide near-normal beam incidence and provided an included angle of surface exposure of about 60” (Fig. 2). RCF specimens were rotated in 5” increments instantaneously, the intervals between rotation being l/72 of the total duration. Implantation parameters included a constant accelerating energy of 40 keV and calculated fluences of 1 X lo”, 2 X 10” and 4 X 101’ Nz+ ions cmp2. 2.3. Rolling contact fatigue tests RCF tests were conducted at the National Aeronautics and Space Administration (NASA) Marshall Space Flight Center (MSFC), Huntsville, AL, using a standard ball-rod type of RCF test machine which is described in detail in ref. 14. The 440C steel rod was loaded between three 1.27 cm (0.5 in) diameter hardened 52100 steel balls which had a roughened surface finish of 0.089 pm (3.5 pin) roughness average. Roughened balls were used to obtain a statistically significant number of failures within a suspension time limit of 200 h. Parameters used in the RCF tests included the following: (1) two stress levels of 4.04 GPa (586 klbf inp2) and 5.42 GPa (786 klbf inp2) (calculated maximum hertzian stress) with seven tests per stress level; (2) a rotational velocity of 3600 rev min-‘; (3) t ur b ine engine oil lubrication (MIL-L-78-8J) at 6 - 8 drops min-‘. Tests were conducted with alternating low and high stress levels on each end of the RCF rod to avoid adverse statistical results
ION BEAM
RCF CYLINDER
0.5 cm
(a)
GRAPHITE
MASK
WATER COOLED, COPPER HEAT SINK FIXTURE
1 RCFCYUNDER
1
I
THERMOCOUPLE
FATIGUE EES
1
I
I---
1
1
I
I t--
I4
:CC
,ONBEAM
ATION ~‘---~---~~
(b) Fig. 2. Schematic diagram of the apparatus (a) top view; (b) side view.
!-----I GRAPHITE
MASK
used for implantation
processing
of RCF
rods;
due to implantation variations along the length of the processed area. Termination of a test occurred when vibration, sensed by a close-proximity accelerometer, exceeded a pre-set level. The signal from the accelerometer triggered an automatic shut-off device. Excessive vibration was produced by the emergence of a fatigue spa11 on the specimen surface. 2.4. Statistical analyses Statistical analyses, using a Weibull analysis computer program, were performed to evaluate the effects of nitrogen implantation on the RCF lifetime of 440C steel. Included was determination of BlO (10% of specimens predicted to fail) and B50 or mean (50% of specimens predicted to fail) Weibull fatigue lifetimes, Weibull slope and confidence levels. The last parameter is useful in determining the significance of the difference between the fatigue lifetimes of the nitrogen-implanted specimens and the unimplanted specimens [ 151. This confidence number is calculated by taking into account three things: the slope of the Weibull line, the sample size and the B10 or mean life of the populations tested. However, the degree of confidence need not be constant from one level to another. For example, it
5
is possible to have a statistically significant improvement at the B50 life but to have no statistically significant improvement at the BlO life. 2.5. Surface analysis Auger electron spectroscopy (AES) was performed to determine the atomic concentration-depth profiles for the different implanted doses. Data were taken during argon ion sputter depth profiling and the following sensitivity factors were used to quantify the peak height data: N(379 eV), 0.23; C(272 eV), 0.28 (carbide); O(503 eV), 0.40 [16, 171. Sputter rates were determined from previous profilometry measurements of crater depths on polished 440C steel specimens [ 181. It was found that actual sputter rates on 440C steel were 10% lower than those calculated for the TazO, standard material.
3. Results 3.1. Implantation parameters 3.1.1. Current density Selection of an appropriate ion beam current density is critical during processing of materials sensitive to thermal excursions. For high carbon alloy steels such as 44OC, slow cooling through or tempering in the range 260 350 “C can result in tempered martensite embrittlement (TME) [19], a phenomenon which can reduce fracture toughness [20]. To prevent this temperature exposure during implantation processing, a study was performed to establish an upper limit on the ion beam current density. A 440C steel RCF specimen was equipped with a thermocouple near the cylinder endpoint just below (within 0.1 mm of) the surface, to monitor near-surface temperatures (Fig. 2). Nitrogen implantations were performed to determine the ion beam current density at which a maximum temperature of 260 “C was reached. As shown in Fig. 3, current densities greater than 107 /..LA cmP2 resulted in a near-surface temperature of 260 “C in a finite time. For a current density of 107 PA cmh2, a near-surface temperature of 260 “C was never reached. As a result, this current density was used for subsequent implantation processing. It should be noted that the temperature at the bottom of the cylinder represents a maximum temperature because of the geometry of the heat sink fixture employed (Fig. 2). 3.1.2. Exposure time From previous RCF tests of coated 440C steel specimens, NASA-MSFC has determined that a bulk hardness loss of greater than 2 Rockwell C points, produced by the thermal exposure from coating treatments, resulted in reduced RCF lifetimes [21]. This trend was also observed for improperly implanted 440C rods where high processing temperatures were suggested [ 61. To establish the maximum exposure time at which such a hardness loss occurred, a tempering study was performed on 440C flat specimens heat
6
75
107
125
175
225
275
ION BEAM CURRENT DENSITY (p A/CM*)
Fig. 3. Time to reach showing the optimum was never exceeded.
the lower temperature limit for TME during nitrogen implantation, current density of 107 PA cmP2 at which a temperature of 260 “C
TIME (MIN)
Fig. 4. Hardness changes of fully tempered 440C steel after post-temper ing small hardness reductions for annealing at 255 “C.
annealing,
show-
treated according to the same schedule used for the RCF cylinders. As shown in Fig. 4, tempering at 300 “C (within the TME range) for times greater than about 30 min resulted in a hardness loss greater than 2 Rockwell C points. In contrast, for a 255 “C temper, exposure times of up to 120 min can be tolerated without significant hardness reductions (Fig. 4). From the results of these parameter development studies, a current density of 107 /.LA cme2 and implantation processing times of up to 120 min could be used without degradation of near-surface mechanical properties. Three RCF rods were subsequently implanted with N,+ using a current density of 107 PA cmP2 to calculated total doses of 1 X 10i7, 2 X 10” and 4 X 1017 N2+ cmM2. All implantations were performed at 40 keV accelerating energy. Time of processing varied from 15 min for the lowest dose to 60 min for the highest dose.
7
3.2. Surface analysis by Auger electron spectroscopy Increasing the implantation dose produced nitrogen profiles with similar shapes (Fig. 5(a)) and nearly constant maximum nitrogen concentrations of about 31 at.% (Table 1). This constant peak concentration suggests a sputter-limited condition which was confirmed by additional AES analyses within the first 40 nm of the surface (Fig. 5(b)). It was observed that nitrogen concentrations of 75% - 80% of the peak were reached within 2 nm from the surface for the 10” and 2 X 101’ N2+ cm-* dose specimens with the very-near-surface region being oxygen rich. For the highest dose (4 X 10” N2+ cm-*) specimen, 75% of the peak concentration was reached at a greater depth of about 13 nm as a result of the deeper range of the implanted nitrogen. In a similar manner, the first 40 nm for this specimen consisted of a thickened oxygen-rich layer. Thus, in general, all the profiles could be characterized as consisting of an oxygen-rich zone (presumably an oxide), extending to about 4 nm for the lower dose specimens and to about 40 nm for the highest dose specimen, followed by a constant peak concentration of nitrogen, then the outward leg of the gaussian peak which varied according to the implanted dose. It should be noted that these near-surface AES analyses (Fig. 5(b)) were performed at the opposite end of the RCF rod to that used for the average area profiles (Fig. 5(a)), which may account for the difference in the peak nitrogen concentrations observed. For example, ratios of the peak nitrogen concentrations for the near-surface AES analyses to the average area analyses are 0.89 (10” N2+ cm-*), 1.02 (2 X 10” N2+ cm-*) and 0.89 (4 X 101’ N,” cm-*). Thus reproducible peak nitrogen concentrations within about 11% were found from AES analyses. Other modifications in the profiles with increased dose were (1) variations in total integrated areas, (2) larger integrated area fractions on the bulk side of the nitrogen mean and (3) a significantly enhanced maximum nitrogen penetration depth for the highest dose specimen. Ratios of the total integrated areas for the three implantation doses, 1.0:0.82: 1.81, were much lower than the calculated intended doses of 1:2: 4 supporting the loss of nitrogen by sputter removal of material from the surface [22]. The increased distribution of nitrogen on the substrate side of the mean appears to be due to knock-in effects, such that the previously implanted ions are impacted to deeper depths. This effect is graphically illustrated for the highest dose specimen (4 X 10 ” N2+ cm-‘) (Fig. 5(a)) as a threefold increase in the total nitrogen penetration depth compared with that for the lowest dose specimen. However, other effects, such as radiation-enhanced diffusion (effective at temperatures greater than about 200 “C [23]) may also contribute, especially considering the similarity of the nitrogen profile trail for the high dose implantation (Fig. 5(a)) to the enhanced penetration observed for high current density nitrogen-implanted 304 stainless steel (Fig. 1) [24]. Nitrogen concentrations at the surface decreased as the calculated dose level was increased, i.e. 6.3 at.% for 10” N2+ cm-*, 5.7 at.% for 2 X 10” N,+
0
75
225
150
DEPTH [NM) (a)
0
10
20
30
40
50
DEPTH (NM) (b)
Fig. 5. AES profiles for nitrogen-implanted nitrogen penetration depth for the highest nitrogen concentrations in very-near-surface
TABLE
W2+cm
showing (a) enhanced specimen and (b) high
I
Summary of RCF cylinders Dose
440C steel specimens dose (4 X 1O1’ Nz cm-‘) regions.
-2
1
nitrogen
concentrations,
Maximum N concentration (at.%)
ranges
Maximum N penetration (to 1 at.% N) (nm)
and
integrated
areas
Integrated area (at.% nm X 10d3) Surface
to N mean
for
three
implanted
(% of total) N mean
to bulk
Total
10”
32.1
108
0.50 (31.4%)
1.09 (68.6%)
1.59
2 x 10”
30.6
126
0.36 (27.2%)
0.94 (72.3%)
1.30
4 x 101’
31.0
360
0.72 (25.1%)
2.15 (74.8%
2.81
9
cmp2 and 2.4 at.% for 4 X 10” N,+ cm-*. These different surface nitrogen concentrations modified the oxide composition from the normal M20, type (where M 1s Fe,.,, Cr0.14) of oxide common for 440C steel [25], except for the high dose (4 X 10” N,+ c m-*) specimen which exhibited an oxide resembling the unimplanted material (but with some N substitution). The other implanted specimens exhibited an (M30,)N,,55 type of oxide with similar iron-to-chromium ratios as for unimplanted 440C steel. 3.3. Rolling contact fatigue test results Tables 2 and 3 list the calculated BlO and B50 Weibull lifetimes, fatigue life ratios (normalized with respect to unimplanted 440C steel fatigue lifetimes) and confidence levels for the 4.04 GPa (586 klbf in-*) and 5.43 GPa (786 klbf in*) hertzian stress tests respectively. For the lower stress level of 4.04 GPa (586 klbf in*) (Table 2), all the B50 fatigue lifetimes (which TABLE
2
Weibull tests
fatigue
lifetimes,
life
ratios
and confidence
Confidence level (%)
levels
for
4.04
B50 life (X106 cycles)
GPa hertzian
Life ratiob
Confidence level (%)
N2+ dose -2 (ions cm )
BlO life (X106 cycles)
Life ratioa
Unimplanted
3.4
-
specimen 10” 2 x 10” 4 x 10’7
9.Q 5.4 6.6
2.65 1.59 1.94
88 71 77
15.9 16.0 16.0
2.21 2.22 2.22
99 99 99
All specimens
7 .O
2.1
78
16.0
2.2
99
aNitrogen-implanted bNitrogen-implanted
TABLE
3
Weibull tests
fatigue
lifetimes,
life ratios
-
7.2
B10 us. unimplanted B50 us. unimplanted
BlO. B50.
and confidence
levels
for
5.43
GPa hertzian
N2+ dose -2 (ions cm )
BlO
life (x106 cycles)
Life ratioa
Confidence level (%)
B50 life (X106 cycles)
Unimplanted specimen 10” 2 x 10” 4 x 10”
1.5
-
-
3.0
1.9 2.2 1.7
1.27 1.47 1.13
61 65 55
5.0 8.2 8.6
1.67 2.73 2.87
97 99 99
All specimens
1.93
1.3
60
7.3
2.43
99
aNitrogen-implanted bNitrogen-implanted
B10 us. unimplanted B50 us. unimplanted
stress
BlO. B50.
Life ratiob
stress
Confidence level (%) -
10
exhibited increases of about 2.2X over that for unimplanted 440C steel) were statistically significant with confidence levels of 99%. Levels of confidence below about 80% are not considered as statistically “significant” improvements. These results are encouraging, but tempered by the fact that BlO lifetimes carry more impact for aerospace bearing systems. For the BlO values, only the low dose (1 X 10 l7 N,+ cme2) specimen showed statistical significance, and its lifetime improvement ratio of 2.65X was dramatic. From the results presented in Table 2, it appears that fatigue lifetimes are insensitive to retained nitrogen concentration and maximum nitrogen penetration depth for the B50 lifetimes. In contrast, for the BlO values, the smallest lifetime improvement was observed for the nitrogen-implanted specimen with the lowest retained nitrogen concentration (i.e. 2 X 1017 N2+ cmp2). Since there did not appear to be any clear correlation of RCF lifetime with implanted N,+ dose or penetration depth, statistical analyses were performed on all the nitrogen-implanted specimens taken as one set. From the analysis for the 4.04 GPa tests, an extrapolated BlO lifetime of 7.0 X lo6 cycles and a B50 lifetime of 16.0 X lo6 cycles were determined. These values represent increases of 2.1X and 2.2X respectively in the BlO and B50 RCF lifetimes for the nitrogen-implanted specimens compared with that of the unimplanted specimen. Confidence levels for the nitrogen-implanted BlO and B50 lifetimes were 78% and 99% respectively, indicating that only the mean lifetime improvements were statistically significant. For the higher hertzian stress level tests (Table 3), the magnitude of the BlO lifetime improvements was lower than that for the lower stress level tests and not a single increase was considered statistically significant. As for the lower stress test results, the B50 values were all statistically significant and there appears to be a slight correlation of fatigue lifetime with maximum nitrogen penetration depth. For example, increasing the implanted nitrogen dose increased the maximum nitrogen depth (Table 1) which also coincided with increased RCF lifetimes (Table 3). For the 5.43 GPa stress level, BlO and B50 fatigue lifetimes for the nitrogen-implanted specimens, taken as a single population, were 1.93 X lo6 cycles and 7.3 X lo6 cycles respectively. The BlO lifetime improvement of 1.3X was more modest than that observed for the lower stress test results. In addition, the confidence levels for the nitrogen-implanted specimens taken as a whole were 60% for the BlO and 99% for the B50 lifetimes, which indicates the statistical significance for the BlO lifetime was much lower for the higher stress level tests.
4. Discussion The observed improvements in RCF lifetimes for nitrogen-implanted 440C steel reported here are in contrast with the conventional notion that only surface modifications to the depth of maximum cyclic plasticity (and thus maximum shear stresses) are effective for pure rolling and rolling plus sliding conditions [26,27]. For the latter conditions, early studies at Battelle
11
[28] predicted that reductions in the coefficient of friction would lower the peak subsurface octahedral shear stress and move the planes of octahedral shear stress to deeper locations. These modifications favor longer RCF lifetimes from (1) increased crack initiation periods from the reduced shear stresses and (2) longer crack propagation periods because of the increased ligament length to the surface. For nitrogen-implanted 440C steel, unlubricated test results have shown (1) no reductions in friction or wear for a 52100 steel us. nitrogen-implanted 440C steel wear couple [18], (2) 25% reductions in friction and wear for a 440C steel vs. nitrogen-implanted 440C steel wear couple [29] and (3) 0.8 reduction in starting torque for a nitrogen-implanted 440C steel bearing ball and race assembly [ 301. All these results were for unlubricated wear couples. However, friction and wear data are lacking for the exact test conditions used in the RCF test in this study, i.e. 52100 steel against nitrogenimplanted 440C steel under oil-lubricated conditions. Other studies of unimplanted vs. implanted material couples tested with synthetic polyester lubrication have shown reduced wear volumes and wear variability for unimplanted 4140 steel and O-6 steel against 52100 steel implanted with titanium or titanium plus carbon [31]. These wear improvements were observed in spite of actual increases in the coefficients of friction [ 311. Microstructural modifications from nitrogen implantation of 440C steel include 1251 (1) formation of nitride ((Fe, Cr),+,N(x = 0.47 - 1.2)) and/or carbonitride ((Fe, Cr),.,(C,.,N,.,)) compounds, (2) reduced retained austenite content from radiation damage and induced stresses from volume expansion associated with nitride formation, and (3) nitrogen incorporation in and modification of the oxide stoichiometry. Formation of hard nitride particles would not be expected to add a hardening increment, since 440C steel exhibits high intrinsic hardness (i.e. a Rockwell C hardness of 59 or higher). The effects of austenite content on RCF resistance are inconclusive at this time [32]. The last effect, that of oxide modification, has been speculated to be very important by Japanese investigators [ 301, who suggest that nitrogen incorporation in the oxide increases its adhesion to the substrate and thus promotes self-lubrication. Thus the speculated reason for the increased RCF lifetimes from nitrogen implantation of 440C steel appears to be maintenance of a low-friction oxide film which reduces the magnitude of the subsurface shear stresses which in turn increases the crack initiation time. 5. Summarizing
remarks
Parameter development studies were performed to establish more optimum implantation conditions for 440C steel, to prevent excursion into the temperature range for tempered martensite embrittlement and to prevent bulk tempering. These conditions were identified as a current density of 107 /LA cmp2 and exposure times of up to 120 min at 255 “C. RCF tests on 440C steel specimens implanted under these conditions showed reproducible imnrovements in BlO lifetimes with larger increases (up to 2.65X) for the
12
lower stress level tests. No apparent dependence of lifetime on retained concentration was observed, although a slight correlation of higher B50 lifetimes with increased nitrogen penetration was found. The consistent improvements in RCF life for nitrogen-implanted 440C steel appear to be due to modification of oxide composition, promoting lubricity and enhanced oxide adhesion.
References 1 I. L. Singer, MRS Symp. Proc., 27 (1984) 585. 2 K. V. Jata and E. A. Starke, Jr., J. Met., 35 (1983) 23. 3 W. W. Hu, H. Herman, C. R. Clayton, J. Kozubowski, R. A. Kant, J. K. Hirvonen and R. K. MacCrone, in C. M. Preece and J. K. Hirvonen (eds.), Ion Implantation Metallurgy, The Metallurgical Society of AIME, Warrendale, PA, 1980, p. 92. Naval Research Laboratory, 4 G. K. Hubler, Materials Modification Branch, Washington, DC, personal communications. 5 F. M. Kustas, M. S. Misra and P. Sioshansi, MRS Symp. Proc., 27 (1984) 686. 6 F. M. Kustas and M. S. Misra, Progress Rep. R85-48681-002, 1985 (Martin Marietta Aerospace). 7 N. E. W. Hartley, J. Vat. Sci. Technol., 12 (1975) 485. 8 M. N. Gardos, in A. G. Gray and R. T. Nach (eds.), Conservation and Substitution Technology for Critical Metals in Bearings and Related Components for Industrial Equipment and Opportunities for Improved Performance, BuMines OFR 21-85, 1985, p. 370 (U.S. Bureau of Mines, Washington DC). 9 L. 0. Daniels and P. J. Wilbur, Nucl. Instrum. Methods Phys. Res. B, 19 20 (1987) 221. 10 M. Rangaswamy, D. Farkas and H. L. Sobel, Nucl. Instrum. Methods Phys. Res. B, 19 - 20 (1987) 196. 11 F. M. Kustas, M. S. Misra, S. R. Smith and W. S. Sampath, to be published. 12 Heat Treatment Specification DCN 28433 (NASA Marshall Space Flight Center). 13 P. J. Wilbur and L. 0. Daniels, Vacuum, 136 (1 3) (1986) 5. 14 D. Glover, in T. C. Hoo (ed.), Rolling Contact Fatigue Testing of Bearing Steels, ASTM, Philadelphia, PA, 1982, p. 107. 15 R. B. Abernethy, J. E. Breneman, C. H. Medlin and G. L. Reinman, Weibull Analysis Handbook, Government Rep. AFWAL-TR-83-2079, 1983, p. 1093. 16 L. E. Davis (ed.), Handbook of Auger Electron Spectroscopy, Physical Electronics Division, Perkin-Elmer, Eden Prairie, MN, 2nd edn., 1978. 17 J. F. Moulder, Physical Electronics Division, Perkin-Elmer, personal communication, 1986. 18 F. M. Kustas and M. S. Misra, Progress Rep. R85-48681-002, 1985 (Martin Marietta Denver Aerospace). 19 G. Krauss (ed.), Principles of Heat Treatment of Steel, American Society for Metals, Metals Park, OH, 1980, p. 188. 20 B. Low and B. L. Averbach, Metall. Trans. A, 14 (1983) 1899. 21 R. L. Thorn, NASA Marshall Space Flight Center, Huntsville, AL, personal communication, 1986. 22 I. Manning, in F. A. Smidt (ed.), The Use of Ion Implantation for Materials Processing, NRL Memo. Rep. 4527, 1981, p. 27 (Naval Research Laboratory, Washington, DC). 23 H. Wiederesich, Nucl. Instrum. Methods Phys. Res. B, 7 - 8 (1985) 1. 24 L. 0. Daniels, M.S. Thesis, Colorado State University, 1986.
13 F. M. Kustas, D. L. Williamson and M. S. Misra, Nucl. Instrum. MethodsPhys. Res. B, 31 (1988) 393. 26 V. Bhargava, G. T. Hahn and C. A. Rubin, Metall. Trans. A, 18 (1987) 827. 27 G. T. Hahn, V. Bhargava, G. Ham, A. Kumar and C. A. Rubin, Proc. ASM Session on Tribological Mechanisms and Wear Problems in Materials, 1987. 28 T. G. Johns, S. G. Sampath, J. C. Bell and K. B. Davies, Engineering Analysis of Stresses in Railroad Rail, Phase I, Battelle Columbus Laboratories, 1977 (FRA Contract DOT-TSC-1038). 29 D. M. Follstaedt, F. G. Yost and L. E. Pope, MRS Symp. Proc., 27 (1984) 655. 30 M. Hirano and S. Miyake, Extension of bearing endurance life by ion implantation, Appl. Phys. Lett., 49 (1986) 779. 31 P. Sioshansi and J. J. Au, Improvements in sliding wear for bearing-grade steel implanted with titanium and carbon, Mater. Sci. Eng., 69 (1985) 161. 32 E. V. Zaretsky, Selection of Rolling-Element Bearing Steels for Long-Life Application, NASA Tech. Memo. 88881, 1986 (NASA Lewis Research Center, Cleveland, OH).
25
and Coatings
Technology,
37 (1989)
1
1
- 13
HIGH CURRENT DENSITY ION IMPLANTATION PROCESSING OF 440C STEEL PART 1: IMPLANTATION PARAMETERS AND ROLLING CONTACT FATIGUE IMPROVEMENT F. M. KUSTAS,
M. S. MISRA
Martin
Astronautics
Marietta
and S. R. SMITH Group,
Denver,
CO (U.S.A.)
P. J. WILBUR Department (U.S.A.)
of Mechanical
Engineering,
Colorado
State
University,
Fort
Collins,
CO
R. L. THOM NASA
Marshall
(Received
Space
August
Flight
Center,
Huntsville,
AL
(U.S.A.)
3, 1987)
High alloy steels, such as 440C steel, are commonly used for high speed, high load turbopump bearing applications. The low fatigue lifetimes of this steel have been attributed to cracking of the large Fe-Cr carbide particles and subsequent spalling. Surface modification by high current density (107 PA cm-‘) ion implantation processing with nitrogen (N2+) was performed to assess the effects of ion implantation on rolling contact fatigue performance. Process parameter studies showed that implantation using a current density of 107 /..LAcme2 prevented excessive thermal exposure and substrate tempering. Rolling contact fatigue tests were performed under lubricated conditions at two different stress levels: 4.04 GPa (586 klbf in2) and 5.42 GPa (786 klbf inM2)hertzian stress. Test results showed improvements in the rolling contact fatigue lifetimes for the N,+-implanted specimens compared with those for unimplanted 440C steel.
1. Introduction Ion implantation is a proven technique to increase the tribological properties of engineering alloys. Significant improvements in wear resistance [l] and fatigue resistance [2, 31 have been reported for ion-implanted low alloy steels. For the severe application of pure rolling, common for high speed, high load turbopump bearings, inconsistent test results have been reported for ion-implanted bearing steels. A review of rolling contact fatigue (RCF) results on implanted bearing steels, compiled by Hubler [4], showed increases in RCF lifetimes for nitrogen-implanted M50 and 52100 steels but 0257-8972/89/$3.50
@ Elsevier
Sequoia/Printed
in The Netherlands
2
inconsistent results for nitrogen-implanted 440C steel [5, 61. For the latter material, Kustas and coworkers showed increased RCF lifetimes for low dose nitrogen implantation [5] and reduced lifetimes for higher dose specimens
[61.
One of the possible reasons for the non-reproducible RCF results for nitrogen-implanted 440C steel is the use of non-optimal implantation parameters, resulting in degradation of near-surface mechanical properties. For example, improper heat sink fixturing may allow excessive temperature excursions, and abnormally high doses can blister the surface, inducing plastic deformation [ 71. The reduced RCF lifetimes observed for higher dose nitrogen-implanted 440C steel were attributed to a hardness reduction of about 2.5 Rockwell C points, presumably because of excessive tempering during implantation. Another concern with implantation of quenched and tempered martensitic steels is the preferential sputtering of the matrix and exposure of the hard Fe-Cr carbide particles which can raise the shear stresses near the surface during hertzian contact [8]. In this investigation, systematic studies were performed to define better a more optimum range of implantation parameters to prevent degradation of near-surface material properties. Implantation was performed using high ion beam current densities (greater than 100 PA cm-*) to reduce processing time and potentially to increase depth of penetration [9] (Fig. 1) from radiationenhanced diffusion effects [lo]. RCF tests were performed to evaluate the effects of the more optimum implantation conditions. A companion paper [ll] presents failure analyses of RCF specimens to establish the modification in the fatigue failure mode from N,+ implantation processing.
0
t
100
200 DEPTH
300
400
(NM)
Fig. 1. AES concentration-depth profiles high ion beam current density implantation
showing enhanced (from ref. 9).
nitrogen
penetration
from
2. Experimental 2.1.
details
Test specimens
A 6.35 cm (2.5 in) diameter annealed bar of 440C steel (nominal composition in weight per cent: Cr, 17.0; C, 1.08; Si, 1.0 max; Mn, 1.0 max; MO, 1.0; Fe, balance) was rough machined into oversized solid cylindrical rods and heat treated according to the following NASA specification [ 121: (1) austenitize at 1054 “C for 1 h; (2) oil quench (50 “C); (3) temper at 163 “C for 1 h; (4) air cool to ambient temperature, (5) liquid nitrogen soak for 0.5 h; (6) temper at 163 “C for 1 h. Specimen surfaces were ground to final dimensions (0.953 cm diameter by 8.26 cm long) and then buffed to produce a surface finish of 100 nm (4 pin) r.m.s. 2.2. Ion implantation Implantation processing was performed in the Mechanical Engineering Department of Colorado State University, Fort Collins, CO, using a broadbeam system developed from ion thruster technology [13]. A gaseous nitrogen source, emitting primarily molecular N,+ ions, was accurately controlled to maintain uniform dose exposures to within *lo% of the desired levels. High current densities of up to 1500 PA cm-’ can be achieved with this broad-beam system, although current densities of this magnitude were not utilized in this preliminary study. Selection of the most optimum ion current density to prevent excessive tempering is discussed in Section 3. RCF specimens were mounted in a water-cooled heat sink fixture as illustrated in Fig. 2. A graphite mask was used to provide near-normal beam incidence and provided an included angle of surface exposure of about 60” (Fig. 2). RCF specimens were rotated in 5” increments instantaneously, the intervals between rotation being l/72 of the total duration. Implantation parameters included a constant accelerating energy of 40 keV and calculated fluences of 1 X lo”, 2 X 10” and 4 X 101’ Nz+ ions cmp2. 2.3. Rolling contact fatigue tests RCF tests were conducted at the National Aeronautics and Space Administration (NASA) Marshall Space Flight Center (MSFC), Huntsville, AL, using a standard ball-rod type of RCF test machine which is described in detail in ref. 14. The 440C steel rod was loaded between three 1.27 cm (0.5 in) diameter hardened 52100 steel balls which had a roughened surface finish of 0.089 pm (3.5 pin) roughness average. Roughened balls were used to obtain a statistically significant number of failures within a suspension time limit of 200 h. Parameters used in the RCF tests included the following: (1) two stress levels of 4.04 GPa (586 klbf inp2) and 5.42 GPa (786 klbf inp2) (calculated maximum hertzian stress) with seven tests per stress level; (2) a rotational velocity of 3600 rev min-‘; (3) t ur b ine engine oil lubrication (MIL-L-78-8J) at 6 - 8 drops min-‘. Tests were conducted with alternating low and high stress levels on each end of the RCF rod to avoid adverse statistical results
ION BEAM
RCF CYLINDER
0.5 cm
(a)
GRAPHITE
MASK
WATER COOLED, COPPER HEAT SINK FIXTURE
1 RCFCYUNDER
1
I
THERMOCOUPLE
FATIGUE EES
1
I
I---
1
1
I
I t--
I4
:CC
,ONBEAM
ATION ~‘---~---~~
(b) Fig. 2. Schematic diagram of the apparatus (a) top view; (b) side view.
!-----I GRAPHITE
MASK
used for implantation
processing
of RCF
rods;
due to implantation variations along the length of the processed area. Termination of a test occurred when vibration, sensed by a close-proximity accelerometer, exceeded a pre-set level. The signal from the accelerometer triggered an automatic shut-off device. Excessive vibration was produced by the emergence of a fatigue spa11 on the specimen surface. 2.4. Statistical analyses Statistical analyses, using a Weibull analysis computer program, were performed to evaluate the effects of nitrogen implantation on the RCF lifetime of 440C steel. Included was determination of BlO (10% of specimens predicted to fail) and B50 or mean (50% of specimens predicted to fail) Weibull fatigue lifetimes, Weibull slope and confidence levels. The last parameter is useful in determining the significance of the difference between the fatigue lifetimes of the nitrogen-implanted specimens and the unimplanted specimens [ 151. This confidence number is calculated by taking into account three things: the slope of the Weibull line, the sample size and the B10 or mean life of the populations tested. However, the degree of confidence need not be constant from one level to another. For example, it
5
is possible to have a statistically significant improvement at the B50 life but to have no statistically significant improvement at the BlO life. 2.5. Surface analysis Auger electron spectroscopy (AES) was performed to determine the atomic concentration-depth profiles for the different implanted doses. Data were taken during argon ion sputter depth profiling and the following sensitivity factors were used to quantify the peak height data: N(379 eV), 0.23; C(272 eV), 0.28 (carbide); O(503 eV), 0.40 [16, 171. Sputter rates were determined from previous profilometry measurements of crater depths on polished 440C steel specimens [ 181. It was found that actual sputter rates on 440C steel were 10% lower than those calculated for the TazO, standard material.
3. Results 3.1. Implantation parameters 3.1.1. Current density Selection of an appropriate ion beam current density is critical during processing of materials sensitive to thermal excursions. For high carbon alloy steels such as 44OC, slow cooling through or tempering in the range 260 350 “C can result in tempered martensite embrittlement (TME) [19], a phenomenon which can reduce fracture toughness [20]. To prevent this temperature exposure during implantation processing, a study was performed to establish an upper limit on the ion beam current density. A 440C steel RCF specimen was equipped with a thermocouple near the cylinder endpoint just below (within 0.1 mm of) the surface, to monitor near-surface temperatures (Fig. 2). Nitrogen implantations were performed to determine the ion beam current density at which a maximum temperature of 260 “C was reached. As shown in Fig. 3, current densities greater than 107 /..LA cmP2 resulted in a near-surface temperature of 260 “C in a finite time. For a current density of 107 PA cmh2, a near-surface temperature of 260 “C was never reached. As a result, this current density was used for subsequent implantation processing. It should be noted that the temperature at the bottom of the cylinder represents a maximum temperature because of the geometry of the heat sink fixture employed (Fig. 2). 3.1.2. Exposure time From previous RCF tests of coated 440C steel specimens, NASA-MSFC has determined that a bulk hardness loss of greater than 2 Rockwell C points, produced by the thermal exposure from coating treatments, resulted in reduced RCF lifetimes [21]. This trend was also observed for improperly implanted 440C rods where high processing temperatures were suggested [ 61. To establish the maximum exposure time at which such a hardness loss occurred, a tempering study was performed on 440C flat specimens heat
6
75
107
125
175
225
275
ION BEAM CURRENT DENSITY (p A/CM*)
Fig. 3. Time to reach showing the optimum was never exceeded.
the lower temperature limit for TME during nitrogen implantation, current density of 107 PA cmP2 at which a temperature of 260 “C
TIME (MIN)
Fig. 4. Hardness changes of fully tempered 440C steel after post-temper ing small hardness reductions for annealing at 255 “C.
annealing,
show-
treated according to the same schedule used for the RCF cylinders. As shown in Fig. 4, tempering at 300 “C (within the TME range) for times greater than about 30 min resulted in a hardness loss greater than 2 Rockwell C points. In contrast, for a 255 “C temper, exposure times of up to 120 min can be tolerated without significant hardness reductions (Fig. 4). From the results of these parameter development studies, a current density of 107 /.LA cme2 and implantation processing times of up to 120 min could be used without degradation of near-surface mechanical properties. Three RCF rods were subsequently implanted with N,+ using a current density of 107 PA cmP2 to calculated total doses of 1 X 10i7, 2 X 10” and 4 X 1017 N2+ cmM2. All implantations were performed at 40 keV accelerating energy. Time of processing varied from 15 min for the lowest dose to 60 min for the highest dose.
7
3.2. Surface analysis by Auger electron spectroscopy Increasing the implantation dose produced nitrogen profiles with similar shapes (Fig. 5(a)) and nearly constant maximum nitrogen concentrations of about 31 at.% (Table 1). This constant peak concentration suggests a sputter-limited condition which was confirmed by additional AES analyses within the first 40 nm of the surface (Fig. 5(b)). It was observed that nitrogen concentrations of 75% - 80% of the peak were reached within 2 nm from the surface for the 10” and 2 X 101’ N2+ cm-* dose specimens with the very-near-surface region being oxygen rich. For the highest dose (4 X 10” N2+ cm-*) specimen, 75% of the peak concentration was reached at a greater depth of about 13 nm as a result of the deeper range of the implanted nitrogen. In a similar manner, the first 40 nm for this specimen consisted of a thickened oxygen-rich layer. Thus, in general, all the profiles could be characterized as consisting of an oxygen-rich zone (presumably an oxide), extending to about 4 nm for the lower dose specimens and to about 40 nm for the highest dose specimen, followed by a constant peak concentration of nitrogen, then the outward leg of the gaussian peak which varied according to the implanted dose. It should be noted that these near-surface AES analyses (Fig. 5(b)) were performed at the opposite end of the RCF rod to that used for the average area profiles (Fig. 5(a)), which may account for the difference in the peak nitrogen concentrations observed. For example, ratios of the peak nitrogen concentrations for the near-surface AES analyses to the average area analyses are 0.89 (10” N2+ cm-*), 1.02 (2 X 10” N2+ cm-*) and 0.89 (4 X 101’ N,” cm-*). Thus reproducible peak nitrogen concentrations within about 11% were found from AES analyses. Other modifications in the profiles with increased dose were (1) variations in total integrated areas, (2) larger integrated area fractions on the bulk side of the nitrogen mean and (3) a significantly enhanced maximum nitrogen penetration depth for the highest dose specimen. Ratios of the total integrated areas for the three implantation doses, 1.0:0.82: 1.81, were much lower than the calculated intended doses of 1:2: 4 supporting the loss of nitrogen by sputter removal of material from the surface [22]. The increased distribution of nitrogen on the substrate side of the mean appears to be due to knock-in effects, such that the previously implanted ions are impacted to deeper depths. This effect is graphically illustrated for the highest dose specimen (4 X 10 ” N2+ cm-‘) (Fig. 5(a)) as a threefold increase in the total nitrogen penetration depth compared with that for the lowest dose specimen. However, other effects, such as radiation-enhanced diffusion (effective at temperatures greater than about 200 “C [23]) may also contribute, especially considering the similarity of the nitrogen profile trail for the high dose implantation (Fig. 5(a)) to the enhanced penetration observed for high current density nitrogen-implanted 304 stainless steel (Fig. 1) [24]. Nitrogen concentrations at the surface decreased as the calculated dose level was increased, i.e. 6.3 at.% for 10” N2+ cm-*, 5.7 at.% for 2 X 10” N,+
0
75
225
150
DEPTH [NM) (a)
0
10
20
30
40
50
DEPTH (NM) (b)
Fig. 5. AES profiles for nitrogen-implanted nitrogen penetration depth for the highest nitrogen concentrations in very-near-surface
TABLE
W2+cm
showing (a) enhanced specimen and (b) high
I
Summary of RCF cylinders Dose
440C steel specimens dose (4 X 1O1’ Nz cm-‘) regions.
-2
1
nitrogen
concentrations,
Maximum N concentration (at.%)
ranges
Maximum N penetration (to 1 at.% N) (nm)
and
integrated
areas
Integrated area (at.% nm X 10d3) Surface
to N mean
for
three
implanted
(% of total) N mean
to bulk
Total
10”
32.1
108
0.50 (31.4%)
1.09 (68.6%)
1.59
2 x 10”
30.6
126
0.36 (27.2%)
0.94 (72.3%)
1.30
4 x 101’
31.0
360
0.72 (25.1%)
2.15 (74.8%
2.81
9
cmp2 and 2.4 at.% for 4 X 10” N,+ cm-*. These different surface nitrogen concentrations modified the oxide composition from the normal M20, type (where M 1s Fe,.,, Cr0.14) of oxide common for 440C steel [25], except for the high dose (4 X 10” N,+ c m-*) specimen which exhibited an oxide resembling the unimplanted material (but with some N substitution). The other implanted specimens exhibited an (M30,)N,,55 type of oxide with similar iron-to-chromium ratios as for unimplanted 440C steel. 3.3. Rolling contact fatigue test results Tables 2 and 3 list the calculated BlO and B50 Weibull lifetimes, fatigue life ratios (normalized with respect to unimplanted 440C steel fatigue lifetimes) and confidence levels for the 4.04 GPa (586 klbf in-*) and 5.43 GPa (786 klbf in*) hertzian stress tests respectively. For the lower stress level of 4.04 GPa (586 klbf in*) (Table 2), all the B50 fatigue lifetimes (which TABLE
2
Weibull tests
fatigue
lifetimes,
life
ratios
and confidence
Confidence level (%)
levels
for
4.04
B50 life (X106 cycles)
GPa hertzian
Life ratiob
Confidence level (%)
N2+ dose -2 (ions cm )
BlO life (X106 cycles)
Life ratioa
Unimplanted
3.4
-
specimen 10” 2 x 10” 4 x 10’7
9.Q 5.4 6.6
2.65 1.59 1.94
88 71 77
15.9 16.0 16.0
2.21 2.22 2.22
99 99 99
All specimens
7 .O
2.1
78
16.0
2.2
99
aNitrogen-implanted bNitrogen-implanted
TABLE
3
Weibull tests
fatigue
lifetimes,
life ratios
-
7.2
B10 us. unimplanted B50 us. unimplanted
BlO. B50.
and confidence
levels
for
5.43
GPa hertzian
N2+ dose -2 (ions cm )
BlO
life (x106 cycles)
Life ratioa
Confidence level (%)
B50 life (X106 cycles)
Unimplanted specimen 10” 2 x 10” 4 x 10”
1.5
-
-
3.0
1.9 2.2 1.7
1.27 1.47 1.13
61 65 55
5.0 8.2 8.6
1.67 2.73 2.87
97 99 99
All specimens
1.93
1.3
60
7.3
2.43
99
aNitrogen-implanted bNitrogen-implanted
B10 us. unimplanted B50 us. unimplanted
stress
BlO. B50.
Life ratiob
stress
Confidence level (%) -
10
exhibited increases of about 2.2X over that for unimplanted 440C steel) were statistically significant with confidence levels of 99%. Levels of confidence below about 80% are not considered as statistically “significant” improvements. These results are encouraging, but tempered by the fact that BlO lifetimes carry more impact for aerospace bearing systems. For the BlO values, only the low dose (1 X 10 l7 N,+ cme2) specimen showed statistical significance, and its lifetime improvement ratio of 2.65X was dramatic. From the results presented in Table 2, it appears that fatigue lifetimes are insensitive to retained nitrogen concentration and maximum nitrogen penetration depth for the B50 lifetimes. In contrast, for the BlO values, the smallest lifetime improvement was observed for the nitrogen-implanted specimen with the lowest retained nitrogen concentration (i.e. 2 X 1017 N2+ cmp2). Since there did not appear to be any clear correlation of RCF lifetime with implanted N,+ dose or penetration depth, statistical analyses were performed on all the nitrogen-implanted specimens taken as one set. From the analysis for the 4.04 GPa tests, an extrapolated BlO lifetime of 7.0 X lo6 cycles and a B50 lifetime of 16.0 X lo6 cycles were determined. These values represent increases of 2.1X and 2.2X respectively in the BlO and B50 RCF lifetimes for the nitrogen-implanted specimens compared with that of the unimplanted specimen. Confidence levels for the nitrogen-implanted BlO and B50 lifetimes were 78% and 99% respectively, indicating that only the mean lifetime improvements were statistically significant. For the higher hertzian stress level tests (Table 3), the magnitude of the BlO lifetime improvements was lower than that for the lower stress level tests and not a single increase was considered statistically significant. As for the lower stress test results, the B50 values were all statistically significant and there appears to be a slight correlation of fatigue lifetime with maximum nitrogen penetration depth. For example, increasing the implanted nitrogen dose increased the maximum nitrogen depth (Table 1) which also coincided with increased RCF lifetimes (Table 3). For the 5.43 GPa stress level, BlO and B50 fatigue lifetimes for the nitrogen-implanted specimens, taken as a single population, were 1.93 X lo6 cycles and 7.3 X lo6 cycles respectively. The BlO lifetime improvement of 1.3X was more modest than that observed for the lower stress test results. In addition, the confidence levels for the nitrogen-implanted specimens taken as a whole were 60% for the BlO and 99% for the B50 lifetimes, which indicates the statistical significance for the BlO lifetime was much lower for the higher stress level tests.
4. Discussion The observed improvements in RCF lifetimes for nitrogen-implanted 440C steel reported here are in contrast with the conventional notion that only surface modifications to the depth of maximum cyclic plasticity (and thus maximum shear stresses) are effective for pure rolling and rolling plus sliding conditions [26,27]. For the latter conditions, early studies at Battelle
11
[28] predicted that reductions in the coefficient of friction would lower the peak subsurface octahedral shear stress and move the planes of octahedral shear stress to deeper locations. These modifications favor longer RCF lifetimes from (1) increased crack initiation periods from the reduced shear stresses and (2) longer crack propagation periods because of the increased ligament length to the surface. For nitrogen-implanted 440C steel, unlubricated test results have shown (1) no reductions in friction or wear for a 52100 steel us. nitrogen-implanted 440C steel wear couple [18], (2) 25% reductions in friction and wear for a 440C steel vs. nitrogen-implanted 440C steel wear couple [29] and (3) 0.8 reduction in starting torque for a nitrogen-implanted 440C steel bearing ball and race assembly [ 301. All these results were for unlubricated wear couples. However, friction and wear data are lacking for the exact test conditions used in the RCF test in this study, i.e. 52100 steel against nitrogenimplanted 440C steel under oil-lubricated conditions. Other studies of unimplanted vs. implanted material couples tested with synthetic polyester lubrication have shown reduced wear volumes and wear variability for unimplanted 4140 steel and O-6 steel against 52100 steel implanted with titanium or titanium plus carbon [31]. These wear improvements were observed in spite of actual increases in the coefficients of friction [ 311. Microstructural modifications from nitrogen implantation of 440C steel include 1251 (1) formation of nitride ((Fe, Cr),+,N(x = 0.47 - 1.2)) and/or carbonitride ((Fe, Cr),.,(C,.,N,.,)) compounds, (2) reduced retained austenite content from radiation damage and induced stresses from volume expansion associated with nitride formation, and (3) nitrogen incorporation in and modification of the oxide stoichiometry. Formation of hard nitride particles would not be expected to add a hardening increment, since 440C steel exhibits high intrinsic hardness (i.e. a Rockwell C hardness of 59 or higher). The effects of austenite content on RCF resistance are inconclusive at this time [32]. The last effect, that of oxide modification, has been speculated to be very important by Japanese investigators [ 301, who suggest that nitrogen incorporation in the oxide increases its adhesion to the substrate and thus promotes self-lubrication. Thus the speculated reason for the increased RCF lifetimes from nitrogen implantation of 440C steel appears to be maintenance of a low-friction oxide film which reduces the magnitude of the subsurface shear stresses which in turn increases the crack initiation time. 5. Summarizing
remarks
Parameter development studies were performed to establish more optimum implantation conditions for 440C steel, to prevent excursion into the temperature range for tempered martensite embrittlement and to prevent bulk tempering. These conditions were identified as a current density of 107 /LA cmp2 and exposure times of up to 120 min at 255 “C. RCF tests on 440C steel specimens implanted under these conditions showed reproducible imnrovements in BlO lifetimes with larger increases (up to 2.65X) for the
12
lower stress level tests. No apparent dependence of lifetime on retained concentration was observed, although a slight correlation of higher B50 lifetimes with increased nitrogen penetration was found. The consistent improvements in RCF life for nitrogen-implanted 440C steel appear to be due to modification of oxide composition, promoting lubricity and enhanced oxide adhesion.
References 1 I. L. Singer, MRS Symp. Proc., 27 (1984) 585. 2 K. V. Jata and E. A. Starke, Jr., J. Met., 35 (1983) 23. 3 W. W. Hu, H. Herman, C. R. Clayton, J. Kozubowski, R. A. Kant, J. K. Hirvonen and R. K. MacCrone, in C. M. Preece and J. K. Hirvonen (eds.), Ion Implantation Metallurgy, The Metallurgical Society of AIME, Warrendale, PA, 1980, p. 92. Naval Research Laboratory, 4 G. K. Hubler, Materials Modification Branch, Washington, DC, personal communications. 5 F. M. Kustas, M. S. Misra and P. Sioshansi, MRS Symp. Proc., 27 (1984) 686. 6 F. M. Kustas and M. S. Misra, Progress Rep. R85-48681-002, 1985 (Martin Marietta Aerospace). 7 N. E. W. Hartley, J. Vat. Sci. Technol., 12 (1975) 485. 8 M. N. Gardos, in A. G. Gray and R. T. Nach (eds.), Conservation and Substitution Technology for Critical Metals in Bearings and Related Components for Industrial Equipment and Opportunities for Improved Performance, BuMines OFR 21-85, 1985, p. 370 (U.S. Bureau of Mines, Washington DC). 9 L. 0. Daniels and P. J. Wilbur, Nucl. Instrum. Methods Phys. Res. B, 19 20 (1987) 221. 10 M. Rangaswamy, D. Farkas and H. L. Sobel, Nucl. Instrum. Methods Phys. Res. B, 19 - 20 (1987) 196. 11 F. M. Kustas, M. S. Misra, S. R. Smith and W. S. Sampath, to be published. 12 Heat Treatment Specification DCN 28433 (NASA Marshall Space Flight Center). 13 P. J. Wilbur and L. 0. Daniels, Vacuum, 136 (1 3) (1986) 5. 14 D. Glover, in T. C. Hoo (ed.), Rolling Contact Fatigue Testing of Bearing Steels, ASTM, Philadelphia, PA, 1982, p. 107. 15 R. B. Abernethy, J. E. Breneman, C. H. Medlin and G. L. Reinman, Weibull Analysis Handbook, Government Rep. AFWAL-TR-83-2079, 1983, p. 1093. 16 L. E. Davis (ed.), Handbook of Auger Electron Spectroscopy, Physical Electronics Division, Perkin-Elmer, Eden Prairie, MN, 2nd edn., 1978. 17 J. F. Moulder, Physical Electronics Division, Perkin-Elmer, personal communication, 1986. 18 F. M. Kustas and M. S. Misra, Progress Rep. R85-48681-002, 1985 (Martin Marietta Denver Aerospace). 19 G. Krauss (ed.), Principles of Heat Treatment of Steel, American Society for Metals, Metals Park, OH, 1980, p. 188. 20 B. Low and B. L. Averbach, Metall. Trans. A, 14 (1983) 1899. 21 R. L. Thorn, NASA Marshall Space Flight Center, Huntsville, AL, personal communication, 1986. 22 I. Manning, in F. A. Smidt (ed.), The Use of Ion Implantation for Materials Processing, NRL Memo. Rep. 4527, 1981, p. 27 (Naval Research Laboratory, Washington, DC). 23 H. Wiederesich, Nucl. Instrum. Methods Phys. Res. B, 7 - 8 (1985) 1. 24 L. 0. Daniels, M.S. Thesis, Colorado State University, 1986.
13 F. M. Kustas, D. L. Williamson and M. S. Misra, Nucl. Instrum. MethodsPhys. Res. B, 31 (1988) 393. 26 V. Bhargava, G. T. Hahn and C. A. Rubin, Metall. Trans. A, 18 (1987) 827. 27 G. T. Hahn, V. Bhargava, G. Ham, A. Kumar and C. A. Rubin, Proc. ASM Session on Tribological Mechanisms and Wear Problems in Materials, 1987. 28 T. G. Johns, S. G. Sampath, J. C. Bell and K. B. Davies, Engineering Analysis of Stresses in Railroad Rail, Phase I, Battelle Columbus Laboratories, 1977 (FRA Contract DOT-TSC-1038). 29 D. M. Follstaedt, F. G. Yost and L. E. Pope, MRS Symp. Proc., 27 (1984) 655. 30 M. Hirano and S. Miyake, Extension of bearing endurance life by ion implantation, Appl. Phys. Lett., 49 (1986) 779. 31 P. Sioshansi and J. J. Au, Improvements in sliding wear for bearing-grade steel implanted with titanium and carbon, Mater. Sci. Eng., 69 (1985) 161. 32 E. V. Zaretsky, Selection of Rolling-Element Bearing Steels for Long-Life Application, NASA Tech. Memo. 88881, 1986 (NASA Lewis Research Center, Cleveland, OH).
25