Use of pulsed high power ion beams to enhance tribological properties of stainless 5Al/a--98- I J b LC- steel, Ti, and Al C O ~ F -5sofa9--

D. Cowell Senft, T.J. Renk, M.T. Dugger

Sandia National Laboratories, PO Box 5800, Albuquerque, NM 87185-0340

K.S. Grabowski

Naval Research Laboratory, Washington, DC 20375

M.O. Thompson

Department of Materials Science, Cornell University, Ithaca, New York 14853 /

Abstract

Enhanced tribological properties have been observed after treatment with pulsed high power

ion beams, which results in rapid melting and resolidification of the surface. We have ueated and

tested 44oc martensitic stainless steel (Fe-17 Cr-1 C). Ti and Al samples were sputter coated and

ion beam treated to produce surface alloying. The samples were beated at the RHEPP-I facility at

Sandia National Laboratories (0.5 M Y , 0.5-1 p at sample location, 4 0 J/cm2, 1-5 jm ion range).

We have observed a reduction in size of second phase particles and other microstructural changes in

44OC steel. The hardness of treated 44oC increases with ion beam fluence and a maximum

hardness increase of a factor of 5 is obtained. Low wear rates are observed in wear tested of treated

44OC steel. Surface alloyed Ti-Pt layers show improvements in hardness up to a factor of 3 over

untreated Ti, and surface alloys of AI-Si result in a hardness increase of a factor of two over

untreated Al. Both surface alloys show increased durability in wear testing. Rutherford

I

Backscattering (RBS) measurements show overlayer mixing to the depth of fhe melted layer. X-ray

Diffraction (XRD) and TEM confirm the existence of metastable states within the treated layer.

Treated layer depths have been measured from 1 - 10 pm.

h

DISCZAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or use- fulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any spe- cific commercial product, process, or service by trade name, trademark, manufac- turer, or otherwise docs not necessarily constitute or imply its endorsement, mom- mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

Introduction

Metastable microstructures produced by the rapid solidification of liquid metals have been

shown to produce enhanced wear resistance, corrosion resistance, and increases in hardness.[ 11

Rapid melt and resolidification (RMR) of metals surfaces by pulsed laser treatment[2,3] can be

used to produce hardened surfaces or chemically different surface alloyed layers. However, laser

treatment has inefficient coupling to metal surfaces, produces surface roughening due to

vaporization, and is confined to small treatment spot sizes. Treatment and surface alloying by

pulsed high power ion beams (HPIB) can produce rapid melt and resolidification without these

restrictions [4,5].

We report on sample treatment with the RHEPP- facility at Sandia National Laboratories

(0.5 MV, 0.5-1 p at sample location, 1-5 p ion range).[6] Fluences of 1-10 J/cm2 are produced

at the sample surface. The ions, which can be single species OM the MAP (Magnetically confined

Anode Plasma) source or a mixed species beam (C and protons) from the flashover source,

impinge on the surface to produce rapid heating. Conduction of the heat into the bulk of the sample

results in rapid solidification at a rate of lo9 Ws [7] Implantation effects due to the ion beam are

negligible since only 3 ~ 1 0 ' ~ ions/pulse are required to melt the surface, amounting to aimplanted

ion concenmtion of less than atomic percent over the ion range.

The surface can be alloyed by applying an overlayer to the sample before treatment.

Convective forces mix the overlayer into the melted substrate during treatment. The resulting

interface between the surface alloy and the substrate is ,graded; eliminating the adhesion problems

associated with highly stressed coatinghubstrate interfaces.

2

HPlB treatment results in surfaces that are topographically wavy with the magnitude and

waveIength dependent upon the.fluence of the treatment. The escape of low vapor pressure

impurities, when present, produces microcraters on the treated surface. Treating the surface with .

multiple pulses can reduce the number of microcraters. The differential cooling rate, in which the

contraction of the melted layer is hindered by the substrate, produces a tensile residual stress in the

treated layer.

Procedure

The effect of RMR on the hardness and tribological properties of Al(6061-T6), Ti-6Al4V

(an a+p titanium alloy), and martensitic stainless steel 44OC @e-- 17Cr- 1C) has been investigated.

Aluminum has been surface alloyed with sputter co-deposited layers of 60% Al - 40% Si and 80%

Al - 20% Si with the intention of creating wear resistant surfaces. Titanium has been surface

alloyed with alayerof 90% Ti - 10% Ptprimarily to increase corrosion resistance, but also has

shown improvements in tribological properties. The effect of RMR on corrosion resistance is

described elsewhere.[8]

The samples were polished to a mirror finish before treatment with an avewze roughness,

R, of 20 nm or better. The 44OC samples were initially heat-treated, quenched, and tempered for

maximum hardness followed by polishing. The hardness of the samples was measured by

nanoindentation with a load of 0.2 g. The depths of the indents was approximately 90 nm for the

44OC and Ti samples and less than 200 nrn for the thicker treated layers on the Al samples.

Hardnesses were also measured on untreated polished control samples for comparison.

Wear testing was primarily done with a linear reciprocating tribometer with a ball-on-flat

geometry. This apparatus allowed the testing of small, uniform fluence areas of the sample. The

44OC steel samples were also tested on a pin-on-disk tribometer. The counterface material for the

tests was a 44OC steel ball bearing for the reciprocating tribometer and a&C steel pin for the pin-

on-disk tribometer. Testing was done in ambient air. The nominal Hertzian contact pressure was

0.8 GPa for the Si samples and 1.3 GPa for the Ti and440C samples. Wear volumes could not be

measured due to the roughness of the treated samples and the low wear rates. Instead the number of

wear cycles to failure (defined as occurring when a friction coefficient of 0.5 was reached) was

3

. , . ~. . . . . . "

. .

measured as an indication of the durability of the surface. Measurements were also made on the

untreated, polished samples for comparison.

Results

Martensitic stainless steel

RMR treatment of 44OC resulted in a melt depth of 2.5 p. Figure 1, showing SEM

images of the surface before and after treatment at a fluence of 4 J/cm2, indicates that second phase

particles have been dispersed by the RMR. A martensitic microstructure predominates on the treated

surface, and microcratering is visible. The dark areas in the treated micrograph, Fig. l(b), are holes

probably introduced from the tensile residual stress in the treated layer. The average roughness of

the treated surface is -600 nm. The hardness is plotted as a function of the ion beam fluence in Fig.

2 showing increases in hardening with fluence. The results are compared with ameasured hardness

of 1.2GPaforan untreated, polished440C control sample. The error bars on the hardness values

are given by the standard deviation of the results, and the increase in spread of the hardness values

with increasing fluence is due to the roughening of the surface caused by RMFL Fiopre 2 illustrates

that treatment at a fluence of 4 J/cm2 produces a factor of 5 increase in the surface hardness of 44OC

stainless steel. Nanoindentation testing on the treated cross section indicates that hardening is

-.

confined to the treated layer.

Untreated and RMR treated martensitic stainless steel 44OC samples were subjected to pin-

on-disk wear testing. Figure 3 shows a plot of the coefficient of friction as a function of the number

of wear cycles for the 4 J/cm2 treated and untreated samples. The treated sample maintains a

si,onificantly lower friction coefficient than the untreated sample. Figure 4 shows scanning electron

micro,orphs of the wear tracks formed on the samples during testing. The untreated surfaces

showed evidence of substantial wear compared to the RMR treated samples. Inspection of the 44OC

pins used as counterfaces on the tests of the treated samples showed wear of the pin surface. Wear

of the RMR surface was negligible.

Aluminum

Al(6061-T6) samples with overlayers of either 80% AI - 20% Si or 60% Al - 40% Si have

been treated with either the flashover ion source or the MAP ion source using nitrogen gas. The 1.1

4

., , . .. ,

-

pm overlayers were sputter co-deposited. Cross sectioning of the 80 % Al - 20 % Si overlayer

sample treated with the flashover source shows that the thickness of the treated layer ranges from 7

pm at the high fluence end to c1 pm at the low fluence end. The Al samples were 5.5 cm - long

and were treated with a range of fluences from 1.3 - 2.9 J/crn2 by the flashover source and from

1.6 - 3.7 J/cm2 by the MAP nitrogen source. The average roughness of the treated surfaces was

700 nm. The hardness, measured by nanoindentation was 1.3 k 0.1 GPa compared to 0.6 GPa for

untreated Al6061. The hardness measurements were independent of treatment fluence with the

exception of the extreme low fluence ends of the sample where the indentation results were

influenced by the substrate because of the thinness of the treated layer.

Wear test results for the 60% Al - 40% Si sample treated with the MAP nitrogen diode and

the 80% Al-20% Si sample treated with the flashover diode are shown in Figs. 5 and 6. The

durability of the surface alloyed Al depends strongly on the fluence of the beam treatment. The

maximum effect is produced with a fluence of 1.7 J/cm2, independent of ion beam source and.

composition. The durability of the- Al-Si surface alloyed layers is much lower for both higher and

lower beam fluences, but s t i l l exceeds the performance of untreated Al6061. In the best case, the

durability of the Al-Si surface alloy exceeds that of untreated aluminum by a factor of 100.

Cross-sectional TEM (transmission electron microscopy) was used to examine the high

durability region of the 60% M40% Si overlayer sample. Figure 7 shows a TEM image of the

interface between the treated and untreated regions. Second phase particles present in the lower,

untreated region are absent from the melted layer. The darkfield TEii image of the melted layer in

Fig. 8 shows small Si-rich particles approximately 10 nm in diameter. These particles are found in

the top 1.5 pm of the 3 pn - deep melted layer. Microprobe analysis indicates that the Si

concentration in this region of'the treated layer is - 3 wt.%, which exceeds @e solid solubility limit

for Si in Al. These metastable precipitates form during the rapid solidification of the treated layer.

The presence of'these small particles may account for the improvement in durability seen in this

portion of the sample by improving the fatigue properties of the material.

. '- I. .

Titanium

RMR treatment of Ti Grade 2 (commercially pure a-Ti) and Ti Grade 5 (Ti-6Al-4V) alters

the microstructure as shown in cross-section micrographs in Fig. 9. Homogenization of the surface

occurs. Note the absence of structure, particularly in the Ti Grade 2 sample. The treated layer is

clearly visible, and the distinct grains appear to stop at that layer. While less pronounced in the Ti

Grade 5 sample, there is s t d l evidence of homogenization of the treated layer, with less pronounced

grain boundaries and beta phase Ti present Titanium exhibits less microcratering than ferrous or

aluminum materials with treated average surface roughnesses of 200 nm.

RBS measurements on a RMR Ti Grade 2 sample with a 180 nm pure Pt overlayer confirm

that the overlayer is thoroughly mixed into the treated layer. There is a maximw R concentration

of 24-30 a t 96 at the surface, decaying to a concentration of 5 at. 96 at a depth of 1 p m X-ray

difhction from the treated layer shows unidentifiable lines, presumably due to metastable states in

the treated layer.

Hardness of the surface alloyed samples was improved by a factor of 3 in some cases.

Wear testing of a Ti Grade 5 sample with a sputter co-deposited overlayer of 90% Ti - 10% Pt and

treated with the M A P nitrogen ion source shows increased durability as compared to both untreated

Ti Grade 5 and treated, unalloyed Ti Grade 5 as shown in Fig. 10. The surface oxide layer of the

untreated Ti Grade 5 is breached in less than 10 cycles as indicated by the rapid rise in friction

coefficient. In the best case, the treated surface maintains a friction coefficient below 0.3 for 1800

cycles. The wide variation in the resurts for the surface alloy may be due to the roughness of the

treated sample. The depth of the treated layer for this sample was 2.5 p n

SEM photos of the wear tracks on the untreated Ti Grade 5 sample, untreated Ti Grade 5

sample with the 90% Ti- 10% Pt overlayer, and the treated, surface alloyed 90% Ti- 10% Pt

overlayer sample are shown in Fig. 12 (a), (b), and (c), respectively. The wear track in Fig. 12(c) .'

conesponds to a wear test 1u11 for 2000 cycles and shows negligible wear.

6

Conclusions

We have used the technology of repetitive pulsed intense ion beams to rapidly heat and cool

the surface of three types of metal alloys - aluminum, iron, and titanium. We have also investigated

surface alloying, where the beam is used to mix a predeposited thin-film coating into the melted

layer of the substrate. Improvements to surface hardness and durability have been demonstrated for

44OC steel and Al-Si and Ti-Pt surface alloys. Low wear has also been observed on these RMR

surfaces.

Acknowledgements

This work was supported by the United States Department of Energy under Contract DE-ACW

94AL85000. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed

Martin Company, for the United States Department of Energy.

We would like to acknowledge the work of David Schmale, Alice Kilgo, Elizabeth Sorroche, Paul

Hlava, Thomas Headley, and Bonnie McKenzie for sample analysis.

References

1. T. R. Anantharaman and C. Suryanarayana, in Rauidv Solidified Metals: A Technolo$cal Overview (Trans Tech Publications, Switzerland, 1987, Rauidlv Solidified Allovs, ed. by Howard H. Liebermann (Marcel Dekker, Inc., New York, 1993).

2. J. M. Rigsbee, Surface Engineering by Laser-and Physical Vapor Deposition Techniques, J. Met., Aug 1984, p. 31.

3. C. W. Draper, Laser Surface Alloying: The State of the Art, J. Met., June 1982, p. 24.

4. A.D. Pogrebnyak, Phys. Stat. Sol. (a) 117, 17 (1990).

_’ 5. M. Nastasi, , R. Fastow, J. Gyulai, J. W. Mayer, S. J. Plimpton, E. D. Wolf, and B. Manfred Ullrich, Nucl. Instr; Meth. Phys. Res. B7/8, 585 (1985).

6. R. W. Stinnett, R. G. Buchheit, F. A. Greulich, C. R. Hills, A. C. Kilgo, D. C. McIntryre, J. B. Greenly, M. 0. Thompson, G. P. Johnston, and D. E Rej, Mat. Res. SOC. Symp. Proc. 316, 521 (1994)..

7. M. 0. Thompson and T.J. Renk, “Numerical Modeling and Experimental Verification During Pulsed Ion Beam Surface Treatment”, submitted to MRS Proceedings.

8. T.J. Re&, R. G. Buchheit, N.R. Sorensen, D. Cowell Senft, M. 0. Thompson, and K. S. Grabowski, to be published, Physics of Plasmas, May 1998.

- ,-

Figure 1. (a) SEM image showing the untreated 44OC surface and the size of second phase particles. (b) An SEM image of the martensitic 44OC surface after treatment The dark areas are holes.

7

6

2

1

1 I 1 - - -

0

Control t 4 J/cm2 increasing fluence level

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8

Figure 2. The hardness of 44OC steel can be increased by a factor of 5 by RMR. Hardness increases with the fluence level of the treatment. The sample at the second lowest fluence received 15 shots and the others received 1 shot. Hardness values are relative.

0.6

0.5

0.4

0.3

0.2

0.7 1 I -.

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9

Figure 3. Variation of the coefficient of friction with increasing number of wear cycles in the pin on disk test indicating improved wear resistance for the RMR treated sample of 44oC stainless steel.

- 1OcLm/100 jlm (upper)/(lower)

Figure 4. Scanning electron micrographs of wear traces from pin on disk tests for untreated (left) and FWR treated (right) 44OC martensitic stainless steel samples. The wear tracks for the RMR treated samples are barely visible indicating improved wear resistance.

10

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Wear Cycles

Figure 5. Wear test results for-60% Al - 40% Si surface alloyed by the MAP nitrogen diode and shown for the control sample and three fluence treatment levels.

. Figure 6. Wear test results for 80% Al - 20% Si surface alloyed by the flashover diode and shown for the control sample and three fluence treatment levels..

I f

Figure 7. Transmission electron micrograph of the interface between the treated and untreated layers in the 80% Al - 20% Si surface alloy. The precipitates in the lower, untreated region are absent from the upper, treated region.

Figure 8. DarkfTeld transmission electron micrograph showing the small silicon-rich precipitates found in the top 1.5 pn of the treated layer.

12:

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- - -.Untreated Ti -----1- U ntreated Ti-Pt- -

-Treated. Ti-Pt

-

-

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Grade 2 Ti Grade 5 Ti

Figure 9. Electron microprobe images of Ti-2 and Ti-5 cross-sections. Note the absence of structure in the treated layer.

0 " " " " " " " " " ' 400 800 1200 1600 2000

Wear Cycles

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