Surface mechanical properties - effects of ion implantation

Nuclear Instruments and Methods 182/183 (1981) 887-898 © North Holland Publishing Company

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S U R F A C E MECHANICAL PROPERTIES - EFFECTS OF ION I M P L A N T A T I O N

Herbert HERMAN Department of Materials Science and Engineering, State University of New York, Stony Brook, New York 11794, USA

Ion implantation has been used to modify the mechanical properties of a wide range of metals and alloys. The affected properties which have been studied include friction and wear, erosion and fatigue. Both BCC and FCC systems have been examined, with the major effort being directed at the former, due to the strong influence of interstitial implantants on mechanical properties of BCC and because of the industrial utility of these alloys. In seeking the microstructural origins of these sometimes dramatic effects, researchers have employed numerous surface analysis techniques, including backscattering and electron spectroscopy, TEM, SEM, X-ray and MSssbauer analysis and internal friction measurements. The interactions of surface dislocation structures with implantation-induced imperfections, surface alloying, and precipitation phenomena are discussed. A review is given of the current status of activities as represented by a number of research groups.

1. Introduction A central goal of physical metallurgy is to improve physical properties through the control of microstructure. Thermal and/or mechanical treatment can have dramatic effects on mechanical properties; the microstructural changes induced by these processes are discernible with standard electron and optical microscope techniques. Herein lies the frustration of explaining the significant changes in mechanical properties which are frequently observed when ions are implanted into only the outer several hundreds of ~mgstroms of metals. Crystalline materials become stronger when the dislocations are made more difficult to move and multiply, and in some complex way the implantation products interact with dislocations. We wish to seek relations between implantationinduced structure (e.g., defects or new phases) and dislocations on operative glide planes. The earliest work originates from Harwell, where Dearnaley and co-workers demonstrated that the implantation of nitrogen into steel increases fatigue life and improves mechanical wear properties [ 1 - 5 ] . Since those earliest studies, a number of research groups have examined the influence on mechanical properties of a range of implanted species in BCC and FCC host metals. Studies have been extended to various surface mechanical properties including different forms of wear, erosion, fatigue in all of its facets, and more subtle features of dislocation motion as manifested by internal friction phenomena. It is clear from these many studies that implantation of

energetic ions does indeed significantly modify the above mentioned properties. The origins of these effects has been the motivating force behind many of the recently carried out experimental programs. In this paper will be discussed some important features of implantation4nduced changes in fatigue properties, wear, and erosion. The bases for these effects can, in principle, be determined from microstructural analysis, but the results to date have been difficult to obtain, to explain, and to correlate. It is interesting that those mechanical properties which can be affected by ion implantation are those mainly influenced by surface condition, and it has been just these properties which have been so difficult to understand generaUy. For example, there have been literally thousands of papers published on the origins of fatigue failure in metals and alloys, with any number of suggested models of crack initiation. But the field remains unsettled, with deep controversies still raging [6]. Into this situation enters ion implantation, which itself is by no means well understood. Furthermore, some of the most significant implantation-induced effects are observed in commercially alloys. Here one is attempting to explain complex mechanical behavior, effected by difficultto-characterize implantation processes, in highly impure, structurally and chemically inhomogeneous alloys. (This situation, it is important to note is consistent with the best tradition of industrial physical metallurgy.) More satisfying explanations of the effects of implantation on mechanical properties will require great VII. ION-IMPLANTED METALS

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care artd control as well as high purity, single crystal host specimens. Such work is starting to be carried out, as will be noted in this review.

2. Fatigue * Fatigue represents a singularly dangerous mode of material failure, in that no obvious prior warning is given of impending fracture. Generally, such failure occurs upon the cyclic loading at some stress below the fracture stress. High loading amplitudes give rise to short lifetimes (!0w-c_ycle fatigue), whereas relatively low loads yield longer lifetimes (high-cycle fatigue). In the case of ferrous alloys there is a stress level, referred to as the endurance limit below which failure never occurs, with lifetimes exceeding 108 cycles. The commonly employed rotating-bend test, involving a specimen of circular cross-section with a reduced central section, is seen in fig. 1. In this test, one end of the specimen is gripped in a chuck and loaded at the other end. As the specimen rotates, the stress at a given point on the surface of the reduced section varies sinusoidally, being in tension at the upper region and compression in the lower region. The maximum stress amplitude is always the same; therefore this test is referred to as one of constant stress. Any work-hardening which occurs during cycling will give rise to a decreased strain. In the constant strain test, the strain amplitude is constant, so that the stress amplitude will generally vary. While constant stress tests are the most common, constant strain amplitude tests are used as well, especially in studies of dislocation phenomena. Fatigue fracture represents the initiation and propagation of a crack in a sequential manner [6,7]. It is thought generally that crack initiation occurs at or near the specimen's surface. Crack propagation is a macroscopic phenomenon which gives rise to failure when the fracture toughness of the material ultimately is exceeded. For our purposes, crack initiation will be investigated as the effectuating process by which ion implantation operates. Dislocation mechanisms are thought tb give rise to those defects which will finally form the fatigue crack. The means by which dislocations are effected by implantation will thus be the central question on the matter of fatigue * Excellent reviews on fatigue can be found in refs. [6] and [71.

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failure as well as any surface-controlled failure mechanism. Let us briefly review the results of several investigations. The preliminary work on fatigue of ion implanted metals was carried out by the Harwell group showing the influence of nitrogen implantation into low carbon steel [1]. The early experiments showed significant extensions of fatigue life, and were indeed dramatic. This prompted other workers to attempt to quantify this effect. As part of a Stony B r o o k - N R L program, Hu et al. implanted molecular nitrogen at I

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Fig. 2. Fatigue lifetime in ambient air for three different rods of AISI 1018 steel (0.18 wt.% C) for as-received material (unimplanted), implanted with molecular nitrogen at 150 keV to a dose of 2 X 1017 ions/cm2 ' and implanted and aged. The rotated bending mode had constant stress amplitude at 5000 rpm.

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Fig. 3.SIN plot for ?,ISI 1018 steel. See fig. 2 fo~ details. 150 keV into AISI 1018 steel (0.18 wt.% C) to doses of 2 × l017 ions/cm 2 and observed increases in fatigue life [8,9]. However, major fatigue lifetime enhancement was not observed until the spechnen was aged for several months at room temperature ("naturally aged") or for 6 h at 100°C ("artificia.Uy aged"). These res~dts are reviewed in fig. 2 for three speehnen conditions. A common way of displaying fatigue behavior is with an SIN plot, such as seen for these experiments in fig. 3. Each data point represents the lifetime (in cycles to failure, i.e., the occurrence of a crack) for a given stress amplitude. O|" particular interest is the tlofizontal regime at high cycles, the end'arance linut, below which no failure occurs. It ~hould be noted that the SIN plot represents the division between safe and unsafe regimes of operation, though notches and other surface imperfections will significantly modify the plot for a wide range of metals and alloys. [on implantation is most likely to affect crack irfitiafion and not crack propagation. The SIN plot is actually a reflection of the latter, so that its modification with implantation should be treated with caution. Clearly, nitrogen implantation into a low carbon steel will improve fatigue resistance, especially following a low temperature anrte',ding treatment. For this commercial material, it must be noted that the metallurgical considerations are most complex, the alloy being comprised of a relatively soft fertile phase into whJ.ch is embedded pcarlite, a duplex structure made up of alternate layers of iron carbide (Fe~C) arid forrite. Fig. 4 is an optical micrograph of a polished and etched sample of as-received AISI 1018 steel showing the duplex structure of tire pearlite. In addition, prior

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Fig. 4. Polished and etched surface of as-received stock of MSI 1018 steel. The whit~ backgwund gram stxucture in BCC (ferritc) iron and the dark islands z~te pcaxlitc colonies, comprised of alternate layers of fertile and. carbide, Fe3C (cemcntite). Light micrograph. (Courtesy or' W.W. Hu).

Fig. 5. Tzansmission electron rtucrogral~b of pearlite coiorky showing dark ribbons of carbide (Fe..-3C) within, a fertile background, a. Ax-rccei',ed. b. Following implantation with molecular nilwgen with 2 × 10 t 7 ions/era 2 at 150 keV. VII. ION-IMPLANTED 5IETALS

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H. Herman /Surface mechanicalproperties

to implantation, carbon will be dissolved in the ferrite matrix. Nitrogen, gives rise to considerable radiation damage at the surface (<1000 A), and it is also likely that the pearlitic structure is disrupted on irradiation. Some indication of the extent of change that occurs within the pearlite can be seen in figs. 5a and 5b, which are TEM photos taken from a 1018 stee! specimen thinned from one side. Fig. 5a represents an unimplanted specimen, and 5b is for a specimen that has been implanted (molecular nitrogen to a dose of 2 X 1017 ion/cm 2 at 150 keV) and artificially aged. The latter shows a very high degree of phase decomposition and defected structure, which would be expected to modify dislocation behavior. The extent and type of this disruption needs further study. Though it is not apparent from the micrograph presented here, it would certainly be possible that a portion of the Fe3C near the surface will be dissolved and in some manner reprecipitated during the process of implantation. Nitrogen implantation at the energies employed in the Stony B r o o k - N R L work is expected to damage the metal and carbide lattices and to modify structure. Supersaturation of nitrogen will attend implantation, but as a result of the expected high interstitial solute diffusion at the implantation temperature, and damage-induced precipitate nucleation, phase decomposition is likely to occur. The aging phenomenon indicates that thermodynamic relief of excess free energy has not occurred during implantation. Precipitates and clusters which form during implantation are expected to be redissolved through ion collision with these phases giving, in effect, a steady state precipitate distribution. The steady-state metastable structures which are implantation4nduced can, in fact, decay to more stable phases, involving, for example, the decomposition of supersaturated nitrogen martensite (formed through implantation) into a metastable nitride. Evidence from transmission electron microscopy studies supports this concept: The as-nitrogen4mplanted steels contain 100 A particles of Fel6N2 at the surface, whereas annealing gives rise to an additional size range: a fine (~20 A) precipitate of Fea6N2, plus a mottled background indicating the precipitation of some electron contrast-inducing phase associated with an acicular product, i.e., martensite. It is suggested that the martensite was formed during implantation, but was not observed until decorated by the precipitating nitride (and/or carbide) [91. The central question to resolve is the manner in

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Fig. 6. Nitrogen profiles from nuclear reaction analyses employing the 147N(d, a) 1~C nuclear reaction using high resolution magnetic spectrometer to energy analyze emitted a particles. (Courtesy of R.A. Kant and J.K. Hixvonen and after Hu et at. ref. [9]). which the products of nitrogen implantation can have so significant an influence on fatigue behavior. A simple working model can be suggested: the fine precipitates of Fea6N2 act to both strengthen the ferrite phase and to make dislocation motion, and, consequently, surface-emerging slip more homogeneous. Major slip inhomogeneities are thus reduced, and fatigue lifetime is extended. Nitrogen profiles have been determined from nuclear reaction analysis of the fatigue specimens [9]. Fig. 6 shows three of these plots: (1) as-implanted; (2) implanted, aged and stress-cycled; (3) unaged and cycled. It can be seen that cycling in the absence of prior aging gives rise to a shift of carbon to the surface, whereas less of a shift occurs for the aged specimen. More integrated dislocation motion (i.e., accumulated strain) is expected to occur in the unaged case. More nitrogen is apparently being redistributed in the unaged specimen as well. Since the surface is expected to be softer for the unaged case, more dislocations will be present at this surface (as compared with the implanted-and-aged case) and, hence, greater pipeline diffusion would be expected (or alternatively, more interstinal-dislocation association). In an effort to examine the possible association of implantation product and dislocations, experiments

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H. Herman / Surface mechanical properties

were carried out by Hu et al. on the internal friction of the same type of steel, subjected to the identical thermal treatments and implantation conditions [9]. The experiments were carried out at temperatures from cryogenic to over 100°C. While a complex modification of the entire internal friction spectrum is achieved following implantation and implantationand-aging, of particular interest is the greater than 40% decrease in mechanical damping at room temperature (at a 1 kHz resonant frequency), due to implantation with 2 X 10 x7 ions/cm 2 of nitrogen. Furthermore, the damping is linearly dependent upon dose (from the lowest levels used), and, to a first approximation, appears related to an amplitudedependent dislocation-based mechanism. Aging, on the other hand, while being very influential on fatigue lifetime, has somewhat less of an effect on mechanical damping. It may simply be that the aging gives further, more substantial segregation of nitrogen to form nitrides at dislt~cations, thereby having a major effect on fatigue, but not on internal friction. If, for example, much segregation is already at hand, it is not likely that further segregation of nitrogen to dislocations will modify overall pinning density. The above fatigue model may be premature, and will certainly be open to criticism in that it does not consider all the previous models to explain fatigue behavior of materials. However, the salient features are indisputable: a fine precipitate forms on the aging of the implanted steel, and improved fatigue resistance is associated with this treatment * In another program on interstitials, workers at NRL have implanted nitrogen and carbon into the commercial titanium-based alloy, T i - 6 A 1 - 4 V and have studied fatigue resistance [10]. These results, in the form of SIN plots, are shown in fig. 7, where it can be seen that nitrogen implantations, with or without annealing, have only modest effects on fatigue behavior. Carbon, on the other hand, has a most important effect on lifetime. Annealing does not appear to enhance the effect unlike the case for carbon implantation into steel. Titanium and vanadium are known to be strong carbide formers, and implantation is expected to yield high quality carbides, and thus strengthen the surface region, thereby * We have planned in situ implantationqtigh voltage electron microscopy experiments at Argonne National Laboratory so that the evolving nitride can be more readily evaluated. This next phase of our studies will involve pure iron so that the implantation can be carried out into single-phase ferrite, without the attendant presence of pearlite.

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Fig. 7. SIN plot for carbon and nitrogen implanted Ti6A1-4V alloy. (After ref. [10] .) increasing fatigue resistance. A number of experiments have been carried out on implantation-modified fatigue resistance of FCC metals. There is a large amount of literature on normal fatigue studies of single and polycrystal aluminum and copper, both high purity and alloyed [7]. A major goal in many of these studies has been to evaluate the influence of alloying additions on earlystage crack initiation. For example, alloying elements can change the stacking fault energy in copper, thus modifying cross-slip dynamics and changing work hardening behavior. This last feature of the mechanical behavior is thought to play an important role in cyclic-strain-hardening and, therefore, fatigue. Precipitation of a metastable second phase or a compound can also have a great influence on dynamic mechanical properties. In a program connected with ion plating, workers at the Georgia Institute of Technology have examined ion implantation into copper, using X-ray and microscopic methods to evaluate damage and solute distribution [11]. Much of their work concentrated on crack initiation in single crystals of copper implanted with aluminum or argon. In addition to fatigue studies, they carried out X-ray analyses using a double-crystal technique to examine diffuse scattering and Bragg peak distortion. They were thus able to evaluate the state of stress at the surface and to detect the occurrence of lattice defects and their products of coagulation. In one study, using the X-ray method, a sub-surface area was detected in aluminum4mplanted copper, showing heavy radiation VII. IONqMPLANTEDMETALS

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H. Herman /Surface mechanical properties

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damage. These workers conclude that annealing gives rise to migration of the aluminum implantant out of the area of damage but the latter remains intact [12]. This group has also found an increase of fatigue lifetime for both high cycle and low cycle fatigue. This improvement in fatigue behavior is attributed to a decrease in stacking fault energy and the introduction of surface compressive stress [13]. They attribute the improvement of fatigue behavior to a reduced tendency of crack initiation as a consequence of a reduced tendency to form "persistent slip bands" (thought, generally, to be the crack initiator [6,7]). In addition, implanted copper shows less fatigue hardening than does the unimplanted specimen. Fig. 8 shows an SIN plot for copper with and without implantation. The fatigue properties and microhardness of boron4mplanted single and polycrystal copper [14] and nickel [15] have been studied by Preece et al. Although the equilibrium solubility is low for both metals, the estimated boron concentration in the implanted surface layers was of the order of 15 at.%. Comprehensive analysis were carried out of the implanted surfaces using the techniques of Rutherford backscattering, channelling, and transmission electron microscopy. In these experiments copper and nickel single crystals were used for the channelling studies, whereas polycrystals were used for mechanical property measurements. Implantation doses were in the range 2.3 to 18.5 X 1016 ions/cm 2 at 25 to 150 keV. Ion beam channelling studies for boron implanta-

tion into copper indicate that many radiation-induced imperfections (eg., dislocation loops are formed, but that crystaUinity is maintained. Boron implantation into nickel, on the other hand, as determined by RBS and channelling, indicates the formation of an amorphous or microcrystalline layer [14]. For boron implanted into copper, tl{ere was no significant effect on microhardness. It should be noted, however, that this is not particularly surprising since the hardness indentor will penetrate to depths much greater than that of the implanted layer, and, therefore, if the hardness is not greatly enhanced, no effect of implantation will be detected. Fatigue behavior is, on the other hand, significantly enhanced, showing increased lifetimes when the amount of boron exceeds 60%. Scanning electron micrographs of the fatigued implanted copper show no extruded and intruded regions, indicating that gross dislocation motion did not penetrate through the implanted surface. Implantation had the effect of strengthening the surface and, therefore, limiting dislocation egress and the formation of persistent slip bands and other crack-initiating surface defects. This result is consistent with the work reported above on the implantation of aluminum into copper [13]. For the case of boron implanted into nickel, there is an increase in microhardness which varies with depth of penetration and with load. A highly disordered structure is obtained by the implantation of boron into nickel, and this reflected in the observed hardness changes in which the penetrator breaks through a brittle, non-plastic layer [14]. Of special interest is the fatigue behavior of nickel following implantation. Here, one observes a better than 100% increase in fatigue lifetime. A hard, amorphous (or microcrystalline) layer is presumed here to be limiting the establishment of persistent slip bands [151. Burr et al. have used the limitation of formation of persistent slip bands to explain some interesting implantation experiments in copper [16]. A number of species have been implanted into copper at MeV energies to doses ranging from 10 is to 5 × 1017 ions/ cm 2. Increases in lifetime were found for boron, chlorine, helium, nickel, nitrogen and neon. It can be surmised that once again a hardened regime at or somewhere below the surface has been established through which glide dislocations cannot easily penetrate, thus limiting the formation of obvious and intense persistent slip bands. Burr et al. also implanted 150 keV beryllium ions

H. Herman /Surface mechanical properties

into copper to doses from 1017 to 1.5 × 1018 ions/ cm 2. A modest increase in fatigue lifetime was observed. On annealing for one hour at 800°C, however, there was a major loss of fatigue resistance. This effect may result from. cyclic stress softening due to dislocations cutting and thus redissolving and dispersing fine coherent precipitates that may have formed during annealing. Conversely, it may be that large, incoherent particles (e.g., B%Cu) have formed, permitting deformation inhomogeneity in the middle of the grain, thus engendering the easy formation of persistent slip bands, leading to cracking. Careful transmission electron microscopy is needed to resolve these questions. Very little consideration has been given to the establishment of residual stresses on ion implantation. Residual stresses, especially compressive surface stresses, are known to have an important influence on fatigue resistance. Shot peening, for example, is a straightforward way of introducing such stresses, and thus improving fatigue properties of engineering alloys. In general, little effect on fatigue is found by implantation with inert ion species, which would be expected to give surface residual stresses. Implantation4nduced stresses are assumed to be of little consequence in enhancing fatigue resistance, a point which requires more study.

3. Wear Friction and wear, in all their facets, are complex phenomena involving surface and near-surface metallurgical properties. These properties constitute the tribological characteristics of engineering materials and have direct and important utility. Tribologicallyinvolved deterioration mechanisms include scuffing, abrasive wear, fretting, erosion (solid and liquid particle, and cavitation-erosion), rolling-contact fatigue, etc. Wear may be lubricated or non-lubricated, the former condition introducing hydrodynamic, pressure-induced phenomena, as well as chemical effects. Theories of friction and wear abound, and practitioners will readily admit that, not unlike the case for fatigue, the situation is not satisfactory. The reader is advised to consult recent reviews of this field [17]. Tribologicai properties can be modified through ion implantation. There are a number of excellent overviews of the field [ 2 - 4 , 1 8 - 1 9 ] . Nitrogen implantation into plain carbon steel can have a dramatic influence on reducing lubricated sliding wear. Carbon

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and boron are equally effective, with a particularly large benefit in stainless steel. In addition t o lubricated wear of the pin-on-disc variety, a more recent experiment using a 4-ball tester has been carried out to demonstrate the effectiveness in improving rolling fatigue (which is actually a wear process) through nitrogen4mplantation into steel [20]. In abrasive wear a hard micro-sized particle digs into the surface causing scratching and material loss, and gives an interesting measure of implantation effects on mechanical properties (e.g., ref. [21 ]). Dearnaley has given a number of examples of wear improvement in engineering components, where lifetimes have been importantly improved by various implantation treatments: e.g., steel press tools, high temperature mill rolls, steel cutters, injection molding screws for plastics, wire-drawing dies [5,19]. Clearly, ion implantation has an important role to play in fabrication and processing industries. Table 1 lists a number of interesting examples of wear modification through ion implantation. Much needs still to be done to elucidate the role of the implantant. Singer and Bolster [21] of the Naval Research Laboratory have nitrogen-implanted a low-carbon alloy steel and AISI type-304 stainless steel, and have examined abrasive wear with micron-sized particles. The analysis is augmented through surface electron spectroscopy, enabling careful determinations of implantant location. Since it is known that abrasive wear is related to hardness, the authors assert that this measurement can yield information on implantation-modified hardness. On implanting 1017 ions/cm 2 at 40 keV into the quenched and tempered steel, the relative wear resistance shows significant increases, whereas implantation yields the opposite results for stainless steel. In fact, austenitic stainless steel, which is known to transformation-harden, shows considerable softening, which these authors suggest may be due to the nitrogen acting to stabilize the austenite. It was also found that the abrasion hardening effect as reflected in the relative abrasive wear resistance observed in carbon steel was limited to the 'depth of implantation. For the stainless steel, on the other hand, the softening effect persisted, the nitrogen apparently being forced deeper during the wear process. In another study from NRL, Carosella et al. [29] examined friction and wear in a martensite-bearing alloy steel (AISI 52100) following implantation with a number of different elements. Friction and wear VII. ION4MPLANTED METALS

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Table 1 Host material

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EN40B Various A1, steel Stainless steel, type 304 and 416 38NCD4 alloy steel Various steels Various steels Steel Au, Cu, A1 Be Alloy steel Stainless steel, type-304 Stainless steel, austenitic Mild steel Copper 52100 steel 52100 52100 52100 Steel bearing, EN31

N, Mo N, C N, C, Ar N, Co N N, C, B Ne÷, Ar+ B, C, N, Ne B B N N N N, Mo B N Ti N B N

Pin-on-disc, lubricated Pin-on-disc, lubricated Crossed cylinder-on-cylinder Dry sliding wear Pin-on-disc, lubricated Pin-on-disc, lubricated Pin-on-disc, lubricated Pin-on-disc with abrasive grit Pin-on-disc, lubricated Abrasive wear Abrasive wear Sliding wear Pin-on-disc, lubricated Pin-on-disc, unlubricated Ball-on-cylinder Sliding wear, lubricated Sliding wear, lubricated Sliding wear, lubricated Rolling 4-ball lubricated test

10-30X 10-200X 10X 20-100× <3X <200× <10× Only Cu showed improvement Improvement Decreased wear rate IncIeased wear rate Improved < 10× ~2 × 2?< 10× Increased wear r~tte No effect 2×

[4] a) [4] b) [22] [23] [24] [2] [2] [25 ] [26] [ 18 ] [21] [21] [27] [3 ] [3 ] [28 ] [29 ] [29] [29] [20]

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tests using various teclmiques were performed in conjunction with Auger analysis and nuclear reaction profiling. The rates of sliding wear can be diminished in a range of carbon and stainless steels through ion implantation with nitrogen, carbon, boron, molybdenum, and titanium. These authors sought to determine if the same sorts of improvements in tribological properties could be obtained for through-hardened steels. Of the above elements, only implanted titanium significantly improves friction and wear properties. Of central importance was th~ observed improvement in b o t h friction and wear in the steel following high dose implantation of titanium (4.6 × 1017 ions/ cm 2 at 190 keV). Analysis indicates that under these conditions of implantation, a concentration o f 2 2 - 3 0 at.N titanium is achieved in the range from the surface to 100 nm. Furthermore, a large excess of carbon is seen to exist at the surface region, amounting to atomic concentrations ranging from 20N at the surface to 4N at 100 nm. Also, Auger line shape analysis indicates that the titanium is in the form of a carbide. The authors point out that Knapp et al. found an amorphous surface is formed when " p u r e " iron is implanted with titanium [30]. The titanium getters the carbon, forming a carbide. Thus, these sig-

nificantly improved tribological properties are attributed to a thin amorphous skin on the titaniumimplanted steel. These results are consistent [29] with the improved corrosion resistance o f titaniumimplanted M50 bearing steel [31 ]. The enhancement of wear properties through ion implantation which has been observed for many metals, though somewhat surprising, is b y no means inexplicable. Most ions for the energies normally considered will give high implantant concentrations as welt as significant radiation damage to depths of about 1000 A below the surface. The implantant species will interact with the radiation-generated defect distribution (e.g., dislocation loops) to yield second phases and/or a stabilized defect structure. Such fine arrays are bound to interact with near-surface dislocations, giving rise to hardening and, hence, improved wear resistance. The exact mechanism of this enhanced wear resistance will be difficult to resolve. One issue which consistently arises in wear studies of implanted specimens is the very surprising "depthof-effect" to which the implantation operates: the reduction in wear survives the wear4nduced removal of the original layer that was implanted, and, depending on the wearing process, penetrates to depths some

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100 times the range of the implantant ion [4,18]. The temperature rise which occurs during wear induces interstitial implantant diffusion away from the surface more deeply into the bulk of the material. The Harwell School [4,5,19] offers a model employing dislocation pipelines to permit deformationinduced redistribution of implantant species; i.e., interstitials move away from the surface to influence wear at depths well beyond their original emplacement. This model is furthermore consistent with internal friction results mentioned above [9], where a significant (~40%) increase in mechanical damping is obtained on the implantation of nitrogen. Adding to the credence of a mobile interstitial model is a M6ssbauer study carried out by Longworth and Hartley [32] who found resolution of iron nitrides (formed by implantation) by heaing to 270°C, a temperature which is probably below that attained in sliding wear processes. While much of the mobile interstitial model is scientific conjecture it is consistent with simple models of dislocation structure in metals [5]. It is still too early to ascertain if an interstitial-pipeline model, such as the one discussed here for ferrous alloys, is more widely applicable.

(at a dose twice that of the Mo) into the prior Motreated surface. In this case, the conjoint effect of Mo and S is to further decrease the friction. This duplex implant treatment is provocative indeed. The effect of implantation should be evaluated on both friction and wear in carefully controlled experiments on single crystals, together with detailed high resolution TEM and SEM.

5. Cavitation-erosion Cavitation-erosion is a dynamic form of surface wear, and it should be expected that this technique will be helpful in evaluating the surface mechanical properties associated with ion implantation. This process of erosion occurs in liquid, and is the result of the rapid formation of fine bubbles and their subsequent collapse near a surface. The large degree of damage which occurs arising from micro-shock waves or from repetitious loading is thought to give rise to

POWERSUPPLY

4. Friction The coefficient of friction, a property related to wear, can frequently be modified by ion implantation [3,33]. The effects, however, are most complex. Hartley et al. [3], implanted Ag, S, In, Ag, Sn, Pb, Mo and an inert gas, Kr into EN352, an alloy steel. Friction was measured on polished discs implanted to doses of the order of 1016 ions/cm 2. Implantation with Kr showed no influence on frictional properties, indicating that surface contamination, if it did occur, was not the cause of the effect. Significant changes in friction did occur, both increases and decreases, on the implantation of the other ions. The effects have been attributed to modified surface plasticity and/or oxidation state changes. For example, implantation with Pb shows a large dose dependency, the friction increasing by some 40% for the highest dose studied. On the other hand, Mo indicates a decrease in friction of 20% for the same dose level. It is argued that oxidation of the substrate plays a role for the Pb implantations, whereas Mo acts as a metallurgical "dry lubricant", perhaps strengthening surface features. Of particular interest, is the effect of an implantation of S

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896

H. Herman / Surface mechanical properties

fatigue failure. Only recently a fuller appreciation of the metallurgical factors has been achieved notably by Preece [34]. The resistance to cavitation-erosion is related to hardness, though this is by no means the only criterion to be considered. That ion implantation can effect wear processes means in fact that it will influence cavitation-erosion behavior as well. A number of experiments have been carried out using a standard technique to generate a large density of bubbles within distilled water. The technique involves an acoustic horn, with a small WC tip vibrating in fluid at a frequency of 20 kHz and an amplitude of 50/~m. The specimen, in the form of a 1 cm diameter disc, is situated 0.60 mm away from the tip. Using an interruption }echnique, the specimen is weighed at given intervals, and the total weight loss or rate of weight loss is plotted against time. SEM studies are carried out concurrently with the weight loss measurements. The experimental setup is displayed in fig. 9. A series of experiments were carried out on AISI 1018 steel, with and without implantation with nitrogen [9,35]. The results of these experiments are given in fig. 10, where it is seen that an incubation time is observed. This time can be used as a measure of cavitation-erosion resistance, and is commonly seen for most metals. Of significance is the shift to longer times on implantation, and longer times still on the aging of the implanted specimen. It is believed that during the incubation time shock4nduced work hardening of the surface occurs, and that large weight loss finally ensues when the surface roughness becomes sufficiently great for the thus-developed 5!

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asperities to undergo a form of "fatigue failure". SEM studies shows surface features consistent with this model. In a study on the implantation of boron into copper and nickel, Preece observed an increase in incubation time which was not unlike that seen for steel [15]. Of special interest in this study, however, was • the fact that boron implanted in nickel showed a very significant change in incubation time (from less than 30 min for pure nickel to about 1 h for the implanted specimen); whereas boron implanted into copper yielded a change of incubation time of only about 5 rain. These results are consitent with changes of fatigue life for these systems in which the iifetime extension for boron into nickel was almost double that for the case of boron into copper [14]. Preece et al. attribute the differences in property modifications to result from nickel becoming amorphous on being implanted with boron, but copper maintaining its crystallinity, and this was confirmed by channelling experiments in single crystals of boron4mplanted nickel and copper [14]. The improvement of cavitation-erosion behavior achieved on implantation and on implantation-andaging (for steel) can be attributed to enhanced shockresistance as well as to better "fatigue resistant-like" properties. Similar to the case of fatigue, dislocation dynamics are modified in a way such as to reduce the rate of work-hardening.

6. Conclusions Surface-controlled mechanical properties can be significantly modified through ion implantation. Those properties thus modified include fatigue, friction and wear, and erosion. The complex nature of these phenomena in engineering materials, make most difficult any evaluation of the effect of implantation. Certain conclusions are, however, forthcoming and, in some cases, generalizations can now be made. Interstitial implantation into BCC metals will give rise to high density second phases (e.g., nitrides, carbides), which interact with glide plane dislocations limiting, in the case of fatigue, surface egress. Implantation into FCC host metals is a means of surface alloying, and much of what occurs following implantation can be viewed as being due to dislocation structure modification and dynamics (e.g., modification of stacking fault energy) and the formation of second phases.

H. Herman / Surface mechanical properties All of the mechanical properties dealt with here are sensitive to the near-surface dislocation behavior under the application of wear-originated stress or cyclic stressing. Each case must be considered separately. In the case of fatigue, there is a large amount of literature for studies of the response of materials to cyclic deformation. Again, it is important to recognize that an SIN plot is an indication of regimes of survivability, and as such at best indicates something of crack propagation. While ion implantation will clearly influence fatality statistics, it will likely do so by controlling crack initiation through an interaction with mobile dislocations. We should more properly be examining cyclic hardening rate on single crystals, which, together with high resolution metallography, will enable us to understand crack initiation in ion implanted surfaces. Both cyclic hardening and softening need to be examined and effects of implantation noted to learn something of dislocation behavior in ion implanted materials. Surface analysis using high resolution implantant profiling techniques, and high resolution transmission electron microscopy are needed to study implantation-induced radiation damage and new metallurgical phases, as well as the variety of interactions between them. The vast literature on fatigue must be studied with special reference to pertinent experiments. Can, for example, internal friction (which can be connected to the oscillatory dislocation motion) contribute to our understanding of fatigue crack initiation? More sensitive techniques must be devised for measuring implantation-modified surface hardness and monotonic crystal strength. With respect to the latter, there exist microplastic deformation procedures (using, for example, hollow torsion specimens) to assess the effects of surface condition on gross mechanical properties. Of special interest is the potential conjoint effect between the environment (chemistry) and mechanical properties (see, for example, C.R. Clayton, this proceedings). Corrosion fatigue and stress corrosion are particularly dangerous surface-controlled failure modes in high strength engineering alloys. Implantation with ions can aid in both improving and understanding such surface sensitive properties. Following an initial period of discovery, studies of the effects of ion implantation on mechanical properties have expanded quickly, and continue to do so. Larger numbers of implantation machines, designed with surface modification uses more fully in mind

897

will permit a broadening of the field, and will make available to physical metallurgy researchers a wider range of specimens for study. The science of implantation as well as its industrial applications are bound to benefit. The Stony Brook program on ion implantation metallurgy has been supported by the US Office of Naval Research.

References [1] N.E.W. Hartley, in: Ion Implantation, ed., J.K. Hirvonen, Vol. 18, Treatise on Materials Science and Technology, (Academic, New York, 1980). [2] N.E.W. Harfley and R.E.J. Watkins, in: G. Dearnaley, Ion Implantation for Improved Resistance to Wear and Corrosion, Mat. Eng. Appl. 1 (1978) 28. [3] N.E.W. Hartley, Tribology Int. (April, 1975) p. 65. [4] N.E.W. Hartley, in: Applications of Ion Beams to Materials, eds., G. Carter, J.S. Colligen and W.A. Grant (Institute of Physics, Bristol, 1976) p. 123,210; N.E.W. Hartley, Ion Implantation Case Studies, Proc. Conf. Surface Treatments for Protection, London (1978). [5] G. Dearnaley, Practical Applications of Ion Implantation, Proc. Mat. Res. Soc. Ann. Meeting, Cambridge, Mass., 1979, to be published. [6] M.E. Fine, Metall. Trans. A, l l A (1980) 365. [7] S. Kocanda, Fatigue Failure of Metals (Sijthoff and Noordhoff, Leiden, 1978). [8] W.W. Hu, C.R. Clayton, H. Herman and J.K. Hirvonen, Scripta Met. 12 (1978) 697. [9] H. Herman, W.W. Hu, C.R. Clayton, J.K. Hirvonen, R. Kant and R.K. MacCrone, IPAT-79 (London, 1979) p. 255; W.W. Hu, H. Herman, C.R. Clayton, J. Kozubowski, R. Kant, J.K. Hirvonen and R.K. MacCrone, Surface Related Mechanical Properties of NitrogenImplanted 1018 Steel, Proc. Mat. Res. Soc. Ann. Meetflag, Cambridge, Mass., (1979) to be published. [10] R.G. Vardiman, R. Kant, T.W. Crooker amd B.B. Rath, as quoted in: Thin Solid Films 63 (1979) 5. [11] S.B. Chakrabortty, S. Spooner and E.A. Starke, Jr., Technical Report No. 2, ONR Contract N00014-78-C0270 (1 March 1979-29 February 1980). [12] S. Spooner and K. Legg, Proc. Mat. Res. Soc. Ann. Meeting, Cambridge, Mass., (1979) to be published. [13] A. Kujore, S.B. Chakrabortty, E.A. Starke, Jr. and K.O. Legg, Proc. Mat. Res. Soc. Ann. Meeting, Cambridge, Mass. (1979) to be published. [14] C.M. Preece, E.N. Kaufman, A. Staudinger and L. Buene, Proc. Mat. Res. Soc. Ann. Meeting, Cambridge, Mass. (1979) to be published. [ 15] C.M. Preece, private communication. [16] C.R. Burr, H. Bakhru and W. Gibson, Proc. Mat. Res. Soc. Ann. Meeting, Cambridge, Mass. (1979) to be published. [17] D. Scott, ed., in: Treatise on Materials Science and Technology, Vol. 13 (Academic, New York, 1979); VII. ION-IMPLANTED METALS

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[18] [19]

[20] [21] [22]

[23]

[24] [25] '[26]

H. Herman /Surface mechanical properties Source Book on Wear Control Technology, Amer. Soc. Metals, (1978); Tribology Handbook, ed., M.J. Neale (Newnes-Buttersworth, London, 1973). J.K. Hirvonen, J. Vac. Soc. Tech. 15 (1978) 1662. G. Dearnaley, P.D. Goode, N.E.W. Hartley, G.W. Proctor, J.F. Turner and R.E.J. Watkins, IPAT-79 (London, 1979) p. 243. G. White and G. Dearnaley, to be published in: Wear. I.L. Singer and R.N. Bolster, Proc. Mat. Res. Soc. Ann. Meeting, Cambridge, Mass. (1979). V.A. Parlor, P.V. Parlor, E.I. Zorin and D.1. Teteibaum, in: Proc. of All Soviet Meeting on Ion Beam Physics (Kiev, 1974) p. 114. J.K. Hirvonen, J.W. Butler, J.P. Smith, R.A. Kant and W.C. Wescott, in: J.K. Hirvonen, J. Vac. Soc. Tech. 15 (1978) 1662. S. Lo Russo, P. Mazzoldi, I. Seotoni, C. Tosello and S. Tosto, App. Phys. Lett. 34 (1979) 627. S.E. Harding, MSc Thesis, Brighton Polytechnic (1977). J.Y." Robic, J. Piaguet and J.P. Gaillard, Report LETINCE/78427/JP/IJ (1978); these Proceedings, p. 919.

[27] J.K. Hirvonen, C.A. Carosella, R.A. Kant, I.L. Singer, R. Vardiman and B.B. Roth, Thin Solid Films 63 (1979) 5. [28] J.K. Hirvonen, Naval Research Laboratory, private munication. [29] C.A. Carosella, I.L. Singer, R.C. Bowers and C.R. Gossett, Proc. Mat. Res. Soc. Ann. Meeting, Boston, 1979; these Proceedings, p. 923. [30] J.A. Knapp, D.M. Follstaedt and S.T. Picraux, Proe. Mat. Res. Soc. Ann. Meeting, Cambridge, Mass. (1979) to be published. [31] Y.-F. Wang, C.R. Clayton, G.K. Hubler, W.H. Lucke and J.K. Hirvonen, Thin Solid Films 63 (1979) I 1. [32] G. Longworth and N.E.W. Hartley, Thin Solid Films 48 (1978) 95. [33] N.E.W. Hartley, W.E. Swindlehurst, G. Dearnaley and J.F. Turner, J. Mat. Sci. 8 (1973) 900. [34] C.M. Preece, in: Erosion, ed., C.M. Preece, Vol. 16 Treatise on Materials Science and Technology (Academic, New York, 1979) p. 249. [35] W.W. Hu, C.R. Clayton, H. Herman and J.K. Hirvonen, J. Mat. Sci. and Eng. (1980) in press.