J.K. Hirvonen

Improvement of metal properties by ion implantation

Abstract Ion implantation is being investigated as a technique for the beneficial modification of surface-sensitive and life-limiting properties of metals including resistance to wear and fatigue. Ion implantation is a process of accelerating ions to high velocities and directing them into the near-surface regions of materials ( e.g. alloys) to produce in essence a different material (alloy) in the near-surface region. Ion implantation can produce a graded alloy from the surface to the unchanged underlying bulk alloys so that both the surface and the bulk alloys can be independently optimized. The implanted layer is typically hundreds to thousands of angstroms deep with implanted atom concentrations of up to fifty atomic per cent or more. The sliding-wear rate between various steel alloys was significantly reduced by implanting one of the surfaces with selected elemental species such as nitrogen, carbon and titanium. Experiments were conducted on a number of materials including stainless steel, too steel, bearing alloys and silicon nitride. The implantation technique has also been reported to increase fatigue lifetimes in low-carbon steel by a factor of 50–100. Experimentation is now being directed towards other materials of major technological interest such as titanium alloys. The effect of ion implantation on (1) the wear and fatigue properties and (2) the microstructural characteristics of implanted materials with selected examples is discussed.

Applications of ion implantation for the improvement of localized corrosion resistance of M50 bearing steel

Abstract Pitting corrosion of M50 alloy steel bearings used in turbojet engines has been found to be a severe problem. The difficulty arises when salt-spray condensates accumulate in the engine lubricants of aircraft not in regular use. Ion implantation was applied to this problem because in the early stages of this work it was shown to be able to maintain both the dimensional stability and the contact fatigue lifetime of the M50 bearings. Qualitative tests, which simulated the geometry and thermal cycle conditions leading to pitting of the M50 bearing surface, were performed using oil containing 3 ppm NaCl. Initially it was found that chromium surface alloys containing 20–25% chromium substantially reduce the level of attack. Prior to further corrosion simulation tests, potentio-kinetic studies were carried out on M50 implanted with chromium, molybdenum and titanium in order to screen both the passivating tendency of the surface alloys formed and their resistance to localized forms of corrosion. Singular additions of chromium, molybdenum and titanium were found to increase the resistance of M50 to localized breakdown significantly. The highest resistance to localized breakdown was found for a multi-implantation of chromium and molybdenum.

Wear testing under high load conditions: The effect of “anti-scuff” additions to AISI 3135, 52100 and 9310 steels introduced by ion implantation and ion beam mixing

Abstract There is a need to eliminate the sudden onset of severe adhesive wear (“scuffing”) in high performance hardened steels (e.g. AISI 9310) under arduous load conditions. We have investigated the friction and wear behavior of three ion implanted and ion beam mixed steels under simulated scuffing conditions using a Falex friction and wear tester. This machine enabled tests to be carried out at a load of 700 lb (318 kg), corresponding to a mean contact pressure of approximately 20 000 psi (i.e., 1×108 N/m2) which was sufficient to induce scuffing. A series of lower load tests at 200 lb (91 kg) load (5.2 × 107 N/m2) enabled the longer term wear performance of various ion/substrate combinations to be measured. The frictional force experienced during wear testing was used to assess the degree of scuffing, and the amount of material worn away was measured on the Falex tester or by subsequent weight loss determinations, depending on the type of test. The following ions were implanted: C+, N+, P+, Ti+, Cr+, Mo+, and Ta+, chosen in order to evaluate the effects of intermetallic additions (C, N, P), alloys elements (Ti, Cr), and anti-scuff elements (Mo, Ta). In addition some thin ( ∼1000 A ) vacuum evaporated layers of Si, V, Ni, Nb, Sn, Mo, Ta and W were prepared, and in some cases intermixed with N+ ions at a fluence of typically 2×1017/cm2, to compare with the effects of ion implantation. Under the low load conditions the wear rate of AISI 3135 steel (1.5% Ni, 0.65% Cr alloy tool steel) was found to be reduced by a factor 3 as a result of N+ implantation under low load, in agreement with previous work reported elsewhere, whereas other ions gave inconclusive results. The 52100 steel (a through-hardened martensitic bearing steel) showed marked improvements after Ti+ implantation, revealing a sensitivity to fluence which correlated with known dry sliding behaviour of this steel modified by titanium implantations. Ta+ and Mo+ implantations into 9310 steel (a case-hardened gear steel with 3.0% Ni, 1.4% Cr and 0.55% Mn) reduced the wear rate (in μg per second) under the low load parameters from 4.2 to 0.15 and 0.26 respectively. Under simulated scuffing, Ta+ was effective whereas Mo+ was not. SEM examination of the implanted AISI-9310 steel wear pins tested to 200 lb (91 kg) showed extensive smoothing in the implanted worn zones, implying a different wear process. On all the steels, mixed overlayers showed improved friction and wear behavior compared to unmixed layers. The results are discussed in terms of the probable influence of the various added elements on the metallurgical wear mechanisms of the respective steels.

Surface hardening of beryllium by ion implantation

Abstract The effectiveness of ion implantation for the production of a hard wear-resistant surface on instrument grade beryllium of high strength (HP-40) was explored. Samples of beryllium were implanted with boron and were subjected to microhardness tests in both the as-implanted state and after annealing. The implanted region was examined using Rutherford backscattering to determine the depth distribution of the implanted boron. By using ion implantation to produce a buried layer containing boron, the limitations imposed by solubility and diffusivity are avoided and much greater boron concentrations than those attainable with conventional thermal treatments are generated.

Electrochemical and A.E.S. studies of FeCr surface alloys formed on AISI 52100 steel by ion beam mixing

Abstract FeCr surface alloys have been formed on AISI 52100 steel by ion beam mixing in order to improve localized corrosion resistance. Three implant species were considered in this work, namely: Cr + , Xe 2+ and Kr 2+ . Ion implanation was carried out on Cr surface layers, of 30 and 50 nm, deposited onto 52100 steel coupons. The resulting surface alloys were chemically characterized by Auger electron spectroscopy. Electrochemical pitting studies were carried out in deaerated 0.01 M NaCl solution buffered to pH 6. Each of the surface alloys formed by the ion beam mixing technique exhibited a significantly higher resistance to localized corrosion compared to 52100 steel. The possbile influence on the localized corrosion resistance of the surface alloys of oxidation, carbide formation and sputtering, resulting from the ion implantation treatments, is discussed.

Production of high-current metal ion beams

Abstract The increasing usage of ion implantation to modify the chemical and surface sensitive mechanical properties of materials typically requires fluences of 10 17 ions/cm 2 . In order to implant test hardware (e.g., ball bearings) to such high influences in a reasonable period of time it is necessary to employ ion beam currents with intensities of a hundred-microampere or more. This paper will describe the modifications made in the ion source of the NRL Extrion/Varian 200-20A2F ion implanter to produce high current metal ion beams utilizing an internal chlorination technique. Several charge material source geometries and carrier gas combinations have been tried and compared. Beam currents of 100 μA or more intensity have been routinely obtained for most metal species attempted, with 500–1000 μA intensities obtained for some commonly run species (e.g., 52 Cr + and 48 Ti + ). This paper will present (1) a compilation of ion species and intensities obtained, (2) typical ion source operating parameters and lifetimes, and (3) evidence of ion source “poisoning effects”.

Fatigue-life enhancement of steel by nitrogen implantation

Abstract The fatigue-life of low carbon steel was found to be significantly enhanced following implantation of 150 keV N+ 2. The observed lifetimes were found to be dependent on the thermal history of the samples.

Interpretation of electrochemical behavior of nickel-implanted type-430 stainless steel using x-ray photoelectron spectroscopy and transmission electron microscopy☆

Abstract Multiple implantations of Ni + ions into annealed type-430 stainless steel were carried out at energies of 25 and 75 keV in order to produce a surface alloy. X-ray photoelectron spectroscopy, coupled with argon ion etching, was used to characterize the surface composition and chemical depth profiles of the implanted species prior to electrochemical studies. Radiation damage and alloy phase structure were evaluated using transmission electron microscopy. Potentio-kinetic studies were carried out in 1 N H 2 SO 4 deaerated by hydrogen. Nickel implantations reduced both the critical and passive current densities. The open-circuit potential was not altered and a change was observed in the primary passivation potential only at the highest concentration of nickel. Potentio-kinetic studies in hydrogen-saturated 1 M NaCl revealed a general enhancement of pitting resistance due to nickel implantation.

Cavitation erosion of ion-implanted 1018 steel

Abstract The mechanical damage produced by ultrasonic cavitation erosion in 75 keV nitrogen-implanted 1018 steel (0.18%C) was studied using weight loss measurements and scanning electron microscopy observations. Total weight loss curves were obtained for three different conditions as a function of exposure time. The results indicate that there is an incubation period prior to damage which is prolonged by a factor of about 1.5 through ion implantation alone and by a factor of 3 through ion implantation followed by thermal aging. A roughened surface topography was observed to evolve for continued cavitation exposure. The increase in incubation time and the changes in the topographical features are discussed in terms of the formation of nitride when the steel is nitrogen implanted and aged.

Rutherford backscattering investigation of thermally oxidized tantalum on silicon

Abstract The Rutherford backscattering technique utilizing 2 MeV He+ ions was used for studying Ta and thermally grown Ta oxide films on Si substrates. Significant impurity effects were observed for the as-deposited Ta films and are attributed to gettering during deposition. Partially oxidized Ta films exhibit a surface Ta2O5 layer with substantial oxygen incorporation in the underlying Ta film. In contrast with anodic Ta2O5 films on tantalum, there is no sharp boundary between Ta and Ta2O5. Tantalum oxide films on silicon are, to a first approximation, stoichiometric. Their apparent density, as determined from the areal density of Ta atoms, increases with thickness (from 4.7 to 7.3 g cm-3) as do their refractive indices. This supports the contention that incorporation of silicon is responsible for these effects and that they are not merely due to a change in stoichiometry.