F. G. Yost

The effects of N+ implantation on the wear and friction of type 304 and 15-5 PH stainless steels

Abstract Ion implantation of N + into mechanically polished type 304 and 15-5 PH stainless steels was studied to determine its effect on dry wear and friction behavior. Implantation of 4.0 × 10 17 N + cm -2 at 50 keV yielded a depth profile with a peak concentration of about 45 at.% at a depth of 70 nm which dropped to about 10 at.% at 120 nm. Wear and friction were studied in an unlubricated pin-on-disc configuration using type 304 and 440C stainless steel pins. Both N + -implanted steels exhibited reduced wear at low loads but no significant reduction in the coefficient of friction was found. At the lowest normal load studied (12.3 gf), the average maximum wear depth of the implanted 15-5 PH stainless steel disc (about 0.1 μm) was reduced to approximately 10% of that for the corresponding unimplanted pin-on-disc pair after 1000 cycles. At normal loads of 50 gf or above (corresponding to hertzian stresses of 1160 MPa or higher) all beneficial effects were gone. Vacuum heat treatment at 923 K for 1.8 ks of an identically implanted type 304 stainless steel specimen eradicated the benificial effects of the nitrogen implantation. The N + -implanted discs show similar reductions in wear to discs implanted with titanium and carbon, but the N + -implanted discs do not exhibit the reductions in the coefficient of friction seen with the discs implanted with titanium and carbon.

Effects of ion‐implanted C on the microstructure and surface mechanical properties of Fe alloys implanted with Ti

The microstructural and tribological effects of ion implanting C into Ti‐implanted, Fe‐based alloys are examined and compared to the influence of C introduced by vacuum carburization during Ti implantation alone. The amorphous surface alloy formed by Ti implantation of pure Fe increases in thickness when additional C is implanted at depths containing Ti but beyond the range of carburization. Pin‐on‐disc tests of 15‐5 PH stainless steel show that implantation of both Ti and C reduces friction significantly under conditions where no reduction is obtained by Ti implantation alone; wear depths are also less when C is implanted. All available experimental results can be accounted for by consideration of the thickness and Ti concentration of the amorphous Fe‐Ti‐C alloy. The thicker amorphous layer on samples implanted with additional C extends tribological benefits to more severe wear regimes.

The amorphous phase and surface mechanical properties of 304 stainless steel implanted with Ti and C

The wear tracks resulting from unlubricated pin‐on‐disc tests of 304 stainless steel which was ion implanted with Ti and C have been examined with transmission electron microscopy, scanning electron microscopy, and energy dispersive x‐ray spectroscopy. At light pin loads, where the maximum wear depths were reduced by the implantation from ∼1.5 to ∼0.15 μm, nearly continuous amorphous layers containing Ti were found to exist across the wear tracks. However, the amorphous phase was not observed in deeper wear tracks (≳1 μm) produced by higher loads. This correlation of the presence of the amorphous layer with reduced wear demonstrates that this layer is responsible for the reduction in wear produced by implantation of Ti and C.

Thermodynamics of Wetting by Liquid Metals

The wetting of metal surfaces by molten solder is usually considered to be driven solely by an interfacial energy imbalance. The effect of chemical reactions on the wetting process is neglected, although the growth of an intermetallic layer in the wetted interface is commonly observed. In this work, the energy release during the incremental advance of a spreading solder droplet due to the interfacial energy imbalance and the formation of the intermetallic layer is calculated. The free energy of formation, ΔG, of the intermetallic layer is shown to be an important driving force for solder wetting. This approach to wetting has been applied to three systems, Cu-Sn, Cu-Sb and Cu-Cd. Liquid Sn, Sb and Cd react with solid Cu to form Cu 6 Sn 5 (η), Cu 2 Sb (γ) and CuCd 3 (e), respectively. The free energy of formation, ΔG, for these intermetallic compounds is unknown experimentally, but can be calculated from the phase diagrams and other solution data using classical thermodynamics. These thermochemical calculations yield ΔG(η) = 465–3.09T for Cu-Sn, ΔG(γ) = −2500+0.54T for Cu-Sb and ΔG(e) = −825+0.44T for Cu-Cd (cal/mole). These relations were evaluated at the respective melting temperatures and compared with the interfacial energy exchange. In all three cases the ΔG contribution was approximately two orders of magnitude larger than the interfacial energy exchange making it the dominant driving force for wetting kinetics.

The microstructure of type 304 stainless steel implanted with titanium and carbon and its relation to friction and wear tests

Abstract We have used transmission electron microscopy to examine the microstructure of type 304 stainless steel which was ion implanted with high doses (2 × 1017 atoms cm-2 of titanium and carbon. We find that the resulting surface alloy is an amorphous phase similar to that observed when pure iron is identically implanted. This result is important for identifying the mechanisms by which the coefficient of friction and the wear deoth are reduced in unlubricated pin-on-disc tests of type 304 stainless steel implanted with titanium and carbon. We have also examined the effect of temperature on the amorphous alloy during annealing in the microscope. We find that devitrification begins after 15 min at 500 °C and that the alloy fully crystallizes into f.c.c., b.c.c. and TiC phases after 15 min at 650 °C. A comparison of mechanical test results from devitrified specimens with results from amorphous specimens demonstrates that the reduction in the coefficient of friction correlates with the presence of the amorphous layer, whereas the reduction in the wear depth is obtained for both amorphous and crystalline alloys.

Nuclear microprobe analysis of wear tracks on 14N-implanted steels

Abstract Two nuclear microbeam analysis techniques [3,7 MeV(α,p) and 6 MeV(α, α)] have been used to determine the local areal density of 14 N which remains in wear tracks resulting from pin-on-disc testing of nitrogen implanted 15-5 PH and 304 stainless steels. The microbeam analysis shows that the extent of N migration into the 15-5 substrate was to depths ≲ 0.5 μm, but perhaps to ≲ 1.0 μm in 304. The as-implanted layer in 15-5 PH contains up to 40–45 at.% N and consists principally of ∼ 2 μm particles of (Fe, Cr) 2 N 1−x . When sufficient wear has occurred in 304 to lower the N content below 10 17 N/cm 2 , an O buildup to 2 × 10 17 O/cm 2 is observed; however the presence of N does not correlate with low O levels in the wear tracks of 15-5 PH.

Friction and wear reduction of 440C stainless steel by ion implantation

Friction and wear tests on ion-implanted 440C stainless steel discs have been extended to high Hertzian stresses (less than or equal to 3150 MPa). Implantation of 2 x 10/sup 15/ Ti/mm/sup 2/ (180 to 90 keV) and 2 x 10/sup 15/ C/mm/sup 2/ (30 keV) into 440C reduces friction (approx. 40%) and wear (> 80%) for Hertzian stresses as large as 2900 MPa, stresses which significantly exceed the yield strength of 440C (approx. 1840 MPa). Implantation of 4 x 10/sup 15/ N/mm/sup 2/ (50 keV) into 440C reduces friction slightly (approx. 25%) for Hertzian stresses < 1840 MPa but provides little or no reduction in wear. The amount of Ti remaining in the wear tracks correlates with the reductions in friction and wear. The implantation of Ti and C produces an amorphous surface layer which is believed to reduce friction and wear, whereas N implantation is expected to produce hard nitride particles which probably do not modify the hardness of 440C (KHN = 789) significantly.

Microstructures of Stainless Steels Exhibiting Reduced Friction and Wear After Implantation with Ti and C

Implantation of Ti and C into stainless steel discs of Types 304, 15-5 PH, Nitronic 60 and 440C has previously been reported to reduce wear depths by up to approx. 85% and friction by approx. 50% in unlubricated pin-on-disc tests. Our earlier studies relating microstructure to friction and wear results in Type 304 are first summarized: these indicate that the improvements in the surface mechanical properties are due to an amorphous surface layer, similar to the amorphous layer observed in pure Fe implanted with Ti and C. We have now examined the other three implanted steels and found similar amorphous layers. These results strongly suggest that the amorphous surface alloy is responsible for reduced friction and wear in all the steels. 6 figures.

Effect of ion implantation species on the tribological response of stainless steel surfaces

The friction and wear properties of 304 and 15–5 PH stainless steels which were ion implanted with P and with P plus C have been examined and are compared with the properties of the same steels implanted with N and with Ti plus C. While benefits are obtained with the P and the P plus C implantation treatments, the N and the Ti plus C treatments give greater reductions in wear, which extend to more severe wear regimes; with Ti plus C, friction is also reduced. Transmission electron microscopy shows that the P and the P plus C implantations (with 20 to 30 at. pct metalloid concentrations) produce surface alloys with amorphous phases, as do Ti plus C treatments (approximately 20 at. pct each). The greater benefits obtained with the Ti plus C amorphous phase imply that this phase is mechanically superior to the amorphous phase with P plus C, even though the latter has been shown to have excellent mechanical properties when produced by melt quenching. Based on the above studies, Ti and C were selected for implantation into a 15–5 PH discriminator wheel of an electromechanical device for comparison with standard solid film lubrication (MoS2). In comparison to a solid film lubricated wheel, the implanted wheel (unlubricated) performed equally well with respect to time of operation, number of cycles, and tolerance control; in addition, the implanted wheel produced less debris. An alternative ion beam method was used to introduce high surface concentrations of Au for control of fretting corrosion and debris generation of a journal bearing in a different electromechanical device: a sputterdeposited Au film (50 nm) on 15–5 PH stainless steel was ion bombarded with 300 keV Xe-. This treated surface was compared with a sputtered Au film without ion beam treatment, with electrodeposited Au, and with a solid lubricating film. The ionmixed Au surface bearings had the least corrosion and debris.

Friction and Wear of Stainless Steels Implanted with Ti and C

Friction and wear tests were completed on Fe and stainless steels of the type 304, 15-5 pH, Nitronic 60, and 440C implanted with Ti and C. Samples were mechanically polished prior to ion implantation to fluences of 2 x 10/sup 17/ Ti/cm/sup 2/ (90 to 180 keV) and 2 x 10/sup 17/ C/cm/sup 2/ (30 keV); the implantation profiles of the two elements overlapped to 0.1 ..mu..m. Light load wear conditions with no lubrication were evaluated in a pin-on-disc configuration. Ion implantation significantly reduced the friction coefficient by up to 75% and decreased the maximum wear depth by up to 95%, but both effects were material and load dependent. Only stainless steel 304 had both friction and wear reduced by implantation for all loads examined. Fine-scale parallel grooves were present in the wear track for light loads, but this wear pattern was transformed to a galled structure at high loads. A correlation existed between decreased friction, reduced wear, the wear track morphology and Ti in the wear track.