L.E. Pope

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.

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.

Amorphization of stainless steels by carbon implantation

The ion implantation of C has been discovered to amorphize stainless steels, in contrast to the formation of crystalline carbides for pure Fe. Examination of C‐implanted Fe(Cr) alloys indicates that the amorphous phase is stabilized by 12–18 at. % Cr in the steels. Reduced friction is found for C‐implanted stainless steels, but an amorphous layer with C alone is more easily worn away than one with both Ti and C. This and other comparisons indicate that both Ti and C are essential for the extended wear resistance and low friction of amorphous Fe(Ti,C) surface alloys.

Atomic oxygen-MoS2 chemical interactions

Abstract The present study shows that an an O-atom translation energy of 1.5 eV, SO 2 is generated and outgases from an anhydrous MoS 2 surface with an initial reactivity nearly 50% that of kapton. The reaction of atomic oxygen with MoS 2 has little or no translational energy barrier, i.e. thermally generated atomic oxygen reacts as readily as that having 1.5 eV of translational energy. For MoS 2 films sputter-deposited at 50–70 °C, friction measurements showed a high initial friction coefficient (up to 0.25) for MoS 2 surfaces exposed to atomic oxygen, which dropped to the normal low values after several cycles of operation in air and ultrahigh vacuum. For MoS 2 films deposited at 200 °C, the friction coefficient was not affected by the O-atom exposure.

Microstructure and composition of 304 stainless steel implanted with Ti and C

Abstract The microstructure and composition of surface alloys formed by implanting Ti and C into 304 stainless steel are examined for a range of Ti fluences, both with and without additional implanted C. The resulting amorphous layers are found to contain TiC precipitates, apparently with some Cr on Ti lattice sites, when the metal-atom fraction of Ti + Cr exceeds 55%. The depth profiles of Ti and C are measured, and the amounts of C incorporated into the alloys during Ti implantation are determined. Small amounts of H are also incorporated during the high-fluence Ti implantations. Thicker amorphous layers than those resulting from Ti implantation alone can be formed when additional C is implanted either before or after the Ti.

Reactive evaporation of thin titanium nitride films in ultrahigh vacuum and their friction and wear behavior as a function of contact stress

Abstract Titanium nitride films have been deposited near room temperature in an ultrahigh vacuum environment using a reactive evaporation method which gives reproducible and very clean films of predictable stoichiometry. Details of the kinetics of incorporation of nitrogen into the film are strongly affected by the presence of contaminants, particularly carbon. Films were obtained with stoichiometries from Ti to TiN 1.2 and films of TiN 1.1 to TiN 1.2 were analyzed by transmission electron microscopy and friction and wear testing. The films exhibited a uniform grain size near 10nm, with a preferred orientation with (111) planes nearly parallel to the surface. In laboratory air the friction coefficient (0.15–0.2) and effective wear life was independent of the contact stress for films deposited on 304 stainless steel. The coatings failed suddenly by fracture and spallation. The presence of a titanium underlayer resulted in negligible wear of the coating without failure at low contact stress, although the friction coefficient was the same. In ultrahigh vacuum, the friction coefficient was a factor of three higher (0.6) with rapid film failure.

Effects of laser processing and doping on the lubrication and chemical properties of thin MoS2 films

Abstract We have examined the effect of dopant incorporation (nickel and gold) in sputter-deposited MoS 2 thin films. The nickel was codeposited during d.c. triode sputter deposition, whereas the gold was laser mixed into films after deposition. The films were characterized by Raman, Auger and Rutherford backscattering spectroscopy before and after laser treatment. The gold in the laser-mixed MoS 2 films was distributed through the film and did not coalesce into islands on the surface. In addition to gold incorporation, laser doping produced changes in the Roman spectra characteristic of increased crystallinity. Specifically, the spectra after treatment were nearly identical to those from a MoS 2 sputtering target, whereas thin films before laser processing were characterized by increased S-S bonding and in some cases the presence of MoO 3 . We measured the friction coefficients both in laboratory air and in ultrahigh vacuum; in both ambients the friction coefficients of laser-treated films (with and without gold) were less than those of the deposited films.