Amitabh Jain

The sliding wear of titanium nitride coatings

Abstract Although the mechanisms by which titanium nitride (TiN) coatings can affect the abrasive wear resistance of coated components are well documented, relatively little is known about the sliding wear behaviour of these materials. Sphere-on-disc tests have been performed on sputtered TiN coatings, deposited at a range of bias voltages, using both coated and uncoated spheres. The amount of sphere and disc wear decreases with substrate bias, though the wear rate for the disc increases at the very highest bias voltages. The coefficient of sliding friction is approximately constant except for the case where coated spheres slide on coated flats. The mechanisms of adhesive wear are based on the local transfer of iron, which subsequently becomes trapped in the open regions within the coating microstructure, increasing the sphere-coating adhesion at these positions. Consequently, best sliding wear properties result from hard, dense coatings as produced at high substrate bias voltages, although oxidative wear becomes increasingly important as the bias is increased.

Improvement of adhesion of TiN coatings on stainless steel substrates by high energy heavy ion irradiation

Abstract The performance of tribological coatings depends greatly on the adhesion of the coating to the substrate. Various techniques are known for surface modification. Ion irradiation is a technique which has been used to modify the adhesion of thin films and the intrinsic stresses in films. Sputtered TiN coatings on stainless steel were irradiated with MeV Ag ions and the coatings were studied before and after irradiation. Scratch adhesion tests were performed on the unirradiated and irradiated films. The failure mode changes from a predominantly spallation situation to a purely buckling situation indicating enhancement of adhesion of the coating to the substrate. There is also an enhancement of compressive stress in the film as revealed by X-ray diffraction.

Annealing behaviour of nitrogen-ion-implanted 304 stainless steel☆

Abstract We studied the modification of 304 stainless steel following nitrogen ion implantation. We used grazing incidence X-ray diffraction. Nitride formation is enhanced in specimens containing a significant amount of martensite formed by surface finishing. We report for the first time an increase in surface hardness at depths larger than the implantation depth following annealing of such implanted specimens. This effect correlates with the release of nitrogen from nitrides. The nitrogen appears to diffuse inwards and strengthen the martensite phase.

High energy heavy ion irradiation of chromium films

Abstract We have studied the modification of chromium coatings by nickel ion beams at a high energy (75 MeV) so that the range of the ions is many times the film thickness, which is of the order of 1 μm. At this energy, the rate of energy loss of ions is predicted to be fairly uniform over the entire thickness of the coating. The coating hardness increases significantly, even at moderate doses. The effect is stronger when the film deposition temperature is lower.

Hardening of steel by boron ion implantation—dependence on phase composition

The influence of the initial phase composition on the microhardness of 304 stainless steel before and after boron ion implantation has been studied. The initial phase composition is influenced by the specimen preparation methods used and has been determined using Glancing Angle X-ray Diffraction (GAXD). Electropolishing resulted in purely austenitic (fcc) specimens. Electropolishing followed by mechanical polishing resulted in the introduction of a martensitic (bcc) phase. Elastic Recoil Detection Analyses (ERDA) was used for boron depth profiling in the steel. The peak concentration of implanted boron was found to be higher in martensitic specimens. These specimens exhibit higher microhardness.

The influence of target temperature on the structure and properties of nitrogen-ion-implanted stainless steel

Abstract Nitrogen ion implantation has proved effective in the surface strengthening of metals and alloys. In order to achieve high throughputs industrially, high beam currents must be used. The resulting high target temperature may cause a difference in the structure of implanted material. In this work the effect of varying the target temperature during nitrogen implantation in stainless steel is studied. With a beam power of 0.1 W cm−2 the target temperature is restricted to 150 °C. At a dose of 3.5×1017 N2+ cm−2, glancing-angle X-ray diffraction (GAXD) revealed the formation of ϵ-iron nitride and no nitrogen in solution. The microhardness measured at 10 gf is increased by 15%. With a beam power of 0.7 W cm−2 the target temperature is 370 °C. After the same dose as before, the microhardness increases by 40%. GAXD results suggest that the increased hardness is due to the presence of nitrogen in solid solution as opposed to complete precipitation. Thus an elevated target temperature can have a favourable effect on the final properties. A sample deformed by compression formed an enhanced oxide layer during implantation as indicated by Auger depth profiling. This oxide layer appears to have a capping action against the out-diffusion of nitrogen.

Effect of martensite content on the sliding behaviour of boron-ion-implanted 304 stainless steel

Abstract We have investigated the effects of boron ion implantation on the wear behaviour of 304 stainless steel. The initial phase composition of the specimens was varied by using two different kinds of polishing technique. Electropolishing resulted in purely f.c.c. (austenitic) specimens. Subsequent mechanical polishing resulted in a phase transformation that introduced a b.c.c. (martensite) phase. The specimens studied were either electropolished or fully polished (electropolished and mechanically polished). In general, we observed that boron implantation improves the wear performance of the steel. The result of boron implantation is to inhibit hardening of the surface during wear, as was seen from measurements of the microhardness inside the wear tracks. (Such hardening has previously been shown to occur in studies on unimplanted material, through martensitic transformation, and leads to the formation of a brittle surface sheet that is prone to cracking.) Interestingly, the improvement on implantation and the associated inhibition of hardening in the wear tracks were seen to be much more pronounced when full polishing was employed rather than just electropolishing, i.e. when the specimen contained a small amount of martensite prior to implantation. The measurement of the microhardness inside the wear tracks provided a quick method of assessing the transformation during the wear process, which we then correlated with the friction and wear behaviour in each case. Measurements of the microhardness on as-implanted surfaces showed that a fully polished surface strengthens to a greater extent than an electropolished surface. This appears to result in reduced plastic deformation, leading to the observed reduction in transformation during wear and, thus, could explain the greatly improved behaviour.

Reduction of stresses in thin films by high energy ion beams

Abstract A high degree of tensile stress tends to develop during the growth of refractory metal films. At increased values of film thickness the integrated stress becomes high enough to cause adhesion problems. When chromium films on the order of 1 μm were irradiated with Ni ions of energy 75 MeV, a reduction in strain as measured by X-ray diffraction was observed. Scratch Adhesion measurements demonstrated that this is accompanied by a large improvement in coating to substrate adherence. A new model is proposed to account for the observed effects and provides a basis on which to engineer stresses in thin films using ion beams. The model pictures mobile interstitials that escape from displacement cascades as nucleating into loops in preferred orientations. The preferential orientation is a result of the lattice strain. As these aligned loops grow, they introduce a strain of their own which cancels out the initial strain. A mathematical formulation enables prediction of strain with irradiation dose for a given initial stress.

A physical basis for irradiation-induced modification of thin-film stresses

A mechanism is proposed for the modification of stresses in thin films by irradiation. A tensile stress causes preferential alignment of interstitial loops such that the normal to the plane of the loop is parallel to the direction of the stress. The aligned loops grow and produce a positive strain in the lattice, which relieves the initial stress.

Sliding Behaviour of Boron Ion-Implanted 304 Stainless Steel

Abstract We have studied the influence of boron ion implantation on the friction and wear behaviour of 304 stainless steel. We find an increase in microhardness following implantation. We also observe a reduction in wear and coefficient of friction. We have measured the microhardness, inside the wear tracks and have found a large increase in the values in the unimplanted specimens and only a small increase in the implanted specimens. These observations have thrown light on the change in the wear mechanism between the two cases. We have also used Scanning Electron Microscopy and Energy Dispersive Analysis of X-rays, to characterize the differences in the mode of wear. The change in wear behaviour is brought about by the ability of boron to prevent the surface from transforming into a hard brittle layer during wear.