Paul J. Wilbur

Metastable phase formation and enhanced diffusion in f.c.c. alloys under high dose, high flux nitrogen implantation at high and low ion energies

The use of elevated target temperatures near 400 °C during high flux ion implantation of N2+ at energies ranging from 60 keV to 0.4 keV leads to a metastable, f.c.c. high nitrogen solid solution phase induced in austenitic (f.c.c.) Cr-containing stainless steels. This phase has not been produced in an f.c.c. Ni-Fe alloy containing no Cr. Penetration depths of the N are significantly larger than expected on the basis of known diffusion coefficients of N in Cr-containing stainless steels or in pure f.c.c.-Fe. X-ray diffraction data suggest unusual differences in N penetration and concentration depending on the f.c.c. grain orientation. Consideration is given to possible residual stresses induced by N expansion of the lattice and to the anisotropic elastic constants for austenitic stainless steels. The amount of N in interstitial solid solution approaches 40 at.% under the lower energy, higher flux conditions, but only in the (200) crystallographic planes parallel to the surface. The N-expanded f.c.c. phase with the highest amount of N is found by conversion electron Mossbauer spectroscopy to be magnetic in AISI 304 and 310 stainless steels. A comparison of our results with those from related methods such as conventional plasma ion nitriding and pulsed plasma ion implantation is made to demonstrate that the observed metastable solid solution phase is often produced by other methods provided that appropriate temperatures and processing times are used. The similar penetration depths of N in the f.c.c. Cr-containing stainless steels for the various methods are consistent with thermal diffusion and metastable solubility that are enhanced by the Cr. The metastability is associated with the low Cr mobility below about 450 °C and the strong N-Cr bond. Evidence for vacancy-enhanced diffusion is not found and we see at present no advantage to using ion energies higher than about 2 keV at these elevated temperatures for producing surfaces with optimized tribological behavior based on the extremely high strength, metastable, N solid solution phase.

A tribological investigation of the graphite-to-diamond-like behavior of amorphous carbon films ion beam deposited on ceramic substrates

Abstract A tribological investigation was conducted on the graphite-to-diamond-like behavior of hard carbon films produced on SiC, Si 3 N 4 and ZrO 2 substrates by ion beam deposition. Friction tests were performed on a ball-on-disk machine with pairs of various ceramic balls and disks coated with hard carbon films in dry and humid air, argon and N 2 . The friction coefficients of carbon films sliding against Si 3 N 4 and sapphire balls were in the range 0.02–0.04 in N 2 and argon, but were significantly higher (about 0.15) in humid air. The wear rates of ceramic disks coated with carbon films were unmeasurable, and, depending on the test environment, the wear rates of counterface ceramic balls were reduced by two to four orders of magnitude below those of balls slid against uncoated ceramic disks. Graphite disks were also tested, to obtain friction data that can help us to understand the graphite-to-diamond-like tribological behavior of carbon films. Micro laser Raman spectroscopy and scanning electron microscopy were used to analyze the structure and chemistry of worn surfaces and to elucidate the graphite- and diamond-like tribological behavior of amorphous carbon films.

A comparative study of beam ion implantation, plasma ion implantation and nitriding of AISI 304 stainless steel

Abstract This paper presents the results of a comparative study using beam ion implantation (BII), plasma ion implantation (PII), ion nitriding and gas nitriding of AISI 304 stainless steel. We have demonstrated that under controlled conditions (the same treatment times of 30 and 60 min, and the same treatment temperature of 400 °C), the microstructures produced by all four techniques are similar, being mainly the formation of nitrogen in solid solution (γ N phase). However, the concentrations of nitrogen and the detectable depths of the nitrogen-enriched layers are significantly different, depending on the process. Both BII and PII produce thick nitrogen-enriched layers (greater than 1 μm) at high concentrations (20–30 at.%) compared with either ion nitriding or gas nitriding (layers less than 1 μm thick with low nitrogen concentrations). As a result, the load-bearing capacity after either BII or PII is much greater than after either ion or gas nitriding. It has also been found that high current density implantation is crucial for the formation of the thick N-enriched layers.

Characterization of transfer layers on steel surfaces sliding against diamond-like hydrocarbon films in dry nitrogen

Abstract Carbon-rich transfer layers on sliding contact surfaces play important roles in the tribological performance of diamond-like hydrocarbon (DLHC) films. In this study, we investigated the nature of these layers formed on M50 balls during sliding against DLHC films (1.5 μm thick) prepared by ion-beam deposition. Long-duration sliding tests were performed with steel balls sliding against the DLHC coatings in dry nitrogen at room temperature, approximately 22 ± 1 °C. Results indicated that the friction coefficients of test pairs were initially about 0.12 but decreased steadily with sliding distance to 0.02–0.03 and remained constant throughout the tests, which lasted for more than 250 000 sliding cycles (approximately 30 km). This low-friction regime appeared to coincide with the formation of a carbon-rich transfer layer on the sliding surfaces of M50 balls. Micro-laser Raman spectroscopy and electron microscopy were used to elucidate the structure and chemistry of these transfer layers and to reveal their possible role in the wear and friction behavior of DLHC-coated surfaces.

Characterization of transfer layers forming on surfaces sliding against diamond-like carbon

Abstract Metallic and ceramic surfaces may become covered with a carbon-rich transfer layer during sliding against diamond-like carbon (DLC) films. The presence of such layers at sliding interfaces may dominate the long-term friction and wear performance of these films. In this study, we use Raman and infrared spectroscopies to characterize the chemical structure of such transfer layers forming on the surface of magnesia/partially-stabilized ZrO2 (MgOPSZ) balls. The DLC film (≈ 2 μm thick) was prepared by ion-beam deposition at room temperature, with methane used as the source gas. Tribological tests were performed on a ball-on-disk machine in open air at room temperature (≈22 ± 1°C) and humidity of 30–50%. Sliding velocity ranged from 1 to 6 m s−1 and the tests were continued until the DLC films were effectively worn through. The results showed that the friction coefficients of DLC against MgOPSZ were initially 0.08–0.12 but decreased to 0.05–0.06 after about 200 000–500 000 sliding passes, depending on velocity. They remained constant at 0.05 for the duration of the tests, which was 1.5 million cycles at 6 m s−1 but > 4 million cycles at 1 m s−1. The low friction coefficients observed in each test coincided with the formation of a carbon-rich transfer layer on the rubbing surfaces of MgOPSZ balls. Micro-laser-Raman and Fourier transformed infrared (FTIR) spectroscopies confirmed that these carbon-rich transfer layers had a disordered graphitic structure.

Friction and wear performance of ion-beam-deposited diamond-like carbon films on steel substrates

Abstract In this study, we investigated the friction and wear performance of ion-beam-deposited diamond-like carbon (DLC) films (1.5 μm thick) on AISI 440C steel substrates. Furthermore, we performed a series of long-duration wear tests under 5, 10 and 20 N loads to assess the load-bearing capacity and durability limits of these films under each load. Tests were performed on a ball-on-disk machine in open air at room temperature, about 22 ± 1 °C, and humidity, about 30% ± 5%. For the test conditions explored, we found that (1) the steady state friction coefficients of pairs without a DLC film were in the range 0.7–0.9 and the average wear rates of 440C balls (9.55 mm in diameter) sliding against uncoated 440C disks were on the order of 10−5 mm3 N−1 m−1, depending on contact load; (2) DLC films reduced the steady state friction coefficients of test pairs by factors of 6–8, and the wear rates of pins by factors of 500–2000; (3) the wear of disks coated with a DLC film was virtually unmeasurable whereas the wear of uncoated disks was quite substantial; (4) the DLC films were able to endure the range of loads, 5–20 N, without delamination, and to last over 1 000 000 cycles before wearing out. During long-duration wear tests, the friction coefficients were initially on the order of 0.15, but decreased to some low values of 0.05–0.07 after sliding for 15–25 km, depending on the load, and remained low until wearing out. This low friction regime was correlated with the formation of a carbon-rich transfer film on the wear scar of 440C balls. Microlaser Raman spectroscopy and scanning electron microscopy were used to examine the structure and chemistry of worn surfaces and to elucidate the wear- and friction-reducing mechanisms of the DLC film.

solid solution strengthening of stainless steel surface layers by rapid, high-dose, elevated temperature nitrogen ion implantation

Abstract Extremely strong and wear-resistant surfaces of AISI 304 and 310 stainless steels have been produced by rapid, high-dose nitrogen ion implantation at elevated temperatures. X-ray diffraction, conversion electron Mossbauer spectroscopy, and Auger electron spectroscopy reveal that this behavior is due to thick layers (> 1 μm) of austenite which contain as much as 20 at% N in solid solution after implantation of 2 × 1018 N atoms/cm2 into samples held near 400°C. Much larger retained doses are obtained at the elevated temperatures.

Relative roles of ion energy, ion flux, and sample temperature in low-energy nitrogen ion implantation of FeCrNi stainless steel

Abstract A matrix of nitrogen ion energies (0.4, 0.7 and 1.0 keV) and ion fluxes (1, 2, and 3 mA/cm2) has been selected to investigate the systematics of ion-beam processing of an austenitic FeCrNi stainless steel (AISI 304) at 400°C for 60 min. In addition, the role of temperature was examined over the range from 280°C to 475°C at fixed processing conditions of 0.7 keV, 2 mA/cm2, and 60 min. Characterization of the composition, structure, magnetic nature, thickness and strength of the nitrogen-containing layers was made by a combination of Auger electron spectroscopy, X-ray diffraction, backscatter Mossbauer spectroscopy, and microhardness measurements. A high N content layer, which is a magnetic, fcc phase, is predominant for all conditions and its thickness increases linearly with either ion energy at fixed ion flux or with ion flux at fixed ion energy. This behavior is consistent with layer growth that is controlled primarily by the N supply rate into the first few atomic layers rather than by thermal diffusion.

High current hollow cathode phenomena

Experimental results show that the energies of ions produced near a hollow cathode orifice can be several times the anode-to-cathode potential difference generally considered available to accelerate them. These energies (of order 50 eV) are sufficient to induce substantial sputter erosion rates. Increases in discharge current (to 60 A) cause the energies and current densities of these jet ions to increase substantially. A model describing jet ion generation is proposed. The effects of discharge current on cathode internal pressure are also examined experimentally and described phenomenologically.

Effect of source gas and deposition method on friction and wear performance of diamondlike carbon films

Abstract We investigated the tribological performance of diamondlike carbon (DLC) films derived from methane, acetylene, and hydrogen + methane source gases in a magnetron sputtering system and an ion-beam-deposition system. Films have been deposited on AISI 440C bearing-steel substrates and were tested in a pin-on-disk machine under a wide range of conditions in open air and in dry nitrogen. We found that the films grown in a hydrogen + methane plasma resulted in significantly lower friction coefficients (i.e. 0.01) and ball wear rates during tests in dry nitrogen (N 2 ). The friction coefficients for the methane-derived films were also low (0.02 in dry N 2 but the friction coefficients of the acetylene-produced films were 2- to 10-times higher than those recorded for the methane- and hydrogen + methanegrown films, especially in dry N 2 . The methane- and hydrogen + methane-grown films also resulted in very small wear losses on counterface balls, regardless of the test environments. Raman spectroscopy and electron microscopy were used to elucidate the structural chemistry of each film and correlate the findings with tribological performance.