ICMCTF2010 Session G6: Coatings, Pre-Treatment, Post-Treatment and Duplex Technology

Austenitic stainless steels are attractive materials for various industrial applications where corrosion resistance especially in wet environment is a primary requirement. In order to improve their tribological properties, low thermochemical diffusion treatments as plasma assisted nitriding (PAN), plasma assisted carburizing (PAC) or their combination i.e. plasma assisted nitrocarburizing (PANC), are developed and improved since the 1980s.

PAN and PAC are treatments in which nitrogen and carbon atoms diffuse respectively in a metallic material from moderate to elevated temperatures. Below 420°C, a plasma assisted nitriding treatment of austenitic stainless steel produces a specific phase usually called expanded austenite, S phase, m phase or or ?N phase. Expanded austenite is a metastable nitrogen supersaturated solid solution with a disordered fcc structure and a distorted lattice. This phase is hard, wear resistant and a paramagnetic to ferromagnetic transition takes place for nitrogen contents higher than 14 at%. Other features of the expanded austenite are its high nitrogen content (from 10 to 35 at%) and compressive residual stress in the nitrided layer. Replacing nitrogen by hydrocarbon for conducting a low temperature PAC leads to a carburized layer containing the carbon expanded austenite phase ((?C). These two phases can be obtained simultaneously by combining hydrocarbon and nitrogen in the gas phase through a low temperature PANC.

We perform PAN and PAC treatments by using a multi-dipolar plasma, based on the Distributed Electron Cyclotron Resonance concept. The microwave power is supplied through eight microwave antennas arranged in a two-dimensional network. The resonant magnetic field is produced by permanent magnets located in front of the antennas. The resulting plasma mixture (N₂/H₂/Ar or C₃H₈/H₂/Ar) diffuses towards the substrate-holder that can be independently heated and/or biased.

In this work, we show results obtained with sequential treatments: carburizing followed by nitriding and nitriding followed by carburizing. It was found that nitrogen and carbon atoms can simultaneously be dissolved into the austenite lattice during the nitrocarburising process, forming a dual layer structure with an extremely hard nitrogen-enriched layer near the substrate surface and above a hard carbon-enriched layer. Our investigations indicate that the carbon content is higher when carburizing is followed by nitriding. X-Ray Diffraction is used to identify the presence and structure of the expanded austenite. The layer thickness is observed by optical microscopy and the composition of the layers is determined by SNMS.

In this study, we employed an ultra-fast boriding technique to grow hard, wear, and corrosion resistant boride layers on low carbon steel substrates using an induction furnace at 900°C. The technique utilizes an electrochemical cell in which it is possible to achieve 100 micrometer thick boride layers in about 30 minutes. The effect of the electrolysis time on boride layer thickness, composition, and structural morphology was investigated and an empirical equation was derived for the growth rate of boride layers. In depth analyses of the boride layers were carried out by microscopy and X-ray diffraction (XRD) which revealed that different boride phases (FeB, Fe₂B, Fe₃B) could be formed depending on the process duration. The growth rate of boride layers is nearly linear up to 30 minutes of treatment. However during much longer treatments, the growth rate has a parabolic character which can be represented with the equation (d = 1.4904 (t)^0.5 + 11.712); where d (in µm) is the growth rate, t (in sec) is duration. The mechanical characterization of the borided surfaces in plane and in cross-sections has confirmed hardness values as high 19 GPa at or near the borided surface (where FeB phase is the most dominant) but gradual decrease in hardness to 14 to 16 GPa beneath the surface where Fe₂B phase is present.

An innovative approach to improving behavior of aluminum surfaces and meeting long-term durability requirements of aluminum devices is to design and develop novel systems incorporating duplex diffusion/plasma coating treatments. The composite layers consist of a N-diffusion zone obtained by rf-plasma nitriding, followed by a rf-plasma deposited DLC film. A PACVD processing technology is described that helps to overcome some of the most important limitations of existing technology to generate duplex coatings on technical aluminum substrates. Application of this technique to plasma assisted nitriding of pure aluminum and different aluminum alloys is demonstrated. The influence of the process parameters on the discharge characteristics was examined. Material properties and tribological testing results are shown.

Adhesive bond strength of ceramic components in composite systems is an area of growing interest due to the wide range of applications. The adhesive bond between the ceramic and the adhesive is, in many cases, the point where failure originates. In order to combat this debonding mode of failure, the surfaces of the ceramic materials are modified to promote strong, uniform bonding. The surface pretreatment regiment currently being used includes aggressive surface cleaning with alumina particles (grit blasting) in addition to chemical treatments such as priming and the application of silane coupling agents like glycidoxypropyltrimethoxy silane (GPS). Grit blasting, while effective at removing surface contaminants detrimental to adhesive bonding such as carbon, also acts to reduce flexural strength by exacerbating existing surface flaw populations. As an alternative to grit blasting, atmospheric plasma treatments were used to remove surface contamination and chemically functionalize the surface in one step. Atmospheric plasma processing of polymer systems removes the weak boundary layer residing on the surface, and chemically alters the surface of the material, improving surface wettability and chemical reactivity. It is postulated that ceramic surfaces subjected to a glow discharge would undergo similar improvements in wettability and surface reactivity.

In this study, surface modification of aluminum oxide (Al₂O₃₎) specimens due to helium (He), helium-oxygen (He-O₂), and helium-nitrogen (He-N₂) dielectric barrier discharges was compared to that of as-received and grit blasted specimens. Contact angle goniometry, x-ray photoelectron spectroscopy (XPS) were used to track chemical changes on the surface, while atomic force microscopy (AFM), and flexural strength testing were used to track surface roughness and surface damage, respectively. Wettablility tests showed an improvement in the hydrophilic character of the ceramic surface as the water contact angle decreased by 24% with the He-O₂ plasma. XPS results show a reduction in surface carbon similar for both grit blasted and He-O₂ plasma treated surfaces. Flexural strength data show no measurable reduction in strength due to plasma exposure, while grit blasting reduces the characteristic strength by ~10%.

A duplex surface treatment consisting of a High Temperature Gas Nitriding (HTGN) followed by a Low Temperature Plasma Nitriding (LTPN) was carried out in an UNS 31803 stainless steel. HTGN was performed at 1,200ºC for 8 hours, in a 1 atm N₂ atmosphere while LTPN was carried out in a 75% N₂:25% H₂ atmosphere, at 400ºC for 12 hours. A duplex structure consisting of a ~12 µm thick expanded austenite g_N layer with 1,400 HV0.025 hardness was formed on top of a 1.0 mm thick fully austenitic layer, with 350 HV. The HTGN layer was intended to give mechanical support to the more hard and brittle expanded austenite layer. A less steep hardness gradient was obtained between the expanded austenite layer and the UNS 31803 stainless steel matrix. The duplex treated specimens were tested in vibratory cavitation erosion testing equipment. The incubation time for the onset of cavitation for the duplex treated steel was 1.5 times higher than the HTGN UNS 31803 steel and 20 times higher than the UNS 31803 untreated steel. The mass loss rates decreased from 1.5 mg/h for the untreated stainless steel to 0.25 mg/h for the HTGN steel and 0.15 mg/h for the duplex treated steel.

Duplex stainless steels are nowadays being used in a variety of applications within the refining and petrochemical industry, where high corrosion resistance and mechanical properties are required. The surface properties of these steels may be optimized in order to attain better performance in highly stressed tribological systems. AISI F51 (EN 1.4462) duplex stainless steel was DC-Plasma carburized at 480 ^oC, using CH₄ as carbon carrier gas. Although the carburized layer showed an apparently duplex structure inherited from the matrix, XRD results, using the Rietveld method, showed only peaks of expanded austenite. The carburized layer attained a surface hardness of 1600 HV0.05 and XRD results showed that this strong hardening effect is related to the formation of a highly stressed carbon expanded austenite on the surface of the specimens. When tested in a ball on disk tribometer in dry conditions wear coefficients as low as 4 x 10^-15 m³/(N m) were obtained. The corrosion resistance of the expanded austenite layer in 10% HCl solution was two-fold better than the uncoated F51 steel.

In the machining of hard materials such as glass or stone, cemented carbides have been recently replaced by diamond tools, consisting of a metallic carrier, on to which diamond segments are brazed. One of the most economic ways for the production of diamond segments is the cold compaction of the mixture of a metallic powder and diamond particles. Due to a highly abrasive sliding contact between diamond particles and the die walls, the wear rate of the press dies is very high. As a result of a low lifetime of the press dies, they must be replaced in short time periods. To avoid the costly and time-consuming substitution of the press dies, in this work PVD-coatings were deposited on the inner surface of the pre-plasma nitrided press dies (DUPLEX treatment). Thereby, various high and low alloy tool steels were treated by means of plasma nitriding process. Subsequently, a nanocomposite PVD coating, titanium aluminum supernitride, was deposited by means of a high ionization magnetron sputtering device on nitrided and non-nitrided steel substrates. The mechanical and tribological properties of these coating systems were studied by means of several standard tests such as nanoindentation, ball-on-disc and scratch test. The most wear resistant coating system was chosen to employ on the inner surface of the press dies. The wear resistance of the press dies developed in this study was tested under real loading condition during compaction of the mixture of diamond particles and cobalt powder. It was revealed that employing plasma nitrided tool steels coated with titanium aluminum supernitride enhances the lifetime of the press dies up to 500%. Furthermore it was shown that the press dies with different surface treatments possess different failure modes during the compaction process.

In this paper, the effects of post deposition annealing (PDA) in O₂, N₂, and Ar at 600^oC for 1 min on the resistive switching characteristics of SrZrO₃ (SZO) thin films were investigated. The SZO resistive memory devices exhibited the large dispersion of resistive switching parameters such as turn-on voltage and high resistance state (HRS) current, further restricting their development in the future nonvolatile memory (NVM) applications. The severe dispersion is more greatly reduced by O₂ 600^oC- PDA than N₂ and Ar 600^oC- PDA owing to that the suppression of the oxygen vacancies in thin films could be significantly enhanced by O₂ 600^oC- PDA . Furthermore, device yield is dramatically increased to 90% by O₂ 600^oC- PDA in comparison to the other devices. T he oxygen-annealed SZO memory devices can be successively switched between HRS and low resistance state (LRS) by applying voltage signal over 150 times, and the resistive ratio of LRS and HRS is still kept more than 1000 . The retention characteristics of both HRS and LRS are stable up to 10⁶ s at room temperature (RT) and 85^oC. Besides, the resistive ratio of HRS and LRS still remains nearly 1000 more than 14000 s under successive reading voltage stress at RT and 85^oC. The oxygen-annealed SZO memory device with its superior memory characteristics is a promising candidate for the next-generation NVM applications.