ICMCTF2011 Session G5: Coatings, Pre-Treatment, Post-Treatment and Duplex Technology

In this study, we introduce a new process which results in the formation of a single phase Fe₂B layer on low carbon steel substrates. Although, FeB phase is much harder and more common than Fe₂B in all of the boriding operations, it has very poor fracture toughness, hence can fracture or delaminate easily from the surface under high normal or tangential loading. We call the new process “phase homogenization in electrochemical boriding” (PHEB) in which we perform electrochemical boriding for about 15 min at 950^oC in an electrolyte consisting of 90% borax and 10% sodium carbonate then we turn of power and leave samples in the bath for additional 45 min without any polarization. The typical current density during the electrochemical boriding is about 200 A/m². The total thickness of the resultant boride layer was about 65 micrometer (i.e., 30 micrometer thick FeB and 35 micrometer Fe₂B layers) after 15 min of electrochemical boriding. However, during the additional holding time of 45 min, the thickness of the boride layer increased to 80 micrometer and consisted of only Fe₂B phase as confirmed by glancing angle x - ray diffraction and scanning electron microscope in backscattering mode. The microscopic characterization of boride layers revealed a very dense and homogenously thick boride layer whose microhardness was about 16 GPa. The fracture behavior and adhesion of the boride layer was evaluated using the Daimler-Benz Rockwell C test and found to be excellent, i.e., consistent with an HF1 rating.

DC-based plasma systems have been widely utilized for plasma nitriding in commercial. In addition, pulse bias technique is combined with this DC-based plasma technology for efficient nitriding. In the present study, rf/dc-combined plasma systems are proposed for plasma nitriding. Different from the conventional rf-system, it works around 2 MHz with frequency control for matching. Owing to prompt response in mili-second to varying plasma conditions, nitrogen plasma is easily ignited and controlled at wider range of pressure. Since the sample temperature is controlled independently from plasmas, processing conditions for nitriding are optimized in the broad feasible combinations. Due to this combination of rf-plasma with dc-plasma, sputtering effect by dc bias is reduced in the present system.

Aluminum alloys with A2017 and A2024 are employed to demonstrate the practical feasibility of this system. After pretreatment of samples, they are subjected to pre-sputtering for removal of surface oxide layers; then, they are plasma-nitrided at T = 673 K for various pressure conditions. Their microstructure is characterized by XRD, SEM and XPS to quantitatively identify the formation of AlN and to describe the formation rate of nitrided layer. The effect of pressure or plasma density on the formation rate is also discussed to find an optimum condition for efficient plasma nitriding. Micro-hardness test is performed to evaluate on the mechanical properties of nitrided aluminum alloys.

Metallic Ti and its alloys are widely used in aerospace, bio-medical and chemical industries because of their low density, high specific strength and high corrosion resistance. However, their extended applications are limited by their poor tribological properties. Chemical vapor deposition of wear/corrosion resistant diamond coatings on such substrates will significantly enhance the durability and service performances of these materials. High quality diamond coatings are difficult to deposit on the Ti metal and alloys due to low coating adhesion strength and severe chemical reaction between the gas reactants and the substrate which deteriorates the mechanical properties of the substrate. In this study, a series of plasma surface modification strategies have been carried out to enhance nucleation, growth and adhesion of diamond as well as the prevention of the substrate chemical reaction. The diamond film quality, chemical and structural natures of the interfacial layers and the substrate near-surfaces are characterized.

Financial support of Structural Funds in the Operational Programme - Innovative Economy (IE OP) financed from the European Regional Development Fund - Project No POIG.01.01.02-00-015/09 is gratefully acknowledged.

Thermally sprayed alumina coatings are widely used in a range of industrial applications to improve wear and erosion resistance, corrosion protection and thermal insulation of metallic surfaces. These properties are required for many components for production processes in the paper and printing industry. By means of efficient and adjustable coating processes, long-term use of the refined surfaces is obtained. It can be seen that cost-efficient arc-sprayed Al₂O₃ coatings post-treated by plasma-electrolytic oxidation (PEO) form layers with outstanding hardness, bonding strength, abrasion and corrosion resistance as well as extended service time. The generated layers (arc-sprayed and PEO-converted) show a thickness of up to 250 µm and a microhardness of up to 1600 HV0.1. These coatings are designed to partially replace hard chromium.

The oxide nucleation begins at energetically preferred sites at the surface and forms a non-porous barrier layer. Then the formed oxide layers are partly melted and additional high-temperature phases at the oxide/electrolyte interface, like spinel or mixed oxides, are formed. This rapid oxide formation process stops when the electric field strength falls under a critical value self-induced by the growing layer thickness. In general, the achieved composition and properties depend on the substrate phase composition, the electrolyte composition, and the treatment regimes (temperature, processing time, voltages, current densities, current forms AC/DC etc.).

In conclusion, different substrates coated by thermal spraying of aluminium and other valve metals can be converted by PEO to fully or partially oxidized surfaces with outstanding properties. In the presented article, these coatings will be mechanically and chemically evaluated and compared to standard APS-Al₂O₃ coatings.

Two low temperature plasma surface alloying processes ( low temperature plasma nitriding and low temperature plasma carburising) have been successfully developed for austenitic stainless steels. Both processes significantly enhance their wear resistance without compromising the corrosion resistance [1]. In a previous investigation into the cavitation erosion resistance of ferritic steels, C. Godoy et al. [2] concluded that plasma nitriding followed by PAPVD Cr-N coating was a very effective surface treatment combination to improve their cavitation erosion performance. Austenitic systems are usually preferred for cavitation erosion protection due to their structure with low stalking fault energy. In this work, plasma carburising and a sequential plasma carburising followed by plasma nitriding were carried out in an attempt to further enhance the cavitation erosion resistance of austenitic stainless steels. Five systems were investigated: Uncoated, untreated AISI 316 steel (S1); plasma carburised steel (S2); hybrid plasma treated (i.e. sequentially plasma carburised and plasma nitrided) steel (S3); plasma carburised and PAPVD Cr-Al-N-coated steel (duplex S4) and hybrid plasma treated (as in S3) and PAPVD Cr-Al-N-coated steel (duplex S5). Mass loss plots were obtained as a function of time so that cavitation erosion rates and incubation times could be determined. All duplex systems exhibited a significant increase in the incubation period compared to that of the uncoated steels. Both duplex (S4 e S5) yielded a 25-fold and 22-fold increase, respectively, in the incubation period of the AISI 316 steel. Although uncoated, plasma treated systems also displayed an increase in the incubation time compared to that of the uncoated, untreated AISI 316 steel, they were outperformed by duplex (i.e., plasma treated + coated) systems. The most effective plasma surface treatment without PAPVD Cr-Al-N coating was plasma carburising. Cavitation erosion rates were found to decrease as the depth of hardened layers increased. Plasma carburised systems (S2 and S4) exhibited the lowest erosion rates as a result of deeper hardened layers that were intrinsically produced by that process. Results strongly indicate that PAPVD coatings are of prime importance to increase incubation periods (and therefore delay the onset of cavitation erosion) whilst deep hardened layers are paramount to minimise erosion rates.

[1] Y. Sun, Materials Science and Engineering A 404 (2005) 124–129

[2] C. Godoy et al., Surf. Coat. Technol. 200 (2006) 5370-5378.