P. Steyer

Surface modification of martensitic stainless steels by laser marking and its consequences regarding corrosion resistance

Abstract Marking is of prime importance in the field of biomaterials to allow the identification of surgical tools or implants. Markings are obtained under industrial conditions with a pulsed Nd:YAG laser on a X30C13 martensitic stainless steel. Depending on laser parameters, two modes of marking are considered: a surface oxidisation mode and a matter photoablation mode. Electrochemical behaviour, i.e. passive state, corrosion rate, resistance to pitting, is evaluated in Ringer’s saline solution. Results are then explained on the basis of metallurgical modifications regarding the phases composition and the distribution of alloying elements. In oxidative regime, laser treatment drastically affects both microstructure and chemical composition. Specially, chromium depletion is measured in subsurface, leading to a loss of passive ability. In ablative mode the surface is transformed into a thin favourable layer of austenite. Moreover, the overall chromium distribution is little modified. Therefore, the passive character is maintained, but with nevertheless a shorter passive domain than base material one.

Effect of Ni interlayer on corrosion behaviour of CrN PVD coating deposited onto plastic mould steels

Abstract The plastics industry commonly uses moulds made of steels containing MnS inclusions. These inclusions play the role of heterogeneities, which make manufacturing easier. Moreover, these types of alloy are thermal treatment free, cheap, and easily available. Unfortunately, their use in corrosive conditions is prevented owing to the poor corrosion behaviour of the MnS particles. In order to mask these highly sensitive inclusions and to enhance the surface mechanical properties, hard physical vapour deposition (PVD) CrN coatings, elaborated at two different temperatures, have been used. The coated specimens have been tested in saline corrosive solutions using electrochemical techniques. Corrosion results from dissolution of the anodic substrate through coating porosity, inherent to the PVD process, which leads to the deposit peeling off. The aim of the present study is to determine the corrosion resistance of an underlayer electroless nickel deposited coating on a 40CrMnMo8S substrate coated with PVD CrN.

PVD aluminium alloy coatings: Environmentally friendly alternative to protect steel parts against corrosion

Abstract In the automotive and aeronautic industries, security is of prime importance and so corrosion prevention is essential. Steel parts were once protected with deposits such as cadmium or zinc - nickel produced using a chromatation treatment but recent environmental directives dictate that these methods are no longer acceptable. Aluminium sacrificial coatings have excellent corrosion behaviour but unfortunately they have poor mechanical characteristics and a fast dissolution rate. The present study investigated a possible solution: alloying the aluminium with a more noble element, chromium, in order to decrease the sacrificial galvanic effect of the deposit and therefore improve its lifetime. The corrosion protection afforded by such coatings in relation to their structure and mechanical properties was investigated. The coatings were produced on carbon steel by a vacuum PVD arc evaporation process at a pilot scale. Intrinsic electrochemical properties were determined on pure materials and on layers deposited on glass strip. Several elaboration configurations (pure as well as composite targets) were investigated. In most cases the coatings were stratified and composed of pure aluminium and numerous hardened AlxCry intermetallic phases. Chromium enrichment of aluminium based coatings induces not only a beneficial hardening effect on the surface characteristics (> 700 HV) but also significantly improves the corrosion behaviour of the coated pieces (increasing lifetime by up to three times compared to pure Aluminium).

Electrodeposition of zinc–ceria nanocomposite coatings in alkaline bath

Use of electrodeposited (ED) zinc coatings is a widespread solution to protect steel against corrosion [1]. Many studies were performed in order to enhance the corrosion properties of the ED zinc coatings through alloying elements like nickel, cobalt or iron [2–7]. Mn and Mg additions were also investigated to reduce the release of metals towards the environment [8–12]. However, important discrepancies are reported on how these elements may influence the corrosion properties. Similarly, the improvement of sacrificial properties by reducing the dissolution kinetics of Zn-based coating is still on interest. Metal matrix composites (MMCs) seem to be a promising way to enhance the mechanical properties which are considered as a major drawback of zinc coatings [13]. In such systems, two structures coexist: a soft metallic-based matrix associated with a hard phase. During the past decades, research studies weremainly focused on the electrodeposition of nickel, copper, or cobalt-based composite coatings [14–16] for mechanical and/or corrosion applications, but in a lesser extent on sacrificial zinc matrix [17, 18], subject of our study. Regarding the hardening phase, oxides, like TiO2, SiO2, and Al2O3, are among the most used particles involved in the reinforcement of the metallic matrix [14–16]. Literature presents different mechanisms, based on bi-phased coatings containingmicrometer-sized particles. The most relevant theory is attributable to Guglielmi [19] who proposes an incorporation of particles linked to both the particle concentration in the bath and the current density. However, he does not consider neither hydrodynamic effects nor the particle characteristics. Few years later, Celis et al. improved this model with a more detailed four-step model [20]. Other models have been proposed afterwards but no one can be generalized to all matrix– reinforcement systems. CeO2 nanoparticles, like other rare earth-containing compounds, present promising interests owing to their high hardness [21], low coefficient of thermal expansion, and good corrosion protectiveness. They are attractive in many technological fields for their intrinsic properties such as ionic conduction, reversible oxygen storage, and catalytic activity [22]. CeO2 nanoparticles have already been used to synthesize MMCs by electrodeposition in conventional acidic baths, especially for the nickel [23–25], cobalt [26], and copper systems [27]. However, very few works deal with their incorporation in zinc matrix [28]. Furthermore, according to our knowledge, no studies have been dedicated to the zinc matrix composite coating electrodeposited from an alkaline bath. Deposition of zinc-based composite coatings was described by Azizi et al. [29]. His mechanism implies adsorption of zinc cations on the particle surface. This would favor the development of a positively net charge, susceptible to promote migration towards the negatively polarized substrate, but this approach remains still discussed by other authors [30, 31]. Another important issue to optimize the MMC electrodeposition process concerns the dispersion of particles. Surface L. Exbrayat : C. Rébéré :C. Berziou : C. Savall : J. Creus LaSIE, Université de la Rochelle, Av. Michel Crépeau, 17042 La Rochelle, France

Functional behaviour of TiO2 films doped with noble metals

To evaluate the effects of different concentrations of noble metal in a TiO2 matrix, different films of both Ag/TiO2 and Au/TiO2 systems were prepared. Mechanical and tribological characterisation was carried out to evaluate the coating response as a function of the noble metal composition and (micro)structure of the films. The overall set of results indicates that the amorphous films reveal better results than the crystalline ones. For the amorphous samples, the reduced Youngs modulus and the adhesion critical loads followed similar tendencies in both sets of films. Wear rates were similar for all samples except for the one with the highest silver content. To improve brittleness of TiO2 films, the results seem to indicate that a slight metal doping is preferred, and Au proved to be a better choice than Ag. In fact, the sample with the lowest Au content revealed a better mechanical behaviour than the pure TiO2 film.

Pure & crystallized 2D Boron Nitride sheets synthesized via a novel process coupling both PDCs and SPS methods

Within the context of emergent researches linked to graphene, it is well known that h-BN nanosheets (BNNSs), also referred as 2D BN, are considered as the best candidate for replacing SiO2 as dielectric support or capping layers for graphene. As a consequence, the development of a novel alternative source for highly crystallized h-BN crystals, suitable for a further exfoliation, is a prime scientific issue. This paper proposes a promising approach to synthesize pure and well-crystallized h-BN flakes, which can be easily exfoliated into BNNSs. This new accessible production process represents a relevant alternative source of supply in response to the increasing need of high quality BNNSs. The synthesis strategy to prepare pure h-BN is based on a unique combination of the Polymer Derived Ceramics (PDCs) route with the Spark Plasma Sintering (SPS) process. Through a multi-scale chemical and structural investigation, it is clearly shown that obtained flakes are large (up to 30 μm), defect-free and well crystallized, which are key-characteristics for a subsequent exfoliation into relevant BNNSs.

Electron CHanneling ORientation Determination (eCHORD): An original approach to crystalline orientation mapping

We present a proof-of-concept attesting the feasability to obtain orientation maps of polycrystalline materials within a conventional Scanning Electron Microscope (SEM) using a standard goniometer and Back Scattered Electron (BSE) detector. The described method is based on the analysis of the contrast variation of grains due to the channeling of incident electrons on a rotating sample. On each pixel of the map, experimental intensity profiles as a function of the rotation angle are obtained and compared with simulated ones to retrieve the orientation. From first results on aluminum polycrystals, the angular resolution is estimated to be better than one degree.

Advanced synthesis of highly-crystallized hexagonal boron nitride by coupling PDCs and SPS processes – influence of the crystallization promoter and sintering temperature

Hexagonal boron nitride nanosheets (BNNSs) are promising 2D materials due to their exceptional chemical and thermal stabilities together with their electrical insulation properties. A combined synthesis method involving the polymer-derived ceramics (PDCs) route and the spark plasma sintering (SPS) process is proposed, leading to well-crystallized and pure layered h-BN crystals, prone to be exfoliated into large BNNSs. Here we focus more specifically on the influence of two key parameters of the process to be optimized: the Li3N concentration (0-10 wt%) and the SPS temperature (1200 °C-1950 °C). The presence of Li3N, added as crystal promoter in the pre-ceramic powder, significantly improves the crystallinity level of the product, as evidenced by XRD, SEM and Raman spectrometry. SPS temperature strongly modifies the size of the resulting h-BN flakes. The influence of SPS temperature on both purity and crystallinity is studied using cathodoluminescence. h-BN flakes larger than 200 μm2 (average flake area) are obtained. Few-layered BNNSs are successfully isolated, through exfoliation process.