Subsequent Diffusion Treatment Research Articles

Boriding of HVOF-sprayed Inconel 625 coatings

The efficient application of high-performance materials under harsh environmental conditions requires advanced coating technologies and the combination of different surface processes. Thermal spraying of metallic coatings with a subsequent diffusion treatment results in a unique combination of bulk and surface properties. A hardening of high-velocity-oxygen-fuel-sprayed Inconel 625 coating systems is performed by powder-pack boriding. In addition to the as sprayed condition, the thermochemical treatment was carried out on solution annealed coatings. Delamination did not occur during the post-treatment due to the good coating adhesion. The microstructure evolution and phase formation are analysed in detail. The successful diffusion of boron within a homogeneous precipitation layer is proven. A distinct increase in microhardness and wear resistance could be achieved. The significant improvement in tribological performance was achieved independently of the pre-treatment state. The correlation between structural and mechanical properties contributes to a better understanding of the material behaviour. In the context of current efforts to improve wear resistance, the presented results provide a new approach comprised of a two-step surface engineering process.

Quantitative evaluation of blending and diffusion in high RAP and RAS mixtures

Due to variable aging of recycled asphalt pavement (RAP) and recycled asphalt shingle (RAS), the concerns arise over old-new binder blending during mixture production and subsequent diffusion process. In this study, a modified staged-extraction method was validated and employed to extract binder in approximately equal layers from retrieved virgin and RAP/RAS aggregates after mixing and subsequent diffusion treatment. Quantitative analysis was done on extracted binders in terms of large molecule size percentage LMS (%) from gel permeation chromatography (GPC). The results indicated that a well-blended binder film coated virgin aggregates from 50% RAP mix, while a non-homogeneous binder film was observed on RAP aggregates. The system of binder blend coating the virgin and RAP aggregates with un-mobilized RAP binder was validated. A possible composite binder system was found coating the virgin aggregates in 10% RAS mix. The diffusion study showed that within the mixture silo storage time, binder diffusion could be accomplished in both warm and hot mixes containing 50% RAP, indicating that binder homogeneity may not be an issue in high RAP mix. The binder diffusion in RAS mix was captured in a very slow rate. It was suggested that binder homogeneity can still be critical for high RAS mix.

A novel structure of Ferro-Aluminum based sandwich composite for magnetic and electromagnetic interference shielding

A novel Ferro-Aluminum based sandwich composite for magnetic and electromagnetic interference shielding was designed and fabricated by hot pressing and subsequent diffusion treatment. The microstructure evolution of sandwich composite was characterized. Magnetic and electromagnetic interference shielding properties and mechanisms of the composites were also investigated. Sandwich composite is obtained with pure iron/Fe–Al alloy layer/pure iron structure and the Fe–Al/Fe interface shows good bonding. Al elemental content in reaction layer presents gradient distribution and the Al-riched brittle phase turns into ductile phase with diffusion time increasing. The electromagnetic shielding effectiveness of sandwich composite is higher than that of pure iron plate and increases with diffusion time extension, reaching 70–80dB at the frequency of 30KHz–1.5GHz. The multiple reflection loss in Fe–Al gradient layer is the primary contribution to the shielding effectiveness improvement of sandwich composite. The magnetic shielding effectiveness of sandwich composite can amount to 10dB, about 2.5 times of that of pure iron plate. Fe–Al intermetallic layer, as non-magnetic spacer, is added between two iron plates and the permeable layer in sandwich composite can shunt magnetic field twice to improve shielding effectiveness.

Coercivity enhancement in HDDR near-stoichiometric ternary Nd–Fe–B powders

Anisotropic HDDR near-stoichiometric ternary Nd–Fe–B powders have been prepared. The coercivity of the powders was improved from 208.6 to 980.1kA/m by the subsequent diffusion treatment using the Pr–Cu alloy. For comparison, Nd11.5Fe80.7B6.1Pr1.2Cu0.5 alloy, in which Pr and Cu elements were directly added into the original Nd–Fe–B alloy, was also treated by the same HDDR process and the coercivity was only 557.3kA/m. Microstructural investigations showed that a large area of (Nd, Pr)-rich phases concentrated at triangle regions in the HDDR Nd11.5Fe80.7B6.1Pr1.2Cu0.5 powders, while the (Nd, Pr)-rich phases distributed uniformly in the diffusion treated powders. The uniform grain boundary layer can pin the motion of domain wall more effectively, resulting in a higher coercivity in diffusion treated HDDR Nd–Fe–B powders.

Aluminizing of TiAl-based alloy using thermal spray coating

Coating of aluminide on a TiAl-based alloy (49.1at.% Al) was carried out by thermal spraying pure aluminum and subsequent diffusion treatment at 1100°C. The growth of a Ti–Al intermetallic layer in the coating layers during diffusion treatment for 120–1800s as well as the oxidation resistance of the aluminized TiAl-based alloy was investigated. The outermost surface layer was comprised of Al-rich intermetallic TiAl3 and contained pores. In contrast, an Al concentration gradient layer consisting of Ti2Al5, TiAl2, and Al-rich TiAl containing 55at.% Al was formed between the outermost layer and the substrate. The thickness of the outermost layer decreased with increasing diffusion time, while the thickness of the intermediate layer grew to approximately 30μm. In addition, the coating/substrate interface changed from a wavy to a linear form with the growth of the intermediate layer. The aluminized coating, at all diffusion times, showed good oxidation resistance in cyclic oxidation tests at 900°C in air.

Aluminizing of TiAl-Based Alloy Using Thermal Spray Coating

Aluminizing the surface of a TiAl-based alloy (49.1 at% Al) was carried out by thermally spraying a pure aluminum coating and subsequent diffusion treatment at 1100°C. The influence of the diffusion time for the formation of Ti-Al intermetallic phases in the coating layers and the oxidation resistance of the aluminized TiAl-based alloys were investigated. The layer formed on the outermost surface was comprised of the Al-rich intermetallic T2iAl5 and contained pores. On the other hand, an intermediate layer consisting of TiAl2 and TiAl was formed between the outermost layer and the substrate. The thickness of the outermost layer decreased as the diffusion time increased, while the thickness of the intermediate layer increased. In addition, the coating/substrate interface changed from a wavy to a linear form with the growth of the intermediate layer. The aluminized coating showed good oxidation resistance at 900°C for all diffusion times.

High temperature corrosion behaviour of a gradient NiCoCrAlYSi coating I: Microstructure evolution

A gradient NiCoCrAlYSi coating was prepared by a combined method of arc ion plating and subsequent diffusion treatment. The microstructure of the gradient coating was investigated in the as-annealed condition and after oxidation at 1000 °C. During the oxidation, an in situ diffusion barrier composed of Cr(W)-rich σ phases was formed in the interdiffusion zone, resulting in an effective inhibition of inward diffusion of Al. The formation of the in situ diffusion barrier layer is also discussed.

Preparation and oxidation behaviour of an AlSiY diffusion coating on a Ni-based single crystal superalloy

An AlSiY diffusion coating was developed by a combined approach of arc ion plating and subsequent diffusion treatment. Cyclic and isothermal oxidation behaviour of the coating specimens was tested in a muffle furnace at 1000 °C and 1100 °C. The results confirmed an enhanced performance of the diffusion coating due to the presence of β-NiAl enriched outer layer. The microstructure change and the oxidation behaviour of the diffusion coating are discussed.

Nanocrystallization of aluminized surface of carbon steel for enhanced resistances to corrosion and corrosive wear

Aluminizing is often used to improve steel's resistances to corrosion, oxidation and wear. This article reports our recent attempts to further improve aluminized carbon steel through surface nanocrystallization for higher resistances to corrosion and corrosive wear. The surface nanocrystallization was achieved using a process combining sandblasting and recovery heat treatment. The entire surface modification process includes dipping carbon steel specimens into a molten Al pool to form an Al coat, subsequent diffusion treatment at elevated temperature to form an aluminized layer, sandblasting to generate dislocation network or cells, and recovery treatment to turn the dislocation cells into nano-sized grains. The grain size of the nanocrystallized aluminized surface layer was in the range of 20–100 nm. Electrochemical properties, electron work function (EWF), and corrosive wear of the nanocrystalline alloyed surfaces were investigated. It was demonstrated that the nanocrystalline aluminized surface of carbon steel exhibited improved resistances to corrosion, wear and corrosive wear. The passive film developed on the nanocrystallized aluminized surface was also evaluated in terms of its mechanical properties and adherence to the substrate.

Morphologic, Compositional and Magnetic Characterization of Sputtered CoCr Thin Films for Applications in MTJs as Hard Spin Injectors

AbstractSpin-polarized currents across an insulating tunnel barrier, needed for the development of efficient magnetic tunneling junctions (MTJs), may be obtained using hard spin injector electrodes. Thin films of CoCr solid solutions have been fabricated involving two main steps: 1) deposition of Co/Cr alternated layers via RF magnetron sputtering both onto silicon (100) substrates and thermally oxidized wafers and 2) thermal annealing in a partial Ar pressure of 1 mTorr at 450°C for 1 hour and cooling treatment in a uniform magnetic field (600 Oe). The deposition of stacks of pure elements and subsequent diffusion treatment has been preferred instead of the direct deposition of the native alloy because in the former case the right composition and magnetic bias may be tuned more easily playing on the layer thicknesses and number of repetitions.A detailed numerical correlation of field effect SEM images and EDX micro-maps was used to evaluate the oxygen diffusion on the magnetic film, while an alternating gradient force magnetometer (AGFM) allowed us to evaluate at room temperature both coercivity and magnetic bias obtained after the field cooling treatment. The effect of standard thermal treatment on the homogeneity of the films is discussed, and a possible alternative heating technique is proposed.

Aluminizing and subsequent nitriding of plain carbon low alloy steels for piston ring applications

Nitriding is a case hardening process that is commonly used for increasing the wear life of automotive piston rings. However, special alloy steels are required to achieve high surface hardness and nitrided case depth values required by the automotive industry. The cost of such alloy steels is one of the major components of the total cost of the nitrided piston ring. To address this issue, efforts have been directed towards development of cheaper raw materials as substitutes for nitridable steels. In this study, an attempt has been made to increase the surface hardness of two plain carbon low alloy steels by aluminizing and subsequent diffusion treatment and nitriding. The process parameters for the aluminizing operation are discussed. Results indicate that a near twofold increase in surface hardness is achievable by aluminizing followed by diffusion treatment and nitriding (580–1208 HV for EN32B steel and 650–1454 HV for 15CR3 steel). It has also been found that the nitrided case depth obtained (0.11–0.13 mm for EN32B steel and 0.10–0.14 mm for 15CR3 steel) matches well with the general requirements of the piston ring industry. The diffusion of aluminum into the alloy layer has also been discussed and the theoretical predictions were compared with actual values of Al concentration, as obtained by SEM–EDS system. It is found that Fick’s law gives a fairly good prediction of the actual Al concentration profile, in spite of the complexity of the diffusion path. X-Ray diffraction studies have confirmed the presence of AlN in the alloy layer, which could be instrumental in the significant increase in surface hardness. It is proposed that aluminizing followed by diffusion treatment and nitriding of plain carbon low alloy steels could provide an alternative to the use of expensive nitridable steels for piston ring applications.

Plasma carburization of wear-resistant high chromium iron

A series of 10–34 wt.% Cr alloyed iron (2 wt.% C) prepared by a spray-forming process (the Osprey process) were plasma carburized. The plasma carburization was performed for 2–9 h and 1–3 h at 950 °C and 1040 °C respectively, in a CH 4 gas atmosphere to keep the surface carbon content sufficiently high to form carbide dispersion layers, without subsequent diffusion treatment. Evaluation of these layers was conducted by X-ray diffraction, electron probe microanalysis, optical microscopy, microhardness measurements and wear testing. In typical results of 26 wt.% Cr iron, owing to an increase in fine chromium carbide precipitation during carburization, the surface Vickers hardness reached over 1000 HV and the surface carbon content amounted to about 5 wt.%. The wear loss in the severe sliding wear condition performed by an Ogoshi-type wear test decreased to a quarter of that of uncarburized material. These characteristic values varied according to the chromium content. Further, we discuss the types and quantity of carbides identified by X-ray diffraction methods.

The basic principles of electrophoretic paint deposition and its application to the development of coating systems for metal powders on metal substrates for subsequent diffusion treatment

Electrophoretic coating is analogous to electroplating in that a voltage is applied across electrodes in a bath and insoluble solids are deposited to achieve a coating. However, in the case of electrophoretic coating the electrode coated is usually the anode, although cathodic systems have recently been developed. The anodic system dates back to the 1950s and is currently in use on a large scale in the motor industry for paint application because of its speed, the evenness of coating and the ease of draining. The system works by using water-soluble resins solubilized at pH 7.6 – 8.4 with organic amines. For paint systems pigments and extenders are also added. The paint deposition on the anode is achieved by using a high voltage (50 – 300 V) low current (0.3 – 0.5 A dm −2) d.c. supply. The resin macro-anions are attracted towards the anode by the steep voltage gradient close to the anode, i.e. electrophoresis. At the same time counterflow of the aqueous phase take place, i.e. electroendosmosis. This, together with the build-up of hydrogen ions, also near the anode, which gives an acidified layer, causes the resin to lose solubility and to coagulate on the anode. Where resin deposits it forms a resistive layer so that further deposition occurs at extremities; this is responsible for the high throwing power of the system and the evenness of the coating thickness. Pigment is deposited in a similar fashion since the pigment itself is coated with resin macro-ions. The system was adapted for the deposition of aluminium powder of 6 μm particle size onto Nimonic substrates and turbine blades for the purpose of providing a coating which, after diffusion treatment, was capable of providing a nickel aluminide layer for high temperature oxidation resistance at about 1000 °C. The main problems with the electrophoretic

Fe-Si系合金の陽極分極挙動について

Fe-Si alloys having various Si contents of 0-14% were prepared by sinter siliconizing and subsequent diffusion treatment according to the procedure described in the author's previous report. Anodic polarization behaviors of the prepared specimens in 2N H2SO4 were investigated by the potentiostatic method.For the specimens having Si contents up to 6%, both of the maximum current density at the active state and the corresponding potential indicated each constant value, independent of Si content. At above 6% of Si content, however, the maximum current density decreased with the increase of Si content and the corresponding potential shifted to be less noble.Effects of Si content on the passive current density were quite different from those of abovementioned. That is, the passive current density increased with the increase of Si content up to 6%, but decreased with the increase of Si content above 6%.The anodic polarization was measured in 1 and 0.5N H2SO4 by using specimens having Si contents of 6 and 14%. The polarization for 6% Si specimen was only affected by concentration of H2SO4 and the potential range of the active state was extremely extended. In other words, 6% specimen was much less to be passive than other specimens when H2SO4 solution was diluted to 1 or 0.5N from 2N. It was confirmed by the author's another experiment that the above results mainly depended upon SO4-- concentration in H2SO4 solution.