Comparison of hardmetal and hard chromium coatings under different tribological conditions

Electroplated hard chromium and thermal spray hardmetal coatings are widely used in a variety of applications for wear protection of component surfaces. The two protective coating types are tested in direct comparison for tribological conditions of dry abrasive wear (Taber Abraser test) and dry oscillating wear load. Oscillating wear tests are carried out both with hardened 100Cr6 steel and alumina balls as counterbody. Different types of hardmetal coatings are imparted. Besides HVOF sprayed coatings also coatings sprayed by an APS gun with axial powder feed are tested. For HVOF spraying besides standard WC/Co(Cr) feedstock also coarse (d₅₀ = 5 μm) and fine carbide feedstock (d₅₀ = 0.8 μm) and ultrafine powders, i.e. 2 μm < d < 12 μm, are considered. Use of ultrafine powders is particularly interesting from the economical point of view, as belt grinding can be sufficient for finishing in many cases. The optimum coating solution for wear protection depends on the specific tribosystem. The choice of feedstock, spraying process, equipment and processing conditions does not only depend on the resultant tribological properties. Therefore simultaneous influence on corrosion protection capability and thermal conductivity might have to be considered.     

Sintered stainless steels: Fatigue crack propagation resistance under hydrogen charging conditions

Monophasic and multiphasic (two and three phases) sintered stainless steels were prepared both considering premixes of AISI 316LHC and AISI 434LHC stainless steels powders and using a prealloyed duplex stainless steel 25% Cr, 5% Ni, 2% Mo powder. Their fatigue crack propagation resistance was investigated both in air and under hydrogen charging conditions (0.5 M H₂SO₄ + 0.01 M KSCN aqueous solution; applied potential = −700 mV/SCE), considering three different stress ratios (R = 0.1; 0.5; 0.75). Fatigue crack propagation micromechanisms were investigated by means of fracture surface scanning electron microscope (SEM) analysis.For all the investigated sintered stainless, fatigue crack propagation resistance is influenced by hydrogen charging and an increase of crack growth rates dependent on the steel microstructure is obtained. Experimental results also allow to identify the sintered stainless steel obtained from the prealloyed 25% Cr, 5% Ni, 2% Mo powder as the most resistant to fatigue crack propagation in air and under hydrogen charging conditions.     

Thermo-physical characterization of binder and feedstock for single and multiphase flow of PIM 316L feedstock

A thorough knowledge of the material properties of the feedstock and binder system is essential for successful powder injection moulding (PIM) as well as for numerical simulation. In view of the above, characterization of a developed binder system and feedstock has been reported in this paper for processing of 316L stainless steel powder through PIM route. The binder system consists of paraffin wax, stearic acid and low-density polyethylene. The feedstock comprises of 316L stainless steel powder and the above binder system. The thermal, physical and rheological characteristics of the binder system and feedstock have been investigated separately along with binder removal technique from the injection-moulded green compact. The thermal characterization revealed the semi-crystalline nature having distinct melting and solidification range for both the binder and feedstock. Data from DSC and TGA show that injection of the feedstock should be carried out above 102 °C (i.e. the upper melting temperature) but below 154 °C as beyond which the binder components paraffin wax and stearic acid start degrading and mould temperature should be below 57 °C. The binder and feedstock are found as shear-thinning fluid as viscosity decreases with the increase in shear strain rate and temperature. However, the viscosity of the binder is more sensitive to shear strain rate and temperature compared to that of the feedstock.     

Enhanced sintering, microstructure evolution and mechanical properties of 316L stainless steel with MoSi2 addition

Sintering 316L stainless steel to near full density with an appropriate sintering additive can ensure high mechanical properties and corrosion resistance. We present here a sintering approach which exploits the dissociation of ceramics in steels at high temperatures to activate sintering densification to achieve near full dense 316L stainless steel materials. MoSi₂ ceramic powder was used as a sintering additive for pre-alloyed 316L stainless steel powder. Sintering behavior and microstructure evolution were investigated at various sintering temperatures and content of MoSi₂ as sintering additive. The results showed that the sintering densification was enhanced with temperature and MoSi₂ content. The distribution of MoSi₂ was characterized by XMAPs. It was found that MoSi₂ dissociated during sintering and Mo and Si segregated at the grain boundaries. Excess Mo and Si were appeared as separate phases in the microstructure. Above 98% of theoretical density was achieved when the specimens were sintered at 1300 °C for 60 min with 5 wt.% MoSi₂ content. The stainless steel sintered with 5 wt.% MoSi₂ exhibited very attractive mechanical properties.     

Modeling of nitrogen penetration in polycrystalline AISI 316L austenitic stainless steel during plasma nitriding

The nitrogen depth profile in polycrystalline AISI 316L austenitic stainless steel after plasma nitriding at temperatures around 400 °C is analyzed by the “trapping-detrapping” model. This model considers the diffusion of nitrogen under the influence of trap sites formed by local chromium atoms. Nitrogen depth profiles in polycrystalline AISI 316L steel simulated on the basis of this model are in good agreement with experimental nitrogen profiles. The enhanced nitrogen diffusivity as well as a plateau-type shape of nitrogen depth profile can be explained. The nitrogen diffusion coefficient at 400 °C is found to be D = 4.81 × 10⁻¹² cm²/s and the diffusion pre-exponential factor D₀ (0.837 × 10⁻³ cm²/s) and detrapping activation energy E_B (0.28 eV) were deduced from fitting experimental data. It is known that the nitrogen penetration depth (and nitrogen diffusivity) depends on the crystalline orientation and a tentative to take into account this anisotropy effect and describe nitrogen depth profiles in polycrystalline AISI 316L steel is proposed by using different diffusion coefficients characteristic for each crystallite orientation.     

Thermal sprayed stainless steel/carbon nanotube composite coatings

Stainless steel/carbon nanotube (SS/CNT) composite coating was prepared by thermal spray from the feedstock powder synthesized by chemical vapor deposition at a synthesis temperature and time of 800 °C and 120 min under ethanol atmosphere. Microstructural investigation by TEM and SEM revealed that grown CNTs covering the surface of stainless steel particles were multi-walled type with an average diameter of about 44 nm. Microstructures of pure stainless steel and SS/CNT composite coatings similarly showed splat characteristic and lamellar structure. Incorporation of CNTs was clearly observed in the composite coating. Hardness of SS/CNT composite coating (480 ± 36 HV_0.3) was higher than that of pure stainless steel coating (303 ± 33 HV_0.3). Coefficient of friction of the SS/CNT coating was almost 3 times lower than that of stainless steel coating which resulted in reduction of sliding wear rate of nearly 2 times. This research thus demonstrated a new composite coating with better wear resistive performance compared to a coating deposited by commercially available stainless steel powder.     

Aqueous corrosion behaviour of sintered stainless steels manufactured from mixes of gas atomized and water atomized powders

The electrochemical corrosion improvement of a powder metallurgical (PM) stainless steel is studied in this work. Water atomized (WA) ferritic AISI 434L powders have been mixed with gas atomized (GA) austenitic (AISI 316L type) and ferritic (AISI 430L type) powders and processed through the traditional PM route. The addition of GA powder to the usual WA powder decreases the mean size of the pores of the sintered stainless steels. As the bigger pores are the ones that are able to act as crevices, unlike the smaller ones - that act as closed porosity, reduction in the number of big pores tends to improve the corrosion behaviour of PM stainless steels. Reductions of the corrosion rate (i_corr) and increases of the corrosion potential (E_corr) have been measured in neutral media, with and without chlorides. Moreover, the additional beneficial effect of achieving a duplex microstructure through the addition of GA austenitic powders to the WA ferritic powders has also been verified.     

Corrosion properties of active screen plasma nitrided 316 austenitic stainless steel

AISI 316 austenitic stainless steel has been plasma nitrided using the active screen plasma nitriding (ASPN) technique. Corrosion properties of the untreated and AS plasma nitrided 316 steel have been evaluated using various techniques, including qualitative evaluation after etching in 50%HCl + 25%HNO₃ + 25%H₂O, weight loss measurement after immersion in 10% HCl, and anodic polarisation tests in 3.5% NaCl solution. The results showed that the untreated 316 stainless steel suffered severe localised pitting and crevice corrosion under the testing conditions. AS plasma nitriding at low temperature (420 °C) produced a single phase nitrided layer of nitrogen expanded austenite (S-phase), which considerably improved the corrosion properties of the 316 austenitic stainless steel. In contrast, AS plasma nitriding at a high temperature (500 °C) resulted in chromium nitride precipitation so that the bulk of the nitrided case had very poor corrosion resistance. However, a thin deposition layer on top of the nitrided case, which seems to be unique to AS plasma nitriding, could have alleviated the corrosion attack of the higher temperature nitrided 316 steel.     

Single track formation in selective laser melting of metal powders

Selective laser melting (SLM) is a powder-based additive manufacturing capable to produce parts layer-by-layer from a 3D CAD model. Currently there is a growing interest in industry for applying this technology for generating objects with high geometrical complexity. To introduce SLM process into industry for manufacturing real components, high mechanical properties of final product must be achieved. Properties of manufactured parts depend strongly on each single laser-melted track and each single layer. In this study, effects of the processing parameters such as scanning speed and laser power on single tracks formation are explored. Experiments are carried out at laser power densities (0.3–1.3) × 10⁶ W/cm² by cw Yb-fiber laser. Optimal ratio between laser power and scanning speed (technological processing map) for 50 μm layer thickness is determined for stainless steels (SS) grade 316L (−25 μm) and 904L (−16 μm), tool steel H13 (−25 μm), copper alloy CuNi10 (−25 μm) and superalloy Inconel 625 (−16 μm) powders. A considerable negative correlation is found between the thermal conductivity of bulk material and the range of optimal scanning speed for the continuous single track sintering.     

Mesoscopic simulation model of selective laser melting of stainless steel powder

A 3D mesoscopic model is developed to simulate selective laser melting processes using the ALE3D multi-physics code. We study the laser-induced melting of a random bed of stainless steel 316 particles on a solid substrate (1000 μm × 300 μm × 50 μm) and its solidification into either a continuous track or a discontinuous track as a result of Plateau–Rayleigh instability. Our approach couples thermal diffusion to hydrodynamics and accounts for temperature dependent material properties and surface tension, as well as the random particle distribution. The simulations give new physical insight that should prove useful for development of continuum models, where the powder is homogenized. We validate our approach against the experiment and find that we match the main laser track characteristics.     

Corrosion behaviour of duplex stainless steels sintered in nitrogen

Duplex stainless steels obtained through powder metallurgy (PM) technology from austenitic AISI 316L and ferritic AISI 430L powders were mixed on different amounts to obtain biphasic structures with austenite/ferrite ratio of 50/50, 65/35 and 85/15. Prepared mixes of powders have been compacted at 750 MPa and sintered in N₂-H₂ (95% and 5%) at 1250 °C for 1 h. Corrosion behaviour, using electrochemical techniques such as anodic polarization measurement, cyclic anodic polarization scan and electrochemical potentio-kinetic reactivation test and double loop electrochemical potentio-kinetic reactivation double loop test were evaluated. For duplex stainless steels, when austenite/ferrite ratio increases the corrosion potential shifts to more noble potential and passive current density decreases. The beneficial effect of annealing solution heat treatment on corrosion behaviour was established and was compared with corrosion behaviour of vacuum sintered duplex stainless steels. The results were correlated with the microstructural features.     

Increasing surface hardness of austenitic stainless steels by pack nitriding process

This paper investigated the possibility of increasing the surface hardness of austenitic stainless steels under very low nitrogen dissociation pressures of metal nitride powders using pack nitriding process. Thin sheet of 304 type of stainless steel of approximately 1 mm in thickness was used as a substrate for the study. Based on the results of thermochemical calculations, Cr₂N powder was selected as a nitrogen source from a series of metal nitride powders considered for the pack nitriding process, which included Si₃N₄, Mn₄N, BN, AlN and TiN. The pack nitriding was carried out in a sealed alumina retort at temperatures of 860 °C and 910 °C for up to 48 h. The surface was then characterised using techniques of SEM, XRD and microhardness testing. It was observed that the process used increased the surface hardness of the steel, but it also induced precipitation of chromium nitrides in the matrix even under the nitrogen dissociation pressures below 50 Pa. It was also observed that, in the nitrided layer, the γ phase of the steel was partially transformed to the α phase under the pack nitriding process conditions studied.     

Preparation,characterization and infrared emissivity properties of polymer derived coating formed on 304 steel

High infrared emissivity coatings were prepared on 304 steel by pyrolyzing reactions with poly(hydridomethylsiloxane) (PHMS) and Al/HW powders. The microstructure, phase and chemical composition of the coatings were determined by SEM, XRD and EDS techniques. The infrared emissive properties at wavelength 3–20 μm of the coatings pyrolyzed at 600 and 800 °C on the steels were investigated. It was found that the 800 °C pyrolyzed coating exhibited a slightly higher infrared emissivity value than that of the 600 °C pyrolyzed coating, which was attributed to the complete conversion of Al to Al₂O₃ and pyrolysis of PHMS into SiO₂, as well as the enhancement of photon emission by HW. Comparatively, the uncoated steel indicated a much lower infrared emissivity value about 0.2 in 8–14 μm.     

Effect of the suspension composition on the microstructural properties of high velocity suspension flame sprayed (HVSFS) Al2O3 coatings

Seven different Al₂O₃-based suspensions were prepared by dispersing two nano-sized Al₂O₃ powders (having analogous size distribution and chemical composition but different surface chemistry), one micron-sized powder and their mixtures in a water + isopropanol solution. High velocity suspension flame sprayed (HVSFS) coatings were deposited using these suspensions as feedstock and adopting two different sets of spray parameters.The characteristics of the suspension, particularly its agglomeration behaviour, have a significant influence on the coating deposition mechanism and, hence, on its properties (microstructure, hardness, elastic modulus). Dense and very smooth (Ra ~ 1.3 μm) coatings, consisting of well-flattened lamellae having a homogeneous size distribution, are obtained when micron-sized (~ 1-2 μm) powders with low tendency to agglomeration are employed. Spray parameters favouring the break-up of the few agglomerates present in the suspension enhance the deposition efficiency (up to > 50%), as no particle or agglomerate larger than ~ 2.5 μm can be fully melted. Nano-sized powders, by contrast, generally form stronger agglomerates, which cannot be significantly disrupted by adjusting the spray parameters. If the chosen nanopowder forms small agglomerates (up to a few microns), the deposition efficiency is satisfactory and the coating porosity is limited, although the lamellae generally have a wider size distribution, so that roughness is somewhat higher. If the nanopowder forms large agglomerates (on account of its surface chemistry), poor deposition efficiencies and porous layers are obtained.Although suspensions containing the pure micron-sized powder produce the densest coatings, the highest deposition efficiency (~ 70%) is obtained by suitable mixtures of micron- and nano-sized powders, on account of synergistic effects.     

Tailoring powder processing and thermal treatment to optimize the densification of W–CuO powder mixtures

W–CuO powder mixtures were prepared by attritor mixing/milling of commercial powders. The reduction steps during heating under He/H₂ gas flow were identified by thermogravimetric analysis (TGA). W–Cu powder mixtures with three different O-content were prepared by adjusting the reduction conditions in a furnace. The effect of O-content on the sintering of powder compacts was then especially investigated. Two sintering steps were identified by dilatometry during liquid phase sintering under He/H₂ atmosphere. The first step was associated to rearrangement after copper melting. The second step was related to the presence of W-oxides at the particle surface: shrinkage was enhanced and the second step was shifted to lower temperatures by using initial powders with low oxygen content, by decreasing the heating rate or by introducing a holding time at 1050 °C. This behaviour was related to a gradual reduction of W-oxides from the edge to the bulk of the samples. Microstructural observations were performed at different stages to confirm the analysis. Powder processing and thermal cycle were optimized to obtain materials with 96–97% relative density.     

Characterization and comparison of materials produced by Electron Beam Melting (EBM) of two different Ti–6Al–4V powder fractions

Electron Beam Melting (EBM) has been recognized as a revolutionary technique to produce mass-customized parts to near-net-shape from various metallic materials. The technique produces parts with unique geometries from a powder stock material and uses an electron beam to melt the powder layer-by-layer to fully solid structures. In this study we have investigated the use of two different Ti–6Al–4V powders of different size fractions in the EBM process; a larger 45–100 μm powder, and a smaller 25–45 μm powder. We have also investigated the effects of two build layer thicknesses, 70 μm and 50 μm, respectively. We hypothesize that the smaller powder has the potential to improve surface resolution of parts produced in the EBM process. The EBM as-built parts were investigated regarding surface and bulk chemistry, surface oxide thickness, macro- and microstructure, surface appearance and mechanical properties. We conclude from the results that both powders and both build layer thicknesses are feasible to use in the EBM process. The investigated material properties were not significantly affected by powder size or layer thickness within the studied range of process parameters. However, the surface appearance was found to be different for the samples made with the different powder sizes.     

Producing nanocrystalline bulk Mg-3Al-Zn alloy by powder metallurgy assisted hydriding-dehydriding

Nanocrystalline bulk Mg-3Al-Zn alloy with an average grain size of 48 nm has been prepared by powder metallurgy assisted hydriding-dehydriding. Evolutions of nanograined structure powders and bulk alloy have been investigated by TEM, SEM and XRD, respectively. The results showed that by milling in hydrogen for 60 h, as-hydriding powder possessed an average grain size of 5.9 nm. After a subsequent process of desorption-recombination treatment (at 350 °C) and consolidation process (extruded at 200 °C) resulted in bulk samples with an average crystallite size of 48 nm and MgH₂ was fully turned into Mg. The consolidated samples of 60 h milled powder had a final density of 1.77663 ± 0.006 g/cm³, which corresponded to 97.57 ± 0.3% of theoretical density. The highest microhardness of the nanocrystalline bulk alloy reached about 872.5 MPa, which is about three times higher than that of the coarse-grained AZ31.     

Mechanochemical synthesis of Mo-Cu nanocomposite powders

Mo-Cu nanocomposite powders were successfully synthesized by the mechanochemical (high energy ball-milling) and hydrogen-reduction process at low temperature (650 °C). MoO₃ and CuO powders were used as precursors, which were calcined in air atmosphere to get CuMoO₄-MoO₃ mixtures. The mechanochemical treatment of the CuMoO₄-MoO₃ powder mixtures caused a substantial increase of both the reduction activity of powder mixtures in hydrogen and the refinement of powders. It was accompanied by a transformation from CuMoO₄ to Cu₃Mo₂O₉, playing a critical role in hydrogen reduction process. By optimizing the experimental parameters, Mo-30 wt.% Cu nanocomposite powders with superfine particles with size ranging from 100 to 200 nm could be successfully obtained by mechanochemical-reduction method.     

Effect of nanostructuring and Al alloying on friction and wear behaviour of thermal sprayed WC–Co coatings

Nanostructured WC–Co and WC–Co–Al coatings, with about 300-μm as-deposited coating thickness, were deposited by high velocity oxy-fuel (HVOF) spraying. Agglomerated nanostructured cermet powders produced by the Mechanomade® process was used for HVOF spraying. Dense and well-adherent coatings with crystal sizes below 30 nm were deposited on stainless steel 304 substrate. Porosity was less than 5% and the bond strength with the substrate was around 60 MPa. Experimental data on friction, wear, and abrasion resistance revealed that nanostructured WC–Co based coatings containing some Al as alloying element, exhibit improved tribological characteristics in comparison to nanostructured and micron-sized WC–Co coatings. This was attributed to a carbide particle distribution within the coating revealed by SEM, the absence of brittle W₂C-like phases revealed by XRD, and the presence of Al at particle/matrix boundaries revealed by TEM.