Zum Einfluss von Abkühlverlauf und Tauchdauer auf die Haftfestigkeit und das Bruchverhalten von Zinküberzügen nach DIN EN ISO 1461

The influence of cooling conditions and immersion time on the adhesive power and the nature of the break of zinc coatings in according to EN ISO 1461 Zinc coatings are exposed external and internal mechanical stresses The object of our investigation was to determine the influence on the adhesion test results of the parameters originally determining the structure and the properties of the zinc coatings, such as • chemical composition of steels (Si/P‐content) • galvanizing conditions (zinc melt composition, immersion time) • cooling conditions The results of the adhesion test by the modified pull‐off‐test in according to ISO 4624, metallographic, scanning electron microscope and EDX analytical investigations and also layer thickness measurements make it possible to state that the structure and constitution of zinc coatings are especialy depended on the immersion time and cooling conditions. The reasons for this would be found. The best results for adhesive power of zinc coatings were received by cooling in cold water immediately after the hot dip galvanizing process. Relevant informations for the galvanizing industry are given.     

Diffusion kinetics of borided AISI 52100 and AISI 440C steels

In this study, the case properties and diffusion kinetics of AISI 440C and AISI 52100 steels borided in Ekabor-II powder were investigated by conducting a series of experiments at temperatures of 1123, 1173 and 1223 K for 2, 4 and 8 h.The boride layer was characterized by optical microscopy, X-ray diffraction technique and micro-Vickers hardness tester. X-ray diffraction analysis of boride layers on the surface of the steels revealed the existence of FeB, Fe₂B and CrB compounds.The thickness of boride layer increases by increasing boriding time and temperature for all steels. The hardness of the boride compounds formed on the surface of steels AISI 52100 and AISI 440C ranged from 1530 to 2170 HV_0.05 and 1620 to 1989 HV_0.05, respectively whereas Vickers hardness values of untreated steels AISI 440C and AISI 52100 were 400 HV_0.05 and 311 HV_0.05, respectively. The activation energies (Q) of borided steels were 340.426 kJ/mol for AISI 440C and 269.638 kJ/mol for AISI 52100. The growth kinetics of the boride layers forming on the AISI 440C and AISI 52100 steels and thickness of boride layers were also investigated.     

Development in high-grade dual phase steels with low C and Si design

Cold rolled dual phase steels with low C and Si addition were investigated in terms of combination of composition and processing in order to improve mechanical properties and workability including welding and galvanizing. Mo and Cr could be used as alloying elements to partially replace C and Si to assure enough hardening ability of the steels and also give solute-hardening. Mo addition is more effective than Cr addition in terms of obtaining the required volume fraction of martensite and mechanical strength. The ferrite grain was effectively refined by addition of Nb microalloying, which gives optimized mechanical properties. The experimental results show that it is possible to obtain the required mechanical properties of high grade 800 MPa dual phase steel, i.e., tensile strength>780 MPa, elongation>15%, and yield/tensile strength ratio<0.6 in the condition of low carbon (C<0.11 wt.%) and low silicon design (Si<0.05 wt.%) through adequate combination of composition and processing.     

The influence on immersion time of titanium conversion coatings on electrogalvanized steel

The study aims to replace chromate treatments of electrogalvanized steels by immersing the electrogalvanized steel in a titanium trichloride (TiCl₃) bath. The influence of immersion time on surface morphology and corrosion resistance of the titanium conversion coatings was investigated. The titanium conversion coating prepared for 300 s immersion time with a uniform structure has the most excellent anticorrosive performance. Furthermore, the coating comprises a loose outer layer and a dense inner layer structure with a thickness of 150 nm. X-ray photoelectron spectroscope analysis shows that the coating consisted of Zn and Ti oxides and/or hydroxides.     

锌浴温度对0.49%Si活性钢热浸镀锌层组织的影响

研究了高硅钢(0.49%Si,质量分数)在不同浸镀温度(440,480,520,560,600℃)和不同浸镀时间(1,3,5,10min)下的热镀锌试样镀层组织,探讨了浸镀温度和浸锌时间对镀层组织的影响。SEM/EDS结果表明:440℃和480℃热镀锌,镀层由薄而连续δ层和破碎的块状ζ相组成,无Γ相。ζ相生长过快,镀层过厚、灰暗、黏附性差,镀层生长主要由界面反应机制控制;520℃热镀锌时,镀层由极薄的Γ层、致密δk层和疏松的δp层组成,镀层生长主要受扩散机制控制;560℃和600℃镀锌时,镀层为Γ层和致密的δ相层,部分δ粒子弥散分布在η层中,镀层生长为扩散机制控制并伴随有显著的δ溶解过程。最佳高温镀锌温度应在520~560℃。     

Surface microstructure and cell biocompatibility of silicon-substituted hydroxyapatite coating on titanium substrate prepared by a biomimetic process

Silicon-substituted hydroxyapatite (Si-HA) coatings with 0.14 to 1.14 at.% Si on pure titanium were prepared by a biomimetic process. The microstructure characterization and the cell compatibility of the Si-HA coatings were studied in comparison with that of hydroxyapatite (HA) coating prepared in the same way. The prepared Si-HA coatings and HA coating were only partially crystallized or in nano-scaled crystals. The introduction of Si element in HA significantly reduced P and Ca content, but densified the coating. The atom ratio of Ca to (P + Si) in the Si-HA coatings was in a range of 1.61–1.73, increasing slightly with an increase in the Si content. FTIR results displayed that Si entered HA in a form of SiO₄ unit by substituting for PO₄ unit. The cell attachment test showed that the HA and Si-HA coatings exhibited better cell response than the uncoated titanium, but no difference was observed in the cell response between the HA coating and the Si-HA coatings. Both the HA coating and the Si-HA coatings demonstrated a significantly higher cell growth rate than the uncoated pure titanium (p < 0.05) in all incubation periods while the Si-HA coating exhibited a significantly higher cell growth rate than the HA coating (p < 0.05). Si-HA with 0.42 at.% Si presented the best cell biocompatibility in all of the incubation periods. It was suggested that the synthesis mode of HA and Si-HA coatings in a simulated body environment in the biomimetic process contribute significantly to good cell biocompatibility.     

Structure and Corrosion-Mechanical Properties of the Surface Layers of Steels after Laser Alloying

We perform the thermodynamic analysis of the solubility of alloying elements and stability of possible secondary phases of chromium and chromium-nickel steels in the components of the Li₁₇Pb₈₃ eutectic heat carrier and reveal the negative effect of the lead melt on the long-term strength and plasticity of the major part of these steels. It is proposed to improve the stability of the characteristics of structural steels promising for nuclear and thermonuclear power engineering by the laser alloying (modification) of the surfaces with Zr, Nb, or Si and B. By analyzing the changes in the microhardness, structure, and thermoelectromotive force of the corresponding coatings on holding in the eutectic melt at 350 or 500°C for 10⁴ h, it is shown that the procedure of laser alloying may result in the formation of layers with long-term protective action.     

Dissolution kinetics of iron in liquid zinc

In hot dip galvanizing, steel strip is coated by immersion in a bath of molten zinc. The principal reactions that occur at the steel/liquid zinc interface are (1) dissolution of iron and (2) nucleation and growth of intermetallic compounds. In order to improve the management of industrial galvanizing baths, it is essential to evaluate the flux of dissolved iron that diffuses into the bath from the sheet. For this purpose, a rotating disk device has been developed to study the dissolution and diffusion of iron in pure liquid zinc at the temperature usually employed in galvanizing baths (465°C). Since the dissolution reaction is controlled by diffusion under these conditions, the diffusion coefficient of iron in liquid zinc has been measured and found to be: D _Fe ^Zn(L) = (9.8 ± 0.1) × 10^–10 m²·s^–1     

Optimization of hot-dip galvanizing process of reactive steels: Minimizing zinc consumption without alloy additions

One of the problems encountered when galvanizing reactive steel is the thickness of the zinc layer. The aim of this paper is to apply Doehlert design taking into account the effects of the bath temperature, the immersion time and the withdrawal speed in order to obtain a reduction of the zinc consumption. Our results showed that there is a serious opportunity to optimize the galvanizing process at a minimum thickness playing only on the physical parameters and without resorting to any addition.     

Studies on the presence of copper in the galvanized coating on weathering steel and the adherence characteristic of the protective copper complex formed on galvanized weathering steel

Following the hot-dip process for zinc coating on weathering steel, the galvanizing bath was found to have picked up copper. The galvanizing bath was observed to pick up Cu from the weathering steel at an average rate of 1.83×10^–3% s^–1m^–2 at 452±2C. EDAX/SEM studies exhibited a concentration gradient of copper to exist across the thickness of the galvanized coating on weathering steel. XRD studies revealed the formation of a protective copper complex, {Cu[(OH)₂Cu]₃}SO₄, on galvanized coating containing 0.739% Cu, when exposed in marine and industrial atmospheres. The adherence characteristic of the copper complex to the galvanized coating was found to be very satisfactory.     

Behavior of magnesium and silicon in the formation of MgO/AlN composite

MgO/AlN composites have been fabricated by directed metal nitridation of Al–Si alloy in flowing N₂ at 1473 K. A mixture of magnesia particles and chemically pure magnesium powder was placed on the surface of Al–Si alloy block as reinforcement materials. Mg powder initiates the infiltration and nitridation of Al alloy melt by eliminating protective Al₂O₃ film at the reaction frontier. New Mg vapor from the interface reaction between Al and MgO particles, keeps as continuous deoxidization agent as the added Mg powder. The spinel layer thickness due to the reaction of Al melt with MgO particles is controlled by Mg content. Si not only reduces the surface tension and viscosity of Al alloy melt, but also leads to increase in N₂ content.     

Sol-gel processing of lithium disilicate

Lithium disilicate (Li₂Si₂O₅) coatings were prepared by spin-coating alkoxide solutions on to substrates [Si, SiO₂, polycrystalline (poly) Si, sapphire] and heating isothermally at 500–600 °C. The effects of solution chemistry, coating thickness and substrate type on crystallization behaviour and microstructure development were investigated using atomic force microscopy, X-ray diffraction and transmission electron microscopy. Amorphous dried coatings began to crystallize into Li₂Si₂O₅ at 500–550 °C. Coatings prepared on Si substrates (with a thin native oxide) using Li-Si methoxyethoxide solution crystallized into microstructures with large grain sizes (ca. 2–5 m diameter) as compared with the coating thickness (<0.3 m). Nucleation rate in these coatings could be increased (and hence transformation rate increased and grain size decreased) by: (1) adding H₂PtCl₆to the solution to act as nucleation agent; (2) increasing the thickness of the coating; or (3) using a crystalline substrate (sapphire or poly Si). Coatings prepared using Li-Si ethoxide solution had fine-grained microstructures (0.5 m diameter) for all substrates. Chemical heterogeneity in the ethoxide system may have increased nucleation rate. Nucleation rate in this system could be decreased by using partially hydrolysed tetraethylorthosilicate as the Si precursor. The relationship between solution chemistry and microstructure was used to tailor microstructures in multilayer coatings.     

Protective coatings for commercial particulates

SiC/Al composites are in large-scale production with AlSi alloy matrices. The same composites with pure Al or low Si matrices need diffusion barriers on the SiC reinforcement to control the interfacial reaction. The present paper describes various approaches taken to obtain protective coatings of alumina and zirconia on SiC particulates by sol-gel techniques. Aqueous and organic precursors have been used. The extent of the reaction, i.e., the Si and Al₄C₃ content in the matrix, was determined by differential thermal analysis and X-ray diffraction. The reaction rates of some coated particulates in liquid Al are decreased by as much as one order of magnitude during the first 15 min of immersion. Pretreatments of the SiC surface, the composition and thickness of the coating interphase and heat treatments of the coated materials have been studied, and are discussed in relation to their effect on the control of reaction rates.     

Microstructural size effects on the hardness of nanocrystalline TiN/amorphous-SiNx coatings prepared by magnetron sputtering

It has been postulated that equiaxed nanocrystalline (<10 nm) TiN grains embedded in a thin amorphous silicon nitride (a-SiN_x) phase are a prerequisite to obtain ultrahard TiN/a-SiN_x coatings. The present study correlates hardness and microstructure of TiN/a-SiN_x coatings with Si contents between 0 and 17 at.%. The coatings have been deposited by magnetron sputtering in industrial-scale physical vapour deposition systems. Transmission electron microscopy studies revealed that increasing the silicon content causes the TiN grain size to decrease. This is accompanied by a change in grain morphology: At Si contents lower than 1 at.% TiN grains become columnar, while at Si contents higher than 6 at.% equiaxed grains with diameters of 6 nm form. For silicon contents between 1 and 6 at.%, a transition region with nanocrystalline columnar grains exists. This nanocrystalline columnar microstructure causes maximum hardness values of more than 45 GPa for TiN/a-SiN_x coatings as determined by nanoindentation. The elongated and equiaxed nanocrystalline TiN grains exhibit almost theoretical strength as dislocation-based deformation mechanisms are constrained.     

Large area Ba1 − xSrxTiO3 thin films for microwave applications deposited by pulsed laser ablation

Large area Ba_1 − xSr_xTiO₃ (BST) thin films with x = 0.4 or x = 0.5 were deposited on 75 mm diameter Si wafers in a pulsed laser deposition (PLD) chamber enabling full-wafer device fabrication using standard lithography. The deposition conditions were re-optimized for large PLD chambers to obtain uniform film thickness, grain size, crystal structure, orientation, and dielectric properties of BST films. X-ray diffraction and microstructural analyses on the BST films grown on Pt/Au/Ti electrodes deposited on SiO₂/Si wafers revealed films with (110) preferred orientation with a grain size < 100 nm. An area map of the thickness and crystal orientation of a BST film deposited on SiO₂/Si wafer also showed (110) preferred orientation with a film thickness variation < 6%. Large area BST films were found to have a high dielectric tunability of 76% at an electric field of 400 kV/cm and dielectric loss tangent below 0.03 at microwave frequencies up to 20 GHz and a commutation quality factor of ~ 4200.     

Specific features of the borating of iron—carbon alloys in sodium melt

We describe some specific features of the diffusion borating of steels in Na-B melt. We succeeded in obtaining high-quality single-phase coatings on steels with a total concentration of alloying elements less than 2 mass % and on high-strength cast irons. The maximum microhardness of these coatings is 22–23 GPa. The optimal borating temperatures are 900–950°C. At temperatures lower than 850°C, we observe the dissolution of certain components of steels and cast irons in Na-B melt initiated by boron.Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 40, No. 2, pp. 99–104, March–April, 2004.     

Plasma paste boronizing of AISI 8620, 52100 and 440C steels

In the present study, AISI 8620, 52100 and 440C steels were plasma paste boronized (PPB) by using 100% borax paste. PPB process was carried out in a dc plasma system at temperature of 700 and 800 °C for 3 and 5 h in a gas mixture of 70%H₂–30%Ar under a constant pressure of 4 mbar. The properties of boride layer were evaluated by optical microscopy, X-ray diffraction and Vickers micro-hardness tester. X-ray diffraction analysis of boride layers on the surface of the steels revealed FeB and Fe₂B phases for 52100 and 8620 steels and FeB, Fe₂B, CrB and Cr₂B borides for 440C steel. PPB process showed that since the plasma activated the chemical reaction more, a thicker boride layer was formed than conventional boronizing methods at similar temperatures. It was possible to establish boride layer with the same thickness at lower temperatures in plasma environment by using borax paste.     

Microstructure and wear behaviour of silicon doped Cr–N nanocomposite coatings

Hard Cr–N and silicon doped Cr–Si–N nanocomposite coatings were deposited using closed unbalanced magnetron sputtering ion plating system. Coatings doped with various Si contents were synthesized by changing the power applied on Si targets. Composition of the films was analyzed using glow discharge optical emission spectrometry (GDOES). Microstructure and properties of the coatings were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), and nano-indentation. The harnesses and the elastic modulus of Cr–Si–N coatings gradually increased with rising of silicon content and exhibited a maximum at silicon content of 4.1 at.% and 5.5 at.%. The maximum hardness and elastic modulus of the Cr–Si–N nanocomposite coatings were approximately 30 GPa and 352 GPa, respectively. Further increase in the silicon content resulted in a decrease in the hardness and the elastic modulus of the coatings. Results from XRD analyses of CrN coatings indicated that strongly preferred orientations of (111) were detected. The diffraction patterns of Cr–Si–N coatings showed a clear (220) with weak (200) and (311) preferred orientations, but the peak of CrN (111) was decreased with the increase of Si concentration. The XRD data of single-phase Si₃N₄ was free of peak. The peaks of CrN (111) and (220) were shifted slightly and broadened with the increase of silicon content. SEM observations of the sections of Cr–Si–N coatings with different silicon concentrations showed a typical columnar structure. It was evident from TEM observation that nanocomposite Cr–Si–N coatings exhibited nano-scale grain size. Friction coefficient and specific wear rate (SWR) of silicon doped Cr–N coatings from pin-on-disk test were significantly lower in comparison to that of CrN coatings.     

The effects of zinc bath temperature on the coating growth behavior of reactive steel

The purpose of this work is to identify the influence of zinc bath temperature on the morphology and the thickness of reactive steel (Fe–0.1 wt.%Si alloy) coatings. The Fe–0.1 wt.%Si samples were galvanized for 3 min at temperatures in the range of 450–530 °C in steps of 10 °C. The coatings were characterized by using scanning electron microscopy/energy dispersive X-rays analysis. It was found that the coating thickness reaches the maximum at 470 °C and the minimum at 500 °C, respectively. When the reactive steel is galvanized at temperatures in the range of 450–490 °C, the coatings have a loose ζ layer on the top of a compact δ layer. With the increase of the galvanizing temperature, the ζ layer becomes looser. When the temperature is at 500 °C, the ζ phase disappears. With the increase of temperature, the coatings change to be a diffuse-Δ layer (δ+ liquid zinc).