Non-coated Alloy Research Articles

Quasi-in vivo corrosion behavior of AZ31B Mg alloy with hybrid MWCNTs-PEO/PCL based coatings

This study investigated the effects of multi-walled carbon nanotubes (MWCNTs) and polycaprolactone (PCL) on the quasi- in vivo corrosion behavior of AZ31B Mg alloy treated by plasma electrolytic oxidation (PEO). Thin (∼2 μm, PCTPCL4) and thick (∼60μm, PCTPCL6) PCL layers were applied only onto the MWCNTs-PEO coating (PCT) as it showed better corrosion performance. Findings reveal that incorporation of MWCNTs induced several structural and functional modifications in the PEO coating, such as increased roughness, a thicker inner barrier layer, and reduced hydrophilicity. Quasi- in vivo corrosion testing was carried out under controlled temperature, pH, and fluid flow in simulated body fluid (SBF) by electrochemical impedance spectroscopy (EIS) and hydrogen evolution experiments. EIS results revealed that, after 48 h immersion, a diffusion process controlled hydration of the ceramic coatings. Comparison of the collected hydrogen after 15 days of immersion in the quasi- in vivo environment revealed that the PEO and PCT ceramic coatings decreased hydrogen generation by up to 74% and 91%, respectively, compared to non-coated alloy. PCTPCL6 coating exhibited the lowest amount of collected hydrogen (0.2 mL/cm 2 ). The thick PCL layer delayed the onset of substrate corrosion for at least 120 h, reducing the corrosion rate by 85% compared with the PCT.

Surface Modification of Ti–30Ta Alloy by Deposition of P(VDF-TrFE)/BaTiO3 Coating for Biomedical Applications

This study aims to promote an adequate methodology for coating an experimental Ti-30Ta alloy with P(VDF-TrFE)/BaTiO3. The combination of a copolymer with a ceramic has not been used until now. Ti-30Ta is an excellent choice to replace current alloys in the global market. The composite deposition on the Ti-30Ta substrate was performed by a spray coating process and at low temperature using two different surface modifications: surface acidic etching and surface polishing. Characterization was divided into four areas: (I) the substrate surface treatments used and their influences on the adhesion process were evaluated using surface energy, wettability, and roughness analyses; (II) the properties of the composite film, which were carried out using X-ray diffractometry (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TG), and differential scanning calorimetry (DSC); (III) the study of the adhesion of the film on the substrate, which was performed by a scratch test; (IV) the final product, which was evaluated to determine the surface properties after the coating process. Biofilm formation using Staphylococcus aureus and Staphylococcus epidermidis strains and a hemocompatibility test were performed as biological assays. The results indicated that the P(VDF-TrFE)/BaTiO3 film showed high thermal stability (up to ≈450 °C); the FTIR and DSC tests indicated the presence of the β phase, which means that the material presents a piezoelectric nature; and the scratch test showed that the samples with the polish treatment provided a better adhesion of the film with an adhesion strength of ~10 MPa. From the SEM analysis, it was possible to determine that the spray deposition coating process resulted in a well-applied film as evidenced by its homogeneity. Microbiological tests showed that for Staphylococcus aureus, the bacterial growth in the coated Ti-30Ta presented no significant differences when compared to the alloy without coating. However, for Staphylococcus epidermidis, there was considerable growth on the coated Ti-30Ta, when compared to the non-coated alloy, indicating that the film surface may have favored bacterial growth. The hemolysis assay showed that the coated material presents hemocompatible characteristics when in contact with blood cells. The results obtained indicate that the Ti-30Ta alloy coated with P(VDF-TrFE)/BaTiO3 is a promising alternative for implant applications, due to its biocompatible properties, simplicity, and low cost.

In Vivo Response of Growth Plate to Biodegradable Mg-Ca-Zn Alloys Depending on the Surface Modification

Because Mg-Ca-Zn alloys are biodegradable and obviate secondary implant removal, they are especially beneficial for pediatric patients. We examined the degradation performance of Mg-Ca-Zn alloys depending on the surface modification and investigated the in vivo effects on the growth plate in a skeletally immature rabbit model. Either plasma electrolyte oxidation (PEO)-coated (n = 18) or non-coated (n = 18) Mg-Ca-Zn alloy was inserted at the distal femoral physis. We measured the degradation performance and femoral segment lengths using micro-CT. In addition, we analyzed the histomorphometric and histopathologic characteristics of the growth plate. Although there were no acute, chronic inflammatory reactions in either group, they differed significantly in the tissue reactions to their degradation performance and physeal responses. Compared to non-coated alloys, PEO-coated alloys degraded significantly slowly with diminished hydrogen gas formation. Depending on the degradation rate, large bone bridge formation and premature physeal arrest occurred primarily in the non-coated group, whereas only a small-sized bone bridge formed in the PEO-coated group. This difference ultimately led to significant shortening of the femoral segment in the non-coated group. This study suggests that optimal degradation could be achieved with PEO-coated Mg-Ca-Zn alloys, making them promising and safe biodegradable materials with no growth plate damage.

Cryogenic Treatment of Nimonic Alloy- Hard Turning: State -of-the-art, Challenges and Future Directions

Abstract Nickel based super alloys have been gaining importance day by day and it is being implemented extensively in various sectors like nuclear, aerospace, chemical and defense industries. But these alloys have an increased chemical affinity and low thermal conductivity which makes it difficult to turn it in when it is dry. Hence, Nimonic family of Nickel based super alloy is gradually getting importance in terms of research. Hard turning needs coolants in order to make a hole in the alloy, but since these coolants damage the environment, alternate methods are necessary to perform the turning process without these fluids. Hence, cryogenic process is done to coat the alloys with different components at very low temperatures. Performing the cryogenic process makes the alloy harder and improves the overall structural integrity of the alloy. This paper studies various cryogenic processes done in Nickel based alloys and then focuses on the Nimonic based Nickel alloys and compares the performance and properties of coated alloys with non-coated alloys.

Corrosion protection of epoxy coating with pH sensitive microcapsules encapsulating cerium nitrate

The ability of an epoxy coating with pH sensitive microcapsules (pH-MC) to protect AA2024-T3 aluminum alloy from corrosion in a NaCl solution was investigated. Immersion of the noncoated alloy in the solution revealed that addition of cerium nitrate to the solution led to a decrease in the cathode area or cathodic reactivity to form cerium oxides on copper-enriched sites. Application of the pH-MC containing cerium nitrate to the coated alloy resulted in an increase in the charge transfer resistance with time. Encapsulation of cerium ions in the pH-MC was beneficial for preventing excess elution of cerium ions and self-healing ability.

Chromium Coating as a Surface Protection of Zirconium Alloys

Abstract Increasing of nuclear safety, higher demands for longer nuclear fuel campaign and higher levels of burnout are based on improving the properties of zirconium alloys. Protective coatings appears to be a promising way to reduce their chemical reactivity and increase resistance to hydriding. In this work, a thin chromium coating applied to a standard Zr1Nb zirconium alloy was studied using in-situ impedance spectroscopy. Exposure was carried out at a temperature of 280 ° C and a pressure of 8 MPa in a simulated WWER primary coolant environment. The results show that the chromium-coated Zr1Nb alloy is under these conditions oxidized significantly slower than the non-coated alloy.

In vitro corrosion performance of PEO coated Ti and Ti6Al4V used for dental and orthopaedic implants

The present work describes the characteristics of 5–10 μm-thick plasma electrolytic oxidation (PEO) coatings with graded Ca/P ratio generated on titanium of commercial grades I (c.p. Ti) and V (Ti6Al4V) and some of the aspects of their bioactivity and corrosion resistance in vitro in long-term (up to 8 weeks) normal and short-term (1 week) inflammatory conditions. Simulated body fluid (SBF) and artificial saliva at 37 °C were used as corrosive media; inflammatory conditions were simulated by controlling the pH and introducing hydrogen peroxide (usual metabolism product of inflammation-inducing bacteria). Additionally, the saliva was modified with fluoride ions. DC and AC electrochemical tests were used to characterize the corrosion protection mechanism of the coatings. Metal ion release (Ti4 +, Al3 + and V5 +) from PEO coated materials during their in vitro immersion in normal and inflammatory SBF was evaluated by ICP-MS. PEO coatings on Ti6Al4V alloy inhibited the liberation of titanium compared with non-coated alloy both at short-term inflammatory and long-term normal immersion conditions; but the liberation of aluminium and vanadium was greater from the coated than non-coated alloy at both conditions, due to the presence of these ions in the coatings. The PEO-coated c.p. Ti exhibited considerably higher stability with respect to Ti4 + ion release in SBF at all conditions compared with the PEO-coated Ti6Al4V.

Role of PEO coatings in long-term biodegradation of a Mg alloy

Long-term effect of plasma electrolytic oxidation (PEO) coatings on in vitro corrosion of 3D Mg implant specimens has been investigated in simulated body fluid (SBF). Hydrogen evolution rate in PEO-coated specimens was 35 times lower compared to the non-coated alloy during first 2 weeks of immersion (0.04–0.07mLcm−2d−1) and reached a steady-state rate of 0.3mLcm−2d−1 by week 6 of immersion. Studies of the tribocorrosion behaviour of the PEO-coated alloy disclosed that the coatings rapidly lose their tribocorrosion resistance due to hydration of the coating material and associated drastic decrease of microhardness but exhibit a better tirbocorrosion performance than the non-coated material.

Behaviour of alumina-coated 304L steel in a Waste-to-Energy plant

Pieces of 304L stainless steel, alumina-coated by plasma spraying, were placed during one week at about 1170K in the furnace of the Waste-to-Energy plant of Limoges, France. The resistance against corrosion was only slightly improved compared to non-coated alloy. The origin of this unsatisfactory result lies in the cracking of the coating that occurred when the pieces were introduced in the furnace, due to the great difference in the Coefficient of Thermal Expansion (CTE) of alumina and 304L steel. However, the plasma-sprayed alumina coatings themselves appear as impervious enough to provide an efficient protection of this alloy against the very corrosive atmosphere of the incinerator.

Oxidation of Fe–22Cr Coated with Co3O4: Microstructure Evolution and the Effect of Growth Stresses

The oxidation behavior of a commercially available Fe–22Cr alloy coated with a Co3O4 layer by metal organic—chemical vapor deposition was investigated in air with 1% H2O at 1,173 K and compared to the oxidation behavior of the non-coated alloy. The oxide morphology was examined with X-ray diffraction, electron microscopy, and energy dispersive X-ray spectroscopy. Cr2O3 developed in between the Co3O4 coating and the alloy, while alloying elements of the substrate were incorporated into the coating. Particular attention was devoted to possible sources of growth stresses and the effect of the growth stresses on microstructure evolution in the scales that developed on the non-coated and the coated Fe–22Cr alloy. Microstructural features suggested that scale spallation on coated Fe–22Cr occurred as a result of superimposing thermal stresses during cooling onto the growth stresses, that had developed during oxidation.

Inter-diffusion between Co 3O 4 coatings and the oxide scale on Fe-22Cr

The oxidation behaviour of a commercially available Fe-22Cr alloy coated with a Co 3O 4 layer by spray-painting or plasma-spraying was investigated at 1173 K in air with 1% H 2O and compared to the oxidation behaviour of the non-coated alloy. The oxide morphology was examined with X-ray diffraction, electron microscopy, and energy dispersive X-ray spectroscopy. Cr 2O 3 developed in-between the Co 3O 4 coating and the alloy, whilst alloying elements of the substrate were incorporated in the coating, regardless of the deposition method. The coatings reduced the growth rate of the Cr 2O 3 layer as well as the evolution in contact resistance with time between two Fe-22Cr plates sandwiched around a Pt-mesh. SiO 2 developed as particles within the alloy during oxidation of the Co 3O 4 spray-painted samples, whereas SiO 2 was identified as an interfacial layer between Cr 2O 3 and the alloy after oxidation of the Co 3O 4 plasma-sprayed and the non-coated samples. The difference in morphology is suggested to be an effect of SiO 2 nucleation assisted by Kirkendall void formation.

Effects of HA/TiN Coated Film on the Surface Activation of Bone Plate Alloys

In this study, in order to fabricate dental and medical bone plates, the effects of hydroxyapatite(HA) coating made of tooth ash and TiN coating prepared by electron-beam physical vapor deposition(EB-PVD) method on corrosion resistance and surface phenomena of Ti alloyed bone plates were investigated using various test methods. The coatings were deposited on the Ti-6Al-4V alloy fabricated by using a vacuum induction furnace. The corrosion behaviors of the samples were examined through potentiodynamic method in 0.9% NaCl solutions at 36.5±1°C and corrosion surface was observed using SEM and XPS. The surface roughness of TiN coated bone plates was lower than that of HA coated plates. The structure of TiN and HA coated layers showed the columnar structure. The corrosion potential was observed to be higher on the TiN and HA coated specimens than that of the non-coated alloy. The active current density was drastically lowered by both of TiN and HA coatings. The pit number and pit size of TiN and HA coated alloy were smaller in comparison with those of non-coated alloy. From the experimental results, it is suggested that TiN coated Ti alloy sample with rough surface has good protective propertiy against corrosion as well as HA coated bone plate alloy.

ＷＣ‐Ｃｏ系超硬合金の圧縮疲労特性

As WC-Co based alloys have high hardness, compressive strength and Young's modulus and excellent deformation resistance, they have been widely applied for wear and impact resistance tools. Therefore, it becomes very important to acquire knowledge on compressive fatigue characteristics of these alloys. In this study, the effects of Co content, WC grain size, HIP and coating treatment on compressive fatigue life of medium or coarse-grain WC-7-20mass% Co alloys and micrograin WC-6-l5mass%Co-0.4-1.2mass%Cr3C2 alloys were mainly investigated and discussed with the variation of coercive force during fatigue process.The results obtained were as follows: (1) The fatigue life increased with decreasing Co content in the medium and coarse-grain alloys, but it was almost same with no relation to Co content in the micrograin alloys. The fatigue life showed an upward tendency with decreasing WC grain size. (2) The effect of HIP treatment on fatigue life was not observed at all. The fatigue life of PVD-coated alloy did not change and that of CVD-coated alloy remarkably decreased, comparing with that of non-coated alloy. (3) The coercive force of medium-grain WC-12mass%Co alloy increased with increasing applied maximum stress and repeated cycle, and it became much larger in the repeated compressive fatigue test than static compressive test. This increase of the coercive force was considered to be equivalent to the degree of toughness deterioration due to fatigue hardening in Co phase during fatigue process.

成形用潤滑樹脂をプレコートした６０６１アルミニウム合金板のレーザ溶接材のミクロ組織と力学的性質

In order to reveal the laser weldability of Al–Mg–Si alloys pre-coated with lubricating film, microstructure and mechanical properties were investigated in pre-coated 6061-O and -T4 aluminum alloy joints. Two types of laser (CO2 and YAG) were used for welding. No significant reduction in ultimate tensile strength (UTS) was observed in the pre-coated 6061-O joints regardless of the type of laser. The YAG-laser welds of pre-coated 6061-T4 alloy showed 20% reduction in UTS compared to that of the parent alloy. This amount of reduction was similar to that of the non-coated alloy joints. On the other hand, for the CO2 laser welds, UTS drop of the pre-coated joints was 50%. This was larger than the 30% reduction of the non-coated alloy joints. The reduction of UTS in the CO2 laser weld was attributed to a large number of porosity in the weld metal.

ＷＣ‐１２ｍａｓｓ％Ｃｏ超硬合金の曲げ疲労特性に及ぼすＨＩＰ処理および表面性状の影響

Carbide tools applied for plastic deformation often reach a tool life due to bending fatigue under repeated stress at their use. Therefore, as it is very important to acquire knowledge on bending fatigue of carbide tool, the effects of HIP treatment and surface conditions on the bending fatigue characteristics of WC-12mass%Co alloys were mainly investigated. The results obtained were as follows: (1) The bending fatigue characteristics of HIP-treated alloys was improved at lower repeated cycle, compared with that of normal sintered alloy. The fracture origin observed after fatigue fracture was pore in normal sintered alloy, while it was coarse carbide or Cc pool with a step-shaped form in HIP-treated alloy. (2) The specimens with different surface roughness had almost same bending strength, but the fatigue strength decreased with decreasing surface roughness. The cause of this decrease was considered to be due to the decrease of the surface compressive residual stress. (3) The fatigue strength of PVD-coated alloy with TiN layer decreased at lower. repeated cycle and that of CVD-coated alloy remarkably decreased, compared with that of non-coated alloy. The cause of these decrease in the coated alloys were mainly discussed in relation to the residual stress of the substrate alloy and the coated layer and the adhesion between them.

Silica coatings formed on noble dental casting alloy by the sol-gel dipping process.

The sol-gel dipping process, in which liquid silicon alkoxide is transformed into the solid silicon-oxygen network, can produce a thin film coating of silica (SiO2). The features of this method are high homogeneity and purity of the thin SiO2 film and a low sinter temperature, which are important in preparation of coating films that can protect from metallic ion release from the metal substrate and prevent attachment of dental plaque. We evaluated the surface characteristics of the dental casting silver-palladium-copper-gold (Ag-Pd-Cu-Au) alloy coated with a thin SiO2 film by the sol-gel dipping process. The SiO2 film bonded strongly (over 40 MPa) to Ti-implanted Ag-Pd-Cu-Au alloy substrate as demonstrated by a pull test. Hydrophobilization of Ti-implanted/SiO2-coated surfaces resulted in a significant increase of the contact angle of water (80.5 degrees) compared with that of the noncoated alloy specimens (59.3 degrees). Ti-implanted/SiO2-coated specimens showed the release of many fewer metallic ions (192 ppb/cm2) from the substrate than did noncoated specimens (2,089 ppb/cm2). The formation of a thin SiO2 film by the sol-gel dipping process on the surface of Ti-implanted Ag-Pd-Cu-Au alloy after casting clinically may be useful for minimizing the possibilities of the accumulation of dental plaque and metal allergies caused by intraoral metal restorations.

Electrochemical characteristics of encapsulated metal-hydride-alloy electrodes

Metal hydride electrodes with copper-encapsulated alloys and non-coated alloys were fabricated using suitable conductive and binding agents. The charge-discharge characteristics of three kinds of hydride electrodes were comparatively investigated. The encapsulated alloy electrode is remarkably superior to the non-coated LaNi 5-based one, discharging at a high rate and exhibiting a smaller capacity decay at the stage of cycle tests. The hydride alloy quality of hydride electrodes can be effectively determined by measuring rate capability. The results of vented cell experiments confirm that the capacity decay of non-coated alloy electrodes in sealed cells is not due to the oxidation of oxygen from the nickel hydroxide positive electrodes. The relationship between the equilibrium potential of hydride electrode and the equilibrium hydrogen pressure has been deduced by a succinct thermodynamic method, without consideration of the unknown activity of water and fugacity coefficient of hydrogen.

Controlled reactions between chromia and coating on alloy surface

An electrically conducting Sr-doped lanthanum chromite (LSC) coating has been produced by reacting a coating of fine particles of La oxide and Sr oxide with chromia formed as an external scale on a metallic alloy. In addition to the formation of LSC the coating also resulted in much reduced buckling of the underlying chromia layer compared with a non-coated alloy.