The Effect of Mechanical Pretreatment on the Electrochemical Characteristics of PEO Coatings Prepared on Magnesium Alloy AZ80.

The main objective of this article is to provide new information on the effects of mechanical pretreatment of AZ80 magnesium alloy ground with SiC emery papers of different grain sizes on the plasma electrolytic oxidation (PEO) process and corrosion properties of AZ80 in 0.1 M NaCl solution. Then, the roughness of the coated samples was measured by confocal microscopy. The corrosion properties of the ground and coated surfaces were determined by potentiodynamic polarization (PDP) within 1 h of exposure, and electrochemical impedance spectroscopy (EIS) was performed during 168 h of exposure at laboratory temperature. Consequently, the obtained results of the PDP measurements were evaluated by the Tafel analysis and the EIS evaluation was performed by the equivalent circuit analysis through Nyquist diagrams. The morphology and structure of PEO coatings were observed by scanning electron microscopy (SEM) through the secondary imaging technology, and the presence of certain elements in PEO coatings was analyzed by EDS analysis.

Improving of 100Cr6 Steel Corrosion and Wear Properties in Simulated Sea Water Environment by Tungsten-Doped DLC Coating.

A progressive type of tungsten-doped DLC coating was applied to a quenched and tempered 100Cr6 steel with the aim to improve the wear and corrosion properties in simulated seawater conditions and to compare the performance to conventional DLC coating. Tungsten doping caused a shift of the corrosion potential (Ecorr) to a lower negative value of -172 mV, while the conventional DLC exhibited an Ecorr of -477 mV. In dry conditions, the W-DLC coefficient of friction is slightly higher than that of the conventional DLC (0.187 for the W-DLC vs. 0.137 for the DLC), but in cases of a saltwater environment, this difference becomes almost negligible (0.105 for the W-DLC vs. 0.076 for the DLC). The conventional DLC coating also started to show marks of deterioration when exposed to a combination of wear in a corrosive environment, while the W-DLC layer still maintained its integrity.

Influence of Magnetizing Conditions on Barkhausen Noise in Fe Soft Magnetic Materials after Thermo-Mechanical Treatment.

Low alloyed steels of low, medium, or high strength are frequently used for many applications in the automotive, civil (bridges), aerospace, and petrochemical industries. A variety of thermomechanical regimes, in which these steels can be produced, enable customization of their matrix with respect to their fatigue resistance, resistance against friction and impact wear, fracture toughness, corrosion resistance, etc. This study analyses the influence of magnetising conditions on Barkhausen noise and other extracted parameters. It was found that the increasing magnetising frequency makes Barkhausen noise weaker, especially in the high strength low alloyed steels, as a result of the decreasing magnetic field in a sample. For this reason, increasing fraction of domain walls is unpinned at the higher frequencies. Barkhausen noise for the high strength low alloyed steels at higher frequencies is remarkably attenuated. Moreover, the different behaviour with respect to direction of the sheet rolling and the transversal direction, can be found due to realignment of the domain walls. This study demonstrates that the position of Barkhausen noise envelopes and the number of Barkhausen noise pulses increase in a systematic manner at the lower magnetising frequencies. Those parameters can be employed for distinction of the low alloyed steels, investigated in this study. However, the increasing magnetising frequency makes attenuation of Barkhausen noise more remarkable for the low alloyed steels of the higher strength. Therefore, the effective value of Barkhausen noise, at the magnetising frequency 750 Hz, in the rolling direction exhibits the systematic descent along with the increasing yield strength. This parameter can be used for distinction of the low alloyed steels after their thermomechanical processing, as well.

Fatigue Crack Initiation Change of Cast AZ91 Magnesium Alloy from Low to Very High Cycle Fatigue Region.

Fatigue tests were performed on the AZ91 cast alloy to identify the mechanisms of the fatigue crack initiation. In different fatigue regions, different mechanisms were observed. In the low and high cycle fatigue regions, slip markings formation accompanied with Mg17Al12 particles cracking were observed. Slip markings act as the fatigue crack initiation sites. The size and number of slip markings decreased with decreased stress amplitude applied. When slip markings formation was suppressed due to low stress amplitude, particle cracking became more important and the cracks continued to grow through the particle/solid solution interface. The change of the fatigue crack initiation mechanisms led the S-N curve to shift to the higher number of cycles to the fracture, demonstrated by its stepwise character. A lower fatigue limit of 60 MPa was determined at 20 kHz for 2 × 109 cycles compared to the 80 MPa determined at 60 Hz for 1 × 107 cycles.

Inclined and multi-directional surface impacts accelerate biodegradation and improve mechanical properties of pure iron.

Impact based surface treatments with adequate kinetic energy have favorable effects on promoting cell-substrate interactions, reducing bacterial adhesion, and enhancing fatigue performance of metallic biomaterials. Here, we used both numerical and experimental approaches to evaluate the potential of these treatments for addressing the major issue associated with the application of pure iron in biomedical implants, i.e. its low corrosion rate. Despite the efficiency of impact based surface treatments in modulating the degradation rate of pure iron, the maximum reported depth of the affected surface layer is still limited, even when extreme process parameters are used. To address this issue, herein, two impact based treatments were adjusted to trigger the dislocation activities that facilitate grain refinement in pure iron using multi-directional inclined impacts. An alternative approach of severe shot peening (SSP) was developed and compared with ultrasonic shot peening (USP). The effect of both treatments and variations of their key parameters were analyzed considering the significant role of shear bands and dislocation cells in the grain refinement mechanism of pure α-iron. Microstructural, mechanical and electrochemical properties of the treated material were analyzed. The observations showed extension of the grain refined layers for the specimens subjected to multidirectional oblique impacts compared to the ones treated in the classic manner using normal impacts. The results imply that by adapting peening parameters, it would be possible to effectively create a thick surface layer with properties that can accelerate the biodegradation of pure iron boosting its potential to meet clinical requirements for temporary hard tissue implants.

Accelerated biodegradation and improved mechanical performance of pure iron through surface grain refinement.

Pure iron and its biocompatible and biodegradable alloys have a high potential to be used for temporary load bearing medical implants. Nevertheless, the formation of passive iron oxide and hydroxide layers, which lead to a considerably low degradation rate at the physiological environment, has highly restricted their application. Herein we used numerical and experimental methods to evaluate the effect of severe shot peening, as a scalable mechanical surface treatment, on adjusting the performance of pure iron for biomedical applications. The developed numerical model was used to identify the range of peening parameters that would promote grain refinement on the pure iron surface. Experimental tests were then performed to analyze the gradient structure and the characteristics of the interface free surface layer created on peened samples. The results indicated that severe shot peening could notably increase the surface roughness and wettability, induce remarkable surface deformation and grain refinement, enhance surface hardness and generate high in-depth compressive residual stresses. The increased surface roughness besides the high concentration of micro cracks and dislocation density in the grain refined top layer promoted pure iron's degradation in the biologically simulated environment. STATEMENT OF SIGNIFICANCE: Biodegradable metallic materials with resorbable degradation products have a high potential to be used for temporary implants such as screws, pins, staples, etc. They can eliminate the need for implant retrieval surgery after the damaged tissue is healed, and result in reduced patient suffering besides lowered hospitalization costs. Pure iron is biodegradable and is an essential nutrient in human body; however, its application as biomedical implant is highly restricted by its slow degradation rate in physiological environment. We applied a scalable surface treatment able to induce grain refinement and increase surface roughness. This treatment enhances mechanical performance of pure iron and accelerates its degradation rate, paving the way for its broader applications for biomedical implants.