Role of Zr(OH)4 hard agglomerates in fabricating porous ZrO2 ceramics and the reinforcing mechanisms

Porous ZrO₂ ceramics were fabricated by adding Zr(OH)₄ hard agglomerates to ZrO₂ powder, followed by pressureless sintering. The mechanical properties of porous ceramics sintered from pure ZrO₂ powder were poor. The addition of Zr(OH)₄ increased the strength and fracture toughness of the porous ZrO₂ ceramics for sintered specimens containing lower porosity. However, the Young’s modulus had little change so that the strain to failure of porous ZrO₂ ceramics increased with the incorporation of Zr(OH)₄. Scanning electron microscopy (SEM) observations revealed that microstructures of the green compacts prepared from pure ZrO₂ powder were nonuniform due to the ZrO₂ soft agglomeration, which resulted in a localized nonuniform shrinkage during densification. The localized nonuniform shrinkage led to a weaker grain bonding and degraded the mechanical properties of porous ZrO₂ ceramics. In this work, we found that this microstructure nonuniformity could be eliminated by the addition of Zr(OH)₄, because the bimodal particle size distribution confined the formation of ZrO₂ soft agglomerates due to a space constraint and an internal friction between the Zr(OH)₄ hard agglomerates during compaction. As Zr(OH)₄ decomposed into ZrO₂ grains during heating, the Zr(OH)₄ hard agglomerates disappeared before sintering occurred. The present study indicates that Zr(OH)₄ hard agglomerate is a unique agent to improve the mechanical properties of porous ZrO₂ ceramics.     

Failure of physical vapor deposition/plasma-sprayed thermal barrier coatings during thermal cycling

ZrO₂-7 wt.% Y₂O₃ plasma-sprayed (PS) coatings were applied on high-temperature Ni-based alloys precoated by physical vapor deposition with a thin, dense, stabilized zirconia coating (PVD bond coat). The PS coatings were applied by atmospheric plasma spraying (APS) and inert gas plasma spraying (IPS) at 2 bar for different substrate temperatures. The thermal barrier coatings (TBCs) were tested by furnace isothermal cycling and flame thermal cycling at maximum temperatures between 1000 and 1150 °C. The temperature gradients within the duplex PVD/PS thermal barrier coatings during the thermal cycling process were modeled using an unsteady heat transfer program. This modeling enables calculation of the transient thermal strains and stresses, which contributes to a better understanding of the failure mechanisms of the TBC during thermal cycling. The adherence and failure modes of these coating systems were experimentally studied during the high-temperature testing. The TBC failure mechanism during thermal cycling is discussed in light of coating transient stresses and substrate oxidation.     

Preparation,microstructure, and properties of tungsten alloys reinforced by ZrO2 particles

Tungsten alloys reinforced by in-situ tetragonal zirconia (W–ZrO₂) were developed via the azeotropic distillation method combined with the powder metallurgy method. The microstructure and abrasive wear properties were studied. The in-situ ZrO₂ particles in the tungsten matrix were obtained by the decomposition of zirconium nitrate after liquid–liquid incorporation of (NH₄)₆H₂W₁₂O₄₀ and Zr(NO₃)₄ aqueous solution. The ZrO₂ particles were distributed evenly in the tungsten matrix, which refined tungsten powders and the grains of tungsten alloys significantly. The density and Vickers hardness of the tungsten alloys decreased with increasing ZrO₂ mass fraction. However, the wear resistance increased firstly and then decreased with increasing ZrO₂ mass fraction. The optimal amount of ZrO₂ for improving wear property is 3%, with the wear resistance of W–3% ZrO₂ improving by approximately 20%–40% compared with that of pure tungsten. The proper amount of ZrO₂ particles can efficiently prevent microcutting to protect the tungsten matrix, thereby enhancing the wear resistance of tungsten alloys.     

Characteristics of plasma electrolytic oxide coatings on Mg-Al-Zn alloy prepared by powder metallurgy

Mg-6 wt.%Al-1 wt.%Zn alloy powders were produced by gas atomization, and subsequently compacted and sintered under various conditions of temperature, time, and pressure. The bulk Mg-6 wt.%Al-1 wt.%Zn alloy was coated by the plasma electrolytic oxidation (PEO) method. The optimum condition of compaction and sintering for PEO coatings was established based on the investigation of microstructure, microhardness, and corrosion properties of coatings which were compared to those of cast Mg-6 wt.%Al alloy. The coatings on Mg-6 wt.%Al and Mg-6 wt.%Al-1 wt.%Zn alloys consisted of MgO, MgAl₂O₄, and Mg₂SiO₄. The Mg-6 wt.%Al-1 wt.%Zn alloy compacted at room temperature for 10 min and sintered at 893 K for 3 h showed the most porous and nonuniform coating layer because the coatings had grown through grain boundaries that resulted from poor bonding between powder particles in the substrate. However, the coated Mg-6 wt.%Al-1 wt.%Zn alloy hot-compacted at 593 K for 10 min had the thickest coating layer and the highest microhardness. In addition, it demonstrated the best corrosion resistance as verified by polarization curves in 3.5% NaCl solution.     

Influence of deposition temperature on mechanical properties of plasma-sprayed hydroxyapatite coating on titanium alloy with ZrO<Subscript>2</Subscript> intermediate layer

Hydroxyapatite coatings were plasma sprayed on the Ti6A14V substrate with and without an intermediate ZrO₂ layer; meanwhile the temperatures of substrates were varied at 90, 140, and 200 °C. The coatings were subjected to the standard adhesion test per ASTM C633-79. The purpose of the investigation was to study the effects of those processing variables on the bonding strength and failure behavior of the system. It is found that the bonding strengths of HA/ZrO₂ and HA coatings generally decrease with increasing substrate temperature, except for the HA/ZrO₂ coating deposited at 200 °C. The rationale of the results is attributed to the residual stress reported in the literature. Introducing ZrO₂ bond coat is found to significantly promote the bonding strength of HA coating. The possible strengthening mechanism is the rougher surface of ZrO₂ bond coat and the higher toughness of ZrO₂, which provide the mechanical strengthening effects. The slightly denser HA in 200 °C deposited HA coating cannot explain the high bonding strength of the HA/ZrO₂ coating, nor the mechanical strengthening effect of ZrO₂ intermediate layer should apply. It is believed that a stronger diffusion bonding is formed at the interface of HA and ZrO₂, which increases the bonding between them chemically. The bonding strengths of HA/ZrO₂ and HA coatings are correlated with the area fraction of adhesive failure of the coatings. The correlation explains the findings in this study.     

Measuring Substrate Temperature Variation During Application of Plasma-Sprayed Zirconia Coatings

Substrate temperature variation was measured during plasma spraying of ZrO₂ 7% Y₂O₃ powder using fast-response thermocouples embedded in the stainless steel surface. Coatings were deposited with both stationary and moving torches. The substrate was either kept at room temperature at the start of coating deposition or pre-heated to 270-300 °C. Peak temperature during spraying reached 450 °C for a surface initially at room temperature, and 680 °C for a surface preheated to 300 °C before coating deposition. Preheating the substrate reduced coating porosity by approximately 40%. The porosity at the center of the deposit was significantly lower than that at its periphery since particle temperature and velocity were lower at the edges of the plasma plume than along its axis. When a coating was applied with a moving torch the substrate temperature did not increase above 450 °C, at which temperature heat losses to the ambient equalled the heat supplied by the plasma plume and particles. Coating porosity decreased with distance from the substrate. As sequential layers of coating are applied surface temperature increases and roughness decreases. Both of these factors suppress break-up of particles landing on the substrate and thereby reduce coating porosity.     

Electrolytic MgO/ZrO2 duplex-layer coating on AZ91D magnesium alloy for corrosion resistance

By a two-step fabrication process of electrolytic deposition and annealing treatment, an MgO/ZrO₂ duplex-layer coating has been prepared on AZ91D magnesium alloy as a protective film against corrosion. Owing to the chemical bonding formed after the condensation of precursory hydroxides, the adhesion strength, thickness and compactness of MgO coating on the substrate are significantly enhanced by the intermediate ZrO₂ layer which prevents the formation of corrosion product Mg₂(OH)₃Cl·4H₂O. As a result, the MgO/ZrO₂ duplex-layer coated specimen reveals relatively high corrosion resistance and superior stability in 3.5 wt% NaCl solution with respect to the MgO single-layer coated specimen.     

Synthesis and Characterization of Functionally Gradient Ni-ZrO<Subscript>2</Subscript> Composite Coating

For the first time, functionally ZrO₂ content graded Ni-ZrO₂ composite coating has been successfully co-electrodeposited from a bath with gradually increasing stirring rate. For this, different composite coatings were electroplated in the same bath with different stirring rates to find the optimum stirring rate in which the maximum content with uniform distribution of ZrO₂ particles in the coating can be achieved. To produce ZrO₂ content graded Ni-ZrO₂ composite coating, the stirring rate was continuously increased from 0 to optimum value. By increasing of ZrO₂ particles content, the microhardness increases from interface towards the surface of the coating. The results of wear resistance measurements, Electrochemical impedance spectroscopy and potentiodynamic polarization test revealed that wear and corrosion resistances of functionally graded Ni-ZrO₂ (FGNZ) is higher than that of ordinary Ni-ZrO₂ (ONZ) composite coating. This result has been attributed to lower mechanical mismatch between coating and substrate in the functionally graded composite coating with respect to the uniformly distributed one.     

Ceramic nanocomposites obtained by sol–gel coating of submicron powders

MgAl₂O₄–ZrO₂ nanocomposites were fabricated by conventional sintering of composite powders obtained by sol–gel coating of a submicron spinel powder. In the composite powder the zirconia grains remain narrow sized and completely tetragonal even after being heat treated at temperatures where a free xerogel is completely monoclinic. The sintered material exhibits a dense, fine and highly homogeneous microstructure. The zirconia nanoparticles are located at both inter- and intragranular positions and exhibit heteroepitaxial relationships with the surrounding crystals. Tetragonal zirconia seems to be stabilised by an interface effect. Both the scale of the microstructure and the fraction of intragranular grains were controlled by adjusting the mean grain size of spinel grains before coating and sintering conditions.     

The use of Al-Al2O3 cold spray coatings to improve the surface properties of magnesium alloys

Pure Al and 6061 aluminium alloy based Al₂O₃ particle-reinforced composite coatings were produced on AZ91E substrates using cold spray. The strength of the coating/substrate interface in tension was found to be stronger than the coating itself. The coatings have corrosion resistance similar to that of bulk pure aluminium in both salt spray and electrochemical tests. The wear resistance of the coatings is significantly better than that of the AZ91 Mg substrate, but the significant result is that the wear rate of the coatings is several decades lower than that of various bulk Al alloys tested for comparison. The effect of post-spray heat treatment, the volume fraction of Al₂O₃ within the coating and of the type of Al powder used in the coatings on the corrosion and wear resistance was also discussed.     

Amorphization of ZrO2 + CeO2 Powders Through Mechanical Milling for the Use of Kinetic Spray

The coating formation in a kinetic spray process mainly depends on the impact of inflight particles at a high velocity. The plastic deformation at the impact interface would disrupt the native oxide scale on the particle and the substrate to generate the intimate contact of the atomic structures. Accordingly, it poses a challenge in producing ceramic coating during kinetic spray because of the lack of plasticity of ceramic powders at room temperature. In this study, we proposed to prepare ZrO₂ ceramic coatings using partially amorphized powder with nanometer size in the kinetic spray process. To prepare the powder for the use of the kinetic spray, the amorphization and grain refinement of ZrO₂ powder in mechanical ball milling were studied. The results showed that the amorphization and grain refinement were improved because of the formation of solid solution when the CeO₂ agent was added. Subsequently, a nearly spherical powder was achieved via spray drying using the milled powders. The plasticity of the milled powders was tested in the kinetic spray process using Nitrogen as process gas. A dense ZrO₂-CeO₂ coating with a thickness of 50 μm was formed, whereas spraying milled ZrO₂ powder can only lead to an inhomogeneous dispersion of the destructible particles on the surface of the substrate.     

Comparative study on corrosion protection properties of electroless Ni‐P‐ZrO2 and Ni‐P coatings on AZ91D magnesium alloy

Electroless Ni‐P‐ZrO₂ and Ni‐P coatings on AZ91D magnesium alloy were prepared, and their corrosion protection properties were compared in this paper. The potentiodynamic curves and electrochemical impedance spectroscopy (EIS) of the coated magnesium alloy in 3.5% NaCl solution showed that the corrosion performance of Ni‐P‐ZrO₂ composite coating was superior to that of Ni‐P coating. The same conclusion was obtained with salt spray and immersion tests. The corrosion morphologies of two kinds of coatings with various immersion time intervals in 3.5% NaCl solution indicated that most corrosion products concentrated on the nodules boundaries of Ni‐P coating and blocked corrosion pit was the main corrosion form. For the Ni‐P‐ZrO₂ coating, tortuous nodules boundaries were not the weak sites of the coating and corrosion initiated from the nickel phosphor alloy around the nanometer powders. Open corrosion pits occurred on the composite coating surface, and the coating was corroded gradually. Thus, the Ni‐P‐ZrO₂ coating exhibited better corrosion protection property to magnesium alloy substrate than Ni‐P coating.     

Characterization of AC PEO coatings on magnesium alloys

Optical emission spectroscopy, fast video imaging and coating characterization are employed to investigate AC plasma electrolytic oxidation (PEO) of magnesium alloys. The findings revealed initiation and gradual increase in the number of discharges after 2-4 ms of each anodic pulse once a critical voltage was reached. No discharges were observed during the cathodic half-cycles. The lifetimes of discharges were in the range of 0.05-4 ms. A transition in the voltage-time response, accompanied by a change in the acoustic and optical emission characteristics of discharges, was associated with the development of an intermediate coating layer with an average hardness of 270-450 HV_0.05. The coatings grew at a rate in the range 4.0-7.5 µm min^− 1, depending on the substrate composition. Regardless of the substrate, the coatings consisted of MgO and Mg₂SiO₄, with incorporation of alloying element species. Electrolyte species were mainly present in a more porous layer at the coating surface, constituting 20-40% of the coating thickness. A thin barrier layer consisting of polycrystalline MgO was located next to the alloy. The corrosion rate of the magnesium alloys determined using potentiodynamic polarization in 3.5 wt.% NaCl was reduced by 2-4 orders of magnitude by the PEO treatment.     

Structure and corrosion resistance of ZrO2 ceramic coatings on AZ91D Mg alloys by plasma electrolytic oxidation

The aim of this work is to study the structure and the corrosion resistance of the plasma electrolytic oxidation ZrO₂ ceramic coatings on Mg alloys. The ceramic coatings were prepared on AZ91D Mg alloy in Na₅P₃O₁₀ and K₂ZrF₆ solution by pulsed single-polar plasma electrolytic oxidation (PEO). The phase composition, morphology and element distribution in the coating were investigated by X-ray diffractometry, scanning electron microscopy and energy distribution spectroscopy, respectively. The results show that the coating thickness and surface roughness were increased with the increase of the reaction time. The ceramic coatings were of double-layer structure with the loose and porous outer layer and the compact inner layer. And the coating was composed of P, Zr, Mg and K, of which P and Zr were the main elements in the coating. P in the coating existed in the form of amorphous state, while Zr crystallized in the form of t-ZrO₂ and a little c-ZrO₂ in the coating. Electrochemical impedance spectra (EIS) and the polarizing curve tests of the coatings were measured through CHI604 electrochemical analyzer in 3.5% NaCl solution to evaluate the corrosion resistance. The polarization resistance obtained from the equivalent circuit of the EIS was consistent with the results of the polarizing curves tests.     

Preparation and characterization of silicon-substituted hydroxyapatite coating by a biomimetic process on titanium substrate

A biomimetic method has been used to prepare silicon-substituted hydroxyapatite coatings on titanium substrates. The surface structures of the coatings were characterized by X-ray diffraction, X-ray photoelectron spectroscopy (XPS) and Fourier transformed infrared spectroscopy (FTIR). Si substituted hydroxyapatite (Si-HA) coatings with different Si contents were deposited successfully on the titanium substrate by immersing the pretreated titanium substrate into silicon containing supersaturated solutions (SSS) with different SiO₃²⁻ concentrations. The pretreatment of the Ti substrate in a mixed alkaline (NaOH + Ca(OH₂)) followed by a heat treatment produced a 3D porous surface structure with rutile and CaTiO₃ as main phases, which contributed mainly to the fast precipitation and deposition of Si-HA. FTIR results showed that Si in the Si-HA coating existed in the form of SiO₄⁴⁻ groups. The cross-section microstructure was observed by scanning electronic microscopy and the shear strength was tested. The coating was about 5-10 μm in thickness and no interval was observed at the interface between the coating and the substrate. Shear strength testing showed that Si-HA/Ti exhibited higher shear strength than HA/Ti due to the existence of the SiO₄⁴⁻ group in the coating.     

Microstructure evolution of semi-solid Mg- 14A1-0.5Mn alloys during isothermal heat treatment

A new Mg-14Al-0.5Mn alloy that exhibits a wide solidification range and sufficient fluidity for semi-solid forming was designed. And the rnicrostructure evolution of semi-solid Mg-14Al-0.5Mn alloy during isothermal heat treatment was investigated. The mechanism of the microstructure evolution and the processing conditions for isothermal heat treatment were also discussed. The results show that the microstructures of cast alloys consist of α-Mg,β-Mg17Al12 and a small amount of Al-Mn compounds. After holding at 520 ℃ for 3 min, the phases of β-Mg17Al12 and eutectic mixtures in the Mg-14Al-0.5Mn alloy melt and the microstructures of α-Mg change from developed dendrites to irregular solid particles. With increasing the isothermal time, the amount of liquid increases, and the solid particles grow large and become spherical. When the holding time lasts for 20 min or even longer, the solid and liquid phases achieve a state of dynamic equilibrium.     

Corrosion and wear behavior of thermally sprayed nano ceramic coatings on commercially pure Titanium and Ti–13Nb–13Zr substrates

Surface treatments and coatings are the practical approaches used to extend the lifetime of components and structures especially when the surface is the most solicited part of the considered engineering component. Hard thermally sprayed coating is one of the most wear resistance coating widely used in many practical mechanical applications. In the construction of articulating parts of medical devices, titanium and its alloys have to be surface coated to improve their tribocorrosion behavior. In this way, the use of porous thermal coatings is known to be a strategy for better binding bone or tissue on femoral stem for example. It is, thus, important to evaluate the corrosion and the wear behaviors of such materials for biosecurity considerations in the human body. In this study, we investigate the behavior of new nano ZrO₂ and Al₂O₃-13 wt.% TiO₂ thermal sprayed coatings on commercially pure (cp)-Ti (grade 4) and titanium alloy substrates. Friction and wear tests against Al₂O₃ balls showed that the wear resistance of Al₂O₃-13 wt.% TiO₂ is better than that ZrO₂ coating. Both plasma sprayings have similar abrasive wear behavior; however, the average friction coefficient is higher for alumina–titania coating. Electrochemical tests, open circuit potential monitoring and potentiodynamic polarization, were performed in simulated body conditions (Hank’s solution, 37 °C). Results showed that corrosion resistance was appreciably higher for alumina–titania coating.     

Characterization of planer cathode-supported SOFC prepared by a dual dry pressing method

Cathode-supported solid oxide fuel cells (SOFCs), comprising porous (La_0.75Sr_0.25)_0.95MnO_3−δ (LSM) + Sm_0.2Ce_0.8O_1.9 (SDC) composite cathode substrate and 11 mol%Sc₂O₃-doped ZrO₂ (ScSZ) electrolyte membranes layer, were successfully fabricated via dual dry pressing method. NiO-SDC anode was prepared by slurry coating method. Phase characterizations and microstructures of electrolyte and cathode were studied by X-ray diffraction (XRD) and scanning electronic microscopy (SEM). No interface reaction took place between LSM/SDC cathode substrate and ScSZ electrolyte layer after sintered at 1300 °C. The cell performances were measured at 800 and 750 °C, respectively, by changing the external load. The peak power densities were 0.228 and 0.133 W cm⁻², and the corresponding open-circuit voltages of the cell were 1.092 and 1.027 V at 800 and 750 °C, respectively. Impedance analysis indicated that the performances of the SOFCs were determined essentially by the composition and microstructure of the electrode.     

Comparative study on dry sliding wear and oxidation performance of HVOF and laser re-melted Al0.2CrFeNi(Co,Cu) alloys

Al_0.2CrFeNiCo and Al_0.2CrFeNiCu high entropy alloys were deposited with high velocity oxygen fuel (HVOF) on 316L substrate. Later, a laser re-melting (LR) process was applied to enhancing the coating microstructure. LR process effects on dry sliding wear and oxidation behaviors were investigated. The mixture of powders with free elements led to the formation of inner oxides in HVOF coatings. The oxide and porosity were eliminated using LR. After LR, FCC was the dominant phase in both alloys, while BCC, sigma and Cr₂O₃ phases were observed in Al_0.2CrFeNiCo alloy. The hardnesses of the Al_0.2CrFeNiCo and Al_0.2CrFeNiCu coatings after HVOF were HV 591 and HV 361, respectively. After LR, the hardnesses decreased to HV 259 and HV 270, respectively. Although HVOF coatings were most affected by increased load, they showed the highest wear resistance compared to other samples. The lowest wear resistance could be seen in the substrate. After the oxidation tests, HVOF coating layer was completely oxidized and also, the coating layer was delaminated from the substrate after 50 h oxidation due to its porous structure. LR coatings exhibited better oxidation performance. Al_0.2CrFeNiCo was dominantly composed of Cr₂O₃, exhibiting a slower-growing tendency at the end of the oxidation tests, while Al_0.2CrFeNiCu was composed of spinel phases.     

Corrosion resistance of TiN and MoN coated grey cast iron in acid solutions

Grey cast iron containing 3.39% C, 2.76% Si, 0.37% Mn, 0.05 %, P and 0.005 % S, with different shape of graphite was covered with Mo and Ti nitrides by cathodic are plasma deposition method at 400°C. Incorporation of C from the substrate caused the formation of molybdenum carbide. Formation of diffused interphase layer due to incorporation of Mo into the substrate caused increase in the coating adhesion. Electrochemical measurements showed that H₃PO₄ and H₂SO₄ solutions penetrated to substrate through flaws in coatings, mainly occurred at the graphite particles. The local corrosion degradation of material was accompanied by hydrogen evolution and formation of microcracks. In the case of coated vermicular cast iron merging of microcracks led to falling off the pieces of coated material. The Mo containing coatings exhibited lower susceptibility to crack formation but higher susceptibility to environmental degradation than the Ti containing coatings. The highest protection in acid solutions provided the complex (Mo,Ti)N coating.