Dependency of fracture toughness of plasma sprayed Al<Subscript>2</Subscript>O<Subscript>3</Subscript> coatings on lamellar structure

The fracture toughness of plasma-sprayed Al₂O₃ coatings in terms of critical strain energy release rate G _Ic was investigated using a tapered double cantilever beam (TDCB) approach. This approach makes the fracture toughness be measured only using the critical fracture load disregarding crack length during test. The Al₂O₃ coatings were deposited under different spray distances and plasma powers to clarify the effect of spray parameters on the G _Ic of the coatings. The fracture surfaces were examined using scanning electron microscope. On the basis of an idealized layer microstructure model for thermal sprayed coatings, the theoretical relationship between the cohesive fracture toughness and microstructure is proposed. The correlation between the calculated fracture toughness and observed value is examined. It was found that the fracture toughness of plasma sprayed Al₂O₃ coatings is not significantly influenced by spray distance up to 110 mm, and further increase in spray distance to 130 mm resulted in large decrease in the fracture toughness of the coatings. The G _Ic value predicted based on the proposed model using lamellar interface mean bonding ratio and the effective surface energy of bulk ceramics agreed well with the observed G _Ic data. Such agreement evidently shows that the fracture toughness of thermally sprayed ceramic coatings at the direction along coating surface is determined by lamellar interface bonding.     

Influence of Ceramic Powder Size on Process of Cermet Coating Formation by Cold Spray

Influence of the ceramic particle size on the process of formation of cermet coatings by cold spray is experimentally studied. A specially developed nozzle with separate injection of ceramic and metal powders into the gas stream is used in the experiments. The results obtained demonstrate that fine ceramic powders (Al₂O₃, SiC) produce a strong activation effect on the process of spraying soft metal (Al, Cu) and increase deposition efficiency of the metal component of the mixture compared to the pure metal spraying. At the same time, coarse ceramic powder produces a strong erosion effect that considerably reduces coating mass growth and deposition efficiency of the metal component. It is experimentally shown that the addition of fine hard powder to soft metals as Al and Cu allows to significantly reduce the “critical” temperature (the minimum gas stagnation temperature at which a nonzero particle deposition is observed) for spraying these metals.     

HVOF-Sprayed Coatings Engineered from Mixtures of Nanostructured and Submicron Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-TiO<Subscript>2</Subscript> Powders: An Enhanced Wear Performance

In previous studies, it has been demonstrated that nanostructured Al₂O₃-13 wt.%TiO₂ coatings deposited via air plasma spray (APS) exhibit higher wear resistance when compared to that of conventional coatings. This study aimed to verify if high-velocity oxy-fuel (HVOF)-sprayed Al₂O₃-13 wt.%TiO₂ coatings produced using hybrid (nano + submicron) powders could improve even further the already recognized good wear properties of the APS nanostructured coatings. According to the abrasion test results (ASTM G 64), there was an improvement in wear performance by a factor of 8 for the HVOF-sprayed hybrid coating as compared to the best performing APS conventional coating. When comparing both hybrid and conventional HVOF-sprayed coatings, there was an improvement in wear performance by a factor of 4 when using the hybrid material. The results show a significant antiwear improvement provided by the hybrid material. Scanning electron microscopy (SEM) at low/high magnifications showed the distinctive microstructure of the HVOF-sprayed hybrid coating, which helps to explain its excellent wear performance. This article is an invited paper selected from presentations at the 2007 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Global Coating Solutions, Proceedings of the 2007 International Thermal Spray Conference, Beijing, China, May 14-16, 2007, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2007.     

Effect of a surface treatment for LiNi<Subscript>1/3</Subscript>Co<Subscript>1/3</Subscript>Mn<Subscript>1/3</Subscript>O<Subscript>2</Subscript> cathode material in lithium secondary batteries

LiNi_1/3Co_1/3Mn_1/3O₂ cathode material was surface-treated to improve its electrochemical performance. Al₂O₃ nanoparticles were coated onto the surface of LiNi_1/3Co_1/3Mn_1/3O₂ powder using a sol-gel method. The as-prepared Al₂O₃ nano-particle was identified as the cubic structure of Al₂O₃. XRD showed that the LiNi_1/3Co_1/3Mn_1/3O₂ structure was not affected by the Al₂O₃ coating. With a coating of 3 wt.% Al₂O₃ on LiNi_1/3Co_1/3Mn_1/3O₂, the cyclic-life performance and rate capability were improved. However, heavier coatings (5 wt.%) on LiNi_1/3Co_1/3Mn_1/3O₂ resulted in a considerable decrease of the discharge capacity and rate capability. The thermal stability of LiNi_1/3Co_1/3Mn_1/3O₂ materials was greatly improved by the 3 wt.% Al₂O₃ coating.     

Multiscale Experimental and Numerical Approach to the Powder Particle Shape Effect on Al-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Coating Build-Up

Aluminum (Al) powders with spherical and irregular particle shapes were mixed with two alumina (Al₂O₃) powders with either a spherical or an angular particle shape to achieve high-performance cold-sprayed coatings onto steel. Two effects of the aluminum particle shape were observed. First, coating microstructure observation showed impinging heterogeneity depending on particle shape. Second, particle jet differences depending on particle morphology were shown by velocity maps. From the latter, SEM and XRD, three effects of the alumina particle shape were also shown, i.e., higher in-flight velocity of angular particles, fragmentation of spherical hollow particles and embedding of alumina particles with aluminum. Numerical simulation of particle impacts was developed to study the densification of Al coating due to Al₂O₃ addition through elucidation of Al-Al₂O₃ interaction behavior at the scale of the coating. Al/Al and Al/Al₂O₃ interfaces were investigated using TEM to understand coating strengthening effects due to alumina addition at the scale of the particle. As a whole, Al and Al₂O₃ particle shape effects were claimed to explain coating mechanical properties, e.g., microhardness and coating–substrate bond strength. This study resulted in specifying criteria to help cold spray users in selecting powders for their applications, to meet economic and technical requirements.     

Metallization of Thin Al2O3 Layers in Power Electronics Using Cold Gas Spraying

A successful combination of insulating substrates with conducting metal coatings produced by cold spraying could open new industrial application areas like the fabrication of power electronic components. For minimizing the number of industrial process steps, insulating ceramic layers should ideally be processed by thermal spray techniques. Thus, this study investigates the impact behavior and coating formation of ductile metallic feedstock powders onto brittle ceramic coatings. With respect to high electrical conductivity of the metallic lines and good electrical insulation of the ceramic interlayer, copper was cold gas sprayed on previously thermally sprayed Al₂O₃ coatings. Successful cold coating formation requires different strategies for the activation of the ceramic layer to increase adhesion and to avoid brittle failure. These both can be achieved either by applying a bondcoat on the ceramic layer or using heated substrates during the cold spray process.     

Effect of Nano-Si<Subscript>3</Subscript>N<Subscript>4</Subscript> Additives and Plasma Treatment on the Dry Sliding Wear Behavior of Plasma Sprayed Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-8YSZ Ceramic Coatings

In this paper, the effect of nano-Si₃N₄ additives and plasma treatment on the wear behavior of Al₂O₃-8YSZ ceramic coatings was studied. Nano-Al₂O₃, nano-8YSZ (8 wt.% Y₂O₃-stabilized ZrO₂) and nano-Si₃N₄ powders were used as raw materials to fabricate four types of sprayable feedstocks. Plasma treatment was used to improve the properties of the feedstocks. The surface morphologies of the ceramic coatings were observed. The mechanical properties of the ceramic coatings were measured. The dry sliding wear behavior of the Al₂O₃-8YSZ coatings with and without Si₃N₄ additives was studied. Nano-Si₃N₄ additives and plasma treatment can improve the morphologies of the coatings by prohibiting the initiation of micro-cracks and reducing the unmelted particles. The hardness and bonding strength of AZSP (Al₂O₃-18 wt.% 8YSZ-10 wt.% Si₃N₄-plasma treatment) coating increased by 79.2 and 44% compared to those of AZ (Al₂O₃-20 wt.% 8YSZ) coating. The porosity of AZSP coating decreased by 85.4% compared to that of AZ coating. The wear test results showed that the addition of nano-Si₃N₄ and plasma treatment could improve the wear resistance of Al₂O₃-8YSZ coatings.     

Studies on Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/ZrO<Subscript>2</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> high K gate dielectrics applied in a fully depleted SOI MOSFET

Al₂O₃/ZrO₂/Al₂O₃ gate stacks were prepared on ultrathin SOI (Silicon on insulator) substrates by ultrahigh vacuum electron beam evaporation and post-annealed in N₂ at 450°C for 30 min. Three clear nanolaminate layered structure of Al₂O₃(2.1 nm)/ZrO₂(3.5 nm)/Al₂O₃(2.3 nm) was observed with a high-resolution cross-sectional transmission electron microscope (HR-XTEM). High frequency capacitance voltage (C-V) characteristics of a fully depleted (FD) SOI MOS capacitor at 1 and 5 MHz were studied. The minority carriers determine the high frequency C-V properties, which is opposite to the case of bulk MOS capacitors. The series resistance of the SOI substrate is found to be the determinant factor of the high frequency characteristics of FD SOI MOS capacitors. This article is based on a presentation in “The 7th Korea-China Workshop on Advanced Materials” organized by the Korea-China Advanced Materials Cooperation Center and the China-Korea Advanced Materials Cooperation Center, held at Ramada Plaza Jeju Hotel, Jeju Island, Korea on August 24≈27, 2003.     

Tensile and wear properties of rolled Al5Mg-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> or C particulate composites

Al5Mg alloy matrix composites reinforced with different percentages of Al₂O₃ (60 μm) or C (90 μm) particulates were prepared by the vortex method. The composites were then subjected to hot or cold rolling with different reduction ratios. The microstructures of the rolled composites revealed that the matrix grains moved around the particulate causing deformation. By continuing deformation, the particulates rearranged themselves in the matrix, leading to lensoid distortion. It was found that the addition of Al₂O₃ or C particulates increased the 0.2% proof stress and reduced both the tensile strength and ductility, compared with the monolithic alloy. Scanning electron microscopy (SEM) fractographic examinations showed that the composites reinforced with Al₂O₃ particulates failed through particulate fracture and matrix ligament rupture. However, the failure of the composites reinforced with C particulates was through particulate decohesion, followed by ductile failure of the matrix. Abrasive wear results showed that the wear rate of the Al5Mg alloy decreased with the addition of C particulates. However, increasing the volume fraction of C particulates did not have a prominent effect on the wear rate. The composites reinforced with Al₂O₃ particulates exhibited a higher wear rate than that of the unreinforced alloy. Furthermore, addition of both C and Al₂O₃ particulates into the Al5Mg matrix alloy did not significantly improve the wear resistance. For all composites studied in this work, hot or cold rolling had a marginal effect on the wear results.     

Effect of Surface Roughness and Structure Features on Tribological Properties of Electrodeposited Nanocrystalline Ni and Ni/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Coatings

Metal matrix composite coatings obtained by electrodeposition are one of the ways of improving the surfaces of materials to enhance their durability and properties required in different applications. This paper presents an analysis of the surface topography, microstructure and properties (residual stresses, microhardness, wear resistance) of Ni/Al₂O₃ nanocomposite coatings electrodeposited on steel substrates from modified Watt’s-type baths containing various concentrations of Al₂O₃ nanoparticles and a saccharin additive. The residual stresses measured in the Ni/Al₂O₃ coatings decreased with an increasing amount of the co-deposited ceramics. It was established that the addition of Al₂O₃ powder significantly improved the coatings’ microhardness. The wear mechanism changed from adhesive-abrasive to abrasive with a rising amount of Al₂O₃ particles and coating microhardness. Nanocomposite coatings also exhibited a lower coefficient of friction than that of a pure Ni-electrodeposited coating. The friction was found to depend on the surface roughness, and the smoother surfaces gave lower friction coefficients.     

Electrodeposition of composite chromium coatings from Cr(III) sulfate-oxalate solution suspensions containing Al<Subscript>2</Subscript>O<Subscript>3</Subscript>, SiC,Nb<Subscript>2</Subscript>N,and Ta<Subscript>2</Subscript>N particles

The effect of concentration and nature of particles (dielectric Al₂O₃, broadband semiconducive SiC, and conductive Nb₂N and Ta₂N) on the adhesion of particles to the surface of coatings and the composition of electrochemical composite coatings deposited from Cr(III) sulfate-oxalate solution suspensions of various concentrations is studied. Adding particles of any nature to the solution does not affect the character of the formation of an X-ray amorphous chromium matrix and the content of hydrogen in the coating. The mechanisms of inclusion of Al₂O₃, SiC, Nb₂N + Ta₂N particles in chromium layers are different. In the former case (Al₂O₃ and SiC), the inclusion of particles is determined by kinetic factors (in the absence of current, there is no adhesion of particles), while, in the latter case, the particles demonstrate a strong adhesion in the absence of current. Accordingly, the distribution of SiC and Al₂O₃ particles depends on the depth by contrast to Nb₂N and Ta₂N particles, whose concentrations close to the substrate do not noticeably differ from those in the bulk and surface layers of the coating. An increase in the temperature of solution promotes the codeposition of chromium and these particles.     

Development and Application of Binary Suspensions in the Ternary System Cr<Subscript>2</Subscript>O<Subscript>3</Subscript>-TiO<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> for S-HVOF Spraying

Compositions in the system Cr₂O₃-TiO₂-Al₂O₃ are among the most used ceramic materials for thermally sprayed coating solutions. Cr₂O₃ coatings present good sliding wear resistance; Al₂O₃ coatings show excellent insulation behavior and TiO₂ striking corrosion properties. In order to combine these properties, coatings containing more than one oxide are highly interesting. The conventional spraying process is limited to the availability of binary feedstock powders with defined compositions. The use of suspensions offers the opportunity for tailor-made chemical compositions: within the triangle of Cr₂O₃-TiO₂-Al₂O₃, each mixture of oxides can be created. Criteria for the selection of raw materials as well as the relevant aspects for the development of binary suspensions in the Cr₂O₃-TiO₂-Al₂O₃ system to be used as feedstock for thermal spraying are presented. This formulation of binary suspensions required the development of water-based single-oxide suspensions with suitable behavior; otherwise, the interaction between the particles while mixing could lead up to a formation of agglomerates, which affect both the stability of the spray process and the coating properties. For the validation of this formulation procedure, binary Cr₂O₃-TiO₂ and Al₂O₃-TiO₂ suspensions were developed and sprayed using the S-HVOF process. The binary coatings were characterized and discussed in terms of microstructure and microhardness.     

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.     

Effect of Ti,Nb, and Ti + Nb Coatings on the Bond Strength-Structure Relationship in Al/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Joints

There is a growing interest in metal-ceramic bonding for wide range of applications in electronic devices and high technology industry for fabrication of metal matrix composites and bonding of ceramic components to metals. The object of the work was to study the effect of Ti, Nb, and Ti + Nb thin films deposited by PVD method on alumina substrates on structure and bond strength properties of Al/Al₂O₃ joints. The joints were fabricated using the results of a wetting experiment and the sessile drop method at a temperature of 1223 K in a vacuum of 0.2 MPa for 30 min of contact. The structure of the metal/ceramic interface was investigated using scanning electron microscopy. The elemental distribution at the metal-ceramic interface was analyzed using energy dispersive x-ray spectroscopy. Transmission electron microscopy was also used to investigate some aspects of the metal/ceramic interface. The bond strength properties of joints were measured using shear test. The shear strength results demonstrated significant improvement of shear strength of Al/Al₂O₃ joints due to the application of Ti + Nb thin film on alumina substrate. Microstructural investigations of the interface indicated that Al/coating/Al₂O₃ couples have diffusion transition interface which influences the strengthening of these joints. A conclusion could be drawn that the presence of thin film layers changes the character of interaction and leads to the formation of new reaction products in the bonding layer.     

Structure and electric property comparison between Ge nanoclusters embedded in Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/ZrO<Subscript>2</Subscript>

Metal-insulator-semiconductor (MIS) structures containing Ge nanocrystals embedded in both Al₂O₃ and ZrO₂/Al₂O₃ are fabricated by an ultra-high vacuum electron-beam evaporation method. Secondary ion mass spectroscopy (SIMS) results indicate that Ge embedded in Al₂O₃ diffuses towards the surface of the Al₂O₃ layer after annealing at 800°C in N₂ ambient for 30 min. Ge embedded in ZrO₂/Al₂O₃ is stable, thus inducing less leakage current. Capacitance voltage studies indicate that annealing can effectively passivate the negatively charged trapping centers. Memory effect of the Ge nanoclusters is verified by hysteresis in the C-V curves in the Al₂O₃/Ge+Al₂O₃/Al₂O₃ and ZrO₂/Ge+Al₂O₃/Al₂O₃ samples. This article is based on a presentation in “The 7th Korea-China Workshop On Advanced Materials” organized by the Korea-China Advanced Materials Cooperation Center and the China-Korea Advanced Materials Cooperation Center, held at Ramada Plaza Jeju Hotel, Jeju Island, Korea on August 24–27, 2003.     

Effect of the addition of Sm<Subscript>2</Subscript>O<Subscript>3</Subscript> on the microstructure of laser cladding alloy coating layers

The effects on the microstructures and phases of coating layers by the addition of micron-sized (m) and nano-sized (n) (m&n) Sm₂O₃ powders were investigated. The coating materials, which were prepared by means of 2.0 kW CO₂ laser cladding, consist of a powder mixture of m Ni-based alloy (NBA) powders comprising 1.5 wt.% m Sm₂O₃ and 3.0% n Sm₂O₃ powders. The results indicate that γ-Ni, Cr₂₃C₆ and Ni₃B are the primary phases of the NBA coatings. The Fe₇Sm and Ni₃Si phases are highlighted by the addition of m&n Sm₂O₃ powders. From the substrate, planar crystal layers are first grown in all NBA and m&n Sm₂O₃/NBA coatings. The dendrite growth then occurs as a result of the addition of the m Sm₂O₃ powder, and the equiaxed dendrite growth occurs as a result of the addition of the n Sm₂O₃. With the addition of a rare earth oxide such as Sm₂O₃ powder, the width of the planar crystal becomes smaller than that of the NBA coating.     

Fabrication and Properties of Plasma-Sprayed Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/ZrO<Subscript>2</Subscript> Composite Coatings

Al₂O₃ /xZrO₂ (where x = 0, 3, 13, and 20 wt.%) composite coatings were deposited onto mild steel substrates by atmospheric plasma spraying of mixed α-Al₂O₃ and nano-sized monoclinic-ZrO₂ powders. Microstructural investigation showed that the coatings comprised well-separated Al₂O₃ and ZrO₂ lamellae, pores, and partially molten particles. The coating comprised mainly of metastable γ-Al₂O₃ and tetragonal-ZrO₂ with trace of original α-Al₂O₃ and monoclinic-ZrO₂ phases. The effect of ZrO₂ addition on the properties of coatings were investigated in terms of microhardness, fracture toughness, and wear behavior. It was found that ZrO₂ improved the fracture toughness, reduced friction coefficient, and wear rate of the coatings.     

Preparation and characterization of nanostructured Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-13wt.%TiO<Subscript>2</Subscript> ceramic coatings by plasma spraying

Nanostructured and conventional Al₂O₃-13wt.%TiO₂ ceramic coatings were prepared by plasma spraying with nanostructured agglomerated and conventional powders, respectively. The microstructure and microhardness of the coatings were investigated using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and microhardness measurement. Meanwhile, the friction and wear behaviors were analyzed and compared using a ball-on-disk tribometer. The results show that the conventional coating has lamellar stacking characteristic and has some pores. However, the nanostructured coating shows a bimodal microstructure, which is composed of both fully melted regions and partially melted regions. According to the microstructural difference, the partially melted regions can be divided into liquid-phase sintered regions (a three-dimensional net or skeleton-like structure: Al₂O₃-rich submicron particles embedded in the TiO₂-rich matrix) and solid-phase sintered regions (remained nanoparticles). The microstructural characteristics of the liquid-phase sintered region are formed due to the selective melting of TiO₂ nanoparticles during plasma spraying. On the other hand, the TiO₂ and Al₂O₃ nanoparticles of the solid-phase sintered regions are all unmelted during plasma spraying. Due to the existence of nanostructured microstructures, the nanostructured coating has a higher microhardness, a lower friction coefficient, and a better wear resistance than the conventional coating.     

Tribological Properties of Ni<Subscript>3</Subscript>Al Matrix Composite Sliding Against Si<Subscript>3</Subscript>N<Subscript>4</Subscript>, SiC and Al<Subscript>2</Subscript>O<Subscript>3</Subscript> at Elevated Temperatures

The Ni₃Al matrix self-lubricating composite was fabricated by powder metallurgy technique. The tribological behavior of the composite sliding against commercial Si₃N₄, SiC and Al₂O₃ ceramic balls was investigated from 20 to 1000 °C. It was found that the composite demonstrated excellent lubricating properties with different friction pairs at a wide temperature range, which can be attributed to the synergetic effect of Ag, fluorides, and molybdates formed by oxidations. The Ni₃Al matrix self-lubricating composite/Si₃N₄ couple possessed the stable friction coefficient and wear rate.     

Effect of Feedstock Size and its Distribution on the Properties of Detonation Sprayed Coatings

The detonation spraying is one of the most promising thermal spray variants for depositing wear and corrosion resistant coatings. The ceramic (Al₂O₃), metallic (Ni-20 wt%Cr) , and cermets (WC-12 wt%Co) powders that are commercially available were separated into coarser and finer size ranges with relatively narrow size distribution by employing centrifugal air classifier. The coatings were deposited using detonation spray technique. The effect of particle size and its distribution on the coating properties were examined. The surface roughness and porosity increased with increasing powder particle size for all the coatings consistently. The feedstock size was also found to influence the phase composition of Al₂O₃ and WC-Co coatings; however does not influence the phase composition of Ni-Cr coatings. The associated phase change and %porosity of the coatings imparted considerable variation in the coating hardness, fracture toughness, and wear properties. The fine and narrow size range WC-Co coating exhibited superior wear resistance. The coarse and narrow size distribution Al₂O₃ coating exhibited better performance under abrasion and sliding wear modes however under erosion wear mode the as-received Al₂O₃ coating exhibited better performance. In the case of metallic (Ni-Cr) coatings, the coatings deposited using coarser powder exhibited marginally lower-wear rate under abrasion and sliding wear modes. However, under erosion wear mode, the coating deposited using finer particle size exhibited considerably lower-wear rate.