Microstructure and properties of CVD γ-Al2O3 coatings

This work deals with the microstructures and wear properties of chemical vapour deposited γ-Al₂O₃. The γ-Al₂O₃ coatings were deposited at 800 °C on TiN and Ti(C,N) pre-coated cemented carbide substrates. The microstructures developed in the γ-Al₂O₃ coatings and the influence of the nucleation surface on the growth of γ-Al₂O₃ were characterised using transmission electron microscopy, electron energy-loss spectroscopy and X-ray diffraction. The γ-Al₂O₃ coatings were fine-grained with a high density of {1 1 1} growth twins and contained some residual sulphur. γ-Al₂O₃ was found to grow epitaxially on the investigated substrates. The mechanical properties were evaluated in metal cutting and were compared with those of κ-Al₂O₃ coated tools. As compared with the κ-Al₂O₃ coatings, the γ-Al₂O₃ coatings exhibited slightly worse adhesion and tendency for edge chipping. However, the γ-Al₂O₃ coatings showed better crater wear resistance on the rake face than κ-Al₂O₃ coatings.     

TEM and DFT investigation of CVD TiN/κ-Al2O3 multilayer coatings

This paper investigates the interfacial structure in hot-wall CVD TiN/κ-Al₂O₃ multilayer coatings using both HREM and DFT modeling. Two multilayers with different thicknesses of the TiN layers (50 and 600 nm) separating the κ-Al₂O₃ layers are analyzed. The general microstructure of the two multilayers is relatively similar. The TiN layer in the thicker TiN/κ-Al₂O₃ coating is thick enough to be several TiN grains high. This means that epitaxial columns, which are often found in the thinner TiN/κ-Al₂O₃ coatings, are not present. However, the orientation relationships at the TiN/κ-Al₂O₃ interfaces are the same in both multilayers. The HREM investigations show that κ-Al₂O₃ (001) planes can grow directly on flat (111) TiN faces, without any other phases or detectable amounts of impurities, such as sulphur, present. Where the TiN layers are more curved, γ-Al₂O₃ can be grown, at least partly stabilized by the cube-on-cube orientation relationship between γ-Al₂O₃ and the underlying TiN. The DFT calculations show very similar adsorption strengths for an O monolayer positioned on Ti-terminated TiC(111) and TiN(111) surfaces, with preferred adsorption in the fcc site. O adsorption on N-terminated TiN(111) is much weaker, with preferred adsorption in the top site. Calculated elastic-energy contributions yield a higher stability for κ-Al₂O₃ on TiN(111) than on TiC(111) and a higher stability for κ-Al₂O₃ than for α-Al₂O₃ on both TiC and TiN. This indicates that the observed higher stability of κ-Al₂O₃ on TiC(111) than on TiN(111) is not due to the lattice mismatch, while the preferred epitaxial growth of κ-Al₂O₃ over α-Al₂O₃ can be partly attributed to the mismatch.     

Deposition, microstructure and properties of texture-controlled CVD α-Al2O3 coatings

The influence of nucleation on the microstructure and properties of CVD Al₂O₃ was investigated. The experimental α-Al₂O₃ layers were deposited (a) without nucleation control and (b) with nucleation steps resulting in pronounced , and growth textures. The experimental layers were characterised using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Wear properties of the textured coatings were evaluated in turning. The chemistry of the nucleation surface appeared to be an important factor in pre-determining the phase content and growth textures of the Al₂O₃ layers. Optimised nucleation resulted in substantially improved wear properties and these kinds of α-Al₂O₃ layers were typically composed of relatively small, defect-free grains exhibiting no porosity. The textured α-Al₂O₃ layer showed the best wear resistance.     

Development of moderate temperature CVD Al2O3 coatings

Chemically vapor deposited Al₂O₃ coatings, due to their high hardness and chemical inertness, are currently the state of art in the cutting tool industry. The conventional high deposition temperature of about 1050 °C for Al₂O₃ coatings, based on the water–gas shift process, has to a great extend restricted the development of several hybrid coatings, such as TiC/TiN/TiCN/Al₂O₃. To overcome this limitation, alternate systems to deposit Al₂O₃ at moderate temperatures have been investigated. Systems using NO–H₂, H₂O₂, NO₂–H₂ and HCOOH were identified and thermodynamic calculations were performed to evaluate them as potential sources of oxygen donors to form Al₂O₃ in the moderate temperature range of 700–950 °C. Preliminary results have clearly demonstrated that it is possible to grow moderate temperature alumina (using such alternate sources) on the TiC/TiN coated cemented carbide substrates.     

Partial dislocation configurations in a low-angle boundary in α-Al2O3

The dislocation structures of a low-angle tilt grain boundary in alumina bicrystal were investigated by transmission electron microscopy. The grain boundary was found to consist of two regions: an area with pairs of partial dislocations and an area with groups of odd numbered partial dislocations (multiple-partial-structure). Eight kinds of multiple-partial-structures were found in the fabricated grain boundary. The Burgers vectors of each partial dislocation in the grain boundary can be distinguished by dark-field imaging, and thus the arrangement of partial dislocations in the multiple-partial-structures are determined. It is concluded that a slight twist component of the boundary is the origin of the characteristic multiple-partial-structures.     

CVD TiC/alumina and TiN/alumina multilayer coatings grown on sapphire single crystals

Multilayers of TiC/α-Al₂O₃ and TiN/κ-Al₂O₃, consisting of three (1 mm thick) alumina layers separated by thin TiC or TiN (∼20 nm thick) layers, have been deposited onto c- and r-surfaces of single crystals of α-Al₂O₃ by chemical vapour deposition (CVD). The aim of this paper is to describe and compare the detailed microstructure of the different multilayer coatings by using transmission electron microscopy (TEM).     

Synthesis of crystalline γ-Al2O3 with high purity

Crystalline γ-AlO(OH) was synthesized by the precipitation of sodium aluminate and oxalic acids in aqueous solution. And then γ-AlO(OH) was successfully transferred to γ-Al₂O₃ after subsequent high temperature heat treatment. The effects of reaction conditions on formation of γ-AlO(OH) and γ-Al₂O₃ were further investigated in detail. The XRD analysis shows that the complete formation of crystalline γ-Al₂O₃ is at pH 8–9, reaction temperature of 93–96 °C and calcination temperature of higher than 400 °C. The product of γ-Al₂O₃ contains impurity, including iron, calcium and silicon ion with a low content of about 0.01% and has large specific surface area and high pore volume of 269.9 m²/g and 0.57 mL/g, which can be applied in catalysts and catalyst supports.     

Mechanical properties of the plasma-sprayed Al2O3/ZrSiO4 coatings

The mechanical properties of plasma-sprayed Al₂O₃/ZrSiO₄ coatings were investigated by indentation-based techniques. Two types of feedstock were used to prepare the coatings: spray-dried powders and plasma-spheroidized powders. A 100-kW direct current (d.c.) thermal plasma system was employed. The values obtained were found to exhibit a close relationship with the microstructure of the as-sprayed coatings, which composed of zircon, alumina, amorphous silica and tetragonal zirconia. The coatings produced with plasma-spheroidized powders had higher microhardness, Young's modulus and fracture toughness than that produced with the spray-dried powders. The coatings produced with plasma-spheroidized powders by a 100-kW computerized system at 15 kW of net plasma energy had the best mechanical properties, while those deposited at 19 kW of net plasma energy had the worst properties due to the high density of cracks in the coatings.     

Thermal cycle behavior of plasma sprayed La2O3, Y2O3 stabilized ZrO2 coatings

The effects of La₂O₃ addition on thermal conductivity, phase stability and thermal cycle life of Y₂O₃ stabilized ZrO₂ plasma sprayed coatings were investigated. Although low thermal conductivity as well as high resistance to sintering was achieved by La₂O₃ addition, it tended to also result in lower phase stability and thermal cycle life of the coatings. Optimization of the composition and structure of the coatings improved these properties, and the optimized coatings showed prolonged thermal cycle life.     

Microstructure and tribological behavior of suspension plasma sprayed Al2O3 and Al2O3-YSZ composite coatings

Several alumina and alumina-zirconia composite coatings were manufactured by suspension plasma spraying (SPS), implementing different operating conditions in order to achieve dense and cohesive structures. Temperatures and velocities of the in flight particles were measured with a commercial diagnostic system (Accuraspray®) at the spray distance as a function of the plasma operating parameters. Temperatures around 2000 °C and velocities as high as 450 m/s were detected. Hence, coatings with high amount of α-alumina phase were produced. The microstructure evolution according to the spray parameters was studied as well as the final tribological properties showing efficient wear resistance.     

CVD deposition and characterization of coloured Al2O3/ZrO2 multilayers

Coloured Al₂O₃/ZrO₂ multilayers have been deposited onto WC-Co based inserts by a CVD process. Through physical as well as optical analysis of such multilayers, colour is believed to originate from interference. The coatings are obtained with good process reproducibility. It was found that the ZrO₂ process used in the multilayer, with ZrCl₄ as the only metal chloride precursor, results in a mixture of tetragonal and monoclinic ZrO₂ phases. However by adding a relatively small amount of AlCl₃ during such a process results in ZrO₂ layers being composed of predominantly tetragonal ZrO₂ phase. Corresponding multilayers seem to have a more fine grained and smoother morphology whereas multilayers containing monoclinic ZrO₂ phase seem to be less perfect with existence of larger grains of ZrO₂ which are believed to scatter light and alter the reflectance of such a multilayer. In addition to this, such multilayers were found to be free of or with greatly reduced amount of thermal cracks, normally present in pure CVD grown Al₂O₃ layers.It is believed that, in the studied Al₂O₃/ZrO₂ multilayers, the observed tetragonal ZrO₂ phase is the result of a size effect, where small enough ZrO₂ crystallites energetically favor the tetragonal phase. However as the ZrO₂ crystallite size distribution is shifted to larger sizes it is believed that a mixture of crystallites with both stable and metastable tetragonal phases as well as a stable monoclinic phase is obtained. The proposed metastable tetragonal ZrO₂ phase may in fact explain the absence of thermal cracks in such multilayers through a transformation toughening mechanism, well known in ZrO₂ based ceramics.     

Microstructure and composition analysis in plasma sprayed coatings of Al2O3/ZrSiO4 mixtures

Plasma spraying of Al₂O₃/ZrSiO₄ was performed using spray dried and plasma spheroidised powder feedstock. The mixtures were sprayed using different spray stand-off distances and plasma power levels. X-Ray diffraction (XRD) was used to characterise the phase composition and scanning electron microscopy (SEM) examined the morphology of the sprayed surface and polished cross-sections. The results showed that the plasma spray process parameters played an important role in the final outcome of microstructures of the coatings. The coatings produced with spheroidised powders displayed a much denser structure than those produced with the spray-dried powders. The phase composition analysis showed the presence of amorphous phases in addition to crystalline alumina, zircon and tetragonal (t) zirconia (ZrO₂). Transmission electron microscopy (TEM) showed that amorphous phases and t-ZrO₂ crystals with particle size 100–200 nm could coexist within a single splat due to the relatively low local cooling rate.     

Effects of alloying elements on microstructure and protective properties of Al2O3 coatings formed on aluminum alloy substrates by plasma electrolysis

Hard alumina coatings were formed on three typical Al alloys using a plasma electrolytic oxidation (PEO) process. The microstructure and protective behavior of the coatings were characterized in the context of the elements alloyed in the substrates. The substrates investigated were three commercial aluminum alloys (Al 6061, 2024, 7075) and various Al − x–Mg binary alloys with x = 0.6–4.6 wt.%. The major portion of all coating films consisted of a mixture of γ- and α-alumina on top of a very thin amorphous substrate-based layer. Examination of the coatings on the commercial alloys reveals that the ratio of α- to γ-alumina is inversely proportional to the total concentration of residual Mg, Cu, and Zn ions in the coatings. Additional experiments on Al − x–Mg binary alloy substrates suggest that Mg ions play the most important role in suppressing the transition of γ- to α-alumina. Our results suggest that Mg content ≥ 3 wt.% yields only the γ-alumina. We propose a mechanism that correlates the residual elements in the coatings to the alumina phase transition. A higher content of the dense α-alumina in the coatings yielded greater hardness as well as improved wear tolerance and crack resistance of the coating layers.     

Phase relations in the SrO–Ga2O3–B2O3 system

The subsolidus phase relations in the SrO–Ga₂O₃–B₂O₃ system were investigated. The system contains 10 binary compounds and two ternary compounds, and can be divided into 15 three-phase regions. The new ternary compound SrGaBO₄ has two modifications (- and β-phases), both of which crystallize in the orthorhombic system but with different space groups.     

Microwave ECR plasma CVD of cubic Y2O3 coatings and their characterization

Yttrium oxide (Y₂O₃) thin films are deposited by microwave electron cyclotron resonance (ECR) plasma assisted metal organic chemical vapour deposition (MOCVD) process at a substrate temperature of 350 °C using indigenously developed metal organic precursors (2,2,6,6-tetra methyl-3,5-heptane dionate) yttrium, commonly known as Y(thd)₃ synthesized by ultrasound method. The deposited coatings are characterized by X-ray photoelectron spectroscopy, glancing angle X-ray diffraction, scanning electron microscopy, EDS and infrared spectroscopy. The characterization results indicate that it is possible to deposit non-porous coatings with excellent uniformity of a single phase cubic Y₂O₃ on various substrates by this process at reasonably low substrate temperature that is desirable in various manufacturing processes.     

Effect of nano-Fe2O3 particles on the corrosion behavior of alkyd based waterborne coatings

A nano-composite coating was formed by adding nano-Fe₂O₃ to a specially developed alkyd based waterborne coating system. The interaction of nano-particles with the polymer functionalities was confirmed using FTIR. Curing behavior of the coating was determined by DSC. The uniformity of coating and dispersion of nano-particles in the polymer matrix was investigated using SEM and AFM. The performance of the coating was investigated in terms of their corrosion resistance, UV-resistance, mechanical properties and optical properties. Composite coatings so formed showed enhanced, corrosion resistance, better mechanical properties such as abrasion and scratch hardness and improved UV blocking properties.     

Dissociation of Ti2O3 from titania slag under mechanical activation

In this present study, the effects of mechanical activation on the characterization of titania slag were systematically investigated. The crystal structures, surface chemical functional groups, and microstructure of the samples were characterized before and after mechanical activation using XRD, FT-IR, and Raman spectroscopy techniques, respectively. It was found that untreated titania slag under mechanical activation was mainly composed of Fe₃Ti₃O₁₀ and rutile TiO₂, but that of being treated by mechanical activation was mainly composed of Fe₃Ti₃O₁₀, Ti₂O₃ and rutile TiO₂. Ti₂O₃ is transformed partially from Fe₃Ti₃O₁₀ under moderate mechanical activation conditions for 12 h. The demonstration of mechanical activation techniques can be applied effectively and efficiently to the treatment processing of titania slag.     

Effects of particle size of 8 mol% Y2O3 stabilized ZrO2 (YSZ) and additive Ta2O5 on the phase composition and the microstructure of sintered YSZ electrolyte

The chemical stability and sinterability of nano- and micro-sized yttria stabilized zirconia (YSZ) powders were investigated, and the effect of Ta₂O₅ additive on the sinterability of YSZ electrolyte was also studied. Addition of Ta₂O₅ easily causes phase transformation of YSZ from cubic to monoclinic, and thus unfavorable for the improvement of ionic conductivity of YSZ electrolyte. Nano-sized YSZ easily causes abnormal grain growth and hence readily results in inhomogeneous microstructure of YSZ matrix, while micro-sized YSZ displays relatively low sinterability. Comparatively, mixture with 50 wt.% nano-sized and 50 wt.% micro-sized YSZ powders exhibits high sinterability and could be sintered to homogeneous microstructure, which can be used as starting materials to prepare dense YSZ electrolyte at low temperatures.     

Evolution of hot rolling textures in a two-phase (α2+β) Ti3Al base alloy

Texture development during thermomechanical processing of two-phase (α₂+β) Ti–24Al–11Nb alloy was studied as a function of variables like initial microstructure, rolling temperature, cooling conditions, etc. The evolution of texture in different conditions has been critically analysed. It has been found that unrestricted rolling of primary α₂ at lower temperature leads to a good basal {0001}uvtw texture, while at higher temperatures, the α₂→β→α₂ phase transformation leads to weakening of the basal texture. Texture of secondary α₂ derived from rolled β is generally non-basal. However, the texture of secondary α₂ derived from recrystallized β has a basal character.     

Preparation and properties of a γ-Al2O3 washcoat deposited on a ceramic honeycomb

Washcoat deposited on ceramic honeycomb was prepared using pseudoboehmite, the CeO₂–ZrO₂–La₂O₃ solid solution, pore enlarger and other additives. The microstructures and surface performances of washcoat/honeycomb were investigated by SEM, BET surface area, XRD, ultrasonic vibration and hot shock simulation. The results show that the performance and loading of washcoat are affected obviously by the properties of slurry gel, such as the apparent viscosity, solid content, particle size and its distribution. When the apparent viscosity of slurry is lower, the gel with a narrow particle size distribution and finer particles can be obtained, with which the coating having an excellent performance can be prepared. Adding a small quantity of the CeO₂–ZrO₂–La₂O₃ solid solution can promote the thermal stability of washcoat, such as, after calcined at 1000 °C for 5 h the sample exhibits mainly the γ-Al₂O₃ phases and the θ-Al₂O₃ -Al₂O₃ and κ-Al₂O₃ phases have not been detected in the XRD spectra. It is found also that the washcoat prepared has excellent properties of the vibration-resistant, heat-resistant and its BET surface area reaches 50 m²/g.