The effects of metal coating on the diffusion bonding in Al2O3/Inconel 600 and the modulus of rupture strength of alumina

Alumina with a sputter-deposited metal film was diffusion bonded to Inconel 600. A higher bonding strength and lower joining temperature were obtained with titanium coating compared to that for the non-coated sample. The improved joining behaviour was attributed to an enhanced interface reaction and reduction in the thermal stress. Also, the effect of various coatings of 3 m thickness on the mechanical property of alumina after heat treatment at 1000 °C for 30 min under 10^–6 torr vacuum was evaluated in terms of modulus of rupture (MOR) using a Weibull plot. While the Cu coating did not change MOR strength of alumina, the reactive Ti and Zr metal coatings caused a noticeable reduction in averaged MOR strength. The effect of co-sputtering of Ti-Cu, and bilayer coatings of Cu/Ti and Ti/Cu was also investigated.     

Untersuchung der Haftmechanismen kaltgasgespritzter Al‐Schichten auf Al2O3

The metallization of ceramics by means of cold gas spraying has been in the focus of numerous publications in the recent past. However, the bonding mechanisms of metallic coatings on non‐ductile substrates are still not fully understood. Former investigations of titanium coatings on corundum revealed that a combination of recrystallisation induced by adiabatic shear processes and heteroepitaxial growth might be responsible for the high adhesions strengths of coatings applied on smooth ceramic surfaces. In the present work, it is intended to examine the interface area of cold gas sprayed aluminum on alumina substrates. Besides a variation of powder fraction and substrate temperature, it is investigated if a downstream heat treatment has an influence on tensile strength and hardness of the coatings. The splat formation of single particles is investigated by means of scanning electron microscopy, while a high resolution transmission electron microscope is used to examine the Al/Al₂O₃ interface. First results suggest that mechanical clamping is the primary bonding mechanism on polycrystalline coatings with a sub‐micrometer‐scaled surface roughness, while heteroepitaxial growth is the main bonding mechanism for Al coatings on single‐crystalline, atomically smooth sapphire (α‐Al₂O₃) substrates. Heteroepitaxy is promoted by deformation‐induced recrystallisation of the cold gas‐sprayed aluminum.     

Effect of active metal coatings on the mechanical properties of silicon nitride-based ceramics

The effects of titanium, zirconium, hafnium and tantalum coatings on the mechanical properties of three silicon nitride ceramics were studied. The titanium coatings was found to cause a 50% decrease in the four-point bend strength of one of the silicon nitride ceramics while the effects of the zirconium, hafnium and tantalum coatings on all three silicon nitride ceramics were moderate. The reactions at a high temperature (940–980°C) between titanium and the grain-boundary glassy phase was the major cause for the degradation of the ceramic properties by the titanium coating. Residual tensile stress developed at the reaction interface replaced the glassy grain-boundary phase. Analytical electron microscopy showed the formation of a 180 nm thick Ti₅Si₃ layer and the crystallization of the amorphous grain-boundary phase. An indentation technique was used to measure qualitatively the residual stress developed at the reaction interface.     

Phase stabilization of plasma-sprayed alumina coatings by spraying mechanically blended alumina–chromia powders

A study of phase stabilization is performed on atmospheric plasma-sprayed alumina–chromia coating applied on low-carbon steel substrates. Alumina–chromia mixture with varying amounts of chromia content (1?wt.% to 6?wt.%) is used for this purpose. X-ray diffraction (XRD) analysis is carried out to investigate the different phase compositions of the coatings. Quantification of the different phases present in the coatings is performed by subsequent use of Rietveld refinement method. Surface morphology, microhardness, and wear behavior of the different coatings are also observed. Rietveld analyses performed on coatings ensure the stabilization of metastable phases present in the alumina coatings by chromia addition. Significant increase of the α -alumina and (Al_xCr_1-x)₂O₃ content is observed in alumina coatings with 4?wt.% chromia content. This is supported by the improved hardness and wear-resistant properties of the alumina coatings containing 4?wt.% chromia. The minimum surface roughness of the coating is also observed for alumina coatings with 4?wt.% chromia content among all the alumina coatings with different chromia content. The formation of alumina–chromia solid solution and solidification of α-alumina are found to be the reasons behind the enhancement in mechanical properties of the coating.     

Effects of nano-Fe2O3 powder on thermal emission property of microarc oxidation coating on titanium alloy

Abstract

A ceramic coating was formed on the titanium alloy by microarc oxidation in an electrolyte containing nano-Fe₂O₃, emulsifier OP-10 and sodium phosphate. The composition, surface and cross-sectional morphology and the element compositions of the coatings were characterised by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray analysis system. The spectral emissivity of the coatings was measured by a Fourier transform spectrometer apparatus. The bonding strength between the coating and the titanium alloy was studied by tensile strength test. The thermal shock resistance of the coatings was also evaluated. The results showed that nano-Fe₂O₃ was incorporated into the coating, and the coating had high emission at the wavelength range of 3–20 μm. The bonding strength was 33·2 MPa, and after being subjected to severe thermal shocking for 50 cycles, little peeling-off of the coating occurred.     

Oxidation behaviour of an intermetallic Ti–Al–Cr–Zr bond coat on a γ–TiAl based TNB alloy with 7YSZ thermal barrier coating

Abstract

Intermetallic titanium aluminide alloys are attractive light-weight materials for high temperature applications in automotive and aero engines. The development of γ-TiAl alloys over the past decades has led to their successful commercial application as low pressure turbine blades. The operating temperatures of γ-TiAl based alloys are limited by deterioration in strength and creep resistance at elevated temperatures as well as poor oxidation behaviour above 800 °C. Since improvement in oxidation behaviour of γ-TiAl based alloys without impairing their mechanical properties represents a major challenge, intermetallic protective coatings have aroused increasing interest in the last years.

In this work, a 10 μm thick intermetallic Ti–46Al–36Cr–4Zr (in at.-%) coating was applied on a TNB alloy using magnetron sputtering. This layer provided excellent oxidation protection up to 1000 °C. Microstructural changes in this coating during the high temperature exposure were extensively investigated using scanning and transmission electron microscopy. The coating developed a three-phase microstructure consisting of the hexagonal Laves-phase Ti(Cr,Al)₂, the tetragonal Cr₂Al phase and the cubic τ-TiAl₃ phase. After long-term exposure the three-phase microstructure changed to a two-phase microstructure of the hexagonal α₂-Ti₃Al phase and an orthorhombic body-centred phase, whose crystal structure has not yet been definitely identified. On the coating, a thin protective alumina scale formed. Applying this intermetallic layer as bond coat, thermal barrier coatings (TBCs) of yttria partially stabilized zirconia were deposited on γ-TiAl based TNB samples using electron-beam physical vapour deposition. The results of cyclic oxidation testing (1 h at elevated temperature, 10 min. cooling at ambient temperature) revealed a TBC lifetime of more than 1000 h of cyclic exposure to air at 1000 °C. The ceramic topcoat exhibited an excellent adhesion to the thermally grown alumina scale which contained fine ZrO₂ precipitates.     

Pulsed laser deposition of hydroxyapatite on titanium substrate with titania interlayer

Pulsed laser deposition (PLD) has been used to deposit hydroxyapatite (HA) ceramic over titanium substrate with an interlayer of titania. PLD has been identified as a potential candidate for bioceramic coatings over metallic substrates to be used as orthopedic and dental implants because of better process control and preservation of phase identity of the coating component. However, direct deposition of hydroxyapatite on titanium at elevated temperature results in the formation of natural oxide layer along with some perovskites like calcium titanate at the interface. This leads to easy debonding of ceramic layer from the metal and thereby affecting the adhesion strength. In the present study, adherent and stable HA coating over Ti6Al4V was achieved with the help of an interlayer of titania. The interlayer was made to a submicron level and HA was deposited consecutively to a thickness of around one micron by exposing to laser ablation at a substrate temperature of 400°C. The deposited phase was identified to be phase pure HA by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis, and inductively coupled plasma spectrometry. The mechanical behavior of coating evaluated by scratch test indicates that the adhesion strength of HA coating was improved with the presence of titania interlayer.     

Influence of surface modification of alumina on bond strength in Al2O3/Al/Al2O3 joints

The subject of the work was to study the effect of Ti thin film on alumina ceramic on mechanical strength and fracture character of Al₂O₃/Al/Al₂O₃ joints. The joints were formed by liquid state bonding of alumina substrates covered with titanium thin film of 800 nm thickness using Al interlayer of 30μm thickness at temperature of 973 K in a vacuum of 0.2 mPa for 5 min. The bend strength was measured by four–point bending test at room temperature. Scanning and transmission electron microscopy were applied for detailed characterization of interface structure and failure character of fractured joint surfaces. Result analysis has shown that application of the Ti thin film on alumina leads to decrease of bond strength properties of Al₂O₃/Al/Al₂O₃ joints along with the change either of structure and chemistry of interface or of failure character.     

HAp/Ti<Subscript>2</Subscript>Ni coatings of high bonding strength on Ti–6Al–4V prepared by the eutectic melting bonding method

Eutectic melting bonding (EMB) method is a useful technique for fabricating bioactive coatings with relatively high crystallinity and bonding strength with substrate on titanium substrates. Using the EMB method, hydroxyapatite/Ti₂Ni coatings were prepared on the surface of Ti–6Al–4V at a relatively low temperature (1,050 °C) in a vacuum furnace. The coatings were then characterized in terms of phase components, microstructure, bonding strength and cytotoxicity. The results showed that the coatings were mainly composed of HAp and Ti₂Ni, and the thickness of the coatings was approximately 300 μm. X-ray diffraction analysis showed that the coatings exhibited relatively high crystallinity. The tensile bonding strength between the coatings and the substrates was 69.68 ± 5.15 MPa. The coatings had a porous and rough surface which is suitable for cell attachment and filopodia growth. The cell culture study showed that the number of MG-63 cells increased, and the cell morphology changed with the incubation time. This study showed that the EMB method can be utilized as a potentially powerful method to obtain high quality hydroxyapatite coatings with desired mechanical and biocompatibility properties on Ti-alloy substrates.     

Reactively sputtered oxide optical coatings for inertial confinement fusion laser components

Results to date are presented for a study of reactively sputtered oxide optical coatings which are candidate materials for use in the NOVA Terawatt neodymium glass laser under development at Lawrence Livermore Laboratory, Livermore, California. Experimental details are described for the deposition of TiO₂ on fused silica substrates by r.f. diode sputtering of titanium in a reactive Ar-O₂ atmosphere. Specific deposition procedures are presented for variation of the phase composition and grain size of the coatings over wide ranges. High laser damage thresholds are reported for TiO₂, and glassy coatings appear to be more damage resistant than polycrystalline coatings. Phase composition appears to be less important to damage resistance than grain size. Application of the reactive sputtering process to preparation of damage-resistant oxides of In-Sn and tantalum is also described and damage thresholds obtained to date are reported.     

Porous Titanium Coatings Through Electrophoretic Deposition of TiH2 Suspensions

In the biomedical field, modification of titanium surfaces to improve the osteoinductive and antibacterial behavior is widely investigated. This functionalization can be further ameliorated by providing a porous coating with high loading capacity for bioactive materials and drug delivery carriers at the implant surface. In this work, a new powder metallurgical processing route used to deposit such porous pure titanium coatings on Ti based substrates is presented. The coatings were prepared by electrophoretic deposition (EPD) of TiH₂ powder suspensions followed by dehydrogenation and sintering in vacuum. The use of hydrides allowed to lower the sintering temperature below that of the α–β transition of the Ti6Al4V substrate. Measurement of the tensile bond strength confirmed a strong adhesion of the porous coating. Deposition of powders with different grain sizes resulted in porous titanium coatings with varying thickness, pore morphology, and surface roughness. The possibility to extend this coating technique to complex shaped implants is highlighted.     

Glazing of alumina by a fluoroapatite-containing glass-ceramic

Glazing of alumina substrates was performed in order to prepare bioactive glass-ceramic coatings for biomedical applications. The coating material was a fluoroapatite-containing glass-ceramic (SAF) with a good degree of bioactivity. A careful optimisation of the coating conditions was carried out. The obtained coatings were characterized by means of scanning electron microscopy (SEM), compositional analysis (EDS), X-Ray diffraction (XRD), and in vitro tests (soaking into a simulated body fluid). Direct firing of SAF powders on the ceramic substrates gave unsatisfactory results in terms of crack propagation and bioactivity, due to the nucleation of a non-bioactive Al-rich phase (leucite) with high linear expansion coefficient. The use of an intermediate layer based on a SiO₂-CaO glass (SC) was necessary in order to avoid Al ions diffusion through the coating and thus the formation of undesired additional phases. The coatings obtained with the optimised processing parameters resulted to be adherent, defect-free, and characterized by unchanged composition and structure as well as unmodified bioactivity if compared with the bulk SAF glass-ceramic.     

Nanostructured Al2O3–TiO2 coatings for high-temperature protection of titanium alloy during ablation

Plasma-sprayed nanostructured Al₂O₃–13 wt.%TiO₂ coatings were successfully fabricated on titanium alloys (Ti–6Al–4V) using as-prepared feedstock. Ablation experiments for the titanium alloy samples with or without a coating were carried out using a Metco 9MB plasma gun. The microstructure, phase constituents and mechanical properties of the titanium alloys before and after ablation were investigated by scanning electron microscope (SEM), X-ray diffractometer (XRD) and Vickers hardness tester. The surface morphologies, cross-sectional microstructure and hardness of titanium alloys with coatings are similar before and after ablation. In contrast, the microstructure and mechanical properties of the titanium alloy without coating are significantly changed after ablation. The surface coating is found to serve as a protective coating during ablation.     

Influence of boron doping on mechanical and tribological properties in multilayer CVD-diamond coating systems

Titanium alloy (Ti6Al4V) substrates were deposited with smooth multilayer coatings, by hot filament chemical vapour deposition technique. The effect of boron doping on lattice parameter, residual stresses, hardness and coefficient of friction in multilayer-diamond coating system was studied. The frictional behaviour of the coatings was studied using a ball-on-disc micro-tribometer by sliding the coated samples of titanium alloy (Ti6Al4V) substrates against alumina (Al₂O₃) balls, and increasing normal load from 1 to 10 N. The average friction coefficient decreased from 0.36 to 0.29 for undoped multilayer-diamond coating system and from 0.33 to 0.18 for boron- doped (BD) multilayer-diamond coating system. The average indentation depths for undoped and BD multilayer- diamond coating systems were found to be equal to ～>58 and ～65 nm, respectively, and their hardness values were 60 and 55 GPa, respectively.     

Alkali corrosion resistant coatings for Si3N4 ceramics

The use of ceramic oxide coatings on silicon nitride is one method to improve its alkali corrosion resistance. Four oxide coatings, including (Ca_0.6, Mg_0.4) Zr₄(PO₄)₆ (CMZP), zirconia, mullite and alumina, were examined. These coatings were applied on Si₃N₄ using both sol–gel and dip coating techniques. The coated and uncoated samples were exposed to sodium molten-salt and sodium-containing atmospheres at 1000 °C for 50 h. The weight loss of all the coated samples was less than that of the uncoated Si₃N₄ with CMZP-coated samples exhibiting the smallest weight loss. There was no decrease in the flexural strength of Si₃N₄ after coating with zirconia and CMZP, and a decrease in strength after coating with either mullite or alumina. After alkali exposure, the strength of the CMZP and zirconia coated samples were significantly higher than those of the mullite-coated, alumina-coated, and uncoated Si₃N₄. The observed behaviour is explained in terms of the microstructure and protection mechanisms. This revised version was published online in November 2006 with corrections to the Cover Date.     

The effect of bond coat on mechanical properties of plasma-sprayed Al2O3 and Al2O3-13 wt% TiO2 coatings on AISI 316L stainless steel

In this study, Al₂O₃ and Al₂O₃-13 wt% TiO₂ were plasma sprayed onto AISI 316L stainless-steel substrate with and without Ni-5 wt% Al as bond coat layer. The coated specimens were characterized by optical microscopy, metallography and X-ray diffraction (XRD). Bonding strength of coatings were evaluated in accordance with the ASTM C-633 method. It was observed that the dominant phase was Al₂O₃ for both coatings. It was also found that the hardness of coating with bond coat was higher than that of coating without bond coat. Metallographic studies revealed that coating with bond coating has three different regions, which are the ceramic layer (Al₂O₃ or Al₂O₃-13 wt% TiO₂), the bond coating, and matrix, which is not affected by coating. The coating performed by plasma-spray process without bond coating has two zones, the gray one indicating the ceramic layer and the white one characterizing the matrix. No delamination or spalling was observed in coatings. However, there are some pinholes in coating layer, but they are very rare. The bonding strength of coatings with bond coat was higher than that of coating without bond coat. The strength of adhesion and cohesion was determined by means of a planemeter. It was seen that percentage of cohesion strength was higher than that of adhesion strength.     

Poly(P‐Phenylene Terephthalamide) Fibers Reinforced with Ultrathin Ceramic Coatings

The poly(p‐phenylene terephthalamide) (PPTA) fibers are engineering polymer materials with high strength and stiffness, lightweight, and outstanding fatigue characteristics. However, these materials exhibit drawbacks, namely, anisotropic mechanical strength along the tensile and compressive directions. In particular, they show low compressive strength, and mismatched thermal expansion as well as moisture uptake and oxidative stability along the axial and radial directions. Here, the authors describe a hybrid material approach by integrating ultrathin ceramic coatings (alumina or silica) onto PPTA fibers to enhance their compressive modulus by 2.5 times. In addition, these ceramic coating improve the energy absorption, thermal conductivity, and chemical stability of PPTA fibers, while their flexibility and lightweight are preserved. This strategy provides a new route for the preparation of high performance polymer fibers for advanced engineering applications.

     

Bioactive glass-derived trabecular coating: a smart solution for enhancing osteointegration of prosthetic elements

In this work, the use of foam-like glass–ceramic scaffolds as trabecular coatings on ceramic prosthetic devices to enhance implant osteointegration is proposed. The feasibility of this innovative device was explored in a simplified, flat geometry: glass–ceramic scaffolds, prepared by polymeric sponge replication and mimicking the trabecular architecture of cancellous bone, were joined to alumina square substrates by a dense glass coating (interlayer). The role played by different formulations of starting glasses was examined, with particular care to the effect on the mechanical properties and bioactivity of the final coating. Microindentations at the coating/substrate interface and tensile tests were performed to evaluate the bonding strength between the sample’s components. In vitro bioactive behaviour was assessed by soaking in simulated body fluid and evaluating the apatite formation on the surface and inside the pores of the trabecular coating. The concepts disclosed in the present study can have a significant impact in the field of implantable devices, suggesting a valuable alternative to traditional, often invasive bone-prosthesis fixation.     

Industrially-styled room-temperature pulsed laser deposition of titanium-based coatings

Titanium-based compounds are widely used as coating materials for mechanical, tribological, electrical, optical, catalytic, sensoric, micro-electronical applications due to their exceptionally physical and chemical properties. Recently, the trend of using temperature-sensitive materials like polymers and tool steels with the highest hardness demands new low-temperature coating techniques for protective surface finishing as well as for surface functionalization, but up to now there is lack of industrially scaled vacuum coating techniques at temperatures below 50 °C. An alternative for overcoming this problem is the pulsed laser deposition (PLD) technique, which was up-scaled for industrial demands at Laser Center Leoben of JOANNEUM RESEARCH Forschungsgesellschaft mbH.The current paper summarizes the application of the industrially-scaled PLD technique on the deposition of the presently most important Ti-based coatings: metallic titanium, titanium nitride (TiN), titanium oxide (TiO₂) and titanium carbonitride (TiCN). PLD coating allows, even at room temperature, the formation of film structures of Zone-T type of Thornton's structure zone model, both on substrates aligned normal and parallel to the incident vapor flux. The high-energetic deposition conditions are revealed by the occurrence of (2 2 0) textures for the fcc TiN-based films. The dense grown structure affects advantageously the tribological behavior—generally, low wear rates and (for TiCN) very low friction coefficients were found. For TiO₂ coatings, growing as a mixture of β-TiO₂ and amorphous phases, the easily reproducible change of deposition parameters in the room-temperature PLD allows large differences in the optical transmission and electrical resistance.     

Hardness,wear resistance and bonding strength of nano structured functionally graded Ni-Al2O3 composite coatings fabricated by ball milling method

Using initial powder mixtures with different Ni:Al₂O₃ weight ratios ranging from 1:1 to 16:1, nano structured Ni-Al₂O₃ composite coatings were deposited onto an aluminum plate by means of a planetary ball mill. It was shown that initial charges with Ni:Al₂O₃ weight ratios of 4:1 and greater, yielded well-compact coatings. Coating deposited from the powder charge with Ni:Al₂O₃ weight ratio of 4:1, contained 20?vol% of alumina particles in the Ni matrix and submitted the highest hardness value (657?±?28?Hv) and wear resistance. Nevertheless, composite coating containing smallest amount of alumina particles showed the highest cohesive strength of 9.8?±?0.3?MPa. In the next step, nano structured functionally graded composite coatings were produced by the deposition of two separate layers containing different amounts of alumina particles. Although the graded coating showed superior hardness and wear resistance compared with the non-graded coatings, it suffers from low cohesive strength attributed to the presence of alumina particles at the interface region between the two layers. To overcome the poor adhesion between two layers, a thin intermediate plain Ni one was deposited between two layers leading to 80% and 30% improvement in the adhesion strength and wear resistance, respectively.