Crystalline Coatings Research Articles

Lanthanum hexaaluminate — novel thermal barrier coatings for gas turbine applications — materials and process development

Lanthanum hexaaluminate (LHA) with a magnetoplumbite structure is a promising competitor to yttria partially stabilized zirconia (Y-PSZ) as a thermal barrier coating (TBC), since most zirconia coatings age significantly, including undesired densification at temperatures exceeding 1100 °C. The microstructure of calcined lanthanum hexaaluminate powders and thermally sprayed coatings show a platelet structure. The magnetoplumbite structure is characterized by the highly charged La 3+ cation located in an oxygen position in the hexagonal close-packed structure of oxygen ions. Ion diffusion is strongly suppressed vertical to the crystallographic c-axis, thus hindering sintering densification. In contrast to the oxygen ion conducting zirconia, lanthanum hexaaluminate permits operating temperatures above 1300 °C because of its thermal stability and electrically insulating properties. This study describes the optimization of powder preparation for thermal spraying by spray drying and the development of parameters for atmospheric plasma spraying (APS) in order to produce homogeneous crystalline coatings with controlled micro-porosity and residual stresses. The phases were characterized by X-ray diffraction (XRD).

Effect of calcium phosphate coating crystallinity and implant surface roughness on differentiation of rat bone marrow cells.

In this study, we examined the effect of calcium phosphate (Ca-P) coating crystallinity and of surface roughness on growth and differentiation of osteogenic cells. Grit-blasted titanium substrates were provided with Ca-P coatings of different crystallinities. Rat bone marrow (RBM) cells were cultured on these substrates and on noncoated rough and smooth titanium substrates. After specific culture times, expression of osteogenic markers by the cells was studied. Cells cultured on crystalline coatings and on titanium substrates proliferate, express alkaline phosphatase, osteocalcin (OC), and show mineralization of the extracellular matrix. Rough titanium substrates only express low OC levels. Significantly higher OC levels were expressed on smooth titanium, and even higher levels on the crystalline Ca-P coating. No difference was found in calcification between smooth and rough titanium. The crystalline coating showed more calcification than the titanium substrates. When substrates without cells were incubated in medium, precipitation of calcium was found. On the titanium substrates, this precipitate disappeared after prolonged incubation. The precipitate on the crystalline coating was stable and increased with longer incubation times. On the amorphous coatings, no proliferation and differentiation of RBM cells were found. After longer culture periods, substrates showed extensive dissolution. Cells on the amorphous coatings did express high levels of prostaglandin E2. In contrast, prostaglandin E2 expression was low for the other substrates. We conclude that crystalline Ca-P coatings stimulate differentiation of RBM cells, to a higher extent than titanium substrates. Surface roughness only has a limited effect on phenotype expression of the cells. In contrast, thin amorphous coatings show negative effects on the growth and differentiation of cultured RBM cells.

Cell adhesion and proliferation on biomimetic calcium-phosphate coatings produced by a sodium silicate gel methodology.

The present study describes a methodology to produce bioactive coatings on the surface of starch based biodegradable polymers or other polymeric biomaterials. As an alternative to the more typical bioactive glass percursors, a sodium silicate gel is being employed as a nucleating agent, for inducing the formation of a calcium-phosphate (Ca-P) layer. The method has the advantage of being able to coat efficiently both compact materials and porous 3D architectures aimed at being used on tissue replacement applications and as bone tissue engineering scaffolds. This treatment is also very effective in reducing the incubation periods, being possible to observe the formation of an apatite-like layer, only after 6 h of immersion in a simulated body fluid (SBF). The influence of the SBF concentration on the formation of the apatite coating was also studied. The apatite coatings formed under different conditions were analyzed and compared in terms of morphology, chemical composition and structure. After the first days of SBF immersion, the apatite-like films exhibit the typical cauliflower like morphology. With increasing immersion times, these films exhibited a partially amorphous nature and the Ca/P ratios became very closer to the value attributed to hydroxyapatite (1.67). The obtained results are very promising for pre-calcifying bone tissue engineering scaffolds. Therefore, in order to study cell behavior and response to these apatite coatings, adhesion, morphology, and proliferation of a human osteoblast cell line (SaOS-2) was also analyzed after being cultured in the coatings formed after 15 days of immersion in SBF. Results indicate a good correlation between crystallinity of the apatite like coatings formed in these conditions and respective cell spreading and morphology. In general, higher cell proliferation was observed for higher crystalline Ca-P coatings.

Evaporative Deposition of Aluminum Single Crystals

ABSTRACTThe classic zone model for growth of evaporated deposits serves a starting point for selecting the process conditions to produce dense crystalline coatings. The columnar structure intrinsic to vapor deposits evolves from the nano-to-millimeter scale as the substrate temperature increases to the melting point. In this paper, the deposition conditions are reviewed for the evolution of a single crystal structure in metal coatings. Experimental results are presented for the electron-beam deposition of aluminum coatings up to 100 μm in thickness. The deposition of a single crystal occurs when the coating temperature exceeds 530 °C, i.e. 85% of the absolute melt point.

B–C–N coatings prepared by microwave chemical vapor deposition

Ternary B–C–N amorphous and crystalline coatings were deposited by microwave chemical vapor deposition using a mixture of NH 3, CH 4, B 2H 6 and H 2 gases at substrate temperatures in the range 800–1350°C, and a gas pressure in the range (9.0–12)×10 3 Pa. We studied the influence of deposition conditions (i.e. gas composition, substrate temperature and material) on the coating structure, chemical composition and surface morphology. Both amorphous and polycrystalline coatings were deposited. Amorphous coatings were usually formed at lower substrate temperatures and were non-homogeneous across the coating thickness. Nanocrystalline inclusions of boron carbide, boron nitride and diamond were observed in amorphous B–C–N compounds. Polycrystalline coatings were generally represented by both diamond and boron nitride phases. In one case, a polycrystalline coating with the composition of B 2CN 4 was fabricated. The experimental results are discussed in detail.

Mullite interfacial coatings for SiC fibers

A process for depositing CVD mullite on SiC fibers for enhanced oxidation and corrosion protection and/or to act as an interfacial barrier coating has been developed. Process optimization via systematic investigation of system parameters yielded uniform, crystalline mullite coatings on SiC fibers. Structural characterization has allowed for tailoring of coating structure and therefore properties. Preliminary high temperature oxidation/corrosion testing of the optimized coatings has shown that the coatings remain adherent and protective for extended periods.

Electrochemical synthesis of crystalline and amorphous manganese coatings

Electrochemical synthesis of crystalline and amorphous manganese coatings

Mechanical properties of calcium phosphate coatings deposited by laser ablation

Amorphous calcium phosphate and crystalline hydroxyapatite coatings with different morphologies were deposited onto Ti–6Al–4V substrates by means of the laser ablation technique. The strength of adhesion of the coatings to the substrate and their mode of fracture were evaluated through the scratch test technique and scanning electron microscopy. The effect of wet immersion on the adhesion was also assessed. The mechanisms of failure and the critical load of delamination differ significantly depending on the phase and structure of the coatings. The HA coatings with granular morphology have higher resistance to delamination as compared to HA coatings with columnar morphology. This fact has been related to the absence of stresses for the granular morphology.

Structural and morphological study of pulsed laser deposited calcium phosphate bioceramic coatings: influence of deposition conditions, laser parameters, and target properties.

Calcium phosphate (CaP) bioceramics, especially hydroxyapatite (HA), have been used as coatings on implants owing to their biocompatible properties. The commercial practice for applying HA coating, plasma spraying, has some disadvantages which limit the long-term stability of the implants. Pulsed laser deposition (PLD) is being investigated as an alternative technique. The purpose of this research was to systematically study the effect of various parameters of the PLD process on the properties of CaP coatings. In this study, three types of HA targets and two laser wavelengths were used to make six categories of coatings. Predominantly crystalline HA coatings were produced under all six categories at optimum conditions, although small amounts of minor phases sometimes were found. Sufficient coating/substrate bond strength was also obtained. A wide variety of coating morphologies was obtained, from rather dense and uniform to rough and porous. The important factors that affected the morphology included target properties, vacuum level, deposition temperature, and laser wavelength and energy density. PLD's ability to produce both amorphous and crystalline, and both smooth/dense and rough/porous coatings may be a unique advantage.

Recent progress in the synthesis and characterization of amorphous and crystalline carbon nitride coatings

This review summarizes our most recent findings in the structure and properties of amorphous and crystalline carbon nitride coatings, synthesized by reactive magnetron sputtering. By careful control of the plasma conditions via proper choice of process parameters such as substrate bias, target power and gas pressure, one can precisely control film structure and properties. With this approach, we were able to produce amorphous carbon nitride films with controlled hardness and surface roughness. In particular, we can synthesize ultrathin (1 nm thick) amorphous carbon nitride films to be sufficiently dense and uniform that they provide adequate corrosion protection for hard disk applications. We demonstrated the strong correlation between ZrN(111) texture and hardness in CNx/ZrN superlattice coatings. Raman spectroscopy and near-edge x-ray absorption show the predominance of sp3-bonded carbon in these superlattice coatings.

Bone growth on and resorption of calcium phosphate coatings obtained by pulsed laser deposition.

Three different calcium phosphate coatings of crystalline hydroxyapatite (HA), alpha- and beta-tricalcium phosphate (alpha+beta-TCP), or amorphous calcium phosphate (ACP) obtained by pulsed laser deposition on Ti-6Al-4V were incubated in a potentially osteogenic primary cell culture (rat bone marrow) in order to evaluate the amount and mode of mineralized bone matrix formation after 2 weeks with special emphasis on the type of interfacial structure that was created. Evaluation techniques included fluorescence labeling and scanning electron microscopy. The resistance to cellular resorption by osteoclasts was also studied. Bone matrix delaminated from the ACP coatings, while it remained on the HA and the alpha+beta-TCP coatings even after fracturing. A cementlike line was seen as the immediate contiguous interface with the nondegrading dense HA surface and with the surface of the remaining porous beta-TCP coating. Highly dense and crystalline HA coatings do not dissolve but are capable of establishing a strong bond with the bone matrix grown on top. Chemical and mechanical bonding were considered in this case. Cellular resorption was practically not observed on the HA coatings, but it was observed on the alpha+beta-TCP coatings. Resorption took place as dissolution that was due to the acidic microenvironment.

Preparation of zeolite coatings by direct heating of the substrates

A novel method is proposed for the preparation of zeolite coatings on substrates, which suppresses the reaction in the bulk and promotes that on the substrate by applying a temperature difference between the reaction mixture and the substrate. The substrate is heated directly, with a soldering resistance in this study, while the reaction mixture is kept at a lower temperature by means of a water bath. The method is tested for the case of zeolite 4A synthesis on stainless steel plates from a clear aluminosilicate solution. As a result of suppressed activity, the composition of the solution remains nearly constant and the solution side of the coating always experiences a lower temperature. As a result, the phase transformations of the metastable zeolites can be delayed for extremely long periods of time, depending on the volume of the reaction mixture. The deposition of crystals from the solution onto the surface of the coating is also avoided. Crystalline coatings of zeolite 4A could be prepared by the appropriate selection of the temperatures of the water bath and the substrate. A final treatment, which involves an increase in the temperature of the water bath, may be applied to the coatings at the end of their preparation, in order to remove the sparse amount of impurities observed at certain synthesis times and to increase the crystallinities when necessary. The mass of the zeolite coated on the substrate is shown to increase significantly with respect to that obtained by conventional synthesis when the proposed method is utilized. The method can thus be useful for some applications where the employment of relatively thicker zeolite coatings of metastable phases may be desired.

Comparative characterization of alumina coatings deposited by RF, DC and pulsed reactive magnetron sputtering

In order to investigate the structure of PVD alumina coatings as a function of substrate temperature and deposition conditions, Al 2 O 3 films have been deposited by reactive magnetron sputtering at deposition temperatures between 100 and 600°C on cemented carbide and high speed steel substrates using aluminum targets, which were sputtered in an argon oxygen plasma in RF, DC and pulsed mode, respectively. The influence of deposition temperature and sputtering conditions upon the crystal structure of the films has been investigated by X-ray diffraction, the hardness by microindentation. Furthermore, a correlation between film growth rate and oxygen partial pressure has been determined. The analyses revealed that the oxygen partial pressure during film deposition had strong influence on the resulting deposition rate and crystallinity, which is also significantly influenced by the deposition temperature. The process window for the deposition of crystalline coatings is very narrow for DC sputtering, while it is larger with RF and pulsed plasma sources. Depending on the process parameters oxygen partial pressure and plasma source, amorphous or crystalline γ-Al 2 O 3 films with thicknesses in the range from 1 to 8 μm and hardness values up to 25 GPa have been deposited. Very low oxygen partial pressures led to the co-deposition of aluminum, while none of the chosen process parameters resulted in the formation of crystalline α-Al 2 O 3 .

Deposition of hard crystalline Al2O3 coatings by pulsed d.c. PACVD

Two kinds of pulsed d.c. plasma assisted chemical vapour deposition (PACVD) processes (unipolar and bipolar excitation) were studied to deposit crystalline Al2O3 coatings on steel or WC/Co cemented carbide inserts using aluminium trichloride (AlCl3), nitrous oxide (N2O) or oxygen (O2) and hydrogen/argon gas mixtures. At substrate temperatures as low as 650–700°C and a gas mixture ratio of 1 for N2O/AlCl3, coatings with different contents of α-Al2O3 and γ-Al2O3 besides amorphous alumina could be obtained under unipolar plasma excitation conditions. The ratio of the crystalline phases depends on plasma power density and substrate temperature, and has been determined qualitatively by X-ray diffraction. The dark grey or transparent coatings have a rough, less well crystallized surface in comparison to surfaces known from the thermal CVD process. The coating thickness differed significantly between the centre and the edges of the samples. The crystallite sizes were estimated to be 9 and 20 nm for the α-Al2O3 and even smaller for γ-Al2O3 containing coatings. By increasing the substrate temperature to 800°C, the content of α-Al2O3 in the coatings was increased. Lower substrate temperatures or conditions with a deficiency of oxygen in the gas mixture lead to the formation of aluminium or aluminium nitride.Crystalline α-Al2O3 or very smooth, transparent γ-Al2O3 were obtained at 700°C dependent on the gas mixture ratio using O2/AlCl3 as precursors and bipolar pulsed d.c. plasma excitation. A clear trend becomes apparent for the formation of γ-A2O3 and amorphous alumina films if the substrate temperature is lowered and the O/AlCl3 gas ratio is changed appropriately. The hardness of the smooth, transparent coatings are found to be in the range 20–24 GPa.The wear properties of some smooth, transparent γ-Al2O3 PACVD coatings were evaluated in a machine cutting test using ball bearing steel and compared with thermal CVD alumina coatings. It could be concluded that the bipolar pulsed PACVD-Al2O3 coatings showed very similar wear properties to thermal CVD coatings.

Deposition and characterization of fine-grained W–Ni–C/N ternary films

W–Ni–C/N ternary films with 3<at.% Ni<14 and 0<at.% C, N<30 were synthesized by reactive sputtering from a W–10 wt.% Ni target with increasing partial pressures of methane or nitrogen and substrate bias. The films have been characterized by electron probe microanalysis, X-ray diffraction, ultramicrohardness and scratch testing. The results show that the degree of structural order of the films decreases with increasing nickel and metalloid element contents. The hardness values obtained vary between 25 and 55 GPa, with a maximum for films with low nitrogen and carbon contents (6–8 at.%), deposited with a substrate bias of −70 V. The Young modulus follows the same trend as hardness, with a maximum modulus value of 550 GPa. The crystalline coatings present higher critical loads than the amorphous coatings. The more adherent coatings have critical loads higher than 70 N.

Deposition of hard crystalline Al2O3 coatings by bipolar pulsed d.c. PACVD

Abstract A bipolar pulsed d.c. PACVD technique was used to deposit crystalline Al2O3 coatings onto WC/Co cemented carbide cutting inserts and onto steel substrates using a gas mixture of AlCl3, H2, O2 and Ar. Different deposition conditions were tested, substrate temperatures (Ts) between 500 and 700°C and a gas mixture ratio O/AlCl3 in the range from 2 to 20. At Ts≥600°C transparent crystalline coatings of the γ-Al2O3 phase were obtained having the best crystallinity for O/Al ratio of 4 and 6. At a substrate temperature of 700°C and an O/AlCl3 ratio of 2, α-Al2O3 or a mixture of α- and γ-Al2O3 phases was formed. The microhardness HV[0.02] of the crystalline coatings was found to be in the range 19–23 GPa. The γ-Al2O3 layers appeared in most cases transparent with smooth surfaces. The wear properties of Al2O3-coated cutting inserts produced by the PACVD method were evaluated and compared with a thermal CVD κ-Al2O3 coating in a machining test in a ball bearing steel. From the test results it could be concluded that the PACVD Al2O3 coatings showed wear properties very similar to the thermal CVD κ-Al2O3 coatings.

Recent advances in the synthesis and properties of amorphous and crystalline carbon nitride†

As a result of the prediction by Liu and Cohen that crystalline β‐C3N4 has mechanical properties similar to diamond, many research groups have attempted to synthesize carbon nitride using various deposition techniques. Most groups succeeded in producing amorphous carbon nitride coatings. These coatings, though softer than diamond, display excellent tribological properties and are now used as protective overcoats for computer hard drive systems. The lack of success to make crystalline coatings suggests that β‐C3N4 is metastable. Using TiN(111) and ZrN(111) as seeding layers (which have similar lattice symmetry and commensurate unit cell dimensions as β‐C3N4(001)), we were able to synthesize fully crystalline carbon nitride composite coatings with hardness in the 50 GPa range, which is in the low end of diamond films.

Deposition of hydroxyapatite thin films by excimer laser ablation

The influence of the pressure of a sole water atmosphere during the pulsed laser deposition of hydroxyapatite thin films on Ti–6Al–4V substrates has been studied. The rest of the technological parameters involved in the process have been fixed near the conditions where the best crystalline coatings are obtained. The pressure of the water atmosphere has been varied between 0.15 and 1.5 mbar. The films properties have been analysed by means of XRD, SEM, FT-IR spectroscopy and SIMS. An optimal region, in order to obtain thin films of highly crystalline hydroxyapatite, has been found near 0.5 mbar for the two excimer laser wavelengths (193 nm and 248 nm) used in this study. These films have a preferential orientation in the (100) direction.

A new coating process integrated in an innovative coating system for production of well-adherent diamond coatings

Coating complex shaped tools with crystalline diamond coatings is technologically and economically an interesting alternative to conventional PCD-techniques. The coating unit CC800D equipped with a u.s.e. ™-Process module for diamond coating enables production of diamond coated tools with reliable quality on an industrial scale. Lifetime tests show excellent performance of diamond coated tools.

Dissolution kinetics of calcium phosphate coatings.

Plasma spray and high velocity oxy-fuel (HVOF) techniques produce coatings with varying composition and amounts of amorphous and crystalline phases. For coatings containing greater amorphous phases, a higher release of calcium ions is evident when samples are placed in Hank's calcium-free balanced salt solutions. Calcium is released from the amorphous phases in the coating, a conclusion that is supported by x-ray powder diffraction (XRD) results. Ion beam sputtering and RF magnetron sputtering under lower energy conditions produce amorphous coatings that will dissolve in a very short time period. When heat treated, crystalline phases are produced in the coatings. Heat-treated coatings are significantly more stable than the amorphous coatings. The dissolution rates of both amorphous and crystalline coatings produced by RF magnetron sputtering have been measured under constant solution conditions at pH 6.50. No reprecipitation is possible under these conditions. The amorphous coating dissolved at a significantly higher rate than the heat-treated coating. Reprecipitation of calcium phosphate onto amorphous coatings is possible in a physiological pH solution. Under these conditions, the dissolution rate of the amorphous coating is four times slower than at the pH 6.50 conditions.