Chromium Oxide Coatings Research Articles

Interfacial Microstructure of Chromium Oxide Coatings

Chromium oxide coatings are chemically inert, have high mechanical strength, hardness and good optical characteristics; therefore, they have been widely used in many applications including corrosion protection, wear resistance, electronics, and optics. Cr2O3 is well suited for wear resistance applications, as it is one of the hardest oxides with 29.5 GPa hardness. Several deposition techniques have been tried for making these coatings. Cr2O3 coating hardness can vary substantially due to compositional and microstructural variations, depending on the deposition method. Hardness of a plasma-sprayed Cr2O3 coating, 50 lm thick was about 14.7 GPa, while a 200 nm thick RF-sputtered chromium oxide coating, stoichiometrically close to Cr2O3, exhibited 30 GPa hardness combined with good scratch resistance. Even for the bulk Cr2O3, hardness values reported were from 9 GPa to 29.5 GPa. Hones et al. investigated a correlation between the hardness and the sputtering deposition parameters, i.e. oxygen partial pressure and substrate temperature, and found favorable deposition conditions with an oxygen partial pressure of about 15–20 % of the total sputtering gas pressure at substrate temperatures exceeding 500 K. Good coating adhesion is required for wear and corrosion resistance applications. Premature failure can occur for many reasons including coating delamination, cracking and plastic deformation. In addition to this, thin ceramic PVD coatings usually have columnar grain structure with micro cracks, pinholes, transient grain boundaries and often high throughcoating porosity, which all lead to accelerated pitting corrosion and failure at the coating/substrate interface, especially in hostile environments. On the other hand, several studies showed that coating thickness plays an important role in enhancing both PVD-coated tool cutting performance and resistance to abrasive and erosive wear. Graded systems have been employed to obtain thicker coatings without losing performance in terms of coating adhesion and toughness. It is likely that thicker coatings will improve corrosion resistance in aqueous environments by eliminating through-thickness pin-hole defects. Coating mechanical, adhesion and wear properties are strongly affected by microstructure. Interfaces with high adhesion are known to ensure prolonged coating life and good wear resistance. Sputtered coating microstructure and physical characteristics depend on the deposition parameters. Also, substrate surface conditions prior to deposition, characterized by surface roughness, stress and oxidation state, play an important role in controlling coating properties. In this paper SEM and TEM techniques were used to characterize thicker chromium oxide coating interfacial microstructure as a step towards developing a unique method for depositing thicker coatings with small grains, smooth surface and low residual stress.

Role of deposition parameters on microstructure and mechanical properties of chromium oxide coatings

Chromium oxide coatings were deposited by reactive magnetron sputtering on high speed steel (HSS) substrate under various oxygen flow rates and radio frequency (RF) powers. The effect of deposition conditions on the microstructure, hardness and critical load of chromium oxide coating failure was studied. The results indicated that a crystalline chromium oxide coating formed at a high oxygen flow rate and a low RF power exhibited a higher hardness and a lower critical load as compared to a chromium oxide coating with an amorphous microstructure.

Preparation and Characterization of Plasma-Sprayed Ultrafine Chromium Oxide Coatings

Ultrafine chromium oxide coatings were prepared by plasma sprayingwith ultrafine feedstock. Processing parameters of plasma sprayingwere optimized. Optical microscope (OM) was used to observe themicrostructure of the ultrafine chromium oxide coatings. Scanningelectron microscopy (SEM) was used to observe the morphology andparticle size of ultrafine powder feedstock as well as to examinethe microstructure of the chromium oxide coating. In addition,hardness and bonding strength of the ultrafine chromium oxidecoatings were measured.The results showed that the optimized plasma sprayingparameters were suitable for ultrafine chromium oxide coating andthe properties and microstructure of the optimized ultrafinechromium oxide coating were superior compared to conventionalchromiumoxide wear resistant coatings.

Correlation between surface roughness of plasma-sprayed chromium oxide coatings and powder grain size distribution: a fractal approach

Attempts were made to correlate the arithmetic mean surface roughness R a and the 10-point height values R z of atmospheric plasma-sprayed (APS) chromium oxide coatings with fractal properties such as the relative area for 2 μm 2 patch size (RAPS), the smooth-rough crossover (SRC) and the area-scaled fractal complexity (ASFC). In particular, the measured surface roughness of the coatings correlates with high statistical probability with the grain size distribution of the selected spray powders. It was found that the SRC value is useful to characterize the low and high cutoff values of the grain size distribution of thermal spray powders. Hence fractal analysis is a potential analytical tool to quantify the geometric complexity of the surface structure of plasma-sprayed coatings as a function of scale of observation.

Aluminum Oxide and Chromium Oxide Coatings on Ceramic Fibers via MOCVD

The overall strength and toughness of ceramic matrix composites is very much affected by the interfacial properties that govern the bonding between the reinforcing fibers and the matrix. The ability to control this interaction is very important. One method involves coating the ceramic fibers with an appropriate material. There is a definite advantage in using lower-temperature processes to produce the coatings: fibers subjected to high-temperature coating processes are more prone to degradation due to microstructure and crystallization changes. Matrix and fiber materials could also react at high temperatures to form a strong bond at the interface making the composite brittle. Metal−organic precursors were used to produce aluminum oxide and chromium oxide coatings on SiC and Al2O3−SiO2 based ceramic fibers. These metal oxides were deposited on the reinforcing fibers by the pyrolysis in air of their respective metal acetylacetonates at temperatures of 500 °C or less. The coated ceramic fibers showed some r...

Oxidation-induced degradation of strength of tungsten carbide-cobalt

Mixed aluminum–chromium oxide coatings in the form of (Al,Cr)2O3 solid solutions have attracted extensive research interest during the past years due to their successful use for challenging wear applications and the comparative ease of their moderate-temperature deposition by physical vapor deposition techniques. During our research into the reactive cathodic arc deposition of this type of coating, we found a previously unobserved transition between two crystalline aluminum–chromium oxide structures. During the early growth stage, films arc-deposited from Al0.55Cr0.45 targets form a first zone, that was found to contain exclusively the metastable cubic fcc-(Al1 − xCrx)2 + δO3 phase. This kinetically favored phase is reproducibly followed by the growth of a second zone made of the initially expected corundum phase, α-(Al1 − xCrx)2 + δO3, as observed by TEM. This dynamic transition has a significant effect on the film properties. XPS studies and structural data show that the formation of fcc-(Al1 − xCrx)2 + δO3 with a (200) preferred orientation arises from the initial presence of a metastable monoxide (M1 − xO) film, which is stabilized by the incorporation of metal vacancies (31%) in the B1 structure. However, as the thickness of coating increases, the thermodynamic aspect becomes more important as compared to kinetics and leads to a loss of structural stability in the cubic layer, which is a kinetically favored phase. As a result, the system will transform into the metastable corundum α-(Al1 − xCrx)2 + δO3, which is thermodynamically more stable than the cubic phase. In this paper, formation of fcc-(Al1 − xCrx)2 + δO3 and its transformation to corundum phase are discussed in detail with respect to the structural and electronic properties of the different phases.

Microstructural Study of Aluminum Phosphate-Sealed, Plasma-Sprayed Chromium Oxide Coating

Microstructural characterization of aluminum phosphate-sealed, plasma-sprayed chromium oxide coating was carried out in order to study the strengthening mechanisms of the aluminum phosphate sealant in the coating. Characterization was performed using x-ray diffractometry, scanning electron microscopy, and analytical transmission electron microscopy. The structure of the sealed coating was lamellar with columnar α-Cr2O3 grains extending through the lamella thickness. Amorphous aluminum phosphate sealant had penetrated into the structural defects of the coating such as cracks, gaps, and pores between the lamellae. The relative composition was 25 at.% aluminum and 75 at.% phosphorus for the sealant in the coating, giving the molar ratio P/Al of 3, which corresponds to that of metaphosphates Al(PO3)3. There is no indication of reaction products from the chemical reactions between the sealant and the coating. Thus, the aluminum phosphate sealing in the chromium oxide coatings can be explained mainly by adhesive binding resulting from the formation of the condensed phosphates with the appropriate adhesive properties to the coating, and not by chemical bonding resulting from the chemical reactions between the sealant and the coating.

Bond Strength of Plasma Sprayed Chromium Oxide Coatings

Plasma spraying is an attractive method to protect the components from severe conditions. Chromium oxide coatings were used to develop the wear resistance because of their good wear properties. One of the problems of plasma sprayed coating is the bond strength. As an effort to improve the bond strength, the Cr 2 O 3 coatings formed on mild steel substrates by the plasma spraying were subjected to annealing and hot press treatment. After each treatment, the bond strengths were compared with the as-sprayed coatings. The bond strength, which was not improved by only annealing, was improved by incorporating the composite layer. Annealed coating after forming the composite layer showed about 14 MPa higher bond strength than that of without the composite layer.

Use of a theoretical model to investigate RF and DC reactive sputtering of titanium and chromium oxide coatings

An analysis of RF and DC reactive sputtering techniques is presented. The transition between a metal sputtering mode and a compound sputtering mode is usually noticed with a metallic target and an argon+oxygen gas mixture. The so-called hysteresis effect often observed for small amounts of reactive gas is explained in recent models. By considering gas kinetics parameters, it is possible to evaluate quite simple relationships between the main processing parameters. These theoretical calculations enable the prediction and aid the understanding of instability phenomena observed in reactive sputtering. In this paper, the effects of some parameters on the position and size of instability regions are discussed, and the difference between DC and RF reactive deposition is investigated. Simulations and experimental results are compared for the case of titanium and chromium oxide thin films prepared by DC and RF reactive sputtering. The influence of sputtering power on the position of the hysteresis loop is analysed theoretically and experimentally, and the changes observed between the reactive sputtering of titanium and chromium oxide materials are also discussed.

Tribological studies of chromium oxide films for magnetic recording applications

Hard coatings are commonly used in magnetic heads and media for protection against corrosion and wear. Chromium oxide coatings have been developed for protection of the tape bearing surface of digital compact cassette (DCC) heads. Based on initial screening tests and lifetime tests, a 50-nm thick rf-sputtered chromium oxide coating exhibits best durability. Surface analytical techniques used for structural and chemical characterization show that chrome oxide coating is polycrystalline and with stoichiometry close to Cr 2O 3. Micromechanical characterization was carried out using a nanoindenter to measure hardness, Young's modulus and scratch resistance. The coatings exhibited hardness as high as 30 GPa combined with a good scratch resistance. A pin-on-disk tester was used to measure friction and wear performance. Since these coatings are hard, scratch and wear resistant (with a low coefficient of friction), they should find many other industrial applications such as magnetic rigid disks and magnetic disk heads.

Wear and corrosion properties of plasma sprayed AI2O3 and Cr2O3 coatings sealed by aluminum phosphates

Plasma-sprayed aluminum oxide (AI2O3) and chromium oxide (Cr2O3) coatings were sealed by aluminum phosphates. Phosphates were formed throughout the coating, down to the substrate, and were verified by scanning electron microscopy and hardness measurements. The sealing increased the hardness of the coatings by 200 to 300 Vickers hardness (HV) units. Abrasion and erosion wear resistances were increased by the sealing treatment. Sealing also substantially closed the open porosity, as shown in electro-chemical corrosion tests. The sealed structures had good resistance against corrosion during 30 days of immersion in both acidic and alkaline solutions with pH values from 0 to 10. No decrease in abrasion wear resistance was observed after immersion.

Electrochemical Chromium(III) Oxide Coatings on Non‐oxide Ceramic Substrates

SiC and TiB2 were electrochemically coated with Cr2O3 from a 0.1M aqueous solution of chromium nitrate hydrate with ethanol additives. On both substrate materials poly‐crystalline Cr2O3 was formed at current densities from 5 to 50 mA/cm2 and deposition durations of 5 to 30 min. The coating weight increased with current density and with deposition time. The as‐deposited coatings contained microcracks due to drying shrinkage. Microstructural observations indicate that sintering of the Cr2O3 coatings on TiB2 at 1100°C for 1 n in a reducing atmosphere in a closed graphite crucible causes the densification of the coating via a liquid phase, which forms by oxidation of TiB2. Under similar conditions, the Cr2O3 coatings on SiC may be sintered via an evaporation–condensation mechanism.

X-ray emission and prebreakdown currents in plain and dielectric bridged vacuum gaps under DC excitation

X-ray activity in both plain and solid insulator bridged-vacuum gaps was investigated under DC stresses. The gap conditions under which X-ray emission takes place, and the correlation between X-ray activity and predischarge current were studied. Steady X-ray activity was observed for a plain gap with electrodes subjected to repeated breakdowns, and for gaps bridged with insulators having low secondary emission yields. The hysteresis in the V-I characteristics of both plain and bridged vacuum gaps is attributed to a regenerative feedback process between X-ray and photoemission processes. The nonresistive component of predischarged current for bridge vacuum gaps obeys the Fowler-Nordheim theory only in the high-field region (>60 kV/cm), and only in the absence of X-ray activity. Chromium oxide coatings on Wesgo AL-300 alumina ceramic gave a substantial improvement (>100%) in the voltage hold-off, but at the expense of increased prebreakdown current and X-ray activity. It has been shown that chromium oxide coatings do not improve the voltage hold-off unless a dense, low porosity, surface is produced by the coating.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Friction and wear behavior of chromium carbide coatings

Chromium carbides, tungsten carbide and chromium oxide have been tested and evaluated as coatings to protect high temperature gas-cooled reactor (HTGR) steam generator and other HTGR components from adhesion, galling associated with sliding wear or from fretting. Tests were performed in commercially pure helium and in helium doped with various gaseous impurities (H 2, H 2O, CH 4, CO) to stimulate the primary coolant of an HTGR. Several types of chromium carbide coatings including Cr 3C 2, Cr 7C 3 and Cr 23C 6 were tested for wear resistance and resistance to long-term spalling. Tungsten carbide and chromium oxide coatings were tested in sliding wear tests. Cr 23C 6-NiCr coatings showed the best performance (from 400 to 816 °C) whether they were applied by detonation gun or plasma gun spraying methods. The presence of the Cr 23C 6-NiCr coatings did not affect the creep rupture properties of alloy 800H substrates at temperatures up to 760 °C. Low-cycle fatigue life of similar specimens at 593 °C was reduced by 10% – 20% when tested in the 1% – 0.6% strain range.

Electrical conductivity in sputter-deposited chromium oxide coatings

The electrical conductivity and thermal stability of sputtered chromium oxide films were investigated in order to assess their potential as protective coatings for high temperature battery electrode materials. Sheet conductivities as high as 10–15 Ω -1 cm -1 were achieved in air at a temperature of 350 °C. Film property changes resulting in reduced electrical conductivity (several orders of magnitude) occurred when the films were heated in vacuum. Electrical conductivity and electron beam methods were used to study the nature of the film instability. It is proposed that the instability is related to both structural and compositional (oxygen) changes in the sputter-deposited films.

Characterization of r.f.-sputter-deposited chromium oxide films

An r.f. sputtering technique with varying sputtering parameters was employed to apply chromium oxide (Cr 2O 3) coatings. The influence of the sputtering parameters on the composition of the coating and on the structure was studied by Auger electron spectroscopy (AES) and X-ray diffraction analyses. The AES technique examines only the surface layer a few ∢ngströms thick (the “altered layer” in the case of multicomponent coatings) and can give erroneous results; however, Auger depth analysis can indicate whether there is any variation in composition throughout the coating. Auger analysis indicated that the coatings examined were uniform throughout their depth and that variations in sputtering parameters produced no variations in composition. In the case of bias-sputtered coatings, no naturally occuring chromium-rich oxide layer appeared at the interface; this condition may be responsible for the poor adhesion. X-ray diffraction analysis showed that the applied coating was amorphous and that it crystallized to Cr 2O 3 after some heat treatment.

The Effect of Chromium Oxide Coatings on Surface Flashover of Alumina Spacers in Vacuum

An experimental investigation of the effect of chromium oxide coatings upon the surface flashover of high-density alumina in vacuum is reported. It is shown that such coatings improve the withstand voltage for dc, 60-Hz, and impulse voltages by significant amounts. The coatings are simply applied, effective, and durable. It is postulated that the improvement in insulation is due to the elimination of positive surface charging by the use of a coating with a secondary-electron emission coefficient less than unity for all incident electron energies.