Sliding wear characteristics of plasma-sprayed Cr2O3 coatings with incorporation of metals and ceramics

Different additives (metals, ceramics, and metal/ceramic assembles) were incorporated into chromium oxide (Cr₂O₃) coatings to improve its wear resistance. The doping or co-doping effects were compared in terms of the microstructure, phase composition, microhardness and sliding wear properties of plasma-sprayed Cr₂O₃-7wt.% Mo, Cr₂O₃-7wt.% Nb₂O₅, Cr₂O₃-4wt.% MoO₃-3wt.% Mo, and Cr₂O₃-4wt.% Nb₂O₅-3wt.% Mo coatings. Under the low sliding loads, the composite coatings mated with WC-Co counterparts have lower friction coefficients than those against Si₃N₄, which are inverse with their microhardnesses, but more obvious fatigue wear characteristics. Under the high sliding loads, Mo/Nb₂O₅ co-doped Cr₂O₃ coating (CNM) has the best wear resistance than other coatings, due to the delaying co-effects of crack formation and propagation on basis of the crack deflection and the toughening effects of Mo additives and the high hardness of Nb₂O₅ additives. As the reciprocating sliding loads exceed the critical stress of brittle Cr₂O₃-based coatings, the coating detachments occur, displaying obvious fatigue wear characteristics.     

Tribological properties of selected ceramic coatings

The paper presents the characteristics of some ceramic coatings obtained by a plasma spray method. The ceramic coatings Al₂O₃, Cr₂O₃ and Cr₂O₃?+?5% TiO₂ were evaluated. Also the influence of the NiCr interlayer on the functional properties of sprayed coatings was studied. Other parameters studied included: thickness; microhardness; adhesion of the coatings; resistance to abrasive wear and thermal cyclic loading. The addition of TiO₂ to the Cr₂O₃ material increased the coating density, but did not substantially reduce the hardness. On the other hand, the lowest loss of material thickness was seen for Cr₂O₃; while the Al₂O₃ and the Cr₂O₃?+?5 wt.% TiO₂ material showed a higher loss. The loss in the case of the latter two was about the same. Relatively, higher values of abrasive wear resistance were observed in the Cr₂O₃ coatings, as compared to the reference material (Al₂O₃ coating), and the highest microhardness values were measured in the Cr₂O₃ coating. Finally, the metal interlayers in all coatings increased their resistance to thermal shock. All the coatings, using the interlayer to reduce differences in coefficients of thermal expansion, were suitable for the purpose of the thermal loading up to 1000?°C.     

Influence of refractory oxide additives on the stability of alumium titanate

Conclusions Batch additives incorporated in the form of various oxides as a means of improving the physicochemical properties of aluminum titanate can be divided into two groups. The first consists of additives which prevent the decomposition of the titanate at temperatures below 1200°C, and stabilize its high-temperature modification. In terms of the degree of activity these oxides are arranged in the series: MgO, TiO₂, SiO₂, Cr₂O₃, La₂O₃. It is possible to assume that their influence is due mainly to the formation of limited solid solutions, and also the development during sintering of impurity grain boundary phases. The second group of oxides (SnO₂, NiO, excess Al₂O₃, etc.) contributes to the sintering of aluminum titanate by forming secondary crystalline phase on the grain boundaries of the ceramic. In this connection in obtaining sintered ceramic materials based on aluminum titanate it is preferable to consider additives of the first group or their combinations with additives of the second group.Translated from Ogneupory, No. 4, pp. 29–31, April, 1987.     

Comparison of dry sliding wear behavior of plasma sprayed FeCr slag coating with Cr2O3 and Al2O3-13TiO2 coatings

The microstructure and dry sliding wear performance of thermally sprayed FeCr slag coating were evaluated in comparison with those of commercially available Al₂O₃-13TiO₂ and Cr₂O₃ ceramic coating powders to assess the applicability of FeCr slag (FS) powder, fabricated from industrial waste, as a ceramic top-coating material against wear. Ceramic top coats and underlying NiCoCrAlY bond coats were deposited on AISI 316L samples via atmospheric plasma spraying (APS), and their tribological properties were assessed using a ball-on-disc test rig at room temperature. As a result, FS coating exhibited the lowest worn volume, although it has the lowest surface hardness. Tribolayer formation was observed on the surface of the samples which were subjected to dry sliding wear tests. Delamination type wear is the dominant wear mechanism for Cr₂O₃ and FS coatings, whereas local spallation areas arising from plastic deformation were observed on the surface of Al₂O₃-13TiO₂ coatings. The results suggested the applicability of FS powder as a candidate ceramic top coating material against wear.     

Effects of Sintering Aids on the Transparency and Conversion Efficiency of Cr4+ Ions in Cr: YAG Transparent Ceramics

Tetravalent chromium‐doped Y₃Al₅O₁₂ ceramics were fabricated by solid‐state reactive sintering method using high‐purity Y₂O₃, α‐ Al₂O₃, and Cr₂O₃ powders as the starting materials. CaO and MgO were co‐doped as the sintering aids. The effects of TEOS and divalent dopants (CaO and MgO) on the optical qualities, the conversion efficiency of Cr⁴⁺ ions, and the microstructure evolutions of 0.1 at.% Cr⁴⁺: YAG ceramics were investigated. Fully dense, dark brown colored Cr⁴⁺: YAG ceramics with an average grain size of 3.1 μm were achieved. The in‐line transmittance of the as‐prepared ceramic at 2000 nm was 85.3% (4 mm thick), and the absorption coefficient at 1030 nm (the characteristic absorption peak of Cr⁴⁺ ions) was as high as 3.7 cm^?1, which was higher than that of corresponding single crystals fabricated by Czochralski method.     

The effect of alumina-based sintering aid on the microstructure,selected mechanical properties,and coefficient of friction of Cf/SiC composite prepared via spark plasma sintering (SPS) method

C_f-SiC air brake discs are being developed due to their high-temperature oxidation resistance compared to conventional C_f/C discs. The C_f-SiC air brake discs should have a coefficient of friction (COF) close to 0.4, a low wear rate, a density higher than 95% of the theoretical density, and flexural strength of more than 200 MPa. To reach the properties of C_f-SiC composite to the required characteristics of the air brake disc, different amounts of alumina-based sintering aid were used. For this purpose, first silicon carbide nanoparticles, sintering aids Al₂O₃–MgO, MgAl₂O₄, Al₂O₃–Y₂O₃, Al₂O₃–SiO₂–MgO, and carbon fiber (20 wt%) with a 5-mm length were prepared. Next, the final composite bulk was created via the SPS method at 1900 °C under a pressure of 50 MPa. The density of the sample sintered with the Al₂O₃–SiO₂–MgO sintering aid was higher than that of other sintering aids. The density value was obtained at 98% and 100% at 8 wt% and 4 wt% respectively. It was also found that the use of 4 wt% of Al₂O₃–SiO₂–MgO offered better mechanical properties compared to 8 wt%, due to the absence of Al₈Si₄O₂₀ phase at 4 wt%. The examination of mechanical properties showed that the hardness (3564 Vickers) and flexural strength (479 MPa) of the sample with the Al₂O₃–SiO₂–MgO sintering aid were higher than those of other sintering aids. The samples with the Al₂O₃–SiO₂–MgO sintering aid with 4 wt% revealed a COF of 0.41, showing the closest feature to the desired indices of aircraft brake discs.     

Volumetric nucleation of crystals catalyzed by Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> in glass based on furnace slags

The features of the volumetric nucleation of crystals in glass obtained by melting furnace slags with the additive of SiO₂, chromium sesquioxide Cr₂O₃, are studied by the methods of differential thermal and Xray phase analysis and optical microscopy. Upon the introduction of Cr₂O₃ as the catalytic additive, two phases are sequentially formed in the glass: magnesiochromite (MgO · Cr₂O₃) and diopside (CaO · MgO · 2SiO₂). The characteristics of homogeneous and heterogeneous crystallization are determined: the stationary nucleation rate, nonstationary nucleation time, crystal growth rate, and their temperature dependences are obtained. Practical recommendations on the use of the obtained glass are given.     

Influence of TiO2, Cr2O3 and ZrO2 on sintering and machinability of fluorophlogopite glass ceramics

Abstract

The influence of Cr₂O₃, TiO₂ and ZrO₂ on the sintering, crystallisation and machinability of SiO₂–Al₂O₃–MgO–K₂O–B₂O₃–F glasses was investigated. Optimum fluoromica glass ceramic compositions with desirable sintering behaviour and machinability were obtained by addition of titanium and chromium oxides to the base glass. Texture and relative intensity of mica phase I_mica/I_Si were determined by XRD analysis and the particle size distribution of chips was studied by drilling. Microhardness and bending strength were also investigated. The relative intensity of mica phase and microhardness were found to be compatible with the experimental results.     

Intensification of Sintering of Mullite-Cordierite Ceramics Using Mineralizers

The effect of various mineralizers on the process of sintering of ceramic materials in the MgO – Al₂O₃ – SiO₂ system is investigated. The possibility of decreasing the firing temperature and expanding the firing interval of cordierite-mullite ceramics by using these mineralizers is demonstrated.     

INVESTIGATION OF REACTIONS OF SIMPLE MAGNESIA SPINELS WITH ALKALINE EARTH ORTHOSILICATES IN THE SOLID STATE *

The spinels MgO Al₂O₃, MgO Cr₂O₃., and MgO Fe₂O₃ and the orthosilicates 2MgO SiO₂, 2CaO SiO₂, 2SrO SiO₂, and 2BaO SiO₂ were synthesized by solid-state reaction of the component oxides or compounds yielding the oxides on ignition. Each of the spinels was tested at various temperatures with each orthosilicate in turn by placing a pellet of each in contact and also by intimately mixing the two. Petrographic microscopy and X-ray diffraction were employed in the identification of reaction products.     

The phase,microstructure evolution and the Nd3+ function in the fabrication process of LuAG transparent ceramics

Nd:LuAG transparent ceramics were fabricated by the solid-state reaction under vacuum sintering using SiO₂ and MgO as sintering aids, commercial Lu₂O₃, Al₂O₃ and Nd₂O₃ as raw materials. The Nd doping concentration was adopted from 0 at. % to 1.3 at. %. The phase transformation and microstructure evolution of 1.3 at. % Nd:LuAG ceramics under different sintering temperature was investigated in detail. Meanwhile, the effects of Nd₂O₃ on the grain growth were surveyed. The results shown that when the samples were sintered at 1780?°C, the 1.3 at. % Nd:LuAG ceramic had clean gain boundary, and the transmittance of it reached 83.8% at 1064?nm.     

Fusion-Cast High-Chrome Refractories Synthesized in the Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> - MgO - SiO<Subscript>2</Subscript> System

The synthesis and comprehensive study of fusion-cast refractories based on the Cr₂O₃ - MgO - SiO₂ system containing more than 60% Cr₂O₃ are reported. Analyzed by x-ray diffractometry and petrography, the synthesized materials display a phase composition represented by a complex spinel and escolaite. Tested for stability by molten alkali-free borosilicate E-glass, the synthesized refractories are shown to be not inferior in corrosion resistance to a chromium oxide-based ceramic refractory. __________ Translated from Novye Ogneupory, No. 10, pp. 68 – 74, October, 2005.     

The Structure of Fusion-Cast High-Chrome Oxide Refractories in the Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> - MgO - Al<Subscript>2</Subscript>O<Subscript>3</Subscript> System

The phase composition and structure of fusion-cast refractories composed of 57.0 – 84.2% Cr₂O₃, 4.3 – 36.1% MgO, 2.0 – 9.7% Al₂O₃, and 2.4 – 6.9% SiO₂ have been studied by petrographic and x-ray spectral microprobe analysis methods. Refractories high in MgO with modulus M = (Cr₂O₃ +Al₂O₃)/MgO = 1.64 – 3.1 are shown to consist of spinel phase Mg(Cr, Al)₂O₄ and silicate glass. Refractory materials (80.8 – 84.2% Cr₂O₃, 4.3 – 4.7% MgO, 2.0 – 9.7% Al₂O₃, and 2.7 – 6.9% SiO₂ with M = 18.7 – 20.2) are three-phase systems composed of spinel, escolaite, and glass phase. These materials, owing to their high corrosion resistance, have promising potentiality for practical applications.__________Translated from Novye Ogneupory, No. 12, pp. 69 – 74, December, 2004.     

Role of Support and Promoter in the Selective Conversion of Isophorone to 3,5-Xylenol over Chromia Catalysts

A detailed study has been undertaken regarding the role of support and promoter towards 3,5-xylenol selectivity improvements in the vapor-phase aromatization of isophorone. The supports used in this study include A1₂O₃, SiO₂, ZrO₂, MgO and carbon-covered alumina for Cr₂O₃ catalysts. K₂O has been used as a promoter for these catalysts. Influence of solvent during Cr₂O₃/MgO catalyst preparation step has also been studied. The parameters governing the aromatization selectivity observed from this study are: moderate acidic sites, coordinatively unsaturated sites of Cr₂O₃ and smaller crystallites of Cr₂O₃.     

Microhardness and wear resistance of Al2O3-TiB2-TiC ceramic coatings on carbon steel fabricated by laser cladding

Al₂O₃-TiB₂-TiC ceramic coatings with high microhardness and wear resistance were fabricated on the surfaces of carbon steel substrates by laser cladding using different coating formulations. The microstructures of these ceramic coatings with the different coating formulations were investigated using X-ray diffraction, scanning electron microscopy, and energy dispersive spectrometer. The wear resistance and wear mechanism were analyzed using Vickers microhardness and sliding wear tests. The results showed that when the amount of independent Al₂O₃ was increased to 30%, the ceramic coatings had a favorable surface formation quality and strong metallurgical bond with the steel matrix. The cladding layer was uniformly and densely organized. The black massive Al₂O₃, white granular TiB₂, and TiC distributed on the Fe substrate significantly increased the microhardness and wear resistance. The laser cladding ceramic coating had many hard strengthening phases, and thus resisted the extrusion of rigid particles in frictional contact parts. Therefore, the wear process ended with a “cutting-off” loss mechanism.     

Effectively controlling the crystal growth of Cr2O3 using SiO2 as the second phase

The inevitable crystal growth of Cr₂O₃ during sintering causes the generation of cracks, which degrades the high-temperature properties. To solve this, SiO₂ is adopted as the second phase and the specimens are sintered at 1200-1500°C under buried carbon condition. The results show that the addition of approximate 20 wt% SiO₂ can effectively control the crystal growth of Cr₂O₃. The Cr₂O₃ particle size can keep uniform ranging from 4 μm to 12 μm even when the temperature increases to 1500°C. The sintering of Cr₂O₃ mainly follows the defect model, which depends on the reaction temperatures and atmospheres. This work should also contribute to the sintering of other oxide refractories with controlled crystal size for practical application.     

Effect of different contents of zirconia and yttrium oxide on microstructure and properties of high alumina ceramics

The effects of zirconia and yttrium oxide addition on microstructure, bulk density, microhardness, flexural strength, and wear resistance of high alumina ceramics (>97 wt% Al₂O₃, MSA ceramics) composed of MgO–SiO₂–Al₂O₃ system have been investigated. The results show that the addition of zirconia makes the mechanical properties and wear properties of ceramics composed of MgO–SiO₂–Al₂O₃–ZrO₂ (MSAZ ceramics) system have been greatly improved compared with MSA ceramics. In addition, the ceramics composed of MgO–SiO₂–Al₂O₃–ZrO₂–Y₂O₃ (MSAZY ceramics) system have better mechanical properties and wear properties than MSAZ ceramics. With the contents of zirconia and yttrium oxide increase, the bulk density, microhardness, and flexural strength of MSAZ and MSAZY ceramics increased at first and then decreased. However, the wear rate shows the opposite. When 0.4 wt% ZrO₂ and 0.6 wt% Y₂O₃ were added to the matrix, the wear rate of MSAZY ceramics reached a minimum of 0.042%, and the wear resistance was improved by about 73.8% compared with MSA ceramics with a wear rate of 0.16%. In addition, the optimum additions of zirconia and yttria are 0.4% and 0.6%, respectively.     

In situ hot pressing/reaction synthesis,mechanical and thermal properties of Lu2SiO5

Lu₂SiO₅ is a promising candidate of environmental barrier coatings (EBC) for silicon based ceramics due to its excellent high temperature stability. However, little information is available for the mechanical and thermal properties of Lu₂SiO₅, which frustrated evaluation of its performances for EBC applications. In this paper, dense Lu₂SiO₅ ceramic is successfully fabricated from Lu₂O₃ and SiO₂ powders by in situ hot pressing/reaction sintering at 1500 °C. Mechanical properties, including Young's modulus, bulk modulus, shear modulus, Poisson's ratio, fracture toughness, Vickers hardness, and bending strength are reported for the first time. Lu₂SiO₅ possesses excellent high temperature mechanical properties up to at least 1300 °C. Thermal stress for the case of Lu₂SiO₅ or Y₂SiO₅ coating on silicon bond coat and thermal stress resistance parameter are also estimated based on the experimental mechanical and thermal properties. The present results suggest that Lu₂SiO₅ has better reliability than Y₂SiO₅ in harsh thermal environment.     

Effect of phosphoric acid concentration on some properties of finely milled refractory materials

Conclusions The concentration of phosphoric acid seriously affects the setting, weakening, and sintering of the finely milled refractory materials.Increasing the acid concentration boosts the setting rate of alumina, MgO·Al₂O₃, chromite, electrocorundum, dunite, and dinas, but hardly affects the setting of MgO, Cr₂O₃, magnesite, zircon, and quartzite.The materials that were studied are almost unaffected at 800°C. Increasing the concentration of H₃PO₄ improves the sintering of alumina, MgO·Al₂O₃, chromite, zircon, and dinas, but impairs the sintering of MgO, Cr₂O₃, magnesite, electrocorundum, dunite, and quartzite.Translated from Ogneupory, No.3, pp. 40–43, March, 1968.     

Facile synthesis of MgO–Mg2SiO4 composite ceramics with high strength and low thermal conductivity

The recycling of solid waste is a win-win solution for humans and nature. For this purpose, magnesite tailings and silicon kerf waste were employed to prepare MgO–Mg₂SiO₄ composite ceramics by solid-state reaction synthesis in the present work. Then, effects of sintering temperature and raw material ratio on as-prepared ceramics were systematically studied. As-prepared ceramics showed improvement in their relative density (from 47.55%–68.12% to 90.96%–95.25%) and cold compressive strength (from 7.34–118.66 MPa to 303.39–546.65 MPa) with the increase in sintering temperature from 1300 to 1600 °C. In addition, it was found that Si promoted synthesis process of Mg₂SiO₄ phase through transient liquid phase sintering and Fe₂O₃ accelerated sintering process through activation sintering. Consequently, the presence of Mg₂SiO₄ phase effectively improved the density and strength of MgO–Mg₂SiO₄ composite ceramic, while reducing its thermal conductivity. This work provides a potential reutilization strategy for magnesite tailings, and as-prepared products are expected to be applied in fields of construction, metallurgy, and chemical industry.