High temperature abradable sealing coating for SiCf/SiC ceramic matrix composites

A study of porous YSZ abradable sealing coating (ASC) plasma-sprayed onto SiC_f/SiC ceramic matrix composites (CMC) through the compatibility of intermediate layers is reported. The multilayer Si/Yb₂Si₂O₇/LaMgAl₁₁O₁₉ thermal-environmental barrier coating (T-EBC) is served as intermediate layers in consideration of its ability to protect the CMC from recession and ease the misfit of the thermal expansivity. Isothermal exposure and thermal shock tests were conducted at 1200°C and led to the decomposition of t'-ZrO₂ phase to t-ZrO₂ and c-ZrO₂ phases in YSZ topcoat, the formation of mud-cracks throughout the entire coating structure and thermally grown oxide (SiO₂), with following an Yb₂Si₂O₇ reaction layer. The measured bond strength of the coated samples was 5.47 ± 0.85 MPa, and the fracture position mainly happened inside the CMC substrate. The Superficial Rockwell Hardness (HR15Y) considered to be an important factor in abradability increased by only 1.34% after 1200°C isothermal exposure for 100 h, showing excellent high temperature hardness stability. The abradability of the ASC was investigated by a sliding wear test, the fatigue wear mainly occurred in worn scar when encountering Si₃N₄ ceramic ball with high hardness and low thermal conductivity, while adhesive wear occurred when GCr15 steel ball with low hardness and high thermal conductivity are encountered.     

Comparative study on wear behavior of plasma sprayed Al2O3 coatings sliding against different counterparts

Although the friction and wear behavior of plasma sprayed aluminum matrix ceramic coatings have been extensively discussed in the last decades, only few researches have been carried out the wear mechanisms sliding against different pairs. The tribological behaviors of plasma sprayed Al₂O₃ coating sliding against ZrO₂, Si₃N₄, Al₂O₃ and stainless steel balls in air were comparatively investigated in this study. It was showed that Al₂O₃ coating sliding against different counterparts exhibited diverse tribological behaviors, which could be mainly ascribed to the different mechanical properties of counterparts. Meanwhile, the tribochemical reactions influenced the friction performances significantly. Moreover, the transform of γ-Al₂O₃ to α-Al₂O₃ occurred during the friction, which was closely related to the coefficient of friction and thermal conductivities of counterparts. The main wear of Al₂O₃ coating sliding against ceramic materials resulted from the brittle fracture and abrasive wear. While it was dominated by adhesive wear when sliding against stainless steel, and accompanied with abrasive wear.     

Tribological properties of polyimide coatings filled with PTFE and surface‐modified nano‐Si3N4

Polyimide (PI) coatings filled with PTFE and nano‐Si₃N₄ were prepared by a spraying technique and successive curing. Nano‐Si₃N₄ particles were modified by grafting 3‐aminopropyltriethoxysilane to improve their dispersion in the as‐prepared coatings. Friction and wear performances and wear mechanisms of the coatings were evaluated. The results show that the incorporations of PTFE and modified nano‐Si₃N₄ particles greatly improve the friction reduction and wear resistance of PI coating. The friction and wear performance of the composite coating is significantly affected by the filler mass fraction and sliding conditions. PI coating incorporated with 20 wt % PTFE and 5 wt % modified nano‐Si₃N₄ displays the best tribological properties. Its wear rate is more than one order of magnitude lower and its friction coefficient is over two times smaller than that of the unfilled PI coating. Differences in the friction and wear behaviors of the hybrid coatings as a function of filler or sliding condition are attributed to the filler dispersion, the characteristic of transfer film formed on the counterpart ball and the wear mechanism of the coating under different sliding conditions. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40410.     

Frictional behavior and wear resistance performance of gradient cermet composite tool materials sliding against hard materials

Gradient cermet composites possessing high surface hardness, flexural strength and interface bonding strength were fabricated using vacuum hot-pressing sintering. Ball-on-disk tests were performed to investigate the tribological properties of the gradient cermet composites against 440 C stainless steel, Al₂O₃ and Si₃N₄ balls at different sliding speed and load in comparison with traditional Ti(C,N) cermets. The tribological behavior was characterized in terms of friction coefficient and wear rate. The results showed that friction coefficient was significantly dependent on the sliding speed and load when sliding against Al₂O₃ and Si₃N₄. However, there was no obvious relation between them during sliding against 440 C stainless steel due to the formation of metal adhesive layer. Gradient cermet composites exhibited a higher friction coefficient but lower wear rate than traditional Ti(C,N) cermets. The main wear mechanism of gradient cermet composites was adhesion wear during sliding against 440 C stainless steel, while abrasion wear was the predominant mechanism during sliding against Al₂O₃ and Si₃N₄. It was expected that gradient cermet composites would be excellent candidates for cutting tool materials.     

Sn-containing Si3N4-based composites for adaptive excellent friction and wear in a wide temperature range

Ceramic design based on reducing friction and wear-related failures in moving mechanical systems has gained tremendous attention due to increased demands for durability, reliability and energy conservation. However, only few materials can meet these requirements at high temperatures. Here, we designed and prepared a Sn-containing Si₃N₄-based composite, which displayed excellent tribological properties at high temperatures. The results showed that the friction coefficient and wear rate of the composites were reduced to 0.27 and 4.88 × 10^?6 mm³ N^?1 m^?1 in air at 800 °C. The wear mechanism of the sliding pairs at different temperatures was revealed via detailed analyses of the worn surfaces. In addition, the tribo-driven graphitization was detected on the wear surfaces and in the wear debris, and the carbon phase was identified by SEM, TEM, and Raman spectrum.     

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.     

Effect of grain size and amount of zirconia on the physical and mechanical properties and the wear resistance of zirconia-toughened alumina

The effects of the ZrO₂ content and the particle size of ZrO₂ powders on the microstructure, phase composition, physical and mechanical properties, and the abrasion wear resistance of advanced Al₂O₃ ceramics and zirconia-toughened alumina (ZTA) composites containing 0 to 30 mass% yttria-stabilised zirconia (YSZ) were investigated. The composite with a ZTA content of 30 mass% of ZrO₂ exhibited the greatest resistance to abrasion wear. α-Al₂O₃ reflex broadening (hkl = 113) as a result of the microstresses in the Al₂O₃ crystal lattice during sandblasting decreased with increasing ZrO₂ amount, where the ZrO₂ particles located along the grain boundaries of Al₂O₃, hindering their growth and deformation. The use of nanodispersed ZrO₂ powder produced by the plasma chemical technique led to a 1.5-fold increase in wear resistance in the resultant ZTA ceramic.     

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 behavior of ceramic materials (Si3N4, SiC and Al2O3) in aqueous medium

The friction and wear behavior of self-mated Si₃N₄, SiC and Al₂O₃ in water were investigated by varying the test conditions of applied load and sliding speed. It was found that, for self-mated Si₃N₄ and SiC ceramics, the tribochemical reaction resulted in surface smoothening with low friction coefficient at high load and high speed condition. Al₂O₃ shows high friction coefficient, but better wear rate (10⁻¹¹ mm²/N) than other ceramic materials.     

Synergistic effect of surface textures and DLC coatings for enhancing friction and wear performances of Si3N4/TiC ceramic

To enhance the tribological properties of Si₃N₄ based ceramics, surface textures of dimples combined with DLC coatings are fabricated on Si₃N₄/TiC ceramic surface by nanosecond laser and plasma enhanced chemical vapor deposition (PECVD). The dry friction and wear performances are evaluated by unidirectional sliding friction tests using a rotary ball-on-disk tribometer. Results reveal that the friction and wear properties of Si₃N₄/TiC ceramics are significantly enhanced by DLC coatings or dimpled textures, and the DLC coatings combined with dimpled textures show the best efficiency in reducing friction, adhesion and wear. This improvement can be explained by the synergistic effect of DLC coatings and surface textures, and the synergistic mechanisms are attributed to the formation of lubrication film and secondary lubrication, debris capture of dimpled textures, increased surface hardness and mechanical interlocking effect, and reduced contact area.     

Tribological and electrical properties of ceramic matrix composites with carbon nanotubes

Tribological behaviur of carbon fibrous phases (nanofibers and nanotubes) containing composites with Si₃N₄, ZrO₂ and Al₂O₃ matrices was studied by pin-on-disk technique in conditions of dry sliding. Coefficients of friction and wear rates were measured, wear damage mechanisms were observed and identified. The resulting tribological behaviur was related to microstructure and mechanical properties of respective materials. Electrical conductivity was measured in wide range of frequencies by two-point method and effect of volume fraction and distribution of CNTs and CNFs on percolation threshold was evaluated. Both coefficient of friction and electrical resistivity decreased with increasing amount of carbon phases, in both cases the nanofibers were more efficient than the nanotubes. The wear resistance in most cases decreased but for Si₃N₄–CNT composite a certain optimum (～5 wt.% CNT) was found.     

Microstructure and improved mechanical properties of Al2O3/Ba-β-Al2O3/ZrO2 composites with YSZ addition

Al₂O₃/Ba-β-Al₂O₃/ZrO₂ composites were fabricated by solid-state reaction sintering of Al₂O₃, BaZrO₃, and yttria stabilized zirconia (YSZ) powders. The effects of YSZ addition on microstructure and mechanical properties have been investigated. The incorporation of YSZ promoted the densification of the composites and formation of tetragonal ZrO₂ phase. The microstructure of the composites was characterized by elongated Ba-β-Al₂O₃ phase and equiaxed ZrO₂ particles including added YSZ and reaction-formed ZrO₂. The Al₂O₃/Ba-β-Al₂O₃/ZrO₂ composites with YSZ addition exhibited improved fracture toughness, as a result of multiple toughening effects including crack deflection, crack bridging, crack branching, and martensitic transformation of ZrO₂ formed by the reactions between Al₂O₃ and BaZrO₃. Moreover, owing to the grain refinement of Al₂O₃ matrix, dispersion strengthening of the added YSZ particles, and an increase in density of the composites, the Vickers hardness and flexural strength of Al₂O₃/Ba-β-Al₂O₃/ZrO₂ composites were dramatically enhanced in comparison with the composites without YSZ addition.     

Effect of load on the friction and wear characteristics of Si3N4-hBN ceramic composites sliding against steels

The tribological behaviors of Si₃N₄-hBN ceramic composites sliding against steels (austenitic stainless steel (ASS) and 45 steel) under dry friction conditions at different loads were investigated by using an MMW-1 type vertical universal friction and wear tester. The experimental results showed that the friction coefficients and wear rates first showed a decrease and then an increase with an increase in the load under dry friction conditions. The better tribological performance was exhibited by the SN10/ASS sliding pair under a load of 20 N (the friction coefficient was as low as 0.27 and the wear rates of both pin and disc had a magnitude of 10⁻⁶ mm³ N⁻¹ m⁻¹). This may be attributed to the formation of a black surface film (consisting of B₂O₃, SiO₂, and Fe₂O₃). For the same sliding pair, when the load was 10 N, the dominating wear mechanism was abrasive wear. Hence, the friction coefficient was higher (0.7). When the load increased to 30 and 50 N, the wear mechanism of the SN10/ASS sliding pair was a combination of abrasive and adhesive wears, and higher friction coefficients (0.48 and 0.72 under loads of 30 and 50 N, respectively) were obtained. On the other hand, the contents of hBN also showed a significant impact on the tribological behaviors of the Si₃N₄-hBN/ASS sliding pairs. When the hBN content was less than 10%, the friction coefficients of the Si₃N₄-hBN/ASS sliding pairs decreased with an increase in the hBN content. On the other hand, at hBN contents of 10% or more, the friction coefficients of the sliding pairs increased with an increase in the hBN content. Under the same experimental conditions, the Si₃N₄-hBN/45 steel pairs showed poor tribological properties as compared with the Si₃N₄-hBN/ASS pairs.     

A Study of the Wear in Ceramic Bearings by a Thin-Layer Activation Method

The wear resistance of ceramic sliding bearings fabricated from sintered Si₃N₄ and ZrO₂ is studied by the thin-layer activation method. Radioisotopic tags were applied by exposing the ceramic components to a proton beam with an energy of 7.6 MeV. Exposure conditions are specified to avoid radiative damage of the surface of ceramic components. The wear under dry-friction conditions was determined by radiometry of the isotopically labeled surface using calibration curves. The running-in time was found to be about 400 min for Si₃N₄-based and 500 – 600 min for ZrO₂-based components, with wear rates of 4 – 5 and 5 – 6 , respectively. The data obtained attest to the high performance characteristics of the ceramic bearings that are used for submersible pumps for pumping crude oil out of wells.     

Thermal cycling behaviour of plasma sprayed lanthanum zirconate based coatings under concurrent infiltration by a molten glass concoction

Thermal barrier coatings (TBCs) used in gas-turbine engines afford higher operating temperatures, resulting in enhanced efficiencies and performance. However, during aero engine operation, environmentally ingested airborne particles, which includes mineral debris, sand dust and volcanic ashes get ingested by the turbine with the intake air. As engine temperatures increase, the finer debris tends to adhere to the coating surface and form calcium magnesium alumino-silicate (CMAS) melts that penetrate the open void spaces in the coating. Upon cooling at the end of an operation cycle, the melt freezes and the infiltrated volume of the coating becomes rigid and starts to spall by losing its ability to accommodate strains arising from the thermal expansion mismatch with the underlying metal. The state-of-the-art ZrO₂-7-weight% Y₂O₃ (YSZ) coatings are susceptible to the aforementioned degradation. Rare-earth zirconates have generated substantial interest as novel thermal barrier coatings (TBC) based primarily on their intrinsically lower thermal conductivity and higher resistance to sintering than YSZ. In addition, the pyrochlore zirconates are stable as single phases at up to their melting point. La₂Zr₂O₇ (LZ) is one among such candidates. Hence, the present study focusses on the comparison of cyclic molten CMAS infiltration behaviour of the base metal Inconel 738 (BM), the bond coat NiCrAlY (BC), the duplex YSZ, the LZ coating and a five layered coated specimen with LZ as top layer. Among those coatings mentioned above, the five layer coated specimen showed excellent CMAS infiltration resistance under thermal cycling conditions.     

Preparation of Y2Si2O7/ZrO2 Composites and Their Composition – Mechanical Properties – Tribology Relationships

In this work, novel Y₂Si₂O₇/ZrO₂ composites were developed for structural and coating applications by taking advantage of their unique properties, such as good damage tolerance, tunable mechanical properties, and superior wear resistance. The γ‐Y₂Si₂O₇/ZrO₂ composites showed improved mechanical properties compared to the γ‐Y₂Si₂O₇ matrix material, that is, the Young's modulus was enhanced from 155 to 188 GPa (121%) and the flexural strength from 135 to 254 MPa (181%); when the amount of ZrO₂ was increased from 0 to 50 vol%, the γ‐Y₂Si₂O₇/ZrO₂ composites also presented relatively high facture toughness (>1.7 MPa·m^1/2), but this exhibited an inverse relationship with the ZrO₂ content. The composition–mechanical property–tribology relationships of the Y₂Si₂O₇/ZrO₂ composites were elucidated. The wear resistance of the composites is not only influenced by the applied load, hardness, strength, toughness, and rigidity but also effectively depends on micromechanical stability properties of the microstructures. The easy growth of subcritical microcracks in Y₂Si₂O₇ grains and at grain boundaries significantly contributes to the macroscopic fracture toughness, but promotes the pull‐out of individual grains, thus resulting in a lack of correlation between the wear rate and the macroscopic fracture toughness of the composites.     

Tribological properties of ethylene–propylene–diene rubber + polypropylene + thermal‐shock‐resistant ceramic composites

This work is part of a program on composites used in thermoelectric devices. Tribological properties of dynamic vulcanizate blends of polypropylene and ethylene‐propylene‐diene rubber filled with 5 wt% of microscale powder have been studied. The microscale thermal‐shock‐resistant ceramic filler contains α‐Al₂O₃, mullite (3Al₂O₃ · 2SiO₂ or 2Al₂O₃SiO₂), β‐spodumene glass‐ceramic and aluminium titanate. We found that our ceramic particles are abrasive; they cause strong abrasion of softer steel ball surfaces during dry sliding friction. To overcome the difficulty of particle dispersion and adhesion, the filler was modified through grafting using three types of organic molecules. Dry sliding friction was measured using four types of counter‐surfaces: tungsten carbide, Si₃N₂, 302 steel and 440 steel. Thermoplastic vulcanizate filled with neat ceramic powder shows the lowest friction compared to composites containing the same but surface‐treated powder. We introduce a ‘bump’ model to explain the tribological responses of our composites. ‘Naked’ or untreated ceramic particles protrude from the polymer surface and cause a decrease of the contact area compared to neat polymer. The ball partner surface has only a small contact area with the bumps. As contact surface area decreases, so does friction and the amount of heat generated during sliding friction testing. Chemical coupling of the ceramic to the matrix smoothens the bumps and increases the contact surface, giving a parallel increase in friction. Copyright © 2012 Society of Chemical Industry     

Wear behaviour of α- and α/β-SiAlON ceramics stabilized with Nd2O3 and Y2O3

α- and α/β-SiAlON compositions, doped with Y₂O₃ or Nd₂O₃, were densified by gas pressure sintering (GPS). The wear and mechanical properties of the materials were investigated and compared to β-Si₃N₄ materials. Microstructure evolution and its change with composition as well as the influence of the microstructural changes on the mechanical and tribological properties were reported. Wear tests were performed using a tribometer with ball-on-plate geometry in reciprocating sliding contact under dry condition. It was observed that α/β-SiAlON compositions, doped with Y₂O₃, have better wear properties in comparison to α-SiAlON and β-Si₃N₄ samples. Field emission scanning electron microscopy (FESEM) was used to analyse the worn surfaces after wear tests. The results showed that α/β-SiAlON and β-Si₃N₄ materials have different wear behaviour.     

Study on thermal shock resistance of Sc2O3 and Y2O3 co-stabilized ZrO2 thermal barrier coatings

In order to reveal the effect of Sc₂O₃ and Y₂O₃ co-doping system on the thermal shock resistance of ZrO₂ thermal barrier coatings, Y₂O₃ stabilized ZrO₂ thermal barrier coatings (YSZ TBCs) and Sc₂O₃–Y₂O₃ co-stabilized ZrO₂ thermal barrier coatings (ScYSZ TBCs) were prepared by atmospheric plasma spraying technology. The surface and cross-section micromorphologies of YSZ ceramic coating and ScYSZ ceramic coatings were compared, and their phase composition before and after heat treatment at 1200 °C was analyzed. Whereupon, the thermal shock experiment of the two TBCs at 1100 °C was carried out. The results show that the micromorphologies of YSZ ceramic coating and ScYSZ ceramic coating were not much different, but the porosity of the latter was slightly higher. Before heat treatment, the phase composition of both YSZ ceramic coating and ScYSZ ceramic coating was a single T′ phase. After heat treatment, the phase composition of YSZ ceramic coating was a mixture of M phase, T phase, and C phase, while that of ScYSZ ceramic coating was still a single T′ phase, indicating ScYSZ ceramic coating had better T′ phase stability, which could be attributed to the co-doping system of Sc₂O₃ and Y₂O₃ facilitated the formation of defect clusters. In the thermal shock experiment, the thermal shock life of YSZ TBCs was 310 times, while that of ScYSZ TBCs was 370 times, indicating the latter had better thermal shock resistance. The difference in thermal shock resistance could be attributed to the different sintering resistance of ceramic coatings and the different growth rates of thermally grown oxide in the two TBCs. Furthermore, the thermal shock failure modes of YSZ TBCs and ScYSZ TBCs were different, the former was delamination, while the latter was delamination and shallow spallation.     

Lubrication mechanism of graphene nanoplates as oil additives for ceramics/steel sliding components

Tribological behavior of Si₃N₄ ceramic/GCr15 steel components under lubrication of graphene was investigated and found that friction reduction and wear resistance of the sliding pairs were improved by 27% and 43%, respectively, after adding 0.075 wt% of graphene into the base oil. Herein, the lubrication mechanism of graphene nanoplates as oil additives was focused on to promote the practical application of graphene in hybrid ceramic bearings industry. In order to investigate the effect of oil film thickness on tribological properties, the viscosity of blended lubricating oil containing different concentration of graphene was measured and the oil film thickness between rubbing surfaces was calculated using a full numerical solution in point contacts. In addition, the rubbing surfaces were analyzed with combination of EDS and Raman technique. The results demonstrated that the viscosity of base oil and oil film thickness increased with addition of graphene. Graphene entered rubbing interface and prevented ceramic/steel pairs from mechanical contact. Lubrication mechanism including interlayer shearing, surface mending and protective film formation contributed to reduce the friction coefficient and worn scar diameter.