Wear behavior of SiC nanowire-reinforced SiC coating for C/C composites at elevated temperatures

To improve the wear resistance of SiC coating on carbon/carbon (C/C) composites, SiC nanowires (SiCNWs) were introduced into the SiC wear resistant coating. The dense SiC nanowire-reinforced SiC coating (SiCNW-SiC coating) was prepared on C/C composites using a two-step method consisting of chemical vapor deposition and pack cementation. The incorporation of SiCNWs improved the fracture toughness of SiC coating, which is an advantage in wear resistance. Wear behavior of the as-prepared coatings was investigated at elevated temperatures. The results show that the wear resistance of SiCNW-SiC coating was improved significantly by introducing SiC nanowires. It is worth noting that the wear rate of SiCNW-SiC coating was an order of magnitude lower than that of the SiC coating without SiCNWs at 800 °C. The wear mechanisms of SiCNW-SiC coating at 800 °C were abrasive wear and delamination. Pullout and breakage of SiC grains resulted in failure of SiC coating without SiCNWs at 800 °C.     

Thermal stability of CVI and MI SiC/SiC composites with Hi-Nicalon™-S fibers

The influence of high-temperature argon heat-treatment on the microstructure and room- temperature in-plane tensile properties of 2D woven CVI and 2D unidirectional MI SiC/SiC composites with Hi-Nicalon?-S SiC fibers was investigated. The 2D woven CVI SiC/SiC composites were heat-treated between 1200 and 1600 °C for 1- and 100-hr, and the 2D unidirectional MI SiC/SiC composites between 1315 and 1400 °C for up to 2000 hr. In addition, the influence of temperature on fast fracture tensile strengths of these composites was also measured in air. Both composites exhibited different degradation behaviors. In 2D woven CVI SiC/SiC composites, the CVI BN interface coating reacted with Hi-Nicalon?-S SiC fibers causing a loss in fast fracture ultimate tensile strengths between 1200 and 1600 °C as well as after 100-hr isothermal heat treatment at temperatures > 1100 °C. In contrast, 2D unidirectional MI SiC/SiC composites showed no significant loss in in-plane tensile properties after the fast fracture tensile tests at 1315 °C. However, after isothermal exposure conditions from 1315° to 1400°C, the in-plane proportional limit stress decreased, and the ultimate tensile fracture strain increased with an increase in exposure time. The mechanisms of strength degradation in both composites are discussed.     

Influence of preheating processes on the microstructure of laser glazed YSZ coatings

Laser glazing is considered to be a promising surface sealing technique for thermal barrier coating. The dense top layer with reduced surface roughness and the segment cracks perpendicular to the surface are considered to be suitable for improving the thermal cycling and hot corrosion resistance of these kind of coatings. In present study, yttria stabilized zirconia ceramic coatings were manufactured by atmospheric plasma spraying and then subjected to a Nd-YAG pulsed laser source. During the laser glazing process, coatings were preheated to 600 °C and 800 °C in order to obtain different microstructure of the laser glazed coatings. The surface morphologies and cross-sections of the coatings were examined by scanning electron microscopy and microhardness measurements of coatings were carried out. The results indicate that preheating process induces a reduction of the grain size of laser glazed coatings in conjunction with an increasing of microhardness and toughness. In addition, preheating also decreases the substrate-coating interface tensile stress which leads to a reduction of crack surface density.     

Influence of annealing temperature on microstructure evolution of TiAlSiN coating and its tribological behavior against Ti6Al4V alloys

TiAlSiN multicomponent coating, owing to its high hardness and excellent high temperature resistance, was widely used in the cutting field of difficult-to-cut materials such as titanium alloys. For machining titanium alloys, high temperature is easy to gather on the tool chips and deteriorate the cutting tools. Moreover, high temperature will also promote the microstructure evolution and make the wear mechanism more complex. In this paper, TiAlSiN coatings were deposited on cemented carbides and annealed at 400 °C, 600 °C and 800 °C respectively for 60 min in air, followed by reciprocating friction tests against Ti6Al4V counterparts. AFM, SEM, EDS and XPS were applied to investigate the microstructure evolution and tribological behavior of TiAlSiN coating after high temperature annealing. The results demonstrated that the oxidation resistance of TiN phase in TiAlSiN coating was worse than Si₃N₄ and AlN phases. These nitrides can be oxidized to TiO₂, SiO_x and AlO_x under 600 °C, and the depth of oxide layer was increased with the rising annealing temperature, resulting in the coarsened microstructure. The wear mechanisms of as-deposited TiAlSiN coating were oxidation wear and adhesion wear. With the rising annealing temperature, abrasive wear was gradually enhanced. For the TiAlSiN coating annealed at 800 °C, abrasive wear became the dominant wear mechanism.     

Thermostability,oxidation, and high-temperature tribological properties of nano-multilayered AlCrSiN/VN coatings

In this study, monolithic AlCrSiN, VN, and nano-multilayered AlCrSiN/VN coatings were deposited using a hybrid deposition system combining arc ion plating and pulsed direct current magnetron sputtering. The microstructure, thermostability, mechanical, oxidation and tribological properties of the coatings were comparably investigated. The multilayered AlCrSiN/VN coating exhibited a face-centered cubic (fcc) structure with (200) preferred orientation and showed the highest hardness (30.7 ± 0.5 GPa) among these three coatings due to the multilayer interface enhancement mechanism and higher compressive stress. The AlCrSiN sublayers effectively prevented the V element from rapid outward diffusion to the surface of AlCrSiN/VN coating at elevated temperatures, which improved the oxidation resistance of the coating. Decomposition of V (Cr)–N bonds occurred at annealing temperatures from 800 °C to 1000 °C and V₂N phase appeared at 1100 °C. The AlCrSiN/VN coating showed excellent tribological performance at high temperatures by combining the merits of VN layers for low friction coefficient and AlCrSiN layers for superior oxidation resistance. Compared to VN and AlCrSiN coatings, AlCrSiN/VN coating showed the lowest wear rate of 2.6×10^-15 m³/N·m at 600 °C and lowest friction coefficient of 0.26 at 800 °C with a relativity low wear rate of 39.4×10^-15 m³/N·m.     

The effect of heat treatment on tensile properties of 2D C/SiC composites

Two-dimensional (2D) carbon fiber reinforced silicon carbide (C/SiC) composites with different initial strength were prepared by chemical vapor infiltration (CVI). After tensile property testing, results exhibited that as the heat-treatment temperature (HTT) increases to 1900°C, the tensile strength and toughness of the low strength specimen (LSS) increased by 110% and 530%, while the high strength specimen (HSS) increased by 5.4% and 550%, respectively. As observed from morphologies, the heat treatment increases the graphitization of the amorphous PyC interphase, which leads to the weakening of interfacial bonding strength (IBS). Meanwhile, the defects arising from heat treatment cause thermal residual stress relaxation. Therefore, the tensile strength and toughness of LSS with relatively high initial IBS increase significantly as HTT increases. For HSS with moderate initial IBS, the heat treatment slightly improves the tensile strength, but significantly improves the toughness. Consequently, the post-heat-treatment tensile properties of 2D C/SiC composites can be regulated by varying HTTs and different initial strength.     

Effect of nano CuO addition on the tribo-mechanical behavior of alumina ceramics in non-conformal contact

Sintering of alumina from 1500°C to 1650°C and tribo-mechanical properties at room temperature had been investigated using nano CuO as a sintering aid. Bulk density gradually increases with sintering temperature from 1500°C to 1600°C and is optimized at 1600°C, beyond this, bulk density does not significantly increase at 1650°C. The addition of 2 wt% CuO showed the best result on densification. Densification of about 97.74% was attained at 1600°C with the incorporation of 2 wt% CuO. Nano CuO at grain boundaries forms CuAl₂O₄ liquid which modifies the morphology of the grain and improves mechanical properties. The formation of self-lubricating tribo-film on the wear track results in a low coefficient of friction <0.2 and reduces specific wear rate. 4 wt% CuO addition increases contact tensile stress (σ_max) by 51.2% and high Hertzian contact pressure (P_max≈1.51 GPa) causes plastic deformation of wear track. The re-solidified strengthening bond phase on the wear track simultaneously increases in friction coefficient and wear resistance with CuO addition. The optimizing effect of CuO addition shows that 2 wt% significantly decreases wear rate, and increases hardness and fracture toughness.     

In-situ synthesis and low-temperature fabrication of B4C/TiB2-based multilayer graded composites with B4C and Ti–Al intermetallics mixtures through transient liquid phase sintering

The seven-layer B4C/TiB2-based graded composites was prepared with B4C and Ti–Al intermetallics through stepped laminating processing and transient liquid phase spark plasma sintering. The sintering strategy of the graded composites was proposed based on the sintering products of monolayer materials with different contents of Ti–Al intermetallics from 5 wt% to 60 wt%. The top three layers and bottom three layers were sintered respectively at 1650 °C and 1500 °C, and then the middle layer was used as the binder to joint the as-preserved two sections at 1550 °C. The apparent density of the as-prepared B4C/TiB2-based multilayer graded composites was 2.94 g/cm3, which was lower than that of most advanced ceramics. With the increase in the addition of Ti–Al intermetallics, the hardness of B4C/TiB2-based multilayer graded composites decreased from 31 GPa (B4C-riched) to 25 GPa (TiB2-riched), whereas the fracture toughness increased from 3.8 MPa·m0.5–6.02 MPa·m0.5. The compressive strength was up to 1100 MPa, displaying the jagged stress-strain curve. Crack propagation resistance mechanisms such as deflection and bridging enhanced the fracture toughness. The B4C/TiB2-based multilayer graded composites fabricated at low temperature possess high front hardness, high rear toughness, high overall strength and low density, and has promising applications in impact-resistant fields such as lightweight ceramic armor.     

Increasing the strength and toughness of a carbon fiber/silicon carbide composite by heat treatment

The effect of heat treatment on the strengthening and toughening of a carbon fiber/silicon carbide composite (C/SiC) with a thin pyrolytic carbon (PyC) interphase was investigated. Tensile strength and modulus were measured using tensile tests, and toughness was obtained by calculating the area under the stress–strain curves. Results show that with increasing heat treatment temperature both the strength and toughness of the C/SiC composite increased, but the modulus decreased. After heat treatment at 1900 °C the tensile strength and toughness increased by a maximum of 42% and 252%, respectively, and the modulus decreased by 48%. X-ray diffraction analysis and microstructural observation confirmed that the heat treatment mainly increased the graphitization of the amorphous PyC interphase, and this was responsible for the property changes observed because it decreased the interfacial sliding resistance associated with long fiber pull-out, relieved the thermal residual stress and lower stress concentrations on the fibers to uniformly share the load for improving the strength and toughness.     

Crystallization behavior and mechanical properties of high strength mica-containing glass-ceramics from granite wastes

The high-strength mica-containing glass-ceramics were prepared from granite wastes by bulk crystallization. The influences of SiO₂/Al₂O₃ molar ratio (S/A = 7.72, 9.62, 12.58, 17.82 and 29.67) on the crystallization behavior, microstructure, mechanical properties and machinability of glass-ceramics were investigated. The results demonstrated that the polymerization degree of the glass network decreased with the S/A ratio increasing, which further caused the decrease in glass transition temperature and crystallization temperatures. The increase in the S/A ratio promoted the precipitation of diopside, hectorite, kalsilite and tainiolite in glass-ceramics when the samples were heated at 750 °C, while inhibiting the precipitation of forsterite. For the glass-ceramics crystallized at 800 and 900 °C, the main crystalline phases transformed from diopside, forsterite, and nepheline to diopside, kalsilite, and tainiolite, with the S/A ratio increasing. As the SiO₂ gradually replaced Al₂O₃, the morphology of crystals changed from lamellar to granular, while the mean size of crystals reduced. The Vickers-Hardness values of glass-ceramics crystallized at 800 and 900 °C ascended with S/A ratio rising, and the values were above 6.30 GPa. The bending strength of most glass-ceramics is stable between 90 and 140 MPa, among which the maximum bending strength is 133.28 ± 14.81 MPa. The fracture toughness of the glass-ceramic crystallized at 800 and 900 °C declined, while that at 700 °C increased with a larger S/A ratio. Glass-ceramics after heat-treated at 900 °C with S/A ratio of 9.62 had the largest fracture toughness of 3.28 ± 0.15 MPa m^1/2. In preliminary tests of machinability, glass-ceramic after heat-treated at 900 °C with S/A ratio of 9.62 showed better results.     

Influence of heat treatment on alternant-layer structure and mechanical properties of Al2O3-TiO2-MgO coatings

Al₂O₃-TiO₂-MgO ceramic alternant layer coatings were prepared by atmospheric plasma spraying and heat treated at 600, 700, 800, 900, and 1000?°C. The influence of heat treatment on microhardness, fracture toughness, and the structural evolution of the coatings on steel were investigated. Heat treatment promoted alternant layer interdiffusion within ceramic coatings, which could result a transformation from a lamellar morphology to mutual pinning. The interfacial diffusion between the bond coating and substrate was clearly demonstrated after heat treatment at different temperatures. Heat treatment also significantly affected the evolution of the hardness and fracture toughness. Temperature strongly affected the microhardness of the specimens, and the hardness arrived to the highest value at 1000?°C. The formation of a new Mg₂Al₆Ti₇O₂₅ phase and alternant layer mutual pinning were beneficial to hardness improvement, and heat treatment also significantly improved fracture toughness.     

High-temperature fracture behaviour of layered alumina ceramics with textured microstructure

Mimicking the damage tolerance of biological materials such as nacre has been realised in textured layered alumina ceramics, showing improved reliability as well as fracture resistance at room temperature. In this work, the fracture behaviour of alumina ceramics with textured microstructure and laminates with embedded textured layers are investigated under uniaxial bending tests at elevated temperatures (up to 1200 °C). At temperatures higher than 800 °C monolithic textured alumina favours crack deflection along the basal grain boundaries, corresponding to the transition from brittle to more ductile behaviour. In the case of laminates, the loss of compressive residual stresses is counterbalanced by the textured microstructure, effective up to 1200 °C. This study demonstrates the potential of tailoring microstructure and architecture in ceramics to enhance damage tolerance within a wide range of temperatures.     

Microstructure effects on fracture failure mechanism of CrAl/CrAlN coating

In this work, the Metal-rich phase Chromium Aluminum (CrAl)/Ceramic phase Chromium Aluminum Nitride (CrAlN) multi-layer coatings were prepared by Arc Ion Plating (AIP). The micro-structure and phase composition of CrAl/CrAlN multi-layer coatings were characterized, and the microstructure, mechanical properties, residual stress and fracture toughness of the coating were emphatically analyzed. It has been found out that the residual stress of the multi-layer coating was only ?0.932 ± 0.065 GPa, which was significantly lower than that of the mono-layer coating for ?1.569 ± 0.093 GPa. At the same time, it was also found that the preferred growth orientation of the coating changed from a mono-layer (111) to a multi-layer (200) crystal plane. The hardness of the multi-layer (22.74 ± 0.57 GPa) is slightly lower than that of the mono-layer (24.92 ± 0.5 GPa), and the adhesion strength (46.2 ± 3.8 N) is obviously higher than that of the mono-layer (37.4 ± 2.4 N), and the fracture toughness is also higher (8.7 ± 0.8 MPa m^1/2). In addition, the mechanism of crack initiation and propagation in stress-induced coatings was studied in detail on the basis of the structure of micro-nano CrAl/CrAlN multi-layer coatings.     

Mechanical properties of LaFeO3 ceramics

The fracture strength, fracture toughness and apparent Young’s modulus of LaFeO₃ ceramics in the temperature region 25–800 °C are reported. The fracture strength of the material was observed to increase from 202 ± 18 MPa at room temperature to 235 ± 38 MPa at 800 °C. The room temperature fracture toughness was 2.5 ± 0.1 MPa m^1/2. The fracture toughness decreased to 2.1 ± 0.1 MPa m^1/2 at 600 °C, followed by an increase to 3.1 ± 0.3 MPa m^1/2 at 800 °C. The temperature dependence of the fracture toughness correlates well with the crystallographic strain, |(a−c)|/(a+c), and ferroelastic toughening of LaFeO₃ materials is inferred. Non-elastic stress–strain behaviour of the LaFeO₃ materials due to ferroelasticity was confirmed by cyclic compression experiments, and residual strain was observed in the material after unloading.     

Mechanical,Tribological, and Thermal Properties of Hot‐Pressed ZrB2‐B4C Composite

The effect of addition of submicrometer‐sized B₄C (5,10 and 15 wt%) on microstructure, phase composition, hardness, fracture toughness, scratch resistance, wear resistance, and thermal behavior of hot‐pressed ZrB₂‐B₄C composites is reported. ZrB₂‐B₄C (10 wt%) composite has V_H1 of 20.81 GPa and fracture toughness of 3.93 at 1 kgf, scratch resistance coefficient of 0.40, wear resistance coefficient of 0.01, and ware rate of 0.49 × 10^?3 mm³/Nm at 10N. Crack deflection by homogeneously dispersed submicrometer‐sized B₄C in ZrB₂ matrix can improve the mechanical and tribological properties. Thermal conductivity of ZrB₂‐B₄C composites varied from 70.13 to 45.30 W/m K between 100°C and 1000°C which is encouraging for making ultra‐high temperature ceramics (UHTC) component.     

Thermal stability and mechanical properties of HVOF/PVD duplex ceramic coatings produced by HVOF and cathodic vacuum arc

Duplex ceramic coatings, consisting of an inner NiCr-Cr₃C₂-based coating and an outmost AlCrN film, were produced on the steel substrate in succession by velocity oxygen-fuel spraying (HVOF) and cathodic vacuum arc methods, and then isochronally annealed at annealing temperatures below 900 °C for 2 h. The thermal stability and mechanical properties of the annealed samples were systematically studied by means of X-ray diffraction, Optical microscope and transmission electron microscope, in association with mechanical property measurements. The results show that the microstructure, phase evolution and mechanical properties of duplex ceramic coatings are significantly dependent on the annealing temperature. Metastable fcc-AlCrN solid solution in AlCrN film first decomposes to rich-Al and rich-Cr domains by spinodal decomposition at 700 °C, leading to a notable increase in hardness due to its smaller grain size and high elastic strain field, and then to equiaxed hcp-AlN and Cr₂N by the nucleation and growth at 900 °C, leading to a notable decrease in hardness due to the recrystallization and the formation of hcp-AlN. Meanwhile, the both decarburization of Cr₃C₂ to Cr₇C₃ occurs at 800 °C, but becomes more intensive at 900 °C, leading to a notable loss in hardness. In addition, the dissolution of Cr₃C₂ produces high density of porosity, which also reduces the hardness. The hardness tests show the following ordering of load-bearing capacity for the duplex ceramic coatings: 700 °C>As-deposited >800 °C>900 °C. Tribological property measurements demonstrate that the wear resistance of the tested duplex ceramic coatings obeys the following ordering: 700 °C>As-deposited >800 °C>900 °C. The improved wear resistance is due to high surface hardness, load-bearing capacity and thermal stability. In addition, the wear mechanisms are shown.     

Characteristics,wear and corrosion properties of borided pure titanium by pack boriding near α → β phase transition temperature

The boride layer characteristics, wear and corrosion properties of borided commercially pure titanium by pack boriding near the α → β phase transition temperature were investigated using X-ray diffraction, scanning electron microscopy, electron probe microanalysis, dry reciprocating friction tests, and electrochemical experiments in this work. The pack boriding was carried out at the temperatures of 860 °C, 880 °C, 900 °C, and 920 °C for 5, 10, 15 and 20 h. The results indicated that, in both α and β phase temperatures, the boride layer is composed of the outer TiB₂ layer and the inner TiB layer. The α→β phase transition temperature of commercially pure Ti in this work is in the range of 882–900 °C, and the growth of TiB layer can be enhanced at this temperature range. Commercially pure Ti borided at 920 °C for 20 h has the best wear resistance and corrosion resistance. Finally, wear and corrosion mechanisms were also discussed.     

Ytterbia–neodymia–costabilized TZP—Breaking the limits of strength–toughness correlations for zirconia?

TZP ceramics were manufactured by hot pressing of pyrogenic zirconia nanopowder which was costabilized by 1 mol% ytterbia and 2 mol% neodymia (1Yb–2Nd–TZP) via the nitrate route. The evolution of microstructure, phase composition and mechanical properties with variation of sintering temperature from 1200 °C to 1400 °C was investigated. 1Yb–2Nd–TZP consists of a bimodal microstructure of small very transformable tetragonal grains and large cubic grains. At intermediate sintering temperature the materials combine a bending strength of 1250 MPa with a fracture resistance >13 MPa √m. The high threshold stress intensity of 7 MPa √m indicates high resistance to subcritical crack growth. An increase in fracture resistance before the crack tip induced by compressive residual stress shifts the strength–toughness correlations to higher values than previously considered possible.     

Residual Stress and Microstructural Evolution in Tantalum Oxide Coatings on Silicon Nitride

Due to its coefficient of thermal expansion (CTE) and phase stability up to 1360°C, tantalum oxide (Ta₂O₅) was identified and investigated as a candidate environmental barrier coating for silicon nitride-based ceramics. Ta₂O₅ coatings were plasma sprayed onto AS800, a silicon nitride ceramic from Honeywell International, and subjected to static and cyclic heat treatments up to 1200°C in air. Cross-sections from coated and uncoated substrates were polished and etched to reveal the effect of heat treatments on microstructure and grain size. As-sprayed coatings contained vertical cracks that healed after thermal exposure. Significant grain growth that was observed in the coatings led to microcracking due to the anisotropic CTE of Ta₂O₅. High-energy X-ray diffraction was used to determine the effect of heat treatment on residual stress and phases. The uncoated substrates were found to have a surface compressive layer before and after thermal cycling. Coating stresses in the as-sprayed state were found to be tensile, but became compressive after heat treatment. The microcracking and buckling that occurred in the heat-treated coatings led to stress relaxation after long heat treatments, but ultimately would be detrimental to the function of the coating as an environmental barrier by affording open pathways for volatile species to reach the underlying ceramic.     

Correlations Between Microstructure and Mechanical Properties of Air Plasma‐Sprayed Thermal Barrier Coatings Exposed to a High Temperature

An indentation method is used to study the variations in Young's modulus, hardness and fracture toughness of air plasma‐sprayed thermal barrier coatings at a high temperature. The coatings were exposed to 1100°C during 1700 h. A sudden increase in Young's modulus for the first 600 h was observed, while the hardness increased after 800 h as a consequence of sintering. Conversely, there was a reduction of 25% in fracture toughness after 1700 h, evidencing the thermal barrier coating degradation. The evolution of these mechanical properties was correlated with microstructural changes. After 1700 h, the thermally grown oxide thickness reached 6.8 μm, the volumetric percentage of porosity was reduced from 6.8% to 4.7% and the amount of monoclinic phase increased to 23.4 wt%. These characteristics are closely related to the stress distribution in the top coat, which promotes cracks nucleation and propagation, compromising the coating durability.