Unlubricated friction and wear behaviors of Al2O3/TiC ceramic cutting tool materials from high temperature tribological tests

The unlubricated friction and wear behaviors of Al₂O₃/TiC ceramic tool materials were evaluated in ambient air at temperature up to 800 °C by high temperature tribological tests. The friction coefficient and wear rates were measured. The microstructural changes and the wear surface features of the ceramics were examined by scanning electron microscopy. Results showed that the temperature had an important effect on the friction and wear behaviors of this Al₂O₃ based ceramic. The friction coefficient decreased with the increase of temperature, and the Al₂O₃/TiC ceramics exhibited the lowest friction coefficient in the case of 800 °C sliding operation. The wear rates increased with the increase of temperature. During sliding at temperature above 600 °C, oxidation of the TiC is to be expected, and the formation of lubricious oxide film on the wear track is beneficial to the reduction of friction coefficient. The wear mechanism of the composites at temperature less than 400 °C was primary abrasive wear, and the mechanisms of oxidative wear dominated in the case of 800 °C sliding operation.     

Wear and friction of TiAlN/VN coatings against Al2O3 in air at room and elevated temperatures

TiAlN/VN multilayer coatings exhibit excellent dry sliding wear resistance and low friction coefficient, reported to be associated with the formation of self-lubricating V₂O₅. To investigate this hypothesis, dry sliding ball-on-disc wear tests of TiAlN/VN coatings on flat stainless steel substrates were undertaken against Al₂O₃ at 25 °C, 300 °C and 635 °C in air. The coating exhibited increased wear rate with temperature. The friction coefficient was 0.53 at 25 °C, which increased to 1.03 at 300 °C and decreased to 0.46 at 635 °C. Detailed investigation of the worn surfaces was undertaken using site-specific transmission electron microscopy (TEM) via focused ion beam (FIB) microscopy, along with Fourier transform infrared (FTIR) and Raman spectroscopy. Microstructure and tribo-induced chemical reactions at these temperatures were correlated with the coating’s wear and friction behaviour. The friction behaviour at room temperature is attributed to the presence of a thin hydrated tribofilm and the presence of V₂O₅ at high temperature.     

Microstructure evolution and densification kinetics of Al2O3/Ti(C,N) ceramic tool material by microwave sintering

The microstructure evolution and densification kinetics of Al₂O₃/Ti(C,N) ceramic tool material during microwave sintering were studied. The density and grain growth significantly increases at the temperatures higher than 1400 °C. The calculated kinetics parameter n indicates that volume diffusion is the main densification mechanism when the sintering temperature is below 1300 °C, while grain boundary diffusion plays a leading role in the densification process when the sintering temperature is higher than 1300 °C. The grain growth activation energy of Al₂O₃/Ti(C,N) composite is 48.82 KJ/mol, which is much lower than those of monolithic Al₂O₃ in the microwave sintering and conventional sintering. The results suggested that the Al₂O₃/Ti(C,N) ceramic tool material with nearly full densification and fine grains can be prepared by two-step microwave sintering.     

Wear mechanism of CrN/NbN superlattice coating sliding against various counterbodies

Wear properties of CrN/NbN superlattice coating deposited on the WC-12Co substrate was investigated while using 100Cr6 steel, SiC and Al₂O₃ ball as counterbodies for friction pairs. The value of friction coefficient and wear rate was lowest at ~ 0.01 and 2.6 × 10^− 7 mm³/Nm, respectively, when coating slides against Al₂O₃ ball. In contrast, friction coefficient and wear rate were increased while sliding with steel and SiC ball. The deviation in friction coefficient was described by mechanical and chemical properties of these balls. Hardness of Al₂O₃ and SiC ball was comparable but significant deviation in friction coefficient was observed. That is related to oxidation resistance of these balls which is high for Al₂O₃ compared to SiC ball as evident by Raman analysis of the wear track. However, hardness and oxidation resistance were low for steel ball which shows oxidational wear mechanism.     

Oxidation behaviour of the Mo–Si–B and Mo–Si–B–Al alloys in the temperature range of 700–1300 °C

The isothermal oxidation kinetics of molybdenum silicide based alloys with composition (in at.%) as 76Mo–14Si–10B (MSB), 77Mo–12Si–8B–3Al (MSB3AL), and 73.4Mo–11.2Si–8.1B–7.3Al (MSB7.3AL) processed by reaction hot pressing of elemental powders, have been investigated in the temperature range of 700–1300 °C in dry air for 24 h. The microstructures of all the alloys have shown the presence of α-Mo, Mo₃Si, Mo₅SiB₂ and SiO₂ or α-Al₂O₃ phases. The oxidation kinetics and the resulting scale characteristics depend on the alloy composition and temperature of exposure. While all the three alloys show unabated loss of mass causing pest disintegration at 700 °C, the MSB3AL and MSB7.3AL alloys undergo large mass loss in the range of 800–900 °C as well. The loss in mass has been attributed primarily to volatilization of MoO₃ as well as spallation. The oxide scales formed in the range of 700–800 °C show SiO₂ and MoO₃, while those formed at 900 °C or above contain Mo, MoO₂ and SiO₂. In addition, α-Al₂O₃ or mullite has been found in the oxide scales of MSB3AL and MSB7.3AL alloys. The oxidation resistance of the Mo–Si–B alloys can be enhanced in the range of 700–800 °C by pre-oxidation treatment at 1150 °C to form a protective scale of B₂O₃–SiO₂.     

SiO2–Al2O3–glass composite coating on Ti–6Al–4V alloy: Oxidation and interfacial reaction behavior

A SiO₂–Al₂O₃–glass composite coating was prepared on Ti–6Al–4V alloy by air spraying and subsequent firing. The oxidation behavior of the specimens at 800 °C and 900 °C for 100 h was studied. The thermal shock resistance of the coating was tested by heating up to 900 °C and then quenching in water. The composite coating acted as an oxygen migration barrier and exhibited good resistance against high temperature oxidation, thermal shock, and oxygen permeation on the Ti–6Al–4V alloy. Coating/alloy interfacial reaction occurred, forming a Ti₅Si₃/Ti₃Al bilayer structure. A thin Al₂O₃ rich layer formed beneath the composite coating during oxidation at 900 °C.     

Oxidation behavior of micro-sized Al2O3–TiC–Co composites prepared from cobalt-coated powders

The oxidation behavior of hot-pressed Al₂O₃–TiC–Co composites prepared from cobalt-coated powders has been studied in air in the temperature range from 200 °C to 1000 °C for 25 h. The oxidation resistance of Al₂O₃–TiC–Co composites increases with the increase of sintering temperature at 800 °C and 1000 °C. The oxidation surfaces were studied by XRD and SEM. The oxidation kinetics of Al₂O₃–TiC–Co composites follows a rate that is faster than the parabolic-rate law at 800 °C and 1000 °C. The mechanism of oxidation has been analyzed using thermodynamic and kinetic considerations.     

Effect of vacuum annealing temperature on tribological behaviors of sintered polycrystalline diamond compact

The sintered polycrystalline diamond compacts (PDCs) were annealed at 200 °C, 300 °C, 400 °C, 500 °C, 600 °C, 700 °C, and 800 °C under vacuum environment. The friction and wear behaviors of the annealed PDCs sliding against Si₃N₄ balls were evaluated by a ball-on-disc tribometer in ambient atmosphere. The compositions, microstructures and surface morphologies of PDC discs and wear scars on Si₃N₄ balls were characterized by energy dispersive spectroscopy (EDS), Raman spectroscopy, and scanning electron microscopy (SEM), respectively. The experimental results demonstrated that the steady friction coefficient decreased at the annealing temperature of 200 °C and increased with annealing temperature increasing. While, the wear rate of PDCs and Si₃N₄ balls increased at 200 °C, and sharply decreased from 300 to 800 °C. The surface morphologies and Raman spectra revealed that the variation law of friction coefficient curves at different annealing temperatures was attributed to carbonaceous transfer films formed on Si₃N₄ balls. The residual stress on PDC surface was reduced after the annealing treatment, thus fine diamond grains were easily extracted from PDC surface onto the contact area during the tribotest which led to the wear of PDC and abrasive wear for both counter parts. These results revealed that the friction and wear behaviors of PDC were significantly affected by the vacuum annealing temperature.     

Oxidation kinetics of the pack siliconized TiAl-based alloy and microstructure evolution of the coating

The cyclic oxidation resistance of a two-phase TiAl-based alloy was remarkably improved with the formation of composite coating by siliconization with mixed powder of 15 wt%Si + 85 wt%Al₂O₃. The composite coating consists of a Ti₅Si₃-based inner layer and an Al₂O₃-based outer layer. The cyclic oxidation test at 900 °C showed that increasing the siliconization temperature benefits the oxidation resistance. The higher the siliconization temperature, the stabler the Ti₅Si₃-based layer and the more Al₂O₃ concentrated in the outer layer. Usually, the oxidation curves consist of three regions, the parabolic, the linear and the quadratic. For the specimen that was siliconized at 1250 °C for 2 h, however, only the parabolic region appeared during the whole cyclic oxidation test at 900 °C up to 1000 h. The weight gain is less than 0.3 mg cm⁻² after cyclic oxidation at 900 °C for 1000 h, corresponding to a parabolic oxidation rate constant, K_p ≈ 6.03 × 10⁻⁵ mg²/(cm⁴ h). Such a low oxidation rate is attributed to the composite layer of the specimen with a stable Ti₅Si₃-based layer and a dense Al₂O₃-based layer.     

Preparation and characterization of Mo/Al2O3 composites

In order to improve the performance of molybdenum, the Mo/Al₂O₃ composites were prepared by using a hydrothermal method for the synthesis of the precursor powders and subsequent powder metallurgical processing. The morphologies of the composite powders and the microstructures and properties of the composites were investigated. Compared with the pure Mo powder, the grains of composite powders are smaller because of the existence of the fine Al₂O₃ particles. The results from the sintered composites show that the fine Al₂O₃ particles are evenly distributed in the Mo matrix and well bonded with the Mo matrix. With increasing Al₂O₃ content, all the values of the micro-hardness, compressive strength and flow stress at 0.08 strain are increased. The strengthening effect is more remarkable at elevated temperatures. At room temperature, the compressive strength and the flow stress at 0.08 strain of the composite with 40 vol.% Al₂O₃ are 1.67 and 2.01 times greater than those of pure molybdenum, respectively, while the values are up to 2.02 and 2.52 at 1100 °C.     

High temperature oxidation and microstructure of MoSi2/MoB composite coating for Mo substrate

The two-layer MoSi₂/MoB composite coatings were developed using the halide activated pack cementation (HAPC) method on Mo substrate. Oxidation resistance property and microstructural evolution of the coatings at high temperatures were investigated. During oxidation exposure, the coatings exhibited a good oxidation resistance property. The mass gains of the coated specimens oxidized at 1200 °C for 100 h and at 1300 °C for 80 h were 0.270 and 0.499 mg/cm², respectively. Compared with the monolithic MoSi₂ coatings, the transformation of MoSi₂ phase in the MoSi₂/MoB composite coatings was more sluggish at elevated temperatures. The growth rate constant of the Mo₅Si₃ layer in the composite coatings was two orders of magnitude lower than that of the Mo₅Si₃ layer in the monolithic coatings at 1300 °C. The microstructural degradation of MoSi₂ in the composite coatings at high temperatures was slowed by the introduced MoB layer. The MoB layer in the composite coatings is useful to prolong the service life of MoSi₂ coatings at high temperatures.     

Effect of hot-dip aluminizing on the oxidation resistance of Ti–6Al–4V alloy at high temperatures

Hot-dip aluminizing and interdiffusion treatment were used to develop a TiAl₃-rich coating on Ti–6Al–4V alloy. Interrupted oxidation at temperatures from 600 to 900 °C and isothermal oxidation at 700 and 800 °C of the coating were conducted. The coating markedly decreases the oxidation rate in comparison with the alloy at temperatures below 800 °C during the interrupted oxidation. The oxidation kinetics follows parabolic relations at 700 and 800 °C during the isothermal oxidation. A layered structure of Al₂O₃/TiAl₃/TiAl₂/TiAl/alloy from the outside to the inside forms after oxidation at 700 °C without changing the main body of the coating.     

Hot corrosion properties of composite coatings in the presence of NaCl at 700 and 900 °C

Cyclic hot corrosion tests have been carried out on three coatings (one NiCoCrAlY and two composite coatings) at 700 and 900 °C. The kinetic curves and evolution of microstructure show that the composite coating with a Cr-base interlayer performs best. The Cr₂O₃ scale is more effective to protect the coating at 700 °C than that at 900 °C. The corrosion process is accelerated by NaCl via forming volatile MCl_x and inducing the formation of molten voids in the coating or extra oxidation at the interface of fusant/oxide scale, determined by the temperature and the compositions of the coating.     

Mechanical and barrier properties of MOCVD processed alumina coatings on Ti6Al4V titanium alloy

This study focuses on the implementation of different aluminum oxide coatings processed by metal-organic chemical vapor deposition from aluminum tri-isopropoxide on commercial Ti6Al4V titanium alloy to improve its high temperature corrosion resistance. Films grown at 350 °C and at 480 °C are amorphous and correspond to formulas AlOOH, and Al₂O₃, respectively. Those deposited at 700 °C are composed of γ-Al₂O₃ nanocrystals dispersed in a matrix of amorphous alumina. Their mechanical properties and adhesion to the substrates were investigated by indentation, scratch and micro tensile tests. Hardness and rigidity of the films increase with increasing deposition temperature. The hardness of the coatings prepared at 350 °C and 480 °C is 5.8 ± 0.7 GPa and 10.8 ± 0.8 GPa respectively. Their Young's modulus is 92 ± 8 GPa (350 °C) and 155 ± 6 GPa (480 °C). Scratch tests cause adhesive failures of the films grown at 350 °C and 480 °C whereas cohesive failure is observed for the nanocrystalline one, grown at 700 °C. Micro tensile tests show a more progressive cracking of the latter films than on the amorphous ones. The films allow maintaining good mechanical properties after corrosion with NaCl deposit during 100 h at 450 °C. After corrosion test only the film deposited at 700 °C yields an elongation at break comparable to that of the as processed samples without corrosion. The as established processing–structure–properties relation paves the way to engineer MOCVD aluminum oxide complex coatings which meet the specifications of the high temperature corrosion protection of titanium alloys with regard to the targeted applications.     

Synthesis and characterization of Al2O3–TiC nano-composite by spark plasma sintering

Al₂O₃–10TiC composite was synthesized by high energy ball milling followed by spark plasma sintering (SPS) process. Microstructure of the sintered composite samples reveals homogeneous distribution of the TiC particles in Al₂O₃ matrix. Effect of sintering temperature on the microstructure and mechanical properties was studied. The sample sintered at 1500 °C shows a measured density of 99.97% of their theoretical density and hardness of 1892 Hv with very high scratch resistance. These results demonstrate that powder metallurgy combined with spark plasma sintering is a suitable method for the production of Al₂O₃–10TiC composites.     

Effect of reinforcement NbC phase on the mechanical properties of Al2O3-NbC nanocomposites obtained by spark plasma sintering

The sintering behavior of Al₂O₃-NbC nanocomposites fabricated via conventional and spark plasma sintering (SPS) was investigated. The nanometric powders of NbC were prepared by reactive high-energy milling, deagglomerated, leached with acid, added to the Al₂O₃ matrix in the proportion of 5 vol% and dried under airflow. Then, the nanocomposite powders were densified at different temperatures, 1450–1600 °C. Effect of sintering temperature on the microstructure and mechanical properties such as hardness, toughness and bending strength were analyzed. The Al₂O₃-NbC nanocomposites obtained by SPS show full density and maximum hardness value > 25 GPa and bending strength of 532 MPa at 1500 °C. Microstructure observations indicate that NbC nanoparticles are dispersed homogeneously within Al₂O₃ matrix and limit their grain growth. Scanning electron microscopy examination of the fracture surfaces of dense samples obtained at 1600 °C by SPS revealed partial melting of the particle surfaces due to the discharge effect.     

Mullite/molybdenum ceramic–metal composites

Dense (>98 th%) homogeneous mullite/Mo (32 vol.%) composites with two different Mo average grain sizes (1.4 and 3 μm) have been obtained at 1650°C in vacuum and in reducing condition. Depending on the Mo grain size and processing atmosphere, the K_IC ranges from 4 to 7 MPa m^1/2 and σ_f from 370 to 530 MPa. The MoO₂–2SiO₂·3Al₂O₃–Mo system was found to be compatible in solid state, and a solid solution of ≈4 wt% of MoO₂ in mullite at 1650°C was detected. A solid state dewetting of MoO₂ from the surface of the Mo particle takes place during sintering. It was found that the absence of MoO₂ in the mullite/Mo composites by processing in reducing conditions increases the strength of the metal/ceramic interface and the plasticity of the Mo metal particles, thus strengthening the composite by a crack bridging mechanism. As a result, the K_IC and the σ_f values of the ceramic–metal composite were found to be ≈4 times and ≈2 times higher than the ones corresponding to the mullite matrix.     

Magnetic properties and their dependence on deposition parameters of Co/Al2O3 multilayers grown by pulsed laser deposition

Co/Al₂O₃ multilayered thin films were grown on Si (111) substrates by pulsed laser deposition (PLD) at temperatures from room temperature (RT) to 600 °C. The Co/Al₂O₃ multilayered thin film grown at RT contains continuous cobalt layers in alumina matrices, with no evidence of island formation. On the other hand, cobalt showed a tendency to form islands in alumina matrices for growth temperatures in the range of 300–600 °C. All the Co/Al₂O₃ multilayered thin films showed ferromagnetic behavior up to RT. It was observed that variations in the deposition parameters can significantly influence the magnetic properties of Co/Al₂O₃ multilayers. Depending on the temperature and pulse rate, RT coercivities in the 50–300 Oe range were observed. Films deposited at 600 °C using a laser pulse rate of 10 Hz exhibited a decrease of coercivity with increasing measurement temperature. On the other hand, films deposited at 600 °C using a reduced pulse rate of 2 Hz demonstrated an “anomalous” relationship between low-temperature coercivity and temperature. In these films, coercivity exhibited a weak tendency to increase with temperature. Squareness (M_r/M_s) of the hysteresis loops and its dependence on the temperature was also shown to be strongly affected by the deposition parameters. These observations have been rationalized on the basis of two competing magnetic anisotropies that act along different directions in the material.     

Tribological properties of Ti-aluminide reinforced Al-based in situ metal matrix composite

For structural application of moving components, the tribological properties (friction and wear) are considered to be one of the major factors controlling the performance. In recent years, lightweight metal matrix composites (MMC) have received wider attention for their technological application, such as automotive parts etc. This paper reports the tribological behavior of Al based composites reinforced with in situ Ti_xAl_y and Al₂O₃ particles. The wear experiments were performed on a newly designed fretting tribometer to evaluate the role of intermetallic particulates on the wear performance of in situ composites against bearing steel under the ambient conditions of temperature (22–25 °C) and humidity (50–55% RH). Based on the topographical observation of the worn surfaces the plausible wear mechanisms are discussed. An important result is that Al-based composites with 20 vol% reinforcement exhibit an extremely low coefficient of friction of 0.2 under unlubricated conditions. Also, around five times lower wear volume is measured with 20 vol% composites when compared to unreinforced Al.     

Glass coatings on stainless steels for high-temperature oxidation protection: Mechanisms

The oxidation behavior of a martensitic stainless steel with or without glass coating was investigated at 600–800 °C. The glass coating provided effective protection for the stainless steel against high-temperature oxidation. However, it follows different protection mechanisms depending on oxidation temperature. At 800 °C, glass coating acts as a barrier for oxygen diffusion, and oxidation of the glass coated steel follows linear law. At 700 or 600 °C, glass coating induces the formation of a (Cr, Fe)₂O₃/glass composite interlayer, through which the diffusion of Cr³⁺ or Fe³⁺ is dramatically limited. Oxidation follows parabolic law.