Tribological characteristics of nanometer Si3N4 filled poly(ether ether ketone) under distilled water lubrication

Nanometer Si₃N₄ filled poly(ether ether ketone) (PEEK) composite blocks with different filler proportions were prepared by compression molding. Their friction and wear properties under distilled water lubrication, as well as under ambient dry conditions, were investigated on a block on ring machine by running a plain carbon steel (AISI 1045 steel) ring against the PEEK composite block. The worn surfaces of nanometer Si₃N₄ filled PEEK and the transfer film were observed by scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The results showed that distilled water could reduce the friction coefficient of nanometer Si₃N₄ filled PEEK but with the sacrifice of a large reduction in wear resistance. The SEM and EPMA pictures of the worn surfaces indicated that the wear mechanisms of nanometer Si₃N₄ filled PEEK under distilled water lubrication and ambient dry rubbing conditions were different. Under water lubrication, the dominant wear mechanism of the filled PEEK was severe abrasive wear with surface fracture. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1394–1400, 2001     

Electromagnetic wave absorption properties of hot-pressed Si3N4-SiC ceramic nanocomposites

High-density Si₃N₄-SiC ceramic nanocomposites have exceptional mechanical properties, but little is known about their electromagnetic wave absorption (EMA) capabilities. In this paper, the effects of sintering temperature and starting material compositions on the dielectric and EMA properties of hot-pressed Si₃N₄-SiC ceramic nanocomposites were investigated. The real and imaginary permittivities of Si₃N₄-SiC ceramic nanocomposites increase with increasing sintering temperature or SiC content, particularly at the sintering temperature of 1850°C and SiC content of 50 wt.%. This is primarily due to the improvement of interfacial and defect polarizations, which is caused by the doping of nitrogen into the SiC nanocrystals during the solution-precipitation process. The real and imaginary permittivities of Si₃N₄-SiC ceramic nanocomposites show decreasing trends as sintering aid content increases. Si₃N₄-SiC ceramic composites have both good EMA and mechanical properties when they are sintered at 1850°C with 30 wt.% SiC and 5–8 wt.% sintering aids. The minimum reflection loss and maximum flexural strength reach -58 dB and 586 MPa, respectively. Materials with multilayered structural designs have both strong and broad EMA properties.     

The influence of annealing on the crystallization and tribological behavior of MWNT/PEEK nanocomposites

In this study, annealing influence on crystallization and scratch behavior of neat and multi‐wall carbon nanotube (MWNT) reinforced poly(ether ether ketone) (PEEK) nanocomposites have been investigated. Crystallization behavior of normal and annealed samples was investigated by using differential scanning calorimeter (DSC). Scratch behavior of normal and annealed samples was investigated by using micro scratch tester. In DSC analysis, it was detected that, melting enthalpy of annealed neat PEEK was increased sharply when compared to neat PEEK. Melting enthalpies of annealed PEEK nanocomposites prepared with addition of up to 1 wt% MWNT were increased with a decreased trend. However, nanocomposites with higher contents of MWNTs (>1 wt%) were dramatically affected by annealing process and melting enthalpy decreased sharply. Friction coefficient values of “annealed MWNT reinforced PEEK composites” were found to be lower than “normal PEEK composites.” Annealing process affects scratch hardness of both annealed and MWNT reinforced PEEK. Annealed nanocomposites with various MWNT concentrations showed higher scratch hardness values than normal PEEK nanocomposites. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

High performance polymer composites on PEEK reinforced with aluminum oxide

A study on high performance poly(ether‐ether‐ketone) (PEEK) composites prepared by incorporating aluminum oxide (Al₂O₃), 0 to 50 wt % by hot compaction at 15 MPa and 350°C was described. Density, thermogravimetric analysis/differential scanning calorimetry, and scanning electron microscopy (SEM) were employed to evaluate their density, thermal stability, crystallinity, and morphology. Experimental density was found higher than theoretical density, which indicates that composite samples are sound. It was found that the addition of micron sized (< 15 μm) Al₂O₃ increased the peak crystallization temperature by 12°C when compared with neat PEEK with insignificant increase in melting temperature. Half‐time of crystallization is reduced from 2.05 min for the neat PEEK to 1.08 min for PEEK incorporated with 30 wt % Al₂O₃ because of the strong nucleation effect of Al₂O₃. The thermal stability of composites in air atmosphere was increased by 26°C. However, thermal stability in nitrogen atmosphere decreases at lower concentration of Al₂O₃ but increases above 20 wt % of Al₂O₃. Uniform dispersion of Al₂O₃ particles was observed in PEEK polymer matrix by SEM. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 4623–4631, 2006     

Effect of Nanosized TiC0.37N0.63 on Unlubricated Wear Responses of Si3N‐Based Nanocomposites Under Low Hertzian Stress

Si₃N₄‐based nanocomposites containing 0–50 wt% TiC_0.37N_0.63 are directly consolidated at 1700°C by spark plasma sintering, and their reciprocal sliding behavior against a Si₃N₄ counterbody is investigated under a maximum Hertzian stress of 1.27 GPa in unlubricated conditions. The average grain widths of Si₃N₄ and TiC_0.37N_0.63 are about 85 and 90 nm, respectively. The decreasing relative densities of the as‐sintered nanocomposites indicate that the nano‐TiC_0.37N_0.63 may introduce pores and reduce the hardness and fracture resistance of the materials. The brittleness index for sliding contacts in all the samples is 25–31, indicating brittle fracture taking place on the wear surface and inducing cavities. When the mean free paths of nano‐TiC_0.37N_0.63 are slightly greater than grain length of Si₃N₄, the best wear resistance is achieved in Si₃N₄ containing 20/30 wt% TiC_0.37N_0.63 due to the process of surface smoothing by triboproducts. Severe wear response can be observed in Si₃N₄ nanocomposites containing 0, 10, 40, and 50 wt% of TiC_0.37N_0.63. The wear responses are explained by considering the microstructural parameters (like grain characteristics for both phases and mean free path of nano‐TiC_0.37N_0.63) and contact‐induced fracturing behavior, as well as tribochemical reactions.     

Study on microhardness,dynamic mechanical,and tribological properties of PEEK/Al2O3 composites

The wear and friction properties of poly (ether‐ether‐ketone) (PEEK) reinforced with 0–33 vol % (60 wt %) micron size Al₂O₃ composites were evaluated at a sliding speed of 1.0 m/s and nominal pressure from 0.5 to 1.25 MPa under dry sliding conditions using a pin‐on‐disk wear tester. The wear resistance of the pure PEEK is 10‐fold higher than that of mild steel under the similar test condition. It is improved to 18‐fold as compared with mild steel at 3.5 vol % Al₂O₃ content. The improvement in wear properties may be attributed to the thin, tenacious, and coherent transfer film formed between the steel countersurface and composite pin. However, the wear resistance of PEEK containing above 3.5 vol % Al₂O₃ was deteriorated, despite their higher hardness and stiffness as compared with that of composites containing lower Al₂O₃ content. This is attributed to the formation of thick and noncoherent transfer film, which does not prevent the wear of the composites from hard asperities of countersurface. Moreover, hard Al₂O₃ particles present in transfer film act as third body wear mechanism. The coefficient of friction of the composites is higher than that of pure PEEK. SEM and optical microscopy have shown that wear of pure PEEK occurs by the mechanism of adhesion mainly whereas of PEEK composites by microploughing and abrasion. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Mechanical properties and sliding wear behavior of potassium titanate whiskers‐reinforced poly(ether ether ketone) composites under water‐lubricated condition

Polyetheretherketone (PEEK) composites reinforced with potassium titanate whiskers (PTW) were compounded using a twin‐screw extruder followed by injection molding. The effects of PTW on the mechanical properties, crystallization performances and wear behaviors of PEEK under water lubrication have been investigated. It was denoted that the yield strength, Young's modulus, and microhardness of the composites increased with increasing whisker content, but the elongation at break and the impact strength showed decreasing trend. It was revealed that the inclusion of PTW could effectively reduce the friction coefficient and enhance the wear resistance of the PEEK. The DSC tests showed that the crystallinity of the composite slightly decreased with the addition of PTW, which might imply that the crystallinity of PEEK was not the dominant factor that influenced the wear properties of the composites. The enhancement on the wear resistance was attributed to the reinforced effect of PTW on PEEK. The wear mechanism changed from fatigue wear into mild abrasive wear when the PTW was added into PEEK. The lowest wear rate 9.3 × 10^?8 mm³/Nm was achieved at 10 wt % PTW content. However, excessive whiskers would cause severe abrasive wear to the composite. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Influence of hBN content on mechanical and tribological properties of Si3N4/BN ceramic composites

Silicon nitride materials containing 1–5 wt% of hexagonal boron nitride (micro-sized or nano-sized) were prepared by hot-isostatic pressing at 1700 °C for 3 h. Effect of hBN content on microstructure, mechanical and tribological properties has been investigated. As expected, the increase of hBN content resulted in a sharp decrease of hardness, elastic modulus and bending strength of Si₃N₄/BN composites. In addition, the fracture toughness of Si₃N₄/micro BN composites was enhanced comparing to monolithic Si₃N₄ because of toughening mechanisms in the form of crack deflection, crack branching and pullout of large BN platelets. The friction coefficient was not influenced by BN addition to Si₃N₄/BN ceramics. An improvement of wear resistance (one order of magnitude) was observed when the micro hBN powder was added to Si₃N₄ matrix. Mechanical wear (micro-failure) and humidity-driven tribochemical reaction were found as main wear mechanisms in all studied materials.     

Study on high‐performance poly(etheretherketone)/Si3N4 nanocomposites: New electronic substrate materials

The morphology, thermal, mechanical, and dielectric properties of high‐performance poly(etheretherketone)/Si₃N₄ nanocomposites fabricated by hot pressing were investigated. It was found that the coefficient of thermal expansion (CTE) and dissipation factor decreased significantly with increasing Si₃N₄ content, whereas thermal stability was affected slightly. A nanocomposite with 30 wt% Si₃N₄ exhibited about 45% and 23% decrease in CTE, below and above T_g, respectively. The glass‐transition temperature (T_g) was increased up to 20°C. Microhardness was improved by 20% at 10 wt% Si₃N₄ content and thereafter it improved slightly. Modified rule of mixture with β = 0.1 or Halpin–Tsai model with ξ = 4 fits well the microhardness. The dielectric constant and loss factor of the nanocomposites are quite low, and thermal stability is much higher compared with commercial products. Various models were also used to correlate CTE and dielectric constant. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers     

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.     

Preparation and mechanical properties of Si3N4 nanocomposites reinforced by Si3N4@rGO particles

A new type of reduced graphene oxide-encapsulated silicon nitride (Si₃N₄@rGO) particle was synthesized via an electrostatic interaction between amino-functionalized Si₃N₄ particles and graphene oxide (GO). Subsequently, the Si₃N₄@rGO particles were incorporated into a Si₃N₄ matrix as a reinforcing phase to prepare nanocomposites, and their influence on the microstructure and mechanical properties of the Si₃N₄ ceramics was investigated in detail. The microstructure analysis showed that the rGO sheets were uniformly distributed throughout the matrix and firmly bonded to the Si₃N₄ grains to form a three-dimensional carbon network structure. This unique structure effectively increased the contact area and load transfer efficiency between the rGO sheets and the matrix, which in turn had a significant impact on the mechanical properties of the nanocomposites. The results showed that the nanocomposites with 2.25 wt.% rGO sheets exhibited mechanical properties that were superior to monolithic Si₃N₄; the flexural strength increased by 83.5% and reached a maximum value of 1116.4 MPa, and the fracture toughness increased by 67.7% to 10.35 MPa·m^1/2.     

Isothermal crystallization behavior of nano-alumina particles-filled poly(ether ether ketone) composites

The isothermal crystallization behavior of nano-alumina particle-filled poly(ether ether ketone) (PEEK) composites has been investigated using differential scanning calorimeter. The results show that all the neat PEEK and nano-alumina-filled PEEK composites exhibit the double-melting behavior under isothermal crystallization. The peak crystallization times (τ_p) for all the neat PEEK and PEEK/aluminum oxide (Al₂O₃) composites increase with increasing crystallization temperature. Moreover, the crystallinity of the PEEK/Al₂O₃ composite with 7.5 wt % nano-filler content reached the maximum value of 44.8% at 290°C, higher than that of the neat PEEK polymer. From the lower value in τ_p and higher value in X_c for the PEEK/Al₂O₃ composites, the inclusion of the nano-alumina into the PEEK matrix favored the occurrence of heterogeneous nucleation. The Avrami exponents n of all the neat PEEK and PEEK/Al₂O₃ composites ranged from 2 to 3, and the n values for PEEK/Al₂O₃ composites were slightly higher than that of the neat PEEK polymer, indicating that the inclusion of the nano-filler made the crystallization mechanism more complex. However, the growth rate of crystallization was lowered as the nano- filler was introduced, and the decrease in growth rate reduced the grain size of the PEEK spherulites because of the lowering of molecule mobility during isothermal crystallization. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

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.     

Polymer-derived SiHfN ceramics: From amorphous bulk ceramics with excellent mechanical properties to high temperature resistant ceramic nanocomposites

Within the present work, additive-free amorphous bulk SiHfN ceramics with excellent mechanical properties were prepared by a resource-efficient low-temperature molding method, namely warm-pressing. As densification mechanism viscous flow has been identified based on cross-linking reaction. The critical problems concerning gas evolution and crystallization inducing bloating and cracking are addressed through controlled thermolysis and pressure. The microstructural evolution of the SiHfN ceramics indicates that the incorporation of Hf in perhydropolysilazane not only increases the ceramic yield (97.4 wt%) and crystallization resistance (1300 °C), but also suppresses the transformation from α-Si₃N₄ to β-Si₃N₄ at high temperatures (1700 °C). Especially, HfN/α-Si₃N₄ nanocomposites converted by the SiHfN ceramics at 1500 °C show a slight weight loss of 3.13 wt%, indicating the high temperature resistance of the ceramic nanocomposites. The method proposed in this work opens a new strategy to fabricate additive-free polycrystalline Si₃N₄- and amorphous Si₃N₄-based (nano)composites.     

A comprehensive study on the influence of the polyorganosilazane chemistry and material shape on the high temperature behavior of titanium nitride/silicon nitride nanocomposites

The high temperature crystallization behavior of polytitanosilazane-derived amorphous SiTiN ceramics was investigated in a nitrogen atmosphere using XRD, Raman spectroscopy, TEM, SEM and BET. At 1400 °C, TiN is the first phase to nucleate in SiTiN ceramics forming nanocomposites with a homogeneous distribution of TiN nanocrystals within an amorphous Si₃N₄ matrix. Above 1400 °C, XRD indicates that the temperature at which Si₃N₄ crystallizes depends on the volume fraction of TiN present in nanocomposites. This is closely related to the chemistry of the polyorganosilazanes used to synthesize polytitanosilazanes. The use of perhydridopolysilazane, the most reactive polyorganosilazane, allows preparing TiN/Si₃N₄ nanocomposites with a remarkable stability of the amorphous matrix up to 1800 °C as mesoporous materials and powders. Dense monoliths crystallize earlier than the powder analogs because of the use of an ammonia pre-treatment before polymer warm-pressing.     

Temperature-driven wear behavior of Si3N4-based ceramic reinforced by in situ formed TiC0.3N0.7 particles

Si₃N₄ as a structural ceramic is desirable for applications in spacecraft, transportation, and energy, but its poor high-temperature properties still do not satisfy the actual requirements. Here, a TiC_0.3N_0.7 reinforced Si₃N₄ ceramic is successfully designed and fabricated via the high-temperature nitridation of TiC_x. It is found that TiC_0.3N_0.7 grains with the size of 1-2 μm are uniformly dispersed in the Si₃N₄ matrix and show a firm bond with substrate. Compared with pure Si₃N₄, the doping of harder TiCN phase can effectively improve ceramic's hardness and fracture toughness at a certain temperature. Importantly, the ceramic material displays extraordinary wear resistance across a wide temperature range (eg, the wear rate of TiC_0.3N_0.7 containing Si₃N₄ over 63 times and 178 times better than pure Si₃N₄ at 600 and 900°C, respectively). More broadly, a correlation between wear mechanism and temperature is established, and the result shows that the mechanical strength and tribochemical oxidation as two key factors determine the wear behavior of the material. These results developed here can provide a springboard for preparation and optimization of multiphase ceramics that serve under high-temperature conditions.     

Self-propagating high temperature synthesis (SHS) of porous Si3N4-based ceramics with considerable dimensions and study on mechanical properties and oxidation behavior

The aim of present work is to fabricate porous Si₃N₄ ceramics with considerable dimensions and homogeneous microstructure by self-propagating high temperature synthesis (SHS) using Si, Si₃N₄ diluent and Y₂O₃ as raw materials. The results indicate that Si₃N₄ diluent with coarse particle sizes and appropriate β-phase content is beneficial to obtain porous Si₃N₄ ceramics with homogeneous microstructure and excellent mechanical property by controlling the shrinkage inside the sample. The produced Si₃N₄ ceramics possessed excellent flexural strength of 168 MPa～259 MPa, and high Weibull modulus of 11.0～17.2. Additionally, BN and SiC are added as second phase to modify the properties of Si₃N₄-based ceramics. Optimum flexural strength of 170 MPa and 137 MPa were obtained with 10 wt.% addition of BN and SiC respectively. After oxidation at 1100 °C～1300 °C, second phase-doped Si₃N₄ ceramics also presented higher residual strength than pure Si₃N₄ ceramics.     

Atomistic analyses of the effect of temperature and morphology on mechanical strength of Si–C–N and Si–C–O nanocomposites

3-D molecular dynamics (MD) analyses of SiC–Si₃N₄ nanocomposite deformation and SiCO nanocomposite deformation are performed at 300 K, 900 K, and 1500 K. In SiC–Si₃N₄ nanocomposites, distribution of second phase SiC particles, volume fraction of atoms in GBs, and GB thickness play an important role in temperature dependent mechanical behavior. The deformation mechanism is a trade-off between the stress concentration caused by SiC particles and Si₃N₄–Si₃N₄ GB sliding. The temperature increase tends to work in favor of GB sliding leading to softening of structures. However, microstructural strength increases with increase in temperature when GBs are absent. In the case of SiCO nanocomposites, findings indicate that temperature change dependent amorphization of nanodomains, the nanodomain wall placement, the nanodomain wall thickness, and nanodomain size are important factors that directly affect the extent of crystallinity and the strength against mechanical deformation.     

Preparation and characterization of Si3N4-based composite ceramic tool materials by microwave sintering

Si₃N₄-based composite ceramic tool materials with (W,Ti)C as particle reinforced phase were fabricated by microwave sintering. The effects of the fraction of (W,Ti)C and sintering temperature on the mechanical properties, phase transformation and microstructure of Si₃N₄-based ceramics were investigated. The frictional characteristics of the microwave sintered Si₃N₄-based ceramics were also studied. The results showed that the (W,Ti)C would hinder the densification and phase transformation of Si₃N₄ ceramics, while it enhanced the aspect-ratio of β-Si₃N₄ which promoted the mechanical properties. The Si₃N₄-based composite ceramics reinforced by 15 wt% (W,Ti)C sintered at 1600 °C for 10 min by microwave sintering exhibited the optimum mechanical properties. Its relative density, Vickers hardness and fracture toughness were 95.73 ± 0.21%, 15.92 ± 0.09 GPa and 7.01 ± 0.14 MPa m^1/2, respectively. Compared to the monolithic Si₃N₄ ceramics by microwave sintering, the sintering temperature decreased 100 °C,the Vickers hardness and fracture toughness were enhanced by 6.7% and 8.9%, respectively. The friction coefficient and wear rate of the Si₃N₄/(W,Ti)C sliding against the bearing steel increased initially and then decreased with the increase of the mass fraction of (W,Ti)C., and the friction coefficient and wear rate reached the minimum value while the fraction of (W,Ti)C was 15 wt%.     

Preparation and radiation shielding properties of Gd2O3/PEEK composites

Gd₂O₃/PEEK (poly ether ether ketone) composites were prepared on a twin‐screw extruder by the incorporation of Gd₂O₃ as a shield against X‐ray to PEEK matrix. The influence of Gd₂O₃ addition and surface treatment of the particles with sulfonated PEEK (SPEEK) on the morphology, thermal and mechanical properties of the composites was investigated by SEM, DSC, TGA and tensile tests respectively. DSC results showed that both the crystallization temperature (T_c) and melting temperature (T_m) of the composites decreased compared with pure PEEK at random filler content, which suggested that Gd₂O₃ hindered the process of PEEK nucleation. The tensile modulus of the composites increased with addition of Gd₂O₃ and the strain to break decreased. But the tensile modulus and strength of modified series were always higher than that of unmodified ones at the same filler content. The X‐ray shielding properties of composites apparently improved with the increment of the Gd₂O₃. The X‐ray transmittance (A) of 45% S₄GPEEK reduced greatly by about three to eight times compared with PEEKs in all energy range measured. POLYM. COMPOS., 36:651–659, 2015. © 2014 Society of Plastics Engineers