In-situ Synthesis of Ag₂S Nanoparticles to Enhance Mechanical and Tribological Properties of PAI Coating

In order to solve the problem of the small size preparation, size control and uniform dispersions of nanoparticles in the coating, the in-situ synthesized method of silver sulfide (Ag₂S) nanoparticles assisted by the thermal decomposition of the single-source precursor in the polyamide-imide (PAI) coating was developed and the particle size was controlled in present paper. X-ray diffractometer and high-resolution field emission scanning electron microscope were used to characterize and analyze the phase structure, morphology, particle size and size distribution of the in-situ synthesized Ag₂S nanoparticles. The influence of Ag₂S nanoparticles on the mechanical properties and mechanical properties of the PAI coatings were investigated in detail. And the enhancement mechanisms and friction-wear mechanisms were discussed.　Firstly, the concentration, particle size and size distribution of the Ag₂S nanoparticles were adjusted by controlling the amount of single-source precursor added in the coating. Generally speaking, the Ag₂S nanoparticles synthesized in-situ were uniformly dispersed in the PAI coating, and had high purity as no other impurity phases were generated. When the single-source precursor content was lower, less Ag₂S nanoparticles were generated and their particle size was smaller in the composite coating. The average particle size of Ag₂S nanoparticles increased as the single-source precursor content increased in the coating. Specifically, when the content of Ag₂S nanoparticles was 1.0%, the particle size of Ag₂S nanoparticles was 24 nm, and the size distribution was relatively uniform. When the weight fraction of Ag₂S nanoparticles increased to 3.0%, 5.0%, 7.0% and 9.0%, Ag₂S nanoparticles showed a bimodal size distribution. The particle size of Ag₂S nanoparticles was 35 nm and 10 nm when the weight fraction was 3.0%, 75 nm and 12 nm when the content was 5.0%, 116 nm and 19 nm when the weight fraction was 7.0%, 230 nm and 26 nm when the weight fraction was 9.0%. These results showed that the amount of single-source precursor added in the coating greatly affected the particle size of Ag₂S nanoparticles synthesized in-situ, that is, the particle size and size distribution of Ag₂S nanoparticles can be effectively controlled by adjusting the added content of the single-source precursor.　Subsequently, the mechanical properties (micro-hardness, elastic modulus, plastic deformation ability and scratch resistance performance) of the PAI composite coatings reinforced by in-situ synthesized Ag₂S nanoparticles with different contents were studied. The results demonstrated that the micro-hardness and elastic modulus of the composite coating were significantly improved after Ag₂S nanoparticles were in-situ introduced. As the content of Ag₂S nanoparticles increased, the micro-hardness and elastic modulus of the nanocomposite coatings both showed a trend of increasing first and then decreasing. In particular, when the weight fraction of Ag₂S nanoparticles was 5.0%, the nanocomposite coating exhibited the largest micro-hardness (H=372 MPa) and elastic modulus (E=6497 MPa), which increased by 39.85% and 95.63% compared with those of the pure PAI coating without Ag₂S nanoparticles (H=266 MPa, E=3321 MPa), respectively. Besides, the addition of in-situ synthesized Ag₂S nanoparticles significantly reduced the critical displacement (corresponds to indentation depth) and residual depth of the coating. The maximum displacement of the pure PAI coating was 1 600.35 nm, and the residual depth was 780.16 nm, which were much higher than those of other coatings containing in-situ synthesized Ag₂S nanoparticles. When the Ag₂S nanoparticle content was 5.0%, the critical displacement and residual depth of the coating decreased most obviously, the maximum displacement dropped to 1 192.20 nm, and the residual depth dropped to 575.44 nm. In other words, the in-situ synthesized Ag₂S nanoparticles greatly affected the plastic deformation resistance and elastic recovery ability of the composite coating, and the enhancement effect was most remarkable when the weight fraction was 5.0%.　The scratch resistance test results of the nanocomposite coating showed that the critical load where coating damaged to begin of the pure PAI coating without Ag₂S nanoparticles was lower, of 7.76±0.12 N. After Ag₂S nanoparticles were introduced in-situ, the critical load for nanocomposite coating increased. The critical load for nanocomposite coatings with Ag₂S nanoparticles weight fraction of 1.0%, 3.0%, 5.0%, 7.0%, 9.0% increased to 9.63±0.06 N, 10.16±0.04 N, 12.64±0.27 N, 11.60±0.05 N and 8.26±0.21 N, respectively. Obviously, when the content of Ag₂S nanoparticles was 5.0%, the critical load of the nanocomposite coating was the largest, which meant that various amount of Ag₂S nanoparticles displayed different enhancement effects on the bonding strength and internal strength of the coating, and the optimal weight fraction of Ag₂S nanoparticles was 5.0%. It can be seen from these results that the in-situ synthesized Ag₂S nanoparticles can effectively enhance the mechanical properties of the nanocomposite coating, and the enhancement effect had a large dependence on their content and size. When the weight fraction was 5.0%, the Ag₂S nanoparticles with a balanced size and bimodal distribution showed better enhancement effect.　The analysis and evaluation of the dynamic thermomechanical properties of the nanocomposite coating showed that the introduction of the in-situ synthesized Ag₂S nanoparticle improved the storage modulus (reflecting the stiffness of the material) in the glassy region of the coating. The storage modulus value of pure PAI coating was lowest (2 424 MPa). As the weight fraction of Ag₂S nanoparticles increased from 1.0% to 9.0%, the storage modulus of the nanocomposite coating increased first and then decreased. When their weight fraction was 5.0%, it showed the highest storage modulus of 3 553 MPa, which was increased by 46.6% compared with that of the pure PAI coating, corresponding to providing a higher carrying capacity. In addition, the glass transition temperature of the pure PAI coating was 153 ℃, and the glass transition temperature of the nanocomposite coating reinforced with Ag₂S nanoparticles synthesized in-situ was greatly increased. Especially for the 5.0% Ag₂S nanoparticle reinforced nanocomposite coating, the glass transition temperature increased to 175 ℃, which was higher than that of other coatings. In addition, the introduction of in-situ synthesized Ag₂S nanoparticles made the peak intensity of the loss factor-temperature curve of the enhanced nanocomposite coating lower than that of the pure PAI coating, and the loss factor value of the 5.0% Ag₂S nanoparticle enhanced nanocomposite coating was the lowest, which showed the stronger energy dissipation capability. All these results demonstrated that the in-situ synthesized Ag₂S nanoparticles greatly affected the dynamic thermomechanical properties of the PAI coating, and their influence trend was consistent with the changes in micro-hardness and elastic modulus.　The enhancement mechanisms of Ag₂S nanoparticles to mechanical properties was that, firstly, the presence of Ag₂S nanoparticles produces a stress concentration effect in the process of material deformation, and the surrounding resin matrix was yielded, which can absorb a large amount of deformation and effectively strengthen the strength and toughness of the coating. Secondly, the presence of rigid Ag₂S nanoparticles also inhibited the cracks propagation during coating deformation, and promoted the passivation and termination of cracks. The reason for crack passivation or termination was that the inorganic Ag₂S nanoparticles would not cause large elongation deformation. Under the action of large tensile stress, the part interface between the Ag₂S nanoparticles and the matrix debonded to form a gap, so that the crack was passivated and ceased to develop into a destructive crack. In addition, the interface debonding caused by yield and stress concentration consumed more energy, thereby enhancing the mechanical toughness and resistance to plastic deformation of the coating. When the Ag₂S nanoparticle weight fraction reached 5.0%, this stress concentration effect and yield deformation effect reached the maximum, therefore, it showed a more significant enhancement to the mechanical properties.　Under dry friction conditions, the friction and wear behaviors of the in-situ synthesized Ag₂S nanoparticles reinforced composite coatings were further studied. The initial friction coefficient of pure PAI coating was relatively large (about 0.245) and quite unstable as the friction coefficient curve fluctuated greatly. After 700 s, the friction coefficient began to rise significantly and wear failure began. At the end of the friction test, the friction coefficient increased to 0.375. Compared with the pure resin system, the in-situ introduction of Ag₂S nanoparticles significantly reduced the friction coefficient of the nanocomposite coating, and the dynamic friction coefficient curve became relatively stable, and there was no sudden failure during the friction test. As the content of Ag₂S nanoparticles gradually raised, the average friction coefficient of the nanocomposite coating increased first and then decreased. For the 5.0% Ag₂S nanoparticle reinforced nanocomposite coating, the average friction coefficient was the lowest (0.210) and the friction coefficient curve was the best stable. In addition, the pure PAI coating exhibited the highest wear rate, i.e. 1.80×10⁻⁴ mm³/(N·m). By introducing in-situ synthesized Ag₂S nanoparticles with different contents, the wear rate of the nanocomposite coating was significantly reduced. As the content of Ag₂S nanoparticles increased, the wear rate of the nanocomposite coating showed a similar trend to the friction coefficient, and the nanocomposite coating reinforced with 5.0% Ag₂S nanoparticles also displayed the lowest wear rate of 9.24×10⁻⁵ mm³/(N·m), which reduced by 47.78% than that of the pure PAI coating. These tribological performance test results showed that Ag₂S nanoparticles synthesized in-situ in the coating by this method can significantly improve the lubricating performance and wear resistance of the polymer coating, and there was an optimal addition amount to offer the best reinforcement effect.　The micro-scale morphologies of the worn surface and the wear scars of dual ball were observed and analyzed to explore the enhancement effects of in-situ synthesized Ag₂S nanoparticles to the tribological properties of the PAI coating. The friction contact area on the pure PAI coating surface was severely worn with a large number of large cracks and worn pits. The counterpart ball was worn more severely as the larger wear scar area, and no transfer film was formed on the counterpart surface. The Ag₂S nanoparticles introduced in-situ significantly inhibited the generation and propagation of defects and cracks on the worn surface, greatly slowed down wear damage of the nanocomposite coating. The worn surface became flat and compact, and the worn spot area of the corresponding dual ball was also reduced. It is worth noting that the nanocomposite coating material transferred at the friction interface, and a friction transfer film was formed on the corresponding dual ball. When less Ag₂S nanoparticles were added (less than 5.0%), the increase of their content can be more conducive to enhancing the wear resistance of the coating. The wear degree of corresponding worn surfaces decreased as the cracks and defects gradually reduced. When their content exceeded 5.0% and further increased, the worn surface cracks and defects of the coating intensified, and more obvious furrows appeared. This was caused by the agglomeration of the excessive Ag₂S nanoparticles. When the Ag₂S nanoparticle weight fraction was 5.0%, the nanocomposite coating exhibited the most excellent wear resistance, its worn surface was polished and crack defects were significantly reduced, and the corresponding worn spot area on the dual ball was also smaller, which was attributed to the improvement of mechanical strength of the coating and the formation of the friction transfer film on counterpart ball.