Study On Microstructure And Properties Of Cold Sprayed Al-Based Metallized Coating On Peek Substrate

The ever increasing demand of lightweight components in aviation,aerospace,automobile,power electronics,petrochemical,energy and other sectors has promoted the application of polymers.PEEK is one of the most frequently used semi-crystalline aromatic thermoplastic polymer due to its excellent mechanical properties,tribological properties,electrical properties,biological properties,thermal stability and chemical stability.It has broad applications in various areas of the national economy.However,low electrical and thermal conductivity and poor erosion wear performance limit the further replacing of metal parts by PEEK materials.In this context,deposition of a metallized coating on PEEK substrate seems a viable solution to improve its surface properties and expand its application range.Cold spray technology is a promising solid-state deposition technology,which can be used to prepare a metal coating at a temperature far below the melting temperature of the feedstock.Further,cold spray technology possesses many advantages when compared with other surface metallization methods（e.g.physical vapor deposition（PVD）,chemical vapor deposition（CVD）,electroless plating and thermal spray technology）,such as high deposition rate,low equipment and use cost,effluent-free and controllable heat input to the substrate and so on.Moreover,Al has excellent corrosion resistance due to the dense oxide film that can be naturally formed on its surface.These characteristics make Al a remarkable candidate for electrically and thermally conductive coatings on the surface of lightweight PEEK parts.In this dissertation,Al-based coatings were prepared on the PEEK substrate by cold spray.In order to overcome the problems of insufficient electrical conductivity and adhesion strength of cold sprayed metallized coating on polymer substrate,the microstructure and properties of metallized coatings were regulated by optimizing the process parameters,adjusting the feedstock design and laser post-processing.Finally,feasible methods for improving the electrical conductivity and adhesion strength of Albased coatings on PEEK substrate were proposed,which provide a foundations for the application of cold spray technology in polymer surface metallization.The main contents are as follow:（1）The process parameters of cold sprayed Al-based coating on PEEK substrate were optimized.The results show that,the properties of the cold sprayed Al coating,such as deposition efficiency,density,adhesive strength,electrical conductivity and thermal diffusivity were improved with the increase in accelerating gas temperature.A dense and thick Al coating was built on PEEK substrate at the accelerating gas temperature of 380℃ in just two nozzle passes.Its electrical conductivity and adhesion strength were 16.0 MS/m and 4.6 MPa,respectively.（2）The bonding properties and deposition behavior of cold sprayed Al coating on PEEK substrate were studied.For the first time,the chemical bonding（Al-O-C）between the cold sprayed metal coating and polymer substrate was confirmed by TEM,FTIR and XPS.The deposition process of cold sprayed Al coating on PEEK substrate can be summarized as follows:building up mechanical interlocking/chemical bonding→bonding between Al-Al splats→building up a dense coating.（3）The qualities of metallized coating were improved to a certain extent by adding hard（i）Al₂O₃（irregular Al₂O₃）particles into feedstock.On the one hand,the ’in-situ tamping effect’ of（i）Al₂O₃ particles on the pre-deposited layer increased the deformation degree of Al particles,which improved the bonding between Al-Al particles and reduced defects（e.g.pores,microcracks）in the coating.On the other hand,the Al₂O₃ particles embedded in the coating/substrate interface were beneficial to the adhesion of the coating.However,the excessive retention of non-conductive Al₂O₃ in Al-（i）Al₂O₃ coating and the Al₂O₃ fragmentation phenomenon limited the further improvement of the electrical conductivity（18.9 MS/m）and adhesion strength（6.4 MPa）of metallized coating.（4）To overcome the limits of（i）Al₂O₃ particle addition method,（s）Al₂O₃（spherical Al₂O₃ with large diameter）particles with size of 80 μm were added into feedstock as hard phases.The results show that,（s）Al₂O₃ particles had a more significantly "in-situ tamping effect",which further improve the bonding between AlAl particles and interlocking effect at coating/substrate interface.In addition,（s）Al₂O₃ retention efficiency in middle coating was very low due to rebound during the cold spray process.Therefore,Adding an appropriate amount of（s）Al₂O₃ particles into feedstock could further improve electrical conductivity（23.7 MS/m）and adhesion strength（11.2 MPa）of Al-based metallized coating,which was improved by 48.1%and 143.5%compared with Al coating.（5）The post-treatment technique of metallized coating on the polymer surface was explored.The microstructure and properties of the Al-based coating on PEEK surface were further regulated by laser post-treatment.Due to the heat input was concentrated on the coating side,laser post-processing process basically eliminated the defects with different scales in the metallized coating and obtained a nearly complete dense Al-based coating.At the same time,PEEK substrate was partially melted by controlling the heat input on interface,which enhanced the chemical bonding between coating/substrate and greatly improved the adhesion of the metallized coating.By this process,the conductivity and adhesion strength of the metallized coating can be further increased to 24.5 MS/m（69%of the bulk Al）and 15.7 MPa,which are 53.1%and 241.3%higher than that of the as-sprayed Al coating.In this dissertation,the Al-based coatings with high conductivity and high adhesion strength were prepared,which have wide application prospects in lightweight fuselage parts,lightweight and efficient electromagnetic shielding parts,electronic conductivity parts and so on.