Application of laser in post-processing of bipolar plate coating

PEMFC bipolar plate materials mainly include three categories: graphite materials, composite materials and metal materials. Graphite bipolar plates have good conductivity and are easy to process, but the material is brittle, has poor mechanical properties and low processing efficiency, making it difficult to achieve commercial mass production.

Composite bipolar plates are made of carbon powder and resin as the main raw materials and are prepared by molding and other methods. They are low in cost, but composite bipolar plates still have problems such as conductivity and gas permeation.

Metal bipolar plates have high strength and electrical and thermal conductivity. They can be produced by mass production methods such as metal sheet stamping and rolling. They are recognized as the first choice for commercialization of fuel cells.

In terms of metal bipolar plates, since fuel cells operate in an acidic environment, coupled with electrical and thermal conditions, fuel cell bipolar plates will corrode in a very short time. Therefore, preparing a coating on the surface of the bipolar plate becomes a feasible solution.

The fuel cell bipolar plate coating is deposited using magnetron construction technology, generally including a transition layer and a surface functional coating. Magnetron sputtering nanoparticles generally range from tens of nanometers to one or two hundred nanometers. This is a unique phenomenon of magnetron sputtering.

After the coating particles are piled up, different gaps will be formed. In the high temperature, high acid and high current environment of the fuel cell, the hydrogen ions and fluorine ions generated by the degradation of the perfluorosulfonic acid resin will penetrate into the substrate through the gaps between the particles, causing corrosion of the transition layer and ultimately functional peeling and failure. This is the main form of failure of the double substrate coating.

Failure mechanism

Columnar crystals in physical vapor deposition

Coating peeling failure

A new technology for heat treatment of the surface of metal materials using the thermal effect generated by the high energy of the laser beam. The working process of this technology is: irradiating the surface of the part with a laser can heat it to above the critical phase change temperature. After removing the laser beam, the surface quickly cools and quenches itself.

This has achieved significant results in improving the wear resistance, corrosion resistance, fatigue resistance and impact resistance of the metal surface. The advantages of laser treatment are that it is pollution-free and belongs to local surface treatment, with low pressure and small deformation, so it has broad application prospects.

Laser heat treatment technology

When the laser power density is low (<10^4w/cm^2) and the irradiation time is short, the laser energy absorbed by the metal can only cause the temperature of the material to rise from the surface to the inside, but maintain the solid phase unchanged. It is mainly used for part annealing and phase change hardening treatment, mostly tools, gears, and bearings; with the increase of laser power density (10^4~10^6w/cm2) and the extension of irradiation time, the surface of the material gradually melts, and with the increase of input energy, the liquid-solid phase interface gradually moves to the deep part of the material. This physical process is mainly used for surface remelting, alloying, cladding and thermal conductivity welding of metals.

Further increase the power density (>10^6w/cm^2) and extend the laser action time. The material surface not only melts, but also vaporizes. The vapor gathers near the material surface and weakly ionizes to form plasma. This rarefied plasma helps the material absorb the laser. Under the pressure of vaporization expansion, the liquid surface deforms to form pits. This stage is used for laser welding, generally in micro-joints within 0.5mm.

Compressive stress during physical vapor deposition

When the laser is used to irradiate the stainless steel surface, the coating is heated to a molten state by the high temperature generated by the laser instantaneously, and then quickly cooled. After melting, the gaps between the particles are reduced, forming a structure similar to a solid solution, which can prevent hydrogen ions and fluorine ions from penetrating into the substrate.

Second, after high-temperature melting treatment, the coating can form a solid solution with the substrate, improving the bonding strength between the coating and the substrate. Especially for stainless steel substrates, the poor bonding strength between the substrate and the coating is a prominent problem. Laser treatment can effectively improve the bonding strength of the coating.

Third, laser irradiation can also reduce the compressive stress formed inside the coating during magnetron sputtering. Through high-temperature heat treatment, the stress inside the coating can be released and the life of the coating can be improved.

Fourth, laser irradiation heat treatment can form a quenching-like effect on the bipolar plate. Improving the strength of the bipolar plate after forming is beneficial to improving the strength of the bipolar plate, especially when the substrate of the bipolar plate of the fuel cell is getting thinner in the future. It provides convenient conditions for the use of 0.075mm or even 0.05mm substrates.

Improvement of coating particle gap by laser heat treatment

Laser treatment of bipolar plate coating has obvious advantages. How to increase the speed of laser treatment is an engineering problem that needs to be solved. There are many bipolar plates and a large area. Fast, low-cost and high-quality processing is the premise for large-scale application in engineering. I believe that we will see more application cases of laser in coating treatment in the future.