Cultivate the blue land! Use the "materials" of a great country to support the "deep blue dream" of the ocean

Produced by: Science Popularization China

Author: Zhang Meiqi (Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences)

Producer: China Science Expo

In November 2020, the "Fendouzhe" manned submersible independently developed by Chinese scientists successfully completed a 10,000-meter deep dive in the Mariana Trench, gradually revealing the mystery of the ocean.

The "Struggler" is filling up with water and diving

(Image source: Institute of Deep Sea Science and Engineering, Chinese Academy of Sciences)

As we all know, the composition of seawater is very complex , containing a large number of cations such as Na+, K+, Ca2+, Mg2+ and Sr2+; anions such as Cl-, SO42-, Br-, HCO3- (CO32-) and F-, plus the huge hydrostatic pressure of the deep sea . The core moving parts of the submersible exposed to seawater face serious wear and corrosion damage problems, which threaten the safety of the submersible at all times . The damage and failure of components mainly come from the surface and subsurface. Through advanced surface engineering technology, putting on a magical "coat" for key moving parts is an effective way to solve the problem of surface damage and failure, which cannot be delayed.

There are three main surface engineering technologies commonly used at present. One is coating technology such as thermal spraying and electroplating . However, due to poor bonding with the substrate, rough surface, and difficult to control coating structure and thickness, they are not suitable for precision moving parts of submersibles. The second is to use electrochemical methods for cathodic protection, that is, to protect the cathode with external current or sacrificial anode. The third is physical vapor deposition (PVD) coating technology, which is a modification technology that generates plasma glow discharge in a closed vacuum environment to form a highly dense, smooth film on the target product. It has the advantages of being green and pollution-free, low deposition temperature and wide applicability. It can significantly improve the hardness, corrosion resistance, oxidation resistance and wear resistance of key components, and is widely used in major national equipment.

Submarine in the deep sea

(Photo source: Veer Gallery)

The materials of a great nation come first! After choosing the PVD surface coating technology, what kind of materials should be chosen to weave this strong and integrated outerwear?

In 1971, scientists successfully synthesized amorphous carbon film materials. You may not know amorphous carbon, but you may be very familiar with diamond and graphite.

Diamond and graphite are both carbon materials, but the atomic orbitals in the microstructure of diamond are sp^3 hybridized, while the atomic orbitals of graphite are sp^2 hybridized, resulting in completely different properties between the two.

Amorphous carbon materials, like graphite and diamond, belong to the carbon family of materials, but they have both the high hardness sp^3 hybrid structure of diamond and the high toughness sp^2 hybrid structure of graphite, and have the advantages of high hardness, friction resistance, and corrosion resistance. According to the different orbital hybridization contents, scientists divide it into diamond-like carbon (DLC, sp^3 hybridization content greater than 50%) and graphite-like carbon (GLC, sp^2 hybridization content greater than 50%). Compared with the harsh conditions such as high temperature and high pressure required for the preparation of diamond, amorphous carbon materials have low preparation temperatures and are easy to produce .

Schematic diagram of graphite layer structure

(Photo source: Veer Gallery)

Schematic diagram of diamond structure

(Photo source: Veer Gallery)

It has been half a century since the discovery of amorphous carbon materials, but scientists have an indelible passion for it because of its special properties. Based on our previous deep-sea hanging experiments, in the corrosion test at 5,000 meters in the deep sea, the stainless steel material was rusted, but the amorphous carbon film made by PVD technology still maintained good performance and the surface was as bright as before.

Amorphous carbon protected plunger in deep sea hydraulic system

(Image source: Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences)

In addition, due to its excellent performance, amorphous carbon materials are also widely used in other fields. For example, in hydrogen proton exchange membrane fuel cells, as a protective coating for bipolar plates, it has good conductivity and corrosion resistance; in the medical field, it is competent for the protective coating of artificial joints with its good tribological properties and excellent corrosion resistance.

It is believed that in the future, amorphous carbon materials will break through bottlenecks such as controlling the interface between the film and the substrate and extending life, and continue to shine in all walks of life.

Medical 3D knee prosthesis

(Photo source: Veer Gallery)

Academician Xue Qunki said: "At present, the world is undergoing major changes unseen in a century, and the ocean is the most important area of ​​international competition. From near seas to far seas, from deep seas to polar regions, there are challenges everywhere." Deep-sea exploration, as an important part of ocean exploration, will continuously promote the innovation and development of my country's manned deep-sea diving, assist in the implementation of my country's strategy of building a strong maritime nation, and greatly enhance our national self-confidence and pride.