Micro Process and Quality Control of Plasma Spraying

Plasma is an ionized gaseous substance composed of negatively charged electrons, positively charged nuclei and atomic clusters after gas ionization. Plasma is conductive because it contains many high-energy free charges.

Plasma spraying consists of three steps of energy conversion. The first step is the conversion from electric energy to internal energy during ionization of working gas under high voltage; the second step is the form of heating transfer, mass transfer and accelerated process after the spray particles interact with the jet; and the final is the energy collaborative dissipation in the spreading and solidification process of molten particles impacting the substrate at high speed.

According to the transfer phenomenon of momentum, heat and mass occur in the interaction process between spraying particles and the plasma jet flow. When they reach the substrate surface, sprayed particles spread and rapidly solidify to form the most basic unit of the coating, a flattened particle called the “splat.” From a macro perspective, the whole of the coating is usually formed when a large number of splats accumulate as particles overlap one another and stack layer by layer. Therefore, it is the spreading and solidification behavior of such flattened molten droplets and the statistical rule of stacking with multiple molten droplets that directly determines the quality of coating (which consists of factors such as content of defects, residual stress, microstructure, mechanical properties, etc.). It was learned from a large number of spray trials that after entering the plasma jet flow, sprayed particles usually have different flight characteristics due to their differences in path, jet characteristics (temperature, velocity, component feature, etc.) and intrinsic particle structure (particle size, material properties, geometric structure, and phase composition). These differences may significantly affect the deposition behavior of a single droplet after it has hit the substrate.

Although coatings that are fabricated with supersonic plasma spraying technology have many advantageous properties, coating is in essence a heterogeneous material formed after sprayed particles are heated and melted at high temperature then spread and solidified on a substrate with the help of high-speed air flow acceleration, and this process inevitably leads to some internal and contact surface problems for the coating.

The preconditions for stable performance of plasma spraying coating are a high bonding strength with the substrate and a low content of defects such as pores, oxidation inclusions, and unmelted particles in the coating itself. Therefore, process modification is needed before and after coating preparation to improve the service performance and service life of coating. In this chapter, typical quality control methods in the preparation and optimization of coatings were introduced from three aspects, i.e., pretreatment, spraying process optimization, and afterprocessing.

The micro processes such as heating, acceleration, impact, spreading and solidification of spray droplets in plasma jet can guide the optimization design of coating more scientifically. Taking typical ceramic, metal and cermet coatings such as BaTiO₃, WC, Fe based amorphous and YSZ as examples, this chapter introduces in detail how to realize rapid optimization of coatings from the micro process.

Due to the excellent coating quality, wide adaptability, convenient process flow, and low manufacturing cost, plasma spraying technology is widely used in strengthening the basic properties of part surfaces, such as wear resistance, corrosion resistance, and high-temperature resistance. This technology also has a broad range of applications that includes super hydrophobicity, stealth absorption, and electric conduction and insulation empowerment.