Process parameters of plasma spraying

       1. Arc plasma



    ·Working gas composition: typical gas is Ar or Ar+H2, Ar+He and Ar+N2 mixed gas, sometimes N2, and N2+H2 mixed gas, usually the gas flow is about 40~50L/min, but some equipment Can reach 80L/min. High-energy spray guns can even achieve gas flow rates of 500 L/min (Jungklaus et al., 1996). Some equipment manufacturers recommend compositions of three gases, eg, Ar+He+H2 or Ar+He+N2. In a plasma jet, each gas has its own role (Janisson et al., 1999):

    Arcs in Ar-stabilized nozzles: He, N2 or H2 have high thermal conductivity, enhancing heat transfer to particles.

    Ternary mixed gas has been available from the market, such as Spral 22TM, Air Liquide. In a water stabilized plasma spray gun, the water vapor forms the plasma gas, and the water consumption is about 5L/h WSP 500, 1997. Reactive plasma-forming gases such as hydrocarbons or carbon dioxide can also be used. But to produce such an active plasma torch, graphite must be used as the cathode (Fridlyand, 1995). The arc current is several hundred amperes, and the voltage depends on two main parameters:

    - the distance between cathode and anode, the longer the distance, the higher the voltage;

    - The type of auxiliary gas (diatomic gas, such as hydrogen, which increases the voltage).

    Plasma spray guns usually operate at a voltage of 30~70V, such as Sulzer Metco's F4 spray gun or PraxairST's SG100 spray gun. These guns generate plasma temperatures around 14,000K, and jet exit speeds of up to 800m/s, water-cooled plasma guns have been reported to reach temperatures of 28,000K, and high-energy plasma arc spray systems can reach speeds of 2900m/s (Plazjet, 1995). The gas mainly affects the degree of melting of the spray particles, the latter being more easily melted when using molecular gases because molecular gases such as water vapor (water-cooled plasma guns) and hydrogen (conventional plasma guns) have higher thermal conductivity than monatomic gases. high. On the other hand, monatomic gases can reach high velocities, which is why mixtures of monatomic gases and molecular gases are usually used for better particle melting and high spraying speed. The monatomic gas He is also used as an auxiliary gas due to its high thermal conductivity and the formation of very narrow spray jets (Ingham and Fabel, 1975).

    ·Power supply, the conventional manuscript spray gun can reach 80kw, but the water-cooled spray gun can reach 200kw (WSP 500, 1997), and the high-energy plasma spray gun can even reach 250kw (Plazjet, 1995).

    · Electrode shape, the cathode head has different shapes for different working gases. The size of the anode affects the shape, temperature and velocity of the plasma flow. For example, the use of convergent-divergent anode nozzles has been reported to reduce arc fluctuations (Schwenk et al., 2003). The arc in contact with the cathode and anode at one or more locations (the root of the arc) can cause wear and tear (Fisher, 1972). In some applications, such as on anilox rolls, it is necessary to monitor this damage to reduce damage caused by the cathode or anode. Metals that transition to coatings. Electrode wear is caused by the high heat flux density in the arc (10 8 W/m2) (Fridlyand, 1995).

    • Stabilization of the plasma jet is usually achieved by using a sheath or vortex of the working gas. Water stabilized plasma spray guns have also been commercialized. (WSP500, 1997)

    2. Powder

    ·Particle size is usually in the range of 20~90υm

    Powder type: The most commonly used are oxide ceramics.

    3. Powder injection

    ·Injection direction:

     - Radial powder feeding, which is the direction of powder feeding in conventional plasma arc spraying, can be placed outside the cathode-anode (see Figures 3-5 and 3-6 and Sulzer-Metco's F4 gun), or inside (iii) (eg PraxairST Corporation SG100 spray gun).

    - Axial powder feeding, to be discussed later, three-cathode spray guns such as Axial IIITM.

    • Injection angle, usually 90 degrees, although 60 degrees backward or 60 degrees forward is sometimes used.

    • Powder feed ports, usually only one, although sometimes two or three are used, and multiple powder feed ports can be used to spray composite coatings.

    · Feeder, usually a rotary scraper type.

    ·Powder feeding rate, usually 50~100g/min, but can reach 200g/min in HPPS. It has been reported that the water stabilized plasma spray gun (WSP 500, 1997) can reach 1650g/min.

    ·The amount of carrier gas is between 3~10L/min.

    4. Main process parameters

    ·Spraying distance: 60~130mm.

    ·The spray atmosphere is normal atmosphere.

    ·The scanning speed of the spray gun relative to the substrate is between 50~2000mm/s.

    Substrate temperature, which is an important parameter when spraying ceramics on metal substrates due to residual stress (the compressed air cooling nozzle in Figure 3-6). In this case, the substrate temperature needs to be kept between 373~473K.

    ·Post-treatment, usually used to improve coating density and other properties, especially worth mentioning: furnace treatment; laser treatment; organic or inorganic sealing agent sealing; discharge plasma treatment.