Preparation method of common TaC coated graphite parts

PART/1
CVD (Chemical Vapor Deposition) method:
At 900-2300℃, using TaCl₅ and CnHm as tantalum and carbon sources, H₂ as reducing atmosphere, Ar₂as carrier gas, reaction deposition film. The prepared coating is compact, uniform and high purity. However, there are some problems such as complicated process, expensive cost, difficult airflow control and low deposition efficiency.
PART/2
Slurry sintering method:
The slurry containing carbon source, tantalum source, dispersant and binder is coated on the graphite and sintered at high temperature after drying. The prepared coating grows without regular orientation, has low cost and is suitable for large-scale production. It remains to be explored to achieve uniform and full coating on large graphite, eliminate support defects and enhance coating bonding force.
PART/3
Plasma spraying method:
TaC powder is melted by plasma arc at high temperature, atomized into high temperature droplets by high-speed jet, and sprayed onto the surface of graphite material. It is easy to form oxide layer under non-vacuum, and the energy consumption is large.

 

Figure . Wafer tray after use in GaN epitaxial grown MOCVD device (Veeco P75). The one on the left is coated with TaC and the one on the right is coated with SiC.

TaC coated graphite parts need to be solved

PART/1
Binding force:
The thermal expansion coefficient and other physical properties between TaC and carbon materials are different, the coating bonding strength is low, it is difficult to avoid cracks, pores and thermal stress, and the coating is easy to peel off in the actual atmosphere containing rot and repeated rising and cooling process.
PART/2
Purity:
TaC coating needs to be ultra-high purity to avoid impurities and pollution under high temperature conditions, and the effective content standards and characterization standards of free carbon and intrinsic impurities on the surface and inside of the full coating need to be agreed.
PART/3
Stability:
High temperature resistance and chemical atmosphere resistance above 2300℃ are the most important indicators to test the stability of the coating. Pinholes, cracks, missing corners, and single orientation grain boundaries are easy to cause corrosive gases to penetrate and penetrate into the graphite, resulting in coating protection failure.
PART/4
Oxidation resistance:
TaC begins to oxidize to Ta2O5 when it is above 500℃, and the oxidation rate increases sharply with the increase of temperature and oxygen concentration. The surface oxidation starts from the grain boundaries and small grains, and gradually forms columnar crystals and broken crystals, resulting in a large number of gaps and holes, and oxygen infiltration intensifies until the coating is stripped. The resulting oxide layer has poor thermal conductivity and a variety of colors in appearance.
PART/5
Uniformity and roughness:
Uneven distribution of the coating surface can lead to local thermal stress concentration, increasing the risk of cracking and spalling. In addition, surface roughness directly affects the interaction between the coating and the external environment, and too high roughness easily leads to increased friction with the wafer and uneven thermal field.
PART/6
Grain size:
The uniform grain size helps the stability of the coating. If the grain size is small, the bond is not tight, and it is easy to be oxidized and corroded, resulting in a large number of cracks and holes in the grain edge, which reduces the protective performance of the coating. If the grain size is too large, it is relatively rough, and the coating is easy to flake off under thermal stress.