Vacuum plasma spraying of titanium (Ti) and hydroxyapatite (HA) coatings; spraying titanium.

Compared with atmospheric plasma arc spraying, vacuum plasma arc spraying features a faster jet velocity, lower temperature, and controllable atmosphere within the spraying chamber. The coatings produced by this method are more dense, have lower oxygen content, and closely match the composition of the powder used.

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Coating spray processing

Thermal spraying of various functional coatings

Tungsten Carbide Spraying and Processing

Plasma spraying processing

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Product Description

Vacuum Plasma Spraying

1. Compared with atmospheric plasma arc spraying, vacuum plasma arc spraying features a faster jet velocity, lower temperature, and controllable atmosphere within the spraying chamber. The coatings produced by this method are more dense, have lower oxygen content, and closely match the composition of the powder used. Vacuum plasma spraying technology can be employed to fabricate high-performance Ti and HA coatings.

2. The vacuum spraying equipment mainly consists of: a vacuum control cabinet, a vacuum tank, a mechanical vacuum pump, a Roots vacuum pump, a dust filter, a cyclone dust collector, and various pipelines, solenoid valves, and sensors.

3. Introduction to some medical applications:

For example, vacuum plasma arc spraying of Ti and HA coatings onto artificial joint surfaces has enabled the clinical application of plasma-sprayed titanium (Ti) and hydroxyapatite (HA) coatings deposited on Ti or its alloy substrates. Plasma arc spraying is a high-temperature process during which the powder undergoes relatively complex physical and chemical transformations. Compared to the original spray powder, plasma-sprayed HA coatings experience changes in both their microstructure and composition, leading to a reduction in coating crystallinity. At temperatures above 500℃, Ti readily reacts with O2, H2, N2, and CO2. Given the extremely high temperature of the plasma arc jet—reaching up to 10,000℃—when plasma arc spraying is performed in an atmospheric environment, Ti will react with the surrounding atmosphere. The reaction products formed during the spraying process often reduce the ductility of the coating and can lead to cracking in the material.

Ti and HA coatings were prepared by vacuum and atmospheric plasma arc spraying on a Ti-6Al-4V substrate. A comparison between the two coatings revealed that the Ti coating produced by atmospheric spraying exhibited severe oxidation, with significant amounts of TiO and TiO2 present in the coating. In contrast, oxidation was effectively avoided during vacuum spraying. The oxidation of the coating adversely affected its bonding to the alloy substrate: the bond strength between the atmospheric plasma-sprayed (APS) Ti coating and the substrate was approximately 37 MPa, whereas the bond strength of the vacuum plasma-sprayed (VPS) Ti coating reached as high as 60 MPa. Compared to the original powder, the plasma-sprayed HA coating contained some amorphous phases and new phases, such as CaO and Ca3(PO4)2. The formation of these amorphous phases was primarily due to the rapid quenching of particles melted at high temperatures when they were sprayed onto the substrate; while the new phases like CaO and Ca3(PO4)2 originated from the decomposition of HA during the spraying process. The vacuum-sprayed HA coating showed a marked reduction in both amorphous phases and new phases, largely because the lower flame temperature used in VPS spraying resulted in improved crystallinity. Crystallinity analysis indicated that the APS HA coating had a crystallinity of only 42%, whereas the VPS HA coating exhibited a crystallinity exceeding 60%, meeting the clinical requirements for coating crystallinity. Figure 2 shows actual components of artificial hip joints coated with Ti-HA using vacuum plasma spraying. Approximately 7,000 coated artificial hip joint implants have already been produced.

HA-based composite coatings were prepared on a Ti-6Al-4V alloy substrate using plasma arc spraying technology. Composite coatings such as HA/Ti, HA/TiO2, and HA/ZrO2 were fabricated, which can effectively enhance the bonding between the coating and the substrate.

　 Combination strength analysis indicates that incorporating components such as metals or ceramics into HA coatings can significantly improve the coating's bonding performance. Taking the HA/ZrO2 composite coating as an example, the combination strength of the composite coating with ωZrO2 = 60% is approximately twice that of the pure HA coating. The enhancement in combination strength is primarily attributed to the fact that the addition of dopants alleviates the mismatch in thermal expansion coefficients between the coating and the substrate, thereby reducing the concentration of residual thermal stresses at the interface caused by this mismatch. Additionally, the higher intrinsic bonding strength of the dopants themselves is another important factor contributing to the improved combination strength of the composite coating.

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