Additive Manufacturing (AM) technology makes possible the integrated forming of complex structures, widely used in the aerospace, biomedical, and other fields. Powdered metal and alloy are the most important factors in determining the performance of printed parts. Ideal metal powders need to be highly spherical, have good flowability, low oxygen content, and excellent interfacial bonding properties. Regrettably, raw powders can become oxidized and create agglomerates when stored, transported, and printed. Luckily, powder performance can be improved by surface modification.

Fig 1. The benefits of powder surface modification

How to Perform Surface Modification on Spherical Powder

Through coating and oxidation treatment, the powder's flowability, oxidation resistance, and interfacial bonding properties can be significantly enhanced.

Particle Coating Technology

Particle Coating Technology is the uniform metallization of spherical metal powder particles by a thin, dense film to improve the physical, chemical, or functional properties.

Purpose

• Prevent oxidation of metal powders such as aluminum, magnesium, and titanium during processing or storage.
• Improve flowability and improve powder spreading uniformity in powder metallurgy and 3D printing.
• Inhibit undesirable excessive sintering or promote low-temperature sintering.
• Functional modification, i.e., inducing magnetic properties, conductivity, catalytic function, or drug sustained-release capability.

Coating Methods

Particle coating can be achieved through physical and chemical methods. Among physical methods, fluidized bed coating technology and dry mechanical coating are useful with metal powders.

• Fluidized bed coating technology: This process primarily depends on the gas flow to fluidize and suspend powder particles and spray coating agents to form an even coating on the particle surface. Aluminum powder and magnesium powder, for example, can be coated with an anti-oxidation SiO₂ coating using this process. The evenness of the coating is excellent, especially suitable for micron-sized particles, especially suitable for industrialization.
• Dry mechanical coating: This method involves physical forces of high energy (high-speed mixing, ball milling) to physically adhere the coating material to the core particle surface. The only variation is that it does not involve solvents and is a solid-state process. This method is also quite common in industry. A few examples include copper-coated graphite for increasing conductivity, WC-Co hard alloy powders, etc.

Fig 2. Fine particles adhere to larger core particles

Chemical coating methods include:

• Chemical Vapor Deposition (CVD): Forms dense coatings on powder surfaces through gas-phase reactions, such as SiC, Al₂O₃, improving oxidation resistance.
• Physical Vapor Deposition (PVD): Deposits films on powder surfaces via sputtering or evaporation, suitable for nanoscale coating preparation.
• Sol-Gel Process: Forms oxide coatings through liquid-phase reactions, featuring low cost and suitability for mass production.

Fig 3. dense coating for powder surface modification

Nano-Coating Technology

Nano-coating technology refers to preparing 1-100 nm thick films or modified layers on substrate surfaces to endow materials with special properties. Compared with traditional coatings, its core features include:

• Ultra-thinness: Nanoscale thickness does not affect powder particle size and flowability;
• High specific surface area: Enhances surface activity and optimizes interfacial bonding;
• Designable functionality: Achieves customized properties (anti-oxidation, conductivity, wear resistance, etc.) by selecting different nanomaterials (ceramics, carbon materials, metals, etc.).

Physical Vapor Deposition (PVD), Chemical Vapor Deposition (CVD), Sol-Gel process, and electrochemical deposition are also methods for acquiring nano-coating technology. Atomic Layer Deposition (ALD) is, however, one of the most precise nano-coating preparation technologies available today.

Atomic Layer Deposition is a thin-film process that deposits material onto a substrate surface layer by layer in the shape of single-atom films. Unlike conventional chemical vapor deposition, ALD divides the complete chemical reaction into multiple half-reactions, thereby offering single-atom-level thin-film control.

Fig 4. Atomic Layer Deposition Process for Powder Surface Modification

Oxidation Treatment

Controlled oxidation is a technology of partial surface oxidation of powders under certain conditions to form a nanoscale oxide layer. Controlled oxidation technology is widely used for titanium alloy and high-temperature alloy powders in order to improve their high-temperature stability.

Titanium powder easily oxidizes in the presence of air to form a TiO₂ layer. However, the nanoscale TiO₂ layer may be a passivation layer. Thin TiO₂ layer effectively covers up oxygen diffusion, reduces powder oxygen increment (O₂ wt%), and can be used to prevent powder failure due to over-oxidation. Similarly, chromium-based powder naturally forms a Cr₂O₃ layer with high passivation capability that protects the substrate.

Fig 5. Cr₂O₃ layer

Effects:

• Forms a passivation layer to prevent further oxidation;
• Improves powder wettability and promotes melt pool homogenization;
• For high-temperature alloys, the Cr₂O₃ layer enhances thermal corrosion resistance.

How Do These Surface Modification Technologies Differ

Now you've read this, you might ask: Don't the three processes all coat up the powder? How are they really any different? Well, although all three technologies form surface layers, they are in fact quite different mechanisms, principles, and objectives of application.

First, their core purposes differ.

Second, their mechanisms differ essentially.

• Particle Coating technology "wraps" heterogeneous materials onto powder surfaces through external forces. The coating has no chemical bonding with the substrate, typically being micron-thick (0.1-10 μm). This method shows obvious layered structures and may contain pores.
• Oxidation treatment induces selective chemical reactions between the base metal and oxygen. The oxide chemically bonds with the substrate (e.g., Ti→TiO₂), with thickness at nanoscale (2-50 nm).
• Nano-coating uses any functional material (e.g., TiN, graphene) for gas/liquid-phase atomic-level deposition. The coating may form covalent bonds with the substrate, with precisely controllable thickness (1-100 nm) and can achieve uniform single-atom-layer coverage.

In terms of cost, nano-coating is the most expensive, followed by oxidation treatment, while particle coating technology is the most economical.

How to Choose for Surface Modification

Based on the above comparison, their differences are clear. The selection is straightforward:

• If your spherical powder is specifically for high-end AM and biomedical implants in precision fields, nano-coating technology is preferred.
• If precision requirements are moderate and for titanium alloys, high-temperature alloys, oxidation treatment is more appropriate.
• If mainly for conventional powder metallurgy and energy storage materials in cost-sensitive mass production scenarios, the economical particle coating technology is most suitable.

Stanford Advanced Materials (SAM) specializes in metal 3D printing materials. Send us an inquiry for more information.