Spray Metallization

"Thermal Spray Coatings"

Thermal spray coatings are surface coatings engineered to provide wear-, heat-, erosion-, abrasion-, and corrosion-resistant coatings for original equipment manufacturers and for the repair and upgrading of in-service equipment. Thermal spray coatings are produced from materials in either wire or powder form. The material is melted in a flame or heat source and projected onto the substrate to form the coating. If the coating is not self-bonding, the surface is first grit blasted. If the coating is a one-step combination self-bond and build-up coating, then it can be applied over a smooth, clean surface.

Original Equipment Manufacture Applications
Thermal spray coatings are used to create high performance parts that would be difficult or costly to produce by other means, to value-engineer new or existing parts by spraying premium material coatings over light-weight or low cost substrates, and to design combination material parts that use the best qualities of the sprayed coating and the structural member.

Rebuilt/Repair Application Advantages
There are advantages to using thermal spray coatings in repair. Parts can be rebuilt faster and at less cost with thermal spray coatings than by using welding, plating, or sleeving. New parts have to be ordered, thermal spray repair can save days of downtime, and the cost can run as low as 10% of replacement cost. A premium material can be used as a build-up coating, thus repaired parts will outlast new ones. This will also reduce spare parts inventories.

The Spray Process
Thermal spray coatings are produced from material in either wire or powder form. Thermal spraying processes deposit finely divided metallic feedstock surfacing materials in a molten or semi-molten condition onto a properly prepared, or grit-blasted, substrate to form a metallic coating. The metallic feedstock material is heated to its plastic or molten state and applied to the prepared substrate by one of three methods: Flame, Electric Arc, or Plasma Spraying. The material, via one of the methods, is accelerated toward the substrate. The particles or droplets strike the surface, flatten and form thin platelets that conform, adhere and interlock with each other and the prepared surface. As the sprayed particles impinge on the substrate, they cool and build up particle by particle into a lamellar-structured coating on the substrate.

The Thermal Spray Coating Bond
Depending on the coating material and the thermal spray method used, the bond to the base substrate may be mechanical, metallurgical, chemical, or a combination of the three.

Most thermal spray coatings are applied directly to the substrate. If the coating is not self-bonding, the surface is first grit-blasted. If the coating is a one-step combination self-bond and build-up coating, then it can be applied over a smooth, clean surface.

Proper surface preparation of the substrate is a critical factor which influences bond strength of the coating. Other factors enhancing bond strength include the type of material being sprayed, the particle size, particle impact velocity, particle temperature, substrate roughness, and substrate surface temperature before, during, and after spraying.

Surface preparation serves two purposes. It removes any contaminants which would interfere with proper bonding of the thermal spray coating and provides a roughened surface which enhances thermal spray coating adhesion.

Thermal Spray Methods
Wire Flame Spraying

The wire flame spraying process feeds the wire into an oxygen-fuel gas combustion flame, melted and then atomized by a blast of compressed air, and projected by the compressed air over a prepared substrate. Types of wire used: Sn/Sb/Cu, Babbitt, Sn/Zn, Tin/Zinc.

Wire Flame Spraying Advantages:
1. All purpose spraying.
2. Coating can be applied rapidly and at a low cost.
3. Sprayed metal is usually harder than the same metal in wrought form and has higher lubrication holding properties.

The materials utilized are Lead, Tin, Cadmium, Nickel, Babbitt, Zinc, Brass, and Bronze.

Powder Flame Spraying
In powder flame spraying, the powder is fed into an oxygen/fuel gas combustion flame, melted and projected by the gas stream onto a prepared substrate. The powder is fed into the gun by gravity or pressurized feed. Types of material used - Aluminum Oxide, Nickel Graphite, Plastic-base powders, and babbitt (Sn/Sb/Cu).

Powder Flame Spraying Advantages:
1. Applies a wide range of coated materials.
2. Well suited for applications requiring protection against heavy wear, corrosion, heat, oxidation, and electrical conductivity.

The materials utilized are Aluminum Oxide, Nickel Graphite, Plastic Base Powders, and Babbitt.

Electric Arc Spraying
Electric ARC spraying brings 2 electrically charged wires together and an ARC is struck between them. Compressed air atomizes the molten material and projects it onto a prepared substrate. Types of materials used for this process are Carbon Steel, Aluminum, Stainless Steel, Zinc, and Babbitt.

Electric Arc Spraying Advantages:
1. Excellent for applications that require heavy coating build-up or for large surfaces.
2. Can be sprayed at extremely high rates of speed.
3. You can adjust coating characteristics, such as coating hardness or surface texture. The materials utilized are Carbon Steel, Aluminum, Babbitt, Stainless Steel, and Zinc.

Plasma Spraying
Plasma spraying is a process where powder is fed into a heat source created by using high voltage electric arc to ionize a plasma-forming gas. The melted particles are projected at high velocity by the plasma gas stream onto a prepared substrate. Typical materials are self-bonding Steels, Aluminum Bronze, and Tungsten. Industries that utilize spray metallization are agriculture, construction, glass, transportation, etc.

Plasma Spraying Advantages:
1. Plasma coatings have higher integrity and give better in-service performance than other coatings.
2. Spray rates are higher, so labor costs are generally lower.
3. Coatings have higher hardness to resist abrasive grains and hard matting surfaces better. The typical materials utilized are Self-Bonding Steels, Aluminum Bronze, and Tungsten.

Material Selection
Once the wear problem has been accurately diagnosed, the service conditions have been assessed, and the process is chosen, a material must be selected on the basis of several conditions. Considerations are machinability, hardness, cost, corrosion-resistance, wear-resistance, application method, bond strength, and conductivity.

Benefits of Fry Grade 2 Babbitts
No laminations that would cause deposition problems. Non-splitting, non-flaking, virtually weld-free surface that provides trouble-free machine feeding. Lead-free composition - meets all federal and legal guidelines. Exceeds ASTM B23 Grade 2 specification - tighter impurity levels specifically for spray metallization. Fry specification produces a soft, pliable wire - for ease of machine feeding. Homogenous structure and tight wire diameter - provides even feeding and flame deposition.

Benefits of 100% Sn
Excellent corrosion resistance
Tin protects by sealing
Low melting point (450°F)
Protects by sealing
Temperature characteristics