March 2018 Archives

(1)Thermal spraying for improving heat resistance

This sounds like a low-tech example, but the steel chimneys are known to corrode when they are exposed to heat under approximately 550 °C. Corrosion occurs not because of the heat itself but for the air and moisture content combined.To form films that can withstand such conditions, aluminum spraying films of approximately 150 to 200 µm thick are applied by wire flame spraying, followed by sealing treatment using silicone-based materials.In the same manner, to achieve heat resistance up to 900 °C, sealing treatment using asphalt-based materials is applied after forming aluminum spraying films.
To withstand temperature of as high as 750 to 1000 °C without sulfur, sealing treatment using asphalt-based materials is applied after forming nickel-chromium alloy spraying films (200 to 300 µm).For the same temperature range conditions where sulfur is present, sealing treatment using asphalt-based materials is applied over nickel-chromium alloy spraying films (350 to 450 µm) + aluminum spraying films (80 to 130 µm).

(2)Thermal spraying for improving heat insulation

Thermal spraying is also used for preventing heat dissipation from the surfaces of machinery and equipment.For this purpose, thermal spraying films having low heat conductivity are used.Ceramics such as magnesium oxide, and zirconium-based materials containing calcium oxide, are the common materials.
To prevent the films from being peeled off due to the difference of heat expansion rates between the substrate and ceramics, spraying materials having enhanced oxidation and corrosion resistance at high temperature, such as nickel-chromium alloy and nickel-chromium-aluminum alloy, are adopted for the undercoat. In addition, cermet containing both of these materials can be sprayed between the undercoat and the ceramic coatings, as a measure of alleviating heat expansion.

(3)Thermal spraying for biomaterials

As the human life expectancy increases, more people start experiencing problems with body parts and organs.To substitute these, developments of artificial organs, such as heart, and artificial bones are becoming more popular than before.
Dentures are the typical example of artificial body parts that have been around quite a while.For a missing tooth, a bridge is mounted between the existing teeth using metals after drilling the surrounding teeth. A ceramic or dental alloy crown is placed over a chipped or discolored tooth after preparing the tooth to fit into it.
If no tooth is left, full dentures are attached over gum lines.If no root is left due to progressed cavities or periodontal diseases, artificial dental roots need to be formed in the gum.This is called implants.Materials for implants should not be harmful to human body. They also need to be corrosion resistant and be strong enough to withstand the biting force, while being compatible with other parts of the human body.
Implants are artificial tooth roots that are mounted on secure jawbones by drilling a hole in them.Eventually, the jawbones grow around the artificial roots and the implanted roots are secured in place.On the artificial roots, artificial teeth are attached with screws.Titanium is the common material for artificial roots.To improve the affinity with tissues, titanium powder and hydroxyapatite, a component of bones, are sprayed over the implants.Thermal spraying films are also used for artificial joints.

This volume introduces the mechanical application examples of thermal spraying.

(1)Thermal spraying for improving corrosion resistance

To add rustproof and anti-corrosion properties to large structures, such as bridges and steel towers, zinc, aluminum, and zinc-aluminum alloy are used for thermal spraying.In general, a thermal spray layer of as thick as 150 to 300 µm is applied after a pretreatment process consisting of sandblasting which is designed to clean and roughen the surface.
The humidity control is an important factor during this process. If the humidity is not below 80%, the corrosion resistance is significantly degraded.Sealing treatment must be applied immediately after thermal spraying (within four hours).This treatment not only seals tiny holes on porous films but also improves adhesiveness of the topcoat because of the anchor effect.
Coatings that are appropriate for the intended operating period are applied over the sealant.In this way, the recoating interval can be drastically extended by applying thermal spraying as an undercoat.

(2)Thermal spraying for improving chemical resistance

Various thermal spraying methods are adopted for applications requiring higher chemical resistance, such as chemical equipment.Spraying materials having high chemical resistance do not necessary form the films that are strong enough to withstand varying chemical agents. The materials should be evaluated by taking test pieces out and exposing them in the operating environment for a long period.
In general, metals or ceramics are adopted for films requiring high chemical resistance, which are processed by wire flame spraying or powder flame spraying.For large structures, spraying is applied on site.
One of the applications is to use nickel chromium, nickel aluminum, tantalum, or molybdenum as the undercoat over the sandblasted substrate and to apply ceramic spraying on the topcoat, followed by sealing treatment.Another method of the applications is to spray cermet, which is a mixture of undercoat metal and topcoat ceramics, between the undercoat and the top coat.
It is also possible to improve chemical resistance by re-melting the multi-layered films with heat and eliminating the pores.

(3)Thermal spraying for improving abrasion resistance

Abrasion occurs in various forms.However, these varying types of the abrasion, including adhesive wear, abrasive wear, fatigue wear, and corrosive wear, hardly occur by itself; one or more types of abrasion and wear are observed concurrently.When a certain degree of abrasion is expected, lubricants are used.
Since the spray coatings are porous, using oil will improve the lubricating properties.Commonly, spraying materials such as high chrome stainless steel and molybdenum are applied over the shafts and role shaft bearings by wire flame spraying or plasma spraying.
Besides this method, plasma spraying of various metals, ceramics, and cermet is widely used for the industrial purposes.

In Vol. 334 "Applications of Thermal Spraying", we introduced examples of thermal spraying adopted in various industrial fields. In this lecture, we will review the application examples of thermal spraying used in fine arts and crafts.

(1) Application to wood

Thermal spraying can be easily applied over wooden materials, such as woodcarvings and wooden signboards.Wood having rough surfaces or that is known to leak oil when being exposed to heat is not suitable for thermal spraying. However, using wood containing little oil and include the surface polishing process, they can be compatible with thermal spraying.The procedure is explained below.
First, apply zinc as undercoat for about 100 µm thick using wire flame spraying.Be careful not to burn the wood during this time.Next, spray the wood using metal as a top coat that will produce a desired color phase, such as aluminum bronze or brass, and deposit it for about 3 mm thick. Use a grinder, buffing, wire brushes, or apply chemical treatment as sulfidizing, to create metallic arts.They are absolutely creative and durable.

(2) Application to sculptures such as relief

Reliefs and shaped articles made of plaster or plastics can be easily transformed into metallic articles using thermal spraying.
To apply thermal spraying to a relief made of plaster, fill the bubbles on the surface first, and then apply a finely textured film made of aluminum or compatible materials for about 150 µm thick, using arc spraying.For plastics, use arc spraying to form a copper film first, followed by chemical treatment such as etching to produce polished look on the surface.

(3) Repairing casted statues

Statues are created by casting metals. However, even small flaws in casting will usually make the workpiece a defective item.Thermal spraying is one of the techniques used for repairing such flaws.
Casting flaws vary, but for build-up layers, the backside can be repaired by thermal spraying. Holes and areas that were not soaked in hot water can be repaired from the back using plaster or plastics, and by spraying metal for about 5 mm thick from the front. Post-processing is adopted for giving a special touch.

(4) Copying by thermal spraying

Zinc spraying is a technique used for copying the surface of an original model faithfully.The thermal spraying film capturing the original model is usually as thick as 5mm. The surface is extremely smooth and glossy.Since the spraying material for the second layer can be selected freely, it can be customized for intended purposes.
Using a relief made of plaster as the original model, metal reliefs having the same pattern can be replicated by wire flame spraying and arc spraying.Any material can be used for the original model as long as the heat conductivity is low.

The surface reforming of material by thermal spraying is applied in various fields in order to improve functionalities, including abrasion, heat, oxidation and corrosion resistance, as well as enhancing electrical conductivity, insulation, dimensional restoration, clearance control, and so on.
Conventionally, the most common thermal spraying methods were the zinc and aluminum spraying which is designed to add rustproof properties to steel structures, and the build-up spraying applied for restoring the worn areas of mechanical components. In recent years, thermal spraying methods are adopted more frequently for improving functionalities during the designing phase.
Taking advantage of the characteristics of spray films, thermal spraying is also popular in the high-tech developments, such as when adjusting the surface roughness or forming porous solid electrolytes.

[Table] Applications of thermal spraying

Ceramic spraying uses ceramics consisting primarily of aluminum oxide, chromium oxide, titanium oxide, or zirconium oxide as spraying materials, in the following methods: plasma spraying, powder flame spraying, and rod flame spraying.

(2)Powder flame thermal spraying films

[Table 2] shows the types and symbols of powder flame thermal spraying films.

[Table 2] Types and symbols of powder flame thermal spraying films