Technology for surface coatings

Coatings are specialized in certain critical surface property characteristic of each application or type of demand, in order to improve it. Some of these demands are the resistance to wear and oxidation, the coefficient of friction and the hardness, which are combined to achieve the coating suitable for their latest service conditions.

A key factor is the temperature of process; the direct relationship between the adhesion of the hard layer and the deformation of the same temperature is known. The phenomena of diffusion between the substrate and the hard parts that form on its surface cause a superior to that obtained in the process adherence to low temperature. The use of new materials with benefits exceeding the conventional basis has been supplemented with improved surface properties.

One of the main characteristics that must present the material to cover is the hardness or possibility to be hardened. All hard layer must be able to transmit efforts that is given to the base material without deformations occur in it. If the hardness of the substrate is insufficient, this layer hard and fragile, and therefore little deformable, breaks to come off as a result of the collapse of the core.

The application of coatings on a substrate allows to play, as in a composite material, with the properties of the layer with the base material. Generally wear-related properties are attributed to the coating and the mechanical properties provides the substrate.

Performed surface coatings are numerous and even so, each day you will discover new applications that bring some improvements in the manufacturing process from different points of view:

• Production in lots of parts, which applies a coating which gives the surface characteristics desired on a more economical substrate or with greater resistance.
• In the repair of large elements of machines whose manufacture is expensive, for being excessively worn or be poorly machined.
• In special and decorative applications.

The thermal projection includes a group of processes where a feed material is heated and designed as a single particle in the form of drops on a surface. Thermal screening equipment generates the necessary heat through combustible gas or electric arc. Materials are heated changing cast or plastic and are compressed and accelerated towards the substrate using compressed gas. Projected particles have an impact on the substrate of cold surface, crushing it and forming thin plates, which increase overlapping particles, adhering to the irregularities of the prepared surface and between themselves, producing structure coating sheet, which is not homogeneous and typically contains a degree of porosity and oxide in the case of projection of metals.

The feed materials may be metallic, ceramic, polymer or a combination of them in the form of powder, wires or bars; they can be applied under normal or special atmospheric conditions. Thermal projection systems are environmentally friendly technologies with environmentally friendly, quick to implement, flexible to technological and advantageous level on the issue of coatings of thickness of 100 µm.

The bond between the substrate and coating can be mechanical, chemical or metallurgy or a combination of both. The process to raise the temperature and melting the material is chemically (combustion) or electrically (ARC); This is achieved substantial improvements in the resistance to corrosion, the resistance to wear and high temperatures. Properties applied to coating or layer dependent on the material of feed, the process of thermal screening, applied parameters and the post-treatment.

-Projection by flame (Flame Spray or FS)

This process is the oldest form of projection thermal so-called 'process cold', concerning the material of the substrate is coated and is where the lowest temperature which remains without changes, changes-metalworkers and distortions. The material is fed continuously into the torch or pistol which is melted in a flame of combustion in the air of a mixture oxidant gas (oxygen or₂) and fuel gas (acetylene C₂H₂, hydrogen (h)₂ and propane (c)₃H₈)to produce a source of heat or flame, using compressed air to project and accelerate the particles in the form of drops molten on the substrate, where they flow together and solidify by providing a layer. The projection by flame is limited to materials that melt at temperatures higher than provided by the flame or materials that break down during the warm-up. Due to the temperature relatively low flame and the velocity of the particle, the layers are generally of lower quality, they have high porosity and low adhesive and cohesive forces, low resistance and limited thickness.

The projection by flame at high speed (High Velocity Oxy-Fuel Spray and HVOF Spray), developed in the early 1980s. Since then, it has become one of the most used techniques and in this last decade has been considerable interest in this technique for screening as a tool of formed parts. This process is based on the generation of a gas flow from high speed (> 1400 m/s) originated in combustion detonation in the interior of the Chamber of combustion of a mixture with high pressure oxygen O2 and fuel gas C3H8(, methane CH4, propylene 2HC = CH - CH3, ethylene C2H4, hydrogen H2, nitrogen N2, acetylene C2H2) or a liquid (kerosene), the mixture of gases is accelerated to supersonic speeds and feeding in the form of powder material is injected into the flame where changes to State cast or semifundido so that the projected particles reach the maximum energy Kinetic impacting against the substrate, forming layers or coatings of high quality with specific properties, extreme density (dense), internal cohesion and adhesion (high bond strength) to the substrate and low porosity.The thickness of layer extends from 0,000013 - 3 mm. A certain oxidation or reduction of low oxides, can occur because the heat source is obtained in less flame temperature, 3000 ° C, and particles by their speed spend less time inside.

Continuous combustion with a series of explosions or controlled intermittent explosions, which fuse the material of layer into particles and project it by impact with high kinetic energy to the substrate, to obtain by accumulation of layers is used in the projection by detonationa certain thickness. These layers are the result of the mixing of oxygen (O2) and fuel gas, acetylene (C2H2), along with the material of layer in the form of powder, commonly introduced or feeds in the Canyon, which consists of an outlet tubeat whose end closed or final is the combustion chamber, and the other is open, cooled by water and cleaned with nitrogen (N2) after each detonation. Depending on the used equipment, there are 1-15 detonations per second. The gas mixture is ignited by a spark in the interior of the House, a spark starts the detonation, resulting in the expansion of the hot gas, which melts and accelerates the particles of the material of the powder in a range of temperatures ranging from 1,100 - 19,000 ° C.

Because of the high impact speed, 600 m/s, in which are deposited materials commonly oxides and carbides, the torches of combustion and detonation Cannon can be used only on metal substrates. Procedure reaches 140 decibels higher levels of noise and, therefore, must take place in rooms to test sound and explosions. The typical layer thicknesses extends 0.05 - 0.5 mm, but they can be obtained from more thin and thick layers. Detonation gun is considered the first process of thermal screening at high speed, where the characteristics of the layers are less sensitive to the angle of deposition than other types of projections. The coatings produced by this method are of excellent quality, with high resistance to abrasion and high temperatures, but with a very high cost.

Aimme and the UPV within the Imoca/2006/51 file of the program of r & d, in cooperation of Impiva (Institute of the small and medium industry Valencian), carried out an investigation which was applied thermal projection by flame with use of dust by HVOFin order to find alternatives in the field of repair of matrices of drawing that are complementary with current technologies, guaranteeing the function for which were intended, before being consumed, during severe labour against the internal sliding operationsfriction, deformation and impact mainly on their edges or edges.

During the investigation carried out were considered various factors of which three variables were fundamental: the composition of coatings of powders micropulverizado; the different morphologies of step threads; and the temperature.

The presented work reported significant original contributions, which include:

• The demonstration of the practical applicability of the technique of thermal screening by flame in the resolution of the problem based on matrix of embuticiones coatings.
• The development of a methodology to carry out through the techniques of thermal screening by flame along a peripheral zone complete or partial and, in addition to bounded height coatings.
• How to use the technique of acoustic emission for the resolution of the problem of subject to shear faults in made coatings through the thermal projection by flame in the industrial processes of tooling applications.
• The usefulness of the statistics in all sorts of problem solving, in this case applied to the research of the characteristics of adhesion at the interface of the substrate with coating.

Bibliography

• Schedule of technological cooperation among centres of research and technology Impiva Imoca/2006/51. Development of ceramic coatings by heat projection nanometric to increase corrosion and wear resistance matrix.
• Vilana., José r., coating of surfaces by heat projection, Revista Habitat, 2002.
• Pawlowski, Lech, The Science and Engineering of Thermal Spray Coatings, 1995.
• Herman, H. and Sampath, S., Thermal Spray Coating, 1996.
• Guilemany C., José M. and Miguel, j. r., the thermal power projection in the engineering of surfaces: possibilities and applications, Conference on coatings and surface treatments of application in moulds and dies, CPT-UB, on June 26, 1997.
• Hermanek, Frank j., Thermal Spraying, 2004.
• I abello Linde, S. a., thermal projection, 2004, pp. 1 20.