History

The origin of thermal spraying dates back to the early 1900s. The invention of a metallizing process by Schoop and Gunenther, 1917) is generally accepted as the date of the technology birth. Initially, the process had been used mainly for deposition of corrosión resistance zinc coatings, followed later by the spraying of hard metals. During the 1950s the broad application of new refractory materials (often related to aerospace industry needs) started and new techniques of termal spraying such as, for example, the D−GUN TM (Poorman et al .,1955) Th estudies of physical backgrounds in spreying processes were initiated in the 1960s (e .g . Engelke ,1962 ;MEyer ,1962∧1969 ;Matting∧Steffens ,1963 ; Scott∧Cannell ,1967). The 1970s witnessed the development of vacuum plasma spraying (VPS), credited to Muehlberger (1973). Finally, during the last decade, termal spraying technology bécame a widely recognized industrial technology. The development of the high velocity oxy-fuel technique by Browning Enginnering Corp. (1980) is another major achievement. Equally active was the research on new materials and on new applications. The accessibility of such modern optic devices as LDV (Laser Doppler Velocimetry) or two-colour fast pyrometry enable advance studies on the physical fundamentals of the spraying process to be carried out, especially investigations of heat and momentum transfer between flame and the sprayed particles (Vardelle et al .1980).

The future development of the technology is related to the economic requirements of the termal spraying market. An analysis of this market in USA, carried out by the Gorham Institute (1990a), shows that the total value of the market in 1990 in the USA s about 610-675 m SU$/ year and is expected to grow to 1800-2000 m US$/year by year 2000. Even if this growth will be less spectacular, due to the present recession, it is still very impressive, especially if one takes into account other dynamic economies impressive, especially if one takes into account other dynamic economies (European Community, Japan and other FAr East countries). This growth could mainly be attributed to new applications of the technology as well as the growing number of such applications which have already been used in prototypes or in a small series of products (automotive industry). The known techniques (especially APS and HVOF) will be further improved by making the spraying processes more repetitive, more user- and especially environment-friendly and less expensive. Finally, the advanced coatings resulting from the deposition of composite powders, from the application of the reinforcements with fibres, whiskers or particles are expected to be widely applied.

THERMAL SPRAYING TECHNIQUES

The selection and order of the thermal spraying techniques described in this chapter are base don the popularity techniques described in this chapter are base don the popularity in the industry rather than on the more academic classification. The description includes the history of the technique development, its principle and some details of the devices cosntruction (if available), the most important process parameters and such sprayed coatings properties as porosity and bond strength as well as their maximal thickness. The properties of these coatings are most important in the practical selection of the spraying technique to be applied. More details about the functional properties of coatings are given in Chapter 8.

In most spraying techniques the gases, combustión gases or plasma are the sources of the thermal and kinetic energy heating the particles and propelling them to the substrate. Only in laser spraying is the energy of photons emitted by a laser used to heat and melt the particles.

Most of the spraying techniques described are commercially available as industrial installations. The important exception is the D−GUN TM technique, which is only available as a coating service by the company PRAXAIR (previously Union Carbide Corporation) (Indianapolis, Indiana, USA.)

It was an arbitrary decisión to limit the detailed discussion to seven spraying techniques. The others are briefly discussed in the last section.

FLAME SPRAYING (FS)

Principles

POWDER FLAME SPRAYING

In the flame-spraying technique, sometimes called combustión flame spraying the chemical energy of combustión of fuel gas in oxygen is used to heat the material. The poder flame spraying torch is shown schematically in Fig 3.1.

The gases inlet is axial (1), the poder (2) can be introduced axially or perpendicularly to the torch (3). Wires or rods can also be used instead of poder(Smith, C. W., 1974) The particles are melted en the flame (6) and accelerated in the direction of the workpiece (4). Modern poder flame-spraying torch is shown in Fig. 3.2.

WIRE FLAME SPRAYING

In the torch used to spray wires or rods (see Fig. 3.3), the flame is generated by combustión of fuel gas (2) in oxygen (1). The flame melts the end of the wire/rod (4), wich is atomized by the compressed air (3) to form droplets. These are forming the stream (5) and successively propelled towards the substrate.

The modern wire/rod spraying torch is shown in Fig. 3.4.

Imágenes de la pistola de alambre

PROCESS PARAMETERS

FLAME

Oxygen to fuel ratio may vary from 1:1 to 1.1:1, resulting in carburizing to oxidizing the atmosphere respectively and the flame temperatura is in the range 3000-3350 K and its velocity is 80-100 m/s.

Fuel gas flow rate and pressure dependo n the torch type. Oxygen flow and pressure dependo n the torch type.

POWDER

Production method (See Chapter 1). Chemical composition. Physical properties (especially: density, melting point, latent heat of fusión, etc.). Shape of the particles: the spherical shape is desired, in order to facilitate poder

handling, and decrease spraying spot. Size distribution: the narrow distribution is desired in order to improve spraying

efficiency. Powder particles sizes: usually in the range 5-100 μm.

RODS OR WIRES

Chemical composition (roads are ceramic materials as Al2O3 ,Cr2O3 and wires are made of metals asMo, Zn∨Al).

Diameter ranges from 3 to 6 mm. The wire feed rate could vary from 80 to 650 g/min.

POWDER INJECTION

In order torches the poder was injected vertically to the torch axis, in modern torches injection is axial.

Powder feeders are gravitional in order torches. In modern installations, the same poder feeders could be used as in th APS installation.

The poder feed rate: vary from 50 to 100 g/min. Transport gas flow rate: 3-5 NI/min.

SPRAY CHARACTERISTICS

Spraying distance is in he range 120-250 mm. Angle of spraying: usually 90°, sometimes differs from the normal owing to shape of

the substrate. Spraying atmosphere: air. Post-spraying treatment: may include fusión of the coating (of suitable composition-

sprayed using self-fluxing powders containing nickel-chromium alloys with boron or silicon fluxing agents) in order to improve its density and the bond strength.

SUBSTRATE

Chemical composition. Surface preparation (see Chapter 2). Surface masking (see Chapter 2). Surface temperature: might be of importance while spraying ceramics on to metals

due to the generation of residual stresses. In this case the temperatura should be kept low (373-473 K). If spraying metal sor alloys this temperatura could rise to higher values, especially while melting the self-fluxing coating to 1000 K.

COATING PROPERTIES

The bond strength can reach 60 MPa for flame-sprayed NiAl coatings, or even 70 MPa for self-fluxing alloys. Typical values are in the range of 15 MPa for ceramics coatings to 30 MPa for other materials.

Porosity is in range of 10-20% (could be no-porosity al all for self-fluxing coatings) and typical thicknesses are in the range 100-2500 μm.

Applicactions of coatings

The different applications of thermally sprayed coatings grows steadily. The available hardware and consumables, the large number of trained development enginners, and technical maturity of many thermal spray techniques are the most important factors of this growth.

For obvious reasons it is not possible to cover all possible applications of thermal spray technology. The selection was made taking into account the industrial importance of the applications . Similary, priority was given to emerging applications, rather than the those already well known.

In this chapter case studies, describing the practical solution of a particular problem may have the following pattern:

Identifiaction of the problem (e.g. scuffing wear, abrasion etc.) Specification of coating properties Proposed solution, namely powder used to spray, spraying technique and eventually

post-spraying treatments. Evaluation of the results from technical and, if data are available, from an economical

point of view.

Such a procedure is used by R&D engineers while developing new coatings. On the other hand, for broad development areas, such as aero-engines and diesel adiabatic engines, the discussion concentrates on the technical achievements and eventual future development steps.

In some successful applications of the coatings, e.g. in marine gas-turbines of power gas-turbines, the technology transfer has ocurred from the manufacture of aero-engines. By analogy, such a transfer is likely to occur in other areas of coating applications.

Thermal spraying is not successful on pieces whit sharp angles. Sharp ends, etc. This is due to the high concentration of thermal stresses in the coating deposited in these places. That is why the pieces to be coated have to be machined prior to deposition. Some indications copncerning such a machining is shown. Fig. 9.1

Similary, the internal part of the holes can be sprayed with the use of typical torches if the ratio hole diameter/hole depth is greater than 1 (Kirner 1978). Otherwise, one of the specially

designed APS or HVOF torches being commercially available has to be used. If the diameter is very small, it is recommended to spray with an insert using free-standing body technology.

9.1 Chemical Industry

Tools in petrol search installation

Tools used in petroleum mining fail due to two-body and three-body abrasion, erosion and corrosion (Keshavan and Kembalyan, 1993) As the modifications of tool design reach their limit, more emphasis is put on the surface properties improvement. The following pieces were reportedly coated: